An optical fiber phase-stable transmission apparatus and method based on balanced optical pulse scheme
By using a fiber optic phase-stabilized transmission device based on a balanced optical pulse scheme, and combining a mode-locked laser and a low-noise microwave source with fiber optic devices, the problems of optical path stability and noise interference in the prior art are solved, and high-sensitivity fiber optic phase-stabilized transmission is achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SHANGHAI ADVANCED RES INST CHINESE ACADEMY OF SCI
- Filing Date
- 2022-12-14
- Publication Date
- 2026-06-12
Smart Images
Figure CN116248183B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of optical pulse transmission technology, and more specifically to an optical fiber phase-stable transmission device and method based on a balanced optical pulse scheme. Background Technology
[0002] Existing optical pulse transmission devices include one type based on optical cross-correlation schemes using nonlinear crystals (PPKTP crystals), such as... Figure 1 As shown, the principle of this scheme is as follows: A mode-locked laser generates and outputs a train of optical pulses in free space. In free space, a half-wave plate adjusts the linear polarization direction of the optical pulse train, and a polarization beam splitter separates the optical pulse train into p-polarized and s-polarized pulse trains. The p-polarized pulses are directly transmitted as reference pulses input into the PPKTP crystal, while the s-polarized pulses are reflected as round-trip pulses. The s-polarized pulses pass sequentially through a delay line, a half-wave plate, and a Faraday rotator, and are then focused by a collimator into a polarization-maintaining fiber. During fiber transmission, they are connected to an fiber extender, a dispersion-compensating fiber, and a bidirectional erbium-doped fiber amplifier. At the end of the optical fiber transmission, a portion of the light is directly output to the client via a fiber optic semi-transparent and semi-reflective transducer, while the other portion is reflected back into free space. After passing through the Faraday rotator again, the original s-polarized light pulse becomes a p-polarized light pulse. Therefore, when it re-enters the polarization beam splitter cube, the round-trip pulse train is directly transmitted and output. After being reflected by a mirror and passing through a quarter-wave plate twice, the round-trip pulse train becomes an s-polarized light pulse again. After passing through the polarization beam splitter cube for the third time, it is reflected into the PPKTP crystal. At this point, the delay time of the round-trip optical pulses is adjusted by optical delay, so that after round-trip transmission, they generate sum-frequency light in the PPKTP crystal along with the reference optical pulse. The far end of the PPKTP is coated with a transflective 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 transflective film and pass through the PPKTP crystal again to generate sum-frequency light a second time. This sum-frequency light is reflected by a dichroic mirror that reflects the sum-frequency light through a fundamental light transmission mirror, and then reflected by a mirror into the other photodetector of the balanced detector. The two photodetectors built into the balanced detector convert the optical signal into an electrical signal, which is then amplified by a transimpedance amplifier. The final output voltage value has a linear relationship with the time re-synthesis of the reference optical pulse and the round-trip optical pulse, and is used as a phase detection signal. This scheme has very high requirements for optical path stability because the re-synthesis adjustment of the reference optical pulse and the round-trip optical pulse in the nonlinear crystal is within the range of a single pulse width, and the pulse width of a single pulse is generally on the order of hundreds of femtoseconds. Therefore, the optical path needs to be placed on a stable optical platform, which places very stringent requirements on the necessary preconditions.
