Optical transmission system

Abstract

The optical filter is configured so as to selectively pass at least one of a plurality of wavelength components included in the input light through the optical filter as output light. The optical filter includes a movable element. The controller is configured to adjust the passband by controlling the movable element on the basis of the power of the light corresponding to the output light measured by the power monitor. The optical filter is configured such that, in a first wavelength band including a center wavelength of a passband, a gain curve corresponds to a convex function having a peak at the center wavelength, and in a second wavelength band adjacent to the first wavelength band, the gain curve exhibits a steeper slope than the convex function.

Description

Technical Field

[0001] The present disclosure relates to optical transmission systems. Background Art

[0002] Conventionally, an optical communication network using wavelength division multiplexing (WDM) optical communication technology is known. In order to compensate for the transmission loss of optical fibers, optical amplifiers are arranged in the optical communication network. As an optical amplifier, for example, an erbium-doped fiber amplifier is known.

[0003] In an optical amplifier, when an optical signal is amplified, amplifier spontaneous emission (ASE) noise is superimposed on the optical signal. Therefore, a wavelength variable filter is configured in an optical communication network to eliminate ASE noise.

[0004] A known variable wavelength filter is provided with a diffraction grating (for example, see patent documents 1 and 2). The variable wavelength filter causes the input light dispersed by the diffraction grating to be reflected by a reflector, thereby coupling only a specific wavelength component contained in the input light with the output optical fiber. The reflector is configured to be able to change the angle of its mirror surface. By adjusting the angle of the mirror surface, the wavelength band coupled with the output optical fiber light is changed.

[0005] Furthermore, there is also known a technique in which the power of light having passed through a variable wavelength filter is measured by a power monitor, and the passband of the variable wavelength filter is controlled based on the measured power (for example, see Patent Document 3).

[0006] The passband of the variable wavelength filter varies with temperature and slowly changes over time due to the external environment. Therefore, the power of light that has passed through the variable wavelength filter is measured by a power monitor, and the angle of the mirror is adjusted based on the measured power.

[0007] For example, the angle of the mirror in the variable wavelength filter is adjusted so that the power measured by the power monitor reaches the maximum. Thus, the passband of the variable wavelength filter is adjusted so that the center of the passband coincides with the center wavelength of the optical signal.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: U.S. Patent Publication No. 2008 / 0085119

[0011] Patent Document 2: Japanese Patent Application Publication No. 2008-203508

[0012] Patent Document 3: Japanese Patent Application Publication No. 2017-063293 Summary of the invention

[0013] Problem that the invention aims to solve

[0014] In optical communication networks, in order to improve the tight data traffic, it is required to improve the efficiency of frequency utilization. In large-capacity networks based on WDM technology that transmits multiple optical signals with different wavelengths using one optical fiber, it is crucial to be able to densely arrange optical signals on the frequency axis.

[0015] In a WDM network with a wavelength of 400Gbps and a spacing of 100GHz, a 400Gbps signal modulated with 64Gbound multi-level modulation occupies a bandwidth of 64GHz with a substantially uniform width on the frequency axis. When this signal is amplified and transmitted in an optical amplifier, ASE noise is generated in adjacent channels. In order to eliminate ASE noise and improve the signal-to-noise ratio (SN ratio) of the WDM signal, a wavelength variable filter is configured downstream of the optical amplifier.

[0016] As a variable wavelength filter, a filter having a Gaussian-shaped amplitude frequency characteristic, in other words, an insertion loss characteristic, is widely used. However, in such a filter, the gain curve, in other words, the slope of the insertion loss curve is relatively gentle. Therefore, it is difficult to pass a WDM signal occupying a 64 GHz bandwidth while significantly attenuating the ASE noise of an adjacent channel located 100 GHz apart on the frequency axis by using a filter.

[0017] On the other hand, if a wavelength variable filter is given a rectangular amplitude frequency characteristic, it is possible to significantly attenuate noise outside the passband. However, in this case, the passband is relatively flat. Therefore, even if the passband is moved near the center wavelength of the optical signal, the power measured by the power monitor will not change significantly. As a result, it is difficult to adjust the center of the passband to be consistent with the center wavelength of the optical signal with good accuracy based on the power.

[0018] Here, according to one aspect of the present disclosure, it is desirable to provide a technology that can significantly attenuate noise outside a passband and appropriately control the passband in an optical transmission system.

[0019] Technical solutions to the problem

[0020] An optical transmission system according to one aspect of the present disclosure includes an optical filter and a controller. The optical filter is configured to selectively allow at least one wavelength component among a plurality of wavelength components included in input light to pass through the optical filter as output light.

[0021] The optical filter includes a movable element for changing a passband. The controller is configured to control the movable element based on the power of light corresponding to the output light measured by the power monitor, thereby adjusting the passband.

[0022] According to one embodiment of the present disclosure, the optical filter is configured as a bandpass filter having a first amplitude-frequency characteristic in a first wavelength band. The first wavelength band is a wavelength band including a center wavelength of the passband. The first amplitude-frequency characteristic is an amplitude-frequency characteristic corresponding to a convex function having a peak value at the center wavelength in a gain curve in the first wavelength band. The convex function can be defined by a Gaussian function or a super-Gaussian function.

