JP3605336B2 - Optical filter - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical component for optical communication or optical information processing, and more particularly to an optical filter for shaping or controlling an optical signal spectrum in the field of optical communication.
[Prior art]
[0002]
2. Description of the Related Art Conventionally, optical filters used in optical communication and the like are used for shaping or controlling the spectrum of an optical signal, and play an important role in an optical wavelength division multiplexing communication network. There are various types of optical filters, for example, an optical fiber Bragg grating type filter, a Mach-Cheinder interferometer type filter, an optical fiber Bragg grating type filter, an optical fiber long period grating type filter and the like are known.
[0003]
FIG. 10 shows an optical fiber long-period grating type filter 100 using a conventional optical fiber long-period grating. The optical fiber long-period grating type filter 100 has a refractive index modulation unit 103 for periodically modulating the refractive index of propagating light at a predetermined position on an optical fiber core 102 provided at a central position of an optical fiber clad 101. Provided at regular intervals.
[0004]
The refractive index modulation unit 103 periodically couples the optical signal incident from the input unit with the cladding mode by light irradiation or the like. Therefore, the optical fiber long-period grating type filter 100 can resonately convert only the wavelength component whose propagation constant difference between the propagation mode and the cladding mode coincides with the period of the refractive index modulation into the cladding mode and radiate it. It is suitable for removing only a specific wavelength component from a plurality of wavelength components. A typical period of the refractive index modulation unit 103 is about several hundred μm.
[0005]
As a filter using an optical fiber long-period grating, a notch-type filter that removes only a desired wavelength component is simpler than a dielectric multilayer filter, an arrayed waveguide grating filter, or a Mach-Cheander interference type filter. In addition, unlike the optical fiber Bragg grating, there is an advantage that an additional optical component such as a circulator is not required because of a transmission type configuration.
[0006]
Therefore, a filter using the optical fiber long-period grating has been put to practical use for gain equalization of an optical fiber amplifier system (for example, A. Vengsarkar, J. Pedrazzani, J. Judkins, P. Lemaile). , N. Bergano, and C. Davidson, "Long-period fiber-grating-based gain equalizers," OSA Optics Letters, vol. 21, pp. 336-382, 1996.).
[0007]
[Problems to be solved by the invention]
However, a conventional filter using an optical fiber long-period grating has the following problems.
(1) Since the optical fiber long-period grating uses the coupling between the propagation mode and the cladding mode, it is sensitive to a change in the material of the outer periphery of the cladding such as the jacket, and the problem that the filter characteristics change due to environmental changes. was there.
[0008]
(2) In an optical fiber long-period grating, since the coupling between the propagation mode and the cladding mode is used, the filter characteristics are sensitive to the cladding outer diameter and the dispersion characteristics of the cladding mode, and it is difficult to realize a precise filter. there were.
[0009]
(3) The optical fiber long-period grating has a problem that the change in the refractive index modulation is attenuated with time to realize the refractive index modulation by light irradiation or the like, and the filter characteristic lacks long-term stability.
[0010]
The present invention has been made in view of the above problems, and an object of the present invention is to provide an optical filter that is stable against environmental changes and elapse of time and that can precisely design filter characteristics.
[0011]
The present invention has been made in view of the above problems, and has an input optical waveguide formed on a substrate, mode coupling means for coupling a fundamental mode and a primary mode, and conversion between a fundamental mode and a primary mode. An optical filter in which a mode conversion unit and an output optical waveguide are coupled in this order, wherein the mode coupling unit is an optical waveguide, and the mode coupling unit is a direction orthogonal to a light propagation direction. A mode switching portion is formed for each interval W, and the interval W and the amount of shift of the optical waveguide width are set so as to satisfy the phase matching condition in which the fundamental mode and the primary mode in the optical wavelength of the input signal resonate. The set optical filter.
