Laser frequency locking circuit

Abstract

The invention relates to the technical field of laser, and provides a laser frequency locking circuit. The laser frequency locking circuit comprises a laser signal acquisition circuit used for acquiring a saturated absorption spectrum signal of light after laser emitted by a laser passes through an optical medium and performing analog-to-digital conversion to obtain an absorption spectrum digital signal; the reference signal source is used for generating a modulation reference signal with a set frequency; the phase sensitive detection circuit is used for obtaining a frequency error signal according to the absorption spectrum digital signal and the modulation reference signal; and the analog PID control circuit is used for performing PID control according to the frequency error signal to obtain a frequency stabilization control signal, and transmitting the frequency stabilization control signal to the control end of the laser, so that the control end of the laser controls the frequency of the laser to be locked in a frequency width range corresponding to the set frequency according to the error signal.

Description

Technical Field

[0001] The present disclosure relates to the field of laser technology, and in particular to a laser frequency locking circuit. Background Art

[0002] Magnetometer is a basic instrument for magnetic detection and ground magnetic field observation. Its performance directly determines the accuracy of magnetic field measurement. OPM (Optical-Pumping Magnetometer) refers to a type of magnetometer that uses the optical pumping effect to polarize atoms and destroys the polarization state by applying an external radio frequency field to achieve magnetic field measurement. The polarized atoms can be cesium atoms.

[0003] When designing an optically pumped magnetometer, a single-mode VCSEL (Vertical-Cavity Surface-Emitting Laser) laser with a small beam divergence angle and low power consumption can be used as the magnetometer pump light source. The single-mode characteristics of the vertical cavity surface emitting laser are stable for a long time, the polarization is stable, there will be no mode hopping and power fluctuation, and it can achieve stable output of single-mode narrow linewidth laser under high temperature conditions. In practical applications, a vertical cavity surface emitting laser with an output power of less than 2W, a spot diameter of 3mm, and an emission wavelength that can resonate with cesium atoms can be selected.

[0004] During operation, the laser will be affected by the external environment temperature, vibration and electromagnetic, which will cause the laser line width to be wider and the center frequency to drift, thus affecting the measurement accuracy of applications in precision measurement fields such as magnetometers. Summary of the invention

[0005] In view of this, an embodiment of the present disclosure provides a laser frequency locking circuit to solve the problems of wide laser line width and drift of center frequency in the prior art.

[0006] To achieve the above objectives, the technical solution adopted by the present disclosure is:

[0007] The disclosed embodiment provides a laser frequency locking circuit, which includes: a laser signal acquisition circuit, which is used to acquire a saturated absorption spectrum signal of the laser light emitted by the laser after passing through an optical medium and perform analog-to-digital conversion to obtain an absorption spectrum digital signal; a reference signal source, which is used to generate a modulated reference signal with a set frequency; a phase-sensitive detection circuit, which is used to obtain a frequency error signal according to the absorption spectrum digital signal and the modulated reference signal; an analog PID control circuit, which is used to perform proportional-integral-differential PID control according to the frequency error signal to obtain a frequency stabilization control signal, and transmit the frequency stabilization control signal to a control end of the laser, so that the control end of the laser controls the laser frequency to be locked within a frequency width range corresponding to the set frequency according to the error signal.

[0008] In one embodiment, the laser comprises a vertical cavity surface emitting laser.

[0009] In one embodiment, the phase sensitive detection circuit includes a differential amplifier and a low pass filter connected to each other.

[0010] In one embodiment, the differential amplifier comprises AD630.

[0011] In one embodiment, the low pass filter comprises an active RC low pass filter.

[0012] In one embodiment, the laser signal acquisition circuit includes a photodetector and an analog-to-digital converter.

[0013] In one embodiment, the analog PID control circuit includes a proportional part implemented by an operational amplifier, an integral part implemented by an integral circuit, and a differential part implemented by an RC circuit.

[0014] In one embodiment, the reference signal source comprises a direct digital frequency synthesis signal generator.

[0015] In one embodiment, the analog-to-digital converter comprises an AD7799.

[0016] In one embodiment, the optical medium includes atomic vapor.

