Wavelength tunable filter

Abstract

Provided is a wavelength tunable filter provided with a transmission-type diffraction grating, a reflective element, and a light selection element. The transmission-type diffraction grating is disposed on a propagation path of input light. The reflective element is disposed facing the transmission-type diffraction grating. The light selection element is disposed on a propagation path of diffracted light, which is input light reflected by the reflection element and passing again through the transmission-type diffraction grating. The light selection element has a selection surface for optically selecting a portion of the diffracted light as output light. The selection surface has a specific geometry having two sides extending in axial symmetry with respect to a central axis intersecting a wavelength dispersion direction of the diffracted light. The diffracted light is displaced relative to the geometrical shape at least in the wavelength dispersion direction depending on the angle of the reflective surface.

Description

Technical Field

[0001] This disclosure relates to a wavelength-tunable filter. Background Technology

[0002] In the prior art, optical communication networks using wavelength division multiplexing (WDM) optical communication technology are known. Optical amplifiers are configured in these networks to compensate for transmission losses in the optical fiber. Within these amplifiers, ASE (Amplified Spontaneous Emission) noise is generated in the optical signal. Therefore, wavelength-tunable filters are configured in the optical communication network to eliminate ASE noise.

[0003] Known wavelength-tunable filters incorporate diffraction gratings (e.g., see Patent Documents 1, 2, 3). These wavelength-tunable filters reflect diffracted light from the diffraction grating corresponding to the input light via mirrors, thereby optically coupling only the specific wavelength components contained in the input light to the output fiber.

[0004] Existing technical documents Patent Literature Patent Document 1: U.S. Patent Publication No. 2008 / 0085119 Patent Document 2: Japanese Patent Application Publication No. 2008-203508 Patent Document 3: Japanese Patent Application Publication No. 2020-122936 Summary of the Invention The problem the invention aims to solve There is a need for miniaturization in optical communication equipment with built-in wavelength-tunable filters such as optical transceivers. To achieve miniaturization, the wavelength-tunable filter needs to be placed in a confined space. To enable the function of the wavelength-tunable filter in a confined space, effective wavelength dispersion is required. Therefore, in order to place the wavelength-tunable filter in a confined space, it is necessary to set a relatively large incident angle for the light to enter the diffraction grating.

[0005] However, the input light incident on the diffraction grating is a beam with a diameter. Therefore, if the incident angle is set large, the optical path length at different points in the beam will vary significantly as it passes through the diffraction grating, is reflected by the mirror, and then passes through the diffraction grating again. Due to these differences, the diffracted beam output from the diffraction grating has a tilted or rotated cross-section relative to the input beam. For example, the input beam may be an elliptical beam. In this case, a diffracted beam will be produced with its major axis tilted relative to the input beam.

[0006] When the aforementioned tilting or rotation occurs, existing techniques reduce the selectivity of the wavelength components in the optical selection element (e.g., a mirror), which selects the wavelength components from the diffracted beam to be optically coupled to the output fiber. Therefore, existing techniques cannot achieve steep filter slopes when the wavelength-tunable filter is configured in a confined space.

[0007] In recent wavelength division multiplexing (WDM) networks, optical signals are densely arranged along the frequency axis to improve frequency utilization efficiency. Therefore, failure to properly set the filter slope or bandwidth increases the likelihood of optical signals interfering with adjacent channels.

[0008] Therefore, according to one aspect of this disclosure, it is desirable to provide a new technique involving wavelength-tunable filters for appropriately setting the filter slope or bandwidth.

[0009] Problem Solving Methods According to one aspect of this disclosure, a wavelength-tunable filter is provided. The wavelength-tunable filter includes a transmission-type diffraction grating, a reflective element, and a light-selective element.

[0010] A transmission-type diffraction grating is disposed in the propagation path of the input light. A reflective element is disposed opposite to the transmission-type diffraction grating to reflect the input light passing through the transmission-type diffraction grating. A light-selecting element is disposed in the propagation path of the diffracted light, wherein the diffracted light is the input light that has been reflected by the reflective element and passes through the transmission-type diffraction grating again.

[0011] 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 a portion of the diffracted light selected by the selection surface to the light output path, and not guide the remaining light other than the portion of the diffracted light to the light output path.

[0012] The reflecting element has a reflective surface with an adjustable angle. Diffracted light comprises a beam of light with multiple wavelength components aligned along the wavelength dispersion direction. Each wavelength component in the diffracted light is an elliptical beam, with its major axis tilted relative to a direction orthogonal to the wavelength dispersion direction. The major axis can be tilted at an acute angle relative to the direction orthogonal to the wavelength dispersion direction, for example.

