Modulation transfer spectrum frequency stabilization light path structure capable of increasing absorption coefficient

Abstract

The invention discloses a modulation transfer spectrum frequency stabilization light path structure capable of increasing an absorption coefficient. A laser light source is divided into pump light and detection light through an optical fiber electro-optical modulator, an optical fiber collimator, a half-wave plate and a polarization splitting prism. Detection light passes through the half-wave plate, the achromatic zoom beam expander, the wedge-shaped air chamber and the polarization splitting prism and finally reaches the photoelectric detector. Pump light passes through the reflecting mirror, the Glan-Thompson prism, the space photoelectric light modulator, the reflecting mirror and the achromatic zoom beam expander, and finally enters the wedge-shaped air chamber through the polarization splitting prism. And the temperature of the wedge-shaped air chamber is controlled through an electric heater. And the pump light and the detection light complete four-wave mixing in the wedge-shaped air chamber. The atomic absorption coefficient is increased by setting the laser spectrum width, the laser power, the beam diameter and the gas chamber temperature. After the absorption coefficient is increased, a larger modulation transfer signal can be obtained, and the long-term frequency stability of the laser can be ensured.

Description

Technical Field

[0001] The invention belongs to the technical field of alkali metal D1 line laser frequency stabilization in a SERF magnetometer, and in particular relates to a modulation transfer spectrum frequency stabilization optical path structure for increasing an absorption coefficient. Background Art

[0002] SERF atomic magnetometers have important applications in magnetic field measurement, magnetoencephalography and basic physics. SERF atomic magnetometers usually use semiconductor lasers, which need to be stabilized at the alkali metal D1 line. In SERF magnetometers, the laser frequency directly affects the measurement accuracy, and higher laser frequency stability will lead to higher measurement accuracy. In existing laser frequency stabilization technology, the laser frequency needs to be locked to a high-stability, anti-interference frequency reference, such as atomic / molecular absorption transition spectral lines.

[0003] Among the frequency stabilization methods based on atomic transition spectral lines, modulation transfer spectroscopy has a large discrimination signal and is an ideal method for laser frequency. Modulation transfer spectroscopy frequency stabilization utilizes the nonlinear four-wave mixing process. After the modulation information of the pump light interacts with the detection light in the gas chamber, the modulation information is transferred to the detection beam. After the photodetector receives the detection signal, it demodulates and obtains the modulation transfer signal. The modulation transfer process of the D1 line strictly occurs in atoms with zero Doppler velocity, so the modulation transfer signal has no Doppler background and has extremely high sensitivity and fineness. There is no need for direct feedback of the laser current and temperature, which reduces the interference of the spectral modulation signal and is very suitable for frequency stabilization.

[0004] Since the alkali metal D1 line is a non-cyclic transition line, in the process of laser pumping atoms, the alkali metal D1 line is an open energy level system, and optical pumping occurs between low energy levels and high energy levels, but they will relax to other energy levels, which can be regarded as the in and out channels between the atomic transition system and the external environment are open. The number of particles at the low energy level is small, resulting in fewer particles that can transition, which leads to a small D1 line absorption coefficient.

[0005] Under a small absorption coefficient, the obtained modulation transfer signal is small, the long-term stability of the laser is poor, and the frequency noise suppression effect is poor. Under a larger modulation transfer signal, the frequency noise suppression effect is better. It is necessary to optimize the optical parameters in the optical path, including laser line width, optical power, spot diameter, gas chamber temperature, detection light polarization state, etc. After increasing the absorption coefficient, a larger modulation transfer spectrum signal can be obtained, making the laser frequency more stable. Summary of the invention

[0006] In order to solve the problem of small absorption coefficient in the existing D1 line modulation transfer spectrum, the present invention proposes a modulation transfer spectrum frequency stabilization optical path structure which increases the absorption coefficient.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a modulation transfer spectrum frequency stabilization optical path structure with increased absorption coefficient, the optical path structure comprising: a laser, an optical fiber electro-optic modulator, an optical fiber collimator, a first half-wave plate, a first polarization beam splitter prism, a second half-wave plate, a first achromatic zoom beam expander, a wedge-shaped gas chamber, an electric heater, a second polarization beam splitter prism, a first reflector, a Glan-Thompson prism, an electro-optic modulator, a second reflector, a second achromatic zoom beam expander, a plano-convex lens, and a photodetector; a laser light source first obtains a laser spectrum width modulated laser through an optical fiber electro-optic modulator, After passing through the fiber collimator, the laser is converted into spatial light, and then divided into pump light and detection light through the first half-wave plate and the first polarization beam splitter prism. The detection light passes through the second half-wave plate, the first achromatic zoom beam expander, the wedge-shaped gas chamber, the polarization beam splitter prism and finally reaches the photodetector. The pump light passes through the first reflector, the Glan-Thompson prism, the electro-optic modulator, the second reflector, the second achromatic zoom beam expander, and finally enters the wedge-shaped gas chamber through the second polarization beam splitter prism. The temperature of the wedge-shaped gas chamber is controlled by an electric heater. The pump light and the detection light complete four-wave mixing in the wedge-shaped gas chamber to obtain a modulated transfer signal.

