Miniature atomic clock based on external cavity semiconductor laser and implementation method thereof

Abstract

The invention discloses a miniature atomic clock based on an external cavity semiconductor laser and an implementation method thereof.The atomic clock comprises the external cavity semiconductor laser, a Bias-Tee bias device, a first optical loop and a second optical loop, and the external cavity semiconductor laser is used for generating laser beams; the Bias-Tee bias device is used for coupling direct current and microwaves and inputting the direct current and the microwaves to the external cavity semiconductor laser, the first optical loop divides a laser beam generated by the external cavity semiconductor laser into two beams, one beam is used for carrying out laser wavelength locking, and the other beam is used for carrying out CPT signal locking through the second optical loop; according to the invention, the external cavity semiconductor laser is used as a light source for generating CPT signals, so that the laser noise is reduced; a half-wave modulation mode is adopted to directly modulate the miniature external cavity semiconductor laser, on the basis of not additionally adding a frequency modulation or frequency shift module, the effective action light proportion is improved, and a small-sized or miniaturized high-performance CPT atomic clock is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of atomic clocks, and in particular relates to a micro atomic clock based on an external cavity semiconductor laser and a method for realizing the same. Background Art

[0002] Atomic clocks are timekeeping instruments based on the precision of atomic transition frequencies. They are currently the most accurate time devices in the world and are therefore widely used in various electronic systems. Currently, miniature atomic clock products have emerged, represented by miniature rubidium atomic clocks and coherently placed trapped (CPT) atomic clocks. Miniature CPT atomic clocks, in particular, offer small size, low power consumption, and light weight, as well as superior timekeeping performance compared to traditional oven-controlled crystal oscillators (OCRs). These clocks are the preferred choice for high-precision time and frequency systems in portable terminals and small electronic systems. However, traditional CPT atomic clocks use vertical-cavity surface-emitting lasers (VCSELs) as their light source, which have a linewidth of approximately 100 MHz, limiting further improvements in their accuracy. Improving the performance of miniature atomic clocks while maintaining miniaturization is a key technical challenge facing these types of atomic clocks.

[0003] In order to realize a miniaturized high-performance CPT atomic clock, the document "High-performance coherent population trapping atomic clock with direct-modulation distributed Braggreflector laser", Metrologia, 58(4): 045001(2021)" proposes a CPT atomic clock based on directly modulated DBR laser. The microwave frequency is used to directly modulate the DBR laser to achieve direct modulation of the narrow linewidth laser to prepare coherent laser. Compared with the VCSEL in the traditional scheme, the DBR laser linewidth is about 1MHz, which greatly reduces the FM-AM noise caused by the laser linewidth. However, the microwave power required for this scheme is large (26.9dBm), which increases the power consumption of the circuit. At the same time, the laser after frequency stabilization also requires an additional frequency shift module (such as AOM, etc.) to compensate for the atomic transition frequency shift caused by the buffer gas, which increases the complexity of the physical system and the package size, which is not conducive to miniaturization design. In order to simplify the frequency shift module, the paper "Laser Frequency Shift Locking of Coherently Populated Trapped Atomic Clocks", Chinese Journal of Lasers, 2024, 51(22), 2211002, proposed to perform additional RF modulation on the laser current on the basis of half-wave modulation. After the generated polychromatic light interacts with the atoms in the atomic gas chamber, a Doppler-free spectrum corresponding to the modulation sideband frequency can be obtained in the transmission spectrum. This spectrum can be used to achieve frequency stabilization of the laser. However, this method requires the addition of an additional frequency modulation module, and after adding modulation, the laser spectrum becomes more diffuse, reducing the effective light ratio, which in turn reduces the signal contrast. Summary of the Invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a miniature atomic clock based on an external cavity semiconductor laser and its implementation method. By adopting a directly modulated external cavity semiconductor laser with a narrow linewidth (less than 1MHz), the laser noise caused by the laser linewidth is further reduced; by directly modulating the external cavity semiconductor laser using a half-wave modulation method, the effective action light ratio is increased without adding an additional frequency modulation or frequency shift module, thereby realizing a high-performance CPT atomic clock that can be miniaturized or miniaturized.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A miniature atomic clock based on an external cavity semiconductor laser comprises an external cavity semiconductor laser 1, a Bias-Tee bias device 24, a first optical loop and a second optical loop. The external cavity semiconductor laser 1 is used to generate a laser beam. The Bias-Tee bias device 24 is used to couple a direct current and microwaves and input them into the external cavity semiconductor laser 1. The first optical loop splits the laser beam generated by the external cavity semiconductor laser 1 into two beams, and uses one beam for laser wavelength locking, while the other beam is locked to a CPT signal through the second optical loop.

