Polarization polarization enhanced high-transmission ultra-narrow bandwidth cold atomic optical filter and implementation method

Abstract

The application discloses a polarization enhancement type high-transmission ultra-narrow bandwidth cold atom optical filter and an implementation method. The application reduces the influence of Doppler effect on the transmission bandwidth of the atom filter by laser cooling technology to slow down atoms. In combination with the polarization pumping technology, the atoms generate unidirectional atomic population far from the Boltzmann distribution on the ground state Zeeman sub-energy level. The unidirectional atomic population makes one circularly polarized light component in linearly polarized probe laser be absorbed, and the other component is hardly absorbed, thereby bringing the rotation of the polarization plane of the linearly polarized probe laser, forming a large angle and high transmittance optical rotation. According to the above polarization enhancement principle, the application finally realizes the improvement of the transmittance of the cold atom filter by nearly 15 times. The high-transmission ultra-narrow bandwidth cold atom filter realized by the polarization pumping technology does not need to apply an optical rotation magnetic field, which is a significant technical innovation different from the traditional Faraday atomic filter.

Description

Technical Field

[0001] This invention belongs to the fields of laser-cooled atomic technology and atomic filter technology, and particularly relates to a polarization-enhanced high-transmission ultra-narrow bandwidth cold atomic filter and its implementation method. Background Technology

[0002] High peak transmittance and narrow bandwidth are the two main advantages of Faraday atom filters, which have long been pursued by scientists. Research on ultra-narrow bandwidth Faraday atom filters has been continuously advancing for the past decade or so. However, limited by the Doppler broadening effect of hot atoms, current technology has almost reached the limit of atomic physics in narrowing the transmission bandwidth of hot atom Faraday filters. 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, some research has proposed combining cold atoms with Faraday atom filters. By using laser cooling technology to slow down the atoms and reduce the influence of the Doppler effect, a super-narrow bandwidth cold atom filter with a transmission bandwidth close to the natural linewidth of atomic transitions can be achieved.

[0003] However, the above research scheme also has a significant problem: while narrowing the transmission bandwidth of the atomic filter, it greatly sacrifices transmittance. This is mainly because in the Faraday atomic filter realized using the thermal atom scheme, the number of atoms in the atomic gas chamber can reach 10. 12 The order of magnitude is larger than that of atomic filters based on cold atom methods, which capture only about 10 cold atoms using laser cooling technology. 8 The loss of transmittance, on the order of magnitude of the number of atoms, has an extremely detrimental effect on the transmittance of atomic filters. In summary, how to maximize transmittance while maintaining the ultra-narrow bandwidth of atomic filters is currently a key research area in this field. Summary of the Invention

[0004] This invention addresses the aforementioned defects and shortcomings in existing transmission bandwidth narrowing technologies for cold atom Faraday filters. For the first time internationally, it innovatively proposes a polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter and its implementation method. Laser cooling technology slows down the atoms, reducing the impact of the Doppler effect on the narrowing of the atomic filter's transmission bandwidth. Combined with polarization pumping technology, this results in a unidirectional atomic population at the ground-state Zeeman sublevel, far removed from the Boltzmann distribution. This unidirectional atomic population causes the absorption of one circularly polarized component of the linearly polarized probe laser, while the other is almost unabsorbed. This leads to a rotation of the polarization plane of the linearly polarized probe laser, resulting in a larger angle and higher transmittance optical rotation. Based on the aforementioned principle of enhanced polarization, this invention ultimately achieves a nearly 15-fold increase in transmittance for cold atom filters. Furthermore, the high-transmission, ultra-narrow bandwidth cold atom filter achieved using polarized light pumping technology has another advantage: its optical rotation is achieved through circularly polarized laser pumping, resulting in an asymmetric population of atoms at the ground-state Zeeman sublevels, rather than through a magnetic field causing the Zeeman sublevels to split. Therefore, the high-transmission, ultra-narrow bandwidth cold atom filter achieved using polarized light pumping technology in this invention does not require the application of an optical rotation magnetic field, which is a significant technological innovation distinguishing it from traditional Faraday atom filters. In summary, the implementation of this invention effectively solves the problem of significantly reduced transmittance due to the loss of atomic numbers in existing cold atom Faraday filter narrowing techniques, achieving a cold atom filter that simultaneously possesses ultra-narrow transmission bandwidth and high transmittance. Moreover, it 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 numerous fields.

[0005] The polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of the present invention includes: a cooled laser frequency stabilization system, a re-pumped laser frequency stabilization system, a probe laser frequency stabilization system, a fifth half-wave plate, a sixth half-wave plate, a fifth polarization beam splitter, a first fiber coupler, a 1-to-6 fiber beam splitter, a first fiber collimator, a second fiber collimator, a third fiber collimator, a fourth fiber collimator, a fifth fiber collimator, a sixth fiber collimator, a first gradient magnetic field coil, a second gradient magnetic field coil, a vacuum system, an input end of a second fiber coupler, an output end of a second fiber coupler, an attenuator, a first GlanTeller prism, a second GlanTeller prism, a first photodetector, a sixth reflector, a fourth acousto-optic modulator, a seventh reflector, a seventh half-wave plate, a sixth polarization beam splitter, a beam expander, a quarter-wave plate, and a semi-transparent, semi-reflective mirror. The cooling laser frequency stabilization system includes: a 780nm cooling laser, a first isolator, a first half-wave plate, a first polarizing beam splitter, a second half-wave plate, a first reflector, a second polarizing beam splitter, a first saturated absorption spectrum frequency stabilization module, and a first acousto-optic modulator; the re-pumped laser frequency stabilization system includes: a 780nm re-pumped laser, a second isolator, a third half-wave plate, a third polarizing beam splitter, a second reflector, a second saturated absorption spectrum frequency stabilization module, a second acousto-optic modulator, and a third reflector; the probe laser frequency stabilization system includes: a 780nm probe laser, a third isolator, a fourth half-wave plate, a fourth polarizing beam splitter, a fourth reflector, a modulation transfer spectrum frequency stabilization module, a fifth reflector, and a third acousto-optic modulator.

[0006] In the cooled laser frequency stabilization system, the output laser from the 780nm cooled laser is first transmitted to the first isolator to isolate the optical feedback from the rear optical path. After the first isolator, it is split into two beams by the first polarization beam splitter: one beam is split again by the second polarization beam splitter and then transmitted to the first saturated absorption spectrum frequency stabilization module for frequency stabilization. The saturated absorption spectrum signal generated by frequency stabilization is fed back to the 780nm cooled laser for frequency locking (specifically, the 780nm cooled laser of this invention locks the target atom to transition to the target ground state energy level). 87 Rb 5 2 S 1 / 2 F=2 to the target upper energy level 5 2 P 3 / 2 (F' = 3 transition); another beam is frequency-shifted by the first acousto-optic modulator, producing a red detuning of approximately one to two times the natural linewidth of the atomic transition, which is the natural linewidth of the aforementioned locked target atomic transition; the frequency-shifted laser is called the cooling laser, output by the cooling laser frequency stabilization system, used to decelerate moving atoms in the vacuum system; the final output frequency of the cooling laser from the cooling laser frequency stabilization system is red-detuned relative to the atomic resonance frequency in the vacuum system, with a detuning amount of approximately one to two times the natural linewidth of the atomic transition. The first half-wave plate and the second half-wave plate are used to adjust the beam splitting ratio of the laser beam split by the first polarizing beam splitter and the second polarizing beam splitter, respectively.

