Composite phase-locked loop for linear frequency sweep laser

Abstract

The invention provides a composite phase-locked loop for a linear frequency sweep laser. The composite phase-locked loop comprises a frequency modulation continuous wave source, a Mach-Zehnder interferometer module, a balance detector and an optical phase-locked loop module. Wherein the frequency modulation continuous wave source is used for generating frequency modulation continuous waves; the Mach-Zehnder interferometer module is arranged on a laser light path and receives a laser beam through an optical fiber patch cord; the balance detector is arranged on a laser light path and is used for receiving the laser beam, converting the laser beam into a beat frequency electric signal and inputting the beat frequency electric signal into the optical phase-locked loop module; the optical phase-locked loop module is electrically connected to the frequency modulation continuous wave source and the Mach-Zehnder interferometer module, and the optical phase-locked loop module, the frequency modulation continuous wave source and the Mach-Zehnder interferometer module form a feedback loop. According to the composite phase-locked loop architecture, the performance of a single phase-locked loop can be further improved, and due to the unique connection mode of the auxiliary loop, the scheme can eliminate the influence of environmental noise or vibration on the system stability to a certain extent.

Description

Technical Field

[0001] The invention belongs to the technical field of digital signal processing, and in particular relates to a composite phase-locked loop for a linear frequency-sweeping laser. Background Art

[0002] Compared with single-photon lidar, frequency modulated continuous wave lidar has obvious advantages such as high ranging accuracy, eye safety, low power and sensitive detection. Therefore, it is widely used in civilian fields such as autonomous driving, mobile target tracking, drone mapping, optical coherence tomography, etc. There are various ways to generate frequency modulated continuous wave sources. At present, the economical generation scheme is mainly direct modulation (current, temperature, cavity length modulation) of semiconductor lasers. This scheme is simple, effective and can generate frequency modulated continuous wave signals with a sweep width of GHz to THz, but there are some key problems to be solved, such as: phase noise and sweep nonlinearity of the source, which makes it difficult for this scheme to be widely used in some precision measurement scenarios. Phase noise will limit the coherent ranging distance of the source, and sweep nonlinearity will affect the ranging accuracy within the coherent distance. Therefore, they are key issues in the frequency modulated continuous wave scheme based on direct modulation of semiconductor lasers.

[0003] At present, the solutions available to solve these problems include pre-distortion technology and phase-locked loop technology. Patent CN116626655A proposes an optoelectronic phase-locked loop linear correction device, which achieves the correction of frequency sweep nonlinearity through a single optoelectronic phase-locked loop. Patent US20190190234A1 proposes multiple digital feedback pre-distortion architectures. These solutions all use errors as real-time feedback loop control signals to achieve adaptive frequency sweep signal pre-distortion operations.

[0004] The optimization of phase noise or frequency sweep nonlinearity by a single phase-locked loop or pre-distortion structure has reached saturation, and any device in the loop, especially the Mach-Zehnder interferometer, will deteriorate in correction effect if it is continuously disturbed by the environment. Summary of the invention

[0005] The purpose of the present invention is to solve the defects of the above-mentioned prior art and provide a composite phase-locked loop for a linear frequency-sweeping laser. On the basis of the main phase-locked loop, a secondary phase-locked loop for phase locking is introduced from the error signal output to the phase modulator in the Mach-Zehnder interferometer. The present invention can further improve the phase-locking performance, and the present invention can improve the anti-interference of the frequency-sweeping signal to environmental vibration and environmental noise.

[0006] The present invention adopts the following technical solution:

[0007] A composite phase-locked loop for a linear frequency-sweeping laser comprises a frequency-modulated continuous wave source, a Mach-Zehnder interferometer module, a balanced detector and an optical phase-locked loop module.

[0008] Wherein, the frequency modulated continuous wave source is used to generate a frequency modulated continuous wave.

[0009] The Mach-Zehnder interferometer module is set in the laser optical path, receives the laser beam through the optical fiber jumper, and transmits the converged beam to the balanced detector.

