In the fabrication of highly-birefrinqent, e.g. "Bow-Tie", optical-fibre it is required to maximise the stress, and hence the birefringence, within the core of the preform. Conversely, the stresses in other regions of the preform should be minimised to prevent the preforms shattering. This is particularly true for the so-called 'Stress-Guide' prefoms in which the refractive index difference in the core is obtained by using the stress-optic effect, rather than by the more usual method of incorporatinq a dopant. The ability to measure the stress distribution within such preforms would therefore be extremely useful
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