CN107389560A - Multiband all -fiber high spectral resolution total atmospheric spectral transmittance simultaneous measuring apparatus and measuring method - Google Patents

Abstract

The invention discloses a multi-band all-fiber high-spectral resolution whole-layer atmospheric transmittance simultaneous measurement device and measurement method. A sun tracking device is used to mount a plurality of optical coupling modules to couple signal light into an optical fiber. The beamer combines the local oscillator light and the signal light to realize the multi-band all-fiber high-resolution whole-layer atmospheric transmittance measurement. The device uses light transmission and beam combining in the optical fiber to replace the traditional free space transmission and beam combining, which greatly improves the stability and enhances the anti-interference ability of laser heterodyne; at the same time, it uses the sun tracking device to mount multiple optical coupling modules , combined with multiple local oscillators and heterodyned, the laser heterodyne signals of multiple bands can be obtained at the same time, and the real-time measurement of the high-resolution full-layer atmospheric transmittance within the band range of the local oscillator light source is realized. It can be used in fields such as meteorological research, air pollution monitoring and atmospheric optical characteristic measurement.

Description

Multi-band all-fiber high-spectral resolution whole-layer atmospheric transmittance simultaneous measurement device and Measurement methods

technical field

The invention relates to the field of laser atmospheric transmission transmittance measurement, in particular to a device and measurement method for simultaneous real-time measurement of multi-band high-resolution full-layer atmospheric transmittance based on laser heterodyne method combined with sun tracking, belonging to the field of optics.

Background technique

The whole-layer atmospheric transmittance is an important atmospheric parameter in the fields of atmospheric radiation, remote sensing of earth resources, air quality monitoring, space target monitoring, laser atmospheric transmission, etc., especially the measurement of high-resolution whole-layer atmospheric transmittance, in addition to In addition to the above research, it can also be applied to long-distance or whole-layer atmospheric transmittance measurement, multi-component greenhouse gas profile inversion and other fields.

At present, the commonly used devices for measuring the transmittance of the whole layer of the atmosphere mainly include pyranometers and Fourier transform spectrometers. The pyranometers are more portable, but the spectral resolution depends on the bandwidth of the filter used. Generally speaking, it is relatively wide (approximately several nm or even wider); Fourier transform spectrometers can meet the requirements of high resolution, but the spectral resolution of Fourier transform spectrometers is inversely proportional to the moving distance of the moving mirror. The higher the resolution, the larger the volume and poor portability. It is difficult to take into account the field experiment requirements of portability and high spectral resolution.

Using laser heterodyne technology to measure the solar spectrum can achieve high spectral resolution and high detection sensitivity and the instrument is small in size, which can meet the needs of miniaturization and field experiments. However, the beam combining process of local oscillator laser and signal light in common laser heterodyne devices mostly adopts free space optical transmission, which is easily affected by the external environment (such as vibration, atmospheric turbulence, etc.), which affects the stability of laser heterodyne devices . Based on the above reasons, and considering that some field applications need to measure the whole-layer atmospheric transmittance of multiple bands at the same time, a new method for measuring the full-layer atmospheric transmittance of multi-band all-fiber optics with high spectral resolution is invented. , the method can meet the requirements of portable mobility required for field experiments, and can be applied to meteorological research, air pollution monitoring and atmospheric optical characteristic measurement and other fields.

Contents of the invention

In order to make up for the defects of the prior art, the present invention uses the optical coupling module to couple the sunlight captured by the sun tracking device into the optical fiber, realizes the full optical fiber of the optical path of the laser heterodyne device, and introduces multiple beams by adding an optical coupling module. The signal light and the corresponding local oscillator light are combined to generate multiple sets of heterodyne signals after frequency mixing. After calibration and inversion of these heterodyne signals, the transmittance information of the whole layer of the atmosphere with multi-band high spectral resolution can be obtained. The design The advanced measuring device and measuring method have the advantages of high precision, portability and high stability.

