CN112098330B - Atomic concentration measuring device and method for alkali metal vapor laser - Google Patents
Atomic concentration measuring device and method for alkali metal vapor laser Download PDFInfo
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Abstract
The invention belongs to the technical field of photoelectric measurement, and particularly relates to an atomic concentration measuring device and method based on an alkali metal vapor laser, wherein the device comprises a light splitting module, a detection module and a calibration module; the light splitting module is used for splitting the probe light into three beams, wherein the two beams of light are respectively used as detection light and reference light to enter the detection module; and the third beam of light enters the calibration module as calibration light, and the detection light is monitored to determine whether the wavelength of the laser is in the transition line type of the alkali metal atoms. The invention has the advantages that: using nS1/2‑(n+1)P1/2、3/2The energy level transition wavelength of the optical fiber is used for detecting the atomic concentration of the gain medium in the alkali metal vapor laser, compared with the traditional method of using nS1/2‑nP1/2、3/2The energy level transition wavelength detection has smaller absorptivity, can realize linear direct detection on the atomic concentration in the vapor pool under high temperature and high pressure, and is favorable for better analyzing the performance of the laser.
Description
Technical Field
The invention belongs to the technical field of photoelectric measurement, and particularly relates to an atomic concentration measuring device and method for an alkali metal vapor laser.
Background
A semiconductor-pumped Alkali-metal vapor Laser (DPAL), which was proposed in 2001 by the bevermol laboratory W Krupke in usa, has the characteristics of full electric drive, high quantum efficiency, gas circulation flow heat dissipation and single-aperture scaling amplification, and has become a next-generation high-energy Laser. The atomic concentration in alkali metal vapor, which is one of the most basic parameters of DPAL, directly affects the power and operating state of the laser, if compared to solid-state lasers and fiber lasers, corresponding to the doping concentration of rare-earth ions.
The most common methods currently used to determine the atomic concentration in alkali metal vapors are: (1) estimated by the saturated vapor pressure equation and the ideal gas law. The saturated vapor pressure formula is an empirical formula, and means that at a certain temperature, the vapor of the substance exists on the surface of the liquid (solid), and when the pressure of the vapor is equal to the pressure on the surface of the liquid (solid), the vapor is in an equilibrium state, and the pressure is called the saturated vapor pressure. According to the principle, the saturated vapor pressure formula is a stable formula and can be used only near the liquid level of gas-liquid coexistence, in actual work, alkali metal vapor needs to flow rapidly under the condition of high-power pumping so as to dissipate heat, so that the conditions frequently deviate from the saturated vapor pressure formula, the particle number density distribution is uneven, the absorption of vapor medium to pumping light is reduced, the working efficiency of a laser is influenced, and the saturated vapor pressure formula is not applicable any more; (2) the atomic concentration is detected by taking a 795nm or 780nm tunable single-frequency laser as detection light, and because the absorption rate of alkali metal vapor to the two wavelengths is too large, when the spectrum is strictly aligned, probe light is difficult to pass through an alkali metal vapor pool, so that a transmittance curve cannot be measured, the light source is often required to be greatly detuned to carry out wide spectrum scanning in actual measurement, so that far wings of absorption lines are swept out, and then the atomic concentration is fitted by a uniform widening formula. This presents great difficulties for practical measurements, high requirements on the light source and limits the speed of the measurement. (3) And (3) adding a uniform electric field outside the alkali metal vapor pool, and measuring the Faraday rotation angle by utilizing the Faraday magneto-optical effect to obtain the corresponding atomic concentration in the alkali metal vapor. This is difficult to achieve in practical applications of DPAL. (4) Since the wavelength of the probe light is in the vicinity of the absorption line of the alkali metal vapor, it is strongly absorbed and shows a very strong nonlinearity, and therefore the atomic concentration in the alkali metal vapor is determined by measuring the refractive index thereof by an interferometer. But this approach is inconvenient to implement in practical applications.
Disclosure of Invention
In order to accurately measure the atomic concentration in the alkali metal vapor, the invention provides a novel atomic concentration measuring device and method, which utilize the alkali metal atom nS1/2-(n+1)P1/2、3/2And (3) taking a narrow-linewidth laser with corresponding wavelengths (lithium atoms when n is 4, rubidium atoms when n is 5, cesium atoms when n is 6 and 459 nm) obtained by energy level transition as a probe light source, obtaining the absorption transmittance of the alkali metal vapor pool by a two-dimensional light beam scanning mode, and finally calculating the concentration of the alkali metal vapor atoms by using the beer lambert law.
