JP3349938B2 - Optical cross connect system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical cross-connect system used in a wavelength division multiplexing optical communication network.

[0002]

2. Description of the Related Art A wavelength division multiplexing transmission system modulates a plurality of optical carriers having different wavelengths with different signals, and multiplexes the plurality of modulated optical carriers (a plurality of optical signals) into one optical fiber. Is propagated. For this reason, at a relay point where a plurality of optical fibers are gathered, an optical cross-connect system that can arbitrarily replace a plurality of optical signals wavelength-multiplexed in each optical fiber irrespective of the wavelength is required.

FIG. 18 shows a configuration example of a conventional optical cross-connect system using a wavelength multiplexing technique. Here, 4
It is assumed that a pair of input and output optical fibers are provided, and four optical signals (λ _{1 to} λ ₄ ) are wavelength-multiplexed in each optical fiber. In FIG. 18, 11 to 14 are input optical fibers, 21 to 24 are 1 × 4 optical demultiplexers, and 30 is 16 × 16
Optical switches 711 to 744 are wavelength converters and 81 to 8
4 is a 4 × 1 optical multiplexer, and 91 to 94 are output optical fibers.

[0004] Wavelength-division multiplexed light propagating through the input optical fibers 11 to 14 is demultiplexed by the optical demultiplexers 21 to 24 according to the wavelength. The 16 optical signals demultiplexed by the four optical demultiplexers 21 to 24 are guided to any one of the 16 wavelength converters 711 to 744 by the optical switch 30. Here, the optical signals guided to the wavelength converters 711 to 714 are converted into optical signals of predetermined wavelengths λ _{1 to} λ ₄ , multiplexed by the optical multiplexer 81, and output from the output optical fiber 91. Is output to The same applies to other wavelength converters and optical multiplexers. Thus, the optical signal of each wavelength propagating in the plurality of input optical fibers can be freely distributed to the plurality of output optical fibers regardless of the original wavelength.

However, with the configuration shown in FIG. 18, it is difficult to configure the optical switch 30 when the number of input / output optical fiber pairs and the number of wavelengths are large. In particular, according to demand,
For example, to be able to add one pair of input and output optical fibers,
It is difficult to divide and configure the optical switch portion.
An optical cross-connect system that solves such a problem is described in Japanese Patent Application Laid-Open No. 3-219793 (wavelength division optical switch). Hereinafter, the conventional optical cross connect system will be briefly described. The principle of operation of the optical switch portion is that the wavelength-division multiplexed light propagating through m input optical fibers (m is an integer of 2 or more) is distributed to a large number of m × 1 optical switches without being demultiplexed. After selecting any one of the wavelength division multiplexed light, any one of the optical signals is selected by the variable wavelength selection element.

FIG. 19 shows an example of the configuration of a conventional optical cross-connect system using this optical signal selecting means. Here, it is assumed that four pairs of input / output optical fibers are provided, and four optical signals (λ _{1 to} λ ₄ ) are wavelength-multiplexed in each optical fiber. In FIG. 19, 11 to 14 are input optical fibers, 31 to 34 are 1 × 16 optical splitters,
511 to 544 are 4 × 1 optical switches, 611 to 644
Is a variable wavelength selection element, 711 to 744 are wavelength converters, 8
1 to 84 are optical multiplexers, and 91 to 94 are output optical fibers.

The wavelength-division multiplexed light propagating through the input optical fibers 11 to 14 is converted into the wavelength-division multiplexed light by the optical splitters 31 to 31.
At 34, the light is branched into 16 light and guided to 16 light switches 511 to 544. For example, the optical switch 511
, One output of each of the optical splitters 31 to 34 is guided.

First, an output from any one of the optical splitters is selected by 4 × 1 optical switches 511 to 544, and then four wavelength multiplexed wavelengths are multiplexed among the selected outputs by variable wavelength selection elements 611 to 644. A desired optical signal is selected from among the optical signals. The optical signals selected via the optical switches 511 to 514 and the variable wavelength selection elements 611 to 614 are respectively guided to the corresponding wavelength converters 711 to 714, where the optical signals of the wavelengths λ _{1 to} λ ₄ set in advance are set. Is multiplexed by the optical multiplexer 81 and output to the output optical fiber 91. The same applies to other wavelength converters and optical multiplexers. In this way, the optical signal of each wavelength propagating in the plurality of input optical fibers can be freely distributed to the plurality of output optical fibers regardless of the wavelength.

In the conventional cross-connect system shown in FIG. 19, optical splitters 31 to 34, 4 × 1 optical switches 511 to 544, and variable wavelength selection elements 611 to 644 are used to connect the optical splitters 21 to 24 in FIG. Optical switch 3
0 functions are realized. 4 × 1 optical switches 511-
544 has a simpler configuration than the 16 × 16 optical switch 30. Further, the optical switches 511 to 544 and the variable wavelength selection elements 611 to 644 can be added, for example, for each input / output optical fiber pair. That is, the number of optical switches can be increased stepwise according to demand.

[0010]

However, according to the above-mentioned publication, the 4 × configuration of the optical cross-connect system shown in FIG.
Crosstalk with one optical switch or variable wavelength selection element,
No consideration is given to the optical switch drive circuit. FIG.
The 4 × 1 optical switches 511 to 544 shown in FIG.
As shown in the figure, three 2 × 1 optical switch elements 151 to 15
3 can be configured by connecting two stages in a tree shape. When any one of the inputs 1 to 4 is selected, it is necessary to switch two 2 × 1 optical switch elements. That is, at least one optical switch drive circuit 311 and 312 is required for each stage. In general, a 2 ^P × 1 optical switch is configured by connecting a total of (2 ^P −1) 2 × 1 optical switch elements in p stages, and requires at least p optical switch driving circuits. For this reason, the size and power consumption of the optical switch drive circuit part increase.

Since the optical switches 511 to 544 select the wavelength multiplexed light as it is, the crosstalk includes a component whose wavelength coincides with the selected optical signal. This situation is shown in FIG.
Shown in In FIG. 21, thick arrows indicate the flow of the selected wavelength multiplexed light. As shown in FIG. 21, when there is a crosstalk component whose wavelength coincides with the optical signal to be selected other than the wavelength-division multiplexed light selected (see a broken arrow), beat noise is generated and the SN ratio of the optical signal is significantly deteriorated. . Therefore, the optical switches 511 to 544 are required to have a high extinction ratio.

