ЛЕК Устр и действ / Устройство и действие Л-15

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The ТЕА-laser resonator is stable-type with Fr 1 and optical aperture 12 mm. The resonator provides transversal single-mode generation and near- diffraction-limited divergence. The resonator’s mechanical structure is based on a four-pass layout with two roof mirrors. Those mirrors provide relative insensitivity to vibration and temperature disturbances. Rotating diffraction grating serves as a back mirror. The output mirror has a central hole for cw beam injection.

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2

1 – output mirror, 2 – rotated grating, 3 – fold mirror, 4, 5 – roof mirrors.

Inter-mode interval of a stable singlemode TEA-laser’s resonator is about 10

MHz, which is much less than the gain contours width at each of the utilized 60 lines of VR spectrum.

AFC system operation

1 – voltage at the piezo-drive controlling the TEA resonator length, 2 – reflected signal measured by the pyro-sensor

TEA laser pulse profile, VR line 9R18, а – free generation mode; b – injection mode

Pulse generation in the presence of injected single-frequency radiation occurs in a singlefrequency mode under the condition of quite accurate coincidence between the TEA laser resonator’s longitudinal mode and the injector’s radiation frequency. Allowable deviation depends on the injector’s power and typically does not exceed 2 3 MHz. That is why before the pumping of TEA laser it is necessary to perform longitudinal mode tuning. It is executed with the help of an interferometer assembled on a pyro-sensor 5. The TEA-laser resonator’s length is tuned by a piezo-drive. With an accurate enough agreement between the resonator mode and the injected radiation frequency the power of the radiation reflected by the TEA-laser resonator is minimal. Within the piezo-drive operation range four operational points are realized and each of the points can be used for single-frequency pulse generation. The TEA-laser resonator’s tuning is performed automatically under the control of the AFC system. The tuning process takes about 30 ms and is repeated before each pulse as a part of 10-Hz operational cycle of TEA laser.

De, A – telescope aperture diameter and area F, R – focusing and sounding distance

B – bandwidth

E – pulse energy

( ) – backscattering volume efficiency r0 – coherence length

– total extinction

Heterodyne detection of the scattered radiation obtained with the help of a wide-aperture receiving telescope allows increasing SNR as well as the lidar system operational distance considerably. The required pulse energy and accessible sounding distance can be estimated as follows:

SNR value obtained experimentally:

the real lidar system spatial resolution cτ/2 is defined by effective duration of a sounding pulse τ

signal oscillations with time scale less than τ are assumed as noise admittedly. If there is a singlepulse lidar signal, then the noise current RMS value

Range, km

Range, km

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A sample of a window FFT analyses for the Doppler wind

lidar data is presented at the top-left. A time window of duration of 5 ms travels in time, intermediate frequency signal amplitude is rated and after that autocorrelation function is calculated as well as its Fourier transform. Consequently an undimentional energy spectrum of a lidar signal fraction is obtained. The spectrum is protracted on a line parallel to the axis of ordinates. The line’s position on the axis of abscissas corresponds with the center of a time window. Figure demonstrates peak in the range of 21 MHz

which corresponds with the wind velocity (2120) 106/200 103=5 m/s. Some signal spectrum widening can be noticed as the distance increases and the widening apparently is caused by atmospheric turbulence as well as limited coherence of the local oscillator.

Figure at the bottom presents the distribution of the wind velocity axis projection along the sounding range reconstructed by the Window-Fourier-transform.

Substance concentration calculation is carrying out through the differentiation

Without appropriate measures such an operation causes considerable noise increasing, which in its turn makes it impossible to perform lidar measurements even at the distances where the SNR for the initial lidar signals is much higher than one. In order to resolve this problem the special post-processing filter is applied to decrease noise level by the coarsening of spatial resolution.

Thus, the spatial resolution of a differential absorption СО2 heterodyne lidar is limited by two factors:

finite duration of a sounding laser pulse and

required accuracy of the concentration measurement

Spatial resolution (DR), SNR, differential absorption cross-section (Ds) and minimal resolved concentration Cmin are interrelated between each other. Thus, lidar sensitivity at the best spatial resolution can be estimated. For example, for lon - 10R12, loff - 9R30 differential cross section of ammonia (NH3) is Ds=55.8 atm-1cm-1. At the SNR values, which are achieved in the MLC’s receiving channel at the extreme distance and at spatial resolution limited by a pulse duration only, the

minimal revealed ammonia concentration at a range of 15 kilometers makes 1 ppm. At closer distances sensitivity increases according SNR growth.

The typical result of the sector scanning of the pollution in the real city

environment is shown at the bottom.

Turbulence (Cn2) x 1e-15

Wavelength:

532 nm

Pulse duration:

10 ns