Femtosecond laser ranging device based on two-color asynchronous optical sampling

Abstract

The invention discloses a femtosecond laser distance measuring device based on double-color asynchronous optical sampling, and relates to the technical field of precise distance measuring devices. The device comprises two femtosecond laser frequency combs, a light path module, a non-linear frequency conversion module of a periodically poled non-linear optical crystal adopting 0-class phase matching, a narrow-band filtering module and a photoelectric detection and processing module. The device uses a pair of optical frequency comb lasers with different central wavelengths and small repetition frequency difference to obtain detection light and sampling light, and utilizes a nonlinear optical efficient double-color class 0 phase matching sum frequency effect of a nonlinear optical crystal to convert echo light pulses carrying distance information into a background-free sum frequency signal. And finally, high-sensitivity and high-precision absolute distance measurement without complex phase locking is realized, and the system has the advantages of simple structure, high signal-to-noise ratio, strong applicability and the like.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of precision ranging devices, and in particular to a femtosecond laser ranging device based on dual-color asynchronous optical sampling. BACKGROUND

[0002] Distance metrology is the cornerstone of modern scientific research and industrial manufacturing, with applications ranging from precision manufacturing, semiconductor lithography to geodesy and satellite navigation. In recent years, with the development of laser technology, optical frequency comb (OFC) as a revolutionary tool has been introduced into distance measurement, achieving unprecedented accuracy. Among the many optical frequency comb ranging schemes, dual-comb ranging technology has attracted much attention due to its ability to simultaneously achieve high accuracy, large unambiguous distance and fast data acquisition time. Traditional dual-comb ranging systems rely on strict phase coherence between the two optical frequency combs. Specifically, the repetition frequencies (f r ) and carrier envelope offset frequencies (f ceo) all need to be precisely locked, and the time information of the optical signal is converted to the radio frequency domain for processing by coherent interference, so as to achieve nanometer-level ranging accuracy. However, the realization and maintenance of such strict phase coherence require complex and expensive phase-locked electronic systems, which greatly limits its application in industrial environments and other practical applications. In order to simplify the system and reduce the stringent requirements for phase coherence, researchers have proposed a Time-of-Flight (ToF) measurement method based on nonlinear Asynchronous Optical Sampling (ASOPS), which has shown great potential in applications requiring only micron-level accuracy. For example, some technical solutions use Type-II phase-matched nonlinear crystals (such as PPKTP, BBO) for cross-correlation measurement, but this requires two lasers with orthogonal polarization states, which has special requirements for optical path design [Prior Art 1: Zhang H, Wei H, Wu X, et al. Absolute distance measurement by dual-comb nonlinear asynchronous optical sampling [J]. Optics Express, 2014, 22(6): 6597-604. DOI: 10.1364 / OE.22.006597.][Prior Art 2: Application No. CN202111331708.1, entitled: An optical frequency comb detection system]. Another solution utilizes the Two-Photon Absorption (TPA) effect in semiconductors, eliminating the need for nonlinear crystals and further simplifying the system structure [Prior Art 3: Hollie W, Jinghua S, David M, et al. Two-photon dual-comb LiDAR [J]. Optics Express, 2021, 29(23): 37037-37047.]. However, due to linear interference between the two optical frequency combs, there is a constant background noise in the measurement signal, which reduces the signal-to-noise ratio, especially when the return light signal is very weak, which can severely affect the measurement accuracy. SUMMARY

[0003] The technical problem to be solved by the present application is how to provide a femtosecond laser ranging device that can avoid complex phase locking, overcome background noise interference, and ensure high conversion efficiency and high sensitivity.

