JP4007977B2 - Light switch - Google Patents

Description

  The present invention relates to an optical switch used for long-distance large-capacity optical fiber communication or the like that switches controlled light with control light.

  In order to effectively use limited communication line resources and realize large-capacity optical fiber communication, means for increasing the number of channels that can be transmitted and received and increasing the communication speed are required.

  As means for increasing the number of channels, a multiplex communication method such as time division multiplexing (TDM) has been studied. TDM time-multiplexes multiple channels and transmits them as time-division multiplexed signals, and separates the individual channel information from the time-division multiplexed signals by the gate signal generated from the clock signal on the receiving side. This communication employs a method of individually retrieving and receiving.

  In order to increase the communication speed of the TDM described above, it is desirable to realize all the demultiplexing means by optical means. In other words, it is desirable to realize an optical switch that can execute a switching operation for blocking / transmitting an optical pulse constituting an optical pulse signal by using only an optical control signal without using an electrical means.

  The optical Kerr effect developed in an optical fiber is a phenomenon in which the refractive index of the optical fiber changes due to the propagation of strong light through the optical fiber. The response speed of the optical Kerr effect is several femtoseconds (fs). That is, if an optical switch is configured using the optical Kerr effect, an optical switch capable of switching an optical pulse signal of several hundred Gbit / s or more may be realized. By the way, with a conventional switch that converts an optical pulse signal into an electrical pulse signal, which is an electrical signal, and switches the electrical pulse signal back to an optical pulse signal after switching with an electronic device, the bit rate is about 40 Gbit / s. It was the limit to switch the optical pulse signal.

  As an optical switch using the optical Kerr effect, a non-linear optical loop mirror (NOLM) has been studied (for example, see Non-Patent Document 1 and Patent Document 1). The basic configuration of NOLM disclosed in Non-Patent Document 1 and Patent Document 1 includes an optical demultiplexer / synthesizer, an optical coupler, and an optical fiber loop. The basic operation of these NOLMs is summarized as follows.

  The time-division multiplexed optical pulse signal to be switched is equally divided into two by the optical demultiplexer / synthesizer. Each of the time-division multiplexed optical pulse signals divided into two propagates clockwise and counterclockwise in the optical fiber loop, and is input again to the optical demultiplexer / synthesizer to be coupled. At this time, if no control light is input from the optical coupler, the time division multiplexed optical pulse propagating clockwise in the optical fiber loop and the time division multiplexed optical pulse propagating counterclockwise are Combined with the same phase. As a result, the time division multiplexed optical pulse signal input to the NOLM is output to the same input / output terminal as the input / output terminal input to the optical demultiplexer / synthesizer. The time-division multiplexed optical pulse signal (light) output to the input / output terminal is sometimes called loop reflected light.

  On the other hand, when control light is input from the optical coupler, an optical Kerr effect appears in the optical fiber loop and the refractive index changes. Therefore, each of the time-division multiplexed optical pulse signals divided into two propagates clockwise and counterclockwise in the optical fiber loop, and when they are input again to the optical demultiplexer and combined, Is off. If the length of the optical fiber loop and the intensity of the control light are adjusted so that this phase shift amount is π, the time-division multiplexed optical pulse signal input to the NOLM can be input / output to the optical demultiplexer / synthesizer. It is output to the other input / output terminal paired with the terminal. The time-division multiplexed optical pulse signal (light) output to the other input / output terminal is sometimes called loop transmitted light.

  As described above, the time-division multiplexed optical pulse signal is output to the same input / output terminal as the input / output terminal input to the optical demultiplexer / synthesizer by the control light, or is input to the optical demultiplexer / synthesizer. A switching operation such as output to the other input / output terminal paired with the output terminal is realized.

In addition, in NOLM, in order to combine an optical pulse signal propagating clockwise through an optical fiber loop and an optical pulse signal propagating counterclockwise by an optical demultiplexer / synthesizer, both of them can be interfered with each other. It is necessary to match the polarization direction of the optical pulse signal. As a method for that purpose, a method of forming the above-described optical fiber loop with a polarization-maintaining optical fiber has been studied (for example, see Non-Patent Document 2).
"Modeling of NOLM Demultiplexers Employing Optical Soliton Control Pulse," Z. Ghassemlooy, CY Cheung & AK Ray, Microwave and Optical Technology Letters, vol. 21, No. 3 pp. 205-208, 1999. European Patent Application EP 0 892 516 A2. "Ultrafast polarisation-independent all-optical switching using a polarisation diversity scheme in the nonlinear optical loop mirror," K. Uchiyama, H. Takara, S. Kawanishi, T. Morioka and M. Saruwatari, Electronics. Letters. Vol. 28, No. 20, pp. 1864-1866, 1992.

  The magnitude of the change in the refractive index that occurs in the optical fiber due to the optical Kerr effect is determined by the dimensional requirements such as the length of the optical fiber, the material, and the intensity of the propagating light. That is, if the type of optical fiber constituting the optical fiber loop of the optical switch is selected, the length of the optical fiber loop and the control light required to obtain the above-described phase shift amount required for switching are selected. The strength of is determined. The type of optical fiber constituting the optical fiber loop is determined depending on the use of the optical switch. That is, it is a matter determined by the wavelength of the optical carrier wave of optical communication.

The amount of phase shift φ required for the above switching is given by the following equation (1).
φ = 2γPL (1)
Here, P (W) is the power of the control light, and L (km) is the length of the optical fiber constituting the optical fiber loop. γ (W ⁻¹ km ⁻¹ ) is a nonlinear optical constant based on the optical Kerr effect, and a value normalized by the effective cross-sectional area of the optical fiber is used.

γ (W ^-1 km ^-1 ) is a value of about ^{1 to} 2 W ^-1 km ^-1 for a normal optical fiber, but a value of about several tens to several hundreds W ^-1 km ^-1. A special optical fiber called a highly nonlinear optical fiber with a reduced effective area has also been developed.

  Accordingly, an object of the present invention is to reduce the power of the control light and configure the optical fiber loop under the condition that the type of optical fiber used for the optical fiber loop and the intensity of the control light are determined as design parameters. It is an object of the present invention to provide a compact optical switch that can shorten the length of an optical fiber. In general, when an optical switch is used in time-multiplexed optical communication, the optical fiber material that can be used or the intensity of control light may be determined in advance as a design precondition depending on various conditions such as the wavelength of the optical carrier wave and its intensity. Many.

The optical switch of the reference example includes an optical demultiplexer, a first optical fiber, a second optical fiber, and an optical nonlinear controller. The optical demultiplexer combines the first port to input the signal light that is controlled light, the second port to connect the first optical fiber, the third port to connect the second optical fiber, and the switched signal light 4th port, and the light intensity branching ratio is adjusted to 1: 1. The first optical fiber has an optical coupler that is connected at one end to the second port of the optical demultiplexer and that receives control light. One end of the second optical fiber is connected to the third port of the optical demultiplexer.

  The optical nonlinear controller includes a polarization separation / combination module, a third optical fiber, a fourth optical fiber, and a polarization plane rotating unit. The polarization beam splitting / combining module includes first to fourth input / output terminals. One end of the third optical fiber is coupled to the third input / output end opposite to the first input / output end to which the other end of the first optical fiber of the polarization splitting / combining module is coupled. The fourth optical fiber has one end coupled to the second input / output end facing the fourth input / output end to which the other end of the second optical fiber of the polarization splitting / combining module is coupled. The polarization plane rotation unit connects the other end of the third optical fiber and the other end of the fourth optical fiber.

  Preferably, one end of the fifth optical fiber is connected to the fourth port of the optical demultiplexer / synthesizer, and an optical bandpass filter is connected to the other end of the fifth optical fiber to extract the switched signal light. It is good to do. Preferably, the first to fourth optical fibers are polarization plane preserving optical fibers.

In order to achieve the above object, an optical switch of the present invention includes an optical demultiplexer / synthesizer, a first optical fiber, a second optical fiber, and an optical nonlinear controller. The optical demultiplexer includes a first port for inputting signal light, a second port for connecting the first optical fiber, a third port for connecting the second optical fiber, and a fourth port for outputting the switched signal light. In particular, the light intensity branching ratio is adjusted to 1: 1. One end of the first optical fiber is connected to the second port of the optical demultiplexer / synthesizer and includes an optical coupler for inputting control light. One end of the second optical fiber is connected to the third port of the optical demultiplexer / synthesizer, and includes a first polarization plane rotating unit that rotates the plane of polarization by 90 °.