[0003] Another existing optical pulse phase-stabilized transmission device is based on a radio frequency phase detection scheme, such as... Figure 2 As shown, this scheme generates an optical pulse train using a mode-locked laser and outputs it through an optical fiber. A fiber optic splitter is then connected to one port of the splitter, which is connected to a photodetector to convert the optical pulse signal into an electrical pulse signal. The output of the photodetector is connected to a filter to filter out the corresponding first harmonic signal in the electrical pulse signal. This first harmonic signal is amplified by a low-noise amplifier and then input to one port of a phase detector. The other port of the splitter is connected to a fiber optic circulator, a unidirectional three-port device with irreversible transmission. The output of the circulator is connected to a fiber delay line. Following the delay line are, in sequence, a fiber extender, a dispersion-compensating fiber, a transmission fiber, a bidirectional erbium-doped fiber amplifier, and a fiber semi-transparent / semi-reflective unit. The fiber semi-transparent / semi-reflective unit outputs a portion of the light directly to the client, while the other portion returns along the same path, passing back through the bidirectional erbium-doped fiber amplifier, fiber extender, and fiber delay line before entering the circulator. At this point, the circulator output is directly connected to a photodetector to convert the optical pulse signal into an electrical pulse signal. The output of the photodetector is connected to a filter to filter out the second harmonic signal with the same frequency as the first harmonic signal. This second harmonic signal is amplified by a low-noise amplifier and then input to another port of the phase detector. When the first and second harmonic signals are simultaneously input to the phase detector, a voltage signal with a linear relationship to the phase difference between the two signals is generated. The feedback controller uses this voltage signal to control the fiber delay line and fiber extender. Although this scheme is simpler in structure and transmits within the fiber, making it more stable, the amplitude and phase conversion of the optical pulse signal using a photodetector introduces additional and difficult-to-eliminate noise. Furthermore, the RF-domain-based phase detector is technically limited and cannot achieve high phase detection sensitivity. Summary of the Invention
[0004] To address the problems in the prior art, this invention provides a fiber optic phase-stable transmission device and method based on a balanced optical pulse scheme. This device and method have a more stable structure, require simpler experimental conditions, and possess high phase detection sensitivity while avoiding noise introduced by amplitude-phase conversion.
[0005] This invention provides a fiber optic phase-stabilized transmission device based on a balanced optical pulse scheme, comprising a mode-locked laser and a low-noise microwave source connected to each other. Along the transmission direction of the optical pulses output by the mode-locked laser, a first half-wave plate, a polarization cube, a first collimator, a circulator, a coupler, and an electro-optic modulator are arranged sequentially. One side of the polarization cube has a quarter-wave plate and a reflector arranged sequentially along the optical pulse transmission direction, while the other side has a round-trip optical pulse transmission system. The coupler is connected to a balanced detector, the electro-optic modulator is connected to the low-noise microwave source, and both the round-trip optical pulse transmission system and the balanced detector are connected to a feedback controller.
[0006] Furthermore, the circulator is connected to the first photodetector.
[0007] Furthermore, the coupler is connected to a second photodetector.
[0008] Furthermore, the round-trip optical pulse transmission system includes a delay line, a second half-wave plate, a Faraday rotator, a second collimator, a dispersion compensation fiber, an extender, a transmission fiber, a bidirectional erbium-doped fiber amplifier, and a semi-transparent, semi-reflective device connected in sequence. The delay line is connected to the polarization cube and the feedback controller, respectively, and the extender is connected to the feedback controller.
[0009] Furthermore, 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.
[0010] Furthermore, both the first port and the third port are connected to the balance detector, and the second port is connected to the circulator.
[0011] Furthermore, both the fourth port and the sixth port are connected to the electro-optic modulator, and the fifth port is connected to the second photodetector.
[0012] This invention also provides a method for stable phase transmission in optical fibers based on a balanced optical pulse scheme, comprising:
[0013] Step S1: Provide the fiber optic phase-stabilized transmission device based on the balanced optical pulse scheme described above.
[0014] Step S2: The light pulses output by the mode-locked laser are sequentially transmitted to the first half-wave plate and the polarization cube. The first half-wave plate is adjusted so that the light pulses entering the polarization cube are s-polarized light.
[0015] In step S3, the polarization cube reflects the s-polarized light to the round-trip optical pulse transmission system. Of the optical pulses entering the round-trip optical pulse transmission system, part is directly emitted and distributed to the designated client, while the other part is reflected back along the original path.
[0016] In step S4, the light pulse returning along the original path is converted into p-polarized light, which is transmitted from the polarization cube to the quarter-wave plate and the mirror. The mirror reflects the p-polarized light back to the quarter-wave plate, and the light pulse returns to the s-polarized state.
[0017] In step S5, the light pulse enters the polarization cube for the third time and is reflected to the first collimator. The light pulse is then transmitted sequentially through the first collimator to the circulator, coupler, and electro-optic modulator.
[0018] Step S6: The microwave reference signal output by the low-noise microwave source modulates the optical pulse through the electro-optic modulator, and the modulated optical pulse is transmitted to the coupler.