[0004] To solve the above technical problems, the technical scheme adopted by the present application is: a femtosecond laser ranging device based on dual-color asynchronous optical sampling, comprising: a first optical frequency comb and a second optical frequency comb, the detection light emitted by the first optical frequency comb is divided into two paths after passing through a first optical fiber beam splitter, one of the two paths enters an optical fiber ring after collimation by a first collimator and irradiates onto a target to be measured, the echo light reflected by the target to be measured passes through the first collimator and the optical fiber ring again to form a measurement light pulse carrying time-of-flight information and is guided to one input end of a first optical fiber coupler, the other path of light divided from the first optical fiber beam splitter directly enters one input end of a second optical fiber coupler as a reference light pulse;

[0005] The local sampling light emitted by the second optical frequency comb is also divided into two paths after passing through a second optical fiber beam splitter and is sent to the other input end of the first optical fiber coupler and the other input end of the second optical fiber coupler respectively; in the first optical fiber coupler, the measurement light pulse carrying the time-of-flight information and the local sampling light pulse are combined, and in the second optical fiber coupler, the reference light pulse and the local sampling light pulse are combined;

[0006] The mixed light output from the first optical fiber coupler focuses into a first nonlinear optical crystal after passing through a second collimator and a first focusing lens in sequence; the mixed light output from the second optical fiber coupler focuses into a second nonlinear optical crystal after passing through a third collimator and a second focusing lens in sequence;

[0007] In the first nonlinear optical crystal and the second nonlinear optical crystal, the light pulse and the sampling light pulse undergo a dual-color 0-type phase matching and frequency generation process to generate a sum frequency signal, the generated sum frequency signal is filtered to remove residual fundamental frequency light through a first band-pass filter and a second band-pass filter respectively, and the sum frequency light signals of the two channels are received by a first photodetector and a second photodetector respectively and are converted into electrical signals, which are input into a digitizing instrument for processing and calculation.

[0008] The beneficial effects produced by the above technical scheme are: (1) The device uses a nonlinear optical crystal and utilizes its highest nonlinear coefficient d_33 for 0-type phase matching, greatly improving the sum frequency conversion efficiency. This enables the device to achieve high-precision measurement when the echo light power is only in the order of microwatts (μW), with extremely high sensitivity.

[0009] (2) By placing a narrow-band filter in front of the photodetector that only allows the sum frequency signal (such as 780nm) to pass through, the signals of the two fundamental frequency lasers (such as 1540nm and 1580nm) and their linear interference terms are completely filtered out, fundamentally eliminating background noise and significantly improving the signal-to-noise ratio.

[0010] (3) The present application does not need to strictly phase-coherently lock two optical frequency comb lasers, only needs to maintain the stability of the repetition frequency difference, greatly reduces the complexity and cost of the system.

[0011] (4) Combined with the advantages of asynchronous optical sampling, the ranging accuracy of microns (μm) can be achieved within a collection time of milliseconds (ms), and the dynamic tracking capability for moving targets can be achieved. BRIEF DESCRIPTION OF DRAWINGS

[0012] The present application will be further described below in conjunction with the drawings and specific embodiments.

[0013] Figure 1 is a principle block diagram of the device described in embodiment one of the present application;

[0014] Figure 2 is an asynchronous optical sampling waveform diagram in the device described in embodiment one of the present application;

[0015] Figure 3 is a principle block diagram of the light source of the device described in embodiment one of the present application;

[0016] Figure 4 is a principle block diagram of the device described in embodiment two of the present application;

[0017] Figure 5 is an asynchronous optical sampling waveform diagram in the device described in embodiment two of the present application;

[0018] Figure 6 is a principle block diagram of the light source of the device described in embodiment two of the present application; wherein: 1, first signal optical frequency comb; 2, first signal optical frequency comb; 3, first optical fiber beam splitter; 4, second optical fiber beam splitter; 5, optical fiber ring; 6, first collimator; 7, first optical fiber coupler; 8, second optical fiber coupler; 9, second collimator; 10, third collimator; 11, first focusing lens; 12, second focusing lens; 13, first nonlinear optical crystal; 14, second nonlinear optical crystal; 15, first narrowband filter; 16, second narrowband filter; 17, first photodetector; 18, second photodetector; 19, digitizer; 20, repetition frequency phase-locked loop; 21, target to be measured. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be described clearly and completely in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0020] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0021] Example 1

[0022] like Figure 1 As shown, this embodiment of the invention discloses a femtosecond laser ranging device based on dual-color asynchronous optical sampling, including a first signal optical frequency comb 1, a first signal optical frequency comb 2, and an optical path module consisting of a first fiber beam splitter 3, a second fiber beam splitter 4, a fiber circulator 5, a first collimator 6, a first fiber coupler 7, a second fiber coupler 8, a second collimator 9 and a third collimator 10, a first focusing lens 11 and a second focusing lens 12, a first nonlinear optical crystal 13 and a second nonlinear optical crystal 14, a first narrowband filter 15 and a second narrowband filter 16, etc., and a signal conversion and processing module consisting of a first photodetector 17 and a second photodetector 18, a digitizing instrument 19, a repetition frequency phase-locked loop 20, etc.

