Method for generating frequency tunable ultra-stable laser based on modulation sideband frequency locking and laser system

Abstract

The application provides a frequency-adjustable ultra-stable laser generation method and a laser system based on modulation sideband frequency locking, which lock first-order sideband light of electro-optical modulation laser to the resonance frequency of an ultra-stable optical resonant cavity, and then utilize frequency synthesis technology to ensure constant modulation-demodulation phase relationship in the frequency modulation process of the electro-optical crystal, so as to realize the ultra-stable laser with frequency locking function. With the application, short-term ultra-stable laser with a frequency adjustment range of 100 MHz and high spectral purity can be obtained through simple optical structure and control process, so as to meet the demand of precise spectral measurement and quantum communication and other application scenarios for ultra-stable laser signals with a large adjustment range.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser frequency stabilization, and particularly relates to a frequency tunable ultra-stable laser generation method and a laser system based on modulation sideband frequency locking. BACKGROUND

[0002] Ultra-stable cavity frequency stabilization is a common method for generating ultra-high short-term stability laser signals, and the short-term frequency stability is usually in the order of E -15 -E -16 , and the highest can reach E -17 , which is better than all other technical means.

[0003] At present, the frequency locking of the stable cavity generally adopts the PDH (Pound-Drever Hall, resonance and non-resonance light interference frequency locking) technology commonly used internationally. In the PDH technology, the original laser frequency is matched with the resonance frequency of the ultra-stable optical cavity by modulating the laser with an electro-optic modulator, and the cavity reflection laser signal or the laser frequency deviation signal from the resonance cavity frequency is demodulated by a photoelectric detector, and then fed back to the laser to realize the laser frequency stabilization. However, since the length of the ultra-stable optical cavity is very stable, the resonance frequency thereof cannot be adjusted, and therefore, the frequency stabilization can only be realized at a specific frequency (i.e. the resonance frequency of the ultra-stable optical cavity) in the prior art. However, many application scenarios, including precise spectral measurement and quantum communication, require fine scanning or control of the laser frequency within a certain range (e.g. hundreds of megahertz), and the ultra-stable cavity frequency stabilization scheme based on the PDH technology cannot simultaneously meet the requirements of flexible laser frequency tuning and frequency stable laser output.

[0004] In order to obtain the frequency tuning function, the prior art proposes a solution of adding laser frequency tuning technology to the frequency non-tunable ultra-stable laser. For example, another independent laser is locked to the ultra-stable laser at a fixed frequency, and the frequency locking reference frequency is locked by adjusting the heterodyne beat frequency, or the frequency is shifted by using an optical modulator. However, these schemes have certain deficiencies in the frequency tuning range, spectral purity and the like, and cannot fully meet the requirements of the application scenarios such as precise spectral measurement and quantum communication. SUMMARY

[0005] In view of the above problems of the prior art, the present application proposes a frequency tunable ultra-stable laser generation method and a laser system based on modulation sideband frequency locking, which locks the first sideband light on the laser signal modulated by an electro-optic phase modulator to the resonance frequency (reference frequency) of the ultra-stable optical resonance cavity, and then uses frequency synthesis technology to ensure the constant modulation-demodulation phase relationship in the frequency modulation process of the electro-optic crystal, so as to realize the ultra-stable laser with frequency locking function.

[0006] The first aspect of the present application relates to a method for generating a frequency tunable ultra-stable laser based on modulation sideband frequency locking, which comprises a frequency signal generating step, a laser signal modulating step, a reference cavity coupling step, a photoelectric detection step, a frequency down-conversion step, an error signal generating step, and a feedback locking step; wherein,

[0007] In the frequency signal generating step, a first frequency signal with a first frequency f1, a second frequency signal with a second frequency f2, and a third frequency signal with a third frequency f3 are generated, the first frequency f1 and the second frequency f2 are tunable and have a frequency difference △f, and the third frequency f3 = △f.

[0008] In the laser signal modulating step, the laser signal is phase-modulated based on the first frequency signal, the laser signal is generated by a single-frequency laser source and has a frequency v.

[0009] In the reference cavity coupling step, the phase-modulated laser signal is coupled into an ultra-stable optical resonant cavity.