Reference 1 (Goldstein, et al., "Scaling li
mitations in transparent opticalnetwork due to low
-level crosstalk ", IEEE Photonics Technology Lette
According to rs, Vol. 7, pp. 93-94, 1995), when a beat noise is generated, a crosstalk ε _b [d which causes a power penalty pp [dB] of sensitivity at a certain bit error rate.
B] is given by ε _b [dB] = 10 log {(1−10− ^{PP / 5} ) / (4Q ² )} (1). Here, Q is a coefficient uniquely determined according to the bit error rate. For example, when the bit error rate is 10 ⁻¹² , Q = 7. Therefore, for example, a bit error rate of 10
^In order to suppress the power penalty at ^-12 to within 0.5 dB, it is necessary to suppress the crosstalk to -30 dB or less. When a 4 × 1 optical switch is configured by connecting 2 × 1 optical switch elements in two stages, crosstalk is added for each stage. It needs to be smaller than -33 dB.

Further, depending on the type of the optical switch, it may be difficult to achieve this extinction ratio. For example, a quartz waveguide Mach-Zehnder interferometer type 2 using the thermo-optic effect
In the x1 optical switch element, a large crosstalk occurs in one of the two input ports due to a manufacturing error of the directional coupler. This problem is described in Document 2 (Kominato et al., “Waveguide-type WDM Circuit Consisting of Mach-Zehnder Interferometer”, IEICE Transactions on Information, CI, Vol. 73-CI, No. 5, pp.
354-359, 1990).

FIG. 22 shows a basic configuration of a 2 × 1 optical switching element of the quartz waveguide Mach-Zehnder interferometer type. This optical switch element is composed of two directional couplers 161 and 162 and two single-mode waveguides 163 and 164 whose waveguide lengths are L and L + △ L, respectively. A thin film heater 165 is mounted on one of the waveguides, thereby changing the temperature in the vicinity of one of the waveguides and changing the equivalent refractive index by the thermo-optic effect for switching.

Assuming that the transmittance of the optical switch element from port 1 to port 3 is T ₁ and the transmittance from port 2 to port 3 is T ₂ , T ₁ = [{(1-k ₁ ) (1 -k ₂ )} ^1/ _2- (k ₁ k ₂ ) ^1/2 ] ² +4 {k ₁ k ₂ (1-k ₁ ) (1-k ₂ )} ^1/2 sin ² (πn △ L / λ _s ) ・ ・ ・ ・ ・ (2) T ₂ = [{k ₂ (1-k ₁ )} ^1/ _2- {k ₁ (1-k ₂ )} ^1/2 ] ² +4 {k ₁ k ₂ (1-k ₁ ) (1-k ₂ )} ^1/2 cos ² (πn △ L / λ _s ) (3) Here, k ₁ and k ₂ are the directional couplers 16.
The coupling ratio of the light intensity at 1,162, n △ L is the effective optical path length difference, and λ _s is the wavelength of the optical carrier. The propagation loss of the waveguide is ignored because it is sufficiently small.

By the way, the coupling ratios k ₁ and k ₂ of the directional couplers 161 and 162 depend on the relative refractive index difference of the waveguide and the waveguide interval, and deviate from the design value of 0.5 due to a manufacturing error. I will. However, k ₁ = k ₂ = k is relatively easily achieved because these fabrication errors affect the two directional couplers almost equally. At this time, the above equations (2) and (3) are respectively expressed as T ₁ = (1-2k) ² + 4k (1-k) sin ² (πn △ L / λ _s ) (4) T ₂ = 4k (1-k) cos ² (πn △ L / λ _s ) (5)

Here, the effective optical path length difference n △ L of the waveguide is
In a state where the thin film heater is not driven, for example, a design is made so as to satisfy n △ L = λ _s / 2 (6). In this case, the transmittance T ₁ of the ports 1 to port 3, the transmittance T ₂ of the from port 2 to port 3, respectively, regardless of the value of both coupling factor k T
₁ = 1 ... (7) T ₂ = 0 ... (8) That is, if driving the thin film heater, this optical switching element outputs regardless of the wavelength as long as the light in the vicinity of lambda _s input to the port 1. At this time, crosstalk from port 2 does not exist in principle.

On the other hand, the thin-film heater is driven to increase the temperature in the vicinity of one of the waveguides, and the effective refractive index of the waveguide is changed to change the effective optical path length difference n △ L to n △ L = λ _s. if & (9), the transmittance T ₁ of the ports 1 to port 3, the transmittance T ₂ of the from port 2 to port 3, respectively ^{_{T 1 = (1-2k) 2 ·····}} (10 ) T ₂ = 4k (1-k) (11) That is, by driving the thin film heater, this optical switch element selects and outputs the optical signal input to the port 2. However, at this time, if the value of the coupling ratio k does not exactly match 0.5, the crosstalk from the port 1
(1-2k) ² results. In the above-mentioned document 2, for example, the waveguide interval of the directional coupler is 20
It has been reported that the crosstalk is degraded to about -16 dB only by shifting by%.

On the other hand, as shown in FIG. 23, a wavelength multiplexed light is demultiplexed by an optical demultiplexer 601 as a variable wavelength selecting element of a cross connect system which is often used in the past, and one optical signal is converted into a 4 × 1 optical signal. Some are selected by the optical switch 602 and output. Also in FIG. 23, thick arrows indicate the flow of the selected optical signal. If the 4 × 1 optical switch 602 is configured by connecting 2 × 1 optical switch elements in multiple stages in a tree shape, the size and power consumption of the optical switch driving circuit portion also increase.

However, since the optical switch 602 selects the demultiplexed signal, the crosstalk does not include a component having the same wavelength as the selected optical signal, so that no beat noise is generated. Therefore, the power penalty caused by the crosstalk needs to consider only the influence as the intensity noise. At this time, a crosstalk ε causing a power penalty pp [dB] at a certain bit error rate
_i [dB] is given by ε _i [dB] = 5 Log {(1−10− ^{pp / 5} ) / Q ² } (12) For example, to suppress the power penalty at a bit error rate of 10 ⁻¹² to within 0.5 dB, the crosstalk may be reduced to −12 dB or less. For example, when the 4 × 1 optical switch is configured by connecting 2 × 1 optical switch elements in two stages, the crosstalk of the 2 × 1 optical switch element alone may be suppressed to −15 dB or less.

