JP7809389B2 - Tunable wavelength filter - Google Patents

Description

This disclosure relates to a wavelength tunable filter.

Conventionally, optical communication networks using wavelength division multiplexing (WDM) optical communication technology are known. Optical amplifiers are installed in optical communication networks to compensate for transmission losses in optical fibers. Optical amplifiers generate amplified spontaneous emission (ASE) noise in optical signals. For this reason, tunable wavelength filters are installed in optical communication networks to remove ASE noise.

Known wavelength tunable filters include a diffraction grating (see, for example, Patent Documents 1, 2, and 3). These wavelength tunable filters use a mirror to reflect diffracted light from the diffraction grating corresponding to the input light so that only specific wavelength components contained in the input light are optically coupled to the output optical fiber.

U.S. Patent Publication No. 2008/0085119 Japanese Patent Application Laid-Open No. 2008-203508 Japanese Patent Application Laid-Open No. 2020-122936

There is a demand for miniaturization of optical communication devices that incorporate tunable filters, such as optical transceivers. To achieve this, the tunable filter must be placed in a small space. Effective wavelength dispersion is required to achieve the function of a tunable filter in a small space. Therefore, in order to place a tunable filter in a small space, the angle of incidence of light on the diffraction grating must be set large.

However, the input light to a diffraction grating is a light beam with a diameter. Therefore, if the incident angle is set large, a non-negligible difference occurs in the optical path length at each point of the light beam as it passes through the diffraction grating, reflects off the mirror, and passes through the diffraction grating again. Due to this difference, the diffraction grating outputs a diffracted light beam whose cross section is tilted or rotated relative to the input light beam. The input light beam is, for example, an elliptical beam. In this case, a diffracted light beam is generated whose major axis is tilted relative to the input light beam.

When such tilt or rotation occurs, in conventional methods, the wavelength component selection ability of the optical selection element (e.g., a mirror) that selects the wavelength component from the diffracted light beam to be optically coupled to the output optical fiber is reduced. For this reason, when placing a wavelength tunable filter in a narrow space, conventional methods cannot achieve a steep filter slope.

In recent WDM networks, optical signals are densely spaced along the frequency axis to improve spectral efficiency. Therefore, failure to properly set the filter slope or bandwidth increases the likelihood of optical signals interfering with adjacent channels.

Therefore, according to one aspect of the present disclosure, it is desirable to provide a novel technology related to wavelength tunable filters for appropriately setting the filter slope or bandwidth.

According to one aspect of the present disclosure, there is provided a wavelength tunable filter. The wavelength tunable filter includes a transmission diffraction grating, a reflective element, and a light selection element.

The transmissive diffraction grating is positioned in the propagation path of the input light. The reflective element is positioned opposite the transmissive diffraction grating so as to reflect the input light that has passed through the transmissive diffraction grating. The light selection element is positioned in the propagation path of the diffracted light, which is the input light that has been reflected by the reflective element and passed through the transmissive diffraction grating again.

The light selection element has a selection surface for optically selecting a portion of the diffracted light as output light. The light selection element is configured to guide the portion of the diffracted light selected by the selection surface to the light output path, and not guide the remaining light, excluding the portion of the diffracted light, to the light output path.

The reflective element has a reflecting surface with a variable angle. The diffracted light includes beams of multiple wavelength components aligned in the wavelength dispersion direction. Each wavelength component beam included in the diffracted light is an elliptical beam with its major axis tilted relative to a direction perpendicular to the wavelength dispersion direction. The major axis can be tilted, for example, at an acute angle relative to the direction perpendicular to the wavelength dispersion direction.

The selection surface has a predetermined geometric shape with two sides extending symmetrically about a central axis that intersects with the wavelength dispersion direction of the diffracted light. The diffracted light is displaced relative to the geometric shape in at least the wavelength dispersion direction depending on the angle of the reflecting surface. The wavelength component of the diffracted light selected as output light is determined by the relative position of the diffracted light with respect to the geometric shape.

According to one aspect of the present disclosure, the selection surface is arranged so that the central axis of the geometric shape and the major axis of the beam of each wavelength component contained in the diffracted light are parallel. In a wavelength-tunable filter configured in this manner, the major axis of the beam is parallel to the central axis of the geometric shape. The wavelength dispersion direction is a direction intersecting the central axis. Therefore, for example, when two sides of the geometric shape are parallel to the central axis, or when the two sides of the geometric shape extend axially symmetrically from the central axis, different wavelength components of the diffracted light are well separated between the inside and outside of the area surrounded by the two sides of the geometric shape.

