Optical comb absolute ranging system

Abstract

The application discloses an optical comb absolute distance measurement system, comprising a dual optical comb generation module, a power amplifier module, an asynchronous optical sampling module and a measurement control unit, wherein the dual optical comb generation module is used for generating dual-wavelength dual optical comb mode-locked pulses; the power amplifier module is used for power amplifying and narrow-band spectral filtering the dual-wavelength dual optical comb mode-locked pulses and outputting the dual-wavelength dual optical comb mode-locked pulses through a power amplifier first output end and a power amplifier second output end; the asynchronous optical sampling module is used for sampling the dual-wavelength dual optical comb mode-locked pulses after power amplifying and narrow-band spectral filtering, generating interference signals or cross-correlation signals with distance information; and the measurement control unit is used for calculating distance measurement values carried in the interference signals or the cross-correlation signals based on a repetition frequency of a local sampling pulse output by the power amplifier second output end, and is also used for correcting a working state of the dual optical comb generation module based on the interference signals or the cross-correlation signals output by an output end of the asynchronous optical sampling module.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser ranging technology, and in particular to an optical comb absolute ranging system. BACKGROUND

[0002] The dual-comb system is a Fourier transform spectroscopy tool without mechanical delay line in recent years. In the time domain, the pulse sequences of two optical combs with slightly different repetition frequencies generate similar cross-correlation interference signals. In the frequency domain, two optical combs with different longitudinal mode intervals are down-converted to a single radio frequency comb of MHz through multi-heterodyne interference. Therefore, it is widely used in high-resolution spectroscopy, femtosecond laser ranging, three-dimensional imaging and other fields.

[0003] At present, the absolute distance measurement system based on single-cavity dual-comb usually selects a material saturable absorber as a mode-locked device of the dual-comb light source. The dual-comb light source is provided by a single-cavity bidirectional output mode-locked laser, which still needs to be phase-locked controlled by a piezoelectric ceramic, and the balanced cross-correlation structure is very complex, so that the whole system is too large. In addition, a pair of optical combs with offset repetition frequency is generated in a single resonant cavity, and the commonly used material saturable absorber has the disadvantages of easy deliquescence, low damage threshold, long relaxation time and the like. Moreover, such dual-comb is difficult to ensure short-term or long-term stability, and it is difficult to start dual-wavelength mode locking, which seriously affects the application in the field of absolute distance measurement. SUMMARY

[0004] The present application provides an optical comb absolute ranging system, which can feedback and adjust the polarization state of the electrically controlled polarization element in the dual-comb generation module in real time, so as to adjust the generated mode-locked pulse in real time, ensure the correct and stable state of the dual-comb mode-locked pulse, speed up the starting speed of the ranging system, and improve the measurement accuracy.

[0005] To achieve the above-mentioned purpose, the present application provides an optical comb absolute ranging system, comprising:

[0006] A dual-comb generation module for generating dual-wavelength dual-comb mode-locked pulses;

[0007] A power amplification module comprising a power amplification input end, a power amplification first output end and a power amplification second output end, the power amplification input end is connected with the output end of the dual-comb generation module, for power amplification and narrow-band spectral filtering of the dual-wavelength dual-comb mode-locked pulses, and output through the power amplification first output end and the power amplification second output end;

[0008] An asynchronous optical sampling module includes a local sampling end and a to-be-tested sampling end, the local sampling end is connected with the power amplification second output end, the to-be-tested sampling end is connected with the power amplification first output end, and is used for sampling the double-wavelength double-comb mode-locked pulse after power amplification and narrow-band spectral filtering to generate an interference signal or a cross-correlation signal with distance information;

[0009] A measurement control unit is connected with the double-comb generation module, the power amplification second output end and the output end of the asynchronous optical sampling module respectively, is used for calculating the distance measurement value carried in the interference signal or the cross-correlation signal based on the repetition frequency of the local sampling pulse output by the power amplification second output end, and is used for correcting the working state of the double-comb generation module based on the interference signal or the cross-correlation signal output by the output end of the asynchronous optical sampling module.

[0010] Optionally, the double-comb generation module includes:

[0011] A first pump source, a first wavelength division multiplexer, a first gain optical fiber, a first electrically-controlled polarization element, a polarization-dependent isolator, a second electrically-controlled polarization element, a polarization-maintaining optical fiber and an output coupler are connected in sequence; wherein the input end of the output coupler is connected with the signal injection end of the first wavelength division multiplexer, the first output end of the output coupler is connected with the polarization-maintaining optical fiber, and the second output end of the output coupler is connected with the power amplification input end.

[0012] Optionally, the second electrically-controlled polarization element and the polarization-maintaining optical fiber constitute a filter of the double-comb generation module, and the filter bandwidth of the filter satisfies:

[0013] wherein, is the birefringence of the polarization-maintaining optical fiber, is the length of the polarization-maintaining optical fiber, is 1550 nm, is the filter bandwidth.

