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.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).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.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.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 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%).At fast buildup rates (lO-nsec half-width) 
peak power pulses of SO MW at twice the axial mode 
frequency are obtained.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).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 pump cavity was an elliptical reflector 
with dielectric coating at the absorption bands of 
ruby ..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.Six different cavity configurations have been in-
vestigated, the geometric layout and dimensions of 
which are given in Fig.A 7.S-cm Fabry-Perot with mirrors of 9S% re-
flectivity was used to measure the mode structure 
of the giant pulse.The relative position of the passive Q switches and 
also the ruby rod with respect to the mirrors were 
varied.University of Minnesota 
Minneapolis, Minnesota 
(Received 20 September 1965) 
the detection system was O.S nsee.Consecutive orders were spaced 
by 2 kMc, the resolution was of the order of SO Me. 
Figure 1 shows this experimental setup.The 
self beat of these modes leads to output intensity 
fluctuations.45 % 
100 % REFLECTOR 1.5cm 
REFLECTOR 
4 SATURABLE 
FILTER 
~ 1 ~'~;~~ ..Conclusions are that the 
mode structure established at the point of opening 
the shutter will also be found in the giant pulse.A10cker 
Systems and Research Department 
Honeywell, Inc.St. Paul, Minnesota 
R. J. Collins 
Dept.Elec.Eng.2 and Table I respectively.The total rise time of 
270 
}fans ~.