[0013] The selection surface has a specific geometry that has two sides extending axially symmetrically relative to a central axis intersecting the wavelength dispersion direction of the diffracted light. The diffracted light undergoes a relative displacement with respect to the geometry, at least in the wavelength dispersion direction, depending on the angle of the reflecting surface. The wavelength component selected as the output light in the diffracted light is determined based on the relative position of the diffracted light with respect to the geometry.

[0014] According to one aspect of this disclosure, the surface is configured such that the central axis of the geometry is parallel to the long axis of the beam containing each wavelength component of the diffracted light. According to the wavelength-tunable filter constructed above, the long axis of the beam is parallel to the central axis of the geometry. The wavelength dispersion direction is the direction intersecting the central axis. Therefore, in cases where, for example, the two sides of the geometry extend axially symmetrically from the central axis (e.g., when the two sides of the geometry are parallel to the central axis), the different wavelength components of the diffracted light are well separated between the interior and exterior of the region enclosed by the two sides of the geometry.

[0015] Therefore, according to one aspect of this disclosure, a wavelength-tunable filter can be provided that can set the filter slope to an appropriate shape. According to the aforementioned wavelength-tunable filter, even when the long axis of the beam is not aligned in a direction orthogonal to the wavelength dispersion direction and the long axis of the beam is tilted, the filter slope can still be set to a steep shape.

[0016] According to another aspect of this disclosure, a wavelength-tunable filter can be provided, which includes a transmission diffraction grating, a reflective element, and a light selection element, wherein the transmission diffraction grating is disposed in the propagation path of the input light; the reflective element is disposed opposite to the transmission diffraction grating to reflect the input light passing through the transmission diffraction grating, and the wavelength-tunable filter has the following features.

[0017] An optical selection element is disposed in the propagation path of the diffracted light, which is the input light reflected by a reflecting element and then passed again through a transmission-type diffraction grating. The optical selection element has a selection surface for optically selecting a portion of the diffracted light as the output light. The optical selection element is configured to guide a portion of the diffracted light selected by the selection surface to the optical output path, and not to guide the remaining light other than the selected portion of the diffracted light to the optical output path.

[0018] The reflecting element has an adjustable-angle reflecting surface. The diffracted light comprises a beam of light with multiple wavelength components aligned along the wavelength dispersion direction. The selection surface has a specific geometry that has two axially symmetrical sides extending relative to a central axis intersecting the wavelength dispersion direction of the diffracted light, and these sides are not parallel to the central axis. The diffracted light undergoes a relative displacement with respect to the geometry depending on the angle of the reflecting surface. The wavelength component selected as the output light in the diffracted light is determined based on the relative position of the diffracted light with respect to the geometry.

[0019] The reflective element is configured to rotate the reflective surface around multiple rotation axes to change the angle of the reflective surface, so that the light beam containing each wavelength component in the diffracted light can move along the central axis of the selected surface and the light beam can move in the wavelength dispersion direction.

[0020] The wavelength-tunable filter constructed as described above allows for the alteration of the output wavelength by moving the beam along the wavelength dispersion direction. When the selection surface has two sides that are not parallel to the central axis, the width of the selection surface in the direction perpendicular to the central axis changes along the central axis. Therefore, the bandwidth can be changed by moving the beam along the central axis.

[0021] Therefore, according to another aspect of this disclosure, a new technique involving wavelength-tunable filters can be provided, which can appropriately set the bandwidth. According to the aforementioned wavelength-tunable filter, the wavelength and bandwidth can be appropriately changed. According to another aspect of this disclosure, the beam containing each wavelength component in the diffracted light can all be elliptical beams, with their major axes tilted relative to a direction orthogonal to the wavelength dispersion direction.

[0022] According to another aspect of this disclosure, the selection surface can be configured such that the central axis of the geometry is parallel to the major axis of the beam containing each wavelength component of the diffracted light. With this configuration, the filter slope can be set to a steep shape.

[0023] According to another aspect of this disclosure, the selection surface can be configured such that the central axis of the geometry is tilted relative to the major axis of the beam containing the individual wavelength components in the diffracted light. With this configuration, the filter slope can be set to an appropriate shape by adjusting the tilt.

[0024] According to another aspect of this disclosure, one of the multiple rotation axes can be configured such that the direction of the beam movement caused by the rotation of the reflecting surface around the rotation axis is parallel to the central axis. Based on the above configuration of the rotation axis, the bandwidth can be adjusted by rotating the reflecting surface around the rotation axis without changing the center wavelength of the filter.

[0025] According to another aspect of this disclosure, one of the multiple rotation axes can be set such that the direction of the beam movement caused by the rotation of the reflecting surface around the rotation axis is consistent with the wavelength dispersion direction. Based on the above setting of the rotation axis, the center wavelength of the filter can be changed by rotating the reflecting surface around a rotation axis while maintaining the bandwidth of the filter. According to another aspect of this disclosure, the plurality of rotation axes may include a first rotation axis and a second rotation axis. The first rotation axis may be configured such that the direction of movement of the light beam caused by the rotation of the reflecting surface around a rotation axis is parallel to the central axis. The second rotation axis may be configured such that the direction of movement of the light beam caused by the rotation of the reflecting surface around a rotation axis is consistent with the wavelength dispersion direction. Based on the above configuration of the rotation axes, different rotation axes can be used to adjust the center wavelength and bandwidth of the filter. Attached Figure Description

[0026] Figure 1 This is a block diagram showing the structure of an optical transmission system.