[0023] In a specific embodiment of the present invention, the first optical frequency comb 1 is used as a signal optical frequency comb, which is a mode-locked erbium fiber laser, and its output probe light pulse sequence λ1 is 1540nm with a repetition frequency of f. r1 The second optical frequency comb 2 is used as a local optical frequency comb. It is also a mode-locked erbium fiber laser, and its output local sampled optical pulse sequence λ2 is 1580 nm with a repetition frequency of f. r2 The center wavelengths of the two optical frequency combs are adjusted by using an intracavity filter, ensuring a difference in center wavelength between the output light from the two frequency combs greater than 30 nm. The repetition rate difference between the two lasers is on the order of MHz, with a repetition rate difference Δf. r =f r2 -f r1 Locked by a repetition frequency phase-locked loop 20, it remains on the order of kHz. This tiny repetition rate difference is the basis for asynchronous optical sampling, which amplifies the minute time delay in the optical domain to the radio frequency domain for precise measurement through the time stretching effect.

[0024] The working principle of the device is as follows: the probe light emitted by the first optical frequency comb 1 is split into two paths after passing through the first optical fiber beam splitter 3, and enters the measurement optical path and the reference optical path respectively.

[0025] (1) Measurement light path: one of the lights enters the fiber-optic circulator 5, collimates through the first collimator 6, and irradiates on the target 21 to be measured. The echo light reflected from the target passes through the first collimator 6 and the fiber-optic circulator 5 again, forms a measurement light pulse carrying time-of-flight information, and is guided to one input end of the first fiber-optic coupler 7;

[0026] (2) Reference light path: the other light from the first fiber-optic beam splitter 3 directly enters one input end of the fiber-optic coupler 8 as a reference light pulse.

[0027] Meanwhile, the local sampling light emitted by the second optical frequency comb 2 is also divided into two paths through the second fiber-optic beam splitter 4, and is sent to the other input end of the first fiber-optic coupler 7 and the other input end of the second fiber-optic coupler 8, respectively.

[0028] In the first fiber-optic coupler 7, the measurement light pulse carrying time-of-flight information is combined with the local sampling light pulse. In the second fiber-optic coupler 8, the reference light pulse is combined with the local sampling light pulse. In this way, two independent signal processing channels are formed: a measurement channel and a reference channel.

[0029] (1) Measurement channel: the mixed light output from the first fiber-optic coupler 7 passes through the second collimator 9 and the first focusing lens 11 in turn, and is focused into the first nonlinear optical crystal 14;

[0030] (2) Reference channel: the mixed light output from the second fiber-optic coupler 8 passes through the third collimator 10 and the second focusing lens 12 in turn, and is focused into the second nonlinear optical crystal 14.

[0031] The nonlinear optical crystal includes but is not limited to a periodically poled lithium niobate crystal, a PPLN bulk crystal, a waveguide, and other nonlinear crystals available for type-0 phase matching, etc.

[0032] In the first nonlinear optical crystal 13 and the second nonlinear optical crystal 14, the 1540 nm light pulse and the 1580 nm sampling light pulse undergo an efficient bichromatic type-0 phase matching and sum frequency generation (SFG) process, generating a sum frequency signal with a wavelength of about 780 nm. The generated sum frequency signal is filtered by the first band-pass filter 15 and the second band-pass filter 16 to remove the residual fundamental frequency light. This design physically eliminates background noise, so that the first photodetector 17 and the second photodetector 18 can only receive the effective sum frequency signal in an extremely low background, thereby realizing high signal-to-noise ratio detection. The photodetector can use a silicon-based avalanche photodiode (Si-APD) or a superconducting nanowire single-photon detector (SNSPD).

