One urgent problem to solve in realizing optical networks is monitoring the signal quality in the transmission fiber. A comprehensive and precise monitoring method is needed to determine failure location. Conventional digital networks commonly perform parity checking of the total bit sequence at each regenerator repeater for this purpose. One benefit of optical networks is their bit-rate or signal coding transparency [1], so the last measure we want to adopt is equipping line repeaters with bit-error monitoring or parity checking circuits at every monitoring point. The transparency would be lost because digital monitoring is workable only with predetermined bit-rates and frame formats. The new optical method proposed herein makes it possible to monitor the signal quality or signal-to-noise ratio (SNR) of a digital signal regardless of its bit-rate, frame format or even modulation format. This method utilizes the sum-frequency-generation (SFG) optical sampling technique with the temporal resolution of 1 ps [2], and measures amplitude histograms of optical signals all-optically. Therefore, it can monitor the averaged quality (Q-factor) of optical signals in the optical networks with any bit-rate or coding. We also demonstrate averaged Q-factor monitoring of RZ or NRZ 10 Gbit/s optical signals using the SFG optical sampling method.
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