A device and method for stable phase transmission of optical pulses based on all-e optical cross-correlation

Abstract

The present invention relates to a phase-stable transmission device of optical pulses based on all-e-optical cross-correlation, including a mode-locked laser, the mode-locked laser is connected to a ring structure, the ring structure is respectively connected to a beam splitter and a round-trip optical pulse transmission system, the beam splitter is connected to a balanced detector through a first sum frequency optical generation system and a second sum frequency optical generation system, and the round-trip optical pulse transmission system and the balanced detector are both connected to a feedback controller. The present invention also provides a corresponding method. The present invention designs a special ring structure and uses a type 0 PPLN crystal to generate a sum frequency signal, which can achieve a higher conversion efficiency, improve the sensitivity of phase detection, and achieve the effect of delay adjustment outside the loop. At the same time, through the symmetrical structure of the present invention, the common mode noise on the sum frequency optical path can be suppressed, thereby having a better signal-to-noise ratio.

Description

Technical Field

[0001] The present invention relates to the technical field of optical pulse transmission, and more specifically to an optical pulse phase-stable transmission device based on all-e-optical cross-correlation. Background Art

[0002] The existing optical pulse transmission device is an optical cross-correlation scheme based on type II periodically poled potassium titanyl phosphate (PPKTP) crystals, such as Figure 1 As shown in the figure, the principle of this scheme is as follows: the mode-locked laser outputs a linearly polarized light pulse train in free space, the linear polarization direction of the light pulse train is adjusted by rotating the half-wave plate, and the light pulse train is separated into a light pulse train in the p-polarization state and a light pulse train in the s-polarization state by the polarization beam splitter cube, wherein the light pulse in the p-polarization state is directly transmitted as a reference path light pulse and input into the PPKTP crystal, and the light pulse in the s-polarization state is reflected as a round-trip path light pulse. The light pulse in the s-polarization state passes through the delay line, the half-wave plate and the Faraday rotator in sequence, and is focused by the collimator into the polarization-maintaining fiber, and is connected to the fiber extender and the bidirectional erbium-doped fiber amplifier during the fiber transmission process. At the end of the optical fiber transmission, a part of the light is directly output for use by the client through an optical fiber semi-transmitter and semi-reflector, and the other part of the light is reflected back along the original path and enters the free space again. After passing through the Faraday rotator again, the original s-polarized light pulse becomes a p-polarized light pulse. Therefore, when entering the polarization beam splitter cube again, the round-trip pulse train is directly transmitted and output, and is reflected by the reflector and passes through the 1 / 4 wave plate twice. The round-trip pulse train becomes an s-polarized light pulse again, and is reflected into the PPKTP crystal after passing through the polarization beam splitter cube for the third time. If the delay time of the round-trip optical pulse is adjusted through optical delay so that it generates sum-frequency light with the reference optical pulse in the PPKTP crystal after round-trip transmission, the far end surface of the PPKTP is coated with a reflective film, and the generated sum-frequency light is directly output into one of the photodetectors of the balanced detector. The reference pulse and the round-trip pulse are reflected by the reflective 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 that transmits fundamental frequency light and reflects sum-frequency light, and is reflected by a reflector into another photodetector of the balanced detector. The two photodetectors built into the balanced detector convert the optical signal into an electrical signal and amplify the signal through a transimpedance amplifier. The final output voltage value is linearly related to the time coincidence of the reference optical pulse and the round-trip optical pulse, which is used as a phase-locked signal.

[0003] However, the sum frequency conversion efficiency of the PPKTP crystal is low, and the polarization of the reference pulse and the round-trip pulse must be orthogonal, resulting in low phase detection sensitivity. Summary of the invention

[0004] To solve the above problems in the prior art, the present invention provides an optical pulse phase-stable transmission device based on all-e-optical cross-correlation, which can improve the efficiency of generating sum-frequency light for reference path pulses and round-trip path pulses, thereby further improving the sensitivity of phase detection.

[0005] The present invention provides an optical pulse phase-stable transmission device based on all-e-light cross-correlation, comprising a mode-locked laser, wherein the mode-locked laser is connected to a ring structure, wherein the ring structure is respectively connected to a beam splitter and a round-trip optical pulse transmission system, wherein the beam splitter is connected to a balanced detector via a first sum frequency optical generation system and a second sum frequency optical generation system, and the round-trip optical pulse transmission system and the balanced detector are both connected to a feedback controller.

