A CPT atomic clock
By using lenses and optical cavities in the CPT atomic clock to adjust the frequency and amplitude of the laser signal and eliminate high-order sidebands, the problem of low signal-to-noise ratio of the frequency discrimination signal was solved, thereby improving the signal-to-noise ratio and reducing power consumption.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- ZHEJIANG SAISI ELECTRONICAL TECHNOLOGY CO LTD
- Filing Date
- 2023-03-24
- Publication Date
- 2026-07-24
AI Technical Summary
In existing CPT atomic clocks, the high-order sidebands generated by phase modulation result in an excessively low signal-to-noise ratio for the frequency discrimination signal, affecting its stability and accuracy.
By replacing attenuators with lenses and optical cavities, and adjusting the amplitude of different frequencies in the laser signal, higher-order sidebands that do not participate in the CPT effect are eliminated, and the carrier and +1 or -1 order sidebands are attenuated to the same amplitude, so as to achieve coherent population trapping effect and improve signal-to-noise ratio.
It improves the signal-to-noise ratio of the frequency discrimination signal of the CPT atomic clock, reduces optical frequency shift, and has the advantages of small size and low power consumption.
Smart Images
Figure CN116300373B_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of atomic frequency standards, and in particular to a CPT atomic clock. Background Technology
[0002] Coherent population trapping (CPT) is a quantum interference phenomenon generated by the interaction between lasers and atoms. CPT atomic clocks are a new type of atomic frequency standard based on this phenomenon and are widely used in various time and frequency systems. Generating coherent population trapping requires excitation of two sidebands. Generally, two methods are used: +1st-order sideband and -1st-order sideband, or 0th-order sideband and +1st-order sideband (or 0th-order sideband and -1st-order sideband). These two excitation methods are called full-width modulation excitation and half-width modulation excitation, respectively. Currently, the physical structure of a CPT atomic clock mainly includes: a vertical cavity surface emitting laser (VCSEL), an attenuator, a quarter-wave plate (1 / 4λ waveplate), an atomic gas cell assembly, a photodetector, and a frequency-locking circuit.
[0003] However, in current attenuator-based schemes, regardless of whether half-width or full-width modulation is used, at least one frequency of the laser, either the fundamental frequency (0th order) or ±1st order, does not participate in the CPT effect. The unparticipated laser sidebands become the noise floor of the error signal, leading to optical frequency shift. This phenomenon is the main factor contributing to the deterioration of the CPT error signal's signal-to-noise ratio. Therefore, the high-order sidebands generated in current phase modulation result in an excessively low signal-to-noise ratio for the CPT atomic clock's frequency discrimination signal.
[0004] Therefore, overcoming the phenomenon that the signal-to-noise ratio of the CPT atomic clock frequency discrimination signal is too low due to the high-order sidebands generated in phase modulation is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention
[0005] The purpose of this application is to provide a CPT atomic clock to overcome the phenomenon that the signal-to-noise ratio of the CPT atomic clock frequency discrimination signal is too low due to the high-order sidebands generated in phase modulation.
[0006] To address the aforementioned technical problems, this application provides a CPT atomic clock, comprising: a VCSEL, a 1 / 4λ waveplate, a lens, and an optical cavity; wherein the VCSEL is used to generate the laser required for the optical pumping of the CPT atomic clock;
[0007] The lens and the optical cavity are disposed between the VCSEL and the 1 / 4λ waveplate; the lens is disposed on the side closer to the VCSEL, and the lens is used to converge the diverging light emitted from the VCSEL into Gaussian light;
[0008] The optical cavity is located on one side close to the 1 / 4λ waveplate; the inner surface of the optical cavity includes a mirror for adjusting the amplitude of different frequencies in the laser signal to eliminate noise in the laser that does not participate in the CPT effect.
[0009] Preferably, the design parameters of the optical cavity are adjusted according to the laser frequency and / or microwave frequency of the CPT atomic clock.
[0010] Preferably, the design parameters include at least one of the following: the reflectivity of the mirror, the radius of curvature of the mirror, and the cavity length of the optical cavity.
[0011] Preferably, the 1 / 4λ waveplate makes a 45° angle with the laser when it is in operation.
[0012] Preferably, the reflector is a pair of plano-concave mirrors.