[0007] In the re-pumped laser frequency stabilization system, the output laser light from the 780nm re-pumped laser is first transmitted to the second isolator to isolate the optical feedback from the rear optical path. After the second isolator, it is split into two beams by a third polarization beam splitter: one beam is reflected by the second mirror and transmitted to the second saturable absorption spectrum frequency stabilization module for frequency stabilization. The saturated absorption spectrum signal generated by frequency stabilization is fed back to the 780nm re-pumped laser for frequency locking (specifically, the 780nm re-pumped laser in this invention is locked to...). 87 Rb 5 2 S 1 / 2 F = 1 to 5 2 P 3 / 2 F' = 1 transition (up); another beam is frequency-shifted by the second acousto-optic modulator to 157MHz to 5. 2 P 3 / 2 F' = 2, used to cause the vacuum system to fall to the ground state 5 under the action of the cooling laser. 2 S 1 / 2 Atoms at the F=1 energy level can return to the cooling cycle and contribute again; the laser after frequency shifting is called a re-pumped laser, which is output by the re-pumped laser frequency stabilization system; the third half-wave plate is used to adjust the splitting ratio of the two split laser beams.

[0008] The 780nm cooled laser, frequency-shifted by the first acousto-optic modulator, and the 780nm re-pumped laser, reflected by the third mirror, are transmitted to the fifth polarization beam splitter for beam combining. The fifth and sixth half-wave plates are used to adjust the optical power of the combined laser. The combined 780nm cooled laser and 780nm re-pumped laser are coupled into an optical fiber by the first 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, respectively. The first and second gradient magnetic field coils carry currents in opposite directions, providing an anti-Helmholtz magnetic field with zero magnetic field strength at the intersection of the six laser beams and linearly increasing towards the edges. The vacuum system contains a rarefied magnetic field. 87 Rb atomic vapor is trapped in a magneto-optical trap formed by a cooling laser, a re-pumped laser, and a gradient magnetic field, with the atoms converging at the center of the six laser beams.

[0009] In the probe laser frequency stabilization system, the output laser from the 780nm probe laser is first transmitted to the third isolator to isolate the optical feedback from the subsequent optical path. After the third isolator, it is split into two beams by the fourth polarization beam splitter: one beam is reflected by the fourth mirror and then transmitted to the modulation transfer spectrum frequency stabilization module for precise laser frequency locking (specifically, the 780nm probe laser in this invention is locked to...). 87 Rb 5 2 S1 / 2 F = 2 to 5 2 P 3 / 2 F'=3 transition), the resulting servo signal is fed back to the frequency feedback control ports of the 780nm probe laser, and frequency compensation is performed through feedback to achieve frequency stabilization; another beam is reflected by the fifth mirror and transmitted to the third acousto-optic modulator, which shifts the frequency by 267MHz to 5. 2 P 3 / 2 F' = 2; the laser after frequency shifting is called the probe laser, which is output by the probe laser frequency stabilization system; the fourth half-wave plate is used to adjust the splitting ratio of the two split laser beams.

[0010] The 780nm probe laser, frequency-shifted by the third acousto-optic modulator, is first coupled into the optical fiber through the input end of the second fiber coupler, and after transmission through the fiber, is output from the output end of the second fiber coupler. The 780nm probe laser output from the second fiber coupler is then transmitted to an attenuator to adjust the laser power, and subsequently passes through the first Glan Taylor prism and is confined in the vacuum system. 87 The Rb cold atom cluster, after passing through the second GlanTell prism, is received by the first photodetector. The first photodetector is used to detect the transmission signal of the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter.

[0011] Another laser beam, generated by the second polarization beam splitter in the cooled laser frequency stabilization system, is reflected by the sixth reflecting mirror and then transmitted to the fourth acousto-optic modulator, where it is frequency-shifted by 267 MHz. The frequency of the shifted laser corresponds to... 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 The F'=2 transition occurs; the frequency-shifted laser, as the pump laser of this invention, is reflected by the seventh reflecting mirror, and then sequentially passes through the seventh half-wave plate, the sixth polarizing beam splitter, the beam expander, and the quarter-wave plate before being reflected by a semi-transparent mirror; the pump laser reflected by the semi-transparent mirror coincides with the probe laser in opposite directions and passes through the vacuum system. 87 Rb cold atom clusters; the seventh half-wave plate and the sixth polarization beam splitter are used to adjust the optical power of the pump laser and to convert the pump laser into pure linearly polarized light; the beam expander is used to increase the spot area of ​​the pump laser beam so that the pump laser can completely contain the cold atom clusters when passing through the vacuum system, thereby improving the optical pumping efficiency; the quarter-wave plate is used to convert the linearly polarized pump laser into standard σ-wave light. + Circularly polarized pump laser.

[0012] The 780nm cooled laser, stabilized by the cooled laser frequency stabilization system, and the 780nm re-pumped laser, stabilized by the re-pumped laser frequency stabilization system, are combined by the fifth polarization beam splitter. The combined cooled and re-pumped lasers are then coupled into an optical fiber by the first fiber coupler and split into six beams of equal power by a 1-to-6 fiber beam splitter. These six 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, increasing linearly towards the edges. This gradient magnetic field, within the vacuum system, creates a magnetic field... 87 Rb atoms are subjected to a light scattering force that points towards the center. Due to the scattering force of the cooling laser, the slower atoms in the Maxwell velocity distribution are stably trapped at the center where the six laser beams converge, while the faster atoms outside the trap redistribute their velocity, forming a new Maxwell velocity distribution, generating new slower atoms that are then trapped by the magneto-optical trap. Then, by introducing a corresponding laser beam... 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F' = 2 transition σ + Circularly polarized pump lasers pump all atoms to their ground state 5. 2 S 1 / 2 F=2m F At the highest sub-level; after stabilizing the frequency using the probed laser frequency stabilization system, the corresponding... 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 The 780nm probe laser for the F'=2 transition and σ + The circularly polarized pumped lasers, in opposite directions, coincidentally pass through the first Glan Taylor prism and are then trapped in the vacuum system. 87 Rb cold atom clusters, the second Glan Taylor prism, and then the particles are inserted into the first photodetector; located in the ground state 5. 2 S 1 / 2 F = 2, m F Atoms at the +2 energy level can only absorb σ in linearly polarized probe lasers. - The circular polarization component thus transitions to 5 2 P 3 / 2 F' = 2, m F’ =+1, while σ + The circularly polarized light component is hardly absorbed, and this asymmetric absorption causes the polarization plane of the probe laser to rotate, thereby enabling the ultra-narrow bandwidth cold atom filter to rotate light at a large angle and with high transmittance.

[0013] Furthermore, the principle of cooling laser deceleration of 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 further in this invention. The atomic cooling method employed in this invention... 87 Rb atoms 780nm 5 2 S 1 / 2 Up to 5 2 P 3 / 2 The natural linewidth of the transition is 6.1 MHz. Therefore, this invention utilizes a first acousto-optic modulator to generate a red detuning of about 10-15 MHz by frequency shifting the cooling laser, which is used to decelerate the moving atoms in the vacuum system by the cooling laser.

[0014] Furthermore, when there are other energy levels in the atomic ground state, atoms that transition to a higher energy level under the action of a cooling laser may spontaneously transition to other energy levels in the ground state that are outside the cooling cycle, causing the atoms to accumulate at that level and thus ceasing their interaction with the cooling laser. Therefore, this invention uses a 780nm re-pumped laser to cause atoms that have fallen to the ground state 5 to... 2 S 1 / 2 Atoms at the F=1 energy level can return to the cooling cycle and contribute again; the frequency of this re-pumped laser is tuned to the corresponding... 87 Rb 5 2 S 1 / 2 F = 1 to 5 2 P 3 / 2 F' = 2 transition.

