Continuous cold atom light filter based on long-strip-shaped diffuse reflection cooling and implementation method

Abstract

The invention discloses a continuous cold atom light filter based on long-strip-shaped diffuse reflection cooling and an implementation method. Diffuse reflection laser cooling is an isotropic cooling mode without a magnetic field, the influence of a gradient magnetic field in a magneto-optical trap on an optical rotation atomic energy level can be effectively avoided, and a cold atom light filter which continuously operates is achieved. Long-strip-shaped diffuse reflection cooling based on the long-strip-shaped vacuum glass tube is technical innovation compared with traditional diffuse reflection cooling, the cold atomic group with the meter-scale length can be achieved, the interaction distance between light and atoms is greatly increased, and compared with a millimeter-scale magneto-optical trap cold atomic group, the optical thickness of the cold atoms is also remarkably increased. According to the invention, the problem that the cold atom optical filter works in a pulse mode due to the limitation of a magnetic field is solved remarkably and effectively, the diffuse reflection cooling technology is applied to the construction of the atom optical filter, and outstanding substantial progress and a remarkable continuous operation effect are achieved.

Description

Technical Field

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

[0002] Atomic filters have advantages such as narrow bandwidth and excellent out-of-band suppression, and have been widely studied in theory and experiment. In the past decade, research on ultra-narrow bandwidth atomic filters has been continuously advancing, but due to the influence of hot atom Doppler broadening, the transmission bandwidth of atomic filters is difficult to narrow to the order of natural line width.

[0003] The proposal of cold atom filters provides a new idea for further narrowing the transmission bandwidth of atomic filters [application number: CN202210671258.9, CN202210672698.6]. At present, ultra-narrowband filtering of the order of magnitude of natural line width has been achieved based on magneto-optical traps. However, considering that the gradient magnetic field of the magneto-optical trap will affect the optically active atomic energy levels, the existing cold atom filters all work in pulse mode: while using the magneto-optical trap to capture atoms, the optically active effect cannot be detected. It is necessary to turn off the magneto-optical trap after capturing enough cold atoms, and then turn on the circularly polarized pump laser (or optically active magnetic field) and the detection laser to measure the transmission signal of the atomic filter, and based on this time sequence, continuously cycle to realize a pulsed cold atom filter. Although the above-mentioned cold atom filter has the narrowest transmission bandwidth, it also limits its application in fields that require continuous narrow bandwidth filtering (such as: lidar, optical communication, narrowband quantum light source, etc.).

[0004] In summary, solving the problem of cold atom filters working in pulse mode and realizing continuous ultra-narrow bandwidth cold atom filters are the key research areas in the current technology field. Summary of the invention

[0005] In view of the above-mentioned defects and shortcomings existing in the existing working mode of the cold atom filter, the present invention innovatively proposes a continuous cold atom filter based on long strip diffuse reflection cooling and an implementation method. Diffuse reflection laser cooling can cool atoms within a wide speed range, and does not require a gradient magnetic field in the magneto-optical trap configuration, which can well avoid the influence of the magnetic field on the energy level of optically active atoms. In addition, the present invention uses a meter-sized vacuum glass tube coated with a diffuse reflection coating to prepare cold atoms, which can achieve a meter-sized length of cold atom clusters. Compared with the millimeter-scale magneto-optical trap cold atom clusters, the distance of light-atom interaction in the present invention is greatly improved, and the optical thickness of the cold atoms is also significantly improved. The realization of the continuous cold atom filter of the present invention, in addition to bringing substantial and significant progress to the filter based on cold atoms, will also bring great potential development to many fields (such as: self-stabilizing lasers, active optical clocks, etc.).

[0006] The technical solution of the present invention is:

[0007] A continuous cold atom filter based on long strip diffuse reflection cooling includes: a cooling laser source, a re-pumping 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 pumping laser source, a first attenuation plate, a beam expander, a quarter-wave plate, a reflective mirror, a detection laser source, an isolator, a fifth half-wave plate, a fourth polarization beam splitter prism, a thermal atom saturated absorption spectrum module, a second attenuation plate, a second reflector, a first Glan-Taylor prism, a second Glan-Taylor prism, and a photodetector.

