Effect of the residual doping on the performance of InN epilayers as saturable absorbers for ultrafast lasers at 155µm

We report on the improvement of performance of InN-based saturable absorbers in fiber lasers operating at 1.55 µm by reducing the residual doping, due to the lower Burstein-Moss effect. The improved tuning of the band-to-band transition with respect to the operation wavelength leads to an enhancement of nonlinear optical effects, resulting in 30 % of modulation depth. We introduce the development of an ultrafast mode-locked fiber laser using an improved InN-based saturable absorber that incorporates a buffer layer between the active layer and the substrate. The laser delivers output pulses with a temporal width of ∼220 fs, a repetition rate of 5.25 MHz, and high-pulse energy of 5.8 nJ.