[0027] Figure 2 This is a conceptual diagram illustrating the optical configuration of a wavelength-tunable filter.

[0028] Figure 3 This is a diagram showing the specific configuration of the wavelength-tunable filter.

[0029] Figure 4 It is a diagram illustrating the geometry of the reflecting surface of a mirror that selects light.

[0030] Figure 5A It is a diagram showing the surrounding structure of a transmission diffraction grating as observed from a specific direction; Figure 5B This is a diagram showing the surrounding structure of a transmission-type diffraction grating as observed from other directions.

[0031] Figure 6 It is a graph showing the relationship between wavelength and transmittance.

[0032] Figure 7 This is a diagram illustrating the change in the relative position of the diffracted beam with respect to the light-selective mirror.

[0033] Figure 8 This is a diagram illustrating the change in the relative position of the diffracted beam with respect to the light-selective mirror.

[0034] Figure 9 This is a diagram illustrating the schematic structure of an adjustable-angle reflector with two rotation axes.

[0035] Figure 10 This is a diagram illustrating the geometry of the reflecting surface of the light-selective mirror in the modified example.

[0036] Description of Reference Signs 1… Optical transmission system; 10… Optical amplifier; 30… Wavelength-tunable filter; 31… Optical input / output unit; 32… First optical system; 33… Lens array; 34… Beam expander prism group; 35… Transmission diffraction grating; 36… Adjustable angle mirror; 36A… Reflecting surface; 37… Second optical system; 38… Lens; 39… Wave plate; 40… Light-selective reflector; 40A… Reflecting surface; 40B… Reflecting surface; 50… Controller; 311… Input fiber; 315… Output fiber; A1… First rotation axis; A2… Second rotation axis; C… Central axis. Detailed Implementation

[0037] Exemplary embodiments of this disclosure will now be described with reference to the accompanying drawings.

[0038] Figure 1 The optical transmission system 1 of this embodiment is configured such that an optical signal flowing in a wavelength division multiplexing (WDM) network is amplified by an optical amplifier 10 and transmitted downstream. The optical transmission system 1 includes a wavelength-tunable filter 30 downstream of the optical amplifier 10 to eliminate ASE noise generated during the amplification of the optical signal by the optical amplifier 10.

[0039] The wavelength-tunable filter 30 is configured as a bandpass filter capable of changing the signal passband. The wavelength-tunable filter 30 includes an adjustable-angle reflector 36 as a movable element. The adjustable-angle reflector 36 has a reflective surface 36A with an adjustable angle. The wavelength-tunable filter 30 is configured to change the signal passband by changing the angle of the reflective surface 36A.

[0040] Specifically, the wavelength-tunable filter 30 is configured to change the center wavelength and bandwidth of the signal passband. In the following text, the center wavelength will be referred to as the filter center wavelength, and the bandwidth as the filter bandwidth.

[0041] The optical transmission system 1 also includes a controller 50, which serves as a component for controlling the center wavelength and bandwidth of the filter. The controller 50 is configured to control the center wavelength and bandwidth of the filter by controlling the adjustable angle reflector 36.

[0042] The controller 50 is able to control the angle of the reflective surface 36A of the adjustable angle reflector 36 to achieve the specified filter center wavelength and filter bandwidth based on a measurement signal from, for example, a power monitor (not shown).

[0043] A power monitor is installed, for example, downstream of the wavelength-tunable filter 30. Light passing through the wavelength-tunable filter 30 can be split and input to the power monitor. The power monitor can measure the power of the light passing through the wavelength-tunable filter 30 based on the input split light.

[0044] Figure 2 The optical configuration of an exemplary wavelength-tunable filter 30 is conceptually illustrated. Figure 2 The concept shows that when from Figure 3 The configuration of each component in the wavelength-tunable filter 30 is observed when looking in the direction of the middle arrow R1. Figure 3 The specific configuration of the wavelength-tunable filter 30 is shown.

[0045] like Figure 2As shown, the wavelength-tunable filter 30, in addition to an adjustable-angle reflector 36, also includes an optical input / output unit 31, a first optical system 32, a transmission-type diffraction grating 35, a second optical system 37, and a light-selective reflector 40. The optical input / output unit 31 includes an input optical fiber 311 and an output optical fiber 315. Figure 3 As shown, the first optical system 32 includes a lens array 33 and a beam expander prism group 34. The second optical system 37 includes a lens 38 and a waveplate 39.

