We present an ultrafast depolarizer based on four-wave mixing (FWM) in
highly nonlinear optical fiber (HNLF), in which a completely polarized laser beam with
any fixed state of polarization (SOP) mixed with an unpolarized pump can be
instantaneously converted into an unpolarized idler wave. The degree-of-polarization
(DOP) of the idler wave is experimentally measured to be 0.33 when the DOPs of the
polarized signal and unpolarized pump are 0.986 and 0.036, respectively. We analyze the
beam couplings between a polarized beam and a completely unpolarized pump in the
ultrafast depolarizer by using vector FWM theory. The nonzero DOP of the idler wave
ascribes to the polarization dependence of the FWM conversion efficiency. The speed of
the depolarizer is determined by the intrinsic nonlinear response of silica, which could be
just a few femtoseconds.
The measurements on the intensity of the diffracted beam of transmission gratings were done. The gratings with different center-to-center separation were illuminated by the light beams with different polarizations, the ratio between the intensities of P-polarization and S-polarization shows clearly the vector properties of grating.