[0010] In the photoelectric detection step, the light signal reflected from the ultra-stable optical resonant cavity with respect to the laser signal is photoelectrically detected, and a photoelectric detection signal is generated.

[0011] In the frequency down-conversion step, the second frequency signal is mixed with the photoelectric detection signal to obtain a frequency down-converted signal.

[0012] In the error signal generating step, the frequency down-converted signal is demodulated using the third frequency signal to generate a frequency error signal.

[0013] In the feedback locking step, the frequency v of the single-frequency laser source is adjusted based on the frequency error signal to lock the first-order sideband frequency on the phase-modulated laser signal at the resonant frequency of the ultra-stable optical resonant cavity.

[0014] Further, the first and second frequency signals are respectively independently generated by a frequency signal source, and the third frequency signal is generated by mixing the first and second frequency signals; and / or, the frequency difference △v is a fixed value.

[0015] Further, the phase modulation based on the first frequency f1 is high-frequency modulation with respect to the resonant spectral line width of the ultra-stable optical resonant cavity.

[0016] Further, the photoelectric detection step further comprises a pre-amplification step of the photoelectric detection signal, and / or a step of filtering the photoelectric detection signal to filter out frequency components other than the first frequency f1.

[0017] Further, the second frequency signal and the photo-detection signal for mixing have the same time delay; and / or, the step of down-conversion further comprises a step of filtering the down-converted signal to filter out frequency components other than the third frequency; and / or, the step of down-conversion further comprises a step of amplifying the down-converted signal.

[0018] Further, the step of error signal generation further comprises a step of adjusting the time delay or phase of the third frequency signal to maximize the frequency error signal.

[0019] Further, the step of feedback locking further comprises a step of performing proportional-integral operation on the frequency error signal to generate a feedback control signal.

[0020] Further, in the step of feedback locking, the frequency v of the single-frequency laser source is adjusted based on the frequency error signal by a combination of one or more of the following: laser cavity temperature control, laser pump driving current control, piezoelectric ceramic, or frequency tunable driving of an electro-optical modulator.

[0021] A second aspect of the present application relates to a frequency tunable ultra-stable laser system based on modulation sideband frequency locking, comprising a single-frequency laser source, a frequency signal generation unit, an electro-optical modulation unit, an optical coupling unit, an ultra-stable optical resonator, a photo-detection and error signal generation unit, and a laser frequency control unit, wherein:

[0022] the single-frequency laser source is configured to generate a laser signal, and is frequency tunable;

[0023] the frequency signal generation unit is configured to generate a first frequency signal with a first frequency f1, a second frequency signal with a second frequency f2, and a third frequency signal with a third frequency f3, wherein the first frequency f1 and the second frequency f2 are tunable and have a frequency difference △f, and the third frequency f3 = △f;

[0024] the electro-optical modulation unit is configured to phase modulate the laser signal based on the first frequency signal;

[0025] the optical coupling unit is configured to couple the modulated laser signal into the ultra-stable optical resonator, and to transmit a reflected light signal from the ultra-stable optical resonator to the photo-detection and error signal generation unit;

[0026] The photoelectric detection and error signal generation unit is configured to photoelectrically detect the reflected light signal to generate a photoelectric detection signal, mix the photoelectric detection signal with the second frequency signal to generate a down-converted signal, and demodulate the down-converted signal with the third frequency signal to generate a frequency error signal.

[0027] The laser frequency control unit is configured to adjust the frequency of the single-frequency laser source based on the frequency error signal to lock a first-order sideband frequency on the modulated laser signal at the resonance frequency of the super-stable optical resonant cavity.

[0028] Further, the frequency signal generation unit comprises first and second frequency signal sources for generating the first and second frequency signals respectively, and a down-conversion component for mixing the first and second frequency signals to generate the third frequency signal; and / or, the frequency difference Δf is a fixed value.

[0029] Still further, the frequency signal source comprises a tunable frequency synthesizer; and / or, an amplifier and an impedance matching element are provided between the frequency signal generation unit and the electro-optical modulation unit.

[0030] Preferably, the electro-optical modulation unit comprises a wideband electro-optical modulator without resonant circuit.

[0031] Preferably, the super-stable optical resonant cavity comprises a v-P cavity.