[0014] Optionally, the power amplification module includes:

[0015] A filter, a first power amplification assembly and a second power amplification assembly, the input end of the filter is connected with the output end of the double-comb generation module, and the output end of the filter is connected with the input end of the first power amplification assembly and the input end of the second power amplification assembly respectively;

[0016] The first power amplification component and the second power amplification component each comprise a fiber Bragg grating, a second pump source, a polarization-independent isolator, a second wavelength division multiplexer, a second gain optical fiber and a three-port circulator connected in sequence; wherein the first end of the three-port circulator is connected to the second gain optical fiber, the second end of the three-port circulator is connected to the local oscillator sampling end or the device under test sampling end, and the third end of the three-port circulator is connected to the fiber Bragg grating.

[0017] Optionally, the parameters of the fiber Bragg gratings in the first power amplification component and the second power amplification component are the same.

[0018] Optionally, the asynchronous optical sampling module comprises a local oscillator sampling unit and a device under test sampling unit.

[0019] The local oscillator sampling unit comprises a second collimator, a second half-wave plate, a third half-wave plate, a second polarization beam splitter, an avalanche diode and an interference signal generation unit or a cross-correlation signal generation unit; and the device under test sampling unit comprises a first collimator, a first half-wave plate, a first quarter-wave plate, a first polarization beam splitter, a reference mirror, a second quarter-wave plate and a target mirror.

[0020] The first mode-locked pulse output by the power amplification first output end passes through the first collimator and the first half-wave plate to be divided into a first polarization mode-locked pulse and a second polarization mode-locked pulse, the first polarization mode-locked pulse reaches the reference mirror after being reflected by the first polarization beam splitter and passing through the first quarter-wave plate, and the second polarization mode-locked pulse reaches the target mirror after being transmitted by the first polarization beam splitter and passing through the second quarter-wave plate; the first polarization mode-locked pulse reflected by the reference mirror and the second polarization mode-locked pulse reflected by the target mirror are combined after passing through the first polarization beam splitter and then incident on the local oscillator sampling unit through the second half-wave plate.

[0021] The local oscillator sampling unit comprises a second collimator, a second half-wave plate, a third half-wave plate, a second polarization beam splitter, an avalanche diode and an interference signal generation unit or a cross-correlation signal generation unit.

[0022] The second mode-locked pulse output by the power amplification second output end passes through the second collimator, the third half-wave plate and the second polarization beam splitter, is combined with the combination of the first polarization mode-locked pulse and the second polarization mode-locked pulse, and then reaches the avalanche diode after passing through the interference signal generation unit or the cross-correlation signal generation unit, and the avalanche diode performs sampling.

[0023] Optionally, the interference signal generation unit comprises a fourth half-wave plate and a third polarization beam splitter.

[0024] Optionally, the cross-correlation signal generation unit comprises a focusing lens and a sum frequency crystal.

[0025] Optionally, control ends of the measurement control unit are connected with the first electrically controlled polarization element and the second electrically controlled polarization element respectively, and an input end of the measurement control unit is connected with an output end of the asynchronous optical sampling module; wherein the measurement control unit generates a voltage signal based on the interference signal or the cross-correlation signal output by the asynchronous optical sampling module, and outputs the voltage signal to the first electrically controlled polarization element and the second electrically controlled polarization element.

[0026] Optionally, the first electrically controlled polarization element and the second electrically controlled polarization element are both electrically controlled polarization controllers or electrically controlled wave plates, and each of the electrically controlled polarization controller or the electrically controlled wave plate comprises eight voltage input pins, forming four direct current voltage channel drives.

[0027] In summary, the optical comb absolute distance measurement system according to the embodiment of the present application comprises a double optical comb generation module, a power amplification module, an asynchronous optical sampling module and a measurement control unit, wherein the double optical comb generation module is used to generate double-wavelength double optical comb mode-locked pulses; the power amplification module is used to power amplify and narrow-band spectrum filter the double-wavelength double optical comb mode-locked pulses, and output the double-wavelength double optical comb mode-locked pulses through a power amplification first output end and a power amplification second output end in two ways; the asynchronous optical sampling module is used to sample the double-wavelength double optical comb mode-locked pulses after power amplification and narrow-band spectrum filtering, and generate interference signals or cross-correlation signals with distance information; and the measurement control unit is used to calculate a distance measurement value carried in the interference signals or the cross-correlation signals based on a repetition frequency of the local oscillation sampling pulses output by the power amplification second output end, and is also used to correct a working state of the double optical comb generation module based on the interference signals or the cross-correlation signals output by the output end of the asynchronous optical sampling module. Thus, the distance measurement system can feed back and adjust in real time a polarization state of an electrically controlled polarization element in the double optical comb generation module, so as to adjust the generated mode-locked pulses in real time, ensure the state of the double optical comb mode-locked pulses to be correct and stable, accelerate the start speed of the distance measurement system, and improve the measurement precision.

[0028] It should be understood that the content described in this part is not intended to identify key or important features of the embodiments of the present application, nor to limit the scope of the present application. Other features of the present application will become apparent from the following description. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to make the technical solutions in the embodiments of the present application clearer, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0030] Figure 1 is a structural schematic diagram of an optical comb absolute distance measurement system provided by the embodiments of the present application.

[0031] Figure 2 is a spectrum diagram of the stable dual-wavelength mode-locked pulse provided by the embodiments of the present application.

[0032] Figure 3 is a principle schematic diagram of the interferometric distance measurement provided by the embodiments of the present application.

