Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

The present work describes the development of an organic photodiode (OPD) receiver for high‐speed optical wireless communication. To determine the optimal communication design, two different types of photoelectric conversion layers, bulk heterojunction (BHJ) and planar heterojunction (PHJ), are compared. The BHJ‐OPD device has a −3 dB bandwidth of 0.65 MHz (at zero bias) and a maximum of 1.4 MHz (at −4 V bias). A 150 Mbps single‐channel visible light communication (VLC) data rate using this device by combining preequalization and machine learning (ML)‐based digital signal processing (DSP) is demonstrated. To the best of the authors' knowledge, this is the highest data rate ever achieved on an OPD‐based VLC system by a factor of 40 over the previous fastest reported. Additionally, the proposed OPD receiver achieves orders of magnitude higher spectral efficiency than the previously reported organic photovoltaic (OPV)‐based receivers.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/advs.202203715

Small Molecule Based Organic Photo Signal Receiver for High‐Speed Optical Wireless Communications

Visible Light Communication is a very promising candidate for future optical wireless technology by solving the problem of crowded RF spectrum, supporting high data rate systems and reducing the cost of installation. Modulation schemes reported for VLC are mainly intensity modulation which introduces flickering and limits the efficiency of transmitted data. Use of external modulator can solve above stated problems. This article proposes modeling and simulation of Lithium Niobate based Mach-Zehnder Modulator (MZM) at 680 nm, 640 nm, and 670 nm which can be considered as future options for VLC systems. The designs of MZMs are simulated using the beam propagation method and validated through numerical modeling. The maximum obtained extinction ratio is 30 dB for MZM at 640 nm. BER results of VLC systems with all proposed devices are compared and analyzed.

Modeling of lithium niobate based Mach-Zehnder modulator for visible light communication system with BER analysis

Current and future horizon of optics and photonics in environmental sustainability

As one of the most promising candidates for signal carrier sources in visible light communication, GaN-based green micro-light emitting diodes ( μ-LEDs) exhibit a limited modulation bandwidth. In this work, we propose an approach to accelerate carrier recombination rate in green μ-LEDs and, thus, improve the modulation bandwidth by enhancing p-type conductivity to allow more efficient hole injection into an active region. The polarization-induced p-type doping with graded AlGaN enhances the p-type layer conductivity to 2.5 × 10−2 S/m, which is about 4 times in magnitude higher than that of the conventional p-type GaN layer (0.6 × 10−2 S/m). 16 × 16 green μ-LEDs arrays using such graded p-AlGaN exhibit a light output power of 4.4 mW and a modulation bandwidth of 130 MHz, both showing an improvement of about 45% as compared with the ones using a pure p-GaN layer. The polarization-induced p-type doping in graded AlGaN would accelerate the application of GaN-based μ-LEDs in visible light communication.

Modulation bandwidth improvement of GaN-based green micro-LEDs array by polarization-induced p-type doping

Fluorescent planar concentrators have been proposed as optical concentrators that can have both a wide field of view and a high optical gain stemming from a large collection area for optical wireless communications. However, the fluorescent concentrators with such a large collection area often lead to a low light coupling efficiency due to the edge coupling mechanism leading to a considerable optical power loss. In this work, an analysis of the light coupling efficiency enhancement in the electrical power gain is presented. In particular, a practical method to improve the coupling efficiency by introducing edge and back reflection using Lambertian-, specular-, and retro-reflectors is presented. It is demonstrated that by choosing the optimal reflectors, the received signal strength can be improved by more than a factor of two. Also demonstrated with the proposed method is a data rate more than 1.12 Gbps with bit error rate less than 3.8 × 10−3 using a DC-biased optical orthogonal frequency division multiplexing. This is, to the best of our knowledge, the first Gbps class demonstration using a commercial fluorescent planar concentrator.

Reflection based coupling efficiency enhancement in a fluorescent planar concentrator for an optical wireless receiver.

In this letter, a practical structure enabling both high-speed visible light communication (VLC) and high-quality white-light is presented. The proposed structure directly mixes red-green-blue colors from semiconductor laser diodes (LD) without dichroic mirrors. The three colors are mixed by a thermally stable off-the-shelf phosphor plate. The broad yellow photoluminescence spectrum excited by a portion of blue LD light fills the vacant spectrum, leading to a high color rendering index (CRI). It is experimentally verified that the phosphor plate works as an excellent diffuser even for the green and red channels. Consequently, an incoherent Lambertian source suitable for indoor lighting is created. It is also found that the impact of the broad yellow spectrum on the green and red communication channels is insignificant. A proof-of-concept demonstration shows a 90 CRI (3503 K) white-light delivering 11 Gb/s of wavelength division multiplexing VLC link, using channel adaptive DC-biased optical orthogonal frequency division multiplexing.

High CRI RGB Laser Lighting With 11-Gb/s WDM Link Using Off-the-Shelf Phosphor Plate

Indoor smart-farming based on artificial grow lights has gained attention in the past few years. In modern agricultural technology, the growth status is generally monitored and controlled by radio-frequency communication networks. However, it is reported that the radio frequency (RF) could negatively impact the growth rate and the health condition of the vegetables. This work proposes an energy-efficient solution replacing or augmenting the current RF system by utilizing light-emitting diodes (LEDs) as the grow lights and adopting visible light communications and optical camera communication for the smart-farming systems. In particular, in the proposed system, communication data is modulated via a 24% additional green grow LED light that is also known to be beneficial for the growth of the vegetables. Optical cameras capture the modulated green light reflected from the vegetables for the uplink connection. A combination of white ceiling LEDs and photodetectors provides the downlink, enabling an RF-free communication network as a whole. In the proposed architecture, the smart-farming units are modularized, leading to flexible mobility. Following theoretical analysis and simulations, a proof-of-concept demonstration presents the feasibility of the proposed architecture by successfully demonstrating the maximum data rates of 840 b/s (uplink) and 20 Mb/s (downlink).

Utilization of LED Grow Lights for Optical Wireless Communication-Based RF-Free Smart-Farming System.

With the remarkable advances in vertical-cavity surface-emitting lasers (VCSELs) in recent decades, VCSELs have been considered promising light sources in the field of optical wireless communications. However, off-the-shelf VCSELs still have a limited modulation bandwidth to meet the multi-Gb/s data rate requirements imposed on the next-generation wireless communication system. Recently, employing machine learning (ML) techniques as a method to tackle such issues has been intriguing for researchers in wireless communication. In this work, through a systematic analysis, it is shown that the ML technique is also very effective in VCSEL-based visible light communication. Using a commercial VCSEL and bidirectional long short-term memory (Bi-LSTM)-based ML scheme, a high-speed visible light communication (VLC) link with a data rate of 13.5 Gbps is demonstrated, which is the fastest single channel result from a cost-effective, off-the-shelf VCSEL device, to the best of the authors’ knowledge.

/pdf/bi-lstm-augmented-deep-neural-network-for-multi-gbps-vcsel-3aeiwn4m.pdf

Seonghyeon Cho

Papers

Small Molecule Based Organic Photo Signal Receiver for High‐Speed Optical Wireless Communications

Reflection based coupling efficiency enhancement in a fluorescent planar concentrator for an optical wireless receiver.

High CRI RGB Laser Lighting With 11-Gb/s WDM Link Using Off-the-Shelf Phosphor Plate

Utilization of LED Grow Lights for Optical Wireless Communication-Based RF-Free Smart-Farming System.

Bi-LSTM-Augmented Deep Neural Network for Multi-Gbps VCSEL-Based Visible Light Communication Link