[0007] The first optical loop includes a first half glass slide 2, a laser beam generated by an external cavity semiconductor laser 1 is incident on a first polarization beam splitter 3 through the first half glass slide 2, and is split into two laser beams with perpendicular polarization directions by the first polarization beam splitter 3, a laser beam is incident on a second polarization beam splitter 5 through a second half glass slide 4, and is again split into two laser beams with perpendicular polarization directions by the second polarization beam splitter 5, a laser beam passes through a laser frequency-stabilized atomic gas cell 9, a first quarter glass slide 10, and a first reflector 11 in sequence, and light reflected by the first reflector 11 is used as detection light, which passes through the first quarter glass slide 10, the laser frequency-stabilized atomic gas cell 9, and the second polarization beam splitter 5 in sequence, and is reflected by the second polarization beam splitter 5 into a first photodetector 6, a current signal output end of the first photodetector 6 is connected to a current signal input end of a laser lock-in amplifier 22, a voltage signal output end of the laser lock-in amplifier 22 is connected to a voltage signal input end of a laser power supply 23, and a DC current signal output end of the laser power supply 23 is connected to a DC current signal input end of a Bias-Tee bias device 24;

[0008] Another laser beam split by the first polarization beam splitter 3 in the first optical loop is reflected by the second reflector 12 in the second optical loop, and then passes through the second quarter glass 13 and the clock frequency atomic gas chamber 14 in sequence, and is incident on the second photodetector 18. The current signal output end of the second photodetector 18 is connected to the current signal input end of the microwave lock-in amplifier 19, the voltage signal output end of the microwave lock-in amplifier 19 is connected to the voltage signal input end of the local oscillator 20, the frequency output end of the local oscillator 20 is connected to the frequency input end of the microwave frequency synthesizer 21, and the microwave signal output end of the microwave frequency synthesizer 21 is connected to the microwave signal input end of the Bias-Tee bias device 24.

[0009] The resonant cavity length of the external cavity semiconductor laser 1 is in the order of millimeters and the modulation bandwidth is greater than 3.4 GHz.

[0010] The external cavity semiconductor laser 1 includes a semiconductor laser chip 25 for generating a polychromatic laser beam; a collimating lens 26 for collimating the polychromatic laser beam; an interference filter 27 for optical frequency selection to select the desired narrow linewidth laser; a partial reflector 28 for reflecting part of the narrow linewidth laser back to the semiconductor laser chip 25; and an output coupling lens 29 for transmitting and collimating part of the narrow linewidth laser for output.

[0011] The laser frequency stabilization atomic gas chamber 9 is filled with alkali metal atoms; the clock frequency discrimination atomic gas chamber 14 is filled with alkali metal atoms and buffer gas.

[0012] The alkali metal atoms are in gaseous state 87 Rb atoms, and the buffer gas is a mixed gas of N2 and Ar.

[0013] The ratio of N2 and Ar in the buffer gas is adjusted so that the absorption spectrum frequency of the clock frequency detection atomic gas cell 14 corresponds to the laser stabilization spectrum line frequency of the laser frequency stabilization atomic gas cell 9.

[0014] A first magnetic shielding component 7 is provided on the outer surface of the laser frequency-stabilized atomic gas chamber 9 to shield the laser frequency-stabilized atomic gas chamber 9 from interference from the external environmental magnetic field; a first temperature control component 8 is provided on the outer surface of the first magnetic shielding component 7 to heat and maintain the operating temperature of the laser frequency-stabilized atomic gas chamber 9 to a preset temperature.

[0015] The outer surface of the clock frequency atomic gas chamber 14 is wound with a magnetic field coil 15, which is used to provide a constant magnetic field parallel to the laser optical axis to the clock frequency atomic gas chamber 14; the outer surface of the magnetic field coil 15 is provided with a second magnetic shielding component 16, which is used to shield the clock frequency atomic gas chamber 14 from interference from the external environmental magnetic field; the outer surface of the second magnetic shielding component 16 is provided with a second temperature control component 17, which is used to heat and maintain the working temperature of the clock frequency atomic gas chamber 14 to a preset temperature.