[0046] The optical signal from the optical amplifier 10 is input to the wavelength-tunable filter 30 via the optical input / output unit 31, specifically via the input optical fiber 311. Hereinafter, the optical signal input to the wavelength-tunable filter 30 via the input optical fiber 311 will be referred to as the input light.

[0047] The input light propagates through the first optical system 32 to the transmission-type diffraction grating 35. During its passage through the first optical system 32, the input light is collimated by the beam-expanding prism group 34. That is, after being collimated by the first optical system 32, the input light propagates to the transmission-type diffraction grating 35. The input light enters the transmission-type diffraction grating 35 in the form of a beam with an elliptical cross-section.

[0048] A transmissive diffraction grating 35 is positioned in the propagation path of the collimated input light. The diffraction angle of the transmissive diffraction grating 35 depends on the wavelength. Due to this wavelength dependence, the transmissive diffraction grating 35 spatially separates the collimated input light into multiple wavelength components. The input light from the first optical system 32 passes through the transmissive diffraction grating 35 and propagates to the adjustable angle mirror 36.

[0049] The adjustable-angle mirror 36 functions as a reflective element. The adjustable-angle mirror 36 is configured opposite to the transmission diffraction grating 35 to reflect the input light passing through the transmission diffraction grating 35.

[0050] The input light propagating to the adjustable-angle mirror 36 is reflected at the reflecting surface 36A of the adjustable-angle mirror 36 and passes through the transmission-type diffraction grating 35 again. The input light that passes through the transmission-type diffraction grating 35 again is diffracted light as multiple wavelength components dispersed in space, and propagates to one side of the light-selective mirror 40 through the second optical system 37. The second optical system 37 and the light-selective mirror 40 are arranged in the propagation path of the diffracted light.

[0051] Figure 2 and Figure 3 The solid arrow shown conceptually illustrates the propagation of the wavelength component of the input light that is located within the signal passband and optically coupled to the output fiber 315 as the output light. Figure 2 The dashed arrow shown conceptually illustrates the propagation of wavelength components located outside the signal passband.

[0052] The adjustable-angle reflector 36 is configured to change the angle of the reflecting surface 36A under the control of the controller 50. The adjustable-angle reflector 36 is a tilting mirror with multiple rotation axes, specifically, a tilting mirror with two rotation axes. For example, the adjustable-angle reflector 36 can be a MEMS mirror (Micro-Electro-Mechanical System mirror).

[0053] Lens 38 functions as a telecentric lens, converting the diffracted light from the transmissive diffraction grating 35, specifically each of the spatially dispersed wavelength components contained in the diffracted light, into a beam whose propagation direction is parallel to the optical axis passing through the center of lens 38.

[0054] Wave plate 39 is a half-wave plate, which can be selectively disposed between the transmission diffraction grating 35 and the light selective mirror 40. Wave plate 39 can be disposed between the transmission diffraction grating 35 and the light selective mirror 40 with its optical axis tilted at 45° relative to the grating axis of the transmission diffraction grating 35, so as to suppress the influence of polarization dependence on diffraction efficiency in the transmission diffraction grating 35.

[0055] Due to the presence of the waveplate 39, the polarization state of the light incident on the transmission diffraction grating 35 in the reverse optical path is orthogonal to the polarization state of the light in the forward optical path. Therefore, 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 via the output fiber 315 can be eliminated. The waveplate 39 is configured to act on the diffracted light in one of the optical paths (e.g., the forward optical path) in either the forward or reverse optical path.

[0056] If the transmission diffraction grating 35 is a substantially polarization-independent diffraction grating, then the waveplate 39 may not be required. However, the waveplate 39 may also be provided to compensate for the polarization-dependent loss of the wavelength-tunable filter 30 caused by the polarization dependence of the residual diffraction efficiency in the transmission diffraction grating 35.

[0057] The light-selective reflector 40 is configured to reflect a specific wavelength component of the beam, aligned by the adjustable-angle reflector 36, from the beam of light containing multiple wavelength components in the arriving diffracted light. The light-selective reflector 40 functions as a light-selecting element.

[0058] Figure 4 The geometry of the light-selective reflector 40 and the arrangement of the multiple wavelength components contained in the diffracted light are shown. The light-selective reflector 40 has a rectangular reflecting surface 40A, whose two sides E1 and E2 extend axially symmetrically with respect to the central axis C. The central axis C is located at... Figure 4 The Chinese character is represented by a single-dot dash.

[0059] Reflector 40A and Figure 4 The corresponding section line area. Figure 4 Each of the multiple elliptical objects shown corresponds to a beam of light with a wavelength component. The beam of light in the band located within the profile line region is reflected as output light by the light-selective reflector 40.

