JP4885995B2 - Dispersion compensator - Google Patents

Description

  The present invention relates to a dispersion compensator used in optical fiber communication.

  With the explosive spread of the Internet, wavelength division multiplexing (WDM) communication is moving from a conventional point-to-point system to a ring-mesh system. This is because a ring mesh type system can flexibly cope with changes in communication demand between nodes by using a transparent wavelength selective switch or the like that processes an optical signal in an optical state. However, in a ring mesh type network, the dispersion value of the path dynamically changes as the optical path is switched. For this reason, adaptability is also required for dispersion compensation of optical communication paths. Conventional dispersion compensators are mainly of a type that collectively compensates for a plurality of channels. However, in a ring mesh type network using a wavelength selective switch, the distance of the path that passes is different for each wavelength. For this reason, there has been a demand for setting a different dispersion value for each WDM wavelength.

  As an adaptive dispersion compensation technique that meets such a demand, for example, a technique using a spectroscope and a mirror array (Patent Document 1), a technique using a waveguide (Patent Document 1, Non-Patent Document 1), a three-dimensional mirror, The thing by a spectroscopic element (nonpatent literature 2) etc. was proposed.

  In addition, a variable dispersion compensator with a smaller size and lower cost using an LCOS (Liquid Crystal on Silicon) type spatial phase modulator by combining an arrayed waveguide grating (AWG) and a spatial optical system is also proposed. Has been. The tunable dispersion compensator using AWG has a feature that a large dispersion value can be set by using a high order diffraction order from several tens to several thousand based on the flexible optical design of AWG. . If the relationship between the bandwidth of the communication channel and the AWG FSR (Free Spectral Range) design value is appropriately set, dispersion compensation can be performed for a plurality of channels at once. By combining an AWG with an LCOS type spatial phase modulation element, it was possible to set a more flexible dispersion compensation value.

  FIG. 16 is a diagram illustrating a configuration example of a variable dispersion compensator according to the related art. FIG. 16A shows the configuration of a tunable dispersion compensator (TODC) in which one WDM communication channel and the AWG FSR value are matched. The TODC is composed of an AWG, a condensing lens, a spatial phase modulation element, and the like. The AWG FSR is configured to correspond to one WDM communication channel. According to the configuration of FIG. 16A, dispersion compensation can be performed collectively for 40 WDM communication channels from λ1 to λ40, for example.

  FIG. 16B is a diagram showing the light transmittance of the tunable dispersion compensator having the configuration shown in FIG. It can be seen that on the optical frequency axis, signal processing is collectively performed for 40 communication channels that coincide with the cycle of the FSR. However, dispersion compensation cannot be performed independently for each channel.

  FIG. 17A is a diagram illustrating a configuration example of a dispersion compensator capable of performing dispersion compensation independently for each communication channel. This tunable dispersion compensator includes an AWG 34, a cylindrical lens 35, a condenser lens 36, an LCOS element 37, and the like. This configuration is a reflection-type variable dispersion compensator in which an optical signal is reflected by an LCOS element and a single AWG 34 serves as both demultiplexing and multiplexing of the optical signal. The optical signal group subjected to dispersion compensation is input to the input port 38 of the optical circulator 11. The dispersion-compensated optical signal group is output from the output port 39 of the optical circulator 11. A plurality of pixel groups arranged in the spectral axis direction (x-axis) of the AWG 34 on the LCOS element 37 are divided into six groups, and one WDM communication channel (ch1,... Ch6) is assigned to each group of pixel groups. (Non-patent Document 3). Dispersion compensation can be performed independently for each channel by giving a different phase distribution φ (x) to each group of pixels on the LCOS element. FIG. 17B shows group delay characteristics set independently for each channel.

  FIG. 18 is a diagram illustrating an example of setting the phase distribution for one WDM communication channel in the variable dispersion compensator having the configuration of FIG.

JP 2002-303805 (pages 5-7, FIG. 1 and FIG. 11)

K. Takiguchi, K. Okamoto, and T. Goh, “Dispersion slope equalizer on planar Lightwave circuit for 40Gbit / s based WDM transmission,” Electron. Lett, 37 (24), p.1469-1470, 2001. National Institute of Information and Communications Technology, “Research and Development Report of FY 2004 Research and Development of Highly Functional Integrated Optical Switching Nodes to Realize Economical Optical Networks,” 2006 K. Seno, K. Suzuki, K. Watanabe et al., “Channel-by-channel tunable optical dispersion compensator consisting of arrayed-waveguide grating and liquid crystal on silicon” OWP4, Proceeding of OFC2008

  However, the conventional variable dispersion compensator using the spatial phase modulation element has the following problems. As described above, according to the configuration of FIG. 16A, dispersion compensation could not be performed independently for each WDM communication channel. In addition, according to the configuration of FIG. 17A, dispersion compensation can be performed independently for a plurality of WDM communication channels, but the required spatial phase modulation element becomes large. There was a problem. That is, in order to realize dispersion compensation for many channels, a spatial phase modulation element that is long in a direction corresponding to the spectral axis direction of the AWG is required.

  For example, in the configuration of FIG. 17A, dispersion compensation is performed for six communication channels with a bandwidth of 100 GHz. Here, when the number of communication channels increases, the size of the LCOS element in the spectral axis direction (x-axis direction) increases. More specifically, the number of LCOS elements corresponding to one WDM communication channel is 128, and the pixel arrangement pitch is 8 μm. At this time, the length of the LCOS element per channel in the spectral axis (x-axis) direction needs to be about 1000 μm. Therefore, the total length of the LCOS element exceeds 40 mm. If the size of the LCOS element in the direction of the spectral axis is to be shortened, there is a concern that characteristics such as a dispersion compensation amount and a channel transmission band are degraded. Therefore, depending on the required chromatic dispersion value and the number of channels, an LCOS element having a large number of pixels and a very long side is required.

  As is generally known, as the chip size increases, the mass productivity decreases and the cost increases. An increase in the size of the spatial phase modulation element is not preferable in terms of mass productivity and manufacturing cost. Therefore, in a multi-channel variable dispersion compensator using a spatial phase modulation element, it has been required to realize mass productivity and low cost.

  In addition, when a tunable dispersion compensator is configured using a spatial phase modulation element, there is a problem that light transmission characteristics are deteriorated at the optical frequency (wavelength) corresponding to the end of the AWG FSR. There is also a problem in using a pixel corresponding to the end of the FSR in the pixel group of the spatial phase modulation element.

  The present invention has been made in view of the above-described problems, and an object thereof is to realize a multi-channel variable dispersion compensator that realizes higher mass productivity and low cost. Furthermore, a multi-channel variable dispersion compensator with a more flat light transmission characteristic is realized by effectively using pixels of the spatial phase modulation element corresponding to the FSR. Maintenance of the tunable dispersion compensator is simplified and the cost is reduced. Further, a variable dispersion compensator that can be applied to a communication system that uses an interleaver as a group demultiplexing filter and has a flat light transmission band is also realized.

In order to achieve such an object, the present invention provides a plurality of optical signal groups of N consecutive communication channels including a plurality of channels that are discretely selected at a predetermined channel interval. In a dispersion compensator for an interleaved communication system that performs signal processing by branching to a plurality of wavelength groups, the optical signal groups of the N communication channels are each b (maximum (a-1) communication channels). Each of the b wavelength groups, and b dispersion compensator blocks corresponding to each of the b wavelength groups, each dispersion compensator block including the b number of dispersion compensator blocks. a corresponding one of the optical demultiplexing means further demultiplexes the optical signal of the wavelength group of the wavelength group, when the bandwidth of one communication channel is B, has a FSR of (B × a) , Each one communication Comprising a number b of output ports having an optical frequency difference corresponding to Yan'neru bandwidth B, the number b of optical coupler-input-output ports corresponding to the corresponding one of the wavelength groups of the input and output ports are chosen A spatial phase modulation element including a wave unit and a plurality of element elements arranged in a direction of a demultiplexing axis of the optical multiplexing / demultiplexing unit, wherein each of the plurality of element elements corresponds to one communication channel And one of the a sections includes a spatial phase modulation element that is not used for optical signal processing, and a and b are coprime, and a ≧ 3, b ≧ The dispersion compensator is selected so as to satisfy the relationship of 2 and N ≦ (a−1) × b.

The invention according to claim 2 is the dispersion compensator according to claim 1, wherein in each of the spatial phase modulation elements, the distance of the demultiplexing axis is independent of each of the sections corresponding to each of the optical signals. And a phase defined by a function of second order or higher is set as a parameter.

  The invention according to claim 3 is the dispersion compensator according to claim 1 or 2, wherein the multiplexing / demultiplexing means is an arrayed waveguide diffraction grating (AWG), and the spatial phase modulation element is LCOS. Features.

  A fourth aspect of the present invention is the dispersion compensator according to any one of the first to third aspects, wherein b is a power of 2.

A fifth aspect of the present invention is the dispersion compensator according to any one of the first to fourth aspects, wherein the spatial phase modulation element reflects an incident optical signal corresponding to the communication wavelength, and wave means, said group branching filter is characterized by a reflective structure for multiplexing the dispersion compensated optical signal.