[0023] According to one embodiment of the present disclosure, the optical filter is configured as a bandpass filter having a second amplitude-frequency characteristic in a second wavelength band. The second wavelength band is a wavelength band adjacent to the first wavelength band and further from the center wavelength than the first wavelength band. The second amplitude-frequency characteristic is an amplitude-frequency characteristic in which a gain curve exhibits a steeper inclination than the above-mentioned convex function in the second wavelength band.

[0024] By configuring an optical filter having the above frequency characteristics, it is possible to reduce signal attenuation within the passband and increase noise attenuation outside the passband. In other words, it is possible to effectively reduce noise outside the passband from a wavelength band near the passband.

[0025] Furthermore, according to the optical transmission system, since the frequency characteristic of the passband is convex, the deviation between the center of the passband and the center of the signal caused by the change of the passband is easily reflected in the change of the power measured by the power monitor. Therefore, it is possible to perform control with better accuracy to adjust the center wavelength of the passband to coincide with the center wavelength of the optical signal.

[0026] According to one embodiment of the present disclosure, the controller can control the movable element in the following manner, that is, the movable element is controlled so that the power measured by the power monitor presents a peak value, thereby adjusting the central wavelength of the passband to be consistent with the central wavelength of the signal component included in the input light. According to the optical transmission system including the above-mentioned control of the movable element, the central wavelength of the passband can be adjusted to be consistent with the central wavelength of the optical signal with better accuracy.

[0027] According to one aspect of the present disclosure, the optical filter may have an amplitude frequency characteristic in which a gain curve corresponds to a Gaussian function or a super Gaussian function having an order of 1 or more and less than 3 in the first wavelength band.

[0028] According to one aspect of the present disclosure, the optical filter may have an amplitude frequency characteristic in which a gain curve corresponds to a super-Gaussian function of order 3 or greater in the second wavelength band.

[0029] As described above, by providing a difference in the inclination of the gain curve between the first wavelength band and the second wavelength band, it is possible to effectively suppress noise outside the passband from a wavelength band near the passband.

[0030] According to one embodiment of the present disclosure, the optical filter may include a transmission-type diffraction grating and a reflective mirror in addition to the movable element. The transmission-type diffraction grating may be arranged on the propagation path of the input light. The reflective mirror may have a reflective surface.

[0031] The reflector may be configured to reflect, as output light, a wavelength component incident on the reflective surface under a predetermined angle condition among a plurality of dispersed wavelength components of input light included in the transmitted diffraction light from the transmission type diffraction grating. The movable element may be configured to change the passband by changing the relative propagation path of the plurality of dispersed wavelength components with respect to the reflective surface under the control of the controller.

[0032] According to one embodiment of the present disclosure, the movable element may be a variable angle mirror. The variable angle mirror may be configured to change the wavelength component incident on the reflection surface of the reflection mirror among the dispersed multiple wavelength components by changing the angle of the mirror surface that reflects the transmitted diffracted light from the transmission type diffraction grating toward the reflection mirror.

[0033] According to one embodiment of the present disclosure, a telecentric optical system can be set between the reflector and the transmissive diffraction grating, and the telecentric optical system includes at least a lens, which is used to adjust the propagation directions of each of the dispersed multiple wavelength components contained in the transmissive diffraction light propagating from the transmissive diffraction grating toward the reflector to be parallel to the optical axis.

[0034] According to one embodiment of the present disclosure, in an optical transmission system provided with a telecentric optical system, a reflector may have a reflective surface having a convex shape with a location where a wavelength component corresponding to the center wavelength of a passband is incident as a vertex, and formed so that an optical filter has a first amplitude-frequency characteristic and a second amplitude-frequency characteristic. By forming the reflective surface into the above-mentioned shape, the above-mentioned frequency characteristic can be imparted to the optical filter.

[0035] According to one embodiment of the present disclosure, in an optical transmission system provided with a telecentric optical system, a reflector may have a reflective surface that is given a reflectivity distribution characteristic so that an optical filter has a first amplitude-frequency characteristic and a second amplitude-frequency characteristic. By giving the above-mentioned reflectivity distribution characteristic, the above-mentioned frequency characteristic may be given to the optical filter.

[0036] According to one aspect of the present disclosure, a lens may be provided between the reflective mirror and the transmissive diffraction grating, the lens being used to adjust the propagation directions of each of a plurality of wavelength components included in the transmissive diffraction light propagating from the transmissive diffraction grating toward the reflective mirror.

[0037] In this case, the lens can be configured to generate the following aberrations, that is, the propagation direction of the wavelength component of the wavelength band that is farther away from the central wavelength of the passband is more inclined relative to the optical axis, thereby making the optical filter have the first amplitude-frequency characteristic and the second amplitude-frequency characteristic. The above-mentioned aberration can be used to give the optical filter the above-mentioned frequency characteristics.

[0038] According to one aspect of the present disclosure, a lens may be provided between the reflective mirror and the transmissive diffraction grating, the lens being used to adjust the focal lengths of each of a plurality of wavelength components included in the transmissive diffraction light propagating from the transmissive diffraction grating toward the reflective mirror.

[0039] In this case, the lens can be configured to generate the following aberration, that is, the wavelength component of the wavelength band that is farther away from the central wavelength of the passband has a focus farther away from the reflection surface, thereby making the optical filter have the first amplitude frequency characteristic and the second amplitude frequency characteristic. The above-mentioned aberration can be used to give the optical filter the above-mentioned frequency characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a block diagram showing the structure of an optical transmission system.

[0041] Figure 2 It is a diagram showing the structure of a variable wavelength filter.

