  We propose a novel laser configuration that can output 3.5-$\mu$m nanosecond
laser pulses based on a simple and monolithic fiber structure.
Cascade-gain-switching, which converts the wavelength of nanosecond pulses from
1.55 $\mu$m to 3.5 $\mu$m by two successive gain-switching processes. Instead
of using expensive pump sources at special wavelengths and bulky active or
passive modulation elements for Q-switching or mode-locking, the cascade
gain-switching only requires the pumping of an electric-modulated 1.55-$\mu$m
pulsed laser and two continuous-wave (CW) 975-nm laser diodes. They are all
standard products for fiber optic communication applications, which can greatly
lower the cost of mid-infrared laser pulse generation. To investigate the
feasibility of this configuration, we numerically simulated the
cascade-gain-switching processes by comprehensive rate-equation models. In
single-shot regime, for stable 3.5-$\mu$m pulsed lasing, the CW 975-nm pump
should be turned on at least $\sim500$ $\mu$s ahead of the 1.55-$\mu$m pulsed
pump. It shows that the pulse width of the 1.55-$\mu$m pump has major impact on
the temporal shape of the intermediate 1.97-$\mu$m pulse while has neglected
influence on the generated 3.5-$\mu$m pulse. On the other hand, increasing the
CW pump power can significantly improve the output peak power and shorten the
pulse when the pump power is less than $\sim4$ W. In the repetitive-pulse
regime, we found the 3.5-$\mu$m pulse train can be stably outputted when the
repetition rate is $<=100$ kHz. As the repetition rate increases, the duration
of reaching the stable operation increases. When the repetition rate is large
than 100 kHz, the stable operation cannot be established because the rate of
consuming the population on the $^4I_{11/2}$ level of $Er^{3+}$ ions is faster
than the rate of building the population.