[0019] Step S7: The optical pulse output by the coupler is transmitted to the balanced detector, and the output signal of the balanced detector is transmitted to the feedback controller. The parameters of the feedback controller are adjusted so that the phase of the optical pulse in electro-optic modulation is always aligned with the maximum slope point of the microwave source signal.
[0020] The phase detection component of this invention is entirely integrated within the fiber optic device, thus simplifying the required experimental conditions and providing a more stable structure due to the inherent properties of fiber optic devices. This invention performs phase detection between a long-distance transmitted round-trip optical pulse and a reference signal output from a low-noise microwave source, effectively suppressing errors introduced by the reference signal transmission. Furthermore, this invention can mitigate phase detection errors introduced by amplitude-phase conversion and exhibits high phase detection sensitivity. Attached Figure Description
[0021] Figure 1 This is a schematic diagram of the structure of an existing optical pulse transmission device.
[0022] Figure 2 This is a schematic diagram of another optical pulse transmission device in the prior art.
[0023] Figure 3 This is a schematic diagram of the optical fiber phase-stable transmission device based on the balanced optical pulse scheme according to the present invention. Detailed Implementation
[0024] The preferred embodiments of the present invention are given below with reference to the accompanying drawings and described in detail.
[0025] like Figure 3As shown, the fiber optic phase-stabilized transmission device based on a balanced optical pulse scheme provided by this invention includes a mode-locked laser 10 and a low-noise microwave source 20 connected to each other. The mode-locked laser 10 is a passively mode-locked laser with extremely low jitter. Since the resonant cavity of the mode-locked laser is easily affected by the environment, its long-term stability is poor. On the other hand, the low-noise microwave source has excellent long-term stability. Therefore, the phase of the laser pulse output by the mode-locked laser 10 is locked to the phase of the low-noise microwave source 20 through phase-locked loop technology (i.e., the phases of the two are fixed), so that the locked laser pulse output has long-term stability and extremely low noise performance.
[0026] 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 sequence along its optical pulse transmission direction. A quarter-wave plate 31 and a reflector 32 are arranged in sequence on one side of the polarization cube 12 along the optical pulse train transmission direction, and a reciprocating optical pulse transmission system 40 is provided on the other side of the polarization cube 12. The circulator 14 is connected to a first photodetector 51, the coupler 15 is connected to a second photodetector 52 and a balanced detector 60, the electro-optic modulator 16 is connected to a low-noise microwave source 20, and a feedback controller 70 is connected to the reciprocating optical pulse transmission system 40 and the balanced detector 60.
[0027] The mode-locked laser 10 outputs linearly polarized light pulses in free space, and the first half-wave plate 11 is used to adjust the linear polarization direction of the light pulses. Taking advantage of the characteristic of the polarization cube 12 to reflect s-polarized light and transmit p-polarized light, the first half-wave plate 11 is adjusted so that the light pulses entering the polarization cube 12 are in a state of total internal reflection, that is, so that all s-polarized light is reflected to the round-trip optical pulse transmission system 40.
[0028] 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, an extender 46, a transmission fiber 47, a bidirectional erbium-doped fiber amplifier 48, and a semi-transparent, semi-reflective element 49 connected in sequence. The delay line 41 is connected to the polarization cube 12, so that the s-polarized light reflected from the polarization cube 12 passes through the delay line 41, the second half-wave plate 42, and the Faraday rotator 43, and is then coupled into the optical fiber by the second collimator 44. The delay line 41 has a range of approximately 4 ns. The second half-wave plate 42 is used to adjust the polarization direction of the optical pulse to maximize the coupling ratio when the optical pulse is coupled from free space into the optical fiber through the second collimator 44. The dispersion-compensating fiber 45 is used to counteract pulse broadening caused by dispersion during transmission in the optical fiber. The extender 46 is used to adjust the delay of the optical pulse. The transmission fiber 47 is a long-distance transmission fiber, through which optical pulses can be transmitted over long distances to a designated working point. Since the splicing of different fiber optic devices introduces insertion loss (i.e., power loss), and the power of the optical pulse will also attenuate during long-distance transmission in the fiber, a bidirectional erbium-doped fiber amplifier 48 is set between the transmission fiber 47 and the semi-transparent semi-reflector 49 to amplify the power of the optical pulses output from the transmission fiber 47.
