Optical fiber group delay measurement based on laser beat frequency

A new method for optical fiber group delay measurement is proposed and experimentally demonstrated based on laser beat frequency. When the laser wavelength in the optical fiber resonator changes, the time delay through the optical fiber under test will change accordingly, leading to a change of round trip time in the optical fiber laser resonator. While the round trip time is the reciprocal of the minimum beat frequency, the group delay of the optical fiber under test which is placed in the fiber laser resonator can be easily obtained by measuring the beat frequency variation via the radio frequency spectrum analyzer. The optical fiber laser resonator utilized in experiments is composed of two reflectors: one is a sagnac loop composed of a coupler, the other is a composite structure composed of an optical circulator connected with a Fiber Fabry-Perot Tunable Filter. The group delay of the single-mode fiber and the dispersion compensating fiber are measured respectively in a relatively wide spectral range. Experimental results show that this method is feasible. Furthermore, this method is of very simple structure and low cost compared with other conventional methods.