  We report on the experimental observation of period doubling of multiple
dissipative-soliton-resonance (DSR) pulses in an all-normal-dispersion fiber
laser based on a nonlinear amplifying loop mirror. By increasing the pump
power, in addition to the typically linearly pulse broadening under fixed pulse
peak power, the jump from a single DSR pulse to multiple DSR pulses was
observed. During this process, the period doubling appears, that is, the DSR
pulses can exhibit the characteristics of period doubling in a multi-pulse
state. The typical DSR performance of linearly pulse duration change versus
pump power varying can be maintained when the period doubling of multiple DSR
pulses appears.
