Method for improving signal-to-noise ratio of double-optical-comb interferometer based on harmonic pulse coherence enhancement

Abstract

The invention discloses a method for improving the signal-to-noise ratio of a double-optical-comb interferometer based on harmonic pulse coherence enhancement, and belongs to the field of precise laser measurement. The optical frequency comb light source of which the pulse repetition frequency is higher than the target system resolution is used and is matched with periodic or quasi-periodic pulse phase regulation and control, so that the optical frequency comb light source can work at the target spectral resolution; and coherent enhancement of harmonic pulses on a frequency spectrum is utilized to replace a traditional coherent averaging technology to improve the signal-to-noise ratio of an interferogram, so that the measurement time is greatly shortened, and the requirement of a mutual dryness locking system of an optical comb light source is reduced. And finally, the time resolution of the system is greatly improved, and the hardware cost and requirements are reduced.

Description

Technical Field

[0001] The present invention belongs to the field of precision optical comb spectroscopy, and in particular relates to a method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement. Background Art

[0002] Dual-comb interferometer (DCI), or dual-comb spectroscopy (DCS) technology, is a new type of Fourier transform interferometer technology, first proposed by Schiller of Germany in 2002 (see Schiller, S. Spectrometry with frequency combs. Opt. Lett. 27, 766 (2002)). Its principle diagram is as follows: Figure 1 As shown. Different from the traditional Fourier transform infrared spectroscopy (FTIR), the core of DCS technology is two instruments with a small repetition frequency difference (Δf s ) coherent optical comb (i.e., a highly phase-stable mode-locked laser) can obtain a complete interferogram (IGM) by automatic linear sampling between pulses without the need for scanning of a mechanical moving mirror. And since the acquisition time of one interferogram depends on the inverse of the difference in repetition frequency of the two optical combs (1 / Δf s ), by appropriately setting a larger Δf s In theory, sampling can be completed once on the μs or even lower scale (see Hoghooghi, N., Cole, RK & Rieker, GB11-μs time-resolved, continuous dual-comb spectroscopy with spectrally filtered mode-locked frequencycombs. Appl. Phys. B 127, 17 (2021)), which has extremely high time resolution potential. Combined with the advantages of wide spectral coverage, high detection sensitivity, high resolution, fast measurement and high frequency accuracy, DCS technology exhibits extremely advantageous comprehensive performance compared to traditional FTIR and other technologies, and is considered to be a technology with great potential to replace the traditional and widely used FTIR in the future (see Coddington, I., Swann, W. & Newbury, N. Coherent dual-comb spectroscopy at high signal-to-noise ratio. Phys. Rev. A 82, 043817 (2010) and Coddington, I., Newbury, N. & Swann, W. Dual-comb spectroscopy. Optica, OPTICA 3, 414–426 (2016)).

[0003] However, since typical DCS uses an optical frequency comb based on a mode-locked laser with extremely high pulse peak power, the detector can enter a nonlinear response or saturation range at very low average optical power, so the signal-to-noise ratio (SNR) of a single-frame interferogram is very low. This leads to the need for long-term averaging or sampling to improve the measurement signal-to-noise ratio (SNR gain brought by coherent averaging). (where N is the average number of interferograms.) This not only reduces the temporal resolution of the spectral system and degrades the high temporal resolution potential of DCS, but also requires a more sophisticated locking system to ensure a longer coherence time, significantly increasing system complexity and cost.

[0004] In addition, since the spectral resolution of the traditional dual-comb spectroscopy system is determined by the comb pulse repetition frequency f rep However, low repetition rate and high performance optical combs are often difficult to obtain due to technical limitations, which limits the flexibility and high resolution potential of dual-comb systems.