[0033] Figure 4 is a principle schematic diagram of the cross-correlation distance measurement provided by the embodiments of the present application. DETAILED DESCRIPTION

[0034] In order to make the technical solutions in the embodiments of the present application clearer, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0035] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and do not necessarily mean a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion.

[0036] Figure 1 is a structural schematic diagram of an optical comb absolute distance measurement system provided by the embodiments of the present application. As shown in Figure 1 the distance measurement system 100 includes a dual optical comb generation module 110, a power amplification module 120, an asynchronous optical sampling module 130 and a measurement control unit 140.

[0037] The double optical comb generation module 110 is configured to generate double-wavelength double optical comb mode-locked pulses; the power amplification module 120 includes a power amplification input end, a power amplification first output end and a power amplification second output end, the power amplification input end is connected with the output end of the double optical comb generation module 110, and is configured to perform power amplification and narrow-band spectral filtering on the double-wavelength double optical comb mode-locked pulses and output the double-wavelength double optical comb mode-locked pulses in two paths through the power amplification first output end and the power amplification second output end; the asynchronous optical sampling module 130 includes a local sampling end and a to-be-measured sampling end, the local sampling end is connected with the power amplification second output end, the to-be-measured sampling end is connected with the power amplification first output end, and the asynchronous optical sampling module 130 is configured to sample the double-wavelength double optical comb mode-locked pulses after power amplification and narrow-band spectral filtering to generate an interference signal or a cross-correlation signal carrying distance information; and the measurement control unit 140 is connected with the double optical comb generation module 110, the power amplification second output end and the output end of the asynchronous optical sampling module 130 respectively, and is configured to calculate a distance measurement value carried in the interference signal or the cross-correlation signal based on a repetition frequency of the local sampling pulse output by the power amplification second output end, and correct the working state of the double optical comb generation module 110 based on the interference signal or the cross-correlation signal output by the output end of the asynchronous optical sampling module 130.

[0038] It should be noted that the double optical comb generation module 110 generates double optical comb mode-locked pulses, the power amplification module 120 outputs the double optical comb mode-locked pulses in two paths after power amplification and filtering, the asynchronous optical sampling module 130 samples the double optical comb mode-locked pulses in two paths after filtering and power amplification to output an electrical signal including distance information, and the measurement control unit 140 can adjust the double optical comb generation module 110 according to whether the electrical signal output by the asynchronous optical sampling module 130 is abnormal, so that the double optical comb generation module 110 generates stable and correct double optical comb mode-locked pulses. In addition, when the electrical signal output by the asynchronous optical sampling module 130 is normal, the measurement control unit 140 can calculate distance information carried in the electrical signal according to the electrical signal output by the asynchronous optical sampling module 130.

[0039] The measurement control unit 140 can pre-store a normal electrical signal and record the number of signals (pulse envelopes / interference fringes) in a down-sampling period, and then compare the number of electrical signals output by the asynchronous optical sampling module 130 in a period with the number of pre-stored normal electrical signals to determine whether the electrical signal output by the asynchronous optical sampling module 130 is abnormal. For example, the number of electrical signals in a period under a normal state is usually 2, and other cases are abnormal states, and the voltage applied to the electrically controlled polarization element in the double optical comb generation module 110 needs to be fine-tuned.

[0040] Therefore, the distance measuring system can feed back and adjust the double optical comb mode-locked pulse generated by the double optical comb generation module in real time, ensure the correct and stable state of the double optical comb mode-locked pulse, speed up the starting speed of the distance measuring system, and improve the measurement accuracy.

[0041] Optionally, as shown in Figure 1 The double optical comb generation module 110 includes:

[0042] The first pump source 1, the first wavelength division multiplexer 2, the first gain optical fiber 3, the first electrically controlled polarization element 4, the polarization-dependent isolator 5, the second electrically controlled polarization element 6, the polarization maintaining optical fiber 7, and the output coupler 8 are connected in sequence; wherein the input end of the output coupler 8 is connected with the signal injection end of the first wavelength division multiplexer 2, the first output end of the output coupler 8 is connected with the polarization maintaining optical fiber 7, and the second output end of the output coupler 8 is connected with the power amplification input end.

[0043] The first pump source 1, the first wavelength division multiplexer 2, the first gain optical fiber 3, the first electrically controlled polarization element 4, the polarization-dependent isolator 5, the second electrically controlled polarization element 6, the polarization maintaining optical fiber 7, and the output coupler 8 are connected in sequence; wherein the input end of the output coupler 8 is connected with the signal injection end of the first wavelength division multiplexer 2, the first output end of the output coupler 8 is connected with the polarization maintaining optical fiber 7, and the second output end of the output coupler 8 is connected with the power amplification input end.

