An ultranarrow-bandwidth atomic filter based on cold atoms and a method for implementing the same

Abstract

The application discloses an ultranarrow-bandwidth atomic filter based on cold atoms and an implementation method thereof. The application combines cold atoms with an atomic filter, slows down atoms through laser cooling technology, reduces the influence of Doppler effect, and further realizes an ultranarrow-bandwidth atomic filter with a transmission bandwidth close to the natural line width of atomic transition. The implementation of the application can solve the problem that the influence of thermal atomic Doppler broadening causes the transmission bandwidth to be unable to be further narrowed in the existing transmission bandwidth narrowing technology of a Faraday atomic filter, and can expand the research of a traditional thermal atomic Faraday filter to the field of cold atoms in modern science, provide new possibilities for substantial and significant progress and construction of a filter based on cold atoms, and bring extremely potential development to many fields. The application overcomes the limitation of thermal atomic Doppler broadening on the further narrowing of the transmission bandwidth of a Faraday atomic filter, and brings a significant bandwidth narrowing effect.

Description

Technical Field

[0001] This invention belongs to the fields of laser-cooled atomic technology and atomic filter technology, and particularly relates to an ultra-narrow bandwidth atomic filter based on cold atoms and its implementation method. Background Technology

[0002] Faraday anomalous dispersive atomic filters based on the resonant Faraday effect possess advantages such as high peak transmittance, narrow bandwidth, and excellent out-of-band suppression, and have played a crucial role in fields such as lidar, remote sensing, optical communication, quantum key distribution, and narrowband quantum light sources. With the increasing application demands for Faraday atom filters, thermal atom Faraday filter technology is now quite mature. Over the past two decades, thermal atom Faraday filters in various wavelength bands have been developed, such as 423nm (calcium), 455nm (cesium), 532nm (rubidium), 694nm (potassium), 780nm (rubidium), 852nm (cesium), and 1529nm (rubidium).

[0003] However, due to the Doppler broadening effect of hot atomic motion, the transmission bandwidth of hot atom Faraday filters is typically limited to the gigahertz range. Even though some studies have reported narrowing the transmission bandwidth of Faraday atom filters using the saturation absorption effect of optical pumping, the Faraday rotation effect, and the principle of photodichroism [Patent Nos.: CN201611007662.7, CN201921696691.8], it is still far greater than the natural linewidth of atomic transitions. Furthermore, the research schemes involved in the aforementioned patents have a significant problem: they require an additional high-power pump laser locked to the atomic transition spectral line. The center frequency of the Faraday atom filter depends on this pump laser rather than the atomic transition itself, which is highly detrimental to the application of ultra-narrow bandwidth Faraday atom filters in laser frequency stabilization. In summary, the Doppler broadening of hot atoms becomes the physical limit for further narrowing the transmission bandwidth of Faraday atom filters. Existing technologies have almost reached the atomic physical limit in narrowing the transmission bandwidth of hot atom Faraday filters. Summary of the Invention

[0004] This invention addresses the aforementioned deficiencies and shortcomings of existing transmission bandwidth narrowing technologies for hot-atom Faraday filters. For the first time internationally, it innovatively proposes an ultra-narrow bandwidth atomic filter based on cold atoms and its implementation method. Considering that cold atoms can overcome the Doppler effect caused by atomic motion and other factors related to atomic velocity during the interaction between light and atoms, this invention innovatively combines cold atoms with an atomic filter. By using laser cooling technology to slow down the atoms, the influence of the Doppler effect is reduced, thereby achieving an ultra-narrow bandwidth atomic filter with a transmission bandwidth close to the natural linewidth of atomic transitions. The realization of this invention can effectively solve the problem of the Doppler broadening effect of hot atoms in existing transmission bandwidth narrowing technologies for Faraday atomic filters, which prevents further narrowing of the transmission bandwidth. It also extends the research of traditional hot-atom Faraday filters to the modern scientific field of cold atoms, providing new possibilities for substantial and significant progress and construction of subsequent cold-atom-based filters, and bringing highly promising developments to many fields.

[0005] This invention relates to an ultra-narrow bandwidth atomic filter based on cold atoms, comprising: a cooled laser frequency stabilization system, a re-pumped laser frequency stabilization system, a probe laser frequency stabilization system, a fourth half-wave plate, a fifth half-wave plate, a fourth polarization beam splitter, an optical fiber coupler, a 1-to-6 optical fiber beam splitter, a first optical fiber collimator, a second optical fiber collimator, a third optical fiber collimator, a fourth optical fiber collimator, a fifth optical fiber collimator, a sixth optical fiber collimator, a first gradient magnetic field coil, a second gradient magnetic field coil, a vacuum system, an attenuator, a twelfth reflecting mirror, a first GlanTell prism, a first optically rotating uniform magnetic field coil, a second optically rotating uniform magnetic field coil, a second GlanTell prism, and a first photodetector. The cooling laser frequency stabilization system includes: a cooling laser, a first reflector, a first isolator, a first half-wave plate, a first polarizing beam splitter, a second reflector, a first saturated absorption spectrum frequency stabilization module, a first acousto-optic modulator, a third reflector, and a second acousto-optic modulator; the re-pumped laser frequency stabilization system includes: a re-pumped laser, a second isolator, a fourth reflector, a second half-wave plate, a second polarizing beam splitter, a second saturated absorption spectrum frequency stabilization module, a fifth reflector, a sixth reflector, a third acousto-optic modulator, and a seventh reflector; the probe laser frequency stabilization system includes: a probe laser, a third isolator, a third half-wave plate, a third polarizing beam splitter, an eighth reflector, a modulation transfer spectrum frequency stabilization module, a ninth reflector, a fourth acousto-optic modulator, a tenth reflector, a fifth acousto-optic modulator, and an eleventh reflector.

[0006] In the cooled laser frequency stabilization system, the output laser from the cooled laser is reflected by the first mirror and transmitted to the first isolator to isolate the optical feedback from the rear optical path. After the first isolator, the laser is split into two beams by the first polarization beam splitter: one beam is reflected by the second mirror and transmitted to the first saturated absorption spectrum frequency stabilization module for frequency stabilization, and the saturated absorption spectrum signal generated by the frequency stabilization is fed back to the cooled laser for frequency locking; the other beam is frequency-shifted by the first and second acousto-optic modulators, producing a red detuning of about one to two times the natural linewidth of the atomic transition, which is the natural linewidth of the locked atomic transition mentioned above; the frequency-shifted laser is called the cooled laser and is output by the cooled laser frequency stabilization system to decelerate the moving atoms in the vacuum system; the first half-wave plate is used to adjust the splitting ratio of the two split laser beams.

