Scanning high-resolution double-optical-comb spectrum system based on Raman gain modulation

Abstract

The invention discloses a scanning high-resolution double-optical-comb spectrum system based on Raman gain modulation. The method is suitable for real-time spectrum detection with a wide spectrum range and high spectral resolution. The system comprises a single-frequency laser and driver, an optical coupler, a mode-locked laser, a rare earth doped amplifier, a wavelength division multiplexer, a Raman gain fiber, a sample to be tested, a photoelectric detector, a data processor and a wavemeter. The single-frequency laser couples part of light to the wavemeter, and the remaining light is divided into two paths to be coupled with the two amplified mode-locked lasers to enter the Raman gain optical fiber for Raman gain modulation and is converted into high-mutual-dryness double optical combs. The double combs pass through a to-be-measured sample in a combined manner, and interference signals are measured by the photoelectric detector and received by the data processor. And meanwhile, the processor sends a tuning signal to the single-frequency laser driver, so that the double combs scan in a wide spectrum range to obtain high-resolution spectrum information. Electrical feedback locking is not needed, and high-precision spectrum acquisition with megahertz resolution in a terahertz range can be completed within a few minutes.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of high-resolution spectral detection, ultrafast fiber laser, optical frequency comb and precise measurement, and particularly relates to a scanning high-resolution dual-comb spectral system based on Raman gain modulation. BACKGROUND

[0002] Dual-comb spectral technology has the advantages of wide spectral range, high precision and fast detection, and is regarded as an ideal spectral detection technology. Its potential application in spectral detection and gas remote sensing has been proved in experiments.

[0003] However, the spectral resolution of the traditional dual-comb spectral technology is limited by the repetition frequency of the optical comb, which is typically 40 MHz to 1 GHz, and it is difficult to achieve a resolution of ~MHz. Reducing the repetition frequency to ~MHz will cause problems such as increased system complexity, significantly reduced acquisition speed, and narrower non-aliasing spectral range per acquisition.

[0004] Scanning the comb teeth of the dual-comb in the frequency spectrum and filling the gaps between the sampling points of the dual-comb is an effective method to improve the spectral resolution of the dual-comb. The main scanning method is to tune the optical frequency comb based on electro-optic modulation, but the spectral coverage of the electro-optic modulation optical frequency comb is narrow, and it is difficult to take advantage of the wide spectral range of the dual-comb spectral technology. Another scanning method is frequency shift scanning based on an acousto-optic modulator, but this method has a narrow scanning bandwidth and it is difficult to improve the resolution to the order of ~MHz.

[0005] Therefore, it is a difficult problem to design a simple and efficient dual-comb spectral system to improve the resolution to the order of ~MHz in the case of wide spectral range and fast acquisition. SUMMARY

[0006] In order to solve the problem that the current dual-comb spectral technology cannot achieve high spectral resolution of the order of ~MHz in the case of wide spectral range and high-speed acquisition, the present application provides a novel method to achieve the above requirements. The dual-comb spectral system based on Raman gain modulation guarantees the long-term mutual coherence of the dual-comb and the resolution of the comb teeth by the Raman gain modulation process. The dual-comb based on Raman gain modulation expands the sidebands around the single-frequency laser, and the overall translation of the comb teeth is completely determined by the frequency position of the single-frequency laser. By accurately controlling the frequency of the continuous light output by the single-frequency laser, high-precision spectral information with a spectral range of ~THz and a spectral resolution of ~MHz can be obtained within a few minutes, and the system does not require electrical feedback locking and algorithm correction assistance. This method makes it possible to apply dual-comb spectral technology to low-pressure or cold molecular spectral detection, dense absorption region resolution, isotope resolution, hyperfine structure analysis and other directions.