Therefore, according to one aspect of the present disclosure, it is possible to provide a wavelength-tunable filter that can set the filter slope to an appropriate shape. With this wavelength-tunable filter, the filter slope can be set to a steep shape even when the major axis of the beam is not aligned in a direction perpendicular to the wavelength dispersion direction and tilts.

According to another aspect of the present disclosure, there may be provided a tunable filter comprising a transmissive diffraction grating arranged in the propagation path of input light, a reflective element arranged opposite the transmissive diffraction grating so as to reflect the input light that has passed through the transmissive diffraction grating, and a light-selecting element, wherein the tunable filter has the following characteristics:

The light selection element is positioned in the propagation path of the diffracted light, which is input light that has been reflected by the reflecting element and then re-transmitted through the transmission diffraction grating. The light selection element has a selection surface for optically selecting a portion of the diffracted light as output light. The light selection element is configured to guide the portion of the diffracted light selected by the selection surface to the light output path, and not guide the remaining light, excluding the portion of the diffracted light, to the light output path.

The reflecting element has a reflecting surface with a variable angle. The diffracted light includes beams of multiple wavelength components aligned in the wavelength dispersion direction. The selection surface has a predetermined geometric shape with two sides extending symmetrically about a central axis that intersects with the wavelength dispersion direction of the diffracted light, and two sides that are non-parallel to the central axis. The diffracted light is displaced relative to the geometric shape depending on the angle of the reflecting surface. The wavelength component of the diffracted light selected as output light is determined by the relative position of the diffracted light with respect to the geometric shape.

The reflective element is configured to be able to change the angle of the reflective surface by rotating the reflective surface around multiple rotation axes so that the beams of each wavelength component contained in the diffracted light can be moved along the central axis of the selection surface and so that the beams can be moved in the wavelength dispersion direction.

With a wavelength tunable filter configured in this manner, the output wavelength can be changed by moving the beam in the wavelength dispersion direction. If the selective surface has two sides that are not parallel to the central axis, the width of the selective surface in the direction perpendicular to the central axis varies along the central axis. Therefore, the bandwidth can be changed by moving the beam along the central axis.

Therefore, according to another aspect of the present disclosure, a new technology can be provided for a wavelength-tunable filter that can appropriately set the bandwidth. This wavelength-tunable filter allows the wavelength and bandwidth to be appropriately changed. According to another aspect of the present disclosure, the beams of each wavelength component contained in the diffracted light may be elliptical beams whose major axes are tilted relative to a direction perpendicular to the wavelength dispersion direction.

According to another aspect of the present disclosure, the selection surface can be arranged so that the central axis of the geometric shape is parallel to the major axis of the beam of each wavelength component contained in the diffracted light. This arrangement allows the filter slope to be set to a steep shape.

According to another aspect of the present disclosure, the selection surface may be arranged so that the central axis of the geometric shape is inclined with respect to the major axis of the beam of each wavelength component contained in the diffracted light. With such an arrangement, the filter slope can be set to an appropriate shape by adjusting the inclination.

According to another aspect of the present disclosure, one of the multiple rotation axes can be set so that the direction of movement of the beam caused by rotation of the reflecting surface around the rotation axis is parallel to the central axis of the geometric shape. By setting the rotation axis in this way, the bandwidth can be adjusted without changing the center wavelength of the filter by rotating the reflecting surface around the rotation axis.

According to another aspect of the present disclosure, one of the multiple rotation axes may be set so that the direction of movement of the beam caused by rotation of the reflecting surface around the rotation axis coincides with the wavelength dispersion direction. By setting the rotation axis in this way, it is possible to change the center wavelength of the filter while maintaining the filter bandwidth by rotating the reflecting surface around the rotation axis.

According to another aspect of the present disclosure, the multiple rotation axes may include a first rotation axis and a second rotation axis, the first rotation axis being set so that the direction of movement of the beam caused by rotation of the reflecting surface about the first rotation axis is parallel to the central axis of the geometric shape, and the second rotation axis being set so that the direction of movement of the beam caused by rotation of the reflecting surface about the second rotation axis is aligned with the wavelength dispersion direction. By setting the rotation axes in this way, the center wavelength and bandwidth of the filter can be adjusted using the individual rotation axes.

