Dual comb absolute distance measurement system based on raman gain modulated single frequency laser

Abstract

This invention discloses a dual-comb absolute distance measurement system based on Raman gain modulation of a single-frequency laser. The system includes an optical frequency comb, a single-frequency laser, a rare-earth-doped amplifier, an optical coupler, a wavelength division multiplexer, a Raman gain fiber, an optical circulator, an optical collimator, a target, an optical bandpass filter, and a photodetector. The single-frequency laser is split into two paths, each coupled to an amplified optical frequency comb and then into the Raman gain fiber, generating two highly coherent Raman gain modulated dual combs: a signal comb and a local oscillator comb. The signal comb is divided into two groups; one group is reflected from the target and carries distance information, coupled to the other group and then to the local oscillator comb. After aliasing is eliminated by an optical bandwidth filter, the signal is collected by the photodetector, and the phase is de-encapsulated to calculate the distance. Due to the Raman gain modulation process, simple optical longitudinal mode stabilization and carrier wavelength readout can be achieved, ultimately using interferometry to achieve nanometer-level ranging accuracy.

Description

Technical Field

[0001] This invention relates to the fields of optical ranging technology, ultrafast lasers, optical frequency combs, and precision measurement, specifically to a dual-comb absolute distance measurement system based on Raman gain modulated single-frequency laser. Background Technology

[0002] Dual-comb ranging technology combines the large unambiguous distance and rapid measurement capability of time-of-flight method with the nanometer-level ranging accuracy of interferometric ranging, and is expected to be widely used in satellite formation flight, high-precision industrial production, micro-nano fabrication and inspection and other fields.

[0003] Achieving interferometric dual-comb ranging requires stable repetition frequencies of both combs and extremely low carrier wavelength jitter, as well as precise reading and calibration of the optical carrier wavelength. Traditional dual-comb ranging methods typically employ complex multi-channel electrical feedback systems to simultaneously lock the repetition frequencies of the two optical combs and the carrier envelope offset frequency onto a single ultra-stable optical reference cavity in order to achieve the accuracy of interferometric ranging. This locking scheme makes the system extremely complex and costly, and highly sensitive to environmental disturbances (such as temperature changes and vibrations), resulting in poor stability. Furthermore, minute jitter in the optical carrier wavelength directly translates into ranging error, and real-time monitoring and effective compensation for this wavelength jitter remains a challenge in this field.

[0004] Furthermore, existing technologies, such as Chinese patent application CN121453191A, disclose a scanning high-resolution dual-comb spectral system based on Raman gain modulation. This system utilizes Raman gain modulation to generate highly coherent dual optical frequency combs, but its system structure is specifically designed for spectral detection, with a unidirectional optical path and lacking a reference arm and a separate ranging optical path for transmitting and receiving. Therefore, this system cannot be directly applied to scenarios requiring absolute distance measurement of targets.

[0005] Therefore, traditional methods and existing dual-comb systems still have significant limitations in terms of system simplicity, robustness, and carrier wavelength control for absolute distance measurement, which restricts the application of dual-comb ranging technology in complex environments such as industrial sites. Summary of the Invention

[0006] To address the challenges of optical carrier wavelength jitter and extraction in dual-comb ranging technology, this invention provides an efficient method for interferometric dual-comb ranging. This invention transforms a single-frequency laser into a dual-comb laser by simultaneously applying two Raman gain modulations, directly inheriting the high mutual coherence, low phase noise, and carrier wavelength of the single-frequency laser.

