Pulse stretching in a Q-switched ruby laser for bubble chamber holography G. Harigel, C. Baltay, M. Bregman, M. Hibbs, A. Schaffer, H. Bjelkhagen, J. Hawkins, W. Williams, P. Nailor, R. Michaels, and H. Akbari In testing a holgraphic particle track recording system for the Fermilab 15-ft bubble chamber, it was shown that the peak power of Q-switched laser pulses (50-ns duration) at the required energy gave rise to boiling during the chamber expansion. A pulse stretching technique is described which was developed to reduce the peak power. Applied to a ruby laser (oscillator and three amplifiers) with a maximum Q-switched output of 30 J, pulses of up to lOO-us duration with coherence up to and exceeding 11 mat 2.5 Aswere produced. These pulses were amplified to -5 J without shape degradation. The considerably increased coherence length will find applications in many fields of pulsed holography, and its use with fiber optics is particularity promising. 1. Introduction was demonstrated that, at a given energy (5 J), the An initial test of holographic recording of particle boiling is suppressed by using a free-lasing pulse (1 tracks in a 35-M3 cryogenic bubble chamber at CERN ms) instead ofthe Q-switched pulse (<50 ns). Howev- (BEBC) was successful.' However, use of a powerful er, the 1-ms operation mode of the laser is unsuitable Q-switched ruby laser produced as an unwanted after- for our purpose because of the bubble movement and effect boiling of the chamber liquid which adversely size variation during illumination. Furthermore, it affects the quality of the conventional photographs may not provide reliably enough the necessary coher- taken some 10 ms later. This boiling is in all probabili- ence length. We, therefore, have to aim for an inter- ty due to the absorption of light on small impurities mediate pulse duration with good beam quality, as (with diameters of a few micrometers or even fractions proposed earlier.' It is the purpose of this paper to of a micrometer), which float in the bubble chamber describe our 2 4 technique, which reduces the instanta- liquid. - Their heating gives rise to bubble nucle- neous power at constant energy and solves, at least ation during the expansion cycle. In this first test it partially, the boiling problem. Parallel approaches, not the subject of this paper, consist of the reduction of the overall energy requirement by the increase of the sensitivity of existing holographic emulsions5 and the decrease of impurities by filtering the liquid.6 The stretched pulse system was G. Harigel is with CERN, CH-1211 Geneva 23, Switzerland. used in the Fermilab 15-ft When this work was done C. Baltay, M. Bregman, M. Hibbs, and A. bubble chamber to record high energy neutrino inter- Schaffer were with Columbia University, Nevis Laboratories, Ir- actions. vington, New York 10533; M. Bregman and M. Hibbs are now with Various methods for obtaining stretched pulses have IBM T.J. Watson Research Center, Yorktown Heights, New York been reviewed both from the experimental and theo- 7 10598, and A. Schaffer is now with CERN, CH-1211 Geneva 23, retical point of view (sixty references). Any applica- Switzerland. H. Bjelkhagen, J. Hawkins, and W. Williams were all tion requires a specific shape and a certain energy of with Fermilab, Batavia, Illinois 60510 when this work was done; H. stretched pulses, which may then be obtained with one Bjelkhagen is now with Northwestern University, Evanston, Illinois of the following techniques. 60201.2 When this work was done P. Nailor was with Imperial Lengthening of the cavity College, London, of a Q-switched laser, SW7, U.K.; he is now with P. A. Technology, introducing nonlinear materials into its Cambridge Laboratory, Melbourn, Herts. SG8 6DP, U.K. When cavity, or use this work was done R. Michaels was with Rutgers University, New of a feedback loop to control switching of an electroop- Brunswick, New Jersey 08903; he is now with Hewlett Packard, Palo tic shutter was used to obtain long pulses. The first Alto, California 94304. H. Akbari is with Tufts University, Med- two methods produce light pulses whose time variation ford, Massachusetts 02155. is either almost Gaussian or very asymmetric and fairly Received 13 February 1986. short: they will not be considered further, since the 0003.6935/86/22110209$02.00/0. pulse for our application must be reasonably flat over © 1986 Optical Society of America. at least several microseconds, and the rise and decay 4102 APPLIEDOPTICS / Vol. 25, No. 22 / 15 November 1986 To bubble chamber ~~~~~~~~~~~~~~~I . --- Amplifiers and beam expander - - -Oscillator - o-*1 Fig. 1. Layout of the KORAD OS- I B cillator stage with feedback optics M3: I Al; A3 A2 BE Al iOE M and four amplifiers (schematic): OE, output etalon; MS, mode