[0010] The balanced detector is arranged on the laser optical path, and is used for receiving the laser beam and converting the laser beam into a beat frequency electrical signal and inputting it into the optical phase-locked loop module.

[0011] The optical phase-locked loop module is electrically connected to the frequency modulated continuous wave source and the Mach-Zehnder interferometer module, and the optical phase-locked loop module, the frequency modulated continuous wave source, and the Mach-Zehnder interferometer module form a feedback loop for forming a main phase-locked loop and a sub-phase-locked loop. The optical phase-locked loop module receives the beat frequency electrical signal and uses the two loops to phase-lock and correct the sweep frequency error signal.

[0012] In one embodiment, the Mach-Zehnder interferometer module includes a fiber coupler I, a fiber delay line, a phase modulator, a fiber coupler II, and a fiber coupler III.

[0013] The optical fiber coupler I is used to receive a part of the light beam and transmit the light beam to the optical fiber coupler II.

[0014] The optical fiber delay line receives another part of the light beam and transmits the light beam to the optical fiber coupler III.

[0015] The optical fiber coupler II receives the light beam transmitted by the optical fiber coupler I and transmits the light beam to the optical fiber coupler III; another port of the optical fiber coupler II is used to output the linear frequency sweep signal after error correction.

[0016] The optical fiber coupler III is used to merge the light beam transmitted from the optical fiber delay line and the light beam transmitted from the optical fiber coupler II, and transmit the merged light beam to the balanced detector.

[0017] In one embodiment, the optical phase-locked loop module includes a mixer, an arbitrary function generator, a power divider, an electrical amplifier, an integrator, and an electrical filter.

[0018] The electric amplifier includes electric amplifier I and electric amplifier II.

[0019] The electric filter includes electric filter I and electric filter II.

[0020] Arbitrary function generator, used to generate reference signals.

[0021] The mixer receives the beat frequency electrical signal output by the balanced detector and the reference signal generated by the arbitrary function generator, and mixes the two signals to generate an error signal.

[0022] The power divider receives the error signal generated by the mixer and transmits it to the main phase-locked loop and the auxiliary phase-locked loop.

[0023] In the main phase-locked loop:

[0024] The electric amplifier I receives the error signal generated by the mixer, amplifies the error signal and transmits it to the integrator.

[0025] The integrator integrates the amplified error signal and transmits the integrated error signal to the electric filter I.

[0026] The electric filter I performs filtering on the error signal after the integration process, and then feeds back the error signal after the filtering process to the frequency modulated continuous wave source.

[0027] In the secondary phase-locked loop:

[0028] The electrical amplifier II receives the error signal generated by the mixer, amplifies the error signal and transmits it to the electrical filter II.

[0029] The electric amplifier II is used to adjust the loop gain of the secondary phase-locked loop.

[0030] The electric filter II receives the error signal after amplification and transmits the amplified error signal to the phase modulator of the Mach-Zehnder interferometer after filtering.

[0031] Electrical filter II is used to adjust the loop bandwidth of the secondary phase-locked loop.

[0032] In one embodiment, the frequency modulated continuous wave source is a current modulated distributed feedback semiconductor laser.

[0033] In one embodiment, the splitting ratios of the optical fiber coupler I and the optical fiber coupler III are both 5:5, the splitting ratio of the optical fiber coupler II is 1:9, and the optical fiber delay line is a delay line with a length of 10 m.

[0034] In one embodiment, the bandwidth of the balanced detector is DC-500 MHz, and the bandwidth of the mixer is 40 KHz to 400 MHz.

[0035] In one embodiment, the phase noise of the electrical signal output by the arbitrary function generator within the frequency range of 0.1 KHz-10 MHz is -125 dBc / Hz@10 kHz.

[0036] In one embodiment, the electric power distribution ratio of the power divider is 5: 5. The electric amplifier I and the electric amplifier II are devices of the same model, providing an adjustable amplification / reduction factor of -10dB-10dB.

