JP3037833B2 - Multi-terminal optical delay compensation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for compensating for optical delay of a multi-terminal optical switch having a function of arbitrarily changing the order of optical paths of a plurality of light beams propagating in space.

[0002]

2. Description of the Related Art A two-input / two-output unit switch is connected in multiple stages, and a one-to-one connection is established between N input terminals and N output terminals.
As a switch to be connected, there is a matrix switch based on a crossbar type configuration. FIG. 1 shows a wiring diagram of a matrix switch having N inputs and N outputs (specifically, 8 inputs and 8 outputs) as an example. In FIG. 1, a solid line indicates a wiring in the network.
The rectangles in the figure are unit switches having two inputs and two outputs, which are controlled by an external device to connect input-output 1, input 2-output 2 (through), input 1-output 2, and input 2-output. One of two connection forms (cross) of the output 1 is adopted. In order to connect the input signal from the i-th input terminal in the drawing to the j-th output terminal using this switch, a unit switch (see FIG. 1) located at the intersection of the i-th input terminal and the j-th output terminal In the drawing, the hatched unit switches) are in a cross state, and the other unit switches connected to the i-th input terminal or the j-th output terminal (unit switches in the area within the dashed line frame in the figure) ) May all be in a through state.

This matrix switch has an advantage that a switch network having N input terminals and N output terminals can be realized with a simple configuration, and a signal propagation path in the network can be set easily and unblocked. However, since N ² unit switches are required for the number of terminals N, the number of components becomes very large as N increases. In addition, when the signal is attenuated when passing through the unit switch, the number of unit switches passing therethrough becomes very large as N increases, so that there is a disadvantage that the passage loss increases accordingly.

[0004] In contrast, the unit switches with two inputs and two outputs connected cascade, to realize all combinations of connecting one-to-one between the 2 ^N pieces of input terminals and the 2 ^N output terminals that A switch network configuration method has been devised. FIG.
Is a wiring diagram of a switch network of 32 inputs and 32 outputs based on the above configuration method. The solid line in FIG. 2 indicates the wiring in the network. The rectangle is a unit switch having two inputs and two outputs, and a connection form (through) between input 1 and output 1 and input 2 and output 2 and input 1 and output 2 and input 2 and output 1 are controlled by external control. It takes either one of the connection forms (cross).

The circuit network shown in FIG. 2 has a configuration in which switch stages in which unit switches are arranged in an array and wiring networks connecting adjacent switch stages are alternately arranged. In the case of FIG.
The switch network is composed of nine switch stages and eight wiring networks. In general, the switch network of the form is extended to easily constructed having the 2 ^N input terminals and the 2 ^N output terminals. At this time, the switch network has 2 ^N-1 per stage.
(2N-1) unit switch arrays each including one unit switch, and one of the outputs of each unit switch for one, two, and four switches in the next-stage switch array. Wiring networks connected to the unit switches whose positions are spatially shifted by ^N-2 pieces are formed by a total of (2N-2) wiring networks in two stages.

As a multi-terminal optical switch optically realizing the above-mentioned switch network, an academic paper, “Relocationable multi-channel free-space optical switch (A based on a multi-stage network configuration) (A)
rearrangeable multichannel free-space opticalswitc
h based on multistage network configuration) ", IEEEE J. Lightwave Tec
h.), Vol. 9, pages 1726-1732.

