George W. Sutton

Beam-jitter measurements of turbulent aero-optical path differences.

We have developed new a technique for measuring aero-optical aberrations in flowing turbulent fluids. The rms optical path difference power spectral density is obtained from angular beam-jitter measurements. We tested the technique in an airflow in which there was a temperature discontinuity at a turbulent interface. It was validated by comparison with the data from a pulsed interferometer.

Fraunhofer diffraction patterns from uniformly illuminated square output apertures with noncentered square obscurations

Theoretical Fraunhofer diffraction patterns are presented for uniformly illuminated square apertures with noncentered square obscurations. The energy within a given subtended solid angle in the far field is calculated. It is shown that the cornered-off-axis obscuration provides much more far-field energy in a given spot size than the centered obscuration for the same clear aperture area and total energy, for example, 82% more far-field energy in the first Airy square for 50% obscuration, thus providing superior performance for practical systems.

Fog hole boring with pulsed high-energy lasers: an exact solution including scattering and absorption.

The exact time dependent solution for hole boring through fog is obtained for pulsed high energy lasers, which includes scattering, absorption, and the effect of realistic particle distributions. The results are applied to both haze burnout and fog hole boring. For the latter, at 10.6 microm, the calculated result is that 50% more laser pulse energy is required than that calculated from thermodynamic considerations alone. An absolute upper bound of 100% is also established. The results are compared to shorter wavelengths, e.g., 3.8 microm for which much larger fluences are required to evaporate fog or haze, and are also compared to other recent approximate results, which overestimate the required energy by a larger factor.

Effect of inhomogeneous turbulence on imaging through turbulent layers

An analysis of the effect of inhomeogeneous turbulence on the point-spread function for imaging through a turbulent layer has been performed. The formulation retains the concept of the modulation-transfer function that has been used in the past for homogeneous turbulence layers, but the integration now depends on more than just the separation distance ρ in the plane of the aperture. An example is chosen in which the rms optical path difference increases by a factor of 3.51 from one side of the aperture to the other in the flow direction. It was found that the results were insensitive to the angle between the separation-distance vector ρ and the flow direction. Surprisingly it was found that the detailed resultswere within a few percent of those obtained with constant-turbulence parameters evaluated at the center line of the aperture.

Mode-locked Nd:YLF laser for precision range-Doppler imaging

We report the application of an actively stabilized CW mode-locked Nd:YLF laser to the problem of range-Doppler imaging. A laboratory demonstration of cm range and cross-range resolution is described. The effect of target speckle and glint on signal-to-noise ratio (SNR) and signal processing are discussed. Because of the wide range of mode-locked pulse and pulse train durations achievable with the solid state laser system, the single shot signal-to-noise ratio can be maximized. A Nd:YLF oscillator and Nd:glass amplifier laser system has distinct advantages over other high power laser systems in range-Doppler applications.

Experimental measurements of the optical path difference of a 4-m dual aerocurtain

The Gound-Based FEL Technology Integration Experiments high power laser requires a beam director-dome window to protect the optics from dust accretion and reduce the optical aberrations occurring in the air interface between the dome interior and exterior. One half of the dual aerocurtain designed for this purpose is heated 18 C above ambient temperature, while the other is not heated; this results in the development of two mixing zones, (1) between the heated and unheated halves, and (2) between the latter and the room air. This aerocurtain configuration is shown to reduce the Strehl ratio loss through the window to several percent.

Frequency stability of a solid state mode-locked laser system

The authors have stabilized the frequency of a cw, lamp-pumped, mode-locked Nd:YLF oscillator. The output pulse duration is 50 ps and, simultaneously, the mode jitter is less than 25 kHz relative to a stable, high-finesse Fabry-Perot. The frequency shifts and broadening of the oscillator modes caused by pulsed amplification in a flashlamp-pumped Nd:glass rod with a gain of 100 have been measured. Broadening and frequency shifts on the order of 10 kHz are induced by the amplifier. The changing population of the active transition in the amplifier is the dominant source of the frequency shifts. For frequency-stable amplification, significant saturation of the gain medium must be avoided.

Effect of a spatial distribution of aerosols on multiple pulse aureole scattering

The scattering cross section for regular spatial distribution of aureoles created by continuous multiple pulse radiative heating of aerosols is analyzed and compared with that for a single isolated aerosol aureole.