Cold atom optical frequency standard based on long-strip-shaped diffuse reflection cooling and implementation method of cold atom optical frequency standard

Abstract

The invention discloses a cold atom optical frequency standard based on long-strip-shaped diffuse reflection cooling and an implementation method of the cold atom optical frequency standard. The device comprises a cooling laser source, a heavy pump laser source, a long-strip-shaped vacuum glass tube, a narrow linewidth laser source and a photoelectric detector. Wherein atoms are decelerated through a laser cooling technology, and Doppler frequency shift and collision frequency shift related to the atomic movement speed are greatly inhibited; the diffuse reflection laser cooling does not need a gradient magnetic field in a magneto-optical trap configuration, so that the influence of the magnetic field on the energy level of the frequency stabilization atomic can be well avoided; the cold atomic group with the meter-scale length, which is realized by using the long-strip-shaped vacuum glass tube, can also greatly increase the interaction distance between light and atoms, and effectively improve the signal-to-noise ratio of an atomic transition spectral line. The cold atom optical frequency standard based on long-strip-shaped diffuse reflection cooling is achieved, substantial and remarkable progress is brought to medium-and-long-term frequency stability of the optical frequency standard, the optical frequency standard can be further developed into an absolute wavelength standard, and a reference is provided for numerous precise measurement applications.

Description

Technical Field

[0001] The invention belongs to the technical field of diffuse reflection laser cooling atoms and the technical field of optical frequency quantum frequency standard, and in particular relates to a cold atom optical frequency standard based on long strip diffuse reflection cooling and a realization method thereof. Background Art

[0002] Ultrastable lasers play an important role in atomic, molecular, and optical physics (AMO), and are of great significance to the development of ultracold atoms, gravitational wave detection, high-resolution spectroscopy, and basic physics. At present, some sub-Doppler laser spectroscopy techniques, such as saturation absorption spectroscopy, polarization spectroscopy, modulation transfer spectroscopy, and frequency modulation spectroscopy, are widely used in laser frequency stabilization. By passively locking the laser frequency to the transition spectrum of atoms or molecules, optical frequency standards can obtain higher frequency stability and absolute quantum frequency reference. Among all these spectral frequency stabilization techniques, modulation transfer spectroscopy (MTS) and frequency modulation spectroscopy (FMS) have the advantages of high signal-to-noise ratio, high sensitivity, and no Doppler background. Hot atom optical frequency standards based on MTS and FMS frequency stabilization have achieved better than 10 in different atomic and molecular ensembles. -14 The short-term frequency stability of the optical frequency standard is of the order of magnitude [patent number: CN201710192626.0, CN201910943197.5]. However, due to the influence of the high-speed motion of hot atoms, the Doppler frequency shift caused by frequent collisions, the collision frequency shift and other factors related to the atomic speed, the medium- and long-term frequency stability of the above optical frequency standard cannot be calculated according to τ -1 / 2 If we want to further improve the frequency stability, we must break through the limitations of factors such as Doppler frequency shift and collision frequency shift, which is the key to the current research in the technical field. Summary of the invention

[0003] In view of the above-mentioned defects and shortcomings existing in the hot atom optical frequency standards based on MTS and FMS frequency stabilization, the present invention innovatively proposes a cold atom optical frequency standard based on long strip diffuse reflection cooling and its implementation method for the first time in the world. The atoms are decelerated by laser cooling technology, and the Doppler frequency shift and collision frequency shift related to the atomic movement speed are greatly suppressed; and diffuse reflection laser cooling does not require the gradient magnetic field in the magneto-optical trap configuration, which can well avoid the influence of the magnetic field on the frequency-stabilized atomic energy level; the meter-level length of the cold atomic cluster achieved by the long strip vacuum glass tube can also greatly increase the interaction distance between light and atoms, and effectively improve the signal-to-noise ratio of the atomic transition spectrum. The realization of the cold atom optical frequency standard based on the long strip diffuse reflection cooling of the present invention not only brings substantial and significant progress to the medium- and long-term frequency stability of the optical frequency standard, but can also be further developed into an absolute wavelength standard, providing a benchmark for many precision measurement applications.

[0004] The technical solution of the present invention is:

[0005] The cold atom optical frequency standard based on long strip diffuse reflection cooling includes: a cooling laser source, a heavy pump laser source, an acousto-optic modulator, a first reflector, a first half-wave plate, a second half-wave plate, a first polarization beam splitter prism, a third half-wave plate, a second polarization beam splitter prism, a first fiber coupling head, a second fiber coupling head, a fourth half-wave plate, a third polarization beam splitter prism, a third fiber coupling head, a fourth fiber coupling head, a long strip vacuum glass tube, a narrow linewidth laser source, an isolator, a fifth half-wave plate, a fourth polarization beam splitter prism, a thermal atom saturated absorption spectrum module, an attenuation plate, an electro-optic phase modulator, a second reflector, a photodetector, a laser phase detector and a high-speed servo control circuit.

