Short-coherence frequency stabilization optical path device applied to interferometer

Abstract

The invention discloses a short-coherence frequency stabilization light path device applied to an interferometer. A tunable laser unit generates continuous tunable laser; the phase regulation and control unit mixes a low-frequency reference signal and a high-frequency radio-frequency signal and then loads the mixed signal to a tunable laser phase; the signal subjected to phase regulation and control outputs short coherent light through a nonlinear regulation and control unit, and the short coherent light comprises an optical fiber amplifier, a high nonlinear optical fiber and a collimation output device which are sequentially arranged on the same optical axis; the saturable absorption unit screens laser with required power, inputs the screened laser into the saturable absorption gas chamber to generate a characteristic absorption spectral line, and provides a reference signal; and the feedback control unit comprises a photoelectric detector, a resolving module and an electrical control system which are arranged in sequence, converts a reference signal into a time domain electric signal, mixes the time domain electric signal with a reference signal, resolves to obtain a frequency error signal, and outputs a voltage feedback signal to the tunable laser unit through a control algorithm to realize frequency stabilization. According to the invention, laser frequency stabilization and short coherent light generation are realized at the same time.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser technology, and in particular to a short-coherence frequency stabilization optical device applied to an interferometer. BACKGROUND

[0002] Laser technology has been widely used in many fields due to its high directivity, high brightness, and high monochromaticity. However, the frequency of a laser is easily affected by external factors such as temperature changes, mechanical vibrations, air disturbances, etc., resulting in frequency drift over time and environmental conditions. This makes it difficult for the laser to meet the requirements of high-precision measurement. Therefore, to ensure the stability of the laser wavelength, frequency stabilization technology has emerged as a key means to improve the performance of the laser. The core idea of frequency stabilization technology is to lock the output frequency of the laser to a highly stable reference frequency, which is usually derived from the specific transition spectrum of atoms or molecules.

[0003] Currently, common frequency stabilization techniques include atomic bidirectional dispersion frequency stabilization technology (DAVLL), polarization spectrum frequency stabilization technology, Pound-Drever-Hall (hereinafter referred to as PDH) frequency stabilization technology, saturated absorption spectrum frequency stabilization technology, frequency modulation spectrum frequency stabilization technology (Frequency Modulation Spectroscopy, hereinafter referred to as FMS), and modulation transfer spectrum frequency stabilization technology (Modulation Transfer Spectroscopy, hereinafter referred to as MTS). Among them, the FMS technology uses an electro-optic modulator (Electro-optic Modulator, hereinafter referred to as EOM) or an acousto-optic modulator to externally modulate the laser, thereby avoiding the additional noise introduced by direct frequency modulation. FMS technology has the advantages of high sensitivity and fast response speed, and because of its simple optical path and relatively low signal processing difficulty, it has become one of the ideal choices for laser frequency stabilization technology.

[0004] Traditional frequency stabilization schemes are usually designed for single-frequency lasers, with only one longitudinal mode supported in the laser cavity. Single-longitudinal-mode lasers themselves have a relatively narrow spectral linewidth and strong coherence. Short-coherence light has a wider spectral linewidth and greater frequency fluctuations. The purpose of frequency stabilization of the laser is to ensure the stability of the laser frequency. If the frequency fluctuation of the light source itself is large, it is difficult to achieve the goal of frequency stabilization. In addition, frequency stabilization usually requires feedback control to lock the laser frequency to a fixed reference. The frequency fluctuation of short-coherence light is severe, making it difficult and unstable to lock, and the feedback system is difficult to effectively act. Therefore, traditional frequency stabilization light sources are basically high-coherence light. In the field of high-precision interferometric measurement, traditional laser interferometers are prone to interference fringes due to the high coherence of the reflected light from the upper and lower surfaces of a transparent object. By using a short-coherence light source, the short-coherence length of the light source can be effectively utilized to eliminate this problem. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a short-coherence frequency-stabilized light path device applied to an interferometer, which has the short-coherence characteristic and can realize super-high-precision lens interference measurement.

[0006] The specific technical solutions are as follows:

[0007] A short-coherence frequency-stabilized light path device applied to an interferometer, comprising a tunable laser unit, a phase control unit, a nonlinear control unit, a saturable absorption unit and a feedback control unit connected in sequence along an optical axis.