[0016] A method for realizing a micro atomic clock based on an external cavity semiconductor laser comprises the following steps:

[0017] Step 1: Temperature control is performed on the external cavity semiconductor laser 1, the laser frequency stabilization atomic gas chamber 9 and the clock frequency discrimination atomic gas chamber 14 so that their operating temperatures reach a preset temperature and are in a stable state;

[0018] Step 2: The laser lock-in amplifier 22 outputs a scanning voltage signal, controls the laser power supply 23 to output a laser scanning current, and scans the output wavelength of the external cavity semiconductor laser 1; the first photodetector 6 converts the light intensity signal passing through the laser frequency-stabilized atomic gas chamber 9 during laser wavelength scanning into a current signal and feeds it back to the laser lock-in amplifier 22 for modulation and demodulation processing. When the laser frequency corresponds to the atomic transition frequency, a laser frequency stabilization error signal is obtained, and the laser power supply 23 is controlled according to the laser frequency stabilization error signal to adjust the operating current of the laser, so as to lock the laser wavelength to the cross absorption peak;

[0019] Step 3: After the laser wavelength is locked, a scanning voltage signal is output through the microwave phase-locked amplifier 19 to control the output frequency change of the local oscillator 20, thereby realizing the scanning of the output frequency of the microwave frequency synthesizer 21; the second photodetector 18 converts the light intensity signal passing through the clock frequency atomic gas chamber 14 during microwave frequency scanning into a current signal and feeds it back to the microwave phase-locked amplifier 19 for modulation and demodulation processing to obtain a clock locking error signal, and controls the output frequency of the local oscillator 20 according to the clock locking error signal, and finally locks the microwave frequency output by the microwave frequency synthesizer 21 to the center of the clock locking error signal to realize atomic clock locking.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. A miniaturized external cavity semiconductor laser with a laser wavelength of 795 nm is applied to the CPT atomic clock as the light source for generating CPT signals. The narrow line width of the directly modulated external cavity semiconductor laser is utilized.

[0022] (less than 1MHz) and good stability, further reducing the FM-AM noise caused by the laser linewidth, improving the coherent light characteristics of the CPT signal, and thus improving the stability performance of the micro atomic clock.

[0023] 2. Use half-wave modulation to directly modulate the micro external cavity semiconductor laser, and use the good direct modulation characteristics of the external cavity semiconductor laser to achieve the preparation of coherent laser. 87 The first optical loop was built in the Rb atomic gas cell to prepare Doppler-free frequency-stabilized spectrum. The saturation absorption peak generated by the ±1st order sideband was used to lock the laser frequency to 87 Rb atom 5 2 S1 / 2 ,F=1&2→5 2 P 1 / 2 ,F'=1 and 5 2 S 1 / 2 ,F=1&2→5 2 P 1 / 2 ,F'=2 cross absorption peak, the spectrum is the same as F=1&2→5 2 P 1 / 2 The absorption spectrum frequency difference of F'=2 is approximately 407 MHz. Simultaneously, by adjusting the ratio of N2 to Ar in the buffer gas within the clock-frequency atomic cell, a collision frequency shift of approximately 407 MHz is achieved, i.e., the clock spectrum line frequency of the clock-frequency atomic cell corresponds to the laser-stabilized spectrum line frequency of the laser-stabilized atomic cell. After locking the laser frequency to the stabilization spectrum line using the laser-stabilized atomic cell and a laser lock-in amplifier, the laser beam is converted into circularly polarized light, which interacts with the clock-frequency atomic cell to generate a CPT signal. The atomic clock is locked through signal detection and a microwave lock-in amplifier. The atomic clock designed in this invention not only requires low microwave power, as low as approximately 15 dBm, reducing circuit power consumption, but also eliminates the AOM required for frequency shifting in traditional schemes, enabling miniaturized design. Furthermore, the modulated laser's ±1st-order sidebands are all effective light. Compared to the scheme described in the literature "Laser Frequency Shift Locking of Coherently Populated Trapped Atomic Clocks," the effective light ratio is higher, resulting in higher signal contrast. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a structural schematic diagram of a miniature atomic clock based on an external cavity semiconductor laser provided by the present invention.

[0025] Figure 2 This is a schematic structural diagram of the external cavity semiconductor laser provided by the present invention.

[0026] In the figure: 1-external cavity semiconductor laser, 2-first half glass slide, 3-first polarization beam splitter, 4-second half glass slide, 5-second polarization beam splitter, 6-first photodetector, 7-first magnetic shielding assembly, 8-first temperature control assembly, 9-laser frequency-stabilized atomic gas chamber, 10-first quarter glass slide, 11-first reflector, 12-second reflector, 13-second quarter glass slide, 14-clock frequency-discrimination atomic gas chamber, 15-magnetic field coil, 16-second magnetic shielding assembly, 17-second temperature control assembly, 18-second photodetector, 19-microwave lock-in amplifier, 20-local oscillator, 21-microwave frequency synthesizer, 22-laser lock-in amplifier, 23-laser power supply, 24-Bias-Tee bias device, semiconductor laser chip 25, collimating lens 26, interference filter 27, partial reflector 28, output coupling lens 29, temperature control substrate 30.

[0027] Figure 3 This is a schematic diagram of the laser stabilization spectrum line and the clock detection spectrum line in the half-wave modulation state provided by the present invention.