[0012]
With such a configuration, the mode coupling unit forms the mode switching unit at a position where the optical waveguide is formed so as to be shifted in a direction orthogonal to the light propagation direction. When the fundamental mode and the first-order mode are handled, for example, when the fundamental mode symmetrical with respect to the center of the optical waveguide enters the mode switching unit of the mode coupling unit, the optical waveguide after the mode switching unit has the center of the optical waveguide. And a part of the fundamental mode can be switched to a first-order mode that is asymmetric with respect to the center of the optical waveguide. The phase matching condition in the mode coupling unit 4 is such that, assuming that the mode switching unit 4a has a constant interval Λ, the beat length between the fundamental mode and the primary mode of the mode coupling unit 4 at the optical wavelength of the input signal is Lb. Then, Λ = Lb / 2.
[0013]
Further, an input optical waveguide formed on a substrate, a first mode conversion means for converting the fundamental mode and first-order mode, and mode coupling means for coupling the fundamental mode and first-order modes, the fundamental mode and first-order mode a second mode converter for converting the output optical waveguide is an optical filter coupled in this order, the mode coupling means is an optical waveguide by shifting in a direction perpendicular to the propagation direction of the light The formed mode switching portion is provided for each interval W, and the interval W and the amount of shift of the optical waveguide width are set so as to satisfy a phase matching condition in which a fundamental mode and a primary mode in a predetermined optical wavelength component resonate. Optical filter.
[0014]
With such a configuration, the plurality of optical signals are respectively converted into the fundamental mode or the primary mode by the first mode converter. The mode coupling means forms the optical waveguide so as to be shifted in a direction orthogonal to the propagation direction, so that the mode coupling unit is provided at the position of the shifted optical waveguide. Therefore, when the fundamental mode enters the mode switching unit, the optical waveguide after the mode switching unit becomes left-right asymmetric with respect to the center of the optical waveguide, and a part of the fundamental mode is switched to the primary mode. Further, a part of the primary mode is replaced with the fundamental mode.
[0015]
The optical filter of each configuration may have a configuration in which the mode coupling ratio in the mode coupling unit is set low at the end of the mode coupling unit and high at the center. With this configuration, the spatial distribution of the mode coupling coefficient can be set low at the end of the mode coupling unit and high at the center of the mode coupling unit. Therefore, it is possible to reduce the side lobe of the optical filter characteristic given by the Fourier transform of the spatial distribution of the mode coupling coefficient.
[0016]
Further, the mode conversion means may be constituted by a bent optical waveguide, or may be constituted by a multi-mode interference optical coupler. By using the bent optical waveguide, a higher-order mode including the first-order mode can be easily dissipated by radiation, and the fundamental mode can be hardly dissipated. Further, by using the multi-mode interference optical coupler, the number of optical waveguides connected to the input optical waveguide or the output optical waveguide can be treated as plural. In addition, the optical waveguide that propagates light from the input optical waveguide to the output optical waveguide may be a silica-based optical waveguide. By using a silica-based optical waveguide, a stable and highly accurate optical waveguide can be realized.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram illustrating an optical filter according to the first embodiment, FIG. 2 is a schematic diagram illustrating an optical filter according to the second embodiment, and FIG. 3 is a schematic diagram of a main part illustrating a configuration of a mode coupling unit. FIG. 4A is a schematic diagram illustrating another configuration of the mode coupling unit, FIG. 4B is a schematic diagram illustrating another configuration of the mode coupling unit, and FIG. 5 illustrates a mode conversion unit using a bent optical waveguide. FIG. 6 is a schematic diagram showing a mode conversion unit using a multi-mode interference optical coupler, and FIG. 7 is a principle diagram showing an operation of the optical filter according to the second embodiment.
[0018]
As shown in FIG. 1, an optical filter 1 includes a substrate 2 such as a silicon substrate, an input optical waveguide 3 formed at a predetermined position on the substrate 2, and a mode coupling formed continuously with the input optical waveguide 3. A mode conversion means 5 formed continuously with the mode coupling means 4; and an output optical waveguide 6 formed continuously with the mode conversion means 5. Here, as an example, a case where the number of output optical waveguides 6 is three (first to third output optical waveguides 6a, 6b, 6c) will be described. The number of the output optical waveguides 6 may be one, two, or four or more.