The technical scheme adopted by the invention is as follows:
an atomic concentration measuring device for an alkali metal vapor laser comprises a light splitting module 1, a detection module 2 and a calibration module 3; the light splitting module 1 is used for splitting the probe light into three beams, wherein two beams of light respectively serve as detection light and reference light to enter the detection module 2, the detection light is used for direct measurement, and the reference light is used for eliminating the noise background of a light source; the third light enters the calibration module 3 as calibration light, and the detection light is monitored to determine whether the wavelength of the laser is within the transition line of the alkali metal atom.
The light splitting module 1 comprises a narrow linewidth semiconductor laser 11, a first half-wave plate 12, a first polarization beam splitter prism 13 and a thin film beam splitter 14; the narrow-linewidth semiconductor laser 11 emits linearly polarized laser, the linearly polarized laser is converted into circularly polarized light through the first half-wave plate 12, the circularly polarized light is divided into two beams of vertical line polarized light and parallel line polarized light with the same light intensity by the first polarization splitting prism 13, the vertical line polarized light is used as detection light and enters the detection module 2, the transmission direction of the parallel line polarized light is deflected for 90 degrees, the transmission light passing through the film beam splitter 14 is used as calibration light and enters the calibration module 3, and the reflected light after being reflected by the film beam splitter 14 is used as reference light and enters the detection module 2;
the detection module 2 comprises a second half-wave plate 21, a second polarization splitting prism 22, a quarter-wave plate 23, a scanning device 24, an alkali metal vapor pool 25 to be detected, a plane mirror 26, a first right-angle prism 27, a first attenuation plate 28A, a first convex lens 29A, a second attenuation plate 28B, a second convex lens 29B, a balanced amplification detector 210 and a data acquisition module 211; the vertical linear polarized light obtained by the first polarization splitting prism 13 in the light splitting module 1 is used as detection light, is changed into circular polarized light again after passing through the second half-wave plate 21 and then passes through the second polarization splitting prism 22 to obtain parallel linear polarized light, the parallel linear polarized light is deflected for 180 degrees in the deflection direction through the quarter-wave plate 23 to obtain vertical linear polarized light, the vertical linear polarized light is scanned by the scanning device 24 and enters the alkali metal vapor pool 25 to be detected, the vertical linear polarized light is reflected back when encountering the plane reflecting mirror 26 after passing through the alkali metal vapor pool 25 to be detected, and double optical path is realized, so that the resolution of the measured transmittance is improved; the reflected light passes through the alkali metal vapor pool 25 to be detected, the scanning device 24 and the quarter wave plate 23 again, due to the effect of the quarter wave plate 23, the returned light is changed into parallel linear polarized light to be transmitted through the second polarization beam splitter prism 22, the light intensity is weakened through the first attenuation sheet 28A after the parallel linear polarized light is deflected for 90 degrees in the transmission direction of the first right-angle prism 27, and then the light is focused through the first convex lens 29A and then is input into the balanced amplification detector 210 through the first input port in the balanced amplification detector 210 for detection; meanwhile, reflected light obtained after being reflected by the thin film beam splitter 14 in the light splitting module 1 is used as reference light, the light intensity is weakened through the second attenuation sheet 28B, the reference light is focused through the second convex lens 29B and then is input into the balanced amplification detector 210 through the second input port of the balanced amplification detector 210, the first output port and the second output port of the balanced amplification detector 210 are respectively connected with two signal input ends in a DAQ data acquisition card 211-1 in the data acquisition module 211, and data acquired through the DAQ data acquisition card 211-1 are displayed on a display screen of a computer 211-2 in the data acquisition module 211; the balanced amplification detector 210 is configured to convert the reference optical signal and the detection optical signal into electrical signals, and output the electrical signals to the data acquisition module 211; the data acquisition module 211 comprises a DAQ data acquisition card 211-1 and a computer 211-2, and is used for acquiring signals of the balanced amplification detector 210 and displaying a two-dimensional image.
