JP2021021747A - Optical filter - Google Patents

Abstract

To provide a narrow band tunable filter array that integrates multiple optical filter functions.SOLUTION: A narrow-band optical filter uses an integrated WSS array including a plurality of WSS in which reflected light from the spatial light modulation means is folded back into the spatial optical system and connected in multiple stages, each WSS including: an optical input/output unit; spectroscopic means for wavelength-dividing multiplexing light signal; condensing means that condenses the dispersed light signals into each one; and spatial light modulation means that photomodulates each of the condensed light signals.SELECTED DRAWING: Figure 10

Description

The present invention relates to an optical filter that filters the wavelength component of input light, and more particularly to an optical filter used for an optical communication node of an optical communication network, for example.

In the current optical communication network, a large capacity is realized by using a wavelength division multiplexing (WDM) technology in which optical signals of a plurality of different wavelengths are simultaneously mounted on a single optical fiber cable. In an optical communication network using WDM technology, optical communication nodes are connected by light of different wavelengths to virtually set an optical communication path (path) for any connection (hereinafter, the optical communication path is simply referred to as a path). Call). The configuration of the wavelength assignment of light used for these paths was operated almost fixedly.

On the other hand, with the development of video-on-demand and social media in recent years, the traffic demand on the Internet is fluctuating from moment to moment. Optical communication networks are also required to efficiently and flexibly handle such time-varying traffic. For example, it is preferable to be able to flexibly change the capacity of the path between the optical communication nodes and the wavelength configuration forming the path in response to a change in traffic demand. However, constructing a new path or changing the communication capacity of the existing path is affected by the configuration of the existing path. For example, it is affected by the start node and end node of the existing path, the wavelength used, the wavelength bandwidth, and the like. Therefore, the wavelength resources in the communication band of optical communication cannot always be used efficiently, and the traffic accommodation efficiency of the optical communication network cannot be maximized.

FIG. 1 is a diagram illustrating an example of a traffic time change in a conventional optical communication network. FIG. 1A shows an optical communication node configuration of an optical communication network, and FIG. 1B conceptually shows an optical signal on each optical communication node or path on the wavelength axis.

In FIG. 1A, in an optical communication network consisting of four nodes, node A, node B, node C, and node D, traffic from node A to node C via node B is considered. Now consider the situation where the path is set as follows.

With reference to (b) of FIG. 1, first, signals 1 to 6 are added from the add unit 1 at the node A of FIG. 1 (a) (fourth line of FIG. 1 (b), A: Add), and the node B It is transmitted in the direction. At node B, signal 2 and signal 4 of signals 1 to 6 are dropped from the drop portion 2 of node B (third line of FIG. 1B, B: Drop). Further, the remaining signals 1, 3, 5, and 6 reach the node C and are dropped from the Drop unit 3 (first line of FIG. 1 (b), C: Drop).

In such a situation, it is assumed that a new traffic demand is generated and it becomes necessary to set the path of the signal 7 from the node B to the node C (second line of FIG. 1 (b), B: New). The WDM wavelength band from node B to node C has a sufficient margin as a whole. However, since the wavelength band 7 of the signal 7 overlaps the entire band 5 of the signal 3 and a part of the band 6 of the signal 5, the node B cannot accommodate the signal 7. After all, even if the wavelength band as a whole has a vacancy and a margin, the traffic cannot be accepted depending on the mode of the path for new traffic. In order to solve such a problem, for example, Non-Patent Document 1 proposes wavelength defragmentation.

FIG. 2 is a diagram illustrating the concept of wavelength defragmentation in Non-Patent Document 1. Non-Patent Document 1 discloses an example of performing wavelength defragmentation by changing the state of three Steps 1 to 3 from left to right in a linear network consisting of nodes A to D as shown in the lower part of FIG. There is.

In the initial state of Step 1 at the left end of FIG. 2, Signal 1 to Signal 4 are transmitted between node A and node D. In each Step, the vertical direction represents the wavelength λ, and the horizontal axis direction represents the node position (transmission direction). Signal 1 (broadband signal) having a wide bandwidth is transmitted between nodes A and C, and Signal 4 (narrow band signal) having a narrow bandwidth is transmitted between nodes A to B.

The state of Step 2 in the center of FIG. 2 is a state in which Signal 3 in which a path is set from node C to node D in Step 1 in the initial state is abolished. Here, the wavelengths of Signal2 (10a) and Signal1 (9a) can be gradually changed downward (for example, on the short wavelength side).

As a result, in the state of Step 3 at the right end of FIG. 2, an empty wavelength band 8 can be provided in the wavelength band originally occupied by Signal 1, and the traffic accommodation efficiency in the wavelength resource can be maximized. Non-Patent Document 1 discloses an example of successful transmission of a 100 G double-polarized four-phase shift keying (DP-QPSK) signal.

Kyosuke Sone, Xi Wang, Shoichiro Oda, Goji Nakagawa, Yasuhiko Aoki, Inwoong Kim, Paparao Palacharla, Takeshi Hoshida, Motoyoshi Sekiya, and Jens C. Rasmussen, “First Demonstration of Hitless Spectrum Defragmentation using Real-time Coherent Receivers in Flexible Grid Optical Networks, ”ECOC2012, _Th.3.D.1 (PD)

However, the wavelength defragmentation disclosed in Non-Patent Document 1 has a problem that the control for carrying out the defragmentation is complicated and troublesome. According to the method of Non-Patent Document 1, it is necessary to change the wavelength of the signal light on the transmitting node side and the wavelength of the locally oscillating light for the receiver on the receiving node side in synchronization between these nodes at remote locations. There is. Further, it is necessary to synchronously change the wavelength of the transmission band used by the wavelength selection switch in the optical communication node through which the signal light passes.

FIG. 3 is a diagram illustrating an outline of a control operation required for each optical communication node in the wavelength defragmentation of the prior art. FIG. 3 shows control when the wavelength of the signal light is switched in synchronization between the node A11, the node B12, and the node C13. First, it is assumed that a signal is input from the ad circuit 14 of the node A11, the signal light of the wavelength λ1 is transmitted, and the signal is output from the drop circuit 15 of the node C13 via the node B12.

