A bismuth-doped germanosilicate fiber laser was experimentally demonstrated to engage in self-Q-switched mode-locking at a wavelength of 1470 nm. Mode-locked pulse bursts were generated from a ring cavity without using a saturable absorber when the cavity length was properly adjusted. At a cavity length of 65 m, repetition rates of ~31.5 kHz and ~3.088 MHz and temporal widths of ~4.7 μs and ~1.2 ns were measured for the Q-switching envelopes and modelocked pulses under the Q-switching envelopes, respectively. The laser operating regime was observed to change from Q-switching to Q-switched mode-locking as cavity length increases by adding a segment of anomalously dispersive fiber. The output pulse characteristics such as pulse width and repetition rate of the Q-switching envelopes and mode-locked pulses were also investigated with respect to the cavity length.

We report the results of our theoretical investigation into the impact of the Q-switch rise time on three key parameters of the output pulses in an actively Q-switched fiber laser, namely the multipeak phenomenon, pulse energy, and pulse peak power. Our investigation is conducted over a Q-switch time range of up to 12 times the cavity round-trip time (τ round ). We show that when a longer Q-switch rise time is applied to the cavity, a trade-off exists between multipeak phenomenon suppression and pulse energy maintenance. The results show that the Q-switch rise time (τ rise ) for the efficient suppression of multipeak phenomenon without loss of pulse energy varies, depending on the cavity parameters. In an exemplary, actively Q-switched fiber laser operating under the condition of a repetition rate of 15 kHz, τ rise is estimated to be approximately five times larger than τ round .

We demonstrate the use of an ion-doped fiber type saturable absorber for mode-locking of a fiber laser for the first time, to the best of the authors' knowledge. More specifically, it is experimentally demonstrated that an unpumped segment of bismuth doped germanosilicate fiber can readily be used as a mode-locker operating at 2 µm wavelengths. Incorporating a bismuth-doped germanosilicate fiber as a saturable absorber in a thulium/holmium codoped fiber ring cavity, stable mode-locked pulses with a temporal width of ~3.94 ns were easily achieved at a wavelength of 1927.6 nm with a repetition rate of ~6.806 MHz. A series of output characterizations were conducted to clarify whether the outputs are Q-switched or mode-locked pulses.

We proposed a multimode interference (MMI) fiber based saturable absorber using bismuth telluride at ∼2 μm region. Our MMI based saturable absorber was fabricated by fusion splicing with single mode fiber and null core fiber. The MMI functioned as both wavelength fixed filter and saturable absorber. The 3 dB bandwidth and insertion loss of MMI were 42 nm and 3.4 dB at wavelength of 1958 nm, respectively. We have also reported a passively mode locked thulium doped fiber laser operating at a wavelength of 1958 nm using a multimode interference. A temporal bandwidth of ∼46 ps was experimentally obtained at a repetition rate of 8.58 MHz.