The optical nonlinear controller includes a first polarization separation / combination module including a first optical fiber and a second optical fiber, and first to third input / output terminals, and first to third input / output terminals. A second polarization separation / combination module, a third optical fiber, and a fourth optical fiber. The third optical fiber has one end coupled to the second input / output end facing the first input / output end of the first polarization separation / combination module, and is connected to the third input / output end of the second polarization separation / combination module. The other end is coupled to the first input / output end on the opposite side. The fourth optical fiber includes a second polarization plane rotating unit that rotates the polarization plane by 90 °, and one end of the fourth optical fiber is coupled to the third input / output end of the first polarization separation / combination module. its other end to the third input and output ends of the side facing the first input and output terminals are coupled.

  The other end of the first optical fiber is coupled to the first input / output end of the first polarization separation / combination module, and the second input / output end of the second polarization separation / combination module is connected to the second optical fiber. The other end is connected.

  Preferably, one end of the fifth optical fiber is connected to the fourth port of the optical demultiplexer / synthesizer, and an optical bandpass filter is connected to the other end of the fifth optical fiber to extract the switched signal light. It is good to do. Preferably, the first to fourth optical fibers are polarization plane preserving optical fibers.

According to the optical switch of the reference example , the signal light, which is the controlled light, is input from the first port to the optical demultiplexer / synthesizer and branched into the first signal light and the second signal light. The second signal light is output from the second port and input to the first optical fiber, and the second signal light is output from the third port and input to the second optical fiber.

  The first signal light propagates through the first optical fiber and is then input to the optical nonlinear controller. That is, it is input from the first input / output end of the polarization splitting / combining module, output from the third input / output end opposite to the first input / output end, and input to the third optical fiber to propagate through the third optical fiber. Then, the polarization plane is rotated by 90 ° at the polarization plane rotation unit, propagates through the fourth optical fiber, and is input to the second input / output end of the polarization separation / combination module. Then, it is output from the third input / output terminal of the polarization separation / combination module and propagates again through the third optical fiber, and the polarization plane is rotated again by 90 ° at the polarization plane rotating section, and propagates through the fourth optical fiber and polarized. It is input to the second input / output terminal of the wave separation / synthesis module. Then, the signal is output from the fourth input / output end facing the second input / output end, propagates through the second optical fiber, and returns to the third port of the optical demultiplexer / synthesizer.

  On the other hand, the second signal light propagates through the second optical fiber and is then input to the optical nonlinear controller. In other words, it is input to the fourth input / output end of the polarization separation / combination module, output from the second input / output end opposite to the fourth input / output end, and input to the fourth optical fiber to propagate through the fourth optical fiber. Then, the polarization plane is rotated by 90 ° at the polarization plane rotation unit, propagates through the third optical fiber, and is input to the third input / output end of the polarization separation / combination module. Then, it is output from the second input / output end of the polarization separation / combination module and propagates again through the fourth optical fiber, and the polarization plane is rotated again by 90 ° at the polarization plane rotating section and propagates through the third optical fiber. Input from the third input / output end of the wave separation / combination module, output from the first input / output end opposite to the third input / output end, input to the first optical fiber, and propagate through the first optical fiber. Return to port 2 of the optical demultiplexer.

  The control light is input to the first optical fiber via an optical coupler installed in the middle of the first optical fiber. In addition, output light is output from the fourth port of the optical demultiplexer / synthesizer. The control light input to the first optical fiber causes the optical Kerr effect in the first to fourth optical fibers, and the refractive index for the signal light propagating through the first to fourth optical fibers changes. Since the length of the third and fourth optical fibers is much longer than the length of the first and second optical fibers, the contribution to the phase change of the signal light is mainly brought about by the third and fourth optical fibers. It is. That is, the phase change of the signal light becomes more remarkable as the lengths of the third and fourth optical fibers are longer.

If the lengths of the third and fourth optical fibers are adjusted according to the intensity of the control light so that the phase change amount of the signal light is π, the signal light input to the optical switch of the reference example is Only when the control light is input from the optical coupler included in the first optical fiber, the control light is output as the loop transmitted light to the fourth port paired with the first port input to the optical demultiplexer / synthesizer. On the other hand, when no control light is input, the signal light is reflected to the first port as loop reflected light.

  As described above, the first and second signal lights propagate through the third and fourth optical fibers twice, respectively. Although details will be described later, this is realized by the fact that the polarization plane is rotated by 90 ° in the polarization plane rotation section and the polarization plane selectivity of the polarization separation / combination module.

  When the first and second signal lights propagate twice through the third and fourth optical fibers, respectively, the effect is substantially the same as setting the lengths of the third and fourth optical fibers to double. There is. That is, if the optical fiber material used as the third and fourth optical fibers and the intensity of the control light are determined as design parameters, the signal light required to realize the optical switch operation This leads to a reduction in the lengths of the second and third optical fibers for obtaining the phase change.

  That is, shortening the lengths of the third and fourth optical fibers constituting the optical switch is effective for making the optical switch compact. In addition, since the lengths of the third and fourth optical fibers can be shortened, they are less susceptible to the influence of the ambient temperature and the like, which is effective for stabilizing the operating state of the optical switch.

Further, according to the optical switch of the present invention, the signal light is input from the first port to the optical demultiplexing synthesizer and branched into the first signal light and the second signal light, and the first signal light is transmitted from the second port. It is output and input to the first optical fiber, and the second signal light is output from the third port and input to the second optical fiber.

  The first signal light propagates through the first optical fiber and is then input to the optical nonlinear controller. That is, the third light is input from the first input / output end of the first polarization splitting / combining module, output from the second input / output end opposite to the first input / output end, and input to the third optical fiber. It propagates through the fiber, is input from the first input / output end of the second polarization separation / combination module, is output from the third input / output end opposite to the first input / output end, and is input to the fourth optical fiber. Then, the polarization plane is rotated by 90 ° in the second polarization plane rotation unit provided in the middle of the fourth optical fiber after propagating through the fourth optical fiber, and the third input / output of the first polarization separation / combination module Input at the end. Then, it is output from the second input / output end of the first polarization separation / combination module, propagates again through the third optical fiber, is input to the first input / output end of the second polarization separation / combination module, and is input to the second input / output end. The polarization plane is rotated by 90 ° at the first polarization plane rotation unit that is output from the end, propagates through the second optical fiber, and is provided in the middle of the second optical fiber. Return to the port.

  On the other hand, the second signal light propagates through the second optical fiber, and after the polarization plane is rotated by 90 ° by the first polarization plane rotation unit provided at a position in the middle of the second optical fiber, the optical nonlinear control unit Is input. That is, the first polarization separation / combination module is input from the second input / output end of the second polarization separation / combination module, output from the first input / output end, input to the third optical fiber, and propagates through the third optical fiber. Are input from the second input / output terminal, output from the third input / output terminal, and input to the fourth optical fiber. Then, the polarization plane is rotated by 90 ° at the second polarization plane rotating section that is propagated through the fourth optical fiber and is provided in the middle of the fourth optical fiber, and the third input / output of the second polarization separation / combination module Input at the end. Then, it is output from the first input / output end on the side opposite to the third input / output end of the second polarization separation / combination module and propagates again through the third optical fiber, and the second input / output of the first polarization separation / combination module Is input to the end, output from the first input / output end opposite to the second input / output end, propagates through the first optical fiber, and returns to the second port of the optical demultiplexer / synthesizer.

When the control light is input to the first optical fiber through the optical coupler installed in the middle of the first optical fiber, the output light is output to the outside as loop transmitted light from the fourth port of the optical demultiplexer / combiner The point is common to the optical switch of the reference example . In the optical switch of the present invention, the first and second signal lights are propagated twice through the third optical fiber. In this case as well, the third embodiment has the same effect as that of setting the length of the third optical fiber to be doubled, and can provide a compact optical switch in common with the optical switch of the reference example . In addition, since the third optical fiber can be shortened to be less susceptible to the influence of the ambient temperature or the like, it is also effective for stabilizing the operation state of the optical switch, and is common to the optical switch of the reference example .

