Robust interferometer for the routing of light beams carrying orbital angular momentum

We have developed an interferometer requiring only minimal angular alignment for the routing of beams carrying orbital angular momentum. The Mach–Zehnder interferometer contains a Dove prism in each arm where each has a mirror plane around which the transverse phase profile is inverted. One consequence of the inversions is that the interferometer needs no alignment. Instead the interferometer defines a unique axis about which the input beammust be coupled. Experimental results are presented for the fringe contrast, reaching a maximum value of 93± 1%. The orbital angular momentum (OAM) carried by light is an extremely useful optical characteristic, with applications in many areas of optics. It was Allen et al who recognized that a helically phased light beam with a phase cross section of exp(i `φ) carries an OAM of `h̄ per photon [1, 2]. An example is Laguerre–Gaussian (LG) beams which have a helical phase structure. The integer ` is unbounded, giving a large state space in which to encode information [3–5]. The use of diffractive elements containing an `-fold fork dislocation has become commonplace for the generation of helically-phased beams [6, 7]. The fork diffraction grating, when illuminated with a Gaussian beam, for example from a single-mode fibre, produces the helical mode in the first diffraction order. This grating can also be used in reverse to couple light with a helical phase into a single-mode fibre [8]. Sequentially changing the dislocation in the fork allows a range of ` values to be measured, but checking for N states requires at least N photons [8]. Recently it has been shown that two diffractive optical elements can New Journal of Physics 13 (2011) 093014 1367-2630/11/093014+06$33.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft