BG113445A - A method and a broad-spectrum lidar for the sounding of atmospheric methane - Google Patents

Abstract

The present invention relates to a method and a broad-spectrum LIDAR for the sounding of atmospheric methane, and more particularly to a method and a broad-spectrum differential absorption LIDAR for atmospheric methane sounding by means of a powerful pulsed laser diode, and would be applied in the area of LIDARs for atmospheric remote sensing, in meteorology, climatology, ecology, power engineering and agriculture as well as in the exploration of deposits of energy resources. The atmospheric methane sounding method is based on the differential absorption in the atmospheric gases of pulses emitted by laser diodes, and uses direct detection of the reflected laser pulses emitted by a broad-spectrum LIDAR equipped with powerful pulsed laser diodes. The laser radiation entering the receiving optical circuit of the broad-spectrum LIDAR is split into two spectral channels. One of them operates at a wavelength of 1.667 µm at linewidth 4 nm and falls in the spectral range of methane, and the other spectral channel transmits some part of the laser line excluding the 4 nm wide central spectral range. When sounding for atmospheric methane, the parasitic background absorption in the atmospheric water vapors that matches the absorption spectra of methane is segregated, and the parasitic absorption in the atmospheric water vapors is balanced in the two spectral channels of the LIDAR. The broad-spectrum LIDAR for atmospheric methane sounding includes one transmitting and one receiving optical circuit. The LIDAR consists of an emitter optical circuit with a powerful pulsed laser diode positioned within the focal length of an optical collimator. The receiving optical circuit is arranged in an optical axis parallel to that of the emitter circuit. It consists of a large-aperture optical lens and a translucent mirror positioned in the optical axis at some distance from the optical lens. The laser beam splitting ratio is inversely proportional to the ratio of the incoming spectral intensities, which defines two spectral channels. One of these spectral channels comprises a narrowband filter with linewidth 4 nm at the basic wavelength of 1.667 µm, and a photoreceiver positioned within the focus of the laser radiation at some distance after the filter. The other spectral channel includes a band-pass filter of linewidth 15 nm at 1.667 µm wavelength, a stopband filter positioned as some distance from the band-pass filter followed by a photoreceiver positioned at some distance and in the focus of the laser radiation.

Description

METHOD AND WIDE SPECTRUM LIDAR FOR PROBING THE

ATMOSPHERIC METHANE

FIELD OF ENGINEERING

The invention relates to a method and broad-spectrum lidar for probing atmospheric methane, in particular, to a method and broad-spectrum differential absorption lidar for probing atmospheric methane with a powerful, pulsed laser diode, with application in the field of lidars for atmospheric, remote monitoring , in meteorology, climatology, ecology, energy, agriculture and the exploration of energy deposits.

PRIOR ART

Lidars are known for recording atmospheric greenhouse gases, using tunable lasers with a narrow spectral line combined with an optical power amplifier (master oscillator - power amplifier), [1-3]. Also known are point gas sensors based on laser diodes with a narrow spectral line [4], which have low energy parameters unsuitable for lidar applications.

A common disadvantage of such lidars is that they require very high thermal and frequency stabilization, and in many cases coolants and energy-intensive refrigeration devices are used. In addition, a frequency readjustment is required to select suitable absorption lines, as well as in cases of their saturation at higher gas concentrations.

In the prior art [5-6], powerful laser diodes were considered unsuitable for spectroscopic applications and gas analysis due to their low coherence properties. Their characteristic broadened laser line is modulated by an integral spectrum of the detected gas consisting of multiple absorption lines. The disadvantage of the method is that the integral spectrum is often mixed with absorption lines of other atmospheric gases, especially with the intense spectra of water vapor in the near and mid-infrared range.

-3 The proposed method utilizes the spectral and energy properties of powerful pulsed laser diodes for the registration of atmospheric methane. The main advantage of the method is that the lidar parameters are independent of atmospheric conditions, such as pressure, temperature and humidity. Also, the parasitic absorption from water is completely balanced in the two spectral channels.

A method using an optimal wavelength of 1.667rt in the region of strong absorption spectrum of methane extends the current industrial laser diode technologies, the proposed invention significantly increases the lidar track. This spectral range is also favorable due to the availability of highly sensitive, uncooled photodetectors.