  As described above, according to the present invention, a plurality of variable dispersion compensator blocks (hereinafter referred to as TODC blocks) using spatial phase modulation elements in which the size in the direction corresponding to the spectral axis of the AWG is kept short are combined, and more A multi-channel variable dispersion compensator that realizes good mass productivity and low cost can be realized. Maintenance and operation of the tunable dispersion compensator can also be simplified and reduced in cost. Furthermore, the type of TODC block can be reduced by setting the AWG FSR used in the TODC block to be equal to the entire bandwidth of the WDM communication channel group covered by the TODC block. Furthermore, by setting the FSR so as to have a predetermined relationship with the total bandwidth of the WDM communication channel group covered by the TODC block, the light transmission characteristics can be flattened.

  Furthermore, even when an interleaved group demultiplexing filter is used, the number of wavelength groups and the FSR are appropriately selected, the channel is not overlapped with the cell on the LCOS (spatial phase modulation element), and the gap The light transmission characteristics can be flattened.

1 is a block diagram illustrating an overall configuration of a tunable dispersion compensator according to Embodiment 1. FIG. FIG. 3 is a diagram illustrating light transmittance characteristics of the tunable dispersion compensator according to the first embodiment. 6 is a configuration diagram of a TODC block used in the variable dispersion compensator of Embodiment 2. FIG. It is a figure which shows the light transmittance of the TODC block which concerns on Example 2. FIG. FIG. 6 is a block diagram illustrating an overall configuration of a tunable dispersion compensator according to a second embodiment. 6 is a configuration diagram of a TODC block used in the variable dispersion compensator of Example 3. FIG. 6 is a configuration diagram of an LCOS element used in a TODC block used in a tunable dispersion compensator of Example 3. FIG. FIG. 9 is a block diagram illustrating an overall configuration of a tunable dispersion compensator according to a third embodiment. FIG. 10 is a configuration diagram of a TODC block used in the variable dispersion compensator of Example 4. FIG. 10 is a block diagram illustrating an overall configuration of a tunable dispersion compensator according to a fourth embodiment. FIG. 10 is a block diagram illustrating an overall configuration of a variable dispersion compensator according to a fifth embodiment. FIG. 10 is a diagram illustrating a configuration of an AWG used in the variable dispersion compensator according to the fifth embodiment. It is a figure explaining the channel allocation in the variable dispersion compensator of Example 5. FIG. It is a figure explaining the channel allocation in the variable dispersion compensator of Example 5. FIG. FIG. 10 is a diagram showing the relationship between the total number of wavelengths and the number of LCOS elements applicable to the tunable dispersion compensator of Example 5. It is a figure which shows the structure of the variable dispersion compensator of the prior art using a spatial phase modulation element. It is a figure which shows the other structure of the variable dispersion compensator of the prior art using a spatial phase modulation element. It is a figure which shows the example of the phase setting in the variable dispersion compensator of the prior art using a spatial phase modulation element.

  In the present invention, a multi-channel variable dispersion compensator is configured by combining a plurality of variable dispersion compensator blocks (TODC blocks) using spatial phase modulation elements whose size in the direction corresponding to the spectral axis of the AWG is kept short. Has characteristics. Further, the AWG FSR used in the TODC block is set to be equal to the entire bandwidth of the WDM communication channel group covered by the TODC block. Further, by setting the FSR so as to have a predetermined relationship with the entire bandwidth of the WDM communication channel group covered by the TODC block, the light transmission characteristics can be flattened. A communication system using an interleaved group demultiplexing filter can also be supported. Hereinafter, the present invention will be described in detail with examples.

FIG. 1 is a block diagram showing an overall configuration of a tunable dispersion compensator according to Embodiment 1 of the present invention. The tunable dispersion compensator 100 is composed of a plurality of TODC blocks and a group demultiplexing filter. For example, an optical signal in which an optical signal group having 40 wavelengths from λ ₁ to λ ₄₀ is multiplexed is input to the input port In of the first group demultiplexing filter 101. The multiplexed optical signal is divided into four wavelength groups (G1, G2,...) From λ ₁ to λ ₁₀ , λ ₁₁ to λ ₂₀ , λ ₂₁ to λ ₃₀ and λ ₃₁ to λ ₄₀ by the first group demultiplexing filter 101. G3, G4) are group-demultiplexed. The optical signal group of each wavelength group output from the port 1, port 2, port 3, and port 4 of the wavelength group demultiplexing filter 101 is input to the TODC blocks 103a, 103b, 103c, and 103d through optical fibers or the like, respectively. . The optical signal groups of the wavelength groups G1 to G4 are compensated for dispersion by the TODC blocks 103a, 103b, 103c, and 103d, respectively, and are connected to the ports 1 to 4 of the second wavelength group demultiplexing filter 102. Optical signal group of the four wavelength groups after dispersion compensation (G1, G2, G3, G4 ) are multiplexed by the second wavelength group branching filter 102, 40 again having a central wavelength of from lambda ₁ to lambda ₄₀ Are multiplexed on the optical signal of the communication channel and output from the output port out.

  In the block diagram shown in FIG. 1, it is described that there are two wavelength group demultiplexing filters 101 and 102. However, when a reflection type configuration is used as the TODC block, a reflection plate is provided. An equivalent function can be realized by one wavelength group demultiplexing filter capable of generating waves and demultiplexing. The wavelength group demultiplexing filters 101 and 102 can be composed of, for example, a dielectric multilayer film.

  As each TODC block 103 of the tunable dispersion compensator of the present invention, for example, the same configuration as that of the tunable dispersion compensator shown in FIG. Therefore, the input and output of each TODC block in FIG. 1 correspond to the input port 38 and the output port 39 of the optical circulator 11 in FIG. In FIG. 17A, a variable chromatic dispersion compensator having a reflection type is shown, but it should be noted that it can also be realized by a transmission type configuration in which two AWGs are arranged.

  For the individual TODC blocks 103a, 103b, 103c, 103d, the configuration parameters of the AWG are set so that the center wavelength of the spectroscopic operation corresponds to the center wavelength of each band of G1-G4, and the predetermined number of communication channels in each band Each is set so that the linear dispersion value conforms to. The basic configuration of each TODC block can be exactly the same. As shown in FIG. 17A, the phase can be set independently for each channel by a plurality of pixels arranged in the direction of the spectral axis of the AWG on the LCOS element included in each TODC block. . In the case of the configuration of FIG. 1, dispersion compensation can be performed independently for 10 WDM communication channels in one wavelength group band.

  Therefore, with the configuration shown in FIG. 1, the size corresponding to the spectral axis of each LCOS element can be shortened by suppressing the number of communication channels for performing dispersion compensation in each TODC block. That is, it is possible to realize a multi-channel variable dispersion compensator excellent in mass productivity and cost while suppressing an increase in size of the LCOS element. In particular, when the number of channels requiring dispersion compensation is very large, a smaller LCOS element is used compared to the case where one TODC block using only one LCOS element having a large side is used. It is cheaper to configure with a plurality of TODC blocks.

  More preferably, in each TODC block of the present embodiment, the AWG FSR can be set to be equal to the entire bandwidth of the communication channel included in one wavelength group. Specifically, the AWG FSR can be set to an optical frequency width corresponding to 10 channels of the WDM communication channel. For example, if one WDM communication channel width is 100 GHz, the FSR is set to 1000 GHz.

  By setting the AWG FSR and the total bandwidth of a plurality of communication channels included in one wavelength group to the same value, the configurations of the TODC blocks 103a, 103b, 103c, and 103d may be exactly the same. it can. That is, the tunable dispersion compensator 100 shown in FIG. 1 can be configured with only one type of TODC block having the same design specifications. Since the types of components of the tunable dispersion compensator 100 can be reduced, the manufacturing cost of the tunable dispersion compensator can be reduced. Furthermore, from the viewpoint of maintenance and operation of the tunable dispersion compensator, maintenance work can be simplified and cost can be reduced. In order to cope with the failure of the tunable dispersion compensator, it is not necessary to provide a plurality of types (for example, four types) of TODC blocks, and only one type of TODC block may be provided for maintenance. Note that maintenance and replacement can be simplified.

  FIG. 2 is a diagram showing an example of the light transmittance characteristic of the variable dispersion compensator of the present invention. In this example, the AWG FSR is set to the same value as the total bandwidth of the communication channels included in one wavelength group. The horizontal axis indicates the optical frequency, and the vertical axis indicates the light transmission characteristics as a dispersion compensator. The dispersion compensation characteristic is repeated for each FSR due to the circulatory property of the AWG. As for the light transmittance, band characteristics are repeated for each FSR. One FSR corresponds to the wavelengths of ten communication channels in one wavelength group (G1, G2, G3, G4).

  Although the variable dispersion compensator having the configuration shown in FIG. 1 has been described as an example in which 10 WDM communication channels are included in one wavelength group, the present invention is not limited to this. Similarly, the total number of channels of the system subject to dispersion compensation is 40, the number of wavelength groups is 4, and the number of corresponding TODC blocks is 4. However, these numbers are not limited at all.

