JP4732156B2 - Polarization direction conversion apparatus and polarization direction conversion method - Google Patents

Description

  The present invention relates to a polarization direction conversion device and a polarization direction conversion method for converting the polarization direction of incident light.

  In the field of communication and measuring instruments, polarization scramblers whose polarization state is scrambled periodically are used (for example, see Non-Patent Documents 1 and 2). The polarization scrambler is disposed at the input end in long-distance optical transmission and is used for the purpose of eliminating the influence of an EDFA (Erium Doped Fiber Amplifier) PHB (Polarization Hole Burning) disposed at a predetermined interval. The polarization scrambler is used for the purpose of averaging the polarization state of the back-scattered light in OTDR (Optical Time Domain Reflectometer) measurement for detecting loss and distortion of an optical fiber from a single direction.

  A TE-TM converter that performs TE-TM mode conversion used for a polarization scrambler or the like is also known. The TE-TM converter has an optical waveguide of lithium niobate crystal and a comb-shaped electrode provided along the light propagation direction of the optical waveguide. The TE-TM converter converts a TE mode component of light guided through the optical waveguide into a TM mode component by applying a control voltage to the comb-shaped electrode, or converts a TM mode component into a TE mode component. Convert to

Shimotsu Shinichi et al., “Low DOP with Built-in Polarizer: LiNbO3 Polarization Scrambler”, 1995 IEICE General Conference, C-281, P281 Fred Heismann, "Integrated-Optic Polarization Transformer for Reset Free Endless Polarization Control", IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol.25, No8, 1998, P.1898-1906

  By the way, when the TE-TM converter is used for a long period of time, polarization occurs in the lithium niobate crystal and the conversion efficiency changes. For this reason, the TE-TM converter causes DC drift due to long-term use, and does not output light having the same polarization direction even when the same control voltage is applied.

  Then, an object of this invention is to provide the polarization direction conversion apparatus and polarization direction conversion method which can solve said subject. This object is achieved by a combination of features described in the independent claims. The dependent claims define further advantageous specific examples of the present invention.

  In order to solve the above-mentioned problem, in the first embodiment of the present invention, a TE-TM converter that enters light having a TE mode component or TM mode and emits light polarized in accordance with an applied control voltage. And a voltage application unit that applies a control voltage to the TE-TM converter, and the voltage application unit is predetermined from the TE-TM converter during an application period including a timing at which light is incident on the TE-TM converter. Polarized light in which a first level control voltage is applied to emit light having the polarized light, and a second level control voltage having a sign different from the first level is applied to at least a part of the non-application period other than the application period. A direction changing device is provided.

  The voltage application unit may apply a control voltage of a second level that brings the average value of the application period and the control voltage applied in the non-application period close to 0, at least in part of the non-application period. The voltage application unit may apply a second level control voltage that has an average value of the application period and the control voltage applied during the non-application period to approximately 0, at least in part of the non-application period. The voltage application unit may set the control voltage to the first level over a period longer than the pulse width of light incident on the TE-TM converter.

  The voltage application unit may apply a second level control voltage in which the average voltage of the application period and the non-application period is substantially zero in the non-application period following the application period. The voltage application unit may provide one non-application period in which the control voltage is set to the second level corresponding to the plurality of application periods, and may bring the average value of the plurality of application periods and the control voltage in the non-application period close to zero. .

  The voltage application unit is a pulse voltage generation unit that generates a pulse voltage that is synchronized with light incident on the TE-TM converter, and an AC that couples the pulse voltage to the TE-TM converter as a control voltage. And a coupling circuit. The voltage application unit includes a pulse voltage generation unit that generates a pulse voltage synchronized with light incident on the TE-TM converter, a reverse level DC voltage source that generates a DC voltage opposite to the pulse voltage, and a pulse voltage. And an addition circuit that adds a DC voltage at a reverse level to the TE-TM converter as a control voltage.

