Modeling of the nonlinear response of the intrinsic HgCdTe photoconductor by a two-level rate equation with a finite number of carriers available for photoexcitation

A description of the nonlinear response of a HgCdTe photoconductor on the incident optical radiation is derived from a two-level rate equation for the photoexcited carriers. The model accounts for a limited number of electrons available for photoexcitation, and the possibility of photostimulated relaxation of an already excited photoelectron. The detector response as well as the level of incident optical power where saturation occurs is directly related to the physical constants of the detector material. The saturation of the detector is the same phenomenon as is known for the saturable absorbers-the detector material becomes transparent when the incident optical power exceeds the saturation power. Even though the description given here is applied to the HgCdTe detector, the general model can be applied to other detector materials as well. The parameters for the model are fitted to a thermoelectrically cooled HgCdTe detector and afterwards employed to predict the detector noise performance in a heterodyne setup. Unlike for liquid-nitrogen-cooled detectors, shot-noise-limited detection cannot be obtained with use of only thermoelectrical cooling (226 K) of the detector. However, the detector performance can be optimized to be able to perform Doppler measurements from aerosol backscatter by use of the model presented to optimize the applied optical power in the reference wave.

Vertical variability of aerosol backscatter from an airborne-focused continuous-wave CO2 lidar at 9.1-microm wavelength

Atmospheric aerosol backscatter measurements taken with a continuous-wave focused Doppler lidar at 9.1-microm wavelength were obtained over western North America and the Pacific Ocean from 13 to 26 September 1995 as part of a NASA airborne mission. Backscatter variability was measured for approximately 52 flight hours, covering an equivalent horizontal distance of approximately 30,000 km in the troposphere. Some quasi-vertical backscatter profiles were also obtained during various ascents and descents at altitudes that ranged from approximately 0.1 to 12 km. Similarities and differences for aerosol loading over land and ocean were observed. A midtropospheric aerosol backscatter background mode near 3 x 10(-11) to 1 x 10(-10) m(-1) sr(-1) was obtained, which is consistent with those of previous airborne and ground-based data sets.

Lidar calibration technique using laboratory-generated aerosols

A new calibration technique for continuous-wave Doppler lidars that uses an aerosol scattering target has been developed. Calibrations with both single- and many-particle scattering were performed at the same lidar operating conditions as in atmospheric measurements. The calibrating targets, simulating atmospheric aerosols, were laboratory-generated spherical silicone oil droplets with known complex refractive indices and sizes, hence with known single-particle backscatter cross sections as obtained from Mie theory. Measurements of lidar efficiency with the conventional hard target calibration method were consistently higher by a factor of ~2 than measurements with the aerosol calibration technique. This result may have important implications for lidar backscatter estimates both for aerosol modeling efforts and for optimal design of future lidar systems. The aerosol calibration method provides a validation of basic lidar theory for particle scattering for coherent detection.

Optimum optical local-oscillator power levels for coherent detection with photodiodes

The use of an optical local oscillator for coherent detection with a photodiode can significantly reduce the responsivity of the detector because of saturation effects. Consequently, local-oscillator shot-noise-limited operation of the detector may not be possible. This effect is analyzed and formulations are developed for the optimum optical local-oscillator power level and the resultant maximum possible signal-to-noise ratio in terms of parameters derived from the photodiode current versus the optical power response curve. An effective heterodyne responsivity that can be used as a part of the specifications when one is procuring photodiodes for use in coherent detection systems is defined.

Laser Doppler optical air-data system: feasibility demonstration and systems specifications

A compact laser Doppler velocimeter with diode-pumped solid-state laser technology at 1.06 µm and a novel solid-block beam-splitting interferometer was demonstrated. This system allows for accurate Doppler velocity measurements even in a vibration environment. A comparison of the backscatter measurements with a backscatter sonde from the University of Wyoming showed excellent agreement. Based on demonstrated performance and technology projection, the system specifications of an optical air-data system at 2.015 µm are determined, and a detailed design concept is presented. In addition, the potential for a multifunctional sensor that can determine air data and detect wind shear and wake vortices is addressed.