[0006] Among them, the output frequency of the cooling laser source has been stabilized in advance to the selected cooling transition frequency (the light frequency absorbed when the lower energy level particle of the cooling transition energy level transitions to the upper energy level of the cooling transition energy level is called the cooling transition frequency) through frequency stabilization methods such as saturation absorption spectrum or modulation transfer spectrum. If it is a rubidium atom, it is stabilized to the hyperfine transition frequency of the ground state F=2 of the rubidium atom to the first excited state F'=3 of the rubidium atom; if it is a cesium atom, it is stabilized to the hyperfine transition frequency of the ground state F=4 of the cesium atom to the first excited state F'=5 of the cesium atom; the laser output after frequency stabilization is frequency-shifted by an acousto-optic modulator to produce a red detuning of about two to three times the natural line width of the atomic transition, and the natural line width is the natural line width of the above-mentioned frequency-stabilized atomic transition; the above-mentioned frequency-shifted laser is called a cooling laser, which is used to achieve the deceleration of the moving atoms in the long strip vacuum glass tube.

[0007] Similarly, the output frequency of the heavy pump laser source has been stabilized in advance to the selected heavy pump transition frequency (the frequency of light absorbed when the lower energy level particle of the heavy pump transition energy level transitions to the upper energy level of the heavy pump transition energy level is called the heavy pump transition frequency) through frequency stabilization methods such as saturation absorption spectrum or modulation transfer spectrum. If it is a rubidium atom, it is stabilized to the hyperfine transition frequency of the rubidium atom ground state F=1 to the rubidium atom first excited state F'=2; if it is a cesium atom, it is stabilized to the hyperfine transition frequency of the cesium atom ground state F=3 to the cesium atom first excited state F'=4; the laser output after frequency stabilization is called a heavy pump laser, which is used to maintain continuous cooling in the long strip vacuum glass tube.

[0008] The re-pumping laser and the cooling laser reflected by the first reflecting mirror are transmitted to the first polarization beam splitter prism for beam combining, and the first polarization beam splitter prism divides the combined laser into two beams, and the first half-wave plate and the second half-wave plate are used to adjust the optical power of the split laser; the above two beams of laser light after the splitting are further divided into four beams by the second polarization beam splitter prism and the third polarization beam splitter prism, and the third half-wave plate and the fourth half-wave plate are used to adjust the splitting power of the second polarization beam splitter prism and the splitting power of the third polarization beam splitter prism respectively; the four beams of laser light after the splitting are respectively coupled into four one-to-two multimode optical fibers by the first optical fiber coupling head, the second optical fiber coupling head, the third optical fiber coupling head, and the fourth optical fiber coupling head, and eight laser beams containing the cooling laser and the re-pumping laser are obtained at the output ends of the four one-to-two multimode optical fibers.

[0009] The long strip of vacuum glass tube is a hollow cylinder with a length of 1 meter and an inner diameter of 2 centimeters. It contains a thin alkali metal atomic vapor inside and the outer surface is coated with a diffuse reflection coating with high reflectivity to the above-mentioned cooling laser and heavy pumping laser, with a reflectivity of more than 98%; there is no coating attached to the two circular outer end faces of the long strip of vacuum glass tube, and eight small holes not covered by the coating are reserved on the side surface of the cylinder, with a diameter of 3 mm, as incident windows for injecting the eight laser beams emitted by the above-mentioned four one-to-two multimode optical fibers; the above-mentioned eight laser beams are reflected multiple times in the tube, and under the action of the radiation pressure brought by the cooling laser, the alkali metal atoms in the long strip of vacuum glass tube are cooled, and the entire 1-meter-long tube is filled with cold atomic clusters.

[0010] The output laser of the narrow linewidth laser source is first transmitted to the isolator for isolating the optical feedback of the rear optical path; after the isolator, the fifth half-wave plate and the fourth polarization beam splitter prism are matched and adjusted to adjust the splitting power and split the laser into two beams: one beam is transmitted to the hot atom saturation absorption spectrum module, and the hot atom saturation absorption spectrum obtained by detection is used as the frequency reference of the cold atom optical frequency standard; the other beam is sent to the electro-optical phase modulator for phase modulation after the laser power is adjusted by the attenuation plate, and the modulation signal is generated by the laser phase detection and high-speed servo control circuit; the above phase-modulated laser is reflected by the second reflector The optical fiber is injected into the long strip of vacuum glass tube through a circular light-transmitting end face, passes through the cold atomic cluster in the long strip of vacuum glass tube, and is emitted from the other circular end face; the photodetector transmits the received cold atom transition spectrum signal to the laser phase detection and high-speed servo control circuit, which filters, amplifies, and demodulates the signal, and feeds back the demodulated dispersion linear servo signal to the fast feedback port and slow feedback port of the narrow linewidth laser source for high-speed full-bandwidth locking, thereby finally realizing the output of high-frequency stability cold atom optical frequency standard laser from the narrow linewidth laser source.