  The voltage application unit adjusts the DC bias value of the pulse voltage so that the error signal becomes substantially zero, and a pulse voltage generation unit that generates a pulse voltage synchronized with light incident on the TE-TM converter, A bias adjustment circuit that is applied to the TE-TM converter as a control voltage, an integration circuit that integrates the control voltage, and an inverting amplification circuit that generates an error signal by inverting the output voltage of the integration circuit and multiplying by a predetermined gain. You may have. The voltage application unit adjusts the peak value and DC bias value of the pulse voltage based on the pulse voltage generation unit that generates a pulse voltage synchronized with the light incident on the TE-TM converter, and the average control voltage, You may have a peak DC bias adjustment circuit applied to a TE-TM converter as a control voltage, and an integration circuit which produces | generates an average control voltage by integrating a control voltage.

  The voltage application unit includes a pulse generation circuit that generates a timing pulse indicating a generation timing of light emitted to the TE-TM converter, and a period from when the timing pulse is generated until light is incident on the TE-TM converter. A delay circuit that delays the timing pulse according to time may include a voltage generation unit that generates a control voltage to be applied to the TE-TM converter based on the timing pulse delayed by the delay circuit.

  The voltage application unit receives light incident on the TE-TM converter, generates a timing pulse indicating the generation timing of the light, and generates a timing pulse after the timing pulse is generated. A delay circuit that delays a timing pulse according to a time until it enters the converter, and a voltage generator that generates a control voltage to be applied to the TE-TM converter based on the timing pulse delayed by the delay circuit; May be included.

  The TE-TM converter may emit light having the same TE mode component and TM mode component by applying a control voltage so that the TE mode component and the TM mode component are equal. The TE-TM converter may emit light whose polarization direction periodically changes between the TE mode and the TM mode when a periodically changing control voltage is applied.

  The polarization direction conversion device may further include a phase modulator that emits light that is incident on the light emitted from the TE-TM converter and that changes the phase difference between the TE mode and the TM mode at a predetermined period.

  In the second embodiment of the present invention, light having a TE mode component or a TM mode component is incident on the TE-TM converter and a control voltage is applied, and polarization corresponding to the control voltage is applied from the TE-TM converter. In the polarization direction changing method for emitting the emitted light, a first level control voltage for emitting light having a predetermined polarization from the TE-TM converter during the application period including the timing when the light is incident on the TE-TM converter. And applying a second level control voltage having a different sign from the first level to at least a part of the non-application period other than the application period.

  The above summary of the invention does not enumerate all the necessary features of the present invention, and sub-combinations of these feature groups can also be the invention.

  According to the present invention, the polarization direction can be converted with high accuracy.

  Hereinafter, the present invention will be described through embodiments of the invention. However, the following embodiments do not limit the invention according to the scope of claims, and all combinations of features described in the embodiments are included. It is not necessarily essential for the solution of the invention.

  FIG. 1 shows a configuration of a polarization scrambler 10 according to the present embodiment, together with a light source 11 and a polarization maintaining fiber 12. The polarization scrambler 10 receives pulsed light including a transmission signal from a light source 11 via a polarization maintaining fiber 12, and linearly polarized light → circularly polarized light → diagonal linearly polarized light → reverse circularly polarized light → linearly polarized light, etc. The light in the polarization state that goes around the Poincare sphere is emitted. That is, the polarization scrambler 10 periodically scrambles the polarization state and emits light that is averaged to be non-polarized light.

The polarization scrambler 10 includes a polarizer 21, a TE-TM converter 22, a first voltage application unit 23, a phase modulator 24, and a second voltage application unit 25.
The polarizer 21 transmits TE mode or TM mode light among the pulsed light incident from the polarization maintaining fiber 12, that is, light having a component whose polarization direction is 0 degree or 90 degrees.

The TE-TM converter 22 receives light having a TE mode component or TM mode transmitted through the polarizer 21 and is applied with a control voltage. The TE-TM converter 22 emits light having a predetermined polarization direction by rotating the incident TE mode light in the TM mode or TE mode direction according to the control voltage. Specifically, the TE-TM converter 22 converts a part or all of the TE mode component or TM mode propagating in the optical waveguide into the TM mode or the TE mode according to the applied control voltage. The light of the TE mode component and the TM mode component adjusted to a predetermined intensity is emitted.
For example, the control voltage is applied to the TE-TM converter 22 so that the TE mode component and the TM mode component have the same intensity. The TE-TM converter 22 may emit light that periodically changes between the TE mode and the TM mode.

