JP4798244B2 - Light modulator - Google Patents

Description

The present invention relates to an optical modulator, for example, is used for long-distance large-capacity optical fiber communication or the like, it may be applied to the all-optical optical modulator to provide intensity modulation and or phase modulation to the controlled light by the control light Is.

  With the spread of the Internet and the like, demands for increasing the communication capacity of optical fiber communication have been increasing in recent years. In recent years, the increase in communication capacity of optical fiber communication has increased the number of wavelength channels that can be transmitted and received (wavelength multiplex communication, WDM: Wavelength Division Multiplexing), and the communication speed for each wavelength channel has been increased. It is made by.

  The increase in the number of wavelength channels by the WDM system is approaching the limit from the viewpoint of increasing the complexity of the system and the complexity of system maintenance. Therefore, increasing the communication speed per wavelength channel has recently been an important research and development issue. It has become a challenge. As one means, a multiplex communication method such as time division multiplexing (TDM) has been studied and put into practical use.

  The TDM scheme is a scheme for increasing the communication speed per wavelength channel by using a time division multiplexed signal obtained by time division multiplexing a plurality of channels. In the TDM system, information on individual channels is individually received and received by demultiplexing means for separating individual channels from the time division multiplexed signal by a gate signal generated from a clock signal on the receiving side.

  Conventionally, the TDM method used has been a method of generating a time division multiplexed signal and demultiplexing the time division multiplexed signal at the electronic device level. This method is called electric TDM. In order to increase the communication speed of electrical TDM, it is necessary to increase the speed of electronic devices and optoelectronic devices such as photodiodes and semiconductor lasers for performing photoelectric conversion and electro-optical conversion. The communication speed is limited to a bit rate of about 40 Gbit / s.

  In order to further increase the communication speed of the TDM system, it is desirable that all the means for generating the time division multiplexed signal and demultiplexing the time division multiplexed signal are realized by optical means. This method is called optical TDM. In the optical TDM system, a time division multiplexed optical pulse signal is generated using an optical circuit combined with an optical coupler or the like. Further, it is desirable that the demultiplexing on the receiving side is performed using an all-optical type optical switch that is gated with an optical control signal that is control light. Furthermore, optical signal control technologies such as optical repeaters for long-distance transmission and the like, wavelength conversion and modulation optical signal generation at optical network nodes, and optical signal regeneration operations are also applied to control optical signals. It is desirable to perform this using an all-optical wavelength converter / optical modulator that performs wavelength conversion of a controlled optical signal, generation of a modulated optical signal, signal regeneration, and the like.

  That is, in the optical TDM system, in order to perform demultiplexing at the receiving end or optical signal regeneration in the optical repeater, the control optical signal performs the switching operation of the controlled optical signal and the modulation signal generating operation. An all-optical type optical switch / modulator is required.

  As a method for realizing an all-optical type optical switch / modulator, a method using the optical Kerr effect that appears in an optical fiber is a preferable example. The optical Kerr effect that appears 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, and the response speed is several femtoseconds (fs). In other words, if an optical switch or an optical modulator is configured using the optical Kerr effect of an optical fiber, an optical switch / optical modulator capable of switching and modulating an optical pulse signal of several hundred Gbit / s or more can be realized. There is sex.

  As an optical switch using the optical Kerr effect, an optical switch using the optical Kerr effect developed in a polarization-maintaining single-mode optical fiber has been studied (for example, see Non-Patent Document 1).

  In the optical switch disclosed in Non-Patent Document 1, in order to achieve practical operational stability against environmental temperature changes and the like, first, the birefringence of the polarization-maintaining single-mode optical fiber is canceled. In addition, it is necessary to make the fiber lengths of the two polarization-maintaining single-mode optical fibers exhibiting the optical Kerr effect exactly the same. On the other hand, the fiber length of each optical fiber usually has a length of several hundreds to several kilometers. Therefore, the fiber length is strictly matched when considering actual device fabrication. Accuracy (approximately 1 mm or less) is necessary, and there is a problem in feasibility. Furthermore, there is a problem of instability of the switch operation due to the polarization crosstalk component that exists in the actual polarization-maintaining single-mode optical fiber disclosed in Non-Patent Document 2.

  As a method for solving these problems, the inventors of the present application have previously proposed the optical switch disclosed in Non-Patent Document 3. That is, Non-Patent Document 3 does not require adjustment of the fiber length of the polarization-maintaining single-mode optical fiber constituting the optical switch, and as a polarization-maintaining single-mode optical fiber that causes the optical Kerr effect, An optical switch is disclosed in which instability of switch operation due to polarization crosstalk components does not occur even when a long one is used.

  On the other hand, various optical signal encoding formats used in an optical communication system have been proposed and used so far. A typical example is an amplitude modulation system that represents a binary digital signal based on the magnitude of the peak intensity of the optical signal, and a phase modulation system that represents a binary digital signal based on the optical phase difference of the optical carrier wave of the optical signal.

  It is desirable that the amplitude modulation method and the phase modulation method are each selected as appropriate so as to satisfy the required specifications of the network as much as possible. The optical communication network has a form in which a plurality of networks having various specifications are interconnected. That is, in an optical communication network, it is desirable that various optical signals encoded in various modulation formats such as an amplitude modulation method and a phase modulation method are mixedly operated at appropriate positions.

  In view of such a situation, the optical modulator for generating the encoded optical signal has versatility so that it can be used for both the amplitude modulation method and the phase modulation method. It is desirable.

  The all-optical type optical switch disclosed in Non-Patent Document 1 and Non-Patent Document 3 is amplitude-modulated by using an optical pulse signal that is signaled by an amplitude modulation method based on the magnitude of its peak intensity as control light. It can be used as an all-optical light intensity modulator that generates a modulated optical signal.

  However, it is difficult to use the all-optical optical switch disclosed in Non-Patent Document 1 or Non-Patent Document 3 as an all-optical optical phase modulator that generates a phase-modulated modulated optical signal.

  The cross-phase modulation effect by the optical Kerr effect, which is the operation principle of the all-optical type optical switch disclosed in Non-Patent Document 1 and Non-Patent Document 3, can be directly used as the operation principle of the phase modulator. An optical optical phase modulator can be provided.

However, in this case, in order to support both the amplitude modulation method and the phase modulation method, separate all-optical optical modulators corresponding to the respective methods are prepared. This causes problems such as an increase in the size of the apparatus, an increase in cost, and an increase in power consumption.

  If an all-optical type intensity / phase modulator that can generate optical signals of either amplitude modulation system or phase modulation system can be generated by simple adjustment means using a single device, the above problem can be solved. it can. At this time, it is practically desirable that no significant change in optical signal quality occurs, such as a significant change in optical loss due to a change in modulation format. Furthermore, if the highly stable operating characteristics can be secured without changing the characteristics even if the signal light wavelength or ambient temperature changes, there will be no increase in parts, cost and power consumption related to stabilization control, and there will be significant practical benefits. become able to.

  On the other hand, the inventors of the present application have proposed an all-optical intensity / phase modulator disclosed in Non-Patent Document 4 so far. In the method disclosed in Non-Patent Document 4, the format can be easily switched between the amplitude modulation method and the phase modulation method by a simple method of rotating the optical axis of a λ / 2 wavelength plate incorporated in the apparatus. is there. In addition, highly stable operating characteristics are ensured in which the characteristics do not change even if the signal light wavelength or the environmental temperature changes.

T.A. Morioka, M.M. Saruwatari and A.M. Takada, “Ultrafast optical multi / demultiplexer utility optical Kerr effect in polarization-maintaining single-mode fibers”, Electronic Letters. 23, no. 9, pp. 453-454, 1987 Arai, Saito, Oyama, Nakamura, Yokomizo, and Sozoe, “Polarization-maintaining optical fiber”, Furukawa Electric Times, No. 109, pp. 5-10, 2002 S. Arahira, H .; Murai and Y.M. Ogawa, “Modified NOLM for stable and improved 2R operation at ultra-high bit rates”, IEICE Trans. Commun. , Vol. E89-B, no. 12, pp. 3296-3305, 2006 S. Arahira, H .; Murai and K.M. Fujii, “All-Optical Modulation-Formal Converter Deploying Polarization-Rotation-Type Nonlinear Optical Fiber LooperLolterVoltLoltsV”. 20, no. 18, pp. 1530-1532, 2008

  By the way, when the generated modulated optical signal is put into practical use as an optical signal in a long-distance optical fiber communication system, one of the important performance indexes is the magnitude of frequency chirping of the modulated optical signal.

  From the viewpoint of frequency chirping, the following problems exist in the amplitude modulation system and phase modulation system optical modulation signal generated based on the techniques disclosed in Non-Patent Documents 1, 3 and 4 described above. .

  The techniques disclosed in Non-Patent Documents 1, 3 and 4 described above are based on the principle of phase shift due to the mutual phase modulation effect based on the optical Kerr effect in the nonlinear optical fiber. This phase shift is proportional to the light intensity of the control optical signal in the simple case where loss in the optical fiber, group velocity dispersion, or other higher-order nonlinear optical effects can be ignored. That is, the time waveform of the phase shift is proportional to the intensity time waveform of the control light signal.

  In many cases, the peak light intensity of the control light signal is made to correspond to the maximum phase shift amount π to obtain the light modulation operation.

  On the other hand, the phase shift caused by the control light signal continuously changes from 0 to π until the control light signal reaches its peak light intensity.

  In other words, the phase shift caused by the control light signal in principle causes so-called frequency chirping in which the phase shift amount changes with time at the rising and falling portions.

  That is, the modulated optical signal of the amplitude modulation method and the phase modulation method generated based on the techniques disclosed in Non-Patent Documents 1, 3 and 4 described above is a modulated optical signal having frequency chirping in principle.

  Temporarily, the signal light that is the controlled light is an optical pulse train composed of optical pulse signals instead of continuous light, and the pulse width is sufficiently larger than the pulse width of the optical pulse signals constituting the control optical signal. If it is narrow, it is possible to execute the optical modulation operation with the signal light intensity sufficiently attenuated at the rising and falling portions of the control optical signal where frequency chirping occurs. As long as this is the case, it is possible to suppress frequency chirping in the optical modulator.

  On the other hand, when the signal light pulse width is wide or continuous light, the modulated light signal to be generated has a large frequency chirping particularly at the rising and falling parts of the signal light pulse. .

  This can be generated based on the technology disclosed in Non-Patent Documents 1, 3 and 4 described above, and an optical modulation signal of an amplitude modulation method or a phase modulation method with a small frequency chirping is a so-called return-to-zero. It means that it is limited to the modulated optical signal in the (RZ) format. In particular, a so-called non-return-to-zero (NRZ) format optical modulation signal that is often used in optical communication systems can be generated. There is a problem that the modulated optical signal has frequency chirping.

  The present invention has been made in view of the above points, and it is possible to generate a modulated optical signal of any format of an amplitude modulation method or a phase modulation method by a simple adjustment means, and its operation is very stable. An optical modulator is provided. In addition, the present invention aims to provide an optical modulator capable of generating a modulated optical signal with a small frequency chirping regardless of whether it is an RZ format or an NRZ format.

The optical modulator of the present invention includes (1) a first closed-loop optical path section (for example, 22 and 26 in FIG. 1) that forms a polarization-preserving first closed-loop optical path, and (2) peak intensity. An optical pulse train in which aligned optical pulses are arranged at equal time intervals, or a first signal light having a linearly polarized first wavelength, which is continuous light, is divided into two linearly polarized first and second components. A first and second component dividing unit (for example, 14 and 18 in FIG. 1 correspond), and (3) intensity modulated light having intensity patterns corresponding to “0” and “1” of the signal, An intensity-modulated light having a linearly polarized first control optical signal having two wavelengths and an intensity pattern complementary to the intensity pattern of the first control optical signal, wherein the second wavelength or the vicinity thereof is A second polarization plane orthogonal to the polarization plane of the first control optical signal. A control light generator for outputting a control light signal (for example, 50 corresponds to FIG. 1), and (4) input / output of the first signal light to and from the first closed loop optical path, A first closed-loop first signal light input / output unit (for example, corresponding to 18 and 24 in FIG. 1) that inputs the first component and the second component to the first closed-loop optical path so that the circulation direction is reversed; (5) Input / output the first control optical signal and the second control optical signal to / from the first closed-loop optical path, and the circulation direction of the first control optical signal is the first The first control light signal and the second control light signal are the same as the first component, and the second control light signal has the same cyclic direction as the second component. A first closed-loop control light input / output unit (for example, 51, 18 and 2 in FIG. 1) that inputs to the closed-loop optical path. And (6) changing the optical phase of the first component according to the first control light signal traveling in the same direction and the second control light signal traveling in the opposite direction. (7) The second control light signal traveling in the same direction and the second component traveling in the opposite direction and the first component optical phase shift unit (for example, corresponding to 22 in FIG. 1) interposed in the closed loop optical path of A second component optical phase shift unit (for example, corresponding to 22 in FIG. 1) interposed in the first closed-loop optical path for changing the optical phase of the second component in accordance with one control optical signal; 8) A first signal light extraction unit (for example, for separating the first component and the second component output from the first closed-loop optical path from the first control light signal and the second control light signal) 1 corresponds to 51), and (9) the first closed-loop first signal light input / output unit. A first forward / reverse optical path separating unit (for example, separating the optical path between the first signal light traveling toward the first signal light and the first component and the second component of the return light output from the first closed loop first signal light input / output unit) , 10 or 30 in FIG. 1), and (10) provided in any place where the return light output from the first closed-loop first signal light input / output unit travels, the first component and the second A first signal light component multiplexing unit (for example, corresponding to 14 in FIG. 1) for combining the polarization planes of the components, and (11) the first component and the second component of the first signal light travel. A first optical phase difference providing unit (for example, corresponding to 40 in FIG. 1) provided at any location and providing a relative optical phase difference to the first component and the second component, (12) A polarization plane preserving optical path is applied to an optical path other than the first closed loop optical path, and the first signal light component combination is applied. Parts and the multiplexing and optical signal optical path separation was made of the first positive backlighting path separating portion, and outputs a modulated optical signal, (13) the first closed-loop first signal light input unit, a polarization beam splitter or The first and second component dividers and the first closed-loop control light input / output unit, which are polarizing prisms, serve as the first closed-loop optical path unit. It includes an in-loop polarization plane converter that rotates the wavefront by 90 ° .

  According to the first aspect of the present invention, there is provided an optical modulator that can generate a modulated optical signal of any format of an amplitude modulation system and a phase modulation system and that is very stable in operation by a simple adjustment means. In addition, it is possible to provide an optical modulator capable of generating a modulated optical signal with small frequency chirping, whether it is in the RZ format or the NRZ format.

