Wavelength-variable filter

Abstract

This wavelength-variable filter comprises a transmissive diffraction grating, a reflective element, and a light selection element. The transmissive diffraction grating is disposed in the path of propagation of input light. The reflective element is disposed opposite from the transmissive diffraction grating. The light selection element is disposed in the path of propagation of diffracted light, which is input light that has been reflected by the reflective element and has been re-transmitted through the transmissive diffraction grating. The light selection element has a selection surface for optically selecting part of the diffracted light as output light. The selection surface has a prescribed geometric shape having two sides that extend so as to be axisymmetric with respect to a central axis which intersects the wavelength dispersion direction of the diffracted light. The diffracted light is relatively displaced with respect to the geometric shape in at least the wavelength dispersion direction, in accordance with the angle of the reflection surface.

Description

Tunable Filter

[0001] The present disclosure relates to tunable filters.

[0002] 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 filters are installed in optical communication networks to remove ASE noise.

[0003] 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 that corresponds to input light so that only specific wavelength components contained in the input light are optically coupled to an output optical fiber.

[0004] US Patent Publication No. 2008 / 0085119 Japanese Patent Application Laid-Open No. 2008-203508 Japanese Patent Application Laid-Open No. 2020-122936

[0005] There is a demand for miniaturization of optical communication devices that incorporate tunable filters, such as optical transceivers. To achieve this miniaturization, the tunable filter must be placed in a small space. To achieve the function of the tunable filter in a small space, effective wavelength dispersion is required. Therefore, in order to place the tunable filter in a small space, the angle of incidence of light on the diffraction grating must be set large.

[0006] 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 whose major axis is tilted relative to the input light beam is generated.

[0007] When such tilt or rotation occurs, in the conventional method, the wavelength component selection ability of the optical selection element (e.g., mirror) that selects the wavelength component to be optically coupled to the output optical fiber from the diffracted light beam is reduced, and therefore, when the wavelength tunable filter is placed in a narrow space, the conventional method cannot realize a steep filter slope.

[0008] In recent WDM networks, optical signals are densely arranged on the frequency axis to improve spectral efficiency, so inability to appropriately set the filter slope or bandwidth increases the possibility of optical signals interfering with adjacent channels.

[0009] Therefore, according to one aspect of the present disclosure, it is desirable to provide a new technology related to a wavelength tunable filter for appropriately setting the filter slope or bandwidth.

[0010] According to one aspect of the present disclosure, there is provided a tunable filter, the tunable filter comprising a transmission grating, a reflective element, and a light-selective element.

[0011] The transmission grating is disposed in a propagation path of the input light, the reflection element is disposed opposite the transmission grating so as to reflect the input light that has transmitted through the transmission grating, and the light selection element is disposed in a propagation path of the diffracted light that is the input light that has been reflected by the reflection element and transmitted again through the transmission grating.

[0012] The light selection element 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 the light output path and not guide the remaining light except for the portion of the diffracted light to the light output path.

[0013] 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 with respect to a direction perpendicular to the wavelength dispersion direction. The major axis can be tilted, for example, at an acute angle with respect to the direction perpendicular to the wavelength dispersion direction.

[0014] The selective 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 at least in 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.

[0015] 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.

[0016] Therefore, according to one aspect of the present disclosure, it is possible to provide a tunable filter that can set the filter slope to an appropriate shape. With this tunable filter, it is possible to set the filter slope to a steep shape even when the major axis of the beam is not aligned in a direction perpendicular to the chromatic dispersion direction and tilts.

[0017] According to another aspect of the present disclosure, there may be provided a tunable filter including a transmissive diffraction grating arranged in a 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 features:

[0018] The light selection element is disposed in a propagation path of the diffracted light, which is input light reflected by the reflecting element and transmitted again 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.

[0019] The reflecting element has a reflecting surface whose angle can be changed. The diffracted light includes beams of multiple wavelength components aligned in the wavelength dispersion direction. The selection surface has a predetermined geometric shape having two sides that extend symmetrically with respect to 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 according to the angle of the reflecting surface. The wavelength component of the diffracted light that is selected as output light is determined by the relative position of the diffracted light with respect to the geometric shape.