The scanning device 24 consists of a two-dimensional galvanometer 24-1 and a concave paraboloid reflector 24-2, when a light beam passes through the two-dimensional galvanometer 24-1, an x-axis scanning head and a y-axis scanning head of the two-dimensional galvanometer 24-1 are controlled to move along the x-axis direction and the y-axis direction respectively to realize two-dimensional scanning, and the light beam is reflected by the concave paraboloid reflector 24-2 to form a parallel light beam to enter the alkali metal vapor pool 25 so as to realize the real-time monitoring of the atomic concentration in the alkali metal vapor pool 25 to be detected; the two-dimensional galvanometer 24-1 can also be replaced by a rotating mirror, an acousto-optic deflector, a digital micromirror array, a spatial light phase modulator or a mode of forming a large light beam by an optical system and detecting the whole distribution of light intensity by a camera.
The calibration module 3 comprises a second right-angle prism 31 and a vacuum vapor pool 32; transmitted light obtained in the thin film beam splitter 14 in the light splitting module 1 is transmitted through the second right-angle prism 31 to turn an optical path and then enters the vacuum vapor pool 32, and after the vacuum vapor pool 32 is heated, fluorescence is generated in the vacuum vapor pool 32, so that calibration can be realized, and detection light is monitored to determine whether the wavelength of the laser is in a transition line type of alkali metal atoms; taking metallic rubidium as an example, under vacuum condition, rubidium atom is from 5S1/2Energy level to 6P1/2The wavelength of the transition spectral line of the energy level is 421.673nm, namely the absorption wavelength. When the calibration light enters the heated vacuum vapor cell 32 at this wavelength, the outer electrons of the alkali metal atoms will transition from the ground state to the excited state, and 5S will be generated as they fall1/2-6P1/2Light with energy level transition spectral line wavelength, so a beam of obvious fluorescence appears in the vacuum vapor cell 32; when the laser wavelength deviates from the line type, the phenomenon can not occur, and the phenomenon is used as an indicating signal for wavelength calibration;
the narrow-linewidth semiconductor laser 11 is a 420nm narrow-linewidth semiconductor laser, and at the moment, the corresponding alkali metal atom is a rubidium atom;
the narrow-linewidth semiconductor laser 11 is a 459nm narrow-linewidth semiconductor laser, and the corresponding alkali metal atom is a potassium atom at this time;
the narrow-linewidth semiconductor laser 11 is a 404nm narrow-linewidth semiconductor laser, and at this time, the corresponding alkali metal atom is a cesium atom.
The invention also provides a method for measuring the atomic concentration of the alkali metal vapor laser used for the device, which comprises the following steps:
s1 linear polarization laser is generated through a narrow linewidth semiconductor laser 11 in a light splitting module 1, the linear polarization laser is firstly changed into circular polarization light through a first half-wave plate 12, then the circular polarization laser is divided into two beams of vertical line polarization light and parallel linear polarization light with equal light intensity through a first polarization splitting prism 13, the vertical linear polarization light is used as detection light to enter a detection module 2, transmission light which is deflected by 90 degrees in the transmission direction of the parallel linear polarization light and passes through a film beam splitter 14 is used as calibration light to enter a calibration module 3, and reflected light which is reflected by the film beam splitter 14 is used as reference light to enter the detection module 2;
s2 takes the parallel linear polarized light obtained by the first polarization splitting prism 13 in S1 as the detection light in the detection module 2, the detection light passes through the second half-wave plate 21 and then becomes circular polarized light again, and then the two linear polarized lights with equal intensity are obtained by the second polarization splitting prism 22 in the detection optical path 2:
s2.1, linearly polarized light reflected by the second polarization beam splitter prism 22, namely the propagation direction of parallel linearly polarized light is deflected for 90 degrees, then enters the quarter-wave plate 23, the polarization angle of the linearly polarized light is changed into 180 degrees and then is changed into vertical linearly polarized light, and then the linearly polarized light enters the scanning device 24:
s2.1.1 the vertical linear polarized light with 180 degree polarization angle changes passes through the two-dimensional galvanometer 24-1 in the scanning device 24, which includes two scanning heads, the two scanning heads are vertical to each other and controlled by two same motors respectively, when the motor controls the x scanning head to deflect, the light spot will move along the x axis direction, when the motor controls the y scanning head to deflect, the light spot will move along the y axis direction, therefore, the transmittance of any position in the alkali metal vapor pool can realize scanning by controlling the deflection of the x scanning head and the y scanning head by the motor, after the light beam is reflected by the concave paraboloid reflector 24-2 to form parallel light beam, the parallel light beam enters the alkali metal vapor pool 25 to be measured;
