CN112180394B - A multi-longitudinal mode high spectral resolution laser radar interferometer frequency locking system - Google Patents

Abstract

The invention discloses a multi-longitudinal mode high spectral resolution laser radar interferometer frequency locking system, which comprises a multi-longitudinal mode laser, a half wave plate, a polarization beam splitter prism, an acousto-optic modulation module, a spectrum frequency discrimination interferometer, a converging lens, a photoelectric detector, a peak value holding circuit and a proportion-integration-differential servo control module; the multi-longitudinal mode pulse laser output by the multi-longitudinal mode laser is subjected to beam splitting to obtain a low-power beam after passing through a half-wave plate and a polarization beam splitting prism; the low-power light speed is shifted in frequency by the acousto-optic modulation module and then enters the spectrum frequency discrimination interferometer, and the emergent light is received by the photoelectric detector after passing through the converging lens; after the electric signal output by the photoelectric detector is transmitted to the peak value holding circuit for processing, the output signal is transmitted to the proportional-integral-derivative servo control module, and the output tuning voltage acts on the spectrum frequency discrimination interferometer to realize resonance frequency locking. The invention can meet the requirement of multi-longitudinal mode HSRL on the frequency stability of the spectrum frequency discrimination interferometer.

Description

A multi-longitudinal mode high spectral resolution laser radar interferometer frequency locking system

Technical field

The invention belongs to the technical field of atmospheric laser radar, and in particular relates to a multi-longitudinal mode high spectral resolution laser radar interferometer frequency locking system based on a peak hold circuit.

Background technique

High Spectral Resolution Liar (HSRL) is currently one of the most accurate atmospheric aerosol detection systems. Its advantage lies in the use of filters with high spectral resolution to detect atmospheric backscattering signals in the atmosphere. The aerosol meter scattering signal is separated from the Rayleigh scattering signal of atmospheric molecules, so that the characteristics of the aerosol backscattering coefficient and extinction coefficient can be accurately inverted.

In this context, considering that the single longitudinal mode laser currently used in HSRL has poor adaptability to the environment and is bulky, research on a new HSRL system using multi-longitudinal mode lasers is ongoing. The principle of multi-longitudinal mode HSRL is basically the same as that of HSRL, that is, an interferometer frequency discriminator with a periodically distributed transmittance curve is used to perform spectral frequency discrimination on the atmospheric backscattered signal of each longitudinal mode component.

The premise of the above implementation is to ensure that the relative position of the frequency of each laser longitudinal mode and the frequency discriminator characteristic curve is stable. Therefore, the interferometer needs to be frequency locked so that its frequency discrimination curve distribution follows the longitudinal mode movement to lock the two.

Currently, the frequency stability of the interferometer discriminators used by HSRL, such as field-widening Michelson interferometer and Fabry-Perot interferometer, is slightly poor. Temperature control methods are mainly used to maintain frequency stability. Although this method can achieve long-term frequency stability, the slow response temperature adjustment method is destined to have poor short-term stability and extremely high temperature accuracy requirements. Therefore, a new frequency locking method is needed to make all longitudinal modes The atmospheric echo signal strictly matches the frequency identification curve of the device. The multi-longitudinal mode laser in multi-longitudinal mode HSRL has a simple structure and outputs pulsed laser light directly from the resonant cavity. Therefore, the conventional continuous laser frequency locking method is no longer applicable in this system.

Contents of the invention

The invention provides a multi-longitudinal mode high spectral resolution lidar interferometer frequency locking system, which improves the spectral frequency identification quality of multi-longitudinal mode high spectral resolution lidar for atmospheric detection, and at the same time solves the problem of direct multi-longitudinal mode laser The control difficulty caused by the characteristic of outputting pulsed light.

