We have analyzed the temperature-dependent birefringence of a polarization-maintaining photonic crystal fiber. The temperature-coefficient of birefringence, dB/dT, for a commercially available polarization-maintaining photonic crystal fiber was found to be approximately -2.0 X 10~(-9) K~(-1). This is 35 times smaller than that of a standard polarization-maintaining fiber, which was approximately -7.0 X 10~(-8) K~(-1). Based on numerical analysis of the fiber structures, we determined that the temperature-dependence of the birefringence of the polarization-maintaining photonic crystal fiber is mainly due to a combination of temperature-dependent structure and index change. Due to thermal expansion of the silica, the photonic crystal structure of the fiber deforms, resulting in the shifting of the photonic band gap, which results in the birefringence change. Temperature-dependent refractive index change of the silica also causes a shift in the photonic band gap and thus a birefringence change. The numerically calculated dB/dT of the photonic crystal fiber was found to be approximately (-4+-2) X 10~(-9) K~(-1), and is in excellent agreement with the experimental results.
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