Development of differential absorption LIDAR Technologies

Summary

We are advancing optical methods and developing deployable technologies for performing differential absorption LIDAR (LIght Detection And Ranging), commonly known as DIAL. The laser-based standoff technique is one of the only methods that enables volumetric (3 dimensional) sensing with high specificity. When coupled with precise wind velocity measurements, DIAL can report on the amount of a specific chemical released per unit time (flux) over a sampled region.

Description

The basic processes involved in elastic backscatter LIDAR are as follows. A laser emits a pulse of light (typically a few nanoseconds), and as the pulse propagates, the photons interact with molecules and aerosol particles. Some of these, the interactions with aerosols, such as Mie and Rayleigh scattering, result in photons backscattered to the detector, while interactions with molecules involve absorption of the light. The photons that are backscattered are collected by the detector are recorded as function of time. This time-of-flight data has a direct correspondence with the range (distance) at which the scattering event occurred.

Differential absorption LIDAR (DIAL) is based on the same principal, but operates at two wavelengths, one where light is absorbed by the molecule of interest (on-resonance) and one that is not absorbed (off-resonance). The technique is self-calibrating. The number density (concentration) of the absorber over the beam path can be determined directly from the difference between these on- versus off-resonance absorption signals.

Laser system:

The DIAL system operates at wavelengths in the near infrared (near-IR) spectral region (1 μm to 2 μm in wavelength) where (i) the transmitted laser light is the most eye-safe and (ii) cheap and portable telecom technology can be leveraged for wavelength tuning and control. The tunable near-IR laser light is generated using a specially designed optical parametric oscillator (OPO) and pumped with a 100 Hz Nd:YAG laser. The OPO is based on the Rotated Image Singly-Resonant Twisted RectAngle (RISTRA) design which is engineered for high conversion efficiency, large peak pulse energy, and excellent beam quality. The DIAL system also implements heterodyne detection and takes advantage of acoustic-optical-modulator (AOM) and electro-optic-modulator (EOM) telecom technology to recover multiple points across the absorption line shape for each backscatter return signal using a single seed laser and a high-speed detector (400 MHz InGaAs APD).