[0017] Compared with the prior art, the beneficial effects of the embodiments of the present disclosure are as follows: the laser frequency locking circuit of the embodiments of the present disclosure performs phase-sensitive detection based on the saturated absorption spectrum signal and the modulation reference signal to obtain a frequency error signal, and adjusts the laser frequency based on the frequency error signal, thereby achieving frequency locking of the laser, thereby solving the problems of wide laser line width and drift of center frequency, and thus improving the measurement accuracy of the magnetometer.

[0018] Specifically, the laser frequency locking circuit in the technical solution of the embodiment of the present disclosure includes a laser signal acquisition circuit, a reference signal source, a phase-sensitive detection circuit and an analog PID control circuit. Among them, the laser signal acquisition circuit can detect light and perform analog-to-digital conversion, the reference signal source can generate a modulated reference signal, the phase-sensitive detection circuit can obtain a frequency error signal based on the absorption spectrum digital signal and the modulated reference signal combined with differential spectroscopy technology; the analog PID control circuit can perform PID control on the frequency error signal, obtain and output a frequency stabilization control signal, thereby realizing the laser frequency stabilization function. This solution adopts intelligent frequency locking technology, and monitors and dynamically adjusts laser parameters in real time through negative feedback to ensure the stability of the laser output frequency. When the vertical cavity surface emitting laser is used in a magnetometer, the accuracy of magnetic field measurement can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present disclosure, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present disclosure. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 is a structural schematic diagram of a laser frequency locking circuit provided by an embodiment of the present disclosure;

[0021] Figure 2 is a schematic diagram of laser signal acquisition provided by an embodiment of the present disclosure;

[0022] Figure 3 is a schematic diagram of phase-sensitive detection provided by an embodiment of the present disclosure;

[0023] Figure 4 It is a structural diagram of an analog PID control circuit provided in an embodiment of the present disclosure. DETAILED DESCRIPTION

[0024] The disclosed embodiments will now be described more fully with reference to the accompanying drawings in which the embodiments are shown. However, the disclosed embodiments may be implemented in many different forms and should not be construed as being limited to the embodiments specifically set forth herein; rather, the purpose of providing these embodiments is to make the disclosure exhaustive and complete and to fully convey the concepts of the embodiments to those skilled in the art so that they can implement the invention. The same reference numerals in the drawings represent the same elements, and therefore their descriptions will be omitted.

[0025] It should be understood that when an element is referred to as being connected to another element or being connected with another element, the element can be directly connected to another element, or there may be intervening elements between them. On the contrary, when an element is referred to as being directly connected to another element, there are no intervening elements.

[0026] Unless otherwise defined, all terms (including technical terms and scientific terms) used herein have the same meaning as those commonly understood by those skilled in the art to which the disclosed embodiments belong. It should also be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning consistent with their meaning in the context of the relevant field, and unless explicitly defined as such in this article, will not be interpreted in an idealized or overly rigid sense.

[0027] The term "device" used in the embodiments may be a software component or a hardware component configured to perform a specific function. The hardware component may include a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC). A software component may refer to data used by an executable code and / or data stored in an addressable storage medium and used by the executable code. Therefore, a software component may be, for example, an object-oriented software component, a class component, and a working component, and may include a process, a function, a property, a procedure, a subroutine, a program code segment, a driver, firmware, microcode, a circuit, data, a database, a data structure, a table, an array, or a variable.

[0028] The words used herein are for the purpose of describing specific embodiments and are not intended to be limiting of the present disclosure. It should be understood that the terms "include", "comprises", "comprising", "having", etc., when used in this specification, indicate the presence and characteristics, quantities, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other characteristics, quantities, steps, operations, elements, components and / or groups thereof.

[0029] The laser frequency locking circuit according to the embodiment of the present disclosure will be described in detail below with reference to the accompanying drawings.

[0030] Figure 1 is a structural schematic diagram of a laser frequency locking circuit provided by an embodiment of the present disclosure; Figure 2 is a schematic diagram of laser signal acquisition provided by an embodiment of the present disclosure; Figure 3 is a schematic diagram of phase-sensitive detection provided by an embodiment of the present disclosure; Figure 4 : is a schematic diagram of the structure of the analog PID control circuit provided by the embodiment of the present disclosure. Figures 1 to 4 The laser frequency locking circuit provided by the embodiments of the present disclosure is described together.

