We reported on the design, demonstration, and analysis of white lighting systems based on GaN laser diodes. Compared to light-emitting-diodes (LEDs), lasers have been proposed for the development of high-power light sources for many potential advantages, including circumventing efficiency droop, reduced light emitting surface, directional beam characteristics. Laser-based white light sources are also attractive for visible light communication (VLC) applications that enabling lighting and communication dual functionalities. In this work, we detailed the color-rendering index (CRI), correlated color temperature (CCT), and luminous flux analysis of laser white light sources by using the GaN laser diode exciting color converters at various driving conditions. By using a blue-emitting laser exciting a yellow YAG phosphor crystal, a luminous flux greater than 600 lm has been achieved with a moderate CRI of 67.2. By constructing a white lighting system using phosphor crystal array based on a reflection configuration, an improved CRI of 74.4 and a luminous flux of ~400 lm with a CCT of 6425 K was obtained at 3A. Using a novel ceramic phosphor plate as color converter, the CRI for the white light source has been further improved to ~ 84.1 with a CCT of ~ 4981 K, which suggests that the laser-based white light source is capable of high-quality illumination applications. The CCT of the white laser sources can be engineered from 5000 K to 6500 K and a potential approach to use laser array for high power white lighting is discussed.

High-speed visible light communications (VLC) has been identified at an essential part of communication technology for 5G network. VLC offers the unique advantages of unregulated and secure channels, free of EM interference. Compared with the LED-based VLC transmitter, laser-based photonic systems are promising for compact, droop-free, and high-speed white lighting and VLC applications, ideal for ultra-fast 5G network and beyond. Besides the potential for achieving high data rate free-space communication links, i.e. the Li-Fi network, laser-based VLC technology can also enable underwater wireless optical communications (UWOC) for many important applications. In this paper, the recent research progress and highlights in the fields of laser-based VLC and UWOC have been reviewed with a focused discussion on the performance of various light sources, including the modulation characteristics of GaNbased edge emitting laser diodes (EELDs), superluminescent diodes (SLDs) and vertical-cavity surface-emitting lasers (VCSELs). Apart from the utilization of discrete components for building transceiver in VLC systems, the development of III-nitride laser-based photonic integration has been featured. Such on-chip integration offers many advantages, including having a small-footprint, high-speed, and low power consumption. Finally, we discuss the considerations of wavelength selection for various VLC and UWOC applications. Comparison of infrared (IR) and visible lasers for channels with high turbulence and the study of ultraviolet (UV) and visible lasers for non-line-of-sight (NLOS) communications are presented.

The use of autonomous underwater vehicles (AUVs) is highly desirable for collecting data from seafloor sensor platforms within a close range. With the recent innovations in underwater wireless optical communication (UWOC) for deep-sea exploration, UWOC could be used in conjunction with AUVs for high-speed data uploads near the surface. In addition to absorption and scattering effects, UWOC undergoes scintillation induced by temperature- and salinity-related turbulence. However, studies on scintillation have been limited to emulating channels with uniform temperature and salinity gradients, rather than incorporating the effects of turbulent motion. Such turbulent flow results in an ocean mixing process that degrades optical communication. This study presents a turbulent model for investigating the impact of vehicle-motion-induced turbulence via the turbulent kinetic energy dissipation rate. This scintillation-related parameter offers a representation of the change in the refractive index (RI) due to the turbulent flow and ocean mixing. Monte Carlo simulations are carried out to validate the impact of turbulent flow on optical scintillation. In experimental measurements, the scintillation index (SI) and signal-to-noise ratio (SNR) are similar with (SI = 0.4824, SNR = 5.56) and without (SI = 0.4823, SNR = 5.87) water mixing under uniform temperature channels. By introducing a temperature gradient of 4 °C, SI (SNR) with and without turbulent flow changed to 0.5417 (5.06) and 0.8790 (3.40), respectively. The experimental results show a similar trend with the simulation results. Thus, turbulent flow was shown to significantly impact underwater optical communications.

Single and multiple wavelength laser systems are presented that employ self-injection locked InGaN/GaN green laser diodes in an external cavity configuration with a partially reflective mirror. A stable and simultaneous locking of up to four longitudinal Fabry–Perot modes of the system cavity is demonstrated with appreciable signal-to-noise-ratio of ∼13 dB and average mode linewidth of ∼150 pm. The multi-wavelength spectrum exhibited a flat-top emission with nearly equal power distribution among the modes and an analogous mode spacing of ∼0.5 nm. This first demonstration of multi-wavelength generation source is highly attractive in a multitude of cross-disciplinary field applications besides asserting the prospects of narrow wavelength spaced multiplexed visible light communication. Moreover, an extended two-stage self-injection locked near single wavelength visible laser system is also presented. An ultra-narrow linewidth of ∼34 pm is realized at 525.05 nm locked wavelength from this innovative system, with ∼20 dB side-mode-suppression-ratio; thus signifying a paradigm shift toward semiconductor lasers for near single lasing wavelength generation, which is presently dominated by other kinds of laser technologies.