[0037] In one embodiment, the electric filter I is a low-pass filter with a cut-off frequency of 2 MHz, and the electric filter II is a low-pass filter with a cut-off frequency of 4 MHz. The integration time of the integrator is 10 KHz.

[0038] Beneficial effects of the present invention:

[0039] The composite phase-locked loop for linear frequency sweeping laser of the present invention can obtain linear chirp which is better than that of a single phase-locked loop. In the whole system, the laser output is 60mW.

[0040] 1. The present invention can further improve the performance of a single phase-locked loop and improve the saturation of the performance of a single phase-locked loop. Referring to equations (1.1) and (1.2) in the specific implementation method, equation (1.1) describes the phase noise output by a single phase-locked loop after phase locking, wherein the first term on the right side of the equation is understood as the product of a low-pass function with a cutoff frequency of F and the sum of the reference signal phase noise and the Mach-Zehnder interferometer phase noise; the second term on the right side can be understood as the product of a high-pass function with a starting frequency of F and the laser phase noise. The final output phase noise, the low-frequency part (0-F) is determined by the reference signal phase noise and the Mach-Zehnder interferometer phase noise, and the high-frequency part (F-end) is determined by the laser phase noise. Comparing (1.1) and (1.2), it can be seen that after the introduction of the composite loop, the phase noise within the loop bandwidth is controlled by the two loops, so the phase-locked performance can be further improved.

[0041] 2. The correction effect of the Mach-Zehnder interferometer in the present invention is better than that of a single-loop phase-locked loop and can correct environmental noise and vibration. Referring to the specific embodiment equation (1.2), the environmental noise and vibration noise phase noise terms introduced by the Mach-Zehnder interferometer can be added to the first term on the right side of the equation through the properties of additive noise. Therefore, this structure can further optimize the impact of this type of noise on system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a structural schematic diagram of the present invention.

[0043] Figure 2 The figures are comparative diagrams of the columnar statistics of the residual errors after linear fitting of the upper and lower chirp swept frequency signals in the single-loop phase-locked loop and the composite phase-locked loop of the embodiment and the comparative example, wherein (a) represents the columnar statistics of the residual error of the upper chirp swept frequency signal, and (b) represents the columnar statistics of the residual error of the lower chirp swept frequency signal.

[0044] Figure 3 1 is a time series comparison diagram of the residual errors after linear fitting of the upper and lower chirp swept frequency signals in the single-loop phase-locked loop and the composite phase-locked loop of the embodiment and the comparative example, wherein (a) represents the time series of the residual error of the upper chirp swept frequency signal, and (b) represents the time series of the residual error of the lower chirp swept frequency signal.

[0045] Figure 4 The figures are comparison diagrams of the average noise power spectral density of ten acquisitions within 30 minutes of the residual errors of the upper and lower chirp swept frequency signals after linear fitting in the single-loop phase-locked loop and the composite phase-locked loop of the embodiments and the comparative examples, wherein (a) represents the average noise power spectral density of the residual error of the upper chirp swept frequency signal, and (b) represents the average noise power spectral density of the residual error of the lower chirp swept frequency signal.

[0046] Figure 5 It is a schematic diagram of the structure of a single-loop phase-locked loop in the comparative example.

[0047] Figure 6 The following is the effect of the single-loop PLL and the composite PLL on the performance of the same FMCW signal obtained by theoretical simulation. (a) is the loop gain fixed to 1×10 9 rad / s; (b) is the phase-locked result obtained when the loop gain is fixed at 1×10 10 rad / s to obtain the phase-locked result.

[0048] In the figure: 1-frequency modulated continuous wave source, 2-fiber type coupler I (splitting ratio 5:5), 3-fiber delay line, 4-phase modulator, 5-fiber type coupler II (splitting ratio 1:9), 6-fiber type coupler III (splitting ratio 5:5), 7-balanced detector, 8-mixer, 9-arbitrary function generator, 10-power divider (electrical power distribution ratio 5:5), 11-electric amplifier, 12-integrator, 13-electric filter;

[0049] 111-electrical amplifier I, 112-electrical amplifier II;

[0050] 131-Electric filter I, 132-Electric filter II.

