CN104316186B - A kind of spectral measurement method of optically-based frequency comb - Google Patents

Abstract

The invention discloses a spectrum measurement method based on an optical frequency comb. In the measurement method, an optical frequency comb controllable in the time domain and frequency domain is used to actively modulate a ring laser resonator based on a phase modulator, so that it becomes a This optical frequency comb has a second optical frequency comb with a slight difference in repetition frequency, and performs dual optical frequency comb spectral detection to obtain an interference signal carrying the information of the sample to be tested; The frequency laser is used to beat the frequency, and the difference frequency signal of the two beat frequency signals obtained is mixed with the interference signal, and the signal obtained by the detection and mixing is used as a spectral signal for Fourier analysis to restore the optical information of the sample to be tested. The invention has the advantage that it can eliminate the error caused by the phase drift of the double optical comb system itself to the spectrum detection, thereby improving the resolution and detection precision of the spectrum measurement.

Description

A Spectral Measurement Method Based on Optical Frequency Comb

technical field

The invention belongs to the field of ultrafast laser science and technology, and in particular relates to a spectral measurement method based on an optical frequency comb.

Background technique

The basic principle of spectral analysis is that under the action of laser, each atom will absorb or emit its own characteristic spectral lines, so the substance can be identified and its chemical composition can be determined according to the spectrum. Spectral measurement technology is an important embodiment of the application of laser, an abstract physical quantity, in production and life. It is widely used in many fields such as explosives detection, biological sample analysis, and metal object detection. At present, the relatively mature spectral measurement method is based on the principle of Michelson interferometer, which divides the input laser into two beams, one of which reaches the moving mirror moving at a constant speed, and the other reaches the fixed mirror; the two beams of light The optical path difference is generated after being reflected by the fixed mirror and the moving mirror respectively, and the formed interference light passes through the sample to be tested, and then the corresponding spectral information is obtained by detecting the interference light signal containing the sample information and performing Fourier transformation on it. For example, the common Fourier transform infrared spectrometer (Fourier transform infrared spectrometer) Transform Infrared Spectrometer, abbreviated as FTIR Spectrometer) follows this working principle. However, this method inevitably requires the movement of mechanical parts to complete the spectral measurement, and needs to cooperate with multiple scans of the sample to achieve high measurement accuracy, which undoubtedly has a negative impact on the stability and measurement speed of the entire measurement process.

On the other hand, in recent years, the emerging optical frequency comb technology has been successfully applied in spectral measurement due to its advantages of providing highly stable ultrashort pulse lasers in the time domain and frequency domain. There are two main methods for using femtosecond optical frequency combs in precision spectral measurements. One is to use optical frequency combs as frequency scales to calibrate continuous lasers and use them for spectral measurements. The other is to use optical frequency combs directly for Spectral measurement, that is, a dual-comb system composed of two optical frequency combs with slightly different repetition frequencies, in which the output light of the first optical comb passes through the sample cell and enters the detector together with the output light of the second optical comb , and perform Fourier analysis on the obtained interference signals of the two optical combs, that is, obtain the spectral information of the sample to be measured by measuring the amplitude and phase information of an optical comb passing through the sample. The dual-comb spectral measurement method has the obvious advantages of fast measurement speed, high spectral resolution, and high signal-to-noise ratio. However, in the actual measurement process, the construction and precise control methods of two optical frequency combs are cumbersome and complicated. The maintenance of the entire system Difficult and expensive.

Contents of the invention

The object of the present invention is to provide a spectrum measurement and method based on an optical frequency comb according to the deficiencies of the above-mentioned prior art. probing.

The object of the present invention is realized by the following technical solutions:

A spectral measurement method based on an optical frequency comb, involving a sample to be measured, characterized in that the measurement method at least includes the following steps: using an optical frequency comb I controllable in the time domain and frequency domain, and converting a part of the output light The signal modulates a ring laser resonator composed of a phase modulator, so that the ring laser resonator becomes an optical frequency comb II with a stable repetition rate.