[0007] In the re-pumped laser frequency stabilization system, the output laser from the re-pumped laser is first transmitted to the second isolator to isolate the optical feedback from the rear optical path. After being reflected by the fourth mirror, it is split into two beams by the second polarization beam splitter: one beam is transmitted to the second saturated absorption spectrum frequency stabilization module for frequency stabilization, and the saturated absorption spectrum signal generated by frequency stabilization is fed back to the re-pumped laser for frequency locking; the other beam is reflected by the fifth and sixth mirrors and then shifted to the desired atomic transition energy level by the third acousto-optic modulator. The laser after frequency shifting is called the re-pumped laser and is output by the re-pumped laser frequency stabilization system; the second half-wave plate is used to adjust the splitting ratio of the two split laser beams.

[0008] The cooled laser reflected by the third mirror and the re-pumped laser reflected by the seventh mirror are transmitted to the fourth polarization beam splitter for beam combining. The fourth and fifth half-wave plates are used to adjust the optical power of the combined laser. The combined cooled laser and re-pumped laser are coupled into an optical fiber by an optical fiber coupler, and then split into six laser beams of equal power by a 1-to-6 fiber beam splitter. The six laser beams are output in pairs by the first, second, third, fourth, fifth, and sixth fiber collimators. The first and second gradient magnetic field coils carry currents in opposite directions, providing an anti-Helmholtz magnetic field with a magnetic field strength of zero at the intersection of the six laser beams and linearly increasing towards the edges. The vacuum system contains a thin atomic vapor, and under the action of the magneto-optical trap formed by the cooled laser, the re-pumped laser, and the gradient magnetic field, the atoms are trapped at the intersection of the six laser beams.

[0009] In the probe laser frequency stabilization system, the output laser from the probe laser is first transmitted to the third isolator to isolate the optical feedback from the rear optical path. After the third isolator, it is split into two beams by the third polarization beam splitter: one beam is reflected by the eighth mirror and transmitted to the modulation transfer spectrum frequency stabilization module for precise laser frequency locking. The servo signal generated by locking is fed back to the frequency feedback control ports of the probe laser; the other beam undergoes positive frequency shifting by the fourth acousto-optic modulator and negative frequency shifting by the fifth acousto-optic modulator. Controlling the frequency shift of the fourth and fifth acousto-optic modulators causes the laser to produce positive and negative frequency detuning near the locked atomic resonance frequency (the resonance frequency corresponds to the transition frequency from the target ground state energy level to any upper energy level). (Using this resonance frequency as zero, the frequency is controlled...) The fourth and fifth acousto-optic modulators are configured, one with a positive frequency shift and the other with a negative frequency shift, thereby generating positive and negative frequency detuning near the resonant frequency. The frequency detuning range is approximately the natural linewidth of the aforementioned locked atomic transition. The frequency-shifted laser is called the probe laser and is output by the probe laser frequency stabilization system. The atoms in the vacuum system are decelerated under the action of the cooling laser and distributed on the ground state energy level of the cooling laser transition. The probe laser detects the atoms located on the ground state energy level of the cooling laser transition, and their transition frequency can be tuned to any transitionable upper energy level from the ground state energy level of the cooling laser transition. The third half-wave plate is used to adjust the splitting ratio of the two split laser beams. The ninth and tenth reflectors are used to adjust the direction of the optical path.

[0010] The detection laser reflected by the eleventh mirror is first transmitted to the attenuator to adjust the laser power, and then reflected by the twelfth mirror. The reflected detection laser then passes sequentially through the first Grangelle prism, the cold atom cluster trapped in the vacuum system, and the second Grangelle prism before being received by the first photodetector. The first and second optically rotating uniform magnetic field coils carry currents in the same direction to generate the uniform magnetic field required for the Faraday optical rotation effect. The direction of the uniform magnetic field is along the direction in which the detection laser is incident on the first photodetector.

[0011] The cooled laser, stabilized by the cooled laser frequency stabilization system, and the re-pumped laser, stabilized by the re-pumped laser frequency stabilization system, are combined by the fourth polarization beam splitter. The combined cooled laser and re-pumped laser are then coupled into an optical fiber by an optical fiber coupler and split into six beams of equal power by a 1-to-6 fiber beam splitter. These six laser beams are then output in pairs by the first, second, third, fourth, fifth, and sixth fiber collimators. A first and second gradient magnetic field coil, carrying currents in opposite directions, provide a non-uniform gradient magnetic field with zero strength at the center of the six laser beams' intersection and linearly increasing towards the edges. This causes the atoms in the vacuum system to experience a uniform magnetic field. The light scattering force points towards the center; relying on the effect of the cooling laser scattering force, the slow atoms in the Maxwell velocity distribution are stably trapped at the center where the six laser beams converge, while the higher-velocity atoms outside the trap redistribute their velocity, forming a Maxwell velocity distribution again, generating new slow atoms that are then trapped by the magneto-optical trap; then, the probe laser, after being frequency stabilized by the probe laser frequency stabilization system, passes sequentially through the first Glan-Taylor prism, the cold atom cluster trapped in the vacuum system, the second Glan-Taylor prism, and then enters the first photodetector; by using laser cooling technology to slow down the atoms, combined with the effect of Faraday rotation, this invention ultimately realizes an ultra-narrow bandwidth atomic filter with a transmission bandwidth close to the natural linewidth of atomic transitions.

[0012] Furthermore, the principle of how cooling lasers decelerate moving atoms lies in the following: When an atom moves within a pair of cooling laser beams whose frequencies are detuned to the energy level difference of atomic transitions and propagate in opposite directions, due to the Doppler effect, the atom tends to absorb photons opposite to its direction of motion, thereby generating a damping force opposite to the direction of atomic motion, thus slowing down the atom's movement. This part pertains to the basic principle of Doppler cooling and will not be elaborated upon further in this invention.

[0013] Furthermore, when there are other ground state energy levels besides the target ground state energy level contained in the target transition energy level, atoms that transition to the target upper energy level contained in the target transition energy level under the action of the cooling laser may spontaneously transition to other ground state energy levels (which may be higher or lower than the target ground state energy level), and accumulate in these other ground state energy levels, thereby escaping the cooling cycle corresponding to the target transition energy level and ceasing to interact with the cooling laser. Therefore, the frequency of the repump laser of this invention needs to be tuned to the transition from other ground state energy levels to the excited upper energy level outside the cooling cycle, in order to enable atoms in other ground state energy levels to return to the cooling cycle and participate in the contribution. The "cooling cycle" refers to the target transition energy level of the cooling laser, which includes the target upper energy level and the target ground state energy level (i.e., the target lower energy level); the frequency of the repump laser corresponds to the transition from the ground state energy level to the upper energy level outside the target transition energy level of the cooling laser.