[0007] The solution of the present application is as follows: A scanning high-resolution double-comb spectrum system based on Raman gain modulation, comprising a single-frequency laser and a driver, a first optical coupler, a second optical coupler, a third optical coupler, a first mode-locked laser, a second mode-locked laser, a first rare-earth doped amplifier, a second rare-earth doped amplifier, 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, a sample to be measured, a photodetector, a processor, and a wavemeter. The single-frequency laser and the driver generate single-frequency continuous laser, a small part of which is first split by the first optical coupler to the wavemeter for accurate wavelength detection, and the remaining light is then split by the second optical coupler into a first single-frequency laser and a second single-frequency laser; the first mode-locked laser and the second mode-locked laser respectively generate mutually incoherent ultrafast pulses with different repetition frequencies, the pulses output by the first mode-locked laser are amplified by the first rare-earth doped amplifier and then coupled into the first Raman gain fiber with the first single-frequency laser via the first wavelength division multiplexer; the pulses output by the second mode-locked laser are amplified by the second rare-earth doped amplifier and then coupled into the second Raman gain fiber with the second single-frequency laser via the third wavelength division multiplexer; in the first and second Raman gain fibers, the first and second single-frequency lasers act as pump light and respectively perform Raman gain modulation on the amplified ultrafast pulses, thereby generating two high-intercorrelation Raman optical frequency combs; the two optical combs are separated from the remaining pump light and pulses via the second wavelength division multiplexer and the fourth wavelength division multiplexer; the high-intercorrelation double comb obtained by Raman gain modulation is combined by the third optical coupler, and double-comb spectrum detection is performed by the sample to be measured, the interference signal is detected by the photodetector, and the processor receives and processes the signal; the processor synchronously receives accurate wavelength data from the wavemeter and simultaneously sends a tuning signal to the single-frequency laser and the driver, so that the double comb is continuously translated in the frequency domain as a whole, the comb tooth sampling interval is constantly filled, and the purpose of high-resolution spectrum acquisition is achieved.

[0008] The first mode-locked laser and the second mode-locked laser are rare-earth doped mode-locked lasers, and the first mode-locked laser and the second mode-locked laser respectively generate ultrafast pulse lasers with different repetition frequencies, the repetition frequency being 40 MHz to 1 GHz, and the repetition frequency difference being 100 Hz to 20 kHz.

[0009] The single-frequency laser and the driver generate single-frequency continuous laser with a line width <1 MHz and a frequency jitter standard deviation <5 MHz within 5 seconds.

[0010] The first rare-earth doped amplifier and the second rare-earth doped amplifier are doped fiber amplifiers, which can amplify the average power of the ultrafast pulses generated by the first mode-locked laser and the second mode-locked laser from mW to >1 W, and the pulse width is <100 ps.

[0011] The first Raman gain fiber and the second Raman gain fiber are small-core silicon-based fibers, and the 3dB Raman gain bandwidth is 9-15 THz, and the length used in the system is 1-20 m.

[0012] The photoelectric detector is an indium gallium arsenide photoelectric detector with a bandwidth of ≥200MHz.

[0013] The processor comprises a data acquisition card, the sampling rate of the data acquisition card is ≥200MHz, and the continuous acquisition capacity under the 200MHz sampling rate is ≥10s.

[0014] The wavelength resolution of the wavemeter is ≤10fm, and the acquisition interval is ≤100ms.

[0015] All the fiber devices in the system are polarization maintaining devices.

[0016] Compared with the prior art, the technical effects of the present application are: 1) The present application can obtain high-precision spectral information with ~THz spectral range and ~MHz spectral resolution within a few minutes by performing precise frequency scanning on the dual-comb spectrum system based on Raman gain modulation.

[0017] 2) The present application can work in real time without any electrical feedback locking and correction algorithm assistance by performing precise frequency scanning on the dual-comb spectrum system based on Raman gain modulation.

[0018] 3) The dual-comb spectrum system based on Raman gain modulation in the present application is composed of a full polarization maintaining fiber structure, and the system is easy to integrate, robust and has high environmental stability.

[0019] 4) The dual-comb spectrum system based on Raman gain modulation in the present application realizes frequency shift through stepwise control scanning of the temperature or driving current of the single-frequency laser, which is easy to control and simple to operate.

[0020] 5) The dual-comb spectrum system based on Raman gain modulation in the present application does not need to accurately control the single tuning position, only needs to stepwise tune according to the predetermined number of times, so that the comb teeth step in the spectrum, and then the data is recorded by the wavemeter for accurate inversion.

[0021] 6) The dual-comb spectrum system based on Raman gain modulation in the present application, the highest spectral resolution is limited by the frequency jitter standard deviation of the single-frequency laser, so if a single-frequency laser with better performance is replaced or the single-frequency laser is locked and tuned, the highest resolution will be further broken through to ~kHz or lower.

[0022] 7) The double optical comb spectrum system based on Raman gain modulation in the application can keep high mutual coherence laser output of several watts while precise spectrum control, and can be applied to remote high resolution spectrum remote sensing detection. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is the structural schematic diagram of the scanning high resolution double optical comb spectrum system based on Raman gain modulation in the application.