Here, consider a case where the wavelength of the light used is switched from λ1 to λ2 and the wavelength is rearranged. At this time, the Tx frequency (wavelength) of the transmitter 16 for the signal at the node A11, the transmission wavelength of the plurality of wavelength selection switches of the nodes A to C (only the wavelength selection switch 18 of the node B12 is displayed), and the transmission wavelength of the node C13. It is necessary to synchronously control the frequency (wavelength) of the local oscillator 17 for the receiver. Therefore, when rearranging the wavelength of the signal light from λ1 to λ2, complicated control of the optical transmission device is required over three nodes at remote locations.

In addition, due to the difference in wavelength bandwidth, wavelength rearrangement in which wavelengths intersect, for example, wavelength rearrangement in which the relationship between Signal 1 and Signal 4 in FIG. 2 is exchanged cannot be performed. Optical transmission equipment arranged in an optical communication node often uses a laser light source. When changing the wavelength of the laser light source, it is not possible to change the wavelength digitally instantly, and it is necessary to change the resonance length of the laser in an analog manner to stabilize it.

In many optical transmission devices, it takes a certain amount of time to change the wavelength for communication to an actually usable state. In reality, from the user's point of view, the signal light is completely stopped, and communication is interrupted. If the other signal contains the wavelength in use in the band between the two wavelengths to be switched, the signal in use may be disturbed or communication may be interrupted while the wavelength is switched. After all, changing the wavelength of light during the operation of an actual optical communication network requires troublesome and complicated control, and the range that can be changed is limited, so that wavelength switching can be performed without interruption of service or deterioration of communication quality. It was difficult to do.

For such a situation, a wavelength defragmentation technique using a wavelength conversion technique and a wavelength filtering technique is promising as described later, but it is necessary to use a plurality of wavelength variable filters in a narrow transmission band that transmit only the necessary optical signal. There is. However, narrowing the band of the optical filter cannot be easily realized. In general, a spatial optical system is often used as an optical filter that supports the recent wavelength multiplicity and has a variable band. The principle configuration of the operation of the optical filter using the spatial optical system is as represented by LCOS (Liquid crystal on silicon) by wavelength-dividing (spectroscopically) the light emitted from the input / output optical system into the free space by a diffraction grating. In general, a spatial light modulator capable of phase modulation with a spatially fine pattern controls so that only light of a desired wavelength is coupled to the output port.

It is well known that in order to realize further narrowing of the band with this configuration, it is necessary to increase the ratio of wavelength dispersion (coordinate movement amount on LCOS per unit wavelength) to the beam diameter on LCOS. Has been done. However, in order to reduce the beam diameter, it is necessary to apply a lens with high NA and large diameter, and there is a risk of cost increase and deterioration of beam quality due to aberration due to the increase in size of the optical element including the diffraction grating. is there. On the other hand, the approach of increasing the wavelength dispersion requires fundamental performance improvement of the wavelength dispersion element such as the number of grooves of the diffraction grating, which is also not easy. Therefore, it is difficult to realize narrow band characteristics only by changing the optical design.

Further, in an optical communication node to which this wavelength defragmentation technology is applied, a plurality of such tunable optical filters are required, so that introduction cost, deployment space, and power consumption become new issues.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a narrowband wavelength tunable filter array in which a plurality of optical filter functions are integrally integrated.

One aspect of the present invention is characterized by providing the following configurations in order to achieve such an object.

(Structure 1)
Group C (C is an integer of 2 or more) optical input / output unit, which includes A (A is an integer of 1 or more) input ports and B (B is an integer of 1 or more) as one group.
A spectroscopic means for wavelength division multiplexing light signals emitted from the output port, and
A condensing means for condensing optical signals dispersed for each wavelength by the spectroscopic means,
A spatial light modulation means that photomodulates and reflects each of the light signals collected by the light collection means is included.
The optical signal input from any of the input ports belonging to the M group (M is an integer from 1 to C) has only the first wavelength range (λ _M1 ≤ λ ≤ λ _M2 ) by the spatial optical modulation means. After being filtered
The reflected light from the spatial light modulation means is folded back and again filtered by the spatial light modulation means only in the second wavelength range (λ _N1 ≤ λ ≤ λ _N2 ).
An optical filter characterized in that the optical signal is output from any of the output ports belonging to the Nth group (N is an integer from 1 to C, where M ≠ N).
An optical filter having a WSS array configuration, characterized in that either λ _M1 ≠ λ _N1 or λ _M2 ≠ λ _N2 holds.

(Structure 2)
After the optical signal input from any of the input ports belonging to the M group is filtered by the spatial optical modulation means only in the first wavelength range (λ _M1 ≤ λ ≤ λ _M2 ),
The reflected light from the spatial light modulation means is reflected by the mirror and folded back, and only the second wavelength range (λ _N1 ≤ λ ≤ λ _N2 ) is filtered again by the spatial light modulation means.
The optical filter according to configuration 1, wherein the optical signal is output from any of the output ports belonging to the Nth group.

(Structure 3)
It is characterized in that there are wavelengths included in the two wavelength ranges in the first wavelength range (λ _M1 ≤ λ ≤ λ _M2 ) and the second wavelength range (λ _N1 ≤ λ ≤ λ _N2 ). The optical filter according to configuration 1 or 2.

(Structure 4)
A configuration 1 characterized in that a plurality of WSS connecting portions or mirrors for folding back the reflected light from the spatial light modulation means and connecting them to another WSS are provided to form a multi-stage optical filter using a multi-stage WSS array having three or more stages. The optical filter according to any one of 3 to 3.

As described above, according to the present invention, it is possible to provide a narrow band wavelength tunable filter array in which a plurality of optical filter functions are integrally integrated.

It is a figure explaining the traffic time change of an optical communication network. It is a figure explaining the wavelength defragmentation of the prior art. It is a figure explaining the wavelength defragmentation control of the prior art. It is a figure which shows the schematic structure of the optical communication node. It is a figure which shows the structure of the pump light generation means of an optical communication node. It is a figure which shows the spectrum of each part in a pump light generating means. It is a figure which shows the structure of the wavelength conversion means of an optical communication node. It is a figure explaining the spectrum of each part in a wavelength conversion means. It is a figure explaining the operation of the wavelength conversion which exchanges two optical signals. It is a figure explaining the basic principle of the narrow band optical filter of this invention. It is a figure which shows the structural example of Example 1 of the narrow band optical filter of this invention. It is a figure which shows the structural example of Example 2 of the narrow band optical filter of this invention.

