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 (>109) voxel count scenes is accommodated with a portable, scalable software architecture that supports single processors to fine-grained parallel supercomputers.
In this paper, the performance of a compact, eyesafe, all-solid-state, mid-wave IR (MWIR) transceiver for data communication through low visibility conditions is discussed. The transceiver was developed for Multiple Integrated Laser Engagement System (MILES) application. The MWIR wavelengths are derived using a passively q-switched Nd:YAG laser pumped periodically poled lithium niobate based optical parametric oscillator. MILES weapon code transmission for small and heavy weapon platforms have been demonstrated through dense theatrical fog. With 2 μJ/pulse at ~4 mm and a room temperature IR detector, greater than 5 km range has been successfully demonstrated. A bit map image transmission at MWIR wavelengths was also accomplished using this device. Test images consisting of 50x40 pixels and 100x80 pixels were successfully transmitted through free space.
High power laser based electro-optic sensor systems are generally bulky. Currently, several of these systems have to be located near exit apertures on airborne platforms due to lack of high damage resistant flexible optical conduits offering high mode fidelity and transmission efficiency. In several cases, system functionality has to be scaled down in a space-constrained environment. Coherent Technologies, Inc. has developed flexible, efficient, metallic rectangular hollow waveguides for use with high-energy laser systems such that they can be re-located to a convenient, safe, and protective location. These ribbon-like waveguides provide >96%/m transmission efficiency with near diffraction-limited performance and high mode fidelity besides transporting high-energy optical radiation. In this paper, damage threshold measurement carried out using aluminum based rectangular hollow waveguides is reported. Measurements at 2 micrometers and 10.6 micrometers wavelengths in 1-m long waveguides with aperture heights of 100 to 250 micrometers have been performed. Damage thresholds greater than 1 GW/cm2 has been measured at 2 micrometers wavelength. Being thin and flexible, these waveguides are low cost, easy to fabricate, and are amenable for integration into fuselage of an airplane. It is anticipated that distributed aperture coherent ladar systems and high power optical directed energy on space/airborne platforms would benefit from this technology.
We are developing a novel 2D focal plane array (FPA) with read-out integrated circuit (ROIC) on a single chip for 3D laser radar 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 64 by 64 element, 100-micrometers pixel-size detector array that is directly bump bonded to a low-noise 64 by 64 array silicon CMOS-based ROIC. The architecture is scalable to 256 by 256 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 3D or 4D 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 micrometers wavelength and cooled HgCdTe PIN device for MWIR imaging at 3.8 micrometers wavelength. We are also investigating concepts using multi-color detector arrays for simultaneous imaging at multiple wavelengths 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.
Current established solid state Raman laser (SSRL) materials tend to be oxides or tungstates, which have low thermal conductivity and therefore inherently limited power scaling potential. We have tested the Raman material bulk undoped gallium phosphide (GaP), which has excellent thermal and mechanical properties, and assessed its ability to power scale. Pumping GaP with 1.06 and 1.3 micron Q- Switched Nd:YAG lasers has resulted in outputs of up to 14 mJ, the highest pulse energy GaP Raman laser known to date. Conclusions from laboratory tests and finite element modeling indicate that this Raman laser material can scale to kW average output power levels. We are currently investigating Raman lasers that will improve the pump laser spatial beam quality during Raman conversion. This could be developed into an add-on 'kit' that would improve beam quality in Nd:YAG industrial lasers with power output levels of over 100 W. We will present our latest GaP Raman laser laboratory results and discuss power scaling performance estimates.
Doppler lidar sensors provide a unique capability to generate high resolution 3D distributions of wind and aerosol data. Appropriately processed, these data can yield useful detection, tracking and short-term prediction information relating to the extent, density and location of potentially dangerous isolated aerosol plumes. The aerosol data are analyzed to detect above-threshold inhomogeneities and the wind and turbulence data are used to provide short- term prediction of plume propagation. Broadcast of these data to plume dispersion models can enable robust prediction of dispersion and propagation over longer time periods. Performance predictions are given for both wind and aerosol measurements. Sample processed field data results are presented for Doppler lidars operating at the eyesafe 2 micron wavelength.
The large lunar telescope is a proposed moon-based telescope which incorporates a sixteen-meter segmented primary mirror. An error budget is developed for the active control system of the primary mirror. A control methodology for the primary mirror is then described which utilizes piston sensors for measuring the relative piston error between adjacent segments as well as a separate sensor which measures the tilt of each segment with respect to the pointing direction of the telescope. A trade study is conducted in which the following types of tilt sensors are examined to determine their applicability to this program: stellar wavefront sensors, such as a Hartmann-Shack or a shearing interferometer; holographic optical elements; interferometers; scanning systems; and some nonoptical systems which electronically measure the relative tilt between adjacent segments. In addition, two independent methods of quantitatively verifying the performance of the telescope using either a phase retrieval algorithm or an image sharpening technique, both of which are based on the quality of a stellar image, are presented.