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We propose a novel laser configuration that can output 3.5-$\mu$m nanosecondlaser pulses based on a simple and monolithic fiber structure.Cascade-gain-switching, which converts the wavelength of nanosecond pulses from1.55 $\mu$m to 3.5 $\mu$m by two successive gain-switching processes. Insteadof using expensive pump sources at special wavelengths and bulky active orpassive modulation elements for Q-switching or mode-locking, the cascadegain-switching only requires the pumping of an electric-modulated 1.55-$\mu$mpulsed laser and two continuous-wave (CW) 975-nm laser diodes. They are allstandard products for fiber optic communication applications, which can greatlylower the cost of mid-infrared laser pulse generation. To investigate thefeasibility of this configuration, we numerically simulated thecascade-gain-switching processes by comprehensive rate-equation models. Insingle-shot regime, for stable 3.5-$\mu$m pulsed lasing, the CW 975-nm pumpshould be turned on at least $\sim500$ $\mu$s ahead of the 1.55-$\mu$m pulsedpump. It shows that the pulse width of the 1.55-$\mu$m pump has major impact onthe temporal shape of the intermediate 1.97-$\mu$m pulse while has neglectedinfluence on the generated 3.5-$\mu$m pulse. On the other hand, increasing theCW pump power can significantly improve the output peak power and shorten thepulse when the pump power is less than $\sim4$ W. In the repetitive-pulseregime, we found the 3.5-$\mu$m pulse train can be stably outputted when therepetition rate is $<=100$ kHz. As the repetition rate increases, the durationof reaching the stable operation increases. When the repetition rate is largethan 100 kHz, the stable operation cannot be established because the rate ofconsuming the population on the $^4I_{11/2}$ level of $Er^{3+}$ ions is fasterthan the rate of building the population.

Cascade-gain-switching for generating 3.5-um nanosecond pulses from monolithic low-cost fiber lasers