阅读说明：本技术 一种中红外光学频率梳产生的方法及装置 (Method and device for generating intermediate infrared optical frequency comb ) 是由 黎遥 王枫秋 徐永兵 孟亚飞 戴瑞宏 秦嘉嵘 王晟 于 2020-06-09 设计创作，主要内容包括：本发明提供了一种中红外光学频率梳产生的方法,1.5μm超短脉冲光纤激光激发近红外非线性光纤产生2μm中心波长的超短脉冲激光源；2μm超短脉冲源经过掺铥光纤放大器后激发中红外非线性光纤获得倍频光谱宽度的中红外超连续光谱；中红外超连续光谱通过f-2f自参考系统,获得频率梳的载波偏移频率f<Sub>0</Sub>；通过反馈控制锁模种子源的泵浦光功率,实现偏移频率f<Sub>0</Sub>锁定；进一步将1.5μm超短脉冲激光与精密稳定的1.5μm连续激光拍频获得重复频率f<Sub>r</Sub>的误差信号；该误差信号反馈控制激光器的腔长,实现中红外光频梳的重复频率锁定；f<Sub>0</Sub>和f<Sub>r</Sub>精密锁定后中红外超连续谱光源即为中红外的光频梳。(The invention provides a method for generating a mid-infrared optical frequency comb, wherein 1.5 mu m ultra-short pulse optical fiber laser excites a near-infrared nonlinear optical fiber to generate an ultra-short pulse laser source with the central wavelength of 2 mu m; after passing through a thulium-doped optical fiber amplifier, a 2-micron ultrashort pulse source excites a mid-infrared nonlinear optical fiber to obtain a mid-infrared supercontinuum with a frequency doubling spectral width; the carrier offset frequency f of the frequency comb is obtained by the intermediate infrared super-continuum spectrum through an f-2f self-reference system 0 (ii) a The offset frequency f is realized by feedback control of the pump light power of the mode locking seed source 0 Locking; further mixing the 1.5 mu m ultrashort pulse laser withPrecise and stable 1.5 mu m continuous laser beat frequency obtaining repetition frequency f r The error signal of (2); the error signal feedback controls the cavity length of the laser to realize the repetition frequency locking of the intermediate infrared optical frequency comb; f. of 0 And f r And the middle infrared supercontinuum light source is the middle infrared light frequency comb after the precise locking.)

1. A method for generating a mid-infrared optical frequency comb is characterized in that 1.5 mu m ultra-short pulse optical fiber laser excites a near-infrared nonlinear optical fiber to generate an ultra-short pulse laser source with 2 mu m central wavelength; after passing through a thulium-doped optical fiber amplifier, a 2-micron ultrashort pulse source excites a mid-infrared nonlinear optical fiber to obtain a mid-infrared supercontinuum with a frequency doubling spectral width; the carrier offset frequency f of the frequency comb is obtained by the intermediate infrared super-continuum spectrum through an f-2f self-reference system₀(ii) a The offset frequency f is realized by feedback control of the pump light power of the mode locking seed source₀Locking; further beating the 1.5 mu m ultrashort pulse laser and the precise and stable 1.5 mu m continuous laser to obtain the repetition frequency f_rThe error signal of (2); the error signal feedback controls the cavity length of the laser to realize the repetition frequency locking of the intermediate infrared optical frequency comb; f. of₀And f_rAnd the middle infrared supercontinuum light source is the middle infrared light frequency comb after the precise locking.

2. The method of claim 1, wherein the 1.5 μm ultrashort pulse fiber laser is provided by a saturable absorber, a nonlinear ring mirror, or a nonlinear polarization rotation mode-locked erbium doped fiber laser.

3. The method of claim 2, wherein 1.5 μm ultrashort pulse fiber laser is subjected to optical power amplification by a dispersion managed erbium doped fiber amplifier; the amplified 1.5 mu m ultrashort pulse laser is incident to the near-infrared high nonlinear optical fiber, and generates ultrashort pulse laser with 2 mu m wavelength through spectral self-phase modulation or Raman process.

