Laser frequency stabilization method and device based on saturated fluorescence spectrum

Abstract

The invention discloses a laser frequency stabilization method and device based on a saturated fluorescence spectrum, and belongs to the technical field of laser frequency stabilization. According to the method, a frequency discrimination signal of a low atomic density or low absorption system is generated by using a saturated fluorescence spectrum; the saturated fluorescence spectrum of atoms is sunken at the resonance position by utilizing opposite laser, so that the influence of Doppler effect is eliminated; a derivative of a saturated fluorescence spectrum is extracted as a frequency discrimination signal by using a method of modulating pump light and then demodulating a fluorescence signal; and a lens and a reflecting mirror are arranged on the opposite side of the PD to collect the fluorescence and then reflect the fluorescence, so that the PD can collect the fluorescence with double intensity. According to the scheme, a fluorescence signal which is enhanced from zero is directly detected, so that the signal-to-noise ratio is extremely high, and meanwhile, the problem of long-term frequency drift is eliminated; and the hot atom device is adopted, a high vacuum environment is not needed, the complex precise temperature control requirement is also avoided, and the device adopting the scheme is simpler in structure and low in cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser frequency stabilization, and particularly relates to a laser frequency stabilization method and device based on saturated fluorescence spectrum, which is suitable for precise frequency stabilization of low-density or weak-absorption samples (such as rare isotopes). BACKGROUND

[0002] In the prior art, there are two technical solutions for realizing laser frequency stabilization. One is PDH frequency stabilization technology based on super-stable cavity. After the laser interacts with the super-stable cavity, a frequency discrimination signal is generated. The narrow linewidth and ultra-high stability of the super-stable cavity are used to lock the laser frequency at the resonance frequency of the cavity for a long time. The other solution is laser frequency stabilization technology based on saturated absorption spectrum. The laser frequency is locked at the saturated absorption peak formed by atomic / molecular energy level transition, a frequency discrimination signal is generated to feedback control the frequency of the laser, and high-precision frequency stabilization is realized.

[0003] For the solution of stabilizing frequency by using super-stable cavity, the locking precision is highly dependent on the fineness of the super-stable cavity and strict environmental isolation. The solution requires the super-stable cavity to be in a high-vacuum environment and be equipped with a precise temperature control system, and is extremely sensitive to external vibration. In addition, the locking frequency of the super-stable cavity will inevitably drift over time. The solution of stabilizing frequency based on saturated absorption spectrum realizes locking by detecting the slight change of light intensity. The solution has low signal-to-noise ratio and is not suitable for low-atomic-density or weak-absorption systems.

[0004] Therefore, there is an urgent need for a new laser frequency stabilization solution that can provide high signal-to-noise ratio, no long-term frequency drift and frequency stabilization means suitable for low-density samples without complex environmental control. SUMMARY

[0005] In view of the technical bottlenecks of the existing super-stable cavity frequency stabilization and saturated absorption spectrum frequency stabilization, the present application provides a laser frequency stabilization method and device based on saturated fluorescence spectrum, which can at least solve part of the above technical problems. The frequency discrimination signal is generated by the saturated fluorescence spectrum of atoms, the influence of the counter-pumping laser on the Doppler effect is eliminated, and the function of accurately locking the laser frequency at the atomic resonance frequency is realized.

[0006] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0007] In a first aspect, the present application provides a laser frequency stabilization method based on saturated fluorescence spectrum, which comprises the following steps:

[0008] Modulation signal generation: the laser to be stabilized is frequency-modulated by an acousto-optic modulator and enters a vacuum cavity in a horizontal direction as pump light to irradiate a fluorescent light emitting material in the vacuum cavity, thereby generating a fluorescent signal; the pump light returns to the vacuum cavity via the first mirror to form counter-propagating laser light, thereby eliminating the influence of Doppler effect; the fluorescent signal is received by a photodetector on the side of the vacuum cavity and converted into an electric signal;

[0009] Frequency discrimination signal generation and laser locking: a signal with the same frequency as the modulation frequency is input into a mixer together with the electric signal to generate a mixed signal, and the mixed signal is input into a low-pass filter to generate a frequency discrimination signal, which is used to generate a feedback voltage signal and fed back to the laser to stabilize the frequency of the laser.