1 is a layout diagram illustrating a configuration of an all-optical light modulator according to a first embodiment. FIG. It is explanatory drawing which shows the internal structural example of the 1st optical phase bias circuit in 1st Embodiment. It is sectional drawing which shows the schematic structure of the cross section cut | disconnected perpendicularly | vertically with respect to the propagation direction of the light of the panda type | mold optical fiber which is a polarization-maintaining optical fiber. It is explanatory drawing which shows the structural example of the 1st polarization plane conversion part in 1st Embodiment. It is explanatory drawing which shows the structural example of the 2nd polarization plane conversion part in 1st Embodiment. It is explanatory drawing of the polarization state of the signal light in the 1st polarization plane conversion part in 1st Embodiment. It is explanatory drawing of the phase shift produced with respect to 1st signal light by the control light signal and 1st optical phase bias circuit in 1st Embodiment. FIG. 5 is a layout diagram illustrating a configuration of an all-optical light modulator according to a second embodiment. It is explanatory drawing of the polarization state of the signal light in the 3rd polarization plane conversion part in 2nd Embodiment. It is explanatory drawing of the phase shift produced with respect to 2nd signal light by the control light signal and 2nd optical phase bias circuit in 2nd Embodiment.

(A) First Embodiment Hereinafter, a first embodiment of an optical modulator according to the present invention will be described with reference to the drawings. The optical modulator of the first embodiment is an all-optical optical modulator.

(A-1) Configuration of First Embodiment FIG. 1 is a layout diagram showing a configuration of an all-optical light modulator 6 according to the first embodiment.

  In FIG. 1, the all-optical optical modulator 6 according to the first embodiment includes a first polarization separation / combination module 10, a first polarization plane preserving optical fiber 12, a first polarization plane converter 14, Two polarization plane preserving optical fibers 16, a second polarization separation / combination module 18, a third polarization plane preserving optical fiber 22, a second polarization plane converter 24, a fourth polarization plane preserving optical fiber 26, and a first An optical phase bias circuit 40, a first optical bandpass filter 51, and a fifth polarization-maintaining optical fiber 52 are provided.

  The signal light (first signal light) 1 in the first embodiment is controlled light having a wavelength λs. The first signal light 1 is a so-called optical pulse train in which continuous light or optical pulses having uniform peak intensities are arranged at equal time intervals. If the data format of the desired modulated optical signal is the NRZ format, continuous light is used as the first signal light 1, and if it is the RZ format, an optical pulse train is used as the first signal light. When an optical pulse train is used, the pulse period is 1 / f [s] where the bit rate of the desired modulated optical signal is f [bit / s].

  On the other hand, the first control light signal 2 and the second control light signal 3 have a wavelength λp different from the wavelength λs of the first signal light 1. The bit rates of the first control optical signal 2 and the second control optical signal 3 coincide with the bit rate f [bit / s] of the desired optical modulation signal.

  The first control light signal 2 and the second control light signal 3 are intensity modulation signals (intensity modulation light) whose intensity time waveforms are in a complementary relationship with each other. That is, when the intensity time waveform of the first control light signal 2 and the intensity time waveform of the second control light signal 3 are added on the time axis, the continuous light-like time waveform whose intensity does not change on the time axis. It becomes.

  The first control light signal 2 and the second control light signal 3 that are in a complementary relationship are, for example, data light and inverted data light of a modulated light signal generated in the NRZ format. Also, for example, so-called constructive output and destructive output are output from the optical interferometer when the modulated optical signal generated in the RZ format and continuous light having the same wavelength are caused to interfere by the optical interferometer. As described above, the first control light signal 2 and the second control light signal 3 may be NRZ format intensity modulation signals or RZ format intensity modulation signals.

  The first polarization separation / combination module 10 includes a first input / output end 10-1 to which one end of an input optical fiber 32-2 for inputting the first signal light 1 is coupled, and a first input / output end. A second input / output end 10-2, to which one end of the first polarization-maintaining optical fiber 12 is coupled, and a modulated signal light (hereinafter referred to as a modulated optical signal). And a third input / output terminal 10-3.

  The second polarization separation / combination module 18 is located on the side opposite to the first input / output end 18-1 and the first input / output end 18-1 that couples one end of the second polarization plane-maintaining optical fiber 16. A second input / output end 18-2 for coupling one end of the three polarization-maintaining optical fiber 22, a third input / output end 18-3 for coupling one end of the fourth polarization-maintaining optical fiber 26, and a third input / output end A fourth input / output terminal 18-4 for outputting a polarization crosstalk component located on the side facing 18-3 is provided.

  Since the fourth input / output terminal 18-4 does not input / output light from there, it is necessary to connect optical components for optical signal input / output interfaces such as optical fiber pigtails and optical connectors, for example. There is no. In the first embodiment, the fourth input / output terminal 18-4, as will be described later, in order to explain how the polarization crosstalk generated in the third polarization-maintaining optical fiber 22 is removed, It is provided only for convenience and is not an essential component of the first embodiment.

  One end of the first polarization-maintaining optical fiber 12 is coupled to the second input / output terminal 10-2 of the first polarization separation / combination module 10, and the second polarization-maintaining optical fiber 16 is provided with the second polarization separation. One end is coupled to the first input / output end 18-1 of the combining module 18, and the other end of the first polarization plane preserving optical fiber 12 and the other end of the second polarization plane preserving optical fiber 16 are converted to the first polarization plane. It is connected via a portion 14 (set to a position indicated as A in FIG. 1).

  The third polarization-preserving optical fiber 22 is the first controlled light having the wavelength λs due to the optical Kerr effect caused by the control optical signals having the wavelength λp (the first control optical signal 2 and the second control optical signal 3). This is a so-called nonlinear optical fiber in which a phase shift occurs due to the mutual phase modulation effect with respect to the signal light 1. One end of the third polarization-maintaining optical fiber 22 is coupled to the second input / output end 18-2 of the second polarization separation / combination module 18.

  The fourth polarization plane preserving optical fiber 26 has one end coupled to the third input / output end 18-3 of the second polarization separation / combination module 18. The other end of the third polarization plane preserving optical fiber 22 and the other end of the fourth polarization plane preserving optical fiber 26 are set to a second polarization plane converter 24 (set at a position indicated by B in FIG. 1). Connected through.

  The first optical phase bias circuit 40 is inserted at an arbitrary position in the middle of the third polarization-maintaining optical fiber 22 or the fourth polarization-maintaining optical fiber 26 (in the middle of the fourth polarization-maintaining optical fiber 26 in FIG. 1). Is shown in the example). FIG. 2 is an explanatory diagram showing an internal configuration example of the first optical phase bias circuit 40.

  The first optical phase bias circuit 40 includes a first Faraday rotator 278 that rotates the plane of polarization of linearly polarized light by + 45 °, a second Faraday rotator 280 that rotates the plane of polarization of linearly polarized light by −45 °, The first Babinet Soleil compensator 282 has optical axes X and Y, and the phase difference applied between both optical axes is variable. In FIG. 2, for the sake of convenience, the first optical phase bias circuit 40 is inserted in the middle of the fourth polarization-maintaining optical fiber 26. However, as described above, the insertion location is not limited thereto.

  The optical axis direction of the first optical phase bias circuit 40 is set as follows. That is, now, linearly polarized light parallel to the fast axis direction of the fourth polarization plane preserving optical fiber 26 is output from the left end 276 of the fourth polarization plane preserving optical fiber 26 on the right side in FIG. When it is inserted into 40, first, it passes through the second Faraday rotator 280 and the polarization direction rotates by −45 °. The first Babinet Soleil compensation plate 282 is arranged so that the polarization direction of the linearly polarized light whose polarization plane has been rotated coincides with one of the optical axis directions (the Y axis in FIG. 2). This light passes through the first Babinet Soleil compensator 282 as a linearly polarized wave parallel to the Y axis, and is then input to the first Faraday rotator 278. Then, in the first Faraday rotator 278, the plane of polarization is rotated by + 45 °. Then, this light is converted into a linearly polarized wave whose polarization direction is parallel to the fast axis of the fourth polarization plane preserving optical fiber 26 on the left side in FIG. 2, and the right end of the fourth polarization plane preserving optical fiber 26 on the left side in FIG. 274 and propagates through the fourth polarization-maintaining optical fiber 26 again.

  On the other hand, linearly polarized light parallel to the fast axis direction of the fourth polarization plane preserving optical fiber 26 is output from the right end 274 of the fourth polarization plane preserving optical fiber 26 on the left side in FIG. When inserted, this light first passes through the first Faraday rotator 278 and the polarization direction rotates by + 45 °. At this time, the polarization direction of the linearly polarized light whose polarization plane is rotated coincides with the X-axis direction of the first Babinet Soleil compensation plate 282. This light passes through the first Babinet Soleil compensator 282 as a linearly polarized wave parallel to the X axis, and is then input to the second Faraday rotator 280. Then, in the second Faraday rotator 280, the plane of polarization is rotated by −45 °. Then, this light is converted into a linearly polarized wave whose polarization direction is parallel to the fast axis of the fourth polarization plane preserving optical fiber 26 on the right side in FIG. 2, and the left end of the fourth polarization plane preserving optical fiber 26 on the right side in FIG. 276 and propagates through the fourth polarization-maintaining optical fiber 26 again.

  The first optical bandpass filter 51 is inserted at an arbitrary position in the middle of the optical path of the second polarization-maintaining optical fiber 16. The first optical bandpass filter 51 has at least three light input / output terminals. That is, the first signal light 1 having the wavelength λs passes through the first input / output terminal 51-1 and the second input / output terminal 51-2 in both directions. On the other hand, the first control optical signal 2 and the second control optical signal 3 having the wavelength λp pass through the second input / output terminal 51-2 and the third input / output terminal 51-3 bidirectionally. The optical path connecting the first input / output end 51-1 and the second input / output end 51-2 constitutes a part of the optical path of the second polarization-maintaining optical fiber 16. On the other hand, the other end of the fifth polarization-maintaining optical fiber 52 is connected to the third input / output end 51-3.

  As the first optical bandpass filter 51, for example, an optical bandpass filter using a dielectric multilayer film can be applied. That is, a dielectric multilayer optical bandpass filter having a transmission center wavelength of λs and a predetermined transmission band is prepared, and the first input / output end 51-1 and the second input / output end are provided in an optical path through which the transmitted light passes. 51-2 is prepared, and the third input / output terminal 51-3 may be prepared in the optical path through which the reflected light passes. The predetermined transmission band here means that the transmission band is sufficiently wider than the wavelength band of the first signal light 1 and exceeds the wavelength band of the control light signals (control light signals 2 and 3). It means a band that is narrow enough not to wrap.

  The fifth polarization plane preserving optical fiber 52 has one end coupled to a second input / output end 50-2 of a third polarization separation / combination module 50 described later, and a third input / output end of the first optical bandpass filter 51. The other end is connected to 51-3.

  As a polarization-maintaining optical fiber suitable for use as the first to fifth polarization-maintaining optical fibers 12, 16, 22, 26, 52, a panda type optical fiber as shown in FIG. This panda type 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. 3 is a cross-sectional view showing a schematic structure of a cross section cut perpendicularly to the light propagation direction of the panda optical fiber.

Two stress applying portions 144 are formed in a clad 140 surrounding the core 142 through which light is guided so as to sandwich the core 142. For example, the clad 140 is made of SiO ₂ , the core 142 is made of SiO ₂ doped with GeO ₂ , and the stress applying portion 144 is made of SiO ₂ doped with B ₂ O ₃ .

  By forming in this way, in FIG. 3, in the direction of the slow axis set in a plane perpendicular to the light propagation direction of the panda optical fiber, and the direction of the fast axis orthogonal to the slow axis The equivalent refractive index for light guided through the core 142 is different. That is, since the stress applying portion 144 having a refractive index higher than the refractive index of the clad 140 is disposed near the core 142, the equivalent refraction with respect to light in which the vibration direction of the electric field vector of light is parallel to the slow axis direction. The refractive index is higher than the equivalent refractive index for light whose electric field vector oscillation direction is parallel to the fast axis direction. Due to the asymmetry of the equivalent refractive index, the polarization plane of the light input to the panda optical fiber is preserved and propagated.

  That is, in the panda type optical fiber, when the polarization plane of linear polarization is input according to the slow axis (or fast axis) shown in FIG. 3, it propagates in the panda type optical fiber while maintaining the polarization state. Even at the emission end, it is possible to obtain only a linearly polarized light component whose polarization plane coincides with the slow axis (or fast axis).

  In the following description, for the sake of convenience, when light enters the polarization separation / combination module such as the first polarization separation / combination module 10 and the second polarization separation / combination module 18, A component corresponding to the vibration direction of the electric field vector with respect to the polarization plane selective reflection surface is defined as follows. That is, a component whose electric field vector oscillates in a direction parallel to the incident surface of the incident light incident on the polarization plane selective reflection surface is a p component (also referred to as a p-polarized component or p wave), and a direction perpendicular to the incident surface of the incident light The component in which the electric field vector vibrates is referred to as an s component (also referred to as s polarization component or s wave).

  For example, when light is incident on the first polarization separation / combination module 10, the electric field in a direction parallel to the incident surface with respect to the polarization plane selective reflection surface 10 </ b> R of the polarization separation / combination element constituting the first polarization separation / combination module 10. The component that the vector vibrates is the p component, and the component that the electric field vector vibrates in the direction perpendicular to the incident surface of the incident light is the s component. The same applies to the second polarization separation / combination module 18 and the third polarization separation / combination module 50 described later.

  In the first polarization separation / combination module 10, the p-polarization component input from the first input / output terminal 10-1 is output to the second input / output terminal 10-2, and from the second input / output terminal 10-2. The input s-polarized component is output to the third input / output terminal 10-3. Further, the p-polarized component input from the second input / output terminal 10-2 is output to the first input / output terminal 10-1.

  As the polarization separation / combination module such as the first polarization separation / combination module 10, a suitable one can be selected from commercially available polarization beam splitters. Further, the polarizing beam splitter is not limited to the type using the thin film assumed in the above description, and a so-called polarizing prism using a birefringent crystal can also be used.

  In addition, each input / output end of the polarization separation / combination module such as the first polarization separation / combination module 10 and the input / output ends of the polarization-maintaining optical fibers such as the first to fifth polarization-maintaining optical fibers coupled thereto It is assumed that the polarization direction of the p-wave or s-wave of the polarization separation / combination module is joined to the direction of the slow axis or fast axis of the polarization-maintaining optical fiber. In the following description, for the sake of convenience, it is assumed that the polarization direction of the p wave of each polarization separation / combination module is joined so that the direction of the slow axis of each polarization plane preserving optical fiber matches. . Note that the present invention is not limited to this, and some joints are joined so that the polarization direction of the p-wave of the polarization separation / combination module matches the direction of the fast axis of the polarization-maintaining optical fiber. Even if it is done, the effect of the present invention can be realized.