Separately split the output lasers of the optical frequency combs I and II; beat a part of the output lasers of the optical frequency combs I and II respectively with the continuous frequency-stabilized laser to obtain the optical frequency comb I The difference frequency signal f _error of the carrier envelope phase drift frequency information; the other part of the optical frequency comb I outputs laser light as the probe light and enters the sample cell to measure the sample to be tested and then enters the detection module, while the optical frequency comb II The other part of the output laser light directly enters the detection module as a reference light, and the detection module outputs the interference signal f' of the two; the frequency difference signal f _error is mixed with the interference signal f', so that the The frequency instability generated by the optical frequency comb II in the interference signal f' is offset to obtain the spectral signal f _signal , and then the spectral signal f _signal is subjected to Fourier transform analysis to restore the sample to be tested optical properties.

The optical frequency comb II, that is, the ring laser resonator is composed of a laser pump source, a phase modulator, a gain fiber, a wavelength division multiplexer and a coupling output device, wherein the phase modulator, gain fiber, wavelength division The multiplexer and the coupler are sequentially connected end to end to form a loop, the laser pump source is connected to the wavelength division multiplexer; a beam splitter is arranged between the optical frequency comb II and the optical frequency comb I device, a photodetector, a frequency divider and a sum-frequency component, and the sum-frequency component is connected to the phase modulator.

The laser beam output by the optical frequency comb I is split by the beam splitter, one of the laser beams is selected to detect its repetition frequency signal through the photodetector, and its fundamental frequency signal fr ₁ is filtered, and the fundamental frequency signal fr 1 is filtered. A part of the frequency signal fr ₁ is input to the frequency divider to obtain a difference signal f _Δ , where f _Δ = fr ₁ /n, n is the frequency division multiple; after that, the difference signal f _Δ is combined with the base frequency signal _The other part of fr1 is input to the sum frequency component together, and the superposition value of the two frequencies is generated as the repetition frequency modulation signal of the optical frequency comb II, which is loaded on the phase modulator of the optical frequency comb II, At the same time, the laser output by the laser pump source enters the ring laser resonator through the wavelength division multiplexer, and the phase modulator uses the repetition frequency modulation signal to modulate the input laser to generate ₂ = fr ₁ + f _Δ is the spaced sideband signal, and then passes through the multiple gain and oscillation of the gain fiber to form a frequency comb with a fixed repetition frequency, and output the repetition frequency fr ₂ = fr ₁ through the coupler +f _Δ , so that the output repetition frequencies of the optical frequency comb I and the optical frequency comb II have a difference f _Δ .

filtering a part of the output laser of the optical frequency comb I and the beat frequency signal f _beat1 generated by the beat frequency of the continuous frequency-stabilized laser, and splitting the continuous frequency-stabilized laser beam and a part of the output laser of the optical frequency comb II Perform a beat frequency, and filter out the beat frequency signal f _beat2 generated by the comb teeth at the same position of the optical frequency comb II and the optical frequency comb I and the continuous frequency stabilized laser; then the beat frequency signal f _beat1 Perform beat frequency detection with f _beat2 to obtain a beat frequency signal f _error =|f _beat1 - f _beat2 |.

The advantages of the present invention are:

(1) In the process of realizing dual-comb spectrum measurement, the present invention only needs to precisely lock an optical frequency comb in the time domain and frequency domain to complete the construction of the dual-comb system, thereby greatly reducing the entire The size and maintenance difficulty of the system; at the same time, due to the relatively mature development of electrical components, this method can reduce the economic cost of the system;

(2) The structure of the second optical comb of the present invention is flexible, as long as it is a mode-locked laser based on active modulation technology, it can meet the system requirements, so the frequency modulation process can be performed by electro-optic modulators, acousto-optic modulators, etc. The components are completed, and the structure of the ring laser resonator can choose an all-fiber laser or a block solid-state laser with a spatial structure;

