Jeffrey W. Grantham

Characterization of scannerless ladar

Scannerless laser radar (LADAR) is the next revolutionary step in laser radar technology. It has the potential to dramatically increase the image frame rate over raster-scanned systems while eliminating mechanical moving parts. The system presented here uses a negative lens to diverge the light from a pulsed laser to floodlight illuminate a target. Return light is collected by a commercial camera lens, an image intensifier tube applies a modulated gain, and a relay lens focuses the resulting image onto a commercial CCD camera. To produce range data, a minimum of three snapshots is required while modulating the gain of the image intensifier tubes microchannel plate (MCP) at a MHz rate. Since November 1997 the scannerless LADAR designed by Sandia National Laboratories has undergone extensive testing. It has been taken on numerous field tests and has imaged calibrated panels up to a distance of 1 km on an outdoor range. Images have been taken at ranges over a kilometer and can be taken at much longer ranges with modified range gate settings. Sample imagery and potential applications are presented here. The accuracy of range imagery produced by this scannerless LADAR has been evaluated and the range resolution was found to be approximately 15 cm. Its sensitivity was also quantified and found to be many factors better than raster- scanned direct detection LADAR systems. Additionally, the effect of the number of snapshots and the phase spacing between them on the quality of the range data has been evaluated. Overall, the impressive results produced by scannerless LADAR are ideal for autonomous munitions guidance and various other applications.

Instabilities of a microcavity laser with a weak injected signal

An external cw laser signal in injected into a microcavity laser, and the dynamics of the resulting coupled oscillator system are studied. By variation of the injection detuning and intensity, interesting nonlinear behavior and injection locking are experimentally observed. A theoretical model of this system based on coupled rate equations and including many-body gain effects is presented and yields good agreement with experiment.

A low-cost, high-resolution, video-rate imaging optical radar

Sandia National Laboratories has developed a unique type of portable low-cost range imaging optical radar (laser radar or LADAR). This innovative sensor is comprised of an active floodlight scene illuminator and an image intensified CCD camera receiver. It is a solid-state device (no moving parts) that offers significant size, performance, reliability, and simplicity advantages over other types of 3D imaging sensors. This unique flash LADAR is based on low- cost, commercially available hardware, and is well suited for many government and commercial uses. This paper presents an update of Sandias development of the Scannerless Range Imager technology and applications, and discusses the progress that has been made in evolving the sensor into a compact, low cost, high-resolution, video rate Laser Dynamic Range Imager.

Laser radar in adverse weather

Laser radar images of an outdoor target scene were collected in adverse weather such as rain and fog during the course of one year. Included in this collection is imagery in fogs with visibilities less than 2 km and rains with rain rates of up to 60 mm/hr. The targets were calibrated panels at 510 m and 1 km. The laser radar system used was a direct- detection 1.06 micrometers system designed to operate at 2 km in clear weather. For the purposes described here, though, the maximum range gate was set to 1.5 km. The system used a correlation technique for detection and discrimination, which significantly reduced the number of false returns in fog. Using these collected images, dropout pixels and false returns were correlated with rain rate and visibility. Extinction coefficients for 1.064 micrometers laser light were also calculated as a function of rain rate and visibility in fog and rain conditions. These coefficients were found to be consistent with those measured previously at 0.55 micrometers , 0.63 micrometers and 10.6 micrometers . These coefficients can be used to predict the performance of any circular polarized 1.064 micrometers LADAR system in rain and fog conditions.

Control of a phase-locked laser diode array using piston micromirrors

A ten element piston micromirror array has been used to produce a single irradiation spot in the far-field of a ten element laser diode array operating in the fundamental out- of-phase supermode and to steer the far-field irradiance pattern. A strong single lobe was obtained for several different laser injection currents. The produced lobe with full width at half maximum of 0.167 degree(s) was narrower than the original far-field lobes. Steering of the single lobe to 12 separate spots over a 0.807 degree(s) range was demonstrated.

Power broadening of coherent energy transfer in semiconductor gain media

Coherent energy transfer gives rise to a new peak and dip in the probe gain spectrum that move proportionally with the intracavity injected power, showing that stimulated emission and absorption significantly speed up the semiconductor response.

Spatial solitary waves, solitons, and vortices

Self-focusing leads to bifurcations of transverse solitary waves in sodium vapor (2D) and to second-order spatial solitons in a GaAs planar waveguide gain medium (1D). Transverse patterns in vertical-cavity surface-emitting lasers are shown to contain field vortices under some conditions. Good agreement is found between experimental data and computations.

Injection-induced instabilities and local gain modification in VCSELs

Injection of a cw narrow-band laser beam into a lasing vertical- cavity surface-emitting laser results in the appearance of new frequencies on the way to injection locking as predicted by our theoretical model. Injection also causes a local asymmetric modification of the lasing line, resulting in a new gain peak at a lower frequency and a dip on the high-frequency side. The peak and dip move out directly as the intracavity injected power, as predicted by our quantum mechanical theory.

Bifurcations of optical transverse solitary waves

Bifurcations of optical transverse solitary waves are studied for one-way propagation through a sodium vapor cell. Two types of phase encoding seed transverse bifurcations resulting in cell- exit profiles with beauty rivaling that of a kaleidoscope. The cell-exit profiles are stationary in time, reproduce completely when the power or frequency is scanned, and agree well with one- way computations. Temporal and longitudinal development of the cell-exit profiles is shown, demonstrating both the instability nature of this phenomenon and its solitary wave nature. The first evidence is also presented for a double-peaked Raman gain