[0033] Finally, the sum frequency light signals of the two channels are received by the first photodetector 17 and the second photodetector 18 respectively and converted into electrical signals, which are input into a digitizing instrument 19. The digitizing instrument 19 records the signal waveforms of the two channels and accurately calculates the absolute distance of the target to be measured by measuring the time delay between the reference channel pulse and the measurement channel pulse.

[0034] Figure 2 The schematic diagram is a two-color asynchronous optical sampling. The pulses of the first optical frequency comb 1 and the second optical frequency comb 2 overlap in the time domain, thereby generating sum frequency pulses.

[0035] The two optical frequency combs have a repetition frequency difference of the order of kHz. This repetition frequency difference causes the local pulses (the second optical frequency comb 2) to gradually scan the signal pulses, effectively amplifying the femtosecond time scale interval t d The optical scanning in the time domain is performed in steps of Δt and is represented as:

[0036]

[0037] The two pulses are scanned in fixed steps of Δt. After N = f r / Δf r steps, a complete down-sampling cycle is completed, and the pulses overlap again. Therefore, the generated sum frequency signal has a period of T update = 1 / Δf r . The original time interval t d is amplified to T d = N·t d . The two-color asynchronous optical sampling (ASOPS) pulse signals from the reference light path and the measurement light path are detected by two silicon photodetectors, respectively, and then input into a digitizing instrument or a signal acquisition device. The time at which the centroid of the sum frequency signal obtained by the photodetector is fitted, and then the time of flight and the absolute distance are calculated.

[0038] The distance L is calculated according to the following formula (with a repetition frequency phase-locked loop 20):

[0039]

[0040] In the formula, n g represents the group refractive index of air, and m is an integer used to calculate the distance ambiguity, which can be obtained by other distance measuring devices.

[0041] Embodiment Two

[0042] As Figure 4As shown, the difference between the second embodiment of the application and the first embodiment is that the second embodiment discloses a femtosecond laser ranging device based on dual-color asynchronous optical sampling, which does not have a repetition frequency phase-locked loop 20, and other devices are the same as those of the device described in the first embodiment. In this embodiment, the distance L is calculated according to the following formula:

[0043]

[0044] T is the time of the signal light pulse detected by the probe, tar T is the time of the signal light pulse detected by the probe, ref1 T is the time of the first reference light pulse, ref2 T is the time of the immediately adjacent reference light pulse. Wherein, Figure 5 is the asynchronous optical sampling waveform diagram of the device described in the second embodiment of the application; Figure 6 is the optical source principle diagram of the device described in the second embodiment of the application.

[0045] In summary, the device described in the present application uses a pair of optical frequency comb lasers with different center wavelengths and a small repetition frequency difference to obtain detection light and sampling light, utilizes the high-efficiency dual-color 0-type phase matching and frequency doubling effect of a nonlinear optical crystal to convert the echo light pulse carrying distance information into a background-free sum frequency signal, and finally realizes high-sensitivity and high-precision absolute distance measurement without complex phase locking, has the advantages of simple system structure, high signal-to-noise ratio, strong applicability, etc.

Claims

1. A femtosecond laser ranging device based on two-color asynchronous optical sampling, characterized in that The application relates to a kind of optical frequency comb absolute distance measurement systems, comprising: First optical frequency comb (1) and second optical frequency comb (2), the probe light emitted by first optical frequency comb (1) is divided into two ways after passing through first optical fiber beam splitter (3), one of the ways of light enters optical fiber circulator (5) and is collimated by first collimator (6) after collimating and irradiates to the target to be measured (21), the echo light reflected by the target to be measured (21) passes through first collimator (6) and optical fiber circulator (5) again, forms the measuring light pulse carrying time-of-flight information, and is guided to one input end of first optical fiber coupler (7), the other way of light divided from first optical fiber beam splitter (3) directly enters one input end of second optical fiber coupler (8) as reference light pulse; Second optical frequency comb (2) emits local sampling light, which is also divided into two ways after passing through second optical fiber beam splitter (4) and is sent to the other input end of first optical fiber coupler (7) and the other input end of second optical fiber coupler (8) respectively;In first optical fiber coupler (7), the measuring light pulse carrying time-of-flight information is combined with the local sampling light pulse, and in second optical fiber coupler (8), the reference light pulse is combined with the local sampling light pulse; The mixed light output from first optical fiber coupler (7) passes through second collimator (9) and first focusing lens (11) in turn and is focused into first nonlinear optical crystal (13);The mixed light output from second optical fiber coupler (8) passes through third collimator (10) and second focusing lens (12) in turn and is focused into second nonlinear optical crystal (14); In first nonlinear optical crystal (13) and second nonlinear optical crystal (14), the light pulse and the sampling light pulse generate sum frequency signal through bichromatic 0-type phase matching and frequency mixing process, and the generated sum frequency signal is filtered to remove residual fundamental frequency light through first band-pass filter (15) and second band-pass filter (16) respectively, and the sum frequency light signals of the two channels are received by first photodetector (17) and second photodetector (18) and converted into electrical signals, which are input into digitizing instrument (19) for processing and calculation.

