JP3982702B2 - Automatic polarization adjuster and polarization adjustment method - Google Patents

Description

  The present invention relates to an automatic polarization adjuster and a polarization adjustment method for maintaining and outputting light whose polarization state changes every moment in a constant polarization state.

  In communication systems and sensors, optical fibers are actively used. However, when stress is applied to the optical fiber, the polarization state of light propagating through the optical fiber changes. Therefore, in a system that uses the polarization state of light, a polarization adjuster is indispensable. However, most polarization adjusters are manually adjusted and must be adjusted by a person while observing the polarization state of light by some means. Often not.

As an apparatus for coping with such a problem, a product called an auto-tracking polarization controller is commercially available (for example, see Non-Patent Document 1). This auto-tracking polarization controller converts light with an arbitrary polarization state into linearly polarized light and outputs it, and always outputs linearly polarized light with a constant intensity even when the polarization state of incident light changes. It is possible to continue.
The configuration of this automatic tracking polarization controller is shown in FIG. The light incident on the controller passes through a quarter-wave plate (QWP) 1000 and a half-wave plate (HWP) 1001, and then is polarized by a polarizing beam splitter (Polarizing-Beam Splitter). PBS) 1002. The PBS 1002 is an element having a function of transmitting P-polarized light and reflecting S-polarized light. Here, P-polarized light is linearly polarized light having only an electric field component parallel to the paper surface of FIG. 15, and S-polarized light is linearly polarized light having only an electric field component perpendicular to the paper surface.

  The light incident on the controller is generally elliptically polarized and has both S and P polarizations. Therefore, the S-polarized light is reflected by the PBS 1002 and enters the photo detector (PD) 1004, and the P-polarized light passes through the PBS 1002 and enters the beam splitter (BS) 1003. Part of the P-polarized light is reflected by the BS 1003 and detected by the PD 1005. The P-polarized light transmitted through the BS 1003 is extracted as output light from the controller. The control circuit 1006 compares the output signal strengths of the PDs 1004 and 1005, and adjusts the angle between the QWP 1000 and the HWP 1001 so that the output signal strength of the PD 1004 is sufficiently smaller than the output signal strength of the PD 1005. If the output signal intensity of the PD 1004 is sufficiently small, the light that has passed through the QWP 1000 and the HWP 1001 should be almost P-polarized light, and thus linearly polarized light can be obtained by such a method.

The applicant has not yet found prior art documents related to the present invention by the time of filing other than the prior art documents specified by the prior art document information described in this specification.
“Auto-tracking polarization controller”, [online], Opt-Quest Inc., [searched on January 27, 2004], Internet <http://www.optoquest.jp>

As described above, in order to convert light of an arbitrary polarization state into complete linearly polarized light, an automatic tracking polarization controller that is already on the market is effective.
However, this automatic tracking polarization controller is not universal for any system that requires polarization control. For example, consider a device having a function of “outputting high-speed polarization-modulated light using as input light polarization-modulated light whose polarization state changes with time and a plurality of frequencies of the change exists”. The input light at this time includes a low-speed polarization change and a high-speed polarization change that is faster than this. The device only cancels the slow polarization change contained in the input light. Accordingly, it is possible to output high-speed polarization-modulated light based on linearly polarized light such that a specific linear polarization state is observed when observed at a low frequency and a change in polarization state is observed when observed at a high frequency.

  When such high-speed polarization-modulated light based on linearly polarized light is transmitted through an optical fiber, elliptically polarized light is observed, which shows an elliptical polarization state when observed at a low frequency, and changes in the polarization state are observed when observed at a high frequency. In many cases, this high-speed polarization-modulated light based on elliptically polarized light is converted again into high-speed polarization-modulated light based on linearly polarized light. It will be necessary to convert it again.

  Accordingly, there is a need for a polarization controller that converts high-speed polarization-modulated light with elliptically polarized light as a reference into high-speed polarization-modulated light with linearly polarized light as a reference. The conventional automatic tracking polarization shown in FIG. The controller can convert high-speed polarization-modulated light based on elliptically polarized light into perfect linearly-polarized light, but cannot convert it to high-speed polarized light modulated based on linearly polarized light. . The reason is that the PBS 1002 is used to extract monitoring light in the automatic tracking polarization controller. This is because the polarization-modulated light that has entered the automatic tracking polarization controller is converted into intensity-modulated light by passing through the PBS 1002, and thus the polarization-modulated light cannot be output.

The present invention has been made to solve the above problems, and an automatic polarization adjuster and polarization adjustment capable of converting light of an arbitrary polarization state into light of another arbitrary polarization state It aims to provide a method.
The present invention also provides an automatic polarization adjuster and polarization adjustment method capable of converting high-speed polarization-modulated light based on an arbitrary polarization state into high-speed polarization-modulated light based on another arbitrary polarization state. The purpose is to provide.

The automatic polarization adjuster of the present invention includes a first quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized light component of input light having an arbitrary polarization state, and the first quarter-wave plate. A first half-wave plate that gives a phase difference of 180 degrees to independent linearly polarized light components of the light that has passed through, and a first separation that splits part of the output light that has passed through the first half-wave plate A second quarter wavelength that is rotated at a constant frequency f with the element and an axis parallel to the traveling direction of the light branched by the first separation element as a rotation axis, and is polarized by passing the branched light. A plate, a second separation element that separates light that has passed through the second quarter-wave plate into P-polarized light and S-polarized light, a first photodetector that photoelectrically converts the P-polarized light, and the S-polarized light A second photodetector that performs photoelectric conversion, an output electrical signal of the first photodetector and an output electrical signal of the second photodetector And the angle of the first half-wave plate is controlled so that the level of the DC component included in the output electrical signal of the differential amplifier becomes a preset value. First control means and second control means for controlling the angle of the first quarter-wave plate so that the component of the frequency 2f included in the output electric signal of the differential amplifier becomes zero. Is.
The automatic polarization adjuster of the present invention includes a first quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized light component of input light having an arbitrary polarization state, and the first quarter-wavelength. A first half-wave plate that gives a phase difference of 180 degrees to an independent linearly polarized light component of the light that has passed through the plate, and a first that splits part of the output light that has passed through the first half-wave plate. And a second 1/2 that rotates at a constant frequency f about an axis parallel to the traveling direction of the light branched by the first separation element, and polarizes and modulates the branched light by passing through the branched light. A four-wave plate, a second separation element that separates light that has passed through the second quarter-wave plate into P-polarized light and S-polarized light, a first photodetector that photoelectrically converts the P-polarized light, and A second photodetector for photoelectrically converting S-polarized light, an output electrical signal of the first photodetector, and an output of the second photodetector A differential amplifier for differentially amplifying the air signal and a sum or difference between the level of the direct current component included in the output electric signal of the differential amplifier and the amplitude of the component of the frequency 4f are obtained, and the sum or difference becomes zero. The first control means for controlling the angle of the first half-wave plate and the first 1 so that the component of the frequency 2f included in the output electric signal of the differential amplifier becomes zero. And a second control means for controlling the angle of the / 4 wavelength plate.

The automatic polarization adjuster of the present invention includes a first quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized light component of input light having an arbitrary polarization state, and the first quarter-wavelength. A first half-wave plate that gives a phase difference of 180 degrees to an independent linearly polarized light component of the light that has passed through the plate, and a first that splits part of the output light that has passed through the first half-wave plate. And a second 1/2 that rotates at a constant frequency f about an axis parallel to the traveling direction of the light branched by the first separation element, and polarizes and modulates the branched light by passing through the branched light. A four-wave plate, a second separation element that separates light that has passed through the second quarter-wave plate into P-polarized light and S-polarized light, a first photodetector that photoelectrically converts the P-polarized light, and A second photodetector for photoelectrically converting S-polarized light, an output electrical signal of the first photodetector, and an output of the second photodetector The differential amplifier that differentially amplifies the air signal, and the absolute value of the ratio between the level of the DC component contained in the output electric signal of this differential amplifier and the amplitude of the component of the frequency 4f is below a predetermined threshold The first control means for controlling the angle of the first half-wave plate and the ratio between the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the output electrical signal of the differential amplifier And a second control means for controlling the angle of the first quarter-wave plate so that is less than or equal to the threshold value.
The automatic polarization adjuster of the present invention includes a first quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized light component of input light having an arbitrary polarization state, and the first quarter-wavelength. A first half-wave plate that gives a phase difference of 180 degrees to an independent linearly polarized light component of the light that has passed through the plate, and a first that splits part of the output light that has passed through the first half-wave plate. And a second 1/2 that rotates at a constant frequency f about an axis parallel to the traveling direction of the light branched by the first separation element, and polarizes and modulates the branched light by passing through the branched light. A four-wave plate, a second separation element that separates light that has passed through the second quarter-wave plate into P-polarized light and S-polarized light, a first photodetector that photoelectrically converts the P-polarized light, and A second photodetector for photoelectrically converting S-polarized light, an output electrical signal of the first photodetector, and an output of the second photodetector A differential amplifier for differentially amplifying the air signal, and a sum or difference between the level of the DC component contained in the output electric signal of the differential amplifier and the amplitude of the component of the frequency 4f, and the level of the sum or difference First control means for controlling the angle of the first half-wave plate so that the absolute value of the ratio with the amplitude of the component of the frequency 4f is equal to or less than a predetermined threshold, and the output of the differential amplifier Second control means for controlling the angle of the first quarter-wave plate so that the ratio of the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the electrical signal is equal to or less than the threshold value. Are provided.

