All-Optical Modulation Characteristics of a Microfiber Coupler Combined Sagnac Loop

In this paper, we theoretically and experimentally investigate the all-optical modulation characteristics of a microfiber coupler combined Sagnac loop (MCSL). We inject pump lights with different intensities into the MCSL and utilize the thermo-optical effect from the microfiber coupler to control the phase relationship between the counter-propagating beams inside the Sagnac loop. The spectral characteristics from the Sagnac loop can be controlled accordingly, and the rate of the frequency comb (interference characteristic wavelength) shift can be 1 pm/mW. In addition, at a specific wavelength (e.g., 1550 nm), the MCSL can be used as an intensity modulator as well as a reflectivity-tunable Sagnac reflector. The reflectivity variation of the Sagnac reflector can reach about 10% when the pump light is at milliwatt level. The intensity modulation depth can be 9.3% at 100 Hz when the intensity-modulated 980 nm pump light imposed on the MCSL is 5.5 mW. Based on the thermo-optical effect, the MCSL can achieve the all-optical intensity modulation and all optically tunable filtering functions. The device is expected to play an important role in the development of tunable, multi-wavelength, and compact fiber lasers.