[0028] Figure 4 This is the CPT signal spectrum generated by the micro atomic clock provided by the present invention.

[0029] Figure 5 The half-wave modulation spectrum generated by the micro atomic clock provided by the present invention.

[0030] Figure 6 Half-wave modulation and RF modulation spectra provided for the paper "Laser Frequency Offset Locking of Coherently Populated Trapped Atomic Clocks". DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0032] like Figure 1 As shown, a miniature atomic clock based on an external cavity semiconductor laser operates at a suitable and constant temperature point when the atomic clock is in operation, comprising: an external cavity semiconductor laser 1, a Bias-Tee bias device 24, a first optical loop, and a second optical loop; the external cavity semiconductor laser 1 is used to generate a laser beam, the Bias-Tee bias device 24 is used to couple a direct current and microwaves and input them into the external cavity semiconductor laser 1, the first optical loop splits the laser beam generated by the external cavity semiconductor laser 1 into two beams, and uses one beam for laser wavelength locking, while the other beam is locked to a CPT signal through the second optical loop, wherein:

[0033] The first optical loop is used to lock the laser wavelength, including the first half glass slide 2, the first polarization beam splitter 3, the second half glass slide 4, the second polarization beam splitter 5, the first photodetector 6, the first magnetic shielding component 7, the first temperature control component 8, the laser frequency-stabilized atomic gas chamber 9, the first quarter glass slide 10, the first reflector 11, the laser lock-in amplifier 22, and the laser power supply 20.

[0034] The laser beam generated by the external cavity semiconductor laser 1 passes through the first half glass slide 2 and enters the first polarization beam splitter 3, where it is split into two laser beams with perpendicular polarization directions. After one laser beam passes through the second half glass slide 4, the laser beam split by the second polarization beam splitter 5 passes through the laser frequency-stabilized atomic gas chamber 9, the first quarter glass slide 10, and the first reflector 11 in sequence. The light reflected by the first reflector 11 passes through the first quarter glass slide 10 in sequence and enters the laser frequency-stabilized atomic gas chamber 9. The other laser beam split by the second polarization beam splitter 5 is reflected and becomes a useless beam of the system. The laser frequency-stabilized atomic gas chamber 9 is filled with alkali metal atoms, which interact with the incident laser to prepare a Doppler-free frequency-stabilized spectrum.

[0035] The light reflected by the first reflector 11 is used as the detection light and passes through the first quarter wave plate 10, the laser frequency-stabilized atomic gas cell 9, and the second polarization beam splitter 5 in sequence along the same path but in the opposite direction. The laser passes through the first quarter wave plate 10 twice, and the polarization direction is rotated 90 degrees. It is reflected by the polarization beam splitter 5 and enters the first photodetector 6 to obtain an atomic transition signal for spectral line detection of laser wavelength locking. The current output by the first photodetector 6 is related to the intensity of the laser passing through the laser frequency-stabilized atomic gas cell 9, reflecting the interaction between the laser and the atoms.

[0036] The current signal output end of the first photodetector 6 is connected to the current signal input end of the laser lock-in amplifier 22, converting the detected light intensity signal into a current signal and sending it to the laser lock-in amplifier 22; the voltage signal output end of the laser lock-in amplifier 22 is connected to the voltage signal input end of the laser power supply 23. The laser lock-in amplifier 22 calculates and amplifies the current signal and outputs a voltage signal to the laser power supply 23. The laser power supply 23 scans and locks the laser wavelength based on the voltage signal; the DC current signal output end of the laser power supply 23 is connected to the signal input end of the Bias-Tee bias device 24, converting the voltage signal from the laser lock-in amplifier 22 into a DC current signal through the laser power supply 23 as a control signal for the external cavity semiconductor laser 1, and sending it to the Bias-Tee bias device 24; the signal output end of the Bias-Tee bias device 24 is connected to the signal input end of the micro external cavity semiconductor laser 1, and the output signal of the Bias-Tee bias device 24 controls the operating current of the external cavity semiconductor laser 1.

[0037] In the first optical loop, when the laser lock-in amplifier 22 controls the laser power supply 23 to scan the laser wavelength, the following is obtained: Figure 3 As shown in the laser frequency stabilization line, the laser lock-in amplifier 22 obtains the laser frequency stabilization error signal through modulation and demodulation processing, and locks the laser wavelength to the cross saturation absorption peak.

[0038] The second optical loop is used to lock the CPT signal based on the locked laser wavelength, and includes: a second reflector 12, a second quarter glass slide 13, a clock frequency atomic gas chamber 14, a magnetic field coil 15, a second magnetic shielding assembly 16, a second temperature control assembly 17, a second photodetector 18, a microwave lock-in amplifier 19, a local oscillator 20, and a microwave frequency synthesizer 21.

