Page 161 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 141

t
RED SHIFT #
1.1 w
SLOPE DETECTOR OUTPUT -8
\ i
*

1.1 w
SLOPE DETECTOR OUTPUT +9

f
-1
0 20 40 60 80 100 120 140 160
PRESSURE (rnTorr)
FIGURE 20 Two runs with the grating positions deliberately offset in order to produce 00th
"blue" and "red" shifts. Note that these "instrumental" pseudo pressure shifts ma) easily dominate
over me pressure shift, especially for pressures greater than about 60 mTorr. (Repnnted 111th per-
mission from SooHoo et a1 [76]. 0 1985 IEEE.)

dispersion due to the laser configuration, cavity alignment, components, and lasing
and absorption medium. Even with an ideal cavity configuration, there are physical
and mechanical limitations on designing and building a perfectly centered and a
perfectly aligned laser cavity, especially since the PZT, with a nonlinear hysteresis
response to a symmetric signal, can easily distort any alignment of the cavity as a
function of the applied voltage, and may also introduce dither-caused asymmetry
in the derivative signal. In grating-controlled lasers, such as are used in our system.
there is the additional inherent dispersion of the grating itself. Consequently, the
laser power peak for any J line will almost never coincide perfectly with the corre-
sponding saturated resonance dip. and the error will depend on the existing laser
power profile and cavity configuration. It turns out that for each J line there is a
certain angular tuning range of the grating for which that line and a particular lon-
gitudinal mode dominate the laser gain. Because the gain profile depends on the
cavity arrangement, including the grating position, slightly tilting the grating cre-
ates a different cavity configuration and consequently a different gain profile,
which generally varies from J line to J line. Figure 20 is an illustration of both
blue and red ''instrumental" pseudo pressure shifts that were obtained by deliber-
alely offsetting the grating positions first in one and then in the other direction.
Note that the power slope offset error varies quadratically with pressure and its