[0020] 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 as to be able to move the beams of each wavelength component contained in the diffracted light along the central axis of the selection surface and to be able to move the beams in the wavelength dispersion direction.

[0021] With a wavelength tunable filter configured in this way, the output wavelength can be changed by moving the beam in the wavelength dispersion direction. When the selective surface has two sides non-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.

[0022] Therefore, according to another aspect of the present disclosure, it is possible to provide a novel technology related to a wavelength tunable filter capable of appropriately setting a bandwidth. With this wavelength tunable filter, it is possible to appropriately change the wavelength and bandwidth. 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 with respect to a direction orthogonal to the wavelength dispersion direction.

[0023] According to another aspect of the present disclosure, the selection surface may be arranged such that the central axis of the geometric shape is parallel to the major axis of the beam of each wavelength component included in the diffracted light. With this arrangement, it is possible to set the filter slope to a steep shape.

[0024] According to another aspect of the present disclosure, the selection surface may be arranged such that the central axis of the geometric shape is inclined with respect to the major axis of the beam of each wavelength component included in the diffracted light. With such an arrangement, the filter slope can be set to an appropriate shape by adjusting the inclination.

[0025] According to another aspect of the present disclosure, one of the rotation axes may be set so that the direction of movement of the beam caused by rotation of the reflecting surface about the rotation axis is parallel to the central axis of the geometric shape. By setting the rotation axis in this manner, the bandwidth can be adjusted by rotating the reflecting surface about the rotation axis without changing the center wavelength of the filter.

[0026] According to another aspect of the present disclosure, one of the rotation axes may be set so that the direction of movement of the beam caused by rotation of the reflecting surface about the rotation axis coincides with the wavelength dispersion direction. By setting the rotation axis in this manner, it is possible to change the center wavelength of the filter while maintaining the bandwidth of the filter by rotating the reflecting surface about the rotation axis.

[0027] According to another aspect of the present disclosure, the plurality of 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.

[0028] FIG. 5 is a block diagram showing the configuration of an optical transmission system. FIG. 5 is a diagram conceptually showing the optical configuration of a wavelength-tunable filter. FIG. 5 is a diagram showing a specific configuration of a wavelength-tunable filter. FIG. 5A is a diagram showing the peripheral configuration of a transmission type diffraction grating viewed from a predetermined direction, and FIG. 5B is a diagram showing the peripheral configuration of the transmission type diffraction grating viewed from another direction. FIG. 5B is a graph showing the relationship between wavelength and transmittance. FIG. 5 is a diagram showing changes in the relative position of a diffracted light beam with respect to the light selection mirror ... a schematic configuration of an angle-variable mirror having two rotation axes. FIG. 5 is a diagram showing the geometric shape of the reflecting surface of a light selection mirror in a modified example.

[0029] 1...optical transmission system, 10...optical amplifier, 30...wavelength tunable filter, 31...light 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 selection 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.

[0030] Exemplary embodiments of the present disclosure will now be described with reference to the drawings.

[0031] 1 is configured to amplify an optical signal flowing through a wavelength division multiplexing (WDM) network using an optical amplifier 10 and transmit the amplified signal downstream. This optical transmission system 1 includes a tunable filter 30 downstream of the optical amplifier 10 to remove ASE noise that occurs in the optical signal during amplification by the optical amplifier 10.

[0032] The tunable filter 30 is configured as a bandpass filter capable of changing the signal pass band. The tunable filter 30 includes a variable-angle mirror 36 as a movable element. The variable-angle mirror 36 has a reflecting surface 36A whose angle is variable. The tunable filter 30 is configured so that the signal pass band can be changed by changing the angle of the reflecting surface 36A.

[0033] 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.

[0034] The optical transmission system 1 further includes a controller 50 as a component for controlling the filter center wavelength and the filter bandwidth. The controller 50 is configured to control the filter center wavelength and the filter bandwidth by controlling the angle-variable mirror 36.