[0019]
The input optical waveguide 3 is configured to transmit the transmitted optical signal to the mode coupling means 4.
The mode coupling means 4 has an optical waveguide width for stably guiding the fundamental mode and the first-order mode. As shown in FIG. 1 and FIG. 3, the mode switching part (connection point where the optical waveguides are shifted and joined) 4a of the mode coupling means 4 is provided between the optical waveguides at a constant interval Λ in the light propagation direction. (Five locations in the drawing). The mode switching unit 4a of the mode coupling unit 4 configures the shift amount D of the optical waveguide width through which the optical signal can propagate to a constant (or according to a certain function) state so that the transmitted fundamental mode and primary mode This causes a coupling between the modes.
[0020]
Phase matching condition in the mode coupling means 4, when the mode interchanging unit 4a is constant intervals lambda, the beat length between the fundamental mode and first-order modes of the mode coupling means 4 in the optical wavelength of the input signal is a L _b Sometimes, it is determined by Λ = L _b / 2. Therefore, only the wavelength component of the light that satisfies the phase matching condition is resonantly converted to the first-order mode.
[0021]
Therefore, the optical filter 1 can be configured to remove only a predetermined wavelength component if only the fundamental mode is extracted by the mode coupling means 4, or if the primary mode is extracted only by the mode conversion means 5, It can be configured to extract only the wavelength component.
[0022]
The mode switching unit 4a of the mode coupling unit 4 may be configured as shown in FIGS. 4 (a) and 4 (b). That is, as shown in FIG. 4 (a), the optical waveguides on the input optical waveguide 3 side and the output optical waveguide 6 side are formed so as to have a small mode coupling amount and to have a large mode coupling amount at the central portion. It is composed. By configuring the mode coupling means 4 as shown in FIG. 4A in this way, it is possible to perform the apotizing process performed by the optical fiber grating.
[0023]
Further, as shown in FIG. 4B, the mode switching unit 4a of the mode coupling unit 4 may be configured to be shifted in the same direction. In the configuration of FIG. 4 (b), the phase matching condition for the fundamental mode and first-order mode resonance is given by Λ = L _b.
[0024]
On the other hand, the mode conversion means 5 discriminates between the fundamental mode and the primary mode sent from the mode coupling means 4 and guides them to the output optical waveguide 6.
[0025]
The configuration of the mode conversion means 5 may be the configuration shown in FIGS. 5 and 6 as an example. In other words, the mode conversion means 5A (5) forms a waveguide 5b ₁ curved continuously from the input port 5a as shown in FIG. 5, so as to have an output port 5c to the curved rear end of the waveguide 5b ₁ It is configured. The curvature of the curved waveguide 5b ₁ is a fundamental mode is propagated almost without loss, the primary mode is set to be lost by radiation. In the case where the bent waveguide 5b ₁ shown in FIG. 5 is used, the input port 5a and the output port 5b correspond one-to-one with the optical waveguide. It is configured to be connected to one central optical waveguide 6b.
[0026]
Therefore, the mode converting means 5A can be used as a mode converting means for converting an input optical signal into a fundamental mode or a mode converting means for guiding only a fundamental mode of an input optical signal to an output. it can. Incidentally, the layout of the waveguide 5b ₁ bend shown in Figure 5 is not limited in particular is an example.
[0027]
Further, as shown in FIG. 6, another mode conversion means 5B (5) may be used. That is, the mode conversion means 5B uses a multi-mode interference optical coupler. The mode conversion means 5B includes an input port 5a formed on one end, a multimode interference optical waveguide 5b ₂ which is formed continuously to the input port 5a, formed continuously to the other end of the multimode optical waveguide 5b ₂ Output port 5c. The output port 5c is a drawing to correspond to the output optical waveguides 6 (see FIG. 1), the first output port 5c _1, the second output port 5c _2, and a third output port 5c _3.