  According to the variable chromatic dispersion compensator of the first embodiment described above, the multiplexed WDM optical signal is separated into a plurality of wavelength groups, and a TODC block corresponding to each wavelength group is provided, so that the spectral axis direction can be provided. LCOS elements with a short size can be used. A multi-channel variable dispersion compensator excellent in mass productivity and cost can be realized without requiring a large LCOS element. In addition, by setting the AWG FSR and the total bandwidth of the communication channels included in one wavelength group to the same value, it is possible to reduce the cost of the variable dispersion compensator, simplify the maintenance, and reduce the cost. it can.

  In the present embodiment, a tunable dispersion compensator is shown in which a TODC block having completely the same specification can be used and the light transmission characteristics are flattened. In the first embodiment, the basic configuration of each TODC block can be the same. However, unless the AWG FSR and the total bandwidth of the communication channels included in one wavelength group are set to the same value, it is necessary to prepare a dedicated TODC block corresponding to each wavelength group. That is, for each wavelength group, it is necessary to set the configuration parameters of the AWG included in the TODC block so that the center wavelength of the spectral operation of the AWG corresponds to the center wavelength of each band of G1 to G4. The AWG chip constituting the TODC block must be different for each TODC block.

When the AWG FSR and the total bandwidth of the communication channels included in one wavelength group are set to the same value, one type of TODC block can be used. However, as shown in FIG. 2, it is inevitable that the light transmittance decreases in the communication channel corresponding to both ends of the FSR. When the AWG FSR and the entire bandwidth of the communication channel included in one wavelength group are set to the same value, the transmittance of the dispersion compensator also shows a characteristic that repeats for each FSR. As shown in FIG. 2, in general, the transmittance decreases in the communication channels at both ends of one wavelength group. For example, in the communication channels corresponding to λ ₁ and λ ₁₀ , there is a problem that the light transmittance is reduced as compared with the communication channel in the center of one wavelength group. In this embodiment, this problem is further improved.

  FIG. 3 is a diagram illustrating a configuration of a TODC block used in the variable dispersion compensator according to the second embodiment. The TODC block 103 is used as a component of a tunable dispersion compensator according to a second embodiment which will be described later with reference to FIG. The basic configuration of the TODC block 103 shown in FIG. 3 is the same as the configuration of the tunable dispersion compensator in the prior art shown in FIG. Further, as in the phase distribution shown in FIG. 18, it is defined by a function of second order or higher with the distance of the demultiplexing axis as a parameter. The AWG is different from the configuration of the prior art in that the AWG has two input / output waveguides connected to positions separated by 0.5 FSR on the boundary surface of the slab waveguide.

  Referring to FIG. 3, the TODC block includes a spatial phase modulation element 8 including an AWG 1, a cylindrical lens 6, a condenser lens 7, and an LCOS element. The AWG 1 includes a slab waveguide 3 and an arrayed waveguide 4. The multiplexed optical signal group is demultiplexed from one end of the arrayed waveguide 4 at an emission angle corresponding to the wavelength of the optical signal, and emitted from the end face A of the AWG substrate. On the LCOS element 8, a plurality of pixels are arranged along the spectral axis (x-axis direction) of AWG 1 corresponding to a plurality of WDM communication channels (for example, 10 channels). The TODC block 103 has a reflective configuration, and each optical signal emitted from the AWG 1 is reflected after being given a predetermined phase in the LCOS element 8 and returns to the AWG 1 again.

  The AWG 1 of the present TODC block has two input / output waveguides 16 and 17 that can input and output optical signal groups of a plurality of different wavelength groups. The first input / output waveguide 16 is connected to the slab waveguide 3 at a point on the boundary surface B opposite to the connection surface of the slab waveguide 3 to the arrayed waveguide 4. The first input / output waveguide 16 is connected to the first optical circulator 11 via an optical fiber or the like. The first optical circulator 11 functions as a port A, and an optical signal of a predetermined wavelength group is input (Ain) and output (Aout). Similarly, the second input / output waveguide 17 is connected to the slab waveguide 3 at a point b on the boundary surface B of the slab waveguide 3. The second input / output waveguide 17 is connected to the second optical circulator 14 via an optical fiber or the like. The second optical circulator 14 functions as a port B, and an optical signal having a wavelength group different from that of the port A is input (Bin) and output (Bout).

  Note that the points a and b to which the two input / output waveguides 16 and 17 are connected are shifted by a distance corresponding to 1/2 of the FSR set in the AWG 1. Therefore, the transmission band of the optical signal input / output from the port A and the transmission band of the optical signal input / output from the port B are shifted by 0.5 FSR on the optical frequency axis.

  FIG. 4 is a diagram illustrating the light transmittance of the TODC block according to the second embodiment. The light transmittance of the optical signal input / output using the port A is indicated by a solid transmittance line 21, and the light transmittance of the optical signal input / output using the port B is the dotted line transmittance. This is indicated by the line 22. Since all the transmittance lines have the same AWG transmittance, the transmittance characteristics with the FSR as a repetition period are shown. Note, however, that the center of the transmission band is shifted by 0.5 FSR between the two ports.

  In the transmittance characteristic by the port A, a flat portion at the center of the transmission band can be allocated for use of the optical signal of the wavelength group G1. In order to use the optical signal of the wavelength group G2 adjacent to the wavelength group G1, a flat portion at the center of the transmission band in the transmittance characteristic by the port B can be used. Further, for the wavelength group G3 adjacent to the wavelength group G2, in the transmittance characteristics by the port A, a flat portion at the center of the next transmission band that is FSR away from the transmission band used for the wavelength group G1 can be used. Similarly, for the wavelength group G4 adjacent to the wavelength group G3, the flat portion at the center of the next transmission band that is FSR away from the transmission band used for the wavelength group G2 can be used again in the transmittance characteristics by the port B.

  As described above, the ports used in the TODC block shown in FIG. 3 are alternately selected for each wavelength group, so that the light transmission characteristics are flat for a continuous communication band including a number of WDM communication channels. Dispersion compensation can be realized. At this time, the FSR set in the AWG 1 in the TODC block is twice the total bandwidth of the channels in one wavelength group. In the first embodiment, as shown in FIG. 2, the AWG FSR and the total bandwidth of the channels in one wavelength group are set to be the same. That is, in the first embodiment, the AWG FSR value of the TODC block is set to match the wavelength separation interval of the group demultiplexing filters 101 and 102. On the other hand, in Example 2, it should be noted that the AWG FSR value of the TODC block is set to twice the separation interval of the group demultiplexing filter. Next, the overall configuration of the tunable dispersion compensator using the TODC block shown in FIG. 3 will be further described.

FIG. 5 is a block diagram showing the overall configuration of the tunable dispersion compensator according to the second embodiment of the present invention. Similar to the first embodiment, an optical signal obtained by multiplexing optical signals having 40 wavelengths from λ ₁ to λ ₄₀ is divided into four wavelength groups (G1, G2, G3, G4). Dispersion compensation is performed using four TODC blocks. The second embodiment is different from the first embodiment in that the ports connected to the two group demultiplexing filters in the TODC block are alternately selected for each wavelength group between the A port and the B port. .

An optical signal in which an optical signal group having 40 wavelengths from λ ₁ to λ ₄₀ is multiplexed is input to the input port in of the first group demultiplexing filter 201. The multiplexed optical signal is converted into four wavelength groups (G1, G2) from λ ₁ to λ ₁₀ , λ ₁₁ to λ ₂₀ , λ ₂₁ to λ ₃₀ and λ ₃₁ to λ ₄₀ by the first group demultiplexing filter 201. , G3, G4). The optical signal group of each wavelength group output from the port 1, port 2, port 3, and port 4 of the wavelength group demultiplexing filter 201 is sent to the TODC blocks 203a, 203b, 203c, and 203d via optical fibers, respectively. Entered. The optical signals of each wavelength group are dispersion-compensated by the TODC blocks 203a, 203b, 203c, and 203d, respectively, and input to the respective ports of the second wavelength group demultiplexing filter 202. The optical signal groups of the four wavelength groups (G1, G2, G3, G4) that have been dispersion-compensated are combined by the second wavelength group demultiplexing filter 202, and each of them has a central wavelength from λ ₁ to λ ₄₀ again. It is multiplexed into an optical signal including optical signal groups of 40 communication channels.

  The port A is used in the TODC 203a that performs dispersion compensation on the optical signal group of the wavelength group G1. The port B is used in the TODC 203b that performs dispersion compensation for the optical signal group of the wavelength group G2 adjacent to the wavelength group G1. Further, the port A is used again in the TODC 203c that performs dispersion compensation for the optical signal group of the wavelength group G3 adjacent to the wavelength group G2. In the TODC 203d that performs dispersion compensation for the optical signal group of the wavelength group G4 adjacent to the wavelength group G3, the port B is used again. As described above, the A port and the B port are used alternately for each wavelength group from G1 to G4. As a result, as shown in FIG. 4, it is understood that the transmission bands corresponding to the port A and the port B are alternately used for the adjacent wavelength groups. At this time, as the chromatic dispersion compensator, only the central region having a flat light transmittance is used in one transmission band corresponding to the FSR on the optical frequency axis. For this reason, only a flat central region is used without using a region corresponding to both ends of the FSR and being at both ends of the transmission band where the light transmittance decreases.