4. The method of claim 3, wherein the ultra-short pulse laser of 2 μm wavelength is optically power amplified by a dispersion managed thulium doped fiber amplifier; the intermediate infrared super-continuum spectrum is generated by exciting a fluorine-series or sulfur-series intermediate infrared optical fiber by using amplified 2-micron-wavelength ultrashort pulse laser, and the range of the generated intermediate infrared super-continuum spectrum covers 2-5 microns.

5. The method of claim 1, wherein the f-2f self-reference system beats the long wavelength component in the mid-infrared spectrum with the short wavelength component in the continuum to obtain a carrier offset frequency; the carrier offset frequency f₀F is realized by modulating pumping light current of 1.5 mu m mode-locked fiber laser₀Locking; the specific process is as follows: the middle infrared super-continuum spectrum passes through a dichroic mirror, the long-wavelength component is reflected, and the short-wavelength component is transmitted; the long wavelength component generates light with the same wavelength as the short wavelength branch optical path after frequency multiplication of the frequency multiplication PPLN crystal; after the two paths of light are combined, the light is incident to a detector through a filter (2 mu m band-pass) to perform beat frequency; the detector is connected to an RF radio frequency spectrometer; carefully adjusting the time delay of the 2-micrometer branch optical path, and observing a difference frequency radio frequency signal in an RF (radio frequency) spectrometer, wherein the signal is the carrier offset frequency f0 of the intermediate infrared frequency comb; after obtaining the offset frequency f0, the frequency of f0 is locked to a stable radio frequency signal source by a phase-locked loop.

6. Such asThe method of claim 1, wherein the carrier offset frequency f is stabilized₀The stability of the mid-infrared optical frequency comb is only determined by the repetition frequency f_rThe influence of (a); to f_rAnd locking is carried out, namely the locking of the whole mid-infrared optical frequency comb is completed. The specific process is as follows: the beat frequency locking f is carried out on the pulse light source to be stabilized and the continuous laser with stable wavelength_r(ii) a Namely, a part of output of the 1.5-micron mode-locked pulse seed source and a precise frequency-stabilized 1.5-micron continuous laser are subjected to beat frequency; the beat frequency derived frequency difference signal is then locked to a stable radio frequency source; the variable of locking adjustment is the laser cavity length of the mode locking pulse seed source with the diameter of 1.5 mu m; the cavity length feedback adjustment of the seed laser is realized by adopting an electrically driven piezoelectric ceramic (PZT) to stretch an optical fiber; the frequency precision of the locked intermediate infrared optical frequency comb can reach the mHz magnitude.

7. The method of claim 3, wherein the ultra-short pulse of 1.5 μm has strong self-phase modulation and Raman effect in HNLF, and the nonlinear optical fiber (HNLF) is a negative dispersion germanium-doped nonlinear single-mode fiber or a photonic crystal fiber.

8. The method of claim 4, wherein the pulse power amplification of the 2 μm component seed light employs a double-clad thulium-doped fiber amplifier; the process is as follows: the 2 mu m signal light is coupled to the amplifier system through the optical fiber isolator; a pumping source of the amplifier adopts a 793nm multimode semiconductor laser; the double-clad optical fiber beam combiner couples the 2-micron signal light and the 793nm pump light into the double-clad thulium-doped optical fiber; the signal light is amplified by 793nm pump light in a cladding in the process of transmitting in the fiber core of the thulium-doped optical fiber; by managing the dispersion of the optical fiber link of the amplifier, the average power of the output 2 mu m pulse light reaches W magnitude, and the pulse width is 200 fs.

9. The method of claim 4, wherein to obtain the super-continuum spectrum of the mid-infrared, the amplified 2 μm ultrashort pulse laser is incident on a mid-infrared nonlinear optical fiber; selecting a chalcogenide fiber or a ZBLAN fiber with a high nonlinear coefficient from the intermediate infrared nonlinear fiber; the spectral broadening produces a supercontinuum in the spectral range of 2-5 μm; the longest wavelength of the intermediate infrared super-continuum spectrum is more than 2 times of the shortest wavelength, namely the spectrum range of the frequency doubling layer is obtained.