FIG. 3 shows the configuration of the multi-terminal optical switch described in the above-mentioned document. This switch is an example of a polarization multiplexing type 32 × 32 channel switch corresponding to the circuit network shown in FIG. In FIG. 3, reference numerals 1-1 and 1-2 denote collimated optical signal input terminals for inputting the collimated optical signal to the switch body. Reference numerals 2-1 and 2-2 denote polarization beam splitters that transmit a horizontally polarized wave (P wave) as it is and reflect a vertically polarized wave (S wave) in a 90 ° direction on an internal reflection surface. Reference numerals 3-1 to 3-9 denote liquid crystal spatial light modulator arrays, each of which has a function of switching the rotation angle of the polarization plane of light transmitted through each cell to 0 ° or 90 ° for each cell.
Reference numerals 4-1 to 4-8 denote polarization routing elements, which spatially shift the incident beam according to the polarization state thereof, and perform the same function as the wiring network of FIG. Reference numerals 5-1 and 5-2 denote collimated optical signal receiving terminals for receiving a collimated optical beam transmitted through the switch. This switch uses each cell of the liquid crystal spatial light modulator array 3-1 to 3-8 as a unit switch, and inputs and outputs to and from the unit switch are two linearly polarized light beams orthogonal to each other, so that the polarization plane of the passing light beam is changed. Is made to correspond to the operation of the unit switch. Accordingly, the liquid crystal spatial light modulator arrays 3-1 to 3-9 correspond to the switch stages of FIG. 2, and the polarization routing elements 4-1 to 4-8.
Corresponds to a wiring network.

[0008] The P-wave optical signal emitted from the collimated optical signal input terminal 1-1 and the S-wave optical signal emitted from the collimated optical signal input terminal 1-2 are converted into a polarization beam splitter 2-1.
And incident on the switch body. The polarization plane of the optical signal incident on the switch body is controlled by the liquid crystal spatial light modulator array 3, the optical path is shifted by the polarization routing element 4 depending on the polarization state, and finally has the target position and polarization state. The light exits from the switch body as a collimated light beam. The light beam emitted from the switch body is separated into a P-wave optical signal and an S-wave optical signal by the polarization beam splitter 2-2, and the P-wave component and the S-wave component are collimated optical signal receiving terminals 5-1 and 5 respectively. -2
Received by

As the routing element 4 applied to the present switch, the elements shown in FIGS. 4A and 4B are disclosed. In FIG. 4, reference numeral 12 denotes a polarization plane conversion element array, which includes an element having a function of exchanging horizontal polarized light and vertical polarized light such as a half-wave plate and an optical rotator (hatched portion in the figure) and a normal glass plate or the like. It has a structure in which elements having no polarization plane exchange function (open areas in the figure) are alternately arranged. 13
Is a polarizing beam splitter, 14 is a polarizing beam splitter 13
It is the same size glass rod. FIG. 4 (a) has a structure in which the polarization plane conversion element array 12 is inserted between the polarizing beam splitters 13 which are stacked to form an array. FIG. 4B shows a first polarizing beam splitter array in which polarizing beam splitters 13 are stacked, and a second polarizing beam splitter 13 in which polarizing beam splitters 13 and glass rods 14 are alternately stacked.
And a polarizing beam splitter array.

FIG. 4 shows an optical path in the element of the incident horizontally polarized light (solid line) and vertically polarized light (dashed line). Regarding any of the elements, it can be seen that one polarized light component passes through the element without being shifted, and the other polarized light component is shifted and the optical paths are exchanged with each other. The magnitude of the shift of the optical path at this time is equal to the length of one side of the polarizing beam splitter 13. Therefore, by changing these sizes, the shift amount of the optical path can be adjusted, and a routing element having an interval between adjacent beams twice, four times,... Can be manufactured.

[0011]

By the way, as is apparent from FIG. 4, the light beams passing through the routing element have different distances in the element depending on the incident position and the plane of polarization. Therefore, when these routing elements are used, the present optical switch has a different propagation time for each incident light beam. For this reason, a phase difference corresponding to the propagation time difference is generated in the optical signal passing through the switch. At this time, if the optical signal passing through the switch has a high bit rate and the signals are synchronized with each other, the above-described phase difference causes the synchronization to be lost, which adversely affects the transmission characteristics.

[0012]

According to the present invention, as described above, the present invention is applied to a high-speed optical signal by applying an optical delay compensation circuit for removing a propagation time difference of an optical signal passing through the inside of a switch. It is intended to realize an optical switch that operates stably without being affected by jitter.