[0044] It should be noted that the first gain optical fiber 3 can be one of a ytterbium-doped fiber, an erbium-doped fiber, an erbium-ytterbium co-doped fiber, or a thulium-doped fiber, which can be selected according to the specific band requirements. It should be noted that the type of the first gain optical fiber 3 includes but is not limited to the above examples, and a person skilled in the art can select according to the actual situation. The first pump source 1 includes a semiconductor laser with a center wavelength of 976 nm. It should be noted that the type of the first pump source 1 and the center wavelength are not specifically limited in this embodiment, and a person skilled in the art can select the type of the first pump source 1 and its corresponding center wavelength according to the actual situation. The working wavelength of the first wavelength division multiplexer 2 includes 980 / 1550 nm. The cavity length of the ring cavity of the dual-comb generation module 110 is 19.8 m. The cavity length of the ring cavity of the dual-comb generation module 110 is not limited to 19.8 m, and a person skilled in the art can set it according to the actual situation. The spectral diagram of the dual-wavelength mode-locked pulse generated by the dual-comb generation module 110 is shown in the example of Figure 2 Figure 2 The repetition frequencies of the two peaks are 10.1222 MHz and 10.1232 MHz, respectively, corresponding to a repetition frequency difference of 1 kHz.

[0045] It can be understood that the gain spectrum of the dual-comb generation module 110 is provided by the first pump source 1 pumping the first gain optical fiber 3 through the first wavelength division multiplexer 2. The first electrically controlled polarization element 4, the polarization-dependent isolator 5 (PD-ISO), and the second electrically controlled polarization element 6, for example, an electrically controlled polarization controller (EPC), together form an artificial saturable absorber as a passive mode-locked device.

[0046] The first electrically controlled polarization element 4 and the second electrically controlled polarization element 6 are both electrically controlled polarization controllers or electrically controlled wave plates to reduce the impact of environmental and mechanical vibrations on stability. The electrically controlled polarization controller or the electrically controlled wave plate includes 8 voltage input pins to form four direct current voltage channel drivers.

[0047] In addition, in this embodiment, the second electrically controlled polarization element 6 and the polarization maintaining optical fiber 7 form a Lyot filter structure, so that the gain spectrum has two or more gain peaks, and the corresponding filter bandwidth can be adjusted by selecting the length of different polarization maintaining optical fibers. That is, the first electrically controlled polarization element 4, the polarization-dependent isolator 5, the second electrically controlled polarization element 6, and the polarization maintaining optical fiber 7 in this embodiment constitute a passive mode-locked device and a filter structure, and simultaneously play the dual roles of an artificial saturable absorber and a comb filter, so that the laser generates stable dual-wavelength mode-locked pulses in a suitable polarization state.

[0048] In this embodiment, the second electrically controlled polarization element 6 and the polarization maintaining optical fiber 7 constitute a filter of the dual-comb generation module 110, and the filter bandwidth of the filter satisfies: ​

[0049] wherein, is the birefringence of the polarization maintaining fiber, is the length of the polarization maintaining fiber, is 1550 nm, is the filter bandwidth. That is, the length of the polarization maintaining fiber 7 corresponds to the bandwidth of the comb filter, which is determined by the above formula, and an exemplary length of the polarization maintaining fiber 7 is 15 cm, corresponding to a filter bandwidth of 34 nm, wherein the filter position and the modulation depth are adjusted by the second electrically controlled polarization element 6.

[0050] In addition, in this embodiment, the output coupler 8 outputs a fixed proportion of the light to the power amplification module 120 (not described here) described in subsequent embodiments, and the remaining light continues to circulate in the resonant cavity. Among them, the output proportion is selected according to the needs.

[0051] Exemplarily, the first pump source is a 976 nm semiconductor laser, and the pump light pumps the erbium-doped fiber (first gain fiber 3) through the 980 / 1550 nm first wavelength division multiplexer 2. The splitting ratio of the output coupler is 1:9, that is, 10% of the light is output, and the remaining 90% of the light is coupled into the wavelength division multiplexer to form a ring cavity. The polarization-dependent isolator allows the signal light to run in the ring cavity in only one direction. The signal light generated by stimulated radiation at the erbium-doped fiber passes through the composite structure composed of the polarization-dependent isolator 4, the polarization maintaining fiber 5 and the electrically controlled polarization controller (electrically controlled polarization element 6) in turn, and returns to the first wavelength division multiplexer 2 from the 90% end of the output coupler. The dual effects of nonlinear polarization evolution mode-locking and comb filtering make the laser produce stable dual-wavelength mode-locked pulses in a suitable polarization state. The cavity length of the laser ring cavity is 19.8 m. The length of the polarization maintaining fiber 5 corresponds to the bandwidth of the comb filter, which is determined by wherein, Δn is the birefringence of the polarization maintaining fiber 5, LPMF is the length of the polarization maintaining fiber 5, the length of the polarization maintaining fiber 5 used in this embodiment is 15 cm, λ is 1550 nm, and Δλ is the filter interval, wherein the filter bandwidth corresponding to 34 nm, the filter position and the modulation depth are adjusted by the electrically controlled polarization controller. By controlling the electrically controlled polarization controller 4 DC voltage channels through an algorithm, all possible polarization states on the Poincare sphere can be globally searched, and a set of voltage values that can achieve dual-wavelength mode-locking can be found. When this set of voltage values is fixed, the mode-locked state also remains unchanged, thus having high stability. The voltage values corresponding to the mode-locked state are different for different lasers.

[0052] Therefore, the dual-optical comb generation module 110 adopts a ring cavity structure based on a nonlinear polarization rotation artificial saturable absorber. This mode-locked structure has lower quantum noise than material saturable absorbers, which is beneficial to obtaining higher measurement accuracy in laser ranging applications.