The technical field is as follows:

the invention belongs to the technical field of laser, and particularly relates to a method and a device for generating a mid-infrared optical frequency comb.

Background art:

the mode-locked ultrafast pulse laser is a periodic ultrashort pulse train in the time domain and shows a frequency comb spectrum with equal intervals in the frequency domain. Any frequency line on the femtosecond optical frequency comb can be represented as f_n＝n×f_r+f₀Wherein f is_rFor the repetition frequency of mode-locked pulsed laser, f₀Is the carrier offset frequency of the spectrum. If two parameters (f) in the spectral line frequency_rAnd f₀) Are precisely locked, all frequency components of the optical frequency comb can be determined. In this case, the optical frequency comb is like a reference ruler that one uses, and it can be used for precise measurement of unknown spectra. Therefore, the optical frequency comb based on the ultrafast laser plays an important role in national defense safety, industrial application and social life. Especially, the method has a very high application prospect in the application of atmospheric environment monitoring. The rotational-vibrational transition energy levels of a large number of molecules in air are concentrated in the mid-infrared band. Gas component such as CO₂CO and NO₂Has strong absorption lines in the 2.8 μm, 2.4 μm and 2.9 μm wave bands. Various hydrocarbons, hydrochloric acid compounds and organic solvents exhibit strong absorption characteristics in the range of 3.2 to 3.6 μm. Therefore, in order to detect greenhouse gases, toxic gases, pollutants and medicines in the atmospheric environment, a stable mid-infrared optical frequency comb with a waveband range of 2-5 μm is urgently needed.

The current common optical frequency comb mainly passes through the mode locking of the near-infrared wave bandYtterbium fiber laser (1 μm) and mode-locked erbium-doped fiber laser (1.5 μm). The two lasers are injected into the nonlinear optical fiber, the range of the generated frequency comb can cover the range of 500nm-2 mu m, and the requirement of the wavelength range of the mid-infrared optical frequency comb cannot be met. An indirect approach is to use an ytterbium-doped fiber laser excited Optical Parametric Oscillator (OPO). OPO can realize mid-infrared optical frequency comb in 2-5 μm range. However, the optical parametric oscillation cavity of the OPO is very sensitive, and needs to be precisely locked with the cavity mode of the pump laser, which increases the difficulty of use. In addition, only laser light in one wavelength band can be output under a specific phase matching condition. By adopting the optical Difference Frequency (DFG) method, the mid-infrared optical frequency comb in the range of 3-5 μm can be obtained. However, the disadvantages of spatial light path and complex system are inevitable. The stability of the resulting mid-infrared optical frequency comb is also affected. The direct method is that the long wavelength pulse laser is adopted to excite the nonlinear optical fiber of the middle infrared, and a light source with the wave band of 2-5 mu m can be obtained. Further locking the carrier offset frequency and the repetition frequency, the mid-infrared optical frequency comb can be obtained. For example, excitation of a nonlinear fiber by 2 micron thulium-doped mode-locked fiber laser or 2.8 μm band Er-ZEBLAN mode-locked fiber laser is expected to meet the requirements of the mid-infrared optical frequency comb. However, compared with erbium-doped and ytterbium-doped fiber lasers, thulium-doped or Er-ZEBLAN fiber lasers have the disadvantages of poor mode locking stability, low conversion efficiency and immature devices. And the optical fiber presents larger negative dispersion in a long wavelength band, and an effective dispersion compensation mechanism is lacked. The pulse width of the thulium-doped or Er-ZEBLAN fiber laser is difficult to compress, the generation of a supercontinuum and the carrier offset frequency f₀The detection of (2) brings more difficulties.

Therefore, in view of the advantages of high stability, mature devices, narrow pulse width and the like of the current mode-locked ytterbium-doped fiber laser and mode-locked erbium-doped fiber laser, whether the mid-infrared frequency comb can be generated based on the near-infrared mode-locked fiber laser is a problem to be solved in the technical field of laser.

