During the last decade, the demand for wireless network services has witnessed an explosive growth owing to the decrease in the cost of mobile electronic devices. The fifth-generation (5G) wireless system requires larger bandwidth to support more users simultaneously with lower latency and higher data rates for better user experience. The wireless communication system based on conventional radio frequency (RF) uses the frequency spectrum from 3 kHz to 300 GHz, and most of this range has been allocated in such a way that limits the capacity of the network. To realize an enhanced mobile broadband wireless network, light-emitting diode (LED)-based visible light communication (VLC), also known as Li-Fi[
|Year||Transmitter||Data Rate (Gbps)||Distance (m)||Modulation||Refs.|
|2018||Violet LD||24||10||64-QAM DMT||[|
Table 1. Recent Progress in VLC Systems
Figure 1.Recent advances in nitride-based LED and LD-based VLC[
The potential applications of Li-Fi are in traffic-light-to-vehicle communications, VLC in hospitals, and underwater communication. Traffic-light-to-vehicle communication is enabled by existing lighting devices that can be integrated with communication systems. With VLC-based traffic-light-to-vehicle communications, the functions of collision warnings, pre-crash sensing, turning assistance, and self-driving guidance are feasible[
Sign up for Chinese Optics Letters TOC Get the latest issue of Advanced Photonics delivered right to you！Sign up now
Figure 2.Recent research progress in UWOC[
In this paper, we summarize the recent progress of VLC systems in free-space environments. Section
2. BASIC CONCEPTS AND TERMINOLOGIES
For the realization of a VLC system, the following parameters are imperative in determining the overall performance of a VLC link. In this section, we summarize some of the key terms that are closely associated with the VLC system. The terminology can be characterized into (1) white lighting characteristics, (2) performance of communication systems, and (3) photodetector (PD) characteristics.
A. White lighting characteristics
B. Performance of communication systems
C. Photodetector characteristics
Figure 3.Typical spectral response of Si-based photodetector.
3. DEVICES IN LASER-BASED VLC SYSTEMS
As a key component of VLC, the spotlight on light emitters for enabling high data rate and modulation bandwidth is pivotal. In particular, the continuous improvement of device technology related to laser and the associated SLD is imperative. The first demonstration of the GaN-based LD grown on (0001) sapphire substrates was reported by Nakamura
Figure 4.Recent advances in III-nitride-based LDs and SLD for enabling high data rate VLC systems[
Figure 5.(a) Electroluminescence emission spectrum of the semipolar violet-emitting LD at an injection current of 400 mA. (b) Schematic of the small-signal modulation response measurement setup. (c) Small-signal modulation response of the violet-emitting LD at an injection current of 400 mA. The LD shows a
1. Semipolar InGaN laser diode
The development of GaN-based violet and blue LD technology in the past two decades is typically grown on
|Wavelength ||Substrate||Waveguide Design||Facet||Optical Power (mW)||Threshold ||Modulation Bandwidth||Refs.|
|395||Broad area||Cleaved, uncoated||10–180 (Pulse)||3.2–3.6||–||[|
|410||2–10 µm ridge||Cleaved, ||10–75 (CW)||–||2.5 GHz and 1.38 GHz||[|
|410||Semipolar ||2 µm and 3 µm ridge||RIE, uncoated||20–128 (CW)||6.25||5 GHz||[|
|445||Semipolar ||2.5–15 µm ridge||CAIBE, uncoated||100–1100 (CW)||2.2||–||[|
|450 (commercial)||–||–||10–70 (CW)||–||1.8 GHz||[|
|453||Semipolar ||Ridge||Polished, uncoated||5–35 (Pulse)||8.6||–||[|
|457||Semipolar ||2 µm and 4 µm ridge||Polished, uncoated||1–10 (Pulse)||13.0 and 12.6||–||[|
|518||Semipolar ||Ridge||RIE, uncoated||5–18 (Pulse)||40||–||[|
|520 (commercial)||–||–||10–80 (CW)||–||200–1000 MHz||[|
|536.6||Semipolar ||2 µm ridge||Cleaved, Coated||10–90 (CW)||5.9||–||[|
Table 2. Summary of InGaN-based Laser Diode Design and Performance
The design and fabrication of lasers should consider the active region design, optimization of the confinement factor, waveguide design, contact formation, and facet optimization. Most published works explored the utilization of asymmetric InGaN/InGaN MQW active regions[
Here, we characterize a 410 nm emitting semipolar InGaN/GaN QW LD. Violet-emitting LDs have a similar epitaxial structure to blue-emitting LDs[