[0014] Furthermore, the saturated absorption spectrum stabilization module includes: a sixth half-wave plate, a thirteenth reflecting mirror, a fifth polarizing beam splitter, a first atomic gas cell, a second photodetector, a fourteenth reflecting mirror, a seventh half-wave plate, a fifteenth reflecting mirror, and a sixteenth reflecting mirror. The laser incident on the saturated absorption spectrum stabilization module is reflected by the thirteenth reflecting mirror and then split into two beams by the fifth polarizing beam splitter: the weaker beam serves as the probe laser, passes through the first atomic gas cell, and is reflected by the sixth polarizing beam splitter to the second photodetector; the stronger beam serves as the pump laser, is reflected sequentially by the fourteenth, fifteenth, and sixteenth reflecting mirrors, and then overlaps with the probe laser in the opposite direction before passing through the first atomic gas cell. The sixth half-wave plate is used to adjust the splitting ratio of the probe laser and the pump laser; the seventh half-wave plate is used to adjust the optical power of the pump laser; the saturated absorption spectrum signal received by the second photodetector is fed back to the laser for frequency locking. Depending on the characteristics of the different atoms used in the specific invention, the first atomic gas cell needs to be controlled at different temperatures.

[0015] Furthermore, the modulation transfer spectrum stabilization module includes: an eighth half-wave plate, a seventh polarization beam splitter, a second atomic gas cell, a seventeenth reflecting mirror, a third photodetector, an eighteenth reflecting mirror, an electro-optic phase modulator, a ninth half-wave plate, a mixer, a signal generator, and a proportional-integral-differential locked circuit module. The laser incident on the modulation transfer spectrum stabilization module is split into two beams by the seventh polarization beam splitter: the weaker beam serves as the probe laser, passes through the second atomic gas cell, and is reflected by the seventeenth reflecting mirror to the third photodetector; the stronger beam serves as the pump laser, is reflected by the eighteenth reflecting mirror to the electro-optic phase modulator for phase modulation, and is then reflected sequentially by the eighth polarization beam splitter and the seventeenth reflecting mirror before merging with the probe laser in the opposite direction and passing through the second atomic gas cell. The eighth half-wave plate is used to adjust the splitting ratio of the probe laser and the pump laser, and the ninth half-wave plate... A waveplate is used to adjust the optical power of the pump laser. The modulated pump laser and the unmodulated probe laser interact with atoms in the second atomic gas chamber, transferring the modulation of the pump laser to the probe laser through the atoms. The modulated probe laser is then received by a third photodetector, whose modulation sideband beats the main frequency. The resulting beat signal has obvious spectral characteristics at the modulation frequency applied by the signal generator. A mixer demodulates the signal with these spectral characteristics, and the demodulated spectral line shape has obvious dispersion characteristics with respect to the detuning amount of the laser relative to the atom center frequency. Then, using a proportional-integral-differential locking circuit module, the laser frequency is locked near the zero point of the dispersion spectral line, and the servo signal generated by the locking is fed back to the frequency feedback control ports of the laser, thus achieving frequency stabilization of the modulation transfer spectrum corresponding to the atom transition frequency of the laser. In addition, depending on the characteristics of different atoms used in the specific invention, the temperature of the second atomic gas chamber needs to be controlled at different times.

[0016] Furthermore, the first, second, third, fourth, fifth, and sixth fiber collimators each contain a collimating lens with a specific focal length and a quarter-wave plate. The collimating lens with the fixed focal length serves to expand and collimate the six opposing laser beams; the quarter-wave plate ensures that the two opposing laser beams have the same frequency but opposite circular polarization directions.

[0017] Furthermore, the polarization directions of the first and second Grange Taylor prisms are orthogonal to each other.

[0018] Furthermore, this invention establishes a temporal sequence between the atom trapping in the magneto-optical trap and the detection of the Faraday rotation effect. On the one hand, the detection laser and the optically uniform magnetic field may affect the atoms trapped in the magneto-optical trap; on the other hand, the gradient magnetic field of the magneto-optical trap may affect the energy levels of Faraday-rotating atoms. Therefore, after capturing a sufficient number of cold atoms, this invention shuts off the current supply to the cooling laser, the re-pumped laser, the first gradient magnetic field coil, and the second gradient magnetic field coil to release the cold atoms. Immediately afterwards, it turns on the current supply to the detection laser, the first optically uniform magnetic field coil, and the second optically uniform magnetic field coil to measure the transmission signal of the cold atom filter, thus continuously cycling the timing sequence.

[0019] The timing control of each component in this invention is based on LabVIEW software, but other instrument control software can also be used. In addition, the timing of the cooling laser, the re-pumping laser and the probe laser is turned on and off by frequency hopping using the second acousto-optic modulator, the third acousto-optic modulator and the fourth acousto-optic modulator, respectively.

[0020] Furthermore, unlike hot atom Faraday filters, if a probe laser and a uniformly rotating magnetic field are applied to cold atoms for a prolonged period, a force will be exerted on the cold atoms, causing the cold atom clusters to heat up and diffuse, and the number of cold atoms and optical thickness will also change. Therefore, this invention cannot obtain the transmission spectrum of the cold atom filter by continuously scanning the probe laser frequency near the resonance frequency. Instead, it is necessary to use a fourth and a fifth acousto-optic modulator to shift the frequency near the resonance frequency to generate positive and negative frequency detuning. The transmittance values ​​at multiple frequency points are then discretely measured by a first photodetector, and finally combined to plot a complete transmittance variation spectrum.

[0021] Furthermore, it should be noted that the ultra-narrow bandwidth atomic filter based on cold atoms trapped in a magneto-optical trap currently realized in this invention operates in pulse mode. If the cold atoms are prepared using an optical agglomeration method, the ultra-narrow bandwidth atomic filter can achieve continuous operation.

[0022] Another objective of this invention is to propose a method for implementing an ultra-narrow bandwidth atomic filter based on cold atoms.

[0023] The method for implementing an ultra-narrow bandwidth atomic filter based on cold atoms according to the present invention specifically includes the following steps:

[0024] 1) The laser output from the cooled laser passes through the first isolator and is then matched and regulated by the first half-wave plate and the first polarizing beam splitter to split the laser into two beams: one beam is transmitted to the first saturated absorption spectrum stabilization module for frequency stabilization, and the saturated absorption spectrum signal generated by the frequency stabilization is fed back to the cooled laser for frequency locking; the other beam is frequency-shifted by the first acousto-optic modulator and the second acousto-optic modulator to produce red detuning of about one to two times the natural linewidth of the atomic transition, which is the natural linewidth of the locked atomic transition mentioned above; the laser after frequency shifting is called the cooled laser, which is output by the cooled laser frequency stabilization system and is used to decelerate moving atoms in the vacuum system;

[0025] 2) After passing through the second isolator, the laser output from the re-pumped laser is sequentially matched and regulated by the second half-wave plate and the second polarization beam splitter to split the laser into two beams: one beam is transmitted to the second saturated absorption spectrum frequency stabilization module for frequency stabilization, and the saturated absorption spectrum signal generated by frequency stabilization is fed back to the re-pumped laser for frequency locking; the other beam is frequency-shifted by the third acousto-optic modulator, and the frequency-shifted laser is called the re-pumped laser, which is output by the re-pumped laser frequency stabilization system;

[0026] 3) The frequency-stabilized cooling laser from step 1) and the frequency-stabilized re-pumped laser from step 2) are transmitted to the fourth polarization beam splitter for beam combining. The optical power of the combined laser is adjusted by the fourth half-wave plate and the fifth half-wave plate respectively. The combined cooling laser and re-pumped laser are coupled from space light to fiber, and then split into six laser beams with completely equal power by a one-to-six fiber beam splitter.