  In addition, the first polarization plane converter 14 that connects the other end of the first polarization-maintaining optical fiber 12 and the other end of the second polarization-maintaining optical fiber 16 is configured to convert the polarization of the input linearly-polarized light. It has a function of outputting linearly polarized light whose wave direction is rotated by 45 °. Specifically, as shown in FIG. 4A, such a function is achieved by the slow axes of the first and second polarization-maintaining fibers 12 and 16 at the other end faces 74 and 76 facing each other. This can be realized with a structure in which the members are fusion-spliced so as to be rotated by 45 °. Further, as shown in FIG. 4B, the slow axes may coincide with each other, and a half-wave plate 114 may be inserted into the joint. The above-described function can be realized by arranging the optical axis directions of the half-wave plates 114 so as to be rotated by 22.5 ° from each other's slow axis. Hereinafter, for the sake of convenience, the description will be made assuming that a half-wave plate 114 as shown in FIG. 4B is inserted in the first polarization plane converter 14.

  In addition, the second polarization plane converter 24 that connects the other end of the third polarization-maintaining optical fiber 22 and the other end of the fourth polarization-maintaining optical fiber 26 is configured to apply the polarization to the input linearly polarized light. It has a function of outputting linearly polarized light whose wave direction is rotated by 90 °. As shown in FIG. 5, the slow axes of the third and fourth polarization-maintaining fibers 22 and 26 are rotated by 90 ° between the end surfaces 174 and 176 of the other ends facing each other. It can be realized by a fusion spliced form, in other words, a structure in which the slow axis and the fast axis are fused and connected to each other. Similarly to the case of the first polarization plane converter 14, the slow axes are made to coincide with each other, and the optical axis direction of the joint is rotated by 45 ° from each slow axis. It can also be realized by inserting a two-wave plate.

  In the following description, the length of the path from the second input / output terminal 10-2 of the first polarization separation / combination module 10 to the first polarization plane converter 14, that is, the length of the first polarization-preserving optical fiber 12 is described. L1 (also referred to as path L1), the length of the path from the first polarization plane converter 14 to the first input / output end 18-1 of the second polarization separation / combination module 18, that is, the second polarization plane The length of the storage optical fiber 16 is L2 (also referred to as a path L2), the length of the path from the second input / output end 18-2 of the second polarization separation / combination module 18 to the second polarization plane converter 24, That is, the length of the third polarization plane preserving optical fiber 22 is L3 (also referred to as a path L3), and the second polarization plane conversion unit 24 is connected to the third input / output terminal 18-3 of the second polarization separation / combination module 18. The length of the path to be reached, that is, the fourth polarization preserving optical fiber (Also referred to as path L4) to the length of the driver 26 and L4.

  The first polarization plane preserving optical fiber 12, the second polarization plane preserving optical fiber 16, the fourth polarization plane preserving optical fiber 26, and the fifth polarization plane do not particularly contribute to the occurrence of the phase shift due to the optical Kerr effect. Even if all or part of the storage optical fiber 52 is realized not by an optical fiber but by a spatial optical system, the same effect can be obtained.

In addition to the basic configuration as described above, the all-optical optical modulator 6 according to the first embodiment includes a third polarization separation / combination module 50, a first optical circulator 30, a second optical bandpass filter 28, and A third optical bandpass filter 38 is provided.

  The third polarization separation / combination module 50 includes a first input / output end 50-1 to which one end of the first control light input optical fiber 31 for inputting the first control light signal 2 is coupled, The second input / output end 50-2, which is disposed on the side facing the first input / output end 50-1 and to which one end of the fifth polarization-maintaining optical fiber 52 is coupled, and the second control optical signal 3 And a third input / output end 50-3 to which one end of the second control light input optical fiber 33 for input is coupled.

  The first and second control light signals 2 and 3 pass through the first control light input optical fiber 31 and the second control light input optical fiber 33, respectively. The first input / output terminal 50-1 and the third input / output terminal 50-3 are input.

  In order to minimize the optical loss of the control optical signals 2 and 3 in the third polarization separation / combination module 50 from the polarization separation / combination characteristics of the polarization separation / combination module described above, the first control optical signal 2 is p. It is desirable that the linearly polarized light is polarized in the polarization direction, and the second control light signal 3 is linearly polarized light polarized in the s-polarization direction. In order to realize this, it is desirable that the first control light input optical fiber 31 and the second control light input optical fiber 33 described above are also polarization-maintaining optical fibers. Alternatively, each of the first and second control optical signals 2 and 3 is an optical path (for input) to the first input / output end 50-1 and the third input / output end 50-3 of the third polarization splitting / combining module 50. A polarization plane controller is inserted into any part of the optical fibers 31 and 33) so that the polarization states of the control optical signals 2 and 3 are adjusted to a desired polarization state. Also good.

  The first and second control optical signals 2 and 3 introduced into the third polarization separation / combination module 50 are combined and output from the second input / output terminal 50-2 of the third polarization separation / combination module 50. . At that time, due to the property of the polarization separation / combination module, the first and second control optical signals 2 and 3 in the combined output are output in a state of linearly polarized light and in a state of being orthogonal to each other. The

  The combined output light is coupled to the fifth polarization-maintaining optical fiber 52 and maintains a linearly polarized light state of mutually orthogonal polarizations, and then the third input / output end 51-of the first optical bandpass filter 51. 3, output from the second input / output terminal 51-2 of the first optical bandpass filter 51, and input to the first input / output terminal 18-1 of the second polarization separation / combination module 18.

  Since the first control optical signal 2 is input as linearly polarized light in the p-polarization direction to the first input / output terminal 18-1 of the second polarization separation / combination module 18, the second polarization separation / combination module 18. Are then output from the second input / output terminal 18-2, reach the third polarization-maintaining optical fiber 22 that causes the optical Kerr effect, and propagate through the third polarization-maintaining optical fiber 22.

  On the other hand, since the second control optical signal 3 is input as linearly polarized light in the s-polarization direction to the first input / output terminal 18-1 of the second polarization separation / combination module 18, the second polarization separation / combination is performed. After being output from the third input / output terminal 18-3 of the module 18, the polarization plane is 90 ° at the second polarization plane converter 24 via the fourth polarization plane preserving optical fiber 26 and the first optical phase bias circuit 40. After the rotation, the optical fiber reaches the third polarization-preserving optical fiber 22 that causes the optical Kerr effect, and propagates through the third polarization-preserving optical fiber 22 against the first control optical signal 2.

  As described above, the first and second control optical signals 2 and 3 propagate in the same polarization direction while going backward in the third polarization-maintaining optical fiber 22. Both of them propagate in the third polarization plane-maintaining optical fiber 22 in reverse directions, and are polarized waves of two first signal light components (S1 component and S2 component) described later in the operation section. It also matches the direction. For example, both of them propagate through the third polarization-maintaining optical fiber 22 as linearly polarized light parallel to the slow axis of the third polarization-maintaining optical fiber 22.

  The first optical circulator 30 is connected to the second input / output end 30-2 for the first signal light 1 input from the input optical fiber 32-2 connected to the first input / output end 30-1. Output to the input optical fiber 32-2 and input to the first input / output terminal 10-1 of the first polarization separation / combination module 10. The first optical circulator 30 is output from the first input / output terminal 10-1 of the first polarization splitting / combining module 10 and input to the second input / output terminal 30-2 via the input optical fiber 32-2. The output light is output to the output optical fiber 37 connected to the third input / output terminal 30-3.

  The optical band-pass filter 38 is a predetermined band of light output from the third input / output end 30-3 of the first optical circulator 30 to the output optical fiber 37 (the center wavelength coincides with the wavelength λs of the signal light). In other words, only the control light component having the wavelength λp is cut off and output to the output optical fiber 39.

  The optical bandpass filter 28 is a predetermined band of light output from the third input / output terminal 10-3 of the first polarization splitting / combining module 10 to the output optical fiber 27 (the center wavelength matches the wavelength λs of the signal light). In other words, the control light component having the wavelength λp is cut off and output to the output optical fiber 29.

(A-2) Operation of the First Embodiment Next, the operation of the all-optical optical modulator 1 according to the first embodiment having the above configuration will be described.

  The first signal light 1 that is the controlled light having the wavelength λs is input to the input optical fiber 32-2 and reaches the first input / output terminal 10-1 of the first polarization separation / combination module 10. Here, as the first signal light 1, continuous light is used when the data format of the desired modulated optical signal is the NRZ format, and an optical pulse train is used when the data format is the RZ format. When an optical pulse train is used, the pulse period is the reciprocal of the bit rate of the desired optical modulation signal. For example, the intensity-modulated or phase-modulated optical modulation signal having a data rate of 10 gigabits per second (Gbit / s) is finally obtained. If desired, the pulse time interval of the first signal light, which is an optical pulse train, is 100 picoseconds, and its repetition frequency is 10 gigahertz (GHz).

  The polarization direction of the first signal light 1 reaching the first input / output terminal 10-1 of the first polarization separation / combination module 10 is adjusted so as to be linearly polarized light parallel to the p polarization component. Yes. As a result, the first signal light 1 is output as linearly polarized light from the second input / output terminal 10-2 of the first polarization separation / combination module 10, and then the first polarization plane preserving optical fiber 12 It propagates as linearly polarized light parallel to the slow axis and reaches the first polarization plane converter 14 from the path on the left side in FIG.

  As described above, the first polarization plane preserving optical fiber 12 and the second polarization plane preserving optical fiber 16, which are input / output ports to the first polarization plane converter 14, are disposed between the opposing fiber end faces 74 and 76 as described above. When the two-wavelength plate 114 is inserted and the first polarization plane converter 14 is configured (see FIG. 4B), the following adjustment is made.

  That is, adjustment is made so that the slow axis directions of the fiber end faces 74 and 76 coincide with each other (FIG. 6A). Furthermore, the optical axis of any one of the first polarization plane converter 14 is tilted by 22.5 ° from the slow axis of the first polarization plane preserving optical fiber 12 (FIG. 6).

  At this time, the polarization direction of the first signal light 1 coupled to the second polarization-maintaining optical fiber 16 via the first polarization-plane conversion unit 14 is aligned with the slow axis of the second polarization-maintaining optical fiber 16. On the other hand, it becomes linearly polarized light inclined by 45 ° (FIG. 6A). Thereafter, the first signal light 1 propagates through the second polarization-maintaining optical fiber 16 in a manner that is divided into a linearly polarized light component parallel to the slow axis and a linearly polarized light component parallel to the fast axis direction. In the middle, after inputting / outputting the first input / output end 51-1 and the second input / output end 51-2 of the first optical bandpass filter 51, the first input / output end 18- 1 is input. Here, it is assumed that the first signal light 1 does not cause any polarization conversion before and after passing through the first optical bandpass filter 51. That is, the component of the first signal light 1 that has propagated through the second polarization-maintaining optical fiber 16 on the left side of the first optical bandpass filter 51 in FIG. 1 passes through the first optical bandpass filter 51 and then passes through the second polarization plane preserving optical fiber 16 on the right side of the first optical bandpass filter 51 in FIG. It propagates as a wave component. Similarly, the first signal light 1 that has propagated as a linearly polarized light component parallel to the fast axis in the second polarization-maintaining optical fiber 16 on the left side of the first optical bandpass filter 51 in FIG. After passing through the first optical bandpass filter 51, the component passes through the second polarization plane preserving optical fiber 16 on the right side of the first optical bandpass filter 51 in FIG. 1 and is also a straight line parallel to the fast axis direction. It propagates as a polarized light component.

  Although detailed description is omitted, even when the first polarization plane converter 14 is configured as shown in FIG. 4A, the polarization plane of the first signal light 1 is shown in FIG. 6A. Can be converted.

  In the following, the linearly polarized light component of the first signal light 1 that is coupled from the first polarization plane converter 14 to the second polarization-maintaining optical fiber 16 and parallel to the slow axis is the S1 component, and is parallel to the fast axis direction. The linearly polarized light component of the first signal light 1 is defined as the S2 component. The intensity ratio of the S1 component and the S2 component is such that the polarization direction of the first signal light 1 that is linearly polarized light coupled to the second polarization plane-maintaining optical fiber 16 is slow in the second polarization plane-maintaining optical fiber 16. It is 1: 1 because it is inclined by 45 ° with respect to the axis.

  In the following, in order to represent the polarization direction and optical phase state of an optical signal for convenience, vector notation as exemplified in FIGS. 6A and 6B will be used.

  That is, the polarization state of the signal light when the first signal light 1 is input from the first polarization-maintaining optical fiber 12 to the second polarization-maintaining optical fiber 16 is expressed as shown in FIG. The The first signal light 1 propagating through the first polarization plane preserving optical fiber 12 is linearly polarized light parallel to the slow axis, which is indicated by an upward arrow in FIG. When the first signal light 1 is input to the second polarization-maintaining optical fiber 16, the polarization direction is rotated by 45 ° clockwise relative to the slow axis of the second polarization-maintaining optical fiber 16. Yes. Therefore, the S1 component is shown as an upward arrow in FIG. 6A, and the S2 component is shown as a right arrow in FIG. 6A. The amplitudes of the S1 component and the S2 component are equal. Also, at this stage, there is no relative phase difference between them.

  Thereafter, the S1 component and the S2 component of the first signal light 1 are both input to the first input / output terminal 18-1 of the second polarization splitting / combining module 18 and input to the input / output terminals 18-2 and 18-3, respectively. Branch output. That is, the S1 component is output to the input / output terminal 18-2, and the S2 component is output to the input / output terminal 18-3.

  The first and second control light signals 2 and 3 have a wavelength λp different from the wavelength λs of the first signal light 1 described above. The bit rates of the first and second control optical signals 2 and 3 coincide with the bit rate f [bit / s] of the desired optical modulation signal. For example, as described above, if the bit rate of the desired optical modulation signal is 10 gigabits per second, the first and second control optical signals 2 and 3 are modulated optical signals having a bit rate of 10 gigabits per second.

