With the rapid development of ultra-dense large capacity coherent WDM optical communication networks, the monitoring of in-band optical signal-to-noise ratio (OSNR) plays an essential role to ensure signal qualities. Different from the classic polarization-nulling method, we proposed and experimentally demonstrated a novel fiber-based programmable in-band OSNR monitoring method for flexgrid coherent transmission system, the OSNR monitor is based on linearly chirped fiber Bragg grating (LCFBG) and commercial thermal print head (TPH). For the coherent communication system, when the output power of the pre-amplifier at the receiving terminal is constant, degraded OSNR leads to decreased signal power and elevated ASE noise. Therefore, if the central spectrum (signal and in-band noise) is filtered by an ultra-narrow bandwidth optical filter, the output optical power is in proportional to the OSNR value, the influence of the filtered in-band ASE noise will be negligible with relatively high OSNR and the ultra-narrow bandpass filter is the key element for this technique. Based on the thermo-optic effect of the LCFBG. we used the in-house developed driver circuits and a Lab VIEW based software to implement a programmable ultra-narrow passband optical filter for OSNR monitoring. Linear monitoring range of 9-27dB OSNR values with wavelength ranging from 1530.6 to 1538 nm is achieved. The OSNR monitor has advantages of low cost, low insertion loss, large wavelength tunability and compatible with current optical fiber communication system.
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