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A compact module including a polarization-selective electrooptic modulator and a light-emitting diode (LED) sensor is utilized for optical signal-to-noise-ratio (OSNR) monitoring and channel recognition. The modulator acts as a polarizer and provides signal dithering to improve the detection sensitivity, while the LED is used as a channel sensor. The channel sensor can monitor a wide wavelength range that is rapidly adjustable and can have much relaxed tolerance on temperature control. The combination of a polarization controller/rotator and the polarized modulator can accurately measure the OSNR between 5 and 35 dB with an error less than 0.5 dB.