[0060] As described above, the wavelength component selected as the output light in the diffracted 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, which optically selects a portion of the diffracted light as the output light. The light-selecting mirror 40 functions to guide a portion of the diffracted light selected by the reflecting surface 40A to the optical output path (output fiber 315), while not guiding the remaining light other than this portion of the diffracted light to the optical output path.

[0061] It is worth noting that, Figure 4 In this context, the elliptical cross-section of the beam is tilted relative to the wavelength dispersion direction. Figure 4 In this case, the wavelength dispersion direction is up and down, and it is the direction in which the multiple wavelength components contained in the diffracted light are arranged. Figure 4 The image shows a beam with an elliptical cross-section, whose major axis is tilted at an acute angle relative to a direction orthogonal to the wavelength dispersion direction. This tilt is caused by the different optical path lengths at different points in the beam, which is the optical path length between the transmission diffraction grating 35 and the reflecting surface 36A of the adjustable angle mirror 36.

[0062] Figure 5A Showing from Figure 3 The configuration of the transmission diffraction grating 35 and the adjustable angle mirror 36 as observed in the direction of the middle arrow R1. Figure 5A The propagation paths of the optical signals, corresponding to the two endpoints of the major axis of the ellipse, are further illustrated by the double-dotted lines.

[0063] Figure 5B The cross-sectional shape F1 of the light beam propagating from the first optical system 32 through the transmission diffraction grating 35 to the adjustable-angle mirror 36 is shown, along with the endpoints P11 and P12 of the major axis of the ellipse. Furthermore, Figure 5B The cross-sectional shape F2 of the beam reflected from the adjustable angle mirror 36 and then passing through the transmission diffraction grating 35 is shown, as well as the endpoints P21 and P22 of the major axis of the ellipse.

[0064] The double-dotted line extending from endpoint P11 to endpoint P21 conceptually illustrates the propagation path of the optical signal from endpoint P11 to endpoint P21. The double-dotted line extending from endpoint P12 to endpoint P22 conceptually illustrates the propagation path of the optical signal from endpoint P12 to endpoint P22.

[0065] exist Figure 5A In the diagram, the optical path length in the propagation path of the optical signal from endpoint P11 to endpoint P21 is represented by the value L1, and the optical path length in the propagation path of the optical signal from endpoint P12 to endpoint P22 is represented by the value L2.

[0066] exist Figure 5B In the diagram, the distance from endpoint P11 along the surface of the transmission diffraction grating 35 to endpoint P21 is represented by the value ΔX1, and the distance from endpoint P12 along the surface of the transmission diffraction grating 35 to endpoint P22 is represented by the value ΔX2. Since the optical path length L1 is longer than the optical path length L2, the distance ΔX1 is longer than the distance ΔX2.

[0067] from Figure 5A and Figure 5B It can be understood that the beam incident on the transmission diffraction grating 35 is a beam with thickness or diameter, so the optical path length at each point of the beam is different. As a result, the beam cross section of the diffracted light that passes through the transmission diffraction grating 35 again and is directed toward the second optical system 37 rotates relative to the beam cross section when it is incident on the transmission diffraction grating 35.

[0068] In this embodiment, the transmission-type diffraction grating 35 is configured such that the wavelength is within... Figure 2 and Figure 4 The light is dispersed in the vertical direction. On the other hand, the input light from the first optical system 32 is reflected in the horizontal direction on the reflecting surface 36A and propagates as diffracted light to the second optical system 37.

[0069] Therefore, as Figure 4 As shown, the beam of light containing each wavelength component in the diffracted light propagates as an elliptical beam to the light-selective reflector 40, the major axis of which is relative to the wavelength dispersion direction ( Figure 2 and Figure 4 The major axis of the elliptical cross-section of the beam is tilted relative to the wavelength dispersion direction (the vertical direction within the beam). In other words, if no rotation occurs, the major axis of the beam's elliptical cross-section is located in a direction orthogonal to the wavelength dispersion direction, but due to rotation, this major axis is tilted relative to the wavelength dispersion direction.

[0070] Figure 5B The direction of the major axis of the ellipse shown is... Figure 4 The reason why the directions of the major axes of the ellipse shown differ by 90 degrees is that, in this embodiment, during the process of the diffracted light propagating through the lens 38 and the waveplate 39 to the light-selective reflector 40, the elliptical cross section is rotated by 90 degrees by the waveplate 39.

[0071] In this embodiment, such as Figure 4As shown, the reflecting surface 40A of the light-selective reflector 40 is configured such that the central axis C of the reflecting surface 40A of the light-selective reflector 40 is parallel to the major axis of the ellipse of each wavelength component of the light beam. By setting the direction of the central axis C as described above, the filter slope of the wavelength-tunable filter 30 is set to a steep shape.

