Page 181 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 159

In another project at Lincoln Laboratory, we demonstrated the equivalent of a
programmable and highly accurate tunable IR synthesizer, as shown in Fig. 27
[146,147,56]. In Figure 27 the IR synthesizer is derived from a lead-salt TDL; a
small portion of the TDL output is heterodyned against a line-center stabilized
grating-controlled CO, or CO molecular laser. 4 high-speed HgCdTe varactor pho-
todiode detects the beat note of the two lasers. The detected beat frequency, which
is generally in the 0- to 18-GHz range, is further heterodyned to some convenient
intermediate frequency (IF) through the use of readily available commercial
rf/microwave-frequency synthesizers and wideband double-balanced mixers. The
IF output is amplified and amplitude limited by means of low-noise wideband
amplifiers and limiters. The limiter output is. in turn. used as input to a wideband
delay-line-type frequency discriminator (200- to 600-MHz typical bandwidth). 4
servoamplifier/integrator further amplifies the output of the frequency disciimina-
tor, and the amplified output is then used to control the TDL current, which deter-
mines the TDL output frequency. Closing the servoloop in this fashion frequency-
offset-locks the TDL output to the combination of CO, (or CO) laser. rf/microwave
synthesizer, and the center frequency of the wideband IF discriminator. which a
frequency counter accurately monitors.
A computer controls the entire IR synthesizer system shown in Fig. 27. If.
for instance, the microwave synthesizer is frequency swept under cemputer con-
trol. the IR output frequency of the TDL would also be swept in synchronism
with the microwave synthesizer because the frequency-offset-locking servoloop
forces the TDL output to maintain the following frequency relationship:

Either the operator or the computer program predetermines the frequency of the
rf/microwave synthesizer in Eq. (22). The IF is very accurately measured, and
averaged if so desired, even in the presence of appreciable frequency modula-
tion, which may be necessary to line-center-lock either one or both lasers. Thus
to a great extent the absolute accuracy of the TDL output frequency fmL will
depend on the absolute accuracy, resettability, and long-term stability of the
reference molecular gas laseifs). To date. the most accurate results have been
achieved with the use of CO, reference lasers.

14. SPECTROSCOPIC USE OF CO, LASERS OUTSIDE THEIR FUNDA-
MENTAL 8.9- TO 12.4-pM WAVELENGTH RANGE

We can utilize harmonics and the difference frequencies of CO, lasing
transitions to synthesize precisely knomm reference frequencies well beyond the
8.9- to 12.4-ym range of the C0,-isotope laser transition frequencies illustrated