[0042] Figure 3A This is a diagram for explaining a telecentric optical system between a transmission-type diffraction grating and a reflecting mirror; Figure 3B This is a diagram for explaining the focal position of light incident on a reflecting mirror.

[0043] Figure 4 This is a diagram for explaining the reflectivity distribution in the reflector.

[0044] Figure 5 It is a graph showing the insertion loss characteristics of the wavelength variable filter.

[0045] Figure 6 is a diagram showing an insertion loss curve defined by a super Gaussian function.

[0046] Figure 7 It is a diagram showing the insertion loss characteristics of the wavelength variable filter and the spectrum of the input light.

[0047] Figure 8 This is a diagram for explaining the change in power measured by the power monitor due to a change in the passband.

[0048] Fig. 9 This is a diagram for explaining the structure of a reflecting mirror in the first modification.

[0049] Fig.10It is an explanatory diagram regarding the focal position of the lens in the second modification example.

[0050] Fig.11 It is an explanatory diagram regarding the propagation direction of light having passed through the lens in the third modification example.

[0051] Fig.12 It is a diagram showing the insertion loss characteristics of a conventional variable wavelength filter and the spectrum of input light.

[0052] Description of Reference Numerals

[0053] 1…optical transmission system; 10…optical amplifier; 30…wavelength variable filter;

[0054] 31 ... optical input and output unit; 33 ... collimator; 35 ... transmission type diffraction grating;

[0055] 37, 371, 373…lenses; 39, 391, 393…reflectors;

[0056] 39A, 391A, 393A…reflecting surface; 40…variable angle mirror; 40A…mirror surface;

[0057] 60…power monitor; 70…controller. DETAILED DESCRIPTION

[0058] Exemplary embodiments of the present disclosure are described below with reference to the accompanying drawings.

[0059] (1) Main Implementation Method

[0060] Figure 1 The optical transmission system 1 of the present embodiment shown is configured to amplify an optical signal in a WDM network by an optical amplifier 10 and transmit the amplified optical signal to a downstream.

[0061] In the optical transmission system 1 , in order to eliminate ASE noise that is superimposed on the optical signal when the optical signal is amplified by the optical amplifier 10 , a wavelength variable filter 30 as an optical filter is provided downstream of the optical amplifier 10 .

[0062] The tunable filter 30 includes a variable angle mirror 40 as a movable element. The wavelength variable filter 30 can change the passband by changing the angle of a mirror surface 40A of the variable angle mirror 40 .

[0063] The optical transmission system 1 further includes a power monitor 60 and a controller 70 as components for controlling the passband of the variable wavelength filter 30. The power monitor 60 is provided downstream of the variable wavelength filter 30. Light having passed through the variable wavelength filter 30 is split and input to the power monitor 60.

[0064] The power monitor 60 measures the power of the light having passed through the wavelength variable filter 30 based on the input branched light. A measurement signal of the power measured by the power monitor 60 is input to the controller 70.

[0065] The controller 70 is configured to control the passband of the variable wavelength filter 30 by controlling the variable angle mirror 40. Specifically, the controller 70 controls the angle of the mirror surface 40A of the variable angle mirror 40 based on the measurement signal from the power monitor 60 so that the center of the passband of the variable wavelength filter 30 coincides with the center wavelength of the optical signal of the transmission object.

[0066] Even when the mirror surface 40A of the variable angle mirror 40 is fixed, the passband of the variable wavelength filter 30 may slightly change due to environmental factors such as temperature. That is, after the center of the passband is made consistent with the central wavelength of the optical signal, if the angle of the mirror surface 40A is not adjusted, the center of the passband will deviate from the central wavelength of the optical signal.

[0067] The controller 70 controls the variable angle mirror 40 to correct the above-mentioned deviation. Specifically, the controller 70 controls the angle of the mirror surface 40A so that the power measured by the power monitor 60 presents a peak value. Thus, the passband of the variable wavelength filter 30 is adjusted so that the center of the passband coincides with the center wavelength of the optical signal, which is the main component of the light input to the variable wavelength filter 30.

[0068] Figure 2 The following is a diagram showing an exemplary configuration of a variable wavelength filter 30 . The variable wavelength filter 30 further includes a light input / output unit 31 , a collimator 33 , a transmission type diffraction grating 35 , a lens 37 , and a reflection mirror 39 .

[0069] The optical input / output unit 31 includes an optical circulator 311, and an input optical fiber 313, an output optical fiber 315, and an internal optical fiber 317 connected to the optical circulator 311. Light from the optical amplifier 10 is input to the variable wavelength filter 30 via the input optical fiber 313.

[0070] The input light input to the variable wavelength filter 30 via the input optical fiber 313 is transmitted to the internal optical fiber 317 via the optical circulator 311. The collimator 33 is disposed at the end of the internal optical fiber 317, and the input light is collimated by the collimator 33 and then propagates to the transmission type diffraction grating 35.

[0071] The transmission type diffraction grating 35 is provided on the propagation path of the collimated input light. The diffraction angle of the transmission type diffraction grating 35 depends on the wavelength of the incident light. The transmission type diffraction grating 35 spatially separates the collimated input light into a plurality of wavelength components using this wavelength dependency. The input light from the collimator 33 is transmitted through the transmission type diffraction grating 35, and is propagated to the variable angle mirror 40 as a transmission diffraction light spatially dispersed in a plurality of wavelength components.

