Complementary (Balanced) Heterodyne Receiver
In order to ensure the lowest possible noise-floor, it is important to suppress intensity noise from the optical local oscillator.
The schematic 9006-027 shows a balanced heterodyne receiver which provides complementary photodetection and high common-mode rejection of residual intensity noise on the local oscillator laser. The diagram shows the system for "looking" at only one plane of polarization, but it may be extended to cover polarization modulation schemes.
The beat signal outputs of each photodetector are 180o out of phase, so that by taking the difference between the two signals, twice the signal amplitude is obtained, and the local-oscillator induced intensity noise common to both signals is essentially canceled. This technique further improves on the Relative Intensity Noise (RIN) reduction imposed by the noise eater.
The output of the differential amplifier goes to the mixer and bank of filters as shown in the full system diagram 9006-026. The same electronic parallel signal processing technology developed for Microwave SETI may be applied to this optical counterpart because the Intermediate Frequency (I.F.) produced is similar.
The Columbus Optical SETI Observatory