Lakhasly

7, Number 10 APPLIED PHYSICS LETTERS 15 November 1965
MODE COMPETITION AND SELF -LOCKING EFFECTS IN A
Q-SWITCHED RUBY LASER
(pulse-regenerative oscillator; cavity length effect; E)
Since the advent of the passive Q switch,t-3 it was
recognized early that only a few modes could be
found in the giant pulse. 405 A recent theoretical con-
sideration by Sooy6 indicates that the narrower
output frequency of passive Q-switched laser is due
to independent growth of single axial modes in
many passes prior to reaching the saturation point
of the switching material. Conclusions are that the
mode structure established at the point of opening
the shutter will also be found in the giant pulse.
Much recent work has been directed towards achiev-
ing single-mode operation in a ruby laser Q-switched
by a saturable absorber by mode-selection tech-
niques. 7os
Due to gain and loss discrimination the saturable
filter acts as a mode selector allowing only a few
modes to build up to the high power level. The
self beat of these modes leads to output intensity
fluctuations. We have investigated mode competi-
tion and mode coupling on a multi megawatt peak
power and nanosecond time scale as a function of
cavity length, initial absorption and position of the
passive Q switch.
The results obtained indicate that at fast buildup
rates mode competition takes place such that the
center mode in a mode triplet is completely ex-
tinguished. Furthermore with large mirror spacings
and with the ruby at one side and the passive filter at
the opposite side of the cavity, self 10cking9 of the
phases of many cavity modes has been obtained and
the laser is operated as a pulse regenerative os-
cillator. At fast buildup rates (lO-nsec half-width)
peak power pulses of SO MW at twice the axial mode
frequency are obtained. At slow buildup rates
(6(}-80-nsec half-width) a self-locked train of pulses
each 1 nsec in half-width is obtained with a spacing
up to 10 nsec and a peak power level of S MW.
The output intensity fluctuations were observed
with a biplanar photodiode and a traveling wave
oscilloscope (Tektronix SI9). The total rise time of
270
}fans ~. A10cker
Systems and Research Department
Honeywell, Inc.
St. Paul, Minnesota
R. J. Collins
Dept. Elec. Eng.
University of Minnesota
Minneapolis, Minnesota
(Received 20 September 1965)
the detection system was O.S nsee. The ruby rod
(600 orientation, flat to 1/10 wave and parallel to
2 sec of arc) was 6 in. long by 3/8 in. in diam with
the surfaces cut at Brewsters angle. It was sub-
merged in water and pumped by a flashtube (EGG
FX-67B). The pump cavity was an elliptical reflector
with dielectric coating at the absorption bands of
ruby ..
A 7.S-cm Fabry-Perot with mirrors of 9S% re-
flectivity was used to measure the mode structure
of the giant pulse. Consecutive orders were spaced
by 2 kMc, the resolution was of the order of SO Me.
Figure 1 shows this experimental setup.
The cavity was Q switched by: a) a Kerr cell (with
straight and Brewster windows), b) a liquid passive
Q switch (LQ) cryptocyanine in methanol (low power
absorption a = 24%), and c) two filter glasses (FG):
Schott RG8 (a = 6S%) and Schott RG10 (a = 98%).
Six different cavity configurations have been in-
vestigated, the geometric layout and dimensions of
which are given in Fig. 2 and Table I respectively.
The relative position of the passive Q switches and
also the ruby rod with respect to the mirrors were
varied.
45 %
100 % REFLECTOR 1.5cm
REFLECTOR
4 SATURABLE
FILTER
~ 1 ~'~;~~ ..
BEAM ~
e
TRAVELLING WAVE
OSCILLOSCOPE
TEKTRONIX "9
SPLITTER FILM
BIPLANAR PHOTODIODE
KORAO KOI
Fig. 1. Schematic diagram of the experimental apparatus.