  The first and second control light signals 2 and 3 are intensity modulation signals whose intensity time waveforms are complementary to each other. That is, the intensity time waveform of the first control light signal 2 as shown in FIG. 7A and the intensity time waveform of the second control light signal 3 as shown in FIG. 7C on the time axis. When added together, a continuous light-like time waveform with no change in intensity on the time axis is obtained. Incidentally, in the example of FIG. 7, when the optical signal (the first control optical signal 2 or the second control optical signal 3) is “1”, the peak intensity is Ip (≠ 0), and the signal is “0”. A so-called On-Off Keying (OOK) optical signal is assumed in which the peak intensity of the signal is zero. In the following description, the first and second control light signals 2 and 3 are OOK signals unless otherwise specified. However, the control light signal is not limited to the OOK signal. For example, it may be a general Amplitude Shift keying (ASK) signal in which the peak intensity when the signal is “0” is not zero.

  The first control optical signal 2 is input to the first input / output terminal 50-1 of the third polarization separation / combination module 50 via the input optical fiber 31. The first control optical signal 2 is desirably linearly polarized light polarized in the p polarization direction. The second control optical signal 3 is input to the third input / output terminal 50-3 of the third polarization separation / combination module 50 via the input optical fiber 33. The second control optical signal 3 is preferably linearly polarized light polarized in the s polarization direction.

  Here, one optical pulse signal included in the first control optical signal 2 and one optical pulse signal included in the second control optical signal 3 that is complementary to the first optical signal 2 are obtained by the first signal light 1. Before the first polarization plane conversion unit 14 separates the S1 component and the S2 component (that is, at the right end of the first polarization plane preserving optical fiber 12), the first signal light 1 at the same time position For the polarization components S 1 and S 2, the temporal timing is adjusted so as to cause a phase shift based on the mutual phase modulation in the third polarization-maintaining optical fiber 22. For this purpose, the temporal timings at which the first and second control optical signals 2 and 3 are respectively input to the input optical fibers 31 and 33 are adjusted by an optical delay circuit (not shown) or the like. Further, when the first signal light 1 is an optical pulse train, one optical pulse signal included in the control optical signal (2 or 3) and one optical pulse signal included in the first signal light 1 are included. The delay time of the control light signal (2 or 3) or the first signal light 1 is adjusted so as to coincide with the time when the light is input to the third polarization-maintaining optical fiber 22.

  In the third polarization-maintaining optical fiber 22 that generates the optical Kerr effect, when there is a walk-off effect between the control light signal and the first signal light due to group velocity dispersion, the cross-phase modulation effect due to the optical Kerr effect. In some cases, the optical pulse position of the control light and the optical pulse position of the first signal light are input with a slight offset.

  The first and second control optical signals 2 and 3 are combined and output from the second input / output terminal 50-2 of the third polarization separation / combination module 50. At this time, the first and second control optical signals 2 and 3 are output in the state of linearly polarized light and in the state of being orthogonal to each other.

  The combined output is coupled to the fifth polarization-preserving optical fiber 52 and maintains a linearly polarized light state of mutually orthogonal polarizations, and then the third input / output end 51-3 of the first optical bandpass filter 51. And output to the second input / output terminal 51-2.

  Thereafter, these two control optical signals 2 and 3 are input to the first input / output terminal 18-1 of the second polarization separation / combination module 18. Then, since the first control optical signal 2 is input to the first input / output terminal 18-1 of the second polarization separation / combination module 18 by the linear polarization in the p polarization direction, the second polarization separation / combination is performed. It is output to the second input / output terminal 18-2 of the module 18, and then reaches the third polarization-maintaining optical fiber 22 that causes the optical Kerr effect, and propagates through the third polarization-maintaining optical fiber 22. On the other hand, since the second control optical signal 3 is input to the first input / output terminal 18-1 of the second polarization separation / combination module 18 as a linearly polarized wave in the s polarization direction, the second polarization separation / combination is performed. After being output to the third input / output terminal 18-3 of the module 18, the polarization plane is 90 ° at the second polarization plane converter 24 via the fourth polarization plane preserving optical fiber 26 and the first optical phase bias circuit 40. After the rotation, the light reaches the third polarization-maintaining optical fiber 22 that causes the optical Kerr effect, and propagates through the third polarization-maintaining optical fiber 22 in the direction opposite to the first control optical signal 2. That is, the first and second control optical signals 2 and 3 propagate in the same polarization direction while going backward in the third polarization-maintaining optical fiber 22.

  The first and second control optical signals 2 and 3 are both two polarization components (S1 component) of the first signal light 1 that propagates in the third polarization plane-maintaining optical fiber 22 in the opposite directions. , S2 component) also coincides with the polarization direction. That is, both the S1 component of the first control light signal 2 and the first signal light 1 are linearly polarized light parallel to the slow axis of the third polarization-maintaining optical fiber 22 and are transmitted through the third polarization-maintaining optical fiber 22. Will propagate in the same direction. In addition, both the S2 component of the second control light signal 3 and the first signal light 1 are linearly polarized light parallel to the slow axis of the third polarization-maintaining optical fiber 22 in the third polarization-maintaining optical fiber 22. Will propagate in the same direction. The traveling directions of the S1 components of the first control light signal 2 and the first signal light 1 in the third polarization-maintaining optical fiber 22 are the same as those of the second control light signal 3 and the first signal light 1. The direction of travel in the third polarization-maintaining optical fiber 22 of the S2 component is reversed.

  Next, the modulation operation in the all-optical modulator 6 according to the first embodiment will be specifically described.

  Here, for the sake of convenience, the finally desired intensity-modulated optical signal is a light intensity that represents “0” when the first control optical signal 2 is “0” (that is, intensity zero), the first control The description will be made on the assumption that the optical signal 2 has a light intensity representing “1” (that is, the intensity is not zero) when the optical signal 2 is “1”. Similarly, the desired phase-modulated optical signal corresponds to the optical phase “0” when the first control optical signal 2 is “0”, and corresponds to the optical phase “π” when the first control optical signal 2 is “1”. It is assumed that the signal is

  For convenience, as described above, the first control light signal 2 has an infinite peak intensity corresponding to the signal “0” to 0 with respect to the peak intensity corresponding to the signal “1”. It is assumed that the signal is an OOK signal having an infinite extinction ratio.

  Further, when the first control light signal 2 is “0”, the first signal light 1 is not subjected to any phase shift due to cross phase modulation. This state is assumed to be an optical signal subjected to phase modulation of optical phase “0”.

  First, consider the case where the first and second control optical signals 2 and 3 are not input. That is, consider a case where the first signal light 1 is not subjected to any phase shift due to cross phase modulation. For the sake of convenience, the discussion will proceed without considering the optical phase bias effect described later in the optical phase bias circuit 40 for the time being.

  The S1 component and the S2 component of the first signal light 1 output from the first polarization plane converter 14 are the second polarization separation / combination module 18, the third polarization plane preserving optical fiber 22, and the second polarization plane converter 24. , The fourth polarization plane preserving optical fiber 26 and the first optical phase bias circuit 40 are passed through a closed loop clockwise or counterclockwise, passed through the second polarization plane preserving optical fiber 16 and again the first polarization plane. Input to the wavefront converter 14.

  Here, the S1 component and the S2 component are input from the left end of the second polarization-maintaining optical fiber 16, that is, the end surface 76 shown in FIG. 4, and the second polarization separation / combination module 18, the third polarization-maintaining optical fiber 22, It passes through a closed loop composed of the second polarization plane converter 24, the first optical phase bias circuit 40, and the fourth polarization plane preserving optical fiber 26, and at the left end (ie, end face 76) of the second polarization plane preserving optical fiber 16. Consider the optical path length to reach again. The optical path length is a value obtained by multiplying the physical length of an optical medium such as an optical fiber by a refractive index.

  The S1 component propagates through the second polarization-maintaining optical fiber 16 as linearly polarized light parallel to its slow axis direction, and then passes through the third polarization-maintaining optical fiber 22 as linearly polarized light parallel to its slow axis direction. Propagate as. Further, after passing through the second polarization plane converter 24, the fourth polarization plane preserving optical fiber 26 is propagated as a linearly polarized wave parallel to the fast axis direction. Then, the second polarization-maintaining optical fiber 16 is propagated as a linearly polarized wave parallel to the fast axis direction, and the first input / output end 51-1 is connected from the second input / output end 51-2 of the first optical bandpass filter. After elapses, the light reaches the left end of the second polarization-maintaining optical fiber 16.

Therefore, the total optical path length through which the S1 component passes can be expressed by equation (1). Here, the refractive index of the slow axis of each polarization-preserving optical fiber is n _s , and the refractive index of the fast axis is n _f .

n _s L2 + n _s L3 + n _f L4 + n _f L2 (1)
On the other hand, the S2 component propagates through the second polarization plane preserving optical fiber 16 as a linearly polarized wave parallel to the fast axis direction, and then passes through the fourth polarization plane preserving optical fiber 26 to a straight line parallel to the fast axis direction. Propagates as polarization. Further, after passing through the second polarization plane converter 24, the third polarization plane preserving optical fiber 22 is propagated as a linearly polarized wave parallel to the slow axis direction. Then, the second polarization-maintaining optical fiber 16 is propagated as a linearly polarized wave parallel to the slow axis direction, and the first input / output end 51-1 extends from the second input / output end 51-2 of the first optical bandpass filter. After elapses, the light reaches the left end of the second polarization-maintaining optical fiber 16.

  Therefore, the total optical path length through which the S2 component passes can be expressed by equation (2).

n _f L2 + n _f L4 + n _s L3 + n _s L2 (2)
Given in.

  From the equations (1) and (2), the S1 component and the S2 component are input from the left end of the second polarization-maintaining optical fiber 16 to the second polarization-maintaining optical fiber 16, and It can be seen that the optical path length until reaching the left end again is exactly the same. That is, when the first and second control optical signals 2 and 3 are not input or both are “0” signals, the second polarization-maintaining optical fiber 16 is between the S1 component and the S2 component. There is no relative optical phase difference until the left end is reached again. Therefore, the vector representation of the S1 component and the S2 component when reaching the left end of the second polarization-maintaining optical fiber 16 again is as shown in FIG. That is, as in the case of FIG. 6A, it is expressed as an upward arrow and a right arrow. However, since the polarization direction of the S1 component is parallel to the fast axis and the polarization direction of the S2 component is parallel to the slow axis, the state when the second polarization plane preserving optical fiber 16 is input to the left end as shown in FIG. Compared with each other, they are replaced with each other. When the optical phase bias effect in the first optical phase bias circuit 40 is not taken into account, the S1 component and the S2 component pass through the same optical path, so that no relative optical phase difference occurs at this stage.

  These S1 component and S2 component pass through the first polarization plane converter 14 in the opposite direction again. As a result, the S1 component and the S2 component when coupled again to the right end of the first polarization-maintaining optical fiber 12 are respectively 45 ° (−45) counterclockwise from the slow axis direction of the first polarization-maintaining optical fiber 12. Is a linearly polarized wave inclined clockwise by 45 ° (assumed to be + 45 °). That is, in terms of a vector, the S1 component is represented as an upward arrow in the diagonal 45 ° direction, and the S2 component is represented as an upward arrow in the diagonal 45 ° direction. Then, since there is no relative optical phase difference between the S1 component and the S2 component for the above reason, as a result, when reaching the right end of the first polarization-maintaining optical fiber 12, the S1 component and the S2 component The first signal light 1 obtained by combining is linearly polarized parallel to the slow axis of the first polarization-maintaining optical fiber 12 (FIG. 6B).

  The first signal light 1 is then input again to the second input / output end 10-2 of the first polarization splitting / combining module 10, and the polarization direction is the p-polarization direction. -1 is output. That is, it is not output to the third input / output terminal 10-3.

  As described above, when there is no control light signal, all components of the first signal light are output from the first input / output terminal 10-1 of the first polarization separation / combination module 10, and the third input / output terminal 10 No output from -3.

  Next, consider a case where the first and second control optical signals 2 and 3 are input. In this case, a phase shift occurs due to the mutual phase modulation based on the optical Kerr effect with respect to the first signal light 1.

  Here, for the sake of simplicity, an RZ format OOK signal that is intensity-modulated with the signal pattern “1011” is considered as the first control light signal 2. FIG. 7A is a schematic diagram of the intensity time waveform. It is assumed that the peak light intensity when the signal is “1” is Ip (> 0), and the minimum light intensity when the signal is “0” is 0. FIG. 7C shows an intensity time waveform of the second control light signal 3 complementary to the first control light signal 2 given in FIG. Since the second control light signal 3 is complementary to the first control light signal 2, when the first control light signal 2 is the signal “1”, a dip (indentation) in which the light intensity decreases to 0 is obtained. And has a constant value of the light intensity Ip when the first control light signal 2 is a signal “0”. Further, the rising and falling time waveforms of the optical signal when the first control optical signal 2 is “1” are respectively the second control optical signal 3 complementary to the first control optical signal 2. It is symmetrical with respect to the time waveform of the fall and rise of dip.

  Now, assume that when the light intensity of the control light signal (2 or 3) is Ip, a phase shift of aπ occurs with respect to the first signal light 1 by cross-phase modulation. The symbol a is uniquely determined by the type of optical fiber used. Here, a assumes a positive value (a> 0).

  Hereinafter, the phase shift that occurs with respect to the first signal light 1 (the S1 component and the S2 component thereof) when the first and second control light signals 2 and 3 and the first optical phase bias circuit 40 are present. Consider.

  The phase shift generated with respect to the first signal light 1 is the sum of the phase shift due to the mutual phase modulation by the control light signal and the phase shift generated by the first optical phase bias circuit 40.

  Here, the effect of the phase shift due to the mutual phase modulation by the control light signal has two contributions: the effect from the control light signal propagating in the same direction as the first signal light 1 and the effect from the reverse control light signal. Exists. That is, for the S1 component of the first signal light 1, there is a contribution from the first control light signal 2 propagating in the same direction and a contribution from the backward second control light signal 3. On the other hand, for the S2 component of the first signal light 1, there is a contribution from the second control light signal 3 propagating in the same direction and a contribution from the first control light signal 2 going backward.

  As described above, when the light intensity of the first control light signal 2 and the second control light signal 3 is Ip, the effect from the control light signal propagating in the same direction is aπ for each of the S1 component and the S2 component. Given in.

  On the other hand, when the fiber length of the third polarization-maintaining optical fiber 22 is a practical length of several tens of meters to several kilometers, the effect of the phase shift due to the mutual phase modulation from the reverse control optical signal is as follows. It is a time-independent continuous phase shift given by the same amount for each pulse of the signal light 1, and the amount is determined by the average intensity of the reverse control light signal. It is made clear in the literature. In practical application, the effect of phase shift due to cross-phase modulation from the reverse control optical signal is obtained for each of the continuous light or optical pulse signal sequence constituting the S1 component and S2 component of the first signal light 1. Can be considered as a time-independent continuous phase shift given by the same amount.