[0015] Furthermore, the saturated absorption spectrum stabilization module includes: an eighth half-wave plate, an eighth reflector, a seventh polarizing beam splitter, a first atomic gas cell, a second photodetector, a ninth reflector, a ninth half-wave plate, a tenth reflector, and an eleventh reflector. The laser incident on the saturated absorption spectrum stabilization module is reflected by the eighth reflector and then split into two beams by the seventh polarizing beam splitter: the weaker beam serves as the probe laser, passes through the first atomic gas cell, and is reflected by the eighth polarizing beam splitter to the second photodetector; the stronger beam serves as the pump laser, is reflected sequentially by the ninth, tenth, and eleventh reflectors, and then overlaps with the probe laser in the opposite direction before passing through the first atomic gas cell. The eighth half-wave plate is used to adjust the splitting ratio of the probe laser and the pump laser; the ninth 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. The rubidium atomic gas cell used in this invention can observe the saturated absorption spectrum at room temperature.

[0016] Furthermore, the modulation transfer spectrum stabilization module includes: a tenth half-wave plate, a ninth polarizing beam splitter, a second atomic gas cell, a twelfth reflecting mirror, a tenth polarizing beam splitter, a third photodetector, a thirteenth reflecting mirror, an electro-optic phase modulator, an eleventh 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 ninth polarizing beam splitter: the weaker beam serves as the probe laser, passes through the second atomic gas cell, and is reflected by the twelfth reflecting mirror to the third photodetector; the stronger beam serves as the pump laser, is reflected by the thirteenth reflecting mirror to the electro-optic phase modulator for phase modulation, and is then reflected sequentially by the tenth polarizing beam splitter and the twelfth reflecting mirror before colliding with the probe laser in the opposite direction and passing through the second atomic gas cell. The tenth half-wave plate is used to adjust the splitting ratio of the probe laser and the pump laser. A half-wave plate 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 second atomic gas chamber needs to be controlled at different temperatures. The rubidium atomic gas chamber used in this invention can obtain the modulation transfer spectrum at room temperature.

[0017] Furthermore, this invention establishes a temporal sequence between the atom trapping in the magneto-optical trap, the preparation of the optically pumped atomic energy level, and the detection of the transmission signal from the atomic filter. The pump laser is applied to the cold atoms after the magneto-optical trap is closed and before the detection laser is turned on. The optimal duration of the pump laser's on-time can be determined by the maximum value of the subsequent atomic filter transmittance measurement. This ensures that the entire cold atom cluster is effectively pumped to the desired Zeeman sublevel, while also avoiding significant impacts from the pump laser on the number and temperature of the cold atom cluster, which could affect the detection.

[0018] In this invention, the timing of the cooling laser, re-pumping laser, pump laser, and probe laser being turned on and off is achieved by frequency hopping of the first, second, fourth, and third acousto-optic modulators, respectively. The frequency hopping refers to controlling the frequency of the radio frequency drive signal input to the acousto-optic modulator. If the frequency exceeds the resonant frequency range of the acousto-optic modulator, the acousto-optic modulator will not output diffracted light, and the laser will be turned off. In addition, the timing control of each component in this invention is based on LabVIEW software, but other instrument control software can also be used.

[0019] Furthermore, this invention employs a corresponding beam before detecting the optical rotation signal. 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F' = 2 transition σ + A circularly polarized pump laser is injected into a cold atom cluster, and according to the selection rule Δm... F =m F’ -m F =+1, ground state m F Atoms at smaller energy levels will be gradually pumped to m F At higher energy levels, until finally all atoms are pumped down to level 5. 2 S 1 / 2 F = 2, m F = +2 energy level and remain at this energy level, no longer interacting with σ + The interaction between the circularly polarized pump laser and the target laser completes the state selection for polarized light pumping; however, it is important to note that the target laser... + Circularly polarized pump laser pumped to 5 2 P 3 / 2 Atoms in the F'=2 energy level can potentially transition to the ground state through spontaneous emission. 2 S 1 / 2 At the F=1 energy level, atoms accumulate at that level, causing losses. In this case, the present invention keeps the 780nm re-pumped laser for capturing cold atoms in the magneto-optical trap continuously on, keeping the ground state 5... 2 S 1 / 2 Atoms in the F=1 energy level return to level 5. 2 S 1 / 2 F=2 energy level.

[0020] Furthermore, the optical rotation effect generated by this invention places strict requirements on the magnetic quantum energy levels of the working transition level. The maximum magnetic quantum number of the upper energy level corresponding to the transition of the probe laser cannot be greater than the maximum magnetic quantum number of the lower energy level. This is necessary to introduce absorption asymmetry and thus achieve rotation of the linear polarization plane of the probe laser. The 780nm probe laser corresponding to the transition in this invention (… 87 Rb5 2 S1 / 2 F = 2 to 5 2 P 3 / 2 The maximum magnetic quantum number of the upper energy level of the F'=2 transition is equal to the maximum magnetic quantum number of the lower energy level, which satisfies the requirement.

[0021] Furthermore, unlike hot atom Faraday filters, if the probe laser is applied to cold atoms for a prolonged period, it exerts a force on the cold atoms, causing the cold atom clusters to heat up and diffuse, thus changing the number of cold atoms and the optical thickness. Therefore, this invention cannot obtain the transmission spectrum of the cold atom filter by continuously scanning the probe laser frequency near its resonant frequency; it requires the use of a third acousto-optic modulator at the probe laser resonant frequency (i.e.,...). 87 Rb5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 Frequency shifting near the resonant frequency (F' = 2) produces positive and negative frequency detuning. Transmittance values ​​at multiple discrete frequency points are measured and then combined to plot a complete transmittance variation spectrum. The range of frequency detuning is the natural linewidth of the transition energy level corresponding to the resonant frequency.

[0022] 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 specific 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.

[0023] Furthermore, the polarization directions of the first and second Grange Taylor prisms are orthogonal to each other.

[0024] Furthermore, based on the principle of enhanced polarization polarization, this invention ultimately achieves a nearly 15-fold increase in transmittance of the ultra-narrow bandwidth cold atom filter, from 1.14% to 16.5%.

[0025] Furthermore, the 780nm magneto-optical trap system of the present invention can be replaced with any other wavelength that can achieve cold atom trapping; the 780nm pump laser of the present invention can be replaced with any other wavelength that can achieve atomic energy state preparation according to the selection rule; the 780nm probe laser of the present invention can be replaced with any other transition wavelength that meets the magnetic quantum number requirement of the transition energy level.

[0026] Furthermore, it should be noted that the high-transmission ultra-narrow bandwidth cold atom filter currently realized by the magneto-optical trap in this invention operates in pulse mode. If the cold atoms are prepared by optical agglomeration, the high-transmission ultra-narrow bandwidth cold atom filter can achieve continuous operation.

[0027] Another objective of this invention is to provide a method for implementing a polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter.

