Method and system for estimating a relative phase difference between light beams

A method of controlling optical signals defining an optical path, the method comprising:

• phase-locking optical signals at a first location of the optical path up to a given time-varying offset,
• estimating a relative phase difference between a first light beam and a second light beam guided through the optical path to determine a time-varying offset, the first light beam having a first wavelength and the second light beam having a second wavelength different from the first wavelength, by at least:
• performing one or more measurements on the first light beam and the second light beam to measure a value indicative of a physical pathlength variation,
• determining the physical pathlength variation based on the value indicative of the physical pathlength variation,
• determining a relative phase difference between the first light beam and the second light beam at a predetermined location of the optical path based on the determined physical pathlength variation,
• guiding a third light beam through the optical path, the third light beam having a third wavelength that is different from the first and second wavelengths, and
• determining a value indicative of the physical pathlength variation based on a relative phase difference between the third light beam and a particular light beam guided through the optical path, and
• generating a phase-lock without offset at a second location of the optical path.