First, the configuration of an optical communication node capable of assuming wavelength defragmentation will be described. At least one in the middle of the path between the input port (WSS (Wavelength Selective Switch) and Add port for demultiplexing for path switching) and the output port (WSS for combining) in the optical communication node. Provide a wavelength conversion means. Of the paths from one input port to the output ports directed to multiple directions, some paths are direct connections and wavelength conversion means are provided on some of the remaining paths. Wavelength conversion means may be provided in all paths. Pump light used for wavelength conversion is supplied to the wavelength conversion means from the pump light generating means. An efficient and integrated configuration for supplying pump light to the wavelength conversion means is also disclosed. Hereinafter, the overall configuration of the optical communication node to which the optical filter of the present invention is applied, its operation, and the specific configuration of each component will be described in detail with reference to the drawings.

[Configuration of optical communication node capable of wavelength defragmentation]
FIG. 4 is a diagram showing a schematic configuration of an optical communication subsystem. The optical communication subsystem 20 of FIG. 4 corresponds to the node B and the node D having the number of directions N = 3 in FIG. Typically, the optical communication subsystem 20 corresponds to an optical communication node in, for example, a ROADM (Reconfigurable Optical Add / Drop Multiplexer) system. Hereinafter, for the sake of simplicity, the optical communication subsystem 20 of FIG. 4 will be referred to as an optical communication node. The optical communication node of FIG. 4 has a configuration in which a functional block required for wavelength defragmentation is added in addition to the configuration of a general optical communication node. The optical communication node 20 of FIG. 4 shows an example of the configuration of an optical network node having the number of directions N = 3, but the number of directions N is not limited to 3, and can be configured in a scalable manner by adapting to different numbers of directions N. ..

In FIG. 4, wavelength division multiplexing signals (hereinafter referred to as WDM signals) from inputs (In1, In2, In3) of different directions are divided into wavelength division multiplexing switches 21-1 to 21-3 (WSS: Wavelength Selective Switch) (WSS: Wavelength Selective Switch) (WSS). It is input to Ingress1 to Ingress3). The input WDM signal is wavelength demultiplexed by each of the demultiplexing WSSs 21-1 to 21-3, and is routed to one of the desired directions (Out1 to Out3) for each wavelength. The routed optical signal is wavelength-combined by the corresponding combined wave WSS29-1 to 29-3 (Egress1 to Egress3) in each direction (Out1 to Out3). The optical signal wavelength-multiplexed by WSS29-1 to 29-3 for combine wave is output as a WDM signal from each output (Out1 to Out3) of WSS29-1 to 29-3.

In the routing in the optical communication node 20 described above, the input side demultiplexing WSS21-1 to 21-3 and the output side combine WSS29-1 to 29-3 are the paths shown by the dotted lines in FIG. 4, respectively. It is connected by the path shown by the solid line. Here, a path between WSSs on the same route, such as between In1 and Out1, is meaningless because the signal is routed to the same route and an optical signal is returned. Therefore, in the optical communication node 20, there is no connection between the demultiplexing WSS and the combined WSS in the same direction.

The paths 23-1 to 23-3 shown by the dotted lines in FIG. 4 are paths that directly connect the demultiplexing WSS on the input side and the combined WSS on the output side. On the other hand, the paths 22-1 to 22-3 shown by the solid lines are paths newly added in the optical communication node 20, and the wavelength conversion means is inserted on each path. In the conventional optical communication node, the WDM signal input from the route In1 is wavelength-divided via the demultiplexing WSS21-1, and the combined wave WSS29-2 via the dotted path 23-1. Directly connected to 29-3. In the optical communication node 20, when the wavelength is demultiplexed via the demultiplexing WSS21-1 and the wavelength is demultiplexed via the solid line path 22-1, the wavelength conversion means 24a to 24d are passed through, respectively, and the combined wave WSS29-2, It is connected to 29-3. The optical signal demultiplexed by the demultiplexing WSS21-1 is wavelength-converted by the wavelength conversion means 24a to 24d, and then routed to a desired direction via the demultiplexing WSS29-2 to 29-3. For the path 22-2 from the demultiplexing WSS21-2 to the combined wave WSS29-1 and 29-3, and for the path 22-3 from the demultiplexing WSS21-3 to the combined wave WSS29-1 and 29-2. Also, a similar wavelength conversion means is installed.

In a specific connection path (for example, In1 → Out2), q wavelength conversion means are installed. Therefore, q is the number of paths including the wavelength conversion means in a specific connection path. In the configuration example of the optical communication node in FIG. 4, q = 2, and the value of q may be 1 or 3, but the larger the q, the higher the degree of freedom when performing wavelength defragmentation in the optical communication node. In the optical communication node 20, the value of q may be different for each connection path (routing route) for the WSS for demultiplexing one route. Further, for some other reason, a part of the optical communication node 20 may include a route where q = 0, that is, a wavelength conversion means is not installed.

The simplest configuration, as shown in FIG. 4, includes the same number of paths with wavelength conversion means installed in all connection paths connecting the demultiplexing WSS and the combined WSS between different routes. Just do it. In the following description, the configuration of the wavelength conversion means used in the optical communication node will be described on the premise of the configuration of FIG.

[Structure of wavelength conversion means]
As the wavelength conversion means 24a to 24d in the optical communication node of FIG. 4, an all-optical wavelength conversion means can be used. Here, the all-optical wavelength conversion means is one that performs wavelength conversion only on the optical layer without performing light-electricity-light conversion (O / E / O conversion). As will be described later, the wavelength conversion means 24a to 24d in the optical communication node perform wavelength conversion by utilizing the nonlinear optical phenomenon of the optical layer. Since the all-optical wavelength conversion means does not involve an electric layer, it has merits in all aspects such as deployment cost, power consumption, component failure rate, and complicated operation.

Referring to FIG. 4 again, for example, the signal light 78 is input to one wavelength conversion means 24a, and the pump light 77 for causing wavelength conversion is input. The pump light 77 is generated by the pump light generating means 25-1 as one of the plurality of pump lights, and is input to the wavelength converting means 24a to 24d via the WSS: 26-1 having an M × p configuration. .. The pump light is also called an excitation light, and the pump light generating means corresponds to the excitation light generating means. Hereinafter, a WSS having a configuration in which the number of input ports is M and the number of output ports is p is referred to as M × p WSS. The number of output ports p in M × p WSS26-1 is p = (N-1) × q. Here, N is the number of routes in the optical communication node, and q is the maximum number of routes including wavelength conversion means in the connection path from one route to the other.