2. The dual-color asynchronous optical sampling based femtosecond laser range finder of claim 1, wherein: The first optical frequency comb (1) is used to act as a signal optical signal light frequency comb, which is a mode-locked erbium fiber laser, whose output probe light pulse sequence has a center wavelength of 1540 nm and a repetition frequency of f r1 .

3. The dual-color asynchronous optical sampling based femtosecond laser range finder of claim 2, wherein: The second optical frequency comb (2) is used as a local optical frequency comb for a mode-locked erbium fiber laser, whose output local sampling optical pulse sequence has a center wavelength of 1580 nm and a repetition frequency of f r2 .

4. The dual-color asynchronous optical sampling based femtosecond laser range finder of claim 3, wherein: The difference in repetition rate of the first optical frequency comb (1) and the second optical frequency comb (2) is: Δf r = f r2 - f r1 is locked by a repetition rate phase-locked loop (20).

5. The dual-color asynchronous optical sampling based femtosecond laser range finder of claim 1, wherein: In first nonlinear optical crystal (13) and second nonlinear optical crystal (14), the light pulse of 1540 nm and the sampling light pulse of 1580 nm generate sum frequency signal of 780 nm through bichromatic 0-type phase matching and frequency mixing process, and the generated sum frequency signal is filtered to remove residual fundamental frequency light through first band-pass filter (15) and second band-pass filter (16) respectively.

6. The dual-color asynchronous optical sampling based femtosecond laser range finder of claim 1, wherein: The digitizing instrument (19) records the signal waveforms of the measurement channel and the reference channel, and calculates the absolute distance of the target to be measured (21) by measuring the time delay between the reference channel pulse and the measurement channel pulse.

7. The dual-color asynchronous optical sampling based femtosecond laser range finder of claim 1, wherein: The absolute distance L of the target to be measured (19) is calculated by the following steps: The first optical frequency comb (1) and the second optical frequency comb (2) have a repetition frequency difference in the order of kHz, which difference in repetition frequency causes the second optical frequency comb (2) to gradually scan the signal pulse, amplifying the femtosecond time scale interval t d The optical scan in time domain is performed with a step of At and is represented as: Two pulses are scanned with a fixed step of Δt, and N = f r / Δf r steps later, a complete down-sampling cycle is completed, at this time the pulses overlap again, therefore, the generated sum frequency signal has a period of T update = 1 / Δf r ; the original time interval t d is amplified to T d = N·t d , the double optical comb asynchronous optical sampling pulse signals from the reference light path and the measurement light path are detected by the first photodetector (17) and the second photodetector (18) respectively, and then input into the digitizing instrument (19), the time of the centroid of the sum frequency signal obtained by the fitting detector is fitted, and then the time of flight and the absolute distance are calculated; The absolute distance L is calculated according to formula (2) or formula (3): where n g denotes the group refractive index of air, m is an integer used to calculate distance ambiguity, obtained by ranging equipment; T tar is the time of the signal light pulse detected by the detector, T ref1 is the time of the first reference light pulse, T ref2 is the time of the immediately preceding reference light pulse.

8. The dual-color asynchronous optical sampling based femtosecond laser range finder of claim 1, wherein: First photodetector (17) and second photodetector (18) are silicon-based avalanche photodiode Si-APD or superconducting nanowire single-photon detector SNSPD.

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