[0011] After modulation in the electro-optic phase modulator, extracting the laser frequency noise information at high frequency can effectively reduce the influence of the loop low-frequency noise, because low-frequency noise is an important factor that limits the signal-to-noise ratio of the spectrum line in non-modulation spectroscopy technology. The modulation signal of the electro-optic phase modulator is generated by the laser phase detection and high-speed servo control circuit. The laser carrying the modulation signal is received by the photodetector and further transmitted to the laser phase detection and high-speed servo control circuit for demodulation. By controlling the phase difference between the demodulated signal and the detector signal, the demodulated frequency modulation spectrum line type has obvious dispersion characteristics about the frequency detuning amount at the atomic transition. The laser frequency is locked at the zero crossing point of the dispersion line type, and the frequency stabilization of the corresponding atomic transition frequency is achieved. The present invention takes the frequency modulation spectrum as an example to realize the cold atom optical frequency standard based on the cold atom frequency modulation spectrum stabilization, which can also be replaced by other frequency stabilization technologies such as modulation transfer spectrum.

[0012] Furthermore, the principle of cooling laser decelerating the moving atoms is that when atoms move in a cooling laser whose frequency is detuned to the energy level difference of atomic transition, due to the Doppler effect, the atoms tend to absorb photons in the opposite direction of the atomic motion, thereby generating a damping force in the opposite direction of the atomic motion, slowing down the motion of the atoms. This part belongs to the basic principle of Doppler cooling and will not be repeated in the present invention. In the specific implementation process, the cooling effect is best when the frequency of the cooling laser is detuned by about two to three times the natural line width of the atomic transition. For the natural line width of 6.1MHz of the D2 transition line from the ground state of the rubidium atom to the first excited state, the red detuning is about 12-18MHz; for the natural line width of 5.2MHz of the D2 transition line from the ground state of the cesium atom to the first excited state, the red detuning is about 10-15MHz.

[0013] Furthermore, the atoms that are transitioned to the upper energy level by the cooling laser may transition to other ground state energy levels other than the lower energy level of the cooling transition through spontaneous radiation, and the atoms accumulate at the other ground state energy levels, thereby escaping from the cooling cycle corresponding to the cooling transition energy level and stopping the interaction with the cooling laser. Therefore, the frequency of the re-pumping laser of the present invention needs to be tuned to the transition from other ground state energy levels outside the cooling cycle to the upper energy level of the excited state, in order to enable atoms at other ground state energy levels to return to the cooling cycle and participate in the contribution.

[0014] Furthermore, there are no special requirements for the length and inner diameter of the long vacuum glass tube, which can be selected at will. However, the longer the tube is, the longer the interaction distance between light and atoms is, and the higher the signal-to-noise ratio of the cold atom transition spectrum, which is more beneficial to the frequency stability of the cold atom optical frequency standard.

[0015] Furthermore, there is no special requirement for the splitting power ratio after the cooling laser and the re-pumping laser are combined, and it can be adjusted and optimized according to the experimental phenomena; the number of laser beams can also be adjusted according to the length of the long vacuum glass tube. For example: a 1-meter-long vacuum glass tube is injected with eight split laser beams, and a 0.5-meter-long vacuum glass tube is injected with six split laser beams. There is no special requirement for the number of laser beams, and it can meet the experimental needs.

[0016] Furthermore, as the final output of the cold atom optical frequency standard, the narrow linewidth laser source can be selected according to the corresponding cold atoms and user needs. Taking cold rubidium 87 atoms as an example, the narrow linewidth laser source can be stabilized to 780nm5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F'=3 (F'=2, F'=1) transition, and 420nm 5 2 S 1 / 2 F = 2 to 6 2 P 3 / 2 F'=3 (F'=2, F'=1) transition, etc. Taking cold cesium atoms as an example, the narrow linewidth laser source can be stabilized to 852nm 6 2 S 1 / 2 F = 4 to 6 2 P 3 / 2 F'=5 (F'=4, F'=3) transition, and 455nm 6 2 S 1 / 2 F = 4 to 7 2 P 3 / 2 F'=5 (F'=4, F'=3) transition, etc.

[0017] Furthermore, the hot atom saturation absorption spectrum module is used to calibrate the output frequency of the narrow linewidth laser source in order to observe the cold atom transition spectrum corresponding to the atomic hyperfine transition frequency.

[0018] Furthermore, the laser phase detection and high-speed servo control circuit includes functions such as generation of RF modulation signals, demodulation, filtering and amplification of error signals, and high-speed servo feedback control.

[0019] Furthermore, the present invention currently adopts the frequency modulation spectrum frequency stabilization technology, which can also be replaced by other frequency stabilization technologies such as modulation transfer spectrum.

[0020] Another object of the present invention is to provide a method for realizing a cold atom optical frequency standard based on long strip diffuse reflection cooling.

