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We propose and experimentally demonstrate a new technique for monitoring optical signal-to-noise ratio (OSNR). The noisy signal to be measured is split into two paths. With offset optical bandpass filters and balanced radio-frequency subtraction after detection, the signal is separated from the beat noise. OSNR can then be monitored by analyzing this beat noise. An experiment for a 10-Gb/s system shows that for OSNR in the range of 10-30 dB, the proposed OSNR monitoring scheme produces errors of less than 0.5 dB.