BG4239U1 - Broad spectrum lidar for atmospheric methane sounding - Google Patents

Abstract

The utility model relates to a broad spectrum atmospheric methane sounding lidar, particularly a broad-spectrum differential absorption lidar, for sounding atmospheric methane with a powerful, pulsed laser diode (1), with applications in the field of atmospheric lidars, remote monitoring, meteorology, climatology, ecology, energy, agriculture, and in the field of exploration of deposits of energy sources. The broad spectrum lidar for atmospheric methane sounding consists of transmitting and receiving optical circuits. The transmitting optical circuit with a powerful pulsed laser diode (1), which is located at the focal length of an optical collimating lens (2). The receiving optical circuit is arranged on an optical axis parallel to that of the transmitting optical circuit comprises a large aperture optical lens (3),as on the optical axis at a distance therefrom is arranged a translucent mirror (4). The laser beam splitting ratio is inversely proportional to the ratio of the incoming spectral intensities, which sets two spectral channels. One of the spectral channels comprises a narrowband filter with a linewidth of 4 nm at the fundamental wavelength of 1.667 micrometers (5), after which a recording photoreceiver (6) is located at a distance within the focus of the laser radiation. The other spectral channel comprises a bandpass filter (7) with a linewidth of 15 nm at a wavelength of 1.667 micrometers, as at a distance therefrom is arranged a limiting filter (8), after which a photoreceiver (9) at a distance within the focus of the laser radiation.

Description

Field of technique

The utility model relates to a broad-spectrum lidar for atmospheric methane probing, in particular, a broad-spectrum differential absorption lidar for atmospheric methane probing, consisting of a high-power, pulsed laser diode emitting circuit and a dual spectral channel receiving circuit, 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, frequency retuning 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 broad-spectrum lidar is that the integral spectrum is often mixed with absorption lines of other atmospheric gases, especially the intense spectra of water vapor in the near and mid-infrared range.

Lidars of direct detection of reflected laser pulses for gas analysis with laser diodes in the near-infrared spectral range and those based on pseudorandom modulation [4] or correlation spectroscopy [6] are known.

A broad-spectrum differential absorption lidar for probing atmospheric methane, consisting of a powerful, pulsed laser diode emitting circuit and a receiving circuit with two spectral channels, is not known.

Technical nature of the utility model

Broad-spectrum lidar for probing atmospheric methane is based on differential absorption by atmospheric gases by means of laser diodes, which uses direct detection of reflected laser pulses by means of broad-spectrum lidar with powerful, pulsed laser diodes, with pulse energies from 1 pJ to 10 pJ. The laser radiation in the receiving optical scheme of the wide-spectrum lidar is divided into two spectral channels. One of them has a wavelength of 1.667 pm with a line width of 4 nm and falls in the spectral region of methane, and the other spectral channel passes a part of the laser line excluding the central spectral region with a width of 4 nm.

When probing atmospheric methane, the background spurious absorption from atmospheric water vapor that coincides with the methane absorption spectra is distinguished by balancing the spurious absorption from atmospheric water vapor in the two spectral channels of the lidar.

The broad-spectrum lidar for probing atmospheric methane consists of the emitting optical circuit with a powerful pulsed laser diode, which is located at the focal length of an optical collimating lens. The receiving optical circuit is located on a parallel optical axis with that of the emitting circuit. It consists of an optical lens with a large aperture, and a translucent mirror is located on the optical axis at a distance from it. The splitting ratio of the laser beam is inversely proportional to the 0.25 ratio of the input spectral intensities, which are -- which sets two spectral channels. 0.75

One spectral channel includes a narrowband filter with a line width of 4 nm at the fundamental wavelength of 1.667 pm, after which a recording photodetector is located at a distance from the focus of the laser radiation. The other spectral channel includes a bandpass filter with a line width of 15 nm at a wavelength of 1.667 pm, with a limiting filter located at a distance from it, after which a photodetector is located at a distance at the focus of the laser radiation.

The proposed broad-spectrum lidar exploits the spectral and energy properties of powerful pulsed laser diodes for the registration of atmospheric methane. The main advantage of wide-spectrum lidar is that 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.

Broad-spectrum lidar using an optimal wavelength of 1.667 pm in the strong absorption spectrum region of methane expands the application of modern industrial laser diode technologies. This spectral range is also favorable due to the availability of highly sensitive, uncooled photodetectors.

The proposed utility model of wide-spectrum lidar significantly increases the lidar track, with vibration-resistant scanning in mobile or airborne applications. At the same time, the need for a time delay between the laser pulses of the two spectral channels is eliminated, which avoids parasitic interference from the electrical network, atmospheric turbulence and mode noise.

Explanations of the attached figures

The useful model 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 1.667 pm:

(a) absorption spectrum of methane, at a lidar track of 2 km, composed of: absorption overtone Q 2 ν 3, 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 4 nm linewidth narrowband filter at a wavelength of 1.667 tsh;

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

Examples of implementation of the utility model

According to an exemplary embodiment of the utility model, the wide-spectrum lidar for recording atmospheric methane consists of a transmitting and receiving optical circuit.

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 to the ratio 0.25 of the input spectral intensities which are-- specifying two spectral channels.

0.75

One spectral channel includes a narrow-band filter with a line width of 4 nm at the main wavelength 1.667 tsh 5, after which a recording photodetector 6 is located at a distance in the focus of the laser radiation.

The other spectral channel includes a bandpass filter 7 with a line width of 15 nm at a wavelength of 1.667 tsh, 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 broad-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 1.667 Tsh modulated by a molecular absorption spectrum at a lidar track of 2 km, composed of an absorption overtone Q - 2 ν 3 of methane d at a mixing ratio with atmospheric air of 10 parts per million (ppm) and (Fig. 2b) from the absorption spectrum of water vapor at a mixing ratio of 10 gm ^-3 , 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 circuit of the lidar, where it is introduced into two lidar spectral channels by means of a translucent mirror 4. The spectral line of the laser diode 1 is divided at a ratio inversely proportional 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 narrowband filter 5, the output spectrum of which has a line width of 4 ps at a wavelength of 1.667 pm (Fig. 3a), after which it is focused on the input aperture of a photodetector 6, which registers the signal of methane absorption.

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

The spectrum of the output of the limiting filter 8 isolates the spectral region with a width of 4 psh at a wavelength of 1.667 tsh, passing the rest of the spectral line of the laser diode 1, passing the rest of it, after which it is focused on the input aperture of a photodetector 9 , recording the signal outside the methane absorption.

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

Application of the utility model

The wide-spectrum atmospheric methane sounding lidar has applications in the field of atmospheric remote monitoring lidars, meteorology, climatology, ecology, energy, agriculture, and exploration of energy sources.

Claims (1)

Claims 1. A wide-spectrum lidar for probing atmospheric methane, including emitting and receiving optical circuits, characterized in that the emitting optical circuit consists of 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, consisting of an optical lens with a large aperture (3), with a translucent mirror (4) located on the optical axis at a distance from it, with a separation ratio of the laser beam inversely proportional to the ratio of the inputs 0.25 spectral intensities, which are defining two spectral channels, and one spectral channel includes a narrowband filter with a line width of 4 ps at the main wavelength of 1.667 pm (5), after which a recording photodetector is located at a distance in the focus of the laser radiation (6 ), and the other spectral channel includes a bandpass filter (7) with a line width of 15 ps at a wavelength of 1.667 μηι, and a limiting filter (8) is located at a distance from it, after which a photodetector is located at a distance in the focus of the laser radiation (9).