[0032] Further, the photoelectric detection and error signal generation unit comprises a photoelectric detection part and an error signal generation part; the photoelectric detection part comprises a photoelectric detector; the error signal generation part comprises a down-conversion component for generating the down-converted signal, and a mixing and demodulation component for generating the frequency error signal.

[0033] Still further, the photoelectric detection part further comprises a first signal amplifier configured to amplify the photoelectric detection signal, and / or a first band-pass filter configured to filter the photoelectric detection signal to filter out components other than the first frequency; and / or,

[0034] The down-conversion component further comprises a second band-pass filter configured to filter the down-converted signal to filter out components other than the third frequency, and / or a second signal amplifier configured to amplify the down-converted signal; and / or,

[0035] The photoelectric detection and error signal generation unit further comprises a component for adjusting the time delay or phase of the third frequency signal, and / or a component for adjusting the time delay of the second frequency signal.

[0036] Further, the laser frequency control unit is further configured to perform a proportional integral operation on the frequency error signal to generate a feedback control signal. BRIEF DESCRIPTION OF DRAWINGS

[0037] The specific embodiments of the present application will be further described below with reference to the drawings.

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0039] Figure 1 A block schematic diagram of the frequency tunable ultra-stable laser system based on the modulation sideband frequency locking of the present application is shown.

[0040] Figure 2 A block schematic diagram of the frequency signal generation unit in the frequency tunable ultra-stable laser system based on the modulation sideband frequency locking of the present application is shown.

[0041] Figure 3 A block schematic diagram of the photoelectric detection and error signal generation unit in the frequency tunable ultra-stable laser system based on the modulation sideband frequency locking of the present application is shown. DETAILED DESCRIPTION

[0042] In the following, the exemplary embodiments of the present application will be described in detail with reference to the drawings. The following embodiments are provided by way of example in order to fully convey the spirit of the present application to those skilled in the art to which the present application pertains. Therefore, the present application is not limited to the embodiments disclosed herein.

[0043] In order to obtain a frequency tunable ultra-stable laser, the present application proposes to lock the first sideband of the phase-modulated laser signal to the resonance frequency (reference frequency) of the ultra-stable optical resonant cavity, to obtain a frequency tunable short-term ultra-high stable laser by adjusting the electro-optical phase modulation frequency through the relationship between the first sideband frequency and the laser signal frequency and the phase modulation frequency.

[0044] Specifically, the frequency tunable ultra-stable laser generation method based on the modulation sideband frequency locking according to the present application can include a frequency signal generation step, a laser signal modulation step, a reference cavity coupling step, a photoelectric detection step, a frequency down-conversion step, an error signal generation step, and a feedback locking step.

[0045] In the frequency signal generation step, a first frequency signal having a first frequency f1, a second frequency signal having a second frequency f2, and a third frequency signal having a third frequency f3 are generated. Wherein, the first frequency f1 and the second frequency f2 are tunable and have a fixed frequency difference △f therebetween, and the third frequency f3 is f1-f2=△f.

[0046] As an example, the first and second frequency signals can be generated by means of independent frequency signal sources (e.g. tunable frequency synthesizers) respectively, and the third frequency signal can be generated by mixing the first frequency signal and the second frequency signal.

[0047] In the laser signal modulation step, the laser signal having a fourth frequency v is phase-modulated based on the first frequency signal to form a modulated laser signal. As an example, the laser signal can be generated by a single-frequency laser source.

[0048] As known by those skilled in the art, the phase-modulated laser signal mainly includes the laser signal (carrier) having the fourth frequency v, and a pair of first sideband lights having the first sideband frequencies of v±f1 and opposite phases respectively.

[0049] In the reference cavity coupling step, the phase-modulated laser signal is coupled into an ultra-stable optical resonant cavity.

[0050] In the present application, the ultra-stable optical resonant cavity can be a v-P reference cavity placed in a vacuum environment for providing a reference frequency for frequency locking; the first frequency f1 is selected to be a high frequency modulation based on the resonant spectral line width of the reference cavity, so as to allow a large (e.g. hundreds of MHz) frequency adjustment range to be provided.

[0051] In the photoelectric detection step, the reflected light signal from the ultra-stable optical resonant cavity is photoelectrically detected to generate a photoelectric detection signal.