[0013] Preferably, the method further includes: adjusting the waist spot position and waist spot radius of the laser emitted by the VCSEL according to the design parameters so that the laser emitted by the VCSEL is consistent with the waist spot position and waist spot radius of the optical cavity.
[0014] Preferably, the position and radius of the laser waist spot are adjusted by adjusting the focal length of the lens.
[0015] Preferably, the cavity linewidth of the optical cavity is greater than the frequency difference between the two lasers that cause the CPT effect, but less than twice the frequency difference.
[0016] Preferably, it further includes: controlling the VCSEL to perform real-time frequency correction on the output laser according to the frequency locking signal to achieve laser frequency stability.
[0017] Preferably, the optical cavity is a Fabry-Perot cavity.
[0018] The CPT atomic clock provided in this application includes a VCSEL, a quarter-λ waveplate, a lens, and an optical cavity. The VCSEL generates the laser light required for the CPT atomic clock's optical pump. The lens and optical cavity are positioned between the VCSEL and the quarter-λ waveplate. The lens, located closer to the VCSEL, focuses the laser light emitted from the VCSEL into Gaussian light. The optical cavity, located closer to the quarter-λ waveplate, has a mirror on its inner surface for adjusting the amplitude of different frequencies in the laser signal to eliminate higher-order sidebands. Specifically, the mirror in the optical cavity can adjust the amplitude of different frequencies in the optical signal, eliminating higher-order sidebands, while simultaneously attenuating the carrier wave and the +1 (or -1) order sidebands to the same amplitude for coherent population trapping with basic atoms. The solution provided in this application, using a lens and optical cavity instead of the attenuator in current solutions, achieves specific frequency filtering of multi-sideband modulated light in the CPT atomic clock, improving the signal-to-noise ratio of the frequency discrimination signal, and possesses advantages such as small size, low power consumption, and high signal-to-noise ratio. Attached Figure Description
[0019] To more clearly illustrate the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0020] Figure 1 A schematic diagram of the structure of a CPT atomic clock provided in an embodiment of this application;
[0021] Figure 2 This is a schematic diagram of the laser amplitude in two different sidebands.
[0022] Figure 3 This is a schematic diagram of the laser amplitude of two different sidebands under the CPT atomic clock of this application, provided as an embodiment of the present application. Detailed Implementation
[0023] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of this application.
[0024] The core of this application is to provide a CPT atomic clock to overcome the phenomenon that the signal-to-noise ratio of the CPT atomic clock frequency discrimination signal is too low due to the high-order sidebands generated in phase modulation.
[0025] Coherent quantum interference (CPT) is a quantum interference phenomenon generated by the interaction between lasers and atoms. CPT atomic clocks are a new type of atomic frequency standard based on this phenomenon, widely used in various time and frequency systems due to their small size and low power consumption. According to the basic principle of the CPT effect, realizing a CPT atomic clock requires microwaves of a certain power to be mixed with DC current through a capacitor and inductor circuit. The mixed signal is injected into a VCSEL to generate multi-sideband light. The fundamental frequency of the VCSEL is controlled by the injected current and temperature. The frequency difference between adjacent sidebands is equal to the frequency of the modulating microwave, and the light intensity of each sideband satisfies a Bessel function. Generating the coherent population trapping phenomenon requires the excitation of two sidebands. Generally, two methods are used: +1st-order sideband and -1st-order sideband, or 0th-order sideband and +1st-order sideband (or 0th-order sideband and -1st-order sideband). These two excitation methods are called full-width modulation excitation and half-width modulation excitation, respectively. According to the definition of the Bessel function, the light intensities of +1st-order and -1st-order light are theoretically completely equal and represent the pair of sidebands with the highest power in the spectrum. Therefore, full-width modulation excitation is commonly used in the development of CPT atomic clocks. The light intensity required by the CPT atomic clock is smaller than that of the VCSEL. Therefore, the laser output by the VCSEL needs to be attenuated using an attenuator to control the light intensity entering the atomic bubble.
