RU2567469C2 - Lidar of differential absorption on mobile medium - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: device comprises a laser radiator, a laser radiation phase randomizer, a transceiving optical tract, mirrors that forward radiation collected with a telescope to a photodetector module, and a light filter that reduces photodetector exposure due to a narrow band, and a pilot laser. The transmitting part of the tract comprises a system of guide mirrors, and the receiving part - from the receiving telescope. At the same time the transmitting channel generates directed radiation with time-space and spectral characteristics, which exclude impact of a speckle structure at results of lidar measurements.

EFFECT: elimination of error due to use of a system of randomization of a laser radiation phase for speckle-picture correction, which provides for higher accuracy of measurements.

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Description

The invention relates to the field of measurement technology and can be used for remote sensing of various impurities in the atmosphere, in particular natural gas, vapors of organic liquids.

Mobile lidars are widely used, which are devices for remote sensing and operational detection of areas of natural gas leaks from pipelines, installed on vehicles, both land and air, and measuring the concentration of methane impurities in air using laser radiation. The sensitivity and speed of lidars depends on many factors, in particular on the design of the photodetector path.

Known systems are firms Nanoplus GmbH, German Aerospace Center DLR and Adlares GmbH, E. ON Ruhrgas AG and others.

One of the closest to the claimed system in its technical characteristics is the SELMA system manufactured by Pergam-suisse AG (http://www.pergam-suisse.ch/), which provides remote control of natural gas leaks in urban areas, taken as one of the prototypes . The claimed system is mounted on the roof of a vehicle, such as a car, which requires special conversion of it and is a disadvantage. Remote sensing from a vehicle occurs only during a certain movement or when the vehicle is completely stopped, as well as only in places accessible for travel, which creates inconvenience in the measurement process.

Therefore, the LMD06A mobile system was created in the same company, which is smaller in overall dimensions, accordingly convenient to carry by the operator, does not require any conversion of the vehicle and is suitable for measurements in slow motion (foot step). By the claimed technical solution, this system is close in its overall dimensions, but has the technical characteristics for remote measurements at short distances, which is not always convenient.

The design of the above systems is based on lidar, which works on the principle of differential absorption. In the Russian Federation and in the world community there are a lot of different lidars working on the same principle (see patents WO 2007062810 dated November 28, 2006 “MOBILE REMOTE DETECTION OF FLUIDS BY A LASER”; US 5,015,099 dated May 14, 1991 “DIFFERENTIAL ABSORPTION LASER RADAR GAS DETECTION APPARATUS HAVING TUNABLE WAVELENGTH SINGLE MODE SEMICONDUCTOR LASER SOURCE "; US 5,202,570 dated 04/13/1994" GAS DETECTION DEVICE "; US 7,075,653 dated 06/11/2006" METOD AND APPARATUS FOR LASEME DASERE BASED - . The laser wavelength changes during the measurement cycle. Based on the difference in the values of the optical signal, a laser, for example, a semiconductor laser with a tunable wavelength in the spectral region of light absorption of the detected substance, can be used as a radiation source. It is also possible to use two or more lasers tuned to some fixed wavelengths corresponding to the maximum and minimum light absorption by the molecules of the detected substance.

The main disadvantage of lidar systems operating on the principle of differential absorption, including both systems of the Pergam-suisse AG company, is the measurement error arising due to the formation of a speckle structure, otherwise a random interference pattern of the reflected laser radiation. The cause of the speckle structure is the mutual interference of coherent waves upon reflection from a randomly inhomogeneous surface or when light passes through an optically inhomogeneous medium. The presence of the speckle structure leads to an uneven distribution of the intensity of the reflected light and to an error in lidar measurements (see Fig. 1 the speckle structure of laser radiation reflected from an arbitrary surface).

The speckle structure is manifested in the form of graininess of the reflected radiation and affects the magnitude of the light flux incident on the photodetector devices. The effect of speckle structure formation affects the functioning of laser measuring devices, not only lidars, but also such devices as a laser scanner, interferometer, bicycle meter and others. Also, the speckle structure affects the operability of devices where lasers are used as light sources, for example, in laser projectors, microscopes, as it affects the visual perception of the observed images.

Various options for eliminating the speckle structure and solving this problem are reflected in patents US 6952435 dated 04.10.2005. "SPEKLE FREE LASER PROBE BEAM"; US 7567349 dated June 28, 2009 "SPEKLE REDUCTION IN OPTICAL COHERENCE TOMOGRAPHY BY PATH LENGTH ENCODED ANGULAR COMPOUNDING"; US 7114393 dated 10/03/2006 "METOD AND APPARATUS FOR LASER VIBROMETRY".

However, the proposed speckle structure correction system to date has never been used with respect to differential absorption lidars.

The objective of the invention is the creation of a mobile lidar of differential absorption of high sensitivity with a system to eliminate the influence of the speckle effect on lidar measurements.