Technical scheme of the present invention is as follows:

The multi-band all-fiber high-spectral resolution whole-layer atmospheric transmittance simultaneous measurement device is characterized in that it includes: a sun tracking device mounted with more than one optical coupling module, which respectively couples the received sunlight into the optical fiber as signal light output , the optical coupling module is composed of a converging lens and an optical fiber installed on a fixed bracket, wherein the end face of the optical fiber is located at the focal point of the lens;

More than one local oscillator light device, which respectively couples the local oscillator light of different bands generated by the laser into the optical fiber for output;

More than one optical fiber beam combiner, respectively combine the signal light with the local oscillator light of different bands;

More than one converging lens, converging and outputting the combined beams;

The heterodyne signal detection system includes more than one heterodyne detector, more than one intermediate frequency filter, and more than one square law detector. The converged optical signal is first incident on the corresponding heterodyne detector for optical mixing. The output mixed frequency signal is followed by bandpass filtering and square law detection to obtain the corresponding heterodyne signal;

The signal acquisition system, including a data acquisition card and an acquisition computer, is used to acquire heterodyne signals.

The optical coupling module is mounted on the sun tracking device in parallel.

The local oscillator light device adopts a highly coherent narrow-linewidth laser as a local oscillator light source.

The method for simultaneously measuring the transmittance of the whole layer of the atmosphere with multi-band all-fiber high-spectral resolution is characterized in that, the details are as follows: after the sunlight passing through the atmosphere is captured by the sun tracking device, it is coupled into the optical fiber through an optical coupling module, and the signal light is output. Then use a highly coherent narrow-linewidth laser to emit the corresponding local oscillator light, and couple the local oscillator light into the optical fiber to output the corresponding local oscillator light signal, and then use the fiber beam combiner to combine this group of signal light with the local oscillator light , the combined beams are converged by the converging lens and incident on the heterodyne detector for optical mixing, and the mixed frequency signal obtained is passed through the intermediate frequency filter and the square law detector in turn, and then sent to the bandpass filter for filtering. In addition to noise, computer acquisition is finally used to obtain the corresponding heterodyne signals generated by the sunlight penetrating the atmosphere and the local oscillator laser. After calibration and inversion of these heterodyne signals, the height of sunlight penetrating the entire atmosphere can be obtained. The spectral resolution signal realizes the simultaneous measurement of the multi-band all-fiber optical transmittance of the entire layer of the atmosphere.

The heterodyne signal contains the transmittance information of sunlight through the entire layer of the atmosphere, and the heterodyne signals of different bands are obtained by increasing or decreasing the number of optical coupling modules on the sun tracking device, so as to realize the multi-band entire layer of atmosphere The purpose of measuring transmittance.

The acquisition of the hyperspectral resolution of the multi-band whole layer of the atmosphere is to remove the high-frequency signal through the filter to remove the high-frequency signal from the optical mixing signal of the multi-band combined beam obtained by the heterodyne detector through electronic filtering, and only retain the intermediate frequency signal The bandwidth of the filter used is within 100 megahertz, and the corresponding spectral resolution is better than 0.005cm ^-1 .

The amplitude of the heterodyne signal is positively correlated with the power of the local oscillator light, and the weak signal light to be measured can be amplified by increasing the power of the local oscillator light.

The calibration method described is the Langley calibration method.

The working principle of the present invention is as follows:

The measurement method is realized based on laser heterodyne technology. Laser heterodyne is to combine the measured signal light with the local oscillator laser beam and then incident on the fast response photodetector, using the coherence of the two light fields on the photosensitive surface of the detector. The square-law response characteristics of the subsequent square-rate detector and the subsequent optical frequency mixing are also called optical heterodyning. The detection principle is shown in Figure 1. The photocurrent output by the photodetector contains the high-frequency signal generated by the beat frequency of the local oscillator light and the signal light—that is, the mixing signal. Utilizing the controllability and amplification of the local oscillator light, the amplitude, frequency and phase information of the signal light can be obtained by measuring and processing the heterodyne signal.