2. Two-section laser and photonics integration
GaAs- and InP-based photonics integration has been achieved in optical telecommunication wavelength regimes for many commercial applications[
In order to achieve a high-efficiency electroabsorption modulator (EAM) on a III-nitride platform and reduce the required modulation bias for low voltage operations, the waveguide modulator based on semipolar
Figure 6.Comparison of photocurrent versus wavelength spectra in (a) semipolar
As a building block of the high-performance transmitter in a VLC system, a dual-section SOA-LD was demonstrated and characterized[
Figure 7.(a) Three-dimensional (3D) illustration of the 405 nm emitting dual-section SOA-LD on a semipolar GaN substrate. The device involves four pairs of
Figure 8.Comparison of output power versus current in the LD and photocurrent from the WPD at zero bias versus current in the LD[
The combination of both active and passive photonic components enables the on-chip integration of optoelectronic devices with versatile functionalities[
3. Superluminescent diode
In recent years, nitride-based SLDs have also received significant attention, owing to their optical characteristics, i.e., broadband spectrum similar to that of LEDs and high spatial coherence similar to that of LDs. Owing to its unique advantages, SLD is typically used as the source of broadband light in short-wavelength optical coherence tomography and fiber optic gyroscope systems[
For nitride-based SLD, Feltin
The droop-free and speckle-free high optical power InGaN-based SLD for SSL was first, to the best of our knowledge, demonstrated by Shen
|Spectral width (FWHM)||40 to 80 nm||0.1 to 5 nm||6 to 20 nm|
|Modulation bandwidth||Up to tens of MHz||Up to few GHz||Up to hundreds of MHz|
|Cost||Low||High||Moderate to high|
Table 3. Comparison of LEDs, Laser Diodes, and SLDs for SSL-VLC Systems
Figure 9.(a) 405 nm SLD grown on semipolar (
B. Phosphor for SSL-VLC
Phosphor is a material that can release secondary optical emissions at a certain wavelength (
Conventional phosphors for SSL or VLC are based on inorganic materials. They contain an inorganic host material doped with an optically active element. Garnet (
Figure 10.Emission spectrum of Ce-doped yttrium aluminum garnet (
However, the long decay time for the YAG phosphor, which is on the order of microseconds, hinders the progress from being applied in the VLC system[
To overcome the bandwidth bottleneck in the YAG-phosphor-based SSL-VLC system, organic materials, such as boron-dipyrromethene and poly[2,5-bis(
Recently, there has been an emerging interest in lead halide perovskites [
Figure 11.(a) Measured frequency response of the perovskite-based VLC system. (b) BER of the perovskite-based VLC system at different data rates and the eye diagram of 2 Gbps data rate showing a clear open eye[
The laser-based SSL-VLC system requires new phosphor geometries to handle the greater light flux from the laser. Spark plasma sintering (SPS), which allows for compositional modulation and phase fraction controlling, provides the ability to create new desired phosphor geometries. Using SPS to prepare YAG phosphors combined with a chemically compatible and thermally stable oxide, α-Al2O3, Cozzan
4. SIGNAL PROCESSING IN VLC
A. Modulation Technology
After preprocessing, coding, and modulation, the LD (or LED) is driven by the original binary bit stream, and electrical signals are converted into optical signals with intensity modulation. Each modulation technique has a finite number of symbols in which data can be encoded. Having more symbols allows the representation of more bits by a single symbol. For example, if an eight-symbol modulation technique is used, each symbol can represent a set of three bits because each set can have one of eight possibilities. In general, each symbol of an
1. On–off keying
In OOK, the signal can be written as
2. Phase-shift keying
Instead of modulating the amplitude of the carrier, we can modulate its phase. This method is called phase-shift keying (PSK). One variation of PSK is binary PSK (BPSK), in which a “0” is represented by a phase shift of 180° in the carrier, and a “1” is represented by not changing the phase of the carrier[
Figure 12.Signal waveforms of different modulation techniques: (a) NRZ-OOK and (b) RZ-OOK.