[0028] The method for implementing the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of the present invention specifically includes the following steps:

[0029] 1) The laser output from the 780nm cooled laser passes through the first isolator and is then matched and split into two beams by the first half-wave plate and the first polarizing beam splitter. One beam is split again by the second polarizing beam splitter and then transmitted to the first saturated absorption spectrum stabilization module for frequency stabilization. The saturated absorption spectrum signal generated by frequency stabilization is fed back to the 780nm cooled laser for frequency locking. 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F' = 3 transition); another beam is frequency shifted by the first acousto-optic modulator to produce red detuning (10-15MHz) of about one to two times the natural linewidth of the atomic transition, which is the natural linewidth of the aforementioned locked atomic transition; the frequency-shifted laser is called the cooled laser, which is output by the cooled laser frequency stabilization system and is used to decelerate the moving atoms in the vacuum system;

[0030] 2) The laser output from the 780nm re-pumped laser passes through the second isolator and is then 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 second saturable absorption spectrum stabilization module for frequency stabilization, and the saturable absorption spectrum signal generated by frequency stabilization is fed back to the 780nm re-pumped laser for frequency locking. 87 Rb 5 2 S 1 / 2 F = 1 to 5 2 P 3 / 2 F' = 1 transition); another beam is frequency-shifted by 157MHz to 5 by the second acousto-optic modulator. 2 P 3 / 2 F' = 2; The laser after frequency shifting is called a re-pumped laser, which is output by a re-pumped laser frequency stabilization system;

[0031] 3) The frequency-stabilized 780nm cooled laser from step 1) and the frequency-stabilized 780nm re-pumped laser from step 2) are transmitted to the fifth polarization beam splitter for beam combining. The optical power of the combined laser is adjusted by the fifth half-wave plate and the sixth half-wave plate respectively. The combined 780nm cooled laser and 780nm re-pumped laser are coupled from space light to fiber, and then split into six laser beams of completely equal power by a 1-to-6 fiber beam splitter.

[0032] 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; and opposite currents are applied to the first and second gradient magnetic field coils to form an anti-Helmholtz magnetic field with zero magnetic field strength at the intersection center of the six laser beams and linearly increasing towards the edges; under the action of the cooling laser, the re-pumped laser, and the magneto-optical trap formed by the gradient magnetic field, 87 Rb atoms are trapped at the center where six laser beams converge;

[0033] 5) The 780nm probe laser output laser, after passing through the third isolator, is sequentially matched and adjusted by the fourth half-wave plate and the fourth 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. 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F' = 3 transition), that is, locking the frequency to the target transition frequency, and the servo signal generated by locking is fed back to the frequency feedback control ports of the 780nm probe laser; another beam is shifted 267MHz to 5 by the third acousto-optic modulator. 2 P 3 / 2 F' = 2; The frequency-shifted laser mentioned above is called the probe laser, which is output by the probe laser frequency stabilization system;

[0034] 6) The frequency-stabilized 780nm probe laser from step 5) is transmitted to an attenuator to adjust the laser power, and then sequentially passes through the first Glan Taylor prism and the vacuum system containing the laser. 87 Rb cold atom clusters, the second Glan Taylor prism, and then the particles are inserted into the first photodetector;

[0035] 7) The other laser beam generated by the second polarizing beam splitter in step 1) is transmitted to the fourth acousto-optic modulator and frequency-shifted by 267MHz to 5. 2 P 3 / 2 F' = 2; the frequency-shifted laser, used as the pump laser of this invention, passes sequentially through the seventh half-wave plate, the sixth polarizing beam splitter, the beam expander, and the quarter-wave plate before being reflected by a semi-transparent mirror; the σ reflected by the semi-transparent mirror... + Circularly polarized pump laser and 780nm probe laser coincide in opposite directions as they pass through a vacuum system. 87 Rb cold atom group;

[0036] 8) Establish a time sequence between atom trapping in the magneto-optical trap, the preparation of optically pumped atomic energy levels, and the detection of optical rotation effects. By observing the fluorescence signal emitted by the cold atom cluster using an additional fluorescence detector, determine when enough cold atoms have been trapped (i.e., when the fluorescence signal intensity reaches saturation). Then, turn off the current supply to the 780nm cooled laser, the first gradient magnetic field coil, and the second gradient magnetic field coil, releasing the cold atoms. Immediately afterwards, turn on the corresponding... 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / σ of 2F' = 2 transitions + Circularly polarized pump lasers pump all atoms to the target ground state energy level (5). 2 S 1 / 2 The maximum m of F=2) F Magnetic level (m F =+2) on; if σ is used - Laser, corresponding to pumping to the smallest m F At the magnetic level.

[0037] 9) After completing the preparation of the optically pumped atomic level, immediately turn off σ. + Circularly polarized pump laser, open the corresponding 87 Rb5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 A 780nm probe laser with F'=2 transition; located in ground state 5 2 S 1 / 2 F = 2, m F Atoms at the +2 energy level can only absorb σ from 780nm linearly polarized probe laser. - The circular polarization component thus transitions to 5 2 P 3 / 2 F' = 2, m F’ =+1, while σ + The circularly polarized light component is hardly absorbed, and the asymmetric absorption causes the rotation of the polarization plane of the 780nm probe laser, thereby enabling the ultra-narrow bandwidth cold atom filter to rotate the light at a large angle and with high transmittance.

[0038] 10) Using the third acousto-optic modulator in step 5), positive and negative frequency detuning are generated by frequency shifting near the resonant frequency. The transmittance values ​​at multiple frequency points are measured discretely, and then combined to draw the complete transmission spectrum of the high-transmission ultra-narrow bandwidth cold atom filter, realizing a high-transmission ultra-narrow bandwidth cold atom filter with a transmission bandwidth close to the natural linewidth of atomic transition and high transmittance.

[0039] In steps 1) and 2), the saturated absorption spectrum stabilization module consists of an eighth half-wave plate, an eighth reflector, a seventh polarizing beam splitter, a first atomic gas cell, a second photodetector, a ninth reflector, a ninth half-wave plate, a tenth reflector, and an eleventh reflector. The laser incident on the saturated absorption spectrum stabilization module is sequentially split into two beams by the eighth half-wave plate and the seventh 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.

[0040] 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.

[0041] In step 5), the modulation transfer spectrum frequency stabilization module consists of the tenth half-wave plate, the ninth polarization beam splitter, the second atomic gas cell, the twelfth reflector, the tenth polarization beam splitter, the third photodetector, the thirteenth reflector, the electro-optic phase modulator, the eleventh half-wave plate, the mixer, the signal generator, and the proportional-integral-differential locked circuit module. The laser incident on the modulation transfer spectrum stabilization module is split into two beams by the matching adjustment of the 10th half-wave plate and the 9th 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.

[0042] In step 6), the polarization directions of the first Glan Taylor prism and the second Glan Taylor prism are orthogonal to each other.

[0043] In step 7), the seventh half-wave plate and the sixth polarization beam splitter adjust the optical power of the pump laser and convert it into pure linearly polarized light; the beam expander increases the spot area of ​​the pump laser beam, ensuring that the pump laser completely contains cold atom clusters when passing through the vacuum system, thereby improving the optical pumping efficiency; the quarter-wave plate converts the linearly polarized pump laser into standard σ-wave light. + Circularly polarized pump laser.

[0044] In step 8), the preparation of the optically pumped atomic level requires keeping the 780nm heavy-pump laser continuously on, so that the energy level falling to the ground state 5... 2 S 1 / 2 Atoms in the F=1 energy level return to level 5. 2 S 1 / 2 F=2 energy level.

[0045] In steps 8) and 9), the 780nm cooling laser, the 780nm re-pumped laser, and σ... + The timing of the circularly polarized pump laser and the 780nm probe laser being turned on and off is achieved by frequency hopping using the first acousto-optic modulator in step 1), the second acousto-optic modulator in step 2), the fourth acousto-optic modulator in step 7), and the third acousto-optic modulator in step 5), respectively.

[0046] In step 9), the maximum magnetic quantum number of the upper energy level corresponding to the transition of the 780nm probe laser cannot be greater than the maximum magnetic quantum number of the lower energy level. This is necessary to bring about the asymmetry of absorption and thus achieve the rotation of the linear polarization plane of the probe laser.