(3) The generation mechanism of the repetition frequency difference of the dual optical comb system in the present invention is simple, flexible in operation, and high in stability; it is different from the traditional method of adjusting the laser cavity length or scanning the piezoelectric ceramic crystal with the external cavity to generate a slight difference in repetition frequency In contrast, the present invention only needs to detect the repetition frequency signal fr ₁ of the first optical comb with a photodetector, and cooperate with the electrical sum frequency and frequency division technology to load the synthesized fr ₁ + fr ₁ /n as a modulation signal on the On the phase modulator of the second optical comb, the precise output of the repetition frequency of the second optical comb can be realized; at the same time, due to the high stability of the output repetition frequency of the first optical comb, the second optical frequency The repetition rate of the comb can also be locked and controlled;

(4) The present invention transmits the frequency instability of the optical comb system by beating with a continuous frequency-stabilized laser and filtering out one of the beating signals; the signal is combined with the interference signal obtained by the double-comb sample Do frequency mixing to offset the error caused by the jitter of the optical comb itself to the spectral detection, thus avoiding the frequency locking of the carrier envelope phase drift of the second optical frequency comb, and the whole operation process is simple and easy;

(5) The spectral measurement method combining dual optical frequency comb technology with cavity-enhanced absorption spectroscopy and Fourier transform spectroscopy. Compared with other spectral measurement technologies, the measurement system does not require any mechanical movement, the measurement speed is faster, and the spectral resolution , higher signal-to-noise ratio;

(6) Dual optical frequency comb spectroscopy technology can use the full spectrum of the optical comb for measurement, similar to the use of countless frequency and phase-stabilized narrow-linewidth lasers, and its spectral resolution is limited by the linewidth of a single comb tooth, usually On the order of kilohertz to sub-hertz;

(7) The optical frequency comb system can realize a high-power optical frequency comb with an average power of tens to hundreds of watts through a single-stage or continuous-stage optical amplifier. The measurement of the spectrum is completed when the seed source pulse has a low repetition rate; at the same time, the high-power optical frequency comb is conducive to the widening of the frequency domain of the optical comb, which can expand the output frequency band of the optical comb to cover ultraviolet, visible and infrared Wide range, so as to realize the spectral measurement of different substances, broaden the application field of the present invention.

Description of drawings

Fig. 1 is the structural representation of the output repetition frequency of the second optical frequency comb accurately controlled by the first optical frequency comb in the present invention;

Fig. 2 is a schematic structural diagram of a method for improving spectral measurement accuracy by means of electrical frequency mixing in the present invention;

Fig. 3 is a schematic diagram of a device for measuring the absorption spectrum of water molecules using a near-infrared dual-comb system in the present invention.

detailed description

The features of the present invention and other relevant features are described in further detail below in conjunction with the accompanying drawings through the embodiments, so as to facilitate the understanding of those skilled in the art:

As shown in Figure 1-3, the marks 1-30 in the figure are: optical frequency comb 1, optical frequency comb 2, beam splitter 3, photodetector 4, frequency divider 5, crystal oscillator 6, and frequency components 7, wave Demultiplexer 8, gain fiber 9, phase modulator 10, output coupler 11, sample to be tested 12, spectral detection device 13, data acquisition module 14, continuous frequency stabilized laser emission module 15, optical comb and continuous laser beat frequency Signal processing module 16, electrical frequency mixing and filtering module 17, Fourier transform analyzer 18, optical frequency comb 19, optical frequency comb seed source 20, continuous frequency stabilized laser 21, photonic crystal fiber 22, adjustable bandpass filter 23 , polarizer 24, polarizing beam splitter 25, water molecule sample cell 26, infrared band half mirror 27, infrared band full mirror 28, optical comb and continuous laser beat frequency and signal filtering module 29, optical comb Beat frequency and signal filtering module 30 with continuous laser;

Among them, LD is the laser pumping source.

