RU126851U1 - POLARIZATION LIDAR FOR ATMOSPHERIC PROBING - Google Patents

Abstract

A polarizing lidar for sensing the atmosphere, including a linearly polarized radiation source, a receiving telescope with a quarter-wave phase plate located on the optical axis with the possibility of rotation of the axis of greatest transmission, a polarizing splitter analyzer and two photodetectors with a recording unit, characterized in that the output of the polarizing radiation source a quarter-wave phase plate with the possibility of synchronous rotation with the phase plate of the telescope is installed.

Description

The utility model relates to the field of meteorology and atmospheric optics, is used to measure the optical and microphysical parameters of the atmosphere and can be used to control the level of air pollution, recognition of crystalline and hail clouds.

Known optical polarization devices for sensing the atmosphere, consisting of a linearly polarized radiation source, photoelectric detectors, a recording unit and an optical system containing polarizing filters dividing the backscattered radiation into two mutually orthogonal components, one of which is parallel to the plane of polarization of the emitted light flux [1,2].

In these devices, a plane-polarized light beam is directed to the medium and the degree of depolarization is measured, which is a criterion for the boundaries of the multiple light scattering region. Two telescopes with photodetectors, in front of which polarizing filters are installed, are used as radiation detectors.

The disadvantage of such devices is the need to use two receiving telescopes, which complicates the design of the locator and makes it difficult to accurately

joint adjustment of telescopes to one scattering volume. In addition, the presence of only polarization filters in the receiving path allows one to determine only the degree of depolarization without receiving information about the state of the polarization form of the reflected wave.

An optical polarizing device is also known, consisting of a linearly polarized light source, one receiving telescope with a polarizing splitter (Wollaston prism) and two photodetectors.

In this device, the Wollaston prism is oriented so that at its output one of the components of the echo signal is parallel to the plane of polarization of the emitted light flux, and the second is orthogonal to it [3].

The disadvantage of this device is the difficulty of highlighting in meteorological regions, consisting of particles of non-spherical shape and an indication of their preferred spatial orientation. This is due to the fact that the probe radiation and the polarization splitter have a strictly fixed spatial orientation of the polarization planes, which does not allow us to fix the experimentally observed observation due to the non-sphericity of particles of rotation of the polarization plane.

The closest technical solution to the utility model is a polarization lidar for sensing the atmosphere, including a polarization radiation source, a receiving telescope with a quarter-wave phase plate located on the optical axis, with the possibility of rotation of the axis of greatest transmission, a polarization splitter analyzer and two photo detectors with a recording unit [4 ].

The main disadvantage of this device is the difficulty in determining the presence of specularly reflecting particles when measuring the degree of depolarization in the scattering volume of the atmosphere. This is due to the fact that when using the initial linear laser polarization, the measured degree of depolarization substantially depends on the orientation of the lidar reference plane with respect to the direction of particle orientation.

The purpose of the utility model is the detection of specularly reflecting layers in the atmosphere formed by particles mainly oriented in the horizontal plane.

This goal is achieved by the fact that at the output of the linearly polarized radiation source a similar quarter-wave phase plate is installed with the possibility of synchronous rotation with the phase plate of the receiving telescope. This makes it possible to obtain circular polarization at the output of the radiation source, which is a necessary condition for detecting atmospheric regions with a predominant orientation of particles in the horizontal plane.

Figure 1 schematically shows a block diagram of a polarization lidar for sensing the atmosphere.

The lidar contains a source 1 of linearly polarized radiation (laser), the output of which is a quarter-wave quartz plate 2 with the ability to set the fast axis of the phase plate at an angle of 0 or ± 45 ⁰ to the reference plane. This allows you to form a light beam with linear or circular polarization of radiation.

Next to the transmitter is a receiving optical telescope 3 with an angle of view covering the entire light beam directed by the source into the atmosphere. At the output of the optical telescope, a quarter-wave quartz phase plate 4 is also installed, with the possibility of synchronous rotation with the phase plate of the radiation transmitter. After the phase plate 4, an optical splitter-analyzer 5 (Wollaston prism) is located on the optical axis, which divides the light beam into two components with orthogonal polarization (parallel and perpendicular). At the output of the analyzer 5, photodetectors 6 and 7 are installed on the path of the split beams, which convert the light signals into electric ones, which are digitized in the recording unit 8 and processed.