[0008] 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.

[0009] 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 of the moving atoms in the long vacuum glass tube.

[0010] 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.

[0011] 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 circular windows 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.

[0012] Like the above-mentioned cooling laser source and re-pumping laser source, the output frequency of the pumping laser source has been stabilized in advance to the selected pumping transition frequency by means of frequency stabilization methods such as saturation absorption spectrum or modulation transfer spectrum (the frequency of light absorbed when the lower energy level particle of the pumping transition energy level transitions to the upper energy level of the pumping transition energy level is called the pumping transition frequency). 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 or F'=2; 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 or F'=4; the laser output after frequency stabilization is called the pumping laser, which passes through the first attenuator, beam expander, and quarter-wave plate in sequence and is reflected by the reflector; the above-mentioned first attenuator is used to adjust the optical power of the pumping laser; the beam expander is used to increase the spot area of ​​the pumping laser beam to improve the optical pumping efficiency; the quarter-wave plate is used to convert the linearly polarized pumping laser into a standard σ + (or σ - ) circularly polarized pump laser; the above-mentioned beam expansion σ after being reflected by the reflective mirror + (or σ - ) The circularly polarized pumping laser is injected into the cold atom group from one circular end face of the long vacuum glass tube and emitted from the other circular end face, pumping all the cold atoms passing through to the lower energy level of the pumping transition energy level. F At the largest (or smallest) sub-energy level.

[0013] The output laser of the detection laser source is first transmitted to the isolator for isolating the optical feedback of the rear optical path; after the isolator, it is divided into two beams by the fourth polarization splitter prism: 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 filter, and the other beam is reflected by the second reflector after the laser power is adjusted by the second attenuation plate; the fifth half-wave plate is used to adjust the optical power of the split laser; the above-mentioned reflected detection laser is used to detect the transmission spectrum of the cold atom filter, and is received by the photoelectric detector after passing through the first Glan Taylor prism, the cold atom cluster in the long vacuum glass tube, the reflective mirror, and the second Glan Taylor prism in sequence, and is used to detect the transmission spectrum of the cold atom filter, which can directly output a spectral signal; in the long vacuum glass tube, the detection laser and the pumping laser overlap in reverse; the polarization directions of the first Glan Taylor prism and the second Glan Taylor prism are orthogonal to each other.

[0014] The cooling laser and the re-pumping laser are combined by the first polarization beam splitter and then divided into four beams, which are coupled into four one-to-two multimode optical fibers respectively. The eight laser beams output by the optical fiber are injected into the cylindrical side surface of a long strip vacuum glass tube coated with diffuse reflection paint. The cooling laser and the re-pumping laser are reflected multiple times in the long strip vacuum glass tube to form diffuse reflection laser. Under the effect of the radiation pressure brought by the diffuse reflection laser, the alkali metal atomic vapor in the long strip vacuum glass tube is cooled to form a 1-meter-long cold atomic cluster. Then, a σ beam stabilized to a specific atomic transition energy level is injected from a circular light-transmitting end face of the long strip vacuum glass tube. + (or σ - ) Circularly polarized pump laser pumps all atoms to the magnetic quantum number m below the pump transition energy level F The maximum (or minimum) sub-energy level; the detection laser with the hot atom saturation absorption spectrum as the frequency reference is injected from the other circular light-transmitting end face of the long strip vacuum glass tube in the opposite direction to the pumping laser, and passes through the first Glan-Taylor prism, the cold atom group in the long strip vacuum glass tube, the reflective mirror, the second Glan-Taylor prism, and then enters the photoelectric detector; the frequency of the detection laser is continuously scanned, and the magnetic quantum number m at the lower energy level of the transition is F Atoms at the maximum (or minimum) sub-energy level can only absorb the linearly polarized probe laser σ - (or σ + )The circularly polarized component thus transitions to the upper energy level, and σ + (or σ - ) The circularly polarized light component is almost not absorbed, and the asymmetric absorption causes the rotation of the linear polarization plane of the detection laser. That is, the cold atoms formed by diffuse laser cooling produce ultra-narrow band transmission under the action of the pump laser and the detection laser, thus realizing a continuous cold atom filter.