After the signal light and the local oscillator light are combined by the beam combiner, they are converged by the lens and incident on the photosensitive surface of the detector to achieve frequency mixing and generate sum frequency and difference frequency electrical signals. The frequency response of general photodetectors is far from keeping up with the extremely high frequency optical frequency and sum frequency, and can only respond to the difference frequency signal. The difference frequency signal is amplified by the preamplifier to output an analog amplified heterodyne electrical signal with a certain bandwidth range. For the convenience of analysis, it is assumed that the two beams of light are ideal monochromatic light, namely: local oscillator laser E _lo =A _lo cosω _lo t, signal light E _s =A _s cosωst. Then the output current detected by the heterodyne detector is:

i(t)＝α(A _lo cosω _lo t+A _s cosω _s t) ² (1)

where α is a constant of proportionality including the quantum efficiency of the detector. Assuming that the amplitude of the signal light and the local oscillator light does not change with time, the above formula is further simplified and reasoned with the trigonometric formula, we can get:

In the above formula, cos2ω _lo t, cos2ω _s t and cos(ω _lo +ω _s )t are all far larger than the bandwidth of the photodetector circuit, so the output after integration is zero. If ω _lo and ω _s are close enough, the frequency difference between the two is within the response range of the heterodyne detector. At this time, ignoring the high-frequency items that the heterodyne detector cannot detect, the detected intermediate frequency current signal (Intermidiate Frequency) i _IF is further simplified to:

i _IF =αA _LO A _s cos(ω _LO -ω _s )t (3)

Signal Power:

P _IF ＝(i _IF ) ² R＝2α ² P _LO P _S R (4)

The above analysis is the heterodyne between the two laser beams, but for the heterodyne between the laser and the broadband light source (such as sunlight), the broadband light source can be regarded as the superposition of a series of narrow linewidth laser light sources, and the analysis process Similar to the above analysis. Through the above analysis, it can be seen that the laser heterodyne detection has the following characteristics: (1) The frequency of the intermediate frequency signal i _IF is equal to the frequency difference between the local oscillator light and the signal light (this is also the reason why the beat frequency is called heterodyne), such as The frequency of the array light and the signal light are similar, and the frequency of the heterodyne signal will drop from the infrared range to the intermediate frequency range, so that it is easy to be measured by a fast-response detector; (2) The intermediate frequency signal i _IF is proportional to the signal light and the local oscillation It is not only related to the power of the signal light, but also related to the power of the local oscillator light. The greater the optical power of the local oscillator, the greater the amplitude of the heterodyne signal. By increasing the power of the local oscillator, the weak signal light to be measured can be amplified; (3) The frequency of the heterodyne signal depends on the local The frequency difference between the vibration light and the signal light, when the signal light is a narrow linewidth laser light source, the frequency of the heterodyne signal is the difference between the frequency of the local oscillator light and the signal light; when the signal light is a broadband light source, the frequency of the heterodyne signal As a broadband signal, the bandwidth depends on the response bandwidth of the detection circuit and the subsequent filter circuit.

When measuring the transmittance of the whole layer of the atmosphere, choose sunny and stable weather, and use the Langley calibration method to calibrate the measured heterodyne signal. Assuming that the atmosphere is composed of several layers of parallel planes, at a given position, according to the Beer-Lambert law, the irradiance P(λ) of direct solar radiation with a wavelength of λ measured by the laser heterodyne system on the ground can be Expressed as:

P(λ)＝P ₀ (λ)(d ₀ /d) ² exp[-m(θ)τ(λ)] (5)

Among them, P ₀ (λ) is the system measurement value corresponding to the solar irradiance at the top of the atmosphere, (d ₀ /d) ² is the sun-earth distance correction factor, which is used to correct the solar irradiance at the upper boundary of the atmosphere, m (θ) is the relative air mass on the optical path along the zenith angle θ, and τ(λ) is the optical thickness of the atmosphere pointing to the zenith at a wavelength of λ. Atmospheric slope transmittance is:

T(λ)＝P(λ)/[P ₀ (λ)(d ₀ /d) ² ] (6)

Take the logarithm on both sides of (2) to get:

ln[P(λ)/(d ₀ /d) ² ]＝lnP ₀ (λ)–m(θ)τ(λ) (7)

Take the air mass m(θ) as the independent variable and ln[P(λ)/(d ₀ /d) ² ] as the variable in the Cartesian coordinate system. If τ(λ) is constant, a straight line is obtained, and the intercept is lnP ₀ (λ), the slope is τ(λ) which is the total optical thickness, and the intercept can be obtained by linear fitting and extrapolation by the least square method. This calibration method is also the most commonly used calibration method in related research. After calibration, it can be used for high-resolution whole-layer atmospheric transmittance measurement. Under ideal calibration weather conditions, the calibration can obtain a high enough accuracy, and at the same time, the calibration accuracy can be verified by multiple calibrations and comparisons with each other.