[0035] The controller 50 can control the angle of the reflecting surface 36A of the angle-variable mirror 36 based on a measurement signal from, for example, a power monitor (not shown) so that a specified filter center wavelength and filter bandwidth are achieved.

[0036] The power monitor is installed, for example, after the wavelength-tunable filter 30. Light that has passed 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 has passed through the wavelength-tunable filter 30 based on the input branched light.

[0037] Fig. 2 conceptually illustrates the optical configuration of an exemplary wavelength-tunable filter 30. Fig. 2 conceptually illustrates the arrangement of each component in the wavelength-tunable filter 30 when viewed from the direction of arrow R1 in Fig. 3. Fig. 3 illustrates a specific configuration of the wavelength-tunable filter 30.

[0038] 2, the tunable filter 30 includes a light input / output unit 31, a first optical system 32, a transmission type diffraction grating 35, a second optical system 37, and a light selection mirror 40 in addition to the variable angle mirror 36. 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.

[0039] 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 an 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.

[0040] The input light passes through the first optical system 32 and propagates to the transmission grating 35. In the process of passing through the first optical system 32, the input light is collimated by the beam expanding prism group 34. That is, the input light is collimated by the first optical system 32 and then propagates to the transmission grating 35. The input light is incident on the transmission grating 35 as a light beam with an elliptical cross section.

[0041] The transmission grating 35 is disposed in the propagation path of the collimated input light. The diffraction angle at the transmission grating 35 depends on the wavelength. The transmission grating 35 spatially separates the collimated input light into multiple wavelength components due to this wavelength dependency. The input light from the first optical system 32 passes through the transmission grating 35 and propagates to the variable-angle mirror 36.

[0042] The variable angle mirror 36 functions as a reflecting element and is disposed opposite the transmission grating 35 so as to reflect the input light that has passed through the transmission grating 35.

[0043] The input light propagated to the variable-angle mirror 36 is reflected by the reflecting surface 36A of the variable-angle mirror 36 and passes again through the transmission diffraction grating 35. The input light that passes again through the transmission diffraction grating 35 passes through the second optical system 37 as diffracted light in which multiple wavelength components are spatially dispersed, and propagates toward the light-selecting mirror 40. The second optical system 37 and the light-selecting mirror 40 are disposed on the propagation path of the diffracted light.

[0044] 2 and 3 conceptually show the propagation of wavelength components within the signal pass band of the input light that are optically coupled as output light to the output optical fiber 315. The dashed arrows in Fig. 2 conceptually show the propagation of wavelength components outside the signal pass band.

[0045] The variable-angle mirror 36 is configured to change the angle of the reflecting surface 36A under the control of the controller 50. The variable-angle mirror 36 is a tilt mirror having multiple rotation axes, specifically, two rotation axes. The variable-angle mirror 36 may be, for example, a MEMS mirror.

[0046] The lens 38 functions as a telecentric lens and converts the diffracted light from the transmission diffraction grating 35, more 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 the lens 38.

[0047] The wave plate 39 is a half-wave plate, and is optionally provided between the transmission 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 transmission diffraction grating 35, the wave plate 39 can be disposed between the transmission diffraction grating 35 and the light-selecting mirror 40 with its optical axis tilted 45 degrees with respect to the grating axis of the transmission diffraction grating 35.

[0048] Due to the presence of this wave plate 39, the polarization state of light incident on the transmission grating 35 on the return path is orthogonal to the polarization state of light on the outgoing path, so even if the transmission grating 35 has polarization dependency, it is possible to remove the polarization dependency of the output light that is output to the outside of the wavelength-tunable filter 30 through the output optical fiber 315. The wave plate 39 is arranged so as to act on the diffracted light on either the outgoing path or the return path (for example, the outgoing path).

[0049] When the transmission grating 35 is a substantially polarization-independent diffraction grating, the wave plate 39 may not 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 dependency of the diffraction efficiency remaining in the transmission grating 35.

