A method for stable control of laser frequency in CPT magnetometer

Abstract

The present invention provides a method for stable control of laser frequency of CPT magnetometer. Under the joint action of microwave and modulated current on VCSEL laser, phase-sensitive detection is used to demodulate the transmitted laser of atomic gas chamber to obtain frequency discrimination signal and take the average value, and then the digital frequency discrimination signal under the same phase step and different laser driving current is compared, the first zero crossing point of the maximum value of the frequency discrimination signal is quickly and accurately found, and input into the digital PID controller to realize stable control of laser frequency. The present invention solves the problem that the zero point of the frequency discrimination signal is difficult to distinguish and the control accuracy is low when using scanning laser current, and has the advantages of easy automatic control and strong anti-interference ability.

Description

Technical Field

[0001] The invention belongs to the technical field of laser frequency stabilization, and in particular relates to a method for stabilizing the laser frequency of a CPT magnetometer. Background Art

[0002] The CPT (Coherent Population Trapping) magnetometer has the characteristics of small size, low power consumption, high precision, and no directional blind spot. It is expected to replace the optically pumped magnetometer and be used in space magnetism, ocean magnetism, and aviation magnetism.

[0003] The CPT magnetometer measures the magnetic field intensity based on the electromagnetically induced transparency phenomenon caused by the quantum interference effect between the laser field and atoms. The basic condition for achieving the electromagnetically induced transparency phenomenon is a two-color coherent laser that meets the energy level transition frequency, and the drift of the laser frequency will directly affect the accuracy of the magnetic field measurement. At present, there are generally two ways to achieve frequency stability control of two-color coherent lasers. First, one laser is first stabilized using the atomic absorption signal, and then the two-color coherent laser is realized with another laser using the beat frequency phase-locking technology. This method has a complex optical path, is difficult to operate, and is not easy to integrate, and basically cannot guarantee the practical application of the CPT magnetometer; second, the VCSEL (Vertical-cavity Surface-emitting Laser) of the CPT magnetometer is microwave modulated to produce a series of sidebands. When there are ±2 and ±1 level sidebands, 7 absorption peaks will be generated by scanning the laser current with a sawtooth wave or a triangle wave. The extreme point of the middle absorption curve is used as the frequency stabilization point to achieve frequency control of the two-color laser, such as Figure 1 As shown. Due to the multiple absorption peaks, the zero point of the discrimination signal is not easy to distinguish in this method, and the zero point control accuracy is not high. Based on the second method, it is necessary to propose a method for CPT magnetometer laser frequency stabilization control to achieve VCSEL laser frequency stabilization control. Summary of the invention

[0004] In order to overcome the deficiencies in the prior art, the inventors have conducted intensive research and provided a method for stable control of the laser frequency of a CPT magnetometer. The method compares the digital discrimination signals under different laser driving currents, accurately finds and controls the laser frequency stabilization point, and realizes stable control of the dual-color coherent laser frequency, thus solving the problem that the zero point of the discrimination signal is difficult to distinguish and the control accuracy is low when using the scanning laser current method.

[0005] The technical solution provided by the present invention is as follows:

[0006] A method for stabilizing the laser frequency of a CPT magnetometer comprises the following steps:

[0007] The precision driving current source generates a driving current of magnitude I0 and acts on the VCSEL laser of the CPT magnetometer. The corresponding output frequency of the VCSEL laser is f L Laser

[0008] Let the frequency be f m The high-frequency microwave signal is input into the VCSEL laser, causing the VCSEL laser to generate a series of sidebands, which are symmetrically distributed at the frequency f L On both sides of the laser, the laser frequency contains f L 、f L ±f m and f L ±2f m ;

[0009] The amplitude superimposed on the VCSEL laser driving current I0 is A m , frequency f w The laser is incident on the atomic gas chamber of the CPT magnetometer, and the frequency discrimination signal is demodulated by the phase-sensitive detection method through the power of the transmitted laser.

[0010] The demodulation signal within the time t is averaged and recorded as E0, and the demodulation signal E0 and the driving current I0 are stored;

[0011] The drive current is set to the step current I s Gradually increase, driving current I0+n*I s and I0+(n+1)*I s The corresponding frequency discrimination signals are E n and E n+1 , where n = 0, 1, 2…; if E n+1 >E n When (I0+n*I s , E n ) is replaced by (I0+(n+1)*I s , E n+1 ), otherwise, the driving current continues to increase without replacing the frequency discrimination signal; if there is E m =0, then (I0+m*I s , E m ) are stored separately, where m = 0, 1, 2... and m∈n;

[0012] The comparison process ends, in the corresponding E m =0 current value I0+m*I s The value is greater than the current value corresponding to the maximum value of the demodulation signal I0+(n+1)*I s And with the current value I0+(n+1)*I sThe current value with the smallest absolute difference is taken as the laser frequency stabilization point, and the current value corresponding to the laser frequency stabilization point is used as the driving current to act on the VCSEL laser, and the generated frequency discrimination signal is input into the PID controller of the CPT magnetometer in real time, and the output of the PID controller is applied to the laser current, thereby implementing stable control of the laser frequency.

