Measurements of the Birefringence and Verdet Constant in an Air-Core Fiber

In order to determine the Verdet constant of an air-core photonic bandgap fiber (PBF), the birefringence of the fiber needs to be accurately known. We used two methods to measure the linear and circular birefringence of a commercial PBF around 1.5 mum. The linear birefringence beat length is found to vary significantly with wavelength, ranging from 6.8 plusmn 0.2 cm at 1526.8 nm to 9.5 plusmn 0.4 cm at 1596.8 nm. The circular birefringence is observed to be weaker by a factor of at least ten. The Verdet constant of this fiber, measured using a Faraday-effect measurement, is 6.1 plusmn 0.3 mrad/T/m. This value is in broad agreement with the prediction of a numerical model, and it confirms that the Verdet constant of this fiber is dominated by the residual spatial overlap of the mode with silica. It is also found to be 90 times weaker than the Verdet constant of a solid-core fiber (SMF-28) measured by the same process (0.55 plusmn 0.01 rad/T/m, in agreement with published values). This significantly reduced susceptibility to magnetic fields points to yet another benefit of air-core fibers in the fiber optic gyroscope.