Compared with the prior art, the present invention has the advantages of:

Device of the present invention has realized:

(1) The all-fiber transmission and beam combining of light replaces the traditional free space transmission and beam combining, which greatly improves the stability of optical transmission and enhances the anti-interference ability of laser heterodyne;

(2) Use the sun tracking device to mount multiple optical coupling modules to realize simultaneous measurement of multiple bands.

The measurement method of the present invention not only inherits the advantages of the laser heterodyne measurement technology, such as: it has an amplification effect on weak signal light and has high spectral resolution. In particular, the use of signal light and local oscillator light in all-fiber transmission replaces common free-space light transmission, which has good characteristics of anti-environmental interference. At the same time, using the sun tracking device to hang on multiple optical coupling modules can meet the needs of simultaneous measurement of multiple wavebands, making this method more important in application value.

Description of drawings

Figure 1 is a schematic diagram of laser heterodyne detection.

Fig. 2 is a schematic diagram of the optical coupling module in the present invention.

Among them, 1. lens; 2. fixed bracket; 3. optical fiber;

Fig. 3 is a structural schematic diagram of the measuring device used in the embodiment.

Among them, 4. Sun tracking device; 5. Local oscillator light one; 6. Local oscillator light two; 7. Local oscillator light three; 8. Fiber beam combiner; 9. Converging lens; 10. Heterodyne detector; 11. Intermediate frequency filter Circuit; 12. Square rate detector; 13. Data acquisition card; 14. Acquisition computer.

detailed description

The present invention will be further described below in combination with specific embodiments.

As shown in Figure 3, measuring device of the present invention comprises:

The optical coupling module (Fig. 2) consists of three parts (1-3). The converging lens (1) is installed on a fixed bracket parallel to the sun tracking device (4). The received sunlight is converged by the converging lens (1) And incident on the end face of the optical fiber (3), the end face of the optical fiber is at the position of the focal point of the lens.

The three beams of sunlight signals output by the three optical coupling modules (Figure 2) mounted on the sun tracking device (4) are then combined with the local oscillator light (5-7) of different emission wavelengths via the beam combiner (8) The beam is incident on the heterodyne detector (9), and after the detected signal passes through the intermediate frequency filter (10) and the square law detector (11), the high-resolution heterodyne signal is obtained and input to the data acquisition card (12) and Finally collected by computer (13).

The specific measurement steps are as follows:

The sunlight passing through the atmosphere is captured by the sun tracking device (4) and coupled into the optical fiber through three optical coupling modules to output three beams of signal light, and then use a highly coherent narrow linewidth laser to emit three beams of local oscillator light with different wavelengths (5-7), and the local oscillator light is coupled into the optical fiber to output three beams of local oscillator light signals, and then the three beams of signal light are combined with the three beams of local oscillator light by using the fiber combiner (8). The final light beam is converged by the converging lens (9) and then incident on the heterodyne detector (10), and the detected signal is passed through the intermediate frequency filter (11) and the square law detector (12) to obtain the heterodyne detector (12) with high spectral resolution The difference signal is input to the data acquisition card (13) and finally collected by the computer (14). The collected signal is the heterodyne signal generated by the combination of sunlight and local oscillator light, and then calibrated by the Langley calibration method, which realizes Multi-band high-spectral resolution measurement of the entire layer of atmospheric transmittance; in the case of suitable atmospheric conditions (ideal calibration weather conditions), the calibration can obtain high enough accuracy, and at the same time, the calibration can be verified by multiple calibrations and mutual comparisons precision.

