Design of an eyesafe Mie scattering lidar using the third harmonic frequency of an injection seeded single mode Nd:YAG laser

Abstract

With the growing demand for automatic and mobile operation of lidars, laser safety becomes more and more important. Rather than installing or developing expensive surveillance and shut down instrumentation, it is advisable to make the lasers themselves eyesafe. In practice, to be eyesafe at short ranges from the transmitter and still maintain a reasonable pulse energy output, the source wavelength must be shorter than 400 nm or longer than 1.4 μm where the human eye is transparent. Since the laser scattered light is composed mainly of Rayleigh (molecules) and Mie (aerosols) scattering, eyesafe lidar investigations of the atmospheric properties of aerosols are commonly carried out in the near infrared. We designed an eyesafe Mie scattering lidar system using the third harmonic frequency of an injection seeded Nd:YAG laser. For UV wavelengths, Rayleigh scattering dominates the signal because of λ^-4 dependence of the molecular volume backscattering coefficient of the atmosphere. To reduce this, a solid etalon, with a bandwidth comparable to the spectral linewidth of the aerosol, is placed before the detector. The spectral distribution of the laser signal backscattered from the atmosphere consists of the thermally broadened Rayleigh component and a much narrower signal that corresponds to the heavier aerosol particles. The spectral intensity distributions of both the Rayleigh and aerosol returns at a range R can be represented by a Gaussian profile. After passing through a narrow bandwidth solid etalon, the spectral intensity of the lidar return signal will be the convolution of the transmission function of the etalon and the Gaussian spectral intensity distribution of the Rayleigh and aerosol returns. At the central fringe, the transmission function of the solid etalon may be expressed as T(𝜈) = T_max/[1 + (𝜈-𝜈₀)²/α²] where α is the half bandwidth of the etalon.