[0027] 4) The six laser beams with completely equal power generated in step 3) are output in pairs by the first, second, third, fourth, fifth, and sixth fiber collimators respectively; and opposite currents are applied to the first and second gradient magnetic field coils to form an anti-Helmholtz magnetic field with a magnetic field strength of zero at the center of the intersection of the six laser beams and linearly increasing towards the edge; under the action of the cooling laser, the re-pumped laser, and the gradient magnetic field forming a magneto-optical trap, the atoms are trapped at the center of the intersection of the six laser beams;

[0028] 5) After passing through the third isolator, the laser output from the probe laser is sequentially matched and regulated by the third half-wave plate and the third polarization beam splitter to split the laser into two beams: one beam is transmitted to the modulation transfer spectrum frequency stabilization module for precise laser frequency locking, and the servo signal generated by locking is fed back to the frequency feedback control ports of the probe laser; the other beam is forward-shifted by the fourth acousto-optic modulator and negative-shifted by the fifth acousto-optic modulator, controlling the frequency shift of the fourth and fifth acousto-optic modulators to cause positive and negative frequency detuning of the laser near the locked atomic resonance frequency. The frequency detuning range is approximately the natural linewidth of the locked atomic transition; the laser after frequency shifting is called the probe laser, and it is output by the probe laser frequency stabilization system.

[0029] 6) The frequency-stabilized probe laser from step 5) is transmitted to an attenuator to adjust the laser power, and then passes sequentially through the first Glan Taylor prism, the cold atom cluster trapped in the vacuum system, and the second Glan Taylor prism before being injected into the first photodetector; currents in the same direction are applied to the first and second optically rotating uniform magnetic field coils to form a uniform magnetic field required for the Faraday optical rotation effect along the direction in which the probe laser is incident on the first photodetector.

[0030] 7) Establish a timing sequence between the atom trapping in the magneto-optical trap and the detection of the Faraday rotation effect. After capturing enough cold atoms, turn off the current supply to the cooling laser, the re-pumped laser, the first gradient magnetic field coil, and the second gradient magnetic field coil to release the cold atoms. Then turn on the current supply to the detection laser, the first optical rotation uniform magnetic field coil, and the second optical rotation uniform magnetic field coil. Using the fourth and fifth acousto-optic modulators in step 5), positive and negative frequency detuning is generated near the resonance frequency by frequency shifting. The transmittance values ​​at multiple frequency points are measured discretely, and then combined to draw the complete transmission spectrum of the cold atom filter, realizing an ultra-narrow bandwidth atom filter with a transmission bandwidth close to the natural linewidth of the atomic transition.

[0031] In steps 1) and 2), the saturated absorption spectrum stabilization module consists of a sixth half-wave plate, a thirteenth reflecting mirror, a fifth polarizing beam splitter, a first atomic gas cell, a sixth polarizing beam splitter, a second photodetector, a fourteenth reflecting mirror, a seventh half-wave plate, a fifteenth reflecting mirror, and a sixteenth reflecting mirror. The laser incident on the saturated absorption spectrum stabilization module is sequentially split into two beams by the sixth half-wave plate and the fifth polarizing beam splitter, with the weaker beam serving as the probe laser and the stronger beam as the pump laser. The two laser beams (pump laser and probe laser) converge in opposite directions through the first atomic gas cell, where the second photodetector receives the probe laser and feeds back the obtained saturated absorption spectrum signal to the laser for frequency locking.

[0032] In step 4), the first, second, third, fourth, fifth, and sixth fiber collimators contain collimating lenses and quarter-wave plates with a certain focal length, which can realize the expansion and collimation of six pairs of laser beams, as well as the opposite circular polarization directions of the two pairs of laser beams.

[0033] In step 5), the modulation transfer spectrum frequency stabilization module consists of the eighth half-wave plate, the seventh polarization beam splitter, the second atomic gas cell, the seventeenth mirror, the eighth polarization beam splitter, the third photodetector, the eighteenth mirror, the electro-optic phase modulator, the ninth half-wave plate, the mixer, the signal generator, and the proportional-integral-differential lockout circuit module. The laser incident on the modulation transfer spectrum stabilization module is split into two beams by matching and adjusting the splitting power of the eighth half-wave plate and the seventh polarization beam splitter: the weaker beam is used as the probe laser, and the stronger beam is used as the pump laser. The two beams (pump laser and probe laser) are transmitted to the electro-optic phase modulator for phase modulation. The two laser beams (pump laser and probe laser) overlap in opposite directions and pass through the second atomic gas cell, where they interact with the atoms. The modulation of the pump laser is transferred to the probe laser through the atoms. The modulated probe laser is received by the third photodetector, and the received signal is transmitted to the mixer and proportional-integral-differential locking circuit module for demodulation, phase detection, and servo feedback locking, ultimately achieving modulation transfer spectrum stabilization of the laser's corresponding atomic transition frequency.

[0034] In step 6), the polarization directions of the first Glan Taylor prism and the second Glan Taylor prism are orthogonal to each other.

[0035] In step 7), the timing of turning on and off the cooling laser, the re-pumping laser, and the probe laser is achieved by frequency hopping using the second acousto-optic modulator in step 1), the third acousto-optic modulator in step 2), and the fourth acousto-optic modulator in step 5), respectively.

[0036] Compared with existing transmission bandwidth narrowing techniques for Faraday atom filters, the novelty and inventiveness of this invention are reflected in:

[0037] 1. This invention provides an ultra-narrow bandwidth atomic filter based on cold atoms and its implementation method. Considering that cold atoms can overcome the Doppler effect caused by atomic motion and other factors related to atomic velocity during the interaction between light and atoms, this invention innovatively combines cold atoms with an atomic filter. Laser cooling technology is used to slow down the atoms, reducing the influence of the Doppler effect, thereby achieving an ultra-narrow bandwidth atomic filter with a transmission bandwidth close to the natural linewidth of atomic transitions. This invention significantly and effectively overcomes the limitation imposed by the Doppler broadening of hot atoms on further narrowing the transmission bandwidth of Faraday atomic filters. The application of laser cooling technology to the construction of atomic filters demonstrates outstanding substantial progress and significant bandwidth narrowing effect, which has not been reported in domestic or international patents.