[0047] Compared with existing transmission bandwidth narrowing techniques for cold atom Faraday filters, the novelty and inventiveness of this invention are reflected in:

[0048] 1. This invention provides a polarization-polarization-enhanced high-transmittance ultra-narrow bandwidth cold atom filter and its implementation method. By using laser cooling technology to slow down atoms, the impact of the Doppler effect on the narrowing of the transmission bandwidth of the atom filter is reduced. Combined with polarization pumping technology, the atoms generate a unidirectional atomic population at the ground-state Zeeman sublevel, far from the Boltzmann distribution. This unidirectional population causes one circularly polarized component of the linearly polarized probe laser to be absorbed, while the other is almost unabsorbed, resulting in a rotation of the polarization plane of the linearly polarized probe laser, forming a larger angle and higher transmittance optical rotation. Based on the above principle of polarization-polarization enhancement, this invention ultimately achieves a nearly 15-fold increase in the transmittance of the cold atom filter, from 1.14% to 16.5%. The implementation of this invention significantly and effectively solves the problem of drastically reduced transmittance due to the loss of atomic numbers in existing cold atom Faraday filter narrowing technologies, realizing a cold atom filter with both ultra-narrow transmission bandwidth and high transmittance. An example of simultaneously applying laser cooling technology and polarized light pumping technology to the construction of atomic filters has yielded outstanding substantial progress and significant transmittance improvements that have not been reported in domestic or international literature and patents.

[0049] 2. This invention utilizes the principle of polarization polarization enhancement to achieve a high-transmission, ultra-narrow bandwidth cold atom filter. Its optical rotation is achieved through circularly polarized laser pumping, resulting in an asymmetric population of atoms at the ground-state Zeeman sublevels, rather than through a magnetic field causing the splitting of Zeeman sublevels. Therefore, this invention's high-transmission, ultra-narrow bandwidth cold atom filter, achieved using the principle of polarization polarization enhancement, does not require the application of an optical rotation magnetic field, a significant technological innovation distinguishing it from traditional Faraday atom filters.

[0050] 3. This invention not only provides a creative new approach to realizing atomic filters with both ultra-narrow transmission bandwidth and high transmittance, 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

[0051] Figure 1 This is a schematic diagram of the structure of an embodiment of the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of the present invention;

[0052] Wherein: 1—Cooled laser frequency stabilization system, 2—Re-pumped laser frequency stabilization system, 3—Detector laser frequency stabilization system, 4—Fifth half-wave plate, 5—Sixth half-wave plate, 6—Fifth polarization beam splitter, 7—First fiber 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 11—Second gradient magnetic field coil, 12a—Second fiber optic coupler input, 12b—Second fiber optic coupler output, 13—Attenuator, 14—First GlanTell prism, 15—Second GlanTell prism, 16—First photodetector, 17—Sixth reflector, 18—Fourth acousto-optic modulator, 19—Seventh reflector, 20—Seventh half-wave plate, 21—Sixth polarizing beam splitter, 22—Beam expander, 23—Quarter-wave plate, 24—Half-transparent and half-reflective mirror;

[0053] 101—780nm cooled laser, 102—first isolator, 103—first half-wave plate, 104—first polarizing beam splitter, 105—second half-wave plate, 106—first reflector, 107—second polarizing beam splitter, 108—first saturated absorption spectrum frequency stabilization module, 109—first acousto-optic modulator;

[0054] 201—780nm heavy-pumped laser, 202—second isolator, 203—third half-wave plate, 204—third polarization beam splitter, 205—second mirror, 206—second saturated absorption spectrum frequency stabilization module, 207—second acousto-optic modulator, 208—third mirror;

[0055] 301—780nm detector laser, 302—third isolator, 303—fourth half-wave plate, 304—fourth polarization beam splitter, 305—fourth reflector, 306—modulation transfer spectrum frequency stabilization module, 307—fifth reflector, 308—third acousto-optic modulator.

[0056] Figure 2 This is a schematic diagram of the structure of the (first and second) saturated absorption spectrum stabilization modules in the embodiment of the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of the present invention;

[0057] Among them: 401—eighth half-wave plate, 402—eighth reflecting mirror, 403—seventh polarizing beam splitter, 404—first atomic gas cell, 405—eighth polarizing beam splitter, 406—second photodetector, 407—ninth reflecting mirror, 408—ninth half-wave plate, 409—tenth reflecting mirror, 410—eleventh reflecting mirror.

[0058] Figure 3 This is a schematic diagram of the modulation transfer spectrum stabilization module in an embodiment of the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of the present invention;

[0059] Among them: 501—Tenth half-wave plate, 502—Ninth polarizing beam splitter, 503—Second atomic gas cell, 504—Twelfth reflecting mirror, 505—Tenth polarizing beam splitter, 506—Third photodetector, 507—Thirteenth reflecting mirror, 508—Electro-optic phase modulator, 509—Eleventh half-wave plate, 510—Mixer, 511—Signal generator, 512—Proportional-integral-differential locked circuit module.

[0060] Figure 4 The 780nm cooled laser, 780nm re-pumped laser, and σ laser are used in the embodiment of the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of the present invention. + Energy level structure diagrams of circularly polarized pump lasers and 780nm probe lasers.

[0061] Figure 5 The figure shows the test results of the cold atom filter in the embodiment of the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of the present invention, which shows a nearly 15-fold increase in transmittance from 1.14% to 16.5%. Detailed Implementation

[0062] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0063] like Figure 1 The polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of 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 fifth half-wave plate 4, a sixth half-wave plate 5, a fifth polarization beam splitter 6, a first fiber coupler 7, a one-to-six fiber beam splitter 8, a first fiber collimator 9a, a second fiber collimator 9b, a third fiber collimator 9c, a fourth fiber collimator 9d, a fifth fiber collimator 9e, a sixth fiber collimator 9f, and a fifth fiber collimator 9e. A gradient magnetic field coil 10a, a second gradient magnetic field coil 10b, a vacuum system 11, a second fiber optic coupler input 12a, a second fiber optic coupler output 12b, an attenuator 13, a first Glan Taylor prism 14, a second Glan Taylor prism 15, a first photodetector 16, a sixth reflector 17, a fourth acousto-optic modulator 18, a seventh reflector 19, a seventh half-wave plate 20, a sixth polarizing beam splitter 21, a beam expander 22, a quarter-wave plate 23, and a semi-transparent, semi-reflective mirror 24. The cooled laser frequency stabilization system 1 further includes: a 780nm cooled laser 101, a first isolator 102, a first half-wave plate 103, a first polarizing beam splitter 104, a second half-wave plate 105, a first reflector 106, a second polarizing beam splitter 107, a first saturated absorption spectrum frequency stabilization module 108, and a first acousto-optic modulator 109; the re-pumped laser frequency stabilization system 2 further includes: a 780nm re-pumped laser 201, a second isolator 202, and a third half-wave plate 203. 03, third polarization beam splitter 204, second reflector 205, second saturated absorption spectrum stabilization module 206, second acousto-optic modulator 207, third reflector 208; the probe laser stabilization system 3 further includes: 780nm probe laser 301, third isolator 302, fourth half-wave plate 303, fourth polarization beam splitter 304, fourth reflector 305, modulation transfer spectrum stabilization module 306, fifth reflector 307, and third acousto-optic modulator 308.