[0005] In addition, there is a type of dual-comb spectroscopy technology that uses a self-correction algorithm to eliminate phase noise on the interferogram rather than closed-loop locking to restore the high mutual coherence of the dual comb. This is generally called computational self-correction dual-comb spectroscopy. In this technology, since only one central burst in an interferogram cycle can provide effective phase sampling information (see Hebert, NB, Michaud-Belleau, V., Deschenes, J.-D. & Genest, J. Self-Correction Limits in Dual-Comb Interferometry. IEEE J. Quantum Electron. 55, 1–11 (2019)), the other areas of the interferogram = due to the lack of signal or the signal-to-noise ratio is too low to produce effective sampling, so the self-correction algorithm's tracking rate of phase noise is limited to the interferogram refresh rate, that is, Δf rep This limits the noise correction bandwidth of the self-correction algorithm and also restricts the application of this free-running scheme. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement. By using an optical frequency comb with a high pulse repetition frequency to perform periodic timing phase control, it is converted into multiple coherent harmonic pulse sequences, and the signal-to-noise ratio of the measured spectrum is improved through the spectral pattern enhancement mechanism between these coherent harmonics.

[0007] The technical solutions of the present invention are as follows:

[0008] A method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement, characterized by comprising:

[0009] An optical frequency comb module is used to generate at least two pulse sequences with different repetition frequencies, wherein the repetition frequency of at least one of the pulses is close to an integer multiple of the target spectral resolution.

[0010] a timing phase control module, connected to the output end of the optical frequency comb module, and applying periodic or quasi-periodic phase modulation to the pulse sequence with a repetition frequency close to an integer multiple of the target spectral resolution;

[0011] a photodetector for collecting optical signals of at least two optical frequency combs and converting their interference electric fields into interference pattern electrical signals;

[0012] A data acquisition and processing system is used to process the interferogram electrical signal to extract information such as the spectrum; wherein the optical frequency comb module generates a pulse sequence with a repetition frequency close to an integer multiple of the target spectral resolution and the modulation of the timing phase control module can make the central burst on the generated interferogram (1) have a repetition frequency that is a harmonic frequency of the mode interval on its spectrum, that is, an integer multiple of the latter; or (2) have an enhanced amplitude, thereby ultimately improving the signal-to-noise ratio.

[0013] Preferably, it is characterized in that when the central burst repetition frequency on the interference pattern generated by the system is a harmonic of the mode interval, the interference pattern has multiple central burst structures in one period, which have different and nonlinearly sliding carrier-envelope phases.

[0014] Preferably, it is characterized in that when periodic or quasi-periodic phase modulation is applied to an optical frequency comb, the interference pattern has multiple central burst structures in one period, and the phase modulation applied to the optical pulse is transferred to the interference pattern through an asynchronous optical sampling process.

[0015] Preferably, the timing phase control module includes an electro-optical phase modulator and a radio frequency signal generator, wherein the periodic or quasi-periodic electrical signal output by the radio frequency signal generator drives the electro-optical phase modulator to achieve timing phase control of the optical frequency comb pulse.

[0016] Preferably, it is characterized in that the period of the phase modulation signal is an integer multiple of the repetition period of the modulated optical frequency comb pulse, and the repetition frequency is the target spectral resolution.

[0017] Preferably, the feature is that the interference pattern has multiple central burst structures in one period, and the algorithm tracking rate of the phase noise on the interference pattern can be increased to an integer multiple of the period frequency of the interference pattern.

[0018] Preferably, it is characterized in that it also includes a clock synchronization step for synchronizing the clocks of the optical frequency comb and the phase control module to ensure time base consistency, and adjusting the relative phase between the optical frequency comb pulse and the modulation signal through the phase delay module.

[0019] The beneficial effects of the present invention are:

[0020] (1) The present invention performs periodic or quasi-periodic pulse phase control on a high-pulse repetition frequency optical frequency comb, converting it into a pulse sequence with coherent harmonic characteristics. Compared to traditional DCS at the same spectral resolution, the coherence enhancement of harmonic pulses in the spectrum can be used to replace the traditional coherent averaging technology to improve the signal-to-noise ratio of the interferogram, thereby significantly reducing measurement time and lowering the requirements for the mutual coherence locking system of the optical comb light source. Ultimately, the system's temporal resolution is significantly improved, and hardware costs and requirements are reduced.