[0006] Furthermore, the annular structure includes a splitter, a first delay line, a combiner and a circulator connected in sequence, and the splitter is connected to the circulator. The annular structure is connected to the beam splitter through the combiner and connected to the round-trip optical pulse transmission system through the circulator.

[0007] Furthermore, the round-trip optical pulse transmission system includes a first collimator, a second delay line, a second collimator, a transmission optical fiber and a semi-transmitting and semi-reflecting device which are connected in sequence.

[0008] Furthermore, the round-trip optical pulse transmission system is connected to a feedback controller via its second delay line, and an extender connected to the feedback controller is provided between the second collimator and the transmission optical fiber.

[0009] Furthermore, a bidirectional erbium-doped fiber amplifier is provided between the transmission optical fiber and the semi-transmissive and semi-reflective device.

[0010] Furthermore, the first sum frequency light generating system and the second sum frequency light generating system both include a third collimator, a first half-wave plate, a beam splitter cube, a second half-wave plate, a focusing mirror, a type 0 PPLN crystal and a filter connected in sequence, the beam splitter cube is connected to the photodetector, and the third collimator is connected to the beam splitter.

[0011] Furthermore, the balanced detector includes a first photodetector and a second photodetector, the first photodetector is respectively connected to a first resistor and a first operational amplifier, and the first resistor is connected to a positive voltage pole; the second photodetector is respectively connected to a second resistor and a second operational amplifier, and the second resistor is connected to a negative voltage pole, and the first photodetector and the second photodetector are both connected to a transimpedance amplifier and a third operational amplifier in sequence.

[0012] The present invention also provides a method for phase-stable transmission of optical pulses based on all-e-optical cross-correlation, comprising:

[0013] Step S1, providing an optical pulse phase-stable transmission device based on all-e-optical cross-correlation as described in claims 1-7;

[0014] Step S2, the optical pulse generated by the mode-locked laser is divided into a first optical pulse and a second optical pulse through a splitter, the first optical pulse enters a first delay line as a reference optical pulse, and the second optical pulse enters the circulator;

[0015] Step S3, after passing through the circulator, the second optical pulse is emitted into the second delay line through the first collimator, and after passing through the second delay line, is focused into the transmission optical fiber through the second collimator;

[0016] Step S4, the optical pulse outputted by the transmission optical fiber passes through a semi-transmitting and semi-reflecting device, a part of which is directly emitted and distributed to the designated client, and the other part is reflected and returned along the original path, and then passes through the bidirectional erbium-doped fiber amplifier, the transmission optical fiber, the extender, the second collimator, the second delay line and the first collimator in sequence, and then enters the circulator to generate a round-trip optical pulse;

[0017] Step S5, the reference optical pulse output by the first delay line and the round-trip optical pulse output by the circulator enter the combiner for combination, and the combined optical pulses are equally divided into a first beam of combined optical pulses and a second beam of combined optical pulses by a beam splitter;

[0018] Step S6, after the first beam of combined light pulses and the second beam of combined light pulses are focused by a third collimator, a small amount of light pulses are reflected to a photodetector by rotating the first half-wave plate and combining with a beam splitting cube to monitor the power of the light pulses;

[0019] Step S7, after the light pulse emitted from the beam splitter cube is adjusted by the second half-wave plate, it is focused by the focusing lens to the PPLN crystal of type 0, and the fundamental frequency light is filtered out by the filter to obtain the upper sum frequency light and the lower sum frequency light;

[0020] Step S8, the upper path sum frequency light and the lower path sum frequency light are respectively incident on a balanced detector, and a final phase detection signal is obtained through a differential structure inside the balanced detector.

[0021] The present invention designs a special ring structure and uses a type 0 PPLN crystal to generate a sum frequency signal, which can achieve a higher conversion efficiency, improve the sensitivity of phase detection, and achieve the effect of delay adjustment outside the loop. At the same time, through the symmetrical structure of the present invention, the common mode noise on the sum frequency optical path can be suppressed, thereby having a better signal-to-noise ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the structure of an optical pulse transmission device in the prior art.

[0023] Figure 2 It is a schematic structural diagram of an optical pulse phase-stable transmission device based on all-e-optical cross-correlation according to the present invention.