[0015] 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.

[0016] Furthermore, the atoms that are transitioned to the upper energy level by the cooling laser may transition to other ground state energy levels through spontaneous radiation, and the atoms accumulate at the other ground state energy levels, thereby breaking away from the cooling transition 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 to the excited state upper energy level except the lower energy level of the cooling transition, in order to enable the atoms at other ground state energy levels to return to the cooling transition and maintain continuous cooling.

[0017] Furthermore, there are no special requirements for the length and inner diameter of the long vacuum glass tube, which can be selected at will, but the longer the tube is, the longer the interaction distance between light and atoms, and the higher the optical thickness of the cold atoms, which is more beneficial to the transmittance of the cold atom filter.

[0018] 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.

[0019] Furthermore, σ + (or σ - ) The atomic state preparation of circularly polarized pump laser follows the selection rule Δm F =m F’ -m F = +1(or Δm F =m F’ -m F =-1). When the pump laser is σ + When circularly polarized, the ground state magnetic quantum number m F Atoms at smaller energy levels will be gradually pumped to the magnetic quantum number m F At a larger energy level; when the pump laser is σ - When circularly polarized, the ground state magnetic quantum number m F Atoms at larger energy levels will be gradually pumped to magnetic quantum number m F At a smaller energy level, the state selection of polarized light pumping is achieved.

[0020] Furthermore, there is no special requirement for the transmittance-reflection ratio of the transflective mirror, and lenses with different parameters such as T (transmittance): R (reflectivity) = 9:1, T:R = 7:3, T:R = 5:5, etc. can be selected by yourself. However, considering that the transflective mirror is located in the optical path structure of the cold atom filter of the present invention, it will directly affect the transmittance of the cold atom filter. Therefore, it is recommended to select a transflective mirror with a higher transmittance, for example: T:R = 9:1.

[0021] Furthermore, the hot atom saturation absorption spectrum module is used to calibrate the output frequency of the detection laser source in order to observe the cold atom filter transmission spectrum corresponding to the atomic hyperfine transition frequency.

[0022] Furthermore, for a certain atom, there are multiple options for the frequency of the detection laser according to the different transition energy levels, but the ultra-narrowband transmission of the cold atom filter can only be obtained when the detection laser frequency corresponds to the atomic hyperfine transition frequency, and the maximum magnetic quantum number of the upper energy level corresponding to the transition of the detection laser cannot be greater than the maximum magnetic quantum number of the lower energy level, so as to bring about absorption asymmetry and realize the rotation of the linear polarization plane of the detection laser.

[0023] Another object of the present invention is to provide a method for realizing a continuous cold atom filter based on long strip diffuse reflection cooling.

[0024] The method for realizing the continuous cold atom filter based on long strip diffuse reflection cooling of the present invention specifically comprises the following steps:

[0025] 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 laser beams is adjusted by the first half-wave plate and the second half-wave plate respectively;

[0026] 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;

[0027] 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 incident windows not covered by the coating are reserved as incident windows for cooling laser and re-pumping laser; eight laser beams output by 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;

[0028] 4) After the frequency stabilization, the output laser of the pumping laser source passes through the first attenuator, beam expander, and quarter-wave plate in sequence and is reflected by the reflective mirror; the σ reflected by the reflective mirror + (or σ -) The circularly polarized pumping laser is injected into the cold atom group from one circular end face of the long strip vacuum glass tube in step 3) and emitted from the other circular end face, pumping all the cold atoms passing through to the transition lower energy level magnetic quantum number m F At the largest (or smallest) sub-energy level;

[0029] 5) After the output laser of the detection laser source passes through the isolator, the splitting power is matched and adjusted by the fifth half-wave plate and the fourth polarization splitter prism in turn, and the laser is divided 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 filter; the other beam is reflected by the second reflector after the laser power is adjusted by the second attenuation plate, and then passes through the first Glan-Taylor prism, the cold atom group trapped in the long vacuum glass tube, the reflector, the second Glan-Taylor prism in turn, and then enters the photodetector;