[0021] (2) The present invention eliminates the coupling relationship between the optical comb pulse repetition frequency and the spectral resolution of the dual-comb spectroscopy system, and can flexibly set the spectral resolution of the spectroscopy system by controlling the period of the timing phase.

[0022] (3) The present invention can realize multiple central burst structures in one interferogram period, thereby directly improving the phase noise sampling rate of the self-correction algorithm, thereby increasing its correction bandwidth, and ultimately promoting the application of free-running dual-comb spectroscopy technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the principle of the traditional dual-comb spectroscopy technology architecture;

[0024] Figure 2 Schematic diagram of a device for a method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement according to embodiment 1 of the present invention;

[0025] Figure 3 Schematic diagram of the technical architecture of embodiment 1 of the method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement of the present invention;

[0026] Figure 4 2 is a schematic diagram of a device according to embodiment 2 of the present invention for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement;

[0027] Figure 5 Schematic diagram of the apparatus of Embodiment 3 and Embodiment 4 of the method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement of the present invention;

[0028] Figure 6 Schematic diagram of the technical architecture of embodiment 3 of the method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement of the present invention;

[0029] Figure 7 It is a schematic diagram of the device of Example 5 of the method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement of the present invention.

[0030] Figure 8 It is a schematic diagram of the device of Example 6 of the method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement of the present invention.

[0031] Figure 9 It is a schematic diagram of the device of Example 7 of the method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement of the present invention.

[0032] Figure 10 It is a schematic diagram of the device of Example 8 of the method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement of the present invention. DETAILED DESCRIPTION

[0033] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0034] [Example 1]

[0035] The schematic diagram of the device in this embodiment is as follows Figure 2 As shown, the principle diagram is as follows Figure 3 The method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement includes an optical frequency comb module, a timing phase control module, a coupling module, a sample to be measured, a photoelectric detection module, and data acquisition and processing.

[0036] The optical frequency comb module in this embodiment 1 includes a first optical frequency comb 1 and a second optical frequency comb 2. The first optical frequency comb 1 uses a pulse repetition frequency of 250 MHz (f ms ) near a mode-locked fiber laser with a repetition frequency of f rep1 =249.999MHz(corresponding principle diagram Figure 3 f in ms -Δf s ); The second optical frequency comb 2 uses a higher pulse repetition frequency, a repetition frequency of 4 times the target spectral resolution of the system rep2 =1GHz(corresponding principle diagram Figure 3 4f in ms ) mode-locked fiber lasers. Both have a central wavelength of 1550 nm, cover a spectral range of 1500-1650 nm, and have their repetition rate and carrier-envelope offset frequency locked to a radio frequency reference.

[0037] The timing phase control module includes a first electro-optical phase modulator 3 (EOPM, bandwidth 10 GHz) and a radio frequency signal generator 4. The radio frequency signal generator 4 generates an amplitude V pp =100mV, cycle frequency is f mod =250MHz, 25% duty cycle square wave signal, the driver integrated in the first electro-optical phase modulator 3 rectifies the signal into a periodic [V π ,0,0,0] signal (where V π is the half-wave voltage of the electro-optical phase modulator). The clock of the RF signal generator 4 is synchronized with the clocks of the locking servo circuits of the first and second optical frequency combs 1 and 2, ensuring long-term matching between the modulation signal and the optical comb pulses. Given the extremely low duty cycle of the optical frequency comb pulses in the time domain, the modulation signal generated by the signal generation module 4 can be non-strictly periodic, i.e., quasi-periodic—as long as the portion that coincides with the pulses in time is periodic, periodic phase modulation can be imparted to the pulse sequence.