As described above, in the conventional optical cross-connect system, it is necessary to selectively use optical switches having a high extinction ratio as the optical switches 511 to 544, which increases the cost. In addition, as described with reference to FIG. 23, if an optical switch is also used for the variable wavelength selection element, since two types of optical switches having different requirements for the extinction ratio are included, integration of the optical switch is difficult, and miniaturization and It is difficult to reduce the cost. Furthermore, there is a possibility that the number of necessary optical switch driving circuits is large, the size and power consumption are increased, and the size cannot be reduced.

According to the present invention, a complicated m × 1 optical switch is not required, the extinction ratio of each optical switch may be low in addition to a simple gate optical switch, and a small number of optical switch drives may be used. It is an object of the present invention to provide an economical optical cross-connect system that operates on a circuit.

[0023]

In order to solve the above-mentioned problems, the present invention provides a method for reconstructing an optical signal constituting wavelength-division multiplexed light propagating through m (m is an integer of 2 or more) input optical fibers. An optical cross-connect system for wavelength-multiplexing and switching and outputting to m output optical fibers, wherein (1) the wavelength-multiplexed light input from each of the input optical fibers is split into a plurality of wavelength-multiplexed lights and output. And (2) a plurality of optical signal selecting means to which a plurality of wavelength-division multiplexed lights split by the optical splitting means are inputted. Wavelength multiplexed light from the input fiber is input, respectively, and the optical signal selecting means selects one of the input m wavelength multiplexed lights, and further converts an optical signal of one wavelength from the selected wavelength multiplexed light. Optical signal selection means for selecting and outputting (3) a plurality of wavelength converting means for converting the optical signals output from the respective optical signal selecting means into optical signals of a predetermined wavelength, respectively; and (4) the plurality of wavelength converting means connected to the m output optical fibers, A plurality of optical multiplexing means for multiplexing the optical signals output from the wavelength conversion means and outputting the wavelength multiplexed light to these output optical fibers, wherein each of the optical signal selecting means comprises: first optical switching means having m light passages corresponding to the m wavelength multiplexed lights, passing only the wavelength multiplexed light passing through one of the light passages, and blocking the remaining light passages; 2-2) The first optical system has m input ports and p output ports (p is an integer equal to or greater than the wavelength multiplexing number of the wavelength multiplexed light) connected to the m light passage paths, respectively. Wavelength multiplexed light passing through optical switching means has a plurality of different wavelengths A wavelength router that splits optical signals and outputs these split optical signals to different output ports, wherein the output destination of each split optical signal has a different routing characteristic for each input port; , (2-3) having p optical passages connected to p output ports of the wavelength router, passing only optical signals passing through one of them, and blocking the other optical passages (2-4) An optical cross-connect system comprising: (2-4) an optical converging unit that couples the p optical passages of the second optical switching unit to one optical passage. .

According to the above configuration, in each optical signal selecting means, only one wavelength multiplexed light is selected via the first optical switching means, and only one of the wavelength multiplexed lights is selected via the second optical switching means. An optical signal is selected. Further, of the crosstalk caused by the m optical passages (optical waveguides) in the first optical switching means, the component having the same wavelength as the optical signal selected by the second optical switching means is converted into a second component by the wavelength router. Since the light is guided to a different optical path (optical waveguide) of the optical switching means, no beat noise is generated. Therefore, both the first and second optical switching means can be constituted by optical switches having a low extinction ratio. Each of these two optical switching means has a plurality of light passages,
Since it has a similar function of allowing only one of them to pass, it can be configured with the same type of optical switch.

A technique that can suppress the generation of beat noise by the combination of the first optical switching means and the wavelength router is described in reference 3 (Ishida et al., "Optical parallel connection multi-wavelength star network (POI
MS Net) ”, 1996 IEICE Communications Society Conference B-1072). This document 3 discloses a plurality of optical branching units, a first optical switching unit having an optical passage connected to each output of the optical branching unit, and a wavelength having input ports respectively connected to these optical passages. An optical network comprising a router and a plurality of optical receivers respectively connected to a plurality of output ports of the wavelength router is described. A similar configuration is also described in detail in JP-A-9-247179 (optical receiver and optical network using the same).

Therefore, the present invention provides a conventional m × 1 optical switch and a variable wavelength selecting means (the configuration shown in FIG. 19) by adding a second optical switching means and an optical combining means to the first optical switching means and the wavelength router. Thus, it can be said that an optical signal selecting means different from m = 4) is realized. That is, the first
The combination of the optical switching means and the wavelength router does not function as an m × 1 optical switch as it is, but the function of the variable wavelength selection means is added by skillfully utilizing the routing characteristics of the wavelength router. It is made applicable to the means.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION

(Basic Configuration of Optical Cross Connect System) FIG. 1 shows a basic configuration of an optical cross connect system of the present invention. Here, an optical cross-connect system is shown in which four pairs of input / output optical fibers are connected, and four optical signals having different wavelengths are wavelength-multiplexed on each optical fiber.

In FIG. 1, 11 to 14 are input optical fibers, 31 to 34 are 1 × 16 optical splitters, 411 to 4
44 is an optical signal selecting means, 711 to 744 are wavelength converters,
Reference numerals 81 to 84 denote 4 × 1 optical multiplexers, and reference numerals 91 to 94 denote output optical fibers. The wavelength multiplexed light propagating in each of the input optical fibers 11 to 14 is branched into 16 by each of the optical splitters 31 to 34 as the wavelength multiplexed light, and the 16 optical signal selecting means 4
11 to 444, respectively. For example, the optical signal selecting means 411 has four 1 × 16 optical splitters 31 to
One output of each of the 34 is derived. On the other hand, the optical signals selected by the respective optical signal selecting means 411 to 414 are guided to wavelength converters 711 to 714, where they are converted into optical signals of preset wavelengths λ _{1 to} λ ₄ and then optically multiplexed. Multiplexed by the optical device 81 and output to the output optical fiber 91.
The same applies to other wavelength converters and optical multiplexers.