The optical switch of the present invention is different from the optical switch of the reference example is the following. That is, since the polarization separation / combination module is configured in two places as the first and second polarization separation / combination modules, the selectivity of the polarization plane of the switched signal light is doubled. That is, in the first to fourth optical fibers that are signal light transmission paths, when polarization crosstalk occurs, the optical switch of the reference example configured by setting the polarization demultiplexing / combining module at only one place In comparison, the polarization crosstalk component can be removed more effectively. In addition, even when the polarization plane selection of the polarization separation / combination module is incomplete, setting the polarization separation / combination module at two locations substantially improves the polarization plane selectivity of the polarization separation / combination module. There is also an effect. Thereby, an optical switching operation can be realized more effectively.

In the reference switch and the optical switch of the present invention, the first and second signal lights are combined by the optical demultiplexer / synthesizer to cause interference, so that the first and second signals in the optical demultiplexer / synthesizer are switched. The polarization plane of light must be aligned. According to the optical fiber in which the central symmetry of the core and clad structure is completely guaranteed, the polarization plane of the light propagating through the optical fiber is preserved. In general, however, the central symmetry of the core and cladding structures is rarely guaranteed. Therefore, if polarization plane preserving optical fibers are used as the first to fourth optical fibers, the polarization planes of the first and second signal lights can be reliably aligned in the optical demultiplexer / synthesizer.

  Also, control light is output from the fourth port of the optical demultiplexer that outputs the switched signal light. Therefore, when using an optical switch for applications where it is inconvenient if the control light is output together, one end of the fifth optical fiber is connected to the fourth port of the optical demultiplexer / synthesizer. If an optical band-pass filter is connected to the other end, the control light can be removed and only the switched signal light can be extracted.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Each figure shows one configuration example according to the present invention, and only schematically shows the arrangement relationship of each component to the extent that the present invention can be understood. It is not limited to. In the following description, specific equipment and conditions may be used. However, these materials and conditions are only one of preferred examples, and are not limited to these. In addition, overlapping description of the same components in each drawing may be omitted.

< Reference example >
The structure and operation of the optical switch of the reference example will be described with reference to FIG.

(Construction)
The optical switch includes an optical demultiplexer / synthesizer 10, a first optical fiber 16, a second optical fiber 18, and an optical nonlinear controller 20.

As described above, the first optical fiber 16 and the second optical fiber 18 in the optical switch of the reference example have the polarization planes of the control light and the signal light that is the controlled light not being propagated through these optical fibers. It is desirable to use a polarization-maintaining optical fiber so that it does not fluctuate regularly. As a polarization-maintaining optical fiber, a panda (PANDA: Polarization-maintaining AND Absorption-reducing) type optical fiber shown in FIG. 2 is representative. This optical fiber has a polarization maintaining property by forming a stress applying portion in the vicinity of the core and applying a strong stress to the core.

FIG. 2 is a diagram showing a schematic structure of a cross section cut perpendicular to the light propagation direction of a PANDA type optical fiber, which is a polarization-maintaining optical fiber. A stress applying portion 44 is formed in a clad 40 surrounding the core 42 through which light is guided so as to sandwich the core 42. For example, the cladding 40 is made of SiO ₂ , the core 42 is made of SiO ₂ doped with GeO ₂ , and the stress applying portion is made of SiO ₂ doped with B ₂ O ₃ .

  By forming in this way, in FIG. 2, the direction of the slow axis set in the plane perpendicular to the light propagation direction of the PANDA type optical fiber and the fast ( In the direction of the fast axis, the equivalent refractive index for light guided through the core 42 is different. That is, since a stress applying portion having a refractive index higher than the refractive index of the clad 40 is placed near the core 42, the equivalent refractive index for light in which the vibration direction of the electric field vector of light is parallel to the slow axis direction is The oscillation direction of the electric field vector of light becomes higher than the equivalent refractive index for light parallel to the fast axis direction. Due to the asymmetry of the equivalent refractive index, the polarization plane of light input to the PANDA type optical fiber is preserved and propagated.

  In other words, in the PANDA type optical fiber, if the polarization plane of linearly polarized light is input in accordance with the slow axis (or fast axis) shown in Fig. 2, it propagates through the PANDA type optical fiber while maintaining the polarization state. Even at the output end, it is possible to obtain only linearly polarized light whose polarization plane coincides with the slow axis (or fast axis).

As the optical demultiplexer / synthesizer 10, a directional optical coupler or the like can be used. In particular, to configure the optical switch of the reference example , it is desirable to use a directional optical coupler that can switch the optical path while the plane of polarization of light is determined in a certain direction. A polarization-maintaining optical coupler shown in FIG. 3 has been developed as a directional optical coupler that preserves such a plane of polarization. FIG. 3 is a schematic configuration diagram of a typical polarization-maintaining optical coupler. This polarization-preserving optical coupler is manufactured by fusing and stretching after the slow axes (or fast axes) of two PANDA optical fibers are precisely aligned and aligned in parallel. When it is confirmed that the desired characteristics as a directional optical coupler (such as a light intensity branching ratio of 1: 1) can be obtained, stretching is completed and the substrate is fixed to a reinforcing substrate and completed. Is done.

In FIG. 3, a fusion drawing part 58 is a part produced by fusing and drawing precisely the slow axes (or fast axes) of two PANDA type optical fibers. The polarization-maintaining optical coupler shown in FIG. 3 can be used as the optical demultiplexer / synthesizer 10 of the optical switch of the reference example . That is, a mechanism for coupling the optical fiber and the polarization-maintaining optical coupler may be configured so that the input / output end face 50 can be used as the first port 10-1 for inputting the signal light having the wavelength λs. What is necessary is just to comprise the mechanism which couple | bonds an optical fiber and a polarization-maintaining optical coupler so that the input / output end faces 52, 54, and 56 can be similarly used as the second port, the third port, and the fourth port.

  If a product designed to have a light intensity branching ratio of 1: 1 as a polarization-maintaining optical coupler, the light intensity branching ratio required for the optical demultiplexer / combiner 10 is 1: 1. The condition of being can be satisfied.

In the following description, for the sake of convenience, in the schematic configuration diagram of the optical switch shown in FIG. 1, the polarization direction of light propagating through an optical transmission line such as an optical fiber or an optical element such as an optical demultiplexer is shown as follows. It is prescribed as follows. Polarization in which the vibration direction of the electric field vector of light is perpendicular to the paper surface is called TE (Transverse-Electric Modes) polarization, and the direction perpendicular to the paper surface is called the TE direction. Also, polarized light in which the vibration direction of the electric field vector of light is parallel to the paper surface is called TM (Transverse-Magnetic Modes) polarized light, and the direction parallel to the paper surface is called TM direction. Of course, the possibility of using the optical switch of the reference example is not limited to the above case.

  The first optical fiber 16 includes an optical coupler 12 that is connected at one end to the second port 10-2 of the optical demultiplexer / synthesizer 10 and that receives control light having a wavelength λp. One end of the second optical fiber 18 is connected to the third port 10-3 of the optical demultiplexer / synthesizer 10. The optical coupler 12 is preferably a type of optical coupler that preserves the plane of polarization. For example, the setting may be performed using three input / output terminals among the input / output terminals of the polarization-preserving optical coupler described later with reference to FIG.

  The optical nonlinear controller 20 includes a polarization splitting / combining module 14, a third optical fiber 22, a fourth optical fiber 24, and a polarization plane rotating unit 26. The polarization separation / combination module 14 includes a first input / output terminal 14-1, a second input / output terminal 14-2, a third input / output terminal 14-3, and a fourth input / output terminal 14-4. The third optical fiber 22 has one end connected to the third input / output end 14-3 on the side facing the first input / output end 14-1 to which the other end of the first optical fiber 16 of the polarization splitting / combining module 14 is coupled. Are combined. The fourth optical fiber 24 has one end connected to the second input / output end 14-2 on the side facing the fourth input / output end 14-4 to which the other end of the second optical fiber 18 of the polarization splitting / combining module 14 is coupled. Are combined. The polarization plane rotating unit 26 connects the other end of the third optical fiber 22 and the other end of the fourth optical fiber 24.

