Precise measurement of polarization maintaining fiber length based on sawtooth wave phase modulation

A new method of sawtooth wave phase modulation is proposed to improve the measurement accuracy of polarization maintaining fiber length. We convert the measurement of polarization maintaining fiber length into the measurement of time delay. The optical signal is phase-modulated by the sawtooth wave modulation signal with modulation frequency of 2fp. The demodulated error signal is related to the frequency offset of the sawtooth wave and fp. The frequency of the sawtooth wave is adjusted to make the value of the error signal be zero, so that we can obtain the value of fp and calculate the delay time. Using this method, a set of polarization maintaining fiber length measuring system is designed and the fibers of 350 m, 570 m, 600 m, 920 m and 950 m are measured. The measurement accuracy of optical fiber length is about 8 ppm, which is lower than 10 ppm. Compared with other methods of measuring length of the polarization maintaining fiber, the structure and principle of this system are simple, and the experimental results show that this method has high precision and is easy to realize.