[0024] Figure 2 is the schematic diagram of the scanning high resolution double optical comb spectrum system based on Raman gain modulation in the application, in which the wavelength of the single frequency laser output changes with time during the multi-group step scanning process.

[0025] Figure 3 is the spectrum diagram obtained by Fourier transform of the signal collected by the scanning high resolution double optical comb spectrum system based on Raman gain modulation in the application in a single acquisition, and the resolution thereof is equal to the repetition frequency of the optical comb.

[0026] Figure 4 is the comparison diagram of the high resolution absorption spectrum and the single detection spectrum of the scanning high resolution double optical comb spectrum system based on Raman gain modulation in the application after 27 times of step scanning.

[0027] BRIEF DESCRIPTION OF DRAWINGS 1-single frequency laser and drive, 201-first optical coupler, 202-second optical coupler, 203-third optical coupler, 301-first mode-locked laser, 302-second mode-locked laser, 401-first rare earth doped amplifier, 402-second rare earth doped amplifier, 501-first wavelength division multiplexer, 502-second wavelength division multiplexer, 503-third wavelength division multiplexer, 504-fourth wavelength division multiplexer, 601-first Raman gain optical fiber, 602-second Raman gain optical fiber, 7-sample to be detected, 8-optoelectronic detector, 9-processor, 10-wavelength meter. DETAILED DESCRIPTION

[0028] The application will be further described below in conjunction with an example and the drawings, but the protection scope of the application should not be limited thereby.

[0029] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise defined, 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 belongs.

[0030] It should be noted that the terms used herein are only for the purpose of describing specific embodiments, and are not intended to be limiting.

[0031] It will be understood that where a unit is referred to herein as being "connected" or "coupled" to another unit, that other unit can be directly connected or coupled to the unit or intervening units can be present. Conversely, where a unit is referred to as being "directly connected" or "directly coupled" to another unit, intervening units are not present.

[0032] The present example provides a scanning high-resolution dual-comb spectroscopy system based on Raman gain modulation. The system structure is the same as Figure 1 The single-frequency laser and driver 1 generate a single-frequency continuous laser. A small part of the laser is first split by the first optical coupler 201 to the wavemeter 10 for accurate wavelength detection, and then the remaining laser is split by the second optical coupler 202 into a first single-frequency laser and a second single-frequency laser. The first mode-locked laser 301 and the second mode-locked laser 302 generate mutually incoherent ultrafast pulses with different repetition frequencies, respectively. The pulses output by the first mode-locked laser 301 are amplified by the first rare-earth-doped amplifier 401 and coupled with the first single-frequency laser into the first Raman gain fiber 601 via the first wavelength division multiplexer 501. The pulses output by the second mode-locked laser 302 are amplified by the second rare-earth-doped amplifier 402 and coupled with the second single-frequency laser into the second Raman gain fiber 602 via the third wavelength division multiplexer 503. In the first Raman gain fiber 601 and the second Raman gain fiber 602, the first single-frequency laser and the second single-frequency laser act as pump light and perform Raman gain modulation on the amplified ultrafast pulses, respectively, thereby generating two high-intercorrelation Raman optical frequency combs. The two high-intercorrelation dual combs obtained by Raman gain modulation are combined by the third optical coupler 203, pass through the sample 7 to be measured for dual-comb spectroscopy detection, the interference signal is detected by the photodetector 8, and the processor 9 receives and processes the signal. The processor 9 synchronously receives accurate wavelength data from the wavemeter 10 and sends a tuning signal to the single-frequency laser and driver 1 at the same time, so that the dual-comb is continuously translated as a whole in the frequency domain, the comb tooth sampling interval is constantly filled, and high-resolution spectrum reconstruction is achieved.

[0033] The single-frequency laser and driver 1 are used to generate continuous laser with accurate adjustable frequency. In this embodiment, the single-frequency laser used is a 1645 nm DFB laser produced by LD-PD company, model PL-HDFB-1645, line width 200 kHz, maximum output power 50 mW, and tuning range ~4 nm. The driver used is a low-noise driver produced by Shanghai Xiaoxiao Photonics, model MPLD600mA, which can provide a current control step of 0.01 mA and a temperature control step of 0.001℃.

[0034] The first optical coupler 201, the second optical coupler 202 and the third optical coupler 203 are all optical fiber devices for splitting or combining light. In the embodiment, they are all polarization maintaining optical couplers with splitting ratios of 1 / 99, 50 / 50 and 50 / 50 respectively, which are produced by Optolink.