[0052] In a preferred example, the photoelectric detection step can further include a step of pre-amplifying the photoelectric detection signal, and / or a step of filtering the photoelectric detection signal to filter out frequency components other than the first frequency f1.

[0053] In the down-conversion step, the second frequency signal is mixed with the photodetected signal generated in the photodetection step to obtain a down-converted signal. Thereby, the photodetected signal having the first frequency f1 is down-converted to a down-converted signal having a frequency of f1-f2 = Δf.

[0054] According to the present application, the second frequency signal and the photodetected signal for mixing have the same time delay. As an example, the time delay can be controlled by means of a time delay element.

[0055] In a preferred example, the down-conversion step can further comprise a step of filtering the down-converted signal to filter out frequency components other than the third frequency Δf, and / or a step of amplifying the down-converted signal.

[0056] In the error signal generation step, the down-converted signal is demodulated with the third frequency signal, i.e. the demodulation signal, to generate a frequency error signal indicative of a deviation of the selected first sideband frequency from the reference frequency of the ultra-stable optical resonator.

[0057] According to the present application, the error signal generation step can further comprise a step of adjusting the time delay or the phase of the third frequency signal to maximize the frequency error signal. Thereby, a fixed phase relationship between the demodulation signal and the down-converted signal to be demodulated can be maintained during frequency variations, enabling reliable frequency locking, thus allowing to obtain a reliable frequency tunable ultra-stable laser signal. At the same time, by setting a fixed demodulation frequency, it is also advantageous for e.g. noise filtering and amplification design, ensuring system efficiency and reliability, and improving system performance.

[0058] As an example, the adjustment of the phase or the time delay of the third frequency signal can be achieved by means of a phase shifter or a delay line.

[0059] In the feedback locking step, the frequency of the single frequency laser source is adjusted based on the frequency error signal to achieve frequency locking.

[0060] As an example, the frequency of the single frequency laser source can be adjusted based on the frequency error signal by a combination of one or more of laser cavity temperature control, laser pump driving current control, piezoelectric ceramic or tunable frequency driving electro-optical modulator.

[0061] As an example, the feedback locking step can further comprise a step of performing a proportional-integral operation on the frequency error signal to generate a feedback control signal, wherein the frequency of the single frequency laser source is adjusted based on the feedback control signal.

[0062] It can be seen that, by means of the frequency tunable ultra-stable laser generation method based on modulation sideband frequency locking of the application, the first sideband frequency (for example, v-f1) is locked to the reference frequency of the ultra-stable optical resonant cavity, so that the frequency v of the short-term ultra-stable laser can be simply adjusted by changing the first frequency f1, and the frequency adjustment range can be as high as, for example, 100 MHz. In addition, the method only needs to additionally set a frequency signal source on the basis of the conventional ultra-stable laser system based on the PDH technology, and the implementation structure is simple and the process is easy to control, and the spectral purity of the finally output ultra-stable laser is high.

[0063] In order to better understand the working principle of the application, the frequency tunable ultra-stable laser system based on modulation sideband frequency locking of the application will be described below in conjunction with the examples of Figures 1-3

[0064] Figure 1 The frequency tunable ultra-stable laser system based on modulation sideband frequency locking of the application is schematically shown.

[0065] As shown in Figure 1 , the frequency tunable ultra-stable laser system of the application can include a single-frequency laser source 1, a frequency signal generation unit 2, an electro-optical modulation unit 3, an optical coupling unit 4, an ultra-stable optical resonant cavity 5, a photoelectric detection and error signal generation unit 6, and a laser frequency control unit 7.

[0066] The single-frequency laser source 1 can be any single-frequency laser with a frequency control function, which is used to generate a laser signal with a fourth frequency v.

[0067] As an example, the frequency control function of the single-frequency laser can be realized by means of a combination of one or more of laser cavity temperature control, laser pump driving current control, piezoelectric ceramic, or adjustable frequency driven electro-optical modulator.

[0068] The frequency signal generation unit 2 is used to generate a first frequency signal with a first frequency f1, a second frequency signal with a second frequency f2, and a third frequency signal with a third frequency f3, wherein the first frequency f1 and the first frequency f2 are adjustable and have a fixed frequency difference △f = f1-f2 between them, and the third frequency f3 = △f.

