The modal and group birefringence of a hybrid photonic crystal fiber (hybrid PCF) and the sensitivities of modal birefringence of hybrid PCF to strain and temperature are investigated. The hybrid PCF composes of air-holes and Ge-doped silica rods surrounding a silica core region and light is confined to the core by hybrid index-guiding and photonic bandgap effects. A theoretical model was established and used to calculate these birefringence properties as functions of Ge concentration and diameter of the Ge-doped region. In experiments, the birefringence properties of a hybrid PCF made by University of Bath were measured by using a Sagnac interferometer. The experimental results show that the sensitivities of fringe minimum of this Sagnac interferometer to strain and temperature, at the wavelength of 1550 nm, were 2.01 nm/m$\varepsilon$ and -$0.334 nm/°C, respectively, which agree well with the theoretical predictions. The model may be used to design hybrid PCFs with desired birefringence properties.
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