For the polarization separation / combination module 14, a suitable one can be selected from commercially available polarization beam splitters, for example. The third optical fiber 22 and the fourth optical fiber 24 are preferably polarization plane preserving optical fibers, but it is also desirable that the nonlinear optical effect be large in addition to the property of preserving the polarization plane. In order to increase the nonlinear optical effect, the optical fiber core (corresponding to the core 42 shown in FIG. 2) is heavily doped with GeO ₂ to increase the nonlinear optical constant γ (W ⁻¹ km ⁻¹ ) based on the optical Kerr effect. Or, a device has been devised to increase the optical energy density in the optical fiber by reducing the mode field diameter (MFD) which is the waveguide mode cross-sectional area of the optical fiber. For example, an ordinary optical fiber with an MFD of 8 μm is about γ = 2 km ⁻¹ W ⁻¹ , whereas an optical fiber with an MFD of 3.6 μm and γ = 20 km ⁻¹ W ⁻¹ is an order of magnitude larger. It is commercially available. In addition, optical fibers with high optical nonlinearity have been developed, such as a fiber having a cavity formed in a clad called a “Holey fiber” and a photonic bandgap fiber. In the future, it is naturally expected that the polarization-maintaining optical fiber will incorporate the above-described device, and that an optical fiber having high optical nonlinearity while having the property of maintaining the polarization plane will be developed.

  In the polarization separation / combination module 14, the TM polarization component input from the first input / output terminal 14-1 is output to the third input / output terminal 14-3 and input from the first input / output terminal 14-1. The TE polarization component is output to the fourth input / output terminal 14-4. Similarly, TM polarization components input from the third input / output terminal 14-3, the second input / output terminal 14-2, and the fourth input / output terminal 14-4 are respectively connected to the first input / output terminal 14-1 and the first input / output terminal 14-1. 4 Output to the input / output terminal 14-4 and the second input / output terminal 14-2, and TE polarization components are respectively the second input / output terminal 14-2, the third input / output terminal 14-3, and the first input / output terminal. Output to 14-1.

The length of the path from the second port 10-2 of the optical demultiplexer / synthesizer 10 to the first input / output terminal 14-1 of the polarization splitting / combining module 14, that is, the length of the first optical fiber 16 is defined as l ₁ (path L _1. ), The length of the path from the third port 10-3 of the optical demultiplexer / synthesizer 10 to the fourth input / output terminal 14-4 of the polarization separation / combination module 14, that is, the second optical fiber The length of 18 is l ₂ (also referred to as path L ₂ ), the path from the third input / output end 14-3 of the polarization splitting / combining module 14 to the polarization plane rotating unit 26, that is, the third optical fiber 22. The length is l ₃ (also referred to as path L ₃ ), the path from the second input / output end 14-2 of the polarization separation / combination module 14 to the polarization plane rotation unit 26, that is, the length of the fourth optical fiber 24. Is l ₄ (also referred to as path L ₄ ).

  The polarization plane rotating unit 26 can be configured by using a so-called half-wave plate, or can be realized by the structure described with reference to FIG. FIG. 4 is a diagram for explaining the structure of the polarization plane rotation unit. The optical fiber 70 and the optical fiber 72 are PANDA type optical fibers. In FIG. 4, cross sections 74 and 76 show a schematic structure of a cross section of PANDA type optical fibers 70 and 72 cut perpendicular to the light propagation direction. This cross-sectional structure is the same as the PANDA type optical fiber described above with reference to FIG.

  As shown in the upper part of FIG. 4, the cut surfaces of the PANDA type optical fibers 70 and 72 are arranged so that their slow axes are orthogonal to each other. The polarization plane rotating unit 26 cuts the PANDA type optical fiber at one end, perpendicular to the light propagation direction, and then makes each slow axis (fast axis) perpendicular to each other so that the cores are in close contact with each other. It is formed by fusing.

(Operation)
The operation principle of the optical switch of the reference example will be described with reference to FIG. In the following description, the signal light is described as being TM polarized. Of course, it is also possible to configure an optical switch using the signal light as TE polarized light, but in this case also in the polarization splitting / combining module 14, the input / output terminal from which the output light is obtained with respect to the input from the first to fourth input / output terminals Since the same description can be made only by reversing the relationship of TM polarization and TE polarization, the description is omitted.

According to the optical switch of the reference example , the signal light is input from the first port 10-1 of the optical demultiplexer / synthesizer 10 and branched into the first signal light and the second signal light. The first signal light is output from the second port 10-2 of the optical demultiplexer / synthesizer 10 and input to the first optical fiber 16, and the second signal light is input to the third port 10-3 of the optical demultiplexer / synthesizer 10. To be input to the second optical fiber 18. That is, the first signal light propagates clockwise in the first optical fiber 16 (sometimes referred to as the CW direction), and the second signal light travels counterclockwise in the second optical fiber 18 (also referred to as the CCW direction). ) Is propagated.

  The first signal light propagates through the first optical fiber 16 and is then input to the optical nonlinear controller 20. That is, the third optical fiber is input from the first input / output end 14-1 of the polarization splitting / combining module 14 and output from the third input / output end 14-3 on the side facing the first input / output end 14-1. 22 is propagated through the third optical fiber 22, the polarization plane is rotated by 90 ° at the polarization plane rotating unit 26, and propagated through the fourth optical fiber 24 to be transmitted to the second input / output end of the polarization splitting / combining module 14. Input to 14-2. Then, it is output from the third input / output terminal 14-3 of the polarization separation / combination module 14 and propagates again through the third optical fiber 22, and the polarization plane is rotated 90 ° again by the polarization plane rotation unit 26, and the fourth light The signal propagates through the fiber 24 and is input to the second input / output terminal 14-2 of the polarization beam splitting / combining module 14. Then, it is output from the fourth input / output end 14-4 on the side facing the second input / output end 14-2, propagates through the second optical fiber 18 and passes through the third port 10- of the optical demultiplexer / synthesizer 10. Return to 3.

  On the other hand, the second signal light propagates through the second optical fiber 18 and is then input to the optical nonlinear controller 20. That is, the fourth optical fiber is input to the fourth input / output end 14-4 of the polarization splitting / combining module 14 and output from the second input / output end 14-2 on the side facing the fourth input / output end 14-4. 24 is propagated through the fourth optical fiber 24, the polarization plane is rotated by 90 ° at the polarization plane rotating unit 26, and is propagated through the third optical fiber 22 to be transmitted to the third input / output end of the polarization splitting / combining module 14. Input to 14-3. Then, it is output from the second input / output terminal 14-2 of the polarization separation / combination module 14 and propagates again through the fourth optical fiber 24, and the polarization plane is rotated 90 ° again by the polarization plane rotating unit 26, and the third optical fiber is rotated. 22 is input from the third input / output end 14-3 of the polarization splitting / combining module 14 and output from the first input / output end 14-1 on the side opposite to the third input / output end 14-3. The light is input to the first optical fiber 16 and propagates through the first optical fiber 16 to return to the second port 10-2 of the optical demultiplexer / synthesizer 10.

  The control light is input to the first optical fiber 16 via the optical coupler 12 installed in the middle of the first optical fiber 16. Further, output light is output from the fourth port 10-4 of the optical demultiplexer / synthesizer 10 to the outside. The control light input to the first optical fiber 16 causes an optical Kerr effect in the first optical fiber 16, the second optical fiber 18, the third optical fiber 22, and the fourth optical fiber 24, and propagates through these optical fibers. The refractive index with respect to the signal light changes. Since the lengths of the third optical fiber 22 and the fourth optical fiber 24 are much longer than the lengths of the first optical fiber 16 and the second optical fiber 18, the contribution to the phase change of the signal light is mainly the first. Provided by three optical fibers 22 and a fourth optical fiber 24. That is, the phase change of the signal light becomes more significant as the lengths of the third optical fiber 22 and the fourth optical fiber 24 are longer.

If the lengths of the third optical fiber 22 and the fourth optical fiber 24 are adjusted according to the intensity of the control light so that the phase change amount of the signal light is π, it is input to the optical switch of the reference example . The signal light is a fourth port 10-4 that is paired with the first port 10-1 of the optical demultiplexer / synthesizer 10 only when the control light is input from the optical coupler 12 included in the first optical fiber 16. Is output as loop transmitted light. On the other hand, when the control light is not input, the signal light is reflected to the first port 10-1 of the optical demultiplexer / synthesizer 10 as loop reflected light.

  As described above, the first and second signal lights propagate through the third optical fiber 22 and the fourth optical fiber 24 twice. This is realized because the polarization plane is rotated by 90 ° in the polarization plane rotation unit 26 and the polarization plane selectivity of the polarization separation / combination module. The reason for this is as follows.