[0008] The tunable laser unit is used for generating continuous tunable laser; the phase control unit is used for loading a multi-frequency microwave signal on the phase of the tunable laser, and comprises a phase modulator, a frequency mixer, a low-frequency radio frequency source and a high-frequency programmable source; the output end of the low-frequency radio frequency source is connected with the first input end of the frequency mixer, and the output end of the high-frequency programmable source is connected with the second input end of the frequency mixer; the first input end of the phase modulator is connected with the output end of the tunable laser unit along the optical axis, and the second input end is connected with the output end of the frequency mixer; the low-frequency radio frequency source is used for generating a low-frequency reference signal, and the high-frequency programmable source is used for generating a plurality of high-frequency radio frequency signals, the radio frequency of the high-frequency radio frequency signals being not less than 1GHz.

[0009] The nonlinear control unit is used for outputting short-coherence light, and comprises a fiber amplifier, a high-nonlinear optical fiber and a collimating output device arranged in sequence along the optical axis; the input end of the fiber amplifier is connected with the output end of the phase modulator along the optical axis, and the high-nonlinear optical fiber is used for expanding the signal spectrum.

[0010] The saturable absorption unit comprises a beam splitter and a saturable absorption cell, the beam splitter is connected with the output end of the phase modulator along the optical axis, and is used for screening laser with a required power and inputting the laser into the saturable absorption cell; the saturable absorption cell is used for generating a stable characteristic absorption spectrum line, and provides a reference signal for laser frequency stabilization.

[0011] The feedback control unit comprises an optoelectronic detector, a calculation module and an electrical control system arranged in sequence; the input end of the optoelectronic detector is connected with the output end of the saturable absorption cell along the optical axis, and is used for converting the reference signal into a time-domain electrical signal; the calculation module is used for mixing the time-domain electrical signal with a reference signal to obtain a frequency error signal; and the electrical control system is used for outputting a voltage feedback signal according to the frequency error signal through a control algorithm, and outputting the voltage feedback signal to the tunable laser unit to realize frequency stabilization.

[0012] Further, the nonlinear regulation unit is also used for nonlinear wavelength conversion according to requirements, and the nonlinear regulation unit comprises, which are arranged in sequence along the same optical axis: a fiber amplifier, a collimating and focusing light path, a nonlinear crystal, a collimating and expanding light path, a dichroic mirror and a collimating output device; the input end of the fiber amplifier is connected to the output end of the tunable laser unit along the same optical axis, the laser amplified by the fiber amplifier is output as collimated spatial light through the collimating and focusing light path, and is focused to be incident into the nonlinear crystal for nonlinear frequency conversion; the laser after the nonlinear frequency conversion is input into the dichroic mirror in the form of spatial collimated light after the collimating and expanding light path, and the frequency converted laser is separated to obtain the required frequency of the collimated spatial light after the collimating output device.

[0013] Further, the nonlinear crystal comprises: a β-barium borate crystal, a lithium triborate crystal, a potassium titanyl phosphate crystal and a periodically poled lithium niobate crystal.

[0014] Further, the tunable laser used by the tunable laser unit is selected from any one of a fiber laser, a semiconductor laser and a solid-state laser.

[0015] Further, the tuning speed of the tunable laser unit is not less than 20 kHz, the tuning range is greater than 0.5 nm, and the spectral line width of the output tunable laser is less than 10 kHz.

[0016] Further, the saturable absorption gas in the saturable absorption gas chamber is selected from any one of iodine, cesium, rubidium or acetylene.

[0017] Further, the multi-frequency microwave signal comprises a first radio frequency signal and a second radio frequency signal, the first radio frequency signal is a reference signal, the second radio frequency signal is a high frequency radio frequency signal, and the first and second radio frequency signals have reciprocal frequency characteristics and random phase relationship.

[0018] Further, the high nonlinear optical fiber expands the spectrum through four-wave mixing effect.

[0019] A short coherence frequency stabilization method, which is realized based on the short coherence frequency stabilization optical path device applied to an interferometer, comprises the following steps:

[0020] S1: arranging the short coherence frequency stabilization optical path device applied to an interferometer according to assembly requirements and starting, and the tunable laser unit generates a tunable laser signal input into the phase modulation unit;

[0021] S2: the phase modulation unit loads a multi-frequency microwave signal to the phase of the tunable laser, and widens the laser spectrum;

[0022] S3: the modulated laser signal is input into the nonlinear regulation unit for optical nonlinear broadening to generate short coherence light;

[0023] S4: the modulated laser signal is input into a saturable absorption chamber after power screening to generate a characteristic absorption spectrum line;

[0024] S5: the reference signal is input into a feedback control unit, a demodulation calculation output voltage feedback signal is output to the tunable laser unit, the output frequency of the tunable laser unit is adjusted, and the laser frequency is controlled in a stable range.