FIG. 1 is a block diagram illustrating a configuration of an optical transmission system. FIG. 2 is a diagram conceptually illustrating the optical configuration of a wavelength tunable filter. FIG. 2 is a diagram illustrating a specific configuration of a wavelength tunable filter. 3A and 3B are diagrams illustrating the geometric shape of the reflecting surface of the light-selecting mirror. FIG. 5A is a diagram showing the peripheral configuration of a transmission type diffraction grating as viewed from a predetermined direction, and FIG. 5B is a diagram showing the peripheral configuration of the transmission type diffraction grating as viewed from another direction. The relationship between wavelength and transmittance is shown in a graph. 10A and 10B are diagrams illustrating changes in the relative position of a diffracted light beam with respect to a light selection mirror. 10A and 10B are diagrams illustrating changes in the relative position of a diffracted light beam with respect to a light selection mirror. 1 is a diagram illustrating a schematic configuration of a variable angle mirror having two rotation axes. 10A and 10B are diagrams illustrating the geometric shape of the reflecting surface of the light-selecting mirror in a modified example.

1...optical transmission system, 10...optical amplifier, 30...wavelength tunable filter, 31...optical input/output section, 32...first optical system, 33...lens array, 34...beam expansion prism group, 35...transmission diffraction grating, 36...angle-variable mirror, 36A...reflecting surface, 37...second optical system, 38...lens, 39...wavelength plate, 40...light-selecting mirror, 40A...reflecting surface, 40B...reflecting surface, 50...controller, 311...input optical fiber, 315...output optical fiber, A1...first rotation axis, A2...second rotation axis, C...central axis.

Exemplary embodiments of the present disclosure are described below with reference to the drawings.

The optical transmission system 1 of this embodiment shown in Figure 1 is configured to amplify optical signals flowing through a wavelength division multiplexing (WDM) network using an optical amplifier 10 and transmit them downstream. This optical transmission system 1 includes a wavelength-tunable filter 30 downstream of the optical amplifier 10 to remove ASE noise that occurs in the optical signal when it is amplified by the optical amplifier 10.

The wavelength-tunable filter 30 is configured as a bandpass filter with a variable signal passband. The wavelength-tunable filter 30 includes a variable-angle mirror 36 as a movable element. The variable-angle mirror 36 has a variable-angle reflecting surface 36A. The wavelength-tunable filter 30 is configured so that the signal passband can be changed by changing the angle of the reflecting surface 36A.

Specifically, the tunable filter 30 is configured to be able to change the center wavelength and bandwidth of the signal pass band. Hereinafter, this center wavelength will be referred to as the filter center wavelength, and the bandwidth will be referred to as the filter bandwidth.

The optical transmission system 1 further includes a controller 50 as a component for controlling the filter center wavelength and filter bandwidth. The controller 50 is configured to control the filter center wavelength and filter bandwidth by controlling the variable-angle mirror 36.

The controller 50 can control the angle of the reflecting surface 36A of the angle-variable mirror 36 so that the specified filter center wavelength and filter bandwidth are achieved, for example, based on a measurement signal from a power monitor not shown.

The power monitor is installed, for example, after the wavelength-tunable filter 30. Light that passes through the wavelength-tunable filter 30 can be branched and input to the power monitor. The power monitor can measure the power of the light that passes through the wavelength-tunable filter 30 based on the branched light that is input.

Figure 2 conceptually illustrates the optical configuration of an exemplary wavelength-tunable filter 30. Figure 2 conceptually illustrates the arrangement of each component in the wavelength-tunable filter 30 when viewed from the direction of arrow R1 in Figure 3. Figure 3 illustrates the specific configuration of the wavelength-tunable filter 30.

As shown in FIG. 2, the wavelength-tunable filter 30 includes, in addition to the variable-angle mirror 36, a light input/output unit 31, a first optical system 32, a transmission diffraction grating 35, a second optical system 37, and a light-selecting mirror 40. The light input/output unit 31 includes an input optical fiber 311 and an output optical fiber 315. As shown in FIG. 3, the first optical system 32 includes a lens array 33 and a group of beam-expanding prisms 34. The second optical system 37 includes a lens 38 and a wave plate 39.

The optical signal from the optical amplifier 10 is input to the wavelength-tunable filter 30 through the optical input/output unit 31, specifically through the input optical fiber 311. Hereinafter, the optical signal input to the wavelength-tunable filter 30 through the input optical fiber 311 will be referred to as input light.

The input light passes through the first optical system 32 and propagates to the transmission diffraction grating 35. As it passes through the first optical system 32, the input light is collimated by the beam expansion prism group 34. That is, after being collimated by the first optical system 32, the input light propagates to the transmission diffraction grating 35. The input light is incident on the transmission diffraction grating 35 as a light beam with an elliptical cross section.