[0072] The transmitted diffracted light having propagated to the variable angle mirror 40 is reflected by the mirror surface 40A of the variable angle mirror 40 and again transmits through the transmission type diffraction grating 35. The transmitted diffracted light having again transmitted through the transmission type diffraction grating 35 passes through the lens 37 in a state where a plurality of wavelength components are spatially dispersed and propagates to the reflecting mirror 39 side.

[0073] Figure 2 The solid arrows shown schematically represent the propagation of the optical signal in a state where the center of the passband is adjusted to the center wavelength of the optical signal. Figure 2 The dashed arrows shown schematically represent the propagation of noise components outside the passband.

[0074] The variable angle mirror 40 is configured to change the angle of the mirror surface 40A under the control of the controller 70. The variable angle mirror 40 is, for example, a micro-electromechanical system (MEMS) mirror. Figure 2 The one-dot chain arrow in denoted by FIG. 4 shows that the angle of the mirror surface 40A of the variable angle mirror 40 changes by rotation. Figure 2 The thick arrow in denoted by FIG. 1 shows that the relative position of the transmitted diffracted light propagating to the reflector 39 with respect to the reflector 39 changes along a direction parallel to the reflective surface 39A of the reflector 39 due to the change in the angle of the mirror surface 40A.

[0075] Lens 37 is a telecentric lens, such as Figure 3A As shown, the lens 37 converts the transmitted diffracted light from the transmission type diffraction grating 35 , specifically, the lens 37 converts the plurality of spatially dispersed wavelength components included in the transmitted diffracted light into light rays whose propagation directions are parallel to the optical axis passing through the center of the lens 37 .

[0076] The reflector 39 is arranged in such a manner that the center of the flat reflective surface 39A is arranged on the optical axis passing through the center of the lens 37, and the normal line from the center of the reflective surface 39A coincides with the optical axis, and Figure 3B As shown, the reflection surface 39A is arranged at the focal position of the lens 37 .

[0077] Figure 2 , Figure 3A as well as Figure 3B The same structure of the variable wavelength filter 30 is shown. Figure 3A and Figure 3BIn FIG. 1 , in order to explain the propagation direction and focal position of light, the arrangement of the transmission type diffraction grating 35 , the lens 37 , and the reflection mirror 39 is schematically and simply shown. Figure 3B The dashed lines in the figure correspond to the light of each wavelength. Figure 3B The case where the focal position of the light incident on the reflector 39 is located on the reflective surface 39A has been described.

[0078] That is, the lens 37 forms a telecentric optical system between the transmission type diffraction grating 35 and the reflector 39 so that the transmitted diffraction light from the transmission type diffraction grating 35 is imaged on the reflector 39. The lens 37 adjusts the propagation directions of the multiple wavelength components included in the transmitted diffraction light so that the propagation directions of the multiple wavelength components become perpendicular to the reflection surface 39A of the reflector 39.

[0079] As described above, the transmitted diffracted light from the transmission type diffraction grating 35 is in a state where the multiple wavelength components included in the input light are spatially dispersed, and the reflecting surface 39A of the reflecting mirror 39 is smaller than the spatial spread of the multiple wavelength components included in the input light.

[0080] Therefore, a part of the wavelength components included in the input light corresponding to the angle of the mirror surface 40A of the variable angle mirror 40 is selectively incident on the reflection surface 39A. Specifically, a part of the wavelength components is selectively incident on the reflection surface 39A in such a manner that the wavelength component corresponding to the center of the passband is incident from a direction perpendicular to the reflection surface 39A to the center of the reflection surface 39A corresponding to the optical axis passing through the center of the lens 37 and is focused on the reflection surface 39A.

[0081] The reflector 39 selectively reflects the above-mentioned part of the wavelength components of the multiple spatially separated wavelength components that are vertically incident on the reflecting surface 39A as output light. In addition, the wavelength component vertically incident on the reflecting surface 39A corresponds to the wavelength component incident on the reflecting surface 39A under specified angle conditions.

[0082] Figure 3A and Figure 3B This indicates a situation in which, among wavelength components of wavelengths λl to λz included in input light, wavelength components from wavelength λm to wavelength λn are incident on reflection surface 39A of reflection mirror 39 , while the remaining wavelength components propagate at positions deviating from reflection surface 39A.

[0083] The output light reflected by the reflector 39 propagates in the reverse direction along the path to the reflector 39 and propagates to the optical circulator 311 via the collimator 33. The output light propagated to the optical circulator 311 propagates to the downstream of the optical transmission system 1 through the output optical fiber 315.

[0084] According to the configuration of the variable wavelength filter 30 described above, in the variable wavelength filter 30 , some of the multiple wavelength components included in the input light are selectively coupled to the output optical fiber 315 , and are transmitted downstream from the output optical fiber 315 as output light.

[0085] By controlling the angle of the mirror surface 40A of the variable angle mirror 40 by the controller 70, the relative propagation paths of the plurality of wavelength components with respect to the reflection surface 39A of the reflection mirror 39, in other words, the relative positions of the plurality of wavelength components with respect to the reflection surface 39A of the reflection mirror 39 are controlled along Figure 2 and Figure 3A The wavelength variable filter 30 changes in the direction of the bold arrow shown, thereby adjusting the passband of the wavelength variable filter 30 to slide on the frequency axis.

