An optical homodyne technique to measure photorefractive-induced phase drifts in lithium niobate phase modulators

In this paper, we develop an optical measurement system with capabilities of phase unwrapping, real-time and long-term monitoring for measuring a phase drift caused by photorefractive effects in lithium niobate phase modulators. To extract the phase-drift variations, the measurement setup uses a homodyne interferometer with a phase modulation and a Fast Fourier Transform (FFT) demodulation scheme. The phase-drift characteristics of a traditional Ti-indiffused and a Zn-indiffused phase modulator have been investigated under different applied voltages and throughput powers. These experiments were conducted as a proof-of-concept to demonstrate that the apparatus worked successfully to measure the phase drift of a device in the presence of photorefractive effects. The results indicate that the Zn-indiffused phase modulators have better photorefractive stability than the Ti-indiffused phase modulators.