s2.1.2, after the parallel light beam transmits through the alkali metal vapor pool 25 to be measured, the light beam has information to be measured of the light absorption of the alkali metal vapor in the alkali metal vapor pool for the first time, and after the transmitted light beam is reflected back to the alkali metal vapor pool 25 to be measured through the plane mirror 26, the return light of the information to be measured of the light absorption of the alkali metal vapor in the vapor pool for the second time is obtained, thereby increasing the finally obtained transmittance; the returned light passes through the scanning device 24 to the quarter-wave plate 23 again, the polarization angle is converted by 180 degrees to obtain parallel linear polarized light, and the parallel linear polarized light can directly pass through the second polarization splitting prism 22 because the second polarization splitting prism 22 is totally reflective to the vertical linear polarized light and totally transmissive to the parallel linear polarized light;
s2.1.3 after the parallel-line polarized light passing through the second polarization beam splitter prism 22 is turned to the light beam by the first right-angle prism 27, the light intensity of the light beam is weakened and focused by the first attenuator 28A and the first convex lens 29A, respectively, and then the light beam enters the balanced amplification detector 210 through the first input port of the balanced amplification detector 210;
s2.2, the reflected light obtained by the first thin film beam splitter 14 in S1 is used as reference light, and the reference light enters the balanced amplification detector 210 through a second input port of the balanced amplification detector 210 after the reference light is respectively attenuated and focused on the light intensity of the light beam by the second attenuation sheet 28B and the second convex lens 29B;
s2.3 two output ports of the balanced amplification detector 210 are respectively connected with two signal input ends of a DAQ data acquisition card 211-1 in the data acquisition module 211, so that the return light and the reference light detected by the balanced amplification detector 210 are both collected by the DAQ data acquisition card 211-1 and transmitted to a computer 211-2 connected with the same.
S2.4 the alkali metal vapor pool 25 to be measured is heated, so that alkali metal vapor is generated in the alkali metal vapor pool, and light passing through the alkali metal vapor pool is absorbed, so that measurement is started, and the method specifically comprises the following steps:
s2.4.1, the two-dimensional galvanometer 24-1 is controlled to scan the cross section of the alkali metal vapor pool 25 to be measured line by line, and a return light signal with information to be measured is obtained.
S2.4.2 the balanced amplifying detector 2-10 is used to receive the return light signal and the reference light signal with the information to be measured at the same time, and the two-dimensional distribution diagram of transmittance is obtained after the data acquisition module 211 acquires and processes the signals.
S3, the transmitted light obtained by the thin film beam splitter 14 in S1 is used as calibration light, the propagation direction of the light is turned through the second right-angle prism 31, and then calibration is carried out through the vacuum vapor pool 32 in the calibration module 3, so that whether the wavelength of the laser is in the transition line type of the alkali metal atoms or not is determined;
s4, calculating the atomic concentration in the alkali metal vapor pool 25 to be measured, which comprises the following steps:
s4.1, calculating the absorption cross section of the alkali metal vapor pool 25 to be detected: under the condition that the buffer gas pressure in the alkali metal vapor pool 25 to be detected is higher (1 atm-20 atm), the absorption cross section of the alkali metal atom to be detected is approximately calculated by adopting a single-spectral line Lorentz linear formula:
wherein gL (v) is Lorentz linear, delta (P)ref) Is the pressure drift of the line position, PrefFor the pressure, Δ v, of the buffer gas components chargedLIs half height and half width value of the filled buffer gas, v is output frequency of the narrow linewidth laser0The central frequency corresponding to the central wavelength of the alkali metal atom, and P is the total pressure of the filled buffer gas;
the absorption cross section of the alkali metal atom is calculated using the following formula:
σ(v)＝PSgL(v)
wherein σ (v) is an absorption cross section of an alkali metal atom, and S is a strong absorption line of the alkali metal atom; the absorption cross section of the alkali metal atom at the laser output frequency can be obtained
S4.2, according to the two-dimensional distribution diagram of the transmittance in the alkali metal vapor pool 25 to be measured under the output frequency of the laser measured by S2.4.2, and by using the absorption cross section of the alkali metal atom under the output frequency calculated by S4.1, and under the condition that the length L of the alkali metal vapor pool is known, the atomic concentration Na in the alkali metal vapor laser can be obtained by substituting the following beer Lambert law formula:
where T (v) is the transmittance, a two-dimensional distribution map of the atomic concentration in the alkali metal vapor cell 25 to be measured can be obtained.