[0021] The method for realizing the cold atom optical frequency standard based on long strip diffuse reflection cooling of the present invention specifically comprises the following steps:

[0022] 1) The output laser of the frequency-stabilized cooling laser source is frequency-shifted by an acousto-optic modulator to produce a red detuning of about two to three times the natural line width of the atomic transition, and is transmitted to the first polarization beam splitter prism for beam combining together with the output laser of the frequency-stabilized heavy pump laser source, and the optical power of the two combined lasers is adjusted by the first half-wave plate and the second half-wave plate respectively;

[0023] 2) The two laser beams combined in step 1) are respectively injected into the second polarization beam splitter prism and the third polarization beam splitter prism, and the combined laser beam is split into four beams by the second polarization beam splitter prism and the third polarization beam splitter prism, and are respectively coupled into four one-to-two multimode optical fibers by the first fiber coupling head, the second fiber coupling head, the third fiber coupling head, and the fourth fiber coupling head; the third half-wave plate and the fourth half-wave plate are used to adjust the optical power of the split laser beams;

[0024] 3) A diffuse reflective coating with high reflectivity for cooling laser and re-pumping laser is coated on the cylindrical side surface of the long strip vacuum glass tube, and eight small holes not covered by the coating are reserved as the incident windows for the cooling laser and re-pumping laser; the eight laser beams output by the four one-to-two multi-mode optical fibers in step 2) are shot into the eight incident windows; the eight laser beams are reflected multiple times in the tube, and under the effect of the radiation pressure brought by the cooling laser, the alkali metal atoms in the long strip vacuum glass tube are cooled, and the entire 1-meter-long tube is filled with cold atomic groups;

[0025] 4) The output laser of the narrow linewidth laser source passes through the isolator and is matched and adjusted by the fifth half-wave plate and the fourth polarization beam splitter prism in turn to split the laser into two beams: one beam is transmitted to the hot atom saturation absorption spectrum module, and the hot atom saturation absorption spectrum obtained by detection is used as the frequency reference of the cold atom optical frequency standard; the other beam is transmitted to the electro-optical phase modulator for phase modulation after the laser power is adjusted by the attenuator;

[0026] 5) Let the phase-modulated laser in step 4) pass through the cold atom cluster trapped in the long vacuum glass tube and hit the photodetector; the photodetector transmits the received cold atom transition spectrum signal to the laser phase detection and high-speed servo control circuit, which filters, amplifies and demodulates the signal, and feeds back the obtained servo signal to the fast feedback port and slow feedback port of the narrow linewidth laser source for high-speed full-bandwidth locking, thereby finally realizing high-frequency stability cold atom optical frequency standard laser output.

[0027] In step 4), the hot atom saturation absorption spectrum module is used to calibrate the output frequency of the narrow linewidth laser source in order to observe the cold atom transition spectrum corresponding to the atomic hyperfine transition frequency.

[0028] In step 5), the laser phase detection and high-speed servo control circuit includes functions such as generation of RF modulation signal, demodulation, filtering and amplification of error signal, and high-speed servo feedback control; the laser frequency stabilization method adopts frequency modulation spectrum technology of fast phase modulation, which can also be replaced by other frequency stabilization technologies such as modulation transfer spectrum.

[0029] Compared with the existing thermal atom optical frequency standard based on frequency modulation spectrum stabilization, the novelty and creativity of the present invention are embodied in:

[0030] 1. The present invention provides a cold atom optical frequency standard based on long strip diffuse reflection cooling and its implementation method. Considering that cold atoms can overcome the influence of Doppler frequency shift, collision frequency shift and other factors related to atomic speed caused by atomic motion during the interaction between light and atoms, the present invention takes this as a starting point and innovatively combines cold atoms with optical frequency standards based on frequency modulation spectrum stabilization, and decelerates atoms through diffuse reflection laser cooling technology to reduce the influence of Doppler frequency shift and collision frequency shift, thereby further improving the frequency stability of the optical frequency standard. The implementation of the present invention significantly and effectively overcomes the limitations of the Doppler frequency shift and collision frequency shift of hot atoms on the improvement of the frequency stability of the optical frequency standard, and applies diffuse reflection laser cooling technology to the example of optical frequency standards based on frequency modulation spectrum stabilization. Its outstanding substantial progress and significant indicator improvement effect have not been reported in domestic and foreign patents.

[0031] 2. The diffuse reflection laser cooling adopted in the present invention is an isotropic cooling method that does not require a magnetic field, and can effectively avoid the influence of the gradient magnetic field in the magneto-optical trap on the frequency-stabilized atomic energy level; the long strip diffuse reflection cooling based on the long strip vacuum glass tube is also a technological innovation compared to the traditional diffuse reflection cooling, which can realize cold atomic clusters with a length of meters, greatly improving the interaction distance between light and atoms, thereby effectively improving the signal-to-noise ratio of the cold atomic transition spectrum.