[0025] Further, in S3, the modulated laser signal is input into a nonlinear regulation unit for nonlinear wavelength conversion to output the required frequency of collimated spatial light.

[0026] The present application has the following beneficial effects:

[0027] (1) In the present application, a multi-frequency signal source, phase modulation and nonlinear optical effect are used, and laser frequency stabilization and short coherent light generation are simultaneously realized. The reference signal and high-frequency radio frequency signal are simultaneously loaded on the phase modulator, so that the laser is spectrally broadened while being frequency-stabilized, thereby outputting short coherent light with a stable central frequency.

[0028] (2) The present application uses phase modulation and closed-loop feedback control, which can effectively offset the influence of environmental changes on the laser frequency, ensuring the high stability of the laser output frequency, and the frequency stability can be less than the order of megahertz.

[0029] (3) The present application uses phase modulation and nonlinear optical effect, which can realize controllable expansion of laser spectral width, shortening of coherence length, and generation of short coherent light. This technology can effectively eliminate the interference fringe problem caused by high coherence of laser, and is suitable for high-precision lens interferometry and other occasions. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a structural schematic diagram of a short coherent frequency-stabilized light path device applied to an interferometer in Embodiment One of the present application.

[0031] Figure 2 is a schematic diagram of a short coherent frequency-stabilized light path device applied to an interferometer in Embodiment One of the present application.

[0032] Figure 3 is a structural schematic diagram of a short coherent frequency-stabilized light path device applied to an interferometer in Embodiment Two of the present application.

[0033] Figure 4 is a structural schematic diagram of a short coherent frequency-stabilized light path device applied to an interferometer in Embodiment Three of the present application.

[0034] In the figure, tunable laser 11; phase modulator 21, frequency mixer 22, low-frequency radio frequency source 23, high-frequency programmable source 24; fiber amplifier 31, collimating output device 32, collimating focusing light path 33, nonlinear crystal 34, collimating beam expanding light path 35, dichroic mirror 36, high nonlinear optical fiber 37; light splitting light path 41, saturable absorption cell 42; photodetector 51, solving module 52, electrical control system 53. DETAILED DESCRIPTION

[0035] The purpose and effect of the present application will become more apparent from the following detailed description of the preferred embodiments in conjunction with the accompanying drawings and examples. It should be understood that the specific embodiments described herein are merely intended to explain the present application and are not intended to limit the present application.

[0036] As one of the embodiments, a short coherence frequency stabilization optical path device applied to an interferometer is proposed, as shown in Figure 1 The device includes a tunable laser unit, a phase control unit, a nonlinear control unit, a saturable absorption unit, and a feedback control unit. For convenience of description, the connections between the elements in each unit and between the units are all coaxial connections.

[0037] The tunable laser unit is used to generate continuous tunable laser output, and the output tunable laser propagates in an optical fiber, facilitating subsequent phase modulation and power amplification. The output laser frequency can be quickly tuned by the control signal input into the tunable laser unit, so as to provide a light source with a wide tuning range and high tuning accuracy. The tunable laser unit uses a tunable laser 11 selected from any one of a fiber laser, a semiconductor laser, or a solid-state laser; as shown in Figure 2 In this embodiment, the tunable laser 11 uses a narrow linewidth laser light source based on a fiber laser, and obtains a narrow linewidth laser (i.e., tunable laser) with required tuning speed, tuning range, output linewidth, and output power. The center wavelength can be 1550 nm, 1064 nm, or visible light, and the selection of the operating wavelength depends on the specific application scenario requirements; the tuning speed is not less than 20 kHz to ensure that the feedback regulation system can timely compensate for the frequency disturbance caused by the environment; the tuning range is greater than 0.5 nm to ensure that a suitable absorption gas absorption peak can be found for frequency calibration; the spectral linewidth of the output tunable laser is less than 10 kHz to ensure that the error signal obtained by subsequent solving will not be disturbed by the linewidth of the laser itself. The laser can be quickly fine-tuned in output frequency by loading a voltage on a lead zirconate titanate (PZT).