[0086] The feature of this embodiment is that the reflecting surface 39A of the reflecting mirror 39 does not have a uniform reflectivity as a whole. Figure 4 As shown, it has a non-uniform reflectivity distribution.

[0087] Specifically, the reflective surface 39A has a reflectivity distribution in which the reflectivity decreases stepwise from the center of the reflective surface 39A where light of a wavelength corresponding to the center of the passband is incident toward the direction in which the wavelength is dispersed by the transmission type diffraction grating 35. Figure 5 The insertion loss characteristic of the input light shown, in other words, the reflectivity distribution is determined and given to the reflecting mirror 39 so that the wavelength variable filter 30 exhibits a gain characteristic.

[0088] Figure 5 The insertion loss characteristic diagram shown is a diagram in which the horizontal axis is frequency and the vertical axis is insertion loss (IL), in other words, the vertical axis is gain characteristic. As shown in the figure, the wavelength variable filter 30 is configured as a bandpass filter having a convex insertion loss characteristic corresponding to a combination of a plurality of super-Gaussian functions of different orders.

[0089] The wavelength variable filter 30 exhibits an insertion loss of 0 dB (decibel) at the center frequency fc of the passband, and has a convex insertion loss characteristic in the first wavelength band centered on the center frequency fc, that is, the insertion loss characteristic follows an insertion loss curve defined by a super Gaussian function with a peak at the center frequency fc and an order n=1.

[0090] The variable wavelength filter 30 has an insertion loss characteristic in a second wavelength band adjacent to the first wavelength band across the first wavelength band, which follows an insertion loss curve defined by a super Gaussian function having a peak at a center frequency fc and an order of n = 5. The insertion loss curve corresponds to the gain curve, and the insertion loss characteristic corresponds to the amplitude frequency characteristic of the variable wavelength filter 30.

[0091] The insertion loss curve defined by the super Gaussian function of order n is represented by the following mathematical formula. In the following mathematical formula, fc represents the center frequency, Δν represents the frequency difference from the center frequency fc, and BW3 represents the -3 dB bandwidth.

[0092]

[0093] [Mathematical formula 1]

[0094] Figure 6 Insertion loss curves defined by a super-Gaussian function of order n are shown when the order n=1, 2, 4, and 6, respectively. Figure 6 The insertion loss curve of the order n=1 is represented by a solid line; the insertion loss curve of the order n=2 is represented by a dotted line; the insertion loss curve of the order n=4 is represented by a single-dash line; and the insertion loss curve of the order n=6 is represented by a double-dash line.

[0095] Conventional variable wavelength filters have a Gaussian insertion loss characteristic, and the insertion loss curve follows a Gaussian function or a super-Gaussian function of order n=1. Figure 6 It can be understood that the insertion loss curve following the super Gaussian function shows the following tendency, that is, the larger the order n is, the flatter the curve is around the center frequency fc, and the farther away from the center frequency fc, the greater the insertion loss is.

[0096] exist Fig.12 In the figure, the spectrum of light in a WDM network with a wavelength of 400 Gbps is represented by a solid line on the graph with frequency as the horizontal axis. The 400 Gbps signal that is multi-level modulated with 64Gbound as an optical signal occupies a bandwidth of 64 GHz with a substantially uniform width on the frequency axis. When the signal is amplified and transmitted by the optical amplifier 10, the signal-to-noise ratio of the optical signal (WDM signal) of the adjacent channel is reduced due to the superposition of ASE noise.

[0097] Assuming that the input light corresponding to the optical signal superimposed with ASE noise is filtered using a wavelength-variable filter whose insertion loss curve follows a Gaussian function, the noise component near the adjacent channel separated by 100 GHz can be reduced by more than 40 dB relative to the signal component with a passband width BW3 less than 100 GHz. Fig.12 The dashed line shown in corresponds to the insertion loss curve defined by a Gaussian function.

[0098] However, even if the ASE noise is attenuated by the above-mentioned variable wavelength filter, the ASE noise still exists to a non-negligible degree. If the passband width BW3 is narrowed to reduce the ASE noise, the insertion loss of the signal component will increase.

[0099] Therefore, in this embodiment, the variable wavelength filter 30 is designed to present an insertion loss curve in the first wavelength band following a super-Gaussian function of order n = 1. The first wavelength band corresponds to a passband centered at a center frequency fc where the insertion loss is 0 dB and where the insertion loss reaches -3 dB.

[0100] Furthermore, the variable wavelength filter 30 is further designed to present an insertion loss curve that follows a super-Gaussian function of order n=5 in the second wavelength band. The second wavelength band corresponds to a wavelength band that sandwiches the first wavelength band from both sides and is adjacent to the first wavelength band at a position offset from the center frequency fc, and the insertion loss of the second wavelength band is greater than -3dB. That is, the reflectivity distribution of the reflection surface 39A of the reflector 39 is set so that the variable wavelength filter 30 has an insertion loss characteristic that follows the above insertion loss curve.

[0101] According to the insertion loss curve, the wavelength variable filter 30 as a bandpass filter type has the following insertion loss characteristics, in the second wavelength band, the inclination of the insertion loss is larger than the inclination of the insertion loss of the Gaussian function and / or the super Gaussian function with a smaller order n, wherein the insertion loss characteristics follow the super Gaussian function with different orders in the first wavelength band and the second wavelength band.