[0038] 2. This invention not only provides a creative new approach for further narrowing the transmission bandwidth of Faraday atom filters, but also extends the research of traditional hot atom Faraday filters to the field of cold atoms in modern science, providing new possibilities for substantial and significant progress and construction of subsequent cold atom-based filters, and bringing highly promising development to many fields. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the structure of an embodiment of the ultra-narrow bandwidth atomic filter based on cold atoms according to the present invention;

[0040] Wherein: 1—Cooled laser frequency stabilization system, 2—Re-pumped laser frequency stabilization system, 3—Detection laser frequency stabilization system, 4—Fourth half-wave plate, 5—Fifth half-wave plate, 6—Fourth polarization beam splitter, 7—Fiber optic coupler, 8—One-to-six fiber beam splitter, 9a—First fiber collimator, 9b—Second fiber collimator, 9c—Third fiber collimator, 9d—Fourth fiber collimator, 9e—Fifth fiber collimator, 9f—Sixth fiber collimator, 10a—First gradient magnetic field coil, 10b—Second gradient magnetic field coil, 11—Vacuum system, 12—Attenuator, 13—Twelfth reflecting mirror, 14—First GlanTeller prism, 15a—First optically rotating uniform magnetic field coil, 15b—Second optically rotating uniform magnetic field coil, 16—Second GlanTeller prism, 17—First photodetector;

[0041] 101—Cooling laser, 102—First reflector, 103—First isolator, 104—First half-wave plate, 105—First polarizing beam splitter, 106—Second reflector, 107—First saturated absorption spectrum frequency stabilization module, 108—First acousto-optic modulator, 109—Third reflector, 110—Second acousto-optic modulator;

[0042] 201—Re-pumped laser, 202—Second isolator, 203—Fourth mirror, 204—Second half-wave plate, 205—Second polarization beam splitter, 206—Second saturated absorption spectrum frequency stabilization module, 207—Fifth mirror, 208—Sixth mirror, 209—Third acousto-optic modulator, 210—Seventh mirror;

[0043] 301—Detector laser, 302—Third isolator, 303—Third half-wave plate, 304—Third polarization beam splitter, 305—Eighth reflector, 306—Modulation transfer spectrum frequency stabilization module, 307—Ninth reflector, 308—Fourth acousto-optic modulator, 309—Tenth reflector, 310—Fifth acousto-optic modulator, 311—Eleventh reflector.

[0044] Figure 2 This is a schematic diagram of the saturated absorption spectrum stabilization module (first and second) in the embodiment of the ultra-narrow bandwidth atomic filter based on cold atoms of the present invention;

[0045] Among them: 401—sixth half-wave plate, 402—thirteenth reflecting mirror, 403—fifth polarizing beam splitter, 404—first atomic gas cell, 405—sixth polarizing beam splitter, 406—second photodetector, 407—fourteenth reflecting mirror, 408—seventh half-wave plate, 409—fifteenth reflecting mirror, 410—sixteenth reflecting mirror.

[0046] Figure 3 This is a schematic diagram of the modulation transfer spectrum stabilization module in an embodiment of the ultra-narrow bandwidth atomic filter based on cold atoms of the present invention;

[0047] Among them: 501—Eighth half-wave plate, 502—Seventh polarizing beam splitter, 503—Second atomic gas cell, 504—Seventeenth reflecting mirror, 505—Eighth polarizing beam splitter, 506—Third photodetector, 507—Eighteenth reflecting mirror, 508—Electro-optic phase modulator, 509—Ninth half-wave plate, 510—Mixer, 511—Signal generator, 512—Proportional-integral-differential locked circuit module. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0049] like Figure 1The ultra-narrow bandwidth atomic filter based on cold atoms in this embodiment includes: a cooled laser frequency stabilization system 1, a re-pumped laser frequency stabilization system 2, a probe laser frequency stabilization system 3, a fourth half-wave plate 4, a fifth half-wave plate 5, a fourth polarization beam splitter 6, an optical fiber coupler 7, a one-to-six optical fiber beam splitter 8, a first optical fiber collimator 9a, a second optical fiber collimator 9b, a third optical fiber collimator 9c, a fourth optical fiber collimator 9d, a fifth optical fiber collimator 9e, a sixth optical fiber collimator 9f, a first gradient magnetic field coil 10a, a second gradient magnetic field coil 10b, a vacuum system 11, an attenuator 12, a twelfth reflector 13, a first GlanTell prism 14, a first optically rotating uniform magnetic field coil 15a, a second optically rotating uniform magnetic field coil 15b, a second GlanTell prism 16, and a first photodetector 17. The cooled laser frequency stabilization system 1 further includes: a cooled laser 101, a first reflector 102, a first isolator 103, a first half-wave plate 104, a first polarizing beam splitter 105, a second reflector 106, a first saturated absorption spectrum frequency stabilization module 107, a first acousto-optic modulator 108, a third reflector 109, and a second acousto-optic modulator 110; the re-pumped laser frequency stabilization system 2 further includes: a re-pumped laser 201, a second isolator 202, a fourth reflector 203, a second half-wave plate 204, and a second polarizing beam splitter 205. 5. Second saturated absorption spectrum stabilization module 206, fifth reflector 207, sixth reflector 208, third acousto-optic modulator 209, seventh reflector 210; the probe laser stabilization system 3 further includes: probe laser 301, third isolator 302, third half-wave plate 303, third polarization beam splitter prism 304, eighth reflector 305, modulation transfer spectrum stabilization module 306, ninth reflector 307, fourth acousto-optic modulator 308, tenth reflector 309, fifth acousto-optic modulator 310, eleventh reflector 311.

[0050] The laser output from the cooled laser 101 in the cooled laser frequency stabilization system 1 is reflected by the first reflector 102 and transmitted to the first isolator 103. After the first isolator 103, the first half-wave plate 104 and the first polarizing beam splitter 105 are matched and adjusted to split the laser into two beams: one beam is reflected by the second reflector 106 and transmitted to the first saturated absorption spectrum frequency stabilization module 107 for frequency stabilization. The saturated absorption spectrum signal generated by frequency stabilization is fed back to the cooled laser 101 for frequency locking; the other beam is frequency shifted by the first acousto-optic modulator 108 and the second acousto-optic modulator 110 to generate red detuning of about one to two times the natural linewidth of the atomic transition. This natural linewidth is the natural linewidth of the locked atomic transition mentioned above. The frequency-shifted laser is called the cooled laser and is output by the cooled laser frequency stabilization system 1 to achieve deceleration of moving atoms in the vacuum system 11.

[0051] In the re-pumped laser frequency stabilization system 2, the laser output from the re-pumped laser 201 is first transmitted to the second isolator 202, and then reflected by the fourth mirror 203. After that, the splitting power is matched and adjusted by the second half-wave plate 204 and the second polarization beam splitter 205 to split the laser into two beams: one beam is transmitted to the second saturated absorption spectrum frequency stabilization module 206 for frequency stabilization, and the saturated absorption spectrum signal generated by frequency stabilization is fed back to the re-pumped laser 201 for frequency locking; the other beam is reflected by the fifth mirror 207 and the sixth mirror 208 and then frequency-shifted by the third acousto-optic modulator 209. The frequency-shifted laser is called the re-pumped laser and is output by the re-pumped laser frequency stabilization system 2. The frequency of the above-mentioned re-pumped laser needs to be tuned so that atoms on other ground state energy levels can return to the target ground state energy level and participate in the cooling cycle corresponding to the target transition energy level.