  According to the configuration of the second embodiment, the multiplexed WDM optical signal is separated into a plurality of wavelength groups, and the LCOS element having a short size in the direction of the spectral axis is used by including a plurality of one type of TODC blocks. be able to. A multi-channel variable dispersion compensator excellent in mass productivity and cost can be realized without requiring a large LCOS element. By setting the AWG FSR value of the TODC block to be twice the separation interval of the group demultiplexing filter, a variable dispersion compensator with a flat light transmission band can be realized. It is possible to reduce the cost of the tunable dispersion compensator, simplify the maintenance, and reduce the cost.

  In Example 2 described above, the connection point between the two input / output waveguides and the slab waveguide was shifted by a distance corresponding to 1/2 of the FSR set in the AWG 1. However, if the same technical idea is applied, a configuration including three input / output waveguides connected to the slab waveguide at a position shifted by a distance corresponding to 1/3 of the FSR can be provided. In this case, the TODC block includes three ports, port A, port B, and port C. It can be easily understood that a flat portion of light transmittance can be used in one wavelength group by dividing a plurality of consecutive wavelength groups into three groups that are alternately repeated. At this time, the AWG FSR value of the TODC block is set to three times the separation interval of the group demultiplexing filter. Furthermore, it is possible to expand the number of input / output waveguides and the number of ports to 3 or more based on the same concept.

  In the above-described first and second embodiments, a one-dimensional LCOS element in which pixels are arranged only in the AWG spectral axis direction is used as the LCOS element. However, by arranging the pixels two-dimensionally on the LCOS element and introducing the second spectroscopic element into the TODC block, the configuration of the tunable dispersion compensator can be further simplified and the LCOS element can be reduced in size. First, a TODC block including an LCOS element in which pixels are two-dimensionally configured will be described.

  FIG. 6 is a diagram illustrating the configuration of the TODC block according to the third embodiment of the present invention. Similar to the first and second embodiments, the configuration is a reflection type, but is different in that it further includes a bulk diffraction grating 15 and demultiplexes the optical signal also by the bulk diffraction grating. Furthermore, the present embodiment has a great feature in that when viewed through the optical path, the demultiplexing surfaces of the AWG and the bulk type diffraction grating are orthogonal to each other. Hereinafter, a detailed description will be given focusing on the difference from the TODC block of the first and second embodiments.

  Similar to the TODC block of the first and second embodiments, the optical signal input from the input fiber 10 enters the input waveguide 2 of the AWG 1 via the circulator 11 and the connecting fiber 13. The optical signal incident on the input waveguide 2 propagates to the arrayed waveguide 4 through the slab waveguide 3. In the arrayed waveguide 4, optical signal groups having different wavelengths are demultiplexed. That is, the optical signal emitted from the emission end 5 propagates toward the bulk diffraction grating 15 in the z-axis direction at different emission angles according to the wavelength in the xz plane (demultiplexing plane).

  The optical signal emitted from the emission end 5 is converted into a parallel beam by the cylindrical lens 6 in the thickness direction of the AWG substrate, that is, the y direction. The optical signal emitted from the cylindrical lens 6 can be regarded as parallel light in the yz plane. On the other hand, the x direction in the plane of the AWG substrate is converted into a sufficiently wide parallel beam by the lens action of the slab waveguide 3. That is, when the light is emitted from the emission end 5, the optical signal emitted from the AWG 1 can be regarded as parallel light in the xz plane. By passing through the cylindrical lens 6, the optical signal can be regarded as parallel light in both the x direction and the y direction.

  The configuration of the TODC block according to the third embodiment is characterized in that it includes a bulk diffraction grating that further demultiplexes the optical signal demultiplexed by the AWG 1. The optical signal emitted from the cylindrical lens 6 is further demultiplexed by the bulk diffraction grating 15 whose normal is inclined by θi with respect to the z-axis and the grating vector is set in the yz plane. The optical signal demultiplexed by the bulk diffraction grating 15 is condensed on the spatial phase control element 8 by the condenser lens 7. Here, the dispersion direction of the AWG 1 and the dispersion direction of the bulk diffraction grating 15 are in a relationship in which the two demultiplexing surfaces are orthogonal to each other when the demultiplexing surfaces are viewed along the optical path.

  According to FIG. 6A, the AWG 1 and the spatial phase control element 8 are described so as to be arranged in parallel with each other, but strictly speaking, they need not be parallel. FIG. 6 exemplarily shows a case where a specific bulk diffraction grating described later is used and the incident angle θi is 46.76 °. At this time, the optical path is refracted at about 90 ° in the bulk diffraction grating. For this reason, in the drawing, the AWG and the spatial phase control element are expressed as if they were arranged in a parallel positional relationship. Therefore, in this embodiment, there is no limitation on the refraction angle θi of the bulk type diffraction grating. The TODC block of the present embodiment has a spatial phase control element pixel as a result of the relationship between the demultiplexing surface of AWG1 and the demultiplexing surface of bulk diffraction grating 15 being relatively orthogonal when viewed along the optical path. It is characterized in that it can be configured in two dimensions with anisotropy.

  To explain the relationship between the wavelength (optical frequency) and the position of the focused beam of the optical signal, consider the locus that the focused beam draws on the spatial phase control element when the wavelength is virtually continuously changed. Try. In this embodiment, the angular dispersion of the AWG 1 is set to be sufficiently larger than the angular dispersion of the bulk diffraction grating 15, so that the condensed beam on the spatial phase control element 8 is scanned in a raster shape according to the wavelength of the optical signal. The

  For example, the diffraction order of the bulk diffraction grating 15 may be set to 1, and the FSR of the AWG 1 may be set to be equal to the entire bandwidth of the WDM communication channel of one wavelength group in the communication system to be subjected to dispersion compensation. Such a raster scan of the beam can be performed by using an AWG that has a high degree of freedom in design parameters and can easily realize a desired FSR as the first spectroscopic element. Even if a bulk diffraction grating is used as the first spectroscopic element, a desired angular dispersion cannot be set easily. Note that dispersion compensation operation peculiar to this embodiment is realized by allocating and combining appropriate angular dispersions to the demultiplexing characteristics of the first and second spectroscopic elements. I want to be. Similar to the TODC blocks of the first and second embodiments, the optical signal reflected by the spatial phase control element 8 reverses its optical path and propagates in the direction opposite to the forward path, and passes through the circulator 11 to the output fiber 12. It is emitted from.

  FIG. 7A is a diagram illustrating a configuration of a spatial phase control (LCOS) element suitable for the TODC block according to the third embodiment. The coordinate system on the spatial phase control element 8 is defined as u axis-v axis. As described above, the FSR of AWG 1 is set equal to the entire bandwidth of the WDM communication channel in one wavelength group in the communication system that is the object of dispersion compensation. At this time, an optical signal in the entire band of one wavelength group corresponds to interference light of a certain diffraction order of AWG1. Assuming that the wavelength of the virtually unmodulated optical signal is continuously changed, the position of the focused beam draws a locus on the line segment Lm for the optical signal of diffraction order m by the demultiplexing action of AWG1. .

  This optical signal of diffraction order m corresponds to, for example, an optical signal group in the mth specific wavelength group. Similarly, for an optical signal of diffraction order m + 1, a locus on adjacent Lm + 1 is drawn. This optical signal of diffraction order m + 1 corresponds to an optical signal group in the (m + 1) th wavelength group adjacent to the mth wavelength group. Therefore, the optical signal component within the entire band of one wavelength group corresponds to a pixel column localized on one locus line Lm that is scanned and drawn on the spatial phase control element 8. In other words, each optical signal within the entire band of one wavelength group is independently phased by a plurality of pixel rows arranged in the u-axis direction localized on one locus Lm, and dispersion compensation is performed. Is realized.

  Dispersion characteristics (group delay characteristics) can be set for different wavelength groups for each pixel array arranged in the u-axis direction. That is, the TODC block according to the third embodiment has a characteristic that different dispersion characteristics (group delay characteristics) can be set for each wavelength group by setting different phase distributions independently for each pixel column. In FIG. 6, the direction of the line of intersection between the spectral plane (ie, yz plane) of the bulk type diffraction grating 15 and the pixel formation surface of the spatial phase control element 8 is the dispersion direction by the bulk type diffraction grating 15 on the spatial phase control element 8. It becomes. The dispersion axis defined by this intersection line is taken as the z ′ axis. In FIGS. 7A and 7B, the z′-axis is a direction connecting the end points of the locus lines such as Lm and Lm + 1 or a direction connecting the start points.

With reference to FIG. 7A, the relationship between the focused beam diameter and the pixel structure formed on the u-axis and v-axis planes will be further examined. In FIG. 7A, the shape of each pixel is displayed as a square for simplicity. In the following, it should be noted that the method of setting the phase for each pixel will be described by focusing on the relative relationship between the focused spot beam radius and the pixel pitch in each of the u axis and the v axis. In addition, the shape of the condensed beam corresponding to an optical signal having a certain optical frequency (wavelength) that is not modulated is generally an ellipse according to the characteristics of the condensing lens and the cylindrical lens. Here, the ellipse radius in the v-axis direction is wv. The ellipse radius refers to a radius at which the light intensity of the focused spot is 1 / e ^{2 of the} peak value, that is, a radius at which the light intensity of the focused spot is 13.5% of the peak light intensity.

