Laser fusion pulse shape controller

#The present invention relates to laser systems and, more particularly, to systems for generating an optical pulse of preselected waveform or shape, i.e., of preselected time duration and intensity pattern.

#Apparatus for generating an optical pulse of preselected waveform or shape is shown in the U.S. Pat. No. of Carlton E. Thomas 3,879,109. In the apparatus therein disclosed, a single mode-locked laser output pulse having a duration on the order of thirty picoseconds is selected and fed to a system of beam splitters and mirrors by means of which the laser pulse is split into several separate pulses. The split pulses are selectively attenuated and delayed, and then recombined to form a composite output pulse having a waveform, i.e., duration and intensity pattern, which is the sum, over time, of the individually tailored split pulses. The output pulse or signal from the Thomas apparatus is thus, in effect, a composite of several individually tailored and timed pulses "stacked" upon one another. Hence, the Thomas apparatus has been referred to in the art and is referred to hereinafter as a "pulse stacker".

#The apparatus disclosed in the above-referenced Thomas patent is well suited, as noted therein, for use in controlling the duration and intensity pattern of a pellet ignition pulse in a laser-driven fusion reactor system. Indeed, the Thomas apparatus has been used quite successfully in this application. However, there is a slight "ripple" in the composite output of the Thomas device caused by interference between adjacent stacked pulses. This ripple may have an adverse effect upon controlled laser fusion and is, therefore, deemed to be undesirable.

#The novel features which are considered to be characteristic of the present invention are set forth in particular in the appended claims. The invention itself, however, together with additional objects, features and advantages thereof, may be best understood from the following description when read in conjunction with the accompanying drawing which is a schematic diagram of an exemplary but presently preferred embodiment of the laser fusion pulse shape controller provided by the present invention.

#Referrring to the drawing, the presently preferred embodiment 10 shown therein of the pulse shape controller provided by the present invention generally comprises a laser generator 12 for providing an optical control pulse of a preselected desired shape, i.e., duration in time and intensity pattern, a pulsed laser system 14 triggered by the output from generator 12, and at least one optical Kerr-effect gate and associated control apparatus 16 disposed at the output of laser system 14 and selectively triggered by the control pulse to gate or pass only a selected portion of the pulsed laser output generally corresponding to the shape of the control pulse.

#More specifically, control pulse generator 12 includes a mode-locked laser 18 which provides at its output a series of optical pulses. It will be understood, of course, that the pulse series shown at the output of laser 18, and other pulse series or pulse shapes shown in the drawing and to be discussed hereinafter, are schematic representations of typical outputs, which representations have been provided to facilitate an understanding of the invention and should not be construed to place any undue limitations thereon. The output of laser 18 is fed through a polarizer P1, and is routed by mirrors M1,M2 through a pulse selector 20, which may comprise a conventional Pockels cell for example, through a second polarizer P2 and onto a pulse stacker 22, the input to stacker 22 thereby being a selected pulse from the pulse series presented at the output of laser 18 and being polarized in a selected direction. Stacker 22 is preferably of the type disclosed in the above-referenced Thomas patent, the disclosure of which is incorporated herein by reference, and provides at its output a composite stacked pulse which is the sum of a number of individually tailored pulses. For optimum pellet ignition in a fusion reactor system, as disclosed in the Thomas patent, the initial energy of the composite output of stacker 22 should be low, increasing rapidly as implosion proceeds and then falling off very rapidly after peak power is attained. Composite pulse length may be between 100 and 800 picoseconds. The output of stacker 22 is fed through a suitable amplifier 24 into a mirror M3 and then to a beam splitter BS1. Thus, control pulses which are identical in shape but which may have differing intensities depending upon the reflective-transmittance ratio of beam splitter BS1 are provided by generator 12 to laser system 14 and gate 16.

#Laser system 14 includes a pulsed laser 26 which is triggered by a control signal fed from generator 12 via high voltage switch or trigger generator 28. Laser 26, which may be a Q-switched or cavity dumped neodymium/glass laser for example, provides a broad spectrum pulse at its output which may be on the order of 30 nanoseconds in duration.