CN110456383A - A Molecular Scattering Coherent LiDAR System - Google Patents

Abstract

The present application provides a molecular scattering coherent laser radar system. The molecular scattering coherent laser radar system uses a coherent detection method to detect the molecular scattering spectral lines in the atmospheric backscattering signal. By processing the molecular scattering spectral lines, The simultaneous measurement of wind speed and temperature can be realized.

Description

A Molecular Scattering Coherent LiDAR System

technical field

The invention relates to the technical field of laser radar, and more specifically, relates to a molecular scattering coherent laser radar system.

Background technique

In the field of atmospheric detection, Doppler lidar for atmospheric wind field detection generally adopts two methods: direct detection and coherent detection.

Among them, direct detection uses the elastic scattering of atmospheric molecules or aerosols, converts Doppler frequency changes into intensity changes through molecular absorption or high-precision narrow-band discriminators, and then calculates Doppler frequency shifts by detecting intensity changes; coherent The detection uses the backscattered light of aerosol particles and a continuous local oscillator light to perform beat frequency, and the Doppler frequency shift can be obtained directly by detecting the beat frequency signal.

However, since the aerosol components in the atmosphere are mainly distributed in the lower altitude range such as the boundary layer, the detection height of coherent Doppler lidar is limited to a certain extent. Compared with the narrow-band meter scattering signal of molecular scattering, the spectral width of Rayleigh scattering caused by thermal motion is wider. If it is used in coherent detection, it will bring certain difficulties to the beat frequency and detection of the signal.

For the detection of atmospheric temperature, currently commonly used lidar technologies mainly include rotational Raman technology, integral technology, and resonance fluorescence technology, among which rotational Raman technology utilizes the dependence between temperature and rotational Raman line intensity to invert temperature; integral The technology is to calculate the molecular number density, and then calculate the atmospheric temperature; the resonance fluorescence technology is to use the Doppler broadening of the resonance fluorescence of the metal atom to invert the temperature. In addition, the differential absorption technique and the Brillouin-Doppler technique both measure the temperature through the intensity of the molecular scattering signal or the Doppler broadening effect, rather than directly detecting the spectral width of the molecular scattering signal.

At present, there are few lidar systems capable of simultaneous measurement of wind speed and temperature, and there are no related reports on lidar systems that measure wind speed and temperature simultaneously by directly measuring molecular scattering lines.

Contents of the invention

In view of this, in order to solve the above problems, the present invention provides a molecular scattering coherent laser radar system, the technical solution is as follows:

A molecular scattering coherent lidar system, the molecular scattering coherent lidar system comprising:

Fiber seed laser, polarization maintaining fiber beam splitter, acousto-optic modulator, main laser, beam expander, polarization beam splitter, quarter wave plate, coupling lens, fiber adjustment mount, fiber coupler, balanced photodetector, Amplifying module, analog-to-digital conversion data acquisition device and beam expander telescope;

Wherein, the fiber seed laser is used to output single longitudinal mode continuous light;

The optical fiber beam splitter is used to divide the polarization state of the single longitudinal mode continuous light into the seed light and the local oscillator light according to a preset ratio;

The acousto-optic modulator is used to frequency shift the seed light;

The main laser is used to output pulsed light;

The beam expander is used to expand the pulsed light and compress the divergence angle;

The coupling lens is used to couple the atmospheric backscattered light to the optical fiber coupler;

The polarization splitter prism and the quarter-wave plate form an optical transceiver switch, which only allows P-polarized light to pass through, and S-polarized light to reflect on the splitting surface;

The quarter-wave plate is used to adjust the polarization state of emitted laser light and atmospheric backscattered light;

The fiber coupler receives the atmospheric backscattered light and the local oscillator light, and couples them to the balanced photodetector;

The balanced photodetector is used to convert the beat signal obtained by mixing the atmospheric backscattered light and the local oscillator light into an electrical signal;

The amplifying module is used to amplify the electrical signal;

The analog-to-digital conversion data acquisition device is used to convert the amplified electrical signal into a digital signal.

Preferably, in the above molecular scattering coherent lidar system, the fiber seed laser is used to output single longitudinal mode continuous light with a wavelength of 1064nm.

Preferably, in the above-mentioned molecular scattering coherent lidar system, the optical fiber beam splitter is used to split the single longitudinal mode continuous The light is divided into local oscillator light and seed light;

Among them, 99% of the light is used as the seed light, and 1% of the light is used as the local oscillator light.

Preferably, in the above molecular scattering coherent laser radar system, the acousto-optic modulator is an all-fiber frequency shifting element, and the frequency shift is 1 GHz.