[0035] The first mode-locked laser 301 and the second mode-locked laser 302 are used to generate sequences of ultrashort pulses with slightly different repetition rates. In the embodiment, the first mode-locked laser 301 and the second mode-locked laser 302 are both erbium-doped fiber figure-eight cavity mode-locked lasers. The repetition rate of the first mode-locked laser 301 is 102.35 MHz, and the repetition rate of the second mode-locked laser 302 is 102.3522 MHz. The difference between the repetition rates is about 2.2 kHz, and the spectral width is about 80 nm. The output power of the lasers is about 5 mW.

[0036] The first rare-earth-doped amplifier 401 and the second rare-earth-doped amplifier 402 are used to amplify the ultrashort pulses. In the embodiment, the first rare-earth-doped amplifier 401 and the second rare-earth-doped amplifier 402 are both erbium-doped fiber amplifiers, which are used to amplify the output of the mode-locked lasers to several watts of power.

[0037] 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 all optical fiber devices for separating or combining optical signals with different wavelengths. In the embodiment, they are all polarization maintaining 1550 / 1645 nm coarse wavelength division multiplexers produced by Optolink.

[0038] The first Raman gain fiber 601 and the second Raman gain fiber 602 are both media for providing optical gain through stimulated Raman scattering. In the embodiment, the first Raman gain fiber 601 and the second Raman gain fiber 602 are both small-core silicon-based fibers produced by OFS, and the 3dB Raman gain bandwidth is 9-15 THz.

[0039] The sample 7 to be measured is a hollow-core fiber carbon dioxide gas absorption cell produced by Fudan University.

[0040] The photodetector 8 is used to convert optical signals into electrical signals. In the embodiment, the photodetector 8 is a DET01CFC indium gallium arsenide photodetector produced by THORLABS.

[0041] The processor 9 is a computing unit with high-speed data acquisition and processing functions. In the embodiment, the unit is composed of a high-performance computer equipped with a Kunshi Technology QT1144VG-4 data acquisition card. The sampling rate of the data acquisition card is 1 GHz, and it has the ability to continuously acquire data at this sampling rate.

[0042] The wavelength meter 10 is a device for high-precision measurement of laser wavelength. In this embodiment, the wavelength meter 10 used is a WS6 wavelength meter manufactured by Menlo Systems, with a wavelength resolution of 1 fm and a frequency resolution of approximately 1.11 kHz near 1645 nm.

[0043] The single-frequency laser generated by the laser and driver 1 has a small portion of its wavelength detected in real time by wavelength meter 10, which provides the frequency shift amount to processor 9. The majority of the single-frequency light is converted into a 1645 nm high-interference dual optical frequency comb after Raman gain modulation via the first Raman gain fiber 601 and the second Raman gain fiber 602. This combined beam is then subjected to dual-comb spectral detection on the sample 7. Each detection acquisition time is 3 seconds. After each acquisition, processor 9 sends a tuning command to the single-frequency laser and driver 1. This command causes the driver to control the temperature of the single-frequency laser in 0.001°C steps, thereby tuning the single-frequency laser to approximately 15 MHz. After tuning, wavelength meter 10 accurately measures the frequency shift. Once the frequency stabilizes, a trigger signal is sent to processor 9, which then initiates the next 3-second data acquisition. This process repeats until the predetermined number of scans is reached.

[0044] In this embodiment, the predetermined number of scans is 27, and the step-sweep wavelength recorded by the wavelength meter 10 is as follows: Figure 2 As shown. In this embodiment, the repetition frequency difference of the dual combs is 2.2 kHz, and a single acquisition lasts 3 seconds. Approximately 6600 interferograms are coherently averaged and Fourier transformed to obtain the single-shot spectral information before the next acquisition. It is important to note that the positions of the RF comb teeth are completely consistent during multiple acquisitions, while the spectral information is scanned in the RF domain. Therefore, the processor 9 needs to move the RF comb teeth according to the sweep amount recorded by the wavelength meter 10 to recover the high-resolution spectral information. The RF spectrum after a single acquisition and coherent averaging is shown below. Figure 3 As shown, the spectral coverage is ~7 nm, the signal-to-noise ratio is >30 dB, and the carbon dioxide absorption line is clearly visible. A comparison is made between the high-resolution spectra reconstructed after 27 scans and the spectra obtained from a single detection. Figure 4 As shown, the resolution of a single scan is a repetition frequency of 102.35 MHz, and the average resolution after 27 scans reaches ~3.8 MHz, while simultaneously achieving a wide spectral range and fast acquisition speed. Compared with other methods of similar resolution, this scheme has advantages such as simplicity, efficiency, high degree of freedom, wide spectral range, and strong stability, providing a novel approach for high-resolution dual-comb spectroscopy.