According to the present invention, there are provided means for inputting a plurality of collimated incident signal lights, which are arranged in a grid or in a row and in parallel, into a circuit, and a plurality of collimated incident signal lights which are emitted from the circuit and are arranged in a grid, in a row or in parallel. Means for emitting the collimated light beam out of the circuit, and allowing the two linearly polarized light components orthogonal to each other of the passing light beam to pass as it is,
Alternatively, a polarization plane control element array in which a plurality of polarization plane control elements that are controlled to exchange one of these linear polarization components is arranged, and the polarization of two polarization components whose main direction should give a delay time difference. An optical delay element configured of a transparent medium having birefringence arranged so as to coincide with a plane, thereby allowing each of two mutually orthogonal linearly polarized light components of a passing light beam to pass with different propagation delay times. , Are arranged alternately.

According to the present invention, there is provided a polarizing beam splitter in which a part of the optical delay element transmits one of two linearly polarized beams orthogonal to each other and reflects the other in a direction perpendicular to the entering direction. It is characterized by comprising a polarizing beam splitter array in which a plurality of the reflected linearly polarized lights are stacked in the traveling direction.

The present invention compensates for the propagation time difference of an optical signal generated inside a multi-terminal optical switch, and achieves the above object with a simple and stable configuration by switching light polarization. It is a distinctly different technology.

An embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[0017]

When the routing element shown in FIG. 4 is applied to the multi-terminal optical switch shown in FIG. 3, the difference in propagation delay time of light passing through the routing element is determined by the polarization beam splitter constituting the element. This occurs because the light is reflected in a direction perpendicular to the incident light and the emitted light. The propagation delay time difference due to such reflection is always equal to an integral multiple of the time required for light to travel by the length of one side of the polarizing beam splitter. In the case of the above switch, the length of one side of the polarizing beam splitter is all an integral multiple of the cell interval of the liquid crystal spatial light modulator, so that it is necessary for light to pass through glass having a thickness equal to this cell interval. By compensating for a delay time difference that is an integral multiple of time, the problem of jitter or signal phase difference caused by the delay time difference can be solved.

FIG. 5 shows an embodiment of the present invention. In FIG. 5, reference numerals 21-1 to 21-5 denote liquid crystal spatial light modulator arrays, each of which has a function of switching the rotation angle of the plane of polarization of light transmitted through each cell to 0 ° or 90 ° for each cell. ing.
Reference numeral 22 denotes a birefringent crystal, the main axis direction of which is perpendicular to the light incident direction, and the liquid crystal spatial light modulator array 21-1.
21-5 correspond to the cell arrangement directions. Also, 23-1
23-3 are polarizing beam splitter arrays, and the length of one side thereof is arranged to be 1, 2, 4 times the cell interval of the liquid crystal light modulator arrays 21-1 to 21-5 in order. I have.

The polarizing beam splitter arrays 23-1 to 23-
3 allows the P-wave light to pass directly. On the other hand, the S-wave light is shifted downward by the length of one side of the splitter by the preceding polarization beam splitter, and then shifted upward by the length of one side of the splitter by the subsequent polarization beam splitter. For this reason, the S wave does not seem to be shifted in the optical path, and the polarization beam splitter arrays 23-1 to 23-1 do not appear.
It passes through 23-3, but experiences a propagation delay corresponding to the time it takes to travel twice the length of one side of the splitter compared to the P-wave. Therefore, the polarization beam splitter arrays 23-1 to 23-1
Between the P wave and the S wave passing through 32-3, twice the cell interval of the liquid crystal spatial light modulator arrays 21-1 to 21-5, respectively,
A propagation delay time difference equal to the time required to pass through a glass four or eight times thicker is provided.

The polarization beam splitter arrays 23-1 to 23-
3 can be limited to an even multiple of the time required to pass through glass having a thickness equal to the cell spacing of the liquid crystal spatial light modulator arrays 21-1 to 21-5. For this reason, a propagation time difference equal to the time required to pass through glass having a thickness of one time the cell interval needs to be realized using the birefringent crystal 22.

