William Lucas Austin

Intracavity Raman conversion and Raman beam cleanup

Abstract Transient solutions of the transverse depleted-pump and Stokes field distributions are obtained for an externally-pumped solid-state Raman laser. The results are shown to agree well with experiment measurements made on a Q-switched Nd:YAG pumped solid-state barium nitrate Raman laser. The theory of intracavity Raman conversion of real optical beams is presented, and the mechanisms responsible for Raman beam cleanup and mode confinement are qualitatively discussed and modeled by computer simulations.

Advanced 3D polarimetric flash ladar imaging through foliage

High-resolution three-dimensional flash ladar system technologies are under development that enables remote identification of vehicles and armament hidden by heavy tree canopies. We have developed a sensor architecture and design that employs a 3D flash ladar receiver to address this mission. The receiver captures 128×128×>30 three-dimensional images for each laser pulse fired. The voxel size of the image is 3”×3”×4” at the target location. A novel signal-processing algorithm has been developed that achieves sub-voxel (sub-inch) range precision estimates of target locations within each pixel. Polarization discrimination is implemented to augment the target-to-foliage contrast. When employed, this method improves the range resolution of the system beyond the classical limit (based on pulsewidth and detection bandwidth). Experiments were performed with a 6 ns long transmitter pulsewidth that demonstrate 1-inch range resolution of a tank-like target that is occluded by foliage and a range precision of 0.3” for unoccluded targets.

Solid state Raman laser for MMT sodium guide star

Generation of sodium guide stars for adaptive optics requires very precise control of the frequency and bandwidth of the laser to maximize the brightness of the generated guide star. The ruggedness, efficiency and ease of use of a solid state system has great potential for improving the reliability and power of the laser guide star over the dye laser system currently used. The dearth of solid state transitions at the precise wavelength required for exciting resonance scattering in sodium drives us toward Raman shifting to downshift a nearby solid-state transition line tuned to work with the Raman-shifting material. The system being developed for the 6.5 meter multiple mirror telescope (MMT) takes two approaches to creating the sodium guide star: one uses YGAG to maximize the Raman-shifted output at the sodium D2 resonance. The second approach is to thermally tune the output of YAG to reach the appropriate wavelength for Raman shifting to 589 nm. Initial results from YGAG indicate that it will not be a suitable material for creating the sodium guide star laser. Initial results from the YGAG laser is presented, along with a discussion of the potential of the technology.

Theory of Raman gain spectrum transformations

Abstract A theory describing the spectral characteristics of Raman lasers is presented. The foundation of the theory is anchored in the probabilistic nature of light–matter interactions. The Raman gain spectrum is predicted for the cases when the pump laser is oscillating in both a single longitudinal mode and multi-longitudinal mode configuration. The gain profile predicted in the two previous cases are shown to be equivalent to homogeneously and inhomogeneously broadened laser transitions, respectively. This theory is necessary to predict the behavior of Raman lasers when injection-seeding, mode-locking, or multi-frequency operation is desired. Detailed calculations are performed for the case where the pump laser medium in Nd:YAG and the Raman medium is Ba(NO 3 ) 2 .

Solid-state Raman image amplification

Lite Cycles has developed a new type of eye-safe, range-gated, lidar sensing element based on Solid-state Raman Image Amplification (SSRIA) in a solid-state optical crystal. SSRIA can amplify low-level infrared images with gains greater than 106 with the addition of only quantum-limited noise. The high gains from SSRIA can compensate for low quantum efficiency detectors and can reduce the need for detector cooling. The range-gate of SSRIA is controlled by the pulsewidth of the pump laser and can be as short as 30 - 100 cm for nanosecond pulses and less than 5 mm if picosecond pulses are used. SSRIA results in higher SNR images throughout a broad range of incident light levels, in contrast to the increasing noise factor with reduced gain in image intensified CCDs. A theoretical framework for the optical resolution of SSRIA is presented and it is shown that SSRIA can produce higher resolution than ICCDs. SSRIA is also superior in rejecting unwanted sunlight background, further increasing image SNR, and can be used for real-time optical signal processing. Applications for military use include eye-safe imaging lidars that can be used for autonomous vehicle identification and targeting.