[0029] The optical pulses arriving at the semi-transparent, semi-reflective transducer 49 are divided into two groups. One group, having met the power requirements of the terminal, is directly emitted and distributed to the designated client (not shown). The other group is reflected back along the same path, passing sequentially through a bidirectional erbium-doped fiber amplifier 48, a transmission fiber 47, an extender 46, a dispersion-compensating fiber 45, and a second collimator 44. It then re-enters the Faraday rotator 43, passing sequentially through a second half-wave plate 42 and a delay line 41 before re-entering the polarization cube 12. This process generates a round-trip optical pulse. It should be noted that the polarization direction of the optical pulse passing through the Faraday rotator 43 for the first time changes by 45 degrees. When the optical pulse reverses and passes through the Faraday rotator 43 again, its polarization direction changes by another 45 degrees. Therefore, the polarization direction of the optical pulse after passing through the Faraday rotator 43 is 90 degrees different from that before the first passage through the Faraday rotator 43, meaning they are orthogonal. Since the polarization direction of the optical pulse has now changed to a p-polarized state, the optical pulse re-entering the polarization cube 12 will be directly transmitted. The transmitted p-polarized light passes sequentially through a quarter-wave plate 31 and a reflector 32. The reflector 32 reflects the light pulse back to the quarter-wave plate 31, and this second passage through the quarter-wave plate 31 returns the light pulse to the s-polarized state. When the light pulse enters the polarization cube 12 for the third time, it is reflected to the first collimator 13, which guides the light pulse into the optical fiber. The light pulse then travels through the optical fiber to the circulator 14, where its unidirectional transmission allows it to enter the coupler 15.
[0030] Coupler 15 is a 3×3 coupler with three ports on each side: 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. Ports 151 and 153 are connected to a balanced detector 60 to convert optical pulses input from these ports into electrical pulses, which are then differentially processed and amplified for output. Port 152 is connected to a circulator 14 so that optical pulses output from the circulator 14 are input to coupler 15 through port 152. Ports 154 and 156 are connected to an electro-optic modulator 16 to modulate the amplitude of the optical pulses using a low-noise microwave source 20. Port 155 is connected to a second photodetector 52 to monitor the power of the optical pulses input from port 152.
[0031] Due to the inherent properties of the 3×3 coupler, the optical pulse input from the second port 152 is output from the fourth port 154, the fifth port 155, and the sixth port 156 with equal power distribution and a fixed phase difference. The output optical pulses then enter the electro-optic modulator 16. The reference signal output from the low-noise microwave source 20 modulates the optical pulses through the electro-optic modulator 16, and the phase difference between the optical pulses and the microwave source is converted into the amplitude of the optical pulses. The two outputs of the low-noise microwave source 20, one for the mode-locked laser and the other for modulating the optical pulses, help reduce the phase fluctuation difference of the radio frequency signal introduced by the transmission cable during transmission from the microwave source to the mode-locked laser and to the electro-optic modulator, thereby reducing the error introduced by the microwave source signal transmission.
[0032] The modulated optical pulse re-enters coupler 15 through ports 154 and 156. After being combined, the optical pulses are output from ports 151, 152, and 153 respectively, with a fixed phase difference and equal power distribution. The optical pulse output from port 152 is transmitted to circulator 14, where the power of the modulated optical pulse is monitored by photodetector 51. The optical pulses output from ports 151 and 153 are transmitted to balanced detector 60. The output signal of balanced detector 60 reflects the phase difference between the optical pulse and the microwave source (the change in phase difference between the optical pulse and the microwave source is linearly related to the voltage signal output by balanced detector 60). The output signal of the balanced detector 60 is transmitted to the feedback controller 70. The feedback controller 70 controls the delay line 41 and the extender 46 by adjusting its proportional, integral, differential and other parameters so that the phase of the optical pulse in the electro-optic modulation is always aligned with the maximum slope point of the microwave source signal (i.e., the peak value of the optical pulse is aligned with the zero crossing point of the microwave source signal). This enables the phase of the optical pulse to be locked at a fixed point of the microwave source output signal when the optical fiber is transmitted over a long distance to the terminal output, thereby realizing the phase-stable transmission of the optical pulse train.