Compared with the existing measuring mode, the invention has the advantages that: using nS1/2-(n+1)P1/2、3/2The energy level transition wavelength of the optical fiber is used for detecting the atomic concentration of the gain medium in the alkali metal vapor laser, compared with the traditional method of using nS1/2-nP1/2、3/2The energy level transition wavelength detection has smaller absorptivity, can realize linear direct detection on the atomic concentration in the vapor pool under high temperature and high pressure, and is favorable for better analyzing the performance of the laser.
Drawings
FIG. 1 is a structural view showing the composition of an atomic concentration measuring apparatus based on an alkali metal vapor laser according to the present invention;
FIG. 2 is a schematic diagram of the structure of an x-scan head and a y-scan head of a two-dimensional galvanometer in a scanning device;
FIG. 3 is a flow chart of a measurement method according to the present invention;
FIG. 4 is a two-dimensional distribution diagram of the transmittance in the alkali metal vapor cell to be measured;
FIG. 5 is a two-dimensional distribution diagram of atomic concentration in the alkali metal vapor cell to be measured;
the specific implementation mode is as follows:
the following further describes embodiments of the present invention with reference to the drawings.
FIG. 1 is a structural diagram of an atomic concentration measuring device based on an alkali metal vapor laser, which includes a light splitting module 1, a detection module 2 and a calibration module 3; the light splitting module 1 is used for splitting the probe light into three beams, wherein two beams of light respectively serve as detection light and reference light to enter the detection module 2, the detection light is used for direct measurement, and the reference light is used for eliminating a noise background corresponding to a light source; the third beam of light enters a calibration module 3 as calibration light, and the detection light is monitored to determine whether the wavelength of the laser is in the transition line type of the alkali metal atom;
FIG. 2 is a schematic diagram of the x-scan head and the y-scan head of the two-dimensional galvanometer 24-1 in the scanning apparatus; the two-dimensional high-speed galvanometer 24-1 comprises two scanning heads which are perpendicular to each other and are respectively controlled by two same motors, when the x scanning head is controlled by the motor to deflect, the light spot can move along the x-axis direction, and when the y scanning head is controlled by the motor to deflect, the light spot can move along the y-axis direction, so that the absorption rate of any position in the alkali metal vapor pool can realize two-dimensional scanning by controlling the deflection of the x scanning head and the y scanning head by the motor.
FIG. 3 is a flow chart of a measurement method according to the present invention;
FIG. 4 is a two-dimensional distribution diagram of the transmittance in the alkali metal vapor cell to be measured; each of the long rectangles in the distribution diagram represents a line in the line-by-line scanning of the alkali metal vapor cell 25 to be measured, wherein different colors represent different transmittances, and a darker color means a greater transmittance.
FIG. 5 is a two-dimensional distribution diagram of atomic concentration in the alkali metal vapor cell to be measured; each of the long rectangles in the distribution diagram represents a line in the line-by-line scan of the alkali metal vapor cell 25 to be measured, wherein different colors represent different atomic concentrations, and a darker color means a higher atomic concentration.
The invention is based on the following principle: 420nm laser transition corresponding to rubidium metal 5S1/2-6P1/2、6P3/2The transition has the advantages that the integral absorption ratio 795nm is 1-2 orders of magnitude lower, the sufficient absorption is ensured, the linearity is ensured, the wavelength can be locked through a standard rubidium vapor pool, the scanning is not needed, the time response is improved, and the two-dimensional rapid scanning in the aperture of the vapor pool can be realized by matching with a light beam scanning device. And with the increasing maturity of blue light segment semiconductor lasers, the practical application is facilitated, and the problems that blue light is unstable and is difficult to tune are solvedThe laser beam is divided into three paths of calibration, reference and measurement, which respectively play roles in calibrating wavelength, reducing noise and measuring atomic concentration.
