Bidirectional output wavelength characteristics of asymmetric phase-shifted distributed feedback fiber lasers

The π phase-shifted distributed feedback fiber lasers, in which the power would focus on the phase-shift region if the coupling strength is high enough, is often used to acquire single frequency output. But due to the spatial hole burning, the output wavelength would be different for the two output ports. The fundamental mode focuses on the port near the phase shift region while the ±1 modes would focus on the other port if the phase shift region is far away from the center of the grating. Both the theoretical analysis and experiments are presented.