[0050] The light-selecting mirror 40 is configured to reflect a light beam of a specific wavelength component aligned by the angle-variable mirror 36, out of the light beams of multiple wavelength components contained in the incoming diffracted light. The light-selecting mirror 40 functions as a light-selecting element.

[0051] 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 having two sides E1 and E2 extending symmetrically with respect to a central axis C. The central axis C is indicated by a dashed line in FIG. 4.

[0052] The reflecting surface 40A corresponds to the hatched area in Fig. 4. Each of the multiple elliptical objects shown in Fig. 4 corresponds to a light beam of one wavelength component. The light beam of the wavelength band falling within the hatched area is reflected by the light-selecting mirror 40 as output light.

[0053] 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 of the diffracted light excluding this portion from being guided to the light output path.

[0054] Note that in Fig. 4, the elliptical cross section of the light beam is tilted with respect to the wavelength dispersion direction. In Fig. 4, the wavelength dispersion direction is the up-down direction, which is the direction in which the light beam of multiple wavelength components contained in the diffracted light is aligned. Fig. 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 of the light beam, and is caused by the difference in optical path length between the transmission diffraction grating 35 and the reflecting surface 36A of the angle-variable mirror 36.

[0055] Fig. 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 Fig. 3. Fig. 5A also shows, by two-dot chain lines, the propagation paths of the optical signal corresponding to the two end points of the major axis of the ellipse of the light beam.

[0056] 5B shows the cross-sectional shape F1 of the light beam propagating from the first optical system 32 through the transmission grating 35 to the variable-angle mirror 36, together with end points P11 and P12 of the major axis of the ellipse. Furthermore, FIG. 5B shows the cross-sectional shape F2 of the light beam reflected by the variable-angle mirror 36 and transmitted again through the transmission grating 35, together with end points P21 and P22 of the major axis of the ellipse.

[0057] The dashed two-dot line extending from the end point P11 to the end point P21 conceptually shows the propagation path of the optical signal passing through the end point P11 to the end point P21. The dashed two-dot line extending from the end point P12 to the end point P22 conceptually shows the propagation path of the optical signal passing through the end point P12 to the end point P22.

[0058] In FIG. 5A, the optical path length of the optical signal propagation path from endpoint P11 to endpoint P21 is indicated by value L1, and the optical path length of the optical signal propagation path from endpoint P12 to endpoint P22 is indicated by value L2.

[0059] 5B, the distance from the end point P11 to the end point P21 along the surface of the transmission grating 35 is indicated by a value ΔX1, and the distance from the end point P12 to the end point P22 along the surface of the transmission grating 35 is indicated by a value ΔX2. The distance ΔX1 is longer than the distance ΔX2 because the optical path length L1 is longer than the optical path length L2.

[0060] 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 therefore the optical path length at each point of the light beam is different, so that the cross section of the light beam of diffracted light that passes through the transmissive diffraction grating 35 again and proceeds 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.

[0061] 2 and 4, the transmission type diffraction grating 35 is disposed so that wavelengths are dispersed in the vertical direction. Meanwhile, the 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.

[0062] 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 FIGS. 2 and 4). In other words, the major axis of the elliptical cross section of the light beam would be positioned in a direction 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.

[0063] The reason why the orientation of the major axis of the ellipse shown in FIG. 5B differs by 90 degrees from the orientation of the major axis of the ellipse shown in FIG. 4 is that in this embodiment, the cross section of the ellipse is rotated by 90 degrees by the wave plate 39 during the process of the diffracted light propagating through the lens 38 and the wave plate 39 to the light selection mirror 40.

[0064] 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 direction of the central axis C in this way is to set the filter slope of the wavelength-tunable filter 30 to a steep shape.

[0065] In Fig. 6, the solid lines show the transmittance of each wavelength component of the tunable filter 30 when the central axis C of the reflecting surface 40A of the light-selecting mirror 40 is arranged parallel to the major axis of the ellipse of the light beam. The horizontal axis of the graph shown in Fig. 6 corresponds to wavelength (frequency), and the vertical axis corresponds to filter transmittance.