[0030] 6) Continuously scan the detection laser frequency in step 5) near the specific atomic transition frequency, and with the help of the asymmetric atomic population formed by the pump laser in step 4), the magnetic quantum number m of the lower energy level of the transition F Atoms at the maximum (or minimum) sub-energy level can only absorb the linearly polarized probe laser σ - (or σ + )The circularly polarized component thus transitions to the upper energy level, and σ + (or σ - ) The circularly polarized light component is almost not absorbed, and the asymmetric absorption causes the rotation of the linear polarization plane of the detection laser, and then the ultra-narrow-band transmission of the detection laser corresponding to the atomic transition frequency is observed, realizing a cold atom filter with a transmission bandwidth close to the natural linewidth of the atomic transition and continuous operation.

[0031] In step 4), the function of the first attenuator is to adjust the optical power of the pumping laser; the function of the beam expander is to increase the spot area of ​​the pumping laser beam so that the pumping laser can contain as many cold atomic groups as possible when passing through the long strip vacuum glass tube to improve the optical pumping efficiency; the function of the quarter-wave plate is to convert the linearly polarized pumping laser into a standard σ + (or σ - )Circularly polarized pump laser.

[0032] In step 4) and step 5), the pump laser reflected by the reflective mirror and the detection laser reflected by the second reflection mirror are adjusted to overlap in reverse order to ensure that the pump laser and the detection laser act on the same batch of cold atoms.

[0033] In step 5), the polarization directions of the first Glan-Taylor prism and the second Glan-Taylor prism are orthogonal to each other.

[0034] In step 6), the specific atomic transition frequency means that the maximum magnetic quantum number of the upper energy level corresponding to the transition of the detection laser cannot be greater than the maximum magnetic quantum number of the lower energy level, so as to bring about absorption asymmetry and thus realize the rotation of the linear polarization plane of the detection laser.

[0035] Compared with the existing cold atom filter working in pulse mode, the positive effects of the present invention are:

[0036] 1. The present invention provides a continuous cold atom filter based on long strip diffuse reflection cooling and an implementation method. Diffuse reflection laser cooling is an isotropic cooling method that does not require a magnetic field. It can effectively avoid the influence of the gradient magnetic field in the magneto-optical trap on the energy level of optically active atoms, and realize a continuously operating cold atom filter. Long strip diffuse reflection cooling based on long strip vacuum glass tubes is a technological innovation compared to traditional diffuse reflection cooling. It can realize cold atom clusters of meter length, greatly improving the interaction distance between light and atoms. Compared with millimeter-scale magneto-optical trap cold atom clusters, the optical thickness of cold atoms is also significantly improved. The implementation of the present invention significantly and effectively solves the problem of cold atom filters working in pulse mode due to magnetic field limitations. It is an example of applying diffuse reflection cooling technology to the construction of atomic filters. Its outstanding substantial progress and significant continuous operation effect have not been reported in domestic and foreign patents.

[0037] 2. The present invention not only provides a creative new idea for realizing continuous cold atom filters, but also solves the problem that downstream applications (such as self-stabilizing lasers, active optical clocks, etc.) are limited by cold atom filters and work in pulse mode, providing new possibilities for the subsequent development in many fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic structural diagram of an embodiment of a continuous cold atom filter based on long strip diffuse reflection cooling of the present invention;

[0039] Among them: 1-cooling laser source, 2-re-pumping laser source, 3-acoustic-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-pumping laser source, 18-first attenuation plate, 19-beam expander, 20-quarter wave plate, 21-transflective mirror, 22-detection laser source, 23-isolator, 24-fifth half-wave plate, 25-fourth polarization beam splitter prism, 26-thermal atomic saturation absorption spectrum module, 27-second attenuation plate, 28-second reflector, 29-first Glan-Taylor prism, 30-second Glan-Taylor prism, 31-photodetector. DETAILED DESCRIPTION

[0040] The present invention will be further described below through specific embodiments in conjunction with the accompanying drawings.