In the v-axis direction, the condensing spot raster that describes the locus Lm moves dv for each optical frequency corresponding to the FSR of the arrayed waveguide grating based on the angular dispersion of the bulk diffraction grating 15. Therefore, overlapping of beams of adjacent wavelength groups can be eliminated by satisfying the following equation for the elliptical radius of the focused spot in the v-axis direction.
wv ≦ dv / 2 Formula (1)
The ellipse radius wv in the v-axis direction is a radius at which the light intensity of the focused spot is 1 / e ^{2 of the} peak value. Crosstalk performance can be realized.

  In the configuration of FIG. 7A, the direction of the locus Lm determined by the linear dispersion values of the AWG 1 and the bulk diffraction grating 15 is made to coincide with the u-axis direction of the spatial phase modulation element 8. Furthermore, the pixel pitch in the v-axis direction is set to pSLMv, and pSLMv and dv are matched. With the above-described pixel configuration, each optical signal in the entire band of one wavelength group can correspond to one pixel column aligned in the u-axis direction. As a result, dispersion can be imparted to the optical signals in all wavelength groups using the LCOS having the minimum number of pixels. By reducing the number of pixels, the cost of LCOS can be kept low.

  Below, the example of the TODC block of a present Example is shown with a specific numerical example. The arrayed waveguide grating was fabricated using a silica-based optical waveguide having a relative refractive index difference of 1.5%. The path length difference ΔL of the arrayed waveguide was 202 μm, and the arrayed waveguide pitch at the exit end 5 of the arrayed waveguide was 12 μm. According to this configuration, the free spectral range of the arrayed waveguide grating is approximately 1000 GHz.

  The bulk type diffraction grating 15 uses, for example, a volume phase holographic grating (VPHG) having a grating period of 940 lines / mm. The bulk type diffraction grating 15 is not limited to the VPHG type, and a function similar to that of the VPHG can be realized by using a transmission type blaze diffraction grating, a reflection type holographic diffraction grating, or a reflection type blaze diffraction grating. When the incident angle θi is 46.76 °, the angular dispersion value of VPHG with a grating period of 940 lines / mm is 1.37 mrad / nm. The focal length of the cylindrical lens 6 was 1 mm, and the focal length of the condenser lens 7 was 80 mm.

  In the LCOS type spatial phase control element 8, the number of pixels and the pitch in the u-axis direction are 1280 and 8 μm, respectively, and the number of pixels and the pitch in the v-axis direction are 4 and 920 μm, respectively. Therefore, the size of the region where the LCOS pixels are formed is about 10.2 mm × 3.7 mm. This configuration is merely an example, and the pixel pitch in the u-axis direction may be in the range of 5 μm to 10 μm.

  According to the configuration of the optical system described above, the beam radius wv in the v-axis direction is about 300 μm, and it has been confirmed that the relationship of Expression (1) is satisfied. The linear dispersion value in the v-axis direction on LCOS is found to be 0.11 mm / nm as the product of the above-mentioned angular dispersion value of VPHG and the focal length of the condenser lens 7. Therefore, the position of the focused spot moves dv = 920 μm per 1000 GHz (about 8.4 nm) which is the FSR of the AWG. It was confirmed that both the spot movement amount dv corresponding to the FSR of the arrayed waveguide grating and the pixel pitch pSLMv in the v-axis direction were 920 μm.

  The linear dispersion value in the u-axis direction of the LCOS element is 1.22 mm / nm, and 128 pixels contribute to the phase modulation of an optical signal having a frequency range of 100 GHz. Therefore, the TODC block having the configuration of the present embodiment makes it possible to perform dispersion compensation independently for each optical signal of 10 WDM communication channels arranged at 100 GHz intervals in one wavelength group. Furthermore, dispersion compensation can be performed independently for a plurality of optical signal groups of different wavelength groups for each pixel array arranged in the v-axis direction.

  In the configuration example of the LCOS shown in FIG. 7A, the case where the pSLMv and dv are substantially matched and one pixel row arranged in the u-axis direction corresponds to one wavelength group is exemplified. explained. As another configuration example, a configuration example will be described in which dv is set larger than pSLMv and a plurality of pixels are assigned to one wavelength group in the v-axis direction, as shown in FIG. Also in this case, the crosstalk between adjacent wavelength groups can be kept low by satisfying the relationship of the expression (1). In the pixel configuration of FIG. 7A, it is necessary to make the direction of the locus Lm coincide with the direction of the u axis. On the other hand, in the pixel configuration of FIG. 7B, by using a plurality of pixels included in the width of dv centering on the locus Lm in the v-axis direction for controlling one wavelength group m, The locus Lm and the u axis need not be parallel.

  The advantage of the pixel configuration of FIG. 7B is that an arbitrary optical coupling loss can be added to the optical signal. A plurality of pixels in the v-axis direction can set a phase tilted with respect to the v-axis direction independently of the phase setting with respect to the u-axis direction. Referring to FIG. 6 again, the optical signal reflected by the LCOS and passed through the bulk diffraction grating 15 is tilted with respect to the AWG waveguide eigenmode in the y-axis direction at the output end 5 of the arrayed waveguide grating. The light enters the AWG 1 with a distribution. Therefore, it is possible to add a wavelength-dependent loss to the transmission characteristics of the TODC block and compensate the wavelength dependence of the optical signal intensity.

  As an LCOS for realizing the configuration of FIG. 7B, an LCOS in which pixels are arranged on a general square lattice was used. The pixel pitch is 8 μm in both the u-axis direction and the v-axis direction. The configuration of the optical system other than LCOS was the same as that described with reference to FIG. Since dv = 920 μm, 115 pixels are assigned to one wavelength group in the v-axis direction around the locus Lm. At any wavelength, that is, at any wavelength group number m and any position on the u-axis, the phase value given to 115 pixels assigned in the v-axis direction is a phase change with a maximum inclination angle of 0.3 degrees. Was linearly changed in the v-axis direction corresponding to. With this linearly tilted phase, the optical signal intensity at that wavelength could be controlled in the range of 0 dB to -40 dB.

  The variable dispersion compensator according to the first embodiment shown in FIG. 1 is obtained by using a TODC block including LCOS elements in which pixels shown in FIG. 7A or 7B are two-dimensionally arranged. It can be modified to a simpler configuration.

  In the above description related to LCOS, the pixel width and the space between the pixels are not touched by paying attention only to the pixel pitch, but the space between the pixels is narrow to increase the light control efficiency regardless of the usage. It is preferable to do. In general, 1 μm or less is preferable.

  FIG. 8 is a block diagram illustrating the overall configuration of the dispersion compensator according to the third embodiment. FIG. 8 represents the dispersion compensator divided into blocks focusing on the function as the dispersion compensator. Hereinafter, the comparison relationship between the TODC block shown in FIG. 6 and a specific configuration will be described. The LCOS element 8 in the TODC block of FIG. 6 has a configuration in which pixels are arranged in two dimensions (2-Dimension) as shown in FIGS. 7A and 7B. Called an element.

In the tunable dispersion compensator 300 of FIG. 8, for example, an optical signal in which an optical signal group having a wavelength of 40 from λ ₁ to λ ₄₀ is multiplexed is input to the input port In of the spectrometer 301 and is demultiplexed. And input to the first group demultiplexing filter 302. The multiplexed optical signal is divided into four wavelength groups (G1, G2,..., Λ ₁ to λ ₁₀ , λ ₁₁ to λ ₂₀ , λ ₂₁ to λ ₃₀ and λ ₃₁ to λ ₄₀ by the first group demultiplexing filter 302. G3, G4) are group-demultiplexed. The optical signal group of each wavelength group from the first wavelength group demultiplexing filter 302 is a pixel group 306a in each column (first column, second column, third column, fourth column) on the 2D-LCOS element 305. , 306b, 306c, and 306d. The optical signal groups of the four wavelength groups (G1, G2, G3, G4) after the dispersion compensation are multiplexed by the second wavelength group demultiplexing filter 303 and the spectroscope 304, and again from λ ₁ to λ _40. The signals are multiplexed into the optical signals of 40 communication channels having wavelengths and output from the output port out.

  The two spectrometers 301 and 304 in FIG. 8 correspond to AWG1 in the TODC block in FIG. Here, since the TODC block shown in FIG. 6 has a reflection type configuration, it should be noted that the functions of the spectrometer 301 and the spectrometer 304 can be realized by one AWG. The two group demultiplexing filters 302 and 303 in FIG. 8 correspond to the bulk diffraction grating 15 (second spectroscopic means) in the TODC block shown in FIG. Similar to the AWG 1, the functions of the two group demultiplexing filters 302 and 303 can be realized by one bulk diffraction grating 15.