[0052] The cooling laser reflected by the third mirror 109 and the re-pumped laser reflected by the seventh mirror 210 are transmitted to the fourth polarization beam splitter 6 for beam combining. The optical power of the combined laser is adjusted by the fourth half-wave plate 4 and the fifth half-wave plate 5 respectively. The combined cooling laser and the re-pumped laser are coupled into the optical fiber by the fiber coupler 7, and then split into six laser beams with completely equal power by the one-to-six fiber beam splitter 8.

[0053] The six laser beams with completely equal power generated by the aforementioned beam splitting are output in pairs by the first fiber collimator 9a, the second fiber collimator 9b, the third fiber collimator 9c, the fourth fiber collimator 9d, the fifth fiber collimator 9e, and the sixth fiber collimator 9f. Currents in opposite directions are applied to the first gradient magnetic field coil 10a and the second gradient magnetic field coil 10b to form an anti-Helmholtz magnetic field with a magnetic field strength of zero at the center where the six laser beams converge and linearly increasing towards the edge. The vacuum system 11 contains atomic vapor, and under the action of the cooling laser, the re-pumped laser, and the magneto-optical trap formed by the gradient magnetic field, the atoms are trapped at the center where the six laser beams converge.

[0054] The laser output from the probe laser 301 in the probe laser frequency stabilization system 3 is first transmitted to the third isolator 302. After the third isolator 302, the third half-wave plate 303 and the third polarization beam splitter 304 are matched and adjusted to split the laser into two beams: one beam is reflected by the eighth mirror 305 and transmitted to the modulation transfer spectrum frequency stabilization module 306 for precise laser frequency locking. The servo signal generated by locking is fed back to the frequency feedback control ports of the probe laser 301; the other beam is forward frequency shifted by the fourth acousto-optic modulator 308 and negative frequency shifted by the fifth acousto-optic modulator 310. The frequency shift amount of the fourth acousto-optic modulator 308 and the fifth acousto-optic modulator 310 is controlled to cause positive and negative frequency detuning of the laser near the locked atomic resonance frequency. The frequency detuning range is approximately the natural linewidth of the locked atomic transition. The laser after frequency shifting is called the probe laser and is output by the probe laser frequency stabilization system 3. The ninth mirror 307 and the tenth mirror 309 are used to adjust the direction of the optical path.

[0055] The detection laser reflected by the eleventh reflector 311 is first transmitted to the attenuator 12 to adjust the laser power, and then reflected by the twelfth reflector 13. The reflected detection laser then passes sequentially through the first Glan Taylor prism 14, the cold atom cluster trapped in the vacuum system 11, and the second Glan Taylor prism 16 before being received by the first photodetector 17. The first Glan Taylor prism 14 and the second Glan Taylor prism 16 are orthogonal to each other. Currents in the same direction are applied to the first optically rotating uniform magnetic field coil 15a and the second optically rotating uniform magnetic field coil 15b to form a uniform magnetic field required for the Faraday optical rotation effect. The direction of this uniform magnetic field is along the direction in which the detection laser is incident on the first photodetector 17. The first photodetector 17 is used to detect the transmission spectral lines of the ultra-narrow bandwidth atomic filter.

[0056] A timing sequence is established between the atom trapping in the magneto-optical trap and the detection of the Faraday rotation effect. After enough cold atoms are trapped, the current supply to the cooling laser 101, the re-pumped laser 201, the first gradient magnetic field coil 10a, and the second gradient magnetic field coil 10b is turned off to release the cold atoms. Then, the current supply to the detection laser 301, the first optically rotating uniform magnetic field coil 15a, and the second optically rotating uniform magnetic field coil 15b is turned on.

[0057] By using the fourth acousto-optic modulator 308 and the fifth acousto-optic modulator 310 to shift the frequency near the resonance frequency of the probe laser to generate positive and negative frequency detuning, and discretely measuring the transmittance values ​​at multiple frequency points, the complete transmission spectrum of the cold atom filter is then plotted. This invention ultimately realizes an ultra-narrow bandwidth atomic filter with a transmission bandwidth close to the natural linewidth of atomic transitions.

[0058] like Figure 2The saturated absorption spectrum stabilization modules 107 and 206 in the ultra-narrow bandwidth atomic filter based on cold atoms in this embodiment include: a sixth half-wave plate 401, a thirteenth reflector 402, a fifth polarizing beam splitter 403, a first atomic gas cell 404, a sixth polarizing beam splitter 405, a second photodetector 406, a fourteenth reflector 407, a seventh half-wave plate 408, a fifteenth reflector 409, and a sixteenth reflector 410.

[0059] The laser incident on the saturated absorption spectrum stabilization modules 107 and 206 is reflected by the thirteenth mirror 402 and then matched and regulated by the sixth half-wave plate 401 and the fifth polarization beam splitter 403 to split the laser into two beams: the weaker beam serves as the probe laser, passes through the first atomic gas cell 404, and is reflected by the sixth polarization beam splitter 405 to the second photodetector 406; the stronger beam serves as the pump laser, is reflected by the fourteenth mirror 407, the fifteenth mirror 409, and the sixteenth mirror 410, and then coincides with the probe laser in the opposite direction before passing through the first atomic gas cell 404; the seventh half-wave plate 408 is used to adjust the optical power of the pump laser; the saturated absorption spectrum signal received by the second photodetector 406 is fed back to the laser for frequency locking.

[0060] like Figure 3 The modulation transfer spectrum stabilization module 306 in the ultra-narrow bandwidth atomic filter based on cold atoms in this embodiment includes: an eighth half-wave plate 501, a seventh polarization beam splitter 502, a second atomic gas cell 503, a seventeenth reflector 504, an eighth polarization beam splitter 505, a third photodetector 506, an eighteenth reflector 507, an electro-optic phase modulator 508, a ninth half-wave plate 509, a mixer 510, a signal generator 511, and a proportional-integral-differential locked circuit module 512.