[0024] Figure 3 yes Figure 2 Schematic diagram of the structure of the balanced detector.

[0025] Figure 4 It is a waveform diagram of the voltage signal obtained by the balanced detector. DETAILED DESCRIPTION

[0026] The preferred embodiments of the present invention are given below in conjunction with the accompanying drawings and described in detail.

[0027] like Figure 2 As shown, the optical pulse phase-stable transmission device based on all-e-light cross-correlation provided by the present invention includes a mode-locked laser 1, the mode-locked laser 1 is connected to a ring structure 2, the ring structure 2 is respectively connected to a beam splitter 3 and a round-trip optical pulse transmission system 4, the beam splitter 3 is connected to a balanced detector 7 through a first sum frequency light generating system 5 and a second sum frequency light generating system 6, and the round-trip optical pulse transmission system 4 and the balanced detector 7 are both connected to a feedback controller 8.

[0028] The above components are described in detail below.

[0029] The mode-locked laser 1 is used to generate a periodic optical pulse train, and can adopt menlosystem-1550, NKT-1550, menhir-1550 or other models of high-performance mode-locked lasers.

[0030] The ring structure 2 includes a splitter 21, a first delay line 22, a combiner 23 and a circulator 24 connected in sequence, and the splitter 21 is connected to the circulator 24. The ring structure 2 is connected to the beam splitter 3 through the combiner 23, and is connected to the round-trip optical pulse transmission system 4 through the circulator 24. The optical pulse generated by the mode-locked laser 1 is divided into a first optical pulse and a second optical pulse through the splitter 21. The first optical pulse enters the first delay line 22 as a reference optical pulse, and the second optical pulse enters the circulator 24 to generate a round-trip optical pulse. In addition, the phase delay of the reference optical pulse can be adjusted by adjusting the first delay line 22.

[0031] The round-trip optical pulse transmission system 4 includes a first collimator 41, a second delay line 42, a second collimator 43, a transmission optical fiber 44 and a semi-transmitter 45 connected in sequence. The round-trip optical pulse transmission system 4 is connected to the ring structure 2 through its first collimator 41, so that after the second optical pulse passes through the circulator 24, it is emitted into the second delay line 42 in free space through the first collimator 41, and after passing through the second delay line 42, it is focused into the transmission optical fiber 44 through the second collimator 43. In order to adjust the delay of the optical pulse, the round-trip optical pulse transmission system 4 is connected to the feedback controller 8 through its second delay line 42, and an extender 46 connected to the feedback controller 8 is provided between the second collimator 43 and the transmission optical fiber 44. By changing the voltage applied to the extender 46, the refractive index of the optical fiber in the extender 46 is changed, thereby changing the transmission speed of the optical pulse in the extender 46, and then adjusting the delay of the optical pulse. In order to amplify the optical pulse to achieve a suitable output power, a bidirectional erbium-doped fiber amplifier 47 is provided between the transmission optical fiber 44 and the semi-transparent and semi-reflecting device 45. The optical pulse output by the transmission optical fiber 44 passes through the semi-transparent and semi-reflecting device 45, and a part of it is directly emitted and distributed to the designated client (not shown), and the other part is reflected back along the original path, and enters the circulator 24 after passing through the bidirectional erbium-doped fiber amplifier 47, the transmission optical fiber 44, the extender 46, the second collimator 43, the second delay line 42 and the first collimator 41 in sequence. In this way, a round-trip optical pulse is generated. In addition, the phase delay of the round-trip optical pulse can also be adjusted by adjusting the second delay line 42.

[0032] The reference optical pulse output by the first delay line 22 and the round-trip optical pulse output by the circulator 24 enter the combiner 23 for combination. The combined optical pulses are equally divided into a first beam of combined optical pulses and a second beam of combined optical pulses by the beam splitter 3. The first beam of combined optical pulses and the second beam of combined optical pulses enter the first sum frequency optical generating system 5 and the second sum frequency optical generating system 6 respectively.

