Designing a beam expander for dual-wave laser fluorescence lidar

Abstract

This paper presents the design and the results of an aberration calculation of a beam expander consisting of the transmitter of a dual-wave laser aerosol-fluorescence lidar intended for the detection of contaminants of biogenic origin in the atmosphere. The transmitter is enabled to function at two strongly differing wavelengths by choosing optical materials that operate in the UV and IR regions, and that in combination minimize the spherical and chromatic aberrations. The fabricated transmitter made it possible to reduce the divergence of the laser radiation at wavelengths 1.064 and 0.266 µm by more than a factor of 7, thus ensuring that it can operate without refocusing. It is a critical task to develop and create apparatus that makes it possible to solve the problem of remote detection of sources of discharges of noxious substances into the atmosphere, based on high-energy laser technology. It should be pointed out that the quality of the measurements will depend on the parameters of the laser beam. It is natural in this case that the laser beam emerging into the atmosphere must be characterized by the minimum possible divergence for the working wavelengths and that this beam must be highly parallel with the optical viewing axis of the apparatus complex. As a rule, the divergence of the radiation of even the best commercial lasers on the market is too great to provide lidars with an operating range greater than hundreds of meters; therefore, the radiation must be collimated. The possibility of obtaining the required parameters depends on laser beam expanders, which must be developed to meet the technical requirements on lidar as a whole.The laser-beam expander described below was developed to equip a mobile aerosol-fluorescence lidar intended for efficient detection of particles of biogenic origin in the atmosphere at a range of up to 4 km. OPTICAL LAYOUT OF A DUAL-WAVE LASER-RADIATION TRANSMITTER OF A FLUORESCENCE LIDARIn designing a laser-beam expander, the starting parameters are the specific parameters of the laser beam: its diameter and divergence angle and the radiation wavelength λ of the laser. The divergence angle of the generated radiation for various types of lasers varies from several arc minutes to several degrees. Telescope systems consisting of reverse Galilean or Keplerian systems are used to reduce the divergence angle of the laser beams (in order to collimate them). It is preferable to use a reverse Galilean system, which gives no intermediate real image, since the latter is not acceptable when powerful lasers are used.This paper discusses a beam expander intended for collimating the radiation of an LS-2137/20 solid-state pulsed laser (LOTIS TII, Republic of Belarus) based on neodymium-doped yttrium-aluminum garnet, lasing at the first (λ = 1.064 µm), second (λ = 0.532 µm), and fourth (λ = 0.266 µm) harmonics; all the harmonics are emitted coaxially. The main harmonics in the lidar are the first and fourth, the second being used to visualize the laser beam in the atmosphere; the radiation pul...

Scanning Polarization Lidar LOSA-M3: Opportunity for Research of Crystalline Particle Orientation in the Clouds of Upper Layers

“…The beam is collimated by the 7-fold achromatic expander BE, designed at the IAO SB RAS (Kochanenko et al, 2012).…”