In FIG. 4, the value of q is the same for all connection paths, and N = 3 and q = 2, so the number of output ports p in M × p WSS26-1 in the pump light generating means is 2 × 2 =. It becomes 4. If the number of q is the same in all connection paths as in the optical communication node of FIG. 4, all ports of M × p WSS can be used without excess or deficiency. The number of input ports M in M × p WSS26-1 will be described later. In the optical communication node of FIG. 4, pump light is supplied from one pump light generating means to one connection path from one demultiplexing WSS.

As the all-optical wavelength conversion means, wavelength conversion using four-wave mixing in an optical fiber or a nonlinear crystal can be used. In addition, a method using mutual gain modulation of the semiconductor optical amplifier may be used. The means for performing wavelength conversion has a drawback that power consumption increases because an electric layer is interposed, but OE conversion and EO conversion may be performed without using the all-optical wavelength conversion means. In this case, the pump light generating means 25-1 to 25-3 and the M × p WSS are not required. Next, a more specific configuration of the pump light generating means used together with the wavelength converting means will be described.

[Configuration of pump light generating means]
FIG. 5 is a diagram showing a more specific configuration of the pump light generating means in the optical communication node. The pump light generating means (excitation light generating means) shown in FIG. 5 includes an excitation light generation unit 25a and an excitation light frequency shift unit 25b. The pump light generating means shown in FIG. 5 includes M × p WSS26-1 to 26-3 in FIG. 4 as an excitation light selective distribution unit 26. In the excitation light generation unit 25a of FIG. 5, first, the continuous light (CW light) output by the laser diode light source LD41 is input to the photodetector PD43 via the gas cell 42. The optical signal received by the PD 43 is converted into an electric signal and input to the control circuit 44. The control circuit 44 has a mechanism for controlling the injection current into the LD 41. Therefore, the LD41, the gas cell 42, the PD43 and the control circuit 44 form a feedback loop that controls the output wavelength from the LD41. By applying a weak modulation to the injection current to the LD41 in this loop, a control loop that locks the oscillation wavelength of the LD41 is configured on the absorption line of the gas cell 42. As a result, at point 60, the oscillation wavelength of the output light from the LD41 is locked very precisely to the desired wavelength.

A part of the CW light output from the LD 41 is intensity-modulated at the frequency f by the intensity modulator 45, and a pulse is generated at the point 61. The generated pulsed light is amplified by an Erbium Doped optical fiber amplifier (EDFA) 46 and then input to a highly nonlinear fiber HNLF47. At point 62, HNLF47 produces supercontinuum light (frequency comb) by self-phase modulation. Here, the frequency interval of the frequency comb is the frequency f.

As for the frequency comb generated by the HNLF 47, one CW light having a desired pump frequency is selected from the frequency comb lights by 1 × M WSS48. At point 63, the selected CW light is input to the SSB (Single Side Band) modulator 49. Therefore, the 1 × M WSS48 functions as a comb separation means for spatially demultiplexing the frequency comb light into a single CW light for each comb frequency. At the output of 1 × M WSS48, a maximum of M SSB modulators are provided in parallel. In the SSB modulator 49 driven by the modulation frequency Δf, the frequency of the input CW light is finely adjusted. The finely tuned CW light is amplified by the EDFA 50 and then output from the frequency shift unit 25b. The modulation frequencies Δf of the maximum M SSB modulators provided in parallel may be different from each other.

A maximum of M finely tuned CW lights from the frequency shift unit 25b are first combined by the M × 1 WSS 52 in the excitation light selective distribution unit 26. Further, it is demultiplexed by 1 × p WSS53 and output to predetermined output ports 54a to 54d as excitation light which is CW light of a desired wavelength. The CW light of a desired wavelength output to any of the output ports 54a to 54d is supplied to the wavelength selection means as pump light from the pump light generating means in FIG. The output ports 54a to 54d are connection terminals connected to the wavelength conversion means 24a to 24d in FIG. 4, and the above-mentioned CW light is input to the wavelength conversion means 24a to 24d, respectively. The CW light output from the EDFA 50 of the frequency shift unit 25b is partially branched without passing through the excitation light selective distribution unit 26, and the branch light 51 is used by the pump light generating means 25-1 to 25-3. It may be used as pump light.

FIG. 6 is a diagram showing a spectrum of the pump light generation process in the pump light generation means. Each point in the following description corresponds to points 60-66d of the pump light generating means of FIG. At point 60, which is the output of the wavelength control loop, the oscillation wavelength of the LD41 is locked to one of the absorption spectra of the gas cell and is output as CW light. At point 61, which is the output of the intensity modulator 45, sideband waves are generated by intensity-modulating the CW light. After the intensity-modulated light wave is amplified by EDFA46, self-phase modulation in the highly non-linear fiber produces supercontinuum light. As a result, at point 62, which is the output of HNLF47, the spectrum becomes frequency comb light 67 over a very wide band. Here, the interval of the frequency comb light is the intensity modulation frequency f.

At point 63, which is the output of 1 × M WSS48 of the frequency shift unit 25b, a desired spectrum of the frequency comb light 67 is output to each output port of 1 × M WSS48 one wave at a time. What is output at point 63 is still CW light 68. The frequency of this CW light is shifted by Δf at the point 64, which is the output of the SSB modulator 49, and is finely adjusted. Further, at point 65, which is the combined wave output of the M × 1 WSS 52, a plurality of CW lights from different SSB modulators are combined.

FIG. 6 shows a state in which three waves are combined. Finally, at each of the points 66a to 66d of the output port of the 1 × p WSS53, the pump light, which is the frequency-shifted CW light of the desired wavelength, is distributed one by one. CW light from different SSB modulators is distributed and output to each output port of the 1 × p WSS53 one wave at a time. As shown in FIG. 6, pump lights 69-1, 69-2 of the same wavelength are used. May be distributed to multiple output ports. FIG. 6 shows an example in which CW light 69-1 and 69-2 having the same wavelength are output to the two output ports 54b and 54d of the 1 × p WSS53. The method of distribution output will be described in detail in Examples described later.

