Fiber Polarizer for Telecom Application
Long-period grating in photonic crystal fiber discriminates between polarizations.
Polarizers often are required for fiber optic communications, but those fabricated in fibers usually have a self-defeating temperature dependence. Bulk optical polarizers, the alternative, are large and awkward to integrate into telecom systems, introducing alignment and other problems. Now scientists at Hong Kong Polytechnic University and at Shanghai Jiao Tong University in China have demonstrated how a fiber polarizer with minimal temperature dependence can be created by fabricating a long-period grating in a photonic crystal fiber.
Figure 1. In a photonic crystal fiber supplied by BlazePhotonics, the scientists fabricated a long-period grating by heating one side with a carbon dioxide laser. The grating notches, separated by about 410 μm, were ~10 μm deep and ~60 μm wide. Reprinted with permission of Optics Letters.
The scientists used a carbon dioxide laser to create notches separated by 410 μm along one side of the fiber (Figure 1). The notches formed a long-period grating that preferentially coupled light traveling in the p-polarization out of the core. The scientists had previously reported the same effect using a conventional single-mode fiber, but they found the polarizing mechanism was much stronger in a photonic crystal fiber. They attributed the improvement to the fact that two effects changed the refractive index of the photonic crystal fiber, while only one affected the conventional fiber. In both fibers, residual strains were induced by the rapid heating and cooling of the glass. In the photonic crystal fiber, the collapsing of the airholes induced an additional stress.
Figure 2. Before a longitudinal stress was applied, the discrimination against the p-polarization was ~14 dB. (That is, the p-polarization loss at ~30 dB was ~14 dB greater than the ~16-dB loss to the s-polarization.) The application of a longitudinal stress increased the discrimination to ~23 dB without affecting the loss to the s-polarization.
By applying a longitudinal stress to the fiber, the scientists significantly increased the polarization discrimination of their device (Figure 2), which was apparently the result of bends that appeared in the fiber when it was stretched. The bends had the same period (410 μm) as the long-period grating and resulted because the fiber was physically weakened at the notches. The periodic bends further stressed the glass and increased the stress-induced change in the refractive index.
Figure 3. The 3.9 pm/°C temperature dependence of the photonic crystal polarizer compares very favorably with that of conventional fiber polarizers, which is typically around 60 pm/°C.
The scientists note that, in a packaged commercial device, the longitudinal stress could be built into the packaging, so high discrimination could always be present. They also note that the relatively high ~15 dB loss to the favored s-polarization could readily be compensated with an erbium-doped fiber amplifier.
But the scientists’ goal was to develop an in-fiber polarizer without the temperature dependence of conventional fiber polarizers. The photonic crystal fiber polarizer did indeed satisfy this requirement (Figure 3). Compared with the temperature sensitivity of conventional fiber polarizers — typically around 60 pm/°C — the 3.9 pm/°C was a dramatic improvement.
Optics Letters, May 1, 2007, pp. 1035-1037.
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