The invention content is as follows:

in order to solve the problem of the generation of the mid-infrared frequency comb and change the defects of the existing scheme, the invention aims to provide a method for generating the mid-infrared frequency comb. According to the method, a mid-infrared supercontinuum light source is obtained through near-infrared mode-locked fiber laser, and carrier offset frequency and repetition frequency of the mid-infrared spectrum light source are further locked, so that a precisely locked mid-infrared optical frequency comb is obtained.

The technical scheme of the invention is as follows: a method of mid-infrared optical frequency comb generation, comprising: the ultra-short pulse optical fiber laser with the diameter of 1.5 mu m excites the near-infrared nonlinear optical fiber to generate an ultra-short pulse laser source with the central wavelength of 2 mu m; after passing through a thulium-doped optical fiber amplifier, a 2-micron ultrashort pulse source excites a mid-infrared nonlinear optical fiber to obtain a mid-infrared supercontinuum with a frequency doubling spectral width; the carrier offset frequency f of the frequency comb is obtained by the intermediate infrared super-continuum spectrum through an f-2f self-reference system₀(ii) a The offset frequency f is realized by feedback control of the pump light power of the mode locking seed source₀Locking; further beating the 1.5 mu m ultrashort pulse laser and the precise and stable 1.5 mu m continuous laser to obtain the repetition frequency f_rThe error signal of (2); the error signal feedback controls the cavity length of the laser to realize the repetition frequency locking of the intermediate infrared optical frequency comb; f. of₀And f_rAnd the middle infrared supercontinuum light source is the middle infrared light frequency comb after the precise locking.

The 1.5 mu m ultrashort pulse optical fiber laser is provided by a saturable absorber, a nonlinear annular mirror or a nonlinear polarization rotation mode-locked erbium-doped optical fiber laser. The 1.5 mu m ultrashort pulse optical fiber laser carries out optical power amplification through an erbium-doped optical fiber amplifier managed by dispersion.

The amplified 1.5 mu m ultrashort pulse laser is incident to a near-infrared high nonlinear optical fiber, and ultrashort pulse laser with the wavelength of 2 mu m is generated through spectral self-phase modulation or a Raman process; ultra-short pulse laser with 2 mu m wavelength is subjected to optical power amplification through a dispersion-managed thulium-doped optical fiber amplifier.

The intermediate infrared super-continuum spectrum is generated by exciting a fluorine system or sulfur system intermediate infrared optical fiber by using ultrashort pulse laser with the wavelength of 2 mu m, and the range of the generated intermediate infrared super-continuum spectrum covers 2-5 mu m.

The f-2f self-reference system doubles the frequency of the long wavelength component in the mid-infrared spectrumBeating the short wavelength component in the late and continuous spectrums to obtain the carrier offset frequency f₀. The carrier offset frequency f₀F is realized by modulating pumping light current of 1.5 mu m mode-locked fiber laser₀And (6) locking. The specific process is as follows: the intermediate infrared super-continuum spectrum passes through a dichroic mirror, the long wavelength component is reflected, and the short wavelength component is transmitted. The long wavelength component generates light with the same wavelength as the short wavelength branch optical path after frequency multiplication of the frequency multiplication PPLN crystal; after the two paths of light are combined, the light is incident to a detector through a filter (2 mu m band-pass) to perform beat frequency; the detector is connected to an RF radio frequency spectrometer; carefully adjusting the time delay of the 2 mu m branch optical path, observing a difference frequency radio frequency signal in the RF radio frequency spectrometer, wherein the signal is the carrier offset frequency f of the intermediate infrared frequency comb₀(ii) a At the acquisition of the offset frequency f₀Then f is converted by a phase-locked loop₀The frequency is locked to a stable radio frequency signal source.