  The 2D-LCOS element 305 includes a first column of pixel groups 306a corresponding to the wavelength group G1, a second column of pixel groups 306b corresponding to the wavelength group G2, a third column of pixel groups 306c corresponding to the wavelength group G3, and a wavelength. Dispersion compensation is performed by the pixel group 306d in the fourth column corresponding to the group G4. Here, in the 2D-LCOS element, pixel groups in each column (first column, second column, third column, and fourth column) arranged in the u-axis direction within one element plane are two-dimensionally integrated. Note that the pixels are arranged and arranged. Therefore, the optical signal processing from when the optical signal is emitted from the spectroscope 301 (AWG1) to the space, subjected to the optical signal processing by the bulk diffraction grating and the 2D-LCOS element, and again incident on the spectroscope 304 (AWG1) is performed. This is realized by a set of TODC blocks. That is, the same function as the tunable dispersion compensator that requires the four TODC blocks shown in FIG. 1 can be realized by one TODC block shown in FIG.

  Note that although it has been described that each pixel column of the 2D-LCOS element 305 is from the first column to the fourth column, phase setting is performed using a plurality of pixels in the v-axis direction for one wavelength group. Note that you can also. In this case, a plurality of pixel columns arranged in the u-axis direction are arranged for one wavelength group.

  According to the tunable dispersion compensator according to the first embodiment shown in FIG. 1, separate wavelength group demultiplexing filters 101 and 102 are required in addition to the four TODC blocks. On the other hand, since the tunable dispersion compensator 300 according to the third embodiment can be configured with only one TODC block shown in FIG. 6, the configuration of the tunable dispersion compensator can be greatly simplified.

  Similar to the tunable dispersion compensator of the first embodiment, the tunable dispersion compensator of the second embodiment shown in FIG. 5 can be modified to a simpler configuration by using the 2D-LCOS element shown in FIG. Can do. That is, in the TODC block using the LCOS element in which the pixels are two-dimensionally shown in FIG. 6, the two input / output waveguides 16 and 17 connected to the AWG as in FIG. It is possible to realize variable dispersion compensation having a flat light transmission characteristic for a continuous communication band (wavelength group) including a WDM communication channel.

  FIG. 9 is a configuration diagram of a TODC block used in the variable dispersion compensator of the fourth embodiment. The configuration shown in FIG. 9 is that the second demultiplexing means (bulk type diffraction grating) 15 is added to the TODC block (FIG. 3) used in the variable dispersion compensator of the second embodiment. This is different from the configuration of the TODC block shown in FIG. Further, in terms of having two input / output waveguides 16 and 17 connected to a position equivalent to 0.5 FSR on the boundary surface of the slab waveguide 3, it is common to the configuration of the TODC block of FIG. The difference is that the interleave type group demultiplexing filter 19 is provided. Hereinafter, the description will be given focusing on these differences. In FIG. 9, for simplicity, the optical path bending by the second demultiplexing means 15 is omitted and is shown in a simplified manner.

The TODC block shown in FIG. 9 has a reflective configuration. An optical signal in which an optical signal group having 40 wavelengths from λ ₁ to λ ₄₀ is multiplexed is input to the TODC block via the optical circulator 11. After being compensated for dispersion in the TODC block, it is output from the optical circulator 11 again. The multiplexed input optical signal to the optical circulator 11 is input to the interleave type group demultiplexing filter 19. The interleave type group demultiplexing filter 19 demultiplexes the multiplexed optical signal group including a plurality of wavelength groups into an odd-numbered wavelength group and an even-numbered wavelength group. That is, two wavelength groups (G 1 and G 3) from λ ₁ to λ ₁₀ and λ ₂₁ to λ ₃₀ are group-demultiplexed at the output on the port A side of the interleave type group demultiplexing filter 19. Two wavelengths (G2, G4) from λ ₁₁ to λ ₂₀ and λ ₃₁ to λ ₄₀ are group-demultiplexed at the output on the port B side of the interleaved group demultiplexing filter 19. The optical signal groups from the A port and the B port are input to the two input / output waveguides 16 and 17 connected to the positions separated by 0.5 FSR on the boundary surface of the slab waveguide 3, respectively. Similar to the TODC of the second embodiment, the FSR value of the AWG 1 is set to twice the wavelength group separation interval of the interleave type group demultiplexing filter 19.

  The LCOS element 8 in the TODC block of FIG. 9 is a 2D-LCOS element in which pixels are arranged in a two-dimensional manner as shown in FIG. 7 as in the third embodiment.

  FIG. 10 is a block diagram illustrating an overall configuration of a dispersion compensator according to the fourth embodiment. FIG. 10 represents the dispersion compensator divided into blocks focusing on the function as the dispersion compensator. Hereinafter, description will be made in comparison with a specific configuration of the TODC block shown in FIG.

In the tunable dispersion compensator 400 of FIG. 10, for example, an optical signal obtained by multiplexing optical signal groups having 40 wavelengths from λ ₁ to λ ₄₀ is converted into an odd-numbered wavelength by an interleaved group demultiplexing filter 401. The interleaved group is demultiplexed into a group and an even-numbered wavelength group. The A wavelength group (G1, G3) and the B wavelength group (G2, G4), which are demultiplexed by the interleave group, are input to the A port and the B port of the spectrometer 402, respectively. Here, the spectroscope 402 corresponds to AWG1 in FIG. Further, as in the third embodiment, the first wavelength group demultiplexing filter 403 corresponds to the second demultiplexing means 15 in FIG. The optical signal group of each wavelength group from the first wavelength group demultiplexing filter 403 is a pixel group in each column (first column, second column, third column, fourth column) on the 2D-LCOS element 407. Dispersion compensation is performed by 408a, 408b, 408c, and 408d. The optical signal groups of the four wavelength groups (G 1, G 2, G 3, G 4) after dispersion compensation are combined by the second wavelength group demultiplexing filter 404 and the spectroscope 405. Further, the interleaved group demultiplexing filter 406 interleaves the odd-numbered wavelength group and the even-numbered wavelength group and multiplexes them again into the optical signals of 40 communication channels having the center wavelengths from λ ₁ to λ _40. And output from the output port Out.

  Similarly to the third embodiment, the spectroscopes 402 and 405 and the group demultiplexing filters 403 and 404 are realized by one AWG 1 and one second demultiplexing unit 15 by the TODC block having the reflection type configuration of FIG. Needless to say, you can.

  The 2D-LCOS element 407 includes a first column of pixel groups 408a corresponding to the wavelength group G1, a second column of pixel groups 408b corresponding to the wavelength group G2, a third column of pixel groups 408c corresponding to the wavelength group G3, and a wavelength. Dispersion compensation is performed by the pixel group 408d in the fourth column corresponding to the group G4. Here, it should be noted that in the 2D-LCOS element 407, pixels are arranged in a two-dimensional manner in which pixel groups of each column are integrated in one element plane. Therefore, an optical signal is processed until an optical signal is emitted from the spectroscope 402 (AWG1) to the space, subjected to optical signal processing by the bulk diffraction grating 15 and the 2D-LCOS element, and again incident on the spectroscope 405 (AWG1). Is realized by a set of TODC blocks. That is, it should be noted that one TODC block shown by FIG. 9 can realize the same function as the tunable dispersion compensator shown in FIG.

  In the tunable dispersion compensator according to the second embodiment illustrated in FIG. 5, separate wavelength group demultiplexing filters 201 and 202 are required in addition to the four TODC blocks. On the other hand, since the variable dispersion compensator 400 of the fourth embodiment can be configured with only one TODC block shown in FIG. 9, the configuration of the variable dispersion compensator can be greatly simplified.

  Similar to the second embodiment, by separating the multiplexed WDM optical signal into a plurality of wavelength groups and including only one TODC block, an LCOS element having a short size in the spectral axis direction can be used. A multi-channel variable dispersion compensator excellent in mass productivity and cost can be realized without requiring a large LCOS element. By setting the AWG FSR value of the TODC block to be twice the wavelength group separation interval of the interleave type group demultiplexing filter, a variable dispersion compensator with a flat light transmission band can be realized. It is possible to reduce the cost of the tunable dispersion compensator, simplify the maintenance, and reduce the cost.

  As described above in detail, in the third and fourth embodiments, the number of TODC blocks is further reduced as compared with the variable dispersion compensator having the configuration of the first or second embodiment, so that a multi-channel variable dispersion compensator is obtained. Can be configured. By setting the FSR so as to have a predetermined relationship with the entire bandwidth of the WDM communication channel group covered by the TODC block, the light transmission characteristics can be flattened.

  As described in detail in Example 3 and Example 4, by using a 2D-LCOS element to form a two-dimensional pixel, an LCOS element in which the size in the direction corresponding to the spectral axis of the AWG is shortened can be obtained. Note that you get the same effect as using it.

The dispersion compensator of the second embodiment, the group branching filter 201 and 202, ₁₀ from lambda ₁ lambda, ₂₀ from lambda ₁₁ lambda, 4 two wavelength groups of lambda ₄₀ from lambda ₃₀ and lambda ₃₁ from lambda ₂₁ (G1, G2 , G3, G4), the optical signal group is multiplexed / demultiplexed. In order to realize such multiplexing / demultiplexing of optical signal groups, the optical signals of λ ₁ to λ _{10 in} the wavelength band of the wavelength group G1 are transmitted through the group branching filter with low loss, and adjacent wavelength groups are transmitted. A steep filter characteristic that cuts off optical signals from λ ₁₁ to λ _{20 in} the wavelength band of G2 is required. It is difficult to realize a group demultiplexing filter having such a steep filter characteristic.

