Overview: Wavelength tunable Q-switched fiber lasers have important applications in the fields of communication, medicine, ranging finding and laser processing. Generally speaking, Q-switched fiber lasers can be divided into two types using either active or passive system. There are several kinds of saturable absorber can be used to realize passively Q-switched in fiber lasers, including semiconductor saturable absorber mirrors (SESAMs), transition metal-doped crystals and single-wall carbon nanotubes (SWNTs). However, the method of using nonlinear polarization rotation (NPR) technology which is an artificial saturable absorber to realize passively Q-switched can not only maintain all-fiber structure of fiber lasers, but also change the width of Q-switched pulses by adjusting the states of polarization controllers. By regulating commercial bandpass filter, the central wavelength of the Q-switched pulses can be adjusted continuously in a larger range.

In this paper, a continuously tunable Q-switched all-fiber Er-doped laser based on a 45°-tilted fiber grating and tunable bandpass filter is demonstrated. The 45°-tilted fiber grating is used to achieve the nonlinear polarization rotation (NPR) along with two polarization controllers (PCs). In this experiment, the fiber grating is equivalent to an ideal in-fiber polarizer because it has strong polarization-dependent loss (PDL), then the Q-switched pulses can be easily observed by properly adjusting the polarization controllers. Under the pump power of 655 mW, stable Q-switched pulses with central wavelength of 1548 nm, average output power of 4.45 mW, repetition rate of 105 kHz, and signal to noise ratio (SNR) of 39.89 dB are obtained. Furthermore, the Q-switched optical spectrum can be continuously tuned from 1512 nm to 1552 nm by simply rotating the tunable bandpass filter with 655 mW pump power. As far as we know, this is the widest tunable range of tunable Q-switched fiber Lasers based on nonlinear polarization rotation effect and tunable bandpass filter.