[0051] Note: Frequency-modulated continuous-wave source (FMCW Source), fiber jumper (Fiber), fiber coupler (FC), fiber delay line (DL), phase modulator (PM).

[0052] Balanced detector (BPD, Balanced Photodetector), mixer (Mixer), power divider (PD, Power Divider), arbitrary function generator (AFG, Arbitrary Function Generator), amplifier (AMP, Amplifier), integrator (Integrator), electrical filter (Filter). DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention is described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0054] like Figure 1 As shown, a composite phase-locked loop for a linear frequency-sweeping laser of the present invention comprises a frequency-modulated continuous wave source 1, a Mach-Zehnder interferometer module, a balanced detector 7 and an optical phase-locked loop module.

[0055] Wherein, the frequency modulated continuous wave source 1 is used to generate a frequency modulated continuous wave.

[0056] The Mach-Zehnder interferometer module is set in the laser optical path and receives the laser beam through the optical fiber jumper.

[0057] The Mach-Zehnder interferometer module includes a fiber coupler Ⅰ2, a fiber delay line 3, a phase modulator 4, a fiber coupler Ⅱ5 and a fiber coupler Ⅲ6.

[0058] The optical fiber coupler Ⅰ2 is used to receive a part of the light beam and transmit the light beam to the optical fiber coupler Ⅱ5.

[0059] The optical fiber delay line 3 receives another part of the light beam and transmits the light beam to the optical fiber coupler III6.

[0060] The optical fiber coupler II5 receives the light beam transmitted by the optical fiber coupler I2 and transmits the light beam to the optical fiber coupler III6; another port of the optical fiber coupler II5 is used to output the linear frequency sweep signal after the error is corrected.

[0061] The optical fiber coupler III6 is used to merge the light beam transmitted from the optical fiber delay line 3 and the light beam transmitted from the optical fiber coupler II5, and transmit the merged light beam to the balanced detector 7.

[0062] The balanced detector 7 is arranged on the laser optical path, and is used to receive the laser beam, and convert the laser beam into a beat frequency electrical signal and input it into the optical phase-locked loop module. The balanced detector 7 can improve the influence of relative intensity noise to a certain extent.

[0063] The optical phase-locked loop module is electrically connected to the frequency modulated continuous wave source 1 and the Mach-Zehnder interferometer module, and the optical phase-locked loop module, the frequency modulated continuous wave source 1, and the Mach-Zehnder interferometer module form a feedback loop for forming a main phase-locked loop and a sub-phase-locked loop. The optical phase-locked loop module receives the beat frequency electrical signal and uses the two loops to phase-lock and correct the sweep frequency error signal.

[0064] The optical phase-locked loop module includes a mixer 8 , an arbitrary function generator 9 , a power divider 10 , an electrical amplifier 11 , an integrator 12 , and an electrical filter 13 .

[0065] The electric amplifier 11 includes an electric amplifier I 111 and an electric amplifier II 112. The electric filter 13 includes an electric filter I 131 and an electric filter II 132.

[0066] The arbitrary function generator 9 is used to generate a reference signal. The arbitrary function generator 9 can calculate a standard frequency according to the sweep width of the frequency modulated continuous wave source 1 and the length of the optical fiber delay line 3. The standard frequency is mixed with the frequency output by the balanced detector 7 in the mixer 8 to generate an error signal.

[0067] The mixer 8 receives the beat frequency electrical signal output by the balanced detector 7 and the reference signal generated by the arbitrary function generator 9, and mixes the two signals to generate an error signal. The balanced detector 7 and the mixer 8 serve as frequency and phase detection modules in an analog phase-locked loop.

[0068] The power divider 10 receives the error signal generated by the mixer 8 and transmits it to the main phase-locked loop and the auxiliary phase-locked loop. The power divider 10 is used to divide the error signal equally into the main phase-locked loop and the auxiliary phase-locked loop circuits for feedback control.