  The signal light is input to the polarization-preserving optical fiber as TM polarization (polarization in which the vibration direction of the electric field vector of the light is parallel to the paper surface) and input to the first port 10-1 of the optical demultiplexer / synthesizer 10. All components such as the optical fiber 34, which is the sixth optical fiber connected to the first port 10-1 of the optical demultiplexer / synthesizer 10, and the optical demultiplexer / synthesizer 10 constituting the optical switch are preserved in the polarization plane. It is an element that operates while being secured. That is, the optical fibers such as the first optical fiber 16 all use PANDA type optical fibers, and the optical demultiplexer / synthesizer 10 uses the polarization-maintaining optical coupler shown in FIG.

Consider the first signal light propagating in the CW direction. The first signal light is input to the path L ₁ constituted by a first optical fiber 16 to the first output terminal 14-1 propagates as TM polarization polarization splitting-combining module 14, a third output terminal Output from 14-3. As described above, this is input to the first input / output end 14-1 of the polarization splitting / combining module 14 as TM polarization, so that it goes straight through the polarization splitting / combining module 14 and enters the first input / output end. It is output to the third input / output terminal 14-3 on the side facing 14-1.

  As already explained, when TM polarization is input to the polarization separation / combination module 14, it goes straight through the polarization separation / combination module 14 and is output to the input / output terminal on the side opposite to the input side. . On the other hand, when it is input to the polarization separation / combination module 14 as TE polarization, it is reflected by the reflection surface 14R having polarization plane selectivity existing inside the polarization separation / combination module 14, Output from the input / output terminal on the side. The reflection surface 14R having polarization plane selectivity is formed of a dielectric multilayer film or the like in a polarization beam splitter or the like, and exhibits reflection characteristics having polarization plane selectivity.

The first signal light is output to the third output terminal 14-3 of the polarization splitting-combining module 14, a path L ₃ constituted by the third optical fiber 22 propagates in the TM polarization. Thereafter, the light is converted into TE polarized light by the polarization plane rotating unit 26. As described above with reference to FIG. 4, this occurs because the polarization plane rotating unit 26 is opposed so that the slow axis (fast axis) of the PANDA type optical fiber is orthogonal, so that the core matches. This is due to the fact that they are formed in close contact with each other. For example, when the direction of vibration of the electric field vector of light has been guided as linearly polarized waves parallel to the slow axis, the polarization axis rotating unit 26 changes in the direction rotated by 90 °, The vibration direction of the electric field vector is also rotated 90 ° following this. That is, it can be set to be converted to a TE polarization when the TM polarization passes through the polarization plane rotation unit 26 and converted to a TM polarization when the TE polarization passes.

First signal light converted into the TE polarization from TM polarization by passing through the polarization plane rotating unit 26 propagates through constituted path L ₄ in the fourth optical fiber, the polarization splitting-combining module 14 Since it is input to the second input / output terminal 14-2 as TE polarization, it is output from the third input / output terminal 14-3. Thereafter, the path L ₃ is propagated as TE polarized light and passes through the polarization plane rotating unit 26 to be converted from TE polarized light to TM polarized wave, and is then propagated through the path L ₄ to pass through the second of the polarization separation / combination module 14. Since it is input to the input / output terminal 14-2 as TM polarization, it is output from the fourth input / output terminal 14-4. First signal light output as TM polarization from the fourth output terminal 14-4 propagates the configured path L ₂ in the second optical fiber 18, the third port 10 of the optical demultiplexer 10 -3 to be combined with the second signal light.

When the effective refractive index of the slow axis of the PANDA type optical fiber, which is the first to fourth optical fibers, is n _s , the effective refractive index of the fast axis is n _f, and the direction of the slow axis matches the polarization plane of the TE polarization Then, the first signal light is demultiplexed by the optical demultiplexer / synthesizer 10 and exits from the second port 10-2 of the optical demultiplexer / synthesizer 10 to again the third port 10-3 of the optical demultiplexer / synthesizer 10 The optical path length until the optical demultiplexer / synthesizer 10 multiplexes is added in the order of the propagation path of the first signal light, and is given by the following equation (2).
n _f l ₁ + n _f l ₃ + n _s l ₄ + n _s l ₃ + n _f l ₄ + n _f l ₂ (2)
Here, l ₁ , l ₂ , l ₃ , and l ₄ are the lengths of the paths L ₁ , L ₂ , L ₃ , and L ₄ , respectively.

That is, since the path L ₁ formed by the first optical fiber propagates as TM polarization, the optical path length is n _f l ₁ , and the path L ₂ formed by the second optical fiber also propagates as TM polarization. Therefore, the optical path length is n _f l ₂ . The first signal light is output from the third output terminal 14-3 of the polarization splitting-combining module 14, the propagation path L ₃ as TM polarization (optical path length = n _f l _3), and the path L ₄ Is propagated as a TE polarized wave (optical path length = n _s l ₄ ), the path L ₃ is propagated as a TE polarized wave (optical path length = n _s l ₃ ), and the path L ₄ is propagated as a TM polarized wave (optical path length = n _f l ₄ ) The total optical path length is given by n _f l ₃ + n _s l ₄ + n _s l ₃ + n _f l ₄ . From this, it can be seen that the path L ₃ (third optical fiber) and the path L ₄ (fourth optical fiber) are propagated twice as a TM polarization and a TE polarization, respectively, twice in total.

The second signal light propagating in the CCW direction can also be considered in the same way as the first signal light propagating in the CW direction, and the second signal light is demultiplexed by the optical demultiplexer synthesizer 10 and optical demultiplexing combined Similarly, the optical path length from the third port 10-3 of the optical unit 10 to the second port 10-2 of the optical demultiplexer / synthesizer 10 is multiplexed again by the optical demultiplexer / synthesizer 10 in the same way. It is given by the following equation (3) by adding in the order of the propagation path of the signal light.
n _f l ₂ + n _f l ₄ + n _s l ₃ + n _s l ₄ + n _f l ₃ + n _f l ₁ (3)
The control light follows a path that propagates in the CW direction, which is the same path as the first signal light.

  Comparing the above formulas (2) and (3), the first term, the second term, the third term, the fourth term, the fifth term, the sixth term in the formula (2) and the sixth term, the fifth term in the formula (3), It can be seen that the fourth term, the third term, the second term, and the first term are equal to each other. That is, it can be seen that the first signal light and the second signal light propagate through the same optical path length.

That is, when the control light is not input, the first signal light and the second signal light are combined in the same phase in the optical demultiplexer / synthesizer 10. As a result, the signal light input to the first port 10-1 of the optical demultiplexer / synthesizer 10 is output as loop reflected light to the first port 10-1 of the optical demultiplexer / synthesizer 10. On the other hand, when control light is input from the optical coupler 12, an optical Kerr effect appears in the optical fiber loop, and the refractive index changes. Therefore, the first signal light and the second signal light propagate respectively clockwise and counterclockwise in the optical fiber loop constituted by the paths L _{1 to} L ₄ , and again to the optical demultiplexer / synthesizer 10. When both are input and combined, the phases of both are shifted.

If the intensity of the control light is adjusted or the lengths of the first to fourth optical fibers (particularly the third and fourth optical fibers) are adjusted so that the phase shift amount φ is equal to π, The signal light input to the first port 10-1 of the wave synthesizer 10 is output to the fourth port 10-4 of the optical demultiplexer 10 as loop transmitted light. The phase shift amount φ is proportional to the total length of the optical fiber loop composed of the paths L1 to L4, as given by the above equation (1). In other words, the optical switch of the reference example is configured to propagate twice, a total of two times, each of the path L3 (third optical fiber) and the path L4 (fourth optical fiber) as TM polarization and TE polarization. Therefore, it is substantially equal to the path where the optical Kerr effect is developed longer by the length of the path L3 (third optical fiber) and the path L4 (fourth optical fiber).

The lengths of the third and fourth optical fibers are set to be much longer than those of the first and second optical fibers, so the main contribution to the expression of the phase shift amount φ based on the optical Kerr effect is Third and fourth optical fibers. According to Equation (1), the phase shift amount φ is proportional to the product of the length of the optical fiber constituting the optical fiber loop and the intensity of the control light. Therefore, according to the optical switch of the reference example , Compared with the optical switch, the lengths of the optical fibers corresponding to the third and fourth optical fibers that mainly contribute to the expression of the phase shift amount φ based on the optical Kerr effect are half.