(First Configuration Example of Optical Signal Selection Means) FIG. 2
Shows a first configuration example of the optical signal selection means which is a feature of the present invention. In FIG. 2, the optical signal selecting means includes an optical switch element array 100, an arrayed waveguide grating wavelength router 20.
0, optical switch element array 110, 7 × 1 optical mode combiner 301, optical switch drive circuits 311 and 312,
It is composed of selectors 321 and 322. Also, four and seven optical switch element arrays 100 and 110 are provided, respectively.
It comprises optical switching elements 101 to 104, 111 to 117. Each of the optical switch elements has the configuration shown in FIG. 22. A thin film heater is mounted on the upper surface of one waveguide of the Mach-Zehnder interferometer, and
1 and 322 via optical switch driving circuits 311 and 312
Driven by

FIG. 3 shows an arrayed waveguide grating wavelength router 20.
0 is shown. The wavelength router includes at least four input waveguides (I _{1 to} I ₄ ) 201, a first slab waveguide 202, a waveguide array 203 whose waveguide lengths are different from each other by a fixed value, and a second slab. It comprises a waveguide 204 and seven output waveguides (O _{−3 to} O ₃ ) 205. For example, in the optical signal selection unit 411 of FIG. 1, each of the branch outputs of the optical splitters 31 to 34 is applied to the optical switch elements 101 to 104 of FIG. 2 (here, a Mach-Zehnder interferometer type 2 × 1 optical switch element is applied). (See FIG. 22). The ports 3 of the optical switch elements 101 to 104 are connected to one of the input waveguides 201 of the arrayed waveguide grating wavelength router 200. On the other hand, the seven output waveguides (O _{−3 to} O ₃ ) 205 of the arrayed waveguide grating wavelength router 200 are connected to the ports 2 of the optical switch elements 111 to 117, respectively. The ports 3 of the optical switch elements 111 to 117 are respectively connected to seven inputs of the optical mode combiner 301, and the output thereof is the output of the optical signal selecting means 411.

Next, the operation of each part of the optical signal selecting means shown in FIG. 2 will be described. Optical switch drive circuit 311
Drives the thin film heater of any one of the four optical switch elements 101 to 104 selected by the selector 321 and multiplexes the wavelength multiplexed light input to port 2 of the optical switch element. Lead to port 3. At this time, since the thin film heaters of the remaining three optical switch elements are not driven, no wavelength multiplexed light is output to port 3 of these optical switch elements. Thus, the wavelength multiplexed light is input to only one of the input waveguides 201 of the arrayed waveguide grating wavelength router 200.

Here, the arrayed waveguide grating wavelength router 20
0 is the input waveguide I_i(I = 1 to 4) and the wavelength λ_k(K = 1
4), the output waveguide O_kiNiru
(Ref. 4, Takahashi et al., "T
ransmission Characteristics of Arrayed Waveguide N
× N Wavelength Multiplexer ", Journal of Lightwave
Technology, IEEE, Vol.13, No.3, pp. 447-455, 199
Five). That is, the arrayed waveguide grating wavelength router 200
Has the wavelength routing characteristics shown in FIG. for example
If the input waveguide I_TwoWhen wavelength multiplexed light is input to
λ₁The optical signal of the output waveguide O_-1And the wavelength λ_TwoOptical signal
Force waveguide O₀And the wavelength λ_ThreeOptical signal is output waveguide O₁To
Wavelength λ_FourOptical signal is output waveguide O_TwoSeparated into
Is output. Therefore, one of the four input waveguides
Wavelength-division multiplexed light (λ ₁~ Λ_Four)
Of the seven output waveguides according to the input waveguide and wavelength
The output is output to each of the four lines (see shaded portions).

The optical switch drive circuit 312 drives the thin film heater of any one of the seven optical switch elements 111 to 117 selected by the selector 322, and sets the port 2 of the optical switch element. Is guided to the port 3 of the optical switch element. At this time, since the thin film heaters of the remaining six optical switching elements are not driven, no optical signals are output from these optical switching elements. Thus, an optical signal is input to only one of the seven input waveguides of the optical mode combiner 301.

The optical mode combiner 301 is an optical device that couples an optical signal propagating through seven single-mode optical fibers to a single multi-mode optical fiber (see reference numeral 1000). The configuration is described in “Quartz Waveguide 8 × 1 Optical Mode Combiner”, 1996 IEICE Electronics Society Conference C-160). The coupling loss from each single mode optical fiber to the multimode optical fiber can be reduced to 2 dB or less.

As described above, the optical signal selecting means selects any one of the four input optical fibers by the optical switch element array 100 and guides the wavelength multiplexed light propagating through the input optical fiber to the array. The waveguide grating wavelength router 200 demultiplexes the signal according to the input port and the wavelength, selects one optical signal by the optical switch element array 110, and outputs the multimode optical fiber 1 which is the output destination of the optical mode combiner 301.
000, any one of the total of 16 optical signals propagating through the four input optical fibers is selected and output.

In this configuration example, an optical cross-connect system in which a maximum of four optical signals are wavelength-multiplexed in four pairs of input / output optical fibers has been described. An optical cross-connect system can be configured in which a maximum of n optical signals are wavelength-multiplexed in each output optical fiber. The optical cross-connect system is m
1 × (mn) corresponding to each of the input optical fibers
Optical splitter, mn optical signal selecting means,
It comprises n wavelength converters and n × 1 optical multiplexers corresponding to each of the m output optical fibers.

Each of the optical signal selecting means includes a first optical switching element array composed of m optical switching elements, an arrayed waveguide grating wavelength router, and a second optical switching element composed of (m + n-1) optical switching elements. Two optical switch element arrays and (m
+ N-1) × 1 optical mode combiner.
Further, the arrayed waveguide grating wavelength router has m input waveguides I ₁ -I _m and (m + n−1) output waveguides O _1-m.
ＯOn _-1 and a wavelength λ in the input waveguide I _i (i = ₁ to _m ).
optical signal of _{k (k} = _1~n) is designed to be routed to output waveguide O _ki as they are entered. At this time, the wavelength multiplexed light (n optical signals) input to any one of the m input waveguides is converted into (m + n-1) output waveguides according to the input waveguide and the wavelength. Are demultiplexed into n lines and output.

Next, advantages of the optical signal selecting means shown in FIG. 2 will be described. First, each optical switch element uses only one input port and one output port.
Therefore, in the conventional Mach-Zehnder interferometer type 2 × 1 optical switch element described with reference to FIG. 22, not the port 1 in which crosstalk is likely to occur due to a manufacturing error, but only the port 2 in which crosstalk does not occur in principle. be able to. For this reason, it is not necessary to select and use an optical switch element whose extinction ratio has not deteriorated due to a manufacturing error, which substantially increases the yield and can reduce the cost.