[0069] In the main phase-locked loop:

[0070] The electrical amplifier I111 receives the error signal generated by the mixer 8, amplifies the error signal and transmits it to the integrator 12. The electrical amplifier I111 is used to adjust the loop gain of the main phase-locked loop.

[0071] The integrator 12 integrates the amplified error signal and transmits the integrated error signal to the electric filter I 131. The integrator 12 is used to accumulate error information in the time domain.

[0072] The electric filter I 131 performs filtering on the error signal after the integration process, and then feeds back the error signal after the filtering process to the frequency modulated continuous wave source 1. The electric filter I 131 is used to adjust the loop bandwidth of the main phase locked loop.

[0073] In the secondary phase-locked loop:

[0074] The electric amplifier II 112 receives the error signal generated by the mixer 8, and transmits the error signal to the electric filter II 131 after amplifying the error signal. The electric amplifier II 112 is used to adjust the loop gain of the secondary phase-locked loop.

[0075] The electric filter II 132 receives the error signal after amplification, and transmits the amplified error signal to the phase modulator 4 of the Mach-Zehnder interferometer after filtering. The electric filter II 132 is used to adjust the loop bandwidth of the secondary phase-locked loop.

[0076] In some embodiments, the frequency modulated continuous wave source 1 is a current modulated distributed feedback semiconductor laser.

[0077] In some implementation cases, the splitting ratios of the optical fiber coupler Ⅰ2 and the optical fiber coupler Ⅲ6 are both 5:5, the splitting ratio of the optical fiber coupler Ⅱ5 is 1:9, and the optical fiber delay line 3 is a 10m delay line.

[0078] In some implementation cases, the bandwidth of the balanced detector 7 is from DC to 500 MHz, and the bandwidth of the mixer 8 is from 40 KHz to 400 MHz.

[0079] In some implementation cases, the phase noise of the electrical signal output by the arbitrary function generator 9 within the frequency range of 0.1 KHz-10 MHz is -125 cBc / Hz@10 kHz.

[0080] In some implementation cases, the power distribution ratio of the power divider 10 is 5:5. The electric amplifier I 111 and the electric amplifier II 112 are devices of the same model and can provide an adjustable amplification / reduction factor of -10dB-10dB.

[0081] In some implementation cases, the electric filter I 131 is a low-pass filter with a cut-off frequency of 2 MHz, and the electric filter II 132 is a low-pass filter with a cut-off frequency of 4 MHz. The integration time of the integrator 12 is 10 KHz.

[0082] Example

[0083] The residual error histogram of the composite phase-locked loop for the same frequency modulated continuous wave source

[0084] The frequency modulated continuous wave source 1 selects a current modulated distributed feedback semiconductor laser, the peak-to-peak value of the modulation signal is 20mA, the modulation frequency is 2.5KHz, the output power of the frequency modulated continuous wave source 1 is 60mW, and the operating temperature is 20℃.

[0085] The frequency modulated continuous wave signal is output to the Mach-Zehnder interferometer after passing through the optical fiber jumper.

[0086] The Mach-Zehnder interferometer module includes a fiber coupler Ⅰ2, a fiber delay line 3, a phase modulator 4, a fiber coupler Ⅱ5 and a fiber coupler Ⅲ6. The laser beam transmitted by the fiber jumper is transmitted by the fiber delay line 3 to the fiber coupler Ⅲ6, and the other part of the beam is transmitted by the phase modulator 4 through the fiber coupler Ⅱ5 to the fiber coupler Ⅲ6, and the beams are merged at the fiber coupler 6Ⅲ. The merged output beam is converted into a beat frequency electrical signal by the balanced detector 7 and input into the optical phase-locked loop module.

[0087] The optical phase-locked loop module includes a mixer 8 , an arbitrary function generator 9 , a power divider 10 , an electrical amplifier 11 , an integrator 12 , and an electrical filter 13 .

[0088] The electric amplifier 11 includes an electric amplifier I 111 and an electric amplifier II 112. The electric filter 13 includes an electric filter I 131 and an electric filter II 132.