The transmission grating 35 is positioned in the propagation path of the collimated input light. The diffraction angle at the transmission grating 35 depends on the wavelength. This wavelength dependency allows the transmission grating 35 to spatially separate the collimated input light into multiple wavelength components. The input light from the first optical system 32 passes through the transmission grating 35 and propagates to the variable-angle mirror 36.

The variable-angle mirror 36 functions as a reflective element. The variable-angle mirror 36 is positioned opposite the transmission diffraction grating 35 so as to reflect the input light that has passed through the transmission diffraction grating 35.

The input light propagating to the variable-angle mirror 36 is reflected by the reflecting surface 36A of the variable-angle mirror 36 and passes through the transmissive diffraction grating 35 again. The input light passing through the transmissive diffraction grating 35 again propagates through the second optical system 37 toward the light-selecting mirror 40 as diffracted light in which multiple wavelength components are spatially dispersed. The second optical system 37 and the light-selecting mirror 40 are arranged on the propagation path of the diffracted light.

The solid arrows in Figures 2 and 3 conceptually show the propagation of wavelength components within the signal passband of the input light that are optically coupled to the output optical fiber 315 as output light. The dashed arrows in Figure 2 conceptually show the propagation of wavelength components outside the signal passband.

The variable-angle mirror 36 is configured to be controlled by the controller 50 to change the angle of the reflecting surface 36A. The variable-angle mirror 36 is a tilt mirror with multiple rotation axes, specifically two rotation axes. The variable-angle mirror 36 may be, for example, a MEMS mirror.

Lens 38 functions as a telecentric lens, converting the diffracted light from the transmission diffraction grating 35, specifically each of the spatially dispersed wavelength components contained in the diffracted light, into a light beam whose propagation direction is parallel to the optical axis passing through the center of lens 38.

The wave plate 39 is a half-wave plate and is optionally provided between the transmissive diffraction grating 35 and the light-selecting mirror 40. In order to suppress the influence of the polarization dependency of the diffraction efficiency of the transmissive diffraction grating 35, the wave plate 39 can be placed between the transmissive diffraction grating 35 and the light-selecting mirror 40 with its optical axis tilted 45 degrees with respect to the grating axis of the transmissive diffraction grating 35.

The presence of this wave plate 39 ensures that the polarization state of light incident on the transmission diffraction grating 35 on the return path is orthogonal to the polarization state of the light on the outbound path, so that even if the transmission diffraction grating 35 has polarization dependence, the polarization dependence of the output light output to the outside of the wavelength-tunable filter 30 through the output optical fiber 315 can be eliminated. The wave plate 39 is positioned so that it acts on the diffracted light on either the outbound or return path (for example, the outbound path).

If the transmission diffraction grating 35 is a substantially polarization-independent diffraction grating, the wave plate 39 does not need to be provided. However, the wave plate 39 may be provided to compensate for the polarization-dependent loss of the wavelength-tunable filter 30 due to the polarization dependence of the diffraction efficiency remaining in the transmission diffraction grating 35.

The light-selecting mirror 40 is configured to reflect a light beam of a specific wavelength component aligned by the variable-angle mirror 36 from among the light beams of multiple wavelength components contained in the incoming diffracted light. The light-selecting mirror 40 functions as a light-selecting element.

Figure 4 shows the geometric shape of the light-selecting mirror 40, along with the arrangement of the light beams of multiple wavelength components contained in the diffracted light. The light-selecting mirror 40 has a rectangular reflecting surface 40A with two sides E1 and E2 extending symmetrically about a central axis C. The central axis C is indicated by a dashed line in Figure 4.

The reflecting surface 40A corresponds to the hatched area in Figure 4. Each of the multiple elliptical objects shown in Figure 4 corresponds to a light beam of one wavelength component. The light beam of the wavelength band that falls within the hatched area is reflected by the light-selecting mirror 40 as output light.

In this way, the wavelength component of the diffracted light selected as output light is determined by the relative position of the diffracted light with respect to the reflecting surface 40A. The reflecting surface 40A of the light-selecting mirror 40 functions as a selection surface for optically selecting a portion of the diffracted light as output light. The light-selecting mirror 40 guides a portion of the diffracted light selected by the reflecting surface 40A to the light output path (output optical fiber 315), and functions to prevent the remaining light, excluding this portion of the diffracted light, from being guided to the light output path.