[0028]
The effective optical waveguide width of the input port 5a is set to be equal to the second output port 5c ₂ effective optical waveguide width. Furthermore, the first output port 5c ₁ and the third output port 5c _3, the effective optical waveguide width is set to be 1/2 of the input ports 5a and the second output port 5c ₂ of the effective optical waveguide width I have.
The length L of the multimode interference optical waveguide 5b _2, upon the beat length L _c between the fundamental mode and the first-order mode in the multimode interference optical waveguide 5b _2, substantially L = 3L c / ₄ It is set to be.
[0029]
The mode conversion means 5B is a fundamental mode input from the input port 5a by mode dispersion in multimode interference optical waveguide 5b _2, leading to a second output optical waveguide 6b of output optical waveguides 6, further input port Mode conversion means for guiding the first-order mode input to the first and third output optical waveguides 6a and 6c to the first and third output optical waveguides 6a and 6c can be realized (for example, J. Leutold, JE Gamper, PA Besse, and H). Melchoir, "Multimode interference couplers for the conversion and combining of zero-and first-order models," IEEE / OSA Journal blogging. no. 7, pp. 1228-1239, see 1998).
[0030]
Incidentally, Invert and use an input port 5a and an output port 5c of the mode conversion means 5B, leading to an input port 5a of the light input to the second output port 5c ₂ as a base mode, further, the first and third It is possible to realize a mode conversion unit that guides light input to the output ports 5c ₁ and 5c ₃ to the input port 5a as a primary mode.
[0031]
Next, another embodiment showing an optical filter will be described with reference to FIG. Note that the same members as those in the configuration shown in FIG.
The optical filter 10 includes a substrate 2 such as a silicon substrate, an input optical waveguide 13 formed at a predetermined position on the substrate 2, a first mode conversion unit 17 formed continuously with the input optical waveguide 13, The mode coupling means 4 formed continuously with the first mode conversion means 17, the second mode conversion means 5 formed continuously with the mode coupling means 4, and the mode coupling means 4 And an output optical waveguide 6 formed as described above.
[0032]
The first and second mode converters 17 and 5 have the same configuration as the mode converter 5 already described with reference to FIGS. 1, 5 and 6. The input optical waveguide 13 includes first to third input optical waveguides 13a, 13b, and 13c. The number of the input optical waveguides 13 may be one, two, or four or more. Here, three are set as an example.
[0033]
Since the optical filter 10 has the mode conversion means 17 on the input side, it is set so that the fundamental mode of the optical waveguide is input to the mode coupling means 4 or the primary mode of the optical waveguide is input to the mode coupling means 4. A mode can be set so as to be input, and further, a setting can be made so that the base mode and the primary mode are input to the mode coupling means 4.
[0034]
FIG. 7 shows an operation example of the optical filter 10. Here, for the sake of explanation, the following operations from input to output of an optical signal are performed. That is, the optical signal input to the first input optical waveguide 13a (or the second input optical waveguide 13b) of the input optical waveguide 13 reaches the mode coupling unit 4 as the first mode of the optical waveguide by the mode conversion unit 17. Shall decide. Further, it is assumed that the optical signal input to the second input optical waveguide 13b of the input optical waveguide 13 reaches the mode engaging means 4 as the fundamental mode of the optical waveguide by the mode converting means 17. The first-order mode optical signal output from the mode coupling unit 4 is guided to the first and third output optical waveguides 6 a and 6 c of the output optical waveguide 6 by the mode conversion unit 5. The optical signal of the fundamental mode of the optical waveguide output from the mode coupling means 4 is guided to the second output optical waveguide 6b of the output optical waveguide 6.
[0035]
Now, as shown in FIGS. 2 and 7, when the optical signal 1 and the optical signal 2 having a plurality of wavelength components are input from the first input optical waveguide 13a and the second input optical waveguide 13b, respectively, At 17, the optical signal 1 and the optical signal 2 are converted into the first mode and the fundamental mode of the optical waveguide, respectively, and guided to the mode coupling unit 4. Here, the mode coupling means 4 is set so that coupling between the fundamental mode and the first-order mode occurs periodically, so that only the specific wavelength component is mode-resonantly coupled to form the optical signal 1 and the optical signal 2. Exchanged between Then, the optical signal 1 and the optical signal 2 in which only specific wavelength components are exchanged are converted by the mode converting means 5, and the optical signal 1 is converted into the first output optical waveguide 6a (and the third output optical waveguide 6c). Further, the optical signal 2 is extracted from the second output optical waveguide 6b. Therefore, with such a configuration of the optical filter 10, for example, it is possible to replace the wavelength channels of the wavelength-multiplexed optical signal.