[0038] A phase modulation unit is connected to the output of the tunable laser unit. The phase modulation unit is used to load the multi-frequency microwave signal to the laser phase, so that the tunable laser carries the phase modulation signal, thereby achieving effective widening of the laser spectrum. The process aims to improve the nonlinear response efficiency of subsequent spectrum processing. The phase modulation unit includes a phase modulator 21, a frequency mixer 22, a low-frequency RF source 23, and a high-frequency programmable source 24. The output of the low-frequency RF source 23 is connected to the first input of the frequency mixer 22, the output of the high-frequency programmable source 24 is connected to the second input of the frequency mixer 22, the output of the frequency mixer 22 is connected to the second input of the phase modulator 21, and the first input of the phase modulator 21 is connected to the output of the tunable laser 11.

[0039] The low-frequency signal generated by the low-frequency RF source 23 serves as a reference signal for laser frequency calibration. The RF frequency of the signal does not exceed 50 MHz, and is used to generate a calculation reference frequency for the subsequent error signal. The high-frequency programmable source 24 is used to generate a plurality of high-frequency RF signals, the RF frequency of the signal is not less than 1 GHz, and is used to achieve subsequent spectral nonlinear broadening, helping the laser signal to meet the short coherence requirement. The frequency mixer 22 is used to mix the reference signal and the high-frequency RF signal to obtain a multi-frequency microwave signal, ensuring that multiple frequency signals can be normally and independently loaded into the phase modulator 21. The multi-frequency microwave signal includes a first RF signal (i.e. the reference signal) and a second RF signal (i.e. the high-frequency RF signal), and the first and second RF signals have different frequency characteristics and random phase relationships. The phase modulator 21 is used to load the multi-frequency microwave signal to the laser phase, and the modulation bandwidth of the phase modulator 21 is not less than 5 GHz, so as to ensure the rapid and accurate adjustment of the laser signal. Through the cooperative work of the low-frequency RF source 23 and the high-frequency programmable source 24, the phase modulation unit loads the phase carrier signal of the frequency error signal, and provides conditions for further broadening the spectrum through the four-wave mixing effect and other nonlinear optical effects, thereby being able to provide a stable reference frequency and help the laser signal to meet the short coherence requirement.

[0040] A nonlinear modulation unit is connected to the output of the phase modulation unit, and is used to further broaden the spectrum of the laser signal through optical nonlinear effects, improve the laser power to the nonlinear effect threshold, and perform nonlinear spectrum expansion, effectively shorten the coherence length of the laser, and thereby generate a short coherence broadband light capable of improving the axial resolution and anti-interference capability of the interference system. To achieve this purpose, the nonlinear modulation unit includes, in sequence along the optical axis, a fiber amplifier 31, a high nonlinear optical fiber 37, and a collimating output device 32. The output of the fiber amplifier 31 is connected to the input of the high nonlinear optical fiber 37, and the output of the high nonlinear optical fiber 37 is connected to the input of the collimating output device 32.

[0041] The fiber amplifier 31 is used to increase the power of the laser signal to the threshold of the nonlinear optical effect, and the output power is not less than 1W; the fiber amplifier 31 can be selected from an erbium-doped fiber amplifier (EDFA) or a ytterbium-doped fiber amplifier (YDFA), etc., which has excellent characteristics such as high gain and low noise, and can realize efficient amplification in the process of laser transmission. The high nonlinear optical fiber 37 is used to expand the spectrum through the four-wave mixing effect (Four-Wave Mixing, hereinafter referred to as FWM), so as to effectively shorten the coherence length of the laser and improve the stability of the laser frequency, and ensure the frequency accuracy of the system in long-time operation. By accurately controlling the modulation parameters (frequency and amplitude) of the high-frequency radio frequency signal and the nonlinear characteristics of the optical fiber, the coherence time and spectral characteristics of the laser can be effectively adjusted. The laser output by the high nonlinear optical fiber 37 is output as spatially collimated light (i.e. short coherence light) through the collimating output device 32, and the output meets the application requirements, including performing face detection of a lens.

[0042] FWM is an important nonlinear optical phenomenon, which refers to that when three light waves of different frequencies interact in a nonlinear medium, a fourth new frequency light wave will be generated due to the nonlinear polarization of the medium. This effect is based on the third-order nonlinear polarization characteristics of the medium, and is the result of energy and momentum conservation in the process of interaction between the optical field and the substance.

