All fiber-optic polarization scrambler based on cylindrical PZT modulators was demonstrated. The device was designed to depolarize, on the time average basis, coherent light for an arbitrary input state of polarization.
Two independent polarization modulators were cascaded to form a polarization scrambler. Each polarization modulator was fabricated by wrapping a few tens of turns of single-mode fiber around a cylindrical PZT transducer. This polarizaton modulator is driven at a few hundred kilohertz, where relatively large birefringence modulation takes place due to acoustic pressure generated by the vibration of the PZT wall.
Relatively low birefringence(less than 200˚/m) fiber was selected after measuring and comparing the intrinsic birefringence levels of several fibers. To reduce undesirable effects of the intrinsic birefringence on the performance of polarization modulators, PZT cylinders with small diameter were used and the fibers were coiled with high tension. The polarization modulation depth of each polarization modulator was measured by analyzing the Fourier spectrum of the polarization modulated signal and adjusted to be 2.4 radian.
Two cascaded polarization modulators were deriven at different modulation frequencies. By adjusting the relative angle of the cylinder axis between the two PZT cylinders, polarization independent fiber-optic polarization scrambler was realized. In the experiments with a 1.3㎛ laser source, a reduction in the degree of polarization, P, to 0.01 was achived for arbitrary input state of polarization. The performance degradation from P＝0.01 to P＝0.05 was observed by increasing temperature by 20℃.