A multi-longitudinal mode high spectral resolution lidar interferometer frequency locking system, including a multi-longitudinal mode laser, a half-wave plate, a polarizing beam splitter prism, an acousto-optic modulation module, a spectral frequency discrimination interferometer, a converging lens, a photodetector, a peak value Holding circuit and proportional-integral-derivative servo control module;

The multi-longitudinal mode pulse laser output by the multi-longitudinal mode laser is split into a high-power beam and a low-power beam by a polarizing beam splitter after adjusting the polarization state with a half-wave plate. The high-power beam is used as a laser radar detection signal, and the low-power beam is used as a laser radar detection signal. Frequency locking of spectral frequency discrimination interferometer;

The low-power light speed is frequency-shifted by the acousto-optic modulation module and then incident on the spectral frequency discrimination interferometer that needs to be locked. The light emitted from it passes through the condensing lens and is received by the photodetector;

After the electrical signal output by the photodetector is transmitted to the peak hold circuit for processing, the peak hold signal and sampling trigger signal are output to the proportional-integral-differential servo control module. The proportional-integral-differential servo control module outputs a tuning voltage that acts on spectral frequency discrimination interference. instrument to achieve resonant frequency locking.

Preferably, the acousto-optic modulator is a two-level modulation structure, including a first-level acousto-optic modulator and a second-level acousto-optic modulator; wherein the first-level acousto-optic modulator is used to split the output of a multi-longitudinal mode laser into beams. The low-power beam undergoes frequency shifting processing; the secondary acousto-optic modulator is used to further shift the frequency of the pulsed light that has been frequency shifted. Under the premise of laser and mode matching, the two-level frequency shift makes a series of laser longitudinal modes universally located within the dynamic range of the frequency discrimination curve of the spectral frequency discrimination interferometer.

Further, the peak hold circuit includes a primary peak hold circuit and a secondary peak hold circuit, the primary peak hold circuit includes a transconductance amplifier, a discrimination diode-holding capacitor circuit and a voltage buffer circuit; the second peak hold circuit The first-stage peak hold circuit includes a voltage amplification circuit, a discrimination diode-holding capacitor circuit, a voltage comparator, a monostable trigger circuit, a peak hold switch circuit and a voltage buffer circuit.

Since the laser pulse width in the system is on the order of ns and the intervals between pulses are long, a two-level peak hold structure is adopted.

The first-level peak hold circuit is used to implement a short-time peak hold function and broaden a narrow pulse into a wide pulse.

The secondary peak hold circuit is used to further maintain the long-term peak value of the wide pulse output by the primary peak hold circuit, and generates a fixed-time threshold signal when each pulse arrives, so that the holding capacitor automatically maintains the peak value at the set time. The discharge achieves a fixed peak holding time. In addition, a trigger signal is generated for the AD acquisition process of the proportional-integral-derivative servo control module.

The first-level peak hold circuit is used for preliminary peak hold. The capacitance value used is small, and the internal charge will leak light in a short time. The output hold signal is characterized by a longer falling time and a wider pulse width than the original pulse. Pulse signal.

The secondary peak hold circuit performs true peak hold on the broadened pulse. While using the diode on-off and capacitor charging functions to retain the pulse peak voltage, it uses a voltage comparator and a monostable trigger to generate a fixed-time pulse. The peak hold threshold signal is used to control the switch circuit to release the charge in the holding capacitor after the threshold signal ends to prepare for the peak hold of the next pulse.

The proportional-integral-differential servo control module receives the peak hold signal and the sampling trigger signal, collects the peak hold voltage within the holding time and performs time domain averaging, and obtains the calculated result voltage value through the proportional-integral-differential operation. After level conversion and amplification by the peripheral circuit, a continuous tuning voltage is output, which acts on the piezoelectric ceramic module of the spectral frequency discrimination interferometer to achieve feedback control of the resonant frequency.

The proportional-integral-differential servo module collects each held voltage value multiple times and averages it to improve accuracy when triggered by the rising edge of the signal output by the voltage comparator in the secondary peak hold circuit. Since the pulsed light is detected after passing through the interferometer frequency discriminator, its peak amplitude corresponds to the relative position of each longitudinal mode and the interferometer frequency discriminator curve. When the longitudinal modes of the radar detection signal are all located at the resonant point of the interferometer, the longitudinal modes of the frequency-locked light that has undergone acousto-optic frequency shifting will all be located at the edge of the interference curve. Therefore, the voltage value collected by the module is related to the frequency of the interferometer relative to the laser. Offset related. Accordingly, the proportional-integral-differential operation is performed on the voltage values of a series of pulses to obtain the corresponding continuous output voltage for control. After level shifting and appropriate amplification of this voltage through the peripheral amplification circuit, the tuning voltage that causes the frequency of the interferometer to follow the laser is obtained, thereby keeping each longitudinal mode aligned with each resonance point of the interferometer to ensure that the meter in the atmospheric echo signal Scattered signals are accurately filtered out.