Claims

1. A scanning high-resolution dual-comb spectral system based on Raman gain modulation, characterized in that, Includes a single-frequency laser and driver (1), a first optical coupler (201), a second optical coupler (202), a third optical coupler (203), a first mode-locked laser (301), a second mode-locked laser (302), a first rare-earth-doped amplifier (401), a second rare-earth-doped amplifier (402), 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), a sample to be tested (7), a photodetector (8), a processor (9), and a wavelength meter (10); A single-frequency laser and its driver (1) generate a single-frequency continuous laser. First, a small portion of the light is split off by a first optical coupler (201) and sent to a wavelength meter (10) for precise wavelength detection. Then, the remaining light is split into a first single-frequency laser and a second single-frequency laser by a second optical coupler (202). The first mode-locked laser (301) and the second mode-locked laser (302) generate incoherent ultrafast pulses with different repetition frequencies. The pulse output by the first mode-locked laser (301) is amplified by a first rare-earth-doped amplifier (401) and coupled with the first single-frequency laser through a first wavelength division multiplexer (501) into a first Raman gain fiber (601). The pulse output by the second mode-locked laser (302) is amplified by a first rare-earth-doped amplifier (401) and coupled with the first single-frequency laser through a first wavelength division multiplexer (501) into a first Raman gain fiber (601). After being amplified by the second rare-earth-doped amplifier (402), it is coupled with the second single-frequency laser through the third wavelength division multiplexer (503) into the second Raman gain fiber (602); the first and second single-frequency lasers are used as pump light to perform Raman gain modulation on the amplified ultrafast pulses, thereby generating two highly coherent Raman optical frequency combs; the two optical combs are further separated from the remaining pump light and pulses through the second wavelength division multiplexer (502) and the fourth wavelength division multiplexer (504); the two Raman dual optical combs are combined through the third optical coupler (203) and subjected to dual optical comb spectral detection through the sample under test (7); the interference signal is detected by the photodetector (8) and received and processed by the processor (9); The processor (9) synchronously receives precise wavelength data from the wavelength meter (10) and simultaneously sends a tuning signal to the single-frequency laser and the driver (1) to make the dual optical comb continuously shift in the frequency domain, constantly filling in the sampling interval of the comb teeth, so as to achieve the purpose of high-resolution spectral acquisition.

2. The scanning high-resolution dual-comb spectral system based on Raman gain modulation according to claim 1, characterized in that, The first mode-locked laser (301) and the second mode-locked laser (302) are rare-earth-doped mode-locked lasers, which respectively generate ultrafast pulsed lasers with a repetition frequency of 40 MHz to 1 GHz and a repetition frequency difference of 100 Hz to 20 kHz.

3. The scanning high-resolution dual-comb spectral system based on Raman gain modulation according to claim 1, characterized in that, The single-frequency laser and driver (1) generate a single-frequency continuous laser with a linewidth <1 MHz and a frequency jitter standard deviation <5 MHz within 5 seconds.

4. The scanning high-resolution dual-comb spectral system based on Raman gain modulation according to claim 1, characterized in that, The first rare-earth-doped amplifier (401) and the second rare-earth-doped amplifier (402) are rare-earth-doped fiber amplifiers with an average output laser power >1 W and a pulse width <100 ps.

5. The scanning high-resolution dual-comb spectral system based on Raman gain modulation 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 and a length of 1~20 m used in the system.

6. The scanning high-resolution dual-comb spectral system based on Raman gain modulation according to claim 1, characterized in that, The photodetector (8) is an indium gallium arsenide photodetector with a bandwidth of ≥200MHz.

7. The scanning high-resolution dual-comb spectral system based on Raman gain modulation according to claim 1, characterized in that, The processor (9) includes a data acquisition card with a sampling rate ≥200 MHz and a continuous acquisition capability ≥10 s at a sampling rate of 200 MHz.

8. The scanning high-resolution dual-comb spectral system based on Raman gain modulation according to claim 1, characterized in that, The wavelength resolution of the wavelength meter (10) is ≤10 fm and the acquisition interval is ≤100 ms.

9. The scanning high-resolution dual-comb spectral system based on Raman gain modulation according to claim 1, characterized in that, All fiber optic devices in the system are polarization-maintaining devices.

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