[0066] 6 further shows, by broken lines, the transmittance of each wavelength component of the tunable filter 30 when the central axis C is arranged non-parallel to the major axis of the ellipse of the light beam. In FIG. 4, by broken lines, an example of the arrangement of the reflecting surface 40A when the central axis C is arranged non-parallel to the major axis of the ellipse of the light beam.

[0067] As can be seen from a comparison of the dashed and solid lines in Figure 6, when the central axis C of the reflecting surface 40A of the light-selecting mirror 40 is arranged parallel to the elliptical major axis of the light beam, the filter slope is steeper than when it is arranged non-parallel.

[0068] 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 prevented from spreading in the wavelength dispersion direction, and different wavelength components of diffracted light are effectively separated inside and outside the reflecting surface 40A. 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.

[0069] In recent WDM networks, optical signals are densely arranged on the frequency axis to improve spectral efficiency. That is, adjacent channels are located close to each other on the frequency axis. Therefore, if the filter slope is gentle, the possibility of optical signals interfering with each other on the adjacent channels increases. In this embodiment, to suppress 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.

[0070] Furthermore, 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 FIG. 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 movement direction of the light beam caused by 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.

[0071] 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 movement direction of the light beam caused by rotation of the reflecting surface 36A around the second rotation axis A2 coincides with the wavelength dispersion direction.

[0072] 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.

[0073] 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. Fig. 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.

[0074] 8 illustrates 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, Fig. 8 is merely a diagram illustrating relative displacement. It should be understood that it is not the light-selecting mirror 40 that actually displaces, but the diffracted light, that is, the diffracted light displaces relative to the reflecting surface 40A.

[0075] Fig. 9 shows an example configuration of the reflecting surface 36A of the variable-angle mirror 36. According to Fig. 9, the reflecting surface 36A is supported by a first support part 36B so as to be rotatable about a first rotation axis A1. The first support part 36B is supported by a second support part 36C so as to be rotatable about a second rotation axis A2. This allows the reflecting surface 36A to rotate about the first rotation axis A1 and the second rotation axis A2.

[0076] The above describes the configurations of the optical transmission system 1 and the tunable filter 30 of this embodiment. 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 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 effect of wavelength dispersion.

[0077] However, when the incident angle is set large, the major axis of the elliptical beam corresponding to the input light rotates in the diffracted light due to the difference between the optical path lengths L1 and L2, as shown in Figures 5A and 5B. In this case, when 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 with reference to Figure 6. This gentle filter slope is inconvenient in light of recent demands for improved frequency efficiency.

[0078] On the other hand, in this embodiment, the orientation of the light-selecting 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-selecting mirror 40. Therefore, according to this embodiment, even if the light beam rotates in the diffracted light due to, for example, the placement of the wavelength-tunable filter 30 in a narrow space, it is possible to achieve a steep filter slope and suppress interference of optical signals with adjacent channels.

[0079] Furthermore, in this embodiment, the two sides E1 and E2 of the reflecting surface 40A of the light selection 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.

[0080] The angle-variable mirror 36 is configured as a two-axis tilt mirror. That is, 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 by controlling the angle of the reflecting surface 36A.

[0081] Therefore, the tunable filter 30 can achieve a steep filter slope while changing not only the filter center wavelength but also the filter bandwidth by controlling the angle-variable mirror 36. The tunable filter 30 of this embodiment, which has such characteristics, is very useful for miniaturizing optical communication devices.

[0082] [Other Embodiments] The present disclosure is not limited to the above-described embodiments and can employ various other aspects. In the above-described embodiments, 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 a 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 relative to the major axis.

[0083] For example, if interference with adjacent channels does not occur 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. The inclination of the filter slope can be adjusted by adjusting the angle of the central axis C with respect to the major axis of the light beam. Therefore, the angle of the central axis C with respect to the major axis of the light beam may be adjusted so as to achieve a desired filter slope.