[0064] In the cooled laser frequency stabilization system 1, the output laser from the 780nm cooled laser 101 is first transmitted to the first isolator 102. After the first isolator 102, the first half-wave plate 103 and the first polarizing beam splitter 104 sequentially match and adjust the splitting power, splitting the laser into two beams. One beam is split again by the second polarizing beam splitter 107 and then transmitted to the first saturated absorption spectrum frequency stabilization module 108 for frequency stabilization. The saturated absorption spectrum signal generated by frequency stabilization is fed back to the 780nm cooled laser 101 for frequency locking. 87 Rb5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2F' = 3 transition); another beam is frequency shifted by the first acousto-optic modulator 109 to generate red detuning (10-15MHz) of about one to two times the natural linewidth of the atomic transition, which is the natural linewidth of the aforementioned locked atomic transition; the frequency-shifted laser is called the cooling laser, which is output by the cooling laser frequency stabilization system 1 and is used to decelerate the moving atoms in the vacuum system 11; the second half-wave plate 105 is used to adjust the beam splitting ratio of the second polarization beam splitter 107.

[0065] In the re-pumped laser frequency stabilization system 2, the output laser from the 780nm re-pumped laser 201 is first transmitted to the second isolator 202. After the second isolator 202, the third half-wave plate 203 and the third polarizing beam splitter 204 sequentially match and adjust the splitting power, dividing the laser into two beams: one beam is reflected by the second mirror 205 and then transmitted to the second saturated absorption spectrum frequency stabilization module 206 for frequency stabilization. The saturated absorption spectrum signal generated by frequency stabilization is fed back to the 780nm re-pumped laser 201 for frequency locking. 87 Rb 5 2 S 1 / 2F = 1 to 5 2 P 3 / 2 F' = 1 transition); another beam is frequency-shifted by 157MHz to 5MHz via the second acousto-optic modulator 207. 2 P 3 / 2 F' = 2, used to cause the vacuum system 11 to fall into the ground state 5 under the action of the cooling laser. 2 S 1 / 2 Atoms at the F=1 energy level can return to the cooling cycle and contribute again; the laser after frequency shifting is called a re-pumped laser, which is output by the re-pumped laser frequency stabilization system 2.

[0066] The 780nm cooled laser, after being frequency-shifted by the first acousto-optic modulator 109, and the 780nm re-pumped laser, reflected by the third mirror 208, are transmitted to the fifth polarization beam splitter 6 for beam combining. The optical power of the combined laser is adjusted by the fifth half-wave plate 4 and the sixth half-wave plate 5, respectively. The combined 780nm cooled laser and the re-pumped 780nm laser are coupled into the optical fiber by the first fiber coupler 7, and then split into six laser beams of equal power by the one-to-six fiber beam splitter 8.

[0067] The six laser beams of 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 9e. Opposite currents 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 zero magnetic field strength at the intersection of the six laser beams and linearly increasing towards the edges. The vacuum system 11 contains a rarefied... 87Rb atomic vapor is trapped in a magneto-optical trap formed by a cooling laser, a re-pumped laser, and a gradient magnetic field, with the atoms converging at the center of the six laser beams.

[0068] The 780nm probe laser 301 in the probe laser frequency stabilization system 3 first transmits its output laser to the third isolator 302. After the third isolator 302, the fourth half-wave plate 303 and the fourth polarization beam splitter prism 304 sequentially match and adjust the splitting power, splitting the laser into two beams: one beam is reflected by the fourth mirror 305 and then transmitted to the modulation transfer spectrum frequency stabilization module 306 for precise laser frequency locking. 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F' = 3 transition), the servo signal generated by the lock is fed back to the frequency feedback control ports of the 780nm probe laser 301; another beam is reflected by the fifth mirror 307 and transmitted to the third acousto-optic modulator 308, which shifts the frequency by 267MHz to 5. 2 P 3 / 2 F' = 2, and the laser after frequency shifting is called the probe laser, which is output by the probe laser frequency stabilization system.

[0069] The 780nm probe laser, after frequency shifting by the third acousto-optic modulator 308, is first coupled into the optical fiber through the input end 12a of the second optical fiber coupler, and after transmission through the optical fiber, it is output from the output end 12b of the second optical fiber coupler. The 780nm probe laser output from the output end 12b of the second optical fiber coupler is transmitted to the attenuator 13 to adjust the laser power, and then sequentially passes through the first Glan Taylor prism 14 and is trapped in the vacuum system 11. 87 The Rb cold atom clusters and the second GlanTeller prism 15 are then received by the first photodetector 16; the polarization directions of the first GlanTeller prism 14 and the second GlanTeller prism 15 are orthogonal to each other. The first photodetector 16 is used to detect the transmission spectral lines of the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter.

[0070] The second laser beam generated by the second polarization beam splitter 107 in the cooled laser frequency stabilization system 1 is reflected by the sixth reflecting mirror 17 and then transmitted to the fourth acousto-optic modulator 18 for frequency shifting of 267MHz. The frequency shifted laser frequency corresponds to... 87 Rb 5 2 S 1 / 2F = 2 to 5 2 P 3 / 2 F' = 2 transition; the frequency-shifted laser is reflected by the seventh mirror 19 as the pump laser of the present invention, and then passes sequentially through the seventh half-wave plate 20, the sixth polarizing beam splitter 21, the beam expander 22, and the quarter-wave plate 23 before being reflected by the semi-transparent mirror 24; the σ reflected by the semi-transparent mirror 24... +The circularly polarized pump laser and the 780nm probe laser coincide in opposite directions and pass through the vacuum system 11. 87 Rb cold atom group.

[0071] A time sequence is established between the atom trapping in the magneto-optical trap, the preparation of optically pumped atomic energy levels, and the detection of optical rotation effects. After capturing a sufficient number of cold atoms, the current supply to the 780nm cooled laser 101, the first gradient magnetic field coil 10a, and the second gradient magnetic field coil 10b is turned off, releasing the cold atoms; then, the corresponding... 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / σ of 2F' = 2 transitions + Circularly polarized pump lasers pump all atoms to their ground state 5. 2 S 1 / 2 F = 2, m F =+2 on the sub-level.

[0072] After the optically pumped atomic level was prepared, the σ level was immediately shut off. + Circularly polarized pump laser, open the corresponding 87 Rb5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 780nm probe laser 301 with F'=2 transition; located in ground state 5 2 S 1 / 2 F = 2, m F Atoms at the +2 energy level can only absorb σ from 780nm linearly polarized probe laser. - The circular polarization component thus transitions to 5 2 P 3 / 2 F' = 2, m F’ =+1, while σ + The circularly polarized light component is hardly absorbed, and the asymmetric absorption causes the polarization plane of the 780nm probe laser to rotate, thereby achieving optical rotation with a large angle and high transmittance in the ultra-narrow bandwidth cold atom filter.

[0073] By using the third acousto-optic modulator 308 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 high-transmission ultra-narrow bandwidth cold atom filter is plotted. The present invention ultimately realizes a high-transmission ultra-narrow bandwidth cold atom filter with a transmission bandwidth close to the natural linewidth of atomic transition and high transmittance.

[0074] like Figure 2The saturated absorption spectrum stabilization modules 108 and 206 in the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of this embodiment include: an eighth half-wave plate 401, an eighth reflector 402, a seventh polarization beam splitter 403, a first atomic gas cell 404, an eighth polarization beam splitter 405, a second photodetector 406, a ninth reflector 407, a ninth half-wave plate 408, a tenth reflector 409, and an eleventh reflector 410.