[0010] Preferably, a hot atom device is connected below the vacuum cavity, and the hot atom device has high-temperature atomic vapor inside, which is sprayed out through a collimator to generate a collimated hot atom beam that is vertically upwardly sprayed, and the hot atom generates a fluorescent signal under the irradiation of the pump light.

[0011] Preferably, a lens and a second mirror are arranged on the other side of the vacuum cavity, opposite to the photodetector, and the fluorescent signal radiated to the side is collected by the lens and the second mirror and reflected toward the photodetector.

[0012] Preferably, an RF signal is generated by a signal source and used as the input of the acousto-optic modulator, and the frequency of the RF signal input into the acousto-optic modulator by the signal source satisfies:

[0013] f(t) = f0 + α sin(Ωt)

[0014] wherein f(t) represents the real-time frequency of the RF signal input into the acousto-optic modulator by the signal source at time t, f0 is the fundamental frequency, Ω is the modulation frequency, and α is the modulation amplitude.

[0015] Preferably, the output signal of the photodetector and a sinusoidal signal β sin(Ωt + φ) output by the signal source and having the same frequency as the frequency of the RF signal with the modulation are input into a mixer together to obtain a mixed signal, wherein β is the amplitude of the RF signal, and φ represents the initial phase.

[0016] The mixed signal is input into a low-pass filter to retain only the direct current term, thereby generating a frequency discrimination signal:

[0017] h(t) = 1 / 2 * αβ dI(ω0 + f0) / dω * cos(φ)

[0018] wherein ω0 is the frequency of the laser light emitted by the laser, I(ω0 + f0) represents the intensity of the fluorescent light collected by the lens when the frequency of the laser light irradiating the atom is ω0 + f0, and dω represents the derivative with respect to the frequency.

[0019] When φ = 0, there is:

[0020] h(t) = 1 / 2 * αβdI(ω0+f0) / dω

[0021] That is, the derivative of the saturated fluorescence spectrum is extracted, and h(t) is the frequency discrimination signal.

[0022] Preferably, when ω0+f0 is lower than the resonance frequency, the frequency discrimination signal is negative, and the voltage signal is fed back through the servo feedback module to raise the frequency of the laser; when ω0+f0 is higher than the resonance frequency, the frequency discrimination signal is positive, and the voltage signal is fed back through the servo feedback module to lower the frequency of the laser.

[0023] Preferably, the vacuum cavity is a sealed atomic gas chamber.

[0024] In the second aspect, the embodiment of the present application also provides a laser frequency stabilization device based on saturated fluorescence spectrum, which comprises a modulation signal generation module and a frequency discrimination signal generation and laser locking module, and applies the laser frequency stabilization method based on saturated fluorescence spectrum to stabilize the frequency of the laser.

[0025] The laser frequency stabilization method and device based on saturated fluorescence spectrum provided by the present application have at least the following beneficial effects compared with the prior art:

[0026] 1. The present application generates a frequency discrimination signal through the saturated fluorescence spectrum of atoms, eliminates the influence of the Doppler effect by using the pumped laser, and realizes the function of accurately locking the laser frequency to the atomic resonance frequency.

[0027] 2. It has a higher signal-to-noise ratio. Saturated absorption spectroscopy detects the small change in transmitted light intensity. This change is usually very small relative to the background transmitted light intensity (i.e. the transmitted light intensity when not in resonance). The background light intensity itself may contain laser intensity noise, detector noise, etc. Saturated fluorescence spectroscopy detects the enhanced fluorescence signal starting from zero or low background. Far from resonance, the fluorescence signal is very weak (ideally close to zero). When resonance occurs, the fluorescence intensity increases significantly. Therefore, saturated fluorescence spectroscopy can obtain an order of magnitude higher signal-to-noise ratio than saturated absorption spectroscopy. This is crucial for improving the accuracy and stability of the lock, especially in the case of weak signals or low atomic density.

