Estimation of frequency response for high-speed LiNbO3 optical modulators

A method of directly measuring the optical sideband in modulated output light by using an optical spectrum analyser is described for quantitatively estimating frequency responses of LiNbO₃ optical modulators in a practical large-signal operation. The conventional methods of estimating microwave power from photo-detectors using an electrical network analyser or an electrical sampling oscilloscope are theoretically compared with the method of using an optical spectrum analyser. The frequency responses measured with the method that uses an optical spectrum analyser are shown to correspond consistently to the small-signal modulation for any value of modulation index in measurements and to be useful in estimating precisely the frequency response of ultra-high-speed optical modulators. The responses estimated with the conventional methods mentioned above depend on the modulation index at a larger-signal modulation of m₀>~0.1. In particular, the responses when an electrical sampling oscilloscope is used vary remarkably with the value of m₀. These analyses make possible accurate estimation of the LiNbO₃ modulator´s frequency response, and thus aid in its practical use in large-signal operation of pulse modulation