[0053] Further, the control end of the measurement control unit 140 is connected with the first electrically controlled polarization element 4 and the second electrically controlled polarization element 6 respectively, and the input end of the measurement control unit 140 is connected with the output end of the asynchronous optical sampling module 130; wherein the measurement control unit 140 generates a voltage signal based on the interference signal or the cross-correlation signal output by the asynchronous optical sampling module 130, and outputs to the first electrically controlled polarization element 4 and the second electrically controlled polarization element 6.

[0054] It should be noted that the measurement control unit 140 obtains a new set of voltage values after algorithm processing based on the interference signal or the cross-correlation signal output by the asynchronous optical sampling module 130, and adds the set of voltage values to the electrically controlled polarization element in the cavity, thereby forming a feedback control; wherein the algorithm includes: genetic algorithm, Rosenblueth search algorithm, greedy algorithm, etc.; in addition, the feedback control can be completed by online or offline method. Exemplarily, the number of signals output by the asynchronous optical sampling module 130 is compared with the pre-stored signal number, if they are not consistent, the voltage on the electrically controlled polarization element can be adjusted to make the number of signals output by the asynchronous optical sampling module 130 consistent with the pre-stored signal number.

[0055] Further, by setting four direct current voltage channels to drive, the first electrically controlled polarization element 4 and the second electrically controlled polarization element 6 can generate all possible polarization states on the Poincare sphere, while the ordinary polarization controller often cannot achieve full coverage, so the two electrically controlled polarization elements can sweep the entire polarization space in the laser cavity, and by algorithm controlling the driving voltage of the electrically controlled polarization element, the corresponding polarization state of the dual-wavelength mode-locked can be realized and stabilized, and the starting speed is greatly improved. Compared with the prior art, in the embodiment, the measurement control unit 140 is used to control the electrically controlled polarization element, and then the polarization state of the mode-locked pulse freely running in the resonant cavity is effectively controlled, so as to stabilize the dual optical comb, and greatly improve the short-term and long-term stability of the free-running dual light source; at the same time, the polarization element of the embodiment is an electrically controlled polarization element, as long as the voltage of the electrically controlled polarization element is controlled by the program, the desired pulse can be automatically generated, without manual adjustment, without the need for professional personnel to adjust, which is convenient and practical. And by controlling the voltage applied to the electrically controlled polarization element to change the polarization state, the traditional mechanical polarization control element is replaced, which can better avoid the influence of vibration on the polarization state of the laser cavity and cause instability of the mode-locked state.

[0056] Optionally, with reference to Figure 1 , the power amplification module 120 comprises:

[0057] The filter 9, the first power amplification assembly and the second power amplification assembly, the input end of the filter 9 is connected with the output end of the dual optical comb generation module 110, the output end of the filter 9 is connected with the input end of the first power amplification assembly and the input end of the second power amplification assembly respectively;

[0058] The first power amplification assembly and the second power amplification assembly each comprise: a fiber Bragg grating, a second pump source, a polarization-independent isolator, a second wavelength division multiplexer, a second gain optical fiber and a three-port circulator connected in sequence; wherein the first end of the three-port circulator is connected to the second gain optical fiber, the second end of the three-port circulator is connected to the to-be-measured sampling end or the local oscillation sampling end, and the third end of the three-port circulator is connected to the fiber Bragg grating.

[0059] That is, the power amplification module 120 comprises: a first polarization-independent isolator 10, a second polarization-independent isolator 16, a second wavelength division multiplexer 12, a third wavelength division multiplexer 18, a second gain optical fiber 13, a third gain optical fiber 19, a second pump source 11, a third pump source 17, a first three-port circulator 14, a second three-port circulator 20, a first fiber Bragg grating 15 and a second fiber Bragg grating 21; the parameters of the first fiber Bragg grating 15 and the second fiber Bragg grating 21 are the same; the input end of the filter 9 is connected to the second output end of the output coupler 8, the first output end of the filter 9 is connected to the input end of the first polarization-independent isolator 10, the output end of the first polarization-independent isolator 10 is connected to the signal injection end of the second wavelength division multiplexer 12, the pump input end of the second wavelength division multiplexer 12 is connected to the second pump source 11, the output end of the second wavelength division multiplexer 12 is connected to the first end of the second gain optical fiber 13, the second end of the second gain optical fiber 13 is connected to the first end of the first three-port circulator 14, and the second end of the first three-port circulator 14 is connected to the first fiber Bragg grating 15; the second output end of the filter 9 is connected to the input end of the second polarization-independent isolator 16, the output end of the second polarization-independent isolator 16 is connected to the signal injection end of the third wavelength division multiplexer 18, the pump input end of the third wavelength division multiplexer 18 is connected to the third pump source 17, the output end of the third wavelength division multiplexer 18 is connected to the first end of the third gain optical fiber 19, the second end of the third gain optical fiber 19 is connected to the first end of the second three-port circulator 20, and the second end of the second three-port circulator 20 is connected to the second fiber Bragg grating 21; the third end of the first three-port circulator 14 and the third end of the second three-port circulator 20 output a double optical comb.

[0060] The types of the second gain optical fiber 13 and the third gain optical fiber 19 are the same as the type of the first gain optical fiber 3.

