The figure schematically depicts a lidar system for measuring optical properties of clouds and aerosols at three wavelengths. The system is designed to be operated aboard the NASA ER-2 aircraft, which typically cruises at an altitude of about 20 km — above about 94 percent of the mass of the atmosphere. The system can also be operated aboard several other aircraft, and a version for use on Unmanned Aerial Vehicles (UAVs) is presently under construction. In addition to the requirement for fully autonomous operation in a demanding airborne environment, three other main requirements have governed the design: (1) to make the system eye-safe at the operating altitude; (2) to make the system as lightweight as possible, yet rugged; and (3) to use solid-state photon-counting detectors fiber-coupled to the receiver.
The laser transmitter is based on a Nd:YVO4 laser crystal pumped by light coupled to the crystal via optical fibers from laser diodes that are located away from the crystal to aid in dissipating the heat generated in the diodes and their drive circuits. The output of the Nd:YVO4 crystal has a wavelength of 1064 nm, and is made to pass through frequency- doubling and frequency- tripling crystals. As a result, the net laser output is a collinear superposition of beams at wavelengths of 1064, 532, and 355 nm.
The laser operates at a pulse-repetition rate of 5 kHz, emitting per-pulse energies of 50 μJ at 1064 nm, 25 μJ at 532 nm, and 50 μJ at 355 nm. The transmitted laser beam and the returning laser light backscattered from atmospheric aerosols and molecules pass through a telescope, the primary optical element of which is an off-axis parabolic mirror having an aperture diameter of 20 cm. The combination of the off-axis arrangement and other features is such that none of the transmitting aperture is obscured and only about 20 percent of the receiving aperture is obscured.
The returning light collected by the telescope is separated into wavelength components by use of dichroics and narrowband interference filters suppress solar background. The 1064-nm signal is further separated into parallel and perpendicular polarization components. A half-wave plate is inserted in the 1064-nm path to enable calibration of the parallel- and perpendicular- polarization channels. Each resulting output wavelength component is coupled via an optical fiber to a photodetector.
An important feature of this system is an integrating sphere located between the laser output and the laser beam expander lenses. The integrating sphere collects light scattered from the lenses. Three energy-monitor detectors are located at ports inside the integrating sphere. Each of these detectors is equipped with filters such that the laser output energy is measured independently for each wavelength. The laser output energy is measured on each pulse to enable the most accurate calibration possible.
The 1064-nm and 532-nm photodetectors are, more specifically, single-photon-counting modules (SPCMs). When used at 1064 nm, these detectors have approximately 3 percent quantum efficiency and low thermal noise (fewer than 200 counts per second). When used at 532 nm, the SPCMs have quantum efficiency of about 60 percent. The photodetector for the 355- nm channel is a photon-counting photomultiplier tube having a quantum efficiency of about 20 percent.
The use of photon-counting detectors is made feasible by the low laser pulse energy. The main advantage of photon-counting (in contradistinction to processing of analog photodetector outputs) is ease of inversion of data without need for complicated calibration schemes like those necessary for analog detectors. The disadvantage of photon-counting detectors is that they inherently have narrow dynamic ranges. However, by using photon-counting detectors along with a high-repetition-rate laser, it is possible to obtain wide dynamic range through accumulation of counts over many pulses
This work was done by Matthew McGill and V. Stanley Scott of Goddard Space Flight Center, Luis Ramos Izquierdo of LRI Corp., and Joe Marzouk of Sigma Space Corp.