Laser-based white-light source for high-speed underwater wireless optical communication and high-efficiency underwater solid-state lighting

Laser-Based Mobile Visible Light Communication System

Mobile visible light communication (VLC) is key for integrating lighting and communication applications in the 6G era, yet there exists a notable gap in experimental research on mobile VLC. In this study, we introduce a mobile VLC system and investigate the impact of mobility speed on communication performance. Leveraging a laser-based light transmitter with a wide coverage, we enable a light fidelity (LiFi) system with a mobile receiving end. The system is capable of supporting distances from 1 m to 4 m without a lens and could maintain a transmission rate of 500 Mbps. The transmission is stable at distances of 1 m and 2 m, but an increase in distance and speed introduces interference to the system, leading to a rise in the Bit Error Rate (BER). The mobile VLC experimental system provides a viable solution to the issue of mobile access in the integration of lighting and communication applications, establishing a solid practical foundation for future research.

Plasmonic Nanoparticle Lattice Devices for White-Light Lasing

A plasmonic nanolaser architecture that can produce white-light emission is reported. A laser device is designed based on a mixed dye solution used as gain material sandwiched between two aluminum nanoparticle (NP) square lattices of different periodicities. The (±1, 0) and (±1, ±1) band-edge surface lattice resonance (SLR) modes of one NP lattice and the (±1, 0) band-edge mode of the other NP lattice function as nanocavity modes for red, blue, and green lasing respectively. From a single aluminum NP lattice, simultaneous red and blue lasing is realized from a binary dye solution, and the relative intensities of the two colors are controlled by the volume ratio of the dyes. Also, a laser device is constructed by sandwiching dye solutions between two Al NP lattices with different periodicities, which enables red-green and blue-green lasing. With a combination of three dyes as liquid gain, red, green, and blue lasing for a white-light emission profile is realized.

Flexible broadcast UWOC system using an LCVR-based tunable optical splitter

For underwater wireless optical communication (UWOC) systems, using an omnidirectional light source to construct a broadcast system will require considerable energy due to high geometric loss and water attenuation. In addition, high-sensitivity photon detectors usually have a limited dynamic range, therefore limiting communication distance. In this Letter, a broadcast UWOC system, based on liquid crystal variable retarders (LCVRs) and polarization beam splitters (PBSs), is proposed to allocate user power in accordance with user-specific channel conditions. By adjusting the driving alternating current (AC) voltage of LCVRs to change the input light polarization, different proportions of light can be allocated to different PBS ports before broadcasting to different users. In a dual-user transmitter for the proof-of-concept, the output power dynamic range and the additional insertion loss for the first user are 19.17 dB and 0.91 dB, respectively. For the second user, the performance degrades to 17.33 dB and 1.26 dB, respectively. The step size of power adjustment is less than 0.063 dB. To verify the effectiveness of power adjustment in UWOC systems, a 7-m/243.2-Mbps single-user UWOC system is designed with a water attenuation coefficient ranging from 0.50 dB/m to 2.35 dB/m. All bit error rates (BERs) can decrease to below the forward error correction (FEC) limit by adjusting the LCVR driving voltage. The adjustable range of communication distance could be extended from 4.2 m to 13.19 m with a channel attenuation coefficient of 1.44 dB/m. Finally, a dual-user UWOC experiment is conducted and proves that the proposed system can still work in a multi-user system. The proposed system is proven to be effective for improving the anti-jamming capability and flexibility of UWOC networks.

Investigation of hybrid LD/LED system for UWOC link with depth variations

In this paper, An UWOC system using a hybrid LD/LED link is proposed. A simulation was assumed to study the quality of the received optical signal with different modulation schemes such as 64-PPM, 4-QAM, NRZ-OOK, and RZ-OOK. A total of 64-PPM modulation schemes have enhanced the distance link compared with other modulation schemes. PIN and APD PDs are employed to validate the quality of the received optical signal under different water depths. Various metrics of BER, Q. factor, and data rate was introduced to assess the reliability of the hybrid LD/LED link. The simulation results show the proposed system can be achieved a good link with different water depths. The attenuation coefficient of water varies with depth and restricted the performance link distance by hybrid LD/LED link. The 64- PPM is a suitable modulation scheme for getting a long-distance link. The photodetector APD PD outperforms the PIN PD when 64-PPM and 4-QAM are applied.