Further, one configuration example of the automatic polarization adjuster of the present invention further includes an oscillator that generates a signal of the frequency f, and the second quarter-wave plate is synchronized with a signal generated by the oscillator. And the first control means and the second control means extract the local oscillation wave when the amplitude of the frequency 2f component and the amplitude of the frequency 4f component contained in the output electric signal of the differential amplifier are extracted. As described above, a signal generated by the oscillator is used.
Further, in one configuration example of the automatic polarization adjuster of the present invention, a signal synchronized with a frequency f at which the second quarter-wave plate rotates autonomously is rotated by the second quarter-wave plate. The first control means and the second control means are configured to generate an amplitude of a component of frequency 2f and an amplitude of a component of frequency 4f included in the output electric signal of the differential amplifier. The synchronization signal generated by the synchronization signal generation means is used as a local oscillation wave when extracting the signal.
Further, in one configuration example of the automatic polarization adjuster of the present invention, the synchronization signal generating means includes a light transmitting portion for a quarter turn and a quarter turn for the periphery of the second quarter wavelength plate. The light for generating the synchronization signal is emitted to the two light shielding plates constituting the light shielding unit and the peripheral part of the second quarter-wave plate so that the light shielding units are alternately arranged. It comprises a light source and a third photodetector that photoelectrically converts intensity-modulated light emitted from the light source and passed through the light transmission unit to generate the synchronization signal.

Further, one configuration example of the automatic polarization adjuster of the present invention further includes a third quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized light component of the light that has passed through the first separation element, A second half-wave plate that gives a phase difference of 180 degrees to an independent linearly polarized light component of the light that has passed through the third quarter-wave plate; an ellipticity η that specifies the polarization state desired by the user; Based on the angle ψ, the angle θ _q to be set of the third quarter-wave plate is calculated by θ _q = η so that the angle of the third quarter-wave plate becomes θ _q. The third control means to be controlled and the angle θ _h to be set of the second half-wave plate are calculated by θ _h = η + {(ψ−η) / 2}, and the second 1 / and a fourth control means the angle of the half-wave plate is controlled to be theta _h, wants the second output light which has passed through the half-wave plate of the said user Henhikarijo It is obtained as a light.
In one configuration example of the automatic polarization adjuster of the present invention, the input light incident on the first quarter-wave plate is polarized light whose polarization state changes with time and a plurality of change frequencies exist. When the maximum value of the response frequency of the control by the first control unit and the second control unit is f _C , and an arbitrary frequency among the polarization state change frequencies is f _M , f _M > f _C is satisfied.

The present invention also provides a first quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized component of input light having an arbitrary polarization state, and light that has passed through the first quarter-wave plate. Using the first half-wave plate that gives a phase difference of 180 degrees to each of the independent linearly polarized light components, the polarization adjustment method for maintaining and outputting the input light in a constant polarization state, A first separation procedure for branching a part of the output light that has passed through the half-wave plate, and a second 1 that rotates at a fixed frequency f about an axis parallel to the traveling direction of the branched light. A modulation procedure for polarization-modulating the branched light by a / 4 wavelength plate, a second separation procedure for separating the polarization-modulated light into P-polarized light and S-polarized light, and first light for photoelectrically converting the P-polarized light A detection procedure, a second light detection procedure for photoelectrically converting the S-polarized light, and a first light detection procedure. A differential amplification procedure for differentially amplifying a force electrical signal and an output electrical signal of the second photodetection procedure, and a level of a DC component included in the output electrical signal of the differential amplification procedure to a preset value The first control procedure for controlling the angle of the first half-wave plate and the first frequency so that the frequency 2f component included in the output electrical signal of the differential amplification procedure is zero. And a second control procedure for controlling the angle of the quarter-wave plate.
The polarization adjusting method according to the present invention includes a first separation procedure for branching a part of the output light that has passed through the first half-wave plate, and an axis parallel to the traveling direction of the branched light. A modulation procedure for polarization-modulating the branched light by a second quarter-wave plate rotating at a fixed frequency f as an axis, and a second separation procedure for separating the polarization-modulated light into P-polarized light and S-polarized light A first light detection procedure for photoelectrically converting the P-polarized light, a second light detection procedure for photoelectrically converting the S-polarized light, an output electrical signal of the first light detection procedure, and the second light detection A differential amplification procedure for differentially amplifying the output electrical signal of the procedure, and a sum or difference between the level of the direct current component included in the output electrical signal of the differential amplification procedure and the amplitude of the component of the frequency 4f is obtained. Alternatively, the angle of the first half-wave plate can be controlled so that the difference becomes zero. And a second control procedure for controlling the angle of the first quarter-wave plate so that the frequency 2f component included in the output electrical signal of the differential amplification procedure is minimized. It is.

The polarization adjusting method according to the present invention includes a first separation procedure for branching a part of the output light that has passed through the first half-wave plate, and an axis parallel to the traveling direction of the branched light. A modulation procedure for polarization-modulating the branched light by a second quarter-wave plate rotating at a fixed frequency f as an axis, and a second separation procedure for separating the polarization-modulated light into P-polarized light and S-polarized light A first light detection procedure for photoelectrically converting the P-polarized light, a second light detection procedure for photoelectrically converting the S-polarized light, an output electrical signal of the first light detection procedure, and the second light detection The differential amplification procedure for differentially amplifying the output electrical signal of the procedure, and the absolute value of the ratio between the level of the DC component contained in the output electrical signal of the differential amplification procedure and the amplitude of the component of the frequency 4f is predetermined. A first control for controlling the angle of the first half-wave plate so as to be equal to or less than a threshold value. And the first quarter-wave plate so that the ratio of the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the output electrical signal of the differential amplification procedure is equal to or less than the threshold value. And a second control procedure for controlling the angle.
The polarization adjusting method according to the present invention includes a first separation procedure for branching a part of the output light that has passed through the first half-wave plate, and an axis parallel to the traveling direction of the branched light. A modulation procedure for polarization-modulating the branched light by a second quarter-wave plate rotating at a fixed frequency f as an axis, and a second separation procedure for separating the polarization-modulated light into P-polarized light and S-polarized light A first light detection procedure for photoelectrically converting the P-polarized light, a second light detection procedure for photoelectrically converting the S-polarized light, an output electrical signal of the first light detection procedure, and the second light detection A differential amplification procedure for differentially amplifying the output electrical signal of the procedure, and a sum or difference between the level of the direct current component included in the output electrical signal of the differential amplification procedure and the amplitude of the component of the frequency 4f is obtained. Alternatively, the absolute value of the ratio between the difference level and the amplitude of the frequency 4f component is predetermined. A first control procedure for controlling the angle of the first half-wave plate so as to be less than or equal to a large value, and the amplitude of the frequency 2f component and the frequency 4f included in the output electrical signal of the differential amplification procedure. And a second control procedure for controlling the angle of the first quarter-wave plate so that the ratio of the component amplitude to the threshold value or less.

Further, one configuration example of the polarization adjustment method of the present invention further includes an oscillation procedure for generating a signal of the frequency f, and the modulation procedure is synchronized with the signal generated by the oscillation procedure. The quarter wavelength plate is rotated, and the first control procedure and the second control procedure extract the amplitude of the frequency 2f component and the amplitude of the frequency 4f component contained in the output electrical signal of the differential amplification procedure. In this case, the signal generated by the oscillation procedure is used as a local oscillation wave.
Further, in one configuration example of the polarization adjustment method of the present invention, a signal synchronized with a frequency f at which the second quarter-wave plate rotates autonomously is used to rotate the second quarter-wave plate. The first control procedure and the second control procedure include an amplitude of a component of frequency 2f and an amplitude of a component of frequency 4f included in an output electric signal of the differential amplification procedure. The synchronization signal generated by the synchronization signal generation procedure is used as the local oscillation wave when extracting the signal.
Further, in one configuration example of the polarization adjustment method of the present invention, the synchronization signal generation procedure includes a light transmitting portion for a quarter turn and a quarter turn for the periphery of the second quarter wavelength plate. Light intensity is alternately emitted from the light source to the periphery of the second quarter-wave plate, emitted from the light source, and passed through the light transmission unit. The synchronizing signal is generated by photoelectrically converting the modulated light.

In addition, one configuration example of the polarization adjustment method of the present invention further includes a third quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized light component that has passed through the first separation procedure. Polarization of light by a first adjustment procedure for adjusting the polarization state and a second half-wave plate that gives a phase difference of 180 degrees to an independent linear polarization component of the light that has passed through the third quarter-wave plate. Based on the second adjustment procedure for adjusting the state, the ellipticity η specifying the polarization state desired by the user, and the azimuth angle ψ, the angle θ _q to be set of the third quarter-wave plate is set to θ _q = Η, and a third control procedure for controlling the angle of the third quarter-wave plate to be θ _q and an angle θ _h to be set for the second half-wave plate the calculated by _{θ h = η + {(ψ} -η) / 2}, the said angle of the second half-wave plate is controlled to be theta _h And a control procedure, in which the output light which has passed through the second half-wave plate is set to be light polarization states wishes of the user.
Further, in one configuration example of the polarization adjustment method of the present invention, the input light incident on the first quarter-wave plate has a polarization modulation in which a polarization state changes with time and a plurality of change frequencies exist. a light, said first control procedure and the maximum value f _C of the response frequency of the control by the second control procedure, an arbitrary frequency of the frequency of the change of the polarization state when the f _M, f _M> f It is intended to satisfy _C.