[0061] The above devices are sequentially fused in a single-mode fusion manner, wherein the a port of the first three-port circulator 14 is fused to the output end of the second gain optical fiber 13, and the b port of the first three-port circulator 14 is fused to the first fiber Bragg grating 15; the a port of the second three-port circulator 20 is fused to the output end of the third gain optical fiber 19, and the b port of the second three-port circulator 20 is fused to the second fiber Bragg grating 21.

[0062] In the power amplification module 120 of the embodiment of the present application, the filter 9 divides the dual-wavelength mode-locked pulses output by the output coupler 8 into two paths according to wavelengths, and the filtering position is determined according to the specific dual-wavelength position. The two single optical combs divided by the filter 9 pass through the second gain fiber 13 and the third gain fiber 19 for power amplification, so that the two optical combs have a certain overlapping part in the spectral range, wherein the types of the second gain fiber 13 and the third gain fiber 19 used are determined by the spectral range of the laser output by the resonant cavity. The pulses after the power amplification pass through the three-port circulator (the first three-port circulator 14 and the second three-port circulator 20) and then pass through two identical first fiber Bragg gratings 15 and second fiber Bragg gratings 21. The filtering positions of the first fiber Bragg gratings 15 and the second fiber Bragg gratings 21 are the spectral overlapping part of the two optical combs, and the corresponding filtering bandwidth is usually only a few nanometers to prevent frequency aliasing.

[0063] For example, the dual-frequency pulse output by the second output end (10% end) of the output coupler 7 passes through the filter 9, and the dual-comb with center wavelengths of 1532.78 nm and 1566.72 nm is divided into two paths, and then the two optical combs pass through the erbium-doped fiber for nonlinear amplification, so that the two optical combs have a certain overlapping part in the spectral range. The two paths of light pass through the circulator and then pass through two identical fiber Bragg gratings, and are output at the second port of the circulator, with a filtering bandwidth of 1 nm.

[0064] Optionally, with reference to Figure 1 , the asynchronous optical sampling module 130 includes: a local sampling unit and a to-be-measured sampling unit;

[0065] The to-be-measured sampling unit includes: a first collimator 22, a first half-wave plate 24, a first quarter-wave plate 26, a first polarization beam splitter 25, a reference mirror 28, a second quarter-wave plate 27, and a target mirror 29. The first mode-locked pulse output by the power amplification first output end passes through the first collimator 22 and the first half-wave plate 24 to be divided into a first polarization mode-locked pulse and a second polarization mode-locked pulse. The first polarization mode-locked pulse is reflected by the first polarization beam splitter 25 to the first quarter-wave plate 26 and then reaches the reference mirror 28. The second polarization mode-locked pulse is transmitted through the first polarization beam splitter 25 and then reaches the target mirror 29 after passing through the second quarter-wave plate 27. The first polarization mode-locked pulse reflected by the reference mirror 28 and the second polarization mode-locked pulse reflected by the target mirror 29 are combined after passing through the first polarization beam splitter 25 and then enter the local sampling unit through the second half-wave plate 30.

[0066] The local oscillator sampling unit comprises: a second collimator 23, a second half-wave plate 30, a third half-wave plate 31, a second polarization beam splitter 32, an avalanche diode 35, and an interference signal generating unit 33 or a cross-correlation signal generating unit 34; the second mode-locked pulse output by the power amplifier second output end passes through the second collimator 23, the third half-wave plate 31, and the second polarization beam splitter 32, is combined with the combined beam of the first polarization mode-locked pulse and the second polarization mode-locked pulse, then passes through the interference signal generating unit 33 or the cross-correlation signal generating unit 34, and finally reaches the avalanche diode 35, which performs sampling. The interference signal generating unit 33 comprises a fourth half-wave plate and a third polarization beam splitter. The cross-correlation signal generating unit 34 comprises a focusing lens and a sum-frequency crystal.

[0067] That is, the signal light comb output by the power amplification module 120 outputs spatial light through the first collimator 22, and after passing through the first half-wave plate 24, is divided into reflected vertical polarization light and transmitted horizontal polarization light at the first polarization beam splitter 25, wherein the reflected light serves as the reference light comb, and the transmitted light serves as the target light comb; the reference light comb passes through the first quarter-wave plate 26 and the reference mirror 28 to re-enter the first polarization beam splitter 25 to be transmitted, and the target light comb passes through the second quarter-wave plate 27 and the target mirror 29 to re-enter the first polarization beam splitter 25 to be reflected; then, the two beams of polarization perpendicular to each other pass through the second half-wave plate 30 and are incident on the second polarization beam splitter 32.

[0068] The local oscillator light comb output by the power amplification module 120 outputs spatial light through the second collimator 23, and after passing through the third half-wave plate 31, is incident on the second polarization beam splitter 32; the second half-wave plate 30 and the third half-wave plate 31 are adjusted so that the power of the local oscillator light comb and the signal light comb is equivalent after the second polarization beam splitter 32. The interference signal requires a fourth half-wave plate and a third polarization beam splitter 33, so that the signal light comb and the local oscillator light comb have the same polarization component, thereby obtaining the interference signal at the avalanche diode 35; the cross-correlation signal replaces the fourth half-wave plate and the third polarization beam splitter 33 with a focusing lens and a sum-frequency crystal 34, and the signal light comb and the local oscillator light comb with polarization directions perpendicular to each other are focused by the focusing lens to generate sum-frequency cross-correlation in the sum-frequency crystal (here, a PPKTP crystal is selected), and the cross-correlation signal is measured by the avalanche diode 35.