[0102] Therefore, according to the wavelength variable filter 30, the insertion loss of the passband can be suppressed to a small level, and the ASE noise affecting the adjacent channels can be effectively attenuated. Figure 7 In this way, the noise component in the shadow part can be attenuated more effectively than before. Figure 7 The dotted line shown in corresponds to the insertion loss curve of the wavelength variable filter 30.

[0103] As described in this embodiment, the first wavelength band follows a super Gaussian function of order n=1, and thus, compared to the case where the first wavelength band is defined by a super Gaussian function of the same high order n as the second wavelength band, control can be performed with better accuracy to make the center of the passband consistent with the center wavelength of the optical signal based on the power measured by the power monitor 60.

[0104] As described above, the insertion loss curve following the super Gaussian function with order n less than 3 is relatively convex near the center frequency fc of the passband, while the insertion loss curve following the super Gaussian function with order n greater than or equal to 3 is relatively flat near the center frequency fc.

[0105] In the case where the wavelength variable filter 30 presents a flat insertion loss curve near the center frequency fc, even if the controller 70 attempts to adjust the angle of the mirror 40A of the variable angle mirror 40 so that the power measured by the power monitor 60 presents a peak (i.e., a maximum value), the adjustment cannot be performed with better accuracy.

[0106] This is because, with a flat insertion loss characteristic near the center frequency fc, even if the center of the passband deviates from the center wavelength of the optical signal, the measured power will not change significantly. Even if you try to slide the passband to observe the change in the measured power, such as Figure 8 As shown, even when the insertion loss curve near the center frequency fc follows a super-Gaussian function with an order n of 4 or more, almost no change in power is observed.

[0107] Figure 8 The graph shown is a graph having a horizontal axis representing the deviation δ and a vertical axis representing the power measured by the power monitor 60. The graph shows the change in the measured power with respect to the deviation δ when the insertion loss curve near the center frequency fc follows a super-Gaussian function of order n=1, 2, 3, 4. The deviation δ is the difference of the center frequency of the optical signal on the frequency axis from the center of the passband.

[0108] According to the figure, when the order n is less than 3, the change in power is convex near the deviation δ = 0. However, in reality, even if the order n = 3, it is difficult to observe the peak value of the measured power with good accuracy, and it is difficult to perform control to make the center of the passband consistent with the central wavelength of the optical signal.

[0109] Therefore, in this embodiment, the order n of the first wavelength band is set to 1. As another example, the wavelength variable filter 30 can be designed to present an insertion loss curve that follows a Gaussian function or a super Gaussian function with an order greater than 1 and less than 3 in the first wavelength band, and to present an insertion loss curve that follows a super Gaussian function with an order greater than 3 in the second wavelength band.

[0110] The variable wavelength filter 30 may also be designed to present an insertion loss curve in the first wavelength band that follows a convex function defined by a Gaussian function having a peak at the center frequency fc or a super Gaussian function having a peak at the center frequency fc and an order greater than or equal to 1 and less than 3. The variable wavelength filter 30 may also be designed to present an insertion loss curve in the second wavelength band that is steeper than the slope of the convex function corresponding to the insertion loss curve in the first wavelength band in the second wavelength band. According to the above design, the variable wavelength filter 30 may also be given an insertion loss characteristic suitable for reducing ASE noise and controlling the passband.

[0111] (2) First Modification

[0112] The optical transmission system 1 of the first modified example has the same configuration as that of the main embodiment, except that it includes a reflecting mirror 391 which is different from that of the main embodiment.

[0113] That is, the wavelength variable filter 30 of the first modified example is configured by replacing the reflector 39 in the wavelength variable filter 30 of the main embodiment with a reflector 391 having different characteristics. Fig. 9 The structure of the reflecting mirror 391 will be selectively described, and description of other parts having the same structure as that of the main embodiment will be omitted.

[0114] Unlike the reflector 39 of the main embodiment, the reflector 391 of this modification has a curved reflective surface 391A, and the reflective surface 391A has a uniform reflectivity. As in the main embodiment, the reflector 391 is configured so that the center of the reflective surface 391A is located on the optical axis passing through the center of the lens 37, and the normal line from the center of the reflective surface 391A coincides with the optical axis.

[0115] The telecentric optical system formed upstream of the reflecting surface 391A allows a plurality of wavelength components included in the input light dispersed by the transmission-type diffraction grating 35 to propagate to the reflecting surface 391A side as light rays parallel to the optical axis.

[0116] The reflecting surface 391A is a convex reflecting surface having a vertex at its center. Therefore, among the wavelength components corresponding to the passband, the wavelength components that are spatially farther away from the center of the passband are more likely to be incident on the reflecting surface 391A where the normal line forms an angle with the optical axis. The wavelength component corresponding to the center wavelength of the passband is incident on the vertex of the reflecting surface 391A.

[0117] As in the main embodiment, according to the wavelength variable filter 30 of the first modified example, among the light reflected by the reflector 391, the light propagating parallel to the optical axis is coupled to the output optical fiber 315. The optical coupling rate of the wavelength component incident on the reflection surface 391A and reflected as the output light to the output optical fiber 315 decreases as the wavelength component deviates from the center of the passband. This is because the wavelength component that deviates further from the center of the passband is incident on the portion of the reflection surface 391A where the angle formed by the normal line and the optical axis is larger. The decrease in the optical coupling rate corresponds to the increase in the insertion loss.

[0118] That is, in this modification, the wavelength variable filter 30 is given the same characteristics as those in the main embodiment by intentionally making the reflection surface 391A curved. Figure 5 In other words, the reflection surface 391A is formed so that the wavelength variable filter 30 has Figure 5 Insertion loss characteristics are shown.

