Optical design for an active standoff spectrometer with a broadband source

We describe an optical bench design that efficiently projects broadband infrared energy from a supercontinuum (SC) source onto an extended target and collects the backscattered return. The optical system enables standoff spectral measurements of surfaces at up to 15 m away with a continuous spectral range from 3.6 to 11  μm. The goal of the optical system is to maximize energy transmission and capture under various fundamental and practical constraints imposed by the SC fiber laser source (broad spectrum, high divergence exiting the fiber), spectrometer (light collimation), detectors (etendue limits), and the overall system (size, weight, and cost). The design effort led to an all-reflective, off-axis configuration for both the transmitter and receiver, using custom-designed enhanced gold surfaces. We are able to successfully capture >90  %   of the source energy and transmit it with low loss to the remote target while maintaining a nearly diffraction-limited Gaussian beam. The receiver telescope matches the detector etendue while providing uniform (±2.5  %  ) off-axis signal collection over the target area to enable scanning of the SC laser spot.