CN116248183A - Optical fiber stable phase transmission device and method based on balanced light pulse scheme - Google Patents

Abstract

The invention relates to an optical fiber stable phase transmission device based on a balanced light pulse scheme, which comprises a mode-locked laser and a low-noise microwave source which are connected with each other, wherein a first half wave plate, a polarization cube, a first collimator, a circulator, a coupler and an electro-optic modulator are sequentially arranged along the transmission direction of light pulses output by the mode-locked laser; one side of the polarization cube is sequentially provided with a quarter wave plate and a reflecting mirror along the transmission direction of the light pulse, and the other side of the polarization cube is provided with a round-trip light pulse transmission system; the coupler is connected with a balance detector, the electro-optical modulator is connected with the low-noise microwave source, and the round-trip optical pulse transmission system and the balance detector are connected with a feedback controller. The invention also relates to a corresponding method. The phase-demodulation circuit is more stable in structure, simpler in required precondition experiment conditions, and higher in phase-demodulation sensitivity while avoiding noise introduced by amplitude-phase conversion.

Description

Optical fiber stable phase transmission device and method based on balanced light pulse scheme

Technical Field

The invention relates to the technical field of optical pulse transmission, in particular to an optical fiber stable phase transmission device and method based on a balanced optical pulse scheme.

Background

The existing optical pulse steady phase transmission device is an optical cross-correlation scheme based on nonlinear crystals (PPKTP crystals), as shown in figure 1, and the scheme principle is as follows: the mode-locked laser generates and outputs an optical pulse string in free space, the linear polarization direction of the optical pulse string is regulated through a half-wave plate in the free space, and the optical pulse string is separated into an optical pulse string in p-polarization state and an optical pulse string in s-polarization state through a polarization beam splitting cube, wherein the optical pulse in p-polarization state is directly transmitted and used as a reference path optical pulse to be input into a PPKTP crystal, and the optical pulse in s-polarization state is reflected as a round trip optical pulse. The s polarized light pulse sequentially passes through the delay line, the half wave plate and the Faraday rotator, then enters the polarization maintaining fiber through the collimator in a focusing way, and is connected into the fiber extender, the dispersion compensating fiber and the bidirectional erbium-doped fiber amplifier in the fiber transmission process. At the end of the optical fiber transmission, a part of light is directly output by an optical fiber semi-transparent semi-reflector for use by a client, the other part of light is reflected by the original path and enters the free space again, and after passing through the Faraday rotator again, the original s-polarized light pulse becomes p-polarized light pulse, so that when entering the polarization beam splitting cube again, the round-trip pulse string is directly transmitted and output, and is reflected by the reflecting mirror and passes through the 1/4 wave plate twice, the round-trip pulse string becomes s-polarized light pulse again, and the third time is reflected into the PPKTP crystal after passing through the polarization beam splitting cube. The delay time of the round-trip light pulse is regulated through optical delay, so that the round-trip light pulse and the reference light pulse generate sum frequency light in the PPKTP crystal, the far end surface of the PPKTP is plated with a transmission film, the generated sum frequency light is directly output into one photoelectric detector of the balance detector, the reference light pulse and the round-trip light pulse are reflected by the transmission film and pass through the PPKTP crystal again to generate sum frequency light for the second time, the sum frequency light is reflected by a dichroic mirror for transmitting fundamental frequency light and sum frequency light, and is reflected by a reflecting mirror to enter the other photoelectric detector of the balance detector, the two photoelectric detectors arranged in the balance detector convert light signals into electric signals, and the signals are amplified through a trans-impedance amplifier, and the final output voltage value is in linear relation with the time coincidence of the reference light pulse and the round-trip light pulse, so that the sum frequency light is used as a phase-identifying signal. The scheme has high requirements on the stability of the optical path, because the superposition adjustment of the reference path optical pulse and the round trip optical pulse in the nonlinear crystal is in the single pulse width range, and the single pulse width is generally in the hundred femtoseconds level, the optical path is required to be placed on a stable optical platform, and the required precondition is very strict.