[0031] like Figure 1 As shown, an embodiment of the present disclosure provides a laser frequency locking circuit, which includes: a laser signal acquisition circuit 101, which is used to acquire a saturated absorption spectrum signal of the laser light emitted by the laser after passing through an optical medium and perform analog-to-digital conversion to obtain an absorption spectrum digital signal; a reference signal source 102, which is used to generate a modulated reference signal with a set frequency; a phase-sensitive detection circuit 103, which is used to obtain a frequency error signal according to the absorption spectrum digital signal and the modulated reference signal; an analog PID (Proportional Integral Derivative) control circuit 104, which is used to perform proportional integral differential PID control according to the frequency error signal to obtain a frequency stabilization control signal, and transmit the frequency stabilization control signal to the control end of the laser, so that the control end of the laser controls the laser frequency to be locked within a frequency width range corresponding to the set frequency according to the error signal.

[0032] Specifically, the set frequency may be a preset target center frequency f0 of the laser. The frequency width range corresponding to the set frequency of the laser is the width range of the laser frequency corresponding to the target center frequency f0 (f0-d / 2, f0+d / 2), where d is the size of the frequency width, which can be set to a smaller value to reduce the laser line width. When the laser frequency is locked near the set frequency, the center frequency drift can be reduced. After the laser frequency in the technical solution of the embodiment of the present disclosure is locked, the problem of wide line width and drift of the center frequency of the vertical cavity surface emitting laser can be solved, thereby improving the measurement accuracy of the magnetometer.

[0033] In the related art, in order to obtain the narrowest possible laser line width and ensure the measurement accuracy of precision measurement systems such as magnetometers, it is usually necessary to stabilize the laser frequency, i.e. lock the laser frequency. The laser in the embodiment of the present disclosure includes a vertical cavity surface emitting laser. When the laser is used as the light source of a laser pumped magnetometer, its frequency stabilization performance has a great influence on the measurement accuracy of the laser pumped magnetometer.

[0034] The laser frequency locking circuit in the technical solution of the embodiment of the present disclosure includes a laser signal acquisition circuit, a reference signal source, a phase-sensitive detection circuit and an analog PID control circuit. Among them, the laser signal acquisition circuit can detect light and perform analog-to-digital conversion, the reference signal source can generate a modulated reference signal, the phase-sensitive detection circuit can obtain a frequency error signal based on the absorption spectrum digital signal and the modulated reference signal combined with differential spectroscopy technology; the analog PID control circuit can perform PID control on the frequency error signal, obtain and output a frequency stabilization control signal, thereby realizing the laser frequency stabilization function. The scheme adopts intelligent frequency locking technology, monitors and dynamically adjusts the laser parameters in real time through negative feedback, and ensures the stability of the laser output frequency. When the vertical cavity surface emitting laser is used in a magnetometer, the accuracy of magnetic field measurement can be improved.

[0035] like Figure 2 As shown, in the embodiment of the present disclosure, the laser signal acquisition circuit includes a photodetector 203 and an analog-to-digital converter 204. When the laser output by the laser 201 passes through the atomic vapor in the atomic gas chamber 202, absorption occurs. The photodetector 203 can detect the intensity of the laser light emitted from the atomic gas chamber, thereby realizing the saturated absorption spectrum of the reference gas in the atomic gas chamber and obtaining a saturated absorption spectrum signal. The analog-to-digital converter 204 can convert the saturated absorption spectrum signal into an absorption spectrum digital signal.

[0036] like Figure 3As shown, in the embodiment of the present disclosure, the phase-sensitive detection circuit includes a differential amplifier 1031 and a low-pass filter 1032 connected to each other. The absorption spectrum digital signal output by the laser signal acquisition circuit 101 and the reference modulation signal output by the reference signal source 102 are both input to the differential amplifier 1031. The differential amplifier 1031 multiplies the saturated absorption spectrum digital signal and the modulated reference signal, and inputs the result of the operation into the low-pass filter 1032 for differential calculation to obtain a frequency error signal and input it into the analog PID control circuit. The analog PID control circuit outputs a control voltage according to the frequency error signal, feeds it back to the laser drive circuit, adjusts the output current, and realizes stable control of the laser frequency.