Figure 13.Constellation diagrams of (a) 4-PSK, (b) 8-PSK, (c) 4-QAM, and (d) 16-QAM.
3. Quadrature amplitude modulation
QAM combines both PSK and amplitude-shift keying by changing the two parameters of the carrier. For that reason, this method is also known as amplitude phase keying[
By using multiplexing, the data rate can be significantly increased while maintaining an acceptable error rate. There are many ways in which multiplexing can be achieved in VLC. For example, multiple wavelengths can be used to transmit different data streams simultaneously. It is also possible to use different optical angular momentum modes. Spatial division multiplexing allows the use of multiple transmitters and receivers to communicate parallel data streams. One of the most commonly used multiplexing techniques is OFDM.
OFDM uses different subcarriers with orthogonal frequencies to utilize the available bandwidth efficiently. The block diagram of OFDM transmission and reception is shown in Fig.
Figure 14.OFDM transmission and reception block diagram[
In communication systems, the signal is often distorted when transmitted through a channel. This distortion will cause inter-symbol interference (ISI) and contribute to the increase in the bit error rate. To improve the performance of the system, an equalizer is used to reverse the distortion.
The equalizer is also known as the equalization filter because it is a special filter that reverses the distortion by cutting down some of the unwanted components and boosting some desired components.
There are two approaches that can be used to determine the parameters of the filter: using a training sequence and blind equalization. The training requires sending a given pseudorandom binary sequence or a given code sequence, using this information to determine the distortion of the system, and calculating the inverse filtering parameter to reverse this distortion. Blind equalization, however, requires updating the parameters until the error caused by the distortion is minimized. Therefore, it is also named adaptive filtering. The most widely used adaptive filtering methods include constant module algorithm, least mean squared equalizer, zero forcing equalization, decision feedback equalization, maximum a posteriori probability, and maximum likelihood sequence estimation.
D. Optical Wireless Communication Standards
The IEEE standards are developed to facilitate better communication among market participants and help to accelerate the introduction of products to the market. Users of these standards should consult all applicable laws and regulations. VLC-related standards include IEEE 802.15.7, which is for short-range optical wireless communication using visible light and supports data rates up to 96 Mbps. Another standard is IEEE 802.15.13, which supports up to 10 Gbps for wavelengths ranging from 190 to 10,000 nm, which covers the visible light wavelength, and for a range up to 200 m. One other standard is IEEE 802.11.bb.
While VLC has many advantages over RF communication, it also faces unprecedented challenges that need to be mitigated. One of these challenges is commercialization, in which lighting companies and phone manufacturers have to develop their future devices to accommodate the current VLC technologies for use in future applications[
With the increasing demand for high-speed communication and the inability of existing communication technologies to keep up with the exponentially growing demand, VLC, with its large unregulated bandwidth, has the potential to become the new standard for wireless communication. While it is possible to use Li-Fi, the use of LDs has the additional advantage of being able to transmit data with much higher data rates. Moreover, it can also be used for dual-functionalities in VLC and SSL. This tutorial provides the reader with the necessary information about the general construction of the laser-based VLC system and methods of improving the overall system performance. After introducing the general terminology typically used in VLC systems, we described the state-of-the-art devices used as transmitters, including semipolar InGaN LDs, two-section lasers, and SLDs. Then, we discussed the use of phosphor to generate white light from lasers to cater to dual-functionality SSL-VLC systems. We also presented different modulation schemes used in VLC and the prospect of multiplexing to increase the data rate of the communication link. We also included a discussion of the use of equalization to improve the performance of a VLC system. Finally, we discussed the challenges and prospects concerning the VLC system.
Despite these challenges, VLC technology has a great potential to be the ultimate solution to the impending crisis caused by the increasing demand for wireless communication applications that cannot be met by RF technology. However, there is still much work needed to overcome some of these challenges, including improving the quality of white light produced using stable light converters, misalignment between the transmitter and the receiver that can cause outages, and standardizing the technology. Based on the current trend, we anticipate exponential growth in the field of VLC and envision that the related technology can be expanded in the forthcoming years.
 F.-M. Wu, C.-T. Lin, C.-C. Wei, C.-W. Chen, Z.-Y. Chen, K. Huang. Optical Fiber Communication Conference/National Fiber Optic Engineers Conference 2013, OTh1G.4(2013).