[0033] Based on the above scheme, the present invention also provides a fiber optic phase-stable transmission method based on a balanced optical pulse scheme, comprising the following steps:
[0034] Step S1: Provide the fiber optic phase-stabilized transmission device based on the balanced optical pulse scheme described above.
[0035] In step S2, the 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.
[0036] In step S3, the polarization cube 12 reflects the s-polarized light to the round-trip optical pulse transmission system 40. Of the optical pulses entering the round-trip optical pulse transmission system 40, part is directly emitted and distributed to the designated client, while the other part is reflected back along the original path.
[0037] In step S4, the light pulse returning along the original path is converted into p-polarized light, which is transmitted from the polarization cube 12 to the quarter-wave plate 31 and the mirror 32. The mirror 32 reflects the p-polarized light back to the quarter-wave plate 31, and the light pulse returns to the s-polarized state.
[0038] In step S5, the light pulse enters the polarization cube 12 for the third time and is reflected to the first collimator 13. The light pulse is then transmitted sequentially through the first collimator 13 to the circulator 14, the coupler 15, and the electro-optic modulator 16.
[0039] In step S6, the reference signal output by the low-noise microwave source 20 modulates the optical pulse through the electro-optic modulator 16, and the modulated optical pulse is transmitted to the coupler 15.
[0040] In step S7, the optical pulse output by coupler 15 is transmitted to balance detector 60, and the output signal of balance detector 60 is transmitted to feedback controller 70. The parameters of feedback controller 70 are adjusted so that the phase of the optical pulse in electro-optic modulation is always aligned with the maximum slope point of the microwave source signal.
[0041] Compared to existing optical cross-correlation techniques based on nonlinear crystals, the phase detection part of the optical pulse phase-stable transmission technology of this invention is entirely within the optical fiber device. Therefore, the required experimental conditions are simpler, and the technology exhibits more stable characteristics due to the inherent properties of optical fiber devices. Furthermore, in optical cross-correlation techniques, phase detection is performed between the long-distance round-trip optical pulse and the reference optical pulse, but the transmission path of the reference optical pulse is outside the feedback loop, making it impossible to eliminate errors introduced by the reference optical pulse transmission. In contrast, this invention performs phase detection between the long-distance round-trip optical pulse and the reference signal output from an extremely low-noise microwave source. Therefore, the transmission from the microwave source to the mode-locked laser and from the microwave source to the electro-optically modulated reference signal can be equally canceled by controlling the transmission environment, meaning that errors introduced by the reference signal transmission can be effectively suppressed.
[0042] Compared to existing radio frequency (RF) phase detection (RF) technologies, the optical pulse phase-stabilized transmission technology of this invention effectively avoids the phase detection errors introduced by amplitude-phase conversion in RF phase detection technology by employing a balanced optical pulse-based scheme. Furthermore, this invention is not limited by the material technology of RF phase detection and possesses a phase detection sensitivity far exceeding that of RF phase detection technologies.
[0043] This invention can achieve high-precision phase-stable transmission of optical pulses in the field of accelerators, can achieve signal synchronization of telescope arrays in the field of astronomy, and can also be applied to high-precision distance measurement, etc.
[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention. Various variations can be made to the above embodiments of the present invention. That is, all simple and equivalent changes and modifications made based on the claims and description of this invention fall within the protection scope of the claims of this patent. All aspects not described in detail in this invention are conventional technical content.