In the crosstalk generated in the optical switching element array 100, the optical switching element array 110
Since the component having the same wavelength as the optical signal selected in the step (1) is split by the arrayed waveguide grating wavelength router 200, no beat noise is generated. This is shown in FIG. Here, an example is shown in which the optical switch elements 102 and 113 are driven to select an optical signal of wavelength λ ₁ that has propagated through the input optical fiber 12.

Here, a component that passes through the optical switch element that is not driven causes crosstalk. Crosstalk in optical switching element array 100 is led to the input waveguide I _1, I _3, I ₄ arrayed waveguide grating wavelength router 200 from the optical switch element 101, 103, 104.
The components of the wavelength λ _{1 in} the crosstalk are output waveguides O ₀ , O _-2 , O of the arrayed waveguide grating wavelength router 200.
_-3 is guided to the optical switch elements 114, 112, 111 and is not input to the driven optical switch element 113. As described above, since the crosstalk components having the same wavelength as the selected optical signal do not overlap, no beat noise is generated. Therefore, unlike the conventional example described with reference to FIG. 21, the optical switch element array 100 does not require a high extinction ratio.

In FIG. 5, the optical switch element array 10
Crosstalk per optical switch element at 0,110
E₁, E_TwoIs the same as the selected optical signal.
Crosstalk of the same wavelength is (m-1) · e₁・ E_Two Is
(M is the number of input / output optical fiber pairs). This should be -30dB or less
To make it lower, e is for m = 4₁= E_Two <-18 dB
And it is sufficient. On the other hand, crosstalk of different wavelengths is (m-1) · e₁+ (n-1) ・ e_Two+ (m-1) ・ (n-1) ・ e₁・ E_Two (N is the number of wavelengths), this is set to -12 dB or less.
In the case of n = 4, e₁= E_TwoIf <-20dB
Good. Therefore, in the configuration shown in FIG. ₁= E_Two
<−20 dB, a bit error rate of 10^-12In
Power penalty can be reduced to less than 0.5dB
it can.

Furthermore, in the optical signal selecting means of the present invention shown in FIG. 2, any one of the optical switch element arrays 100 and 110 may be driven to be turned on by driving either one of them. Only two optical switch driving circuits are required, and the size and power consumption can be reduced. Here, for various values of m and n, the optical signal selection means according to the present invention calculates the extinction ratio required for the optical switch element and the required number of optical switch drive circuits, and the results are shown in FIGS. Show. For comparison, in the conventional optical cross-connect system shown in FIG.
The required extinction ratio and the required number of optical switch drive circuits when using an optical switch element were also calculated. In these figures, n indicates the number of wavelengths for each optical fiber. As shown in FIG. 6, the extinction ratio required per optical switch element is relaxed by about 10 dB. Further, as shown in FIG. 7, the optical signal selecting means of the present invention has an extinction ratio of −25 dB to −25 dB.
It can be seen that it can be configured with only one type of optical switch element of about 20 dB. The required number of optical switch drive circuits may be two irrespective of the number m of input / output optical fiber pairs and the number n of wavelengths, so that the size and power consumption can be reduced.

(Another Configuration Example of Optical Splitter) FIG.
A configuration example in which × 16 optical splitters 31 to 34 are divided is shown. This configuration is equivalent to the 1 × 16 optical splitter 31 shown in FIG.
To 34 each of five 1 × 4 optical splitters 35 ₁ to 35 ₁
35 _5, 36 ₁ to 36 _5, 37 ₁ to 37 _5, 38 which is constituted by ₁ to 38 _5. In general, a 1 × (mn) optical splitter can be composed of a 1 × m optical splitter and m 1 × n optical splitters. As a result, optical wiring between the input optical fiber and each optical signal selecting means is simplified.
Further, there is an advantage that an optical splitter and this optical wiring can be added for each pair of input and output optical fibers.

Next, FIG. 24 shows an embodiment of the optical fiber wiring for each of the five locations enclosed by the broken lines in FIG.
A two-dimensional fiber array is formed by stacking four layers of tape-type four-core optical fibers (# 1 to # 4), and two sets of such fiber arrays (# 1 to # 4 and # 1 'to # 4') are interconnected. To 90
The connection can be easily achieved by connecting the wires at an angle. Generally, a two-dimensional fiber array in which the input side n-core optical fiber array is stacked in m layers and a two-dimensional fiber array in which m-core optical fiber arrays are stacked in n layers are tilted by 90 degrees so that they are essentially orthogonal to each other. Desired connection can be easily obtained.

Instead of providing a large number of 1 × 4 optical splitters (as shown in FIG. 8) instead of the 1 × 16 optical splitters 31 to 34 of FIG. 1, FIG. 25 shows 1 × 16 optical splitters 31 to 34.
34 and the two-dimensional fiber arrays 801 to 8 having the above-described configuration.
4 shows an example of a wiring provided with No. 04. Here, each output of the 1 × 16 optical splitters 31 to 34 is constituted by a tape-type four-core optical fiber × 4, and one tape is connected to each of the optical signal selecting means 411 to 444 via a wiring as shown in FIG. A mold 4-core optical fiber is connected. That is, the two-dimensional fiber array 80
By using 1 to 804, the 1 × 16 optical splitter is not divided into a large number of 1 × 4 optical splitters, and an increase in the number of divisions of the optical splitter does not cause an increase in excess loss. Optical wiring becomes possible.

(Exemplary Configuration of Wavelength Converter) Wavelength Converter 711
744 may have a function of converting an input optical signal of an arbitrary wavelength into an optical signal of a preset wavelength. For example, an optical regenerative repeater that drives an optical transmitter with an electrical signal that is identified and reproduced by an optical receiver, or an all-optical repeater that uses a semiconductor element may be used. In addition, a common light source unit for generating a plurality of optical carriers having different wavelengths and distributing the output to each wavelength converter is provided, an optical receiver as each wavelength converter, and an optical modulator for modulating the distributed optical carriers. May be used. FIG. 9 shows an example of this configuration.

