CO2laser preamplifier capabilities for low-level 10.6-µm direct-detection receivers

We examine the potential of CO
2
laser preamplifiers for sensitivity enhancement in low-level, direct-detection 10.6-μm receivers. For the condition in which a gain-dependent competition exists between the background noise and amplifier spontaneous emission noise (assuming negligible thermal noise), the analysis predicts an optimum useful SNR enhancement of only 6 dB for a blackbody background field of 300 K and 4.1 dB for a background of 260 K, when the amplifier gain bandwidth per
p
-line is 100 MHz and the infrared (IR) filter bandwidth is 0.10 μm. Based on preselected choice of gain and bandwidth, a two-stage, water-cooled, flowing-gas amplifier of optimized design was constructed. A maximum gain of 3.12 dB was attained for
P(20)
with a He : CO
2
: N
2
mixture of 5.0 : 1.0 : 0.6 at a coolant temperature of 285 K and a slow gas refresh rate of 0.2 volumes/s. Using a fast-flow system with 12-volume/s refresh rate, we measured an amplifier gain of 3.9 dB, close to the design estimate of 4.1 dB. With a calibrated HgCdTe detector,
f/4
cold shield, and narrow-band (0.25 μm) cold filter, a spontaneous emission flux density of
\sim 1.0 \times 10^{14}
photons/ cm
²
. s was measured at the 3.12-dB gain level, in close agreement with the theoretical estimate. Excess noise resulting from amplifier discharge was undetectable above the basic detector noise.