Preferably, in the above molecular scattering coherent lidar system, the main laser is a seed-injected diode-pumped Nd:YAG laser.

Preferably, in the above-mentioned molecular scattering coherent lidar system, the amplifying module includes: a low-noise preamplifier and a low-noise amplifier;

Wherein, the electrical signal passes through the low noise preamplifier and the low noise amplifier in order to amplify the electrical signal.

Preferably, in the above molecular scattering coherent lidar system, the beam expander telescope is an off-axis reflective beam expander telescope;

Wherein, the reflective surface of the beam expander telescope is coated with a high-reflective dielectric film.

Preferably, in the above molecular scattering coherent lidar system, the beam expander telescope includes: a primary mirror and a secondary mirror.

Compared with the prior art, the beneficial effects realized by the present invention are:

The molecular scattering coherent lidar system uses coherent detection to detect the molecular scattering spectral lines in the atmospheric backscattering signal. By processing the molecular scattering spectral lines, the simultaneous measurement of wind speed and temperature can be realized.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only It is an embodiment of the present invention, and those skilled in the art can also obtain other drawings according to the provided drawings without creative work.

FIG. 1 is a schematic structural diagram of a molecular scattering coherent lidar system provided by an embodiment of the present invention.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

In order to make the above objects, features and advantages of the present invention more comprehensible, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Referring to FIG. 1 , FIG. 1 is a schematic structural diagram of a molecular scattering coherent lidar system provided by an embodiment of the present invention.

The molecular scattering coherent lidar system includes:

Fiber seed laser 1, polarization maintaining fiber beam splitter 2, acousto-optic modulator 3, main laser 4, beam expander 5, polarization beam splitter prism 6, quarter wave plate 7, coupling lens 8, fiber adjustment frame 9, Fiber coupler 10, balanced photodetector 11, amplification module 12, analog-to-digital conversion data acquisition device 15 and beam expander telescope 18;

Wherein, the fiber seed laser 1 is used to output single longitudinal mode continuous light;

The polarization-maintaining fiber beam splitter 2 is used to divide the polarization state of the single longitudinal mode continuous light into seed light and local oscillator light according to a preset ratio;

The acousto-optic modulator 3 is used for frequency shifting the seed light;

The main laser 4 is used to output pulsed light;

The beam expander 5 is used to expand the pulsed light and compress the divergence angle;

The polarization splitter prism 6 and the quarter-wave plate 7 form an optical transceiver switch, which only allows P-polarized light to pass through, and S-polarized light to reflect on the splitting surface;

The quarter wave plate 7 is used to adjust the polarization state of emitted laser light and atmospheric backscattered light;

The coupling lens 8 is used to couple the atmospheric backscattered light to the optical fiber coupler;

The fiber coupler 10 receives the atmospheric backscattered light and the local oscillator light, and couples them to the balanced photodetector;

The balanced photodetector 11 is used to convert the beat frequency signal mixed with the atmospheric backscattered light and the local oscillator light into an electrical signal;

The amplifying module 12 is used to amplify the electrical signal;

The analog-to-digital conversion data acquisition device 15 is used to convert the amplified electrical signal into a digital signal.

In this embodiment, the coherent detection method is used to detect the molecular scattering line of the atmospheric backscattered light, and the simultaneous measurement of wind speed and temperature can be realized by processing the molecular scattering line.

Further, based on the above embodiments of the present invention, the fiber seed laser 1 is used to output single longitudinal mode continuous light with a wavelength of 1064nm.

In this embodiment, the seed laser is a narrow linewidth fiber laser, which is mainly used to output single longitudinal mode continuous light with a wavelength of 1064nm as the seed light source.

Further, based on the above embodiments of the present invention, the main laser 4 is a seed-injected diode-pumped Nd:YAG laser.

In this embodiment, the main laser is used to output pulsed light with a wavelength of 1064nm. Coherent lidar systems usually work in the near-infrared band (1 μm, 1.5 μm-1.6 μm or 2 μm). Because long-wave bands are easier to achieve optical coherent beat frequencies, coherent radars usually use long-wavelengths above 1 μm.

In order to realize that the system of the present application can achieve a higher detection range than traditional coherent radar, it is necessary for the laser to provide an output energy of the order of hundreds of mJ.

Compared with the 1.5μm or 2μm wavelength band, the fiber laser with 1064nm wavelength can provide higher output power and single pulse energy. In addition, because the molecular scattering intensity is inversely proportional to the fourth power of the wavelength, the molecular backscattering intensity at 1064nm wavelength is stronger powerful.