In the pump light generating means shown in FIG. 5, the pump light having a required wavelength can be extracted as CW light by the wavelength filter function of the M × 1 WSS 52 in the excitation light selective distribution unit 26. Therefore, the SSB modulator 49 of the frequency shift unit 25b in FIG. 5 may be replaced with a phase modulator or an intensity modulator that generates bilateral band waves. That is, unnecessary sideband waves and carrier waves generated in the alternative modulator of the SSB modulator 49 can also be removed by the filter function of M × 1 WSS. Next, a specific configuration of the wavelength conversion means 24a to 24d in the optical communication node of FIG. 4 will be described.

[Structure of wavelength conversion means]
FIG. 7 is a diagram showing a configuration example of a wavelength conversion means in an optical communication node. The wavelength conversion means 24 includes an optical merging device 70 that merges the pump light 77 and the signal light 78, and a nonlinear optical medium 72 that converts the wavelength of the signal light. When the intensity of the pump light and the signal light merged by the optical merging device 70 is insufficient to generate the non-linear phenomenon, the light can be optically amplified by the EDFA 71 and then input to the non-linear optical medium 72.

As the optical merging device 70, an optical directional coupler or a wavelength selection switch can be used. Further, as the nonlinear optical medium 72, a highly nonlinear optical fiber, a periodic polarization inversion LiNbO3 waveguide, a semiconductor optical amplifier, or the like can be used. Further, the optical amplifier described as EDFA in each element of the optical communication node may use a semiconductor optical amplifier instead.

FIG. 8 is a diagram illustrating a spectrum of each part in the wavelength conversion means of the optical communication node. Each point of the following description corresponds to points 73 to 76 of the wavelength conversion means 24 of FIG. At points 73 and 74, which are the two inputs of the wavelength conversion means, pump light 81 and signal light 82 having different wavelengths are input, respectively. At point 75, which is the output of the optical confluence 70, the above two waves are combined. Next, at point 76, idler light 84 is generated by degenerate four-wave mixing by the nonlinear optical medium 72. Assuming that the optical frequency of the pump light 81 is fp and the optical frequency of the signal light 82 is fs, the optical frequency fi of the idler light 84 is expressed by the following equation.
fi = 2fp-fs equation (1)

Further, in the nonlinear optical medium 72, the idler light 83 generated by using the signal light as the pump light and the pump light as the probe light is also generated at the same time as the generation of fi.

The wavelength conversion means 24c and 24d in the optical communication node of FIG. 4 are connected to WSS29-2 (Egress2) for combining waves in the optical communication node of FIG. The original pump light 81, signal light 82 and unnecessary idler light 83 in FIG. 8 are removed by the filtering function of WSS29-2 for combined waves, and only the optical signal 84 (idler light) after wavelength conversion is combined. It is output to point 80-2, which is the output port of WSS29-2. In FIG. 8, the spectrum of only the idler light 84 is shown for the purpose of explaining the wavelength conversion, but at point 80-2, the optical signal transmitted without wavelength conversion and the optical signal converted to another wavelength are combined. Is output.

[Structure for wavelength conversion of optical signals of multiple wavelengths]
In the description of the spectrum of FIG. 8 described above, an example is shown in which the pump light is used only for wavelength conversion of an optical signal of one specific wavelength of the optical communication node. However, if the pump light generating means shown in FIG. 5 is used, it can also be used to simultaneously perform wavelength conversion of optical signals having a plurality of wavelengths in the optical communication node 20. In the pump light generating means of FIG. 5, one CW light is selected from the frequency comb light 67 with 1 × M WSS48, and a plurality of SSB modulators are operated in parallel and independently, and pump light of different wavelengths is output at the same time. Because it can be done. This is effective when the wavelengths of two different optical signals are exchanged with each other. That is, it can be applied when an optical signal having an optical frequency of fs2 is converted to fs4 and a signal having an optical frequency of fs4 is converted to fs2.

FIG. 9 is a diagram for comparing and explaining the operation when the wavelength conversion of the two optical signals is performed collectively and the operation when the wavelength conversion is divided by the pump light generating means shown in FIG. FIG. 9A describes an operation in the case where the optical signal of frequency fs2 and the optical signal of fs4 are collectively introduced into the nonlinear optical medium 72 of the wavelength conversion means of FIG. 7 to perform wavelength conversion. At point 75, which is the output of the optical confluence 70, there are three wavelengths, including pump light of frequency fp. The optical spectrum at the point 76, which is the output of the nonlinear optical medium 72, is a mixture of the residual component of the signal light of fs2 and the converted fi4. Similarly, the residual component of the signal light of fs4 and the converted fi2 are mixed. Since these mixed signals have the same wavelength, they cannot be separated by the filtering function of WSS29-2 at point 80-2, which is the output port of, for example, the WSS29-2 for combined waves of the optical communication node 20.

FIG. 9B is a diagram illustrating an operation of performing wavelength conversion with CW light 69-1 and 69-2 having the same wavelength in the pump light generating means shown in FIG. As shown in FIG. 9B, the conversion of fs2 → fi2 (= fs4) and the conversion of fs4 → fi4 (= fs2) are performed using CW light 69-1 and 69-2 of the same wavelength and different wavelengths. It is done by conversion means. In this case, it is possible to avoid mixing the residual component of the signal light and the converted light component of the other signal light described in FIG. 9A. That is, the pump lights 69-1 and 69-2 having an optical frequency of fp are distributed to the output ports 54b and 54d of the 1 × p WSS53, respectively, and are individually performed by the wavelength conversion means 24d and 24b.

The upper figure of FIG. 9B shows a case where the wavelength conversion means 24d converts the signal light fs2 into fs4. The unnecessary signal 85 of the residual component of the pump light fp and the original signal light fs2 is filtered by the wave combining WSS29-2, and only the signal light fs4 after wavelength conversion is extracted at the point 80-2. The lower figure of FIG. 9B shows a case where the wavelength conversion means 24b converts the signal light fs4 into fs2. As in the case of the wavelength conversion means 24d, the unnecessary signal 86 is filtered by the wave combination WSS29-3, and only the signal light fs4 after the wavelength conversion is extracted at the point 80-3. In this way, in the optical communication node 20, the wavelengths of the two signal lights can be easily exchanged. Here, an example of wavelength switching for two waves is shown, but it is also possible to use three pump lights of the same wavelength for mutual wavelength conversion of three or more signal lights.