Polarization lidar works as follows.

для радиации, линейно поляризованной в плоскости референции, и

для правоциркулярной поляризации. Принимаемые сигналы имеют две компоненты: параллельную I_p и ортогональную I_⊥. Направление поворота пластинок выбрано так, что в случае зеркального отражения и линейная и круговая исходная поляризация дают компоненту с максимальной интенсивностью на том же детекторе (I_p>I_⊥). Поворачивая одновременно пластинки перед источником и приемником, мы можем измерять либо второй (Q) либо четвертый (V) вектора Стокса обратно рассеянного излучения.The fast axis λ / 4 of the plates can be set at an angle of 0 or ± 45 ° to the reference plane. The Stokes vectors of laser radiation of unit intensity in this case are written as a column vector

for radiation linearly polarized in the plane of reference, and

for right circular polarization. The received signals have two components: parallel I _p and orthogonal I _⊥ . The direction of rotation of the plates is chosen so that in the case of specular reflection, both the linear and circular initial polarization give the component with the maximum intensity at the same detector (I _p > I _⊥ ). By simultaneously turning the plates in front of the source and receiver, we can measure either the second (Q) or fourth (V) Stokes vector of the backscattered radiation.

From source 1, the radiation enters the phase plate 2. At the initial time, the fast axis of the phase plates 2 and 4 is set at an angle of 0 degrees to the reference plane. Then, linearly polarized radiation is sent from the radiation source 1 through the phase plate 2 to the atmosphere. The radiation scattered in the opposite direction enters the receiving optical telescope 3, is collected in a narrow light beam, collimated and sent to the receiving phase plate 4, the position of the fast axis of which is in phase with the axis of the phase plate 2 of the transmitter. Next, the luminous flux arrives at the polarization splitter-analyzer 5, which divides it into two mutually orthogonal components. The orthogonal components of the light flux are sent to photodetectors 6 and 7, which convert them into electrical signals simultaneously recorded in the recording unit 8. Subsequently, the second Stokes parameter and the degree of depolarization of the backscattered radiation are determined through the measured signal intensities of the orthogonal components.

At the second moment of time, the phase plates 2 and 4 are rotated at an angle of 45 ⁰ and a linearly polarized radiation is sent again from the source 1. In this case, a circularly polarized light beam is formed at the output of the phase plate 2, which is sent to the atmosphere. The optical telescope 2 collects the radiation scattered in the opposite direction by the atmosphere and directs it through the phase plate 4, the splitter 5 to the photodetectors 6 and 7, the electrical signals from which are recorded in block 8.

In this case, during measurements using circular polarization of the radiation sent to the atmosphere, the fourth parameter of the Stokes vector and the degree of depolarization of the backscattered radiation are measured. Analyzing among themselves the obtained parameters of two atmospheric sounding acts, one judges the presence of spherical and nonspherical particles, chaotic or ordered orientation of atmospheric aerosol particles, and the presence of regions of particles with mirror reflection.

Literature.

1. USSR Copyright Certificate No. 373602, cl. G01W 1/00, 1971

2. Abstracts of the IV All-Union Symposium on Laser Sensing of the Atmosphere. Ed. IOA SB AS USSR, Tomsk, 1976, p.236.

3. Toshiyuki Murayama, Hajime Okamoto, Naoki Kaneyasu, Hiroki Kamataki, and Kazuhiko Miura. Application of lidar depolarization measurement in the atmospheric boundary layer: Effects of dust and sea-salt particles // Journal of Geophysical Research, vol. 104, no. d24, pages 31,781-31,792, December 27, 1999.

4. Copyright certificate No. 731410, authors: Balin Yu.S., Kaul B.V., Samokhvalov I.V. “Optical polarization device for sensing the atmosphere”

Claims (1)

A polarizing lidar for sensing the atmosphere, including a linearly polarized radiation source, a receiving telescope with a quarter-wave phase plate located on the optical axis with the possibility of rotation of the axis of greatest transmission, a polarizing splitter analyzer and two photodetectors with a recording unit, characterized in that the output of the polarizing radiation source a quarter-wave phase plate with the possibility of synchronous rotation with the phase plate of the telescope is installed.

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