We have systematically measured the differential stress-optic coefficient, ∆C, in a number of PMMA fibers drawn with different stress, ranging from 2 up to 27 MPa. ∆C was determined in transverse illumination by measuring the dependence of birefringence on additional axial stress applied to the fiber. Our results show that ∆C in PMMA fibers has a negative sign and ranges from -4.5 to -1.5×10 -12 Pa -1 depending on the drawing stress. Increase of the drawing stress results in greater initial

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... r birefringence and lower ∆C. Development of polymer optical fibers has opened new application opportunities in optical communication [1] and sensing [2] because of their low cost, easy handling and very good flexibility. Optical anisotropy in polymers of different types has been investigated for many years due to their potential use in various photonic devices [3, 4] . Although PMMA is a very popular polymer widely used for industry, there are many inconsistencies in the literature concerning its differential stressoptic coefficient ∆C = C 2 -C 1 . Measurements performed in PMMA discs (18mm diameter, 10mm thick) showed ∆C equal to -3.3×10 -12 Pa -1 [4], but in [5] the measurements carried out in discs of different size (13.03 mm diameter, 2.85mm thick) gave a value of ∆C=5.5×10 -10 Pa -1 . Another quite different result was reported in [6], ∆C = -1.08×10 -10 Pa -1 . Very systematic studies of the optical and mechanical properties of bulk PMMA (Plexiglas 55) are presented in [7] . In particular, the strain-optic (σ z >>σ r , σ z >>σ Θ ), which results in: 11. W. Primak, D. Post, "Photoelastic constants of vitreous silica and its elastic coefficient of refractive index," J. of Appl. Physics 30, 779-788 (1958).