[0028] 3. No long drift locking. Traditional PDH frequency stabilization relies on an ultra-stable cavity, but the locking frequency of the ultra-stable cavity is easily affected by external temperature and the aging of its own mechanical structure, leading to slow changes over time. The present application uses the linear property of the saturated fluorescence spectrum of atoms to lock the laser frequency near ω r -f0, where ω r is the atomic resonance frequency, which is a physical constant and can be considered as always unchanged, thus avoiding the problem of long drift in principle.

[0029] Other features and advantages of the present application will be set forth in the description that follows, and in part will be apparent from the description, or can be learned by practice of the application. The purposes and other advantages of the present application will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0030] The technical solutions of the present application are described in further detail below with the help of the accompanying drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and all other drawings that can be obtained by those of ordinary skill in the art without creative work are within the protection scope of the present application.

[0032] The accompanying drawings are used to provide a further understanding of the present application, and constitute a part of the specification, and are used to explain the present application together with the embodiments of the present application, and do not constitute a limitation on the present application.

[0033] Figure 1 The experimental device structure schematic diagram of the laser frequency stabilization scheme based on saturated fluorescence spectrum provided for the embodiments of the present application.

[0034] Figure 2 The saturated fluorescence spectrum and frequency discrimination signal schematic diagram provided for the embodiments of the present application. DETAILED DESCRIPTION

[0035] In order to make the purposes, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely in the following with the help of the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some embodiments of the present application, but not all the embodiments.

[0036] In the description of the present application, it should be noted that: in some processes described in the specification and the accompanying drawings, a plurality of operations appear in a specific order, but it should be clearly understood that these operations can be executed or performed in parallel, or not in the order they appear in this text. In addition, various serial numbers and the like are only for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0037] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but only represents selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art without creative work based on the embodiments in the present application are within the scope of protection of the present application.

[0038] The purpose of the present application is to propose a method and device for frequency stabilization based on atomic saturated fluorescence spectrum, aiming at the technical bottleneck of existing super-stable cavity frequency stabilization and saturated absorption spectrum frequency stabilization. A frequency discrimination signal is generated by the saturated fluorescence spectrum of atoms, and the influence of the Doppler effect is eliminated by the counter-pumped pump laser, so as to realize the function of accurately locking the laser frequency to the atomic resonance frequency.

[0039] The specific implementation and working principle of the method of the present application will be described in detail below.

[0040] Referring to Figure 1 The method of the present application mainly includes the following core parts:

[0041] 1) Modulation signal generation: an RF signal with frequency modulation is generated by a signal source as the electrical input of an acousto-optic modulator (AOM). The first-order diffraction light generated after the laser output by the AOM obtains frequency modulation, and enters the vacuum cavity in the horizontal direction as pump light. After the pump light is emitted from the other end, it is returned by a mirror, forming counter-pumped laser, which is used to eliminate the influence of the Doppler effect. In this embodiment, an atomic furnace is connected below the vacuum cavity, and there is high-temperature atomic vapor inside. The atomic vapor is sprayed out through the collimator at the furnace head, generating a collimated hot atomic beam vertically upward. The hot atoms generate fluorescence signals under the irradiation of the pump light, which are received by the photodetecter (PD) on the side and converted into electrical signals. The lens and mirror on the side of the PD are used to collect the fluorescence radiated to the other side and reflect it back, so that the PD collects twice the fluorescence.

[0042] 2) Frequency discrimination signal generation and laser locking: a signal source outputs a sinusoidal signal with the same frequency as the modulation frequency, which is input into a mixer together with the PD signal to generate a mixed signal. The mixed signal passes through a low-pass filter to generate a frequency discrimination signal. The frequency discrimination signal is input into a servo feedback module, and a feedback voltage signal is generated through the PID parameters of the servo feedback module and fed back to the laser, so as to stabilize the frequency of the laser.