With the optical communication node shown in FIG. 4, more flexible and easy wavelength defragmentation can be realized. By performing wavelength conversion on the path within the optical communication node without changing the frequency of the transmitter at the transmission node, the troublesome and complicated control required across multiple communication nodes is greatly simplified. Node. If the information about the wavelength change is known in the receiving node, the frequency of the local oscillator can be changed to the changed frequency (wavelength) in advance on the receiver side of the receiving node. Even in the pump light generating means, it is sufficient to prepare the pump light having a predetermined frequency in advance before the wavelength change is performed. By changing the transmission wavelength settings of the input side WSS and the output side WSS, the wavelength of the signal light can be changed immediately. On the receiving node side, reception at the wavelength before the change and reception at the wavelength after the change can be performed in parallel. Therefore, the wavelength can be changed without interrupting communication.

As shown above, multiple WSSs are required for an optical communication node capable of wavelength defragmentation using wavelength conversion technology, and the required wavelength can be selected from a large number of pump lights generated by a frequency com light source. Only the pump light that has it needs to be routed to the desired port. The wavelength interval of the frequency comb light source is generally about 10 to 20 GHz, which is even narrower considering that the tunable filter applied to a normal ROADM system operates along a frequency grid of about 37.5 to 50 GHz. It is necessary to realize the operation.

Next, the basic principle of the tunable optical filter having the narrow band characteristic of the present invention used for such an optical communication node will be described.

(Basic Principle of Narrow Band Optical Filter of the Present Invention)
FIG. 10 is a diagram illustrating the basic principle of the narrow band optical filter function of the present invention in which two WSSs (wavelength selection switches) are combined in the case of the optical communication node configuration of FIG.

In FIG. 10 (a), (1) to (4) show an optical filter by light or WSS that passes through the point pointed to by the corresponding upward arrow. FIG. 10B shows the wavelength spectrum of light or the filter characteristics of the optical filter in (1) to (4).

As shown in (1) of FIG. 10 (b), the frequency comb light (1) generated by the excitation light generation unit as a light source is a single fiber as a pump light of nine line spectra of λ1 to 9. It is transparent inside. As shown in FIG. 10A, an excitation light frequency shift portion is connected to the fiber output of the frequency comb light (1), and two WSSs, 1xMWSS1 and Mx1WSS2, are SSB-modulated in order to narrow the band. They are cascaded as a pair with a vessel in between.

As shown in (2) of FIG. 10 (b), the filter characteristics of WSS1 are not narrow enough to take out only one channel of the optical frequency comb light source, and the extent to which a plurality of channels are transmitted is sufficient. Assume. In (2) of FIG. 10 (b), the three channels λ3 to 5 are transmitted.

The filter characteristic (3) of WSS2 arranged in the latter stage is also a filter characteristic to the extent that 3 channels are transmitted, but the set center wavelength of the first stage WSS1 (2) and the set center wavelength of the latter stage WSS2 (3) are slightly changed. It is set so that λ5 to 7 are transparent. An example of the synthetic filter characteristic that has passed through both of them is (4) in FIG. 10 (b). In the composite filter characteristic (4) of WSS1 + 2, only the wavelength region in which the common transmission setting is set for both (2) and (3) can be transmitted.

By using this configuration, as shown in the composite filter characteristics of (4) in FIG. 10 (b), only λ5 can be selectively transmitted, and even a WSS having a conventional transmission characteristic can narrow the band. It will be possible. Of course, the excitation light frequency shift portion described between (2) and (3) in FIG. 10 (a) is not always necessary, and if only one channel is to be extracted in the portion (2), (2) It is also possible to arrange two stages of WSS in the part of.

However, when constructing such a configuration, it is necessary to prepare multiple WSSs, and although the narrow band characteristics that have been a problem in the wavelength defragmentation node can be improved, it is necessary to connect a large number of filtering devices. , Cost and space issues are not resolved. Therefore, consider the application of a WSS array in which a plurality of WSS functions are integrated.

FIG. 11 shows Example 1 in which the narrow band optical filter of the present invention is configured by a multi-unit integrated WSS array in which a plurality of WSS functions are integrally integrated. In the components of the multi-unit integrated WSS array of FIG. 11, the optical waveguide substrate 1101, the diffraction grating 1102, the lens 1103, and the reflective spatial light modulator 1104 are arranged in this order.

On the optical waveguide substrate 1101 on the left side of FIG. 11, an input / output waveguide group 1105, a slab waveguide 1106 to which the input / output waveguide group 1105 is connected, and an array waveguide 1107 connected to the slab waveguide 1106 are composed. A plurality of optical circuits called Spatial Beam Transferrs (SBTs) are provided. In each SBT, all the array waveguides 1107 are designed to have the same length, and depending on which of the input / output waveguides included in the input / output waveguide group 1105 is selected, the array waveguide 1107 passes through the optical waveguide substrate 1101 and becomes a free space. It has a function of determining at what angle and at what beam diameter the emitted light beam is emitted.

The plurality of SBTs on the optical waveguide substrate 1101 are essentially bidirectional optical devices composed of only passive elements, for example, signal light incident from the left end surface of the optical waveguide substrate 1101 in FIG. Focusing on, the light of group C (C is an integer of 2 or more) with A input ports (A is an integer of 1 or more) and B (B is an integer of 1 or more) as one group. It can be said that it is an input / output unit.

The operating principle of the WSS in the optical filter configuration of the first embodiment is as follows. First, the signal light input from the left end to one of the waveguides included in the input / output waveguide group 1105 (for example, the signal light indicated by the shaded arrow incident to the right from the upper left end of FIG. 11) is the uppermost slab. In the waveguide 1106, the slab waveguide 1106 propagates in the Z-axis + direction so as to spread in the Y-axis direction in the YZ plane of the optical waveguide substrate 1101 while being confined in the X-axis direction. Since the wave surface of the spreading signal light has a curvature according to the propagation distance, the end (right end) of the slab waveguide 1106 is configured to have a shape that matches the curvature of the wave surface. Arrayed waveguides 1107 of equal length are connected to the ends of slab waveguides 1106. Here, of the end faces of the optical waveguide substrate 1101, the right end face to which the array waveguide 1107 is connected coincides with the Y-axis direction.