  Further, in a communication system using an interleaver, communication channels are used in a skipped manner, and optical signal groups of wavelength groups corresponding to adjacent communication channels arranged consecutively as in the second embodiment are combined. A wave dispersion compensator cannot be used as it is. Therefore, in this embodiment, a variable dispersion compensator using an interleaver as a group demultiplexing filter and having a flat light transmission band will be described. Note that in the following description, there is a one-to-one correspondence between communication channels and wavelengths. That is, selecting an optical signal having a specific wavelength from the viewpoint of the physical properties of the optical signal is the same as selecting a corresponding channel from the viewpoint of the function for the corresponding communication channel.

  FIG. 11 is a block diagram illustrating an overall configuration of a tunable dispersion compensator according to the fifth embodiment of the present invention. The tunable dispersion compensator 500 of this embodiment has the same overall configuration as the tunable dispersion compensator of Embodiment 1 of FIG. 1 and Embodiment 2 of FIG. In other words, the tunable dispersion compensator 500 is shown as having a transmissive configuration, and a group demultiplexing filter 501 for group-demultiplexing multiplexed optical signals and dispersion compensation for the group-demultiplexed optical signals. The TODC blocks 503a, 503b, 503c, and 503d and a group demultiplexing filter 502 for group-combining the dispersion-compensated optical signals again. If the TODC block has a reflective configuration, it can be configured as a reflective variable dispersion compensator. In this case, it is needless to say that one group demultiplexing filter may be used. The following description will be given focusing on differences from the configuration of the second embodiment.

In the present embodiment, interleaved group group demultiplexing filters are used as the group demultiplexing filters 501 and 502 to group demultiplex the multiplexed optical signals. Hereinafter, the interleave type group demultiplexing filter in this embodiment is called an interleaver for simplicity. In this embodiment, in order to configure a dispersion compensator of 40 channels (channel numbers 1 to 40), the interleavers 501 and 502 are multiplexed with optical signals having 40 wavelengths from λ ₁ to λ ₄₀ . The optical signal is divided into four wavelength groups (G1, G2, G3, G4). In this embodiment, the wavelength of each wavelength group to be _{demultiplexed, G1 (λ 1, λ 5} , λ 9, ··· λ 37), G2 (λ 2, λ 6, λ 10, ··· λ 38 ), G3 (λ ₃ , λ ₇ , λ ₁₁ ,... Λ ₃₉ ), G4 (λ ₄ , λ ₈ , λ ₁₂ ,... Λ ₄₀ ), in the first and second embodiments. It is different from operation. That is, the interleaver demultiplexes or multiplexes all target wavelength groups (here, wavelength groups corresponding to 40 channels) into a predetermined number of wavelength groups (4). Here, it is assumed that the wavelengths (channels) in one wavelength group are selected at equal intervals. For example, the wavelength group G1 of this embodiment has channel numbers 1, 5, 9,... 37, and is selected at intervals of 4 channels.

Here, specifically, the interleaver may be configured by connecting two stages in which every other wavelength is divided into two groups. Alternatively, an array waveguide diffraction _{grating, λ 1, λ 5, λ} 9, may be constituted by those with orbiting of the spacing of · · · lambda _37.

  Four TODC blocks 503a, 503b, 503c, and 503d correspond to each of the four wavelength groups thus demultiplexed. Each TODC block includes an AWG, a condenser lens, a spatial phase modulation element (LCOS), and the like as in the configuration shown in FIG. As will be described below, the TODC block used in this embodiment includes four input / output waveguides at positions corresponding to the difference of one channel. These four input / output waveguides correspond to the four input / output ports (P1, P2, P3, P4) in the TODC block.

  FIG. 12 is a diagram illustrating a configuration of an AWG used in the variable dispersion compensator according to the fifth embodiment. Note that FIG. 5 shows the configuration of the AWG 601 in the reflective TODC block for simplicity. Therefore, the condenser lens and the signal processing element are not displayed. The AWG 601 includes four input / output waveguides 602a, 602b, 602c, 602d, a slab waveguide 603, and an arrayed waveguide 604. The arrayed waveguide 604 is described with the number of waveguides omitted. The input / output waveguides are connected at the boundary surface at one end of the slab waveguide 603, and the connection point is a distance that causes a difference of one channel with respect to the same condensing point of the beam emitted from the AWG. Just away. The ends of the four input / output waveguides 602a, 602b, 602c, and 602d correspond to the connection ports P4, P3, P2, and P1 to the outside of the TODC block.

  In the variable dispersion compensator of this embodiment, an example using LCOS as the spatial phase modulation element will be described. In this embodiment, the number of LCOS element elements (cells) corresponding to one channel (wavelength) and the AWG FSR are associated as described below, and are further selected discretely and periodically. Assign the assigned channel to each cell of LCOS. As a result, it is possible to avoid the effect that the light transmittance decreases at both ends in one transmission band corresponding to the FSR on the optical frequency axis. In the following description, a cell refers to a group of element elements corresponding to signal processing of one communication channel. Taking LCOS as an example, among a plurality of pixels arranged in the direction of the demultiplexing axis, a group of liquid crystal pixels in one section corresponds to a cell. In the following description, it should be noted that signal processing for one communication channel is performed by one cell.

FIG. 13 is a diagram for explaining channel assignment in the variable dispersion compensator of the fifth embodiment. Here, the cell numbers of the LCOS are numbered as viewed from the AWG toward the LCOS. (A) to (D) show how channels are assigned to TODC1 to TODC4, respectively. A channel assigned to TODC 1 will be described below with reference to (A). In the present embodiment, 40 continuous communication channels, that is, 40 wavelengths are demultiplexed into four wavelength groups G1 to G4. For TODC1, LCOS is applied to optical signals of 10 wavelengths in the wavelength group G1 (λ ₁ , λ ₅ , λ ₉ , λ ₁₃ , λ ₁₇ , λ ₂₁ , λ ₂₅ , λ ₂₉ , λ ₃₃ , λ ₃₇ ). Dispersion compensation is performed by each cell (10 cells).

  In the upper part of FIG. 13A, 11 cells arranged on the LCOS are indicated by cell numbers (1) to (11). As described above, the cell number of LCOS is indicated as a cell number (*) by a number with parentheses. On the other hand, below (A), there is shown an assignment table when channels are assigned by sequentially arranging channel numbers 1 to 40 in each cell, starting from a specific cell number (*). Here, the channel is assigned to the same cell in a cycle of 11 LCOS cells. For example, channel numbers 1, 12, 23, and 34 are assigned to cell number (4). In FIG. 13, channel numbers 8, 19, and 30 are assigned to the cell with the cell number (11) at the right end, but a channel that is not incident (not used) on this TODC is assigned so that this cell can be omitted. ing.

  As can be seen from the channel assignment table shown in FIG. 13, this embodiment is characterized in that 11 LCOS cells are assigned to one wavelength group including 10 wavelengths, that is, 10 communication channels. is there. That is, the AWG in each TODC block has an FSR corresponding to 11 channels. Returning to (A) again, the TODC 1 receives the group of optical signals demultiplexed into the wavelength group G1 by the interleaver, and therefore it is only necessary to perform dispersion compensation for the skipped wavelength (channel). The wavelength (channel) corresponding to the wavelength group G1 corresponds to the channel that is hatched in the channel assignment table of (A). As is clear from (A), all wavelengths (channels) included in the wavelength group G1 are assigned to each cell of the LCOS cell number (1) to cell number (10) without overlapping and without any gaps. ing. Similarly, as shown in (B) to (D) of FIG. 13, for TODC2 to TODC4, the wavelengths (channels) of the corresponding wavelength groups G2 to G4 are changed from the cell numbers (1) to (10) of the LCOS. ) Are assigned without overlapping each other and without any gaps.

  Here, it should be noted that the cell number (*) of the cell to which channel number 1 is assigned is shifted by one for each TODC. That is, in TODC1, channel number 1 is assigned from the cell with cell number (4). On the other hand, channel number 1 is assigned from cell number (3) in TODC2, cell number (2) in TODC3, and cell number (1) in TODC4. In order to assign the optical signal of the same channel for each TODC block by shifting the cell number (*) by one channel, the connection position of the AWG input / output waveguide with the slab waveguide is set to one channel for each TODC block. Just shift it considerably. This comprises four input / output waveguides 602a, 602b, 602c, and 602d as shown in FIG. 12, and the connection position at the boundary surface between each input / output waveguide and the slab waveguide 603 is shifted by one channel. Can be realized. When the variable dispersion compensator of FIG. 11 is actually mounted, the connection ports P1 to P4 corresponding to the four input / output waveguides may be selected.