 D. Tsonev, H. Chun, S. Rajbhandari, J. J. D. McKendry, S. Videv, E. Gu, M. Haji, S. Watson, A. E. Kelly, G. Faulkner, M. D. Dawson, H. Haas, D. O. Brien. IEEE Photon. Technol. Lett., 26, 637(2014).
 I. Dursun, C. Shen, M. R. Parida, J. Pan, S. P. Sarmah, D. Priante, N. Alyami, J. Liu, M. I. Saidaminov, M. S. Alias, A. L. Abdelhady, T. K. Ng, O. F. Mohammed, B. S. Ooi, O. M. Bakr. ACS Photon., 3, 1150(2016).
 X. Zhu, F. Wang, M. Shi, N. Chi, J. Liu, F. Jiang. Optical Fiber Communication Conference, M3K.3(2018).
 B. S. Ooi. Pacific Rim Conference on Lasers and Electro-Optics (CLEO-PR) 2018(2018).
 B. S. Ooi, X. Sun, G. Liu, T. K. Ng. Asia Communications and Photonics Conference (ACP)(2018).
 Vehicle Safety Communications Project: Task 3 Final Report - Identify Intelligent Vehicle Safety Applications Enabled by DSRC(2005).
 X.-W. Ng, W.-Y. Chung. Biomed. Eng. Appl. Basis Commun., 24, 155(2012).
 C.-Y. Li, H.-H. Lu, W.-S. Tsai, Z.-H. Wang, C.-W. Hung, C.-W. Su, Y.-F. Lu. IEEE Photon. J., 10, A219(2018).
 H. M. Oubei, J. R. Durán, B. Janjua, H. Wang, C. Tsai, Y. Chi, T. K. Ng, H. Kuo, H. He, M. Alouini, G. Lin, B. S. Ooi. 2016 Conference on Lasers and Electro-Optics (CLEO), SW1F.1(2016).
 H.-Y. Wang, Y.-F. Huang, W.-C. Wang, C.-T. Tsai, C.-H. Cheng, Y.-C. Chi, G.-R. Lin. Optical Fiber Communication Conference, Tu2I.1(2018).
 S. Nakamura, S. Pearton, G. Fasol. The Blue Laser Diode(2000).
 C. Shen, J. Leonard, A. Pourhashemi, H. Oubei, M. S. Alias, T. K. Ng, S. Nakamura, S. P. DenBaars, J. S. Speck, A. Y. Alyamani, M. M. Eldesouki, B. S. Ooi. 2015 IEEE Photonics Conference (IPC), 581(2015).
 G. R. Goldberg, A. Boldin, S. M. L. Andersson, P. Ivanov, N. Ozaki, R. J. E. Taylor, D. T. D. Childs, K. M. Groom, K. L. Kennedy, R. A. Hogg. IEEE J. Sel. Top. Quantum Electron., 23, 2000511(2017).
 J. Piprek. Nitride Semiconductor Devices: Principles and Simulation(2007).
 J. J. Coleman, K. M. Kelchner, A. C. Bryce, S. P. DenBaars, J. S. Speck, C. Jagadish. Semiconductors and Semimetals, 149(2012).
 C. Shen, Y. Guo, X. Sun, G. Liu, K. T. Ho, T. K. Ng, M. S. Alouini, B. S. Ooi. 2017 Opto-Electronics and Communications Conference (OECC) and Photonics Global Conference (PGC), 1(2017).
 J. H. Kang, O. Kruger, U. Spengler, U. Zeimer, S. Einfeldt, M. Kneissl. J. Vac. Sci. Technol. B, 34, 041222(2016).
 C. Lee, C. Zhang, D. L. Becerra, S. Lee, C. A. Forman, S. H. Oh, R. M. Farrell, J. S. Speck, S. Nakamura, J. E. Bowers, S. P. DenBaars. Appl. Phys. Lett., 109, 27(2016).
