Conclusion

Abstract

This is the end of our story about PDA and the detection of ultimately weak optical signal. The diversity of modern PDAs reveals also their unity: by their functional composition they all are detection systems, and incorporate a photodetector, a filteramplifier (integrator), sometimes a discriminator. Multielement PDAs may also include a commutator (multiplexer). The theory of detection conducted the orchestra throughout the book: it set the guidelines for synthesis of the PDA, prompted the ways to minimize its noise, defined the optimal passband of the filter and the rules for making the decision concerning the presence of the signal. All of the signal processing stages are responsible for the threshold characteristics of the device, but the first violin is always played by the photodetector. The input circuits are designed so as to match its impedance and noise level, the cooling system and the power supply maintain the desired temperature and operating regime. Under these conditions the photodetector performs its mission: it detects the optical signal almost without loss, often without introducing any additional noise. Because of this, the thermal background noise prevails in high-quality IR PDAs operating at wavelengths above 2 μm, and the amplifier noise prevails in the short-wave PDAs operating at wavelengths below 1 μm. The noisy amplifier is rescued by the photodetector (avalanche photodiode): it reinforces the signal, and, by avalanche multiplication, drags out both the signal and its own noise from the noise of the input stage. Initiative is persecuted, however: we accuse the APD of introducing the additional multiplication noise.