[0026] Currently, the physical structure of a CPT atomic clock mainly includes: a VCSEL, an attenuator, a quarter-wave plate (1 / 4λ waveplate), an atomic gas cell assembly, a photodetector, and a frequency-locking circuit. CPT atomic clocks exhibit good short-term stability, but their medium- and long-term stability is limited by Zeeman shift, buffer gas shift, and optical shift, among which the optical shift is related to the characteristics of the laser involved in the atomic interaction. Theoretically, CPT clock atoms effectively eliminate the first-order optical shift, but it remains a significant factor affecting the stability of CPT atomic clocks. This is mainly due to the inherent characteristics of the laser and the influence of other sidebands when modulation generates a coherent laser field. Higher-order sidebands and the carrier wave do not participate in the CPT effect, but the photodetector can still receive this portion of the laser, thus contributing to the noise floor of the CPT atomic clock. The inherent characteristics of the laser include its center wavelength, linewidth, luminous intensity, aging characteristics, and temperature coefficient. Electro-optic modulators modulate the laser phase to generate multicolor light. Intensity variations in all sidebands (regardless of whether they participate in resonance) and microwave power variations cause varying degrees of optical frequency shift. The influence of multi-sidebands during modulation is mainly due to changes in their intensity and the modulation microwave power output by the frequency synthesizer. To reduce sidebands that do not participate in the CPT effect, an appropriate modulation depth can be selected, i.e., adjusting the microwave power to minimize the optical intensity of these sidebands. Additionally, active methods such as stabilizing laser power, frequency, and temperature are needed to reduce the noise floor of the CPT signal. However, regardless of whether half-width or full-width modulation is used, at least one frequency in the fundamental frequency (0th order) and ±1 orders does not participate in the CPT effect, and these three frequency bands account for a relatively high proportion of optical power. Therefore, selecting an appropriate modulation depth to reduce the sidebands that do not participate in the CPT effect has limited effect. Furthermore, using active feedback control will additionally increase the power consumption of the CPT atomic clock, which is one of the important performance indicators of a CPT atomic clock. Laser sidebands that do not participate in atomic reactions become the noise floor of the error signal, resulting in optical frequency shift. This phenomenon is one of the main factors causing the deterioration of the signal-to-noise ratio of CPT error signals. Currently, noise can only be partially suppressed, and +1 or -1 order sidebands cannot be suppressed. Therefore, this application provides a scheme to improve the signal-to-noise ratio of CPT atomic clocks, which is mainly applicable to small and micro passive CPT atomic clocks. This scheme can eliminate high-order sidebands, and sidebands that are not within the optical cavity resonant frequency cannot pass through, thereby reducing noise, weakening optical frequency shift, and improving the signal-to-noise ratio.
[0027] To enable those skilled in the art to better understand the present application, the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.
[0028] To address the aforementioned technical problems, embodiments of this application provide a CPT atomic clock. Figure 1 A schematic diagram of the structure of a CPT atomic clock provided in an embodiment of this application; Figure 1The structure shown is only one example of this embodiment, and is not limited to this structure in actual applications, such as... Figure 1 As shown, the CPT atomic clock includes: VCSEL1, lens 2, optical cavity 3, and 1 / 4λ waveplate 4. In reality, a complete CPT atomic clock should also include atomic gas cell assembly, photodetector, frequency locking circuit, and other components. VCSEL1 is used to generate the laser required for the CPT atomic clock's optical pump, typically a 795nm laser. VCSEL1 also performs real-time frequency correction on the output laser based on the frequency locking signal, achieving laser frequency stability. Lens 2 and optical cavity 3 are positioned between VCSEL1 and 1 / 4λ waveplate 4; lens 2 is positioned closer to VCSEL1, and optical cavity 3 is positioned closer to 1 / 4λ waveplate 4. The laser emitted from VCSEL1 is divergent, and lens 2 is used to converge the divergent light emitted from VCSEL1 into Gaussian light. The inner surface of the optical cavity 3 includes a reflector. Specifically, a reflective film can be deposited on the inner surface of the cavity mirror to form a reflector, which is used to adjust the amplitude of different frequencies in the laser signal to eliminate the noise of the laser that does not participate in the CPT effect. That is, adjusting the amplitude of different frequencies in the laser signal eliminates higher-order sidebands, while attenuating the carrier and the +1 (or -1) order sideband to the same amplitude, which is used to interact with basic atoms to produce a coherent population trapping effect, ultimately improving the signal-to-noise ratio of the CPT atomic clock. Optical cavity 3 generally refers to a Fabry-Perot cavity, which can be a Fabry-Perot cavity composed of a pair of plano-concave mirrors and a cavity. Correspondingly, the focal length of lens 2 is equal to the distance between lens 2 and the reflecting surface of the plano mirror in optical cavity 3, in order to achieve mode matching with optical cavity 3. The laser enters through the plano mirror and exits through the concave mirror. The function of optical cavity 3 is to filter out laser light that does not participate in atomic interactions, and based on the characteristics of optical cavity 3, to attenuate the fundamental frequency (0th order) light and +1st order light (or possibly 0th order and -1st order light or +1st order and -1st order light) that participate in atomic activities to similar amplitudes, achieving noise reduction. The 1 / 4λ waveplate 4 is mainly used to change the polarization state of the laser. During operation, the angle between the 1 / 4λ waveplate 4 and the linearly polarized light is approximately 45°, which is the preferred setting, but there may be slight deviations in actual settings. The atomic gas chamber assembly mainly includes an atomic gas chamber that generates the CPT effect between laser and atoms, as well as external devices. The external devices mainly include atomic bubbles that store alkaline atoms and buffer gases, heating devices for heating gases, magnetic solenoids that generate magnetic fields, and magnetic shielding layers.