The first voltage application unit 23 applies a control voltage to the TE-TM converter 22. The first voltage application unit 23 includes a period including the timing when the pulsed light emitted from the light source 11 is incident on the TE-TM converter 22 (application period), and a period other than the application period (non-application period). For example, it is set based on a synchronization signal synchronized with the light emission timing of the light source 11. Then, the first voltage application unit 23 controls the first level V <b> 1 to emit light having a predetermined polarization from the TE-TM converter 22 during an application period including the timing when the light is incident on the TE-TM converter 22. Apply voltage. The first voltage application unit 23 applies, for example, a voltage that emits light in which the TE mode component and the TM mode component have the same intensity to the TE-TM converter 22 as the control voltage of the first level V1. Thereby, the TE-TM converter 22 can emit light having the same intensity of the TE mode component and the TM mode component. For example, the first voltage application unit 23 may apply the control voltage of the first level V1 that periodically changes from 0 (V) to the voltage level at which TM mode light is emitted. As a result, the TE-TM converter 22 can emit light that periodically changes between the TE mode and the TM mode, that is, the polarization direction of 0 degree linearly polarized light → elliptical polarized light → 90 degree linearly polarized light. .
The first voltage application unit 23 applies the control voltage of the second level V _{2 having} a sign different from that of the first level V ₁ to at least a part of the non-application period other than the application period. Thus, the first control voltage level V ₁ applied to the application period is canceled, it is canceled.

The phase modulator 24 receives the linearly polarized light emitted from the TE-TM converter 22 and is applied with a control voltage. The phase modulator 24 periodically changes the phase difference between the TE mode component and the TM mode component in the incident light according to the control voltage. For example, the phase modulator 24 propagates TE mode and TM mode light to an electro-optical element such as a lithium niobate (LiNbO ₃ ) crystal, and generates an electric field in a direction perpendicular to the propagation direction of a sine wave or a triangular wave voltage signal. By being applied, light that emits a phase difference that periodically changes between the TE mode and the TM mode is emitted.

  The second voltage application unit 25 applies a control voltage to the phase modulator 24. Specifically, the second voltage application unit 25 applies a control voltage in which the phase difference between the TE mode and the TM mode periodically changes between −π and π. Thereby, the phase modulator 24 can emit light that is averaged to non-polarized light whose polarization state is periodically scrambled.

FIG. 2 shows a configuration as an example of the TE-TM converter 22.
For example, the TE-TM converter 22 includes an optical waveguide 31 of an electro-optic crystal such as a lithium niobate crystal, and an anode 33 and a cathode 34 at a predetermined pitch Λ along the propagation direction of the guided light in the optical waveguide 31. Comb-shaped electrodes 32 arranged alternately. The optical waveguide 31 receives TE mode or TM mode light transmitted through the polarizer 21 and propagates as guided light. When the control voltage is applied, the comb-shaped electrode 32 applies an electric field that changes in a predetermined cycle determined by the pitch Λ along the propagation direction of the guided light to the optical waveguide 31. When this electric field is applied to the optical waveguide 31, the main axis of the refractive index ellipsoid is generated in the electro-optic crystal, and the polarization direction of the propagating light is rotated. As a result, the TE-TM converter 22 converts the TE mode component into the TM mode component or the TM mode component into the TE mode component when an appropriate control voltage is applied from the first voltage application unit 23. Light having a component ratio between the mode and the TM mode can be emitted.

FIG. 3A shows the power of light incident on the TE-TM converter 22 with respect to time. FIG. 3B shows a control voltage applied to the TE-TM converter 22 when the light shown in FIG. 3A is incident on the TE-TM converter 22.
The first voltage application unit 23 applies the control voltage of the first level (V1) at the timing when the pulsed light is incident on the TE-TM converter 22. The first voltage applying unit 23, the length of the application period is set sufficiently longer than the pulse width T _P of the light incident on the TE-TM converter 22. That is, the first voltage application unit 23 sets the control voltage to the first level (V1) over a period longer than the pulse width of the light incident on the TE-TM converter 22. Thereby, even if some fluctuation | variation generate | occur | produces in the incident timing of pulsed light, a polarization direction can be converted reliably. Note that the first voltage application unit 23 sets the period of the first level (V1) so that at least the non-application period is formed.