[0039] The other laser beam split by the first polarization beam splitter 3 in the first optical loop passes through the second reflector 12 and the second quarter glass 13 in sequence, and is converted into circularly polarized light by the second quarter glass 13 and then enters the clock frequency atomic gas chamber 14. The gas chamber is filled with alkali metal atoms and buffer gas. The laser interacts with the alkali metal atoms to generate a CPT signal, such as Figure 4 As shown; the second photodetector 18 outputs a current signal through photoelectric conversion to realize CPT signal detection. The current output by the second photodetector 18 is related to the intensity of the laser after passing through the clock frequency atomic gas chamber 14, reflecting the interaction between the laser and the alkali metal atoms; the current signal output end of the second photodetector 18 is connected to the current signal input end of the microwave lock-in amplifier 19, and the second photodetector 18 converts the detected CPT signal into a current signal and sends it to the microwave lock-in amplifier 19; the voltage signal output end of the microwave lock-in amplifier 19 is connected to the voltage signal input end of the local oscillator 20, and the microwave lock-in amplifier 19 solves and amplifies the current signal and outputs a voltage signal to the local oscillator 20. The voltage signal can control the output frequency of the local oscillator 20 to scan and lock the microwave frequency; the frequency output end of the local oscillator 20 is connected to the frequency input end of the microwave frequency synthesizer 21, and the microwave frequency synthesizer 21 generates 87 The microwave power of the microwave frequency synthesizer 21 can be as low as about 15dBm, and the laser frequency is half-wave modulated by using the microwave signal. The microwave signal output end of the microwave frequency synthesizer 21 is connected to the microwave signal input end of the Bias-Tee bias device 24, and the DC current signal from the laser power supply 23 and the microwave signal from the microwave frequency synthesizer 21 are superimposed. The superimposed current is used as the driving current of the external cavity semiconductor laser 1, which directly controls the external cavity semiconductor laser 1 to generate multi-color laser.

[0040] The external cavity semiconductor laser 1 generates multi-color laser light, in which the ±1st-order sidebands serve as coherent light for preparing the CPT signal. After the laser frequency is locked through the first optical loop, the microwave phase-locked amplifier 19 scans the output frequency control voltage of the local oscillator 20, and the CPT signal is detected by the second photodetector 18. The microwave phase-locked amplifier 19 obtains the clock locking error signal through modulation and demodulation, and locks the microwave frequency to the center of the error signal to achieve atomic clock locking.

[0041] The resonant cavity length of the external cavity semiconductor laser 1 is only in the order of millimeters and the modulation bandwidth is greater than 3.4 GHz, which supports 3.4 GHz microwave direct modulation to generate multi-color laser with a laser wavelength of 795 nm. Figure 2As shown, the external cavity semiconductor laser 1 includes: a semiconductor laser chip 25 for generating a polychromatic laser beam; a collimating lens 26 for collimating and shaping the polychromatic laser beam emitted by the semiconductor laser chip 25; an interference filter 27 for optical frequency selection to select the required narrow linewidth laser; a partial reflector 28 for reflecting part of the narrow linewidth laser back to the semiconductor laser chip 25; an output coupling lens 29 for transmitting and collimating part of the narrow linewidth laser to output, and adjusting the laser spot emitted by the semiconductor laser chip 25 to match the optical windows of the two atomic gas cells; a temperature control substrate 30, including a substrate and a temperature control component, the semiconductor laser chip 25, collimating lens 26, interference filter 27, partial reflector 28 and output coupling lens 29 are mounted on the substrate surface, and the temperature control component maintains the substrate surface at a constant temperature to ensure the normal and stable operation of other components.

[0042] The laser frequency stabilization atomic gas chamber 9 is only filled with gaseous 87 The clock frequency atomic gas chamber 14 is filled with gaseous 87 In this embodiment, the ratio of N2 to Ar is adjusted to 2:3, and the total pressure is about 50 Torr, which can reduce the 5% of the buffer gas collision frequency. 2 S 1 / 2 , F=1&2→5 2 P 1 / 2 ,F'=2 transition frequency shift is controlled at around 407MHz.

[0043] A first magnetic shielding component 7 is provided on the outer surface of the laser frequency-stabilized atomic gas chamber 9 for shielding the laser frequency-stabilized atomic gas chamber 9 from interference from the external environmental magnetic field. A first temperature control component 8 is provided on the outer surface of the first magnetic shielding component 7 for heating and maintaining the operating temperature of the laser frequency-stabilized atomic gas chamber 9 to a preset temperature.

