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When a coherent (heterodyne or homodyne), single-mixer type optical receiver is combined with an optical preamplifier or in-line amplifier(s), the S/N is deteriorated severely due to the common-mode (signal-ASE and ASE-ASE) beat noise. When a balanced-type coherent receiver is used, the common-mode beat noise can be suppressed and the ideal, so-called LO-ASE beat-noise limited, S/N ratio is attainable. The authors present a clear experimental demonstration of the suppression of common-mode beat noise by a balanced receiver. The minimum required LO power is determined by the thermal noise or the common-mode rejection ratio (CMRR) of the balanced receiver.