[0072] exist Figure 6 The solid lines in the middle show the transmittance of each wavelength component of the wavelength-tunable filter 30 when the central axis C of the reflecting surface 40A of the light selection mirror 40 is set parallel to the major axis of the ellipse of the light beam. Figure 6 The horizontal axis corresponds to wavelength (frequency), and the vertical axis corresponds to the filter's transmittance.

[0073] exist Figure 6 The transmittance of each wavelength component of the wavelength-tunable filter 30 is further shown by dashed lines when the central axis C is set to be non-parallel to the major axis of the ellipse of the beam. Figure 4 The dashed line shows an example of the configuration of the reflecting surface 40A when the central axis C is assumed to be not parallel to the major axis of the ellipse of the beam.

[0074] from Figure 6 The comparison between the dashed and solid lines shows that when the central axis C of the reflecting surface 40A of the light selection mirror 40 is configured to be parallel to the major axis of the ellipse of the beam, the filter slope is steeper than that when it is configured to be non-parallel.

[0075] The major axis of the ellipse of the light beam intersects obliquely with the wavelength dispersion direction. Therefore, when the central axis C is configured parallel to the major axis of the ellipse of the light beam, the diffusion of the light beam in the same wavelength band incident area in the wavelength dispersion direction in the reflecting surface 40A is suppressed, and the different wavelength components of the diffracted light are well separated inside and outside the reflecting surface 40A. Therefore, when the central axis C is configured parallel to the major axis of the ellipse, the filter slope of the wavelength-tunable filter 30 can be set to a steep shape.

[0076] In recent wavelength division multiplexing (WDM) networks, optical signals are densely arranged along the frequency axis to improve frequency utilization efficiency. That is, adjacent channels are located close to each other on the frequency axis. Therefore, if the filter slope is too gentle, the possibility of optical signals interfering with 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 light beam, thereby setting the filter slope to a steep shape.

[0077] Furthermore, in this embodiment, the first rotation axis A1 of the adjustable angle reflector 36 is set such that it rotates around the first rotation axis A1 via the reflecting surface 36A (see reference). Figure 9The light beam is rotated so that it moves parallel to the central axis C along the plane of the reflecting surface 40A of the light selective mirror 40. In other words, the first rotation axis A1 is set such that the direction of the movement of the light beam caused by the rotation of the reflecting surface 36A about the first rotation axis A1 is parallel to the central axis C of the light selective mirror 40.

[0078] In this embodiment, the second rotation axis A2 of the adjustable angle reflector 36 is further configured such that the diffracted beam moves along the wavelength dispersion direction on the plane of the reflector 40A of the light-selective reflector 40 by rotating the reflector 36A around the second rotation axis A2. In other words, the second rotation axis A2 is configured such that the direction of beam movement caused by the rotation of the reflector 36A around the second rotation axis A2 is consistent with the wavelength dispersion direction.

[0079] Therefore, the controller 50 of this embodiment can change the filter bandwidth while keeping the center wavelength of the wavelength-tunable filter 30 unchanged by rotating the reflective surface 36A of the adjustable angle reflector 36 around the first rotation axis A1. Figure 7 This illustrates the situation where the light beam moves along the central axis C of the reflecting surface 40A of the light selection mirror 40 due to the rotation of the reflecting surface 36A about the first rotation axis A1.

[0080] Similarly, the controller 50 can change the center wavelength of the filter while keeping the filter bandwidth of the wavelength-tunable filter 30 constant by rotating the reflective surface 36A of the adjustable angle reflector 36 about the second rotation axis A2. Figure 8 This illustrates the situation where the light beam moves relative to the reflecting surface 40A of the light selective mirror 40 in the wavelength dispersion direction due to the rotation of the reflecting surface 36A about the second rotation axis A2.

[0081] Figure 8 The change in the relative position of the diffracted light with respect to the reflecting surface 40A of the light-selective mirror 40, indicated by a dashed line, illustrates how the rotation of the reflecting surface 36A alters the wavelength dispersion direction of the diffracted light. However, Figure 8 This diagram is only for illustrating the relative displacement. It should be understood that the actual displacement is not caused by the light selective reflector 40, but by the diffracted light; that is, the diffracted light is displaced relative to the reflecting surface 40A.

[0082] Figure 9 An example configuration of the reflecting surface 36A in the adjustable-angle reflector 36 is shown. According to... Figure 9The reflective surface 36A is supported such that it can rotate about a first rotation axis A1 relative to the first support member 36B. The first support member 36B is supported such that it can rotate about a second rotation axis A2 relative to the second support member 36C. Thus, the reflective surface 36A can rotate about the first rotation axis A1 and the second rotation axis A2.

[0083] The configuration of the optical transmission system 1 and the wavelength-tunable filter 30 in this embodiment has been described above. As mentioned above, there is a need for miniaturization of devices such as optical transceivers used in the optical transmission system 1. When miniaturizing the wavelength-tunable filter 30 by arranging it in a narrow space, it is necessary to improve the wavelength dispersion effect by setting the incident angle of light onto the transmission diffraction grating 35 to a larger value.

