4 CO 2 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 2 or CO molecular laser. A 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
if/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). A
servoamplifier/integrator further amplifies the output of the frequency discrimina-
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 2 (or CO) laser, if/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 computer 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:
fTDL ~" fC 0 2/CO "3V fsynthesizer "[" flF counter "
Either the operator or the computer program predetermines the frequency of the
if/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 fTDL will
depend on the absolute accuracy, resettability, and long-term stability of the
reference molecular gas laser(s). To date, the most accurate results have been
achieved with the use of CO 2 reference lasers.
We can utilize harmonics and the difference frequencies of
CO 2
transitions to synthesize precisely known reference frequencies well beyond the
8.9- to 12.4-1am range of the C02-isotope laser transition frequencies illustrated

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