FIG. 6 shows the arrangement of the birefringent crystal 22 with respect to the incident light beam and the path of the incident light beam. In FIG. 6, 31
Indicates the path of the ordinary ray, and 32 indicates the path of the extraordinary ray. An arrow or a black dot attached to each light beam indicates a vibration direction of an electric field of each light beam. Further, thick arrows in the drawing indicate the main axis direction of the birefringent crystal. As shown in this figure, when the vibration direction of the electric field of the extraordinary ray is parallel to the main axis, the ordinary ray and the extraordinary ray do not separate from each other as in ordinary birefringence, but pass through the same path. In the case of a uniaxial crystal, the refractive index for an extraordinary ray having a vibration direction of an electric field parallel to the main axis is n.
is _e, the refractive index for the ordinary ray having the vibration direction of the electric field perpendicular to the main axis is n _o. Therefore, the birefringent crystal 22
When both are arranged as shown in FIG. 6, both the ordinary ray and the extraordinary ray propagate inside the crystal without being shifted in the optical path, and a propagation delay time difference occurs between the ordinary ray and the extraordinary ray.

At this time, if the thickness of the crystal is t, the delay time difference τ _d is

(Equation 1) It is expressed as However, c is the speed of light in a vacuum, n _o, n _e
Represents a refractive index of an ordinary ray and an extraordinary ray, respectively. On the other hand, the cell spacing of the liquid crystal spatial light modulator arrays 21-1 to 21-5 is d,
If the refractive index of the glass forming the polarization beam splitter array 23-1～23-3 and n _g, time tau _g of light takes to pass through the glass thickness equal to the cell spacing,

(Equation 2) It is. Therefore,

(Equation 3) If the thickness of the crystal is set so as to satisfy, a propagation time difference equal to the time required for light to pass through glass having a thickness equal to the cell interval can be given between the ordinary ray and the extraordinary ray.

[0023] As an example, calcite (n _o = 1.658, n _e
= 1.486) and BK7 ( _ng = 1.550) as glass
), The above object can be achieved if t = 9.01d. Note that the above study seeks the thickness of the birefringent crystal that gives a propagation delay time difference equal to the time required for light to pass through the glass having a thickness equal to the cell spacing, but if this thickness is multiplied by n, A birefringent crystal having a propagation time difference equal to the time required to pass through a glass having a thickness of n times the cell interval is obtained. Therefore, even if such a birefringent crystal is used instead of the polarization beam splitter arrays 23-1 to 23-3, an optical delay compensation circuit similar to that of the present embodiment can be achieved.

Here, when cells at the same position of two adjacent liquid crystal spatial light modulator arrays are switched at the same time, polarized light passing through these cells is rotated by 90 ° by the preceding spatial light modulator array. , And pass through a birefringent crystal or polarizing beam splitter array as polarized light orthogonal to the original polarized light. This light is rotated again by 90 ° by the spatial modulator array at the subsequent stage, and becomes the same as the original polarization state. That is, by such switching, the polarization state of light passing through a birefringent crystal or a polarizing beam splitter array sandwiched between two liquid crystal spatial light modulator arrays is switched, and the propagation time is switched. On the other hand, the propagation time difference switched by the birefringent crystal 22 and the polarization beam splitter arrays 23-1 to 23-3 is 1 time, 2 times, 4 (2 ² ) times the cell interval, and so on.
Since it is equal to the time required to pass through glass having a thickness of 8 (2 ³ ) times, the delay time difference from 0 to 15 (2 ⁴ -1) times the cell interval for the entire switch is expressed in units of the cell interval. Can be set in multiples.

FIG. 5 shows a specific example of the above switching.
Table 1 shows the relationship between the switching pattern and the propagation time.