[0013] A method for stabilizing the laser frequency of a CPT magnetometer provided by the present invention has the following beneficial effects:

[0014] (1) The present invention provides a method for stabilizing the laser frequency of a CPT magnetometer, which can effectively improve the accuracy of the frequency discrimination signal by averaging the frequency discrimination signal within a certain period of time;

[0015] (2) The present invention provides a method for stabilizing the laser frequency of a CPT magnetometer, which uses a comparative iteration method to find the maximum value of the discrimination signal and its first zero crossing point, and can quickly and effectively determine the stable frequency point of the laser;

[0016] (3) Compared with the conventional large-amplitude scanning current method, the method for stabilizing the laser frequency of a CPT magnetometer provided by the present invention can eliminate the error caused by the large-amplitude scanning current on the laser stabilization frequency point. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The seven absorption peak signal diagrams produced by the interaction between laser and atom containing 0, ±1 and ±2 sidebands, where ① is the stable frequency point;

[0018] Figure 2 is the demodulation signal diagram, where ① is the stable frequency point;

[0019] Figure 3 The present invention is a flow chart of the laser frequency stabilization control method for a CPT magnetometer. DETAILED DESCRIPTION

[0020] The following detailed description of the present invention will make the features and advantages of the present invention more clear and explicit.

[0021] The word "exemplary" is used exclusively herein to mean "serving as an example, embodiment, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. Although various aspects of the embodiments are shown in the drawings, the drawings are not necessarily drawn to scale unless otherwise noted.

[0022] The present invention provides a method for stabilizing the laser frequency of a CPT magnetometer, such as Figure 3 As shown, the following steps are included:

[0023] Step 1: The precision driving current source generates a driving current of magnitude I0, which acts on the VCSEL laser of the CPT magnetometer. The corresponding output frequency of the VCSEL laser is f L Laser

[0024] In this step, the current I0 generated by the precision driving current source is 0.1 mA to 4 mA, preferably 1.3 mA to 1.4 mA.

[0025] Step 2: Set the frequency to f m The high-frequency microwave signal is input into the VCSEL laser, causing the VCSEL laser to generate a series of sidebands, which are symmetrically distributed at the frequency f L On both sides of the laser, the laser frequency contains f L 、f L ±f m and f L ±2f m .

[0026] In this step, the frequency f of the high-frequency microwave signal m It is 3400MHz~3500MHz, such as 3417.344MHz.

[0027] Step 3: Superimpose an amplitude of A on the VCSEL laser drive current I0 m , frequency f w The laser is incident on the atomic gas chamber of the CPT magnetometer, and the frequency discrimination signal is demodulated by the phase-sensitive detection method through the power of the transmitted laser.

[0028] In this step, the amplitude A of the modulation signal m 0.0001mA~1mA, such as 0.001mA, modulation frequency f w 0.1kHz~100kHz.

[0029] In this step, the main working medium in the atomic gas chamber is alkali metal atoms. 87 Rb(rubidium).

[0030] Step 4: Take the average value of the demodulation signal within the time t, record it as E0, and store the demodulation signal E0 and the driving current I0.

[0031] In this step, the time t is preferably 0.1s to 5s.

[0032] Step 5: Set the drive current to step current I s Gradually increase, driving current I0+n*I s and I0+(n+1)*I s The corresponding frequency discrimination signals are E n and E n+1(where n=0,1,2…), if E n+1 >E n When (I0+n*I s , E n ) is replaced by (I0+(n+1)*I s , E n+1 ), otherwise, the driving current continues to increase without replacing the frequency discrimination signal; if there is E m =0, then (I0+m*I s , E m ) are stored separately (where m=0, 1, 2… and m∈n).

[0033] In this step, the step current I s It is 0.1μA~0.4μA.

[0034] In this step, the step number n is 250 to 1000. Since the wavelength current coefficient of the VCSEL laser is 0.4nm / mA, the laser frequency current coefficient is calculated to be 190MHz / mA, and the range of the 7 absorption peaks is 6.8GHz×2=13.6GHz, so in order to scan out the 7 absorption peaks, at least a scanning current of 71μA is required. Usually, the scanning range in the experiment is 100μA, thus ensuring Is*n=100μA. Among them, the smaller the step Is setting, the larger the step number n, the higher the accuracy of finding the zero point, but the larger the data volume; conversely, the accuracy of finding the zero point is reduced, but the data volume will become smaller. After a large number of experimental verifications, the step I s The step number n is 0.1μA~0.4μA, and the step number n is 250~1000, which reduces the amount of data while ensuring the accuracy and improves the data processing efficiency.