Embodiment 1: This embodiment specifically relates to a spectral measurement method based on an optical frequency comb. The measurement method first utilizes an optical frequency comb controllable in the time domain and frequency domain to actively modulate a ring laser resonator based on a phase modulator , making it a second optical frequency comb with a slight difference in repetition frequency from this optical frequency comb, and performing dual-comb spectral detection to obtain an interference signal carrying the information of the sample to be tested; at the same time, the two optical frequency combs Beat the frequency with the continuous frequency-stabilized laser respectively, and the difference frequency signal of the two beat-frequency signals obtained is mixed with the interference signal, and the signal obtained by detecting the mixed frequency is used as the spectral signal for Fourier analysis to restore the sample to be tested optical information.

As shown in Figure 1, the process of precisely controlling the output repetition frequency of the second optical frequency comb 2 through the first optical frequency comb 1 is as follows:

The laser beam emitted by the optical frequency comb 1 is split by the beam splitter 3, one of the laser beams is selected to detect its repetition frequency signal through the photodetector 4, and the fundamental frequency signal fr ₁ is filtered, and a part of the fundamental frequency signal is input The frequency divider 5 obtains the difference signal f _Δ of the frequency in the radio frequency band (where f _Δ = fr ₁ /n, n is the frequency division multiple), and then uses this signal to drive the crystal oscillator 6 to generate a stable frequency signal f _Δ The f _Δ signal and another part of the base frequency signal fr ₁ are input into the electrical sum frequency element 7 together, and the superposition value of the two frequencies is generated as the repetition frequency modulation signal of the second optical frequency comb 2, which is loaded on the optical frequency comb 2 On the phase modulator 10, the ring laser resonator of the optical frequency comb 2 generates a stable pulse sequence with a repetition frequency of fr ₂ = fr ₁ + f _Δ . The repetition frequency signal of the second optical frequency comb 2 controlled by this method has the same stability as the repetition frequency signal of the first optical frequency comb 1, both of which are controlled by an external standard frequency source, and the frequency value of the difference between the two f _Δ is also characterized by absolute stability.

As shown in Figure 2, the process of improving spectral measurement accuracy through electrical frequency mixing is as follows:

Firstly, split the output laser light of the first optical frequency comb 1 whose repetition frequency and carrier envelope phase frequency are precisely controlled, and a part of the laser light enters the sample cell as the probe light of the dual-comb spectroscopy system to measure the sample 12 to be tested. Enter the spectral detection device 13, another part of laser light and the laser light emitted by the continuous frequency-stabilized laser emission module 15 do beat frequency to obtain the beat frequency signal _fbeat1 ; meanwhile, the output laser light of the second optical frequency comb 2 that only locks the repetition frequency is performed For beam splitting, a part of the laser light directly enters the spectrum detection device 13 as a reference light, and the other part of the laser light is beat with the laser emitted by the continuous frequency-stabilized laser emitting module 15 in the same way to obtain the beat frequency signal f _beat2 .

Afterwards, the initial interference signal f' is obtained through the spectral detection device 13 and the data acquisition module 14. In the optical comb system, the carrier envelope phase drift frequency of the second optical frequency comb 2 is not locked, so the f' signal carries the spectral information of the sample and the carrier envelope phase drift information of the optical frequency comb 2 at the same time. At the same time, the error signal f _error = | f _beat1 - f _beat2 | of the beat frequency signal f _beat1 and the beat frequency signal f _beat2 is generated by the optical comb and the continuous laser beat frequency signal processing module 16, because the f _beat1 signal is absolutely stable, so The carrier envelope phase drift information of the optical frequency comb 2 is transmitted to the error signal f _error through f _beat2 .

