Marc Levenson and John Wallace

According to a team of researchers led by Steve Cundiff of JILA/University of Colorado (Boulder, CO) modelocked soliton lasers can, by properly manipulating their intracavity birefringence, be made to have locked polarizations. This finding may impact optical communications systems where polarization mode dispersion is an issue and causes soliton polarization evolution.

In the experimental setup for evaluating the performance of a soliton fiber laser, the laser was modelocked by a saturable Bragg reflector (SBR) and did not contain any polarizing elements so the pulse polarization was free to evolve under the influence of the intracavity birefringence as it propagated in the cavity. A portion of the cavity was wrapped on two paddles of a fiber polarization controller to manipulate intracavity birefringence. A combination of a linear polarizer and RF spectrum analyzer can determine how fast the polarization evolves, which is denoted as the polarization evolution frequency, D.

In the figure, D is plotted as a function of the angle of the two paddles q1 and q2. In the black and gray regions the output state does not evolve. In theory, D should be directly proportional to the intracavity birefringence; however, for certain angles the polarization spontaneously stops evolving. In some regions this fixed output polarization is linearly polarized, while in others it is elliptically polarized. Comparison of low (a) and high (b) pulse energy operation shows that the size and position of the regions with fixed polarization depend on the pulse energy (see figure). This is clear evidence that a nonlinear process is involved.

According to Cundiff, the elliptically polarized regions result when the nonlinear index exactly compensates the linear birefringence and renders the fiber perfectly isotropic. "This requires a stabilizing mechanism, which is provided by coherent energy exchange between the orthogonally polarized amplitude components," says Cundiff. "The circulating pulse in this case is called a polarization-locked vector soliton."