Our work has mainly been concerned with two complementary methods of creating a Bessel beam, one well known (the axicon lens) and the other a very recent and novel development (a tunable acousto-optical or TAG lens). In both cases the possibility of doing hands-on experimentation has depended on obtaining a specialized optical element from an outside research group.

Both project ideas evolved from an interest in optical vortices and acousto-optics. While searching the literature related to these two seemingly unconnected topics, a paper was found that discussed the transfer of orbital angular momentum (OAM) from an acoustic vortex to an optical vortex in an optical fiber [1]. In a similar way we later learned about recent work in which an tunable acousto-optical lens [2] is used to create Bessel beams. This established a connection to the research of a fellow summer student, Melia Bonomo, who was studing Bessel beam formation through spatial filtering. The TAG lens has been commercialzed and we have been in contact with the company that now produces it (TAG Optics, Inc.) about obtaining a sample device for experimentation. They have agreed to provide a lens but we won't receive it for at least several more weeks.

While the TAG lens is novel and interesting it can't make higher-order Bessel beams. Such beams contain a singularity, similar to an optical vortex. An axicon lens is a conical lens that bends incoming light according to its index of refraction and apex angle [3]. When illuminated by a Gaussian beam, the resulting conical superposition of waves interferes to make a central bright spot with surrounding concentric rings. This is a zero-order Bessel beam. To form a higher-order Bessel beam, one starts with a Laguerre-Gaussian (LG) beam, the topological charge of which is conserved in the transformation.

Axicon lenses are expensive and none was available in our laboratory. Very recently however we have been able to obtain one from a colleage at another university. It is 25.4 mm in diameter and has an axicon angle of 1.0 °. We measured a reflectance of 4.6%, which corresponds to refractive index n = 1.55. We are currently testing the axicon lens with LG beams obtained from an open-cavity HeNe laser and astigmatic mode converter. The LG beam is expanded and collimated with a pair of lenses (telescope) before entering the flat side of the axicon.

This work was supported by the Stony Brook Physics and Astronomy REU program and the Laser Teaching Center. We thank Giovanni Milione at City College of New York for generously providing the axicon lens.

References

[1]   P. Dashti, F. Alhassen, and H.P. Lee, "Observation of Orbital Angular Momentum Transfer between Acoustic and Optical Vortices in Optical Fiber," Phys. Rev. Lett. 96 (2006).

[2]   E. McLeod, C. Arnold. " Multiscale Bessel Beams Generated by a Tunable Acoustic Gradient Index of Refraction Lens," Optics Letters 31, 3155-3157 (2006).

[3]   J. Arlt and K. Dholakia, "Generation of High-Order Bessel Beams by Use of an Axicon," Optics Comm. 177, 297-301 (2000).