Page 110 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 91

averaged over 8.5 hours. The system used a measurement time of T = 10 sec and
A4 = 8 samples for each data point. yielding a measurement accuracy much better
than the approximately f 1-kHz peak-frequency deviation observable in Fig. 16.
The frequency drift was most likely caused by small voltage-offset errors
in the phase-sensitive detector-driven servoamplifier outputs that controlled the
piezoelectrically tunable laser mirrors. Because 500 V was required to tune the
laser one longitudinal mode spacing of 100 MHz, an output voltage error of
i2.5 mV in each channel was sufficient to cause the peak-frequency deviation
of fl kHz that was observed in Fig. 16. By monitoring the piezoelectric drive
voltage with the input to the lock-in amplifier terminated with a 50-SZ load
(instead of connected to the InSb 4.3-ym fluorescence detector), we determined
that slow output-offset voltage drifts were the most probable cause of the il-
kHz frequency drifts observed in Fig. 16. It is important to note that no special
precautions were taken to protect either the lasers or the associated electronic
circuitry from temperature fluctuations in the laboratory. The temperature Wuc-
tuatians were substantial-plus or minus several degrees centigrade. Significant
improvements are possible with more up-to-date electronics and a temperature-
controlled environment.
Perhaps the greatest advantage of the 4.3-ym fluorescence stabilization
method is that it automatically provides a nearly perfect coincidence between the
lasing medium's gain profile and the line center of the saturable absorber, because
they both utilize the same molecule. CO,. Thus every P and R transition of the
(0001 j-[lOOO. 02@0],,,, regular bands and the (Olll)--[Ol@O, 0310],.,, hot bands
[78-811 may be line-center-locked with the same stabilization cell and gas fill.
Furthermore, as illustrated in Fig. 8, the saturation resonance is detected sepa-
rately at the 4.3-pm fluorescence band and not as a fractional change in the much
higher power laser radiation at 8.9 to 12.4 ym. At 4.3 ym, InSb photovoltaic
detectors that can provide very high background-limited sensitivity are available,
However, it is absolutely imperative to realize that cryogenically cooled InSb
photovoltaic elements are extremely sensitive detectors of radiation far beyond
the 4.3-pm CO, fluorescence band. Thus, cryogenically cooled IR- bandpass fil-
ters and field-of-view (FOV) shields. which both spectrally and spatially match
the detector to the CO, gas volume emitting the 4.3-ym fluorescence radiation,
should be used. If this is not done. the detected radiation emanating from other
sources (ambient light, thermal radiation from laboratory personnel and equip-
ment, even electromagnetic emission from motors, transformers, and transmit-
ters) may completely swamp the desired 4.3-ym fluorescence signal. This proce-
dure is a very familiar and standard technique utilized in virtually every sensitive
IR detection apparatus; surprisingly, however, it was only belatedly realized in
several very highly competent research laboratories. because the most commonly
used and least expensive general-purpose IR detectors are bought in a sealed-off
dewar and may not be easily retrofitted with a cryogenically cooled bandpass fil-
ter and FO'V shield.