The spectral frequency identification interferometer is equipped with a piezoelectric ceramic module. The piezoelectric ceramic module is used to control the frequency of the interferometer according to the tuning voltage, including a piezoelectric ceramic driver and a piezoelectric ceramic displacement stage; the piezoelectric ceramic driver is used To amplify the power of the tuning voltage, the piezoelectric ceramic displacement stage is used to control the interference arm length of the interferometer frequency discriminator.

Compared with the prior art, the present invention has the following beneficial effects:

1. The system of the present invention solves the problem that the traditional laser frequency locking method is not suitable for pulse laser light sources, and can meet the frequency stability requirements of multi-longitudinal mode HSRL for spectral frequency identification interferometers in the corresponding band.

2. The equipment involved in the system of the present invention is simple, low in cost, has low requirements on the external environment, and is in line with the low-cost and integrated characteristics of the multi-longitudinal mode HSRL system.

3. The system of the present invention can be applied to other fields that require high frequency stability of interferometer devices, such as the field of interference detection.

Description of drawings

Figure 1 is a schematic diagram of the device of a multi-longitudinal mode high spectral resolution laser radar interferometer frequency locking system of the present invention;

Figure 2 is a schematic diagram of the peak hold circuit in the present invention;

Figure 3 is a signal processing flow chart of the proportional-integral-differential servo module in the present invention.

Detailed ways

The present invention will be described in further detail below with reference to the accompanying drawings and examples. It should be noted that the following examples are intended to facilitate the understanding of the present invention and do not limit it in any way.

As shown in Figure 1, a multi-longitudinal mode high spectral resolution lidar interferometer frequency locking system includes a multi-longitudinal mode laser 1, a half-wave plate 2, a polarizing beam splitter prism 3, an acousto-optic modulation module 4, and spectral frequency discrimination interference Instrument 5, converging lens 6, photodetector 7, peak hold circuit 8, proportional-integral-differential servo control module 9.

The laser output from the multi-longitudinal mode laser 1 is first polarized using a half-wave plate 2 , and then split into a beam of light using a polarizing beam splitter prism 3 and incident on the acousto-optic modulation module 4 . The frequency-locked light is frequency-shifted by the acousto-optic modulation module 4 and then enters the spectral frequency discrimination interferometer 5. The light emitted from it passes through the condensing lens 6 and is received by the photodetector 7. The output electrical signal is first processed by the peak hold circuit 8, and the trigger signal and the pulse signal after peak hold are obtained and transmitted to the proportional-integral-derivative servo control module 9, thereby generating a control voltage for making the interferometer frequency follow the laser frequency. On the piezoelectric ceramic module of the spectral frequency discrimination interferometer 5.

Since the frequency of each longitudinal mode output by the acousto-optic modulation module 4 has moved as a whole relative to the output of the laser, when the frequency of each longitudinal mode of the laser output light is located at the resonance point of the interferometer frequency identification curve, the longitudinal mode of the frequency-locked light is overall is located on the edge of the frequency discrimination curve. The amplitude of the frequency-locked light output from the interferometer can thus represent the degree of frequency shift of each longitudinal mode before frequency shifting relative to the resonant point of the interferometer.

The photodetector 7 receives the frequency-locked light and outputs a narrow pulse signal. Since the pulse signal has an extremely narrow pulse width and a low repetition frequency, with typical values of 10ns and 10Hz, it is difficult to accurately measure its peak voltage with normal AD sampling. Therefore, a peak hold circuit 8 is used here to process the narrow pulse into a rectangular signal, so that the ratio - The integral-differential servo control module 9 can accurately collect the pulse peak value. In addition, when the pulse width is extremely narrow, it is difficult to keep the voltage stable for a long time. Since the charge leakage of the capacitor shows a downward trend, its decreasing speed is not constant related to the pulse amplitude, which will greatly affect the final frequency locking effect. Therefore, the peak hold here The circuit 8 is composed of a primary peak holding part and a secondary peak holding part.

