M.P. Varnham

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.

Coiled-birefringent-fiber polarizers

Experimental results are presented that illustrate the excellent reproducibility and thermal stability of fiber polarizers made by coiling highly birefringent bow-tie fibers. The effective extinction ratio of the polarizer when used in a fiber-optic gyroscope is shown to be 62 dB.

Polarization-maintaining optical fibers with low dispersion over a wide spectral range

The total dispersion characteristics of the doubly clad Panda (or bow-tie) fibers have been investigated. It is shown that the contribution of the photoelastic effect to the total dispersion becomes of the order of several psec/km x nm in the 1.5-1.7-microm wavelength region. By careful adjustment of the cutoff wavelength, the total dispersion is reduced to within +/- 1 psec/km x nm over the 1.38-1.70-microm wavelength region for the HE(11)(x) mode and 1.38-1.68 microm for the HE(11)(y) mode, respectively.