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Optical Engineering

Mode sorter and detector based on photon orbital angular momentum

Baha Jassemnejad, A. Bohannan, John Lekki, Kenneth E. Weiland

Published May 1, 2008• 10 citations
Publisher

Abstract

Photons were generated possessing orbital angular momentum (OAM) in the form of Laguerre-Gaussian (LG) laser modes. Three Mach-Zehnder interferometer systems, with the use of Dove prisms, were constructed to sort and detect the LG modes based on their OAM state. Successful generating, sorting, and detecting of LG modes is the first step towards information encoding and decoding via lasers. It is possible to generate many LG modes, which can be orthogonally decoded. With many modes to encode information with it is possible to transfer more than one bit of information with a single photon using LG encoding. An experimental test of this concept was performed by demonstrating the generation and sorting of LG modes with attenuated intensities averaging below one photon passing through the system at a time. In summary, generating, sorting, and low-power detecting of specific LG modes were all demonstrated.

DOI
10.1117/1.2931686

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Photons were generated possessing orbital angular momentum (OAM) in the form of Laguerre-Gaussian (LG) laser modes. Three Mach-Zehnder interferometer systems, with the use of Dove prisms, were constructed to sort and detect the LG modes based on their OAM state. Successful generating, sorting, and detecting of LG modes is the first step towards information encoding and decoding via lasers. It is possible to generate many LG modes, which can be orthogonally decoded. With many modes to encode information with it is possible to transfer more than one bit of information with a single photon using LG encoding. An experimental test of this concept was performed by demonstrating the generation and sorting of LG modes with attenuated intensities averaging below one photon passing through the system at a time. In summary, generating, sorting, and low-power detecting of specific LG modes were all demonstrated.
Orbital angular momentum (OAM) of photons has both classical and quantum applications due to its feature of optical vortex and infinite dimension. OAM discrimination is one of the basic problems, which has been paid much attention recently. Here we present an interferometer method in which a Sagnac interferometer with a Dove prism is placed on each arm to separate the different OAM of photons into different output ports, namely, OAM sorters. We demonstrate experimentally the feasibility of OAM sorter by dividing different OAM states into different output ports. Using the cascade interferometers, we also sort the superposition state successfully. Experimental results are in good agreement with the theoretical predictions. Compared with other methods, this method is more stable and can be used to separate superposition states into single photon levels. Furthermore, this method can also be used to couple OAM modes with spatial modes, a very important method for manipulating OAM states. It is a useful method and has potential applications in high-capacity optical communication, quantum entanglement, quantum cryptography, quantum computation and quantum information.
A mode sorter separates a set of M orthogonal spatial modes in a shared input channel into M different output channels. Here we present an analytic derivation and experimental validation of a single plane device for sorting spatial modes from a diverse variety of mode families, including Hermite-Gaussian (HG), Laguerre-Gaussian (LG), Bessel-Gaussian (BG), with almost no cross-talk. This sorting capability is required for a wide range of applications that employ classical or quantum light. We also show that applying this design in order to sort a set of Orbital Angular Momentum (OAM) modes with zero radial index reproduces the well-known Fork grating configuration. Furthermore, by taking the limit of M -> inf, we present an analytical expression for sorting all the modes of a given family. By operating this device in reverse, it can be used to generate arbitrary modes, by illuminating it with a Gaussian beam. The power transmission coefficient for this sorter goes as 1/M and we provide a mathematical proof that this is optimal for any typical arrangement of the detector positions. We further study the sorter sensitivity to wavelength and random phase noise.
