Fundamental Methods to Measure the Orbital Angular Momentum of Light

Light is a ubiquitous carrier of information. This information can be encoded in the intensity, direction, frequency and polarisation of the light and, which was described more recently, in its orbital angular momentum. Although creating light beams with orbital angular momentum is relatively easy, measuring this property has proven to be difficult. In this thesis we present two fundamental methods to solve this problem. First, we show that by analysing the interference pattern behind a multi-pinhole interferometer, we can determine the phase and amplitude of the light impinging the pinholes, making it possible to determine the orbital angular momentum of the incoming light beam. A multi-pinhole interferometer can be scaled to arbitrary sizes, making it suitable for studying optical fields that stretch out over large distances, that can be expected in, for instance, astrophysics. The second method is based on transforming the helical wave fronts that are associated with light beams with orbital angular momentum to distinguishable, titled wave fronts that can be easily sorted by a lens. This method works for single photons, making it a key piece in a high-dimensional optical communication scheme. This thesis provides theory, simulations and measurements on both detection methods.LEI Universiteit LeidenUBL - phd migration 201

Method for Probing the Orbital Angular Momentum of Optical Vortices in Electromagnetic Waves from Astronomical Objects

Quantum Matter and Optic

Refractive elements for the measurement of the orbital angular momentum of a single photon

We have developed a mode transformer comprising two custom refractive optical elements which convert orbital angular momentum states into transverse momentum states. This transformation allows for an efficient measurement of the orbital angular momentum content of an input light beam. We characterise the channel capacity of the system for 50 input modes, giving a maximum value of 3.46 bits per photon. Using an electron multiplying CCD (EMCCD) camera with a laser source attenuated such that on average there is less than one photon present within the system per measurement period, we demonstrate that the elements are efficient for the use in single photon experiments

Proton irradiation of liquid crystal based adaptive optical devices

To assess its radiation hardness, a liquid crystal based adaptive optical element has been irradiated using a 60 MeV proton beam. The device with the functionality of an optical beam steerer was characterised before, during and after the irradiation. A systematic set of measurements on the transmission and beam deflection angles was carried out. The measurements showed that the transmission decreased only marginally and that its optical performance degraded only after a very high proton fluence (10<sup>10</sup>p/cm<sup>2</sup>). The device showed complete annealing in the functionality as a beam steerer, which leads to the conclusion that the liquid crystal technology for optical devices is not vulnerable to proton irradiation as expected in space

Unambiguous state discrimination in high dimensions

We propose methods for the unambiguous discrimination of quantum states encoded in the spatial profile of light modes, in particular in the orbital angular momentum of light. Perfect discrimination between quantum states is possible only between orthogonal states of a known basis system. Generalised measurement strategies have been developed for the distinction between non-orthogonal states in a two-dimensional state space, e. g. the polarisation of light. Here we consider information encoded in superpositions of orbital angular momentum states of light, defined within an infinite-dimensional state space. Generalised measurements will be an important addition to quantum communication in high-dimensional spaces