The conventional other optical pulse transmitting and stabilizing phase transmission device is based on a scheme of radio frequency phase discrimination, as shown in fig. 2, the scheme generates an optical pulse string through a mode-locked laser and outputs the optical pulse string through an optical fiber, and then the optical pulse string is connected with an optical fiber branching device, one port of the optical fiber branching device is connected with a photoelectric detector so as to convert an optical pulse signal into an electric pulse signal, the output end of the photoelectric detector is connected with a filter so as to filter out a corresponding first harmonic signal in the electric pulse signal, and the first harmonic signal is amplified by a low-noise amplifier and then is input to one port of the phase discriminator. The other port of the optical fiber branching device is connected with an optical fiber circulator, the optical fiber circulator is a unidirectional three-port device with irreversible transmission, the output end of the optical fiber circulator is connected with an optical fiber delay line, and the rear of the optical fiber delay line is sequentially connected with an optical fiber extender, a dispersion compensation optical fiber, a transmission optical fiber, a bidirectional erbium-doped optical fiber amplifier and an optical fiber semi-transparent semi-returning device. The optical fiber semi-transparent semi-returning device directly outputs a part of light for the client, and the other part of light returns in the original path, reversely passes through the bidirectional erbium-doped optical fiber amplifier, the optical fiber extender and the optical fiber delay line again and then is input into the optical fiber circulator. At this time, the output end of the circulator is directly connected with the photoelectric detector to convert the optical pulse signal into an electric pulse signal, the output end of the photoelectric detector is connected with the filter to filter out a second harmonic signal with the same frequency as the first harmonic signal, and the second harmonic signal is amplified by the low-noise amplifier and then is input to the other port of the phase discriminator. When the first harmonic signal and the second harmonic signal are simultaneously connected to the phase discriminator, voltage signal output in linear relation with the phase difference of the two signals is generated, and the feedback controller feedback-controls the optical fiber delay line and the optical fiber extender through the voltage signals. The scheme is simpler in structure and more stable in transmission in the optical fiber, but when the photoelectric detector is adopted to convert the optical pulse signals into the electric pulse signals, extra noise which is difficult to eliminate can be introduced due to amplitude-phase conversion, and meanwhile, the phase discriminator based on the radio frequency domain is difficult to realize higher phase discrimination sensitivity due to technical limitation.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides the optical fiber stable phase transmission device and the optical fiber stable phase transmission method based on the balanced optical pulse scheme, which have the advantages of more stable structure, simpler required precondition experimental conditions, and higher phase discrimination sensitivity while avoiding noise introduced by amplitude phase conversion.

The invention provides an optical fiber stable phase transmission device based on a balanced light pulse scheme, which comprises a mode-locked laser and a low-noise microwave source which are connected with each other, wherein a first half wave plate, a polarization cube, a first collimator, a circulator, a coupler and an electro-optic modulator are sequentially arranged along the transmission direction of light pulses output by the mode-locked laser; one side of the polarization cube is sequentially provided with a quarter wave plate and a reflecting mirror along the transmission direction of the light pulse, and the other side of the polarization cube is provided with a round-trip light pulse transmission system; the coupler is connected with a balance detector, the electro-optical modulator is connected with the low-noise microwave source, and the round-trip optical pulse transmission system and the balance detector are connected with a feedback controller.

Further, the circulator is connected to a first photodetector.

Further, the coupler is connected with the second photodetector.

Further, the round-trip light pulse transmission system comprises a delay line, a second half-wave plate, a Faraday rotator, a second collimator, a dispersion compensation optical fiber, an extender, a transmission optical fiber, a bidirectional erbium-doped optical fiber amplifier and a semi-transparent half-return device which are sequentially connected, wherein the delay line is respectively connected with the polarization cube and the feedback controller, and the extender is connected with the feedback controller.

Further, the coupler has a first port, a second port, and a third port on one side, and a fourth port, a fifth port, and a sixth port on the other side.

Further, the first port and the third port are both connected with the balance detector, and the second port is connected with the circulator.

Further, the fourth port and the sixth port are both connected to the electro-optic modulator, and the fifth port is connected to the second photodetector.

