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 N_pl < 10^-20 W/m².Hz, the background has no affect on the CNR. When N_pl = 10^-15 W/m².Hz, the CNR reduction is about 32 dB. This CNR impairment is independent of the received signal power P_r or the local oscillator power P_o 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, N_pl can be increased by nearly a factor of 10⁴ before it affects the CNR. Increasing the electrical output bandwidth B_e 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 N_plBo (= P_b) is less that the local oscillator power P_o, so that the photodetector does not become saturated with sunlight, the CNR reduction caused by the background is independent of B_e.