[0038] The first electro-optical phase modulator 3 performs phase modulation on the pulse sequence output by the second optical frequency comb 2. By adjusting the phase of the modulation signal output by the RF signal generator 4 relative to the optical frequency comb pulse, stable phase modulation of each pulse is achieved, so that each pulse no longer carries the same or linearly sliding carrier envelope phase (CEP), but instead has a CEP sudden change of π phase for one pulse in every four pulses.

[0039] The sample to be tested, the gaseous sample 6 used in this embodiment 2, is a greenhouse gas absorption cell, which contains a certain concentration of methane, H2O and carbon dioxide. These gases have intensity absorption or phase perturbation characteristics in the 1550nm spectral band.

[0040] The coupling module, in this embodiment, uses a 2-to-2 fiber coupler 5 with a 50:50 splitting ratio to combine the beams from the first optical frequency comb 1 and the modulated beams from the second optical frequency comb 2. One beam of the combined light passes through the gaseous sample 6 to the first photodetector 7, while the other beam, serving as reference light, is directly transmitted to the second photodetector 8. The first and second photodetectors 7 and 8 are 1 GHz InGaAs biased photodetectors with a wavelength response range of 800-1700 nm. They receive the measurement and reference light and convert them into electrical signals.

[0041] On the detector, due to the difference in pulse repetition frequency between the two optical frequency combs, an asynchronous optical sampling process will occur - the pulse of the first optical frequency comb 1 automatically steps over the four pulses of the second optical frequency comb 2, and within an interferogram period of 1ms (1 / Δf s) generates four central burst structures. And according to the photocurrent formula generated by the interference of two optical fields on the detector during asynchronous optical sampling, the current (or voltage) generated by the interference of the nth pulse pair on the detector is I(n)~<|E1(t-nτ)||E2(t)|cos(Δφ+Δφ m )). Where |E1(t)| is the electric field amplitude envelope of the pulse of the first optical frequency comb, |E2(t)| is the electric field amplitude envelope of the pulse of the second optical frequency comb, τ is the step delay between each pulse pair, Δφ is the original phase difference between the two pulses, Δφ m is the phase difference between the two pulses in the pulse pair applied by the modulator, and <) represents the time average. When the pulse of the first optical frequency comb 1 automatically steps over the four pulses of the second optical frequency comb 2, because the first pulse of the second optical frequency comb 2 is given a phase of π by the electro-optical modulator 3, each position of the interference electric field corresponding to the first central burst is phase-shifted by π. The positions of the other three central bursts are shifted by Δφ due to the phase shift of π. m = 0, then it is not phase shifted. Therefore, these four center burst structures can inherit the [π, 0, 0, 0] carrier envelope phase envelope of the second optical frequency comb 2. Analyze the spectral behavior of the interferogram with the [π, 0, 0, 0] carrier envelope phase envelope: According to the linear property of Fourier transform, its total spectrum is the vector superposition of the spectra of these four center bursts. Since the repetition frequencies of these four center bursts are Δf s , so the mode spacing on its spectrum is 1kHz (Δf s ), its phase is given by: The first term -ω j t k represents the phase shift caused by the relative time shift, ω j =j·2πΔf s represents the angular frequency of the jth mode; t k =(k-1) / 4Δf s represents the time shift of the kth central burst relative to the first one; φ TPM,k This is the introduced [π, 0, 0, 0] carrier envelope phase envelope. According to vector superposition, the final total spectrum is in This means that the pattern spacing of the total spectrum is consistent with the pattern spacing of the spectrum of each central burst, which is 1 kHz (Δf s ) instead of 4kHz(4Δf s ), and the mode amplitude is increased by 2 times compared to a single central burst, and has a non-uniform spectral phase. This result can also be inferred to the second optical frequency comb 2. Combined with the well-known interference pattern in dual-comb spectroscopy, the inversion factor Q = f rep / Δf rep (where f repThat is the optical frequency comb mode spacing f ms , Δf rep is the interferogram spectrum pattern spacing Δf s ), which will ultimately ensure that the measurement resolution of the dual-comb spectrum under this architecture is the target spectral resolution of 250MHz. Figure 1 The conventional DCS technology with a repetition rate of 250 MHz is shown. In one interferogram cycle, there is only one central burst structure. This embodiment can enhance the mode amplitude coherence on the spectrum to twice that of a single central burst (the energy is 4 times, and the amplitude is times), ultimately increasing the measured spectral signal-to-noise ratio by 2 times, thereby reducing the coherent averaging time to 1 / 4 of the original time while achieving the same signal-to-noise ratio. In addition, in this interferogram structure, one cycle is 1ms (1 / Δf s ), there are 4 central bursts that can provide effective phase sampling information, so the tracking rate of the phase noise of the self-correction algorithm can be increased from 1kHz (Δf s ) is increased to 4kHz(4Δf s ), which enables the algorithm to track the phase noise on the interferogram at a rate of 4kHz (4Δf s ) and the interferogram repetition frequency 1kHz (Δf s ) decoupling.