[0069] Figure 2 An example of the frequency signal generation unit 2 according to the application is shown.

[0070] As shown in Figure 2 , the frequency signal generation unit 2 can include a first frequency signal source, a second frequency signal source, and a frequency down-conversion component.

[0071] ​The first and second frequency signal sources are configured to independently generate first and second frequency signals, and the down-conversion component is configured to mix the first and second frequency signals to generate a third frequency signal.

[0072] As an example, the first and second frequency signal sources can comprise tunable frequency synthesizers.

[0073] Further, an amplifier for amplifying the first frequency signal and / or an impedance adaptation element can be provided between the frequency signal generation unit 2 and the electro-optical modulation unit 3.

[0074] The electro-optical modulation unit 3 is configured to phase-modulate the laser signal based on the first frequency signal, thereby generating a laser signal with modulated sidebands.

[0075] As an example, the electro-optical modulation unit 3 can comprise a wideband electro-optical modulator without resonant circuit, which can be driven by a wideband radio frequency signal, for example.

[0076] The optical coupling unit 4 is configured to couple the modulated laser signal to the ultra-stable optical resonator 5 with good spatial mode matching, and to transmit a reflected light signal from the ultra-stable optical resonator to the photodetection and error signal generation unit 6.

[0077] As an example, the optical coupling unit 4 can comprise optical elements for adjusting the beam envelope of the laser signal to the resonator mode of the ultra-stable optical resonator 5 and guiding it into the ultra-stable optical resonator 5.

[0078] The ultra-stable optical resonator 5 can comprise a v-P cavity, for example, placed in a vacuum environment, whose resonant frequency is used as a reference frequency of the ultra-stable laser system.

[0079] The photodetection and error signal generation unit 6 can comprise a photodetection unit and an error signal generation unit, wherein the photodetection unit is configured to photodetect the reflected light signal from the ultra-stable optical resonator to generate a photodetection signal, and the error signal generation unit is configured to demodulate the photodetection signal by means of the second and third frequency signals to generate a frequency error signal representing a deviation between the selected first order sideband frequency and the reference frequency of the ultra-stable optical resonator.

[0080] Figure 3 An example of a photodetection and error signal generation unit 6 according to the present application is shown.

[0081] As Figure 3 shown, the photodetection unit can comprise a photodetector configured to photodetect the reflected light signal from the ultra-stable optical resonator and output a photodetection signal.

[0082] As a preferred example, the photoelectric detection unit can further comprise a first signal amplifier and / or a first band-pass filter. The first signal amplifier is configured to pre-amplify the photoelectric detection signal, and the first band-pass filter is configured to filter the photoelectric detection signal to filter out components other than the first frequency f1, thereby obtaining a photoelectric detection signal containing only the first frequency f1 component.

[0083] The error signal generation unit can comprise a frequency down-conversion component and a frequency mixing demodulation component.

[0084] The frequency down-conversion component is configured to receive the photoelectric detection signal and the second frequency signal, and mix the second frequency signal with the photoelectric detection signal to obtain a frequency down-converted signal having a third frequency.

[0085] As a preferred example, the frequency down-conversion component can further comprise a second band-pass filter and / or a second signal amplifier. The second band-pass filter is configured to filter the frequency down-converted signal to filter out components other than the third frequency, thereby obtaining a frequency down-converted signal containing only the third frequency component, and the second signal amplifier is configured to amplify the frequency down-converted signal. The second signal amplifier can be, for example, a radio frequency amplifier.

[0086] The frequency mixing demodulation component is configured to receive the frequency down-converted signal and the third frequency signal, and demodulate the frequency down-converted signal using the third frequency signal to obtain a frequency error signal.

[0087] According to the present application, the photoelectric detection and error signal generation unit 6 can further comprise a phase adjustment element and / or a time delay element, configured to adjust the time delay or phase of the third frequency signal to maximize the frequency error signal; and / or adjust the time delay of the second frequency signal to make the second frequency signal and the photoelectric detection signal have the same time delay.

[0088] The laser frequency control unit 7 is configured to output a feedback control signal to the single-frequency laser source based on the frequency error signal, to lock the first-order sideband frequency on the reference frequency of the super-stable optical resonator.