“References” Jinno and T.J. Matsumoto, “Nonlinear Sagnac interferometer switch and its applications”, IEEE J. et al. Quantum Electron. , Vol. 28, no. 4, pp. 875-882, 1992
On the other hand, even if the first and second control optical signals 2 and 3 have the same wavelength, the duty ratio and the mark rate are not necessarily the same. That is, even if their peak light intensities are the same at Ip, the average intensities generally do not match. However, one exception is when the first and second control light signals 2 and 3 are NRZ signals with a duty ratio of 0.5 and a mark ratio of 0.5.

  That is, the time-continuous phase shift amount due to the mutual phase modulation by the retrograde control optical signal for the S1 component and the S2 component generally does not match. However, this phase shift is always given to the individual signal lights constituting the S1 component and S2 component by the same amount regardless of the control light signals “1” and “0”.

  Here, it is assumed that the difference in time continuous phase shift amount due to the mutual phase modulation by the retrograde control optical signal as described above, which occurs between the S1 component and the S2 component, is bπ. Here, it is assumed that the phase shift given to the S2 component of the first signal light is larger by bπ than the phase shift given to the S1 component of the first signal light. However, b can take either positive or negative values (including 0) depending on the duty ratio of the control light and the mark rate.

  Next, the phase shift by the first optical phase bias circuit 40 will be considered with reference to FIG. Here, it is assumed that the first optical phase bias circuit 40 is inserted in the middle of the fourth polarization-maintaining optical fiber 26.

  First, as described above, the S1 component is output as a linearly polarized wave parallel to the fast axis from the left end 276 of the fourth polarization plane preserving optical fiber 26 on the right side in FIG. 2, and is output to the first optical phase bias circuit 40. Combined. The first optical phase bias circuit 40 first passes through the second Faraday rotator 280 and rotates the polarization direction by −45 °. The first Babinet Soleil compensator 282, which is a birefringent medium, is arranged so that the polarization direction of the S1 component that has undergone polarization rotation coincides with one of the optical axis directions (the Y axis in FIG. 2). ing. The S1 component passes through the first Babinet Soleil compensator 282 as a linearly polarized wave parallel to the Y axis, and is then input to the first Faraday rotator 278. Then, in the first Faraday rotator 278, the polarization direction is rotated by + 45 °. As a result, the S1 component is a linearly polarized wave whose polarization direction is parallel to the fast axis of the fourth polarization plane preserving optical fiber 26 on the left side in FIG. It is coupled to the right end 274 and propagates through the fourth polarization-maintaining optical fiber 26 again.

  On the other hand, the S2 component is output as a linearly polarized wave parallel to the fast axis from the right end 274 of the fourth polarization plane preserving optical fiber 26 on the left side in FIG. Combined. The first optical phase bias circuit 40 first passes through the first Faraday rotator 278 and rotates the polarization direction by + 45 °. At this time, the polarization direction of the S2 component that has undergone polarization rotation coincides with the X-axis direction of the first Babinet Soleil compensation plate 282. Therefore, the S2 component passes through the first Babinet Soleil compensator 282 as a linearly polarized wave parallel to the X axis. Thereafter, the signal is input to the second Faraday rotator 280. Then, in the second Faraday rotator 280, the deflection direction is rotated by −45 °. As a result, the S2 component is a linearly polarized wave whose polarization direction is parallel to the fast axis of the fourth polarization plane preserving optical fiber 26 on the right side in FIG. It is coupled to the left end 276 and propagates through the fourth polarization-maintaining optical fiber 26 again.

  That is, the S1 component and the S2 component pass through each optical path in the same polarization state as described above except for the insertion of the first optical phase bias circuit 40. That is, there is no change in the optical phase and polarization direction of the S1 component and S2 component of the first signal light 1 other than where the first optical phase bias circuit 40 is inserted.

  On the other hand, the insertion of the first optical phase bias circuit 40 causes the S1 component and the S2 component to be orthogonal to the first Babinet Soleil compensator 282, which is a birefringent medium, disposed in the first optical phase bias circuit 40. Passes in a state of linear polarization parallel to the optical axis (X axis, Y axis). Therefore, an optical phase difference based on the birefringence of the first Babinet Soleil compensator 282 that is a birefringent medium is generated between the two components.

  Therefore, by inserting the first optical phase bias circuit 40 as described above, a time-independent continuous phase difference can be given between the S1 component and the S2 component of the first signal light 1 as a result. it can. This phase difference may later be referred to as a phase offset that occurs in the first optical phase bias circuit 40. If the phase difference to be applied is variable in the first Babinet Soleil compensator 282, an arbitrary phase difference can be imparted between the S1 component and the S2 component of the first signal light 1.

  Now, it is assumed that the phase difference generated between the S1 component and the S2 component described above that occurs in the first optical phase bias circuit 40 is cπ. Here, it is assumed that this phase difference occurs because the phase of the S2 component of the first signal light 1 is larger by cπ than the phase shift of the S1 component of the first signal light 1. However, c is the optical phase difference given by the first Babinet Soleil compensator 282 and can take either positive or negative values (including 0).

  From the above-described considerations, and further, the time waveform of the phase shift by the cross-phase modulation based on the control light signal propagating in the same direction as the first signal light 1 is proportional to the intensity time waveform of the control light signal. Phase shift time waveforms generated for the S1 component and the S2 component of the first signal light 1 are as shown in FIGS. 7B and 7D, respectively.

  That is, for the S1 component, when the first control light signal 2 is the signal “1” (intensity Ip), the phase shift is aπ (peak phase shift), and the first control light signal 2 is the signal. When “0” (intensity 0), the phase shift is zero.

  On the other hand, for the S2 component, when the first control light signal 2 is the signal “1”, the phase shift has a dip that decreases to (b + c) π, and the first control light signal 2 is the signal. When “0”, it has a constant value of phase shift (a + b + c) π. Here, (b + c) π corresponds to a continuous phase offset amount that does not change with time provided by the retrograde control optical signal and the first optical phase bias circuit 40.

  In addition, the rising and falling time waveforms of the phase shift with respect to the S1 component when the first control light signal 2 is “1” are respectively the second control light having a complementary relationship with the first control light signal 2. It is symmetrical in time with the falling and rising time waveforms of the phase shift dip with respect to the S2 component generated by the signal 3.

  Next, referring to the time waveforms of the phase shift of the S1 component and S2 component of the first signal light 1 shown in FIGS. 7B and 7D, the first and second control light signals 2 and 3 are used. A modulation operation that is executed by the all-optical optical modulator 6 of the first embodiment when the signal is input will be described.

  As shown in FIG. 7B described above, the first and second control optical signals 2 and 3 are not input (that is, a = b = 0), and the phase by the first optical phase bias circuit 40 When there is no offset (that is, c = 0), the first signal light 1 is re-input to the first polarization plane preserving fiber 12 as linearly polarized light parallel to the slow axis direction. All of them are output to the first input / output terminal 10-1 of the first polarization separation / combination module 10 and are not output to the third input / output terminal 10-3.

  That is, when no phase shift is given to the S1 component and the S2 component of the first signal light 1, the first signal light 1 is sent to the third input / output terminal 10-3 of the first polarization separation / combination module 10. Is not output.

  In addition, even if some phase shift is given to the S1 component and the S2 component of the first signal light 1, if they are the same amount, the relative relationship between the S1 component and the S2 component of the first signal light 1 Therefore, the first signal light 1 is re-input to the first polarization plane preserving fiber 12 as linearly polarized light parallel to the slow axis direction. It is not output to the third input / output terminal 10-3 of the polarization beam splitting / combining module 10.

  That is, in order for the first signal light 1 to be output from the third input / output terminal 10-3 of the first polarization beam splitting / combining module 10, there is a relative relationship between the S1 component and the S2 component of the first signal light 1. There must be a phase difference.

  1 and 6B, the first signal light 1 is output from the third input / output terminal 10-3 of the first polarization separation / combination module 10 at the maximum output intensity. This is a case where the first signal light 1 is re-input to the first polarization plane preserving fiber 12 as linearly polarized light parallel to the fast axis direction. That is, this is presumed from FIG. 6B, in which the S1 component arrow of the first signal light 1 is leftward and the S2 component arrow is upward, or the first signal light 1 Either the S2 component arrow is pointing downward and the S1 component arrow is pointing to the right. That is, this means that a phase difference of π is relatively generated between the S1 component and the S2 component of the first signal light 1.

  That is, in the first embodiment, when a relative phase difference of π occurs between the S1 component and the S2 component of the first signal light, the third input / output terminal 10 of the first polarization separation / combination module 10 is used. -3 outputs a modulated optical signal.

  In the first embodiment, the first polarization separation is performed by appropriately adjusting the peak intensity Ip of the control light 2 or 3, and hence the phase shift aπ, and the phase offset cπ given by the first optical phase bias circuit 40. From the third input / output terminal 10-3 of the synthesis module 10, a modulated optical signal in any format of an intensity modulated optical signal and a phase modulated optical signal can be output.

(A) Intensity Modulation Operation First, the operation of the all-optical optical modulator 1 according to the first embodiment when outputting an intensity-modulated optical signal will be described with reference to FIG.

  Assume that the intensity-modulated optical signal desired to be generated is an intensity-modulated optical signal having the same code pattern (“1011”) as that of the first control optical signal 2 shown in FIG.

  First, when the first control light signal 2 is the signal “0”, the signal light that is the signal “0” (at this stage) is transmitted from the third input / output terminal 10-3 of the first polarization separation / combination module 10. In order to output an intensity-modulated optical signal, the phase shift of the S1 component (0) and the phase shift of the S2 component (0) corresponding to when the first control optical signal 2 is the signal “0” The phase difference from (a + b + c) π) must be equivalently zero.

  At the same time, when the first control light signal 2 is the signal “1”, the signal light that is the signal “1” is output from the third input / output terminal 10-3 of the first polarization separation / combination module 10. In order to do this, the phase difference between the phase shift (aπ) of the S1 component and the phase shift ((b + c) π) of the S2 component corresponding to when the first control light signal 2 is the signal “1” is , Equivalently π.

  As a, b, and c that satisfy the above-mentioned two conditions and a is the minimum, one satisfying the expression (3) is obtained. Here, the reason why the solution having a minimum a is obtained is that the required light intensities of the first and second control optical signals are minimized.

a = 0.5 b + c = −0.5 (3)
That is, the peak intensities Ip of the first and second control optical signals 2 and 3 and thus the phase shift aπ and the phase offset cπ given by the first optical phase bias circuit 40 are set so as to satisfy the expression (3). Thus, a modulated optical signal in the intensity modulated optical signal format can be output from the third input / output terminal 10-3 of the first polarization separation / combination module 10.

(B) Phase Modulation Operation Next, the operation of the all-optical optical modulator 1 according to the first embodiment when outputting a phase-modulated optical signal will be described with reference to FIG.

  The phase-modulated optical signal to be generated is a phase-modulated optical signal having a phase modulation pattern of “π0ππ” corresponding to the code pattern (“1011”) of the first control optical signal 2 shown in FIG. Suppose there is.

  In this case, regardless of the optical signals “1” and “0” of the first control optical signal 2, signal light (this stage is always output from the third input / output terminal 10-3 of the first polarization splitting / combining module 10). In this case, it is a phase-modulated optical signal). Therefore, the phase difference between the phase shift 0 of the S1 component and the phase shift (a + b + c) π of the S2 component corresponding to when the first control optical signal 2 is the signal “0”, and the first control optical signal The phase difference between the phase shift aπ of the S1 component and the phase shift (b + c) π of the S2 component corresponding to when 2 is the signal “1” must be equivalently π.

  Further, since the output phase-modulated optical signal is the desired phase-modulated optical signal of “π0ππ”, the S1 component corresponding to the case where the first control optical signal 2 is the signal “0”. The phase difference between the phase shift 0 and the phase shift aπ of the S1 component corresponding to when the first control light signal 2 is the signal “1” must also be equivalently π.

  As a, b, and c that satisfy the above-described conditions and a is the minimum, those satisfying the expression (4) are obtained.

a = 1 b + c = −0 (4)
That is, by setting the peak intensity Ip of the control light, and thus the phase shift aπ, and the phase offset cπ given by the first optical phase bias circuit 40 so as to satisfy the expression (4), the first polarization separation / combination module A modulated optical signal in the phase modulated optical signal format can be output from the tenth third input / output terminals 10-3.

  Next, the frequency chirp characteristic of the modulated optical signal generated by the above-described method will be considered.

  As described above, the first control light signal 2 and the second control light signal 3 are intensity-modulated signals in a complementary relationship, and the S1 component when the first control light signal 2 is “1”. The time waveform of the rising part and the falling part of the phase shift is symmetrical in time with the time waveform of the falling part and the rising part of the phase shift dip generated for the S2 component by the second control light signal 3, respectively. Become.

  With reference to FIGS. 7B and 7D, in other words, the time waveform of the phase shift of the S2 component is inverted upside down on the phase shift axis with respect to the time waveform of the phase shift of the S1 component, A waveform obtained by giving a phase offset of (a + b + c) π to it.

Now, the time waveform S ₁ (t) of the phase shift of the S1 component is proportional to the intensity time waveform of the first control light signal 2, and the time waveform I (t) whose peak intensity is normalized to 1 is used. , (5), a time waveform S ₂ (t) of the phase shift of the S2 component is given by (6).

S ₁ (t) = aπI (t) (5)
S ₂ (t) = (a + b + c) π−aπI (t) (6)
At this time, the amplitude time waveform E _mod (t) of the signal light (modulated optical signal) output from the third input / output terminal 10-3 of the first polarization beam splitting / combining module 10 is the same as that of the signal light having the same amplitude. When the interference waveforms of the S1 component and the S2 component are S ₁ (t) = S ₂ (t) = 0, that is, when the phase shifts of the S1 component and the S2 component are both 0, Considering that S ₁ (t) = 0 because the output from the third input / output terminal 10-3 of the polarization splitting / combining module 10 is 0, it is expressed by equation (7) (j Is an imaginary symbol).

E _mod (t) = e ^{jS1 (t)} −e ^{jS2 (t)} (7)
If the right side of the equation (7) is rewritten by dividing it into a real part Re {E _mod (t)} and an imaginary part Im {E _mod (t)}, the equations (8) and (9) are obtained.