[0042] The interferograms generated by the first and second photodetectors 7 and 8 are collected by a data acquisition and processing system 9 (comprising a data acquisition board and computer, with 14-bit vertical accuracy, 250 MHz bandwidth, and a sampling rate of 1 GS / s). This system uses a self-correction algorithm to further eliminate phase noise remaining from the optical frequency comb source and introduced along the optical path. The system then performs coherent averaging and Fourier transform on the corrected interferograms. The spectrum of the measurement path is then normalized using the spectrum of the reference path to ultimately infer the sample absorption spectrum. This system achieves a spectral resolution of 250 MHz and a single measurement time of ≤10 ms.

[0043] [Example 2]

[0044] Based on Example 1, this embodiment adjusts the optical path configuration to achieve phase spectrum measurement, such as Figure 4 As shown:

[0045] In this embodiment, the coupling module uses four 1-to-2 type fiber optic couplers with a splitting ratio of 50:50, namely, the second coupler 10, the third coupler 11, the fourth coupler 12, and the fifth coupler 13.

[0046] The output light of the first optical frequency comb 1 is coupled into the second coupler 10 and split into two beams. One beam passes through the gaseous sample 6 and is received by the fourth coupler 12 ; the other beam is received by the fifth coupler 13 .

[0047] After passing through the electro-optical phase modulation 3 , the output light of the second optical frequency comb 2 is coupled into the third coupler 11 and split into two beams, one of which is received by the fourth coupler 12 , and the other is received by the fifth coupler 13 .

[0048] The outputs of couplers 12 and 13 are received by first photodetector 7 and second photodetector 8. The subsequent process is the same as in Example 1. Since only one of the two optical frequency combs undergoing asynchronous optical sampling on first photodetector 7 passes through the gaseous sample 6, the phase shift caused by the gas can be transmitted to the RF domain during the differential process, thereby measuring the phase spectrum information of the sample.

[0049] [Example 3]

[0050] Based on Example 1, this embodiment increases the pulse repetition frequency of the first optical frequency comb 1 and also applies phase modulation to the pulses it outputs to achieve further enhancement of the spectral signal-to-noise ratio. Figure 5 As shown, the principle diagram is as follows Figure 6 shown.

[0051] The optical frequency comb module in this embodiment 2 includes a first optical frequency comb 1 and a second optical frequency comb 2. The first optical frequency comb 1 and the second optical frequency comb 2 both use a mode-locked fiber laser with a relatively high pulse repetition frequency, which is about 4 times the target spectral resolution of 250 MHz. The repetition frequencies are respectively f rep1 =999.996MHz (corresponding principle diagram Figure 6 4(f ms -Δf s )) and f rep2 =1GHz(corresponding principle diagram Figure 6 4f in ms ). The remaining parameters and settings are the same as in Example 1.