The invention also provides an optical fiber stable phase transmission method based on the balanced light pulse scheme, which comprises the following steps:

step S1, providing the optical fiber stable phase transmission device based on the balanced light pulse scheme;

step S2, light pulses output by the mode-locked laser are sequentially transmitted to a first half-wave plate and a polarization cube, and the first half-wave plate is regulated to enable the light pulses entering the polarization cube to be S-polarized light;

step S3, the polarization cube reflects the S-polarized light to a round-trip light pulse transmission system, and a part of light pulses entering the round-trip light pulse transmission system are directly emitted and distributed to a designated client, and the other part of light pulses are returned by a reflection original path;

s4, converting the light pulse returned by the original path into p-polarized light, transmitting the p-polarized light from the polarization cube to a quarter wave plate and a reflecting mirror, and reflecting the p-polarized light back to the quarter wave plate by the reflecting mirror, wherein the light pulse returns to the S-polarized state again;

step S5, the light pulse enters the polarization cube for the third time and is reflected to the first collimator, and the light pulse is sequentially transmitted to the circulator, the coupler and the electro-optic modulator through the first collimator;

s6, modulating an optical pulse by a microwave reference signal output by a low-noise microwave source through the electro-optical modulator, and transmitting the modulated optical pulse to the coupler;

and S7, transmitting the optical pulse output by the coupler to a balance detector, transmitting an output signal of the balance detector to a feedback controller, and adjusting parameters of the feedback controller to enable the phase of the optical pulse in electro-optical modulation to be always aligned with the maximum slope point of the microwave source signal.

The phase discrimination part of the invention is completely in the optical fiber device, so that the invention has simpler experimental conditions and more stable structure due to the inherent property of the optical fiber device. The invention carries out phase discrimination on the round-trip light pulse transmitted in a long distance and the reference signal output by the low-noise microwave source, and errors introduced by reference signal transmission can be well restrained. Meanwhile, the phase-discrimination error introduced by amplitude phase conversion can be caused, and the phase-discrimination sensitivity is high.

Drawings

Fig. 1 is a schematic diagram of an optical pulse transmission device according to the prior art.

Fig. 2 is a schematic structural view of another optical pulse transmission device according to the prior art.

Fig. 3 is a schematic diagram of a fiber stabilized phase transmission device based on a balanced light pulse scheme in accordance with the present invention.

Detailed Description

Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

As shown in fig. 3, the optical fiber stable phase transmission device based on the balanced light pulse scheme provided by the invention comprises a mode-locked laser 10 and a low-noise microwave source 20 which are connected with each other, wherein the mode-locked laser 10 is a passive mode-locked laser with extremely low jitter, and the resonant cavity of the mode-locked laser is easily affected by the environment, so that the long-term stability of the mode-locked laser is poor. The low-noise microwave source has good long-term stability, so that the laser pulse output by the mode-locked laser 10 is phase-locked to the low-noise microwave source 20 (i.e. the phases of the two are fixed) by the phase-locked loop technology, so that the locked laser pulse output has long-term stability and extremely low noise performance.

The mode-locked laser 10 is provided with a first half-wave plate 11, a polarization cube 12, a first collimator 13, a circulator 14, a coupler 15, and an electro-optic modulator 16 in this order along the transmission direction of the optical pulse. One side of the polarization cube 12 is provided with a quarter wave plate 31 and a reflecting mirror 32 in sequence along the transmission direction of the optical pulse train, the other side of the polarization cube 12 is provided with a round trip optical pulse transmission system 40, the circulator 14 is connected with a first photoelectric detector 51, the coupler 15 is respectively connected with a second photoelectric detector 52 and a balance detector 60, the electro-optical modulator 16 is connected with the low noise microwave source 20, and a feedback controller 70 is respectively connected with the round trip optical pulse transmission system 40 and the balance detector 60.

The optical pulse output by the mode-locked laser 10 in free space is linearly polarized light, and the first half-wave plate 11 is used for adjusting the linear polarization direction of the optical pulse. The first half-wave plate 11 is adjusted to bring the light pulses entering the polarization cube 12 into a totally reflected state, i.e. to totally reflect s-polarized light to the round-trip light pulse transmission system 40, by taking advantage of the characteristics of the polarization cube 12 reflecting s-polarized light and transmitting p-polarized light.