  According to the present invention, a part of the output light that has passed through the first half-wave plate is extracted as sample light by the first separation element, and sampled by the second quarter-wave plate that rotates at a constant frequency. The light is polarization-modulated, the polarization-modulated light is converted into intensity-modulated light by the second separation element, and then photoelectrically converted by the first and second photodetectors to be converted into the first quarter-wave plate and the first Therefore, unlike the conventional automatic tracking type polarization controller, the output light of the automatic polarization adjuster does not need to pass through the polarization beam splitter. Therefore, the polarization-modulated light input to the automatic polarization adjuster can be output as polarization-modulated light with an arbitrary polarization state as a reference.

  Further, the first control means obtains the sum or difference between the level of the direct current component included in the output electric signal of the differential amplifier and the amplitude of the component of the frequency 4f, and the first control means so that the sum or difference becomes zero. The second control means controls the angle of the first quarter-wave plate so that the frequency 2f component contained in the output electrical signal of the differential amplifier becomes zero. By controlling, the first quarter-wave plate and the first half-wave plate can be easily controlled, and S-polarized light or P-polarized light can be easily generated.

  Further, the first control means sets the first value so that the absolute value of the ratio between the level of the DC component included in the output electric signal of the differential amplifier and the amplitude of the component of the frequency 4f is equal to or less than a predetermined threshold value. The angle of the half-wave plate is controlled, and the second control means makes the ratio of the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the output electric signal of the differential amplifier equal to or less than the threshold value. By controlling the angle of the first quarter-wave plate so that the first quarter-wave plate and the first quarter-wave plate are not affected by imperfections in the optical element or noise in the electric circuit. The two-wave plate can be controlled to generate linearly polarized light.

  Further, the first control means obtains the sum or difference between the level of the DC component included in the output electric signal of the differential amplifier and the amplitude of the component of the frequency 4f, and the level of the sum or difference and the component of the frequency 4f The angle of the first half-wave plate is controlled so that the absolute value of the ratio to the amplitude is less than or equal to a predetermined threshold value, and the second control means has a frequency included in the output electric signal of the differential amplifier. By controlling the angle of the first quarter-wave plate so that the ratio between the amplitude of the 2f component and the amplitude of the frequency 4f component is less than or equal to the threshold value, imperfections in the optical element and electrical circuit By controlling the first quarter-wave plate and the first half-wave plate without being affected by noise, S-polarized light or P-polarized light can be generated.

  Also, an oscillator that generates a signal of frequency f is provided, and the second quarter-wave plate is rotated in synchronization with the signal generated by the oscillator, and the first control means and the second control means are generated by the oscillator. By using this signal as a local oscillation wave, it is possible to extract the amplitude of the frequency 2f component and the amplitude of the frequency 4f component contained in the output electric signal of the differential amplifier.

  In addition, by providing synchronization signal generation means for generating the synchronization signal based on the rotation of the second quarter wavelength plate, the second quarter wavelength plate is rotated autonomously at the frequency f. Even so, the amplitude of the frequency 2f component and the amplitude of the frequency 4f component contained in the output electric signal of the differential amplifier can be extracted, and the first quarter-wave plate and the first half-wavelength can be extracted. The angle of the plate can be controlled.

  Further, by providing the third quarter-wave plate, the second half-wave plate, the third control means, and the fourth control means, the output light that has passed through the second half-wave plate Can be the light in the polarization state desired by the user.

Further, when the maximum value of the response frequency of the control by the first control means and the second control means is f _C , and an arbitrary frequency among the change frequencies of the polarization state is f _M , f _M > f _C is satisfied. By doing so, when the polarization state of the input light incident on the first quarter-wave plate changes with time and there are multiple frequencies of the change, the first control means and the first Filtering processing that cancels only the polarization change slower than the maximum value f _C of the response frequency of control by the control means 2 can be realized, and the polarization-modulated light faster than the arbitrary frequency f _M can be output. As a result, it is possible to output high-speed polarization-modulated light that exhibits a constant polarization state when observed at a low frequency and that changes in the polarization state are observed when observed at a high frequency.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[First Embodiment]
FIG. 1 is a block diagram showing a configuration example of an automatic polarization adjuster according to the first embodiment of the present invention. The automatic polarization adjuster of the present embodiment includes a first quarter-wave plate (Quarter-Wave Plate, hereinafter abbreviated as QWP) 100 that gives a phase difference of 90 degrees to an independent linearly polarized component of input light, A first half-wave plate (hereinafter abbreviated as HWP) 101 that gives a phase difference of 180 degrees to an independent linearly polarized light component of light that has passed through the first QWP 100, and light that has passed through the HWP 101 Passes through a beam splitter (Beam Splitter, hereinafter abbreviated as BS) 102, a second QWP 103 for polarization-modulating the light branched by the BS 102, and a second QWP 103. Polarizing beam splitter (hereinafter abbreviated as PBS) 104, which is a second separation element that separates the polarized light into P-polarized light and S-polarized light, and a first that photoelectrically converts two light outputs of PBS 104, Second light Photo detectors (hereinafter abbreviated as PDs) 105, 106, a differential amplifier 107 for differentially amplifying the output electrical signal of PD 105 and the output electrical signal of PD 106, and direct current from the output electrical signal of differential amplifier 107 A low-pass filter (hereinafter abbreviated as LPF) 108 that extracts components, and a first control circuit that controls the angle of the HWP 101 so that the DC component extracted by the LPF 108 has a preset value. 109, an oscillator 110 that generates a signal of frequency f, a frequency doubler 111 that outputs an electric signal having a frequency twice that of the output electric signal of the oscillator 110, and an output electric signal of the differential amplifier 107 that has a frequency of 2f. The first lock-in amplifier 112 that extracts the amplitude of the component, and the amplitude of the component of the frequency 2f extracted by the lock-in amplifier 112 is zero. And a second control circuit 113 for controlling the angle of the first QWP 100. The LPF 108 and the first control circuit 109 constitute first control means, and the frequency doubler 111, the lock-in amplifier 112, and the second control circuit 113 constitute second control means.

  Input light in an arbitrary polarization state (generally elliptically polarized light) passes through the first QWP 100 and the HWP 101 and then enters the BS 102. The BS 102 is an element for extracting a signal necessary for controlling the QWP 100 and the HWP 101, and reflects and branches a part of incident light. The polarization state of the light reflected by the BS 102 and the transmitted light is equal to the polarization state of the light incident on the BS 102. In order to reduce light loss in the automatic polarization adjuster, it is desirable that the intensity of the light branched by the BS 102 is 10% or less of the incident light intensity.

The light reflected by the BS 102 is polarization-modulated by the second QWP 103 rotating at a constant frequency f with the axis parallel to the z direction in FIG.
The relationship between the coordinate axes shown in FIG. 1 and the main axis of the second QWP 103 is shown in FIG. In FIG. 1, the xyz system is a right-handed system, and the light traveling direction (the direction of the index finger when the thumb, index finger, and middle finger of the right hand are perpendicular to each other) is the z axis and vertically upward (the direction of the thumb). ) Is the y-axis, and the horizontal direction (the direction of the middle finger) is the x-axis. Sl and fa in FIG. 2 represent the slow axis and the fast axis of the QWP 103, respectively. An angle formed by the slow axis sl with the x-axis is θ.

At this time, assuming that the intensity of light (P-polarized light) having an electric field component parallel to the x axis is I _x and the intensity of light having an electric field component parallel to the y axis (S polarized light) is I _y , the intensity I _x , I _y can be expressed by the following equation.

However, if the amplitude of the P-polarized component incident on the second QWP 103 is E _x and the amplitude of the S-polarized component is E _y , r is the amplitude ratio (E _y / E) of the S-polarized component and the P-polarized component incident on the QWP 103. _x ), and δ is the phase difference of the S-polarized component with respect to the P-polarized component incident on the QWP 103. Further, the intensity I _x and the S-polarized light intensity I _y of P-polarized light, is standardized light intensity before being separated into P polarized light and S-polarized light as 1 in PBS 104, relationship I _x + I _y = 1 Is true. That is, the intensities I _x and I _y are the intensities of light detected by the PDs 105 and 106, respectively.
Therefore, the normalized output electric signal V _diff of the differential amplifier 107 can be expressed as follows.

The rotation speed of the second QWP 103 is controlled by a rotation mechanism (not shown) based on a signal supplied from the oscillator 110. Here, the rotation mechanism is a mechanism that rotates the QWP 103 at an angular frequency ω = 2πf, where f is the frequency of the electrical signal generated by the oscillator 110. As a result, the output electric signal V _diff of the differential amplifier 107 shown in Expression (5) can be rewritten as a function of time t as follows.

That is, the output electric signal V _diff (t) of the differential amplifier 107 includes three components: a direct current component, a component of angular frequency 2ω (frequency 2f), and a component of angular frequency 4ω (frequency 4f).
By the way, since the phase difference δ between the P-polarized light and the S-polarized light is zero in the linearly polarized light, some control is performed so that δ = 0, that is, the component of the angular frequency 2ω is zero, in order to generate the linearly polarized light. do it. In addition, when the angle of the first QWP 100 is appropriately set, light having an arbitrary polarization state can be converted into linearly polarized light (that is, δ = 0 can be set). Therefore, in order to generate and output linearly polarized light with the automatic polarization adjuster of FIG. 1, the first frequency component is set so that the component of the angular frequency 2ω in the output electric signal V _diff (t) of the differential amplifier 107 becomes zero. What is necessary is just to control the angle of QWP100.