Abstract With the recent exponential developments of high-speed optical communications, traditional optical communications systems are showing their limitations. Over the past decade, the orbital angular momentum (OAM) is attracting more and more research attention, due to its orthogonality between two OAM states, which supplies new degrees of freedom and can be used in fiber or free-space optical communication systems for increasing the communication capacity. Here, we propose a novel OAM de/multiplexing method based on the combination of the March–Zehnder interferometer (MZI) implanted pair of dove prisms (DP) (MZI-DP) and a complex phase gratings (CPG), which can be used to sort and detect multiple OAM modes simultaneously. The fundamental Gaussian beam illuminates the spatial light modulator (SLM) to produce the multiplexed Laguerre–Gaussian modes with different topological charges. The MZI-DP system can sort different OAM modes into odd and even values of topological charges. And then, the designed CPGs perform the concrete measurements of different sorted OAM modes. The propagating directions of different modes are arbitrarily steered by two CPGs in two MZI-DP outputting planes. With this designed system, we can simultaneously sort the OAM modes into odd and even values of topological charges, as well as performing the measurement of a large number of OAM modes without any overlapping between adjacent modes. Furthermore, we have simultaneously demultiplexed 50 different OAM modes and controlled their propagation directions successfully, which shows high-recognition rates and superior performances.
Abstract The orbital angular momentum (OAM) of light has been at the center of several classical and quantum applications for imaging, information processing and communication. However, the complex structure inherent in OAM states makes their detection and classification nontrivial in many circumstances. Most of the current detection schemes are based on models of the OAM states built upon the use of Laguerre–Gauss (LG) modes. However, this may not in general be sufficient to capture full information on the generated states. In this paper, we go beyond the LG assumption, and employ hypergeometric-Gaussian (HyGG) modes as the basis states of a refined model that can be used—in certain scenarios—to better tailor OAM detection techniques. We show that enhanced performances in OAM detection are obtained for holographic projection via spatial light modulators in combination with single-mode fibers (SMFs), and for classification techniques based on a machine learning approach. Furthermore, a three-fold enhancement in the SMF coupling efficiency is obtained for the holographic technique, when using the HyGG model with respect to the LG one. This improvement provides a significant boost in the overall efficiency of OAM-encoded single-photon detection systems. Given that most of the experimental works using OAM states are effectively based on the generation of HyGG modes, our findings thus represent a relevant addition to experimental toolboxes for OAM-based protocols in quantum communication, cryptography and simulation.
We describe a mode sorter for two-dimensional parity of transverse spatial states of light based on an out-of-plane Sagnac interferometer. Both Hermite-Gauss (HG) and Laguerre-Gauss (LG) modes can be guided into one of two output ports according to the two-dimensional parity of the mode in question. Our interferometer sorts HG(nm) input modes depending upon whether they have even or odd order n+m; it equivalently sorts LG(l)(p) modes depending upon whether they have an even or odd value of their orbital angular momentum l. It functions efficiently at the single-photon level, and therefore can be used to sort single-photon states. Due to the inherent phase stability of this type of interferometer as compared to those of the Mach-Zehnder type, it provides a promising tool for the manipulation and filtering of higher order transverse spatial modes for the purposes of quantum information processing. For example, several similar Sagnacs cascaded together may allow, for the first time, a stable measurement of the orbital angular momentum of a true single-photon state. Furthermore, as an alternative to well-known holographic techniques, one can use the Sagnac in conjunction with a multi-mode fiber as a spatial mode filter, which can be used to produce spatial-mode entangled Bell states and heralded single photons in arbitrary first-order (n+m = 1) spatial states, covering the entire Poincar e sphere of first-order transverse modes.
The identification of orbital angular momentum (OAM) as a fundamental property of a beam of light nearly 25 years ago has led to an extensive body of research around this topic. The possibility that single photons can carry OAM has made this degree of freedom an ideal candidate for the investigation of complex quantum phenomena and their applications. Research in this direction has ranged from experiments on complex forms of quantum entanglement to the interaction between light and quantum states of matter. Furthermore, the use of OAM in quantum information has generated a lot of excitement, as it allows for encoding large amounts of information on a single photon. Here, we explain the intuition that led to the first quantum experiment with OAM 15 years ago. We continue by reviewing some key experiments investigating fundamental questions on photonic OAM and the first steps to applying these properties in novel quantum protocols. At the end, we identify several interesting open questions that could form the subject of future investigations with OAM.This article is part of the themed issue 'Optical orbital angular momentum'.