Annex 2: Coherent Balanced Receiver and Method for Noise Suppression
It has been shown that a balanced optical receiver can suppress the excess noise intensity generated from the local oscillator [1,2 and 3]. In a balanced receiver, an optical coupler is employed to mix a weak optical signal with a local oscillator field of average powers of and , respectively. Figure A2.1 shows the generic diagram of a balanced optical receiver under a coherent detection scheme in which two photodetectors and amplifiers are operating in a push–pull mode.
Usually, the magnitude of the optical field of the local oscillator EL is much greater than Es, the signal field. The fields at the output of a 2 × 2 coupler can thus be written as
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