In this application, the detection object of the molecular scattering coherent lidar system is the molecular scattering spectrum, the laser wavelength is 1064nm, the main component of the molecular scattering spectrum is Rayleigh scattering, and the full width at half maximum (FWHM) of the atmospheric backscattering spectrum satisfies the following formula :

Among them, Δv is the full width at half maximum of the atmospheric backscattering spectrum; k is the Boltzmann constant; T is the atmospheric temperature; λ is the laser wavelength; m is the average mass of a single air molecule.

It can be known by calculation that the full width at half maximum of the atmospheric backscattering spectrum is 1.2 GHz, which is much larger than the bandwidth of aerosol scattering.

Further, based on the above-mentioned embodiments of the present invention, the polarization-maintaining fiber beam splitter 2 is a 1×2 polarization-maintaining fiber beam splitter, which is used to ensure that the polarization state of the single longitudinal mode continuous light remains unchanged, The single longitudinal mode continuous light is divided into local oscillator light and seed light according to a ratio of 1:99; wherein, 99% of the light is used as the seed light, and 1% of the light is used as the local oscillator light.

Wherein, the seed light is used to enter the acousto-optic modulator for frequency shifting, and the frequency-shifted light is injected into the main laser to generate laser pulses.

The local oscillator light is used to mix with atmospheric backscattered light to generate a beat frequency signal.

Further, based on the above-mentioned embodiments of the present invention, the acousto-optic modulator 3 is an all-fiber frequency shifting element, which is used to shift the frequency of the seed light so as to determine the wind speed and direction in the application of wind measurement.

Wherein, since the bandwidth of the molecular scattering spectrum to be detected is very wide, the frequency shift of the acousto-optic modulator is designed to be 1 GHz.

Further, based on the above embodiments of the present invention, the beam expander 5 is used to expand the pulsed light output by the main laser, and at the same time compress the divergence angle, and output it to the polarization beam splitter prism after beam expansion and collimation.

Further, based on the above-mentioned embodiments of the present invention, the polarization splitter prism 6 and the quarter-wave plate 7 form an optical transceiver switch. Only p-polarized light is allowed to pass through, and s-polarized light is reflected on the splitting surface.

The quarter-wave plate 7 is used to adjust the polarization state of outgoing laser light and atmospheric backscattered light.

Further, based on the above embodiments of the present invention, the fiber coupler 10 is a 2×2 polarization-maintaining fiber coupler.

Wherein, the optical fiber coupler receives the atmospheric post-scattered light and the local oscillator light, and couples them to the balanced photodetector, and performs signal detection after the surface of the balanced photodetector beats to ensure that the transmitted light The state of polarization remains unchanged.

Further, based on the above-mentioned embodiments of the present invention, the balanced photodetector 11 is used to convert the beat frequency signal obtained by mixing the atmospheric backscattered light and the local oscillator light into an electrical signal.

Among them, the method of balanced detection can eliminate the extra intensity noise of the local oscillator light, strengthen the intermediate frequency signal, and improve the signal-to-noise ratio. Moreover, it can receive all the output optical energy of the optical fiber, and make full use of the optical power of the local oscillator.

Atmospheric backscattered light signal detection is a relatively weak signal detection, and usually requires balanced photodetectors to have high responsivity, transimpedance gain, high saturation power threshold and sufficiently high 3dB corresponding bandwidth.

The full width at half maximum of the Rayleigh scattering spectrum corresponding to the wavelength of 1064nm is about 1.2GHz. Considering the frequency shift of the acousto-optic modulator and the Doppler frequency shift corresponding to the wind speed measurement range, the bandwidth of the balanced detector is finally designed to be 5GHz.

Further, based on the above-mentioned embodiments of the present invention, the amplifying module 12 includes: a low-noise preamplifier 13 and a low-noise amplifier 14;

Wherein, the electrical signal passes through the low noise preamplifier and the low noise amplifier in order to amplify the electrical signal.

In this embodiment, the bandwidth of the molecular scattering spectrum signal obtained by the coherent detection system is very wide, and the signal is very weak, which is likely to be submerged in the noise. In order to observe the complete signal and further effectively extract the spectrum signal, in this The pending low-noise preamplifier and low-noise amplifier feature low noise, wide operating frequency range, and high power gain.

Wherein, the power gain of the low-noise preamplifier is 40dB, and the working frequency range is 10kHz-2GHz.

The power gain of the low noise amplifier is 27dB, and the working frequency range is 50MHz-5GHz.

Further, based on the above-mentioned embodiments of the present invention, the analog-to-digital conversion data acquisition device 15 is used to convert the amplified analog-to-electrical signal into a digital signal.