The round-trip optical pulse transmission system 40 includes a delay line 41, a second half-wave plate 42, a faraday rotator 43, a second collimator 44, a dispersion compensating fiber 45, a spreader 46, a transmission fiber 47, a bidirectional erbium-doped fiber amplifier 48, and a half-transparent half-reflector 49, which are sequentially connected, and the delay line 41 is connected to the polarization cube 12, so that s-polarized light reflected from the polarization cube 12 passes through the delay line 41 in sequence through the second half-wave plate 42 and the faraday rotator 43, and is coupled into the fiber by the second collimator 44. The delay line 41 is a delay line with a range of about 4 ns; the second half-wave plate 42 is used to adjust the polarization direction of the light pulse so as to maximize the coupling ratio of the light pulse when coupled into the optical fiber from free space through the second collimator 44; the dispersion compensating fiber 45 is used to counteract pulse broadening due to dispersion of the optical pulses as they are transmitted in the fiber; the stretcher 46 is used to adjust the delay of the light pulses. The transmission fiber 47 is a long-distance transmissible fiber, and the optical pulse can be transmitted to a specified operating point over a long distance through the transmission fiber 47. Since the fusion of different optical fiber devices introduces insertion loss (i.e. power loss) and the optical pulse is attenuated by long distance transmission when transmitted in the optical fiber, a bidirectional erbium-doped optical fiber amplifier 48 is disposed between the transmission optical fiber 47 and the semi-transparent semi-reflective device 49 to amplify the power of the optical pulse output from the transmission optical fiber 47.

The light pulse reaching the semi-transparent and semi-reflective device 49, a part of which meets the optical power requirement of the terminal, is directly emitted and distributed to a designated client (not shown), and the other part is returned by a reflection path, sequentially passes through the bi-directional erbium-doped fiber amplifier 48, the transmission fiber 47, the extender 46, the dispersion compensating fiber 45 and the second collimator 44, then enters the faraday rotator 43 again, sequentially passes through the second half-wave plate 42 and the delay line 41, and then enters the polarization cube 12 again. Thus, a round trip light pulse is generated. The polarization direction of the light pulse passing through the faraday rotator 43 for the first time is changed by 45 degrees, and when the light pulse passes through the faraday rotator 43 again in the opposite direction, the polarization direction of the light pulse passing through the faraday rotator 43 again is changed by 45 degrees, so that the polarization direction of the light pulse passing through the faraday rotator 43 again is different from the polarization direction before passing through the faraday rotator 43 for the first time by 90 degrees, that is, the two polarization directions are orthogonal. Since the polarization direction of the light pulse has been changed to the p-polarization state at this time, the light pulse re-entering the polarization cube 12 is directly transmitted. The transmitted p-polarized light passes through the quarter wave plate 31 and the reflecting mirror 32 in turn, and the light pulse is reflected back to the quarter wave plate 31 by the action of the reflecting mirror 32, and the polarization state of the light pulse returns to the s-polarized state again by passing through the quarter wave plate 31 twice. When the light pulse enters the polarization cube 12 a third time, it is reflected to the first collimator 13, through which first collimator 13 the light pulse is introduced into the fiber. The light pulses are transmitted through the optical fiber to the circulator 14, and the unidirectional passability of the circulator 14 is exploited such that the light pulses enter the coupler 15.

The coupler 15 is a 3 x 3 coupler, and is provided with three ports on both sides thereof, namely a first port 151, a second port 152 and a third port 153 on one side, and a fourth port 154, a fifth port 155 and a sixth port 156 on the other side. The first port 151 and the third port 153 are connected to the balance detector 60, so that the light pulses input to the balance detector 60 from the first port 151 and the third port 153 are converted into electric pulses, and amplified and output after differential processing. The second port 152 is connected to the circulator 14 such that the optical pulses output by the circulator 14 are input from the second port 152 to the coupler 15. The fourth port 154 and the sixth port 156 are each connected to the electro-optic modulator 16 for modulating the amplitude of the optical pulses using the low noise microwave source 20. The fifth port 155 is connected to the second photodetector 52 to monitor the power of the light pulses input from the second port 152.

Due to the inherent nature of the 3 x 3 coupler, the optical pulses input from the second port 152 are output from the fourth port 154, the fifth port 155 and the sixth port 156, respectively, at a fixed phase difference power, and the output optical pulses enter the electro-optic modulator 16. The reference signal output by the low noise microwave source 20 modulates the optical pulse by the electro-optic modulator 16, and the phase difference between the optical pulse and the microwave source is converted to the amplitude of the optical pulse. Two paths of outputs of the low-noise microwave source 20, one path is used for the mode-locked laser and the other path is used for modulating optical pulses, are beneficial to reducing phase fluctuation difference of radio frequency signals introduced by a transmission cable when the microwave source transmits to the mode-locked laser and transmits to the electro-optical modulator, and therefore reduce errors introduced by signal transmission of the microwave source.

