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

Abstract

The application discloses a kind of femtosecond laser ranging devices based on two-color asynchronous optical sampling, it is related to precise ranging device technical field.The device includes two femtosecond laser frequency comb, optical path module, the nonlinear frequency conversion module of periodic polarization nonlinear optical crystal using 0 type phase matching, narrowband filtering module and photoelectric detection and processing module.The device uses a pair of light frequency comb laser with different center wavelength and small repetition frequency difference to obtain probe light and sampling light, utilizes the nonlinear optical crystal nonlinear optical efficient two-color 0 type phase matching and frequency effect, converts the echo light pulse carrying distance information into background-free sum frequency signal, finally realizes high sensitivity, high precision absolute distance measurement without complex phase-locked, with the advantages of simple system structure, high signal-to-noise ratio, strong applicability etc..

Description

Technical Field

[0001] This invention relates to the field of precision ranging devices, and in particular to a femtosecond laser ranging device based on dual-color asynchronous optical sampling. Background Technology

[0002] Distance metrology is a cornerstone of modern scientific research and industrial manufacturing, with applications spanning numerous fields from precision manufacturing and semiconductor lithography to geodesy and satellite navigation. In recent years, with the development of laser technology, optical frequency combs (OFCs) have been introduced into distance measurement as a revolutionary tool, achieving unprecedented accuracy. Among the many optical frequency comb ranging schemes, dual-comb ranging technology has attracted considerable 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 frequency (f) of the two lasers... r ) and carrier envelope offset frequency (f ceoBoth phase coherence and time-of-flight (ToF) measurement require precise locking. Coherent interference is used to convert the time information of the optical signal into the radio frequency domain for processing, achieving nanometer-level ranging accuracy. However, achieving and maintaining such strict phase coherence requires complex and expensive phase-locked loop (PLL) electronic systems, which significantly limits its application in practical environments such as industrial settings. 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 shows great potential in applications requiring only micrometer-level accuracy. For example, some technical solutions use Type-II phase-matched nonlinear crystals (such as PPKTP and BBO) for cross-correlation measurement, but this requires the two lasers to have orthogonal polarization states, which places special requirements on the 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, title: An optical frequency comb detection system]. Another approach utilizes the two-photon absorption (TPA) effect in semiconductors, eliminating the need for a nonlinear crystal 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 the linear interference between the two optical frequency combs, this approach introduces a constant background noise in the measurement signal, which reduces the signal-to-noise ratio, especially when the echo signal is very weak, severely affecting measurement accuracy. Summary of the Invention

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

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention 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 probe light emitted by the first optical frequency comb is split into two paths after passing through a first fiber beam splitter. One path of light enters a fiber circulator and is collimated by a first collimator before illuminating the target to be measured. The echo light reflected by the target to be measured passes through the first collimator and the fiber circulator again to form a measurement light pulse carrying time-of-flight information, and is guided to one input end of a first fiber coupler. The other path of light split from the first fiber beam splitter directly enters one input end of a second fiber coupler as a reference light pulse.

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

[0006] The mixed light output from the first fiber coupler passes through the second collimator and the first focusing lens in sequence before being focused into the first nonlinear optical crystal; the mixed light output from the second fiber coupler passes through the third collimator and the second focusing lens in sequence before being focused into the second nonlinear optical crystal.

[0007] In the first and second nonlinear optical crystals, the light pulse and the sampled light pulse undergo a two-color type 0 phase matching and frequency matching process to generate a sum-frequency signal. The generated sum-frequency signal is filtered by the first and second bandpass filters to remove residual fundamental frequency light. The sum-frequency light signals of the two channels are received by the first and second photodetectors and converted into electrical signals, which are then input into a digital instrument for processing and calculation.

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

[0009] (2) By placing a narrowband filter in front of the photodetector that only allows sum-frequency signals (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, thus eliminating background noise and significantly improving the signal-to-noise ratio.

[0010] (3) The present invention does not require strict phase coherence locking of the two optical frequency comb lasers, but only needs to maintain the stability of their repetition frequency difference, which greatly reduces the complexity and cost of the system.