[0036]
【Example】
Next, a case where the optical filter shown in the second embodiment is actually formed using a silica-based optical waveguide will be described. First, an SiO ₂ lower cladding layer was deposited on a silicon substrate by a flame deposition method, and then a SiO ₂ glass core layer to which GeO ₂ was added as a dopant was deposited, and then glass-cleared in an electric furnace. Next, the core layer was etched to form a core portion. Finally, an SiO ₂ upper clad layer was deposited, and again glass-cleared in an electric furnace. Here, the relative refractive index of the optical waveguide was 1.5%, and the core thickness was 4.6 μm.
[0037]
In the circuit thus created, the multi-mode interference optical coupler shown in FIG. 6 was used as the mode conversion means. Here, the width of the multimode interference optical waveguide was set to 34 μm and the length was set to 1247.2 μm in order to obtain the mode conversion characteristics. Further, the mode coupling means forms the mode switching part by alternately shifting the optical waveguide perpendicularly to the light propagation direction. The shift amount of this mode switching unit was 0.1 μm, and the interval W (see FIG. 3A) between adjacent mode switching units (connection points) was 136.9 μm. In addition, the number of the mode switching units was 60.
[0038]
In the circuit thus created, the multi-mode interference optical coupler shown in FIG. 6 was used as the mode conversion means. Here, the width of the multimode interference optical waveguide was set to 34 μm and the length was set to 1247.2 μm in order to obtain the mode conversion characteristics. Further, the mode coupling means forms the mode switching part by alternately shifting the optical waveguide perpendicularly to the light propagation direction. The shift amount of this mode switching unit was set to 0.1 μm, and the interval W ( see FIG. 3 ) between adjacent mode switching units (connection points) was set to 136.9 μm. In addition, the number of the mode switching units was 60.
[0039]
Further, FIG. 9 is a configuration of the optical filter, the shift amount D mode interchanging part of the mode coupling means (see Figure 3) was set to be large in the central portion of (e.g., FIGS. 4 (a) Configuration) when the filter characteristics. As shown in FIG. 9, when the light is input to the input optical waveguide 13b, the curve shows the first and third output optical waveguides 6a and 6c and the second output optical waveguide 6b (see FIG. 2). This is a light wavelength dependency indicating transmittance.
[0040]
The profile indicating the shift amount of the mode switching unit of the mode coupling unit was set such that the coupling ratio became a so-called Hanning window function. Then, the maximum displacement amount D here (see Figure 3) is set to be 0.2 [mu] m. From FIG. 9, it can be seen that the wavelength components extracted from the output optical waveguides 6a and 6c have suppressed spectral side lobes.
【The invention's effect】
Since the present invention is configured as described above, it has the following excellent effects.
(1) The optical filter can realize a stable filter characteristic that selectively removes a desired wavelength component by using periodic coupling of the fundamental mode and the first-order mode of the optical waveguide. In addition, when performing periodic coupling between the fundamental mode and the first-order mode of the optical waveguide, the optical filter uses a shift perpendicular to the light propagation direction of the optical waveguide, so that precise filter characteristics are designed. be able to. Then, the optical filter can realize a configuration including a mode conversion unit that is stable against the passage of time and environmental changes.
[0042]
(2) Since the optical filter is provided with the mode conversion means 17 on the input side, the mode coupling means is configured to input the fundamental mode of the optical waveguide or the primary mode of the optical waveguide. Further, it is possible to set so that both the fundamental mode and the primary mode are input. Therefore, the optical filter can replace the wavelength channels of the wavelength-multiplexed optical signal.