9.14-Mbps 64-PPM UWOC system based on a directly modulated MOPA with pre-pulse shaping and a high-sensitivity PMT with analog demodulation

A pulsed fiber master oscillator power amplifier (MOPA), which is combined with second harmonic generation (SHG) and modulated by directly changing the current of the low-power seed laser, is designed in this paper to overcome the 'green gap' of semiconductor lasers and the difficulty of obtaining high-power and wide-bandwidth driving circuits. To decrease the guard slot and increase the data rate of a high-order pulse position modulation (PPM) system, pre-pulse shaping (PPS) is utilized to decrease the fluctuation of pulse power, which is caused by the gain dynamics of multi-order amplification of the MOPA, from 55.6% to 27.5% for 25-ns pulses and from 22.4% to 16.7% for 10-ns pulses, respectively. Moreover, an analog PPM demodulation method is proposed to mitigate the nonlinear effect caused by space charge limitations at dynodes of a photomultiplier tube (PMT) and increase the robustness of the system. In an optical darkroom, a 99-m 64-PPM UWOC transmission, of which the measured link loss is around 13.16 attenuation length (AL), is realized in a water tank with a data rate of 9.14 Mbps. The average received optical power ranges from -60.87 to -52.51 dBm, corresponding to a bit error rate (BER) range of 1.93 × 10^-4 to 2.3 × 10^-3. To further prove the reliability of the proposed system, we implement a 65-m UWOC experiment with the same data rate at a BER of 3.42 × 10^-4 in a 50-m standard swimming pool. The maximum link loss is measured to be 15 AL.

4.0 Gbps visible light communication in a foggy environment based on a blue laser diode

Fog has a strong attenuation effect on the optical channel, which will greatly degrade the performance of visible light communication (VLC). Studying the effect of the fog on communication performance is crucial to realize outdoor VLC for next generation networks, but there is little research on this topic. In this work, the transmission characteristics of visible light band in the foggy channel were measured and a high-speed VLC system based on a 450 nm blue laser diode (LD) and 16-ary quadrature amplitude modulation orthogonal frequency division multiplexing (16-QAM-OFDM) in the artificial fog environment was demonstrated experimentally. Through a foggy channel of 60 cm, a maximum data rate of up to 4 Gbps was achieved at the pass loss of 13.06 dB with a bit error rate (BER) of 3.5 × 10^-3 below the forward error correction (FEC) limit (3.8 × 10^-3), which was the highest data rate ever reported for VLC in the foggy channel. Even at a higher pass loss of 17.32 dB, the proposed system still could achieve a data rate of 2.84 Gbps with a BER of 2.8 × 10^-3. Further extending the distance to 16.9 m for a more practical application, a data rate of 2.0 Gbps was also demonstrated successfully.

Laser-based underwater frequency transfer with sub-picosecond timing fluctuation using optical phase compensation

We demonstrated a sub-picosecond laser-based underwater frequency transfer with an optical phase compensation. With this transfer technique, a highly-stable 500 MHz radio-frequency (RF) signal was disseminated over a 5-m underwater link for 5000 s, and the characteristic of the timing fluctuation and instability for the transfer was analyzed and measured. The experimental results show the total root-mean-square (RMS) timing fluctuation of the transferred RF signal with compensation is about 162 fs with a fractional frequency instability on the order of 2.8 × 10^-13 at 1 s and 2.7 × 10^-16 at 1000 s. The laser-based underwater frequency transfer proposed in this paper has a potential application of transferring atomic clock in water environment as its instability is less than the currently-used commercial Cs or H-master clocks.