[0008] Beneficial effects of the present invention:

[0009] 1. The present invention uses an optical fiber electro-optic modulator to adjust the laser spectrum width and increase the laser absorption coefficient.

[0010] 2. The present invention uses a fiber collimator to convert fiber light into spatial light. The diameter of the spatial light beam is smaller than the aperture of the electro-optic modulator, ensuring that the pump light can be well modulated.

[0011] 3. The present invention uses an electric heater to heat the wedge-shaped gas chamber and precisely control its temperature, thereby increasing the number of active atoms within a certain range.

[0012] 4. The optical power adjustment, spot diameter adjustment, gas chamber stability adjustment, and laser spectrum width adjustment in the present invention can increase the absorption coefficient of the alkali metal to the laser in the gas chamber, and can obtain a modulation transfer signal with a larger amplitude, thereby ensuring the laser frequency stabilization without losing lock and the long-term stability of the laser frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic diagram of the structure of the modulation transfer spectrum frequency stabilization optical path for increasing the absorption coefficient proposed by the present invention.

[0014] The reference numerals in the figure are: laser 101, fiber electro-optic modulator 102, fiber collimator 103, first half-wave plate 104, first polarization beam splitter prism 105, second half-wave plate 106, first achromatic zoom beam expander 107, wedge-shaped gas chamber 108, electric heater 109, second polarization beam splitter prism 110, first reflector 111, Glan-Thompson prism 112, electro-optic modulator 113, second reflector 114, second achromatic zoom beam expander 115, plano-convex lens 116, photodetector 117. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in each embodiment of the present invention described below can be combined with each other as long as they do not conflict with each other. To achieve the above-mentioned purpose, the present invention adopts the following technical scheme.

[0016] This embodiment provides a modulation transfer spectrum frequency stabilization optical path structure that increases the absorption coefficient. Figure 1 As shown, it includes: laser 101, fiber electro-optic modulator 102, fiber collimator 103, first half-wave plate 104, first polarization beam splitter prism 105, second half-wave plate 106, first achromatic zoom beam expander 107, wedge-shaped gas chamber 108, electric heater 109, second polarization beam splitter prism 110, first reflector 111, Glan-Thompson prism 112, electro-optic modulator 113, second reflector 114, second achromatic zoom beam expander 115, plano-convex lens 116, photodetector 117. Laser 101 transmits laser light to fiber electro-optic modulator 102, and the laser light is converted into spatial light through fiber collimator 103 after passing through the fiber electro-optic modulator. Laser light is divided into pump light and detection light through first half-wave plate 104 and first polarization beam splitter prism 105. The detection light first passes through the second half-wave plate 106. The polarization angle of the detection light can be adjusted by rotating the angle of the second half-wave plate 106 to detect the polarization anisotropy of the pump light. After passing through the first achromatic zoom beam expander 107, the detection light enters the wedge-shaped air chamber 108. The temperature of the air chamber is controlled by the electric heater 109, and then the frequency-stabilized laser basic signal is received through the second polarization beam splitter prism 110, the plano-convex lens 116, and the photodetector 117. The pump light first passes through the first reflector 111 to change the beam propagation angle, and then enters the Glan-Thompson prism 112, the electro-optic modulator 113, the second reflector 114, the second achromatic zoom beam expander 115, and finally enters the wedge-shaped air chamber 108 through the second polarization beam splitter prism 110. The detection light and the pump light complete nonlinear four-wave mixing in the air chamber to obtain a modulated transfer signal.

[0017] The fiber electro-optic modulator 102 is used to adjust the laser spectrum width; the modulation frequency and modulation depth of the fiber electro-optic modulator 102 are adjusted to control the laser spectrum width. The half-wave voltage of the fiber electro-optic modulator 102 is less than 2V, the working range is DC-10GHz, it is easy to drive and has a large adjustable range. It uses optical fiber connection, which is easy to build and miniaturize the optical path. In addition, the device avoids the influence of laser polarization fluctuation on the frequency stabilization performance through polarization-maintaining optical fiber transmission.

