R.D. Birch

Analytic solution for the birefringence produced by thermal stress in polarization-maintaining optical fibers

Polarization-maintaining optical fibers are usually made by inducing a large anisotropic thermal stress in the core so that it appears highly birefringent. A simple analytic solution has been found for the birefringence in terms of the cross-sectional distribution of the high-expansion material used to create the thermal stress. The analysis is able to predict optimal structures which efficiently utilize the available stress and thus maximize the birefringence. It is shown that the optimum structure has a cross-sectional geometry resembling a bow-tie. Design rules are given whereby the dimensions may be chosen and these are verified in a simple experiment.

Thermal properties of highly birefringent optical fibers and preforms

Temperature cycling of highly birefringent optical fibers and preforms has been used to investigate the thermal properties of bow-tie and elliptically clad structures. The thermal hysteresis of the birefringence is shown to be a direct consequence of the thermal history of the fiber or preform and has been related to volume changes in the stress-producing borosilicate sections. Annealing increases the axial stress as well as the stress anisotropy and hence the birefringence. Increases of up to a factor of 2 in the birefringence on suitable thermal treatment indicate a new method for further improvement of high birefringence fibers. The implications of the results in the design, fabrication, and use of such fibers are discussed.

Low-cost metal/glass fibre polarisers produced in continuous lengths

Fibre polarisers are essential devices in optical fibre communication and sensor systems whenever the control of polarisation is needed. They are particularly important in fibre-gyro systems where low insertion-loss and high extinction-ratio polarisers are required, A wide polarising spectral-window is another important requirement, since in many practical fibre systems broad-spectrum LEDs are used as the light source. Wide-bandwidth polarisers are also needed in wavelength-multiplexing systems. In the last few years several approaches for fabricating optical-fibre polarisers have been proposed and demonstrated. Most have been based on polishing the fibre to expose the optical field. Amongst these, high extinction ratios have been demonstrated using an overlay of a birefringent crystal or a metal film. However, the exposed-field polishing technique is time consuming and requires considerable skill. Moreover, a high extinction ratio in combination with a wide spectral-range has not been reported. We report here a new approach which yields continuous lengths of polariser fibre and provides a practical low-cost solution. The technique is based on a fibre fabrication process which allows continuous access to the core optical field, as well as providing an extremely-smooth, low-scatter surface at which interactions can be obtained. The design has been used to make metal/glass fibre polarisers in which a metal is incorporated directly into the fibre close to the core, as shown in Figure 1. The result is a high-performance metal/glass fibre polariser which can be produced in continuous lengths and whose extinction ratio can be adjusted to requirements by cutting to a given length