The modulated optical pulses re-enter the coupler 15 from the fourth port 154 and the sixth port 156, and after the optical pulses re-enter the coupler 15 are combined, the optical pulses are output from the first port 151, the second port 152 and the third port 153 with a fixed phase difference equal power, respectively. The optical pulse output from the second port 152 is transmitted to the circulator 14, and the power of the modulated optical pulse is monitored by the first photodetector 51. The optical pulses output from the first port 151 and the third port 153 are transmitted to the balance detector 60, and the output signal of the balance detector 60 is a signal reflecting the phase difference between the optical pulses and the microwave source (the phase difference between the optical pulses and the microwave source changes linearly with the voltage signal output from the balance detector 60). The output signal of the balance detector 60 is transmitted to the feedback controller 70, and the feedback controller 70 controls the delay line 41 and the extender 46 by adjusting parameters such as proportion, integration, difference and the like, so that the phase of the optical pulse in the electro-optical modulation is always aligned to the maximum slope point of the microwave source signal (namely, the peak value of the optical pulse is aligned to the zero crossing point of the microwave source signal), and the optical pulse phase is always locked to the fixed point of the microwave source output signal when the optical fiber is transmitted to the terminal output in a long distance, thereby realizing the stable phase transmission of the optical pulse train.

Based on the scheme, the invention also provides an optical fiber stable phase transmission method based on a balanced light pulse scheme, which comprises the following steps:

step S1, providing the optical fiber stable phase transmission device based on the balanced light pulse scheme;

step S2, light pulses output by the mode-locked laser 10 are sequentially transmitted to the first half-wave plate 11 and the polarization cube 12, and the first half-wave plate 11 is adjusted so that the light pulses entering the polarization cube 12 are S-polarized light;

step S3, the polarization cube 12 reflects the S polarized light to the round-trip light pulse transmission system 40, and one part of the light pulse entering the round-trip light pulse transmission system 40 is directly emitted and distributed to the appointed client, and the other part is returned by the reflection original path;

step S4, the light pulse returned by the original path is converted into p-polarized light, the p-polarized light is transmitted to the quarter wave plate 31 and the reflecting mirror 32 from the polarization cube 12, the reflecting mirror 32 reflects the p-polarized light back to the quarter wave plate 31, and the light pulse returns to the S-polarized state again;

step S5, the light pulse enters the polarization cube 12 for the third time and is reflected to the first collimator 13, and the light pulse is sequentially transmitted to the circulator 14, the coupler 15 and the electro-optic modulator 16 through the first collimator 13;

step S6, the reference signal output by the low-noise microwave source 20 is modulated by the electro-optical modulator 16, and the modulated light pulse is transmitted to the coupler 15;

in step S7, the optical pulse output by the coupler 15 is transmitted to the balance detector 60, the output signal of the balance detector 60 is transmitted to the feedback controller 70, and the parameters of the feedback controller 70 are adjusted so that the phase of the optical pulse in the electro-optical modulation is always aligned to the maximum slope point of the microwave source signal.

Compared with the existing nonlinear crystal-based optical cross-correlation technology, the phase discrimination part of the optical pulse stable phase transmission technology is completely in the optical fiber device, so that the required experimental conditions are simpler, and the optical pulse stable phase transmission technology has more stable characteristics due to the inherent properties of the optical fiber device. In addition, in the optical cross-correlation technique, the round-trip light pulse transmitted over a long distance is phase-discriminated from the reference light pulse, and the transmission path of the reference light pulse is outside the feedback loop, so that the error introduced by the transmission of the reference light pulse cannot be eliminated. The invention carries out phase discrimination on the round-trip light pulse transmitted in a long distance and the reference signal output by the very low noise microwave source, so that the transmission of the reference signal from the microwave source to the mode-locked laser and the transmission of the reference signal from the microwave source to the electro-optic modulation can be cancelled out by controlling the transmission environment, namely, the error introduced by the transmission of the reference signal can be well suppressed.

Compared with the existing technology based on radio frequency phase discrimination, the optical pulse phase-stabilizing transmission technology adopts a scheme based on balanced optical pulses, so that phase discrimination errors, which are introduced by the radio frequency phase discrimination technology due to amplitude phase conversion, are well avoided. Meanwhile, the invention is not limited by the material technology of the radio frequency phase discrimination, and has the phase discrimination sensitivity far higher than that of the radio frequency phase discrimination technology.