Information regarding the angle of the polarization plane of linearly polarized light is included in the amplitude ratio r of the S-polarized component and the P-polarized component incident on the second QWP 103. For example, if the polarization plane is linearly polarized light having an angle of 45 degrees with the x-axis, r = 1. At this time, the direct current component of the output electric signal V _diff (t) of the differential amplifier 107 becomes zero. Further, the HWP 101 can rotate the plane of polarization of linearly polarized light at an arbitrary angle with the z axis as a rotation axis. Therefore, in order to generate and output linearly polarized light whose polarization plane forms an angle of 45 degrees with the x-axis by the automatic polarization adjuster, the DC component of the output electric signal V _diff (t) of the differential amplifier 107 becomes zero. In this way, the angle of the HWP 101 may be controlled.

Further, since the amplitude ratio r = ∞ for linearly polarized light (S-polarized light) having a polarization plane perpendicular to the x-axis, the DC component of the output electric signal V _diff (t) of the differential amplifier 107 is normalized. The level is -0.5 and takes the minimum value. Therefore, in order to generate and output S-polarized light by the automatic polarization adjuster, the angle of the HWP 101 is controlled so that the level of the DC component of the output electric signal V _diff (t) of the differential amplifier 107 is minimized. Good.

In addition, in the case of linearly polarized light (P-polarized light) having a polarization plane perpendicular to the y-axis, the amplitude ratio r = 0, so the DC component of the output electric signal V _diff (t) of the differential amplifier 107 is normalized. The level is +0.5 and takes the maximum value. Therefore, in order to generate and output the P-polarized light by the automatic polarization adjuster, the angle of the HWP 101 is controlled so that the level of the DC component of the output electric signal V _diff (t) of the differential amplifier 107 is maximized. Good.

As described above, in order to generate arbitrary linearly polarized light, the DC component and the angular frequency 2ω component of the output electric signal V _diff (t) of the differential amplifier 107 are extracted, and the first QWP 100 is correspondingly extracted. It is necessary to control the angle of HWP101.
In the present embodiment, the LPF 108 and the first control circuit 109 are used to extract the DC component from the output electric signal V _diff (t) of the differential amplifier 107 and control the HWP 101. That is, the LPF 108 extracts a DC component from the electric signal V _diff (t), and the first control circuit 109 controls the angle of the HWP 101 based on the extracted DC component level. In order to generate the desired linearly polarized light, the HWP 101 may be controlled as described above according to the level of the extracted DC component.

In this embodiment, in order to extract the component of the angular frequency 2ω from the output electric signal V _diff (t) of the differential amplifier 107 and control the first QWP 100, the oscillator 110, the frequency doubler 111, A lock-in amplifier 112 and a second control circuit 113 are used.
The frequency doubler 111 outputs an electric signal having a frequency twice that of the output electric signal of the oscillator 110 as the reference signal Vref (t).

FIG. 3 shows the function of the lock-in amplifier 112. The lock-in amplifier 112 extracts only the component having the same frequency as the reference signal Vret (t) of the frequency doubler 111 from the output electric signal V _diff (t) of the differential amplifier 107, and the amplitude of the extracted component. A signal Vout (t) having a voltage value indicating Therefore, the lock-in amplifier 112 outputs the absolute value (= amplitude) of the coefficient of the term sin2ωt in the equation (6). In FIG. 3, the function of the lock-in amplifier 112 is generalized, and only the component of the angular frequency 2ω of the output electric signal V _diff (t) of the differential amplifier 107 is measured signal Vin (t). It represents as. Further, the angular frequency Ω in FIG. 3 is 2ω in the present embodiment.
The second control circuit 113 controls the angle of the first QWP 100 so that the output signal Vout (t) of the lock-in amplifier 112 becomes zero.

  According to the present embodiment, a part of the output light that has passed through the HWP 101 is extracted as sample light by the BS 102, the sample light is polarization-modulated by the second QWP 103 that rotates at a constant frequency, and this polarization-modulated light is converted by the PBS 104. Since the signals necessary for controlling the first QWP 100 and the HWP 101 are obtained by photoelectric conversion by the PDs 105 and 106 after conversion to intensity-modulated light, the automatic polarization is different from the conventional automatic tracking polarization controller. The output light of the regulator need not pass through the polarization beam splitter. Therefore, the polarization-modulated light input to the automatic polarization adjuster can be output as polarization-modulated light having an arbitrary polarization state.

[Second Embodiment]
Next, a second embodiment of the present embodiment will be described. FIG. 4 is a block diagram showing a configuration example of an automatic polarization adjuster according to the second embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals.
In the present embodiment, the frequency quadruple 114 that outputs an electric signal having a frequency four times that of the output electric signal of the oscillator 110 and the output electric signal of the differential amplifier 107 with respect to the automatic polarization adjuster of FIG. The second lock-in amplifier 115 that extracts the amplitude of the frequency 4f component, and the arithmetic unit 116 that obtains the sum or difference between the level of the DC component included in the output electric signal of the differential amplifier 107 and the amplitude of the frequency 4f component. Are added. The LPF 108, the frequency quadrature 114, the second lock-in amplifier 115, the arithmetic unit 116, and the first control circuit 109 constitute a first control means, and the frequency doubler 111, the lock-in amplifier 112, and the second control circuit 109. The control circuit 113 constitutes a second control means.

In the first embodiment, when generating and outputting S-polarized light or P-polarized light, the HWP 101 is controlled so that the DC component of the output electric signal V _diff (t) of the differential amplifier 107 is minimized or maximized. However, in reality, it is not always easy to keep the level of the electric signal V _diff (t) at the minimum value or the maximum value, and it is easier to keep the signal level at zero.

In the first embodiment, when generating S-polarized light, the normalized level of the direct current component of the output electric signal V _diff (t) of the differential amplifier 107 is −0.5 from Equation (6), and the output electric The normalized amplitude of the component of the angular frequency 4ω of the signal V _diff (t) is +0.5. Therefore, in the case of generating S-polarized light, a component having an angular frequency of 4ω is extracted from the output electric signal V _diff (t) of the differential amplifier 107 using the frequency quadrupler 114 and the second lock-in amplifier 115. Then, the sum of the amplitude of the 4ω component and the level of the DC component may be obtained by the computing unit 116 and controlled so that the level of the output electric signal of the computing unit 116 becomes zero.

That is, the frequency quadrupler 114 outputs an electric signal having a frequency four times that of the output electric signal of the oscillator 110 as a reference signal, and the lock-in amplifier 115 outputs the electric signal V _diff (t) output from the differential amplifier 107. Only a component having the same frequency as that of the reference signal of the frequency multiplier 114 is extracted from the inside, and a signal having a voltage value indicating the amplitude of the extracted component is output. The arithmetic unit 116 obtains and outputs the sum of the electric signal output from the low-pass filter 108 and the electric signal output from the second lock-in amplifier 115, and the first control circuit 109 outputs the electric signal output from the arithmetic unit 116 to zero. The angle of the HWP 101 is controlled so that

In the first embodiment, when generating P-polarized light, the normalized level of the DC component of the output electric signal V _diff (t) of the differential amplifier 107 is +0.5, and the output electric signal V _diff ( The normalized amplitude of the component of the angular frequency 4ω of t) is also +0.5. Therefore, when generating P-polarized light, the arithmetic unit 116 obtains and outputs the difference between the output electrical signal of the low-pass filter 108 and the output electrical signal of the second lock-in amplifier 115, and the first control circuit 109 The angle of the HWP 101 may be controlled so that the level of the output electric signal of the calculator 116 becomes zero.
According to the present embodiment, the angle between the first QWP 100 and the first HWP can be controlled more easily than in the first embodiment, and S-polarized light or P-polarized light can be easily generated.

[Third Embodiment]
Next, a third embodiment of the present invention will be described. FIG. 5 is a block diagram showing an example of the configuration of an automatic polarization adjuster according to the third embodiment of the present invention. The same components as those in FIGS. 1 and 4 are given the same reference numerals.
The automatic polarization adjuster of the present embodiment includes a first QWP 100, an HWP 101, a BS 102, a second QWP 103, a PBS 104, PDs 105 and 106, a differential amplifier 107, an LPF 108, an oscillator 110, The frequency doubler 111, the first lock-in amplifier 112, the frequency quadruple 114, the second lock-in amplifier 115, the level of the DC component extracted by the LPF 108, and the second lock-in amplifier 115 The first control circuit 109a that controls the angle of the HWP 101 and the first lock-in amplifier 112 extract the absolute value of the ratio with the amplitude of the extracted component of the frequency 4f to be equal to or less than a predetermined threshold value. The ratio between the amplitude of the frequency 2f component and the amplitude of the frequency 4f component extracted by the second lock-in amplifier 115 is And a second control circuit 113a for controlling the angle of the first QWP100 to be equal to or less than the threshold. The LPF 108, the frequency quadruple 114, the second lock-in amplifier 115, and the first control circuit 109a constitute a first control means, and the frequency doubler 111, the first lock-in amplifier 112, and the frequency four times. The device 114, the second lock-in amplifier 115, and the second control circuit 113a constitute a second control means.

