The Advanced Ladar Imaging Simulator (ALIS) is a comprehensive multi-dimensional laser radar system simulator that models complex atmospheric scenes combined with high-resolution solid object scenes. The primary functions of ALIS are to serve as a laser radar sensor design tool, data product generator for exploitation.. and a decision aid for implementing system designs. This paper focuses on the software structure of the simulator and the challenges that it presents. The ambient atmospheric scene generation is augmented with built-in approximate plume models or with external large-scale Navier-Stokes computational fluid dynamics plume models. The mixed atmosphere and solid object scene is generated via an adaptively meshed, over-sampled voxel representation predicated jointly on the sensor capabilities and scene complexity. To our knowledge, ALIS goes beyond previous ladar simulators with detailed atmospheric turbulence effects and time-dependent plume dispersion models for direct and coherent detection frequency-agile transceivers. ALIS models a wide range of ladar architectures, treating laser coherence properties, receiver electronics noise/transfer functions, and electronics/photon statistical noise. It provides complex amplitude ladar echo "range cubes" (all range reports along a given line-of-sight) for the composite atmosphere-solid scene. The model complexity and its capability to process large (>10(9)) voxel count scenes is accommodated with a portable, scalable software architecture that supports single processors to fine-grained parallel supercomputers.

We have built a coupled master oscillator and a slave oscillator optical parametric oscillator (OPO) system that provides single-frequency, pulsed radiation in the midwave infrared (MWIR). The direct-diode-pumped master OPO provided narrow-band (<6-MHz, instrument-limited), tunable MWIR cw radiation that was used to seed a higher-peak-power pulsed slave OPO. When seeded directly at the MWIR idler, the pulsed output of the slave OPO was constrained to oscillate on a single longitudinal mode even though the slave OPO is pumped by a multilongitudinal-mode laser source. The linewidth of the pulsed output has been measured to be <220 MHz (instrument limited), which is well suited for coherent differential absorption lidar applications.

We are developing a novel 2-dimensional focal plane array (FPA) with read-out integrated circuit (ROIL) on a single chip for 3 dimensional (3-D) laser radar (ladar) imaging. The ladar will provide high-resolution range and range-resolved intensity images for detection and identification of difficult targets. The initial full imaging-camera-on-a-chip system will be a 64x64 element, 100-mum pixel-size detector array that is directly bump bonded to a low-noise (30 electrons) 64x64 array silicon CMOS-based ROIC. The architecture is scalable to 256x256 or higher arrays depending on the system application. The system will provide all the required electronic processing at pixel level and the smart FPA enables directly producing the three-dimensional (angle-angle-range) or four-dimensional (angle-angle-range-intensity) format data to be captured with a single laser pulse. The detector arrays are made of uncooled InGaAs PIN device for SWIR imaging at 1.5 mum wavelength and cooled HgCdTe PIN device for MWIR imaging at 3.8 mum wavelength. We are also investigating concepts using multicolor detector arrays for simultaneous imaging at multiple wavelengths (active-active and active-passive) that would provide additional spectral dimension capability for enhanced detection and identification of deep-hide targets. The system is suited for flash ladar imaging, for combat identification of ground targets from airborne platforms, flash-ladar imaging seekers, and autonomous robotic/automotive vehicle navigation and collision avoidance applications.

Tunable single-frequency sources in the 2-4 micron wavelength region are useful for remote DIAL measurements of chemicals and pollutants. We are developing tunable single-frequency transmitters and receivers for both direct and coherent detection lidar measurement applications. We have demonstrated a direct-diode-pumped PPLN-based OPO that operates single frequency, produces greater than 10 mW cw and is tunable over the 2.5 - 3.9 micron wavelength region. This laser has been used to injection seed a pulsed PPLN OPO, pumped by a 1.064 micron Nd:YAG laser, producing 50-100 microJoule single-frequency pulses at 100 Hz PRF near 3.6 micron wavelength. In addition, we have demonstrated a cw Cr:ZnSe laser that is tunable over the 2.1 - 2.8 micron wavelength region. This laser is pumped by a cw diode-pumped Tm:YALO laser and has produced over 1.8 W cw. Tm- and Tm,Ho-doped single-frequency solid-state lasers that produce over 50 mW cw and are tunable over approximately 10 nm in the 2 -2.1 micron band with fast PZT tuning have also been demonstrated. A fast PZT-tunable Tm,Ho:YLF laser was used for a direct-detection column content DIAL measurement of atmospheric CO2. Modeling shows that that all these cw and pulsed sources are useful for column-content coherent DIAL measurements at several km range using topographic targets.

We describe stimulus-response measurement techniques based on the photoconductive generation and sampling of picosecond electrical pulses for measuring the high frequency scattering parameters of high speed microwave devices. We compare these techniques with more conventional microwave diagnostic techniques.

Ion-implantation induced amorphization has been used to modify the linearity of response of ultrafast photoconductive switches fabricated on SOS. The extent of amorphization was determined using various materials characterization techniques. TRIM-86 Monte Carlo calculations were used to model the defect densities produced by ion implantation. Linearity of response is critically dependent upon the nature of the semiconductor region under metallic contacts and the character of the response is opposite to that expected from reports in the literature.

Ultrafast photoconductive switches have been used to generate electrical waveforms and to sample both electrical and optical waveforms. Widespread use of these switches is anticipated in conjunction with optical interconnects. We present results of an investigation into the electrical bias dependence and optical intensity dependence of the responsivity of photoconductive switches. The switches were fabricated using standard 50 0 microstrip transmission line technology on silicon-on-sapphire wafers. Ultrafast photoconductive response was produced by ion-implanting the silicon to reduce the carrier lifetime. We find that the performance of the switches is critically dependent upon wafer fabrication and ion-implantation conditions. While all of the switches tested possessed picosecond-scale response, the linearity of response with electrical bias and optical intensity was dependent on the order of the metalization and ion-implantation processing steps. In contrast to reports in the literature, fabrication processes which were expected to yield switches with ohmic contacts instead yielded switches with nonlinear response. We discuss the contributions of nonlinear absorption, carrier transport, charge screening and the build-up of space charge as well as other geometrical effects.

In the interest of increasing the sensitivity of laser spectroscopy, investigators are utilizing high frequency modulation [1,2] and polarization [3] techniques. In the case of highresolution-high-frequency modulation laser spectroscopy, beating between the modulation side bands allows formidable sensitivity [1]. In high frequency modulation pump-probe picosecond spectroscopy [2], the side bands produced are negligibly displaced relative to the laser spectral bandwidth; thus the carrier frequency (or sums and differences of carrier frequencies on different laser beams [4,5]) are detected directly.