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We propose and experimentally demonstrate an asynchronous half-symbol delay-tap sampling technique for in-band optical signal-to-noise ratio (OSNR) monitoring in 38-Gb/s return-to-zero differential quadrature phase-shift-keying systems. The proposed technique enables simple analytical models for the statistics of a signal pulse which avoids system calibration prior to OSNR monitoring. A monitoring range of 11-23 dB has been demonstrated experimentally. Furthermore, the proposed technique also extends the range of calibration-based OSNR monitoring techniques to 5-34 dB.