[0075] The laser incident on the saturated absorption spectrum stabilization modules 108 and 206 is reflected by the eighth reflector 402 and then matched and regulated by the eighth half-wave plate 401 and the seventh 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 eighth polarization beam splitter 405 to the second photodetector 406; the stronger beam serves as the pump laser, is reflected by the ninth reflector 407, the tenth reflector 409, and the eleventh reflector 410, and then coincides with the probe laser in the opposite direction before passing through the first atomic gas cell 404; the ninth 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.

[0076] like Figure 3 The modulation transfer spectrum stabilization module 306 in the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter of this embodiment includes: a tenth half-wave plate 501, a ninth polarization beam splitter 502, a second atomic gas cell 503, a twelfth reflector 504, a tenth polarization beam splitter 505, a third photodetector 506, a thirteenth reflector 507, an electro-optic phase modulator 508, an eleventh half-wave plate 509, a mixer 510, a signal generator 511, and a proportional-integral-differential locked circuit module 512.

[0077] 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 tenth half-wave plate 501 and the ninth 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 twelfth mirror 504 to the third photodetector 506. The stronger beam serves as the pump laser, is reflected by the thirteenth mirror 507 to the electro-optic phase modulator 508 for phase modulation, and is then sequentially reflected by the tenth polarization beam splitter 505 and the twelfth mirror 504, coinciding with the probe laser in the opposite direction before passing through the second atomic gas cell 506. 3; The eleventh 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.

[0078] like Figure 4 In this embodiment, the energy level transition corresponding to the 780nm cooling laser in the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter is: 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F'=3 transition (red detuning 10-15MHz); the energy level transition corresponding to the 780nm re-pumped laser is 87 Rb 5 2 S 1 / 2 F = 1 to 5 2 P 3 / 2 F' = 2 transition; σ + The energy level transition corresponding to circularly polarized pump laser is 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F' = 2 transition; the energy level transition corresponding to the 780nm probe laser is: 87 Rb 5 2 S 1 / 2 F = 2 to 5 2 P 3 / 2 F' = 2 transition.

[0079] like Figure 5The polarization-enhanced high-transmittance ultra-narrow bandwidth cold atom filter in this embodiment achieved a transmittance of only 1.14% before polarization pumping, and after polarization pumping, the transmittance increased to 16.5%, which is nearly 15 times the transmittance improvement.

[0080] Specifically, the polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter in this embodiment of the invention is characterized by combining cold atoms with an atomic filter. Laser cooling technology is used to slow down the atoms, reducing the impact of the Doppler effect on narrowing the transmission bandwidth of the atomic filter. Furthermore, polarization pumping technology is used to create a unidirectional atomic population at the ground-state Zeeman sublevel, far from the Boltzmann distribution. This unidirectional population causes one circularly polarized component of the linearly polarized probe laser to be absorbed, while the other is almost unabsorbed. This results in a rotation of the polarization plane of the linearly polarized probe laser, forming a large-angle, high-transmittance optical rotation. The implementation of this invention significantly and effectively solves the problem of drastically reduced transmittance due to the loss of atomic numbers in existing cold atom Faraday filters, achieving a cold atom filter that simultaneously possesses ultra-narrow transmission bandwidth and high transmittance. Furthermore, the high-transmission ultra-narrow bandwidth cold atom filter achieved by the polarization-enhanced principle in this invention has another advantage: its optical rotation is achieved through the asymmetric population of atoms in the ground-state Zeeman sublevels induced by circularly polarized laser pumping, rather than by the splitting of Zeeman sublevels caused by a magnetic field. Therefore, the high-transmission ultra-narrow bandwidth cold atom filter achieved by the polarization-enhanced principle in this invention does not require the application of an optical rotation magnetic field, which is a significant technological innovation distinguishing it from traditional Faraday atom filters. Finally, this invention not only provides a creative new approach to achieving atomic filters with both ultra-narrow transmission bandwidth and high transmittance, but also extends the research of traditional hot atom Faraday filters to the cold atom field, providing new possibilities for substantial and significant progress and construction of subsequent cold atom-based filters, and bringing highly promising developments to many fields. In this respect, this invention is fundamentally different from current cold atom filter transmission bandwidth narrowing techniques.

[0081] 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.

[0082] The seventh half-wave plate and the sixth polarization beam splitter are used to adjust the optical power of the pump laser and to convert the pump laser into pure linearly polarized light; the beam expander is used to increase the spot area of ​​the pump laser beam so that the pump laser can completely contain cold atom clusters when passing through the vacuum system, thereby improving the optical pumping efficiency; the quarter-wave plate is used to convert the linearly polarized pump laser into standard σ-wave light. + Circularly polarized pump laser.

[0083] In this invention, the fabrication of optically pumped atomic levels requires keeping the 780nm heavy-pump laser continuously on, causing the energy level to drop to the ground state 5. 2 S 1 / 2 Atoms in the F=1 energy level return to level 5. 2 S 1 / 2 F=2 energy level.

[0084] The maximum magnetic quantum number of the upper energy level corresponding to the transition of the 780nm probe laser in this invention cannot be greater than the maximum magnetic quantum number of the lower energy level. This is necessary to bring about absorption asymmetry and thus achieve the rotation of the linear polarization plane of the probe laser.

[0085] The timing control of each component in this invention is implemented using LabVIEW software, but other instrument control software can also be used; for the 780nm cooling laser, 780nm re-pumped laser, σ... + The timing of the circularly polarized pump laser and the 780nm probe laser being turned on and off is achieved by frequency hopping using the first, second, fourth, and third acousto-optic modulators, respectively.

[0086] The high-transmission ultra-narrow bandwidth cold atom filter based on magneto-optical trap currently realized by this invention operates in pulse mode. If the cold atoms are prepared by optical agglomeration, the high-transmission ultra-narrow bandwidth cold atom filter can achieve continuous operation.

[0087] 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 realize a cesium atom magneto-optical trap), or replacing the 780nm pump laser with any other wavelength capable of atomic energy state preparation according to selection rules, or replacing the transition energy level corresponding to the 780nm probe laser with other transition energy levels that can satisfy the magneton energy level requirements of the optical rotation effect for the working transition energy level. 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. A polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter, 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), a pumped laser system, 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 atom transition energy level of the atom. The target atom transition energy level includes the target upper energy level and the target ground state energy level. The pump laser system is used to output a pump laser, wherein the pump laser is σ. + or σ - Circularly polarized laser; the pump laser frequency is the transition frequency from the target ground state energy level to an upper energy level with a magnetic quantum number greater than that of 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 atom transition energy level when there are other ground state energy levels in the ground state of the atoms in the vacuum system besides 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 maximum magnetic quantum number of the upper energy level corresponding to the probe laser transition cannot be greater than the maximum magnetic quantum number of the lower energy level. The probe laser is a linearly polarized laser. The cooling laser and the re-pumped laser are combined by the fifth polarization beam splitter (6) and then coupled into a one-to-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 sequentially through the first Glan Taylor prism (14), the cold atom cluster trapped in the vacuum system (11), and the second Glan Taylor prism (15) before being received by the first photodetector (16). The polarization directions of the first Glan Taylor prism (14) and the second Glan Taylor prism (15) are orthogonal to each other. The pump laser and the probe laser coincide in opposite directions as they pass through the cold atom cluster trapped in the vacuum system (11), causing the atoms uniformly distributed on the target ground state energy level to produce a unidirectional atomic population. Among them, the unidirectionally populated atoms on the target ground state energy level can only absorb the σ in the linearly polarized probe laser. – or σ + The circularly polarized light component then transitions to the corresponding upper energy level of the probe laser, thereby achieving the rotation of the linear polarization plane of the probe laser. The first photodetector (16) is used to detect the transmission spectral lines of a polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter.