[0043] In this embodiment, the frequency discrimination signal is generated by the linear property of the saturated fluorescence spectrum and the modulation-demodulation method, and the specific implementation steps are as follows:

[0044] 1) The frequency of the RF frequency modulation signal input by the signal source to the AOM satisfies: f(t) = f0 + αsin(Ωt), wherein f0 is the fundamental frequency, Ω is the modulation frequency, and α is the modulation amplitude, α << f0. The frequency of the first-order diffraction light passing through the AOM is ω(t) = ω0 + f0 + αsin(Ωt), and ω0 is the frequency of the laser emitted by the laser. Referring to Figure 2As shown, the relationship between the saturated fluorescence signal of the atom and the pump light frequency can be expressed as I(ω), in the case of pump light frequency ω(t), I(ω(t)) can be expanded as I(ω0+f0+αsin(Ωt))=I(ω0+f0)+dI(ω0+f0) / dω*αsin(Ωt). That is, the light intensity signal obtained by the PD is the superposition of the direct current term and the alternating current term.

[0045] 2) The PD output signal and the RF signal βsin(Ωt+φ) output by the signal source with the same frequency and modulation frequency enter the mixer, where β is the RF signal amplitude and φ represents the initial phase. The obtained mixing signal is:

[0046] g(t)=1 / 2*αβdI(ω0+f0) / dω*cos(φ)+βI(ω0+f0)sin(Ωt+φ)+1 / 2*αβdI(ω0+

[0047] f0) / dω*cos(2Ωt+φ)

[0048] That is, the mixing signal contains a direct current term, a frequency Ω term and a frequency 2Ω term.

[0049] After passing through the low-pass filter, only the direct current term h(t)=1 / 2*αβdI(ω0+f0) / dω*cos(φ) is retained. When φ=0, h(t)=1 / 2*αβdI(ω0+f0) / dω, that is, the derivative of the saturated fluorescence spectrum I(ω) is successfully extracted, and h(t) is the frequency discrimination signal.

[0050] As can be seen from Figure 2 , when ω0+f0is less than the resonance frequency ω r , dI(ω0+f0) is less than zero, and the frequency discrimination signal is negative. When ω0+f0is greater than the resonance frequency ω r , dI(ω0+f0) is greater than zero, and the frequency discrimination signal is positive. When the servo feedback module obtains a positive frequency discrimination signal, it increases the voltage fed back to the laser, thereby reducing the frequency of the laser. Similarly, when the servo feedback module obtains a negative frequency discrimination signal, it reduces the voltage fed back to the laser, thereby increasing the frequency of the laser, thereby realizing the function of stabilizing the absolute frequency ω r of the laser near ω -f0.

[0051] In another optional embodiment, the hot atomic beam in the vacuum cavity can be replaced by an atomic gas chamber, both of which can generate saturated fluorescence signals. The hot atomic beam can also be replaced by other fluorescent light emitting materials. In this scheme, the fluorescence signal is enhanced by placing a lens and a mirror on the opposite side to increase the fluorescence collection intensity. Other designs that increase the fluorescence collection intensity within the 4π solid angle are also within the scope of the present patent.

[0052] As described in the above embodiments, those skilled in the art will understand that the present invention provides a laser frequency stabilization method based on saturated fluorescence spectroscopy. In this method, saturated fluorescence spectroscopy is used to generate a frequency discrimination signal for systems with low atomic density or low absorption. By using opposing laser beams, a dip appears in the saturated fluorescence spectrum of the atoms at the resonance point, eliminating the influence of the Doppler effect. The derivative of the saturated fluorescence spectrum is extracted as the frequency discrimination signal by modulating the pump light and then demodulating the fluorescence signal. A lens is placed on the opposite side to collect the fluorescence, which is then reflected back via a mirror, allowing the PD to collect twice the fluorescence intensity. The specific advantages of this method are as follows:

[0053] 1) Higher signal-to-noise ratio. Saturated absorption spectroscopy detects minute changes in transmitted light intensity. This change is typically small relative to the background transmitted light intensity (i.e., the transmitted light intensity at non-resonance). The background intensity itself may contain laser intensity noise, detector noise, etc. Saturated fluorescence spectroscopy detects a fluorescence signal that enhances from zero or low background. Far from resonance, the fluorescence signal is very weak (ideally close to zero). When resonance occurs, the fluorescence intensity increases significantly. Therefore, saturated fluorescence spectroscopy typically achieves a signal-to-noise ratio that is an order of magnitude or more higher than that of saturated absorption spectroscopy. This is crucial for improving the accuracy and stability of locking, especially in cases of weak signals or low atomic density.