  That is, shortening the lengths of the third and fourth optical fibers constituting the optical switch is effective for making the optical switch compact. In addition, since the lengths of the third and fourth optical fibers can be shortened, they are less susceptible to the influence of the ambient temperature and the like, which is effective for stabilizing the operating state of the optical switch.

The operation of the optical switch of the reference example will be specifically described with reference to FIG. 1, taking as an example the case of switching a time division multiplexed optical pulse signal. In FIG. 1, a schematic time waveform of transmitted light and reflected light as a result of switching of signal light, which is an optical pulse signal, control light, and optical pulse signal, is shown with the horizontal axis as the time axis. Assume a binary digital signal corresponding to “0” or “1” corresponding to the presence or absence of an optical pulse. Consider a case in which signal light in which optical pulses of wavelength λs are regularly arranged is controlled (switched) with control light comprising an optical pulse train of wavelength λp representing [1 1 0 1].

  The signal light propagates through the optical fiber 34 as the sixth optical fiber via the optical circulator 28 and is input to the first port 10-1 of the optical demultiplexer / synthesizer 10. Here, the optical fiber 32 is preferably a polarization-maintaining optical fiber such as a PANDA type optical fiber. This is because it is necessary to input the signal light to the first port 10-1 of the optical demultiplexer / synthesizer 10 as TM polarization.

  As described above, the signal light is demultiplexed into the first signal light and the second signal light by the optical demultiplexer / synthesizer 10, and the first signal light is output from the second port 10-2 of the optical demultiplexer / synthesizer 10. It propagates through the first optical fiber 16. The first optical pulse that is the first optical pulse of the signal light (in FIG. 1, the optical pulse that is positioned at the rightmost position on the time axis) passes through the optical coupler 12 and is the first optical pulse that is the first optical pulse of the control light. It is assumed that one optical pulse (in FIG. 1, the optical pulse located on the rightmost side on the time axis) is input from the optical coupler 12. Of course, control light is also input to the first optical fiber as TM polarization.

  The optical pulse (wavelength λs) of the first signal light and the optical pulse of control light (wavelength λp) propagate through the first to fourth optical fibers while running in parallel. For this reason, the effective refractive index of the optical fiber with respect to the optical pulse of the first signal light changes due to the optical Kerr effect generated by the optical pulse of the control light. That is, the optical path of the first signal light and the optical pulse of the control light run in parallel, so that the optical path in which the effective refractive index always changes due to the presence of the optical pulse of the control light passes through the first signal light. The light pulse propagates. On the other hand, the optical pulse (wavelength λs) of the second signal light propagates through the first to fourth optical fibers in the opposite direction to the optical pulse of the first signal light without being affected by the optical pulse of the control light. .

As a result, the effective refractive indexes n _s and n _f appearing in the above equation (2) are different from the effective refractive indexes n _s and n _f appearing in the above equation (3). That is, the effective refractive index values appearing in the above equation (2) are n _s ′ and n _f ′.

Therefore, the length of the path L ₃ (third optical fiber) and the path L ₄ (fourth optical fiber)
(n _f 'l ₁ + n _f ' l ₃ + n _s 'l ₄ + n _s ' l ₃ + n _f 'l ₄ + n _f ' l ₂ ) − (n _f l ₂ + n _f l ₄ + n _s l ₃ + n _s l ₄ + n _f l ₃ + n _f l ₁ ) = λs / 2
Is adjusted so that the phase difference when the optical pulse of the first signal light and the optical pulse of the second signal light are combined in the optical demultiplexer / combiner 10 can be set to π. However, the length from the second port 10-2 of the optical demultiplexer / synthesizer 10 to the optical coupler 12 is sufficiently shorter than the lengths of the third optical fiber and the fourth optical fiber. Ignoring the above formula.

  If the lengths of the third optical fiber 22 and the fourth optical fiber 24 are adjusted as described above according to the intensity of the control light, the optical pulse of the first signal light is changed from the optical coupler 12 by the optical pulse of the control light. Only when it is input to the first optical fiber 16, it is output to the fourth port 10-4 of the optical demultiplexer / synthesizer 10 as loop transmitted light.

  Strictly speaking, since the wavelengths of the control light and the signal light are λp and λs, respectively, the wavelengths are different. Therefore, the group delay time difference between the control light and the signal light due to the group velocity dispersion generated in the second optical fiber 22 and the third optical fiber 24 is the interval of appearance of the optical pulse of the signal light on the time axis, that is, the signal It must be shorter than the time interval occupied by one bit of light (one light pulse). However, this condition can be easily satisfied with almost no difference between the wavelength λp of the control light and the wavelength λs of the signal light. The difference between the wavelength λp of the control light and the wavelength λs of the signal light is determined by an optical bandpass filter set on the output side of the optical switch in order to output only the switched signal light from the optical switch as described later. It is sufficient that the control light can be blocked and the signal light can be transmitted as long as it is separated.

Next, when the second optical pulse next to the first optical pulse of the first signal light passes through the optical coupler 12, there is a second optical pulse that is exactly the next optical pulse of the control light. When the third optical pulse subsequent to the second optical pulse of the signal light passes through the optical coupler 12, there is no third optical pulse that is the optical pulse subsequent to the second optical pulse of the control light. In this case, the third optical pulse of the first signal light is in a state where the optical pulse of the control light does not exist, that is, the optical pulse of the first signal light does not travel in parallel with the optical pulse of the control light. It propagates along the optical path composed of L _{1 to} L ₄ . Therefore, in the optical demultiplexer / synthesizer 10, the optical pulse of the first signal light and the optical pulse of the second signal light are multiplexed in the same phase. The optical pulse of the first signal light is output as loop reflected light to the first port 10-1 of the optical demultiplexer / synthesizer 10, propagates through the optical fiber 34 which is the sixth optical fiber, and passes through the optical circulator 28. The signal light is output toward a transmission line different from the transmission line through which the signal light has propagated.

  As a result, as shown in FIG. 1, the optical pulse train constituting the transmitted light output from the fourth port 10-4 of the optical demultiplexer / synthesizer 10 reflects the optical pulse train constituting the control light. . The reflected light is output to the outside as the loop reflected light through the optical circulator 28 only when the optical pulse of the signal light is present in the time zone in which the optical pulse of the control light does not exist. It becomes a pulse train as shown.

  Further, since the control light having the wavelength λp is also output from the fourth port 10-4 of the optical demultiplexer 10, in order to extract only the signal light having the switched wavelength λs to the outside, the optical demultiplexer One end of the optical fiber 32, which is the fifth optical fiber, is connected to the fourth port 10-4 of 10, and the center of the transmission wavelength is set to λs and the wavelength λp can be cut off at the other end of the optical fiber 32 It is necessary to connect an optical bandpass filter 30 having

  The optical pulse of the first signal light output to the first port 10-1 of the optical demultiplexer / synthesizer 10 as the loop reflected light travels backward on the transmitted transmission line unless the optical circulator 28 is arranged. Will be returned to the sender. In general, in time-multiplexed optical communication, it is not preferable that a part of a transmission signal is reversely transmitted from the receiving side to the transmitting side. Therefore, an optical demultiplexer is used as a loop reflected light by using the optical circulator 28. It is desirable that the optical pulse of the first signal light output to the ten first ports 10-1 be output to a transmission path different from the transmission path through which the signal light has propagated.

<Embodiment of the Invention >
The structure and operation of the optical switch according to the embodiment of the present invention will be described with reference to FIG.

(Construction)
The optical switch of implementation in the form of the invention, the optical demultiplexer 100, a first optical fiber 116, a second optical fiber 118, which comprises an optical nonlinear control unit 120. The optical demultiplexer 100 includes a first port 100-1 for inputting signal light that is controlled light, a second port 100-2 for connecting the first optical fiber 116, and a third port for connecting the second optical fiber 118. A port 100-3 and a fourth port 100-4 for outputting switched signal light are provided, and the light intensity branching ratio is adjusted to 1: 1. One end of the first optical fiber 116 is connected to the second port 100-2 of the optical demultiplexer / synthesizer 100, and includes an optical coupler 112 that inputs control light. The second optical fiber 118 is connected to the third port 100-3 of the optical demultiplexer / synthesizer 100, and includes a first polarization plane rotating unit 132 that rotates the plane of polarization by 90 °.

