Measurement of the fractional topological charge of an optical vortex beam through interference fringe dislocation

Precision measurement of the topological charge of a fractional vortex beam based on angular-grating-diffraction OAM spectrum

Vortex light beams carrying fractional vortices have shown promising applications in many fields such as optical communications, optical encryption, and quantum information processing. Accurate detection of the topological charge of a fractional vortex phase is essential for these applications. In this paper, a simple yet effective method for measuring the fractional topological charge is proposed, which is based on the detection of the orbital angular momentum (OAM) spectrum of a fractional vortex beam diffracted by an angular grating. When the fractional vortex beam passes through an angular grating, the OAM spectrum broadens, increasing the number of detectable OAM components. By applying nonlinear least squares fitting to the broadened spectrum, the topological charge can be detected with high precision. Experimental results demonstrate the detection range for fractional topological charges is from -6 to 6, with a resolution of 0.01 and an error of less than 0.005. Our protocol offers significant potential for high-capacity information transfer.

Characterizing the fractional coherence vortices through the area of the intensity cross-correlation function

We have generated the fractional coherence vortices using the speckle patterns obtained from the scattering of the fractional vortex beams. In this study, we found the topological charge of the fractional vortex beam with a resolution of 0.01 using the area of the coherence function of scattered fractional optical vortex beams. We have also provided accuracy for the measurement of topological charges of fractional vortex beams using the studies of the area of the coherence function. Our experimental results are well matched with the theoretical results. These fractional coherence functions can be used to generate a security key for data authentication and data encryption. In addition, fractional vortex beams have multiple OAM modes and can be used to address the explosive growth in free-space optical communication.

Fractional topological charge measurement through optical correlation

The emerging field of optical vortex beams having fractional topological charges (TCs) is of high interest due to its usefulness in various applications. The efficiency of the result depends on the precise measurement of the orbital angular momentum information tied to the fractional TC. This Letter demonstrates, to our knowledge, a novel and simple technique to measure the fractional TC of optical vortex beams through a hybrid digital-optical correlator with the help of auto-correlation between fork-shaped interference patterns corresponding to integer and fractional TCs. Unlike machine learning-based approaches, the proposed method does not require a complex architecture, which lowers computational cost and enables real-time implementation.

Binary image encryption with a QR code-encoded optical beam having an array of vortices

In recent years, optical information processing has become increasingly important due to its ability to handle large amounts of data efficiently. Amplitude, phase, spatial frequency, wavelength, and polarization are the physical dimensions used for information encoding into the light beam. Information encoding using orbital angular momentum (OAM) carried by a vortex beam is gaining interest in this regard. Such beams provide high-dimensional orthogonal states for encoding and have shown more stability in turbulent media than Gaussian beams, especially in the case of free space propagation. This study demonstrates an image encryption method utilizing an array of OAM light modes. The approach uses a non-interferometric intensity recording-based encoding scheme, which offers ease of implementation. Quick response codes have been utilized for encoding to avoid information loss from non-uniform distribution of light and aberrations. The encryption scheme is highly secure because the encrypted light beam contains only partial information about the object, preventing complete information retrieval without proper key information.

Intersecting of circular apertures to measure integer and fractional topological charge of vortex beams

Diffraction patterns of optical vortex beams (VBs) by differently shaped apertures are used to determine their topological charge (TC). In this paper, we show by simulations and experiments that diffraction of a Laguerre-Gaussian (LG) beam by intersecting circular apertures can be used to reveal the TC. The presented aperture structure has the advantage of the measurement of fractional TC in addition to the integer, sensitivity to the sign of TC, and low sensitivity to adjusting apertures. Accordingly, in addition to the integer TC up to 8, the fractional TC is measured with a step of 0.1 by two intersecting circular apertures (TICA). By examining a wide range of similarity criteria between the diffraction pattern of the fractional TC and the pattern of the lower integer TC, three metrics for measuring the fractional TC are found. Furthermore, the determination of integer TC up to 6 for three intersecting circular apertures (THICA) is demonstrated.

Measurement of the fractional topological charge of an optical vortex beam through interference fringe dislocation: publisher's note

This publisher's note serves to correct Appl. Opt.62, D58 (2023).APOPAI0003-693510.1364/AO.476455.