[0037] Among them, the differential amplifier in the phase-sensitive detection circuit can be a chip AD630, but is not limited to this. The low-pass filter in the phase-sensitive detection circuit can be an active RC low-pass filter, but is not limited to this. As a multiplier in the phase-locked technology, the performance of the phase-sensitive detection circuit determines the detection level of weak signals. In the embodiment disclosed in the present invention, the phase-sensitive detection circuit can be built with a high-performance chip AD630. The AD630 is composed of an operational amplifier A, an operational amplifier B and a comparator. The amplitude of the reference signal determines whether the operational amplifier A or the operational amplifier B works. The input channel isolation of the operational amplifier A and the operational amplifier B exceeds 100dB. After the absorption spectrum digital signal and the reference modulation signal enter the phase-sensitive detection circuit for processing, a DC signal can be obtained after active RC low-pass filtering.

[0038] In addition, the differential amplifier in the phase-sensitive detection circuit can also be AD835. This chip can realize four-quadrant voltage analog multiplication operation, has high input impedance, has ultra-fast settling time, requires few peripheral components, and is cost-effective.

[0039] The active RC low-pass filter combines active components such as resistors (R), capacitors (C) and operational amplifiers to achieve low-pass filtering. The active RC low-pass filter uses the amplification function and feedback characteristics of the operational amplifier to combine the input signal and the feedback signal, and adjusts the frequency characteristics of the input signal by adjusting the values ​​of the capacitor and resistor. When the input signal enters the operational amplifier, the output signal is also amplified due to the amplification characteristics of the operational amplifier. At the same time, according to the combination of capacitors and resistors, the low-pass filter will filter the input signal. Specifically, the high-frequency components of the input signal will be attenuated or filtered out, while the low-frequency components will pass through.

[0040] Compared with traditional RC low-pass filters, active RC low-pass filters can provide higher gain and more precise frequency characteristic control. In active RC low-pass filters, the combination of resistors and capacitors still plays a basic filtering role, but the introduction of active components such as operational amplifiers makes the filter have higher input impedance and lower output impedance, thereby improving the performance of the filter. In addition, active components can also be used to adjust the gain of the low-pass filter, so that the low-pass filter can maintain or enhance the amplitude of the low-frequency signal while filtering out the high-frequency components.

[0041] like Figure 4 As shown, in the embodiment of the present disclosure, the analog PID control circuit may include a proportional part implemented by an operational amplifier 1041, an integral part implemented by an integral circuit 1042, and a differential part implemented by an RC circuit 1043. Among them, an operational amplifier (Operational Amplifier, referred to as op-amp) is an integrated circuit that can be used in applications such as digital-to-analog conversion, filtering, amplification, and signal conditioning, and has high gain and high input impedance. Its working principle is based on the principle of negative feedback. A part of the output signal is fed back to the input end, compared with the input signal, and a difference operation is performed to obtain a difference signal. The difference signal is amplified and output again, and feedback is continuously performed until the output signal and the input signal tend to be consistent.

[0042] Specifically, in the design of analog PID control circuit, the functions of the three parts, namely the proportional part, the integral part and the differential part, need to be implemented by different circuit designs. Among them, the proportional operation circuit including the operational amplifier can realize the proportional part. The integral circuit can realize the integral part, the input of the integral part is the error signal, and the output is the integral control amount. The integral circuit needs to consider the charging and discharging process of the capacitor during the integration process. In the differential part, the error signal is passed through an RC circuit to generate the differential control amount. In the analog PID control circuit, the frequency error signal is compared with the PID given signal, and the output is summed and output through the proportional operation circuit, the integral circuit and the differential circuit. In the analog PID control circuit, the effects of integration and differentiation need to be adjusted repeatedly to achieve better control effects.