[0044] The outer surface of the clock-frequency atomic gas chamber 14 is wrapped with a magnetic field coil 15, which is used to provide a constant magnetic field parallel to the laser optical axis. A second magnetic shielding assembly 16 is provided on the outer surface of the magnetic field coil 15 to shield the clock-frequency atomic gas chamber 14 from interference from the external environmental magnetic field. A second temperature control assembly 17 is provided on the outer surface of the second magnetic shielding assembly 16 to heat and maintain the operating temperature of the clock-frequency atomic gas chamber 14 to a preset temperature. Both the clock-frequency atomic gas chamber magnetic shielding assembly 1 and the clock-frequency atomic gas chamber temperature control assembly 17 have openings on both sides for light to pass through.

[0045] The first magnetic shielding assembly 7 and the second magnetic shielding assembly 16 both adopt a magnetic shielding box structure and are made of Permalloy; the first temperature control assembly 8 and the second temperature control assembly 17 both include temperature sensors for measuring the temperature of the atomic gas chamber in real time and feeding it back to the controller. The controller controls the heating device to heat the atomic gas chamber and maintain it within a preset temperature based on the temperature of the atomic gas chamber.

[0046] Both sides of the first magnetic shielding assembly 7 and the first temperature control assembly 8 are machined with light holes of uniform height for the laser beam to enter the laser frequency stabilization atomic gas chamber 9. Both sides of the second magnetic shielding assembly 16 and the second temperature control assembly 17 are machined with light holes of uniform height for the laser beam to enter the clock frequency atomic gas chamber 14.

[0047] The implementation method of the micro atomic clock provided by the present invention is:

[0048] Step 1: Temperature control is performed on the external cavity semiconductor laser 1, the laser frequency stabilization atomic gas chamber 9 and the clock frequency discrimination atomic gas chamber 14 so that their operating temperatures reach a preset temperature and are in a stable state;

[0049] Step 2: Output scanning voltage signal by laser lock-in amplifier 22, control laser power supply 23 to output laser scanning current, and scan the output wavelength (frequency) of external cavity semiconductor laser 1; The light intensity signal of the first photodetector 6 that passes through the laser frequency-stabilized atomic gas chamber 9 when scanning the laser wavelength is converted into a current signal and fed back to the laser lock-in amplifier 22, and modulation and demodulation are carried out by laser lock-in amplifier 22. When the laser frequency corresponds to the atomic transition frequency, a laser frequency stabilization error signal is obtained by laser lock-in amplifier 22. Laser lock-in amplifier 22 controls laser power supply 23 according to the laser frequency stabilization error signal to adjust the operating current of the laser, and the laser wavelength is locked to 87 Rb atom 5 2 S 1 / 2 ,F=1&2→5 2 P 1 / 2 ,F'=1 and 5 2 S 1 / 2 ,F=1&2→5 2 P 1 / 2 , cross absorption peak of F'=2;

[0050] Step 3: After the laser wavelength is locked, a scanning voltage signal is output through the microwave phase-locked amplifier 19 to control the output frequency change of the local oscillator 20, and then the output frequency of the microwave frequency synthesizer 21 is scanned; the second photodetector 18 converts the light intensity signal passing through the clock frequency atomic gas chamber 14 during microwave frequency scanning into a current signal and feeds it back to the microwave phase-locked amplifier 19, which performs modulation and demodulation processing through the microwave phase-locked amplifier 19, and obtains the clock locking error signal through the microwave phase-locked amplifier 19. The microwave phase-locked amplifier 19 controls the output frequency of the local oscillator 20 according to the clock locking error signal, and finally locks the microwave frequency output by the microwave frequency synthesizer 21 to the center of the clock locking error signal to achieve atomic clock locking.

[0051] The CPT atomic clock light source of the present invention uses a directly modulated external cavity semiconductor laser 1. The linewidth of this laser is smaller than that of reported VCSEL and DBR lasers, that is, it has smaller FM-AM noise. According to the document "Physics of Coherent Population Trapped Atomic Clocks", Wuhan: Huazhong University of Science and Technology, the contribution of the laser linewidth to the signal-to-noise ratio of the CPT atomic clock can be simplified as follows:

[0052]

[0053] Where A represents a parameter related to the signal amplitude and atomic relaxation rate, and Δv represents the laser linewidth; combined with the theoretical formula of atomic clock stability:

[0054]

[0055] Where Q is the quality factor and τ represents the time constant. It can be seen that for an atomic clock system, assuming that other parameters remain unchanged, the stability of the atomic clock is proportional to the 3 / 2 power of the laser line width, that is, the smaller the line width, the better the stability of the atomic clock. The external cavity semiconductor laser 1 used in the present invention has a typical line width of less than 1MHz, which can achieve an atomic clock with higher stability. In addition, the volume of the external cavity semiconductor laser 1 used in the present invention is less than 1cm 3 , which is smaller than traditional narrow-linewidth lasers and can realize chip-level CPT atomic clocks.