[0054] 2) Drift-free locking. Traditional PDH frequency stabilization relies on ultrastable cavities, but the locked frequency of these cavities is easily affected by external temperature and the aging of their mechanical structures, causing it to slowly change over time. This invention utilizes the saturated fluorescence spectral line properties of atoms to lock the laser frequency at ω. r -f0 is nearby. ω r It is the atomic resonance frequency, a physical constant that can be considered to remain unchanged forever, thus avoiding the problem of long drift in principle.

[0055] Furthermore, the present invention also provides a laser frequency stabilization device based on saturated fluorescence spectroscopy. The device mainly includes a modulation signal generation module, a frequency discrimination signal generation module, and a laser locking module, which are applied to a laser frequency stabilization method based on saturated fluorescence spectroscopy in the above embodiments to stabilize the frequency of the laser.

[0056] In one specific implementation, see Figure 1 As shown, the device specifically includes the following components:

[0057] A laser, used to output the initial laser beam;

[0058] An acousto-optic modulator (AOM) is connected to a signal source to receive frequency-modulated RF signals (modulated RF signals) and modulate the frequency of the incident laser.

[0059] A vacuum cavity, an atom source is arranged in the vacuum cavity, the atom source is an atom furnace or a sealed atom gas chamber; wherein, the atom furnace has high-temperature atom vapor inside, the atom vapor is sprayed out through a collimator of a furnace head, a collimated hot atom beam vertically upward is generated, and the hot atom generates a fluorescence signal under the irradiation of pump light.

[0060] A mirror is arranged on the opposite side of the vacuum cavity, and is used for reflecting the pump light in the original path to form a counter laser field.

[0061] A photodetector (PD) is arranged on the side of the atom beam, and is used for receiving the fluorescence signal emitted after the atom is excited and converting the fluorescence signal into an electric signal.

[0062] A lens and mirror assembly is arranged on the opposite side of the photodetector, and is used for collecting and reflecting the fluorescence signal to the PD direction to enhance the detection signal strength.

[0063] A frequency mixer has a first input end connected to the output end of the photodetector and a second input end connected to a reference modulation signal output by a signal source, and is used for generating a mixed signal.

[0064] A low-pass filter is connected to the output end of the frequency mixer, and is used for filtering out high-frequency components and retaining a direct-current frequency discrimination signal.

[0065] A servo feedback module receives the frequency discrimination signal, and outputs a feedback voltage to the laser after PID adjustment, so that frequency closed-loop control is realized.

[0066] The device provided in the embodiment of the application has the same implementation principle and technical effects as the foregoing method embodiment, and for brief description, the part not mentioned in the device embodiment can refer to the corresponding content in the foregoing method embodiment, and will not be described here again.

[0067] In summary, the application provides a new laser frequency stabilization scheme based on atomic saturated fluorescence spectrum. The scheme directly detects the fluorescence signal starting from zero, and thus has a very high signal-to-noise ratio. This characteristic makes it particularly suitable for precise frequency stabilization of low-density or weak-absorption samples (such as rare isotopes). At the same time, the hot atom device used does not require a high-vacuum environment, and also avoids the need for complex precise temperature control. Compared with the super-stable cavity frequency stabilization scheme, the scheme has a significantly reduced cost, eliminates the problem of long-term frequency drift, and has a more simple device structure.

[0068] The embodiments in the specification are described in a progressive manner, and each embodiment focuses on the difference from other embodiments, and the same or similar parts of each embodiment can be referred to each other.