[0119] According to the first modification, the wavelength variable filter 30 can also have the same insertion loss characteristics as the main embodiment. Therefore, the optical transmission system 1 can effectively reduce ASE noise and control the center of the passband to match the center wavelength of the optical signal with good accuracy.

[0120] (3) Second Modification

[0121] The optical transmission system 1 of the second modified example has the same configuration as that of the main embodiment, except that it includes a lens 371 and a reflecting mirror 393 that are different from those of the main embodiment.

[0122] That is, the wavelength variable filter 30 of the second modified example is configured by replacing the lens 37 and the reflector 39 in the wavelength variable filter 30 of the main embodiment with a lens 371 and a reflector 393 having different characteristics. Fig.10 The features of the lens 371 and the reflecting mirror 393 of the second modified example will be selectively described, and description of other portions having the same configuration as that of the main embodiment will be omitted.

[0123] The reflector 393 of this modification has a flat reflective surface 393A. The reflective surface 393A has a uniform reflectivity over its entirety. The reflector 393 is configured so that the center of the reflective surface 393A is located on the optical axis passing through the center of the lens 371, and the normal line from the center of the reflective surface 393A coincides with the optical axis.

[0124] The lens 371 is designed to convert the dispersed multiple wavelength components included in the transmission diffraction light from the transmission type diffraction grating 35 into light parallel to the optical axis passing through the center of the lens 371. And, the lens 371 is further designed so that the wavelength component corresponding to the center wavelength of the passband passing through the center of the lens 371 is focused at the center of the reflection surface 393A. However, the lens 371 is designed so that each position deviating from the center in the wavelength dispersion direction has a different focal length.

[0125] That is, lens 371 is designed to produce the following spherical aberration, namely, the more a wavelength component passes through a position that deviates from the center of lens 371 in the wavelength dispersion direction and is a wavelength component of a wavelength band that is farther away from the center wavelength of the passband, the farther the focal position of the wavelength component is from the reflecting surface 393A in the normal direction.

[0126] According to the above design, the wavelength variable filter 30 of this modified example is designed to utilize the spherical aberration of the lens 371 so that the insertion loss of the wavelength component incident on the reflection surface 393A at a position that deviates from the center of the reflection surface 393A in the wavelength dispersion direction is greater. Specifically, the lens 371 is designed to make the insertion loss curve similar to the main embodiment. Figure 5 The insertion loss curve shown is consistent.

[0127] According to the second modification, the optical transmission system 1 can also effectively reduce ASE noise and can control the center of the passband to coincide with the center wavelength of the optical signal with good accuracy.

[0128] (4) Third Modification

[0129] The optical transmission system 1 of the third modified example has the same configuration as that of the second modified example, except that it includes a lens 373 which is different from that of the second modified example.

[0130] That is, the wavelength variable filter 30 of the third modified example is configured by replacing the lens 371 in the wavelength variable filter 30 of the second modified example with a lens 373 having different characteristics. Fig.11 The features of the lens 373 of the third modification example will be selectively described, and description of other parts having the same configuration as that of the second modification will be omitted.

[0131] Similar to the second modification, the variable wavelength filter 30 of this modification includes a reflector 393 having a flat reflective surface 393A with uniform reflectivity. The reflective surface 393A is disposed so that its center is located on the optical axis passing through the center of the lens 373 .

[0132] Lens 373 is designed to produce the following spherical aberration, namely, the propagation direction of the wavelength component becomes more inclined relative to the optical axis passing through the center of lens 373, as the wavelength component passes through a position that deviates from the center of lens 373 in the wavelength dispersion direction and is in a wavelength band that is farther away from the center wavelength of the passband.

[0133] According to the above design, the wavelength variable filter 30 of this modified example is designed to utilize the spherical aberration of the lens 373 so that the wavelength component incident on the reflection surface 393A at a position deviating from the center of the reflection surface 393A in the wavelength dispersion direction has a lower optical coupling rate to the output optical fiber 315, thereby increasing the insertion loss. Specifically, the lens 373 is designed to make the insertion loss curve similar to that of the main embodiment. Figure 5 The insertion loss curves shown are consistent.

[0134] According to the third modification, the optical transmission system 1 can also effectively reduce ASE noise and can control the center of the passband to coincide with the center wavelength of the optical signal with good accuracy.

[0135] [other]

[0136] The above describes the exemplary embodiments of the present disclosure including the modified examples, however, the present disclosure is not limited to the exemplary embodiments described above, and various embodiments can be adopted. For example, the technology of the present disclosure can be applied to various types of variable wavelength filters, and the configuration of the variable wavelength filter 30 is not limited to the above-mentioned embodiments.

[0137] The technology disclosed in the present invention controls the passband of the wavelength variable filter 30 based on the power of light corresponding to the output light. According to the above-mentioned embodiment, the power of light corresponding to the output light is the power of the output light itself.

[0138] However, the power of the light corresponding to the output light only needs to be related to the power of the output light, for example, it can be the power of the light obtained by subtracting the output light from the input light. In this case, in order to adjust the center of the passband to coincide with the central wavelength of the optical signal, the angle of the mirror surface 40A of the variable angle mirror 40 can be controlled so that the power of the light other than the output light is minimized.