[0084] Furthermore, the rotation axes of the variable-angle mirror 36 are 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 be used as the variable-angle mirror 36.

[0085] 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, or 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.

[0086] However, when 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 around the two axes.

[0087] Alternatively, the reflecting surface 40A of the light-selecting mirror 40 may be modified to have a geometric shape having two parallel sides that are 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.

[0088] 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 may be 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.

[0089] The function 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

1. A wavelength tunable filter comprising: a transmission type diffraction grating arranged in a propagation path of input light; a reflection element arranged opposite to the transmission type diffraction grating so as to reflect the input light that has passed through the transmission type diffraction grating; and an optical selection element arranged in a propagation path of diffracted light that is the input light that has been reflected by the reflection element and passed through the transmission diffraction grating again, the optical selection element having a selection surface for optically selecting a portion of the diffracted light as output light, and 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 except for the portion of the diffracted light to the optical output path, wherein the reflection element has an angle-variable reflection surface, the diffracted light includes beams of a plurality of wavelength components aligned in a wavelength dispersion direction, and the 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, and the selection surface has a predetermined geometric shape having two sides extending axially symmetrically with respect to a central axis that intersects with the wavelength dispersion direction of the diffracted light, a wavelength-tunable filter in which the diffracted light is displaced relative to the geometric shape at least in the wavelength dispersion direction according to the angle of the reflecting surface, a wavelength component of the diffracted light selected as the output light is determined by a relative position of the diffracted light with respect to the geometric shape, and the selection surface is positioned so that the central axis of the geometric shape and a major axis of the beam of each wavelength component included in the diffracted light are parallel to each other.

2. A wavelength tunable filter comprising: a transmission type diffraction grating arranged in a propagation path of input light; a reflection element arranged opposite to the transmission type diffraction grating so as to reflect the input light that has passed through the transmission type diffraction grating; and an optical selection element arranged in a propagation path of diffracted light that is the input light that has been reflected by the reflection element and passed through the transmission diffraction grating again, the optical selection element having a selection surface for optically selecting a portion of the diffracted light as output light, and 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, wherein the reflection element has an angle-variable reflection surface, the diffracted light includes beams of a plurality of wavelength components aligned in a wavelength dispersion direction, and the selection surface has a predetermined geometric shape having two sides that extend axially symmetrically with respect to 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 in accordance with the angle of the reflecting surface, and wavelength components of the diffracted light selected as the output light are determined by the relative position of the diffracted light with respect to the geometric shape, and 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.

3. A wavelength tunable filter according to claim 2, wherein the beams of each wavelength component contained in the diffracted light are elliptical beams with their major axes tilted relative to a direction perpendicular to the wavelength dispersion direction, and the selection surface is positioned so that the central axis of the geometric shape and the major axis of the beams of each wavelength component contained in the diffracted light are parallel.

4. A wavelength tunable filter according to claim 2, wherein the beams of each wavelength component contained in the diffracted light are elliptical beams whose major axes are tilted relative to a direction perpendicular to the wavelength dispersion direction, and the selection surface is arranged so that the central axis of the geometric shape is tilted relative to the major axes of the beams of each wavelength component contained in the diffracted light.

5. A tunable filter according to any one of claims 2 to 4, wherein one of the plurality of rotation axes is set so that the direction of movement of the beam caused by rotation of the reflecting surface around the one rotation axis is parallel to the central axis.

6. A tunable filter according to any one of claims 2 to 4, wherein one of the plurality of rotation axes is set so that the direction of movement of the beam caused by rotation of the reflecting surface around the one rotation axis coincides with the wavelength dispersion direction.

7. A tunable filter according to any one of claims 2 to 4, wherein the plurality of rotation axes include a first rotation axis and a second rotation axis, the first rotation axis is set so that the direction of movement of the beam caused by rotation of the reflecting surface around the first rotation axis is parallel to the central axis, and the second rotation axis is set so that the direction of movement of the beam caused by rotation of the reflecting surface around the second rotation axis coincides with the wavelength dispersion direction.

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