  FIG. 14 is another diagram illustrating channel assignment in each TODC block of the tunable dispersion compensator according to the fifth embodiment in association with the FSR. Similarly to FIG. 13, LCOS cell numbers and channel numbers assigned to each TODC are shown together. Furthermore, the light transmittance characteristics of the AWG used in this embodiment are displayed at the top of FIG. 14 with the wavelength axis and the arrangement direction of the LCOS elements aligned. As shown in FIG. 14, the AWG has FSRs corresponding to 11 channels, and corresponds to the FSR by matching the portion where the light transmittance is reduced with the position of the unused cell number (11) of the LCOS. The influence of a decrease in light transmittance at both ends of one transmission band can be avoided.

  Even if the influence of FSR is reduced by using the configuration of the above-described embodiment, a decrease in light transmittance at both ends of the transmission band may still remain. At that time, a variable optical attenuator (VOA) function can be provided by adding a phase difference or the like in the spatial phase modulation element, so that the optical loss level between channels can be made more uniform.

  In the first embodiment, the transmission band of one wavelength group and the AWG FSR are completely matched. However, in this embodiment, the maximum number of wavelengths (10 wavelengths) included in one wavelength group. Is different in that the AWG has an FSR equivalent to 11 channels with respect to the entire bandwidth (10 channels) corresponding to the. The LCOS cell count (11) also corresponds to the AWG FSR. It should be noted that the present embodiment is different from the first to fourth embodiments in that each wavelength included in one wavelength group is discretely selected by an interleaver.

  By having the FSR having the relationship as described above, it is possible to match the light transmittance decrease region at the end of the transmission band with one unused cell of the LCOS, and to avoid the influence of the light transmittance decrease. Further, in the present embodiment, in one wavelength group that is group-demultiplexed, a plurality of continuous channels are selected in that wavelengths (channels) are discretely and periodically selected by an interleaver. Note the difference from ~ 4. By associating a predetermined number of cells of the LCOS with channels selected discretely and periodically according to a predetermined condition by the interleaver, the channels are not overlapped with all the cells on the LCOS, and the gap Can be assigned without.

The configuration having the characteristics of the fifth embodiment can be generalized more widely. Hereinafter, the relationship between the characteristics of the group demultiplexing filter, the number of wavelength groups demultiplexed by the group demultiplexing filter, and the number of AWG FSR and LCOS cells will be examined. As shown in FIG. 14, the number of wavelength groups demultiplexed by the wavelength group filter is b. In Example 5, b = 4. Here, for simplicity, it is assumed that b is a power of 2, and b = 2, 4, 8, 16, 32. The AWG's FSR is expressed by the following equation, where a is the maximum number of channels included in one wavelength group plus 1 unused cell number, and B is the bandwidth of one communication channel. The
FSR = a × B Formula (2)
At this time, the maximum number of communication channels included in one wavelength group, that is, the number of communication channels multiplexed / demultiplexed by the AWG is (a-1). Under the specific numerical conditions of the fifth embodiment, the total number of channels (number of wavelengths) N processed by the dispersion compensator of the present embodiment is N = (a−1) × b. Even if the total number of channels N is equal to or less than (a-1) × b and does not match , only a cell having no corresponding communication channel is included in some TODC blocks. The effects of flatness and efficient channel allocation to LCOS cells are maintained.

  If Example 5 is made to correspond to a specific example, a = 11, and if the bandwidth B of one communication channel is 100 GHz, the FSR is 11 × 100 GHz = 1100 GHz. The number of LCOS cells is 11, and the maximum number of communication channels is 10. Of the 11 cells arranged on the LCOS, 10 cells are used for signal processing for dispersion compensation. Under the above conditions, when a and b have a prime relationship, channels can be assigned to all cells on the LCOS without overlapping each other and without any gaps.

  In FIG. 13 and FIG. 14, the rightmost cell of the LCOS cell is unused, but the present invention is not limited to this. Any one of 11 LCOS cells may be unused. However, cells in the same position must be unused in all TODC blocks. In FIG. 13, the cell number (*) to which channel number 1 is assigned in each TODC block is shifted by one for each TODC block, but this can be realized by selecting a port. That is, the AWG of each TODC block may have exactly the same configuration, and it is only necessary to select a port corresponding to the wavelength group of each TODC block.

  FIG. 15 is a diagram illustrating the relationship between the total number of wavelengths and the number of LCOS elements applicable to the dispersion compensator according to the fifth embodiment. Depending on the total number of channels (number of wavelengths) for dispersion compensation, the number of wavelength groups (G1, G2,... Gb) multiplexed / demultiplexed by the interleaver and the maximum number of communication channels included in one wavelength group, By selecting an appropriate number of LCOS element cells and corresponding AWG FSR, the conditions of this embodiment can be satisfied.

  By using the dispersion compensator of this embodiment, one unused cell among the plurality of LCOS cells is matched with the region where the transmittance is reduced in the transmission band of the FSR. Can be avoided. With the exception of one unused cell, communication channels can be efficiently allocated to all other cells of the LCOS without waste, and a variable dispersion compensator with a flat light transmission band can be realized. Unused cells can ease the design requirements at the end of the AWG's FSR transmission band. Also in this embodiment, phase characteristics having various shapes as described in the previous embodiments can be given to each cell.

  This embodiment can be applied to both a reflection type and a transmission type dispersion compensator. Further, the present invention is not limited to LCOS, and can be applied to other spatial phase modulation elements including element elements. In addition, the relationship between the number of target communication channels, the number of LCOS cells, and the AWG FSR used in the fifth embodiment is other than the dispersion compensator in that the influence of the decrease in the transmission characteristics related to the AWG FSR can be avoided. The present invention can also be applied to an optical signal processing apparatus that performs the above signal processing.

  As described in detail above, a multi-channel variable dispersion compensator can be configured by further reducing the number of TODC blocks as compared with the variable dispersion compensator having the configuration of the first or second embodiment. Even when an interleaved group demultiplexing filter is used, the light transmission characteristics can be flattened. Further, by setting the FSR so as to have a predetermined relationship with the total bandwidth of the communication channel group covered by each TODC block, it is possible to realize flattening of the light transmission characteristics.

  Also in the fifth embodiment, as described in detail in the third and fourth embodiments, a pixel is two-dimensionally configured using a 2D-LCOS element, so that a direction corresponding to the spectral axis of the AWG can be obtained. The same effect as using an LCOS element with a reduced size can be obtained.

  The present invention can be used for a chromatic dispersion compensator in an optical communication system.

1, 34, 601 AWG
3, 603 Slab waveguide 4, 604 Array waveguide 7, 36 Condensing lens 8, 37 Spatial phase modulation element 11, 14, 30 Optical circulator 15 Bulk diffraction grating 16, 17, 602a, 602b, 602c, 602d Input / output Waveguide 19, 401, 406, 501, 502 Interleave type group demultiplexing filter 100, 200, 300, 500 Variable dispersion compensator 101, 102, 201, 202, 302, 303 Group demultiplexing filter 103, 103a, 103b, 103c 103d, 203a, 203b, 203c, 203d, 400, 503a, 503b, 503c, 503d TODC block 301, 304 duplexer 305, 407 2D-LCOS element

Claims (5)

In a dispersion compensator for an interleaved communication system that performs signal processing by branching optical signal groups of N consecutive communication channels into a plurality of wavelength groups including channels discretely selected at predetermined channel intervals,
An interleaved group demultiplexing filter that multiplexes and demultiplexes the optical signal groups of the N communication channels into b wavelength groups each having a maximum of (a-1) communication channels;
B dispersion compensator blocks corresponding to each of the b wavelength groups,
Each dispersion compensator block is
Optical multiplexing / demultiplexing means for further demultiplexing an optical signal of one corresponding wavelength group among the b wavelength groups , where B is the bandwidth of one communication channel, (B × a) It has FSR, each of which has b input / output ports each having an optical frequency difference corresponding to one communication channel bandwidth B, and corresponds to the corresponding one wavelength group among the b input / output ports Optical multiplexing / demultiplexing means for selecting an input / output port;
A spatial phase modulation element including a plurality of element elements arranged in the direction of a demultiplexing axis of the optical multiplexing / demultiplexing means, wherein the plurality of element elements are divided into a sections each corresponding to one communication channel. And one of the a sections includes a spatial phase modulation element not used for optical signal processing,
A dispersion compensator, wherein a and b are relatively prime, and are selected so as to satisfy the relations of a ≧ 3, b ≧ 2 and N ≦ (a−1) × b. In each of the spatial phase modulation elements, a phase defined by a function of second order or higher is set independently for each section corresponding to each optical signal, with the distance of the demultiplexing axis as a parameter. The dispersion compensator according to claim 1.   3. The dispersion compensator according to claim 1, wherein the multiplexing / demultiplexing means is an arrayed waveguide diffraction grating (AWG), and the spatial phase modulation element is LCOS.   4. The dispersion compensator according to claim 1, wherein a power of 2 is selected for b. The spatial phase modulation element reflects an incident optical signal corresponding to the communication wavelength, and the optical group multiplexing / demultiplexing means and the group demultiplexing filter have a reflection type configuration that multiplexes the dispersion-compensated optical signal. dispersion compensator according to claim 1 to 4, characterized in that.