Micro-LED as a Promising Candidate for High-Speed Visible Light Communication

Visible Light Communication (VLC) technology is an emerging technology using visible light modulation that, in the modern world, will mainly facilitate high-speed internet connectivity. VLC provides tremendous advantages compared to conventional radio frequency, such as a higher transmission rate, high bandwidth, low-power consumption, no health hazards, less interference, etc., which make it more prominent in recent days. Due to their outstanding features, including low cost, low power consumption, etc., µ-light-emitting diodes (LEDs) have gained considerable attention for VLC implementation, but mostly for the ability to be used for lighting as well as communications. In this review paper, we will focus mainly on recent developments in VLC applications and various factors affecting the modulation bandwidth of VLC devices. Numerous factors, such as quantum confined stark effect (QCSE), carrier lifetime, carrier recombination time, crystal orientation, etc. affect the modulation bandwidth of LEDs, and more information will be discussed in the following sections. This paper will focus on VLC applications based on LEDs but mainly on semipolar μ-LEDs and μ-LED-based arrays with high bandwidths. Another important application of VLC is underwater optical wireless communication (UOWC), which has drawn a huge interest in marine exploration and underwater connectivity, but still faces some challenges because visible light is being used. In addition, this paper will focus on how the current VLC system modulation bandwidth can be enhanced. Many methods have been introduced, such as decreasing the active layer thickness or effective active area or using doping, but the bandwidth is restricted by the recombination time when the system configuration reaches its limit. Therefore, it is important to find alternative ways such as optimizing the system, using the blue filter or using the equalization technology, which will be addressed later. Overall, this review paper provides a brief overview of the VLC-based system performance and some of its potential prospects.

Towards a 20 Gbps multi-user bubble turbulent NOMA UOWC system with green and blue polarization multiplexing

We experimentally demonstrated a high-speed multi-user green and blue laser diode based underwater optical wireless communication (UOWC) system using non-orthogonal multiple access (NOMA) with polarization multiplexing. The system affords eight users with a record sum rate of 18.75 Gbps over 2-m underwater plus 0.5-m free-space channel. The modulation bandwidths of four detachable optical paths with different wavelengths and polarization states all exceed 1.5 GHz. The results suggest that the flexible balance according to both user fairness and overall throughput/sum rate can be achieved via an appropriate power allocation strategy. The joint optimization of driving current and user assignment ensures the feasibility of providing stable high-speed UOWC for multiple users. With the proposed OFDMA-NOMA scheme, user scale expands by twice while the sum rate for single path reaches 3.2 Gbps. Finally, the BER performances of NOMA modality in turbulent underwater environment with air bubbles of different flow rates are also discussed in detail.

56-m/3.31-Gbps underwater wireless optical communication employing Nyquist single carrier frequency domain equalization with noise prediction

We propose and experimentally demonstrate an underwater wireless optical communication (UWOC) system using a 520-nm laser diode (LD) and 32-quadrature amplitude modulation (32-QAM) single carrier signals. To mitigate the inter-symbol interference (ISI), a frequency domain equalizer combined with a time-domain decision feedback noise predictor is employed at the receiver. However, this structure cannot apply channel coding conjunctively. Therefore, an interleaver/deinterleaver pair is applied to handle the decoding delay, and thus systematic Reed-Solomon (RS) code can provide reliable feedback signals. With a 3-dB bandwidth of 200 MHz, the proposed system with the frequency domain equalization and noise prediction (FDE-NP) scheme can achieve a maximal net data rate of 3.48 Gbps, which is 17.2% higher than that of orthogonal frequency division multiplexing (OFDM) scheme. At a net data rate of 3.31 Gbps, we have successfully achieved a transmission distance up to 56 m. To the best of our knowledge, this is the first time to employ FDE-NP in UWOC where OFDM conventionally plays a prevailing role for high-speed transmission.

Light transmission characteristic analyses of a laser screen in clear water based on the Monte Carlo method

In order to optimize the design of underwater light screens, the transmission process of the light beam in water is analyzed with the Monte Carlo method. The trajectories and power attenuation of emitted photons, propagating in different types of water from the light source towards the receiver with different initial light powers and transmission distances, are simulated and calculated. The output power can then be obtained to calculate the transmission ratio quantitatively. According to the simulated data, by using the curving fitting method, the exact expression of transmission ratio is derived, which is in the same form as the Beer-Lambert-Bouguer law. Meanwhile, experiments are conducted in a light screen with a vertical line beam laser in clear water to verify the simulations. The experimental results are in agreement with the simulated results, and the differences between them are within ±0.04. The presented results give insight into the design of underwater laser screens.