[0032] 3. The present invention can not only realize optical frequency standards of various wavelengths and frequencies based on different cold atom ensembles to meet the diverse needs of downstream applications, but also develop the cold atom optical frequency standards based on long strip diffuse reflection cooling into absolute wavelength standards, providing a benchmark for many precision measurement applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a structural schematic diagram of an embodiment of a cold atom optical frequency standard based on long strip diffuse reflection cooling of the present invention;

[0034] Among them: 1-cooling laser source, 2-re-pumping laser source, 3-acousto-optic modulator, 4-first reflector, 5-first half-wave plate, 6-second half-wave plate, 7-first polarization beam splitter prism, 8-third half-wave plate, 9-second polarization beam splitter prism, 10-first fiber coupling head, 11-second fiber coupling head, 12-fourth half-wave plate, 13-third polarization beam splitter prism, 14-third fiber coupling head, 15-fourth fiber coupling head, 16-long vacuum glass tube, 17-narrow linewidth laser source, 18-isolator, 19-fifth half-wave plate, 20-fourth polarization beam splitter prism, 21-thermal atomic saturation absorption spectrum module, 22-attenuation plate, 23-electro-optic phase modulator, 24-second reflector, 25-photoelectric detector, 26-laser phase detection and high-speed servo control circuit. DETAILED DESCRIPTION

[0035] The present invention will be further described below through specific embodiments in conjunction with the accompanying drawings.

[0036] like Figure 1 The cold atom optical frequency standard based on long strip diffuse reflection cooling of this embodiment includes: a cooling laser source 1, a heavy pump laser source 2, an acousto-optic modulator 3, a first reflector 4, a first half-wave plate 5, a second half-wave plate 6, a first polarization beam splitter prism 7, a third half-wave plate 8, a second polarization beam splitter prism 9, a first fiber coupling head 10, a second fiber coupling head 11, a fourth half-wave plate 12, a third polarization beam splitter prism 13, a third fiber coupling head 14, a fourth fiber coupling head 15, a long strip vacuum glass tube 16, a narrow linewidth laser source 17, an isolator 18, a fifth half-wave plate 19, a fourth polarization beam splitter prism 20, a hot atom saturated absorption spectrum module 21, an attenuation plate 22, an electro-optic phase modulator 23, a second reflector 24, a photodetector 25, and a laser phase detection and high-speed servo control circuit 26.

[0037] The output frequency of the cooling laser source 1 has been stabilized in advance to the selected cooling transition frequency (the frequency of light absorbed when the lower energy level particle of the cooling transition energy level transitions to the upper energy level of the cooling transition energy level is called the cooling transition frequency) by means of frequency stabilization methods such as saturation absorption spectrum or modulation transfer spectrum. If it is a rubidium atom, it is stabilized to the hyperfine transition frequency of the rubidium atom ground state F=2 to the rubidium atom first excited state F'=3; if it is a cesium atom, it is stabilized to the hyperfine transition frequency of the cesium atom ground state F=4 to the cesium atom first excited state F'=5; the laser output after frequency stabilization is frequency-shifted by the acousto-optic modulator 3 to produce a red detuning of about two to three times the natural line width of the atomic transition, and the natural line width is the natural line width of the above-mentioned frequency-stabilized atomic transition; the above-mentioned frequency-shifted laser is called cooling laser, which is used to achieve the deceleration of the moving atoms in the long strip vacuum glass tube 16;

[0038] The output frequency of the heavy pump laser source 2 has been stabilized in advance to the selected heavy pump transition frequency (the light frequency absorbed when the lower energy level particle of the heavy pump transition energy level transitions to the upper energy level of the heavy pump transition energy level is called the heavy pump transition frequency) by means of frequency stabilization methods such as saturation absorption spectrum or modulation transfer spectrum. If it is a rubidium atom, it is stabilized to the hyperfine transition frequency of the rubidium atom ground state F=1 to the rubidium atom first excited state F'=2; if it is a cesium atom, it is stabilized to the hyperfine transition frequency of the cesium atom ground state F=3 to the cesium atom first excited state F'=4; the laser output after frequency stabilization is called a heavy pump laser, which is used to maintain the cooling cycle in the long strip vacuum glass tube 16;

[0039] The re-pumping laser and the cooling laser reflected by the first reflecting mirror 4 are transmitted to the first polarization beam splitter prism 7 for beam combination, and the first polarization beam splitter prism 7 divides the combined laser into two beams, and the first half-wave plate 5 and the second half-wave plate 6 are used to adjust the optical power of the split laser; the above two beams of laser light after the splitting are further divided into four beams by the second polarization beam splitter prism 9 and the third polarization beam splitter prism 13, and the third half-wave plate 8 and the fourth half-wave plate 12 are used to adjust the splitting power of the second polarization beam splitter prism 9 and the splitting power of the third polarization beam splitter prism 13 respectively; the four beams of laser light after the splitting are respectively coupled into four one-to-two multimode optical fibers by the first optical fiber coupling head 10, the second optical fiber coupling head 11, the third optical fiber coupling head 14, and the fourth optical fiber coupling head 15, and eight laser beams containing cooling laser light and re-pumping laser light are obtained at the output ends of the four one-to-two multimode optical fibers;