  In the optical nonlinear controller 120, the first optical fiber 116 and the second optical fiber 118 are coupled, and the first input / output end 122-1, the second input / output end 122-2, and the third input / output end 122- A first polarization separation / combination module 122 having a first input / output terminal 124-1, a second input / output terminal 124-2, and a third input / output terminal 124-3. 124, a third optical fiber 126, and a fourth optical fiber 128. The third optical fiber 126 has one end coupled to the second input / output end 122-2 opposite to the first input / output end 122-1 of the first polarization separation / combination module 122, so that the second polarization separation / combination is performed. The other end of the module 124 is coupled to the first input / output end 124-1 on the side facing the third input / output end 124-3. The fourth optical fiber 128 includes a second polarization plane rotating unit 130 that rotates the plane of polarization by 90 °, and one end of the fourth optical fiber 128 is coupled to the third input / output end 122-3 of the first polarization separation / combination module 122. One end of the polarization splitting / combining module 124 is coupled to the third input / output end 124-3 facing the first input / output end 124-1.

  The other end of the first optical fiber 116 is coupled to the first input / output end 122-1 of the first polarization separation / combination module 122, and the second input / output end 124 of the second polarization separation / combination module 124 is connected. The other end of the second optical fiber 118 is coupled to -2.

The first optical fiber 116, the second optical fiber 118, the third optical fiber 126, and the fourth optical fiber 128 are configured using polarization-maintaining optical fibers such as PANDA type optical fibers used in the optical switch of the reference example. It is preferable to do this. Further, the optical demultiplexer 100 is also the reference examples and the like can utilize the same optical directional coupler to that used in optical switches, optical couplers 112 also identical to those used in the optical switch of the reference example An optical coupler can be used. Also, the polarization beam splitting / combining modules 122 and 124 can use the polarization beam splitter used in the optical switch of the reference example .

The first polarization separation / combination module 122 and the second polarization separation / combination module 124 are configured to have three input / output terminals, one fewer input / output terminals than the polarization separation / combination module 14 used in the reference example. is there. Since the first polarization separation / combination module 122 and the second polarization separation / combination module 124 have the same structure, the first polarization separation / combination module 122 will be described.

  When TM polarization is input to the polarization separation / combination module 122, the light travels straight through the polarization separation / combination module 122 and is output to the input / output terminal opposite to the input side. On the other hand, when TE polarization is input to the polarization separation / combination module 122, it is reflected by the reflection surface 122R having polarization plane selectivity existing inside the polarization separation / combination module 122, and is reflected on the side toward which reflected light is directed. Output from the input / output terminal on the side.

The roles of the optical circulator 134 and the optical bandpass filter 136 are also the same as in the case of the optical switch of the reference example , and thus description thereof is omitted.

(Operation)
The operation principle of the optical switch according to the embodiment of the present invention will be described with reference to FIG. Here, the signal light is described as being TM polarized. According to the optical switch of the embodiment of the present invention , the signal light is input from the first port 100-1 to the optical demultiplexer / synthesizer 100 and branched into the first signal light and the second signal light, and the first signal The light is output from the second port 100-2 and input to the first optical fiber 116, and the second signal light is output from the third port 100-3 and input 118 to the second optical fiber.

  The first signal light propagates through the first optical fiber 116 and is then input to the optical nonlinear controller 120. That is, it is input from the first input / output end 122-1 of the first polarization splitting / combining module 122, output from the second input / output end 122-2, input to the third optical fiber 126, and the third optical fiber 126. Is transmitted from the first input / output end 124-1 of the second polarization separation / combination module 124, output from the third input / output end 124-3, and input to the fourth optical fiber 128.

  Then, the polarization plane is rotated by 90 ° by the second polarization plane rotation unit 130 that propagates through the fourth optical fiber 128 and is provided in the middle of the fourth optical fiber 128, so that the first polarization separation / combination module 122 The signal is input to the third input / output terminal 122-3. Then, it is output from the second input / output terminal 122-2 of the first polarization separation / combination module 122 and propagates again through the third optical fiber 126, and the first input / output terminal 124-1 of the second polarization separation / combination module 124 is obtained. Is transmitted from the second input / output end 124-2, propagates through the second optical fiber 118, and is deflected by the first polarization plane rotating unit 132 provided in the middle of the second optical fiber 118. The wavefront is rotated 90 ° and returned to the third port 100-3 of the optical demultiplexer 100.

  On the other hand, the second signal light propagates through the second optical fiber 118, and after the plane of polarization is rotated by 90 ° in the first polarization plane rotating unit 132 provided in the middle of the second optical fiber 118, Input to the optical nonlinear controller 120. That is, the signal is input from the second input / output terminal 124-2 of the second polarization separation / combination module 124, output from the first input / output terminal 124-1, and input to the third optical fiber 126 to propagate through the third optical fiber 126. Then, the signal is input from the second input / output terminal 122-2 of the first polarization separation / combination module 122, output from the third input / output terminal 122-3, and input to the fourth optical fiber 128. Then, the polarization plane is rotated by 90 ° in the second polarization plane rotating unit 130 that propagates through the fourth optical fiber 128 and is provided in the middle of the fourth optical fiber. 3 Input to the input / output terminal 124-3. Then, it is output from the first input / output terminal 124-1 of the second polarization separation / combination module 124 and propagates again through the third optical fiber 126, and the second input / output terminal 122-2 of the first polarization separation / combination module 122 is transmitted. , Is output from the first input / output terminal 122-1, propagates through the first optical fiber 116, and returns to the second port 100-2 of the optical demultiplexer / synthesizer 100.

When control light is input to the first optical fiber 116 via the optical coupler 112 installed in the middle of the first optical fiber 116, it is output as loop transmitted light from the fourth port 100-4 of the optical demultiplexer / synthesizer 100. The point that light is output to the outside is common to the optical switch of the reference example . In the optical switch according to the embodiment of the present invention , the first and second signal lights are propagated twice through the third optical fiber. This has the same effect as substantially doubling the length of the third optical fiber.

The optical switch according to the embodiment of the present invention is different from the optical switch of the reference example in the following points. That is, since the polarization separation / combination module is configured in two places as the first polarization separation / combination module 122 and the second polarization separation / combination module 124, the selectivity of the polarization plane of the switched signal light is 2 As a result, the polarization crosstalk component is removed more effectively than the optical switch of the reference example . Moreover, even when the polarization plane selectivity of the polarization separation / combination module is incomplete, the polarization plane selectivity by the polarization separation / combination module is substantially improved.

  Each of the first and second signal lights propagates through the third optical fiber twice. This is realized by the fact that the polarization plane is rotated by 90 ° in the first polarization plane rotation unit 132 and the second polarization plane rotation unit 130, and that the first polarization separation / combination module 122 and the second polarization plane are rotated. This is due to the polarization plane selectivity of the separation / synthesis module 124. The reason for this is as follows.

The signal light is input to the polarization preserving optical fiber 140 as TM polarization (polarization in which the vibration direction of the electric field vector of the light is parallel to the paper surface) and input to the first port 100-1 of the optical demultiplexer 100. Let Here, the first signal light propagating in the CW direction is considered. The first signal light is input to the path L ₁ constituted by the first optical fiber 116 to the first output terminal 122-1 of the first polarization splitting-combining module 122 propagates as TM polarization, a second input Output from the output terminal 122-2. Since this is input to the first input / output terminal 122-1 of the first polarization separation / combination module 122 as TM polarization, it goes straight through the first polarization separation / combination module 122 and enters the first input / output terminal 122- 1 is output to the second input / output terminal 122-2 on the side facing 1.

  As already described, when TM polarization is input to the first polarization separation / combination module 122, the input / output terminal on the side facing the input side is straightened through the first polarization separation / combination module 122. Is output. On the other hand, when it is input to the first polarization separation / combination module 122 as TE polarization, it is reflected by the reflection surface 122R having polarization plane selectivity existing inside the first polarization separation / combination module 122, and the reflected light Is output from the input / output terminal on the side facing the. The same applies to the second polarization separation / combination module 124.

  The first signal light is output to the second input / output terminal 122-2 of the first polarization separation / combination module 122 and propagates through the third optical fiber 126 with TM polarization. Thereafter, the signal is input to the first input / output terminal 124-1 of the second polarization separation / combination module 124 and output from the third input / output terminal 124-3 to propagate through the fourth optical fiber 128 with TM polarization. By passing through the second polarization plane rotation unit 130 installed in the middle of the fourth optical fiber 128, the TM polarization is converted into the TE polarization, and the third input / output end 122 of the first polarization separation / combination module 122 is converted. -3, and since it is a TE polarized wave, it is output from the second input / output terminal 122-2.

