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CNR Versus Background In Coherent Systems

9008-077

 

Graph 9008-076 shows the affect of background radiation on the recoverable CNR in an optical heterodyne receiver. For polarized Planck background spectral density levels Npl < 10-20 W/m2.Hz, the background has no affect on the CNR. When Npl = 10-15 W/m2.Hz, the CNR reduction is about 32 dB. This CNR impairment is independent of the received signal power Pr or the local oscillator power Po if the receiver is quantum-noise limited. We have already seen that at a range of 10 Light Years, the polarized Planckian starlight level is about 2.4 x 10-24 W/m2.Hz, this being about 69 dB below the spectral density level of the 1 kW signal and 35 dB below quantum noise. Thus at this range, Npl can be increased by nearly a factor of 104 before it affects the CNR. Increasing the electrical output bandwidth Be reduces the CNR just as with quantum shot noise without background radiation.

At 10 A.U., the polarized Planckian starlight level is about 9.6 x 10-15 W/m2.Hz. This level of Planck noise will lead to significant CNR reduction, and for this reason, this type of optical receiver will suffer considerable performance impairment when employed within our solar system if the Sun is in direct line-of-sight behind the transmitter. Notice that as long as the optical received background power NplBo (= Pb) is less that the local oscillator power Po, so that the photodetector does not become saturated with sunlight, the CNR reduction caused by the background is independent of Be.

 

 

 

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