Finally, since f' and f _error contain the same amount of frequency drift caused by the second optical comb, the two signals are mixed through the electrical mixing and filtering module 17, and the frequency instability is offset by electrical means , and the obtained spectral signal f _signal passes through the Fourier transform analyzer 18 to obtain the information of the absorption spectral line of the sample to be tested. The method eliminates the error caused by the inaccuracy of the measurement tool to the spectrum measurement result, and realizes the high resolution and high accuracy measurement of the spectrum.

The following is a specific description of the devices mentioned in the above steps:

As shown in Figure 1, the first optical frequency comb 1 (comb ₁ ) precisely controlled in the time-frequency domain is a necessary condition for this embodiment to realize dual-comb spectral measurement. The control process of the optical frequency comb mainly refers to precisely locking its The repetition frequency signal and the carrier envelope phase shift frequency signal are on external standard signal sources, which include commercially available standard atomic clock modules, atomic transition spectral lines or ultra-narrow linewidth continuous frequency-stabilized lasers, etc. A beam splitter 3, a photodetector 4, a frequency divider 5, a crystal oscillator 6, and a frequency component 7 are arranged between the optical frequency comb 1 and the optical frequency comb 2, and the frequency component 7 is connected to the optical frequency comb 2, It is necessary to select the beam splitter 3 and the photodetector 4 of the corresponding band according to the central wavelength of the output spectrum of the optical frequency comb, and detect the repetition frequency signal of the first optical frequency comb 1. According to the specific frequency of the fundamental frequency signal f _r1 of the repetition frequency The numerical values determine the central response frequency and bandwidth of the frequency divider 5, the crystal oscillator 6 and the sum frequency element 7, so as to suppress the electronic noise of the system and improve the detection accuracy.

As shown in Figure 1, the second optical frequency comb 2 (comb2) is an active mode-locked laser based on a phase modulator 10, which can choose an all-fiber structure, a half-space half-fiber structure or a bulk solid-state laser. The pump laser output from the source LD enters the ring laser resonator through the wavelength division multiplexer 8, and the ring laser resonator is mainly composed of the wavelength division multiplexer 8, the gain fiber 9, the phase modulator 10 and the coupler 11, wherein The gain fiber 9 can be fiber or bulk crystal doped with ytterbium, erbium or other rare earth ions according to the center frequency of the spectroscopic measurement system. The frequency of the input laser is modulated by the phase modulator 10 to generate sideband signals with an interval of fr ₂ = fr ₁ +f _Δ , and then through the gain and multiple oscillations of the ring laser resonator, a frequency comb with a fixed repetition frequency is formed The teeth are output through the output coupler 11; the selection of the output coupler 11 can be based on the structure of the laser to select a fiber beam splitter with a corresponding wavelength band or a space coupling mirror with a certain transmittance.

As shown in Fig. 1, both optical frequency comb 1 and optical frequency comb 2 have main output ports for subsequent laser frequency domain expansion, continuous laser beat frequency and spectral detection.

As shown in Figure 2, the sample 12 to be tested can present various physical states such as gaseous state, liquid state or solid state, and in the measurement process, the central wavelength of the output of the optical frequency comb system can be determined according to the approximate position of the optical absorption peak of the sample; meanwhile, in order to obtain The optical information of the sample in a wide frequency range can cooperate to broaden the output spectrum of the optical frequency comb.

As shown in Figure 2, the optical frequency comb 1 is used as signal light to directly enter the sample 12 to be detected for detection, and the output light of the optical frequency comb 2 is used as a reference light without passing through the sample 12 to provide signal reference and data processing for the entire detection system The basis of normalization in the process, the role of the spectrum detection device 13 is to detect the interference signal generated by the beat frequency of the signal light and the reference light, which appears as an irregular peak signal at the lower edge of the entire amplitude envelope in the frequency domain, so that It reflects the absorption of the incident laser light by the sample 12 to be tested. The spectral detection device 13 can be composed of two or more avalanche photodetectors of corresponding wavelength bands built by oneself, or can be composed of commercially available balanced detectors with corresponding circuit control.