[0007] The solution of the present invention is as follows: An absolute distance measurement system based on a single-frequency laser with Raman gain modulation using dual optical combs includes a first optical frequency comb, a second optical frequency comb, a single-frequency laser, a first rare-earth-doped amplifier, a second rare-earth-doped amplifier, a first optical coupler, a second optical coupler, a third optical coupler, a fourth optical coupler, a first wavelength division multiplexer, a second wavelength division multiplexer, a third wavelength division multiplexer, a fourth wavelength division multiplexer, a first Raman gain fiber, a second Raman gain fiber, an optical circulator, an optical collimator, a target under test, an optical bandpass filter, and a photodetector. The first and second optical frequency combs generate pulsed lasers with different repetition frequencies and no coherence. These pulses are amplified into first ultrafast pulses and second ultrafast pulses by a first rare-earth-doped amplifier and a second rare-earth-doped fiber amplifier, respectively. A single-frequency laser generates continuous laser light, which is split into two paths by a first optical coupler. These paths are then coupled to the first and second ultrafast pulses via a first wavelength division multiplexer and a third wavelength division multiplexer, respectively, and fed into the first and second Raman gain fibers for Raman gain modulation. The two highly coherent continuous laser paths output by the single-frequency laser are transformed into highly coherent ultrafast pulses, and the frequency domain is converted into highly coherent dual optical frequency combs. The pulses are then processed by a second wavelength division multiplexer. After the residual pump pulses are removed by the fourth wavelength division multiplexer, they are denoted as signal optical comb and local oscillator optical comb, respectively. The signal optical comb is split into probe light and reference light by the second optical coupler. The probe light is input from the first port of the optical circulator, output from the second port, and illuminates the target under test through the optical collimator. The target under test reflects the probe light and re-enters the optical collimator, carrying distance information. The reflected light is input from the second port of the optical circulator, output from the third port, and combined with the reference light through the third optical coupler. It is then combined with the local oscillator optical comb through the fourth optical coupler to achieve dual-comb ranging. After beam combining, the beam passes through an optical bandpass filter to eliminate aliasing between the two optical combs, and then the absolute distance information is obtained by the photodetector.

[0008] The first optical frequency comb and the second optical frequency comb can be mode-locked lasers, electro-optic modulated optical frequency combs, or microcavity optical frequency combs; their repetition frequency is 20 MHz to 20 GHz, and the repetition frequency difference is 100 Hz to 10 MHz.

[0009] The single-frequency laser has a linewidth ≤ 1 MHz and a power > 10 mW.

[0010] The first and second rare-earth-doped amplifiers are rare-earth-doped fiber amplifiers, which can amplify pulsed lasers with an average power of 10 μW to 10 mW and a pulse width of 200 fs to 100 ps to an average power of >2 W and a pulse width of <50 ps.

[0011] The first Raman gain fiber and the second Raman gain fiber are single-mode silicon-based fibers with a 3dB Raman gain bandwidth of 9~15 THz.

[0012] The target to be tested is a hard target surface with a reverse reflectivity >10%, and the measured distance is the absolute distance between the optical collimator and the target to be tested.

[0013] The photodetector is a photodetector with a bandwidth of ≥1 GHz.

[0014] All fiber optic devices in the system are polarization-maintaining devices.

[0015] Compared with the prior art, the beneficial technical effects of the present invention are: 1) This invention modulates the single-frequency laser with dual-path Raman gain, so that the carrier longitudinal mode of the generated Raman dual optical comb is directly inherited from the single-frequency laser. All longitudinal modes have low phase noise comparable to that of the single-frequency laser. At the same time, the carrier wavelength can be directly determined by the wavelength of the single-frequency laser and the comb tooth number, without the need for complex wavelength measurement and locking devices. Thus, interferometric dual optical comb ranging can be easily realized, achieving nanometer-level ranging accuracy.

[0016] 2) This invention utilizes a dual optical frequency comb generated by stimulated Raman scattering, achieving an energy conversion efficiency of up to 70% and a signal light power significantly higher than traditional methods. The high-power signal light enables effective detection of low-reflectivity, non-cooperative targets, and after eliminating distance ambiguity, supports long-distance, large-area absolute distance measurements.

[0017] 3) The dual-comb ranging system of the present invention adopts a full polarization-maintaining fiber structure, eliminating the need for complex multi-channel electrical feedback locking devices in traditional solutions. The system has a compact structure, is easy to integrate, and has high robustness and environmental stability, making it suitable for complex application scenarios such as industrial sites. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the absolute distance measurement system based on Raman gain modulation of single-frequency laser using a dual-comb optical system according to the present invention.

[0019] Explanation of reference numerals in the attached figures: 101-First optical frequency comb, 102-Second optical frequency comb, 2-Single-frequency laser, 301-First rare-earth-doped amplifier, 302-Second rare-earth-doped amplifier, 401-First optical coupler, 402-Second optical coupler, 403-Third optical coupler, 404-Fourth optical coupler, 501-First wavelength division multiplexer, 502-Second wavelength division multiplexer, 503-Third wavelength division multiplexer, 504-Fourth wavelength division multiplexer, 601-First Raman gain fiber, 602-Second Raman gain fiber, 7-Optical circulator, 01-First port, 02-Second port, 03-Third port, 8-Optical collimator, 9-Target under test, 10-Optical bandpass filter, 11-Photodetector. Detailed Implementation

[0020] The present invention will be further described below with reference to an example and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0021] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0022] It should be noted that the terminology used herein is for the purpose of describing specific implementations only and is not intended to be limiting.