[0040] The long strip vacuum glass tube 16 is a hollow cylinder with a length of 1 meter and an inner diameter of 2 centimeters. It contains a thin alkali metal atomic vapor inside, and the outer surface is coated with a diffuse reflection coating with high reflectivity to the above-mentioned cooling laser and heavy pumping laser, with a reflectivity of more than 98%; the two circular outer end faces of the long strip vacuum glass tube 16 are not attached with paint, and eight small holes with a diameter of 3 mm that are not covered by the paint are reserved on the side surface of the cylinder as incident windows for injecting the eight laser beams emitted by the above-mentioned four one-to-two multimode optical fibers; the above-mentioned eight laser beams are reflected multiple times in the tube, and under the effect of the radiation pressure brought by the cooling laser, the alkali metal atoms in the long strip vacuum glass tube 16 are cooled, and the entire 1-meter-long tube is filled with cold atomic groups;

[0041] The output laser of the narrow linewidth laser source 17 is first transmitted to the isolator 18 for isolating the optical feedback of the rear optical path; after the isolator 18, the fifth half-wave plate 19 and the fourth polarization beam splitter prism 20 are matched and adjusted to split the laser into two beams: one beam is transmitted to the hot atom saturation absorption spectrum module 21, and the hot atom saturation absorption spectrum obtained by detection is used as the frequency reference of the cold atom optical frequency standard; the other beam is adjusted in laser power by the attenuation plate 22 and then injected into the electro-optic phase modulator 23 for phase modulation; the above phase modulated laser is reflected by the second reflector 24 , and then it is injected from a circular light-transmitting end face of the long strip vacuum glass tube 16, and then emitted from the other circular end face after passing through the cold atomic group in the long strip vacuum glass tube 16; the photodetector 25 transmits the received cold atom transition spectrum signal to the laser phase detection and high-speed servo control circuit 26, which filters, amplifies and demodulates it, and feeds back the obtained servo signal to the fast feedback port and slow feedback port of the narrow linewidth laser source 17 for high-speed full-bandwidth locking, and finally realizes the cold atom optical frequency standard laser output with high frequency stability.

[0042] Specifically, the cold atom optical frequency standard based on long strip diffuse reflection cooling in the embodiment of the present invention is characterized by: combining diffuse reflection laser cooling technology with an optical frequency standard based on frequency modulation spectrum stabilization, and utilizing the inhibitory effect of cold atoms on Doppler shift and collision frequency shift to further improve the frequency stability of the optical frequency standard; compared with the magneto-optical trap cooling configuration, the diffuse reflection laser cooling of the present invention does not require a gradient magnetic field, and can well avoid the influence of the magnetic field on the energy level of frequency-stabilized atoms; the meter-level length of cold atomic clusters achieved by long strip vacuum glass tubes can also greatly increase the interaction distance between light and atoms, and effectively improve the signal-to-noise ratio of atomic transition spectral lines. The implementation of the present invention not only significantly and effectively overcomes the limitations of Doppler shift and collision frequency shift on the improvement of the frequency stability of hot atom optical frequency standards, and provides a creative new idea for realizing optical frequency standards with high frequency stability, but also expands a new absolute wavelength standard implementation method, providing new possibilities for the subsequent development of many fields. In this case, the present invention is essentially different from the current optical frequency standard based on hot atom frequency modulation spectrum stabilization.

[0043] When the present invention is implemented, there are no special requirements for the length and inner diameter of the long strip vacuum glass tube, which can be selected at will. However, the longer the tube is, the longer the interaction distance between light and atoms is, and the higher the signal-to-noise ratio of the cold atom transition spectrum is, which is more beneficial to the frequency stability of the cold atom optical frequency standard. The number of laser beams can also be adjusted according to the length of the long strip vacuum glass tube. For example, a 1-meter-long vacuum glass tube is injected with eight split laser beams, and a 0.5-meter-long vacuum glass tube is injected with six split laser beams. There is no special requirement for the number of laser beams, and it only needs to meet the experimental needs.

[0044] As the final output of the cold atom optical frequency standard, the narrow linewidth laser source can be selected according to the corresponding cold atoms and user needs. Taking cold rubidium 87 atoms as an example, the narrow linewidth laser source can be stabilized to 780nm 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F'=3 (F'=2, F'=1) transition, and 420nm 5 2 S 1 / 2 F = 2 to 6 2 P 3 / 2 F'=3 (F'=2, F'=1) transition, etc. Taking cold cesium atoms as an example, the narrow linewidth laser source can be stabilized to 852nm 6 2 S 1 / 2 F = 4 to 6 2 P 3 / 2 F'=5 (F'=4, F'=3) transition, and 455nm 6 2 S 1 / 2 F = 4 to 7 2 P 3 / 2F'=5 (F'=4, F'=3) transition, etc.

[0045] The present invention currently uses frequency modulation spectrum frequency stabilization technology to realize cold atom optical frequency standard, which can also be replaced by other frequency stabilization technologies such as modulation transfer spectrum.

