Patent ID: 7643759

Claim:
A signal-quality evaluation device comprising a converter and an output detector, wherein: an output power P out of the converter has at least one maximum point with respect to an input power P in of a pulse of a digital signal, and the output detector detects an average output power corresponding to a plurality of output pulses converted from the digital signal by the converter and outputs output-power information, when P in is lower than an input power P 1 which corresponds to said at least one maximum point of P out , P out increases with increase of P in , when P in is higher than P 1 , P out decreases with increase of P in , the converter comprises an optical component, the converter includes an optical coupler and a nonlinear optical medium, and the optical coupler receives input light through a first terminal, splits the input light, and outputs the split light beams from a second terminal and a third terminal, and also inputs light through the second terminal and the third terminal and outputs light as the output light from the first terminal, and the nonlinear optical medium has an optical nonlinear effect and is disposed on an optical line extending between the second terminal and the third terminal of the optical coupler.