[0056] like Figure 3 As shown, a schematic diagram of the laser stabilization spectrum line and the clock detection spectrum line generated by the micro atomic clock of the present invention in the half-wave modulation state, wherein the cross saturation absorption peak is the laser stabilization reference peak, and its frequency corresponds to the absorption spectrum line frequency of the clock detection atomic gas chamber 14. 87 Rb atom 5 2 S 1 / 2 ,F=1&2→5 2 P 1 / 2 ,F'=2 spectrum line is about 407MHz red detuned. The present invention proposes to use cross saturation absorption peak to achieve laser frequency stabilization, directly achieve relative87 Rb atom 5 2 S 1 / 2 ,F=1&2→5 2 P 1 / 2 ,F'=2 spectrum line is about 407MHz red detuned, and by controlling the buffer gas ratio, the 5 2 S 1 / 2 ,F=1&2→5 2 P 1 / 2 ,F'=2 transition frequency shift is controlled at around 407MHz. Compared with the existing reported AOM frequency shift scheme, laser frequency stabilization does not require the addition of AOM for frequency shifting, which greatly reduces the cost and size of the system and enables the system to be miniaturized. Moreover, the ±1 level sidebands of the modulated laser are all effective light; Figure 6 As shown in the figure, the scheme of the document "Coherent Population Trapped Atomic Clock Laser Frequency Shift Locking" uses half-wave modulation + radio frequency modulation to achieve frequency stabilization of the laser, and the effective light is ±1 sideband; compared with the scheme of the document "Coherent Population Trapped Atomic Clock Laser Frequency Shift Locking", the present invention only uses half-wave modulation to generate effective light with the same half-wave modulation + radio frequency modulation method used in the document, and the effective light is ±1 sideband, as shown in the figure. Figure 5 As shown; therefore, the micro atomic clock of the present invention can generate a higher proportion of effective light without increasing modulation, the circuit system is simpler, and the obtained signal contrast is higher.

Claims

1. A miniature atomic clock based on an external cavity semiconductor laser, characterized in that: include: An external cavity semiconductor laser (1), a Bias-Tee bias device (24), a first optical loop, and a second optical loop; The external cavity semiconductor laser (1) is used to generate a laser beam, the Bias-Tee bias device (24) is used to couple a direct current and a microwave and input them into the external cavity semiconductor laser (1), the first optical loop divides the laser beam generated by the external cavity semiconductor laser (1) into two beams, and uses one beam to lock the laser wavelength, and the other beam is locked by a CPT signal through a second optical loop.

2. The micro atomic clock based on an external cavity semiconductor laser according to claim 1, characterized in that: The first optical loop comprises a first half glass slide (2); a laser beam generated by an external cavity semiconductor laser (1) is incident on a first polarization beam splitter (3) via the first half glass slide (2); the first polarization beam splitter (3) splits the laser beam into two beams with perpendicular polarization directions; a laser beam is incident on a second polarization beam splitter (5) via a second half glass slide (4); the second polarization beam splitter (5) splits the laser beam into two beams with perpendicular polarization directions; the laser beam passes through a laser frequency stabilization atomic gas chamber (9), a first quarter glass slide (10), and a first reflector (11) in sequence; and the laser beam is reflected by the first reflector (11) The light is used as the detection light, passes through the first quarter glass (10), the laser frequency-stabilized atomic gas chamber (9), and the second polarization beam splitter (5) in sequence, and is reflected by the second polarization beam splitter (5) into the first photodetector (6); the current signal output end of the first photodetector (6) is connected to the current signal input end of the laser lock-in amplifier (22), the voltage signal output end of the laser lock-in amplifier (22) is connected to the voltage signal input end of the laser power supply (23), and the DC current signal output end of the laser power supply (23) is connected to the DC current signal input end of the Bias-Tee bias device (24); Another laser beam split by the first polarization beam splitter (3) in the first optical loop is reflected by the second reflector (12), and then sequentially passes through the second quarter glass (13) and the clock frequency atomic gas chamber (14) to enter the second photodetector (18). The current signal output end of the second photodetector (18) is connected to the current signal input end of the microwave lock-in amplifier (19), the voltage signal output end of the microwave lock-in amplifier (19) is connected to the voltage signal input end of the local oscillator (20), the frequency output end of the local oscillator (20) is connected to the frequency input end of the microwave frequency synthesizer (21), and the microwave signal output end of the microwave frequency synthesizer (21) is connected to the microwave signal input end of the Bias-Tee bias device (24).