[0046] Finally, it should be noted that the purpose of publishing the embodiments is to help further understand the present invention, but those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. The above-mentioned substitutions include substitutions of different atoms and different wavelengths, such as replacing the diffuse reflection cooling of rubidium atoms at 780nm with diffuse reflection cooling of cesium atoms at 852nm, or replacing the 780nm narrow linewidth laser source with any other wavelength such as 420nm that can achieve frequency modulation spectrum stabilization. Therefore, the present invention should not be limited to the contents disclosed in the embodiments, and the scope of protection claimed by the present invention shall be subject to the scope defined in the claims.

Claims

1. A cold atom optical frequency standard based on long strip diffuse reflection cooling, characterized in that: It includes a cooling laser source, a heavy pump laser source, a long vacuum glass tube, a narrow line width laser source and a photodetector; among which, The long strip vacuum glass tube contains alkali metal atomic vapor, the side wall is coated with a diffuse reflection coating with high reflectivity for cooling laser and re-pumping laser, and multiple incident windows are reserved; The output frequency of the cooling laser source is stabilized to a selected cooling transition frequency and then frequency-shifted by an acousto-optic modulator to obtain a cooling laser, which is used to decelerate the moving atoms in the long strip vacuum glass tube; the cooling transition frequency is the light frequency absorbed when particles at the lower energy level of the cooling transition energy level transition to the upper energy level of the cooling transition energy level; The output frequency of the re-pump laser source is stabilized to a selected re-pump transition frequency to obtain a re-pump laser, which is used to maintain continuous cooling in the long strip vacuum glass tube; the re-pump transition frequency is the frequency of light absorbed when a particle at a lower energy level of the re-pump transition energy level transitions to an upper energy level of the re-pump transition energy level; The re-pump laser and the cooling laser reflected by the first reflector are transmitted to the first polarization beam splitter prism for beam combination, and the first polarization beam splitter prism divides the combined laser beam into two beams, wherein one laser beam is divided into two beams by the second polarization beam splitter prism, and the other laser beam is divided into two beams by the third polarization beam splitter prism; the four laser beams after beam splitting are respectively incident on the long strip vacuum glass tube through the incident window to generate diffusely reflected laser beams, so as to cool the alkali metal atoms to form cold atomic clusters; The output laser of the narrow line width laser source is incident on the fourth polarization beam splitter through the isolator and is divided into two beams: one beam is transmitted to the hot atom saturation absorption spectrum module; the other beam is injected into the electro-optic phase modulator for phase modulation after the laser power is adjusted by the attenuation plate, and the phase modulated laser passes through the cold atom group in the long strip vacuum glass tube and then is incident on the photodetector; the hot atom saturation absorption spectrum module is used to use the hot atom saturation absorption spectrum obtained by detection as the frequency reference of the cold atom filter; The photoelectric detector is used to transmit the received cold atom transition spectrum signal to the laser phase detection and high-speed servo control circuit to generate a servo signal which is fed back to the narrow linewidth laser source, so that the narrow linewidth laser source outputs a cold atom optical frequency standard laser with high frequency stability.

2. The cold atom optical frequency standard according to claim 1, characterized in that: The output frequency of the cooling laser source is stabilized to the cooling transition frequency by means of a saturated absorption spectrum or a modulation transfer spectrum. If the alkali metal atom used in the saturated absorption spectrum or the modulation transfer spectrum for stabilizing the output frequency of the cooling laser source is a rubidium atom, the cooling transition frequency is a hyperfine transition frequency from the ground state F=2 of the rubidium atom to the first excited state F'=3 of the rubidium atom. If the alkali metal atom used in the saturated absorption spectrum or the modulation transfer spectrum for stabilizing the output frequency of the cooling laser source is a cesium atom, the cooling transition frequency is a transition frequency from the ground state F=4 of the cesium atom to the first excited state F'=5 of the cesium atom.

3. The cold atom optical frequency standard according to claim 1, characterized in that: The output frequency of the heavy pump laser source is stabilized to the heavy pump transition frequency by means of saturation absorption spectrum or modulation transfer spectrum. If the alkali metal atom used in the saturation absorption spectrum or modulation transfer spectrum for stabilizing the output frequency of the heavy pump laser source is a rubidium atom, the cooling transition frequency is the hyperfine transition frequency from the ground state F=1 of the rubidium atom to the first excited state F'=2 of the rubidium atom. If the alkali metal atom used in the saturation absorption spectrum or modulation transfer spectrum for stabilizing the output frequency of the heavy pump laser source is a cesium atom, the cooling transition frequency is the transition frequency from the ground state F=3 of the cesium atom to the first excited state F'=4 of the cesium atom.

4. The cold atom optical frequency standard according to claim 1, characterized in that: The narrow linewidth laser source is stabilized by saturation absorption spectrum or modulation transfer spectrum. If the alkali metal atom used in the saturation absorption spectrum or modulation transfer spectrum of the output frequency of the narrow linewidth laser source is rubidium atom, the narrow linewidth laser source is stabilized to 780nm 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F' = 3 transition or 420nm 5 2 S 1 / 2 F = 2 to 6 2 P 3 / 2 F'=3 transition; if the alkali metal atom used in the saturation absorption spectrum or modulation transfer spectrum of the output frequency of the narrow linewidth laser source is cesium atom, the narrow linewidth laser source is stabilized to 852nm 6 2 S 1 / 2 F = 4 to 6 2 P 3 / 2 F'=5 (F'=4, F'=3) transition or 455nm 6 2 S 1 / 2 F = 4 to 7 2 P 3 / 2 F'=5 (F'=4, F'=3) transition.