In Figure 4, please note that the elliptical cross section of the light beam is tilted with respect to the wavelength dispersion direction. In Figure 4, the wavelength dispersion direction is the up-down direction, which is the direction in which the light beams of multiple wavelength components contained in the diffracted light are aligned. Figure 4 illustrates a light beam with an elliptical cross section whose major axis is tilted at an acute angle with respect to a direction perpendicular to the wavelength dispersion direction. This tilt of the light beam is the optical path length at each point in the light beam, and is caused by the difference in the optical path length between the transmission diffraction grating 35 and the reflecting surface 36A of the angle-variable mirror 36.

Figure 5A shows the arrangement of the transmission diffraction grating 35 and the variable-angle mirror 36 as viewed from the direction of arrow R1 in Figure 3. Figure 5A also shows, with dashed two-dot lines, the propagation paths of the optical signal corresponding to each of the two endpoints of the elliptical major axis of the light beam.

Figure 5B shows the cross-sectional shape F1 of the light beam propagating from the first optical system 32 through the transmission diffraction grating 35 to the variable-angle mirror 36, along with the endpoints P11 and P12 of the major axis of the ellipse. Figure 5B also shows the cross-sectional shape F2 of the light beam reflected by the variable-angle mirror 36 and re-transmitted through the transmission diffraction grating 35, along with the endpoints P21 and P22 of the major axis of the ellipse.

The two-dot chain line extending from endpoint P11 to endpoint P21 conceptually shows the propagation path of the optical signal passing through endpoint P11 to endpoint P21. The two-dot chain line extending from endpoint P12 to endpoint P22 conceptually shows the propagation path of the optical signal passing through endpoint P12 to endpoint P22.

In Figure 5A, the optical path length of the propagation path of the optical signal from endpoint P11 to endpoint P21 is indicated by value L1, and the optical path length of the propagation path of the optical signal from endpoint P12 to endpoint P22 is indicated by value L2.

In Figure 5B, the distance along the surface of the transmission grating 35 from endpoint P11 to endpoint P21 is indicated by a value ΔX1, and the distance along the surface of the transmission grating 35 from endpoint P12 to endpoint P22 is indicated by a value ΔX2. Because the optical path length L1 is longer than the optical path length L2, the distance ΔX1 is longer than the distance ΔX2.

As can be seen from Figures 5A and 5B, the light beam incident on the transmissive diffraction grating 35 is a light beam having a thickness or diameter, and since the optical path length at each point in the light beam is different, the cross section of the diffracted light beam that passes through the transmissive diffraction grating 35 again and heads toward the second optical system 37 rotates relative to the cross section of the light beam at the time of incidence on the transmissive diffraction grating 35.

In this embodiment, the transmissive diffraction grating 35 is positioned so that wavelengths are dispersed in the vertical direction in Figures 2 and 4. Meanwhile, input light from the first optical system 32 is reflected in the horizontal direction by the reflecting surface 36A and propagates to the second optical system 37 as diffracted light.

For this reason, as shown in Figure 4, the beams of each wavelength component contained in the diffracted light propagate to the light selection mirror 40 as elliptical beams whose major axes are tilted relative to a direction perpendicular to the wavelength dispersion direction (the up and down direction in Figures 2 and 4). In other words, the major axis of the elliptical cross section of the light beam would be positioned perpendicular to the wavelength dispersion direction if there was no rotation, but due to rotation, it is tilted obliquely relative to the wavelength dispersion direction.

The reason why the orientation of the major axis of the ellipse shown in Figure 5B differs by 90 degrees from the orientation of the major axis of the ellipse shown in Figure 4 is that in this embodiment, the cross section of the ellipse is rotated by 90 degrees by the wave plate 39 as the diffracted light propagates through the lens 38 and the wave plate 39 to the light selection mirror 40.

In this embodiment, as shown in Figure 4, the reflecting surface 40A of the light-selecting mirror 40 is set so that the central axis C of the reflecting surface 40A is parallel to the major axis of the ellipse of the light beam of each wavelength component. The reason for setting the orientation of the central axis C in this way is to set the filter slope of the wavelength-tunable filter 30 to a steep shape.

Figure 6 shows the transmittance of each wavelength component of the wavelength-tunable filter 30 with a solid line when the central axis C of the reflecting surface 40A of the light-selecting mirror 40 is positioned parallel to the major axis of the ellipse of the light beam. The horizontal axis of the graph shown in Figure 6 corresponds to wavelength (frequency), and the vertical axis corresponds to filter transmittance.