Marine Raman image amplification

Lite Cycles has developed a new type of range-gated, LIDAR sensing element based on Raman image amplification in a solid-state optical crystal. Marine Raman Image Amplification (MARIA) is a feasible technology for producing high-resolution imagery in an underwater environment. MARIA is capable of amplifying low-level optical images with gains up to 106 with the addition of only quantum-limited noise. The high gains available from MARIA can compensate for low quantum efficiency detectors. The range-gate of MARIA is controlled by the pulsewidth of the amplifier pump laser and can be made as short as 30 - 100 cm, using pump pulses of 2 - 6.7 nsec FWHM. The use of MARIA in an imaging LIDAR system has been shown to result in higher SNR images throughout a broad range of incident light levels, in contrast to the increasing noise factor occurring with reduced gain in ICCDs. The imaging resolution of MARIA in the marine environment can be superior to images produced by a laser line scan or standard range-gated imaging system. MARIA is also superior in rejecting unwanted sunlight background, further increasing the SNR of images. MARIA has the potential of providing the best overall system resolution and SNR, making it ideal for the identification of mine-like objects, even in bright sunlight conditions.

Applications of solid-state Raman lasers

Several laser applications benefit directly from solid-state stimulated Raman scattering (SSRS) technology. These include eye-safe coherent and incoherent lidar, high-resolution 3D marine imaging lidar, laser-corrected adaptive optics, medical lasers, and materials processing. The combination of good beam quality, variable output pulse formats, high conversion efficiency, and frequency agility makes laser sources based on SSRS ideally suited for these applications. In this talk we will discuss specific SSRS laser sources that we have developed for these applications.

Rapid overt airborne reconnaissance (ROAR) for mines and obstacles in very shallow water, surf zone, and beach

Under the Office of Naval Researchs Organic Mine Countermeasures Future Naval Capabilities (OMCM FNC) program, Lite Cycles, Inc. is developing an innovative and highly compact airborne active sensor system for mine and obstacle detection in very shallow water (VSW), through the surf-zone (SZ) and onto the beach. The system uses an innovative LCI proprietary integrated scanner, detector, and telescope (ISDT) receiver architecture. The ISD tightly couples all receiver components and LIDAR electronics to achieve the system compaction required for tactical UAVintegration while providing a large aperture. It also includes an advanced compact multifunction laser transmitter; an industry-first high-resolution, compact 3-D camera, a scanning function for wide area search, and temporally displaced multiple looks on the fly over the ocean surface for clutter reduction. Additionally, the laser will provide time-multiplexed multi-color output to perform day/night multispectral imaging for beach surveillance. New processing algorithms for mine detection in the very challenging surf-zone clutter environment are under development, which offer the potential for significant processing gains in comparison to the legacy approaches. This paper reviews the legacy system approaches, describes the mission challenges, and provides an overview of the ROAR system architecture.

2.5 W Eye-safe Solid-state Raman Laser

We report on a 2.5 W average power 250 mJ/pulse eye-safe solid-state Raman laser with an output wavelength of 1.56 µm.

Fiber Raman laser for sodium guide star

The generation of sodium guide stars through laser excitation imposes very specific requirements on the laser. It has been argued that the most effective sodium guide star laser would operate CW with a mode spacing of less than the 10 MHz Doppler broadening profile of the mesospheric sodium. However, achieving high power in free-space optics with these requirements becomes very difficult because of the limited active gain volume and exceedingly long cavity requirement. The clear solution to these impediments is the development of a fiber raman laser (FRL) to shift the output of a solid state laser to a harmonic of the 589 nm sodium transition. The specific wavelength and bandwidth are selected from the large Raman gain spectrum by application- specific Distributed Bragg Reflectors. The guided-wave nature of the FRL allows for very large interaction volumes, while the long fiber length allows for narrowly-spaced modes which easily fill the sodium D2 continuum. This paper reviews the theory and design of the Fiber Raman Guidestar Laser being developed for Astronomical Adaptive Optical systems.