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The operation mechanism and the pulse property of an actively Q-switched erbium-doped fiber laser based on an acousto-optic modulator (AOM) switch with the injection seeding technique are investigated. Our results show that the Q-switched pulses can be locked to oscillate near a fixed frequency higher than that of the seed laser, though the frequency-shift effect of the AOM impedes stable cavity mode oscillations. The operation mechanism of such Q-switch fiber lasers can be explained by the mutual locking-in among the shifted frequency components originated from the injected coherence seed with the help of the gain dynamics of the Q-switch cavity. Moreover, narrow-linewidth Q-switched pulses with different repetition rates can be obtained with different cavity lengths for incredibly stable output pulses without any use of cavity-stabilized techniques.

A new type and easy-to-fabricate metal–insulator–metal (MIM) waveguide reflector based on Sagnac loop is designed and investigated. The transfer matrix theoretical model for the transmission of electric fields in the reflector is established, and the properties of the reflector are studied and analyzed. The simulation results indicate that the reflectivity strongly depends on the coupling splitting ratio determined by the coupling length. Accordingly, different reflectivities can be realized by varying the coupling length. For an optimum coupling length of 750 nm, the 3-dB reflection bandwidth of the MIM waveguide reflector is as wide as 1.5 μ m at a wavelength of 1550 nm, and the peak reflectivity and isolation are 78% and 23 dB, respectively.