As shown in Figure 2, the first-level peak hold first uses the transconductance amplifier a1 to convert the pulse voltage output by the detector into a pulse current and amplify it. The pulse passes through the diode a2 in the forward direction to charge the holding capacitor a3. The capacitor charge is due to the reverse switching of the diode. The circuit breaker cannot be released quickly, resulting in the phenomenon of pulse peak holding. The capacitor voltage finally passes through the voltage buffer a4 to achieve buffered output. The capacitor used in the first-level peak hold circuit has a small capacitance, and its internal charge leakage is more serious, so the holding voltage will drop to zero quickly. Therefore, the output of the primary peak hold circuit can be regarded as a pulse signal with a wider pulse width than the original pulse, and the secondary peak hold circuit is used to perform peak hold on it in the true sense.

The wide pulse signal is first amplified by the operational amplifier a5, and then divided into two channels. One channel uses the diode a6 and the holding capacitor a7 with a larger value to maintain the peak value. At the same time, a discharge channel controlled by the field effect transistor a8 is set for the capacitor; the other channel The input voltage comparator a9 generates a synchronous logic voltage pulse signal, and its upper edge triggers the monostable trigger a10 to output a fixed-time low-level threshold signal to the gate of the switching circuit field effect transistor to turn it off. At the threshold After the time is over, the field effect transistor is turned on, and the charge in the holding capacitor is quickly released to end the peak hold, thus preparing to process the next pulse. The final peak hold signal is finally output through the voltage buffer a11.

The signal processing flow of the proportional-integral-differential servo module is shown in Figure 3. The proportional-integral-differential servo module 9 receives the output signal of the voltage comparator a9 in the peak hold circuit 8 and the final peak hold signal. Triggered by the rising edge of the comparator output, each held voltage value is collected multiple times and averaged to improve accuracy. This module performs proportional-integral-differential operations on the voltage values of a series of pulses to obtain the corresponding continuous output voltage for control. After level shifting and appropriate amplification of this voltage through the peripheral amplification circuit, the tuning voltage acting on the piezoelectric ceramics of the spectral frequency discrimination interferometer is obtained, so that each longitudinal mode is aligned with each resonance point of the interferometer to ensure atmospheric echoes. Meter scattering signals are accurately filtered out of the signal.

The interferometer frequency locking system of the present invention receives the pulse light of the transmission interferometer and serializes the signal, thereby performing feedback control, and finally locks the resonant frequency of the interferometer to each longitudinal mode frequency of the laser, and can automatically compensate for factors such as ambient temperature. The frequency drift caused by the system has high locking accuracy, the locking process is intelligent and fast, and the cost is low, which creates a fundamental guarantee for the highly stable operation of the interferometer spectral filter in multi-longitudinal mode HSRL, and is helpful for improving the accuracy of HSRL atmospheric detection. A great promotion. In addition, the present invention can also be used to frequency lock pulse lasers for other uses.

In this embodiment, the output wavelength of the multi-longitudinal mode laser 1 is 1064 nm, and the pulse frequency is 10 Hz; the two-stage acousto-optic modulator is driven by 200 MHz to achieve a frequency offset of 400 MHz. Spectral frequency identification interferometer 5 uses a field-widening Michelson interferometer, which can be used for frequency locking in a dynamic range of 1GHz. The photodetector 7 in the system selects an indium gallium arsenic photodetector;

The transconductance amplifier a1 in the first-level peak hold circuit uses the MAX435 chip, the diode a2 uses the Schottky diode BAT17, and the operational amplifier a4 used for buffering the output uses OP07.

The preamplifier in the secondary peak hold circuit uses a wideband amplifier OPA646P, the diode a6 also uses a Schottky diode BAT17, the voltage comparator a9 uses a high-speed differential amplifier LM361, the monostable trigger a10 uses 74HC123, and the field effect transistor a8 uses N The channel enhancement field effect transistor is 2N7000, and the voltage buffer a11 uses BUF601.

Proportional-integral-differential servo module 9 is mainly composed of STM32F103RCT6 microcontroller and peripheral circuits. STM32F103RCT6 has 12-bit AD/DA resolution and a voltage input/output range of 0-3.3V. The peripheral circuit uses dual operational amplifier LM358 to amplify and voltage follow the output of the microcontroller.

In the spectral frequency discrimination interferometer 5, the piezoelectric ceramic driver can use Coremorrow's E00 system, and the displacement stage of the interference arm reflector can use Coremorrow's XP-620 module.