The invention can realize high-precision stable phase transmission of light pulse in the field of accelerators, can realize signal synchronization of telescope arrays in the field of astronomy, and can also be applied to high-precision distance measurement and the like.

The foregoing description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention, and various modifications can be made to the above-described embodiment of the present invention. All simple, equivalent changes and modifications made in accordance with the claims and the specification of this application fall within the scope of the patent claims. The present invention is not described in detail in the conventional art.

Claims (8)

1. The optical fiber stable phase transmission device based on the balanced light pulse scheme is characterized by comprising a mode-locked laser and a low-noise microwave source which are connected with each other, wherein a first half wave plate, a polarization cube, a first collimator, a circulator, a coupler and an electro-optic modulator are sequentially arranged along the transmission direction of light pulses output by the mode-locked laser; one side of the polarization cube is sequentially provided with a quarter wave plate and a reflecting mirror along the transmission direction of the light pulse, and the other side of the polarization cube is provided with a round-trip light pulse transmission system; the coupler is connected with a balance detector, the electro-optical modulator is connected with the low-noise microwave source, and the round-trip optical pulse transmission system and the balance detector are connected with a feedback controller.

2. The balanced optical pulse scheme based fiber optic steady phase transmission device according to claim 1 wherein the circulator is connected to a first photodetector.

3. The balanced optical pulse scheme based fiber optic steady phase transmission device according to claim 1 wherein the coupler is connected to a second photodetector.

4. The balanced optical pulse scheme based optical fiber phase stabilizing transmission device according to claim 1, wherein the round trip optical pulse transmission system comprises a delay line, a second half-wave plate, a faraday rotator, a second collimator, a dispersion compensating optical fiber, an extender, a transmission optical fiber, a bidirectional erbium-doped optical fiber amplifier and a half-transparent half-reflector which are sequentially connected, the delay line is respectively connected with the polarization cube, the feedback controller, and the extender is connected with the feedback controller.

5. The balanced optical pulse scheme based fiber phase stabilization transmission apparatus of claim 1 wherein the coupler has a first port, a second port and a third port on one side and a fourth port, a fifth port and a sixth port on the other side.

6. The balanced optical pulse scheme based fiber optic steady phase transmission device according to claim 5 wherein the first port and the third port are both connected to the balanced detector and the second port is connected to the circulator.

7. The balanced optical pulse scheme based fiber stabilized phase transfer apparatus of claim 5, wherein the fourth port and the sixth port are each connected to the electro-optic modulator and the fifth port is connected to the second photodetector.

8. An optical fiber stable phase transmission method based on a balanced light pulse scheme is characterized by comprising the following steps:

step S1, providing an optical fiber stable phase transmission device based on a balanced light pulse scheme according to one of claims 1 to 7;

step S2, light pulses output by the mode-locked laser are sequentially transmitted to a first half-wave plate and a polarization cube, and the first half-wave plate is regulated to enable the light pulses entering the polarization cube to be S-polarized light;

step S3, the polarization cube reflects the S-polarized light to a round-trip light pulse transmission system, and a part of light pulses entering the round-trip light pulse transmission system are directly emitted and distributed to a designated client, and the other part of light pulses are returned by a reflection original path;

s4, converting the light pulse returned by the original path into p-polarized light, transmitting the p-polarized light from the polarization cube to a quarter wave plate and a reflecting mirror, and reflecting the p-polarized light back to the quarter wave plate by the reflecting mirror, wherein the light pulse returns to the S-polarized state again;

step S5, the light pulse enters the polarization cube for the third time and is reflected to the first collimator, and the light pulse is sequentially transmitted to the circulator, the coupler and the electro-optic modulator through the first collimator;

s6, modulating an optical pulse by a microwave reference signal output by a low-noise microwave source through the electro-optical modulator, and transmitting the modulated optical pulse to the coupler;

and S7, transmitting the optical pulse output by the coupler to a balance detector, transmitting an output signal of the balance detector to a feedback controller, and adjusting parameters of the feedback controller to enable the phase of the optical pulse in electro-optical modulation to be always aligned with the maximum slope point of the microwave source signal.

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