A distributed-feedback (DFB) grating was written in twin-hole fibres with internal electrodes. Due to the intrinsic
birefringence, the grating has two ultra-narrow peaks (~0.41 pm and ~0.27 pm) corresponding to x- and y-polarization.
The separation between them can vary from 40 pm to 104 pm when the temperature increases from room temperature to
96°C. The dominant contributions of the Bragg wavelength shift as the increasing temperature are the change in
refracitive index of the fibre and the expansion of the substrate (largest). Under the current pulses excitation, full on-off
switching with response time ~2.5 ns has been achieved for x-polarization of the DFB grating.
Two methods to achieve dual-wavelength switching in a fibre laser are proposed and two corresponding switchable dual-wavelength fibre lasers based on fibre Bragg grating (FBG) feedback are demonstrated in this paper. In one proposed fibre laser, both Raman and Erbium-doped fibre (EDF) pumps are employed and the dual-wavelength switching is achieved by controlling the power of the Raman pump. In the other proposed fibre laser, an injection technique is used and the dual-wavelength switching is realized by controlling the power of the injection laser. The detailed behavior of the dual-wavelength switching in the two fibre lasers is experimentally studied and the principle is explained physically.
An automatic control algorithm considering the pump interaction for flattening the gain of a fiber Raman amplifier is derived from the Raman scattering equations. A pseudo-inverse gain matrix is introduced to adjust the powers of the pump lasers. The pump-to-pump interaction is compensated by the pump coefficients. It is demonstrated experimentally that the convergence of this algorithm is faster than that of the algorithm without considering the pump-to-pump interaction. In a system with 50-km SMF and 13.4km DCF, flattened Raman gain 15dB with a fluctuation no more than 0.40dB over a 45-nm bandwidth and 26dB with a fluctuation less than 0.8dB are realized in the experiment.