  In addition, the first voltage application unit 23 has a control voltage of the second level (V2) having a sign different from that of the first level (V1), that is, a reverse level in at least a part of the non-application period other than the application period. Apply. For example, when the control voltages applied during the application period and the non-application period are averaged, the control voltage of the second level (V2) that approaches 0 or becomes approximately 0 is applied.

  No matter what control voltage is applied during the non-application period, since the pulsed light is not incident on the TE-TM converter 22, there is no influence on the output light. On the other hand, an electric field applied in the opposite direction to the electric field applied during the application period is applied to the electro-optic crystal constituting the optical waveguide 31. For this reason, even when the TE-TM converter 22 is operated for a long time, the cumulative electric field applied to the electro-optic crystal constituting the optical waveguide 31 approaches 0 or becomes substantially 0. Is small or does not occur. Therefore, according to the TE-TM converter 22, DC drift does not occur even if it is used for a long time, and the polarization direction can be converted with high accuracy.

  Further, according to the polarization scrambler 10 as described above, light having a periodically scrambled polarization state is generated using the TE-TM converter 22 that does not generate DC drift even when used for a long period of time. As a result, the polarization scrambler 10 can emit light that is regarded as non-polarized light with little deterioration in degree of polarization (DOP: Degree of Polarization) even when used for a long period of time.

  FIG. 4 shows a first modification of the application pattern of the control voltage generated by the first voltage application unit 23. The first voltage application unit 23 sets the average control voltage to approximately zero for each cycle when the application period and the non-application period are one period in the non-application period following the application period. A two-level (V2) control voltage is output. The first voltage application unit 23 may apply the control voltage of the second level (V2) to a part of the non-application period as long as the average control voltage becomes substantially 0 every cycle. Thus, by setting the average control voltage to approximately 0 for each cycle, the average control voltage can be easily set to approximately 0 even when light pulses occur irregularly. It should be noted that the first level (V1) and the second level (V2) may have the same absolute value and the same pulse width, or may have a different absolute value and a different pulse width. Good.

  FIG. 5 shows a second modification of the application pattern of the control voltage generated by the first voltage application unit 23. The first voltage application unit 23 provides one non-application period in which the control voltage is set to the second level (V2) corresponding to the plurality of application periods, and the average value of the control voltages in the plurality of application periods and the non-application period. May approach 0. Thus, by applying the control voltage of the second level (V2) every plural cycles, for example, even when the application period of one time is very short, the average control voltage is accurately set to substantially zero. Can do.

  FIG. 6 shows a first example of the configuration of the first voltage application unit 23. For example, the first voltage application unit 23 includes a pulse voltage generation unit 41, a delay circuit 42, and an AC coupling circuit 43. The pulse voltage generator 41 outputs a pulse voltage based on the synchronization signal output from the light source 11. The delay circuit 42 delays the pulse voltage output from the pulse voltage generator 41 until the pulse light is incident on the TE-TM converter 22 after the synchronization signal is generated. As a result, the pulse voltage output from the delay circuit 42 is synchronized with the light incident on the TE-TM converter 22. The AC coupling circuit 43 is a capacitor, for example, and AC-couples and outputs the pulse voltage delayed by the delay circuit 42. The voltage output from the AC coupling circuit 43 is applied to the TE-TM converter 22 as a control voltage.

  According to such a first voltage application unit 23, it is possible to generate a control voltage that is substantially zero when the values of the application period and the non-application period are averaged. In particular, when the optical pulse is generated at a fixed period, it is possible to efficiently generate a control voltage that is substantially zero.

  FIG. 7 shows a second example of the configuration of the first voltage application unit 23. As an example, the first voltage application unit 23 includes a reverse level DC voltage source 51 and an addition circuit 52 instead of the AC coupling circuit 43 of FIG. The reverse level DC voltage source 51 generates a DC voltage having a level opposite to that of the pulse voltage, for example, a second level voltage. The adder circuit 52 adds the DC voltage output from the reverse level DC voltage source 51 to the pulse voltage delayed by the delay circuit 42 and outputs the result. The voltage output from the adder circuit 52 is applied to the TE-TM converter 22 as a control voltage.

