  We prove an alternative pulsing mechanism based on power adiabatic evolution
of pump in Tm-doped fiber laser. A pulsed laser technique, unlike Q-switching
and gain-switching, is explored under the equilibrium between the stimulated
emission and absorption. After the laser reaching a CW steady state, the
temporal fluctuation of pump power is called power adiabatic evolution if it
does not break the balance between the stimulated emission and the absorption.
Under the pump power adiabatic evolution the population densities in the upper
and lower laser levels get clamped to their steady-state values, and the
temporal shape of output laser is identical with that of pump. Based on the
power adiabatic evolution of pump a laser pulse can therefore be generated by a
pulsed pump which is added to a CW pump. Moreover, the temporal profile,
duration and peak power of the laser pulse can all be precisely controlled.
Power adiabatic evolution of pump can be achieved in the three pumping shemes
of Tm-doped fiber laser, provided that the duration of pump pulse is broader
than a certain value. The study on the pulsing mechanism highlights an
alternative pulsed technique possessing several advantages over Q-switching and
gain-switching.