2. The polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter according to claim 1, characterized in that, The cooled laser frequency stabilization system (1) includes a cooled laser, a first isolator, a first half-wave plate, a first polarizing beam splitter, a second polarizing beam splitter, a first saturated absorption spectrum frequency stabilization module, and a first acousto-optic modulator. The laser output from the cooled laser is incident on the first polarizing beam splitter via the first isolator and the first half-wave plate and split into two beams. One beam is split again by the second polarizing beam splitter and then transmitted to the first saturated absorption spectrum frequency stabilization module for frequency stabilization. The saturated absorption spectrum signal generated by frequency stabilization is fed back to the cooled laser for frequency locking. The other beam obtained by splitting the first beam by the first polarizing beam splitter is frequency shifted by the first acousto-optic modulator and then used as the cooled laser output.

3. The polarization-enhanced high-transmission ultra-narrow bandwidth cold atom 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 atomic transition energy level of the atom.

4. The polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter according to claim 2, characterized in that, The second polarization beam splitter generates another laser beam, which is frequency-shifted by the fourth acousto-optic modulator to obtain the pump laser. The pump laser is sequentially incident on the semi-transparent mirror through the seventh half-wave plate, the sixth polarization beam splitter, the beam expander, and the quarter-wave plate. The semi-transparent mirror reflects the pump laser into the vacuum system (11) and overlaps with the probe laser in the opposite direction.

5. The polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter according to claim 1, characterized in that, The re-pumped laser frequency stabilization system (2) includes a re-pumped laser, a second isolator, a third half-wave plate, a third polarizing beam splitter, a second reflector, a second saturated absorption spectrum frequency stabilization module, and a second acousto-optic modulator. The laser output from the re-pumped laser is incident on the third polarizing beam splitter after passing through the second isolator and the third half-wave plate in sequence. The laser beam is then split into two beams: one beam is reflected by the second reflector and transmitted to the second saturated absorption spectrum frequency stabilization module for frequency stabilization. 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 second acousto-optic modulator to obtain the re-pumped laser.

6. The polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter according to claim 1, characterized in that, The probe laser frequency stabilization system (3) includes a probe laser, a third isolator, a fourth half-wave plate, a fourth polarization beam splitter, a fourth reflector, a modulation transfer spectrum frequency stabilization module, a fifth reflector, and a third acousto-optic modulator. The laser output from the probe laser is incident on the fourth polarization beam splitter through the third isolator and the fourth half-wave plate in sequence, and split into two beams: one beam is reflected by the fourth reflector and transmitted to the modulation transfer spectrum frequency stabilization module for laser frequency locking, and the servo signal generated by locking is fed back to each frequency feedback control port of the probe laser; the other beam is reflected by the fifth reflector and transmitted to the third acousto-optic modulator for frequency shifting to obtain the probe laser.

7. The polarization-enhanced high-transmission ultra-narrow bandwidth cold atom 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.

8. A method for implementing a polarization-enhanced high-transmission ultra-narrow bandwidth cold atom filter, 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 atom transition energy level of the atom. The target atom transition energy level includes the target upper energy level and the target ground state energy level. 2) A pump laser is output using a pump laser system; the pump laser is used to induce a unidirectional atomic population by optically pumping atoms uniformly distributed on the target ground state energy level; the pump laser is σ. + or σ - Circularly polarized laser; the pump laser frequency is the transition frequency from the target ground state energy level to an upper energy level with a magnetic quantum number greater than that of the target ground state energy level; 3) The heavy pump laser is output by the heavy pump laser frequency stabilization system (2) when there are other ground state energy levels in the vacuum system other than the target ground state energy level. When the atoms that have transitioned to the target upper energy level by the cooling laser transition to other ground state energy levels other than the target ground state energy level through spontaneous emission, the atoms on other ground state energy levels can return to the target upper energy level and participate in the cooling cycle corresponding to the target atom transition energy level. 4) The cooling laser and the re-pumped laser are combined by the fifth polarization beam splitter (6) and then coupled into a six-beam fiber splitter (8); the six laser beams output by the six-beam fiber 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 pairs, 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 magneto-optical trap formed by the cooling laser, the re-pumped laser and the gradient magnetic field; 5) The laser output from the probe laser of the probe laser frequency stabilization system (3) is sequentially incident on the fourth polarization beam splitter through the third isolator and the fourth half-wave plate, and then split into two beams: one beam is transmitted to the modulation transfer spectrum frequency stabilization module for laser frequency locking, and the servo signal generated by locking is fed back to each frequency feedback control port of the probe laser; the other beam is frequency shifted by the third acousto-optic modulator to obtain the probe laser, which is used to detect atoms located on the target ground state energy level. The maximum magnetic quantum number of the upper energy level corresponding to the probe laser transition cannot be greater than the maximum magnetic quantum number of the lower energy level; wherein, the probe laser is a linearly polarized laser; the probe laser sequentially passes through the first Glan Taylor prism (14), the cold atom cluster trapped in the vacuum system (11), and the second Glan Taylor prism (15) before being received by the first photodetector (16); the polarization directions of the first Glan Taylor prism (14) and the second Glan Taylor prism (15) are orthogonal to each other; 6) The pump laser in step 2) passes through the seventh half-wave plate (20), the sixth polarization beam splitter (21), the beam expander (22), and the quarter-wave plate (23) in sequence and is reflected by the semi-transparent and semi-reflective mirror (24); the circularly polarized pump laser reflected by the semi-transparent and semi-reflective mirror (24) coincides with the probe laser in opposite directions and passes through the cold atom cluster trapped in the vacuum system. 7) Establish a time sequence between the atom trapping in the magneto-optical trap, the preparation of the optically pumped atom energy level, and the detection of the transmission signal through the atom filter; observe the fluorescence signal emitted by the cold atom cluster, and when the fluorescence signal intensity reaches saturation, turn off the current supply to the cooling laser, the first gradient magnetic field coil, and the second gradient magnetic field coil to release the cold atoms; then turn on the pump laser to pump the atoms to the largest or smallest magneton energy level in the target ground state energy level; 8) After the optical pump atomic level is prepared, turn off the pump laser and turn on the probe laser; 9) By using a third 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 measured discretely, and then combined to draw the complete transmission spectrum of the high-transmission ultra-narrow bandwidth cold atom filter; wherein, the resonance frequency corresponds to the transition frequency of the atom from the target ground state energy level to the upper energy level with a magnetic quantum number less than or equal to the magnetic quantum number of the target ground state energy level.

9. The method according to claim 8, 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.

10. The method according to claim 8, characterized in that, The probe laser frequency stabilization system (3) includes a probe laser, a third isolator, a fourth half-wave plate, a fourth polarization beam splitter, a fourth reflector, a modulation transfer spectrum frequency stabilization module, a fifth reflector, and a third acousto-optic modulator. The laser output from the probe laser is sequentially incident on the fourth polarization beam splitter via the third isolator and the fourth half-wave plate, splitting into two beams: one beam is reflected by the fourth reflector and transmitted to the modulation transfer spectrum frequency stabilization module for laser frequency locking, and the servo signal generated by locking is fed back to each frequency feedback control port of the probe laser; the other beam is reflected by the fifth reflector and transmitted to the third acousto-optic modulator, and the third acousto-optic modulator shifts the frequency near the locked atomic resonance frequency to generate positive and negative frequency detuning to obtain the probe laser; wherein, the range of the frequency detuning is the natural linewidth of the transition energy level corresponding to the atomic resonance frequency.

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