As shown in Figure 2, the data acquisition module 14 is the part that combines computer and special testing software and hardware products to realize spectral data acquisition and control, and can select different types of data acquisition cards according to the measurement accuracy of different dual-comb spectral systems to realize flexible , Accurate data recording, get the initial light field interference signal f'.

As shown in Figure 2, the continuous frequency-stabilized laser emission module 15 first needs to meet the requirements that the output continuous laser has an excellent single longitudinal mode characteristic and has an ultra-narrow linewidth in the frequency domain, which is the key to obtain stable beats between the optical comb and the continuous laser. It is also the basic requirement for the spectral measurement system to improve the detection accuracy; secondly, the output wavelength of the continuous laser must coincide with the output center wavelength of the optical frequency comb 1 and 2, so as to ensure that the optical comb and its beat frequency signal have the same For a more ideal signal-to-noise ratio, it is generally necessary to make the amplitude of the signal on the spectrum analyzer greater than 25dB. During the construction of the entire system, a part of the laser obtained by continuous laser beam splitting can be selected as the standard frequency source of the first optical frequency comb 1 to control its time-frequency domain accuracy, which can further reduce the complexity of the system and improve the reliability of the device. stability and compactness.

As shown in Figure 2, the optical comb and continuous laser beat frequency signal processing module 16 generally includes components such as photodetectors, filters, and mixers, and its main function is to detect and filter out the same position in the two optical frequency combs. The two comb teeth are respectively mixed with the two beat frequency signals generated by the continuous laser, and the two beat frequency signals are mixed to obtain the difference frequency signal f _error ; the photodetector generally uses an avalanche photodiode, and the selection of the filter depends on The beat frequency signal depends on the frequency band, and the bandwidth is generally around 10MHz. The mixer needs to have good response rate and noise suppression characteristics.

As shown in Figure 2, the function of the electrical mixing and filtering module 17 is to mix the above-mentioned interference signal f' with the difference frequency signal f _error to make a difference frequency, so as to offset the frequency instability of the optical comb system, and obtain the spectral signal f to be restored _signal .

As shown in FIG. 2 , the function of the Fourier transform analyzer 18 is to restore the obtained spectral signal in the radio frequency band to a time domain signal, so as to read and analyze the relevant optical characteristics of the sample to be tested.

Embodiment two: As shown in Figure 3, this embodiment specifically relates to a method for measuring the absorption spectrum of water molecules using a near-infrared dual-comb system, and the specific steps are as follows:

(1) An erbium-doped fiber optical frequency comb 19 is used as one of the stable light sources of the dual-comb spectral detection system, its output center wavelength is around 1550nm, the average power is on the order of 1W, the pulse width is around 100fs, and the repetition rate is The average value is 250MHz; the optical frequency comb is locked on the ultra-narrow linewidth continuous frequency stabilized laser 21 through the servo feedback circuit system, and the extremely high spectral line precision generated by the atomic transition in the outside world can be transferred to the optical frequency comb system.

(2) The cavity structure of the erbium-doped fiber optical frequency comb seed source 20 based on the phase modulator is shown in the comb ₂ module in Figure 1, in which the laser pump source LD adopts a 980nm fiber output semiconductor laser, through 980/1550nm The optical fiber wavelength division multiplexer 8 couples the pump light into the ring laser resonator, and the gain medium adopts erbium-doped single-mode fiber, and its gain coefficient is about 70dB/m; The compensation fiber manages the intracavity dispersion, so that the optical frequency comb seed source 20 outputs an ultrashort pulse sequence with a time width of about 100 fs.

(3) Erbium-doped fiber optical frequency comb seed source based on phase modulator The phase modulator in the seed source 20 adopts the electro-optic modulator based on lithium niobate crystal, and utilizes the nonlinear optical characteristics of lithium niobate crystal to complete the repetition of the ring laser resonator The active modulation of the frequency, the generation method of the modulation frequency is as mentioned above, according to the precise signal fr ₁ produced by the first optical frequency comb 19, the pulse and pulse frequency interval of the output light of the second optical frequency comb seed source 20 is actively modulated as follows: fr ₂ = fr ₁ + f _Δ ; the output lasers of the two optical frequency combs are both in the state of spatial light.

