FIG. 1: Propagation of oppositely polarized beams inside a screw dislocated circular waveguide. (i) σ = 1 , m = | m | , (ii) σ = − 1 , m = −| m | with λ ( | m | + 1) < b (left) and λ ( | m | + 1) > b (right). this provides an experimental method of determining Burgers vector magnitudes of the order of micrometers. [1] B.E.A. Saleh and M.C. Teich, Fundamentals of photonics, Wiley, New York (1991). [2] L. Allen, M.W. Beijersbergen, R.J.C Spreeuw and J.P. Woerdman, Phys. Rev. A 45 , 8185 (1992). [3] L. Allen, M.J. Padgett and M. Babiker, Prog. Opt. 39 , 291 (1999). [4] A. Maier, Nature (London) 412 , 313 (2001). [5] R.A. Beth, Phys. Rev. 50 , 115 (1936). [6] N.B. Simpson, L. Allen and M. J. Padgett, J. Mod. Opt. 43 , 2485 (1996). [7] M.W. Beijersbergen, L. Allen, H.E.L.O. van der Veen and J.P. Woerdman, Opt. Commun. 96 , 123 (1993). [8] B. Piccirillo and E. Santamato, Phys. Rev. E 69 , 056613 (2004). [9] L. Marrucci, C. Manzo and D. Paparo, Phys. Rev. Lett. 96 , 163905 (2006). [10] M.V. Berry, Proc. R. Soc. A 392 , 45 (1984). 10