[0069] Figure 3 is a schematic diagram of the principle of the interferometric ranging provided by the embodiment of the present application. In the diagram, only the asynchronous optical sampling process of the target light comb and the local oscillator light comb is shown, and a down-sampled interference signal of the target light comb is obtained.

[0070] Figure 4is a principle diagram of the inter-correlation method distance measurement provided by the embodiment of the present application. The local light comb perpendicular to the polarization direction of the signal light comb will generate different frequency (f1-f2) ), the target light comb and the reference light comb are down-sampled, and the inter-correlation signal is generated through the PPKTP crystal. The interval of the two down-sampled electric pulse signals is the time difference Td of the two electric pulses of the target mirror and the reference mirror, and the distance L between the two mirrors is:

[0071] ,

[0072] wherein is the unambiguous range of the measurement system, which is obtained through coarse measurement.

[0073] Finally, the measurement control unit 140 mainly measures the real-time repetition frequency of the local light comb through frequency counting ; and collects the time sequence electric signal. For the interference method distance measurement, the envelope of the interference signal is extracted by using Hilbert transform, and then the time difference Td of the two electric pulses of the target mirror and the reference mirror is calculated. For the inter-correlation method distance measurement, Td is directly calculated. Finally, the electric signal with distance information is fed back to control the electric control polarization element in the double-comb generation module 110, so as to ensure the correctness and stability of the state of the output double comb. For example, the electric signal collected from the avalanche diode 35 can immediately find that the double-comb pulse is abnormal, which is difficult to find in the double-comb pulse sampling process. Subsequently, the electric control polarization element in the double-comb generation module 110 is controlled by feedback to restore to the normal locking state.

[0074] Therefore, the double-comb generation module in the embodiment does not need a complex phase-locked loop as two independent mode-locked lasers with slightly different repetition rates. The asynchronous pulse sequence generated in a single resonant cavity has inherent mutual coherence, so that additional phase locking is not needed, which is conducive to cost control and miniaturization. By using an electric control polarization controller or an electric control wave plate as a polarization element, the polarization state of the mode-locked pulse freely running in the resonant cavity is effectively controlled through on-line / off-line algorithm, which greatly improves the short-term and long-term stability of the free-running double-comb. In addition, the all-fiber structure of the present application generates a double-comb in a single resonant cavity, which replaces two independent single-comb light sources with a single double-comb light source, greatly simplifies the complexity of the system, reduces the cost, and is conducive to the modularization and instrumentation of the double-comb light source. At the same time, the present application does not need a large phase-locked system to lock the repetition frequency and the carrier envelope offset frequency, which reduces the number of auxiliary equipment and reduces the size of the light source system. In addition, the feedback of the distance measurement signal to the polarization element of the light source further improves the stability and reliability of the double-comb distance measurement, and enables long-term uninterrupted measurement.

[0075] In summary, the optical comb absolute distance measurement system according to the embodiment of the present application comprises a dual-comb generation module, a power amplification module, an asynchronous optical sampling module and a measurement control unit, wherein the dual-comb generation module is used to generate dual-wavelength dual-comb mode-locked pulses; the power amplification module is used to power amplify and narrow-band spectrum filter the dual-wavelength dual-comb mode-locked pulses and output them through a power amplification first output end and a power amplification second output end in two ways; the asynchronous optical sampling module is used to sample the dual-wavelength dual-comb mode-locked pulses after power amplification and narrow-band spectrum filtering to generate interference signals or cross-correlation signals with distance information; and the measurement control unit is used to calculate the distance measurement value carried in the interference signals or cross-correlation signals based on the repetition frequency of the local sampling pulses output by the power amplification second output end and correct the working state of the dual-comb generation module based on the interference signals or cross-correlation signals output by the output end of the asynchronous optical sampling module. Thus, the distance measurement system can feed back and adjust the polarization state of the electrically controlled polarization element in the dual-comb generation module in real time to adjust the generated mode-locked pulses in real time, ensure the correct and stable state of the dual-comb mode-locked pulses, speed up the start-up speed of the distance measurement system and improve the measurement accuracy.