[0089] The beat signal output by the balanced detector 7 and the reference signal generated by the arbitrary function generator 9 are mixed in the mixer 8 to generate an error signal. The error signal generated by the mixer 8 is fed back to the frequency continuous wave source 1 by the electric amplifier Ⅰ111, the integrator 12 and the electric filter Ⅰ131. In the secondary phase-locked loop: the error signal generated by the mixer 8 is fed back to the phase modulator 4 in the Mach-Zehnder interferometer by the electric amplifier Ⅱ112 and the electric filter Ⅱ131. The error-corrected linear sweep signal is output through one of the ports of the optical fiber coupler Ⅱ5.

[0090] Comparative Example

[0091] Histogram statistics of residual error of a single-loop phase-locked loop for the same FMCW source.

[0092] (Single-loop phase-locked loop structure such as Figure 5 As shown, in the figure, 1000-frequency modulated continuous wave source, 1002-optical fiber coupler I (splitting ratio 1:9), 1003-optical fiber coupler II, 1004-optical fiber delay line, 1005-optical fiber coupler III, 1006-balanced detector, 1007-mixer, 1008-arbitrary function generator, 1009-electric amplifier, 1010-integrator, 1011-electric filter, compared with Figure 1The single-loop phase-locked loop eliminates the feedback path back to the phase modulator in the Mach-Zehnder interferometer and its corresponding device).

[0093] Comparison results between the examples and the comparative examples: Figure 2 As shown in the figure, compared with the single-loop phase-locked loop and the composite phase-locked loop, the residual error of the frequency sweep signal is greatly reduced, and the maximum residual error is reduced by about 1MHz. Figure 3 As shown in Figure 2, the maximum amplitude of the residual error of the composite phase-locked loop is effectively reduced. Figure 4 As shown in the figure, the residual error of the single-loop phase-locked loop is more unstable than that of the composite loop, and is higher than that of the composite phase-locked loop in the range of 2.5KHz to 4MHz. This shows that the use of a composite phase-locked loop can improve the phase-locked performance and resist environmental interference to a certain extent.

[0094] Simulation test

[0095] like Figure 6 As shown in the figure, the phase-locked performance is characterized by phase noise. It can be seen from the figure that under appropriate loop gain, the performance of the composite phase-locked loop is better than that of the single-loop phase-locked loop.

[0096] The theoretical model of a single-loop phase-locked loop can be described as Equation 1.1:

[0097]

[0098] The theoretical model of the composite phase-locked loop can be described as equation 1.2:

[0099]

[0100] In formula (1.1), φ out represents the output phase noise; K opll_1 Represents the loop gain of a single-loop phase-locked loop; F 1 represents the transfer function of the swept-frequency laser; τ 1 Represents the loop delay of a single-loop phase-locked loop; φ RF represents the reference signal phase noise; φ MZI represents the phase noise introduced by the Mach-Zehnder interferometer due to environmental interference; φ s represents the phase noise of a swept-frequency laser.

[0101] In formula (1.2), the term with subscript 1 represents the relevant parameters of the main phase-locked loop, and the term with subscript 2 represents the relevant parameters of the sub-phase-locked loop. From the whole formula, it can be seen that adding additional sub-loops to form a composite loop can further suppress the noise from the Mach-Zehnder interferometer and the noise of the reference signal.

[0102] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite phase-locked loop for a linear frequency-sweep laser, characterized in that: It includes a frequency modulated continuous wave source, a Mach-Zehnder interferometer module, a balanced detector and an optical phase-locked loop module; Wherein, the frequency modulated continuous wave source is used to generate the frequency modulated continuous wave; The Mach-Zehnder interferometer module is arranged in the laser optical path, receives the laser beam through the optical fiber jumper, and transmits the converged beam to the balanced detector; The balanced detector is arranged on the laser optical path, and is used to receive the laser beam and convert the laser beam into a beat frequency electrical signal and input it into the optical phase-locked loop module; The optical phase-locked loop module is electrically connected to the frequency modulated continuous wave source and the Mach-Zehnder interferometer module, and the optical phase-locked loop module, the frequency modulated continuous wave source, and the Mach-Zehnder interferometer module form a feedback loop for forming a main phase-locked loop and a sub-phase-locked loop. The optical phase-locked loop module receives the beat frequency electrical signal and uses the two loops to phase-lock and correct the sweep frequency error signal.