[0023] It should be understood that when a unit is referred to as "connected" or "coupled" to another unit in this document, it can be directly connected or coupled to another unit, or an intermediate unit may exist. Conversely, when a unit is referred to as "directly connected" or "directly coupled" to another unit in this document, it means that no intermediate unit exists.

[0024] This example provides a dual-comb absolute distance measurement system based on Raman gain-modulated single-frequency laser, the system structure of which is similar to... Figure 1 same.

[0025] The first optical frequency comb 101 and the second optical frequency comb 102 generate pulsed lasers with different repetition frequencies and no coherence. These pulses are amplified into first ultrafast pulses and second ultrafast pulses by the first rare-earth-doped amplifier 301 and the second rare-earth-doped fiber amplifier 302, respectively. The single-frequency laser 2 generates continuous laser light, which is split into two paths by the first optical coupler 401. These paths are coupled to the first and second ultrafast pulses via the first wavelength division multiplexer 501 and the third wavelength division multiplexer 503, respectively, and then fed into the first Raman gain fiber 601 and the second Raman gain fiber 602 for Raman gain modulation. The two highly coherent continuous lasers output from the single-frequency laser 2 are transformed into highly coherent ultrafast pulses, and the frequency domain is transformed into highly coherent dual optical frequency combs. The pulses are then transmitted through the second wavelength division multiplexer 502. After the residual pump pulses are removed by the fourth wavelength division multiplexer 504, they are respectively denoted as signal optical comb and local oscillator optical comb. The signal optical comb is split into probe light and reference light by the second optical coupler 402. The probe light is input from the first port 01 of the optical circulator 7, output from the second port 02, and illuminates the target 9 under test through the optical collimator 8. The target 9 reflects the probe light and re-enters the optical collimator 8, carrying distance information. The reflected light is input from the second port 02 of the optical circulator 7, output from the third port 03, and combined with the reference light through the third optical coupler 403. It is then combined with the local oscillator optical comb through the fourth optical coupler 404 to realize dual-comb ranging. After the beam is combined, it passes through the optical bandpass filter 10 to eliminate the aliasing of the dual optical combs, and is then detected and calculated by the photodetector 11 to obtain the absolute distance information.

[0026] The first optical frequency comb 101 and the second optical frequency comb 102 are pulsed light sources that generate slightly different repetition frequencies. In this embodiment, the first optical frequency comb 101 and the second optical frequency comb 102 used are erbium-doped fiber nine-cavity mode-locked lasers. The repetition frequency of the first optical frequency comb 101 is 102.35 MHz, the repetition frequency of the second optical frequency comb 102 is 102.36 MHz, the repetition frequency difference is ~10 kHz, the spectral center is 1550 nm, the spectral width is ~80 nm, and the output power is ~5 mW.

[0027] The single-frequency laser 2 is used to generate highly coherent single-frequency continuous laser light. In this embodiment, the single-frequency laser used is a 1645 nm DFB laser manufactured by LD-PD, model PL-HDFB-1645, with a linewidth of 200 kHz and a maximum output power of 50 mW.

[0028] The first rare-earth-doped amplifier 301 and the second rare-earth-doped amplifier 302 are used to amplify the power of the pulsed laser. In this embodiment, the first rare-earth-doped amplifier 301 and the second rare-earth-doped amplifier 302 are erbium-doped fiber amplifiers, which adopt a multi-stage chirped pulse amplification structure to amplify the ultrafast femtosecond laser output from the mode-locked laser to an average power of several watts.

[0029] The first optical coupler 401, the second optical coupler 402, the third optical coupler 403, and the fourth optical coupler 404 are used to couple or separate light. In this embodiment, the first optical coupler 401, the second optical coupler 402, the third optical coupler 403, and the fourth optical coupler 404 are all 1645 nm optical couplers manufactured by Optical Technology Co., Ltd., with splitting ratios of 5:5, 7:3, 6:4, and 6:4, respectively.

[0030] The first wavelength division multiplexer 501, the second wavelength division multiplexer 502, the third wavelength division multiplexer 503, and the fourth wavelength division multiplexer 504 are used for coupling or separating two different wavelengths of light. In this embodiment, the first wavelength division multiplexer 501, the second wavelength division multiplexer 502, the third wavelength division multiplexer 503, and the fourth wavelength division multiplexer 504 used are all 1550nm / 1645nm wavelength division multiplexers manufactured by Optical Technology Co., Ltd.