[0061] The laser incident on the modulation transfer spectrum stabilization module 306 is sequentially split into two beams by matching and adjusting the splitting power of the eighth half-wave plate 501 and the seventh polarization beam splitter 502. The weaker beam serves as the probe laser, passes through the second atomic gas cell 503, and is reflected by the seventeenth mirror 504 into the third photodetector 506. The stronger beam serves as the pump laser, is reflected by the eighteenth mirror 507 into the electro-optic phase modulator 508 for phase modulation, and is then sequentially reflected by the eighth polarization beam splitter 505 and the seventeenth mirror 504, coinciding with the probe laser in the opposite direction before passing through the second atomic gas cell 506. 03; The ninth half-wave plate 509 is used to adjust the optical power of the pump laser, and the signal generator 511 is used to apply modulation and demodulation signals; the modulated pump laser and the unmodulated probe laser interact with the atoms in the second atomic gas chamber 503, and the modulation of the pump laser is transferred to the probe laser through the atoms; then the modulated probe laser is received by the third photodetector 506, and the received signal is transmitted to the mixer 510 and the proportional-integral-differential lock circuit module 512 for demodulation, phase detection and servo feedback locking, and finally realizes the frequency stabilization of the modulation transfer spectrum of the laser corresponding to the atomic transition frequency.

[0062] Specifically, the ultra-narrow bandwidth atomic filter based on cold atoms in this embodiment of the invention is characterized by combining cold atoms with an atomic filter, using laser cooling technology to slow down the atoms, thereby reducing the influence of the Doppler effect, and thus achieving an ultra-narrow bandwidth atomic filter with a transmission bandwidth close to the natural linewidth of atomic transitions. This invention not only significantly and effectively overcomes the limitation imposed by the Doppler broadening of hot atoms on further narrowing the transmission bandwidth of Faraday atomic filters, providing a creative new approach to narrowing the transmission bandwidth of Faraday atomic filters, but also extends the research of traditional hot-atom Faraday filters to the field of cold atoms, providing new possibilities for substantial progress and construction of subsequent cold-atom-based filters, and bringing highly promising developments to many fields. This invention is fundamentally different from current Faraday atomic filter transmission bandwidth narrowing technologies in this context.

[0063] In a specific implementation of the present invention, the first, second, third, fourth, fifth, and sixth fiber collimators contain collimating lenses and quarter-wave plates with a certain focal length, which can realize the expansion and collimation of six opposing laser beams, as well as the opposite circular polarization directions of the two opposing laser beams.

[0064] The timing control of each component in this invention is based on LabVIEW software, but other instrument control software can also be used; the timing of the cooling laser, the re-pumping laser, and the probe laser is turned on and off by frequency hopping using the second, third, and fourth acousto-optic modulators, respectively.

[0065] The ultra-narrow bandwidth atomic filter based on cold atoms trapped in a magneto-optical trap, as realized in this invention, operates in pulse mode. If the cold atoms are prepared using an optical agglomeration method, the ultra-narrow bandwidth atomic filter can achieve continuous operation.

[0066] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, 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 aforementioned substitutions include substitutions for different atoms and different wavelengths, such as replacing the 780nm magneto-optical trap system with any other wavelength capable of cold atom trapping (e.g., using an 852nm laser to achieve a cesium atom magneto-optical trap), or replacing the 780nm probe laser with any other wavelength capable of detecting the Faraday rotation effect. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection claimed by the present invention is determined by the scope defined in the claims.

Claims

1. An ultra-narrow bandwidth atomic filter based on cold atoms, characterized in that, It includes a cooled laser frequency stabilization system (1), a re-pumped laser frequency stabilization system (2), a probe laser frequency stabilization system (3), and a vacuum system (11); the vacuum system (11) contains atomic vapor; wherein, The cooling laser frequency stabilization system (1) is used to output a laser that decelerates the moving atoms in the vacuum system (11), and is called a cooling laser; the frequency of the cooling laser corresponds to the set target transition energy level of the atom; the target transition energy level includes the target upper energy level and the target ground state energy level; The re-pumped laser frequency stabilization system (2) outputs a re-pumped laser, which is used to enable atoms that have transitioned to the target ground state energy level under the action of the cooling laser to return to the target ground state energy level and participate in the cooling cycle corresponding to the target transition energy level when there are other ground state energy levels in the ground state of the atoms in the vacuum system that are outside the target ground state energy level. The probe laser frequency stabilization system (3) outputs a probe laser to detect atoms located at the target ground state energy level. The frequency of the probe laser is the transition frequency from the target ground state energy level to any transitionable upper energy level. The cooling laser is incident on the fourth polarization beam splitter (6) through the fourth half-wave plate (4), and the re-pumped laser is incident on the fourth polarization beam splitter (6) through the fifth half-wave plate (5). The fourth polarization beam splitter (6) combines the cooling laser and the re-pumped laser and then couples them into a six-fiber beam splitter (8). The six laser beams output by the one-to-six fiber beam splitter (8) are input into the vacuum system (11) in pairs; wherein, a first gradient magnetic field coil and a second gradient magnetic field coil with opposite currents are respectively set on the two laser beams in the pair, which are used to provide an anti-Helmholtz magnetic field with a magnetic field strength of zero at the intersection of the six laser beams and linearly increasing towards the edge, so that the atomic vapor in the vacuum system (11) is trapped in the intersection of the six laser beams under the action of the cooling laser, the re-pumping laser and the magneto-optical trap formed by the gradient magnetic field; The probe laser passes through a first Grange Taylor prism (14), a first optically rotating uniform magnetic field coil (15a), a cold atom cluster trapped in a vacuum system, a second optically rotating uniform magnetic field coil (15b), and a second Grange Taylor prism (16) before being incident on a first photodetector (17). The polarization directions of the first Grange Taylor prism (14) and the second Grange Taylor prism (16) are orthogonal to each other. Currents in the same direction are applied to the first optically rotating uniform magnetic field coil (15a) and the second optically rotating uniform magnetic field coil (15b) to form a uniform magnetic field required for the Faraday rotation effect along the direction in which the probe laser is incident on the first photodetector (17). The first photodetector (17) is used to detect the transmission spectral lines of the ultra-narrow bandwidth atomic filter.

2. The ultra-narrow bandwidth atomic filter according to claim 1, characterized in that, The cooled laser frequency stabilization system (1) includes a cooled laser, a first reflector, a first isolator, a first half-wave plate, a first polarizing beam splitter, a second reflector, a first saturated absorption spectrum frequency stabilization module, a first acousto-optic modulator, a third reflector, and a second acousto-optic modulator. The laser output from the cooled laser is reflected by the first reflector and then incident on the first polarizing beam splitter via the first isolator and the first half-wave plate, splitting into two beams: one beam is reflected by the second reflector and transmitted to the first saturated absorption spectrum frequency stabilization module for frequency stabilization, and the saturated absorption spectrum signal generated by frequency stabilization is fed back to the cooled laser for frequency locking; the other beam is frequency-shifted via the first acousto-optic modulator, the third reflector, and the second acousto-optic modulator before being output as the cooled laser.

3. The ultra-narrow bandwidth atomic filter according to claim 1 or 2, characterized in that, The frequency of the cooling laser is red-detuned relative to the atomic resonance frequency in the vacuum system, and the red-detuning amount is one to two times the natural linewidth of the atomic transition; the resonance frequency corresponds to the set target transition energy level of the atom.

