Controllable ultrafast light shutter
BACKGROUND OF THE INVENTION 
     In the prior art there have been limitations in obtaining ultra-short laser pulses of selectable pulse duration. As one of the results thereof, it is difficult to obtain extreme accuracy in laser ranging. Another adverse result is the failure to accomplish significant damage studies since the laser pulse utilized therefor is not in the ultra-short category. The present invention, in sharp contrast to the prior art, provides ultra-short laser pulses of selectable pulse duration in the picosecond region. It further provides a light shutter to allow gating ultra-short light pulses. 
     SUMMARY OF THE INVENTION 
     A controllable ultrafast light shutter is provided. The light shutter uses in part a conventional laser triggered spark gap but adds a &#34;tail biter&#34; to terminate the pulse, thus closing the shutter. There are two Pockels cells and two polarizers. When either switch is in the &#34;off&#34; condition the entire switch is off. In the initial static condition the first Pockels cell (PC) polarizer combination causes the switch to be in the &#34;off&#34; condition while the second PC polarizer is &#34;on&#34; due to the fact that the polarizers are crossed. 
     One of the features of the invention includes the use of a spark gap to ground the high voltage electrode of a Pockels cell. Another feature is the use of a second Pockels cell polarizer combination to terminate the light passing through the light switch (&#34;tail biter&#34;). Still another feature permits the selection of light pulse duration in the picosecond region. 
    
    
     DESCRIPTION OF THE DRAWING 
     The single FIGURE partly in schematic and partly in block form shows a preferred embodiment of the invention providing a simple apparatus and method for obtaining ultra-short laser pulses of selectable pulse duration. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now referring in detail to the single FIGURE of this invention, there is shown conventional source 10 of a polarized Q switched laser pulse which passes through conventional Pockels cell 1. Pockels cell 1 and polarizer 1a are a first combination. The polarized light is rotated 90° as it passes through and is deflected by conventional polarizer 1a (Glan prism or equivalent) to lens 3 which focuses the light in spark gap 4. When spark gap 4 breaks down, the high voltage sides of Pockels cells 1 and 8 are grounded causing the remainder of the laser pulse to pass through. 
     Pockels cell 8 and polarizer 8a are a second combination. The second Pockels cell-polarizer combination goes to the off condition, closing the entire light switch. The length of time the entire switch is open is the only time there is an output from polarizer 8a. The on time is dependent on the time differences in grounding the high voltage sides of two Pockels cells 1 and 8 which is dependent on the length difference of two grounding cables L 1  and L 2 . 
     The switch is on for the length of time determined by the equation: Time -- on = (L 2  - L 1 )/V where L 1  equals cable length from spark gap 4 to Pockels cell 1, L 2  equals cable length from spark gap to Pockels cell 8, and V equals velocity in the cable. Further, the delay from breakdown to Pockels cell switching is dependent on the length of L 1  and is determined by: Delay time = L 1  /V. 
     When operated in the mode whereby the high voltage side of the Pockels cell is grounded for switching, there is no concern for impedance matching, therefore, for very short on times sliding bar 9 and moveable contact 9a may be used for fine tuning cable length differences. 
     Sliding bar 9 is for adjusting length differences between L 1  and L 2 . Resistors 5 and 6 are for current limiting and are chosen to be compatible with high voltage supply 7. 
     The embodiment of this invention further includes use of the tail biting in the laser triggered spark gap (LTSG) if the high voltage pulse is directed to a second Pockels cell to terminate the shutter. Further, the embodiment includes use of the light shutter to gate any light (long pulse or CW) irrespective of how the Pockels cells are voltage switched if the on time is dependent on cable length difference. 
     Thus the present invention provides a light shutter using a conventional laser trigger spark gap (LTSG) but adding a tail biter to terminate the pulse, thus closing the switch. There is utilized two Pockels cell-polarizer combinations, one set is used to open the light switch, the other to close the switch. The timing of opening and closing of the switch (i.e., the duration the entire light switch allows light to exit) is dependent on wire length differences. The wire length differences are easily working lengths, as a length difference of 1 cm corresponds to a &#34;switch open&#34; duration of 50 picoseconds (50 × 10.sup. -12  seconds) in typically used cables.