In the first embodiment, for example, when generating linearly polarized light whose polarization plane forms an angle of 45 degrees with the x axis, the direct current component of the output electric signal V _diff (t) of the differential amplifier 107 and the component of the angular frequency 2ω. The first QWP 100 and the HWP 101 are controlled so that both become zero. However, in reality, it may be difficult to make the signal level completely zero due to imperfections in the optical elements or noise in the electric circuit. Therefore, in the present embodiment, adjustment is made so that the DC component and the angular frequency 2ω component of the output electric signal V _diff (t) of the differential amplifier 107 are relatively small with respect to the angular frequency 4ω component. Thus, control equivalent to that of the first embodiment is realized.

That is, in this embodiment, the level of the DC component of the output electrical signal V _diff of the differential amplifier 107 is extracted by LPF 108 (t), the electrical signal V _diff extracted by the second lock-in amplifier 115 (t) The amplitude of the component of the angular frequency 4ω is compared by the first control circuit 109a, and the amplitude of the component of the angular frequency 2ω of the electric signal V _diff (t) extracted by the first lock-in amplifier 112 is compared with the second The amplitude of the component of the angular frequency 4ω extracted by the lock-in amplifier 115 is compared by the second control circuit 113a.

Then, the first control circuit 109a controls the angle of the HWP 101 so that the absolute value of the ratio between the level of the DC component and the amplitude of the component of the angular frequency 4ω is equal to or less than a predetermined threshold value. The control circuit 113a controls the angle of the first QWP 100 so that the ratio of the amplitude of the component of the angular frequency 2ω and the amplitude of the component of the angular frequency 4ω is equal to or less than the threshold value.
According to the present embodiment, linearly polarized light can be generated by controlling the first QWP 100 and the first HWP 101 without being affected by imperfections in the optical element or noise in the electric circuit.

[Fourth Embodiment]
Next, a fourth embodiment of the present invention will be described. In the third embodiment, the case where the direct current component included in the output electric signal of the differential amplifier 107 is set to zero, that is, the case where linearly polarized light whose polarization plane maintains an angle of 45 degrees with the x axis has been described. The second embodiment can be applied to generate S-polarized light or P-polarized light.

In order to generate S-polarized light, as described in the second embodiment, the sum of the level of the DC component of the output electric signal V _diff (t) of the differential amplifier 107 and the amplitude of the component of the angular frequency 4ω is obtained. What is necessary is just to control the angle of HWP101 so that it may become zero. Therefore, in the automatic polarization adjuster of the third embodiment shown in FIG. 5, the first control circuit 109 a determines the level of the DC component of the electric signal V _diff (t) extracted by the LPF 108 and the second lock. The sum with the amplitude of the component of the angular frequency 4ω of the electric signal V _diff (t) extracted by the in-amplifier 115 is obtained by an internal calculator (not shown). Then, the first control circuit 109a controls the angle of the HWP 101 so that the absolute value of the ratio between the sum level obtained by the computing unit and the amplitude of the component of the angular frequency 4ω is equal to or less than a predetermined threshold value. . The operation of the second control circuit 113a is the same as that of the third embodiment. In this way, S-polarized light can be generated.

In order to generate P-polarized light, as described in the second embodiment, the level of the direct current component of the output electric signal V _diff (t) of the differential amplifier 107 and the amplitude of the component of the angular frequency 4ω The angle of the HWP 101 may be controlled so that the difference becomes zero. That is, in the automatic polarization adjuster of the third embodiment, the first control circuit 109a extracts the level of the DC component of the electrical signal V _diff (t) extracted by the LPF 108 and the second lock-in amplifier 115. A difference from the amplitude of the component of the angular frequency 4ω of the electric signal V _diff (t) is obtained by an internal calculator. Then, the first control circuit 109a controls the angle of the HWP 101 so that the absolute value of the ratio between the difference level obtained by the calculator and the amplitude of the component of the angular frequency 4ω is equal to or less than the aforementioned threshold value. . In this way, P-polarized light can be generated.

  According to the present embodiment, S-polarized light or P-polarized light is generated by controlling the first QWP 100 and the first HWP 101 without being affected by imperfections of the optical element or noise of the electric circuit. Can do.

[Fifth Embodiment]
Next, a fifth embodiment of the present invention will be described. FIG. 6 is a block diagram showing a configuration example of an automatic polarization adjuster according to the fifth embodiment of the present invention. The same reference numerals are given to the same configurations as those in FIGS. is there.
The automatic polarization adjuster of the present embodiment includes a first QWP 100, an HWP 101, a BS 102, a PBS 104, PDs 105 and 106, a differential amplifier 107, an LPF 108, a first control circuit 109a, and a first control circuit 109a. A lock-in amplifier 112, a second control circuit 113a, a second lock-in amplifier 115, a second QWP 103a that rotates autonomously at a constant frequency f and polarizes and modulates light branched by the BS 102, A light source 118 that emits synchronization signal generation light to the peripheral portion of the second QWP 103a, and a third that photoelectrically converts intensity-modulated light emitted from the light source 118 and passed through the light transmitting portion around the second QWP 103a. A first band-pass filter (Band-pass filtr, hereinafter referred to as BP) that extracts a frequency 2f component from the synchronization signal output from the PD 119 and the third PD 119. 120) and a second BPF 121 that extracts a frequency 4f component from the synchronization signal output from the third PD 119. The LPF 108, the second BPF 121, the second lock-in amplifier 115, and the first control circuit 109a constitute a first control means, and the first BPF 120, the first lock-in amplifier 112, the second BPF 121, and the like. The second lock-in amplifier 115 and the second control circuit 113a constitute second control means.

  In the present embodiment, the second QWP 103a does not synchronize with the output signal of the oscillator but continues to rotate autonomously at a constant frequency f under the control of a rotation mechanism (not shown). Therefore, in order to obtain a synchronization signal synchronized with the frequency, the light source 118 and PD 119 for acquiring the synchronization signal are used. The structure of the second QWP 103a in this embodiment is shown in FIG.

  Light emitted from the light source 118 travels to a portion between the main body of the QWP 103a and its holder 1030. The holder 1030 is a fixed member that rotatably supports the QWP 103a. In the peripheral part of the QWP 103a, a light shielding part 1031 for 1/4 turn and a light transmission part 1032 for 1/4 turn are alternately arranged. The two light shielding portions 1031 are realized by two light shielding plates. The main body of the QWP 103a and the light shielding plate are integrated, and rotate at an angular frequency ω = 2πf.

  As a result, the output electric signal of the PD 119 obtained as a result of photoelectric conversion of the light emitted from the light source 118 and passing through the light transmitting portion 1032 and reaching the PD 119 is as shown in FIG. In FIG. 8, T is T = 2π / ω = 1 / f and represents the time required for one rotation of the QWP 103a and the light shielding plate.

  The synchronization signal output from the PD 119 includes a frequency component that is an integral multiple of the frequency 2f. Therefore, a component of frequency 2f that is twice the frequency f rotated by the second QWP 103a is extracted by the first BPF 120 as shown in FIG. 8B, and a component of the frequency 4f that is four times the frequency is shown by the second BPF 121. 8 (c) can be extracted. The first BPF 120 outputs the extracted component as a reference signal for the first lock-in amplifier 112, and the second BPF 121 outputs the extracted component as a reference signal for the second lock-in amplifier 115.

The first lock-in amplifier 112 extracts a component having the same frequency 2f as the reference signal of the BPF 120 from the output electric signal V _diff (t) of the differential amplifier 107, and a voltage value indicating the amplitude of the extracted component. The second lock-in amplifier 115 extracts a component having the same frequency 4f as the reference signal of the BPF 121 from the electric signal V _diff (t), and a voltage value indicating the amplitude of the extracted component. The signal is output. The operations of the first control circuit 109a and the second control circuit 113a are as described in the third embodiment or the fourth embodiment.

According to the present embodiment, even if the second QWP 103a is autonomously rotating at a constant frequency f, a synchronization signal is generated based on the rotation of the QWP 103a, and based on the synchronization signal The frequency 2f component and the frequency 4f component included in the output electric signal V _diff (t) of the differential amplifier 107 can be extracted, and the angles of the first QWP 100 and the first HWP 101 can be controlled. .

[Sixth Embodiment]
Next, a sixth embodiment of the present invention will be described. FIG. 9 is a block diagram showing a configuration example of an automatic polarization adjuster according to the sixth embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals.
The automatic polarization adjuster of the present embodiment has a third QWP 122 that gives a phase difference of 90 degrees to the independent linearly polarized light component of the light that has passed through the BS 102 with respect to the automatic polarization adjuster of the first embodiment, A second HWP 123 that gives a phase difference of 180 degrees to an independent linearly polarized light component of light that has passed through the third QWP 122, and the third QWP 122 and the second QWP 122 based on the ellipticity and azimuth that specify the polarization state desired by the user A computer 124 for controlling the angles of the third QWP 122 and the second HWP 123, and an input interface 125 for the user to specify a desired polarization state are added. Thus, not only linearly polarized light but also arbitrary polarized light can be output. The computer 124 and the input interface 125 constitute third control means and fourth control means.

  In general, linearly polarized light can be converted into an arbitrary polarization state by appropriately setting the angles of QWP and HWP. In this embodiment, first, the polarization state of light that the user wants to output is input to the input interface 125. As described in the first embodiment, the amplitude ratio r between the S-polarized component and the P-polarized component incident on the second QWP 103, and the phase difference δ between the P-polarized component and the S-polarized component incident on the second QWP 103, The desired polarization state can be specified by specifying, but the ellipticity η and azimuth angle ψ of the polarization state of the light to be output are specified as parameters that are more intuitive and easier to understand.

