Flat-Field Response And Geometric Distortion Measurements Of Optical Streak Cameras

Abstract

S. Montgomery, R. P. Drake, B. A. Jones, J. D. WiedwaldLawrence Livermore National LaboratoryP.O. Box 5508, L -473, Livermore, California 94550ABSTRACTTo accurately measure pulse amplitude, shape, and relative time histories of opticalsignals with an optical streak camera, it is necessary to correct each recorded image forspatially -dependent gain nonuniformity and geometric distortion. Gain nonuniformitiesarise from sensitivity variations in the streak -tube photocathode, phosphor screen,image- intensifier tube, and image recording system. These nonuniformities may be severe,and have been observed to be on the order of 100% for some LLNL ontical streak cameras.Geometric distortion due to optical couplings, electron -ontics, and sweep nonlinearity notonly affects pulse position and timing measurements, but affects pulse amplitude and shapemeasurements as well.By using a 1.053 -pm, long -pulse, high -power laser to generate a spatially andtemporally uniform source as input to the streak camera, the combined effects offlat -field response and geometric distortion can be measured under the normal dynamicoperation of cameras with S -1 photocathodes. Additionally, by using the same laser systemto generate a train of short pulses that can be spatially modulated at the input of thestreak camera, we can effectively create a two -dimensional grid of equally -spaced pulses.This allows a dynamic measurement of the geometric distortion of the streak camera.We will discuss the techniques involved in performing these calibrations, will presentsome of the measured results for LLNL optical streak cameras, and will discuss softwaremethods to correct for these effects.1. INTRODUCTIONThe Nova laser facility at the Lawrence Livermore National Laboratory (LLNL)1 is ableto perform complex temporal shaping of the laser pulse used to irradiate targets forinertial confinement fusion experiments. LLNL designed streak cameras2'3 are routinelyused to obtain time resolved measurements of the laser power and various target emissionsfor each experiment. For certain experiments, and particularly for pulse -shapedimplosions, it is necessary to know the laser power and target emissions accurately( 50 %) errors in both spatial and temporal measurements. The primary concern of the workpresented in this paper has been to characterize these effects in the optical streakcameras used for laser Power measurements and to apply a correction to the measured data.1.1 Errors in streak camera measurementsThere are several sources of error in making quantitative measurements with a streakcamera system. The largest contributors are:1) Image gain nonuniformities2) Geometric distortion (including sweep nonlinearity)3) Nonuniform background (dark current)4) Spatial and temporal resolution limits5) Random noise6) Dynamic range limitsThis paper deals mostly with the effects of image gain nonuniformities and geometricdistortion. Nonuniform background, although usually a small effect, is accounted for inthis presentation but will not be discussed at length. The last three sources of error,resolution limitations, random noise, and dynamic range limitations, are discussed atlength in other papers in these proceedings4,5 and have been optimized for this work.*This work was performed under the auspices of the U.S. Department of Energy by LawrenceLivermore National Laboratory under contract No. W- 7405- Eng -48.