The repetition frequency f_rThe error signal controls the resonant cavity length of the 1.5 mu m pulse laser through the electronic phase-locked loop, and the repetition frequency locking of the intermediate infrared optical frequency comb is realized. After the carrier offset frequency f0 is stabilized, fr is locked, that is, the whole mid-infrared optical frequency comb is locked. The specific process is as follows: performing beat frequency locking fr on a pulse light source to be stabilized and continuous laser with stable wavelength; namely, a part of output of the 1.5-micron mode-locked pulse seed source and a precise frequency-stabilized 1.5-micron continuous laser are subjected to beat frequency; the beat frequency derived frequency difference signal is then locked to a stable radio frequency source; the variable of locking adjustment is the laser cavity length of the mode locking pulse seed source with the diameter of 1.5 mu m; the cavity length feedback adjustment of the seed laser is realized by adopting an electrically driven piezoelectric ceramic (PZT) to stretch an optical fiber; the frequency precision of the locked intermediate infrared optical frequency comb can reach the mHz magnitude. The self-phase modulation and Raman effect of the 1.5 mu m ultrashort pulse in HNLF are strong, and the nonlinear fiber (HNLF) adopts a germanium-doped nonlinear single-mode fiber or a photonic crystal fiber with negative dispersion.

The pulse power amplification of the 2 mu m component seed light adopts a double-cladding thulium-doped optical fiber amplifier; the process is as follows, 2 μm signal light is coupled to an amplifier system through a fiber isolator; a pumping source of the amplifier adopts a 793nm multimode semiconductor laser; the double-clad optical fiber beam combiner couples the 2-micron signal light and the 793nm pump light into the double-clad thulium-doped optical fiber; the signal light is amplified by 793nm pump light in a cladding in the process of transmitting in the fiber core of the thulium-doped optical fiber; by managing the dispersion of the optical fiber link of the amplifier, the average power of the output 2 mu m pulse light reaches W magnitude, and the pulse width is 200 fs.

The super-continuum spectrum of the intermediate infrared is obtained by enabling amplified 2-micrometer ultrashort pulse laser to be incident into a nonlinear optical fiber of the intermediate infrared; selecting a chalcogenide fiber or a ZBLAN fiber with a high nonlinear coefficient from the intermediate infrared high nonlinear fiber; the spectral broadening produces a supercontinuum in the spectral range of 2-5 μm; the longest wavelength of the intermediate infrared super-continuum spectrum is more than 2 times of the shortest wavelength, namely the spectrum range of the frequency doubling layer is obtained. The device is built based on the method.

Has the advantages that: the intermediate infrared optical frequency comb obtained by the invention uses the mode-locked laser with the diameter of 1.5 mu m as the seed source of the whole system, has better stability, is easier to realize an all-fiber structure and has very high practical value. In addition, the 2-micrometer laser pulse width for exciting the mid-infrared nonlinear optical fiber can be very narrow, so that the noise coefficient of the frequency comb is expected to be smaller, and the accuracy of the mid-infrared optical frequency comb in the field of precision measurement is improved. The repetition frequency f can be obtained_rThe error signal of (2); the cavity length of the laser is controlled in a feedback mode, and the repetition frequency locking of the intermediate infrared optical frequency comb is achieved.

Description of the drawings:

fig. 1 is a functional composition diagram of a mid-infrared optical frequency comb.

FIG. 2 is a schematic diagram of the generation and frequency stabilization process of mid-IR spectrum optical frequency combs. The upper diagram is a schematic wavelength region diagram of the light source; the lower diagram is a schematic diagram of the frequency domain area and the frequency stabilization process of the light source.

FIG. 3 is a schematic diagram of 1.5 μm mode-locked seed laser, fiber amplifier, and 2 μm laser generation.

FIG. 4 is a diagram of a thulium doped fiber amplifier and generation of mid-infrared super-continuum spectrum.

FIG. 5 is a schematic diagram of frequency detection of a mid-infrared supercontinuum f-2f offset from a reference carrier.

FIG. 6 is a schematic diagram of the detection of a repetition frequency error signal of a mid-IR optical frequency comb.

The specific implementation mode is as follows:

in order to make the objects, technical solutions and advantages of the present invention more apparent, the following is a more detailed description of the present invention with reference to the accompanying drawings by way of examples, but the embodiments of the present invention are not limited thereto.