Re {E _mod (t)} = cos (aπI (t)) [1-cos {(a + b + c) π}]
-Sin {(a + b + c) π} sin (aπI (t)) (8)
Im {E _mod (t)} = sin (aπI (t)) [1 + cos {(a + b + c) π}]
-Sin {(a + b + c) π} cos (aπI (t)) (9)
From equations (8) and (9), if a, b, and c satisfy the following equation (10), the real part Re {E _mod (t)} or the imaginary part Im {E _mod (t) }, It can be seen that either one is completely zero.

sin {(a + b + c) π} = 0 (10)
This means that when the expression (10) is satisfied, the amplitude time waveform E _mod (t) of the signal light (modulated optical signal) output from the third input / output terminal 10-3 of the first polarization separation / combination module 10 is satisfied. Means that it is always expressed as a real number or a pure imaginary number regardless of the time t, that is, it means that the modulated optical signal has no frequency chirping.

  The conditions of the above-described formulas (3) and (4) satisfy the conditions of formula (10). That is, according to the first embodiment, the intensity-modulated or phase-modulated modulated signal light output from the third input / output terminal 10-3 of the first polarization separation / combination module 10 may not have a frequency chirp. I understand.

  The first control optical signal 2 after passing through the third polarization plane preserving optical fiber 22 in the clockwise direction is supplied to the third input / output terminal 18-3 of the second polarization separation / combination module 18 as s-polarized polarized light. As a result, the signal is output to the first input / output terminal 18-1 of the second polarization separation / combination module 18 after passing through the third polarization-maintaining optical fiber 22. On the other hand, the second control optical signal 3 after passing through the third polarization-maintaining optical fiber 22 counterclockwise is transmitted to the third input / output terminal 18-2 of the second polarization separation / combination module 18 with p-polarization. As a result, it is input as polarized light, and is output to the first input / output terminal 18-1 of the second polarization separation / combination module 18.

  Thereafter, the first and second control optical signals 2 and 3 are input to the second input / output terminal 51-2 of the first optical bandpass filter 51 and output to the third input / output terminal 51-3, respectively. Here, when the transmission characteristic of the first optical bandpass filter 51 is ideal, that is, when the transmittance of the first optical bandpass filter 51 is 0% at the wavelength λp, the control optical signal is the first optical bandpass filter. 51 is not output to the first input / output terminal 51-1, and as a result, is not output to the third input / output terminal 10-3 of the first polarization separation / combination module 10.

  The control optical signal output to the third input / output terminal 51-3 of the first optical bandpass filter 51 is then passed through the fifth polarization plane preserving fiber 52 and the third polarization separation / combination module 50. There is a risk of adversely affecting the stable operation of the apparatus or the equipment or transmission system outside the apparatus by going back to the original control light input optical fibers 31 and 33. In order to suppress this, the first optical isolator 60 is inserted in the middle of the fifth polarization plane preserving fiber 52 to cut such a retrograde control optical signal.

  On the other hand, the intensity-modulated or phase-modulated signal light (modulated optical signal) is desired via the output optical fiber 27 coupled to the third input / output terminal 10-3 of the first polarization separation / combination module 10. And output as a modulated optical signal. When the transmission characteristics of the first optical bandpass filter 51 are ideal, the control light component is not included in the output light from the output optical fiber 27, and this is ultimately used as a desired modulated optical signal. Can be used practically.

  On the other hand, when the transmission characteristics of the first optical bandpass filter 51 are insufficient, the leakage light of the first and second control optical signals 2 and 3 is caused by the first input / output end 51 of the first optical bandpass filter 51. -1 and the result is output to the third input / output terminal 10-3 of the first polarization beam splitting / combining module 10 and further to the output optical fiber 27.

  In such a case, the second optical bandpass filter 28 whose transmission center wavelength is λs is connected to the output end of the output optical fiber 27, and only the first signal light wavelength component of the wavelength λs is selectively passed. The component of the control optical signal having the wavelength λp is blocked. The optical component having the wavelength λs that has passed through the second optical bandpass filter 28 is coupled to the output optical fiber 29, and finally a desired modulated optical signal is output.

(A-3) Effect of First Embodiment As described above, according to the first embodiment, an electronic device is used by using a cross-phase modulation effect based on an ultrafast optical Kerr effect in an optical fiber. It is possible to generate an ultrahigh-speed modulated optical signal exceeding the speed limit. Further, the birefringence of the polarization-maintaining fiber that is the main element constituting the apparatus has a structure that is automatically canceled as shown in the equations (1) and (2). That is, complicated and high-precision control such as high-precision adjustment of the polarization-preserving fiber length for canceling birefringence is unnecessary. In other words, in the technique disclosed in Non-Patent Document 1, a simpler and more stable configuration of the optical interferometer is possible without requiring special control means for adjusting the optical path of the interferometer.

  Furthermore, all the polarization crosstalk components generated in the third polarization-maintaining optical fiber 22 and the fourth polarization-maintaining optical fiber 26 do not require the second polarization separation / combination module 18 to be coupled to the optical fiber or the like. It is output to the fourth input / output terminal 18-4. Therefore, the polarization crosstalk component generated in the third polarization-maintaining optical fiber 22 is mixed and output to the originally desired modulated optical signal at the third input / output terminal 10-3 of the first polarization separation / combination module 10. It will never be done.

  Therefore, in order to reduce the required peak intensity of the control optical signal, even if the long third polarization plane preserving optical fiber 22 is used, it is possible to suppress the occurrence of operational instability due to polarization crosstalk generated in those fibers. .

  From the above, in the first embodiment, it is possible to suppress the influence of birefringence due to the use of the polarization plane preserving fiber and the operational instability of the light modulation operation due to the occurrence of polarization crosstalk. Therefore, the all-optical light modulator 6 can be provided in which the characteristics do not change even if the signal light wavelength or the environmental temperature changes, and the highly stable operation characteristics are ensured.

  Furthermore, by adjusting the peak intensity of the control optical signal and the phase offset in the first optical phase bias circuit 40 as appropriate, the modulated optical signal in either the intensity modulation scheme or the phase modulation scheme can be obtained with a single device configuration. Can also be generated.

  Furthermore, the frequency chirping of the generated modulated optical signal can be made zero in principle.

  Further, when an intensity modulated optical signal is generated using the optical modulator 6 of the first embodiment, a modulated optical signal that is output from the output optical fiber 29 and logically inverted from the desired modulated optical signal is generated. Are simultaneously output in the form of propagating backward from the first signal light 1 from the input optical fiber 32-2. This logic inversion signal can also be used as a signal quality monitor for a desired positive logic modulation optical signal.

  In this case, as shown in FIG. 1, the first optical circulator 30 is connected to the signal light input end of the input optical fiber 32-2. The first optical circulator 30 includes optical fibers 32-1, 32-2, and 37 connected to the respective input / output ports. The first signal light input from the optical fiber 32-1 is output from the optical fiber 32-2. The other end of the optical fiber 32-2 is connected to the first input / output end 10-1 of the first polarization separation / combination module 10. The polarization direction of the first signal light 1 input to the first input / output terminal 10-1 of the first polarization separation / combination module 10 is adjusted so as to be linearly polarized light parallel to the p polarization component. It is assumed that The logically inverted modulated optical signal is output from the first input / output terminal 10-1 of the first polarization separation / combination module 10, input to the optical fiber 32-2, and output from the optical fiber 37. As in the case of the desired positive logic modulated optical signal, only the wavelength component of the first signal light 1 that passes through the third optical bandpass filter 38 for removing control light is output as a logically inverted modulated optical signal. Output via the optical fiber 39.

  In the above description, the RZ format OOK signal is considered as the first control optical signal 2, but the effect of the first embodiment is that the first control optical signal 2 is intensity modulated in the NRZ format. It can be obtained even in the case of a signal. In this case, if continuous light is prepared as the first signal light 1, an intensity modulated or phase modulated optical signal in the NRZ format can be obtained. If an optical pulse train is prepared as the first signal light 1, an intensity modulated or phase modulated optical signal in the RZ format can be obtained. In either case, it is possible to provide a high-quality modulated optical signal that does not cause frequency chirping.

  The effects of the first embodiment described above are summarized as follows.

  That is, it is possible to provide an all-optical light intensity / phase modulator capable of generating a modulated optical signal of any format of an amplitude modulation system and a phase modulation system using a single device. At that time, it is possible to generate a high-quality modulated optical signal in which frequency chirping is sufficiently suppressed. Furthermore, it is possible to cope with both NRZ format and RZ format signal formats. Furthermore, highly stable operating characteristics can be ensured in which the characteristics do not change even if the signal light wavelength or the environmental temperature changes.

(B) Second Embodiment Next, a second embodiment of the optical modulator according to the present invention will be described with reference to the drawings. In addition to the configuration of the first embodiment, the optical modulator of the second embodiment includes the first and second control optical signals 2 and 2 that are required in the first embodiment and have a complementary relationship with each other. 3 is an all-optical optical modulator having a configuration of an optical circuit having a function of generating 3.

(B-1) Configuration of Second Embodiment FIG. 8 is a layout diagram showing the configuration of the all-optical modulator 7 according to the second embodiment.

  In FIG. 8, the all-optical modulator 7 according to the second embodiment includes the configuration of the first embodiment shown in FIG. 1 (the third polarization separation / combination module 50, the first optical isolator 60, etc.). In addition to 300, a complementary control light generator 8 for generating the first and second control light signals 2 and 3 is provided.

  The complementary control light generator 8 includes a sixth polarization plane preserving optical fiber 312, a third polarization plane converter 314, a seventh polarization plane preserving optical fiber 316, a fourth polarization separation / combination module 318, and an eighth polarization Wavefront preserving optical fiber 322, fourth polarization plane converter 324, ninth polarization plane preserving optical fiber 326, second optical phase bias circuit 340, fourth optical bandpass filter 351, and tenth polarization plane preserving light At least a fiber 352 and a second optical circulator 330 are provided.

  In FIG. 8, the all-optical type optical modulator body 300 is the same as the central configuration of the all-optical type optical modulator described in the first embodiment. In FIG. 8, a portion 300 corresponding to the all-optical type optical modulator of the first embodiment includes a first control optical signal 2 and a second control optical signal that are necessary for the description of the second embodiment. 3 except for one end of the fifth polarization-maintaining optical fiber 52 for combining and inputting 3 and one end portion of the modulated optical signal output optical fiber 27 or 29 serving as an external extraction port for the desired modulated optical signal, The detailed configuration is omitted and shown as one block.

  The complementary control light generator 8 generates the above-described first and second control light signals 2 and 3 from the second signal light 4 and the third control light signal 5. The light modulation configuration is the same as that of the all-optical light modulator of the embodiment.

  The second signal light 4 is controlled light having a wavelength λp, and is here continuous light.

  The third control optical signal 5 is an intensity-modulated optical signal having a wavelength λs ′ different from the wavelength λp of the second signal light 4. The bit rate of the third control optical signal 5 matches the bit rate f [bit / s] of the desired modulated optical signal.

  Fourth polarization separation / combination module 318, sixth to tenth polarization plane preserving optical fibers 312, 316, 322, 326, 352, third polarization plane conversion unit 314, and fourth polarization plane conversion used in the second embodiment The characteristics, configuration, connection form, and role of each optical component of the unit 324, the second optical phase bias circuit 340, and the fourth optical bandpass filter 351 are the second polarization separation / synthesis module used in the first embodiment. 18, first to fifth polarization plane preserving optical fibers 12, 16, 22, 26, 52, first polarization plane converter 14, second polarization plane converter 24, first optical phase bias circuit 40, first optical band The characteristics, configuration, connection form, and role of each optical component of the pass filter 51 are completely equivalent except for the following one point. In other words, in the first embodiment, the first optical bandpass filter 51 is designed such that the first signal light 1 having the wavelength λs passes through the first and second input / output terminals 51-1 and 51-2. However, in the second embodiment, in the fourth optical bandpass filter 351, only the second signal light 4 having the wavelength λp passes through the first and second input / output terminals 351-1 and 351-2. Is different.

  That is, the fourth polarization separation / combination module 318, the sixth to tenth polarization plane preserving optical fibers 312, 316, 322, 326, and 352, the third polarization plane converter 314, and the fourth polarization plane used in the second embodiment. The conversion unit 324, the second optical phase bias circuit 340, and the fourth optical bandpass filter 351 are the second polarization separation / combination module 18 and the first to fifth polarization-preserving optical fibers 12 used in the first embodiment, respectively. 16, 22, 26, 52, the first polarization plane converter 14, the second polarization plane converter 24, the first optical phase bias circuit 40, the first optical bandpass filter 51, and the optical bandpass filter described above. The optical parts are equivalent except for the difference in the pass characteristics of the second embodiment, and the connection and usage of those optical parts in the second embodiment are the same as those in the first embodiment. Detailed functional description of each optical component embodiment will be omitted. The correspondence to each optical component is as follows (however, the passing wavelength is different).

Second polarization separation / combination module 18 → fourth polarization separation / combination module 318
First to fifth polarization-maintaining optical fibers 12, 16, 22, 26, 52
→ Sixth to tenth polarization-maintaining optical fibers 312, 316, 322, 326, 352
First polarization plane converter 14 → third polarization plane converter 314
Second polarization plane converter 24 → fourth polarization plane converter 324
First optical phase bias circuit 40 → second optical phase bias circuit 340
First optical bandpass filter 51 → fourth optical bandpass filter 351
Hereinafter, only the optical components newly added in the second embodiment will be described in detail with respect to the configuration, functions, and the like.

  The second optical circulator 330 has at least three input / output terminals. The second signal light 4 input from the first input / output terminal 330-1 is output to the second input / output terminal 330-2. Further, the light (first and second control light signals 3 and 4) input from the second input / output terminal 330-2 is output to the third input / output terminal 330-3. In the second optical circulator 330, the polarization state of the input / output light is maintained during these input / output operations. That is, light input as linearly polarized light is output as linearly polarized light.

  The input optical fiber 331 is an optical fiber for inputting the third control optical signal 5, and one end thereof is the third input / output end of the fourth bandpass filter 351 of the tenth polarization plane preserving optical fiber 352. It is connected to the other end not connected to 351-3.

  The input optical fiber 332 is an optical fiber for inputting the second signal light 4, and the first input / output end 330-1 of the second optical circulator 330 is connected to one end thereof.

  The output optical fiber 327 is an optical fiber that outputs an intermediate modulated optical signal obtained by modulating the second signal light 4 as the first control optical signal 2 and the second control optical signal 3. One end is connected to the third input / output end 330-3 of the second optical circulator 330, and the other end is connected to one end of the fifth polarization-maintaining optical fiber 52.

  The second input / output end 330-2 of the second optical circulator 330 is connected to the other end of the sixth polarization-maintaining optical fiber 312 that is not connected to the third polarization plane converter 314. .

