Photonic Crystal Fiber Laser Is Highly Polarized

Breck Hitz

A collaboration at the University of Strathclyde in Glasgow, UK, and at Crystal Fibre A/S in Birkerød, Denmark, has resulted in what the experimenters believe is the first photonic crystal fiber laser whose highly polarized output is due to its asymmetric microstructure. The laser produced 3.7 W with a degree of linear polarization in excess of 200:1.

Figure 1. The asymmetric microstructure of the photonic crystal fiber laser forces it to oscillate with a high degree of polarization. The inner-cladding diameter is 140 µm. The edge of the fiber is outside the border of this photograph.

The Yb-doped, double-clad fiber had an inner-cladding diameter of 140 µm, with two large airholes and several smaller airholes defining the 5 × 11-µm core (Figure 1). This structural asymmetry induced a birefringence in the fiber that favored one polarization state over the other. Besides Yb, the core was doped with Ge, Al and F. The Ge-doping enabled Bragg gratings to be photowritten into the fiber, although such gratings were not utilized in this experiment. The F-doping compensated for the increased refractive index that was caused by the other dopants, ensuring that wave confinement in the core was caused by the airholes and not by conventional total internal reflection.

Figure 2. A CCD array distinguished between polarized light emitted from the core and unpolarized light that had leaked into the cladding.

The experimenters pumped a 20-m length of the photonic crystal fiber with the output of a 980-nm diode laser (Figure 2). A high-reflecting mirror provided feedback from one end of the laser, but the 4 percent Fresnel reflection from the fiber facet was the only feedback from the other end. By carefully aligning the fiber's polarization axis with either the s- or p-polarization plane of the dichroic beamsplitter, they avoided any polarization-mixing effects from the beamsplitter.

Figure 3. The polarization ratio -- the ratio of the maximum to minimum transmission through the polarizer as the wave plate was rotated -- was greater than 200:1.

The laser's single-mode output power was approximately 2.9 W when the pump power coupled into the fiber was 5.5 W. By rotating the wave plate in front of the detector, the researchers observed that the polarization ratio -- the ratio of maximum to minimum transmission -- was better than 200:1 (Figure 3).