  The first signal light output as the TE polarization from the second input / output terminal 122-2 propagates again through the third optical fiber 126, and the first input / output terminal 124-1 of the second polarization separation / combination module 124 is obtained. And is output from the second input / output terminal 124-2 and propagates through the second optical fiber 118. Then, by passing through the first polarization plane rotation unit 132 installed in the middle of the second optical fiber 118, it is converted from the TE polarization to the TM polarization, the third port 10- 3 is input and combined with the second signal light.

As in the case of the optical switch of the reference example , the effective refractive index of the slow axis of the PANDA type optical fiber that is the first to fourth optical fibers is ns, the effective refractive index of the fast axis is nf, and the direction of the slow axis is When it matches the polarization plane of TE polarization, the first signal light propagating in the CW direction is demultiplexed by the optical demultiplexer / synthesizer 100 and exits the second port 100-2 of the optical demultiplexer / synthesizer 100. Returning to the third port 100-3 of the optical demultiplexer / synthesizer 100 again, the optical path length until the optical demultiplexer / synthesizer 100 multiplexes is obtained as follows.

The length of the first optical fiber 116 is l ₁ , the length of the third optical fiber 126 is l ₂ , and from the third input / output end 124-3 of the second polarization separation / combination module 124 to the second polarization plane rotation unit 130 The length of the fourth optical fiber 128 is l ₃ , and the length of the fourth optical fiber 128 from the second polarization plane rotating unit 130 to the third input / output end 122-3 of the first polarization separation / combination module 122 is l ₄ , the length of the second optical fiber 118 from the second input / output end 124-2 of the second polarization separation / combination module 124 to the first polarization plane rotation unit 132 is l ₅ , from the first polarization plane rotation unit 132 If the length to the third port 100-3 of the optical demultiplexer / synthesizer 100 is l ₆ , the optical path length is added in the order of the propagation path of the first signal light, and the following equation (4)
n _f l ₁ + n _f l ₂ + n _f l ₃ + n _s l ₄ + n _s l ₂ + n _s l ₅ + n _f l ₆ (4)
Given in.

That is, since the first signal light propagates through the first optical fiber 116 as TM polarization, the optical path length is n _f l ₁ , and the third optical fiber 126 also propagates as TM polarization, so the optical path length is n _f l ₂ . In addition, since the first signal light propagates as the TM polarization in the fourth optical fiber 128 from the third input / output end 124-3 of the second polarization separation / combination module 124 to the second polarization plane rotation unit 130, the optical path length Is n _f l ₃ . Next, since the fourth optical fiber 128 is propagated as TE polarized light from the second polarization plane rotating unit 130 to the third input / output end 122-3 of the first polarization separation / combination module 122, the optical path length is n _s l ₄ . Next, since the third optical fiber 126 propagates again as TE polarized light, the optical path length is n _s l ₂ . Next, since the second optical fiber 118 from the second input / output terminal 124-2 of the second polarization separation / combination module 124 to the first polarization plane rotating unit 132 propagates as TE polarized light, the optical path length is n _s l ₅ It is. Then, the optical path length because the from the first polarization plane rotation portion 132 to the third port 100-3 of the optical demultiplexer 100 propagates as TM polarization is n _f l _6.

  From this, it can be seen that the third optical fiber 126 propagates twice as a TM polarized wave and a TE polarized wave, once in total.

The second signal light propagating in the CCW direction can also be considered in the same way as the first signal light propagating in the CW direction, and the second signal light is demultiplexed by the optical demultiplexer 100 and combined. The optical path length from the third port 100-3 of the multiplexer 100 back to the second port 10-02 of the optical demultiplexer / synthesizer 100 and multiplexed by the optical demultiplexer / synthesizer 100 is the second signal. Add in order of the light propagation path,
n _f l ₆ + n _s l ₅ + n _s l ₂ + n _s l ₄ + n _f l ₃ + n _f l ₂ + n _f l ₁ (5)
Given in.

In the optical switch according to the embodiment of the present invention , the control light follows a path that propagates in the CW direction, which is the same path as the first signal light.

  Comparing the above formulas (4) and (5), the first term, the second term, the third term, the fourth term, the fifth term, the sixth term, the seventh term in the formula (4) and the seventh term in the formula (5), It can be seen that the sixth, fifth, fourth, third, second, and first terms are equal. That is, it can be seen that the first signal light and the second signal light propagate through the same optical path length.

Similar to the optical switch of the reference example , when the control light is not input, the signal light input to the first port 100-1 of the optical demultiplexer / combiner 100 is the first port 100− of the optical demultiplexer / combiner 100. 1 is output as loop reflected light. On the other hand, when the control light is input from the optical coupler 112, the optical Kerr effect appears in the optical fiber loop, and the refractive index changes. For this reason, when the first signal light and the second signal light are input again to the optical demultiplexer / combiner 100 and are combined, the phases of both are shifted.

If the intensity of the control light is adjusted or the length of the first to fourth optical fibers (especially the third optical fiber) is adjusted so that the phase shift amount φ is equal to π, the optical demultiplexer / synthesizer The signal light input to the first port 100-1 of 100 is output as loop transmitted light to the fourth port 100-4 of the optical demultiplexer / synthesizer 100. The optical switch of implementation forms of the present invention, a third optical fiber 126 respectively once each as TM polarization and TE polarization, since it is configured to propagate a total of two times, the length of the third optical fiber 126 This is equivalent to taking a long path for the light Kerr effect.

It is a schematic block diagram of the optical switch of a reference example . It is a schematic sectional drawing of a polarization-maintaining optical fiber. It is a schematic block diagram of an optical demultiplexer. It is a figure where it uses for description of the structure of a polarization plane rotation part. It is a schematic block diagram of the optical switch of embodiment of this invention .

Explanation of symbols

10, 100: Optical demultiplexer
12, 112: Optical coupler
14, 122, 124: Polarization separation and synthesis module
16, 116: First optical fiber
18, 118: Second optical fiber
20, 120: Optical nonlinear controller
22, 126: Third optical fiber
24, 128: Fourth optical fiber
26, 130, 132: Polarization plane rotating part
28, 134: Optical circulator
30, 136: Optical bandpass filter
32, 138: 5th optical fiber
34, 140: 6th optical fiber
40: Clad
42: Core
44: Stress application part
50: First port
52: Second port
54: Third port
56: Port 4
58: Fusion stretch part
70, 72: Polarization-maintaining optical fiber

Claims (4)

Branch of light intensity comprising a first port for inputting signal light, a second port for connecting the first optical fiber, a third port for connecting the second optical fiber, and a fourth port for outputting the switched signal light An optical demultiplexer with a ratio of 1: 1,
A first optical fiber having one end connected to the second port of the optical demultiplexer and having an optical coupler for inputting control light;
One end is connected to the third port of the optical demultiplexer, a second optical fiber including a first polarization plane rotating unit that rotates the polarization plane by 90 °, and
Comprising an optical nonlinear controller that couples the first optical fiber and the second optical fiber,
The optical nonlinear controller includes a first polarization separation / combination module having first to third input / output ends, a second polarization separation / combination module having first to third input / output ends,
One end is coupled to the second input / output end on the side facing the first input / output end of the first polarization separation / combination module, and the first on the side facing the third input / output end of the second polarization separation / combination module. A third optical fiber having the other end coupled to one input / output end;
A second polarization plane rotating unit that rotates the plane of polarization by 90 °; one end coupled to the third input / output end of the first polarization separation / combination module; A fourth optical fiber having the other end coupled to the third input / output end on the opposite side,
The other end of the first optical fiber is coupled to the first input / output end of the first polarization separation / combination module, and the second input / output end of the second polarization separation / combination module is connected to the second light. An optical switch characterized in that the other end of the fiber is coupled. The optical switch according to claim 1 , wherein one end of a fifth optical fiber is connected to the fourth port of the optical demultiplexer / synthesizer, and an optical bandpass filter is connected to the other end of the fifth optical fiber. An optical switch characterized by being connected. 2. The optical switch according to claim 1 , wherein one end of a sixth optical fiber is connected to the first port of the optical demultiplexer / synthesizer, and an optical circulator is connected to the other end of the sixth optical fiber. An optical switch characterized by The optical switch according to any one of claims 1 to 3 , wherein the first to fourth optical fibers are polarization-maintaining optical fibers.

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