Quantum cryptography with structured photons through a vortex fiber

Transmission of optical communication signals through ring core fiber using perfect vortex beams

Orbital angular momentum can be used to implement high capacity data transmission systems that can be applied for classical and quantum communications. Here we experimentally study the generation and transmission properties of the so-called perfect vortex beams and the Laguerre-Gaussian beams in ring-core optical fibers. Our results show that when using a single preparation stage, the perfect vortex beams present less ring-radius variation that allows coupling of higher optical power into a ring core fiber. These results lead to lower power requirements to establish fiber-based communications links using orbital angular momentum and set the stage for future implementations of high-dimensional quantum communication over space division multiplexing fibers.

Estimation of dislocated phases and tunable orbital angular momentum using two cylindrical lenses

A first-order optical system consisting of two cylindrical lenses separated by a distance is considered. It is found to be non-conserving of orbital angular momentum of the incoming paraxial light field. The first-order optical system is effectively demonstrated to estimate phases with dislocations using a Gerchberg-Saxton-type phase retrieval algorithm by making use of measured intensities. Tunable orbital angular momentum in the outgoing light field is experimentally demonstrated using the considered first-order optical system by varying the distance of separation between the two cylindrical lenses.

Suppression of Nonlinear Optical Rogue Wave Formation Using Polarization-Structured Beams

A nonlinear self-focusing material can amplify random small-amplitude phase modulations present in an optical beam, leading to the formation of amplitude singularities commonly referred to as optical caustics. By imposing polarization structuring on the beam, we demonstrate the suppression of amplitude singularities caused by nonlinear self-phase modulation. Our results are the first to indicate that polarization-structured beams can suppress nonlinear caustic formation in a saturable self-focusing medium and add to the growing understanding of catastrophic self-focusing effects in beams containing polarization structure.

Optical vortex tracking algorithm based on the Laguerre-Gaussian transform

Optical vortices are stable phase singularities, revealing a zero-point in the intensity distribution. The localization of this singular point is of significant importance for any application that relies on vortices and their behavior. However, there is still a need for an adaptable, fast, and precise method of singular point localization. Here we show, that the Laguerre-Gaussian transform method can meet those criteria. We compared the performance of this method with two other tracking methods (phase retrieval and weighted centroid) in various conditions. We found out that not only Laguerre-Gaussian transform offers high accuracy, but also does not lose its advantages in the low-contrast regime. The versatility of this algorithm is examined in the optical vortex aberrometry, where we sense two artificially introduced aberrations.

Evaluating the coupling efficiency of OAM beams into ring-core optical fibers

In optical communications, space-division multiplexing is a promising strategy to augment the fiber network capacity. It relies on modern fiber designs that support the propagation of multiple spatial modes. One of these fibers, the ring-core fiber (RCF), is able to propagate modes that carry orbital angular momentum (OAM), and has been shown to enhance not only classical but also quantum communication systems. Typically, the RCF spatial modes are used as orthogonal transmission channels for data streams that are coupled into the fiber using different free space beams. Free space beams commonly used are Laguerre-Gaussian (LG) and perfect vortex (PV) beams. Here, we study the optimal conditions to multiplex information into ring-core fibers in this scheme. We study the beam coupling efficiency using the overlap between free space beams and RCF bound beams and determine which are the most relevant LG beams to be considered and how their coupling efficiency can be maximized by properly adjusting the beam width with respect to the fiber parameters. Our results show that the coupling efficiency depends upon the OAM value and that this can limit the achievable transmission rates in SDM systems. In this regard, we find optimal coupling configurations for LG beams based on the RCF fiber and beam parameters. Further, we study the PV beam that allows for nearly perfect coupling efficiencies for all spatial modes supported by these fibers. PV beams present higher coupling efficiencies than LG beams and negligible dependence on the OAM value, thus offering an attractive solution to multiplex high counts of OAM channels from free space into a ring-core fiber using a single coupling configuration.

Shaping caustics into propagation-invariant light

Structured light has revolutionized optical particle manipulation, nano-scaled material processing, and high-resolution imaging. In particular, propagation-invariant light fields such as Bessel, Airy, or Mathieu beams show high robustness and have a self-healing nature. To generalize such beneficial features, these light fields can be understood in terms of caustics. However, only simple caustics have found applications in material processing, optical trapping, or cell microscopy. Thus, these technologies would greatly benefit from methods to engineer arbitrary intensity shapes well beyond the standard families of caustics. We introduce a general approach to arbitrarily shape propagation-invariant beams by smart beam design based on caustics. We develop two complementary methods, and demonstrate various propagation-invariant beams experimentally, ranging from simple geometric shapes to complex image configurations such as words. Our approach generalizes caustic light from the currently known small subset to a complete set of tailored propagation-invariant caustics with intensities concentrated around any desired curve.