The method mainly aims at the problem that signals cannot be measured at high temperature due to 795nm laser, and improves the detection precision of the concentration of alkali metal vapor atoms. Meanwhile, the method can be further expanded to cesium vapor laser (404nm 6S-7P) and potassium laser (459nm 4S-5P), and is used in occasions such as atomic gyros, polarized gas nuclear magnetic resonance and the like which need vapor and are filled with atmospheric pressure level buffer gas.
Claims (4)
1. An atomic concentration measuring device for an alkali metal vapor laser, characterized in that: the device comprises a light splitting module (1), a detection module (2) and a calibration module (3); the light splitting module (1) is used for splitting the probe light into three beams, wherein the two beams of light enter the detection module (2) as detection light and reference light respectively, the detection light is used for direct measurement, and the reference light is used for eliminating the noise background of a light source; the third beam of light enters a calibration module (3) as calibration light, and the detection light is monitored to determine whether the wavelength of the laser is in the transition line type of the alkali metal atoms;
the light splitting module (1) comprises a narrow-linewidth semiconductor laser (11), a first half-wave plate (12), a first polarization splitting prism (13) and a thin film beam splitter (14); the narrow-linewidth semiconductor laser (11) emits linear polarized laser, the linear polarized laser is converted into circularly polarized light through a first half-wave plate (12), the circularly polarized light is divided into two beams of vertical line polarized light and parallel linear polarized light with the same light intensity by a first polarization splitting prism (13), the vertical linear polarized light is used as detection light and enters a detection module (2), the transmission direction of the parallel linear polarized light is deflected by 90 degrees, transmitted light passing through a film beam splitter (14) is used as calibration light and enters a calibration module (3), and reflected light reflected by the film beam splitter (14) is used as reference light and enters the detection module (2);
the detection module (2) comprises a second half-wave plate (21), a second polarization splitting prism (22), a quarter-wave plate (23), a scanning device (24), an alkali metal vapor pool (25) to be detected, a plane mirror (26), a first right-angle prism (27), a first attenuation plate (28A), a first convex lens (29A), a second attenuation plate (28B), a second convex lens (29B), a balance amplification detector (210) and a data acquisition module (211); the vertical linear polarized light obtained by a first polarization splitting prism (13) in a light splitting module (1) is used as detection light, the detection light is changed into circular polarized light again after passing through a second half-wave plate (21) and then passes through a second polarization splitting prism (22) to obtain parallel linear polarized light, the parallel linear polarized light is deflected for 180 degrees in the deflection direction of a quarter-wave plate (23) to obtain vertical linear polarized light, the vertical linear polarized light enters an alkali metal vapor pool (25) to be detected after being scanned by a scanning device (24), and the vertical linear polarized light is reflected back when encountering a plane reflecting mirror (26) after passing through the alkali metal vapor pool (25) to be detected, so that the double optical path is realized, and the resolution of the measured transmittance is improved; the reflected light passes through the alkali metal vapor pool (25) to be detected, the scanning device (24) and the quarter-wave plate (23) again, due to the effect of the quarter-wave plate (23), the returned light is changed into parallel linear polarized light to be transmitted through the second polarization splitting prism (22), the light intensity is weakened through the first attenuation sheet (28A) after the parallel linear polarized light is deflected for 90 degrees in the transmission direction of the first right-angle prism (27), and then the parallel linear polarized light is focused through the first convex lens (29A) and then is input into the balance amplification detector (210) through the first input port in the balance amplification detector (210) for detection; meanwhile, reflected light obtained after reflection by a thin film beam splitter (14) in a light splitting module (1) is used as reference light, the light intensity is weakened through a second attenuation sheet (28B), the reference light is focused through a second convex lens (29B) and then is input into a balanced amplification detector (210) through a second input port of the balanced amplification detector (210), a first output port and a second output port of the balanced amplification detector (210) are respectively connected with two signal input ends in a DAQ data acquisition card (211-1) in a data acquisition module (211), and data acquired through the DAQ data acquisition card (211-1) are displayed on a display screen of a computer (211-2) in the data acquisition module (211); the balanced amplification detector (210) is used for converting the reference optical signal and the detection optical signal into electric signals and outputting the electric signals to the data acquisition module (211); the data acquisition module (211) comprises a DAQ data acquisition card (211-1) and a computer (211-2) and is used for acquiring signals of the balanced amplification detector (210) and displaying a two-dimensional image;