Claims
1. A fiber optic phase-stabilized transmission device based on a balanced optical pulse scheme, characterized in that, The system includes a mode-locked laser and a low-noise microwave source connected to each other. Along the propagation direction of the optical pulses output from the mode-locked laser, a first half-wave plate, a polarization cube, a first collimator, a circulator, a coupler, and an electro-optic modulator are arranged sequentially. A quarter-wave plate and a mirror are arranged sequentially on one side of the polarization cube along the optical pulse propagation direction, and a round-trip optical pulse transmission system is arranged on the other side. The coupler is connected to a balanced detector, the electro-optic modulator is connected to the low-noise microwave source, and both the round-trip optical pulse transmission system and the balanced detector are connected to a feedback controller. 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. The fourth and sixth ports are both connected to the electro-optic modulator to modulate the optical pulse amplitude using the low-noise microwave source. Thus, a reference signal output from the low-noise microwave source modulates the optical pulse through the electro-optic modulator, and the phase difference between the optical pulse and the microwave source is shifted. The amplitude of the optical pulse is adjusted. The low-noise microwave source has two outputs: one for the mode-locked laser and the other for modulating the optical pulse. This helps reduce the phase fluctuation difference of the radio frequency signal introduced by the transmission cable during transmission from the low-noise microwave source to the mode-locked laser and to the electro-optic modulator, thereby reducing the error introduced by the microwave source signal transmission. The signal transmission from the low-noise microwave source to the mode-locked laser and the reference signal transmission from the low-noise microwave source to the electro-optic modulator are equally canceled by controlling the transmission environment. The first half-wave plate is adjusted to make the optical pulse entering the polarization cube undergo total internal reflection. The output signal of the balanced detector reflects the phase difference between the optical pulse and the reference signal of the low-noise microwave source. The output signal of the balanced detector is transmitted to the feedback controller, and the parameters of the feedback controller are adjusted so that the phase of the optical pulse in electro-optic modulation is always aligned with the maximum slope point of the microwave source signal. This ensures that the phase of the optical pulse during long-distance fiber transmission to the terminal output is always locked to a fixed point of the reference signal of the low-noise microwave source.
2. The fiber optic phase-stabilized transmission device based on the balanced optical pulse scheme according to claim 1, characterized in that, The circulator is connected to the first photodetector.
3. The fiber optic phase-stabilized transmission device based on the balanced optical pulse scheme according to claim 1, characterized in that, The coupler is connected to the second photodetector.
4. The fiber optic phase-stabilized transmission device based on a balanced optical pulse scheme according to claim 1, characterized in that, The round-trip optical pulse transmission system includes a delay line, a second half-wave plate, a Faraday rotator, a second collimator, a dispersion compensation fiber, an extender, a transmission fiber, a bidirectional erbium-doped fiber amplifier, and a semi-transparent, semi-reflective device connected in sequence. The delay line is connected to the polarization cube and the feedback controller, respectively, and the extender is connected to the feedback controller.
5. The fiber optic phase-stabilized transmission device based on the balanced optical pulse scheme according to claim 4, characterized in that, Both the first port and the third port are connected to the balance detector, and the second port is connected to the circulator.
6. The fiber optic phase-stable transmission device based on the balanced optical pulse scheme according to claim 4, characterized in that, The fifth port is connected to the second photodetector.
7. A method for stable phase transmission in optical fibers based on a balanced optical pulse scheme, characterized in that, include: Step S1: Provide an optical fiber phase-stable transmission device based on a balanced optical pulse scheme as described in any one of claims 1 to 6; Step S2: The light pulses output by the mode-locked laser are sequentially transmitted to the first half-wave plate and the polarization cube. The first half-wave plate is adjusted so that the light pulses entering the polarization cube are s-polarized light. In step S3, the polarization cube reflects the s-polarized light to the round-trip optical pulse transmission system. Of the optical pulses entering the round-trip optical pulse transmission system, part is directly emitted and distributed to the designated client, while the other part is reflected back along the original path. In step S4, the light pulse returning along the original path is converted into p-polarized light, which is transmitted from the polarization cube to the quarter-wave plate and the mirror. The mirror reflects the p-polarized light back to the quarter-wave plate, and the light pulse returns to the s-polarized state. In step S5, the light pulse enters the polarization cube for the third time and is reflected to the first collimator. The light pulse is then transmitted sequentially through the first collimator to the circulator, coupler, and electro-optic modulator. Step S6: The microwave reference signal output by the low-noise microwave source modulates the optical pulse through the electro-optic modulator, and the modulated optical pulse is transmitted to the coupler. Step S7: The optical pulse output by the coupler is transmitted to the balanced detector, and the output signal of the balanced detector is transmitted to the feedback controller. The parameters of the feedback controller are adjusted so that the phase of the optical pulse in electro-optic modulation is always aligned with the maximum slope point of the microwave source signal.