Figure 6 also shows, with dashed lines, the transmittance of each wavelength component of the wavelength-tunable filter 30 when the central axis C is positioned non-parallel to the major axis of the ellipse of the light beam. Figure 4 shows, with dashed lines, an example of the positioning of the reflecting surface 40A when the central axis C is positioned non-parallel to the major axis of the ellipse of the light beam.

As can be seen from a comparison of the dashed and solid lines in Figure 6, the filter slope when the central axis C of the reflecting surface 40A of the light selection mirror 40 is arranged parallel to the elliptical major axis of the light beam is steeper than the filter slope when it is arranged non-parallel.

The major axis of the ellipse of the light beam intersects the wavelength dispersion direction at an angle. Therefore, when the central axis C is arranged parallel to the major axis of the ellipse of the light beam, the area on the reflecting surface 40A where light beams of the same wavelength band are incident is suppressed from spreading in the wavelength dispersion direction, and the different wavelength components of the diffracted light inside and outside the reflecting surface 40A are well separated. Therefore, when the central axis C is arranged parallel to the major axis of the ellipse, the filter slope of the wavelength tunable filter 30 can be set to a steep shape.

In recent WDM networks, optical signals are densely spaced on the frequency axis to improve frequency utilization efficiency. In other words, adjacent channels are located close to each other on the frequency axis. Therefore, if the filter slope is gentle, there is a higher possibility that optical signals will interfere with adjacent channels. In this embodiment, to reduce the possibility of such interference, the central axis C is aligned with the major axis of the ellipse of the optical beam, and the filter slope is set to be steep.

In addition, in this embodiment, the first rotation axis A1 of the angle-variable mirror 36 is set so that rotation of the reflecting surface 36A around the first rotation axis A1 (see Figure 9) causes the diffracted light to move parallel to the central axis C along the central axis C in a plane along the reflecting surface 40A of the light selection mirror 40. In other words, the first rotation axis A1 is set so that the direction of movement of the light beam due to rotation of the reflecting surface 36A around the first rotation axis A1 is parallel to the central axis C of the light selection mirror 40.

Furthermore, in this embodiment, the second rotation axis A2 of the angle-variable mirror 36 is set so that rotation of the reflecting surface 36A around the second rotation axis A2 causes the diffracted light to move in the wavelength dispersion direction in a plane along the reflecting surface 40A of the light-selecting mirror 40. In other words, the second rotation axis A2 is set so that the direction of movement of the light beam due to rotation of the reflecting surface 36A around the second rotation axis A2 coincides with the wavelength dispersion direction.

Therefore, the controller 50 of this embodiment can change the filter bandwidth while keeping the filter center wavelength of the wavelength-tunable filter 30 constant by rotating the reflecting surface 36A of the angle-variable mirror 36 about the first rotation axis A1. Figure 7 shows that the rotation of the reflecting surface 36A about the first rotation axis A1 causes the light beam to move along the central axis C of the reflecting surface 40A of the light-selecting mirror 40.

Similarly, the controller 50 can change the filter center wavelength while keeping the filter bandwidth of the wavelength-tunable filter 30 constant by rotating the reflecting surface 36A of the angle-variable mirror 36 about the second rotation axis A2. Figure 8 shows that the rotation of the reflecting surface 36A about the second rotation axis A2 causes the light beam to move in the wavelength dispersion direction relative to the reflecting surface 40A of the light-selecting mirror 40.

Figure 8 explains how the relative position of the reflecting surface 40A of the light-selecting mirror 40 to the diffracted light changes in the wavelength dispersion direction due to the rotation of the reflecting surface 36A, by showing the changed reflecting surface 40A with a dashed line. However, Figure 8 is only a diagram explaining the relative displacement. It should be understood that it is the diffracted light, not the light-selecting mirror 40, that actually displaces; that is, the diffracted light displaces relative to the reflecting surface 40A.

Figure 9 shows an example configuration of the reflecting surface 36A of the variable-angle mirror 36. According to Figure 9, the reflecting surface 36A is supported by the first support part 36B so as to be rotatable around the first rotation axis A1. The first support part 36B is supported by the second support part 36C so as to be rotatable around the second rotation axis A2. This allows the reflecting surface 36A to rotate around the first rotation axis A1 and the second rotation axis A2.

The configuration of the optical transmission system 1 and wavelength-tunable filter 30 of this embodiment has been described above. As mentioned above, there is a demand for miniaturization of devices such as optical transceivers used in the optical transmission system 1. To achieve this miniaturization, when the wavelength-tunable filter 30 is placed in a narrow space, it is necessary to set the angle of incidence of light on the transmission diffraction grating 35 large to enhance the wavelength dispersion effect.

