The noise suppression of the autobalanced photoreceiver devised by Hobbs [Proc. SPIE 1376, 216 (1990)] had been determined to depend on the photocurrent ratio of reference beam to signal beam under the condition of constant signal beam photocurrent, and the best noise cancellation was suggested at a ratio close to 2. But in most applications, the available optical power has a limit. Therefore, to optimize the sensitivity of measurements, we should consider how to allocate the beam power in the case of fixed total optical power. In this paper, we measure the air Faraday rotation at different azimuth angles of beam polarization, which correspond to different photocurrent ratios. The signal-to-noise ratio at each photocurrent ratio is determined, and the best sensitivity appears at the photocurrent ratio of 1. This best sensitivity achieved is 3.02 × 10~(-8) rad Hz~(-1/2), which is about 1.3 times the shot noise limit. Our results are useful for sensitive optical measurements with the autobalanced photoreceiver.
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