[0043] The analog PID control circuit is a feedback loop control circuit composed of three units: proportional-integral-differential. It is widely used in industrial control applications. In the PID algorithm, it is necessary to consider not only the current state value of the control object, but also the state value of the control object in the past period of time and the change of the state value in the recent period of time. The current output control signal is determined based on the values ​​of the current state, historical state and expected state. When the three operations of proportional, integral and differential are combined, the parameters are optimized and adjusted to cooperate with each other. When the system is controlled, the deviation can be adjusted quickly and timely, the control sensitivity can be improved, the system control accuracy can be improved, the steady-state performance can be improved, and the system stability can be improved, so as to achieve a good control effect.

[0044] In the disclosed embodiment, the reference signal source may be a DDS (Direct Digital Synthesis) signal generator. The direct digital frequency synthesis signal generator uses DDS technology to improve the frequency stability and accuracy of the signal generator to the same level as the reference frequency, and can perform fine frequency adjustment within a wide frequency range. The signal source designed using this method can operate in a modulation state, can adjust the output level, and can also output various waveforms.

[0045] DDS is a digital synthesis technology that converts a series of digital signals into analog signals through a digital-to-analog converter. It has two basic synthesis methods: table lookup method and calculation method. When the table lookup method is used for digital synthesis, DDS technology essentially realizes the function of a digital frequency divider. For a continuous sinusoidal signal with a period, the phase / amplitude sampling can be performed along its phase axis at equal phase intervals to obtain a discrete phase amplitude sequence of a periodic sinusoidal signal. The amplitude after quantization of the analog amplitude is encoded with binary data, so that a continuous sinusoidal signal with a period is converted into a series of discrete binary digital quantities, which are then stored in RAM (Random Access Memory). The address of each memory cell is the phase sampling address, and the content of the storage cell is the quantized sinusoidal amplitude. Such a read-only memory constitutes a sinusoidal function table corresponding to 2π periodic phase sampling.

[0046] The DDS signal generator is mainly composed of a standard reference frequency source, a phase accumulator, a waveform memory and a digital-to-analog converter. The DDS signal generator can be AD9832 or AD9850, but is not limited thereto.

[0047] AD9832 is a complete DDS chip with a maximum clock frequency of 25MHz. AD9832 is mainly composed of NCO (Numerically Controlled Oscillator) and phase modulator, sine lookup table and a 10-bit digital-to-analog converter. The numerically controlled oscillator and phase modulator part contains two 32-bit frequency registers, a 32-bit phase accumulator and four 12-bit phase registers. AD9832 can be used in high-frequency testers. It not only has excellent frequency accuracy and stability, but also has small size, light weight and easy operation.

[0048] The maximum clock of AD9850 is 125MHz. It adopts advanced CMOS (Complementary Metal Oxide Semiconductor) technology and is mainly composed of three parts: programmable DDS system, high-performance digital-to-analog converter and high-speed comparator. It can realize frequency synthesis with full digital programming control.

[0049] An analog-to-digital converter is an electronic component that converts an analog signal into a digital signal. A typical analog-to-digital converter can convert an input voltage signal into an output digital signal. In the disclosed embodiment, the analog-to-digital converter may be an AD7799. The AD7799 is a low-power, low-noise, complete analog front end suitable for high-precision measurement applications. It has a built-in low-noise 24-bit Σ-Δ ADC, which contains 3 differential analog inputs and an integrated on-chip low-noise instrumentation amplifier, so that small signals can be directly input. The root mean square (RMS) noise of the AD7799 is 27nV. The on-chip features of the AD7799 include a low-end power switch, reference voltage detection, programmable digital output pins, fuse current control, and an internal clock oscillator. The output data rate can be set by software programming and can vary from 4.17Hz to 470Hz.

[0050] In addition, the analog-to-digital converter can also be a high-precision analog-to-digital conversion chip MS5199T. MS5199T is a low-power, low-noise, three-channel differential input 16bit / 24bit analog-to-digital converter suitable for high-precision measurement applications. It integrates a low-noise input buffer and a low-noise instrument amplifier. When the gain is set to 64 and the update rate is 4.17Hz, the root mean square noise is 10nV. MS5199T also integrates a low-end power switch, programmable digital output, decadent current and internal oscillator. Its output data update rate is 4.17Hz to 470Hz.

