Passively Q-switched erbium fiber laser with a thin crystalline film of bismuth telluride used as a saturable absorber

Performance of a passively Q-switched erbium fiber laser with a narrow bandgap Bi₂Te₃ semiconductor thin film used as a saturable absorber was studied experimentally. In order to obtain repetitively-pulsed lasing, a Q-switcher was fabricated as a section of the standard SMF-28 optical fiber with a short (∼4 mm in length) chemically etched out tapered section laterally covered with the bismuth telluride film. This Q-switcher was installed in a specially designed circuit of a ring-type erbium fiber laser pumped by a laser diode at 975 nm wavelength. The fiber diameter in the middle of the tapered section was 10-15 μm. Two types of taper coverage were studied: Bi₂Te₃ flakes suspended in isopropanol and a ZnTe/Bi₂Te₃ uniform polycrystalline thin-film heterostructure synthesized on a lateral surface of a tapered fiber section via MOCVD. In the first case, we observed laser oscillation at a wavelength of 1565 nm as a train of 7-26 μs pulses with a 86-50 μs repetition period at pump powers varying from 26 to 75 mW. In the second case, CW oscillation in the circuit appeared at a pump power of 30 mW while conversion to the repetitively-pulsed mode with a kilohertz repetition rate occurred at pump powers within the range of 34-40 mW. The results obtained indicate the feasibility of effective interaction of the evanescent field with a bismuth telluride film used as a saturable absorber. This interaction is sufficient to achieve a stable repetitively-pulsed lasing in the circuits of passively Q-switched fiber lasers.