  According to the first voltage application unit 23 as described above, for example, even when the pulsed light has a long period, it is possible to generate a control voltage that is substantially zero when the values of the application period and the non-application period are averaged. . In particular, when the optical pulse is generated at a fixed period, it is possible to efficiently generate a control voltage that is substantially zero.

  FIG. 8 shows a third example of the configuration of the first voltage application unit 23. For example, the first voltage application unit 23 includes a bias adjustment circuit 61, an integration circuit 62, and an inverting amplification circuit 63 instead of the AC coupling circuit 43 of FIG. 6. The bias adjustment circuit 61 adjusts the DC bias value of the pulse voltage delayed by the delay circuit 42. The integration circuit 62 integrates the voltage output from the bias adjustment circuit 61. The inverting amplification circuit 63 inverts the signal output from the integration circuit 62 and multiplies the signal by a predetermined gain to generate an error signal. Then, the bias adjustment circuit 61 adjusts the DC bias value of the pulse voltage so that the error signal becomes substantially zero. Specifically, the bias adjustment circuit 61 maintains the current DC bias if the error signal is approximately zero, and adds the error signal to the current DC bias if the error signal is other than zero. The voltage output from the bias adjustment circuit 61 is applied to the TE-TM converter 22 as a control voltage.

  According to such a first voltage application unit 23, a control voltage is generated such that the integration result is substantially zero. Therefore, when the pulsed light incident on the TE-TM converter 22 has a long period, the application period And the control voltage which becomes substantially 0 when the value of the non-application period is averaged can be generated. In particular, when the optical pulse is generated at a fixed period, it is possible to efficiently generate a control voltage that is substantially zero.

  FIG. 9 shows a fourth example of the configuration of the first voltage application unit 23. The first voltage application unit 23 includes a peak / DC bias adjustment circuit 71 and an integration circuit 72 instead of the AC coupling circuit 43 of FIG. The peak / DC bias adjustment circuit 71 adjusts the peak value of the pulse voltage delayed by the delay circuit 42 and adjusts the DC bias value. The integration circuit 72 integrates the voltage output from the peak / DC bias adjustment circuit 71. Specifically, the bottom / DC bias adjustment circuit 71 amplifies the peak value to the first level (V1), and decreases the DC bias value if the output signal of the integration circuit 72 is larger than zero. If it is smaller than 0, control is performed to increase the DC bias. The voltage output from the peak / DC bias adjustment circuit 71 is applied to the TE-TM converter 22 as a control voltage.

  According to such a first voltage application unit 23, a control voltage is generated such that the integration result is substantially zero, so that the application period and the non-application period can be changed without changing the level of the control voltage applied during the application period. It is possible to generate a control voltage that is substantially zero when the values of are averaged. In particular, even when the light pulse is not periodically generated, it is possible to efficiently generate a control voltage that is substantially zero. Further, according to the first voltage application unit 23, the peak value can be adjusted to the first level (V1) together with the adjustment of the DC bias, so that the polarization direction can be accurately controlled, and the values of the application period and the non-application period can be adjusted. It is possible to accurately generate a control voltage that is substantially zero when.

  FIG. 10 shows a fifth example of the configuration of the first voltage application unit 23. As an example, the first voltage application unit 23 includes a pulse generation circuit 81 and a first amplifier 82 instead of the pulse voltage generation unit 41 of FIG. The pulse generating circuit 81 generates a timing pulse at which the light source 11 emits pulsed light, that is, a timing pulse indicating the generation timing of light emitted to the TE-TM converter 22. The timing pulse is supplied to the light source 11 and to the first amplifier 82. The light source 11 receives the timing pulse and emits light according to the timing pulse. The first amplifier 82 outputs a pulse voltage synchronized with the timing pulse to the delay circuit 42. The delay circuit 42 supplies the input pulse voltage to the peak / DC bias adjustment circuit 71 by delaying the time from when the timing pulse is generated until the pulse light is incident on the TE-TM converter 22. . The delay circuit 42 may be provided before the first amplifier 82 in place of the subsequent stage of the first amplifier 82.

  According to the first voltage application unit 23 as described above, even when the generation timing of the pulsed light by the light source 11 is controlled from the outside, the value is substantially 0 when the values of the application period and the non-application period are averaged. A control voltage can be generated. In particular, even when light pulses are not generated periodically, a control voltage that can be made substantially zero can be generated efficiently.