In this configuration, the optical signal output from the array waveguide 1107 to the space to the right via the slab waveguide 1106 is output as a plane wave whose phase is aligned in the Y-axis direction, and is therefore collimated in the Y-axis direction. Propagates space as a beam. The optical signal that has passed through the optical waveguide substrate 1101 is angularly demultiplexed (spectroscopically) for each wavelength by the diffraction grating 1102, and further passed through the lens 1103, which is a condensing means, to change the angular position for each wavelength. , It is incident on the spatial light modulator 1104 at the right end of FIG.

The optical signals incident on the spatial light modulator 1104 are reflected at arbitrary angles under the control of the spatial light modulator 1104 for each wavelength, and this time, the light signals proceed in the opposite directions from right to left, and the lens 1103 and the diffraction grating 1102 are again moved. It passes through and recombines to the optical waveguide substrate 1101. Finally, the optical signal of the wavelength selected by the control of the spatial light modulator 1104 is emitted from the left end of the optical waveguide substrate 1101 (for example, the signal light indicated by the dotted arrow emitted to the left from the upper left end of FIG. 11). This completes the switching operation for one stage. By controlling each element of the spatial light modulator 1104 according to the light wavelength, it can be operated as a one-stage optical filter.

In this configuration, the point at which the spatial light modulator 1104 collects light at different positions in the Y-axis direction according to the emission angle in the YZ plane when the light beam is emitted from the optical waveguide substrate 1101 into the free space. Is a feature. By variably controlling the spatial light modulator 1104 and deflecting and reflecting each of the light beams focused at different positions on the XY plane of the spatial light modulator 1104 in the Y-axis direction at an arbitrary angle. , Multiple WSS functions can be integrated into one optical system.

Further, in order to connect the WSS arrays in multiple stages, as shown at the left end of the optical waveguide substrate 1101 in FIG. 11, the reflected light from the spatial light modulator 1104 of the different WSS function unit is transmitted to the input / output waveguide group 1105 of the SBT. A WSS connecting portion 1108 that is folded back on the side and connected to another WSS may be provided. Such a WSS connecting portion is formed and connected as an optical fiber to which the input / output waveguide group 1105 is connected outside the optical waveguide substrate 1101 or as an extension portion of the input / output waveguide group 1105 of the optical waveguide substrate 1101. It can be configured as a waveguide.

In this way, the output port of the WSS in the front stage and the input port of the WSS in the rear stage can be directly connected to form a multi-stage optical filter. In the spatial optical system on the right side of FIG. 11, the WSS light beam related to the first-stage optical filter operation is indicated by a solid arrow, and the WSS light beam related to the second-stage optical filter operation is indicated by a dotted arrow. Shown.

As described above, as shown in FIGS. 10 and 11, a multi-unit integrated WSS array in which a plurality of WSS functions are integrally integrated is used, and wiring is performed so as to pass through a plurality of WSS functional units included in the WSS array. By setting the wavelength range of the transmission filter of each WSS function unit to a different setting, it is possible to provide a narrow-band tunable filter that does not increase the introduction cost, deployment space, and power consumption.

In summary, in Example 1 of the narrowband optical filter using the multi-unit integrated WSS array of the present invention as shown in FIG. 11, the plurality of SBTs on the optical waveguide substrate 1101 have A input ports and B. It can be said that it is an optical input / output unit of a plurality of C groups having an output port as one group, and it can be said that the diffraction grid 1102 is a spectroscopic means for wavelength-separating the wavelength-multiplexed optical signal emitted from the output port. The lens 1103 is a condensing means for condensing the optical signals dispersed for each wavelength by the spectroscopic means, and the spatial light modulator 1104 is a light for each of the optical signals condensed by the condensing means. It is a spatial light modulation means that modulates and reflects.

Then, as a function of the WSS (wavelength selection switch) set by the spatial optical modulator 1104, the optical signal input from any of the input ports is transmitted in the first transmission wavelength range (λ _M1 ≤ λ ≤ λ _M2 ). Filtering is possible, and by folding back the reflected light from the spatial light modulation means to another WSS at the WSS connecting portion 1108, only a certain wavelength range (λN1 ≦ λ ≦ λN2) is further filtered by the spatial light modulation means again. It can be made into a narrow band.

That is, the optical filter of the first embodiment of FIG. 11 is
Group C (C is an integer of 2 or more) optical input / output unit, which includes A (A is an integer of 1 or more) input ports and B (B is an integer of 1 or more) as one group.
A spectroscopic means for wavelength division multiplexing light signals emitted from the output port, and
A condensing means for condensing optical signals dispersed for each wavelength by the spectroscopic means,
A spatial light modulation means that photomodulates and reflects each of the light signals collected by the light collection means is included.
Only the first transmission wavelength range (λ _M1 ≤ λ ≤ λ _M2 ) of the optical signal input from any of the input ports belonging to the M group (M is an integer from 1 to C) is transmitted by the spatial optical modulation means. After being filtered
The reflected light from the spatial light modulation means is folded back, and only the second wavelength range (λ _N1 ≤ λ ≤ λ _N2 ) is filtered again by the spatial light modulation means.
It can be said that the optical filter is characterized in that the optical signal is output from any of the output ports belonging to the Nth group (N is an integer from 1 to C, where M ≠ N). ..

In the first embodiment, the WSS functional unit is connected in a plurality of stages via the WSS connecting unit 1108 to transmit an optical signal, thereby realizing a narrow band filter characteristic. On the other hand, in WSS, a certain insertion loss occurs every time one step is transmitted. This loss consists of several factors, for example, the connection / transmission loss of the optical waveguide board 1101, the SBT circuit in the optical waveguide board, the diffraction loss in the diffraction grating 1102 and the spatial light modulator 1104, and the recombination to the optical waveguide board 1101. A typical example is the coupling loss at the time. In particular, since the amount of loss related to the optical waveguide substrate 1101 is large, it is possible to suppress the total insertion loss while maintaining the filter function by suppressing the number of times the optical waveguide substrate 1101 is transmitted.

Example 2 of the present invention of FIG. 12 shows an embodiment of the narrow band optical filter of the present invention configured by a WSS array capable of transmitting a plurality of stages of WSS functional units while reducing the number of transmissions of the optical waveguide substrate 1101. ..