[0031] The first Raman gain fiber 601 and the second Raman gain fiber 602 are used to provide optical gain to the single-frequency laser through stimulated Raman scattering. In this embodiment, both the first Raman gain fiber 601 and the second Raman gain fiber 602 are small-core silicon-based fibers manufactured by OFS, with a 3dB Raman gain bandwidth of 9~15 THz. Testing showed that under excitation by a 1550nm pump pulse, the energy conversion efficiency of both the first Raman gain fiber 601 and the second Raman gain fiber 602 in converting the pump light into a 1645nm signal optical comb can reach 70%.

[0032] The optical circulator 7 is used to receive the backpropagating probe light. In this embodiment, the optical circulator 7 used is a 1645 nm optical circulator manufactured by Optical Circulator Technology Co., Ltd.

[0033] The optical collimator 8 is used to output the fiber laser as a collimated beam. In this embodiment, the optical collimator 8 used is a 1645 nm optical collimator manufactured by Optical Technology Co., Ltd.

[0034] The target to be tested 9 is a reflective target with a distance to be measured. In this embodiment, the target to be tested 9 is a piezoelectric ceramic with a reflective film attached to its surface, and its surface reflectivity is >10%, making it a hard target surface.

[0035] The optical bandpass filter 10 is used for spectral filtering to avoid aliasing between the two optical combs. In this embodiment, the optical bandpass filter 10 used is an adjustable filter with a 1550 nm-1700 nm range and a 3 dB bandwidth of 1.5 nm.

[0036] The photodetector 11 is used to convert optical signals into electrical signals. In this embodiment, the photodetector 11 used is an indium gallium arsenide photodetector with a bandwidth of 1 GHz.

[0037] The single-frequency laser 2, after undergoing two Raman gain modulations, outputs a high-power, highly coherent Raman-modulated dual comb from the second wavelength division multiplexer 502 and the fourth wavelength division multiplexer 504, denoted as the signal comb and the local oscillator comb, respectively. The signal comb is split into two paths by the second optical coupler 402, denoted as the probe beam and the reference beam, respectively. The probe beam is input from the first port 01 of the optical circulator 7, output from the second port 02, and collimated by the optical collimator 8. The probe beam is reflected by the surface of the target 9 and coupled back into the optical collimator 8, carrying a distance and phase delay. It is input from the second port 02 of the optical circulator 7 and output from the third port 03. The probe beam and the reference beam are coupled by the third optical coupler 403, forming two pulse trains with a certain time delay. The probe beam and the reference beam are then coupled to the local oscillator comb through the fourth optical coupler 404, achieving dual-comb heterodyne detection. To avoid aliasing of the dual combs, an optical bandpass filter 10 is used for optical filtering, and then the distance information is detected and calculated by the photodetector 11.

[0038] In this embodiment, the detected signal interferogram and reference interferogram are Fourier transformed and phase information is extracted. After unwrapping the phase information, a linear fit is performed to obtain the phase difference slope, from which the time-of-flight distance information is obtained. Since the carrier longitudinal modes of the Raman gain-modulated dual optical comb are directly inherited from the single-frequency laser, all longitudinal modes have phase noise comparable to that of the single-frequency laser, and the carrier wavelength of each longitudinal mode can be easily calculated using the wavelength of the single-frequency laser and the comb tooth number. Ultimately, nanometer-precision distance information measurement can be achieved using interferometry.

[0039] In a specific measurement of this embodiment, the target 9 was placed approximately 1 meter from the light-emitting end face of the collimator 8. After heterodyne detection by the signal optical comb and the local oscillator optical comb, the interference signal was collected by the photodetector 11. Fourier transforms were performed on the obtained signal interferogram and the reference interferogram, and phase information was extracted. The phase difference was unwrapped and linearly fitted, and the ranging accuracy obtained using the time-of-flight method was better than 800 nanometers. Furthermore, using the known 1645nm carrier wavelength of the single-frequency laser 2, precise measurement using interferometry achieved an accuracy of several nanometers. This demonstrates that the system of this invention successfully achieves high-precision absolute distance measurement.