[0035] Step 6: The comparison process ends, in the corresponding E m =0 current value I0+m*I s The value is greater than the maximum value of the demodulation signal E n+1 The corresponding current value is I0+(n+1)*I s And with the current value I0+(n+1)*I s The current value with the smallest absolute difference, that is, the first zero crossing point of the maximum value of the frequency discrimination signal, is taken as the laser frequency stabilization point (see Figure 2 ), the current value corresponding to the laser frequency stabilization point is used as the driving current to act on the VCSEL laser, and the generated frequency discrimination signal is input into the PID controller of the CPT magnetometer in real time, and the output of the PID controller is applied to the laser current, thereby implementing stable control of the laser frequency.

[0036] The present invention has been described in detail above in conjunction with specific implementations and exemplary examples, but these descriptions cannot be understood as limiting the present invention. Those skilled in the art understand that, without departing from the spirit and scope of the present invention, a variety of equivalent substitutions, modifications or improvements may be made to the technical solution of the present invention and its implementation methods, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be subject to the attached claims.

[0037] The contents not described in detail in the specification of the present invention belong to the common knowledge of those skilled in the art.

Claims

1. A method for stabilizing the laser frequency of a CPT magnetometer, characterized in that: The steps include: The precision driving current source generates a driving current of magnitude I0 and acts on the VCSEL laser of the CPT magnetometer. The VCSEL laser corresponds to an output frequency f L Laser Let the frequency be f m The high-frequency microwave signal is input into the VCSEL laser, causing the VCSEL laser to generate a series of sidebands, which are symmetrically distributed at the frequency f L On both sides of the laser, the laser frequency contains f L 、f L ±f m and f L ±2f m ; The amplitude superimposed on the VCSEL laser driving current I0 is A m , frequency f w The laser is incident on the atomic gas chamber of the CPT magnetometer, and the power of the transmitted laser is used to demodulate the frequency discrimination signal using the phase-sensitive detection method. The demodulation signal within the time t is averaged and recorded as E0, and the demodulation signal E0 and the driving current I0 are stored; The drive current is set to the step current I s Gradually increase, driving current I0+n*I s and I0+(n+1)*I s The corresponding frequency discrimination signals are E n and E n+1 , where n = 0, 1, 2…; if E n+1 >E n When (I0+n*I s , E n ) is replaced by (I0+(n+1)*I s , E n+1 ), otherwise, the driving current continues to increase without replacing the frequency discrimination signal; if there is E m =0, then (I0+m*I s , E m ) are stored separately, where m = 0, 1, 2... and m∈n; The comparison process ends, in the corresponding E m =0 current value I0+m*I s The value is greater than the current value corresponding to the maximum value of the demodulation signal I0+(n+1)*I s And with the current value I0+(n+1)*I s The current value with the smallest absolute difference is taken as the laser frequency stabilization point, and the current value corresponding to the laser frequency stabilization point is used as the driving current to act on the VCSEL laser, and the generated frequency discrimination signal is input into the PID controller of the CPT magnetometer in real time, and the output of the PID controller is applied to the laser current, thereby implementing stable control of the laser frequency.

2. The method for stabilizing the laser frequency of a CPT magnetometer according to claim 1, characterized in that: The current I0 generated by the precision driving current source is 0.1 mA to 4 mA.

3. The method for stabilizing the laser frequency of a CPT magnetometer according to claim 1, characterized in that: The frequency of the high-frequency microwave signal is f m It is 3400MHz~3500MHz.

4. The method for stabilizing the laser frequency of a CPT magnetometer according to claim 1, characterized in that: The amplitude A of the modulated signal m 0.0001mA~1mA, modulation frequency f w 0.1kHz~100kHz.

5. The method for stabilizing the laser frequency of a CPT magnetometer according to claim 1, characterized in that: The main working medium in the atomic gas chamber is alkali metal atoms. 87 Rb.

6. The method for stabilizing the laser frequency of a CPT magnetometer according to claim 1, characterized in that: The time t is 0.1s to 5s.

7. The method for stabilizing the laser frequency of a CPT magnetometer according to claim 1, characterized in that: Step current I s It is 0.1μA~0.4μA.

8. The method for stabilizing the laser frequency of a CPT magnetometer according to claim 1, characterized in that: The maximum value of the step number n is 250 to 1000.

Images