Page 105 - Tunable Lasers Handbook

86 Charles Freed
fm=260Hz 16.4% DIP Ps1.75W; PO
r = 0.1 aec (single pole) p = a034 Torr

FIGURE 1 1 Lamb-dip-like appearance of the resonant change in the 4.3-pm fluorescence. The
magnitude of the dip is 16.4% of the 4.391 fluorescence signal. The pressure in the reference cell
was 0.034 Torr and the laser power into the cell was 1.75 W in the I-P(20) transition. A frequency
dither rate of 260 Hz was applied to the piezoelectric mirror tuner.

with a 0.034-Torr pressure of 12C160, absorber gas. The standing-wave satura-
tion resonance appears in the form of a narrow resonant 16.4% “dip” in the 4.3-
pm signal intensity, which emanates from all the collisionally coupled rotational
levels in the entire (OOOl)+(OOO) band. The broad background curve is due to
the laser power variation as the frequency is swept within its oscillation band-
width. Because collision broadening in the CO, absorber is about 7.5 MHzRorr
FWHM [72], in the limit of very low gas cell pressure the linewidth is deter-
mined primarily by power broadening and by the molecular transit time across
the diameter of the incident beam. The potentially great improvements in SNR,
in reduced power and transit-time broadening, and in short-term laser stability
were the motivating factors that led to the choice of stabilizing cells external to
the laser’s optical cavity. The one disadvantage inherent with the use of external
stabilizing cells is that appropriate precautions must be taken to avoid optical
feedback into the lasers to be stabilized.
For frequency reference and long-term stabilization, it is convenient to
obtain the derivative of the 4.3-pm emission signal as a function of frequency.
This 4.3-pm signal derivative may be readily obtained by a small dithering of
the laser frequency as we slowly tune across the resonance in the vicinity of the
absorption-line center frequency. With the use of standard phase-sensitive detec-
tion techniques we can then obtain the 4.3-pm derivative signal to be used as a
frequency discriminator. Figure 12 shows such a 4.3-pm derivative signal as a
function of laser tuning near the center frequency of the 10.59-pm P(20) transi-
tion. The derivative signal in Fig. 12 was obtained by applying a f200-kHz fre-
quency modulation to the laser at a 260-Hz rate. A 1.75-W portion of the laser’s
output was directed into a small external stabilization cell that was filled with