[0041] like Figure 1 The continuous cold atom filter 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 pumping laser source 17, a first attenuation plate 18, a beam expander 19, a quarter-wave plate 20, a transflective mirror 21, a detection laser source 22, an isolator 23, a fifth half-wave plate 24, a fourth polarization beam splitter prism 25, a hot atom saturated absorption spectrum module 26, a second attenuation plate 27, a second reflector 28, a first Glan-Taylor prism 29, a second Glan-Taylor prism 30, and a photodetector 31.

[0042] The output frequency of the cooling laser source 1 has been stabilized in advance to a 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, which 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 16.

[0043] The output frequency of the heavy pump laser source 2 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 16.

[0044] 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 combining, 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 split laser beams 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 split laser beams are 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 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.

[0045] 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 to the coating, and eight circular windows 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 vacuum glass tube 16 are cooled, and the entire 1-meter-long tube is filled with cold atomic groups.

[0046] The output frequency of the pump laser source 17 has been stabilized in advance to the selected pump transition frequency (the light frequency absorbed when the lower energy level particle of the pump transition energy level transitions to the upper energy level of the pump transition energy level is called the 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=2 to the rubidium atom first excited state F'=3 or F'=2; 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 or F'=4; the laser output after frequency stabilization is called the pump laser, which passes through the first attenuator 18, the beam expander 19, and the quarter-wave plate 20 in sequence and is reflected by the reflective mirror 21; the above-mentioned first attenuator 18 is used to adjust the optical power of the pump laser; the beam expander 19 is used to increase the spot area of ​​the pump laser beam to improve the optical pumping efficiency; the quarter-wave plate 20 is used to convert the linearly polarized pump laser into a standard σ + (or σ - ) circularly polarized pump laser; the expanded beam reflected by the reflective mirror 21 σ + (or σ -) The circularly polarized pumping laser is injected into the cold atom group from one circular end face of the long strip vacuum glass tube 16, and is emitted from the other circular end face, pumping all the cold atoms passing through to the magnetic quantum number m of the energy level below the pumping transition energy level. F At the largest (or smallest) sub-energy level.

[0047] The output laser of the detection laser source 22 is first transmitted to the isolator 23 for isolating the optical feedback of the rear optical path; after the isolator 23, the fifth half-wave plate 24 and the fourth polarization beam splitter prism 25 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 26, and the hot atom saturation absorption spectrum obtained by detection is used as the frequency reference of the cold atom filter, and the other beam is reflected by the second reflector 28 after the laser power is adjusted by the second attenuation plate 27; the above-mentioned reflected detection laser is used to detect the transmission spectrum of the cold atom filter, and is received by the photodetector 31 after passing through the first Glan Taylor prism 29, the cold atom cluster in the long strip vacuum glass tube 16, the reflector 21, and the second Glan Taylor prism 30 in sequence; the polarization directions of the first Glan Taylor prism 29 and the second Glan Taylor prism 30 are orthogonal to each other; in the long strip vacuum glass tube 16, the detection laser and the pumping laser overlap in reverse.

[0048] The frequency of the above-mentioned detection laser is continuously scanned near the specific atomic transition frequency, and the magnetic quantum number m of the lower energy level of the transition is F Atoms at the maximum (or minimum) sub-energy level can only absorb the linearly polarized probe laser σ - (or σ + )The circularly polarized component thus transitions to the upper energy level, and σ + (or σ - ) The circularly polarized light component is almost not absorbed, and the asymmetric absorption causes the rotation of the linear polarization plane of the detection laser, thereby realizing a continuous cold atom filter with an ultra-narrow transmission bandwidth.

[0049] Specifically, the continuous cold atom filter 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 atom filter, and utilizing the characteristic that diffuse reflection cooling does not require a magnetic field to avoid the influence of the gradient magnetic field of the magneto-optical trap on the energy level of optically active atoms, thereby realizing an ultra-narrow bandwidth cold atom filter with a transmission bandwidth close to the natural line width of atomic transition and continuous operation. The implementation of the present invention not only significantly and effectively overcomes the problem that the cold atom filter works in a pulse mode due to magnetic field limitations, but also provides a creative new idea for realizing a continuous cold atom filter, and also solves the problem that its downstream applications (such as self-stabilizing lasers, active optical clocks, etc.) are limited by the cold atom filter and work in a pulse mode, providing new possibilities for the subsequent development of many fields. In this case, the present invention is essentially different from the discontinuous cold atom filter currently realized based on the magneto-optical trap.