[Table 1] The delay time is indicated in units of a time τ _g required to pass through a glass having a thickness equal to the cell interval. The birefringent crystal 22 using a calcite (n _o> n _e), the extraordinary ray is assumed that is arranged such that the P-wave of the polarization beam splitter array 23-1～23-3. The polarization planes of the input light and the output light are polarized beam splitter arrays 23-1.
2323-3 are set as P waves.

[0026] As apparent from Table 1, the case of all the delay time zero to 15Tau _g is achieved. Therefore,
It can be seen that the circuit of FIG. 5 can compensate for the optical delay time difference for each channel generated in the multi-terminal optical switch. By arranging this optical delay compensation circuit at the subsequent stage of the collimated optical signal receiving terminals 5-1 and 5-2 of the multi-terminal optical switch shown in FIG. 3, a multi-terminal that compensates for the optical delay time difference of all channels. An optical switch can be configured. The same effect can also be obtained by disposing the optical delay compensating circuit at a stage before the collimated optical signal input terminals 1-1 and 1-2.

[0027]

As described above, by using the optical delay compensating circuit of the present invention, it is possible to compensate for the propagation delay time difference occurring in the multi-terminal optical switch, and to compensate for the phase difference caused by the propagation delay time difference. This can solve the problems such as out-of-synchronization of signals and jitter.

[Brief description of the drawings]

FIG. 1 is a wiring diagram of an N-input N-output (8 × 8) matrix switch.

FIG. 2 is a wiring diagram of a switch network of a multistage network configuration having 32 inputs and 32 outputs.

FIG. 3 is a diagram showing a configuration of a conventional light beam multi-terminal space switch.

FIG. 4 is a diagram showing a configuration of a routing element applied to the multi-terminal optical switch shown in FIG.

FIG. 5 is a diagram showing one embodiment of the present invention.

FIG. 6 shows an arrangement of a birefringent crystal with respect to an incident light beam and a path of the incident light beam.

[Explanation of symbols]

 1-1, 1-2 Collimated optical signal input terminal 2-1, 2-2 Polarization beam splitter 3-1-3-x Liquid crystal spatial light modulator array 4-1-4-x Polarization routing element 5-1, 5 -2 Collimated light signal receiving terminal 12 Polarization plane conversion element array 13 Polarization beam splitter 14 Glass rod 21-1 to 21-5 Collimation light signal input terminal 22 Polarization beam splitter 23-1 to 21-3 Polarization plane conversion element array 31 Ray path 32 Extraordinary ray path

──────────────────────────────────────────────────続 き Continued on the front page (56) References Journal of Lightwave Technology, Vol. 9 No. 12 (December 1991) pp. 1726-1732 (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/01-1/13 505 G02F 1/29-3/02 H04Q 3/52-3/52 101 H04B 10/00- 10/06

Claims (2)

(57) [Claims] 1. A means for injecting a plurality of collimated incident signal lights arranged in a lattice or row and parallel into a circuit, and a plurality of collimators arranged in a lattice or row and parallel emitted from the circuit Means for emitting a light beam out of the circuit, and controlling either to pass two linearly polarized light components of the passing light beam orthogonal to each other as they are or to exchange these linearly polarized light components with each other A polarization plane control element array in which a plurality of polarization plane control elements to be arranged are arranged, and a birefringent transparent medium arranged so that the main direction thereof coincides with the polarization planes of the two polarization components to give a delay time difference. With this configuration, an optical delay element that passes each of two orthogonal linear polarization components of the passing light beam with different propagation delay times is provided. A multi-terminal optical delay compensation circuit characterized by being arranged alternately. 2. A polarizing beam splitter, wherein a part of the optical delay element transmits one of two linearly polarized beams orthogonal to each other and reflects the other in a direction perpendicular to an entering direction thereof. 2. The multi-terminal optical delay compensating circuit according to claim 1, wherein the multi-terminal optical delay compensating circuit is constituted by a plurality of polarizing beam splitter arrays stacked in the traveling direction of the reflected linearly polarized light.