[0052] A second electro-optical phase modulator 14 (with the same specifications) is added to the timing phase control module. The RF signal generator 4 then generates a new amplitude V pp =100mV, cycle frequency is f mod =249.999MHz, 25% duty cycle square wave signal is used to drive the second electro-optical phase modulator 14. The driver integrated in the second electro-optical phase modulator 14 also rectifies the signal into a periodic [V π , 0, 0, 0] signal (where V π is the half-wave voltage of the electro-optic phase modulator).

[0053] The second electro-optical phase modulator 14 also phase-modulates the pulse sequence output by the first optical frequency comb 1. By adjusting the phase of the modulation signal output by the RF signal generator 4 relative to the optical frequency comb pulse, stable phase modulation of each pulse is achieved, so that each pulse no longer carries the same or linearly sliding carrier envelope phase (CEP), but instead one of every four pulses has a CEP mutation of π phase.

[0054] The other devices of this embodiment are consistent with those of embodiment 1. However, the process and features of generating the interference pattern are different:

[0055] On the detector, two optical frequency combs undergo asynchronous optical sampling processes, such as Figure 6 As shown. Since the first optical frequency comb 1 and the second optical frequency comb 2 have a small pulse repetition frequency difference of 4kHz (4Δf s ), so the relative intervals between the pulses of the two pulse trains will gradually slip, which is manifested as the pulses with a π phase gradually approaching other normal phase pulses and interfering. Finally, the situation within one cycle in the time domain can be divided into four parts - determined by which phase of the pulse of the second optical frequency comb the pulse of the π phase of the first optical frequency comb approaches, and four corresponding central bursts are generated on the interference pattern. Among them, the first central burst forms an interference pulse pair because the pulse of the π phase in the first optical frequency comb just matches the pulse of the π phase in the second optical frequency comb. At this time, the phase pairings in all pulse pairs are π-0 and π-π. According to the photocurrent formula generated by the interference of the two optical electric fields on the detector during the asynchronous optical sampling process introduced in Example 1, at this time all Δφ m = 0, so the phase of the interfering electric field is not affected, resulting in a normal oscillation central burst; while the second and fourth central bursts have two unusual π-0 phase-matched pulse pairs every two normal 0-0 phase-matched pulse pairs. When the pulse pair switches from the normal 0-0 phase matching to the unusual π-0 phase matching, that is, Δφ m From 0 to π, cos(Δφ+Δφ m ) will change from cosΔφ to -cosΔφ. Therefore, the phase of the interference electric field generated at this time will flip twice for every two pulse pairs, which will eventually cause the oscillation of the generated central burst to be irregular and no longer a normal carrier frequency; because the matching rules of the pulse pairs of the third central burst are 0-π, 0-0, π-0, 0-0, its interference electric field will flip once every other point compared to the first central burst area - oscillation faster, that is, a higher carrier frequency. Accordingly, at this time, a radio frequency low-pass filter can be used to filter out other central burst structures except the first central burst on the interference pattern, and the period of the interference pattern will become the same as Figure 1 The same 1ms (1 / Δf s ), the corresponding mode spacing is 1kHz(Δf s). The difference is that this single central burst comes from 4 times the number of pulse pairs, and the corresponding interference electric field is where n0 is Figure 1 The number of pulse pairs contributing to the total number of conventional DCS pulses is shown. Therefore, the central burst amplitude can be increased by a factor of 4 due to the convolution of the detector response and the low-pass filter. Combining the mode spacing of the first optical frequency comb 1 and the second optical frequency comb 2, and the well-known interferogram in dual-comb spectroscopy, the inversion factor Q = f rep / Δf rep This will ultimately ensure that the measurement resolution of dual-comb spectroscopy under this architecture is the target spectral resolution of ~250 MHz, thereby decoupling the dual-comb spectral resolution and the optical frequency comb pulse repetition frequency. This ultimately increases the measured spectral signal-to-noise ratio by a factor of four, reducing the coherent averaging time to 1 / 16 while achieving the same signal-to-noise ratio.