  Here, the length of the path from the second input / output end 330-2 of the second optical circulator 330 to the third polarization plane converter 314, that is, the length of the sixth polarization plane preserving optical fiber 312 is L6 (referred to as path L6). The length of the path from the third polarization plane converter 314 to the first input / output end 318-1 of the fourth polarization separation / combination module 318, that is, the length of the seventh polarization plane preserving optical fiber 316 is set. L7 (also referred to as path L7), the path from the second input / output end 318-2 of the fourth polarization separation / combination module 318 to the fourth polarization plane converter 324, that is, the length of the eighth polarization plane preserving optical fiber 322 L8 (also referred to as path L8), a path from the fourth polarization plane converter 324 to the third input / output end 318-3 of the fourth polarization separation / combination module 318, that is, the ninth polarization plane preserving optical fiber 32 The length and L9 (also referred to as path L9).

  Note that the sixth polarization plane preserving optical fiber 312, the seventh polarization plane preserving optical fiber 316, the ninth polarization plane preserving optical fiber 326, and the tenth polarization plane do not particularly contribute to the occurrence of the phase shift due to the optical Kerr effect. The storage optical fiber 352 can achieve the effects of the second embodiment by using a spatial optical system instead of an optical fiber, as in the first embodiment.

  The second signal light 4 (continuous light) having the wavelength λp is input to the optical fiber 332 for signal light input, and the first input / output end 330-1 and the second input / output end 330-2 of the second optical circulator 330. And is input to the sixth polarization-maintaining optical fiber 312. At that time, the polarization state of the second signal light 4 is appropriately adjusted by a polarization plane controller (not shown) or the like so as to be input as linearly polarized light parallel to the slow axis of the sixth polarization plane preserving optical fiber 312. Shall be entered.

  After that, the second signal light 4 is supplied to the first and second input lights of the sixth polarization plane preserving optical fiber 312, the third polarization plane converter 314, the seventh polarization plane preserving optical fiber 316, and the fourth optical bandpass filter 351. It passes through the output terminals 351-1 and 351-2 and reaches the first input / output terminal 318-1 of the fourth polarization splitting / combining module 318.

  The third control optical signal 5 having the wavelength λs ′ is input to the input optical fiber 331, passes through the tenth polarization plane preserving optical fiber 352, and the third input / output end 351- of the fourth optical bandpass filter 351. 3 and output to the second input / output terminal 351-2, and then to the first input / output terminal 318-1 of the fourth polarization separation / combination module 318. At that time, it propagates through the tenth polarization plane preserving optical fiber 352 and the seventh polarization plane preserving optical fiber 316 as, for example, linearly polarized light parallel to its slow axis, and as a result, the fourth polarization separation / combination module 318 The polarization state of the third control optical signal 5 is appropriately set by a polarization controller (not shown) so that the first input / output terminal 318-1 is input in the state of linearly polarized light polarized in the p polarization direction. It shall be adjusted and input.

  After that, the third control optical signal 5 is input to the first input / output terminal 318-1 of the fourth polarization separation / combination module 318 as linear polarization in the p-polarization direction. 8 is output to the second input / output terminal 318-2 of the module 318, and then reaches the eighth polarization-maintaining optical fiber 322 that causes the optical Kerr effect. The eighth polarization-maintaining optical fiber 322 is rotated clockwise in FIG. That is, it propagates in the same traveling direction as the S3 component of the second signal light 4 described later.

  As in the first embodiment, in the eighth polarization-maintaining optical fiber 322, the polarization direction of the third control optical signal 5 propagates backward in the eighth polarization-maintaining optical fiber 322. Thus, it matches the polarization direction of the polarization components (S3 component, S4 component) of two second signal lights 4 to be described later. For example, both of them propagate through the eighth polarization-maintaining optical fiber 322 as linearly polarized waves parallel to the slow axis of the eighth polarization-maintaining optical fiber 322. The direction in which the third control optical signal 5 propagates through the eighth polarization-maintaining optical fiber 322 is one of the polarization components (S3 component, S4 component) of the second signal light 4 (FIG. 8). Corresponds to the S3 component), and is the reverse of the other (S4 component in FIG. 8).

(B-2) Operation of the Second Embodiment Next, the complementary control light generator 8 of the all-optical modulator 7 according to the second embodiment includes the second signal light 4 and the third control. The operation of generating the first and second control optical signals 2 and 3 from the optical signal 5 will be described.

  The second signal light 4, which is continuous light having the wavelength λp, is input to the input optical fiber 332 and passes through the first input / output end 330-1 and the second input / output end 330-2 of the second optical circulator 330. Then, it is input to the sixth polarization-maintaining optical fiber 312 as a linearly polarized wave parallel to the slow axis of the sixth polarization-maintaining optical fiber 312.

  Thereafter, the second signal light 4 propagates through the sixth polarization plane preserving optical fiber 312 as linearly polarized light parallel to its slow axis, and reaches the third polarization plane converter 314 from the path on the left side in FIG. .

  Similarly to the first signal light 1 in the first embodiment, the polarization direction of the second signal light output from the third polarization plane conversion unit 314 and coupled to the seventh polarization plane preserving optical fiber 316 is The light is linearly polarized light that is inclined by 45 ° with respect to the slow axis of the seven-polarization-preserving optical fiber 316. Thereafter, the second signal light 4 propagates through the seventh polarization-maintaining optical fiber 316 separately into a linearly polarized light component parallel to the slow axis and a linearly polarized light component parallel to the fast axis direction. On the way, after inputting / outputting the first input / output end 351-1 and the second input / output end 351-2 of the fourth optical bandpass filter 351, the first input / output end 318-of the fourth polarization splitting / combining module 318- 1 is input. Similarly to the first signal light 1 in the first embodiment, the second signal light 4 does not undergo any polarization conversion before and after passing through the fourth optical bandpass filter 351.

  Here, in the seventh polarization plane preserving optical fiber 316, the linearly polarized light component of the second signal light 4 parallel to the slow axis is the S3 component, and the second signal light 4 parallel to the fast axis direction is the same. The linearly polarized light component is defined as S4 component. The intensity ratio of the S3 component and the S4 component is 1: 1 as in the first signal light 1 in the first embodiment (see FIG. 9A). FIG. 9 is an explanatory diagram of the polarization state of the signal light in the third polarization plane converter 314 in the second embodiment.

  Thereafter, the S3 component and the S4 component of the second signal light 4 are both input to the first input / output terminal 318-1 of the fourth polarization splitting / combining module 318, and input to the input / output terminals 318-2 and 318-3, respectively. Branch output. That is, the S3 component is output to the input / output terminal 318-2, and the S4 component is output to the input / output terminal 318-3.

  Here, consider a case where the third control light signal 5 is not input. Here, the effect of the phase bias in the second optical phase bias circuit 340 is also ignored.

  As described above, most of the optical components that are additionally used in the second embodiment are equivalent to the corresponding components of the first embodiment. Therefore, the second polarization separation / combination module 318, the eighth polarization plane preserving optical fiber 322, the ninth polarization plane preserving optical fiber 326, the fourth polarization plane converter 324, and the second optical phase bias circuit 340 are configured. In the case of the first embodiment, the optical loop in this embodiment is the same as the optical loop constituted by the optical components corresponding to them.

  Therefore, when the third control light signal 5 is not input, the second signal light 4 passes through the above-described optical loop and passes through the sixth polarization-maintaining optical fiber 312 as in the first embodiment. The second signal light 4 that is obtained again by combining the S3 component and the S4 component is linearly polarized light parallel to the slow axis of the sixth polarization-maintaining optical fiber 312.

  The second signal light 4 is then input to the input / output end 330-2 of the second optical circulator and output from the third input / output end 330-3 while maintaining the polarization direction.

  Next, consider a case where the third control light signal 5 is input. In addition, the effect of the phase offset by the second optical phase bias circuit 340 is also considered.

  The third control optical signal 5 having the wavelength λs ′, which is an intensity modulation signal, is input to the input optical fiber 331, passes through the tenth polarization plane preserving optical fiber 352, and passes through the fourth optical bandpass filter 351. 3 input / output terminal 351-3, output to the second input / output terminal 351-2, and then reaches the first input / output terminal 318-1 of the fourth polarization separation / combination module 318. At that time, it propagates through the tenth polarization plane preserving optical fiber 352 and the seventh polarization plane preserving optical fiber 316 as linearly polarized light parallel to the slow axis, and as a result, the first polarization separation combining module 318 of the first The signal is input to the input / output terminal 318-1 in the state of linearly polarized light polarized in the p polarization direction.

  After that, the third control optical signal 5 is input to the first input / output terminal 318-1 of the fourth polarization separation / combination module 318 as linear polarization in the p-polarization direction. It is output to the second input / output terminal 318-2 of the module 318, and then reaches the eighth polarization-maintaining optical fiber 322 that causes the optical Kerr effect, and propagates through the eighth polarization-maintaining optical fiber 322.

  As in the first embodiment, in the eighth polarization-maintaining optical fiber 322, the polarization direction of the third control optical signal 5 propagates backward in the eighth polarization-maintaining optical fiber 322. Thus, it coincides with the polarization directions of two polarization components (S3 component, S4 component) of the second signal light 4 to be described later. For example, they both propagate through the eighth polarization plane maintaining optical fiber 322 as linearly polarized light parallel to the slow axis of the eighth polarization plane maintaining optical fiber 322. The direction in which the third control optical signal 5 propagates through the eighth polarization-maintaining optical fiber 322 is one of the polarization components (S3 component, S4 component) of the second signal light (in FIG. 8). S3 component) and the other (S4 component in FIG. 8).

  As described in the first embodiment, a phase shift due to the mutual phase modulation effect from the third control optical signal 5 propagating in the same direction occurs for the S3 component. The peak value is represented by a′π according to the first embodiment.

  On the other hand, for the S4 component, the third control light signal 5 propagates in the reverse direction. Unlike the case of the first embodiment, there is no control light traveling in the same direction. Therefore, considering the first embodiment, for the S4 component, the time continuous phase shift (b′π) due to the mutual phase modulation by the retrograde control optical signal is given by the second optical phase bias circuit 340. The same time-continuous phase shift (c′π) occurs.

  As a result, the relationship between the time waveform of the phase shift generated for the S3 component and S4 component of the second signal light 4 and the intensity time waveform of the third control light signal 5 can be shown as in FIG. . FIG. 10 shows the relationship between the time waveform of the phase shift of the S1 component and S2 component of the first signal light 1 and the intensity time waveform of the first and second control light signals 2 and 3 in the first embodiment. It is drawing corresponding to FIG.

  That is, the time waveform of the phase shift of the S3 component is proportional to the intensity time waveform of the third control light signal, and when the third control light signal 5 is the signal “1”, the peak phase shift a′π Thus, when the third control light signal 5 is the signal “0”, the phase shift becomes 0 (FIG. 10B). On the other hand, the time waveform of the phase shift of the S4 component has a phase shift amount of (b ′ + c ′) π, and is a time continuous waveform that does not depend on the passage of time (FIG. 10C).

  Here, the peak light intensity of the third control optical signal 5 and the phase offset (c ′) in the second optical phase bias circuit 340 are set so that a ′ = 1 and b ′ + c ′ = 0. Suppose you are given. In this case, the phase shift amount of the S4 component is always zero.

  Therefore, when the third control light signal 5 is the signal “0”, there is no relative phase difference between the S3 component and the S4 component. As a result, at this time, the second signal light 4 passes through the optical loop and reaches the right end of the sixth polarization-maintaining optical fiber 312 again, as in the case where there is no third control light signal 5 described above. Then, the second signal light 4 obtained by combining the S3 component and the S4 component becomes a linearly polarized wave parallel to the slow axis of the sixth polarization-maintaining optical fiber 312 (FIG. 9B).

  On the other hand, when the signal of the third control light signal 5 is “1”, a maximum phase shift of π (= a′π) occurs in the S3 component. That is, a phase difference of π occurs between the S3 component and the S4 component. At this time, when the second signal light 4 passes through the optical loop and reaches the left end of the seventh polarization-maintaining optical fiber 316 again, the direction of the arrow indicating the S3 component is reversed. As a result, the second signal light 4 passes through the optical loop, reaches the right end of the sixth polarization-maintaining optical fiber 312 again, and is obtained by combining the S3 component and the S4 component. Are linearly polarized waves parallel to the fast axis of the sixth polarization-maintaining optical fiber 312 (FIG. 9C).

  That is, the second signal light 4 that passes through the optical loop and reaches the right end of the sixth polarization-maintaining optical fiber 312 again when the signal of the third control optical signal 5 is “0”. Linearly polarized light parallel to the slow axis of the sixth polarization-maintaining optical fiber 312 and straight line parallel to the fast axis of the sixth polarization-maintaining optical fiber 312 when the signal of the third control optical signal 5 is “1” It becomes a polarized light, that is, a polarization modulation signal in which the polarization direction of the linearly polarized wave is rotated by 90 ° by the signal code of the third control light signal 5.

  Thereafter, the signal light that has become the polarization modulation signal is maintained in the polarization state, while maintaining the polarization state maintaining optical fiber 312, the second input / output end 330-2 of the second optical circulator, and the third input / output end. The light passes through 330-3 and is output to the output optical fiber 327 connected to the third input / output end 330-3 of the second optical circulator.

  The signal light output to the output optical fiber 327 is a polarization modulation signal in which the polarization direction of the linearly polarized wave is rotated by 90 ° by the signal code of the third control optical signal 5. At this time, if the time waveform of the sum of the light intensities of the polarization components orthogonal to each other in the polarization modulation signal is taken and the excess loss is ignored, a time waveform that matches the time waveform of the second signal light 4 that was initially input, That is, it is continuous light. This indicates that the components of the signal light output to the output optical fiber 327 are in a complementary relationship with each other whose polarization is orthogonal to each other. Further, in the example of FIG. 8, the signal pattern of the intensity time waveform of one polarization component, which is output as linearly polarized light parallel to the slow axis of the sixth polarization-maintaining optical fiber 312, is the third control light. It matches the signal pattern of signal 5.

  As described above, the components of the signal light (polarization modulation signal) output to the output optical fiber 327 are orthogonal to each other and are orthogonal to each other, and they are orthogonal to each other. This means that the first control optical signal 2 and the second control optical signal 3 which are two control optical signals complementary to each other and orthogonal to each other, which are required in the first embodiment, It means that the light is output from the output optical fiber 327 in the second embodiment. The signal light (polarization modulation signal) output to the output optical fiber 327 is output from the second input / output end 50-2 of the third polarization separation / combination module 50 of FIG. It is equivalent to the optical signal.