Analysis and experimental demonstration of underwater frequency transfer with diode green laser

We demonstrated an underwater frequency transfer technique with a green diode laser. The characteristic of the timing fluctuation and instability for the transfer technique was analyzed and simulated. With this technique, we had transferred a highly stable 100-MHz frequency signal over an underwater link with distances of 3 m, 6 m, and 9 m for 5000 s, respectively. The experimental results involving the underwater transfer of the 100-MHz radio-frequency signal shows that the rms timing fluctuations are 5.9 ps (3-m link), 6.4 ps (6-m link), and 8.4 ps (9-m link). The calculations also show that the relative Allan deviations for the 3-m, 6-m, and 9-m transmission links are 5.6 × 10^-13 at 1 s and 5.3 × 10^-15 at 1000 s, 5.8 × 10^-13 at 1 s and 1.1 × 10^-14 at 1000, and 6.8 × 10^-13 at 1 s and 1.1 × 10^-14 at 1000 s, respectively. The measured instabilities were lower than Rb and Cs atomic clocks, implying that the proposed frequency transfer scheme can potentially be used to transfer the signals of these atomic clocks over underwater links.

2 Gbps/3 m air-underwater optical wireless communication based on a single-layer quantum dot blue micro-LED

Blue/green light-emitting diodes (LEDs) show great potential in medium/short distance underwater optical wireless communication (UOWC) while suffering limited bandwidth caused by a long radiative recombination carrier lifetime and large resistance-capacitance (RC) constant. We designed, fabricated, and packaged a 75-µm single-layer quantum dot (QD) blue micro-LED with a record high modulation bandwidth up to 1.03 GHz. The single-layer structure of QD reduces the carrier lifetime and RC delay. A data rate of 2 Gbps over a 3-m air-underwater channel using a non-return-to-zero on-off-keying modulation format with a low bit-error rate of 2.03×10^-3 was achieved below the forward error correction limit, which is the highest data rate of micro-LED-based UOWC systems, to the best of our knowledge.

Average capacity of a UWOC system with partially coherent Gaussian beams propagating in weak oceanic turbulence

The average capacity of a single-input single-output (SISO) underwater wireless optical communication (UWOC) system with partially coherent Gaussian beams in a weak oceanic turbulence regime is investigated. An approximate analytical expression of scintillation index is derived mathematically to characterize the impact of oceanic turbulence on the propagation behavior of the partially coherent Gaussian beams. Then, the path loss caused by absorption and scattering in the ocean is numerically simulated with the Monte Carlo method. With consideration for absorption, scattering, and oceanic turbulence, the combined channel fading model is established, and the average capacity of the UWOC system (defined as the maximum mutual information between the input and output) is examined. Results show that the scintillations are reduced by decreases in propagation distance, the dissipation rate of mean-square temperature, and the ratio of the temperature and salinity contributions to the refractive index spectrum. Scintillations are also decreased by increases in source beam width, degree of partial coherence, and the dissipation rate of turbulent kinetic energy per unit mass of fluid. As a result, the average capacity of the UWOC system is enhanced. Moreover, the average capacity of the UWOC system can be promoted with the availability of channel state information at the receiver. This work will benefit the research and development of UWOC systems.

Underwater wireless optical communication system using a 16-QAM modulated 450-nm laser diode based on an FPGA

An underwater wireless optical communication (UWOC) system with a 450-nm laser diode and a 150-MHz PIN photodiode is presented. The 16-ary quadrature amplitude modulation scheme is employed and realized on a field-programmable gate array (FPGA) to efficiently provide a high data rate. Experimental results show that this system can achieve data rates up to 50 Mbps over a distance of 3 m underwater with a bit error rate (BER) of 7.11×10^-4 in artificial seawater at 35‰ salinity, where the attenuation coefficient is 0.481. This BER is below the forward error correction target of 3.8×10^-3. Thus, this system can provide reliable high-speed communications over short to moderate distance point-to-point UWOC links.

Hybrid warm-white organic light-emitting device based on tandem structure

We realized an efficient hybrid tandem warm white organic light-emitting device with extremely stable color purity in forward direction by employing an easy-fabrication charge generation unit (CGU) combining 1, 4, 5, 8, 9, 11-Hexaazatriphenylene-hexacarbonitrile with ultrathin bilayers of LiF and Al. The tandem white device exhibits maximum efficiencies of 37.3 cd/A, 23.3 lm/W and still remains at high efficiencies of 34.4 cd/A, 17 lm/W at the luminance of 1000 cd/m². When the operating current density increases from 5 mA/cm² to 30 mA/cm², the variations in Commission Internationale de l'Eclairage (CIE) coordinates are only (0.007, 0.003). Moreover, we also studied the cause of color variation in tandem white device by performing systematic optical simulation and discovered the origin of alleviation in efficiency roll-off for tandem white device by applying triplet-polaron quenching (TPQ) model.