The wide-spectrum lidar is resistant to vibrations in mobile or airborne applications. There is no need for a time delay between the laser pulses of the two spectral channels, which avoids parasitic interference from the power grid, atmospheric turbulence and mode noise.

DESCRIPTION OF THE ATTACHED FIGURES

The invention is explained in more detail with the attached figures, where:

Figure 1 is an optical schematic of a broad-spectrum lidar for probing atmospheric methane with two spectral channels.

Figure 2 is a laser mode plot of a broad-spectrum, high-power, pulsed laser diode at a wavelength of 1.667pm:

(a) absorption spectrum of methane, at a lidar track of 2km, composed of: absorption overtone Q - 2v3, with a mixing ratio with atmospheric air of 10 parts per million (ppm);

(b) absorption spectrum of water vapor at a mixing ratio of 10 gm' ³ , or 58% relative humidity at 20°C.

Figure 3 is a spectral line plot of a powerful, pulsed laser diode:

(a) spectrum of a narrow-band filter with a line width of 4 nm at a wavelength of 1.667 nm;

(b) spectrum of a cutoff filter that isolates the 4nm-wide spectral region at 1.667pm wavelength, passing the rest of the laser line.

-4 EXAMPLES OF IMPLEMENTATION

According to an exemplary embodiment of the invention, the wide-spectrum lidar for the registration of atmospheric methane consists of emitting and receiving optical circuits.

The emitting optical circuit consists of a powerful pulsed laser diode (1), which is located in the focal length of an optical collimating lens (2).

The receiving optical circuit, located on an optical axis parallel to that of the emitting circuit, consists of an optical lens with a large aperture (3), and on the optical axis at a distance from it a translucent mirror (4) with a set ratio of splitting the laser beam inversely proportional of the ratio of the incoming 0.25 spectral intensities, which are , setting 2 spectral channels.

One spectral channel includes a narrowband filter with a line width of 4nm at the main wavelength of 1.667pm (5), after which a recording photodetector (6) is located at a distance from the focus of the laser radiation.

The other spectral channel includes a band-pass filter (7) with a line width of 15 µm at a wavelength of 1.667 rt, and a limiting filter (8) is located at a distance from it, after which a photodetector (9) is located at a distance in the focus of the laser radiation.

When working with the wide-spectrum lidar for probing atmospheric methane, the emitting optical circuit is turned on, consisting of a powerful pulsed laser diode (1), which is located in the focal length of an optical collimating lens (2). The powerful pulsed laser diode (1), the radiation of which is a Gaussian mode (Fig.2a) at a wavelength of 1.667rt, modulated by a molecular absorption spectrum at a lidar track of 2km, composed of an absorption overtone Q - 2v3 of methane! in relation to mixing with atmospheric air 10 parts per million (ppm) and (Fig.2b) from the absorption spectrum of water vapor in relation to mixing 10 gm', or 58% relative humidity at 20°C.

The beam from the laser diode (1) after reflection from a solid target, the earth's surface, or from the atmosphere, is registered in the receiving optical scheme of the lidar, where it is introduced into two lidar spectral channels by means of a semi-transparent mirror (4). The spectral line of the laser diode (1) is split at an inverse ratio

-5proportional to the ratio of the input spectral intensities 0.25 / 0.75 to the laser line, for the initial equalization of the outputs of the two spectral channels.

The laser diode beam in one spectral channel passes through the narrow-band filter (5), the output spectrum of which has a line width of 4µm at a wavelength of 1.667rt (Fig. 3a), after which it is focused on the entrance aperture of a photodetector (6). , recording the methane absorption signal.

The other spectral channel includes a bandpass filter (7) at a wavelength of 1.667rt with a line width of 15µm, which passes the entire spectral line of the laser diode (1), while limiting background illumination from the atmosphere, as well as a limiting filter (8).

The output spectrum of the cutoff filter (8) isolates the 4nm wide spectral region at a wavelength of 1.667pm, passing the rest of the spectral line of the laser diode (1), passing the rest of it, then focusing on the input aperture of a photodetector (9) registering the signal outside the absorption by methane.

The atmospheric concentration of methane is determined by the ratio of the signals at the outputs of the two spectral channels.