[0139] In addition, the functions of one constituent element in each of the above-mentioned embodiments may be dispersed and arranged in a plurality of constituent elements. The functions of a plurality of constituent elements may also be integrated into one constituent element. A portion of the configuration of the above-mentioned embodiment may be omitted. At least a portion of the configuration of the above-mentioned embodiment may also be added to the configuration of the other embodiments described above, or replaced with the configuration of the other embodiments described above. All modes included in the technical idea determined by the phrases recorded in the claims are embodiments of the present disclosure.

Claims

1. An optical transmission system, characterized in that: have: An optical filter configured to selectively allow at least one wavelength component among a plurality of wavelength components included in input light to pass through the optical filter as output light, and comprising a movable element for changing a passband; as well as a controller configured to control the movable element based on a power of light corresponding to the output light measured by a power monitor, thereby adjusting the passband; and The optical filter is configured as a bandpass filter having a first amplitude-frequency characteristic in a first wavelength band and a second amplitude-frequency characteristic in a second wavelength band, The first wavelength band is a wavelength band including the center wavelength of the passband, The second wavelength band is a wavelength band adjacent to the first wavelength band and farther from the central wavelength than the first wavelength band. The first amplitude-frequency characteristic is an amplitude-frequency characteristic of a gain curve in the first wavelength band corresponding to the following convex function, wherein the convex function has a peak at the center wavelength and is defined by a Gaussian function or a super Gaussian function. The second amplitude-frequency characteristic is an amplitude-frequency characteristic in which the gain curve has a steeper slope than the convex function in the second wavelength band.

2. The optical transmission system according to claim 1, characterized in that: The controller controls the movable element in such a manner that the power exhibits a peak value, thereby adjusting the central wavelength of the passband to coincide with the central wavelength of a signal component included in the input light.

3. The optical transmission system according to claim 1 or 2, characterized in that: The first amplitude-frequency characteristic is an amplitude-frequency characteristic of the gain curve in the first wavelength band corresponding to the Gaussian function or a super-Gaussian function having an order of 1 or more and less than 3 as the super-Gaussian function, The second amplitude-frequency characteristic is an amplitude-frequency characteristic in which the gain curve corresponds to a super-Gaussian function having an order of 3 or more in the second wavelength band.

4. The optical transmission system according to any one of claims 1 to 3, characterized in that: The optical filter includes a transmission-type diffraction grating and a reflecting mirror in addition to the movable element. The transmission type diffraction grating is arranged on the propagation path of the input light. The reflector has a reflective surface and is configured to reflect the following wavelength component as the output light, that is, the wavelength component incident on the reflective surface under a predetermined angle condition among the dispersed plurality of wavelength components of the input light included in the transmission diffraction light from the transmission type diffraction grating, The movable element is configured to change the passband by changing relative propagation paths of the dispersed plurality of wavelength components with respect to the reflection surface under the control of the controller.

5. The optical transmission system according to claim 4, characterized in that: The movable element is a variable angle mirror, The variable angle mirror is configured to change a wavelength component incident on the reflection surface of the reflection mirror among the plurality of dispersed wavelength components by changing an angle of a mirror surface that reflects the transmission diffraction light from the transmission type diffraction grating toward the reflection mirror.

6. The optical transmission system according to claim 4 or 5, characterized in that: A telecentric optical system is provided between the reflector and the transmission type diffraction grating, the telecentric optical system comprising at least a lens for adjusting the propagation direction of each of the plurality of dispersed wavelength components included in the transmission diffraction light propagating from the transmission type diffraction grating toward the reflector to be parallel to the optical axis. The reflecting surface includes a reflecting surface having a convex shape with a location where a wavelength component corresponding to the center wavelength of the passband is incident as a vertex, and is formed so that the optical filter has the first amplitude-frequency characteristic and the second amplitude-frequency characteristic.

7. The optical transmission system according to claim 4 or 5, characterized in that: A telecentric optical system is provided between the reflector and the transmission type diffraction grating, the telecentric optical system comprising at least a lens for adjusting the propagation direction of each of the plurality of dispersed wavelength components included in the transmission diffraction light propagating from the transmission type diffraction grating toward the reflector to be parallel to the optical axis. The reflection surface includes a reflection surface to which a reflectance distribution characteristic is imparted so that the optical filter has the first amplitude-frequency characteristic and the second amplitude-frequency characteristic.

8. The optical transmission system according to claim 4 or 5, characterized in that: The optical transmission system includes a lens between the reflector and the transmission type diffraction grating, the lens being used to adjust the propagation direction of each of the plurality of wavelength components included in the transmission diffraction light propagating from the transmission type diffraction grating toward the reflector. The lens is configured to generate the following aberration, that is, the propagation direction of the wavelength component of the wavelength band farther away from the center wavelength of the passband is more inclined relative to the optical axis, thereby causing the optical filter to have the first amplitude-frequency characteristic and the second amplitude-frequency characteristic.

9. The optical transmission system according to claim 4 or 5, characterized in that: The optical transmission system includes a lens between the reflector and the transmission type diffraction grating, the lens being used to adjust the focal length of each of the plurality of wavelength components included in the transmission diffraction light propagating from the transmission type diffraction grating toward the reflector. The lens is configured to generate the following aberration, that is, the focus of the wavelength component of the wavelength band that deviates from the central wavelength of the passband is farther away from the reflection surface, thereby making the optical filter have the first amplitude-frequency characteristic and the second amplitude-frequency characteristic.

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