The above-described embodiments describe in detail the technical solutions and beneficial effects of the present invention. It should be understood that the above-mentioned are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions and equivalent substitutions should be included in the protection scope of the present invention.

Claims (6)

1. The frequency locking system of the multi-longitudinal-mode high-spectral-resolution laser radar interferometer is characterized by comprising a multi-longitudinal-mode laser, a half wave plate, a polarization beam splitter prism, an acousto-optic modulation module, a spectrum frequency discrimination interferometer, a converging lens, a photoelectric detector, a peak value holding circuit and a proportional-integral-differential servo control module;

the multi-longitudinal mode pulse laser output by the multi-longitudinal mode laser is split into a high-power beam and a low-power beam by a polarization beam splitter prism after the polarization state of the multi-longitudinal mode pulse laser is adjusted by a half-wave plate, the high-power beam is used as a laser radar detection signal, and the low-power beam is used for frequency locking of a spectrum frequency discrimination interferometer;

the low-power light beam is shifted in frequency by the acousto-optic modulation module and then enters the spectrum frequency discrimination interferometer to be locked, and the light emitted from the low-power light beam is received by the photoelectric detector after passing through the converging lens;

after the electric signal output by the photoelectric detector is transmitted to the peak hold circuit for processing, the peak hold signal and the sampling trigger signal are output to the proportional-integral-derivative servo control module, and the proportional-integral-derivative servo control module outputs tuning voltage to act on the spectrum frequency discrimination interferometer to realize resonance frequency locking.

2. The multi-longitudinal mode high spectral resolution laser radar interferometer frequency locking system of claim 1, wherein the acousto-optic modulation module is a secondary modulation structure comprising a primary acousto-optic modulator and a secondary acousto-optic modulator; the primary acousto-optic modulator is used for performing frequency shift processing on low-power light beams obtained by splitting the output of the multi-longitudinal mode laser; the secondary acousto-optic modulator is used for further frequency shifting the pulse light subjected to frequency shifting.

3. The multi-longitudinal mode high spectral resolution lidar interferometer frequency locking system of claim 1, wherein the peak hold circuit comprises a primary peak hold circuit and a secondary peak hold circuit, the primary peak hold circuit comprising a transconductance amplifier, a discriminator diode-hold capacitor circuit, and a voltage buffer circuit; the secondary peak hold circuit comprises a voltage amplifying circuit, a discriminating diode-holding capacitor circuit, a voltage comparator, a monostable trigger circuit, a peak hold switch circuit and a voltage buffer circuit.

4. The multi-longitudinal mode high spectral resolution lidar interferometer frequency locking system of claim 3, wherein the primary peak hold circuit is configured to achieve a shorter peak hold function, stretching the narrow pulses to wide pulses;

the second-stage peak value holding circuit is used for further holding the wide pulse output by the first-stage peak value holding circuit for a long time, and generating a threshold signal of fixed time when each pulse arrives, so that the holding capacitor is automatically discharged at a set time to realize a fixed peak value holding time length, and in addition, a trigger signal is generated for the AD acquisition process of the proportional-integral-differential servo control module.

5. The multi-longitudinal mode high spectrum resolution laser radar interferometer frequency locking system according to claim 1, wherein the proportional-integral-derivative servo control module receives the peak hold signal and the sampling trigger signal, collects the peak hold voltage in the holding time and performs time-domain averaging, obtains a calculated voltage value after the proportional-integral-derivative operation, performs level conversion and amplification by a peripheral circuit, and outputs continuous tuning voltage, and the continuous tuning voltage is applied to the piezoelectric ceramic module of the spectrum frequency discrimination interferometer to realize feedback control of the resonant frequency.

6. The multi-longitudinal mode high spectral resolution laser radar interferometer frequency locking system according to claim 1, wherein the spectral frequency discrimination interferometer is provided with a piezoelectric ceramic module, and the piezoelectric ceramic module is used for controlling the frequency of the interferometer according to the tuning voltage and comprises a piezoelectric ceramic driver and a piezoelectric ceramic displacement table; the piezoelectric ceramic driver is used for amplifying the tuning voltage in power, and the piezoelectric ceramic displacement table is used for controlling the interference arm length of the interferometer frequency discriminator.