the scanning device (24) consists of a two-dimensional galvanometer (24-1) and a concave paraboloid reflecting mirror (24-2), when a light beam passes through the two-dimensional galvanometer (24-1), an x-axis scanning head and a y-axis scanning head of the two-dimensional galvanometer (24-1) are controlled to move along the x-axis direction and the y-axis direction respectively so as to realize two-dimensional scanning, and the light beam is reflected by the concave paraboloid reflecting mirror (24-2) to form a parallel light beam to enter an alkali metal vapor pool (25) so as to realize the real-time monitoring of the concentration of atoms in the alkali metal vapor pool (25) to be detected;
the calibration module (3) comprises a second right-angle prism (31) and a vacuum vapor pool (32); transmitted light obtained in a thin film beam splitter (14) in a light splitting module (1) is transmitted through a second right-angle prism (31) to turn an optical path and then enters a vacuum steam pool (32), when the vacuum steam pool (32) is heated, fluorescence is generated in the vacuum steam pool (32), so that calibration can be realized, and detection light is monitored to determine whether the wavelength of a laser is in a transition line type of an alkali metal atom;
the narrow-linewidth semiconductor laser (11) is a 420nm narrow-linewidth semiconductor laser, and the corresponding alkali metal atoms are rubidium atoms;
the narrow-linewidth semiconductor laser (11) is a 459nm narrow-linewidth semiconductor laser, and the corresponding alkali metal atoms are potassium atoms;
the narrow-linewidth semiconductor laser (11) is a 404nm narrow-linewidth semiconductor laser, and the corresponding alkali metal atoms are cesium atoms.
2. An atomic concentration measuring device for an alkali metal vapor laser according to claim 1, characterized in that: the two-dimensional galvanometer (24-1) can be replaced by a rotating mirror, an acousto-optic deflector, a digital micromirror array, a space optical phase modulator or an optical system to form a large light beam and then detect the whole distribution mode of light intensity by a camera.
3. A method of measuring the atomic concentration of an alkali metal vapor laser for use in an apparatus according to any one of claims 1 to 2, comprising the steps of:
s1 linear polarized laser is generated by a narrow-linewidth semiconductor laser (11) in a light splitting module (1), the linear polarized laser is changed into circular polarized light through a first half-wave plate (12), then the circular polarized laser is divided into two beams of vertical line polarized light and parallel linear polarized light with equal light intensity through a first polarization splitting prism (13), the vertical linear polarized light is used as detection light to enter a detection module (2), transmission light which is deflected by 90 degrees in the transmission direction of the parallel linear polarized light and passes through a film beam splitter (14) is used as calibration light to enter a calibration module (3), and reflected light which is reflected by the film beam splitter (14) is used as reference light to enter the detection module (2);
s2, the parallel linear polarized light obtained by the first polarization splitting prism (13) in the S1 is used as detection light in the detection module (2), the detection light passes through a second half-wave plate (21) and then becomes circular polarized light again, and then two linear polarized lights with equal light intensity are obtained by a second polarization splitting prism (22) in a detection light path:
s2.1, linearly polarized light reflected by the second polarization splitting prism (22), namely the propagation direction of parallel linearly polarized light is deflected for 90 degrees and then enters a quarter-wave plate (23), so that the polarization angle of the linearly polarized light is changed for 180 degrees and then is changed into vertical linearly polarized light, and then the linearly polarized light enters a scanning device (24):
s2.1.1 the vertical linear polarized light with the polarization angle changed by 180 degrees passes through a two-dimensional galvanometer (24-1) in a scanning device (24), the two scanning heads are perpendicular to each other and are respectively controlled by two same motors, when the motor controls the x scanning head to deflect, the light spot can move along the x axis direction, and when the motor controls the y scanning head to deflect, the light spot can move along the y axis direction, therefore, the transmittance of any position in the alkali metal vapor pool can realize scanning by controlling the deflection of the x scanning head and the y scanning head by the motor, and the light beam is reflected by a concave paraboloid reflecting mirror (24-2) to form parallel light beams and then enters the alkali metal vapor pool (25) to be measured;
