PHASE COHERENCE OF OPTICAL WAVE-GUIDES

The effective refractive index of real waveguides is not constant, but fluctuates as a result of variations in composition and waveguide dimensions. Consequently, the accumulated phase during propagation has a component that undergoes a random walk and whose mean square increases with length [DELTAphi2) = 2L/L(coh). These phase fluctuations result in wavelength fluctuations in Mach-Zehnder interferometers, especially in interferometers of low order. By measuring these fluctuations for Mach-Zehnder interferometers of different order, we have verified the above relation and determined that L(coh) almost-equal-to 27 m for our phosphorus-doped core silica on silicon waveguides.