However, when the incident angle is set large, the difference between the optical path lengths L1 and L2 causes the major axis of the elliptical beam corresponding to the input light to rotate in the diffracted light, as shown in Figures 5A and 5B. In this case, if the major axis of the elliptical beam and the central axis C of the reflecting surface 40A of the light-selecting mirror 40 are set non-parallel, the filter slope becomes gentle, as explained using Figure 6. This gentle filter slope is inconvenient in light of the recent demand for improved frequency efficiency.

In contrast, in this embodiment, the orientation of the light selection mirror 40 is set so that the major axis of the elliptical beam is aligned with the central axis C of the reflecting surface 40A of the light selection mirror 40. Therefore, according to this embodiment, even if the light beam rotates in the diffracted light due to the placement of the wavelength-tunable filter 30 in a narrow space, a steep filter slope can be achieved, and interference of optical signals with adjacent channels can be suppressed.

Furthermore, in this embodiment, the two sides E1 and E2 of the reflecting surface 40A of the light-selecting mirror 40 are set as two sides that are axially symmetrical but non-parallel to the central axis C so that the width of the reflecting surface 40A in the direction perpendicular to the central axis C changes in the direction along the central axis C.

The angle-variable mirror 36 is configured as a two-axis tilt mirror. That is, by controlling the angle of the reflecting surface 36A, the wavelength-variable filter 30 can change the relative position of the diffracted light beam with respect to the reflecting surface 40A not only in the wavelength dispersion direction but also in the direction along the central axis C.

As a result, the tunable filter 30 achieves a steep filter slope while also being able to change not only the filter center wavelength but also the filter bandwidth by controlling the variable-angle mirror 36. The tunable filter 30 of this embodiment, which has these characteristics, is extremely useful for miniaturizing optical communication equipment.

[Other embodiments]
The present disclosure is not limited to the above-described embodiment, and various modifications are possible. In the above-described embodiment, the central axis C of the reflecting surface 40A of the light-selecting mirror 40 is aligned with the major axis of the light beam, but such setting of the central axis C is not essential. That is, the central axis C of the reflecting surface 40A of the light-selecting mirror 40 may be arranged non-parallel to the major axis of the light beam, in other words, tilted with respect to the major axis.

For example, if there is no interference with adjacent channels, or if interference with adjacent channels does not need to be considered, the reflecting surface 40A may be arranged so that its central axis C is non-parallel to the major axis of the light beam, as shown by the dashed line in Figure 4. By adjusting the angle of the central axis C relative to the major axis of the light beam, it is possible to adjust the inclination of the filter slope. Therefore, the angle of the central axis C may be adjusted relative to the major axis of the light beam so as to achieve the desired filter slope.

Furthermore, the rotation axis of the variable-angle mirror 36 is not limited to the first rotation axis A1 and the second rotation axis A2 corresponding to the central axis C and the wavelength dispersion direction. For example, the first rotation axis A1 and the second rotation axis A2 may be rotation axes that are perpendicular to each other. A two-axis tilt mirror having two rotation axes that are perpendicular to each other may also be used as the variable-angle mirror 36.

The first rotation axis A1 may be set so that rotation of the reflecting surface 36A around the rotation axis A1 moves the light beam in a direction perpendicular to the wavelength dispersion direction. Alternatively, the second rotation axis A2 may be set so that rotation of the reflecting surface 36A around the rotation axis A2 moves the light beam in a direction perpendicular to the central axis C.

However, if the two rotation axes are orthogonal to each other, rotation of the reflecting surface 36A around one rotation axis can change both the filter center wavelength and the filter bandwidth. Therefore, in order to control the angle-variable mirror 36 so that one of the filter center wavelength and the filter bandwidth is kept constant while the other is changed, it is necessary to precisely and appropriately control the rotation of the two axes.

In addition, the reflecting surface 40A of the light-selecting mirror 40 may be modified to have a geometric shape having two parallel sides that are axially symmetrical with respect to the central axis C. That is, instead of the reflecting surface 40A shown in FIG. 4, the light-selecting mirror 40 may be configured to have a reflecting surface 40B having a geometric shape having two sides E21, E22 that are parallel to the central axis C, as shown in FIG. 10.

When this geometric shape of the reflecting surface 40B is adopted, the filter bandwidth does not change even when the light beam is moved along the central axis C. Therefore, the angle-variable mirror 36 may be changed to a uniaxial tilt mirror or configured with a single rotation axis that can move the diffracted light in the wavelength dispersion direction. This example also provides the advantage of a steeper filter slope.