This problem is solved by using a device for randomizing the phase of laser radiation, which allows you to change the speckle structure in a time shorter than a single measurement cycle.

The magnitude of the optical signal entering the photodetector is equal to the total intensity of the light flux from the speckle structure elements. Due to the nature of the effect, the observed speckle structure changes with a change in the wavelength of light. If the “granularity” of the speckle structure is large enough, when the wavelength of light changes, the photodetector signal may change randomly. Therefore, the speckle effect affects the accuracy of measurements, especially when the lidar is stationary or when moving slowly, since with fast movement, the light flux is averaged over the measurement time.

The phase randomizer used in the lidar carries out a relative phase shift of parts of the wavefront within the cross section of the laser beam, which leads to a multiple change in the speckle structure during the lidar measurement cycle and, as a result, averaging of the light flux. Therefore, the dependence of the magnitude of the light flux incident on the photodetector on fluctuations associated with the speckle grain structure is eliminated.

When conducting measurements of differential absorption lidar, it is necessary that the cycle of the phase change of the optical signal is not synchronized with the cycle of scanning (or changing) the wavelength of the radiation.

By using a randomization system for the phase of the laser radiation and eliminating the influence of the speckle structure, an especially low noise level of the optical signal is achieved, which ensures measurement accuracy.

For a more complete disclosure of the invention provides a graphical description, see figure 2 - lidar differential absorption.

In figure 2 are:

Laser emitter (pos.1.1) is designed to generate radiation with a tunable wavelength in the region of the spectral absorption line of methane or other detectable atmospheric impurities;

The laser phase randomizer (pos. 1.2) based on an electromechanical device containing a rotating phase plate. It is also possible to use other electromechanical devices as a device for phase randomization, including a device based on one or more moving mirrors or lenses, or an electro-optical device based on liquid crystals and other electroactive materials that shift the phase of radiation or change the polarization of radiation.

Transceiver optical path (pos.2) is divided into transmitting (pos.2.1) and receiving (pos.2.2) part. The transmitting part consists of a system of guide mirrors that allow directing the laser radiation along the optical axis of the telescope. We use mirrors having a high reflectance at the laser radiation wavelength. The receiving part of the lidar consists of a receiving telescope, mirrors directing the radiation collected by the telescope to the photodetector module (item 4), and a filter that reduces the illumination of the photodetector due to the narrow passband.

The pilot laser (item 3) is designed to highlight the target and visualize the direction of measurement (item 5), since the radiation of the lidar laser itself is invisible to the lidar operator.

The photodetector module (item 4) is equipped with a cooling system for the photosensitive element, which reduces the noise level of the photodetector.

The control and data processing system is intended for setting parameters of laser radiation, collecting and processing data received at the photodetector.

The built-in GPS receiver is designed to obtain information about the current location of the lidar.

The claimed invention works as follows: Lidar is mounted on a mobile carrier, on a roof, on an open car platform or in the passenger compartment of a car opposite a window, and also on a helicopter so that a target is present in the detection direction (arbitrary objects or the earth’s surface). The direction of detection is controlled using a visible visible pilot laser (a green laser is used in the design, it is possible to use a laser of a different color).

The transmitting channel generates directional radiation with the required spatio-temporal and spectral characteristics, which make it possible to exclude the influence of the speckle structure on the results of lidar measurements.

At the same time, the receiving channel collects the scattered signal reflected from the target located behind the studied area of gas leakage and updates information on the concentration of gas, in particular methane or vapors of organic liquids in the studied volume of the atmosphere.

The detection of gas and other atmospheric impurities, such as vapors of organic substances, can also be carried out behind a surface partially transparent to laser radiation.

Detection can be carried out in motion and when the mobile carrier is stopped.

The detection mode is as follows:

The laser emitter scans the radiation wavelength in the region of the spectral absorption line of atmospheric impurities, methane, another gas or liquid vapor. The signal entering the photodetector is analyzed using the built-in computer. During the measurement, the lidar is self-calibrated, which takes into account the background concentration of the detected gas and the contribution of other atmospheric impurities, including water vapor. All collected information about the gas concentration is recorded by the data storage system, together with information about the current location of the lidar.

The small dimensions and, as a consequence, the low weight of the lidar make it possible to use it installed on any vehicle without the need to make changes to its design, or be held by the operator in his arms. All lidar measurement data is transmitted wirelessly, which allows you to install lidar, if necessary, away from the operator and do without laying data cables.

Claims (1)

The differential absorption lidar on a mobile carrier includes a laser emitter, a laser phase randomizer, a transmit-receive optical path, where the transmitting part consists of a system of guiding mirrors, and the receiving part consists of a receiving telescope, mirrors directing the radiation collected by the telescope to the photodetector module, and a filter that reduces the illumination of the photodetector due to the narrow passband of the pilot laser, and is characterized in that the transmitting channel generates directional radiation from uemymi spatio-temporal and spectral characteristics to eliminate the influence of the speckle pattern results lidar measurements.

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