 J. Yang, Z. Liu, B. Xue, J. X. Wang, J. M. Li. Opt. Quant. Electron., 49, 173(2017).
 S. Takagi, Y. Enya, T. Kyono, M. Adachi, Y. Yoshizumi, T. Sumitomo, Y. Yamanaka, T. Kumano, S. Tokuyama, K. Sumiyoshi, N. Saga, M. Ueno, K. Katayama, T. Ikegami, T. Nakamura, K. Yanashima, H. Nakajima, K. Tasai, K. Naganuma, N. Fuutagawa, Y. Takiguchi, T. Hamaguchi, M. Ikeda. Appl. Phys. Express, 5, 082102(2012).
 J. Yang, D. G. Zhao, D. S. Jiang, P. Chen, J. J. Zhu, Z. S. Liu, X. Li, F. Liang, W. Liu, S. T. Liu, L. Q. Zhang, H. Yang, J. Zhang, M. Li. IEEE Photon. J., 9, 2300108(2017).
 J. Rass, S. Ploch, T. Wernicke, M. Frentrup, M. Weyers, M. Kneissl, 52, 08JG12(2013).
 M. X. Feng, J. P. Liu, S. M. Zhang, D. S. Jiang, Z. C. Li, K. Zhou, D. Y. Li, L. Q. Zhang, F. Wang, H. Wang, P. Chen, Z. S. Liu, D. G. Zhao, Q. Sun, H. Yang. Appl. Phys. Lett., 103, 043508(2013).
 L. A. Coldren, S. W. Corzine, M. L. Mashanovitch. Diode Lasers and Photonic Integrated Circuits(2012).
 C. Shen, C. Lee, T. K. Ng, J. S. Speck, S. Nakamura, S. P. DenBaars, A. Y. Alyamani, M. M. Eldesouki, B. S. Ooi. 2016 IEEE Photonics Conference (IPC), 813(2016).
 C. Shen, C. Lee, T. K. Ng, S. Nakamura, J. S. Speck, S. P. DenBaars, A. Y. Alyamani, M. M. El-Desouki, B. S. Ooi. 2016 IEEE International Electron Devices Meeting (IEDM), 22.4.1(2016).
 F. Kish, R. Nagarajan, D. Welch, P. Evans, J. Rossi, J. Pleumeekers, A. Dentai, M. Kato, S. Corzine, R. Muthiah, M. Ziari, R. Schneider, M. Reffle, T. Butrie, D. Lambert, M. Missey, V. Lal, M. Fisher, S. Murthy, R. Salvatore, S. Demars, A. James, C. Joyner. Proc. IEEE, 101, 2255(2013).
 C. Shen, C. Lee, T. K. Ng, J. S. Speck, S. Nakamura, S. P. DenBaars, B. S. Ooi. 2017 Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR), 1(2017).
 C. Shen, T. K. Ng, C. Lee, J. T. Leonard, S. Nakamura, J. S. Speck, S. P. Denbaars, A. Y. Alyamani, M. M. El-Desouki, B. S. Ooi. Proc. SPIE, 10104, 101041U(2017).
 G. R. Goldberg, P. Ivanov, N. Ozaki, D. T. D. Childs, K. M. Groom, K. L. Kennedy, R. A. Hogg, 10104, 101041X(2017).
 D. C. O’Brien, L. Zeng, H. Le-Minh, G. Faulkner, J. W. Walewski, S. Randel. IEEE 19th International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), 1(2008).
 J. Proakis. Digital Communications(2007).
 B. P. Lathi. Modern Digital and Analog Communication Systems(1995).
 L. N. Binh. Advanced Digital Optical Communications(2015).
 Y. C. Chi, D. H. Hsieh, C. Y. Lin, H. Y. Chen, C. Y. Huang, J. H. He, B. Ooi, S. P. DenBaars, S. Nakamura, H. C. Kuo, G. R. Lin. Sci. Rep., 5, 18690(2015).
 H. Chun, S. Rajbhandari, G. Faulkner, D. Tsonev, H. Haas, D. O’Brien. IEEE International Conference on Communication Workshop (ICCW), 1392(2015).
 N. Chi. LED-Based Visible Light Communications(2018).
 A. Shlomi, J. Barry, G. Karagiannidis, R. Schober, M. Uysal. Advanced Optical Wireless Communication Systems(2012).
 C. W. Chow, C. H. Yeh, Y. F. Liu, P. Y. Huang. IEEE Photon. J., 5, 7900307(2013).
Set citation alerts for the article
Please enter your email address