  Therefore, the signal light output to the output optical fiber 327 is sent to the optical modulator 300 of the first embodiment via the fifth polarization plane preserving optical fiber 52 and the first and second control optical signals 2 and 3 is input, the modulated optical signal without frequency chirping, which is finally intensity-modulated or phase-modulated due to the effect described in the first embodiment, is converted into the optical modulation of the first embodiment. Is output from the output optical fiber 27 or 29 of the device 300.

  Here, the optical intensity of the signal light output to the output optical fiber 327 is set to an appropriate optical intensity as the first and second control optical signals 2 and 3 for operating the optical modulator 300 of the first embodiment. Therefore, an optical amplifier 370 such as an erbium-doped optical fiber amplifier may be inserted into any optical path connecting one end of the output optical fiber 327 and one end of the fifth polarization-maintaining optical fiber 52.

  Furthermore, in order to remove noise light generated in the optical amplifier 370, or in order to remove the wavelength component of the third control optical signal 5 included in the signal light output to the output optical fiber 327, A fifth optical bandpass filter 338 having a transmission center wavelength of the wavelength λp of the second optical signal 4 is inserted into any optical path connecting the output optical fiber 327 and the fifth polarization-maintaining optical fiber 52. May be.

  On the other hand, the third control optical signal 5 after passing through the eighth polarization plane preserving optical fiber 322 is finally the second input / output end 351 of the fourth optical bandpass filter 351, as in the first embodiment. -2 is output to the third input / output terminal 351-3. Here, when the transmission characteristic of the fourth optical bandpass filter 351 is ideal, the third control optical signal 5 is output to the first input / output terminal 351-1 of the fourth optical bandpass filter 351. As a result, the light is not output to the output optical fiber 327 via the third input / output end 330-3 of the second optical circulator 330. On the other hand, if it is not ideal, the wavelength component of the third control optical signal 5 is output via the third input / output end 330-3 of the second optical circulator 330. It is removed by the fifth optical bandpass filter 338 and is not input to the optical modulator 300 of the first embodiment.

  The third control optical signal 5 output to the third input / output terminal 351-3 of the fourth optical bandpass filter 351 is then transmitted to the original control light input via the tenth polarization plane preserving fiber 352. The optical fiber 331 may be reversed to adversely affect the stable operation of the device or the equipment or transmission system outside the device. In order to suppress this, it is preferable to insert the second optical isolator 360 in the middle of the tenth polarization plane preserving fiber 352 to cut such retrograde control light.

  In the above description, the wavelengths of the second signal light 4 and the first and second control light signals 3 and 4 must be the same (λp). However, the wavelength λs of the first signal light 1 and the wavelength λs ′ of the third control light signal 5 may be different from the wavelengths of the second signal light 4 and the first and second control light signals 3 and 4. The wavelength λs of the first signal light 1 and the wavelength λs ′ of the third control light signal 5 are not necessarily required.

  However, the wavelength λs of the first signal light 1 may coincide with the wavelength λs ′ of the third control light signal (λs = λs ′). In this case, the all-optical optical modulator 300 provides a modulated optical signal having the same wavelength as that of the third control optical signal 5. As described above, the usage of performing the optical modulation operation while maintaining the wavelength is particularly important in optical signal regeneration in a long-distance network. From this, it can be said that the second embodiment can be applied as an optical repeater that performs optical signal regeneration in a long-distance network. That is, the modulated optical signal input to the repeater is used as the third control optical signal 5, and the first signal light 1 after the phase modulation or intensity modulation is used as the output light from the repeater. Then, the wavelength of the input light and the output light to the repeater becomes the same, and it is not necessary to set the wavelength of the input light and the output light for each repeater even when the repeater is connected in multiple stages. It becomes.

(B-3) Effects of the Second Embodiment According to the second embodiment, the complementary control necessary for the operation of the first embodiment is performed using one control light signal (third control light signal). Since the optical circuit capable of generating the first and second control optical signals having the objective relationship is also provided, the following effects become more remarkable as compared with the effects of the first embodiment.

  One of the most suitable forms of application of the all-optical type optical modulator is a case where an ultrahigh-speed modulated optical signal exceeding the speed limit of the electronic device is generated. When the apparatus is operated using only the effect of the first embodiment, it is assumed that the speed of the electronic device is limited in the stage of generating the first and second control light signals having a complementary relationship. The

  On the other hand, by adding the configuration described in the second embodiment, the following operation mode is possible. That is, using the optical TDM, a third control optical signal generated at a data rate exceeding the speed limit of the electronic device is prepared, and the speed limit of the electronic device is set using the configuration described in the second embodiment. In this case, complementary first and second control optical signals are generated and used to operate the optical modulator described in the first embodiment. As a result, it is possible to generate an ultrahigh-speed modulated optical signal that exceeds the speed limit of the electronic device.

(C) Other Embodiments In each of the above embodiments, an optical fiber is considered as a medium that exhibits a cross-phase modulation effect based on the optical Kerr effect. However, the effect obtained by the present invention is the use of such an optical fiber. It is not limited. If the optical device has an effect of changing the optical phase of the controlled light by the control light, the effects of the present invention can be produced using various devices depending on the application form. For example, if the operating bit rate is a relatively low bit rate such as 1 Gb / s, a semiconductor optical amplifier or an electroabsorption optical modulator may be used. In addition, a so-called silicon fine wire waveguide having Si as a core and SiO ₂ as a clad can be used.

  In addition, the first control light signal 2 and the second control light signal 3 have been described on the assumption that the wavelengths thereof are the same wavelength λp, but the effect obtained by the present invention is not limited to this. Absent. Specifically, when the wavelengths of the first control optical signal 2 and the second control optical signal 3 are λp1 and λp2, respectively, they are not affected by the transmission wavelength band of the first optical bandpass filter 51. If it is sufficiently far from the signal light wavelength λs, the effect of the present invention can be obtained even if λp1 and λp2 do not coincide.

  Further, the first optical bandpass filter 51 or the like is assumed to have a signal light wavelength as a transmission wavelength, but the effect obtained by the present invention can be realized even when the control light wavelength is a transmission wavelength.

  The complementary control light generation unit 8 in the second embodiment is not only configured to be incorporated in the optical modulator, but may be configured as an independent optical device, which is commercially available. As described above, the complementary control light generator 8 configured as an independent optical device corresponds to the complementary intensity-modulated light generator of the present invention.

  6, 7, 300 ... All-optical modulator, 8 ... Complementary control light generator, 10 ... First polarization separation / combination module, 12 ... First polarization plane preserving optical fiber, 14 ... First polarization plane converter, DESCRIPTION OF SYMBOLS 16 ... 2nd polarization plane preserving optical fiber, 18 ... 2nd polarization separation / combination module, 22 ... 3rd polarization plane preserving optical fiber, 24 ... 2nd polarization plane conversion part, 26 ... 4th polarization plane preserving optical fiber, 28 DESCRIPTION OF SYMBOLS 2nd optical band pass filter, 30 ... 1st optical circulator, 38 ... 2nd optical band pass filter, 40 ... 1st optical phase bias circuit, 50 ... 3rd polarization splitting / combining module, 51 ... 1st optical band pass Filter 52. 5th polarization plane preserving optical fiber 312 6th polarization plane preserving optical fiber 314 3rd polarization plane converter 316 7th polarization preserving optical fiber 318 4th polarization splitting and combining module 322 ... Eighth polarization plane Existing optical fiber, 324... 4th polarization plane conversion section, 326... 9th polarization plane preserving optical fiber, 330... Second optical circulator, 340... Second optical phase bias circuit, 351. 10th polarization-maintaining optical fiber.

Claims (8)

A first closed loop optical path portion forming a polarization plane preserving first closed loop optical path;
An optical pulse train in which optical pulses with uniform peak intensities are arranged at equal time intervals, or a first signal light having a linearly polarized first wavelength, which is continuous light, is converted into two linearly polarized first components and second First and second component dividing sections for dividing into components;
An intensity-modulated light having an intensity pattern corresponding to “0” and “1” of the signal, and a linearly polarized first control light signal having a second wavelength, and the intensity of the first control light signal Second intensity control light having an intensity pattern complementary to the pattern, having the second wavelength or a wavelength near the second wavelength, and a polarization plane orthogonal to the polarization plane of the first control optical signal A control light generator for outputting an optical signal;
The first signal light is input to and output from the first closed loop optical path, and the first component and the second component are input to the first closed loop optical path so that the circulation directions are reversed. A first closed-loop first signal light input / output unit,
The first control light signal and the second control light signal are input to and output from the first closed-loop optical path, and the cyclic direction of the first control light signal is the same as the first component. And the first control light signal and the second control light signal are input to the first closed-loop optical path so that the circulation direction of the second control light signal is the same as that of the second component. A first closed-loop control light input / output unit,
It is interposed in the first closed-loop optical path that changes the optical phase of the first component according to the first control light signal traveling in the same direction and the second control light signal traveling in the opposite direction. A first component optical phase shift unit;
It is interposed in the first closed-loop optical path that changes the optical phase of the second component in accordance with the second control optical signal traveling in the same direction and the first control optical signal traveling in the opposite direction. A second component optical phase shift unit;
A first signal light extraction unit that separates the first component and the second component output from the first closed-loop optical path from the first control light signal and the second control light signal;
An optical path between the first signal light directed to the first closed loop first signal light input / output unit and the first component and the second component of the return light output from the first closed loop first signal light input / output unit. A first forward / reverse optical path separator to be separated;
A first signal light component that is provided at any location where the return light output from the first closed-loop first signal light input / output unit travels and combines the polarization planes of the first component and the second component together. Combining part,
A first optical phase difference providing unit that is provided at any location where the first component and the second component of the first signal light travel and applies a relative optical phase difference to the first component and the second component. And
A polarization plane preserving optical path is applied to an optical path other than the first closed loop optical path, and an optical signal obtained by combining and optical path separation of the first signal light component multiplexing unit and the first forward / reverse optical path separating unit is used. , Output as modulated optical signal ,
The first closed-loop first signal light input / output unit serves as both the first and second component splitting units and the first closed-loop control light input / output unit formed of a polarization beam splitter or a polarizing prism. The one closed-loop optical path unit includes an in-loop polarization plane conversion unit that rotates the polarization planes of the first component and the second component by 90 ° . The first closed-loop optical path is in contrast to the first component or the second component that circulates through the first closed-loop optical path due to the optical Kerr effect by the first control optical signal or the second control optical signal. A first phase-shifting nonlinear optical fiber that generates a phase shift due to a cross-phase modulation effect;
2. The optical modulator according to claim 1, wherein the first phase shift nonlinear optical fiber includes the first component optical phase shift unit and the second component optical phase shift unit. Outside the first loop provided at a position where the first signal light directed to the first closed loop first signal light input / output unit and the return light output from the first closed loop first signal light input / output unit pass. Having a polarization plane converter,
The first closed-loop first signal light in which the polarization plane of the first signal light directed to the first closed-loop first signal light input / output unit is formed by a polarization beam splitter or a polarization prism. The first component passing through the polarization plane selective reflection surface of the input / output unit is converted into a polarization plane having an angle at which the second component to be reflected and the second component are reflected, and output from the first closed loop first signal light input / output unit. The optical modulator according to claim 1 , wherein the optical modulator functions as the first signal light component multiplexing unit that aligns the polarization planes of the first component and the second component of the returned light. The first signal light extraction unit is configured to output the first closed-loop first signal light input / output unit and the first closed-loop control light input / output unit from the output light output from the polarization beam splitter or the polarization prism. 1 components and optical modulator according to claim 1, characterized in that it is composed of a first optical band-pass filter for extracting said second component. The first optical phase difference providing unit is interposed in the first closed loop optical path, and provides a phase difference while maintaining the polarization planes of the first component and the second component. light modulator according to any one of claims 1 to 4. The first forward / reverse optical path separation unit is a polarization beam splitter or a polarization prism different from the polarization beam splitter or the polarization prism constituting the first closed loop first signal light input / output unit. Item 6. The optical modulator according to any one of Items 1 to 5 .   6. The optical modulator according to claim 1, wherein the first forward / reverse optical path separation unit is a circulator. The control light generator is
A second closed-loop optical path section forming a polarization-preserving second closed-loop optical path;
Third and fourth component dividers for dividing the second signal light having the second wavelength of linearly polarized light, which is continuous light, into a third component and a fourth component of two linearly polarized waves;
The second signal light is input to and output from the second closed-loop optical path, and the third component and the fourth component are input to the second closed-loop optical path so that the circulation directions are reversed. A second closed-loop second signal light input / output unit that
Input / output to / from the second closed-loop optical path of linearly-polarized third control optical signal having the same intensity pattern as that of the first control optical signal and having a second wavelength Second closed loop control for inputting the third control optical signal to the second closed loop optical path so that the circulation direction of the third control optical signal is the same as that of the third component. An optical input / output unit;
A third component optical phase shift unit interposed in the second closed loop optical path for changing the optical phase of the third component in response to the third control optical signal traveling in the same direction;
A fourth component optical phase shift unit interposed in the second closed loop optical path for changing the optical phase of the fourth component in response to the third control optical signal traveling in the reverse direction;
A second signal light extraction unit that separates the third component and the fourth component output from the second closed-loop optical path from the third control light signal;
An optical path between the second signal light directed to the second closed loop second signal light input / output unit and the third component and the fourth component of the return light output from the second closed loop second signal light input / output unit. A second forward / reverse optical path separator to be separated;
A second signal light component that is provided at any location where the return light output from the second closed loop second signal light input / output unit travels and combines the polarization planes of the third component and the fourth component together. Combining part,
A second optical phase difference providing unit that is provided at any location where the third component and the fourth component of the second signal light travel, and applies a relative optical phase difference to the third component and the fourth component. And
Applying the polarization plane preserving optical path to the optical path in the control light generating unit other than the second closed loop optical path,
The total phase shift by the third component optical phase shift unit, the fourth component optical phase shift unit, and the second optical phase difference providing unit is “0”, “1” of the signal of the third control optical signal. 0 or π depending on
The optical signal that has been multiplexed and separated by the second signal light component multiplexing unit and the second forward / reverse optical path separating unit is an orthogonal polarization between the first control optical signal and the second control optical signal. Maintain the wavefront and output the same signal as superimposed ,
The second closed-loop second signal light input / output unit is a polarization beam splitter or a polarizing prism, and serves as the third and fourth component splitting units and the second closed-loop control light input / output unit. 8. The optical modulation according to claim 1, wherein the two closed-loop optical path units include an in-loop polarization plane conversion unit that rotates the polarization planes of the third component and the fourth component by 90 °. vessel.

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