[0084] However, when the angle of incidence is set to a large value, such as Figure 5A and Figure 5B As shown, due to the different optical path lengths L1 and L2, the major axis of the elliptical beam corresponding to the input light will rotate in the diffracted light. In this case, if the major axis of the elliptical beam is set to be non-parallel to the central axis C of the reflecting surface 40A of the light-selecting mirror 40, then as described above... Figure 6 As the description shows, the filter slope becomes flat. This flat filter slope is not conducive to the demands for improved frequency efficiency that have emerged in recent years.

[0085] On the other hand, in this embodiment, the direction of the light-selective reflector 40 is set such that the major axis of the elliptical beam is aligned with the central axis C of the reflecting surface 40A of the light-selective reflector 40. Therefore, according to this embodiment, even if the beam rotates in the diffracted light due to the wavelength-tunable filter 30 being placed in a narrow space, a steep filter slope can be achieved, thereby suppressing interference between optical signals in adjacent channels.

[0086] Furthermore, in this embodiment, the two sides E1 and E2 of the reflective surface 40A of the light selective reflector 40 are set to be axially symmetrical but not parallel with respect to the central axis C, so that the width of the reflective surface 40A in the direction perpendicular to the central axis C changes relative to the direction along the central axis C.

[0087] Furthermore, the adjustable angle reflector 36 is configured as a biaxial tilting reflector. That is, by controlling the angle of the reflecting surface 36A, the wavelength adjustable filter 30 can not only change the relative position of the diffracted light beam with respect to the reflecting surface 40A in the wavelength dispersion direction, but also change the relative position of the diffracted light beam with respect to the reflecting surface 40A in the direction along the central axis C.

[0088] Therefore, the wavelength-tunable filter 30 can achieve both a steep filter slope and the ability to change the filter center wavelength and filter bandwidth by controlling the adjustable-angle reflector 36. The wavelength-tunable filter 30 of this embodiment, with the above features, contributes to the miniaturization of optical communication devices.

[0089] [Other Implementation Methods] This disclosure is not limited to the above-described embodiments, and various other methods can be adopted. In the above embodiments, the central axis C of the reflecting surface 40A of the light-selective reflector 40 is aligned with the long axis of the light beam, but the above-described method of setting the central axis C is not mandatory. That is, the central axis C of the reflecting surface 40A of the light-selective reflector 40 can be configured to be non-parallel to the long axis of the light beam, in other words, tilted relative to the long axis.

[0090] For example, when interference with adjacent channels will not occur, and when interference with adjacent channels does not need to be considered, such as Figure 4 As shown by the dashed lines, the reflecting surface 40A can be configured such that its central axis C is not parallel to the major axis of the beam. By adjusting the angle of the central axis C relative to the major axis of the beam, the tilt of the filter slope can be adjusted. Therefore, the angle of the central axis C relative to the major axis of the beam can be adjusted to achieve the desired filter slope.

[0091] Furthermore, the rotation axes of the adjustable angle reflector 36 are not limited to the first rotation axis A1 and the second rotation axis A2, which correspond to the central axis C and the wavelength dispersion direction, respectively. For example, the first rotation axis A1 and the second rotation axis A2 can be rotation axes that are orthogonal to each other. A biaxial tilting reflector with two orthogonal rotation axes can be used as the adjustable angle reflector 36.

[0092] The first rotation axis A1 can be set such that the reflective surface 36A rotates around the rotation axis A1, thereby causing the light beam to move in a direction orthogonal to the wavelength dispersion direction. Alternatively, the second rotation axis A2 can be set such that the reflective surface 36A rotates around the rotation axis A2, thereby causing the light beam to move in a direction orthogonal to the central axis C.

[0093] However, when the two rotation axes are orthogonal, both the filter center wavelength and the filter bandwidth can be changed by rotating the reflective surface 36A around one of the rotation axes. Therefore, in order to control the adjustable angle reflector 36 so that one of the filter center wavelength and the filter bandwidth remains constant while the other changes, the rotation of the two axes needs to be precisely and appropriately controlled.

[0094] Furthermore, the reflecting surface 40A of the light-selective reflector 40 can be modified to have a geometry that is axially symmetric with respect to the central axis C and parallel to both sides of the central axis C. That is, instead of Figure 4The reflective surface 40A shown, and the light-selective reflector 40 can be configured to have, as shown in the figure, reflective surface 40A, light-selective reflector 40. Figure 10 The reflective surface 40B has a geometric shape, wherein the geometric shape has two sides E21 and E22 parallel to the central axis C.