The functions of one component in the above embodiments may be distributed among multiple components. The functions of multiple components may be integrated into one component. Part of the configuration of the above embodiments may be omitted. At least part of the configuration of the above embodiments may be added to or substituted for the configuration of another of the above embodiments. All aspects included in the technical idea identified from the wording of the claims are embodiments of the present disclosure.

Claims (5)

A tunable filter,
a transmission grating disposed in a propagation path of the input light;
a reflective element disposed opposite the transmission grating so as to reflect the input light transmitted through the transmission grating;
a light selection element that is arranged in a propagation path of diffracted light, which is the input light reflected by the reflection element and re-transmitted through the transmission type diffraction grating, has a selection surface for optically selecting a portion of the diffracted light as output light, and is configured to guide the portion of the diffracted light selected by the selection surface to an optical output path and not guide the remaining light of the diffracted light excluding the portion of the diffracted light to the optical output path,
the reflective element has a variable angle reflective surface;
the diffracted light includes beams of a plurality of wavelength components aligned in a wavelength dispersion direction,
the selection surface has a predetermined geometric shape having two sides extending axially symmetrically with respect to a central axis intersecting the wavelength dispersion direction of the diffracted light, the two sides being non-parallel to the central axis;
the diffracted light is displaced relative to the geometric shape in accordance with an angle of the reflecting surface, and a wavelength component of the diffracted light to be selected as the output light is determined by a relative position of the diffracted light with respect to the geometric shape;
the reflecting element is configured to be able to change the angle of the reflecting surface by rotating the reflecting surface around a plurality of rotation axes so as to be able to move beams of each wavelength component included in the diffracted light along the central axis and to be able to move the beams in the wavelength dispersion direction,
beams of each wavelength component included in the diffracted light are elliptical beams whose major axes are tilted with respect to a direction orthogonal to the wavelength dispersion direction,
The selective surface is arranged so that the central axis of the geometric shape is parallel to the major axis of the beam of each wavelength component contained in the diffracted light. A tunable filter,
a transmission grating disposed in a propagation path of the input light;
a reflective element disposed opposite the transmission grating so as to reflect the input light transmitted through the transmission grating;
a light selection element that is arranged in a propagation path of diffracted light, which is the input light reflected by the reflection element and re-transmitted through the transmission type diffraction grating, has a selection surface for optically selecting a portion of the diffracted light as output light, and is configured to guide the portion of the diffracted light selected by the selection surface to an optical output path and not guide the remaining light of the diffracted light excluding the portion of the diffracted light to the optical output path,
the reflective element has a variable angle reflective surface;
the diffracted light includes beams of a plurality of wavelength components aligned in a wavelength dispersion direction,
the selection surface has a predetermined geometric shape having two sides extending axially symmetrically with respect to a central axis intersecting the wavelength dispersion direction of the diffracted light, the two sides being non-parallel to the central axis;
the diffracted light is displaced relative to the geometric shape in accordance with an angle of the reflecting surface, and a wavelength component of the diffracted light to be selected as the output light is determined by a relative position of the diffracted light with respect to the geometric shape;
the reflecting element is configured to be able to change the angle of the reflecting surface by rotating the reflecting surface around a plurality of rotation axes so as to be able to move beams of each wavelength component included in the diffracted light along the central axis and to be able to move the beams in the wavelength dispersion direction,
a beam of each wavelength component included in the diffracted light is an elliptical beam whose major axis is tilted with respect to a direction orthogonal to the wavelength dispersion direction,
The selective surface is disposed so that the central axis of the geometric shape is inclined with respect to the major axis of the beam of each wavelength component contained in the diffracted light. 3. The wavelength tunable filter according to claim 1 ,
One of the plurality of rotation axes is set so that the direction of movement of the beam caused by the rotation of the reflecting surface around the one rotation axis is parallel to the central axis. 3. The wavelength tunable filter according to claim 1 ,
One of the rotation axes is set so that the direction of movement of the beam caused by the rotation of the reflecting surface around the rotation axis coincides with the wavelength dispersion direction. 3. The wavelength tunable filter according to claim 1 ,
the plurality of rotation shafts include a first rotation shaft and a second rotation shaft,
the first rotation axis is set so that a moving direction of the beam caused by the rotation of the reflecting surface around the first rotation axis is parallel to the central axis;
the second rotation axis is set so that a moving direction of the beam caused by the rotation of the reflecting surface around the second rotation axis coincides with the wavelength dispersion direction.