Optical orbital angular momentum (OAM) has become a hot topic in recent years. Encoding an azimuthally dependent phase onto a laser beam imparts angular momentum to its constituent photons. This angular momentum has can be used for a varietly of applications. It can be used directly, as a tool for imparting angular momentum to trapped atoms or particles or for encoding information, or indirectly, by making use of the unique beam shapes of optical vortices for imaging or as optical tweezers.

As with the spin of an electron or the polarisation of a photon, the orbital angular momentum of light can be used to encode quantum information. This basis is unbounded: the orbital angular momentum, unlike spin angular momentum, can be made arbitrarily large (although it is subject to practical constraints).

Exploiting this resource for quantum information requires the efficient generation of beams carrying OAM with a large optical OAM. We have demonstrated that machined reflective diffractive optics are up to the task.

Imparting angular momentum to an an optical field can be done by adding a phase ramp that spirals about the center the beam so that the wavefront varies continuously in a helical pattern. This can be done reflecting a laser beam off of a mirror with a helical spiral imprinted onto its surface. As long as the step that joins the bottom of the spiral with the top is exactly half of the wavelength of light, the surface is, as far as the light is concerned, continuous.

We produce such mirrors by cutting them with an ultra-precision lathe that can operates with nanometer precision. An extremely sharp diamond cutting tip is used to machine the spiral path directly onto an aluminum mirror with an optical quality finish. The result is optics that can have much larger features than any lithographic technique. This allows us to produce optical vortices with an unprecedented combination of high charge (5050), efficiency (92.8%) and quality.



Generation and interferometric analysis of high charge optical vortices
Yong Shen, Geoff T Campbell, Boris Hage, Hongxin Zou, Ben C Buchler and Ping Koy Lam
 Journal of  Optics 15 044005 (2013)

Generation of high-order optical vortices using directly machined spiral phase mirrors
G. Campbell, B. Hage, B. C. Buchler, and P. K. Lam
Applied Optics, Vol. 51, Issue 7, pp. 873-876 (2012)

Flase color image of a charge 100 optical vortex produced by one of our spiral phase mirrors.

False colour image of a charge 100 optical vortex produced by one of our spiral phase mirrors. Vortices with charges of up to 1020 have been verified to carry the intended charge.

The quality of a charge 15 optical vortex is examined by comparing the experimentally produced beam with the theoretical form.

The theoretical evolution of a charge 15 optical vortex is shown in a). Experimental beam cross-sections taken at the plane of the optic (c) and after propagation (b) are compared to the theoretical form.

One of the spiral phase mirrors that we produce.

One of the sprial phase mirrors that we produce. 101 steps, each imparting 50 units of optical angular momentum have been used to limit the total height deviation of the part, in a similar manner to a Fresnel lens. The insets show white-light interferometer measurements of the quality of the steps (c) and the defect (b) that occurs in the centre of the part.


Optical Vortex Generation

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Ben Buchler