Estimation of frequency response for high-speed LiNbO3 optical modulators

A method of directly measuring the optical sideband in modulated output lig ht by using an optical spectrum analyser is described for quantitatively es timating frequency responses of LiNbO3 optical modulators in a practical la rge-signal operation. The conventional methods of estimating microwave powe r from photo-detectors using an electrical network analyser or an electrica l sampling oscilloscope are theoretically compared with the method of using an optical spectrum analyser. The frequency responses measured with the me thod that uses an optical spectrum analyser are shown to correspond consist ently to the small-signal modulation for any value of modulation index in m easurements and to be useful in estimating precisely the frequency response of ultra-high-speed optical modulators. The responses estimated with the c onventional methods mentioned above depend on the modulation index at a lar ger-signal modulation of m(0) > similar to 0.1. In particular, the response s when an electrical sampling oscilloscope is used vary remarkably with the value of m(0). These analyses make possible accurate estimation of the LiN bO3 modulator's frequency response, and thus aid in its practical use in la rge-signal operation of pulse modulation.