In FIG. 9, a common light source 7000 includes a semiconductor laser array 701 and a 1 × 4 optical splitter 702.
To 705. Wavelength converters 711-714
(-744) are each composed of an optical receiver 706 and an optical modulator 707. In this case, since the same optical component can be used for the optical modulator regardless of the wavelength, all the wavelength converters are configured with a common optical component. Therefore, in FIG. 1, if the wavelength converters 711 to 744 corresponding to the respective optical signal selecting means 411 to 444 are accommodated in the same housing, all 16 housings are formed from the same optical component. In addition, any enclosure can be replaced at the time of expansion or failure.

(Another Configuration Example of Optical Multiplexer) Optical multiplexers 81 to 81
84 may have a function of multiplexing four optical signals having different wavelengths into one optical fiber, and has no wavelength dependency.
A small and inexpensive optical coupler may be used. However, in this case, a joining loss of 10 · Log n (n is the number of wavelengths) is incurred. Further, when a failure occurs in the wavelength converter and the wavelength of the output optical signal fluctuates, there is a possibility that other optical signals to be multiplexed are adversely affected.

(Other Configuration Examples and Other Driving Examples of Optical Switching Element) In FIG. 2, the case where a Mach-Zehnder interferometer type optical switching element is used as each optical switching element has been described. Any optical switch which can be used and has a desired extinction ratio as shown in FIG. 6 may be used. For example, a self-holding single-mode optical fiber switch in which an ultra-light magnetic film pipe is mounted on an optical fiber and which is moved by electromagnetic force to intermittently emit light can be used. For details of this optical switch, see Reference 6 (Nagaoka,
"Development of a compact and high-performance self-holding single-mode optical fiber switch", IEICE Technical Report, OQE93-11
9, OCS93-55, pp. 67-72, 1993). Further, a configuration using a Y-branch type optical switch using a thermo-optic effect or a semiconductor optical amplifier as an optical gate may be used.

In FIG. 2, each of the optical switch element arrays 100 and 110 includes an optical switch driving circuit 311 and 3.
12 and the selectors 321 and 322, each optical switch element may be provided with an optical switch drive circuit without the selector. Although the number of optical switch drive circuits increases, the extinction ratio can be improved because the OFF state can be finely adjusted. However, since only one optical switch element is turned on, the power consumption can be reduced even in this case.

(Another Configuration Example of Optical Combining Means) In FIG. 2, an optical mode combiner 301 for coupling an optical signal propagating through a plurality of single mode optical fibers into a single multimode optical fiber is used as the optical combining means. Although used, it may be coupled to a single single mode optical fiber using a normal optical coupler. In this case, the fundamental confluence loss 10
Log (n + m-1) [dB].

(Second Configuration Example of Optical Signal Selection Means) If a circular wavelength router is used as the arrayed waveguide grating wavelength router 200 shown in FIG. 2, the number of optical switching elements in the optical switching element array 110 can be reduced. it can. Here, the circulating wavelength router has n pairs of input / output ports corresponding to the number n of wavelengths, and has circulating properties (Latin square characteristics) in wavelength routing characteristics between the input / output ports. Hereinafter, description will be made with reference to FIGS.

FIG. 10 shows a second example of the configuration of the optical signal selecting means using the circular wavelength router. The first shown in FIG.
The difference from the configuration example of FIG.
0 is provided with four output waveguides O _{0 to} O ₃ , the optical switch element array 110 is made up of four optical switch elements 114 to 117 and provided with a 4 × 1 optical mode combiner 302 as an optical converging means. .

The arrayed waveguide grating wavelength router 210 applies a wavelength λ _k (k = 1 to _k ) to the input waveguide I _i (i = 1 to m).
When the optical signal of 4) is input, it is designed to be routed to the output waveguide O _{(ki) mod 4} . Where x mo
dy represents a remainder (remainder) obtained by dividing x by y. That is, the arrayed-waveguide grating wavelength router 210 has a wavelength-routing characteristic shown in FIG. 11, for example, an input waveguide I ₂
When wavelength-division multiplexed light having wavelengths λ ₁ to λ ₄ is input to the optical waveguide, an optical signal having a wavelength λ ₁ is output to an output waveguide O ₃ , an optical signal having a wavelength λ ₂ is output to an output waveguide O ₀ , and an optical signal having a wavelength λ ₃ is output. Is output to the output waveguide O ₁ , and the optical signal of the wavelength λ ₄ is output to the output waveguide O ₂ . Therefore, the four optical signals input to one of the four input waveguides and wavelength-division multiplexed are demultiplexed into four output waveguides according to the input waveguide and the wavelength and output. Is done. The details and the design of the circulating wavelength router are described in the above-mentioned reference 4.

If a circulating wavelength router is used, generally the maximum
And up to n lights in each of n pairs of input and output optical fibers
The optical cross-connect system in which signals are wavelength-multiplexed
1 can be configured. At this time, n input optical
1 × n corresponding to each bus ^TwoOptical splitter and n^TwoPieces
Optical signal selecting means, and n^TwoWavelength converters and n outputs
N × 1 optical multiplexer corresponding to each power optical fiber
Prepare. Each optical signal selecting means is composed of n optical switch elements.
Optical switching element array comprising a circulating wavelength router
And a second optical switch element comprising n optical switch elements
And an n × 1 optical mode combiner.
You. This circulating wavelength router has n input ports I₁~ I_n
And n output ports O₀~ O_n-1And the input port I
_iWavelength λ_kWhen an optical signal of (k = 1 to n) is input,
Power port O_{(ki) mod n}Designed to be routed to
I do. Therefore, any one of the n input ports
The input wavelength-multiplexed n optical signals are
Output to each of the n output ports according to the port and wavelength.
It is.

As described above, in the second configuration example using the circulating wavelength router, in addition to the effect described in the first configuration example of the optical signal selecting means, there is an effect that the number of optical switch elements can be reduced. . In the second configuration example, the arrayed waveguide grating wavelength router 210 having the wavelength routing characteristics shown in FIG. 11 is used as the wavelength router.
A similar function can be realized by combining a plurality of optical demultiplexers 221 to 224 and a plurality of optical multiplexers 225 to 228 shown in FIG. Optical splitters 221 to 224 and optical multiplexer 2
The wiring between 25 and 228 corresponds to the wavelength routing characteristic shown in FIG. That is, for example, the four outputs of the optical demultiplexer 221 are connected in a loop from the first input port of the optical multiplexer 225 to the fourth input port of the optical multiplexer 228. If the effect of the optical loss can be ignored, the optical splitter may be replaced with an optical splitter, or the optical multiplexer may be replaced with an optical multiplexer.

