Channeled spectrum of a fiber laser

We describe the polarization properties of a long Nd(3+)-doped optical fiber laser. An intracavity prism acts as a dichroic element and fixes the polarization of the laser beam essentially in its plane of incidence. The slight birefringence of the fiber makes a wavelength-dependent phase plate that modifies the state of the polarization. In the laser output spectrum, the azimuth for the elliptically polarized field can thus rotate by several times pi/2 if the fiber is long enough. This gives a channeled spectrum when observed through a polarizer.