s2.1.2, after the parallel light beam transmits through the alkali metal vapor pool (25) to be measured, the light beam has information to be measured of the light absorption of the alkali metal vapor in the alkali metal vapor pool for the first time, the transmitted light beam is reflected back to the alkali metal vapor pool (25) to be measured through the plane mirror (26), so that the return light of the information to be measured with the light absorption of the alkali metal vapor in the vapor pool for the second time is obtained, thereby increasing the finally obtained transmittance; the returned light passes through the scanning device (24) to the quarter-wave plate (23) again, the polarization angle is converted by 180 degrees to obtain parallel linear polarized light, and the parallel linear polarized light can directly pass through the second polarization splitting prism (22) because the second polarization splitting prism (22) is totally opposite to the vertical linear polarized light and totally transmits the parallel linear polarized light;
s2.1.3 after the parallel polarized light passing through the second polarization beam splitter prism (22) is turned to the light beam by the first right-angle prism (27), the light intensity of the light beam is weakened and focused by the first attenuation sheet (28A) and the first convex lens (29A), and then the light beam enters the balanced amplification detector (210) through the first input port of the balanced amplification detector (210);
s2.2, taking reflected light obtained by the first thin film beam splitter (14) in S1 as reference light, and enabling the reference light to enter the balanced amplification detector (210) through a second input port of the balanced amplification detector (210) after the reference light is respectively attenuated and focused on the light intensity of the light beam by a second attenuation sheet (28B) and a second convex lens (29B);
s2.3, two output ports of the balanced amplification detector (210) are respectively connected with two signal input ends in a DAQ data acquisition card (211-1) in the data acquisition module (211), so that return light and reference light which are detected by the balanced amplification detector (210) are collected by the DAQ data acquisition card (211-1) and transmitted to a computer (211-2) connected with the same;
s2.4, heating the alkali metal vapor pool (25) to be measured, generating alkali metal vapor in the alkali metal vapor pool, absorbing light passing through the alkali metal vapor pool, and starting to measure the alkali metal vapor pool, wherein the method comprises the following specific steps:
s2.4.1, controlling a two-dimensional galvanometer (24-1) to scan the cross section of the alkali metal vapor pool (25) to be detected line by line to obtain a return light signal with information to be detected;
s2.4.2, a balanced amplification detector (210) is used for receiving the return light signal with the information to be detected and the reference light signal at the same time, and a two-dimensional distribution graph of the transmittance is obtained after the data acquisition module (211) acquires and processes the return light signal and the reference light signal;
s3, the transmitted light obtained by the thin film beam splitter (14) in S1 is used as calibration light, the propagation direction of the light is turned through a second right-angle prism (31), and then calibration is carried out through a vacuum vapor pool (32) in a calibration module (3) so as to determine whether the wavelength of the laser is in the transition line type of the alkali metal atoms;
s4, calculating the atomic concentration in the alkali metal vapor pool (25) to be measured, which comprises the following steps:
s4.1, calculating the absorption cross section of the alkali metal vapor pool (25) to be detected: under the condition that the gas pressure of buffer gas in an alkali metal vapor pool (25) to be detected is high, the absorption cross section of the alkali metal atoms to be detected is approximately calculated by adopting a single-spectrum Lorentz linear formula:
wherein gL (v) is Lorentz linear, delta (P)ref) Is the pressure drift of the line position, PrefFor the pressure, Δ v, of the buffer gas components chargedLIs half height and half width value of the filled buffer gas, v is output frequency of the narrow linewidth laser0The central frequency corresponding to the central wavelength of the alkali metal atom, and P is the total pressure of the filled buffer gas;
the absorption cross section of the alkali metal atom is calculated using the following formula:
σ(v)＝PSgL(v)
wherein σ (v) is an absorption cross section of an alkali metal atom, and S is a strong absorption line of the alkali metal atom; thereby obtaining the absorption cross section of the alkali metal atom under the laser output frequency;
s4.2, according to the two-dimensional distribution diagram of the transmittance in the alkali metal vapor pool (25) to be measured under the output frequency of the laser measured by S2.4.2, and by using the absorption cross section of the alkali metal atom under the output frequency calculated by S4.1, and under the condition that the length L of the alkali metal vapor pool is known, the atomic concentration Na in the alkali metal vapor laser can be obtained by substituting the following beer Lambert law formula:
wherein T (v) is the transmittance, whereby a two-dimensional distribution map of the atomic concentration in the alkali metal vapor cell (25) to be measured can be obtained.
4. A method of measuring an alkali metal vapor laser atomic concentration according to claim 3, characterized in that: in S4.1, the pressure value of the buffer gas is 1 atm-20 atm.
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