[0011] (4) Combining the advantages of asynchronous optical sampling, it can achieve micrometer (μm) level ranging accuracy in a short acquisition time of milliseconds (ms) and has the ability to dynamically track moving targets. Attached Figure Description

[0012] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0013] Figure 1 This is a schematic block diagram of the device described in Embodiment 1 of the present invention;

[0014] Figure 2 This is a waveform diagram of asynchronous optical sampling in the device described in Embodiment 1 of the present invention;

[0015] Figure 3 This is a block diagram illustrating the light source principle of the device described in Embodiment 1 of the present invention;

[0016] Figure 4 This is a schematic block diagram of the device described in Embodiment 2 of the present invention;

[0017] Figure 5 This is an asynchronous optical sampling waveform diagram in the device described in Embodiment 2 of the present invention;

[0018] Figure 6 This is a block diagram of the light source principle of the device described in Embodiment 2 of the present invention; wherein: 1, first signal optical frequency comb; 2, first signal optical frequency comb; 3, first fiber beam splitter; 4, second fiber beam splitter; 5, fiber circulator; 6, first collimator; 7, first fiber coupler; 8, second 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, digital instrument; 20, repetition frequency phase-locked loop; 21, target under test. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[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 fiber beam splitter 3, and enters the measurement optical path and the reference optical path respectively.

[0025] (1) Measurement optical path: One optical path enters the fiber optic circulator 5, is collimated by the first collimator 6, and then illuminates the target 21. The echo light reflected from the target passes through the first collimator 6 and the fiber optic circulator 5 again to form a measurement optical pulse carrying time-of-flight information, and is guided to one input end of the first fiber optic coupler 7;

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

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

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

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

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

[0031] The nonlinear optical crystals include, but are not limited to, periodically polarized lithium niobate crystals, PPLN bulk crystals, waveguides, and other nonlinear crystals that can be used for zero-type phase matching.

[0032] In the first nonlinear optical crystal 13 and the second nonlinear optical crystal 14, a 1540nm light pulse and a 1580nm sampling light pulse undergo an efficient two-color class 0 phase-matched sum-frequency (SFG) process, generating a sum-frequency signal with a wavelength of approximately 780nm. The generated sum-frequency signal is then filtered by a first bandpass filter 15 and a second bandpass filter 16 to remove residual fundamental frequency light. This design physically eliminates background noise, enabling the subsequent first photodetector 17 and second photodetector 18 to receive only the effective sum-frequency signal against an extremely low background, thereby achieving high signal-to-noise ratio detection. The photodetectors can be silicon-based avalanche photodiodes (Si-APDs) or superconducting nanowire single-photon detectors (SNSPDs).

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

[0034] Figure 2 The diagram shows the principle of 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 a sum-frequency pulse.

[0035] The two optical frequency combs have a repetition frequency difference on the order of kHz. This difference in repetition frequency causes the local pulse (second optical frequency comb 2) to gradually scan the signal pulse, effectively amplifying the femtosecond timescale interval t. d The optical scan in the time domain is performed with a step size of Δt, and is expressed as:

[0036]

[0037] The two pulses scan with a fixed step size of Δt, at N = f r / Δf r After one step, a complete downsampling cycle is completed, at which point the pulses overlap again. Therefore, the resulting sum-frequency signal has T0. update =1 / Δf r The period. The original time interval t d Enlarged to T d =N·t d The asynchronous optical sampling (ASOPS) pulse signals from the reference optical path and the measurement optical path are detected by two silicon photodetectors, and then input into a digitizer or signal acquisition device. The time of the centroid of the sum-frequency signal obtained by the detector is fitted, and then the flight time and absolute distance are calculated.

[0038] The absolute distance L is calculated using the following formula (for a repetition frequency PLL 20):

[0039]

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

[0041] Example 2

[0042] like Figure 4As shown, the difference between Embodiment 2 and Embodiment 1 is that the femtosecond laser ranging device based on dual-color asynchronous optical sampling disclosed in Embodiment 2 does not have a repetition frequency phase-locked loop 20, while other components are the same as those in Embodiment 1. In this embodiment, the distance L is calculated as an absolute distance according to the following formula:

[0043]

[0044] Within a ranging time window following asynchronous optical sampling, T tar T is the time of the signal light pulse detected by the detector. ref1 T is the time of the first reference light pulse. ref2 This is the time of the immediately preceding reference optical pulse. Wherein, Figure 5 This is an asynchronous optical sampling waveform diagram in the device described in Embodiment 2 of the present invention; Figure 6 This is a block diagram illustrating the light source principle of the device described in Embodiment 2 of the present invention.