[0018] The fiber collimator 103 is used to convert the fiber light into spatial light. The diameter of the spatial light spot is smaller than the aperture of the electro-optic modulator 113, thereby ensuring that the pump light is fully modulated.

[0019] The Glan-Thompson prism 112 is used to adjust the polarization state of the pump laser to coincide with the optical axis of the electro-optic modulator 113. The electro-optic modulator 113 has a half-wave voltage of 5V and an operating frequency of 5MHz, and is small in size and easy to install. The first and second achromatic zoom beam expanders 107 and 115 do not change the beam propagation direction when adjusting the spot diameter.

[0020] A first polarization beam splitter prism 105, a second half-wave plate 106, a first achromatic zoom beam expander 107, a wedge-shaped gas chamber 108, and an electric heater 109 are arranged in sequence.

[0021] The first half-wave plate 104 and the first polarization beam splitter prism 105 adjust the power of the pump light and the detection light, and the second half-wave plate 106 is used to change the polarization state of the detection light to achieve the measurement of polarization anisotropy.

[0022] A first reflecting mirror 111, a Glan-Thompson prism 112, an electro-optic modulator 113, a second reflecting mirror 114, and a second achromatic zoom beam expander 115 are arranged in sequence.

[0023] The window material of the wedge-shaped air chamber 108 is ultraviolet fused quartz, and the window angle is 55°.

[0024] The electric heater 109 can accurately control the temperature of the air chamber at 30-80°C through NTC temperature measurement and heating film heating. The electric heater 109 controls the temperature of the air chamber and does not exceed the tolerance temperature of the air chamber. The plano-convex lens 116 focuses the laser on the sensitive surface of the detector.

[0025] The detection light and the pump light are completely aligned and overlapped in the wedge-shaped gas cell 108 .

[0026] The front and rear surfaces of the above-mentioned half-wave plate, polarization beam splitter, achromatic zoom beam expander and electro-optic modulator are all coated with anti-reflection films of corresponding wavelengths.

[0027] The above-mentioned achromatic zoom beam expander can magnify the laser spot by 1-4 times and keep the collimation unchanged when adjusting the spot size.

[0028] The modulation transfer spectrum frequency stabilization optical path structure for increasing the absorption coefficient also includes:

[0029] The processing unit and the photoelectric detector form a modulation transfer spectrum frequency discrimination curve with the demodulation signal after receiving the detection light basic signal.

[0030] The frequency locking unit obtains the control signal from the frequency discrimination curve to achieve the stabilization of the laser frequency.

[0031] A modulation transfer spectrum frequency stabilization optical path structure for increasing absorption coefficient of the present invention further includes:

[0032] By jointly adjusting the optical power, spot diameter and air chamber temperature, the absorption coefficient of the medium in the wedge-shaped air chamber is increased, and the peak-to-peak value and slope of the modulation transfer signal are increased;

[0033] The RF driving unit applies the RF signal to the optical fiber electro-optic modulator and adjusts the laser spectrum width by adjusting the RF signal;

[0034] The frequency locking unit uses the modulation transfer signal as a frequency discrimination signal and feeds the frequency discrimination signal back to the controller, and the controller controls the frequency of the laser to achieve frequency locking.

[0035] The working method of the modulation transfer spectrum frequency stabilization optical path structure with increased absorption coefficient is:

[0036] The laser power emitted from the laser 101 is less than 20mW, which is transmitted to the fiber electro-optic modulator 102. The fiber electro-optic modulator 102 is driven by a high-frequency signal, and the laser spectrum width can be adjusted within a certain range. When the laser spectrum width is smaller than the atomic absorption spectrum width, the absorption coefficient of the atom to the laser is small. When the laser spectrum width increases, the absorption coefficient gradually increases. When the laser spectrum width is larger than the atomic absorption spectrum width, the absorption coefficient no longer increases. The spectrum width adjustment laser is divided into pump light and detection light through the fiber collimator 103. The pump light passes through the reflector 111, the Glan Thompson prism 112, the electro-optic modulator 113, the second reflector 114, the second achromatic zoom beam expander 115, and the second polarization beam splitter prism 110, and then enters the wedge-shaped gas chamber of the rubidium atom. The detection light passes through the first half-wave plate 104, the first polarization beam splitter prism 105, the second half-wave plate 106, and the first achromatic zoom beam expander 107, and then enters the wedge-shaped gas chamber 108 of the rubidium atom. The pump light and the detection light are incident on the wedge-shaped gas chamber 108 of the rubidium atom. After the spectral signal is detected by the photodetector 117, the signal is transmitted to the processing unit, and the spectrum and the discrimination signal can be observed. By adjusting the optical power, the spot size, the gas chamber temperature, and the polarization state of the detection light, the absorption coefficient can be adjusted. The change in the absorption coefficient is fed back to the amplitude and slope of the modulation transfer signal. By changing the above parameters, a larger discrimination signal is adjusted for frequency stabilization. Transmitting the discrimination signal to the frequency locking unit can stabilize the laser frequency to 85 Rb D1 line =3→ =2 in the fine energy level structure.