[0050] 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 optical thickness of the cold atoms is, which is more beneficial to the transmittance of the cold atom filter. 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 can be injected with eight split laser beams, and a 0.5-meter-long vacuum glass tube can be injected with six split laser beams. There are no special requirements for the number of laser beams, and it is sufficient to meet the experimental requirements.

[0051] There is no special requirement for the transmittance-reflection ratio of the transflective mirror in the present invention, and lenses with different parameters such as T:R=9:1, T:R=7:3, T:R=5:5 can be selected by oneself. However, considering that the transflective mirror is located in the optical path structure of the cold atom filter of the present invention, it will directly affect the transmittance of the cold atom filter. Therefore, it is recommended to select a transflective mirror with higher transmittance, for example: T:R=9:1.

[0052] The optical rotation effect produced by the present invention has strict requirements on the working transition energy level of the detection laser. The maximum magnetic quantum number of the upper energy level corresponding to the transition of the detection laser cannot be greater than the maximum magnetic quantum number of the lower energy level. This can bring about absorption asymmetry and further realize the rotation of the linear polarization plane of the detection laser.

[0053] 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 can 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 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 detection laser with any other wavelength such as 420nm that can achieve optical rotation effect detection. 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 continuous cold atom filter based on long strip diffuse reflection cooling, characterized in that: It includes cooling laser source, re-pumping laser source, long vacuum glass tube and pumping laser source; among them, The long strip vacuum glass tube is filled with alkali metal atoms; the side wall of the long strip vacuum glass tube is coated with a diffuse reflection coating with high reflectivity for cooling laser and re-pumping laser and has multiple incident windows; The laser frequency output by the cooling laser source is stabilized to the selected cooling transition frequency and then frequency-shifted by the acousto-optic modulator to obtain 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 the lower energy level particles of the cooling transition energy level transition to the upper energy level of the cooling transition energy level; The re-pumping laser outputted by the re-pumping laser source is frequency-stabilized to a selected re-pumping transition frequency to obtain a re-pumping laser for maintaining continuous cooling of the moving atoms in the long strip vacuum glass tube; the re-pumping transition frequency is the light frequency absorbed when the lower energy level particle of the re-pumping transition energy level transitions to the upper energy level of the re-pumping 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 division are incident into the long strip vacuum glass tube through the incident window respectively to generate diffusely reflected laser beams, so as to cool the alkali metal atoms to form cold atomic clusters; The pump laser source outputs linearly polarized pump laser and stabilizes the frequency to the selected pump transition frequency. The pump laser passes through the first attenuator and the beam expander in sequence and is incident on the quarter-wave plate; the quarter-wave plate is used to convert the linearly polarized pump laser into a standard σ + The circularly polarized pumping laser is incident from one end face of the long strip vacuum glass tube into the cold atom group and then emitted from the other end face of the long strip vacuum glass tube, pumping the cold atoms passing through to the lower energy level of the pumping transition energy level with a magnetic quantum number m F The largest sub-energy level; or the quarter-wave plate is used to convert the linearly polarized pump laser into a standard σ - The circularly polarized pumping laser is incident from one end face of the long strip vacuum glass tube into the cold atom group and then emitted from the other end face of the long strip vacuum glass tube, pumping the cold atoms passing through to the lower energy level of the pumping transition energy level with a magnetic quantum number m F Asymmetric atomic population is formed on the smallest sub-energy level; the pumping transition frequency is the frequency of light absorbed when the lower energy level particle of the pumping transition energy level transitions to the upper energy level of the pumping transition energy level, which is called the pumping transition frequency; The detection laser output by the detection laser source is incident on the fourth polarization splitter prism through the isolator and is divided into two beams: one beam is transmitted to the hot atom saturation absorption spectrum module, and the other beam is adjusted by the second attenuation plate, and then passes through the first Glan Taylor prism, the cold atom cluster in the long strip vacuum glass tube, and the second Glan Taylor prism in sequence before being incident on the photodetector for detecting the transmission spectrum of the cold atom filter; 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; in the long strip vacuum glass tube, the detection laser and the pumping laser overlap in reverse; the polarization directions of the first Glan Taylor prism and the second Glan Taylor prism are orthogonal to each other.