5. The cold atom optical frequency standard according to claim 1, characterized in that: Eight incident windows are reserved on the side wall of the long strip vacuum glass tube; the four split laser beams are coupled into four one-to-two multimode optical fibers through the first optical fiber coupling head, the second optical fiber coupling head, the third optical fiber coupling head, and the fourth optical fiber coupling head respectively, and eight laser beams containing cooling laser and re-pumping laser are obtained at the output ends of the four one-to-two multimode optical fibers, and are respectively incident into the long strip vacuum glass tube through one of the incident windows.

6. The cold atom optical frequency standard according to claim 1, characterized in that: A first half-wave plate is provided between the first reflector and the first polarization beam splitter prism, for adjusting the optical power of the cooling laser incident on the first polarization beam splitter prism; a second half-wave plate is provided between the re-pump laser source and the first polarization beam splitter prism, for adjusting the optical power of the re-pump laser incident on the first polarization beam splitter prism; a third half-wave plate for power adjustment is provided between the first polarization beam splitter prism and the second polarization beam splitter prism, and a fourth half-wave plate for power adjustment is provided between the first polarization beam splitter prism and the third polarization beam splitter prism; a fifth half-wave plate for optical power adjustment is provided between the isolator and the fourth polarization beam splitter prism.

7. The cold atom optical frequency standard according to claim 1, characterized in that: The frequency of the cooling laser is detuned to about two to three times the natural line width of the atomic transition.

8. A method for realizing a cold atom optical frequency standard based on long strip diffuse reflection cooling, the steps comprising: 1) The long vacuum glass tube contains alkali metal atomic vapor, the side wall is coated with a diffuse reflection coating with high reflectivity for cooling laser and re-pumping laser, and multiple incident windows are reserved; 2) stabilizing the output frequency of the cooling laser source to the selected cooling transition frequency and then frequency shifting it through an acousto-optic modulator to obtain a cooling laser, and stabilizing the output frequency of the re-pumping laser source to the selected re-pumping transition frequency to obtain a re-pumping laser; 3) transmitting the re-pump laser and the cooling laser reflected by the first reflector to the first polarization beam splitter prism for beam combination, and the first polarization beam splitter prism divides the combined laser into two beams, wherein one laser beam is divided into two beams by the second polarization beam splitter prism, and the other laser beam is divided into two beams by the third polarization beam splitter prism; the four laser beams after beam splitting are incident into the long strip vacuum glass tube through the incident window respectively to generate diffusely reflected lasers, and the alkali metal atoms are cooled to form cold atomic clusters; 4) The output laser of the narrow line width laser source is incident on the fourth polarization beam splitter through the isolator and split into two beams: one beam is transmitted to the hot atom saturation absorption spectrum module; the other beam is injected into the electro-optic phase modulator for phase modulation after the laser power is adjusted by the attenuation plate, and the phase modulated laser passes through the cold atom group in the long strip vacuum glass tube and then is incident on the photodetector; the hot atom saturation absorption spectrum module is used to use the hot atom saturation absorption spectrum obtained by detection as the frequency reference of the cold atom filter; 5) The photodetector transmits the received cold atom transition spectrum signal to the laser phase detector and high-speed servo control circuit to generate a servo signal which is fed back to the narrow linewidth laser source, so that the narrow linewidth laser source outputs a cold atom optical frequency standard laser with high frequency stability.

9. The method according to claim 8, characterized in that A first half-wave plate is provided between the first reflector and the first polarization beam splitter prism, for adjusting the optical power of the cooling laser incident on the first polarization beam splitter prism; a second half-wave plate is provided between the re-pump laser source and the first polarization beam splitter prism, for adjusting the optical power of the re-pump laser incident on the first polarization beam splitter prism; a third half-wave plate for power adjustment is provided between the first polarization beam splitter prism and the second polarization beam splitter prism, and a fourth half-wave plate for power adjustment is provided between the first polarization beam splitter prism and the third polarization beam splitter prism; a fifth half-wave plate for optical power adjustment is provided between the isolator and the fourth polarization beam splitter prism.

10. The method according to claim 8, characterized in that Eight incident windows are reserved on the side wall of the long strip vacuum glass tube; the four split laser beams are coupled into four one-to-two multimode optical fibers through the first optical fiber coupling head, the second optical fiber coupling head, the third optical fiber coupling head, and the fourth optical fiber coupling head respectively, and eight laser beams containing cooling laser and re-pumping laser are obtained at the output ends of the four one-to-two multimode optical fibers, and are respectively incident into the long strip vacuum glass tube through one of the incident windows.

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