Research Of Passively Q-switched Erbium-doped Fiber Laser Based On Graphene Saturable Absorber

Graphene as a new saturable absorber material, has many excellent properties, which can be used to improve the performance of passive Q-switched fiber lasers. Compared with fiber lasers passively Q-switched by semiconductor saturable absorber mirror, graphene passively Q-switched fiber lasers have many advantages and potential applications. Recently, graphene passively Q-switched fiber lasers have been extensively investigated, which aim at generating pulses with high repetition rate, short pulse width, high pulse energy, or wavelength tuning.This paper mainly analyzed the mechanisms of Q-switching in fiber lasers, and experimentally studied the performance of passively Q-switched fiber laser based on graphene saturable absorber. In theory, we analyzed the basic theory of Q-switching, established Q-rate equation and discussed the influence of spontaneous emission, the fiber structure, Q switch, pump power on output performance of Q-switched fiber lasers. Also we discussed optical properties of graphene, and mechanism of graphene saturable absorber.In the experiment, we demonstrated two graphene passively Q-switched erbium-doped fiber lasers based on a ring cavity structure or a linear cavity structure, and analyzed their Q-switching mechanisms. The Q-switched fiber laser with ring cavity structure produced optical pulses at a wavelength of 1569 nm. With the increase of pump power, the characteristics of output pulses including repetition rate, pulse width, pulse energy and pulse spectrum were measured. The repetition rate of output pulses ranged from 20.32 to 60.97 kHz. The narrowest pulse width was 2.91 μs and the maximum pulse energy was 144.17 nJ. The dual-wavelength Q-switching and self mode-locking phenomena were also observed experimentally. The Q-switched fiber laser with linear cavity structure produced optical pulses at 1550-nm wavelength.With the increase of pump power, the repetition rate of pulses ranged from 51.06 to 85.47 kHz. The narrowest pulse width was 1.17 μs, and the maximum pulse energy was 25.51 nJ. We experimentally observed wavelength tuning from 1521 nm to 1581 nm, by using fiber Bragg gratings(FBGs) with different central wavelengths. The characteristics of output pulses including repetition rate, pulse width, pulse energy and pulse spectrum at different wavelengths were obtained. The broadband saturable absorption and modulation characteristics of graphene were discussed. By using the graphene saturable absorber with different layers, we explored the influence of graphene thickness on output characteristics of passively Q-switched fiber laser. The experimental results showed that monolayer graphene saturable absorber was easy to get higher repetition frequencies, narrower pulse width, higher pulse energy.