2. The composite phase-locked loop for linear frequency sweeping laser according to claim 1, characterized in that: The Mach-Zehnder interferometer module includes a fiber coupler I, a fiber delay line, a phase modulator, a fiber coupler II and a fiber coupler III; Fiber-optic coupler I, used for receiving a portion of the light beam and transmitting the light beam to fiber-optic coupler II; The optical fiber delay line receives another part of the light beam and transmits the light beam to the optical fiber coupler III; The optical fiber coupler II receives the light beam transmitted by the optical fiber coupler I and transmits the light beam to the optical fiber coupler III; another port of the optical fiber coupler II is used to output the linear frequency sweep signal after the error is corrected; The optical fiber coupler III is used to merge the light beam transmitted from the optical fiber delay line and the light beam transmitted from the optical fiber coupler II, and transmit the merged light beam to the balanced detector.

3. The composite phase-locked loop for linear frequency sweeping laser according to claim 1 or 2, characterized in that: The optical phase-locked loop module includes a mixer, an arbitrary function generator, a power divider, an electrical amplifier, an integrator, and an electrical filter; The electric amplifier includes electric amplifier I and electric amplifier II; The electric filter includes electric filter I and electric filter II; Arbitrary function generator, used to generate reference signal; A mixer receives the beat frequency electrical signal output by the balanced detector and the reference signal generated by the arbitrary function generator, and mixes the two signals to generate an error signal; A power divider receives the error signal generated by the mixer and transmits it to the main phase-locked loop and the auxiliary phase-locked loop; Among them, in the main phase-locked loop: The electric amplifier I receives the error signal generated by the mixer, amplifies the error signal and transmits it to the integrator; The integrator integrates the amplified error signal and transmits the integrated error signal to the electric filter I; The electric filter I performs filtering on the error signal after the integration process, and then feeds back the error signal after the filtering process to the frequency modulated continuous wave source; In the secondary phase-locked loop: The electric amplifier II receives the error signal generated by the mixer, and transmits the error signal to the electric filter II after amplifying and processing the error signal; The electric filter II receives the error signal after amplification and transmits the amplified error signal to the phase modulator of the Mach-Zehnder interferometer after filtering.

4. The composite phase-locked loop for linear frequency sweeping laser according to claim 1, characterized in that: The frequency modulated continuous wave source adopts a current modulated distributed feedback semiconductor laser.

5. The composite phase-locked loop for linear frequency sweeping laser according to claim 2, characterized in that: The splitting ratios of the optical fiber coupler I and the optical fiber coupler III are both 5:5, the splitting ratio of the optical fiber coupler II is 1:9, and the optical fiber delay line is a delay line with a length of 10 m.

6. The composite phase-locked loop according to claim 1, characterized in that: The bandwidth of the balanced detector is DC-500MHz, and the bandwidth of the mixer is 40KHz to 400MHz.

7. The composite phase-locked loop for linear frequency sweeping laser according to claim 3, characterized in that: The phase noise of the electrical signal output by the arbitrary function generator is -125dBc / Hz@10kHz within the frequency range of 0.1KHz-10MHz.

8. The composite phase-locked loop for linear frequency sweeping laser according to claim 3, characterized in that: The power distribution ratio of the power divider is 5:

5. The power amplifier I and power amplifier II are the same type of devices, providing an adjustable amplification / reduction factor of -10dB-10dB.

9. The composite phase-locked loop for linear frequency sweeping laser according to claim 3, characterized in that: Electric filter I is a low-pass filter with a cut-off frequency of 2 MHz, and electric filter II is a low-pass filter with a cut-off frequency of 4 MHz; the integration time of the integrator is 10 KHz.

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