Group-III-nitride superluminescent diodes (SLDs) are emerging as light sources for white lighting and visible light communications (VLC) owing to their droop-free, low speckle noise and large modulation bandwidth properties. In this study, we discuss the development of GaN-based visible SLDs, and analyze their electro-optical properties by studying the optical power-bandwidth products (PBPs) and injection current densities. The significant progress in blue SLDs and their applications for white light VLC is highlighted. A blue SLD, with an optical power of > 100 mW and large PBP of 536 mW.nm, is utilized to generate white light, resulting in a high CRI of 88.2. In a modulation experiment designed for an SLD-based VLC system, an on-off keying scheme exhibits a 1.2 Gbps data rate, with a bit error rate (BER) of 1.8 × 10-3, which satisfies the forward error correction (FEC) criteria. A high data rate of 3.4 Gbps is achieved using the same SLD transmitter, by applying the 16-QAM discrete multi-tone (DMT) modulation scheme for high-speed white light communication. The results reported here unequivocally point to the significant performance and versatility that GaN-based SLDs could offer for beyond-5G implementation, where white lighting and high spectral efficiency VLC systems can be simultaneously implemented.

We implemented a tunable dual-longitudinal-mode spacing InGaN/GaN green (521–528 nm) laser diode by employing a self-injection locking scheme that is based on an external cavity configuration and utilizing either a high-or partial-reflecting mirror. A tunable longitudinal-mode spacing of 0.20 – 5.96 nm was accomplished, corresponding to a calculated frequency difference of 0.22–6.51 THz, as a result. The influence of operating current and temperature on the system performance was also investigated with a measured maximum side-mode-suppression ratio of 30.4 dB and minimum dual-mode peak optical power ratio of 0.03 dB. To shed light on the operation of the dual-wavelength device arising from the tunable longitudinal-mode spacing mechanism, the underlying physics is qualitatively described. To the best of our knowledge, this tunable longitudinal-mode-spacing dual-wavelength device is novel, and has potential applications as an alternative means in millimeter wave and THz generation, thus possibly addressing the terahertz technology gap. The dual-wavelength operation is also attractive for high-resolution imaging and broadband wireless communication.

We demonstrate narrow-line green laser emission at 513.85 nm with a linewidth of 31 pm and side-mode suppression ratio of 36.9 dB, operating under continuous-wave injection at room temperature. A high-order (40th) distributed-feedback surface grating fabricated on multimode InGaN-based green laser diodes via a focused ion beam produces resolution-limited, single-mode lasing with an optical power of 14 mW, lasing threshold of 7.27 kA cm−2, and maximum slope efficiency of 0.32 W A−1. Our realization of narrow-line green laser diodes opens a pathway toward efficient optical communications, sensing, and atomic clocks.

To allow for reliable wireless optical links in realistic underwater environments, we study the dependence of turbulence-induced fading on the wavelength using a laser-based white-light interrogator in emulated realistic conditions. We experimentally show that the scintillation index decreases significantly with the increase of wavelength. The results are verified for longer distances using a Monte Carlo simulation. We numerically and experimentally demonstrate that the use of longer wavelengths lowers the bit error ratio by as much as three orders of magnitude. We conclude that using green light is more reliable in turbulent channels than blue. The correlation between different wavelengths under turbulence is studied, which was made possible by the use of the laser-based white-light interrogator.

Underwater wireless optical communication (UWOC) can offer reliable and secure connectivity for enabling future internet-of-underwater-things (IoUT), owing to its unlicensed spectrum and high transmission speed. However, a critical bottleneck lies in the strict requirement of pointing, acquisition, and tracking (PAT), for effective recovery of modulated optical signals at the receiver end. A large-area, high bandwidth, and wide-angle-of-view photoreceiver is therefore crucial for establishing a high-speed yet reliable communication link under non-directional pointing in a turbulent underwater environment. In this work, we demonstrated a large-area, of up to a few tens of cm2, photoreceiver design based on ultraviolet(UV)-to-blue color-converting plastic scintillating fibers, and yet offering high 3-dB bandwidth of up to 86.13 MHz. Tapping on the large modulation bandwidth, we demonstrated a high data rate of 250 Mbps at bit-error ratio (BER) of 2.2 × 10−3 using non-return-to-zero on-off keying (NRZ-OOK) pseudorandom binary sequence (PRBS) 210-1 data stream, a 375-nm laser-based communication link over the 1.15-m water channel. This proof-of-concept demonstration opens the pathway for revolutionizing the photodetection scheme in UWOC, and for non-line-of-sight (NLOS) free-space optical communication.

We report the use of 3D-printed microscale spiral phase plates to generate orbital angular momentum (OAM) carrying beams. We confirm that the generated beams have high purity, and we have successfully tested them to convey data signals with low bit error rates at the wavelength of 980 nm. This method will open new opportunities for generating OAM beams for many applications in optical communications, including free-space optics, as well as underwater, chip-to-chip, and quantum communications.