[0056] [Example 4]

[0057] This embodiment adjusts the periodic frequency of the modulation signal on the basis of embodiment 3 to achieve higher resolution. Figure 5 As shown, the adjustment parts are:

[0058] The repetition frequency of the first optical frequency comb 1 is adjusted to 999.99 MHz.

[0059] The first electro-optical phase modulator 3 is driven by a periodic signal [0, 0, 1, 1, 0, 0, 0, 1, 0, 0] generated by a radio frequency signal generator 4. The signal has a periodic frequency f mod =100MHz, 30% duty cycle square wave signal.

[0060] The second electro-optical phase modulator 14 is driven by the periodic [0, 0, 1, 1, 0, 0, 0, 1, 0, 0] signal generated by the radio frequency signal generator 4. The signal has a periodic frequency f mod =99.999MHz, 30% duty cycle square wave signal.

[0061] Combining the principle analysis in Example 3, it can be seen that at this time, all Δφ in the first central burst area m = 0, so the oscillation frequency remains unchanged, while other regions still have their own Δφ m The final interferogram period will become 10 times that before modulation, and the spectral resolution will become ~100 MHz, which can achieve a 10-fold improvement in signal-to-noise ratio compared to the traditional 100 MHz resolution DCS technology.

[0062] [Example 5]

[0063] This embodiment adjusts the optical frequency comb module based on embodiment 3, using a common cavity dual-comb light source instead of two independent optical frequency combs. The device diagram is shown in FIG. Figure 7 As shown. The adjustment parts are:

[0064] The optical frequency comb module in this embodiment 5 includes a third optical frequency comb 15. The third optical frequency comb 15 uses a common cavity dual-comb mode-locked laser, which can generate two trains of repetition frequencies of f in two polarization directions in one resonant cavity. rep1 =799.99MHz and f rep2 =800MHz mode-locked pulse sequence. The center wavelength of both is 1550nm, and the spectrum coverage range is 1510-1580nm. The RF signal generator 4 generates two amplitudes V pp =100mV, cycle frequency f mod1 =199.9975MHz and f mod2 The first electro-optical phase modulator 3 and the second electro-optical phase modulator 14 are driven by a square wave signal with a frequency of 200 MHz and a duty cycle of 25%.

[0065] The other devices and principles of this embodiment are consistent with those of embodiment 3. Finally, spectrum measurement with a spectrum resolution of 200 MHz can be achieved.

[0066] [Example 6]

[0067] This embodiment adds a phase delay module on the basis of embodiment 3, further improving the long-term stability of pulse phase modulation. The device schematic diagram is shown in FIG. Figure 8 shown.

[0068] The phase delay module in this embodiment includes a first electrical phase shifter 16 and a second electrical phase shifter 17, with a bandwidth of 0-4 GHz and a phase modulation range of 180° / GHz. This module can adjust the phase delay of the modulated signal and thus the relative phase / time delay between the modulated signal and the optical frequency comb pulse sequence, preventing the pulse from being modulated by the rising or falling edge of the square wave signal, thereby improving the long-term stability of the system.

[0069] [Example 7]

[0070] This embodiment is based on the sixth embodiment, in which the electrical phase shifter in the phase delay module is replaced with an adjustable optical delay line. Figure 9 shown.

[0071] The phase delay module in this embodiment includes a first adjustable optical delay line 18 and a second adjustable optical delay line 19, with an operating wavelength of 1550 nm and an optical delay range of 0-500 ps. It can delay the optical pulse to adjust the relative phase / time delay between the modulation signal and the optical frequency comb pulse sequence, preventing the pulse from being modulated by the rising or falling edge of the square wave signal, thereby improving the long-term stability of the system.

[0072] [Example 8]

[0073] This embodiment is based on the embodiment 1, and the transmission measurement of the gaseous sample to be tested is changed to backscattering measurement. Figure 10 shown.