Wherein, the analog bandwidth of the analog-to-digital conversion data acquisition device is greater than the highest frequency of the sampling signal, and then the adopted frequency is determined according to the Nyquist sampling theorem.

The sampling frequency of the analog-to-digital conversion data acquisition device is 10GHz.

Further, based on the above embodiments of the present invention, the beam expander telescope 18 is an off-axis reflective beam expander telescope; wherein, the reflective surface of the beam expander telescope is coated with a high-reflective dielectric film.

The beam expander telescope 18 includes: a primary mirror 17 and a secondary mirror 16, with an effective aperture of 150 mm, used to expand and collimate the output light into the atmosphere and receive atmospheric backscatter signals.

Based on all the above-mentioned embodiments of the present invention, the working principle and flow of the molecular scattering coherent lidar system will be described below by way of example.

As shown in Figure 1, the fiber seed laser emits continuous linearly polarized light with a wavelength of 1064nm, which is split into two beams by a 1×2 polarization-maintaining fiber beam splitter at a ratio of 1:99.

Among them, a small part of continuous light is used as local oscillator light, which is exported from the b port of the 1×2 polarization maintaining fiber splitter, enters the 2×2 polarization maintaining fiber coupler from the d port of the 2×2 polarization maintaining fiber coupler, and the atmosphere After the backscattered light signal is beat, the 2×2 polarization-maintaining fiber coupler splits the beam into the balanced photodetector at a ratio of 1 to 1 for signal detection.

Most of the continuous light is used as seed light, and enters the acousto-optic modulator from the a port of the 1×2 polarization-maintaining fiber beam splitter for frequency shift. The frequency shift is 1 GHz. 1064 pulsed light.

The pulsed light output by the main laser is expanded through a beam expander, and the divergence angle is compressed at the same time.

The installation direction of the polarization beam splitter needs to make the linearly polarized light emitted by the main laser projected, and the transmitted light is guided into the quarter-wave plate, becomes circularly polarized light after passing through the quarter-wave plate, and then enters the beam expander telescope.

The beam expander telescope adopts an off-axis reflective beam expander telescope, which is composed of a secondary mirror and a primary mirror. They are all parabolic mirrors, and their reflective surfaces are coated with high-reflective dielectric film, and the reflectivity is ≥99.5%.

The transmitted light is incident on the secondary mirror for reflection and then incident on the primary mirror. The primary mirror collimates and expands the beam. The expanded parallel light enters the atmosphere and interacts with the atmosphere to generate atmospheric backscattered light.

Atmospheric backscattered light is collected by the beam expander telescope through the same optical path, and then changes from circularly polarized light to linearly polarized light after passing through a quarter-wave plate. Reflection occurs when polarizing a beamsplitter.

The parallel reflected light is incident into the coupling lens and converged, and then coupled into the 2×2 polarization maintaining fiber coupler through the c port of the 2×2 polarization maintaining fiber coupler.

Among them, the fiber adjustment frame is used to adjust the position of the fiber head to couple all the atmospheric backscattered light to the 2×2 polarization-maintaining fiber coupler.

Atmospheric backscattered light and local oscillator light are mixed in a 2×2 polarization-maintaining fiber coupler. The mixed signal is divided into two beams according to the ratio of 1 to 1 and enters the balanced photodetector. The signal is detected after the surface of the balanced photodetector beats. .

The electrical signal output by the balanced photodetector enters the amplification module and enters the signal amplification. It is first amplified by the low-noise preamplifier with a power gain of 40dB, and the output signal is then amplified by the low-noise amplifier with a power gain of 27dB.

The fully amplified signal is collected by an analog-to-digital conversion data acquisition device, which converts the continuous analog electrical signal into a discrete digital signal, and the sampling frequency is 10GHz.

The discrete data signals obtained by sampling are analyzed and processed, and combined with corresponding algorithms, atmospheric parameters such as wind speed and temperature can be obtained.

From the above description, it can be seen that a molecular scattering coherent lidar system provided by this application uses coherent detection to detect molecular scattering spectrum. It is a brand-new detection technology in the field of wind and temperature measurement lidar, and there is no relevant report at home and abroad. .

Moreover, compared with the traditional molecular scattering lidar, it is not necessary to convert the frequency change into an intensity change for measurement, but directly measure the molecular scattering spectrum, analyze the Doppler signal in the frequency domain, and obtain the corresponding parameters after data processing , and its measurement accuracy is higher.

Since the detection object is broad-spectrum molecular scattering, which is different from the narrow-band aerosol scattering measurement of traditional coherent detection, the distribution range of molecular scattering is wider, and the detection height range is no longer limited to the bottom atmosphere such as the boundary layer.