[0033] The first sum frequency light generating system 5 comprises a third collimator 51, a first half wave plate 52, a beam splitter cube 53, a second half wave plate 54, a focusing mirror 55, a PPLN (periodically polarized lithium niobate) crystal 56 of type 0 and a filter 57 connected in sequence, the beam splitter cube 53 is connected to a photodetector 58, and the third collimator 51 is connected to the beam splitter 3. The first beam of combined light pulses enters the free space after being focused by the third collimator 51, and a small amount of light pulses are reflected to the photodetector 58 by the rotation adjustment of the first half wave plate 52 and the beam splitter cube 53 to monitor the power of the light pulses. Here, the rotation adjustment of the first half wave plate 52 means that the power of the light pulse reflected to the photodetector 58 is greater than the minimum input power of the photodetector 58 by adjusting the first half wave plate 52. The beam splitter cube 53 also has the function of washing polarization to ensure the single polarization of the outgoing light pulse and transmit most of the light pulses. The light pulse emitted from the beam splitter cube 53 is adjusted by the second half-wave plate 54 and then focused by the focusing lens 55 into the type 0 PPLN crystal 56. The output surface of the type 0 PPLN crystal 56 is coated with a film that transmits the fundamental frequency light and the sum frequency light, so the filter 57 filters out a very small amount of residual transmitted fundamental frequency light.

[0034] The first sum frequency light generating system 5 and the second sum frequency light generating system 6 are two completely symmetrical systems, including devices of the same model and parameters, and the positions of the same devices in the two systems are symmetrical up and down, so the second sum frequency light generating system 6 is not described in detail. The first sum frequency light generating system 5 and the second sum frequency light generating system 6 respectively generate an upper sum frequency light and a lower sum frequency light, and the two sum frequency lights are respectively incident on the two photodetectors in the balanced detector 7, and the final phase discrimination signal is obtained through the differential structure inside the balanced detector 7.

[0035] The optical pulses transmitted in the first sum frequency optical generation system 5 and the second sum frequency optical generation system 6 include reference optical pulses and round trip optical pulses. In order to utilize the highest nonlinear coefficient of the PPLN crystal, the polarization state (e light, extraordinary light) of the optical pulse input to the PPLN crystal needs to match the dipole moment of the crystal. Specifically, the polarization of the input optical pulse is adjusted by the second half-wave plate 54 so that the voltage signal on the balanced detector 7 reaches the maximum value, and an efficient second harmonic output is obtained. In this way, the polarization state of the input optical pulse is matched with the dipole moment of the PPLN crystal.

[0036] like Figure 3As shown, the balanced detector 7 is a symmetrical structure, which includes a first photodetector 71 and a second photodetector 72. The first photodetector 71 is connected to a first resistor 73 and a first operational amplifier 74, respectively, and the first resistor 73 is connected to the positive pole of the voltage; the second photodetector 72 is connected to a second resistor 75 and a second operational amplifier 76, respectively, and the second resistor 75 is connected to the negative pole of the voltage. In addition, the first photodetector 71 and the second photodetector 72 are connected to a transimpedance amplifier 77 and a third operational amplifier 78 in sequence. Among them, the upper sum frequency light is input to the first photodetector 71, and the first operational amplifier 74 outputs an upper channel monitoring electrical signal for observing the power intensity of the upper sum frequency light. The lower sum frequency light is input to the second photodetector 72, and the second operational amplifier 76 outputs a lower channel monitoring electrical signal for observing the power intensity of the lower sum frequency light. The third operational amplifier 78 outputs a phase detection signal, which is an electrical signal obtained by subtracting the power intensity of the upper sum frequency light from that of the lower sum frequency light.

[0037] The symmetrical structure of the first sum frequency light generating system 5 and the second sum frequency light generating system 6 and the symmetrical structure of the balanced detector 7 itself can suppress the common mode noise on the upper sum frequency light path and the lower sum frequency light path by differential, thereby having a better signal-to-noise ratio.

[0038] Compared with PPKTP (potassium titanyl phosphate) crystals, PPLN crystals have a higher nonlinear coefficient, so the dual-e light solution using PPLN crystals has a higher conversion efficiency than the existing technology. The first sum frequency light generating system 5 and the second sum frequency light generating system 6 have a vertically symmetrical structure. Two equally divided reference light pulses and round-trip light pulses pass through two PPLN crystals to generate two sum frequency lights. After the two sum frequency lights enter the balanced detector 7, a phase detection signal is obtained. The phase detection signal is related to the spatial overlap of the dual e light in the PPLN crystal. By adjusting the phase delay of the reference light pulse or the phase delay of the round-trip light pulse, the relative phase delay of the dual e light in the PPLN crystal is changed, and an "S" wave voltage signal can be obtained. Figure 4 As shown in the figure, the zero-crossing point of the S wave has the largest slope, and the relative phase delay of the double e light in the PPLN crystal is proportional to the magnitude of the phase-locked signal near the zero-crossing point of the S wave, i.e., the phase-locked sensitivity. Therefore, the nonlinear crystal has a higher sum-frequency conversion efficiency, can obtain a higher phase-locked sensitivity, and thus has a higher accuracy in the feedback process.