[0043] Specifically, the tunable laser output by the tunable laser 11 is single-frequency, and the frequency is ω c The tunable laser is phase-modulated by the phase modulator 21 under the driving of the high-frequency radio frequency signal, and a plurality of frequency modulation sidebands are generated around the laser frequency ω c , and the frequency is ω c ±nω k , forming a broadband frequency modulation spectrum. The unmodulated optical field wherein E0 represents the amplitude of the optical field of the tunable laser; assuming that only the first-order sideband is considered, the optical field after modulation by the phase modulator 21 is represented as:

[0044]

[0045] In the formula, β represents the modulation depth, ω m represents the frequency of the first radio frequency signal.

[0046] The modulated signal undergoes FWM and other nonlinear effects in the high nonlinear optical fiber 37, and due to the refractive index modulation of the difference frequency, new frequency components are generated. After the nonlinear effect, a large number of new frequency components with different phases are generated according to the existing frequency components, which is manifested as spectral broadening on the spectrum, thereby realizing the shortening of the coherence distance and the generation of short coherence light. The expressions of the two newly generated frequency components are as follows:

[0047] ν3=ν1-(ν2-ν1)

[0048] v4 = v2 + (v2 - v1)

[0049] wherein v1, v2 are both ω c ±nω k (n = 0, 1, 2…, k = 1, 2, 3…k) arbitrary frequency, ω k represents the kth high frequency radio frequency signal output by the high frequency programmable source 24, and K is the total number of high frequency radio frequency signals output by the high frequency programmable source 24.

[0050] The nonlinear regulation unit can also perform nonlinear wavelength conversion according to application requirements. To achieve this purpose, the same optical axis is sequentially arranged: fiber amplifier 31, collimating and focusing light path 33, nonlinear crystal 34, collimating and expanding light path 35, dichroic mirror 36, and collimating output device 32. The amplified laser light by the fiber amplifier 31 is output as collimated spatial light by the collimating and focusing light path 33 and is focused into the nonlinear crystal 34 for nonlinear frequency conversion to meet different output frequency requirements. The collimating and focusing light path 33 includes a fiber coupler and a focusing light path. The fiber coupler is used to convert the laser light into collimated spatial light, which is then converged by the focusing light path. The nonlinear crystal 34 is coupled at the focal point of the focusing light path. The nonlinear crystal 34 can be selected from BBO (β-barium borate crystal), LBO (lithium triborate crystal), KTP (potassium titanyl phosphate crystal), PPLN (periodically poled lithium niobate crystal), etc. The specific selection depends on the required output wavelength of the specific application and needs to be accurately designed according to the power and efficiency requirements of the system. After nonlinear frequency conversion, the laser light passes through the collimating and expanding light path 35 and is output in the form of spatial collimated light. Then the dichroic mirror 36 separates the laser light before and after frequency conversion. The frequency-converted laser light is output as collimated spatial light by the collimating output device 32 for application requirements, including visible or ultraviolet band interference detection.

[0051] The saturable absorption unit includes a beam splitter 41 and a saturable absorption cell 42, and the saturable absorption cell 42 is filled with a saturable absorption gas, and both ends allow high transmission of laser. The unit is used to make the laser interact with the saturable absorption gas, so as to generate a stable characteristic absorption spectrum line, and to provide a reference signal for laser frequency stabilization. The saturable absorption gas is selected from any one of iodine, cesium, rubidium or acetylene, and the laser signal interacts with the saturable absorption gas to generate a stable characteristic absorption spectrum line, which is very sensitive to the laser frequency and can provide an accurate frequency calibration for the subsequent feedback control unit; the specific selection of which saturable absorption gas depends on the required output wavelength of the specific application. The input end of the beam splitter 41 is connected with the output end of the fiber amplifier 31 of the nonlinear control unit (or directly connected with the output end of the phase modulation unit) on the same optical axis, for transmitting the laser with appropriate power in the appropriate direction into the saturable absorption cell 42, to ensure that the saturable absorption signal (i.e. the reference signal) with appropriate signal-to-noise ratio is generated. In the embodiment, as shown in FIG. 2, the beam splitter 41 is arranged between the fiber amplifier 31 and the collimating and focusing optical path 33, and the beam splitter 41 includes a half-wave plate and a polarization beam splitter prism arranged on the same optical axis. The phase-modulated laser passes through the fiber amplifier 31 and the collimating output device 32 in sequence, passes through the half-wave plate to adjust the polarization direction of the laser, and then enters the polarization beam splitter prism to divide the laser into two beams according to the polarization direction, one of which is input into the saturable absorption cell 42, and the other of which is input into the collimating and focusing optical path 33 for nonlinear wavelength conversion. Figure 2