[0029] The laser emitted from the VCSEL undergoes mode matching through a lens to adjust the waist spot position and radius of the laser, making them consistent with the waist spot of the optical cavity. After the laser enters the optical cavity, laser resonance occurs. The cavity linewidth Γ is greater than the frequency difference Δv between the two lasers exhibiting the CPT effect, but less than twice the frequency difference 2Δy, as shown in formula (1):
[0030] Δv<Γ<2Δv (1)
[0031] When the modulation frequency of the laser is Δv, and both the fundamental frequency and the +1st (or -1st) order laser resonant frequency are within the cavity linewidth Γ, it is possible to isolate irrelevant frequency components and achieve different degrees of attenuation for the two frequency components participating in the CPT effect. According to the basic theory of laser resonance in a Fabry-Perot cavity, the transmitted light intensity I... c With incident light intensity I i The relationship is:
[0032]
[0033]
[0034] Where L is the length of the optical resonant cavity, n is the refractive index of the optical resonant cavity, θ is the angle between the incident light and the normal direction of the cavity mirror, R is the reflectivity of the cavity mirror of the optical resonant cavity, and λ is the resonant laser wavelength of the incident Fabry-Perot cavity. Figure 2 This is a schematic diagram of the laser amplitude in two different sidebands; for example... Figure 2 As shown, the laser amplitudes of the two different sidebands are not the same. Figure 3 This embodiment of the application provides a schematic diagram of the laser amplitude of two different sidebands using the CPT atomic clock of this application. The relationship (1) can be satisfied by selecting an optical cavity with lower precision (low reflection mirror). When the relationship (1) is satisfied, the design parameters of the optical cavity can be changed according to the two laser frequencies that cause the coherent population trapping effect and the microwave frequency that generates the laser, thereby making the laser amplitudes of the two different sidebands equal. Specifically, as shown... Figure 3 As shown.
[0035] When a laser beam enters an optical cavity, the portion of the laser beam within the cavity linewidth enters the cavity and resonates on both surfaces. After passing through the cavity, the components outside the cavity's resonant frequency are isolated, while the two frequency components within the cavity linewidth experience varying degrees of attenuation based on their frequency difference from the cavity's resonant center frequency. The attenuation coefficient is related to the shape of the cavity linewidth. Therefore, when this laser interacts with rubidium atoms in the atomic gas chamber, only one pair of lasers with amplitude symmetry, whose frequency difference between the carrier and sideband is equal to the frequency difference between the two hyperfine energy level structures, interact with the atoms. This reduces the optical frequency shift and improves the signal-to-noise ratio of the frequency discrimination signal.
[0036] CPT atomic clocks typically require optical attenuators to reduce optical power. Since laser light experiences power loss when passing through an optical cavity, this function can be achieved by properly designing the optical cavity parameters, thereby reducing the size of the optical attenuator. This solution also eliminates the need for feedback control, passively eliminating higher-order sidebands that do not participate in the CPT effect without increasing power consumption. Furthermore, by properly designing the optical cavity parameters, it is theoretically possible to completely eliminate the +1st (-1st) order sideband power that does not participate in the CPT effect, further improving the signal-to-noise ratio of the CPT error signal.