se- -o 1-- ---- E --- -- --EnT_* 4 lection aperture; R, ruby rod; PC, 7 I Pockels cell; RM, rear mirror; AO, amplifier; MI, M2, M3, mirrors; I El PO PD, photodiode, El, feedback electronics; Al, A2, A3, A4, ampli- I Feedback loop I -- I fiers; BE, beam expander. times are short compared with the flattop. Therefore, A. KORAD Laser we pursue only the third method and discuss aspects of For the layout of the cavity we used various elements a few earlier technical approaches in view of their from a KORAD K-1000 ruby laser system given to us by suitability. Columbia University's Radiation Laboratory. Figure The longest pulse was achieved with a feedback arrangement. The length of 8 1 shows the geometrical circuit using a Kerr electrooptic shutter. The inten- the cavity is chosen to be 100 cm, similar to the layout sity distribution was highly asymmetric, reaching its described in Ref. 11. To keep the cavity as simple as maximum at -150 s, and fell smoothly to zero at -1.8 possible we extract the light needed for the feedback ms. An attempt to obtain a rectangular pulse of 5-10 loop through the rear 95%reflector rather than from a Asin a feedback circuit, using a Kerr cell,9 resulted only beam splitter placed inside. The outgoing 5% of the in asymmetric pulses of <5-gs duration with heavy light passes through a ruby amplifier to give sufficient intensity instabilities at 0.6 As. A negative feedback intensity and flexibility for operation of the feedback system using a Pockels cell gave pulses of almost 1-as electronics. After only some 30-cm path length (1-ns duration, however, with a spike twice the average in- delay) the light hits a 45-mm diam. phototube (ITT tensity at the beginning.'0 This pulse was amplified FW114A), which acts, through the electronic circuit without shape distortion to an energy of -0.6 J. This shown in Fig. 2(a) on the Pockels cell. The left side of technique was further investigated, both experimen- the circuit serves to adapt the trigger to the KORAD tally and with computer simulation of the electronic power supply. We chose a KD*P Pockels cell (Quan- circuit and of the laser rate equations, aiming for sever- tum Technology model QK-10) with a low quarterwave al hundred nanosecond flattop pulses."1 This concept voltage (2100 V) and low capacitance (6 pF); the con- appeared the most promising with which to obtain flat necting wires were kept short to minimize the stray pulses of tens of microseconds duration. 7 1 capacitance of the circuit. The fire pulse from the In the relevant papers - ' the effect of pulse stretch- laser control system triggers a Kryton switch (EG&G ing on the coherence length and the TEMOO mode, type KN6B), which drops the bias voltage across the important for our application, has not been discussed. Pockels cell to zero in a time of -1 ns. As laser oscilla- tion begins, the phototube starts to conduct. Voltage 1. Feedback-Controlled Laser Systems and is produced across the 200-Q load resistor, rebiasing Experimental Results the Pockels cell. This negative feedback inhibits the For the illumination of <10 m3 of the fiducial vol- buildup of oscillation. To prevent the complete ex- ume of the 15-ft (4.5-m) bubble chamber at Fermilab, a tinction of laser action, positive feedback is applied via system similar to that described in Ref. 1 with light the inductor, causing the differential voltage across the energy up to -30 J is needed. 12 This energy will be Pockels cell to drop again. This allows oscillation to obtained with an oscillator stage followed by several build up, and sustained laser action is ensured by the amplifiers with increasing ruby rod and laser beam balance between the negative and positive feedback. diameter. Since we expect no serious deterioration of For further theoretical details, see Ref. 11. the quality of the initial light pulse going through these Typical stretched pulses (with some mode-beating) amplifiers" (for effects of high pumping of amplifier are shown in the oscilloscope picture of Fig. 3. About rods and backreflections see Appendix), we can limit 20%of all pulses exhibit an overshoot (maximum about ourselves to a description of the oscillator. twice the average height) at the beginning of the pulse, The original development was carried out on a modi- which does not affect significantly our application. fied KORAD laser with a feedback circuit similar to Ref. About 2 s after the Kryton fired, the differential 11 and tested during a technical run of the Fermilab Pockels cell potential rose rapidly closing off all lasing. 4.5-m bubble chamber; the system used later in the The output from the circuit was only marginally effec- physics run was a new design, adapted to the JK Laser tive given the 2.1-kV quarterwave potential of the System 2000.