[0051] In the disclosed embodiment, the optical medium may be atomic vapor of an atomic gas chamber, for example, cesium atomic vapor. Saturation absorption spectroscopy is a method for obtaining laser spectroscopy that eliminates Doppler broadening. The half-width of the saturation absorption spectrum signal is not Doppler broadened, but is determined only by the line width of the atomic energy level transition involved and the laser intensity. When the laser output by the laser passes through the atomic vapor of the atomic gas chamber, absorption occurs. When the frequency of the photon resonates with the hyperfine energy level of the atom, strong resonant absorption occurs. When the detuning is 0, the absorption is maximum. When the atom is stationary, the half-width of the absorption peak is equivalent to the natural line width of the atomic transition line, and the energy level of the atom is very stable, so the resonant absorption peak can be used as an ideal reference frequency for laser frequency stabilization. Because atoms undergo strong thermal motion at room temperature, the motion speed is distributed over a large range, and the Doppler effect is more obvious. For a laser of a certain frequency, the frequency "felt" by atoms of different speeds is different, which results in the frequency of the laser being absorbed by corresponding atoms in a large range, so that the absorption peak is broadened to the Doppler shift frequency order of the average atomic speed, about several hundred MHz. And for transition lines that are close to each other, they will be merged together under this broadening, and the absorption peak will be further broadened. It is precisely because of Doppler broadening that the line width of the atomic absorption spectrum is two to three orders of magnitude larger than the line width of the laser and cannot be used for frequency stabilization. The technical solution of the disclosed embodiment uses a reference modulation signal to eliminate the background of the Doppler absorption curve in the saturated absorption spectrum digital signal through a differential amplifier, which can greatly improve the signal-to-noise ratio of the saturated absorption signal and improve the stability of the laser frequency stabilization.

[0052] According to the laser frequency locking circuit provided in the embodiment of the present disclosure, phase-sensitive detection is performed based on the saturated absorption spectrum signal and the modulation reference signal to obtain a frequency error signal, and the laser frequency is adjusted according to the frequency error signal. The frequency locking of the laser can be achieved, thereby solving the problems of wide laser line width and drift of center frequency, thereby improving the measurement accuracy of the magnetometer.

[0053] The above are only preferred embodiments of the present disclosure and are not intended to limit the present disclosure. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present disclosure should be included in the protection scope of the present disclosure.

Claims

1. A laser frequency locking circuit, characterized in that: The laser frequency locking circuit comprises: The laser signal acquisition circuit is used to acquire the saturated absorption spectrum signal of the laser light emitted by the laser after passing through the optical medium and perform analog-to-digital conversion to obtain the absorption spectrum digital signal; A reference signal source, used for generating a modulation reference signal with a set frequency; A phase-sensitive detection circuit, used for obtaining a frequency error signal according to the absorption spectrum digital signal and the modulation reference signal; The analog PID control circuit is used to perform proportional integral differential PID control according to the frequency error signal to obtain a stable frequency control signal, and transmit the stable frequency control signal to the control end of the laser, so that the control end of the laser controls the laser frequency to be locked within the frequency width corresponding to the set frequency according to the error signal.

2. The laser frequency locking circuit according to claim 1, characterized in that: The laser comprises a vertical cavity surface emitting laser.

3. The laser frequency locking circuit according to claim 1, characterized in that: The phase-sensitive detection circuit includes a differential amplifier and a low-pass filter connected to each other.

4. The laser frequency locking circuit according to claim 3, characterized in that: The differential amplifier includes AD630.

5. The laser frequency locking circuit according to claim 3, characterized in that: The low pass filter comprises an active RC low pass filter.

6. The laser frequency locking circuit according to claim 1, characterized in that: The laser signal acquisition circuit includes a photoelectric detector and an analog-to-digital converter.

7. The laser frequency locking circuit according to claim 1, characterized in that: The analog PID control circuit includes a proportional part implemented by an operational amplifier, an integral part implemented by an integral circuit, and a differential part implemented by an RC circuit.

8. The laser frequency locking circuit according to claim 1, characterized in that: The reference signal source includes a direct digital frequency synthesis signal generator.

9. The laser frequency locking circuit according to claim 7, characterized in that: The analog-to-digital converter includes AD7799.

10. The laser frequency locking circuit according to claim 1, characterized in that: The optical medium includes atomic vapor.

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