In the narrow band optical filter of Example 2 shown in FIG. 12, a mirror formed on the light emission end face side (spatial optical system side) of the optical waveguide substrate 1101 instead of the WSS connecting portion 1108 of FIG. 11 of Example 1 By providing the 1109, the reflected light from the spatial light modulator 1104 is directly returned to the spatial optical system.

As shown by the solid arrow in FIG. 12, the light emitted from the SBT of the optical waveguide substrate 1101 into the free space is focused on the spatial light modulator 1104 according to the emission angle. The light reflected by the spatial light modulator 1104 at a desired angle is focused toward the optical waveguide substrate 1101 according to the reflection angle.

Here, as shown in FIG. 12, when the mirror 1109 is arranged at the condensing position on the right end surface of the optical waveguide substrate 1101 on the spatial optical system side, the reflected light from the spatial light modulator 1104 is transmitted to the optical waveguide substrate 1101. Without being incident, it is reflected by the mirror 1109 as it is, is directly folded back, propagates in free space again, and is focused again on the spatial light modulator 1104. The Y coordinate of the light collection position at this time can be focused at a position of the Y coordinate different from the position where the light light modulator 1104 was first focused by variable control of the spatial light modulator 1104. Then, as a result of performing the second angle deflection reflection in the spatial light modulator 1104, it can be recombined into a desired SBT circuit, and the output port can be determined.

With such a configuration, a narrow band wavelength tunable WSS array capable of suppressing loss due to transmission of the optical waveguide substrate 1101 while being subjected to wavelength filtering twice by the spatial light modulator 1104 is configured.

Such an optical filter of the second embodiment of FIG. 12 replaces the WSS connecting portion 1108 of the first embodiment as a means for returning the reflected light from the spatial light modulator 1104, and is an end face of the optical waveguide substrate 1101 on the spatial optical system side. It is provided with a mirror 1109 formed in.

Therefore, after the optical signal is filtered by the spatial light modulation means only in the first transmission wavelength range (λ _M1 ≤ λ ≤ λ _M2 ), the reflected light from the spatial light modulation means is reflected by the mirror to the spatial optical system. In turn, the optical filter can be narrowed by filtering only the second transmission wavelength range (λ _N1 ≤ λ ≤ λ _N2 ) by the spatial optical modulation means again.

In any of the configurations of the examples, in order to realize a narrow band filter, it is necessary that either λ _M1 ≠ λ _N1 or λ _M2 ≠ λ _N2 is satisfied, and it is included in the two transmission wavelength ranges. There is a wavelength.

Further, the number of times the reflected light from the spatial light modulation means is returned to the spatial optical system is not limited to one, and a plurality of WSS connecting portions that return the reflected light from the spatial light modulation means and connect it to another WSS, or A mirror corresponding to a plurality of reflected lights from the spatial light modulation means (not necessarily a mirror divided into a plurality of mechanically, and may be used in combination with a WSS connecting portion) is provided to provide a multi-stage mirror having three or more stages. It is also possible to configure a multi-stage optical filter with a WSS array, and by adjusting the characteristics of the WSS filter in each stage, it is possible to realize a narrower band optical filter.

As described above, according to the present invention, it is possible to provide a narrow band wavelength tunable filter array in which a plurality of optical filter functions are integrally integrated.

A, B, C, D, 11, 12, 13 Optical communication nodes 1, 4, 14, 27 Add section 2, 3, 15, 28 Drop section 16 Transmitter 18 Wavelength selection switch 17 Local oscillator 20 Optical communication subsystem 21 -1 to 21-3 WSS for demultiplexing
24, 24a to 24i Wavelength conversion means 25, 25-1 to 25-3 Pump light generation means 25a Excitation light generation unit 25b Frequency shift unit 26 Excitation light selection and distribution unit 26-1 to 26-3, 401, 501 M × p WSS
29-1 to 29-3 WSS for combined wave
46, 50, 71 EDFA
47 HNLF
48 1 x M WSS
49 SSB modulator 52 M × 1 WSS
53 1 × p WSS
70 Optical Merger 72 Nonlinear Optical Medium 1101 Optical Waveguide Substrate 1102 Diffraction Grating 1103 Lens 1104 Spatial Light Modulator 1105 Input / Output Waveguide Group 1106 Slab Waveguide 1107 Array Waveguide 1108 WSS Connection 1109 Mirror

Claims (4)

Group C (C is an integer of 2 or more) optical input / output unit, which includes A (A is an integer of 1 or more) input ports and B (B is an integer of 1 or more) as one group.
A spectroscopic means for wavelength division multiplexing light signals emitted from the output port, and
A condensing means for condensing optical signals dispersed for each wavelength by the spectroscopic means,
A spatial light modulation means that photomodulates and reflects each of the light signals collected by the light collection means is included.
The optical signal input from any of the input ports belonging to the M group (M is an integer from 1 to C) has only the first wavelength range (λ _M1 ≤ λ ≤ λ _M2 ) by the spatial optical modulation means. After being filtered
The reflected light from the spatial light modulation means is folded back and again filtered by the spatial light modulation means only in the second wavelength range (λ _N1 ≤ λ ≤ λ _N2 ).
An optical filter characterized in that the optical signal is output from any of the output ports belonging to the Nth group (N is an integer from 1 to C, where M ≠ N).
An optical filter having a WSS array configuration, characterized in that either λ _M1 ≠ λ _N1 or λ _M2 ≠ λ _N2 holds. After the optical signal input from any of the input ports belonging to the M group is filtered by the spatial optical modulation means only in the first wavelength range (λ _M1 ≤ λ ≤ λ _M2 ),
The reflected light from the spatial light modulation means is reflected by the mirror and folded back, and only the second wavelength range (λ _N1 ≤ λ ≤ λ _N2 ) is filtered again by the spatial light modulation means.
The optical filter according to claim 1, wherein the optical signal is output from any of the output ports belonging to the Nth group. It is characterized in that there are wavelengths included in the two wavelength ranges in the first wavelength range (λ _M1 ≤ λ ≤ λ _M2 ) and the second wavelength range (λ _N1 ≤ λ ≤ λ _N2 ). The optical filter according to claim 1 or 2. The claim is characterized in that a plurality of WSS connecting portions or mirrors for folding back the reflected light from the spatial light modulation means and connecting them to another WSS are provided to form a multi-stage optical filter using a multi-stage WSS array having three or more stages. The optical filter according to any one of 1 to 3.

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