Claims (8)

1. multiband all -fiber high spectral resolution total atmospheric spectral transmittance simultaneous measuring apparatus, it is characterised in that including：

The sun tracker of more than one optical coupled module of carry, the sun will be received respectively and is optically coupled into optical fiber conduct Signal light output, described optical coupled module are made up of the plus lens on fixed support and optical fiber, wherein light Fine end face is located at lens focus position；

More than one local oscillator electro-optical device, the local oscillator of different-waveband caused by laser is optically coupled into respectively defeated in optical fiber Go out；

More than one optical-fiber bundling device, respectively by flashlight and the local oscillator combiner of different-waveband；

More than one plus lens, the light beam after conjunction beam is converged into output respectively；

Heterodyne signal detection system, including it is more than one heterodyne detector, more than one intermediate-frequency filter, more than one Square-law detector, the optical signal of plus lens output, which is first incided in corresponding heterodyne detector, carries out optical frequency mixing, exports Mixed frequency signal carry out bandpass filtering successively again and square-law detection obtains corresponding heterodyne signal；

Signal acquiring system, including data collecting card and collection computer, for gathering heterodyne signal.

2. multiband all -fiber high spectral resolution total atmospheric spectral transmittance simultaneous measuring apparatus as claimed in claim 1, its It is characterised by, described optical coupled module parallel carry is on sun tracker.

3. multiband all -fiber high spectral resolution total atmospheric spectral transmittance simultaneous measuring apparatus as claimed in claim 1, its It is characterised by, using highly coherent narrow linewidth laser as local oscillator light source in described local oscillator electro-optical device.

4. multiband all -fiber high spectral resolution total atmospheric spectral transmittance while measuring method, it is characterised in that specific as follows： Through atmosphere sunshine captured by sun tracker after be coupled into optical fiber by optical coupled module, output signal Light, corresponding local oscillator light then is sent using highly coherent narrow linewidth laser, and local oscillator optical coupling is entered in optical fiber, exported Corresponding local oscillator optical signal, followed by optical-fiber bundling device by this group of flashlight and local oscillator combiner, the light beam closed after beam is sharp again Incided with plus lens convergence and optical frequency mixing is carried out in heterodyne detector, obtained mixed frequency signal passes through intermediate-frequency filter successively After square-law detector, it is again fed in bandpass filter and filters out noise, finally using computer acquisition, obtains through big These heterodyne signals are demarcated and inverting by heterodyne signal caused by the sunshine and local oscillator laser of gas-bearing formation, you can are obtained too The high spectral resolution signal of sunlight penetration whole atmosphere, realize the multiband all-fiber of total atmospheric spectral transmittance while survey Amount.

5. multiband all -fiber high spectral resolution total atmospheric spectral transmittance as claimed in claim 4 while measuring method, its It is characterised by, described heterodyne signal passes through the transmitance information of whole atmosphere with sunshine, and by being filled in solar tracking Put and increase and decrease the quantity of optical coupled module to obtain the heterodyne signal of different-waveband, realize multiband total atmospheric spectral transmittance Measure purpose.

6. multiband all -fiber high spectral resolution total atmospheric spectral transmittance as claimed in claim 4 while measuring method, its It is characterised by, the acquisition of described multiband whole atmosphere high spectral resolution is to filter to obtain heterodyne detector by electronics The optical frequency mixing signal of the combined beam light of the multiple wave bands taken removes high-frequency signal by wave filter, only retains intermediate-freuqncy signal to realize , wherein the bandwidth of used wave filter, within hundred megahertzs, spectral resolution correspondingly is better than 0.005cm^-1。

7. multiband all -fiber high spectral resolution total atmospheric spectral transmittance while measuring method as described in claim 4 or 5, Characterized in that, the amplitude of described heterodyne signal and local oscillation optical power positive correlation, can be right by the power for increasing local oscillator light Small-signal light to be measured plays amplification.

8. multiband all -fiber high spectral resolution total atmospheric spectral transmittance as claimed in claim 4 while measuring method, its It is characterised by, described scaling method is bright sharp standardization.