  FIG. 10 shows definitions of the ellipticity η and the azimuth angle ψ of the polarization state of the output light. However, the definition range of the ellipticity η is −π / 4 ≦ η ≦ π / 4. When the ellipticity η is positive, it is defined as clockwise polarization, and when it is negative, it is defined as counterclockwise polarization. The azimuth angle ψ is an angle formed by the major axis of the ellipse and the x-axis, and its domain is −π / 2 ≦ ψ ≦ π / 2.

  By appropriately setting the angle of the third QWP 122, linearly polarized light can be converted into polarized light having an arbitrary ellipticity, but the azimuth angle of the converted polarized light cannot be adjusted. On the other hand, by appropriately setting the angle of the second HWP 123, it is possible to convert the azimuth angle of arbitrary polarization, but the sign (rotation direction) of the ellipticity η is reversed. This is shown in FIG. In this example, linearly polarized light having an electric field component parallel to the x-axis is converted into elliptically polarized light having an ellipticity η and an azimuth angle ψ.

In FIG. 11A, sl _q and fa _q represent the slow axis and fast axis of the third QWP 122, respectively. The angle formed by the slow axis sl _{q and the} x axis is θ _q . As shown in FIG. 11A, first, an angle θ _q formed by the slow axis sl _q of the third QWP 122 and the x axis is set to η, and elliptically polarized light having an ellipticity of −η and an azimuth angle of η is generated. To do.

Next, the azimuth angle is adjusted by the second HWP 123 as shown in FIG. In FIG. 11B, sl _h and fa _h represent the slow axis and fast axis of the second HWP 123, respectively. The angle formed by the slow axis sl _{h and the} x axis is θ _h . The second HWP 123 has a function of converting the azimuth angle ψ of the ellipse to a position symmetric with respect to the slow axis sl _h and inverting the sign of the ellipticity η while maintaining the absolute value of the ellipticity η. In consideration of such functions of the HWP 123 and the QWP 122, θ _q = η, θ _h = η + {(ψ−η) with respect to the ellipticity η and the azimuth angle ψ that are parameters input to the input interface 125. / 2}, polarized light having the input parameters can be obtained.

The calculator 124 calculates the angles θ _q and θ _h as described above based on the ellipticity η and the azimuth angle ψ input from the input interface 125, and calculates the slow axis sl _q of the third QWP 122 and the x axis. The angle of the third QWP 122 is controlled so that the formed angle becomes the calculated angle θ _q , and the angle formed between the slow axis sl _h of the second HWP 123 and the x axis becomes the calculated angle θ _h. The angle of the second HWP 123 is controlled.

Thus, according to the present embodiment, it is possible to stably output light in a polarization state having parameters specified by the user.
Since linearly polarized light with a constant azimuth angle is always incident on the third QWP 122, once the angles θ _q and θ _h are set, the third QWP 122 and the second HWP 123 are connected to the first QWP 100 and the first QWP 100. It is not necessary to control dynamically like 1 HWP101.
In this embodiment, the case where the present invention is applied to the first embodiment has been described as an example. However, the present embodiment is not limited to this, and the present invention is applicable to the second to fifth embodiments. May be.

[Seventh Embodiment]
Next, a seventh embodiment of the present invention will be described. FIG. 12 is a block diagram illustrating a configuration example of a magnetic field sensor according to the seventh embodiment of the present invention.
The magnetic field sensor of FIG. 12 shows an example in which the automatic polarization adjuster of the first to sixth embodiments is applied, and includes a sensor head unit 11 including a magneto-optic crystal 131 and an output of the sensor head unit 11. A signal processing unit 12 for detecting a magnetic field to be measured based on the above, a polarization-maintaining fiber (hereinafter abbreviated as PMF) 13 for transmitting light from the signal processing unit 12 to the sensor head unit 11, and The optical fiber 14 transmits light from the sensor head unit 11 to the signal processing unit 12.

  The sensor head unit 11 propagates through the lens 130 that converts the light beam emitted from the PMF 13 into parallel light and enters the first surface of the magneto-optic crystal 131, the magneto-optic crystal 131, and the magneto-optic crystal 131. A lens 132 that collects the light emitted from the second surface opposite to the first surface of the optical crystal 131 and causes the light to enter the optical fiber 4.

  The signal processing unit 12 includes a light source 200 that emits linearly polarized light, a linearly polarized light generator 201 that converts light transmitted by the optical fiber 14 into linearly polarized light having a polarization plane with a preset angle, and a linearly polarized light generator. PBS 203 that separates the output light of 201 into S-polarized light and P-polarized light, PDs 204 and 205 that photoelectrically convert the two light outputs of PBS 203, and a differential amplifier 206 that differentially amplifies the electrical signals output from PDs 204 and 205, And an electric signal measuring device 207 for detecting an electric field to be measured based on an electric signal output from the dynamic amplifier 206.

In the magnetic field sensor of the present embodiment, the polarization state of the polarization-modulated light transmitted to the linearly polarized light generator 201 by the optical fiber 4 changes randomly due to the temperature change of the optical fiber 4 and the stress applied to the optical fiber 4.
For this reason, it is necessary to cancel the low-speed polarization change caused by the stress applied to the optical fiber 4 or the temperature change of the optical fiber 4 and to output the high-speed polarization change caused by the magnetic field to be measured as it is. The means having such a function is a linear polarization generator 201, and this embodiment uses the automatic polarization adjuster of the first to sixth embodiments as the configuration of the linear polarization generator 201. It was.

The linearly polarized light emitted from the light source 200 is transmitted to the sensor head unit 11 as linearly polarized light by the PMF 13, converted into parallel light by the lens 130 in the sensor head unit 11, and enters the magneto-optical crystal 131. The magneto-optic crystal 131 has a function of rotating the polarization plane of linearly polarized light according to the amplitude of the magnetic field to be measured. The light polarization states at points A and C in FIG. 12 are shown in FIGS. 13 (a) to 13 (d), and the light polarization states at point B in FIG. 12 are shown in FIGS. 13 (e) to 13 (h). Shown in FIG. 13 (i) shows the measured magnetic field H (t) as a function of time t, where H (t ₁ ) = 0, H (t ₂ )> 0, H (t ₃ ) = 0, H (t ₄ ) <0.

  The polarization state of the light at point A in FIG. 12 that has passed through the magneto-optical crystal 131 changes as shown in FIGS. 13A to 13D according to the magnetic field H (t) to be measured. When such polarization-modulated light is transmitted through the optical fiber 14, the polarization state of the light at the point B in FIG. 12 is as shown in FIGS. 13 (e) to 13 (h) in accordance with the measured magnetic field H (t). Change.

The maximum value of the response frequency at which the control mechanism of the automatic polarization adjuster (linear polarization generator 201) described in the first to sixth embodiments works effectively is f _C , and the measured magnetic field H (t). of the a and f _M frequency, If you meet f _M> f _C, since the control of the linearly polarized light generator 201 to the polarization change of light due to the measured magnetic field H (t) can not follow, in FIG. 12 The polarization state of light at point C is equal to the state at point A. The function of the linearly polarized light generator 201 in the present embodiment is to compensate for a static polarization change caused by propagating through the optical fiber 14, so that it is not necessary to increase f _C. It is sufficiently possible to set so as to satisfy _M > f _C.

FIG. 14A shows the waveform of the magnetic field to be measured H (t). Then, when the automatic polarization adjusters of the first to sixth embodiments are used as the linearly polarized light generator 201 of FIG. 12, output electric signals V ₁ (t), V ₂ ( t) and the output electric signal ΔV (t) of the differential amplifier 206 are shown in FIGS. 14 (b), 14 (c), and 14 (d), respectively. The PDs 204 and 205 output electrical signals V ₁ (t) and V ₂ (t) proportional to the intensity of S-polarized light and P-polarized light divided by the PBS 203. Since the output electric signal ΔV (t) of the differential amplifier 206 has the same waveform as the magnetic field to be measured H (t), the electric field ΔV (t) is detected by the electric signal measuring device 207, thereby measuring the magnetic field H to be measured. (T) can be detected.

Further, the output electric signals V ₁ (t) and V ₂ (t) of the PDs 204 and 205 when the automatic polarization adjuster is not used in the configuration of the linear polarization generator 201, and the output electric signal ΔV (t ) Are shown in FIGS. 14 (e), 14 (f) and 14 (g), respectively. Comparing FIG. 14 (d) and FIG. 14 (g), not only can a signal with a large amplitude be obtained by using the automatic polarization adjuster of the present invention, but also an electrical signal Δ that does not contain a DC component (offset). Since V (t) can be obtained, highly sensitive detection can be performed without applying an extra load to the subsequent electrical signal measuring device 207.

As described above, according to the present embodiment, the maximum value of the response frequency of the linearly polarized light generator 201 is f _C , and any frequency among the polarization change frequencies of light incident on the linearly polarized light generator 201 is f _M. In this case, by satisfying f _M > f _C , a filtering process that cancels only the polarization change slower than the maximum value f _C of the response frequency can be realized, and the polarization modulated light faster than the frequency f _M can be realized. Can be output from the linearly polarized light generator 201.

  In the present embodiment, the magnetic field sensor using the magneto-optic crystal has been described. However, the automatic polarization of the first to sixth embodiments is also applied to the electric field sensor using the electro-optic crystal. The adjuster can be applied in the same manner as this embodiment.

  The present invention can be applied to communication systems and sensors.