Fig. 1 shows a schematic diagram of an apparatus for mid-infrared optical frequency comb generation. The device mainly comprises the following components: the system comprises a 1.5 mu m ultrashort pulse mode-locking optical fiber laser seed source 10, a 1.5 mu m waveband erbium-doped optical fiber amplifier 11, a near-infrared nonlinear optical fiber spectrum broadening 12, a thulium-doped optical fiber amplifier 13, a middle-infrared nonlinear optical fiber spectrum broadening 14, an f-2f self-reference system, an f-2f self-reference carrier offset frequency detection system 15 and a repetition frequency error detection system 16.

Fig. 2 shows the method for mid-infrared optical frequency comb generation as follows: the 1.5 μm ultrashort pulse laser 201 excites the near-infrared nonlinear fiber, and generates an ultrashort pulse laser source 202 with a wavelength of 2 μm by spectral broadening, i.e., the spectral broadening 12 of the near-infrared nonlinear fiber. The 2 μm ultra-short pulse source 202 passes through the thulium-doped fiber amplifier and then excites the mid-infrared nonlinear fiber, i.e., the mid-infrared nonlinear fiber spectrum broadening 14, to obtain a mid-infrared super-continuum spectrum 203 with a 2f frequency doubling spectral width. The mid-infrared supercontinuum 203 obtains the carrier offset frequency (f) of the frequency comb through an f-2f self-reference system 204₀)205. And the pump light power of the mode locking seed source is controlled in a feedback mode, so that the carrier offset frequency is locked. Further beating the 1.5 μm pulse seed source laser 201 and the precise frequency stabilized continuous laser 206 to obtain an error signal of the repetition frequency. The cavity length of the laser is controlled in a feedback mode, and the frequency locking of the mid-infrared optical frequency comb is achieved.

FIG. 3 shows a schematic diagram of an embodiment produced by a 1.5 μm seed laser 30, an amplifier 31, and a 2 μm seed laser 32. The mode-locked erbium-doped fiber laser 30 with the wave band of 1.5 mu m is used as a pulse seed source of the middle infrared optical frequency comb. The laser adopts a full-fiber linear cavity structure. The pump light is a single mode 980nm laser 301, which is coupled into the laser cavity by a Wavelength Division Multiplexer (WDM) 302. A semiconductor saturable absorber (SESAM)303 is loaded at one end of the laser to initiate passive mode locking of the laser. The average output power of the mode locking pulse seed source is 2mW, the repetition frequency is 80MHz, the pulse width is 200fs, and the central wavelength is 1550 nm. The seed pulse light further passes through an erbium-doped fiber amplifier 31, and the average power of the pulse is amplified to 100 mW. The fiber amplifier compresses 1.5 μm pulse width to <100fs during amplification by dispersion management and nonlinear process. The amplified 1.5 μm pulse laser light is further incident on a near-infrared nonlinear optical fiber (HNLF) 321. The peak power of the incident pulsed light is high, the nonlinear coefficient of the nonlinear fiber 321 is high, the self-phase modulation and raman effect of the 1.5 μm ultrashort pulse in HNLF is strong, and the output spectrum can be significantly broadened or subjected to long-wavelength frequency shift. In this embodiment, to obtain a large 2 μm wavelength component, the nonlinear fiber (HNLF) is a negative dispersion ge-doped nonlinear single mode fiber or photonic crystal fiber, and the fiber length is about 20 cm. The fourier transform limit pulse width for the 2 μm spectral components generated from phase/raman is less than 100 fs. The component laser is used as a seed source of the next stage for subsequent power amplification.

The pulse power amplification of the 2 mu m component seed light adopts a double-cladding thulium-doped optical fiber amplifier. An exemplary embodiment of a fiber amplifier 40 is shown in fig. 4. The 2 μm signal light is coupled to the amplifier system through a fiber isolator 401. The pump source 402 of the amplifier employs a 793nm multimode semiconductor laser. The double-clad fiber combiner 403 couples the 2 μm signal light and the 793nm pump light into the double-clad thulium-doped fiber 404. The signal light is amplified by 793nm pump light in the cladding during the transmission in the core of the thulium doped fiber. By managing the dispersion of the optical fiber link of the amplifier, the average power of the output 2 mu m pulse light reaches W magnitude, and the pulse width is 200 fs.