The routing characteristics of the wavelength router are not limited to the characteristics shown in FIG.
The same optical signal selection means can be configured with the characteristics shown in FIG. As shown in FIGS. 11 and 13, all different symbols (here, λ ₁ to
The matrix in which [lambda] ₄ ) is arranged is called a Latin square, and it is known that there are many others (Reference 7: RA Barry,
et al., "Latin Routers, Design and lmplementatio
n ", Journal of Lightwave Technology, IEEE, Vol. 1
1, No. 5/6, pp. 891-899, 19931993), any of which may be used.

(Third Configuration Example of Optical Signal Selection Means) FIG.
9 shows a third configuration example of the optical signal selecting means which is a feature of the present invention. The difference from the first configuration example shown in FIG. 2 is that the input waveguide and the output waveguide of the arrayed waveguide grating wavelength router 200 are extracted in the same direction, and the optical switch element array as the first and second optical switching means. Instead of using 100 and 110, a single optical switch element array 120 composed of 11 optical switch elements is used.

In this configuration example, in addition to the advantages described above, only one optical switch element array is required, and the number of optical components is reduced. Further, the connection with the arrayed waveguide grating wavelength router 200 is required at one place, and the number of assembly steps is reduced. Further, according to the present invention, the optical signal selecting means can be constituted only by a single kind of optical switching element having a low extinction ratio, so that the integration ratio of the optical switching elements becomes possible, and miniaturization and cost reduction are realized. Can be planned. In this configuration example, a circular wavelength router similar to that of the second configuration example may be used.

In the optical signal selecting means shown in FIG. 14, the optical switch element array 120, the arrayed waveguide grating wavelength router 200, and the 7 × 1 optical mode combiner 301, all enclosed by a point, are all quartz waveguide planar optical circuits. It can be manufactured with. That is, they can be integrated on a single quartz substrate. In this case, the number of optical components and the assembly process are further reduced, and further reduction in size and cost can be expected.

(Fourth Configuration Example of Optical Signal Selection Means) FIG.
Shows a fourth configuration example of the optical signal selection means which is a feature of the present invention. The difference from the third configuration example shown in FIG. 14 lies in that a reflection type arrayed waveguide grating wavelength router 220 is used instead of the arrayed waveguide grating wavelength router 200. As a result, in addition to the advantages described above, further downsizing can be expected.

Here, the reflection type arrayed waveguide wavelength grating router 220 utilizes the symmetry of the arrayed waveguide grating wavelength router described with reference to FIG. By doing so, a wavelength router is realized using only a single slab waveguide. Hereinafter, FIG.
The principle of the reflection-type arrayed waveguide grating wavelength router 220 will be described with reference to FIGS. For more information,
For example, Reference 8 (Inoue et al., "Optical Branch Circuit with Wavelength Routing Function", IEICE Technical Report, OPE96-2,
pp. 7-12, 1996).

FIG. 16A is an example of the configuration of an 11 × 11 arrayed waveguide grating wavelength router. It has eleven input waveguides I _{1 to} I ₁₁ and eleven output waveguides O _{1 to} O ₁₁ , and an optical signal having a wavelength λ _k is input to the input waveguide I _i (i = 1 to 11). And the output waveguide O _{k-i + 8} . That is, it has the wavelength routing characteristics shown in FIG. At this time, wavelength multiplexed light (four optical signals λ ₁₎ input to any one of I _{1 to} I ₄ of the input waveguides
To λ ₄ ) are separated and output to O _{5 to} O ₁₁ of the output waveguide according to the input waveguide and the wavelength.

Here, the arrayed waveguide grating wavelength router shown in FIG. 16A has a symmetrical structure with respect to a center line indicated by a chain line. Therefore, if the high reflection film 206 is added to the middle point of the arrayed waveguide 203 and then folded, FIG.
As shown in (b), input waveguides I _{1 to} I ₄ and output waveguides O ₅ to O ₅ .
O ₁₁ can be arranged in the same direction. In this way, a wavelength router having desired wavelength routing characteristics can be manufactured in half the size.

[0065]

According to the present invention, in each optical signal selecting means, only one wavelength multiplexed light is selected via the first optical switching means, and only one of the wavelength multiplexed lights is selected via the second optical switching means. Since one optical signal is selected,
The number of drive circuits for the switching operation can be reduced (to two), and the size and power consumption of the drive circuit can be reduced.

Further, m in the first optical switching means
Of the crosstalk caused by the optical path, the component having the same wavelength as the optical signal selected by the second optical switching means is guided to a different optical path of the second optical switching means by the wavelength router. Does not generate beat noise. Therefore, both the first and second optical switching means can be constituted by optical switches having a low extinction ratio. Each of these two optical switching means has a plurality of light passages.
Since it has the same function of allowing only one to pass, it can be composed of the same type of optical switch. As a result, integration becomes easy, and low cost and miniaturization can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of an optical cross-connect system of the present invention.

FIG. 2 is a diagram illustrating a first configuration example of an optical signal selection unit.

FIG. 3 is a diagram showing a configuration of an arrayed waveguide grating wavelength router 200.

FIG. 4 is a diagram showing wavelength routing characteristics of an arrayed waveguide grating wavelength router 200.

FIG. 5 is a diagram for explaining the principle of suppressing generation of beat noise by the arrayed waveguide grating wavelength router 200.

FIG. 6 is a diagram showing a calculation example of a required extinction ratio per optical switch element.

FIG. 7 is a diagram showing a calculation example of a required number of optical switch driving circuits.

FIG. 8 is a diagram showing a configuration example in which the 1 × 16 optical splitters 31 to 34 of FIG. 1 are divided.

FIG. 9 is a diagram illustrating a configuration example of a wavelength converter.

FIG. 10 is a diagram showing a second configuration example of the optical signal selection means.

FIG. 11 is a diagram showing a wavelength routing characteristic of an arrayed waveguide grating wavelength router having a circularity.

FIG. 12 is a diagram showing a configuration of another wavelength router having the wavelength routing characteristics of FIG. 11;

FIG. 13 is a diagram showing an example of another wavelength routing characteristic.

FIG. 14 is a diagram showing a third configuration example of the optical signal selection unit.

FIG. 15 is a diagram showing a fourth configuration example of the optical signal selection means.