[0089] Further, the laser frequency control unit can be configured to perform proportional integral operation on the frequency error signal to generate the feedback control signal.

[0090] Based on the above, the present application, on the basis of a conventional super-stable laser system realized based on the PDH locking technology, changes the phase relationship between the demodulation signal and the photoelectric detection signal by simply changing the structure of the frequency signal generation unit, thereby locking the first-order sideband light generated by the electro-optical modulation laser to the resonance frequency of the super-stable optical resonator, and using the frequency synthesis technology to ensure the constant modulation-demodulation phase relationship in the electro-optical crystal frequency modulation process, i.e. to obtain a short-term super-stable laser with high spectral purity under the premise of taking the resonance frequency of the super-stable optical resonator as the reference frequency (the short-term frequency stability can reach E-16 The frequency adjustment capability of the order of 100MHz is obtained, thereby well meeting the demand of the application scenarios such as precise spectrum measurement and quantum communication for the ultra-stable laser signal with large range adjustment. Moreover, the whole laser system is simple in structure, and compared with the conventional PDH system, all the links remain unchanged except that the adjustment signal process is opposite to the conventional PDH technology, and the frequency signal generation part is changed due to the addition of the frequency modulation function, and the spectrum of the locked laser has no additional modulation signal, which can meet the application demand of the frequency precise scanning and locking. Compared with the combination implementation scheme of the ultra-stable laser without frequency adjustment plus the frequency modulation design, the control process of the present application is simpler based on the regulation and control.

[0091] Although the present application has been described in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that the present application is not limited to the above-described embodiments, but various modifications and equivalents can be made thereto without departing from the spirit and scope of the present application.

Claims

1. A method for generating a frequency tunable ultra-stable laser based on modulation sideband frequency locking, comprising a frequency signal generating step, a laser signal modulating step, a reference cavity coupling step, a photo-detecting step, a down-conversion step, an error signal generating step, and a feedback locking step; wherein, in the frequency signal generating step, a first frequency signal with a first frequency f1, a second frequency signal with a second frequency f2, and a third frequency signal with a third frequency f3 are generated, the first frequency f1 and the second frequency f2 are tunable and have a frequency difference △f, and the third frequency f3 =△f; in the laser signal modulating step, a laser signal is phase modulated based on the first frequency signal, the laser signal is generated by a single frequency laser source and has a frequency v; in the reference cavity coupling step, the phase modulated laser signal is coupled into an ultra-stable optical resonator; in the photo-detecting step, a photo-detection signal is generated by photo-detecting an optical signal reflected from the ultra-stable optical resonator, the reflected optical signal is generated corresponding to the laser signal; in the down-conversion step, the second frequency signal is mixed with the photo-detection signal to obtain a down-converted signal; in the error signal generating step, the down-converted signal is demodulated by the third frequency signal to generate a frequency error signal; in the feedback locking step, the frequency v of the single frequency laser source is adjusted based on the frequency error signal to lock a first order sideband frequency on the phase modulated laser signal at a resonant frequency of the ultra-stable optical resonator. the first and second frequency signals are respectively independently generated by frequency signal sources, and the third frequency signal is generated by mixing the first and second frequency signals; and / or, the frequency difference △f is a fixed value. The phase modulation based on the first frequency f1 is high frequency modulation relative to a resonant spectral line width of the ultra-stable optical resonator. The photo-detecting step further comprises a step of pre-amplifying the photo-detection signal, and / or a step of filtering the photo-detection signal to filter out frequency components other than the first frequency f1. The second frequency signal and the photo-detection signal for mixing have the same time delay; and / or, the down-conversion step further comprises a step of filtering the down-converted signal to filter out frequency components other than the third frequency; and / or, the down-conversion step further comprises a step of amplifying the down-converted signal.

2. The frequency tunable ultra-stable laser generation method of claim 1, wherein, the error signal generating step further comprises a step of adjusting a time delay or a phase of the third frequency signal to maximize the frequency error signal. the feedback locking step further comprises a step of performing a proportional-integral operation on the frequency error signal to generate a feedback control signal.