It is a block diagram which shows the structural example of the automatic polarization adjuster used as the 1st Embodiment of this invention. It is a figure which shows the relationship between the coordinate axis in the 1st Embodiment of this invention, and the principal axis of a 2nd quarter wave plate. It is a figure for demonstrating the function of the lock-in amplifier in the 1st Embodiment of this invention. It is a block diagram which shows the structural example of the automatic polarization adjuster used as the 2nd Embodiment of this invention. It is a block diagram which shows the structural example of the automatic polarization adjuster used as the 3rd Embodiment of this invention. It is a block diagram which shows the structural example of the automatic polarization adjuster used as the 5th Embodiment of this invention. It is a top view which shows the structure of the 2nd quarter wave plate in the 5th Embodiment of this invention. It is a signal waveform diagram of the output electric signal of the 3rd photodetector in the 5th Embodiment of this invention, a 1st band pass filter, and a 2nd band pass filter. It is a block diagram which shows the structural example of the automatic polarization adjuster used as the 6th Embodiment of this invention. It is a figure which shows the definition of the ellipticity and azimuth of the polarization state of output light in the 6th Embodiment of this invention. It is a figure which shows the mode of the conversion of the polarization by the 3rd quarter wavelength plate and the 2nd half wavelength plate in the 6th Embodiment of this invention. It is a block diagram which shows the structural example of the magnetic field sensor which becomes the 7th Embodiment of this invention. It is a figure which shows the polarization state of the light which radiate | emitted the magneto-optic crystal in the 7th Embodiment of this invention, the light which injects into a linearly polarized light generator, and the light which radiate | emitted the linearly polarized light generator. It is a wave form diagram which shows the output electric signal of the photodetector in the 7th Embodiment of this invention, and a differential amplifier. It is a block diagram which shows the structure of the conventional automatic tracking type polarization controller.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... 1st 1/4 wavelength plate, 101 ... 1/2 wavelength plate, 102 ... Beam splitter, 103, 103a ... 2nd 1/4 wavelength plate, 104, 203 ... Polarizing beam splitter, 105, 106, 119 , 204, 205 ... photodetectors, 107, 206 ... differential amplifiers, 108 ... low pass filter, 109, 109a ... first control circuit, 110 ... oscillator, 111 ... frequency doubler, 112 ... first lock-in Amplifier, 113, 113a ... second control circuit, 114 ... frequency quadrature, 115 ... second lock-in amplifier, 116 ... calculator, 118,200 ... light source, 120 ... first band pass filter, 121 ... Second band-pass filter 122 ... third quarter-wave plate 123 ... second half-wave plate 124 ... computer 125 ... input interface 11 ... sen Head, 12 ... signal processing unit, 13 ... polarization-maintaining fiber, 14 ... optical fiber, 130, 132 ... lens, 131 ... magneto-optic crystal, 201 ... linearly polarized light generator, 207 ... electric signal measuring instrument.

Claims (18)