  FIG. 11 shows a sixth example of the configuration of the first voltage application unit 23. The first voltage application unit 23 includes a light receiving element 91, a pulse regeneration circuit 92, and a second amplifier 93 instead of the pulse voltage generation unit 41 of FIG. 9. The light receiving element 91 receives a part of light incident on the TE-TM converter 22 from the light source 11 and converts it into an electrical signal. The pulse regeneration circuit 92 regenerates the timing pulse by performing, for example, I / V conversion and waveform shaping on the electrical signal output from the light receiving element 91. The second amplifier 93 outputs a pulse voltage synchronized with the timing pulse to the delay circuit 42. The delay circuit 42 delays the input pulse voltage from the time the light is received by the light receiving element 91 until the pulse light is incident on the TE-TM converter 22, and the bottom / DC bias adjustment circuit 71. To supply. Note that the delay circuit 42 may be provided in front of the second amplifier 93 instead of in the subsequent stage of the second amplifier 93.

  According to such a first voltage application unit 23, even when the synchronization signal is not supplied from the light source 11 or the like, a control voltage that is substantially zero is generated when the values of the application period and the non-application period are averaged. can do. In particular, even when light pulses are not generated periodically, a control voltage that can be made substantially zero can be generated efficiently.

  As mentioned above, although this invention was demonstrated using embodiment, the technical scope of this invention is not limited to the range as described in the said embodiment. It will be apparent to those skilled in the art that various modifications or improvements can be added to the above-described embodiment. It is apparent from the scope of the claims that the embodiments added with such changes or improvements can be included in the technical scope of the present invention.

A configuration of the polarization scrambler 10 is shown together with a light source 11 and a polarization maintaining fiber 12. The structure as an example of the TE-TM converter 22 is shown. (A) shows the power of light incident on the TE-TM converter 22 with respect to time. (B) shows the control voltage applied to the TE-TM converter 22 when the light shown in (A) is incident on the TE-TM converter 22. The 1st modification of the application pattern of the control voltage which the 1st voltage application part 23 generate | occur | produces is shown. The 2nd modification of the application pattern of the control voltage which the 1st voltage application part 23 generate | occur | produces is shown. The 1st example of a structure of the 1st voltage application part 23 is shown. The 2nd example of a structure of the 1st voltage application part 23 is shown. The 3rd example of a structure of the 1st voltage application part 23 is shown. The 4th example of a structure of the 1st voltage application part 23 is shown. The 5th example of a structure of the 1st voltage application part 23 is shown. The 6th example of a structure of the 1st voltage application part 23 is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Polarization scrambler 11 Light source 12 Polarization-maintaining fiber 21 Polarizer 22 TE-TM converter 23 First voltage application unit 24 Phase modulator 25 Second voltage application unit 31 Optical waveguide 32 Comb-shaped electrode 33 Anode 34 Cathode 41 Pulse Voltage generator 42 Delay circuit 43 AC coupling circuit 51 Reverse level DC voltage source 52 Adder circuit 61 Bias adjustment circuit 62 Integration circuit 63 Inverting amplification circuit 71 Peak / DC bias adjustment circuit 72 Integration circuit 81 Pulse generation circuit 82 First amplifier 91 Light receiving element 92 Pulse regeneration circuit 93 Second amplifier

Claims (14)

