Page 164 - Tunable Lasers Handbook

4 CO, Isotope lasers and Their Applications 143

magnitude will also depend on the power incident on the stabilization cells. Note,
however, that by shychronously detecting the laser output, the power slope can
be monitored and adjusted (by incrementally tilting the diffraction grating) to
obtain as close to zero slope as possible at the center of the Doppler-free saturation
resonance. By using this technique, reliable pressure shift measurements could be
taken without the oveniding errors so frequently encountered as a result of the
power slope variations.
Another way to solve the background slope problem is through the use of
the so-called third derivative detection method. In most saturated absorption
experiments, the laser signal is dithered (frequency modulated) and the first
derivative signal (If) is detected and used as a frequency discriminator. If one
assumes a parabolic power profile, then the background slope error can be elim-
inated if the third derivative signal is detected and used as a frequency discrimi-
nator, This third derivative (3f) method of stabilization has been utilized in s~v-
era1 saturated absorption systems using CH, [113]. OSO,, and SF, [114]. where
the 3f absorption signal is large enough to eliminate or at least reduce the power
slope error without sacrificing the stability provided by the much larger SNR of
the If technique. However. potentially serious errors may be introduced by third
harmonic distortions L115-1171 due to both the motion of the laser mirror
(caused by distortion in the modulation drive voltage or nonlinearities in the
PZT driver) and in the optical detector and associated 3f phase-sensitive elec-
tronics. In our system. the frequency stability using the 3f technique was worse
than that obtained with the If technique. We have, therefore, devised the new
power slope detection method to eliminate the background slope and retain the
SNR advantage of the 1f stabilization technique.
By using the new technique we were able to reliably measure the "true" pres-
sure shifts both in pure CO, and with the admixture of various pertui-ber gases.
Several possible explanations for the anomalous behavior of the pressure
shifts obtained in our experiments were considered [ 1121. none of which could
explain the blue shift.
The effect of different perturber gases on the pressure shift of CO, was also
studied, Here the frequency shift for fixed CO, (20 to 30 mTorr) pressure as a
function of different perturber gas additives (upto about 80-mTorr perturber gas
pressure) including Xe, Ar, N,, He, H2, and CH,F were measured. Xenon. Ar.
N,. and CH,F gave blue shifts, and He and H, gave red shifts. The magnitudes
of the shifts scaled roughly with their corresponding polarizabilities except for
the change in sign.
Similarly anomalous results have been obtained by Bagaev and Chebotayev
[ 118,1191 for a CH,-stabilized HeNe system in which extremely small blue shifts
were measured for CH, perturbed by Xe, He, or Kr at pressures less than 10
mTorr: on the other hand red shifts were measured for the same transitions for
nobel gas perturbers (Xe, Kr. Ar, Ne, He) at pressures greater than 10 Torr 11201.
Again. the blue shift at low pressures was measured using saturated absorption
techniques, whereas linear techniques were used in the high-pressure regime.