4. The ultra-narrow bandwidth atomic filter according to claim 1, characterized in that, The heavy-pumped laser frequency stabilization system (2) includes a heavy-pumped laser, a second isolator, a fourth mirror, a second half-wave plate, a second polarization beam splitter, a second saturated absorption spectrum frequency stabilization module, a fifth mirror, a sixth mirror, a third acousto-optic modulator, and a seventh mirror. The laser output from the heavy-pumped laser passes through the second isolator and the fourth mirror in sequence and is then incident on the second polarization beam splitter, splitting into two beams. One beam is transmitted to the second saturated absorption spectrum frequency stabilization module for frequency stabilization, and the saturated absorption spectrum signal generated by frequency stabilization is fed back to the heavy-pumped laser for frequency locking. The other beam is reflected by the fifth mirror and the sixth mirror in sequence and then incident on the third acousto-optic modulator for frequency shifting to the required atomic transition energy level. The frequency-shifted laser is the heavy-pumped laser output through the seventh mirror.

5. The ultra-narrow bandwidth atomic filter according to claim 3, characterized in that, The probe laser output of the probe laser frequency stabilization system (3) passes through the third isolator and the third half-wave plate in sequence and is then incident on the third polarization beam splitter to split into two beams. One beam is reflected by the eighth mirror and transmitted to the modulation transfer spectrum frequency stabilization module for laser frequency locking. The servo signal generated by locking is fed back to each frequency feedback control port of the probe laser. The other beam is positively frequency shifted by the fourth acousto-optic modulator and negatively frequency shifted by the fifth acousto-optic modulator, thereby generating positive and negative frequency detuning near the resonance frequency to obtain the probe laser. The frequency detuning range is the natural linewidth of the target transition energy level.

6. The ultra-narrow bandwidth atomic filter according to claim 1, characterized in that, Each laser beam output by the 1-to-6 fiber beam splitter (8) is input into the vacuum system (11) via a fiber collimator; the fiber collimator includes a collimating lens and a quarter-wave plate; wherein, the collimating lens is used to expand and collimate the laser beam before inputting it into the quarter-wave plate; the quarter-wave plate is used to make the frequencies of the two opposing laser beams the same but their circular polarization directions opposite.

7. A method for implementing an ultra-narrow bandwidth atomic filter based on cold atoms, comprising the following steps: 1) The laser that decelerates the moving atoms in the vacuum system (11) by using the output of the cooling laser frequency stabilization system (1) is called the cooling laser; the frequency of the cooling laser corresponds to the set target transition energy level of the atom; the target transition energy level includes the target upper energy level and the target ground state energy level; 2) The heavy pump laser is output by the heavy pump laser frequency stabilization system (2) to be used when the ground state of the atoms in the vacuum system exists in other ground state energy levels besides the target ground state energy level. When the atoms that have transitioned to the target energy level under the action of the cooling laser transition to other ground state energy levels outside the target ground state energy level through spontaneous emission, the atoms in other ground state energy levels can return to the target ground state energy level and participate in the cooling cycle corresponding to the target transition energy level. 3) The cooling laser and the re-pumped laser are transmitted to the fourth polarization beam splitter for beam combining. The optical power of the combined laser is adjusted by the fourth half-wave plate and the fifth half-wave plate respectively. The combined cooling laser and the re-pumped laser are coupled into a 1-to-6 fiber beam splitter and divided into six laser beams of equal power. 4) The six laser beams generated in step 3) are input into the vacuum system (11) in pairs; wherein, a first gradient magnetic field coil and a second gradient magnetic field coil with opposite currents are respectively set on the two laser beams in the pair, which are used to provide an anti-Helmholtz magnetic field with a magnetic field strength of zero at the intersection of the six laser beams and linearly increasing towards the edge, so that the atomic vapor in the vacuum system (11) is trapped in the intersection of the six laser beams under the action of the cooling laser, the re-pumping laser and the gradient magnetic field forming a magneto-optical trap; 5) The laser output from the probe laser of the probe laser frequency stabilization system (3) passes through the third isolator and the third half-wave plate in sequence and is then incident on the third polarization beam splitter to split into two beams. One beam is reflected by the eighth mirror and transmitted to the modulation transfer spectrum frequency stabilization module for laser frequency locking. The servo signal generated by locking is fed back to each frequency feedback control port of the probe laser. The other beam is forward frequency shifted by the fourth acousto-optic modulator and negative frequency shifted by the fifth acousto-optic modulator to obtain the probe laser. 6) The probe laser is transmitted to an attenuator to adjust the laser power, and then passes sequentially through a first Glan-Taylor prism, a first optically rotating uniform magnetic field coil, a cold atom cluster trapped in a vacuum system, a second optically rotating uniform magnetic field coil, and a second Glan-Taylor prism before being injected into a first photodetector; wherein currents of the same direction are applied to the first and second optically rotating uniform magnetic field coils to form a uniform magnetic field required for the Faraday optical rotation effect along the direction in which the probe laser is incident on the first photodetector; 7) A time sequence is established between the atom trapping in the magneto-optical trap and the detection of the Faraday rotation effect. After capturing cold atoms within a set time, the current supply to the cooling laser, the re-pumping laser, the first gradient magnetic field coil, and the second gradient magnetic field coil is turned off to release the cold atoms. Then, the current supply to the detection laser, the first optical rotation uniform magnetic field coil, and the second optical rotation uniform magnetic field coil is turned on. Then, positive and negative frequency detuning is generated by frequency shifting near the resonance frequency through the fourth acousto-optic modulator and the fifth acousto-optic modulator. The transmittance values ​​at multiple frequency points are measured discretely, and then combined to draw the complete transmission spectrum of the cold atom filter, realizing an ultra-narrow bandwidth atom filter with a transmission bandwidth close to the natural linewidth of the atomic transition. The resonance frequency corresponds to the transition frequency from the target ground state energy level of the atom to any upper energy level.

8. The method according to claim 7, characterized in that, The frequency of the cooling laser is red-detuned relative to the atomic resonance frequency in the vacuum system, and the red-detuning amount is one to two times the natural linewidth of the atomic transition; the resonance frequency corresponds to the set target transition energy level of the atom.

9. The method according to claim 8, characterized in that, The probe laser output of the probe laser frequency stabilization system (3) passes through the third isolator and the third half-wave plate in sequence and is then incident on the third polarization beam splitter to split into two beams. One beam is reflected by the eighth mirror and transmitted to the modulation transfer spectrum frequency stabilization module for laser frequency locking. The servo signal generated by locking is fed back to each frequency feedback control port of the probe laser. The other beam is positively frequency shifted by the fourth acousto-optic modulator and negatively frequency shifted by the fifth acousto-optic modulator, thereby generating positive and negative frequency detuning near the resonant frequency to obtain the probe laser. The range of this frequency detuning is the natural linewidth of the target transition energy level.

Images