[0052] The feedback control unit is used to receive the reference signal and convert it into a time-domain electrical signal, and mix it with the first radio frequency signal to obtain an error signal, and process it to generate a feedback control signal, which is input into the tunable laser unit to adjust the frequency of the laser signal output by the unit. The feedback control unit includes an optoelectronic detector 51, a calculation module 52 and an electrical control system 53 arranged on the same optical axis in sequence. The optoelectronic detector 51 is used to convert the absorption signal generated by the saturable absorption cell 42 into a time-domain electrical signal, and its response speed is not less than 10 MHz to ensure that the instantaneous intensity and phase change can be captured in a short time to provide an accurate input signal for the calculation module 52. The optoelectronic detector 51 adopts any one of a photodiode, an avalanche photodiode or a photomultiplier tube.

[0053] The calculation module 52 generates an error signal by mixing the time-domain electrical signal received by the optoelectronic detection unit with the first radio frequency signal (i.e. the reference signal) and calculating. The calculation process includes the following processes:

[0054] The expressions of the parameters corresponding to each sideband after phase modulation are as follows:

[0055] ​

[0056] In the formula, T j δ represents the transmission coefficient corresponding to the j-th sideband after phase modulation. j This represents the light intensity attenuation coefficient corresponding to the j-th sideband after phase modulation. This represents the phase shift corresponding to the j-th sideband after phase modulation.

[0057] The electrical signal I generated by the electro-optic detector and |E2(t)| 2 Proportional, the electrical signal can be simplified to:

[0058]

[0059] In the formula, η represents the detection coefficient between the electrical signal generated by the photodetector 51 and the light field intensity.

[0060] In electrical signals, cos(ω) m The t component is proportional to the difference in amplitude loss experienced by the left and right sidebands, while sin(ω) m The t) component is proportional to the difference between the average phase shift experienced by the carrier and the phase shift experienced by the sidebands. Therefore, phase-sensitive detection technology can be used to detect frequencies of ω. m The frequency error signal can be obtained from the photocurrent signal I(t), which reflects the magnitude of the deviation between the laser frequency and the absorption spectral frequency of the saturable absorbable gas. The frequency difference and random phase relationship between the first and second radio frequency signal modules can effectively eliminate possible errors in the optical path device, ensuring the stability of the light source and decoherence.

[0061] The electrical control system 53 receives the frequency error signal and outputs a voltage feedback signal through a control algorithm. This feedback adjusts the output frequency of the tunable laser unit, achieving real-time stable control of the laser frequency. The frequency error signal is kept within a predetermined range (generally near zero), ensuring the laser frequency remains stable near the absorption line of the saturable absorbing gas. This unit employs a precise feedback control mechanism, responding in real-time to laser frequency deviations and automatically adjusting the output frequency of the tunable laser 11. This effectively eliminates the influence of external environmental changes on the laser frequency, ensuring long-term stable operation of the system.

[0062] In the above embodiment (referred to as Embodiment 1), frequency modulation spectrum frequency stabilization technology was used during frequency stabilization. The short coherent frequency stabilization optical path device set up based on this technology has the least impact on the generated short coherent light, and the device structure is simple.

[0063] As a second embodiment, saturable absorption spectroscopy was used for frequency stabilization. The optical path device based on this technology is basically the same as that in the first embodiment, such as... Figure 3As shown, the difference is that the oscillation signal output by the low-frequency radio frequency source 23, i.e. the reference signal, is not input to the mixer 22, but is directly used as the input of the tunable laser 11.