[0037] In summary, the device is used to increase the absorption coefficient in the frequency stabilization process of the D1 line modulation transfer spectrum of rubidium atoms. In the frequency stabilization process, adjusting the laser spectrum width, beam diameter, optical power, gas chamber temperature, and detecting the polarization state of light can affect the absorption coefficient and the amplitude of the modulation transfer spectrum signal. The present invention optimizes them so that the accuracy and robustness of the frequency stabilization can be guaranteed.

[0038] The contents not described in detail in the specification of the present invention belong to the prior art known to the professional and technical personnel in the field. It is easy for the technical personnel in the field to understand that the above description is only the preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement and improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A modulation transfer spectrum frequency stabilization optical path structure for increasing absorption coefficient, characterized in that: The optical path structure includes: a laser, an optical fiber electro-optic modulator, an optical fiber collimator, a first half-wave plate, a first polarization beam splitter prism, a second half-wave plate, a first achromatic zoom beam expander, a wedge-shaped air chamber, an electric heater, a second polarization beam splitter prism, a first reflector, a Glan-Thompson prism, an electro-optic modulator, a second reflector, a second achromatic zoom beam expander, a plano-convex lens, and a photodetector; the laser light source first passes through the optical fiber electro-optic modulator to obtain a laser spectrum width modulated laser, passes through the optical fiber collimator, and the laser is converted into spatial light, and then passes through The first half-wave plate and the first polarization beam splitter prism are divided into pump light and detection light. The detection light passes through the second half-wave plate, the first achromatic zoom beam expander, the wedge-shaped gas chamber, the polarization beam splitter prism and finally reaches the photodetector. The pump light passes through the first reflector, the Glan-Thompson prism, the electro-optic modulator, the second reflector, the second achromatic zoom beam expander, and finally enters the wedge-shaped gas chamber through the second polarization beam splitter prism. The temperature of the wedge-shaped gas chamber is controlled by an electric heater. The pump light and the detection light complete four-wave mixing in the wedge-shaped gas chamber to obtain a modulated transfer signal.

2. A modulation transfer spectrum frequency stabilization optical path structure for increasing absorption coefficient as claimed in claim 1, characterized in that: Adjust the modulation frequency and modulation depth of the fiber electro-optic modulator to control the laser spectrum width.

3. A modulation transfer spectrum frequency stabilization optical path structure for increasing absorption coefficient as claimed in claim 2, characterized in that: The fiber collimator converts the laser into spatial light, and the laser spot diameter is smaller than the aperture of the spatial optoelectronic light modulator.

4. A modulation transfer spectrum frequency stabilization optical path structure for increasing absorption coefficient as claimed in claim 3, characterized in that: The first and second achromatic zoom beam expanders do not change the light beam propagation direction when adjusting the light spot diameter.

5. A modulation transfer spectrum frequency stabilization optical path structure for increasing absorption coefficient as claimed in claim 4, characterized in that: The first half-wave plate and the first polarization beam splitter prism adjust the pump light and the detection light power, and the second half-wave plate is used to change the polarization state of the detection light to achieve the measurement of polarization anisotropy.

6. A modulation transfer spectrum frequency stabilization optical path structure for increasing absorption coefficient as claimed in claim 5, characterized in that: The electric heater controls the temperature of the air chamber so as not to exceed the tolerance temperature of the air chamber.

7. The modulation transfer spectrum frequency stabilization optical path structure for increasing absorption coefficient as claimed in claim 1, characterized in that: The plano-convex lens focuses the laser onto the sensitive surface of the detector.

8. The modulation transfer spectrum frequency stabilization optical path structure for increasing absorption coefficient as claimed in claim 6, characterized in that: Also includes: By jointly adjusting the optical power, spot diameter and air chamber temperature, the absorption coefficient of the medium in the wedge-shaped air chamber is increased, and the peak-to-peak value and slope of the modulation transfer signal are increased; The RF driving unit applies the RF signal to the optical fiber electro-optic modulator and adjusts the laser spectrum width by adjusting the RF signal; The frequency locking unit uses the modulation transfer signal as a frequency discrimination signal and feeds the frequency discrimination signal back to the controller, and the controller controls the frequency of the laser to achieve frequency locking.

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