3. The micro atomic clock based on an external cavity semiconductor laser according to claim 1, characterized in that: The resonant cavity length of the external cavity semiconductor laser (1) is on the order of millimeters and the modulation bandwidth is greater than 3.4 GHz.

4. The micro atomic clock based on an external cavity semiconductor laser according to claim 3, characterized in that: The external cavity semiconductor laser (1) comprises a semiconductor laser chip (25) for generating a polychromatic laser beam; a collimating lens (26) for collimating the polychromatic laser beam; an interference filter (27) for optical frequency selection to select a desired narrow linewidth laser; a partial reflector (28) for reflecting a portion of the narrow linewidth laser back to the semiconductor laser chip (25); and an output coupling lens (29) for transmitting and collimating a portion of the narrow linewidth laser for output.

5. The micro atomic clock based on an external cavity semiconductor laser according to claim 2, characterized in that: The laser frequency stabilization atomic gas chamber (9) is filled with alkali metal atoms; the clock frequency discrimination atomic gas chamber (14) is filled with alkali metal atoms and buffer gas.

6. The micro atomic clock based on an external cavity semiconductor laser according to claim 5, characterized in that: The alkali metal atoms are in gaseous state 87 Rb atoms, and the buffer gas is a mixed gas of N2 and Ar.

7. The micro atomic clock based on an external cavity semiconductor laser according to claim 6, characterized in that: The ratio of N2 and Ar in the buffer gas is adjusted so that the absorption spectrum frequency of the clock frequency-discriminating atomic gas chamber (14) corresponds to the laser stabilization spectrum line frequency of the laser frequency-stabilizing atomic gas chamber (9).

8. The micro atomic clock based on an external cavity semiconductor laser according to claim 2, characterized in that: The outer surface of the laser frequency-stabilized atomic gas chamber (9) is provided with a first magnetic shielding component (7) for shielding the laser frequency-stabilized atomic gas chamber (9) from interference from an external environmental magnetic field; the outer surface of the first magnetic shielding component (7) is provided with a first temperature control component (8) for heating and maintaining the operating temperature of the laser frequency-stabilized atomic gas chamber (9) to a preset temperature.

9. The micro atomic clock based on an external cavity semiconductor laser according to claim 2, characterized in that: The outer surface of the clock frequency discrimination atomic gas chamber (14) is wound with a magnetic field coil (15) for providing a constant magnetic field parallel to the laser optical axis to the clock frequency discrimination atomic gas chamber (14); the outer surface of the magnetic field coil (15) is provided with a second magnetic shielding component (16) for shielding the clock frequency discrimination atomic gas chamber (14) from interference from the external environmental magnetic field; the outer surface of the second magnetic shielding component (16) is provided with a second temperature control component (17) for heating and maintaining the working temperature of the clock frequency discrimination atomic gas chamber (14) to a preset temperature.

10. A method for realizing a micro atomic clock based on an external cavity semiconductor laser according to claims 1 to 9, characterized in that: The following steps are involved: Step 1: Temperature control is performed on the external cavity semiconductor laser (1), the laser frequency stabilization atomic gas chamber (9), and the clock frequency discrimination atomic gas chamber (14), so that their operating temperatures reach a preset temperature and are in a stable state; Step 2: Outputting a scanning voltage signal through a laser lock-in amplifier (22), controlling a laser power supply (23) to output a laser scanning current, and scanning the output wavelength of the external cavity semiconductor laser (1); converting a light intensity signal that passes through the laser frequency-stabilized atomic gas chamber (9) during laser wavelength scanning into a current signal by a first photodetector (6), and feeding the current signal back to the laser lock-in amplifier (22), performing modulation and demodulation processing; obtaining a laser frequency-stabilization error signal when the laser frequency corresponds to the atomic transition frequency, and controlling a laser power supply (23) according to the laser frequency-stabilization error signal to adjust the operating current of the laser, and locking the laser wavelength to the cross-absorption peak; Step 3: After the laser wavelength is locked, a scanning voltage signal is outputted through a microwave lock-in amplifier (19) to control the output frequency change of a local oscillator (20), thereby realizing scanning of the output frequency of a microwave frequency synthesizer (21); a second photodetector (18) converts the light intensity signal that passes through the clock frequency atomic gas chamber (14) during microwave frequency scanning into a current signal and feeds it back to the microwave lock-in amplifier (19), performs modulation and demodulation processing, obtains a clock locking error signal, and controls the output frequency of the local oscillator (20) according to the clock locking error signal, and finally locks the microwave frequency outputted by the microwave frequency synthesizer (21) to the center of the clock locking error signal, thereby realizing atomic clock locking.

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