[0064] As an example three, modulation transfer spectrum frequency stabilization technology was used for frequency stabilization. The optical path device based on this technology is basically the same as that in example one, such as... Figure 4 As shown, the difference lies in the addition of an anti-polarizing laser beam in the saturable absorber chamber 42. The reference signal is only loaded onto the anti-polarizing laser, requiring at least two phase modulators 21. Specifically, the beam splitting path 41 includes two half-wave plates, three polarizing beam splitters, and several mirrors arranged coaxially. After phase modulation, the laser beam passes through the fiber amplifier 31 and the collimation output device 32, then through the first half-wave plate to adjust the polarization direction, and then enters the first polarizing beam splitter, which splits the laser beam into two beams according to the polarization direction. One beam is input to the saturable absorber unit through a mirror, and the other beam is input to the collimation focusing path 33 for nonlinear wavelength conversion. The laser beam input to the saturable absorber unit first passes through the second half-wave plate to adjust the polarization direction again, and then enters the second polarizing beam splitter, which splits the laser beam into two beams according to the polarization direction again. One beam is input to the saturable absorber chamber 42, and the other beam is input to the second phase modulator 21 through another mirror. The reference signal output from the low-frequency radio frequency source 23 is also input to the second phase modulator 21. The output of the second phase modulator 21 is input to the third polarization beam splitter through a mirror, reflecting the laser beam in the polarization direction and inputting it into the saturable absorption chamber 42 (i.e., reverse coincidence input). The output signal of the saturable absorption chamber 42 is input to the feedback control unit. The purpose of this design is to allow the reference signal output from the saturable absorption chamber 42 to carry the modulation signal on the reverse coincidence laser, so that it does not carry the low-frequency radio frequency signal when outputting at the front end, while increasing the signal-to-noise ratio of the saturable absorption line and improving the frequency stabilization accuracy.

[0065] Based on the aforementioned short-coherence frequency-stabilized optical path device applied to interferometers, this embodiment also proposes a short-coherence frequency-stabilized optical path frequency stabilization method, including the following steps:

[0066] S1: Arrange the short coherent frequency-stabilized optical path device for the interferometer according to the assembly requirements, turn on the device, and the tunable laser unit generates a tunable laser signal which is input into the phase modulation unit.

[0067] S2: By loading multi-frequency microwave signals into the tunable laser signal through the phase modulation unit, the laser spectrum is broadened while providing a frequency reference for solving the frequency error signal.

[0068] S3: The modulated laser signal is input into the nonlinear control unit for optical nonlinear broadening to generate short coherent light. Alternatively, the modulated laser signal is input into the nonlinear control unit for nonlinear wavelength conversion to output spatial light of the desired frequency.

[0069] S4: The modulated laser signal is input into the saturable absorption unit to generate characteristic absorption lines.

[0070] S5: The reference signal input feedback control unit calculates the frequency error signal and outputs a voltage feedback signal to the tunable laser unit through the control algorithm to adjust the output frequency of the tunable laser unit so that the laser frequency is controlled within a stable range.

[0071] In summary, this invention loads a low-frequency radio frequency signal, i.e., a reference signal, onto the phase modulator 21, and obtains the frequency error signal through calculation. This signal, combined with an electrical controller, enables frequency locking. Furthermore, by loading a high-frequency radio frequency signal onto the phase modulator 21 and combining it with the nonlinear effect of the highly nonlinear fiber 37, the spectrum is effectively broadened, achieving short-coherent laser output while maintaining frequency stability.

[0072] It will be understood by those skilled in the art that the above descriptions are merely preferred examples of the invention and are not intended to limit the invention. Although the invention has been described in detail with reference to the foregoing examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent substitutions for some of the technical features. All modifications and equivalent substitutions made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A short-coherence frequency-stabilized optical path device for use in an interferometer, characterized in that, It includes a tunable laser unit, a phase control unit, a nonlinear control unit, a saturable absorption unit, and a feedback control unit, all connected sequentially along the optical axis. The tunable laser unit is used to generate continuous tunable laser light; The phase modulation unit is used to load multi-frequency microwave signals onto the tunable laser phase. The unit includes: a phase modulator, a mixer, a low-frequency radio frequency source, and a high-frequency programmable source. The output of the low-frequency radio frequency source is connected to the first input of the mixer, and the output of the high-frequency programmable source is connected to the second input of the mixer; the first input of the phase modulator is connected to the output of the tunable laser unit along the optical axis, and the second input is connected to the output of the mixer; the low-frequency radio frequency source is used to generate a low-frequency reference signal, and the high-frequency programmable source is used to generate multiple high-frequency radio frequency signals, wherein the radio frequency frequency of the high-frequency radio frequency signals is not lower than 1 GHz; The nonlinear modulation unit is used to output short coherent light and includes the following components arranged in sequence along the optical axis: an optical fiber amplifier, a highly nonlinear optical fiber, and a collimation output device; the input end of the optical fiber amplifier is connected to the output end of the phase modulator along the optical axis, and the highly nonlinear optical fiber is used to extend the signal spectrum. The saturable absorption unit includes a beam splitting optical path and a saturable absorption gas cell. The beam splitting optical path is connected to the output of the phase modulator along the optical axis and is used to filter the laser of the required power and input it into the saturable absorption gas cell. The saturable absorption gas cell is used to generate stable characteristic absorption spectral lines and provide a reference signal for laser frequency stability. The feedback control unit comprises, in sequence: a photodetector, a calculation module, and an electrical control system; the input end of the photodetector is connected to the output end of the saturable absorber cell along the optical axis, and is used to convert the reference signal into a time-domain electrical signal; the calculation module is used to perform frequency mixing processing on the time-domain electrical signal and the reference signal to calculate the frequency error signal; the electrical control system is used to output a voltage feedback signal according to the frequency error signal through a control algorithm, and output it to the tunable laser unit to achieve frequency stabilization.