A first quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized component of input light having an arbitrary polarization state;
A first half-wave plate that gives a phase difference of 180 degrees to the independent linearly polarized light component of the light that has passed through the first quarter-wave plate;
A first separation element that branches part of the output light that has passed through the first half-wave plate;
A second quarter-wave plate that rotates at a constant frequency f with an axis parallel to the traveling direction of the light branched by the first separation element as a rotation axis, and that modulates polarization by passing the branched light;
A second separation element that separates light that has passed through the second quarter-wave plate into P-polarized light and S-polarized light;
A first photodetector for photoelectrically converting the P-polarized light;
A second photodetector for photoelectrically converting the S-polarized light;
A differential amplifier for differentially amplifying the output electrical signal of the first photodetector and the output electrical signal of the second photodetector;
First control means for controlling the angle of the first half-wave plate so that the level of the DC component contained in the output electric signal of the differential amplifier becomes a preset value;
Second control means for controlling the angle of the first quarter-wave plate so that the component of the frequency 2f included in the output electric signal of the differential amplifier becomes zero,
An automatic polarization adjuster characterized in that the output light that has passed through the first separation element always becomes a constant linearly polarized light. A first quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized component of input light having an arbitrary polarization state;
A first half-wave plate that gives a phase difference of 180 degrees to the independent linearly polarized light component of the light that has passed through the first quarter-wave plate;
A first separation element that branches part of the output light that has passed through the first half-wave plate;
A second quarter-wave plate that rotates at a constant frequency f with an axis parallel to the traveling direction of the light branched by the first separation element as a rotation axis, and that modulates polarization by passing the branched light;
A second separation element that separates light that has passed through the second quarter-wave plate into P-polarized light and S-polarized light;
A first photodetector for photoelectrically converting the P-polarized light;
A second photodetector for photoelectrically converting the S-polarized light;
A differential amplifier for differentially amplifying the output electrical signal of the first photodetector and the output electrical signal of the second photodetector;
The sum or difference between the level of the direct current component included in the output electric signal of the differential amplifier and the amplitude of the component of the frequency 4f is obtained, and the first half-wave plate of the first half-wave plate is set so that this sum or difference becomes zero. First control means for controlling the angle;
Second control means for controlling the angle of the first quarter-wave plate so that the component of the frequency 2f included in the output electric signal of the differential amplifier becomes zero,
An automatic polarization adjuster characterized in that the output light that has passed through the first separation element always becomes a constant linearly polarized light. A first quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized component of input light having an arbitrary polarization state;
A first half-wave plate that gives a phase difference of 180 degrees to the independent linearly polarized light component of the light that has passed through the first quarter-wave plate;
A first separation element that branches part of the output light that has passed through the first half-wave plate;
A second quarter-wave plate that rotates at a constant frequency f with an axis parallel to the traveling direction of the light branched by the first separation element as a rotation axis, and that modulates polarization by passing the branched light;
A second separation element that separates light that has passed through the second quarter-wave plate into P-polarized light and S-polarized light;
A first photodetector for photoelectrically converting the P-polarized light;
A second photodetector for photoelectrically converting the S-polarized light;
A differential amplifier for differentially amplifying the output electrical signal of the first photodetector and the output electrical signal of the second photodetector;
The angle of the first half-wave plate so that the absolute value of the ratio between the level of the DC component contained in the output electric signal of the differential amplifier and the amplitude of the frequency 4f component is less than a predetermined threshold value. First control means for controlling
The angle of the first quarter-wave plate is controlled so that the ratio of the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the output electric signal of the differential amplifier is equal to or less than the threshold value. Second control means for
An automatic polarization adjuster characterized in that the output light that has passed through the first separation element always becomes a constant linearly polarized light. A first quarter-wave plate that gives a phase difference of 90 degrees to an independent linearly polarized component of input light having an arbitrary polarization state;
A first half-wave plate that gives a phase difference of 180 degrees to the independent linearly polarized light component of the light that has passed through the first quarter-wave plate;
A first separation element that branches part of the output light that has passed through the first half-wave plate;
A second quarter-wave plate that rotates at a constant frequency f with an axis parallel to the traveling direction of the light branched by the first separation element as a rotation axis, and that modulates polarization by passing the branched light;
A second separation element that separates light that has passed through the second quarter-wave plate into P-polarized light and S-polarized light;
A first photodetector for photoelectrically converting the P-polarized light;
A second photodetector for photoelectrically converting the S-polarized light;
A differential amplifier for differentially amplifying the output electrical signal of the first photodetector and the output electrical signal of the second photodetector;
The sum or difference between the level of the DC component contained in the output electric signal of the differential amplifier and the amplitude of the component of the frequency 4f is obtained, and the absolute value of the ratio between the level of the sum or difference and the amplitude of the component of the frequency 4f is obtained. First control means for controlling the angle of the first half-wave plate so as to be below a predetermined threshold;
The angle of the first quarter-wave plate is controlled so that the ratio of the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the output electric signal of the differential amplifier is equal to or less than the threshold value. Second control means for
An automatic polarization adjuster characterized in that the output light that has passed through the first separation element always becomes a constant linearly polarized light. The automatic polarization adjuster according to any one of claims 1 to 4,
Furthermore, an oscillator for generating a signal of the frequency f is provided,
The second quarter-wave plate rotates in synchronization with the signal generated by the oscillator,
The first control means and the second control means are used by the oscillator as a local oscillation wave when extracting the amplitude of the frequency 2f component and the amplitude of the frequency 4f component contained in the output electric signal of the differential amplifier. An automatic polarization adjuster using the generated signal. The automatic polarization adjuster according to any one of claims 1 to 4,
Furthermore, a synchronization signal generating means for generating a signal synchronized with the frequency f at which the second quarter-wave plate rotates autonomously based on the rotation of the second quarter-wave plate,
The first control unit and the second control unit are configured to output the synchronization signal as a local oscillation wave when extracting the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the output electric signal of the differential amplifier. An automatic polarization adjuster using a synchronization signal generated by a generating means. The automatic polarization adjuster according to claim 6, wherein
The synchronization signal generating means includes
Two sheets constituting the light shielding portion so that a light transmitting portion for a quarter turn and a light shielding portion for a quarter turn are alternately arranged on the periphery of the second quarter wavelength plate A light shielding plate,
A light source that emits light for generating a synchronization signal to the periphery of the second quarter-wave plate;
An automatic polarization adjuster comprising: a third photodetector that photoelectrically converts intensity-modulated light emitted from the light source and passed through the light transmission unit to generate the synchronization signal. The automatic polarization adjuster according to any one of claims 1 to 7,
A third quarter-wave plate that gives a phase difference of 90 degrees to the independent linearly polarized light component of the light that has passed through the first separation element;
A second half-wave plate that gives a phase difference of 180 degrees to the independent linearly polarized light component of the light that has passed through the third quarter-wave plate;
Based on the ellipticity η specifying the polarization state desired by the user and the azimuth angle ψ, the angle θ _q to be set of the third quarter-wave plate is calculated by θ _q = η, and the third Third control means for controlling the angle of the quarter-wave plate to be θ _q ;
The angle θ _h to be set of the second half-wave plate is calculated by θ _h = η + {(ψ−η) / 2}, and the angle of the second half-wave plate becomes θ _h And a fourth control means for controlling so that
An automatic polarization adjuster characterized in that output light that has passed through the second half-wave plate becomes light in a polarization state desired by the user. The automatic polarization adjuster according to any one of claims 1 to 8,
The input light incident on the first quarter-wave plate is polarization-modulated light in which the polarization state changes with time and there are a plurality of frequencies of the change.
When the maximum value of the response frequency of the control by the first control means and the second control means is f _C , and any frequency of the polarization state changes is f _M , f _M > f _C is satisfied. Automatic polarization adjuster characterized by A first quarter wave plate that gives a phase difference of 90 degrees to an independent linear polarization component of input light having an arbitrary polarization state, and an independent linear polarization component of light that has passed through the first quarter wavelength plate A polarization adjustment method for maintaining and outputting the input light in a constant polarization state using a first half-wave plate that gives a phase difference of 180 degrees to
A first separation procedure for branching a part of the output light that has passed through the first half-wave plate;
A modulation procedure for polarization-modulating the branched light by a second quarter-wave plate rotating at a constant frequency f about an axis parallel to the traveling direction of the branched light;
A second separation procedure for separating the polarization-modulated light into P-polarized light and S-polarized light;
A first light detection procedure for photoelectrically converting the P-polarized light;
A second light detection procedure for photoelectrically converting the S-polarized light;
A differential amplification procedure for differentially amplifying the output electrical signal of the first light detection procedure and the output electrical signal of the second light detection procedure;
A first control procedure for controlling the angle of the first half-wave plate so that the level of the DC component included in the output electrical signal of the differential amplification procedure becomes a preset value;
A second control procedure for controlling the angle of the first quarter-wave plate so that the component of the frequency 2f included in the output electrical signal of the differential amplification procedure becomes zero,
A polarization adjustment method characterized in that output light that has passed through the first separation procedure is always constant linearly polarized light. A first quarter-wave plate that gives a phase difference of 90 degrees to an independent linear polarization component of input light having an arbitrary polarization state, and an independent linear polarization component of light that has passed through the first quarter-wave plate A polarization adjustment method for outputting the input light while keeping the input light in a constant polarization state using a first half-wave plate that gives a phase difference of 180 degrees to
A first separation procedure for branching a part of the output light that has passed through the first half-wave plate;
A modulation procedure for polarization-modulating the branched light by a second quarter-wave plate rotating at a constant frequency f about an axis parallel to the traveling direction of the branched light;
A second separation procedure for separating the polarization-modulated light into P-polarized light and S-polarized light;
A first light detection procedure for photoelectrically converting the P-polarized light;
A second light detection procedure for photoelectrically converting the S-polarized light;
A differential amplification procedure for differentially amplifying the output electrical signal of the first light detection procedure and the output electrical signal of the second light detection procedure;
The sum or difference between the level of the DC component included in the output electrical signal of the differential amplification procedure and the amplitude of the component of frequency 4f is obtained, and the first half-wave plate is set so that this sum or difference becomes zero. A first control procedure for controlling the angle of
A second control procedure for controlling the angle of the first quarter-wave plate so that the component of the frequency 2f included in the output electrical signal of the differential amplification procedure is minimized,
A polarization adjustment method characterized in that output light that has passed through the first separation procedure is always constant linearly polarized light. A first quarter wave plate that gives a phase difference of 90 degrees to an independent linear polarization component of input light having an arbitrary polarization state, and an independent linear polarization component of light that has passed through the first quarter wavelength plate A polarization adjustment method for maintaining and outputting the input light in a constant polarization state using a first half-wave plate that gives a phase difference of 180 degrees to
A first separation procedure for branching a part of the output light that has passed through the first half-wave plate;
A modulation procedure for polarization-modulating the branched light by a second quarter-wave plate rotating at a constant frequency f about an axis parallel to the traveling direction of the branched light;
A second separation procedure for separating the polarization-modulated light into P-polarized light and S-polarized light;
A first light detection procedure for photoelectrically converting the P-polarized light;
A second light detection procedure for photoelectrically converting the S-polarized light;
A differential amplification procedure for differentially amplifying the output electrical signal of the first light detection procedure and the output electrical signal of the second light detection procedure;
The first half-wave plate is configured so that the absolute value of the ratio between the level of the DC component contained in the output electrical signal of the differential amplification procedure and the amplitude of the frequency 4f component is equal to or less than a predetermined threshold value. A first control procedure for controlling the angle;
The angle of the first quarter-wave plate is set so that the ratio of the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the output electrical signal of the differential amplification procedure is equal to or less than the threshold value. A second control procedure for controlling,
A polarization adjustment method characterized in that output light that has passed through the first separation procedure is always constant linearly polarized light. A first quarter wave plate that gives a phase difference of 90 degrees to an independent linear polarization component of input light having an arbitrary polarization state, and an independent linear polarization component of light that has passed through the first quarter wavelength plate A polarization adjustment method for maintaining and outputting the input light in a constant polarization state using a first half-wave plate that gives a phase difference of 180 degrees to
A first separation procedure for branching a part of the output light that has passed through the first half-wave plate;
A modulation procedure for polarization-modulating the branched light by a second quarter-wave plate rotating at a constant frequency f about an axis parallel to the traveling direction of the branched light;
A second separation procedure for separating the polarization-modulated light into P-polarized light and S-polarized light;
A first light detection procedure for photoelectrically converting the P-polarized light;
A second light detection procedure for photoelectrically converting the S-polarized light;
A differential amplification procedure for differentially amplifying the output electrical signal of the first light detection procedure and the output electrical signal of the second light detection procedure;
The sum or difference between the level of the direct current component included in the output electric signal of this differential amplification procedure and the amplitude of the component of frequency 4f is obtained, and the absolute value of the ratio between the level of this sum or difference and the amplitude of the component of frequency 4f A first control procedure for controlling the angle of the first half-wave plate so that is equal to or less than a predetermined threshold;
The angle of the first quarter-wave plate is set so that the ratio of the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the output electrical signal of the differential amplification procedure is equal to or less than the threshold value. A second control procedure for controlling,
A polarization adjustment method characterized in that output light that has passed through the first separation procedure is always constant linearly polarized light. The polarization adjustment method according to any one of claims 10 to 13,
And an oscillation procedure for generating a signal of the frequency f,
The modulation procedure rotates the second quarter-wave plate in synchronization with the signal generated by the oscillation procedure,
The first control procedure and the second control procedure include the oscillation as a local oscillation wave when extracting the amplitude of the component of frequency 2f and the amplitude of the component of frequency 4f included in the output electric signal of the differential amplification procedure. A polarization adjustment method using a signal generated by a procedure. The polarization adjustment method according to any one of claims 10 to 13,
And a synchronization signal generating procedure for generating a signal synchronized with the frequency f at which the second quarter-wave plate rotates autonomously based on the rotation of the second quarter-wave plate,
In the first control procedure and the second control procedure, the synchronous oscillation is performed as a local oscillation wave when extracting the amplitude of the frequency 2f component and the amplitude of the frequency 4f component included in the output electric signal of the differential amplification procedure. A polarization adjustment method using a synchronization signal generated by a signal generation procedure. The polarization adjusting method according to claim 15, wherein
In the synchronization signal generation procedure, a light transmitting portion for a quarter turn and a light shielding portion for a quarter turn are alternately arranged in the periphery of the second quarter wavelength plate, Light for generating a synchronization signal is emitted from a light source to the periphery of the quarter-wave plate, and the synchronization signal is generated by photoelectrically converting the intensity-modulated light emitted from the light source and passed through the light transmission unit. A polarization adjusting method characterized by the above. The polarization adjustment method according to any one of claims 10 to 16,
A first adjustment procedure for adjusting the polarization state of the light by a third quarter-wave plate that gives a phase difference of 90 degrees to the independent linearly polarized light component of the light that has passed through the first separation procedure;
A second adjustment procedure for adjusting the polarization state of the light by a second half-wave plate that gives a phase difference of 180 degrees to an independent linearly polarized light component of the light that has passed through the third quarter-wave plate;
Based on the ellipticity η specifying the polarization state desired by the user and the azimuth angle ψ, the angle θ _q to be set of the third quarter-wave plate is calculated by θ _q = η, and the third A third control procedure for controlling the angle of the quarter-wave plate to be θ _q ;
The angle θ _h to be set of the second half-wave plate is calculated by θ _h = η + {(ψ−η) / 2}, and the angle of the second half-wave plate becomes θ _h And a fourth control procedure for controlling so that
The polarization adjustment method, wherein the output light that has passed through the second half-wave plate becomes light in a polarization state desired by the user. The polarization adjustment method according to any one of claims 10 to 17,
The input light incident on the first quarter-wave plate is polarization-modulated light in which the polarization state changes with time and there are a plurality of frequencies of the change.
When the maximum value of the response frequency of the control by the first control procedure and the second control procedure is f _C , and any frequency of the polarization state change is f _M , f _M > f _C is satisfied. A polarization adjusting method characterized by the above.

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