[0037] The CPT atomic clock provided in this application includes a VCSEL, a quarter-λ waveplate, a lens, and an optical cavity. The VCSEL generates the laser light required for the CPT atomic clock's optical pump. The lens and optical cavity are positioned between the VCSEL and the quarter-λ waveplate. The lens, located closer to the VCSEL, focuses the laser light emitted from the VCSEL into Gaussian light. The optical cavity, located closer to the quarter-λ waveplate, has a mirror on its inner surface for adjusting the amplitude of different frequencies in the laser signal to eliminate higher-order sidebands. Specifically, the mirror in the optical cavity can adjust the amplitude of different frequencies in the optical signal, eliminating higher-order sidebands, while attenuating the carrier wave and the +1 (or -1) order sidebands to the same amplitude for coherent population trapping with basic atoms. The solution provided in this application, using a lens and optical cavity instead of the attenuator in current solutions, achieves specific frequency filtering effects for multi-sideband modulated light in the CPT atomic clock, improving the signal-to-noise ratio of the frequency discrimination signal, and possesses advantages such as small size, low power consumption, and high signal-to-noise ratio.
[0038] In practical applications, the design parameters of the optical cavity can be adjusted according to the laser frequency and / or microwave frequency of the CPT atomic clock. These design parameters may include at least one of the following: the reflectivity of the mirror, the radius of curvature of the mirror, and the cavity length of the optical cavity. The specific adjustment method is not limited. The reflectivity, radius of curvature of the mirror, and cavity length of the optical cavity can be adjusted according to the laser frequency and / or microwave frequency of the CPT atomic clock. The optical cavity provided in this embodiment can adapt to different laser and microwave frequencies.
[0039] In practical implementation, it is usually necessary to ensure that the waist position and radius of the laser emitted by the VCSEL are consistent with those of the optical cavity. Specifically, the waist position and radius of the laser emitted by the VCSEL can be adjusted according to design parameters (such as the reflectivity of the mirror, the radius of curvature of the mirror, and the cavity length of the optical cavity) to ensure consistency between the emitted laser and the waist position and radius of the optical cavity. In actual adjustment, the waist position and radius of the laser can be adjusted by adjusting the focal length of the lens, ensuring that the focal length equals the distance between the lens and the waist of the optical cavity.
[0040] The CPT atomic clock provided in this application has been described in detail above. The various embodiments in the specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. It should be noted that those skilled in the art can make various improvements and modifications to this application without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims of this application.
[0041] It should also be noted that, in this specification, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes the aforementioned element.
Claims
1. A CPT atomic clock, comprising: VCSEL, 1 / 4λ waveplate; characterized in that it further includes: a lens, an optical cavity; wherein, the VCSEL is used to generate the laser required for the optical pumping of the CPT atomic clock; The lens and the optical cavity are disposed between the VCSEL and the 1 / 4λ waveplate; the lens is disposed on the side closer to the VCSEL, and the lens is used to converge the diverging light emitted from the VCSEL into Gaussian light; the focal length of the lens is equal to the distance between the lens and the reflective surface of the optical cavity mirror, so that the laser emitted from the VCSEL is consistent with the waist spot position and waist spot radius of the optical cavity; The optical cavity is located near the 1 / 4λ waveplate; the 1 / 4λ waveplate forms a 45° angle with the laser during operation; the inner surface of the optical cavity includes a mirror, which is a pair of plano-concave mirrors; the optical cavity is a Fabry-Perot cavity composed of the pair of plano-concave mirrors and the cavity body; the laser enters from the plano mirror and exits from the concave mirror; the cavity linewidth of the optical cavity is greater than the frequency difference between the two lasers that cause the CPT effect, but less than twice the frequency difference; after the laser passes through the optical cavity, components outside the optical cavity resonant frequency are isolated, which is used to adjust the amplitude of different frequencies in the laser signal to eliminate higher-order sidebands, and attenuate the carrier and +1 or -1 order sidebands to the same amplitude, which is used to interact with basic atoms to produce a coherent population trapping effect; the VCSEL is controlled to perform real-time frequency correction on the output laser according to the frequency locking signal to achieve laser frequency stability.
2. The CPT atomic clock according to claim 1, characterized in that, The design parameters of the optical cavity are adjusted according to the laser frequency and / or microwave frequency of the CPT atomic clock.
3. The CPT atomic clock according to claim 2, characterized in that, The design parameters include at least one of the following: the reflectivity of the mirror, the radius of curvature of the mirror, and the cavity length of the optical cavity.