2. The short-coherence frequency-stabilized optical path device for an interferometer according to claim 1, characterized in that, The nonlinear control unit is also used to perform nonlinear wavelength conversion as needed. The nonlinear control unit includes, arranged sequentially along the optical axis: an fiber amplifier, a collimating and focusing optical path, a nonlinear crystal, a collimating and expanding optical path, a dichroic mirror, and a collimating output device. The input end of the fiber amplifier is connected to the output end of the tunable laser unit along the optical axis. The laser amplified by the fiber amplifier is output as collimated spatial light through the collimating and focusing optical path and focused onto the nonlinear crystal for nonlinear frequency conversion. After nonlinear frequency conversion, the laser passes through the collimating and expanding optical path and is input into the dichroic mirror in the form of spatial collimated light. The laser is separated to obtain the frequency-converted laser and outputs collimated spatial light of the required frequency after passing through the collimating output device.

3. The short-coherence frequency-stabilized optical path device for an interferometer according to claim 2, characterized in that, The nonlinear crystals include: barium β-borate crystals, lithium triborate crystals, potassium titanium oxyphosphate crystals, and periodically polarized lithium niobate crystals.

4. The short-coherence frequency-stabilized optical path device for an interferometer according to claim 1, characterized in that, The tunable laser used in the tunable laser unit is selected from any one of fiber lasers, semiconductor lasers, and solid-state lasers.

5. The short-coherence frequency-stabilized optical path device for an interferometer according to claim 1, characterized in that, The tuning speed of the tunable laser unit is not less than 20 kHz, the tuning range is greater than 0.5 nm, and the spectral linewidth of the output tunable laser is less than 10 kHz.

6. The short-coherence frequency-stabilized optical path device for an interferometer according to claim 1, characterized in that, The saturable absorbable gas in the saturable absorbable gas chamber is selected from any one of iodine, cesium, rubidium, or acetylene.

7. The short-coherence frequency-stabilized optical path device for an interferometer according to claim 1, characterized in that, The multi-frequency microwave signal includes a first radio frequency signal and a second radio frequency signal. The first radio frequency signal is the reference signal, and the second radio frequency signal is the high-frequency radio frequency signal. The first and second radio frequency signals have reciprocal frequency characteristics and random phase relationship.

8. The short-coherence frequency-stabilized optical path device for an interferometer according to claim 1, characterized in that, The highly nonlinear optical fiber extends the spectrum through a four-wave mixing effect.

9. A short-coherence frequency stabilization optical path method, implemented based on the short-coherence frequency stabilization optical path device applied to an interferometer as described in any one of claims 1-8, characterized in that, Includes the following steps: S1: Arrange the short coherent frequency-stabilized optical path device for the interferometer according to the assembly requirements and start it. The tunable laser unit generates a tunable laser signal and inputs it into the phase modulation unit. S2: The phase modulation unit loads multi-frequency microwave signals onto the tunable laser phase, thereby broadening the laser spectrum; S3: The modulated laser signal is input into the nonlinear control unit for optical nonlinear broadening to generate short coherent light; S4: The modulated laser signal is power filtered and then input into a saturable absorption cell to generate characteristic absorption lines; S5: The reference signal input feedback control unit demodulates and calculates the output voltage feedback signal to the tunable laser unit, and adjusts the output frequency of the tunable laser unit to keep the laser frequency within a stable range.

10. The short-coherence frequency stabilization optical path frequency stabilization method according to claim 9, characterized in that, S3 further includes: inputting the modulated laser signal into a nonlinear control unit for nonlinear wavelength conversion, and outputting collimated spatial light of the required frequency.

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