Variable sensitivity compensation network for Mach-Zender optical modulator

A compensation network for a Mach-Zender optical modulator comprising

• first and second pilot signal sources emitting first and second pilot signals f.sub.1 and f.sub.2, respectively,
• a signal coupler for combining the first and second pilot signals with an RF information-bearing signal and producing a modulating signal having both information and pilot components, said modulating signal being input to a bias-controlled Mach-Zender optical modulator, wherein a bias control fixes an operating point for said modulator, said optical modulator having a modulated lightwave output comprising the information signal and the sum frequency f.sub.1 +f.sub.2,
• a variable sensitivity feedback loop comprising an optical detector for detecting the pilot signals from the modulated lightwave output of the optical modulator, the output of said detector being the input to a bandpass filter for passing the sum frequency f.sub.1 +f.sub.2, said sum frequency comprising the input to a voltage variable attenuator for adjusting the sensitivity of the feedback loop, a logarithmic compression circuit for compressing the f.sub.1 +f.sub.2 signal level output from the variable attenuator, means for measuring the compressed f.sub.1 +f.sub.2 signal from the compression circuit, means for controlling the variable attenuator, and means for producing an analog bias signal for controlling the operating point of the optical modulator, said bias signal being input to the modulator.