[0039] The feedback controller 8 is used to control the voltage error signal output by the balanced detector 7. The voltage error signal is related to the time of coincidence between the reference optical pulse and the round-trip optical pulse (i.e., related to the phase of the two). The maximum slope point of the voltage error signal waveform output by the balanced detector 7 is Figure 4The zero-crossing point is when the time overlap of the two pulses is the largest. The transmission time of the optical pulses in the delay line 42 and the optical fiber stretcher 46 is feedback-controlled by the feedback controller 8 to ensure that the output of the balanced detection 7 is always at the point with the maximum slope, thereby achieving steady-phase transmission of the optical pulses.

[0040] Based on the above scheme, the present invention also provides a method for phase-stable transmission of optical pulses based on all-e-optical cross-correlation, comprising the following steps:

[0041] Step S1, providing the above-mentioned optical pulse phase-stable transmission device based on all-e-optical cross-correlation;

[0042] Step S2 , the optical pulse generated by the mode-locked laser 1 is divided into a first optical pulse and a second optical pulse by the splitter 21 , the first optical pulse enters the first delay line 22 as a reference optical pulse, and the second optical pulse enters the circulator 24 .

[0043] Step S3 , after the second optical pulse passes through the circulator 24 , it is emitted into the second delay line 42 through the first collimator 41 , and after passing through the second delay line 42 , it is focused into the transmission optical fiber 44 through the second collimator 43 .

[0044] In step S4, the light pulse output by the transmission optical fiber 44 passes through the semi-transmitting and semi-reflecting device 45, a part of which is directly emitted and distributed to the designated client, and the other part is reflected and returned along the original path, and then passes through the bidirectional erbium-doped fiber amplifier 47, the transmission optical fiber 44, the extender 46, the second collimator 43, the second delay line 42 and the first collimator 41 in sequence, and then enters the circulator 24 to generate a round-trip light pulse.

[0045] Step S5, the reference optical pulse output by the first delay line 22 and the round-trip optical pulse output by the circulator 24 enter the combiner 23 for combination, and the combined optical pulses are equally divided into a first beam of combined optical pulses and a second beam of combined optical pulses by the beam splitter 3.

[0046] Step S6, after the first beam of combined light pulses and the second beam of combined light pulses are focused by the third collimator 51, a small amount of light pulses are reflected to the photodetector 58 by the rotation adjustment of the first half-wave plate 52 and the combination of the beam splitter cube 53 to monitor the power of the light pulses.

[0047] Step S7, the light pulse emitted from the beam splitter cube 53 is adjusted by the second half-wave plate 54, and then focused by the focusing lens 55 to the type 0 PPLN crystal 56, and the fundamental frequency light is filtered out by the filter 57 to obtain the upper and lower sum frequency lights.

[0048] Step S8 , the upper path sum frequency light and the lower path sum frequency light are respectively incident on the balanced detector 7 , and the final phase detection signal is obtained through the differential structure inside the balanced detector 7 .

[0049] The basic idea of ​​phase-stable transmission of optical pulses is to phase-detect the round-trip optical pulses and the reference optical pulses in the PPLN crystal, i.e., to spatially overlap them. If the reference optical pulse is delayed, the round-trip optical pulse needs to be delayed accordingly, and the adjustment of the round-trip delay can change the output phase of the optical pulse output by the optical fiber. The present invention designs a ring structure after the mode-locked laser, and introduces a delay line into the ring structure. Since the delay line is on the reference path in the special ring structure, not in the round-trip path (i.e., the so-called inside the ring), the present invention can achieve the effect of delay adjustment outside the ring. In addition, the present invention designs a special ring structure composed of a splitter, a circulator, and a combiner, combined with a polarization beam splitter cube, to ensure that the output of the reference path light and the round-trip light has the same and single polarization state, so as to meet the double e light in the PPLN crystal, thereby achieving the maximum conversion efficiency.

