Dr. Glen Perram has served as a Professor of Physics at the Air Force Institute of Technology since 1989. He earned a B.S. in Applied and Engineering Physics from Cornell University in 1980 and his Ph.D. in Physics from AFIT in 1986. Dr. Perram received the 2013 Air Force Outstanding Civilian Senior Scientist Award for his work on Diode Pumped Alkali Lasers, is a fellow of the Directed Energy Professional Society, a registered professional engineer in the State of Ohio, and received the General Bernard A. Schriever Award for advancing aerospace power in 1995.
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To simulate the effects of multiple-longitudinal modes and rapid fluctuations in center frequency, we use sinusoidal phase modulation and linewidth broadening, respectively. These effects allow us to degrade the temporal coherence of our master-oscillator laser, which we then use to conduct digital holography experiments. In turn, our results show that the coherence efficiency decreases quadratically with fringe visibility and that our measurements agree with our models to within 1.8% for sinusoidal phase modulation and 6.9% for linewidth broadening.
Laser ablation of aluminum, silicon, titanium, germanium, and indium antimonide at 1064 nm in ambient laboratory air with pulse durations ranging from 100 ps to 100 μs has been characterized with optical microscopy. Highly focused spots of 10 μm yields fluences of 0.004 to 25 kJ / cm2 and irradiances spanning 4 × 106-1014 W / cm2. Single pulse hole depths range from 84 nm to 147 μm. A one-dimensional thermal model establishes a set of nondimensional variables for hole depth, fluence, and pulse duration. For pulse durations shorter than the radial diffusion time, the hole depth exceeds the thermal diffusion length by a factor of 1 to 30 for more than 90% of the data. For pulses longer than this critical time, transverse heat conduction losses dominate and holes as small as 10 − 3 times the thermal diffusion depth are produced. For all cases, the ablation efficiency, defined as atoms removed per incident photon, is 10 − 2 or less, and is inversely proportional to volume removed for pulse durations less than 100 ns. At high fluences, more than 10 to 100 times ablation threshold, explosive boiling is identified as the likely mass removal mechanism, and hole depth scales approximately as fluence to 0.3 to 0.4 power. The power-law exponent is inversely proportional to the shielding of the laser pulse by ejected material, and shielding is maximum at the 1-ns pulse duration and minimum near the 1-μs pulse duration for each material. Using the thermal scaling variables, the high-fluence behavior for each material becomes strikingly similar.
First imaging Fourier-transform spectral measurements of detonation in an internal combustion engine
The quest for a high power, electrically driven laser with excellent thermal management, lightweight packaging, and high brightness for tactical military applications may be realized with the advent of the Diode Pumped Alkali Laser (DPAL). The concept of using a gas phase medium for the phasing of large diode arrays via a highly efficient, cyclical photon engine combines the best features of electrically driven lasers with the inherent thermal management advantages of a gas lasers. Indeed, the DPAL concept has sparked great interest within the Directed Energy community resulting in a number of recent low power, highly efficient laser demonstrations. A modest national effort is underway to exploit this technology for military applications.
Early laser demonstrations of the Diode Pumped Alkali Laser achieved output powers of 1-3 W in both rubidium and cesium with slope efficiencies as high as 82%. More recently, cw output powers as high as 145 W with in-band slope efficiencies of 28% have been reported. The system is a three level laser pumped by diode bars on the D2 transition, exciting the first 2P3/2 state of the alkali atom. Collisional relaxation to the 2P1/2 state is accomplished with a spin orbit relaxing gas such as ethane or methane, while pressure broadening of the absorption line has routinely been accomplished with He. The excited alkali atom then lases on the D1 line back to the ground state. Terminating the laser level at the ground state requires the gain volume to be fully bleached before achieving an inversion between the 2P1/2 and 2S1/2 states, resulting in pump threshold values of ~1 kW/cm2.
This course will develop the background spectroscopy and kinetics of the DPAL system, summarize recent laser demonstrations, discuss narrow banding of diode pump sources, develop the key performance and scaling equations, and outline several issues in the development of these devices.
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