[0043]
(3) The optical filter can be configured to reduce the side lobe by changing the mode coupling ratio between the center and the end of the mode coupling means.
[0044]
(4) The optical filter can realize a configuration for selecting the mode of the optical signal with a simple configuration by using a bent optical waveguide as the mode conversion means. Further, by using a multi-mode interference optical coupler as the mode conversion means, it is possible to exchange the wavelength channels of the wavelength-multiplexed optical signal. Further, by using a silica-based optical waveguide as the optical waveguide of the optical filter, a stable and highly accurate optical waveguide can be realized.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating an optical filter according to a first embodiment of the present invention.
FIG. 2 is a schematic diagram illustrating an optical filter according to a second embodiment of the present invention.
FIG. 3 is a schematic diagram of a waist showing a configuration of a mode coupling unit of the present invention;
4A is a schematic diagram showing another configuration of the mode coupling unit, and FIG. 4B is a schematic diagram showing another configuration of the mode coupling unit of the present invention.
FIG. 5 is a schematic diagram showing a mode conversion unit using a bent optical waveguide according to the present invention.
FIG. 6 is a schematic diagram showing mode conversion means using the multi-mode interference optical coupler of the present invention.
FIG. 7 is a principle diagram showing an operation of the optical filter according to the second embodiment of the present invention.
FIG. 8 is a graph showing filter characteristics of an optical filter according to a second embodiment of the present invention.
FIG. 9 is a graph showing filter characteristics when an optical filter according to a second embodiment of the present invention is used as the mode coupling means having the configuration shown in FIG. 3B.
FIG. 10 is a schematic diagram showing a conventional optical filter.
[Explanation of symbols]
1,10 Optical filter 2 Substrate 3,13 Input optical waveguide 4 Mode coupling means (second mode coupling means)
5 Mode conversion means 5a Input port 5b ₁ Bend optical waveguide 5b ₂ Multi-mode interference optical waveguide 5c Output port 5c ₁ First output port 5c ₂ Second output port 5c ₃ Third output port 6, 13 Output optical waveguide 6a First Output optical waveguide 6b Second output optical waveguide 6c Third output optical waveguide 13a First input optical waveguide 13b Second input optical waveguide 13c Third input optical waveguide

Claims (6)

The input optical waveguide formed on the substrate, the mode coupling means for coupling the fundamental mode and the primary mode, the mode conversion means for converting the fundamental mode and the primary mode, and the output optical waveguide were coupled in this order. An optical filter,
The mode coupling means is an optical waveguide, and has a mode switching portion formed for each interval W so as to be shifted in a direction orthogonal to the light propagation direction, and the amount of shift of the interval W and the optical waveguide width is as follows. An optical filter, which is set so as to satisfy a phase matching condition in which a fundamental mode and a first-order mode in an optical wavelength of an input signal resonate . Converting an input optical waveguide formed on a substrate, a first mode conversion means for converting the fundamental mode and first-order mode, and mode coupling means for coupling the fundamental mode and first-order modes, the fundamental mode and first-order mode An optical filter in which the second mode conversion means and the output optical waveguide are coupled in this order,
The mode coupling means is an optical waveguide, and has a mode switching portion formed for each interval W so as to be shifted in a direction orthogonal to the light propagation direction, and the amount of shift of the interval W and the optical waveguide width is as follows. optical filter, characterized in that the fundamental mode and first-order mode definitive a predetermined light wavelength component is set so as to satisfy the phase matching condition for resonance. 3. The optical filter according to claim 1, wherein the mode conversion unit is a bent optical waveguide. 3. The optical filter according to claim 1, wherein the mode converter is a multi-mode interference optical coupler. 5. The mode coupling ratio according to claim 1, wherein a mode coupling ratio in the mode coupling unit is set low at an end of the mode coupling unit and high at a center of the mode coupling unit. Optical filter. The optical filter according to claim 1, wherein the optical waveguide that propagates light from the input optical waveguide to the output optical waveguide is a silica-based optical waveguide.

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