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

Claims

1. A femtosecond laser ranging device based on dual-color asynchronous optical sampling, characterized in that... include: The first optical frequency comb (1) and the second optical frequency comb (2) are used to split the probe light emitted by the first optical frequency comb (1) into two paths after passing through the first fiber beam splitter (3). One path of light enters the fiber circulator (5) and is collimated by the first collimator (6) before illuminating the target (21) under test. The echo light reflected by the target (21) passes through the first collimator (6) and the fiber circulator (5) again to form a measurement light pulse carrying time-of-flight information, and is guided to one input end of the first fiber coupler (7). The other path of light split from the first fiber beam splitter (3) directly enters one input end of the second fiber coupler (8) as a reference light pulse. The local sampling light emitted by the second optical frequency comb (2) is also split into two paths after passing through the second fiber beam splitter (4), and is sent to the other input end of the first fiber coupler (7) and the other input end of the second fiber coupler (8), respectively. In the first fiber coupler (7), the measurement light pulse carrying the time of flight information is combined with the local sampling light pulse, and in the second fiber coupler (8), the reference light pulse is combined with the local sampling light pulse. The mixed light output from the first fiber coupler (7) passes through the second collimator (9) and the first focusing lens (11) in sequence and is then focused into the first nonlinear optical crystal (13); the mixed light output from the second fiber coupler (8) passes through the third collimator (10) and the second focusing lens (12) in sequence and is then focused into the second nonlinear optical crystal (14). In the first nonlinear optical crystal (13) and the second nonlinear optical crystal (14), a 1540 nm light pulse and a 1580 nm sampling light pulse undergo a two-color class 0 phase matching and frequency matching process to generate a frequency signal with a wavelength of 780 nm. The generated frequency signal is filtered by the first bandpass filter (15) and the second bandpass filter (16) to remove residual fundamental frequency light. The frequency light signals of the two channels are received by the first photodetector (17) and the second photodetector (18) and converted into electrical signals, which are then input into the digital instrument (19) for processing and calculation.

2. The femtosecond laser ranging device based on dual-color asynchronous optical sampling as described in claim 1, characterized in that: The first optical frequency comb (1) serves as the optical signal frequency comb and is a mode-locked erbium fiber laser. Its output probe light pulse sequence has a center wavelength of 1540 nm and a repetition frequency of [missing value]. .

3. The femtosecond laser ranging device based on dual-color asynchronous optical sampling as described in claim 2, characterized in that: The second optical frequency comb (2) is used as a local optical frequency comb. It is a mode-locked erbium fiber laser whose output local sampled optical pulse sequence has a center wavelength of 1580 nm and a repetition frequency of . .

4. The femtosecond laser ranging device based on dual-color asynchronous optical sampling as described in claim 3, characterized in that: The repetition frequency difference between the first optical frequency comb (1) and the second optical frequency comb (2) is: It is locked by a repetitive frequency phase-locked loop (20).

5. The femtosecond laser ranging device based on dual-color asynchronous optical sampling as described in claim 1, characterized in that: The digital instrument (19) records the signal waveforms of the measurement channel and the reference channel, and calculates the absolute distance of the target (21) to be measured by measuring the time delay between the pulse of the reference channel and the pulse of the measurement channel.

6. The femtosecond laser ranging device based on dual-color asynchronous optical sampling as described in claim 1, characterized in that: The absolute distance L of the target (21) to be measured is calculated through the following steps: The first optical frequency comb (1) and the second optical frequency comb (2) have a repetition frequency difference on the order of kHz. This difference in repetition frequency causes the second optical frequency comb (2) to gradually scan the signal pulse, amplifying the femtosecond timescale interval t. d The optical scan in the time domain is performed with a step size of Δt, and is expressed as: （1） The two pulses scan with a fixed step size of Δt. After one step, a complete downsampling cycle is completed, at which point the pulses overlap again. Therefore, the resulting sum-frequency signal has... The period; the original time interval t d Enlarged to The asynchronous optical sampling pulse signals from the reference optical path and the measurement optical path are detected by the first photodetector (17) and the second photodetector (18), respectively, and then input into the digital instrument (19) to fit the time of the centroid of the sum-frequency signal obtained by the detector, and then calculate the flight time and absolute distance. The absolute distance L is calculated using formula (2) or formula (3): （2） （3） In the formula, n g The group refractive index of air is represented, and m is an integer used to calculate distance ambiguity, obtained through the ranging device; within a ranging time window after asynchronous optical sampling, The duration of the signal light pulse detected by the detector. For the time of the first reference light pulse, The time of the immediately preceding reference optical pulse.

7. The femtosecond laser ranging device based on dual-color asynchronous optical sampling as described in claim 1, characterized in that: The first photodetector (17) and the second photodetector (18) are silicon-based avalanche photodiodes (Si-APD) or superconducting nanowire single-photon detectors (SNSPD).

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