[0076] The above detailed description does not constitute a limitation on the protection scope of the present application. Those skilled in the art should understand that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent replacement and improvement within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. An optical comb absolute distance measuring system characterized by, The application relates to a distance measurement system. The distance measurement system comprises: a dual-comb generation module for generating dual-wavelength dual-comb mode-locked pulses; a power amplification module comprising a power amplification input end, a power amplification first output end and a power amplification second output end, wherein the power amplification input end is connected with an output end of the dual-comb generation module, the dual-wavelength dual-comb mode-locked pulses are power amplified and narrow-band spectrum filtered, and the power amplified and narrow-band spectrum filtered dual-wavelength dual-comb mode-locked pulses are output through the power amplification first output end and the power amplification second output end; an asynchronous optical sampling module comprising a local sampling end and a to-be-measured sampling end, wherein the local sampling end is connected with the power amplification second output end, the to-be-measured sampling end is connected with the power amplification first output end, the power amplified and narrow-band spectrum filtered dual-wavelength dual-comb mode-locked pulses are sampled, and interference signals or cross-correlation signals with distance information are generated; 2. The optical comb absolute ranging system of claim 1, wherein, a measurement control unit connected with the dual-comb generation module, the power amplification second output end and an output end of the asynchronous optical sampling module respectively, used for calculating distance measurement values carried in the interference signals or the cross-correlation signals based on a repetition frequency of local sampling pulses output by the power amplification second output end, and used for correcting a working state of the dual-comb generation module based on the interference signals or the cross-correlation signals output by the output end of the asynchronous optical sampling module. The dual-comb generation module comprises:

3. The optical comb absolute ranging system of claim 2, wherein, a first pump source, a first wavelength division multiplexer, a first gain optical fiber, a first electrically-controlled polarization element, a polarization-independent isolator, a second electrically-controlled polarization element, a polarization-maintaining optical fiber and an output coupler connected in sequence, wherein an input end of the output coupler is connected with a signal injection end of the first wavelength division multiplexer, a first output end of the output coupler is connected with the polarization-maintaining optical fiber, and a second output end of the output coupler is connected with the power amplification input end. wherein, is the birefringence of the polarization maintaining optical fiber, is the length of the polarization maintaining optical fiber, is 1550 nm, is the filter bandwidth.

4. The optical comb absolute ranging system of claim 1, wherein, The second electrically-controlled polarization element and the polarization-maintaining optical fiber constitute a filter of the dual-comb generation module, and a filter bandwidth of the filter satisfies: The power amplification module comprises: a filter, a first power amplification assembly and a second power amplification assembly, wherein an input end of the filter is connected with an output end of the dual-comb generation module, output ends of the filter are connected with an input end of the first power amplification assembly and an input end of the second power amplification assembly respectively; 5. The optical comb absolute ranging system of claim 4, wherein, The first power amplification assembly and the second power amplification assembly each comprise a fiber Bragg grating, a second pump source, a polarization-independent isolator, a second wavelength division multiplexer, a second gain optical fiber and a three-port circulator connected in sequence, wherein a first end of the three-port circulator is connected with the second gain optical fiber, a second end of the three-port circulator is connected with the to-be-measured sampling end or the local sampling end, and a third end of the three-port circulator is connected with the fiber Bragg grating.

6. The optical comb absolute ranging system of claim 1, wherein, Parameters of the fiber Bragg gratings in the first power amplification assembly and the second power amplification assembly are the same. The asynchronous optical sampling module comprises a local sampling unit and a to-be-measured sampling unit. The local-oscillation sampling unit comprises a second collimator, a second half-wave plate, a third half-wave plate, a second polarization beam splitter, an avalanche diode, and an interference signal generation unit or a cross-correlation signal generation unit. The sampling unit to be measured comprises a first collimator, a first half-wave plate, a first quarter-wave plate, a first polarization beam splitter, a reference mirror, a second quarter-wave plate, and a target mirror. The first mode-locked pulse output by the power amplifier first output end passes through the first collimator and the first half-wave plate to be divided into a first polarization mode-locked pulse and a second polarization mode-locked pulse. The first polarization mode-locked pulse is reflected by the first polarization beam splitter to the first quarter-wave plate and then reaches the reference mirror. The second polarization mode-locked pulse is transmitted by the first polarization beam splitter and then reaches the target mirror after passing through the second quarter-wave plate. The first polarization mode-locked pulse reflected by the reference mirror and the second polarization mode-locked pulse reflected by the target mirror are combined by the first polarization beam splitter and then incident on the local-oscillation sampling unit after passing through the second half-wave plate. The second mode-locked pulse output by the power amplifier second output end passes through the second collimator, the third half-wave plate, and the second polarization beam splitter, is combined with the combined first and second polarization mode-locked pulses, and then reaches the avalanche diode after passing through the interference signal generation unit or the cross-correlation signal generation unit. The avalanche diode performs sampling.

7. The optical comb absolute ranging system of claim 6, wherein, The interference signal generation unit comprises a fourth quarter-wave plate and a third polarization beam splitter.

8. The optical comb absolute ranging system of claim 6, wherein, The cross-correlation signal generation unit comprises a focusing lens and a sum-frequency crystal.

9. The optical comb absolute ranging system of claim 2, wherein, The control end of the measurement control unit is connected with the first electrically-controlled polarization element and the second electrically-controlled polarization element, respectively. The input end of the measurement control unit is connected with the output end of the asynchronous optical sampling module. The measurement control unit generates a voltage signal based on the interference signal or the cross-correlation signal output by the asynchronous optical sampling module and outputs the voltage signal to the first electrically-controlled polarization element and the second electrically-controlled polarization element.

10. The optical comb absolute ranging system of claim 9, wherein, The first electrically-controlled polarization element and the second electrically-controlled polarization element are electrically-controlled polarization controllers or electrically-controlled wave plates. The electrically-controlled polarization controllers or the electrically-controlled wave plates each comprise eight voltage input pins, forming four direct-current voltage channel drives.

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