A TE-TM converter that receives a pulsed light having a TE mode component or a TM mode and emits a polarized light according to an applied control voltage;
A voltage application unit that applies the control voltage to the TE-TM converter;
The voltage application unit includes:
A first level control voltage is applied to emit light having a predetermined polarization from the TE-TM converter during an application period including a timing at which pulsed light is incident on the TE-TM converter;
A period other than the application period, at least a portion of the non-application period in which the pulse light is not incident on the TE-TM converter, wherein the first level is applied to the second-level control voltage of the signs are different A polarization direction changing device that takes substantially zero when the control voltages applied during the application period and the non-application period are averaged . The polarization direction conversion device according to claim 1, wherein the voltage application unit sets the control voltage to a first level over a period longer than a pulse width of light incident on the TE-TM converter. 3. The polarization direction according to claim 1, wherein the voltage application unit applies a second level control voltage in which a mean voltage of the application period and the non-application period is substantially 0 in a non-application period subsequent to the application period. Conversion device. The voltage application unit provides a non-application period in which the control voltage is set to the second level corresponding to a plurality of application periods once, and an average value of the control voltages in the plurality of application periods and the non-application period is substantially zero. The polarization direction conversion device according to claim 1 or 2 . The voltage application unit includes:
A pulse voltage generator for generating a pulse voltage synchronized with light incident on the TE-TM converter;
The polarization direction conversion device according to claim 1, further comprising: an AC coupling circuit that AC-couples the pulse voltage and applies the pulse voltage to the TE-TM converter as the control voltage. The voltage application unit includes:
A pulse voltage generator for generating a pulse voltage synchronized with light incident on the TE-TM converter;
A reverse level DC voltage source for generating a DC voltage having a level opposite to that of the pulse voltage;
5. The polarization direction conversion device according to claim 1, further comprising: an addition circuit that adds the reverse-level DC voltage to the pulse voltage and applies the DC voltage as the control voltage to the TE-TM converter. The voltage application unit includes:
A pulse voltage generator for generating a pulse voltage synchronized with light incident on the TE-TM converter;
A bias adjustment circuit that adjusts the DC bias value of the pulse voltage so that the error signal becomes substantially zero, and that is applied to the TE-TM converter as the control voltage; an integration circuit that integrates the control voltage;
5. The polarization direction conversion device according to claim 1, further comprising: an inverting amplifier circuit that generates the error signal by inverting the output voltage of the integration circuit and multiplying the output voltage by a predetermined gain. The voltage application unit includes:
A pulse voltage generator for generating a pulse voltage synchronized with light incident on the TE-TM converter;
A peak DC bias adjustment circuit that adjusts a peak value and a DC bias value of the pulse voltage based on an averaged control voltage, and applies the control voltage to the TE-TM converter;
The polarization direction conversion device according to claim 1, further comprising: an integration circuit that generates the averaged control voltage by integrating the control voltage. The voltage application unit includes:
A pulse generation circuit for generating a timing pulse indicating a generation timing of light emitted to the TE-TM converter;
A delay circuit that delays the timing pulse according to a time from when the timing pulse is generated to when light enters the TE-TM converter;
5. The polarization direction conversion according to claim 1, further comprising: a voltage generator configured to generate the control voltage to be applied to the TE-TM converter based on the timing pulse delayed by the delay circuit. apparatus. The voltage application unit includes:
A pulse generation circuit that receives light incident on the TE-TM converter and generates a timing pulse indicating a generation timing of the light;
A delay circuit that delays the timing pulse according to a time from when the timing pulse is generated to when light enters the TE-TM converter;
5. The polarization direction conversion according to claim 1, further comprising: a voltage generator configured to generate the control voltage to be applied to the TE-TM converter based on the timing pulse delayed by the delay circuit. apparatus. The TE-TM converter emits light having the same TE mode component and TM mode component when a control voltage is applied so that the TE mode component and the TM mode component are equal to each other. 2. A polarization direction changing device according to item 1 . 12. The TE-TM converter emits light whose polarization direction periodically changes between a TE mode and a TM mode when the control voltage that periodically changes is applied. 12. or the polarization direction converter according to item 1. The TE-TM converter of the emitted light is incident, according to any one of claims 1, further comprising a phase modulator 12 phase difference emits light that varies in a predetermined cycle of the TE and TM modes Polarization direction changing device. In a polarization direction conversion method in which light having a TE mode component or TM mode component is incident on a TE-TM converter and a control voltage is applied, and polarized light corresponding to the control voltage is emitted from the TE-TM converter. ,
Applying a first level control voltage to emit light having a predetermined polarization from the TE-TM converter during an application period including a timing at which light is incident on the TE-TM converter;
A period other than the application period, at least a portion of the non-application period in which the pulse light is not incident on the TE-TM converter, wherein the first level is applied to the second-level control voltage of the signs are different A polarization direction changing method comprising: setting the control voltage applied during the application period and the non-application period to approximately zero when the control voltages are averaged .

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