[0050] The above is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. The above embodiment of the present invention can also be modified in various ways. That is, all simple, equivalent changes and modifications made according to the claims and the description of the present invention fall within the scope of protection of the claims of the present invention. The contents not described in detail in the present invention are all conventional technical contents.

Claims

1. An optical pulse phase-stable transmission device based on all-e-optical cross-correlation, characterized in that: The invention comprises a mode-locked laser, wherein the mode-locked laser is connected to a ring structure, wherein the ring structure is respectively connected to a beam splitter and a round-trip optical pulse transmission system, wherein the beam splitter is connected to a balanced detector via a first sum frequency optical generation system and a second sum frequency optical generation system, and the round-trip optical pulse transmission system and the balanced detector are both connected to a feedback controller. Wherein, the annular structure comprises a splitter, a first delay line, a combiner and a circulator connected in sequence, and the splitter is connected to the circulator, the annular structure is connected to the beam splitter through the combiner, and is connected to the round-trip optical pulse transmission system through the circulator; The round-trip optical pulse transmission system comprises a first collimator, a second delay line, a second collimator, a transmission optical fiber and a semi-transmitter and semi-return device connected in sequence, the round-trip optical pulse transmission system is connected to a feedback controller through its second delay line, and an extender connected to the feedback controller is arranged between the second collimator and the transmission optical fiber; The first sum frequency light generating system and the second sum frequency light generating system both include a third collimator, a first half wave plate, a beam splitter cube, a second half wave plate, a focusing mirror, a type 0 PPLN crystal, and a filter connected in sequence, the beam splitter cube is connected to a photodetector, and the third collimator is connected to the beam splitter; The balanced detector includes a first photodetector and a second photodetector, wherein the first photodetector is connected to a first resistor and a first operational amplifier respectively, and the first resistor is connected to a positive voltage electrode; the second photodetector is connected to a second resistor and a second operational amplifier respectively, and the second resistor is connected to a negative voltage electrode, and the first photodetector and the second photodetector are both connected to a transimpedance amplifier and a third operational amplifier in sequence; Furthermore, the mode-locked laser is connected to the splitter, and the first collimator is connected to the circulator.

2. The optical pulse phase-stable transmission device based on all-e-optical cross-correlation according to claim 1 is characterized in that: A bidirectional erbium-doped fiber amplifier is arranged between the transmission optical fiber and the semi-transmissive and semi-reflective device.

3. A method for phase-stable transmission of optical pulses based on all-e-optical cross-correlation, characterized in that: include: Step S1, providing an optical pulse phase-stable transmission device based on all-e-optical cross-correlation as described in any one of claims 1 to 2; Step S2, the optical pulse generated by the mode-locked laser is divided into a first optical pulse and a second optical pulse through a splitter, the first optical pulse enters a first delay line as a reference optical pulse, and the second optical pulse enters the circulator; Step S3, after passing through the circulator, the second optical pulse is emitted into the second delay line through the first collimator, and after passing through the second delay line, is focused into the transmission optical fiber through the second collimator; Step S4, the optical pulse outputted by the transmission optical fiber passes through a semi-transmitting and semi-reflecting device, a part of which is directly emitted and distributed to the designated client, and the other part is reflected and returned along the original path, and then passes through the bidirectional erbium-doped fiber amplifier, the transmission optical fiber, the extender, the second collimator, the second delay line and the first collimator in sequence, and then enters the circulator to generate a round-trip optical pulse; Step S5, the reference optical pulse output by the first delay line and the round-trip optical pulse output by the circulator enter the combiner for combination, and the combined optical pulses are equally divided into a first beam of combined optical pulses and a second beam of combined optical pulses by a beam splitter; Step S6, after the first beam of combined light pulses and the second beam of combined light pulses are focused by a third collimator, a small amount of light pulses are reflected to a photodetector by rotating the first half-wave plate and combining with a beam splitting cube to monitor the power of the light pulses; Step S7, after the light pulse emitted from the beam splitter cube is adjusted by the second half-wave plate, it is focused by the focusing lens to the PPLN crystal of type 0, and the fundamental frequency light is filtered out by the filter to obtain the upper sum frequency light and the lower sum frequency light; Step S8, the upper path sum frequency light and the lower path sum frequency light are respectively incident on a balanced detector, and a final phase detection signal is obtained through a differential structure inside the balanced detector.

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