LED-based visible light communication for color image and audio transmission utilizing orbital angular momentum superposition modes

Non-orthogonal polarization encoding/decoding assisted by structured optical pattern recognition

The complex vector beams yield up an abundance of polarization information that has not yet been well utilized in information encoding. In this paper, we propose a polarization encoding scheme with the non-orthogonal polarization states using a stationary vector beam. Recognizing those non-orthogonal polarization states is assisted by the structured patterns of the single vector beams under different polarization projections. We show that one can achieve different capacities of encoding bits by changing the step of the polarization angle with the single vector beam. We also demonstrate the non-orthogonal polarization encoding scheme can be well decoded with the machine learning classification algorithm. A 64×64 gray image is successfully transmitted by using 4 bits/symbol encoding-decoding scheme with 99.94 % transmission accuracy. Besides, by extending the encoding-decoding scheme to 8 bits/symbol based on the same single vector beam, we achieve a higher transmission rate with 65.58% transmission accuracy. Our work holds promise for small-angle non-orthogonal polarization encoding for free-space optical communications.

Deep Learning-Based Next-Generation Waveform for Multiuser VLC Systems

Due to the growing number of users, power, and spectral effectiveness, most communication systems are complex and difficult to implement on a large scale. Artificial Intelligence (AI) has played an outstanding role in the implementation of theoretical systems in the real world, with less complexity achieving better results. In this direction, we compare the Non-Orthogonal Multiple Access (NOMA) technique for a multiuser Visible Light Communication (VLC) system with Successive Interference Cancellation (SIC) for two types of detectors: (1) the deep learning-based system and (2) the traditional maximum likelihood (ML) decoder-based system. For multiplexing, we compare the variations of novel Orbital Angular Momentum (OAM) multiplexing and Orthogonal Frequency Division Multiplexing (OFDM) with Index Modulation (IM). In this article, we implement OFDM-IM and OAM-IM for four users for the Gaussian fading MIMO Line-of-Sight (LoS) and Non-Line-of-Sight (NLoS) VLC channels. The suggested systems' bit error rate (BER) performances are compared in simulations for a wide range of Signal-to-Noise Ratios (SNRs), which shows that deep learning-based systems outperform the ML-based system for both users to ensure better decoding at the receiver end, especially at higher SNR values. The detection error is lower in a deep learning-based system at around 20% and around 30% for low SNR and high SNR values, respectively.

Fourth-harmonic generation of orbital angular momentum light with cascaded quasi-phase matching crystals

Orbital angular momentum (OAM) light, combined with the nonlinear process to expand the frequency range, has drawn increasing research interest in recent years. Here, we implement the first, to the best of our knowledge, experimental fourth-harmonic generation of OAM light with two cascaded quasi-phase-matching crystals. A Laguerre-Gaussian beam was transmitted through a duplet crystals system and frequency-doubled twice by two separate second-harmonic generation processes, which transduced the frequency of the OAM beam from telecom band to visible band and then to ultraviolet (UV) band. The topological charge of the OAM beam was increased substantially in the cascaded frequency conversion processes. In this experiment, we verify the OAM conservation by utilizing a specially designed interferometer, and the results correspond well with the numerical simulation. This work provides an effective method for the generation of UV OAM beams with high topological charges.

Seeing infrared optical vortex arrays with a nonlinear spiral phase filter

We demonstrate a new method to detect infrared optical vortex arrays efficiently, which is based on simultaneous up-conversion imaging and spiral phase contrast via second-harmonic generation (SHG) in the Fourier domain. In our experiment, we use a spatial light modulator to prepare a variety of 1064 nm structured vortex arrays and employ a vortex phase plate of different topological charges to serve as the nonlinear orbital angular momentum (OAM) filter. The SHG is done by mixing the Fourier spectra of input-structured vortices with a single OAM beam in a type-II potassium titanyl phosphate crystal. Then we can convert the input invisible vortex arrays into the visible SHG light fields, and the vortex cores are mapped and seen by bright Gaussian spots, revealing both their positions and topological charges. Our work has potential in the field of infrared imaging and monitoring.

Two-dimensional temperature distribution measurement of light-emitting diodes by micro-hyperspectral imaging-based reflected light method

We propose a non-contact measurement method for determining two-dimensional (2D) temperature distribution of light-emitting diodes (LEDs). This method is based on both micro-hyperspectral imaging technology and reflected light method, owning merits of both high efficiency and high spatial resolution. Blue and green bare LEDs are used as LED under test, while red and near-infrared LEDs provide incident light to avoid spectral overlapping so as to reduce measurement error. During data processing, the convolution linear filtering algorithm is employed to improve the measurement accuracy. This proposed method is compared with the micro-thermocouple and infrared thermal imaging, with their respective comparison results in fairly good agreements. For spatial resolution of 2D temperature distribution, this method increases at least one order of magnitude compared with the thermal imaging method.

Information transmission using radial carpet beams

A radial carpet (RC) beam is a new class of beams that is a subfamily of combined half-integer Bessel-like beams, which are a set of solutions for wave equations and introduced recently. In this paper, we propose an indoor optical communication link using RC beams encoding/decoding. By employing spatial light modulators, loaded with radial gratings and fork-shaped gratings as encoders and decoders, respectively, we generate and detect different modes of RC beams as encoding states. In the experiment, we transmit a 2-bit gray-scale image of 50×50 pixels by four different states of RC modes. The system bit error rate (BER) is measured, and a zero BER is achieved after transmitting 2500 modes. We then transmit an 8-bit RGB image of 70×77 pixels by 16 different modes of hexadecimal states to analyze the performance of the communication link in the presence of RC modes with higher RC orders. By defining an optimum threshold value, the maximum improvement of 100% in the fidelity for transmitting 32,340 hexadecimal states in an indoor ideal medium is achieved. The obtained results show acceptable performance for the proposed communication system based on RC beams, especially in the case of lower RC orders.