[0074] The coupling module in this embodiment uses a fourth coupler 12 and a three-port optical circulator 20. The output of the fourth coupler 12 enters port 1 of the optical circulator 20, then passes to port 2 to enter the gaseous sample to be tested. The backscattered light passes through port 2 of the circulator and enters port 3, where it is received by the first detector 7.

[0075] At the same time, the reference background is obtained by first measuring the spectrum without the sample to be tested, so the second detector 8 can be omitted.

[0076] The technical route of the signal-to-noise ratio improvement method of the dual-comb interferometer based on harmonic pulse coherence enhancement provided by the present invention is to make the interference pattern period (the inverse of the pattern interval of the spectrum) become an integer multiple of the original through periodic or quasi-periodic pulse phase regulation, so that the spectral resolution becomes an integer fraction of the optical frequency comb pulse repetition frequency. The decoupling of pulse repetition frequency and spectral resolution is achieved, so that a very high pulse repetition frequency can be retained under fine spectral resolution, and a higher average optical power can be allowed under the same detector dynamic range limit, thereby reducing the coherent averaging time and reducing the complexity of the locking system. At the same time, the spectral resolution of the spectral system can be flexibly set by controlling the period of the timing phase, thereby improving the flexibility and versatility of the instrument.

[0077] Matters not covered by the present invention are known technologies.

[0078] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement, characterized in that: include: An optical frequency comb module for generating at least two pulse trains with different repetition frequencies, wherein at least one of the repetition frequencies is close to an integer multiple of the target spectral resolution; a timing phase control module, connected to the output end of the optical frequency comb module, and applying periodic or quasi-periodic phase modulation to the pulse sequence with a repetition frequency close to an integer multiple of the target spectral resolution; a photodetector for collecting optical signals of at least two optical frequency combs and converting their interference electric fields into interference pattern electrical signals; A data acquisition and processing system is used to process the interferogram electrical signal to extract spectral information and other information; wherein, the optical frequency comb module generates a pulse sequence with a repetition frequency close to an integer multiple of the target spectral resolution and modulates the timing phase control module, so that the repetition frequency of the central burst on the generated interferogram is a harmonic of the interferogram spectrum pattern interval, that is, multiple central bursts are contained within one interferogram period; or adjacent central bursts contain different carrier frequency components, the repetition frequency of the central burst of the fundamental frequency is equal to its pattern interval, and the amplitude is enhanced.

2. The method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement according to claim 1, characterized in that: When the central burst repetition frequency on the interferogram generated by the system is a harmonic of the mode spacing, the interferogram has multiple central burst structures in one period, which have different and nonlinearly sliding carrier-envelope phases.

3. The method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement according to claim 1 or 2, characterized in that: When periodic or quasi-periodic phase modulation is applied to an optical frequency comb, the interferogram has multiple central burst structures in one period, and the phase modulation applied to the optical pulse is transferred to the interferogram through the asynchronous optical sampling process.

4. The method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement according to claim 1, characterized in that: The timing phase control module includes an electro-optical phase modulator and a radio frequency signal generator. The periodic or quasi-periodic electrical signal output by the radio frequency signal generator drives the electro-optical phase modulator to achieve timing phase control of the optical frequency comb pulse.

5. The method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement according to claim 1 or 4, characterized in that: The period of the phase modulation signal is an integer multiple of the repetition period of the modulated optical frequency comb pulse, and the repetition frequency is or is close to the target spectral resolution.

6. The method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement according to claim 1 or 2, characterized in that: The interference pattern has multiple central burst structures in one cycle, and the algorithm tracking rate of the phase noise on the interference pattern can also be increased to an integer multiple of the interference pattern period frequency.

7. The method for improving the signal-to-noise ratio of a dual-comb interferometer based on harmonic pulse coherence enhancement according to claim 1, characterized in that: It also includes a clock synchronization step for synchronizing the clocks of the optical frequency comb and the phase control module to ensure time base consistency, and adjusting the relative phase between the optical frequency comb pulse and the modulation signal through the phase delay module.

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