(4) In the process of measuring the absorption spectrum of a water molecule sample using a dual-comb system, the output light of the optical frequency comb 19 and the optical frequency comb seed source 20 is first input into the frequency domain broadening module based on the photonic crystal fiber 22, the purpose of which is to Broaden the frequency range covered by the optical frequency comb with a wavelength of 1550nm as much as possible to collect the optical properties of the sample in different frequency bands; at the same time, by reasonably adjusting the polarization state, peak power, and pulse width of the photonic crystal fiber 22 incident light And other parameters, so that the two optical frequency combs have a relatively consistent output spectral distribution, which is conducive to improving the coverage and accuracy of the radio frequency spectrum generated in the dual-comb spectral detection, thus affecting the quality of the spectral measurement results.

(5) In order to improve the accuracy of spectral measurement, usually the two optical comb lasers that have been broadened in the frequency domain pass through the adjustable bandpass filter 23 to segment the wider spectral range of the optical frequency comb, and then pass through the sample for detection. Then the spectral measurement results of different frequency bands are combined to restore the entire test image. The central wavelength of the tunable bandpass filter 23 should be consistent with the central output wavelength of the optical comb, which is 1550nm here.

(6) Use the output light of the optical frequency comb 19 as the probe light, and the output light of the optical frequency comb seed source 20 as the reference light to detect the absorption spectrum of the water molecule sample. By using the polarizing beam splitter 25, the water molecule sample cell The detection light of 26 and the detection light that has not passed through the water molecule sample cell 26 all carry out coherent beam combination detection with the reference light, so as to improve the contrast ratio of the detection signal and enhance the authenticity and reliability of the experimental data; the effect of the polarizer 24 is to further Improve the signal-to-noise ratio of the detected spectral signal.

(7) The interference signals of the two optical combs enter the data acquisition module 14 through the spectral detection device 13 built by the balanced detection method for data collection and processing, and obtain the interference signal f' carrying the spectral information of the sample and the frequency drift of the optical combs at the same time .

(8) On the other hand, by splitting the output light of the optical frequency comb 19 and the optical frequency comb seed source 20, the two optical combs are respectively frequency-beated with the output light of the continuous frequency-stabilized laser 21, and the optical comb and the continuous laser The beat frequency and signal filtering modules 29 and 30 both include reflective gratings, pinhole diaphragms, focusing lenses, avalanche photodetectors, narrowband filters and other beat frequency signal detection and filtering components; The pitch angle and relative position make the amplitude of the detected beat frequency signal greater than 25dB. At the same time, the selection of narrowband filters in the two modules needs to filter out the beat frequency signals generated by the optical comb teeth and continuous laser at the same position to ensure The frequency accuracy of the entire system.

(9) The radio frequency signals f _beat1 and f _beat2 respectively generated by the beat frequency and signal filtering modules 29 and 30 of the optical comb and the continuous laser pass through the mixer to obtain the difference frequency signal f _error =|f _beat1 - f _beat2 |, where Contains the frequency jitter of the comb system caused by the unlocked carrier-envelope phase frequency of the second comb.

(10) Mix and filter the two signals, the interference signal f' and the difference frequency signal f _error , which carry the sample spectral information and the frequency drift of the optical comb at the same time. By reasonably selecting the response frequency and bandwidth of the mixer and filter, the The difference frequency signal f _signal of the two signals is extracted, and then the f _signal signal is entered into the Fourier transform analyzer for spectrum restoration, thereby offsetting the error caused by the frequency instability of the optical frequency comb to the spectral measurement result, and realizing A new measurement method to obtain spectral information by controlling and obtaining the amplitude and phase of a femtosecond optical frequency comb passing through the sample to be measured.