To obtain the super-continuum spectrum of the mid-infrared, the amplified 2 μm ultra-short pulse laser is incident into the mid-infrared nonlinear fiber 411. The mid-infrared high nonlinear fiber 411 is a chalcogenide fiber with a high nonlinear coefficient. As is selected in this example₂S₃Optical fiber with nonlinear coefficient of 20W^-1m^-1The zero dispersion point is at 1.8 μm. In addition, the mid-infrared nonlinear fiber 411 in this embodiment may be a fluorine-based fiber with a high nonlinear coefficient, such as a ZBLAN fiber. After the 2 mu m high-power ultrashort pulse laser passes through the intermediate infrared nonlinear fiber, the spectrum is broadened to generate a supercontinuum 42 with the spectral range of 2-5 mu m. The longest wavelength of the mid-infrared super-continuum spectrum 42 is greater than 2 times the shortest wavelength, i.e., the spectral range of the frequency doubling layer is obtained.

After the intermediate infrared supercontinuum of the frequency doubling layer is obtained, the carrier offset frequency f of the intermediate infrared optical frequency comb can be further obtained through a self-reference device of f-2f₀. A typical f-2f self-reference carrier offset frequency measurement device is shown in fig. 5. The mid-infrared supercontinuum 42 passes through a dichroic mirror 501, with long wavelength components (e.g. 4 μm) reflected and short wavelength (2 μm) components transmitted. The 4 μm long wavelength component is frequency-multiplied by the frequency-multiplying PPLN crystal 502 to generate light having the same wavelength as the short wavelength branch. After the two paths of light are combined, the light is incident to a detector 504 through a filter (2 μm band pass) 503 to perform beat frequency. The detector is connected to an RF radio frequency spectrometer. By carefully adjusting the delay 505 of the 2 μm branch path, a radio frequency signal around 20MHz (note: f) less than the repetition frequency (80MHz) can be observed in the RF spectrometer₀In thatRange of repetition frequencies). The signal is the carrier offset frequency (f) of the mid-infrared frequency comb₀)505. At the acquisition of the offset frequency f₀Then, f can be₀The frequency is locked to a stable radio frequency source. Locking f ₀304 can be achieved by feedback control of the pump light power of the 1.5 μm seed laser. Locked carrier offset frequency f₀The precision/line width can reach the mHz magnitude.

At a stabilized carrier offset frequency f₀Then, the stability of the mid-infrared optical frequency comb is only affected by the repetition frequency f_rThe influence of (c). Therefore, the repetition frequency of the mid-infrared frequency comb needs to be further stabilized. The common method is as follows: and carrying out beat frequency locking on the pulse light source to be stabilized and the continuous laser with stable wavelength. The repetition frequency error measuring device is shown in fig. 6.

In this embodiment, a portion of the output 306 of the 1.5 μm mode-locked pulse seed source may be beaten with a precision frequency stabilized (linewidth on the order of mHz) continuous laser 601, since a mid-IR stabilized wavelength continuous laser is not readily available. The resulting beat frequency difference signal 602 is then locked to a stable radio frequency source. The variable of the locking adjustment is the laser cavity length of the 1.5 mu m mode locking pulse seed source. The cavity length feedback adjustment of the laser of the seed is realized by stretching the optical fiber by using an electrically driven piezoelectric ceramic (PZT) 305. The frequency precision of the locked intermediate infrared optical frequency comb can reach the mHz magnitude.

In the above embodiment, the near-infrared 1.5 μm mode-locked pulse laser firstly obtains the supercontinuum light source of the middle infrared through the cascade nonlinear spectrum broadening process. And then the frequency of the mid-infrared supercontinuum light source is precisely locked through the self-reference of f-2f and a repetition frequency measuring device. The line width of the locked mid-infrared optical frequency comb can reach the mHz magnitude. Therefore, the precision of measurement by using the mid-infrared optical frequency comb can reach 10^-18Magnitude. The method and the device for generating the optical frequency comb have important significance in the aspect of precise measurement of mid-infrared molecules.