Moreover, considering the wide bandwidth of the molecular scattering spectrum, the width of the balanced photodetector, the amplification module and the analog-to-digital conversion data acquisition device are all greater than 2 GHz, so that the measurement of wide-spectrum signals can be realized.

And, considering that the molecular scattering signal detected by the coherent detection system is very weak, a two-stage amplification module composed of a low-noise preamplifier and a low-noise amplifier is designed. While suppressing noise, the power gain is also high, and the weak signal is realized. detection and extraction.

The molecular scattering coherent laser radar system provided by the present invention has been introduced in detail above. In this paper, specific examples are used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only used to help understand the present invention. method and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation and application scope. Invention Limitations.

It should be noted that each embodiment in this specification is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. For the same and similar parts in each embodiment, refer to each other, that is, Can. As for the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and for the related information, please refer to the description of the method part.

It should also be noted that in this article, relational terms such as first and second etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that these entities or operations Any such actual relationship or order exists between. Furthermore, the term "comprises", "comprises" or any other variation thereof is intended to cover a non-exclusive inclusion such that elements inherent in a process, method, article, or apparatus comprising a set of elements are included, or are also included as such , method, article or device inherent in the elements. Without further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of additional identical elements in the process, method, article or apparatus comprising said element.

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (8)

1. a kind of molecular scattering coherent lidar system, which is characterized in that the molecular scattering coherent lidar system packet It includes:

Optical fiber seed laser, polarization-maintaining optical fiber beam splitter, acousto-optic modulator, main laser, beam expanding lens, polarization splitting prism, four / mono- wave plate, coupled lens, fiber adjusting mount, fiber coupler, balance photodetector, amplification module, analog-to-digital conversion number According to acquisition device and beam expanding telescope；

Wherein, the optical fiber seed laser is for exporting the continuous light of single longitudinal mode；

The polarization-maintaining optical fiber beam splitter is used in the case where guaranteeing that the continuous polarization state of single longitudinal mode is constant, according to default ratio Example is divided into seed light and local oscillator light；

The acousto-optic modulator is used to carry out frequency displacement to the seed light；

The main laser is for exporting pulsed light；

The beam expanding lens by the pulsed light for being expanded and compressing the angle of divergence；

The polarization splitting prism and quarter-wave plate composition optical transmitting and receiving switch, only allow P-polarized light to penetrate, S polarized light Reflex occurs on light splitting surface；

One of described four parts wave plates are used to adjust the polarization state of transmitting laser and atmospheric backscatter light；

The coupled lens are used to scattering after the atmosphere being optically coupled to the smooth pricker coupler；

The fiber coupler scatters light and the local oscillator light after receiving the atmosphere, and couples the balance photoelectricity for the two Detector；

The balance photodetector is used to that light will to be scattered after the atmosphere and the mixed beat signal of the local oscillator light converts For electric signal；

The amplification module is for amplifying the electric signal；

The analog-digital conversion data acquisition device is used to amplified electric signal being converted to digital signal.

2. molecular scattering coherent lidar system according to claim 1, which is characterized in that the optical fiber seed laser Device is used to export the continuous light of single longitudinal mode of 1064nm wavelength.

3. molecular scattering coherent lidar system according to claim 1, which is characterized in that the fiber optic splitter is used In in the case where guaranteeing that the continuous polarization state of single longitudinal mode is constant, according to 1 to 99 ratio by the continuous light of the single longitudinal mode It is divided into local oscillator light and seed light；

Wherein, 99% light is as seed light, and 1% light is as local oscillator light.

4. molecular scattering coherent lidar system according to claim 1, which is characterized in that the acousto-optic modulator is All -fiber formula shift frequency element, frequency shift amount 1GHz.

5. molecular scattering coherent lidar system according to claim 1, which is characterized in that the main laser is kind The Nd:YAG laser of sub- injection diode pumping.

6. molecular scattering coherent lidar system according to claim 1, which is characterized in that the amplification module packet It includes: low-noise preamplifier and low-noise amplifier；

Wherein, the electric signal successively passes through the low-noise preamplifier and the low-noise amplifier, described in amplification Electric signal.

7. molecular scattering coherent lidar system according to claim 1, which is characterized in that the beam expanding telescope is Off-axis reflection beam expanding telescope；

Wherein, high inverse medium film is coated on the reflecting surface of the beam expanding telescope.

8. molecular scattering coherent lidar system according to claim 1, which is characterized in that the beam expanding telescope packet It includes: primary mirror and secondary mirror.