[0095] When the geometry of the reflective surface 40B is adopted, the filter bandwidth does not change even if the beam is moved along the central axis C. Therefore, the adjustable angle reflector 36 can be transformed into a single-axis tilting reflector, which can be configured to have a single rotational axis capable of moving the diffracted light in the wavelength dispersion direction. Through this example, the advantages of a steeper filter slope can also be realized.

[0096] The function of one component in the above embodiments can be distributed among multiple components. The functions of multiple components can also be integrated into one component. A portion of the structure in the above embodiments can be omitted. At least a portion of the structure in the above embodiments can be added to the structure of other embodiments, or at least a portion of the structure in the above embodiments can be substituted with the structure of other embodiments. All methods included in the technical concept specific to the statements in the claims are embodiments of this disclosure.

Claims

1. A wavelength-tunable filter, characterized in that, have: A transmission diffraction grating, wherein the transmission diffraction grating is disposed in the propagation path of the input light; A reflective element, which is disposed opposite to the transmissive diffraction grating to reflect the input light passing through the transmissive diffraction grating; as well as A light selection element is disposed on the propagation path of the diffracted light and 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 to guide the remaining light other than the selected portion of the diffracted light to the light output path. The diffracted light is the input light that has been reflected by the reflecting element and passed again through the transmission-type diffraction grating. The reflective element has a reflective surface with an adjustable angle. The diffracted light comprises a beam of light with multiple wavelength components arranged along the wavelength dispersion direction. Each wavelength component of the diffracted light is an elliptical beam, with its major axis tilted relative to a direction orthogonal to the wavelength dispersion direction. The selection surface has a specific geometry that has two sides extending axially symmetrically relative to the central axis intersecting the wavelength dispersion direction of the diffracted light. The diffracted light undergoes a relative displacement with respect to the geometry, at least in the wavelength dispersion direction, depending on the angle of the reflecting surface, and the wavelength component selected as the output light in the diffracted light is determined based on the relative position of the diffracted light with respect to the geometry. The selection surface is configured such that the central axis of the geometry is parallel to the long axis of the beam of each wavelength component contained in the diffracted light.

2. A wavelength-tunable filter, characterized in that, have: A transmission diffraction grating, wherein the transmission diffraction grating is disposed in the propagation path of the input light; A reflective element, which is disposed opposite to the transmissive diffraction grating to reflect the input light passing through the transmissive diffraction grating; as well as A light selection element is disposed on the propagation path of the diffracted light and 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 to guide the remaining light other than the selected portion of the diffracted light to the light output path. The diffracted light is the input light that has been reflected by the reflecting element and passed again through the transmission-type diffraction grating. The reflective element has a reflective surface with an adjustable angle. The diffracted light comprises a beam of light with multiple wavelength components arranged along the wavelength dispersion direction. The selection surface has a specific geometry that has two sides extending axially symmetrically relative to a central axis intersecting the wavelength dispersion direction of the diffracted light, and these two sides are not parallel to the central axis. The diffracted light undergoes a relative displacement with respect to the geometry based on the angle of the reflecting surface, and the wavelength component of the diffracted light selected as the output light is determined based on the relative position of the diffracted light with respect to the geometry. The reflective element is configured to allow the reflective surface to rotate about a plurality of rotation axes to change the angle of the reflective surface, so that the light beam containing each wavelength component in the diffracted light can move along the central axis and the light beam can move in the wavelength dispersion direction.

3. The wavelength-tunable filter according to claim 2, characterized in that, The diffracted light contains elliptical beams for each wavelength component, with their major axes tilted relative to a direction orthogonal to the wavelength dispersion direction. The selection surface is configured such that the central axis of the geometry is parallel to the long axis of the beam of each wavelength component contained in the diffracted light.

4. The wavelength-tunable filter according to claim 2, characterized in that, The diffracted light contains elliptical beams for each wavelength component, with their major axes tilted relative to a direction orthogonal to the wavelength dispersion direction. The selection surface is configured such that the central axis of the geometry is tilted relative to the long axis of the beam containing the individual wavelength components in the diffracted light.

5. The wavelength-tunable filter according to any one of claims 2 to 4, characterized in that, One of the plurality of rotation axes is set such that the direction of movement of the light beam caused by the rotation of the reflecting surface about the one rotation axis is parallel to the central axis.

6. The wavelength-tunable filter according to any one of claims 2 to 4, characterized in that, One of the plurality of rotation axes is configured such that the direction of movement of the light beam caused by the rotation of the reflecting surface about the one rotation axis is consistent with the wavelength dispersion direction.

7. The wavelength-tunable filter according to any one of claims 2 to 4, characterized in that, The plurality of rotation axes includes a first rotation axis and a second rotation axis. The first rotation axis is set such that the direction of movement of the light beam caused by the rotation of the reflecting surface about the first rotation axis is parallel to the central axis. The second rotation axis is set such that the direction of movement of the light beam caused by the rotation of the reflecting surface around the first rotation axis is consistent with the wavelength dispersion direction.

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