3. The frequency tunable ultra-stable laser generation method of claim 1, wherein, in the feedback locking step, the frequency v of the single frequency laser source is adjusted based on the frequency error signal by a combination of one or more of a laser cavity temperature control, a laser pump driving current control, a piezoelectric ceramic, or a frequency tunable driving electro-optical modulator.

4. The frequency tunable ultra-stable laser generation method of claim 1, wherein, ​ 5. The frequency tunable ultra-stable laser generation method of claim 1, wherein: ​ ​ ​ 6. The frequency tunable ultra-stable laser generation method of claim 1, wherein, ​ 7. The frequency tunable ultra-stable laser generation method of claim 1, wherein, ​ 8. The frequency tunable ultra-stable laser generation method of claim 1, wherein, ​ 9. A frequency tunable ultra-stable laser system based on modulation sideband frequency locking, comprising a single-frequency laser source, a frequency signal generating unit, an electro-optical modulation unit, an optical coupling unit, an ultra-stable optical resonator, a photo-detection and error signal generating unit, and a laser frequency control unit, wherein: the single-frequency laser source is arranged for generating a laser signal, and is frequency tunable; the frequency signal generating unit is arranged for generating a first frequency signal with a first frequency f1, a second frequency signal with a second frequency f2, and a third frequency signal with a third frequency f3, wherein the first frequency f1 and the second frequency f2 are tunable and have a frequency difference △f therebetween, and the third frequency f3 =△f; the electro-optical modulation unit is arranged for phase-modulating the laser signal based on the first frequency signal; the optical coupling unit is arranged for coupling the modulated laser signal into the ultra-stable optical resonator, and transmitting a reflected light signal from the ultra-stable optical resonator to the photo-detection and error signal generating unit; the photo-detection and error signal generating unit is arranged for photo-detecting the reflected light signal to generate a photo-detection signal, mixing the photo-detection signal with the second frequency signal to generate a down-converted signal, and demodulating the down-converted signal with the third frequency signal to generate a frequency error signal; the laser frequency control unit is arranged for adjusting the frequency of the single-frequency laser source based on the frequency error signal to lock a first-order sideband frequency on the modulated laser signal at a resonant frequency of the ultra-stable optical resonator.

10. The frequency tunable ultra-stable laser system of claim 9, wherein: the frequency signal generating unit comprises first and second frequency signal sources for generating the first and second frequency signals, respectively, and a down-conversion component for mixing the first and second frequency signals to generate the third frequency signal; and / or, the frequency difference △f is a fixed value.

11. The frequency tunable ultra-stable laser system of claim 10, wherein, the frequency signal sources comprise tunable frequency synthesizers; and / or, an amplifier and an impedance adapting element are provided between the frequency signal generating unit and the electro-optical modulation unit.

12. The frequency tunable ultra-stable laser system of claim 9, wherein, the electro-optical modulation unit comprises a wideband electro-optical modulator without resonant circuit.

13. The frequency tunable ultra-stable laser system of claim 9, wherein, the ultra-stable optical resonator comprises a v-P cavity.

14. The frequency tunable ultra-stable laser system of claim 9, wherein, the photo-detection and error signal generating unit comprises a photo-detection component and an error signal generating component; the photo-detection component comprises a photo-detector; the error signal generating component comprises a down-conversion component for generating the down-converted signal, and a mixing demodulation component for generating the frequency error signal.

15. The frequency tunable ultra-stable laser system of claim 14, wherein: the photo-detection component further comprises a first signal amplifier arranged for amplifying the photo-detection signal, and / or a first band-pass filter arranged for filtering the photo-detection signal to filter out components other than the first frequency; and / or, the frequency error signal generating component further comprises a second signal amplifier arranged for amplifying the down-converted signal, and / or a second band-pass filter arranged for filtering the down-converted signal to filter out components other than the third frequency. The down-conversion component further comprises a second band-pass filter arranged for filtering the down-converted signal to filter out components other than the third frequency and / or a second signal amplifier arranged for amplifying the down-converted signal; and / or The photo-detection and error signal generation unit further comprises a component for adjusting the time delay or phase of the third frequency signal and / or a component for adjusting the time delay of the second frequency signal.

16. The frequency tunable ultra-stable laser system of claim 9, wherein, The laser frequency control unit is further arranged to perform a proportional-integral operation on the frequency error signal to generate a feedback control signal.

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