APPLICATION OF THE INVENTION

The method and wide-spectrum lidar for probing atmospheric methane has application in the field of lidars for atmospheric, remote monitoring, in meteorology, climatology, ecology, energy, agriculture and the exploration of energy sources deposits.

-6 LITERATURE

[1] N. Riris, K. Numata, S. Li, S. Wu, A. Ramanathan, M. Dawsey, J. Mao, R. Kawa and J. B. Abshire (2012). Airborne measurements of atmospheric methane column abundance using a pulsed integrated-path differential absorption lidar, AppL Optics, 51, 34, 8296-8305.

[2] J. L. Machol, T. Ayers, K. T. Schwenz, K. W. Koenig, R. M. Hardesty, C. J. Senff, M. A. Krainak, J. B. Abshire, Η. E. Bravo and S.P. Sandberg (2004). Preliminary measurements with an automated compact differential absorption lidar for profiling water vapor, Appl. Optics, 43, 3110-3121.

[3} K.S. Repasky, J.A. Shaw, J.L. Carlsten, M.D. Obland, L.S. Meng and D.S. Hoffman (2004). Diode laser transmitter for water vapor DIAL measurements, IGARSS 2004. IEEE International Geoscience and Remote Sensing Symposium, 3, 1947-1950.

[4] Quatrevalet, M., Ai, X., Perez-Serrano, A., Adamiec, P., Barbero, J., Fix, A., Tijero, J. M. G., Esquivias, 1., Rarity, J. G., & Ehret , G. (2017). Atmospheric CO2 Sensing with a Random Modulation Continuous Wave Integrated Path Differential Absorption Lidar, IEEE Journal of Selected Topics in Quantum Electronics, 23(2), 5300311.

[5] S. Penchev, V. Pencheva, S. Naboko (2012). Atmospheric remote sensing hygrometer employing powerful broad-line laser diodes, Compte Rendus de l'Academie Bulgare des Sciences, 56, 669-674.

[6] B. Thomas, G. David, C. Anselmo, J.-P. Cariou, A. Miffre and P. Rairoux (2013). Remote sensing of atmospheric gases with optical correlation spectroscopy and lidar: first experimental results on water vapor profile measurements, Appl. Physics B, 113, 265-275.

[7] Spectral database HITRAN'2012 (http://hitran.iao.ru/)

Claims (3)

1. A method for probing atmospheric methane based on differential absorption by atmospheric gases by means of laser diodes, characterized by direct detection of the reflected laser pulses by means of a wide-spectrum lidar with powerful, pulsed laser diodes, with a pulse energy from IpJ to ΙΟμΤ, in which the laser radiation in the receiving optical scheme of the broad-spectrum lidar is divided into two spectral channels, one of which has a wavelength of 1.667rt with a line width of 4nm and falls into the region of the methane spectrum, and the other spectral channel passes part of the laser line, excluding the central spectral region with a width of 4nm. 2. A method for probing atmospheric methane according to claim 1, wherein the background parasitic absorption from atmospheric water vapor coinciding with the absorption spectra of methane is distinguished, and the parasitic absorption from atmospheric water vapor is balanced in the two spectral channels of the lidar. 3. Wide-spectrum lidar for probing atmospheric methane according to claim 1 and 2, including emitting and receiving optical circuits, characterized in that it consists of the emitting optical circuit with a powerful pulsed laser diode (1) which is located in the focal distance of an optical collimating lens (2) and a receiving optical circuit located on an optical axis parallel to that of the emitting circuit, which consists of an optical lens with a large aperture (3), and a translucent mirror (4) is located on the optical axis at a distance from it , with a splitting ratio of the laser beam inversely proportional to the ratio of the input spectral 0.25 intensities, which are , which sets 2 spectral channels, one spectral channel including a narrowband filter with a line width of 4nm at the fundamental wavelength 1.667pm (5), after which a recording photodetector (6) is located at a distance in the focus of the laser radiation, and the other spectral channel includes a bandpass filter (7) with a line width of 15nm at a wavelength of 1.667pm, and a limiting filter (8) is located at a distance from it ), after which a photodetector (9) is placed at a distance in the focus of the laser radiation.