[0069] It should be noted that the word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding the

[0070] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and are within the scope of the application as defined by the appended claims. The actual scope of the application is defined by the appended claims. CLAIM

Claims

1. A method for stabilizing a laser frequency based on saturated fluorescence spectroscopy, characterized in that, The method comprises: Modulation signal generation: the laser to be stabilized is frequency-modulated by an acousto-optic modulator and enters a vacuum cavity in a horizontal direction as pump light to irradiate fluorescent light-emitting materials in the vacuum cavity to generate a fluorescent signal; the pump light returns to the vacuum cavity by a first mirror to form counter-propagating laser light to eliminate the influence of the Doppler effect; the fluorescent signal is received by a photodetector on the side of the vacuum cavity and converted into an electrical signal; Frequency discrimination signal generation and laser locking: a signal with the same frequency as the modulation frequency is input into a mixer together with the electrical signal to generate a mixed signal, and a frequency discrimination signal is generated by a low-pass filter, and a feedback voltage signal is generated by the frequency discrimination signal and fed back to the laser to stabilize the frequency of the laser.

2. The method of claim 1, wherein the saturated fluorescence spectrum is generated by a laser. A hot atom device is connected below the vacuum cavity, and high-temperature atomic vapor is in the hot atom device; the atomic vapor is sprayed out through a collimator to generate a collimated hot atom beam that is vertically upwardly sprayed; the hot atoms generate a fluorescent signal under irradiation of the pump light.

3. The method of claim 1, wherein the saturated fluorescence spectrum is generated by a laser. A lens and a second mirror are arranged on the other side of the vacuum cavity, opposite the photodetector, to collect the fluorescent signal radiated to the side and reflect the fluorescent signal to the photodetector.

4. The method of claim 1, wherein the saturated fluorescence spectrum is generated by a laser. An RF signal is generated by a signal source as input of the acousto-optic modulator, and the frequency of the RF signal input into the acousto-optic modulator by the signal source satisfies: f(t)=f0+αsin(Ωt) wherein f(t) represents the real-time frequency of the RF signal input into the acousto-optic modulator by the signal source at t, f0 is a fundamental frequency, Ω is a modulation frequency, and α is a modulation amplitude.

5. The method of claim 4, wherein the saturated fluorescence spectrum is generated by a laser. The output signal of the photodetector and a sine signal βsin(Ωt+φ) with the same frequency as the frequency of the RF signal output by the signal source are input into a mixer to obtain a mixed signal, wherein β is the amplitude of the RF signal, and φ represents an initial phase. The mixed signal is input into a low-pass filter to obtain a frequency discrimination signal: h(t)=1 / 2*αβdI(ω0+f0) / dω*cos(φ) wherein ω0 is the frequency of the laser emitted by the laser, and I(ω0+f0) represents the intensity of the fluorescent light collected by the lens when the frequency of the laser irradiating the atoms is ω0+f0. When φ=0, h(t)=1 / 2*αβdI(ω0+f0) / dω, that is, the derivative of the saturated fluorescence spectrum is extracted, and h(t) is the frequency discrimination signal. When ω0+f0 is lower than the resonance frequency, the frequency discrimination signal is negative, a feedback voltage signal is fed back through a servo feedback module to increase the frequency of the laser; when ω0+f0 is higher than the resonance frequency, the frequency discrimination signal is positive, a feedback voltage signal is fed back through a servo feedback module to decrease the frequency of the laser. The vacuum cavity is a sealed atomic gas chamber.

6. The method of claim 5, wherein the saturated fluorescence spectrum is generated by a laser. The device comprises a modulation signal generation module, a frequency discrimination signal generation and laser locking module, and a laser stabilization method based on saturated fluorescence spectrum according to any one of claims 1-7 to stabilize the frequency of the laser.

7. The method of claim 1, wherein the saturated fluorescence spectrum is generated by a laser. ​ 8. A laser frequency stabilization device based on saturated fluorescence spectrum, characterized in that, ​

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