FIG. 16 is a diagram illustrating the relationship between an 11 × 11 arrayed waveguide grating wavelength router and a reflection type.

FIG. 17 is a diagram showing wavelength routing characteristics of an 11 × 11 arrayed waveguide grating wavelength router.

FIG. 18 is a diagram showing a configuration example of a conventional optical cross-connect system using a wavelength multiplexing technique.

FIG. 19 is a diagram showing a configuration example of a conventional optical cross-connect system using an optical signal selection unit.

FIG. 20 is a diagram showing a configuration example of 4 × 1 optical switches 511 to 544.

FIG. 21 is a view for explaining crosstalk in 4 × 1 optical switches 511 to 544;

FIG. 22: 2 × quartz waveguide Mach-Zehnder interferometer
The figure which shows the basic structure of one optical switch element.

FIG. 23 is a diagram illustrating a configuration example of variable wavelength selection elements 611 to 644 and crosstalk thereof.

FIG. 24 is a diagram showing an example of wiring by a two-dimensional fiber array using a tape-type four-core optical fiber.

FIG. 25 is a diagram showing a configuration example in which wiring is performed between the optical splitter and the optical signal selection unit in FIG. 1 by a two-dimensional fiber array using a tape-type four-core optical fiber.

[Explanation of symbols]

 11 to 14 input optical fiber 21 to 24 1 × 4 optical demultiplexer 30 16 × 16 optical switch 31 to 34 1 × 16 optical splitter 81 to 84 4 × 1 optical multiplexer 91 to 94 output optical fiber 100 , 110, 120 Optical switch element array 101-104, 111-117 Optical switch element 151-153 2 × 1 optical switch element 161, 162 Directional coupler 163, 164 Single mode waveguide 165 Thin film heater 200, 210 Array conductor Waveguide grating wavelength router 201 Input waveguide 202 First slab waveguide 203 Waveguide array 204 Second slab waveguide 205 Output waveguide 206 High reflection film 220 Reflective array waveguide grating wavelength router 221 to 224 Optical splitter 225 to 228 Optical multiplexers 301 and 302 Optical mode combiners 311 and 312 Optical switches Drive circuit 321,322 Selector 411-414,421-424 Optical signal selecting means 431-434,441-444 Optical signal selecting means 511-514,521-524 4 × 1 optical switch 531-534,541-544 4 × 1 optical switch 601 1 × 4 optical demultiplexer 602 4 × 1 optical switch 611-614, 621-624 Variable wavelength selecting element 631 634, 641 644 Variable wavelength selecting element 711 714, 721 724 Converters 731 to 732, 741 to 744 Wavelength converters 701 Semiconductor laser arrays 702 to 705 1 × 4 optical splitters 706 Optical receivers 707 Optical modulators

Continuation of front page (56) References JP-A-8-227059 (JP, A) JP-A-9-247179 (JP, A) JP-A-5-188408 (JP, A) JP-A-3-219793 (JP) JP-A-5-130038 (JP, A) JP-A-8-116562 (JP, A) JP-A-7-75145 (JP, A) Osamu Ishida, et al. , Modular cross-connect system for WDM optical-path networks, ECOC 97, IEEE, September 22, 1997, Conferencing Publication No. 448, p63-66 (58) Fields investigated (Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 H04Q 3/52 G02B 6/00 G02B 6/00 H04Q 11/02 JICST file (JOIS)

Claims (7)

(57) [Claims] 1. An optical cross for wavelength-multiplexing optical signals constituting wavelength-division multiplexed light propagating in m (m is an integer of 2 or more) input optical fibers again and exchanging and outputting the resulting signals to m output optical fibers. In a connect system, a plurality of optical branching means for branching wavelength-multiplexed light input from each of the input optical fibers into a plurality of wavelength-multiplexed lights and outputting the plurality of wavelength-multiplexed lights, a plurality of wavelengths branched by the optical branching means A plurality of optical signal selecting means to which the multiplexed light is inputted, wherein each of the optical signal selecting means is inputted with the wavelength multiplexed light from the m input optical fibers, and the optical signal selecting means is inputted optical signal selecting means for selecting one of the m wavelength multiplexed lights, selecting and outputting an optical signal of one wavelength from the selected wavelength multiplexed light, and output from each of the optical signal selecting means Optical signal A plurality of wavelength converting means for converting the wavelength-converted optical signal into wavelength optical signals, the optical signal output from each of the wavelength converting means connected to the m output optical fibers, and wavelength-multiplexed light is multiplexed on these output optical fibers. A plurality of optical multiplexing means for respectively outputting the light, each of the optical signal selecting means has m light passages corresponding to the m wavelength multiplexed light, and wavelength multiplexing passing through one of them. Let only light pass
First optical switching means for blocking the remaining light passages, m input ports and p output ports respectively connected to the m light passages, where p is equal to or greater than the wavelength multiplexing number of the wavelength multiplexed light. A wavelength router that splits the wavelength multiplexed light that has passed through the first optical switching means into optical signals having a plurality of different wavelengths, and outputs these split optical signals to different output ports. Wherein the output destination of each demultiplexed optical signal comprises a wavelength router having different routing characteristics for each input port, and p optical passages connected to p output ports of the wavelength router. A second optical switching means for passing only an optical signal passing through one of them, and cutting off the remaining optical passages; Light converging means Optical cross-connect system characterized by comprising. 2. The first and second optical signal selecting means of each of said optical signal selecting means.
The optical cross-connect system according to claim 1, wherein the optical switching means is integrated on one substrate. 3. The optical cross-connect system according to claim 2, wherein the wavelength router and the optical converging means of each of the optical signal selecting means are also integrated on the substrate. 4. An input-side m tape-type n-core optical fiber (n is the number of multiplexed wavelengths of the wavelength-division multiplexed light) and an output side as means for coupling the optical branching means and the optical signal selecting means 2. The optical cross-connect system according to claim 1, further comprising a two-dimensional fiber array in which the n tape-type m-core optical fibers are stacked and connected in a direction substantially orthogonal to each other. 5. The optical cross-connect system according to claim 1, wherein the wavelength router of each optical signal selecting means is an arrayed waveguide grating wavelength router. 6. The optical cross-connect system according to claim 5, wherein said arrayed waveguide grating wavelength router is of a reflection type. 7. The optical cross-connect system according to claim 5, wherein the wavelength router of each of said optical signal selecting means is a circular wavelength router.