Abstract

Passive mode-locked fibre lasers using for mode-locking a Nonlinear Optical Loop Mirror (NOLM), called figure- eight lasers (F8L), are simple and attractive devices. A desirable feature of mode-locked lasers is self-starting operation, however the F8L are usually considered as not self-starting. Self-starting operation of a passively mode-locked laser depends on balance between nonlinearity and the intracavity power. In a previous work we studied a scheme including a symmetrical coupler, a highly twisted low-birefringence fiber and a Quarter-Wave (QW) retarder in the loop to break the polarization symmetry [1,2], With this configuration, switching can be obtained through Nonlinear Polarization Rotation (NPR). It is then necessary to create a polarization asymmetry between the counter-propagating beams. This configuration is attractive for application such as a passive mode-locker in fiber lasers because the low-power transmission and the slope of the nonlinear dependence can be easily and precisely adjusted. This adjustment possibility makes it very convenient to find experimentally the conditions for self-starting mode locking operation. In this work we experimentally demonstrate the self-starting operation of the novel F8L based on the power-balanced NOLM with the twisted low-birefringence fiber and the QW retarder in the loop. The experimental setup is shown in Fig. 1. The NOLM is formed by the 51/49 Coupler 1, whose output ports were fusion-spliced with the 220 m loop of the low-birefringence, highly twisted Coming SMF-28 fibre. The QW1 retarder in the loop transfonns the polarization of the counterclockwise beam from circular to linear. Couplers 2 and 3 were used to monitor the laser power. The fibre Bragg grating (FBG) filter was inserted in the laser amplifier section using the circulator. The FBG central wavelength is 1548 mn with 100% peak reflection. The 980 mn laser with 100 mW maximum output power was used to pump the erbium- doped fiber (EDF). The pump power was injected into the system through the WDM coupler. The output signal of the NOLM was amplified by the 10 m EDF and passed through the polarization controller (PC) and the polarization dependent optical isolator (PI) to impose circular polarization at the NOLM input. The PC is adjusted to have maximal transmission through the PI when the laser operates in the cw mode. In this case the polariser PI transmits the NOLM output polarization component that is orthogonal to the NOLM input polarization, as we have described in Ref. [2], When the orthogonal component is selected, the value of the low-power transmission depends of the QW1 position. Experimenting with various angles of the QWl, we found that self-starting is observed only in one position of the QW1. This position corresponds to around 12% of the low-power transmission. The self-starting mode-locking can be found by adjusting only one element, the QW 1 retarder in the NOLM. The adjustment procedure is straightforward; at high pump power we rotated the QWl until the mode-locked operation appears for a particular QWl angle. Once the optimal angle of the QW 1 is determined, after turning on the pump we only have to increase the pump power up to 70 mW to get the self-starting mode-locking operation. The FWHM of the autocorrelation trace is about 30 ps corresponding to a FWHM pulse duration Δτ =21.2 ps giving ΔυΔτ = 2.6 where Δυ is FWHM of the frequency spectrum. Pulses are thus substantially chirped.

© 2007 IEEE