2. The continuous cold atom filter 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.

3. The continuous cold atom filter according to claim 1, characterized in that: After the laser power is adjusted by the second attenuation plate, it passes through the first Glan-Taylor prism, the cold atomic group in the long vacuum glass tube, the reflective mirror, and the second Glan-Taylor prism in sequence and then enters the photodetector; the pumping laser output by the quarter-wave plate is reflected by the reflective mirror into the long vacuum glass tube.

4. The continuous cold atom filter according to claim 3, characterized in that: The ratio of the transmittance T to the reflectance R of the transflective mirror is 9:

1.

5. The continuous cold atom filter according to claim 1, 2 or 3, characterized in that: The frequency of the re-pump laser corresponds to the transition frequency of other ground state energy levels to excited state energy levels outside the cooling cycle corresponding to the cooling transition energy level, and is used to enable atoms on other ground state energy levels to return to the cooling cycle to participate in the contribution.

6. The continuous cold atom filter according to claim 1, 2 or 3, 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 the 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 the transition frequency from the ground state F=4 of the cesium atom to the first excited state F'=5 of the cesium atom.

7. The continuous cold atom filter according to claim 1, 2 or 3, 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.

8. The continuous cold atom filter according to claim 1, 2 or 3, 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.

9. A method for realizing a continuous cold atom filter based on long strip diffuse reflection cooling, the steps comprising: 1) Alkali metal atoms are filled in a long strip vacuum glass tube; the side wall of the long strip vacuum glass tube is coated with a diffuse reflection coating with high reflectivity for cooling laser and re-pumping laser and has multiple incident windows; 2) stabilizing the laser frequency output by the cooling laser source to the selected cooling transition frequency and then frequency shifting the cooling laser through the acousto-optic modulator, and transmitting the cooling laser and the re-pumping laser output by the re-pumping laser source and stabilized to the selected re-pumping transition frequency to the first polarization beam splitter prism for beam combining; 3) Splitting the combined laser beam into two beams and respectively injecting them into the second polarization beam splitter prism and the third polarization beam splitter prism, and then splitting the beams into four beams by the second polarization beam splitter prism and the third polarization beam splitter prism; injecting the four beams of laser beams into the long strip vacuum glass tube through the incident window to generate diffusely reflected laser beams, and cooling the alkali metal atoms to form cold atomic clusters; 4) The pump laser source outputs a linearly polarized pump laser and stabilizes the frequency to the selected pump transition frequency. The pump laser passes through the first attenuator and the beam expander in sequence and is incident on the quarter-wave plate; the quarter-wave plate converts the linearly polarized pump laser into a standard σ + The circularly polarized pumping laser is incident from one end face of the long strip vacuum glass tube into the cold atom group and then emitted from the other end face of the long strip vacuum glass tube, pumping the cold atoms passing through to the lower energy level of the pumping transition energy level with a magnetic quantum number m F The largest sub-level; or the quarter-wave plate converts the linearly polarized pump laser into a standard σ - The circularly polarized pumping laser is incident from one end face of the long strip vacuum glass tube into the cold atom group and then emitted from the other end face of the long strip vacuum glass tube, pumping the cold atoms passing through to the lower energy level of the pumping transition energy level with a magnetic quantum number m F At the smallest sub-energy level, an asymmetric atomic population is formed; 5) The detection laser output by the detection laser source is divided into two beams through the isolator and the fourth polarization splitter prism in turn: 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 filter; the other beam is reflected by the second reflector after the laser power is adjusted by the second attenuation plate, and then passes through the first Glan-Taylor prism, the cold atom cluster trapped in the long vacuum glass tube, the reflector, the second Glan-Taylor prism in turn, and then enters the photodetector to detect the transmission spectrum of the cold atom filter.

10. The method according to claim 9, characterized in that A first half-wave plate is provided before the first polarization beam splitter prism, which is used to adjust 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, which is used to adjust 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.

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