Shadowgraph And Interferometry Experiments With A Ten-Picosecond Light Source At Various Wavelengths

AbstractA very fast (- 10 psec) laser light source has been constructed which can be used tomake shadowgraph, interferometry, and other diagnostic photographs useful in laser fusionstudies. To produce this pulsed light source, a portion of the 100 psec main Nd:glasslaser pulse is split off, sent through CS2 cells to produce a frequency chirp, and thenoptically compressed by diffraction gratings. By frequency converting the output pulse,one can perform studies at frequencies other than the fundamental (i.e., harmonic orRaman -shifted frequencies). The compressed pulse is absolutely time -synchronized with themain laser pulse. Shadowgrams taken using this technique are shown.IntroductionA very important diagnostic technique in laser fusion studies is the use ofsubnanosecond optical observations of laser produced plasmas. Optical techniques utilizinga pulsed light source such as shadowgraphy, Schlieren photography, and interferometry l'2 have proven particularly valuable for measuring the plasma density, shape, and velocity;while the Faraday rotations method has been used to measure the magnetic field generatedin the plasma. These plasmas often have velocities greater than 107 cm /sec, densitiesgreater than 1021 particles /cm3, and steep density gradients, particularly in the vicinityof the critical surface. Thus the use of such techniques usually requires a spatial -resolution of less than 10 u.m and a time -resolution of less than 10 picosec.In this paper we describe a technique by which a very short light pulse can be producedby compressing a portion of the main laser pulse.4 Moreover, the frequency of thecompressed pulse can be modified to serve a particular diagnostic purpose. Although atechnique of synchronizing two independent oscillators to within 100 psec has beenreported, the pulse compression system has the advantage that the probe light signal isabsolutely time -synchronized with the main laser pulse, and has a pulse duration shortenough to prevent time -smearing in the photograph.Pulse CompressionThe technique of pulse compression to reduce optical pulse duration and increase laserpeak power has been used for some time. 8'7 In this technique a relatively long laserpulse is passed through a liquid with a large nonlinear index of refraction to produce afrequency "chirp "; i.e., an actual optical carrier frequency variation within a singlelaser pulse. The chirped pulse can then be temporally compressed by passing it through apair of diffraction gratings.° Compression will take place if those frequencies thatappear earliest in the long pulse are forced to undergo the greatest delay. This isindicated in Fig. 1 where an incoming chirped pulse of FWHM duration At is dispersed sothat the higher frequency component (wo + Aw) is seen to follow a shorter path (AS) afterreflection from the second grating than the lower frequency component (wo - Aw), whichappeared earlier in the incident pulse. The recollimated exiting beam is then time -compressed to width Atc « At if the chirp bandwidth is large and if AS - c(At - Atc),The frequency chirp with initial pulsewidths up to about 100 psec is best produced byself phase modulation in a liquid with a high nonlinear index of refraction,i.e., for CS2n2 = 1.3 x 10 -11 esu. For example, an intense laser pulse passing through a CS2 cell under-goes self phase modulation, which results in an instantaneous frequency displacementfrom the original laser frequency given byHere