Claims (5)

1. the spectral measurement method of an optically-based frequency comb, relate to testing sample, it is characterized in that described measuring method at least comprises the steps: to use a time domain and the controlled optical frequency com I of frequency domain, one loop laser resonance cavity being made up of phase-modulator of a part of optical signal modulation output it, makes described loop laser resonance cavity become the stable optical frequency com of repetition rate II.

The spectral measurement method of a kind of optically-based frequency comb the most according to claim 1, it is characterised in that respectively the Output of laser of described optical frequency com I, II is split；A part of Output of laser of described optical frequency com I and II is done beat frequency with continuous Frequency Stabilized Lasers respectively, obtains carrying the difference frequency signal f of the carrier envelope phase drift frequency information of described optical frequency com I_error；Another part Output of laser of described optical frequency com I enters after sample cell measures described testing sample as detection light and enters back into detecting module, another part Output of laser of the most described optical frequency com II is directly entered described detecting module as reference light, and described detecting module exports both interference signal f '；By described difference frequency signal f_errorIt is mixed with described interference signal f ', to be balanced out by the frequency instability produced by described optical frequency com II in described interference signal f ', obtains spectral signal f_signal, then by described spectral signal f_signalCarry out Fourier transformation analysis, reduce the optical characteristics of described testing sample.

The spectral measurement method of a kind of optically-based frequency comb the most according to claim 1, it is characterized in that described optical frequency com II, the most described loop laser resonance cavity is made up of laser pumping source, phase-modulator, gain fibre, wavelength division multiplexer and coupling follower, wherein said phase-modulator, gain fibre, wavelength division multiplexer and coupling follower head and the tail successively connect and compose loop, and described laser pumping source is connected with described wavelength division multiplexer；Be provided with between described optical frequency com II and described optical frequency com I beam splitter, photodetector, frequency divider and and frequency components and parts, described and frequency components and parts be connected with described phase-modulator.

The spectral measurement method of a kind of optically-based frequency comb the most according to claim 3, it is characterized in that the laser exported by described optical frequency com I is split through described beam splitter, choose a wherein road laser and detect its repetition rate signal, and leaching its fundamental frequency signal fr by described photodetector₁, by described fundamental frequency signal fr₁A part input described frequency divider, to obtain difference signal f_Δ, wherein f_Δ= fr₁/ n, n are frequency dividing multiple；The most described difference signal f_ΔWith described fundamental frequency signal fr₁Another part together input described and frequency components and parts, produce the superposition value repetition rate modulated signal as described optical frequency com II of two frequencies, load on the described phase-modulator of described optical frequency com II, the laser that the most described laser pumping source is exported enters in described loop laser resonance cavity by described wavelength division multiplexer, described phase-modulator utilizes described repetition rate modulated signal to be modulated the laser of input, produces with fr₂= fr₁+f_ΔFor the sideband signals at interval, then through the repeatedly gain of described gain fibre and vibration, form the frequency comb that repetition rate is fixing, export repetition rate fr by described coupling follower₂= fr₁+f_Δ, make the output repetition rate of described optical frequency com I and described optical frequency com II have difference f_Δ。

The spectral measurement method of a kind of optically-based frequency comb the most according to claim 1, it is characterised in that the beat signal f that a part of Output of laser of optical frequency com I described in leaching produces with continuous Frequency Stabilized Lasers beat frequency_beat1, a part of Output of laser of described continuous Frequency Stabilized Lasers beam splitting Yu described optical frequency com II is carried out beat frequency, and the beat signal f that optical frequency com II described in leaching and the comb at described optical frequency com I same position produce with described continuous Frequency Stabilized Lasers simultaneously_beat2；Afterwards by described beat signal f_beat1And f_beat2Carry out difference frequency detection, obtain difference frequency signal f_error=|f_beat1- f_beat2|。