Claims

1. A dual-comb absolute distance measurement system based on Raman gain-modulated single-frequency laser, characterized in that, Includes a first optical frequency comb (101), a second optical frequency comb (102), a single-frequency laser (2), a first rare-earth-doped amplifier (301), a second rare-earth-doped amplifier (302), a first optical coupler (401), a second optical coupler (402), a third optical coupler (403), a fourth optical coupler (404), a first wavelength division multiplexer (501), a second wavelength division multiplexer (502), a third wavelength division multiplexer (503), a fourth wavelength division multiplexer (504), a first Raman gain fiber (601), a second Raman gain fiber (602), an optical circulator (7), an optical collimator (8), a target under test (9), an optical bandpass filter (10), and a photodetector (11); The first optical frequency comb (101) and the second optical frequency comb (102) generate pulsed lasers with different repetition frequencies and no coherence. These pulses are amplified into first ultrafast pulses and second ultrafast pulses by the first rare-earth-doped amplifier (301) and the second rare-earth-doped fiber amplifier (302), respectively. The single-frequency laser (2) generates continuous laser, which is split into two paths by the first optical coupler (401). These paths are coupled to the first ultrafast pulse and the second ultrafast pulse through the first wavelength division multiplexer (501) and the third wavelength division multiplexer (503), respectively, and then fed into the first Raman gain fiber (601) and the second Raman gain fiber (602) for Raman gain modulation. The two highly coherent continuous lasers output by the single-frequency laser (2) are transformed into highly coherent ultrafast pulses, and the frequency domain is transformed into highly coherent dual optical frequency combs. The pulses are then fed into the second wavelength division multiplexer (502) and the fourth wavelength division multiplexer (503). After the multiplexer (504) removes the residual pump pulses, they are respectively denoted as signal optical comb and local oscillator optical comb. The signal optical comb is split into probe light and reference light by the second optical coupler (402). The probe light is input from the first port (01) of the optical circulator (7), output from the second port (02), and illuminates the target (9) through the optical collimator (8). The target (9) reflects the probe light and re-enters the optical collimator (8) carrying distance information. The reflected light is input from the second port (02) of the optical circulator (7), output from the third port (03), and combined with the reference light through the third optical coupler (403). It is then combined with the local oscillator optical comb through the fourth optical coupler (404) to realize dual optical comb ranging. After the beam is combined, it passes through the optical bandpass filter (10) to eliminate the aliasing of the dual optical combs, and is then detected and calculated by the photodetector (11) to obtain the absolute distance information.

2. The dual-comb absolute distance measurement system based on Raman gain-modulated single-frequency laser according to claim 1, characterized in that, The first optical frequency comb (101) and the second optical frequency comb (102) are mode-locked lasers, electro-optic modulated optical frequency combs or microcavity optical frequency combs, with a repetition frequency of 20 MHz to 20 GHz and a repetition frequency difference of 100 Hz to 10 MHz.

3. The dual-comb absolute distance measurement system based on Raman gain-modulated single-frequency laser according to claim 1, characterized in that, The single-frequency laser (2) has a linewidth ≤ 1 MHz and a power > 10 mW.

4. The dual-comb absolute distance measurement system based on Raman gain modulation single-frequency laser according to claim 1, characterized in that, The first rare-earth-doped amplifier (301) and the second rare-earth-doped amplifier (302) are rare-earth-doped fiber amplifiers that can amplify pulsed lasers with an average power of 10 μW to 10 mW and a pulse width of 200 fs to 100 ps to an average power of >2 W and a pulse width of <50 ps.

5. The dual-comb absolute distance measurement system based on Raman gain modulation single-frequency laser according to claim 1, characterized in that, The first Raman gain fiber (601) and the second Raman gain fiber (602) are single-mode silicon-based fibers with a 3dB Raman gain bandwidth of 9~15 THz.

6. The dual-comb absolute distance measurement system based on Raman gain modulation single-frequency laser according to claim 1, characterized in that, The target to be tested (9) is a hard target surface with a reverse reflectivity >10%, and the measured distance is the absolute distance between the optical collimator (8) and the target to be tested (9).

7. The dual-comb absolute distance measurement system based on Raman gain-modulated single-frequency laser according to claim 1, characterized in that, The photodetector (11) is a photodetector with a bandwidth of ≥1 GHz.

8. The dual-comb absolute distance measurement system based on Raman gain modulation single-frequency laser according to claim 1, characterized in that, All fiber optic devices in the system are polarization-maintaining devices.

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