Square-pulse operation in a ring cavity with a single-mode tellurite fiber

We demonstrate the square-pulse operation in a ring cavity with a 207 m single-mode tellurite fiber. The high nonlinearity and birefringence of the tellurite fiber play the key role during the square-pulse formation. The CW operation is obtained when pump power exceeds 80 mW. When the pump power is increased to 140 mW, the single pulse operation with the repetition rate of 689 kHz corresponding to the cavity fundamental frequency is initiated from the ASE noise by the nonlinear polarization rotation (NPR) process. Keeping the pump power at 140 mW and rotating one of the paddles of the polarization controller (PC) toward one direction in a small scale, the pulse number in one packet increases monotonically from 1 to 11. This process continues until the stable square pulse is formed at last. The stable square pulse has the FWHM of 118 ns and the repetition rate of 689 kHz. It is formed by the nonlinear polarization switching. In our cavity, the tellurite fiber is long enough to make the laser operate in the multibeat-length regime, which could lower the threshold for nonlinear polarization switching and clamp the peak power at a low level. This is advantageous for increasing the pulse width because the pulse duration is determined by the intra-cavity energy and the clamped peak power. When the linear phase delay bias in the cavity is set close to the polarization switching point, the third-order harmonic pulses and multiple packets are also observed under different pump levels.