Targeting ubiquitous communication with GaN lasers
Visible communication based on GaN lasers will empower wireless and optical networks.
BY CHAOWEN GUAN, NAN CHI AND CHAO SHEN FROM FUDAN UNIVERSITY
While science fiction often falls short of depicting the current capabilities of technology, it still offers a glimpse into potential future advancements. A prime example is Marvel Comics’ hero Iron Man, whose body armour has evolved since its debut in 1962. In recent iterations, Tony Stark’s suit features nanoscale devices that communicate with one another. For science fiction to become science fact, such body armour would require formidable capabilities in high-speed data transmission.
Although we are unlikely to try and exactly mimic Iron Man, there are moves to more wearables, along with the rapid development of technology for 6G wireless data links. Within this infrastructure, visible light communication (VLC) is poised to play a crucial role. VLC is expected to provide wide-coverage communication across space-air-ground-underwater domains at ultra-high speeds and low latency. In the future, light could even be used to power the Internet of X-things.
What is VLC?
The term ‘visible light communication’ is relatively self-explanatory. It refers to a wireless optical transmission technology that utilises visible light for data communication within the spectral range 380 nm to 790 nm.
In stark contrast to traditional wireless communication, which grapples with a scarcity of spectral resources, visible light communication enjoys an abundance, boasting a bandwidth of around 400 THz. This vast expanse of untapped potential beckons human exploration and innovation. Additional strengths provided by VLC are an immunity to radio interference, an absence of electromagnetic radiation, and high confidentiality. However, the biggest advantage of VLC is its high speed. Today’s lab record for data transmission stands at over 20 Gbit/s, a rate so high a wireless download of a movie in the Ultra HD format takes just a few seconds.
A typical VLC system comprises three main parts: a visible light transmitter, a channel for transmission, and a receiver. Information transfer begins with encoding, modulating, and pre-balancing the signal in the optical transmitter. Transmission then occurs through free space to reach a receiver, where either a p-i-n photodiode or an avalanche photodiode converts the optical signal into an electrical one. This is followed by equalisation, demodulation, and decoding to restore the original transmission signal.
Today there are two significant limitations with VLC systems. One is associated with the transmitter, such as a high-speed blue laser, in intensity-modulation direct-detection systems. This soure is held back by a relatively small bandwidth, as well as a level of maturity that falls far short of its counterparts operating at around 1550 nm. The second limitation is the slow development of key components, such as external modulators, amplifiers and balanced photodetectors in coherent modulation systems.
At Fudan University we are striving to address these all limitations, initially by developing a novel GaN laser that offers a higher modulation rate.
(a) Laser epitaxial structure. (b) Microscopic photographs of the high-speed blue laser diode chip.
High-speed sources
As the ancient Chinese saying goes: A craftsman who wishes to do his work well must first sharpen his tools. We have taken this advice to heart in developing a GaN-based laser, a fundamental component for white light and a promising source for underwater wireless optical communication. At present, GaN-based lasers emitting in the purple, blue, and green are primarily deployed for the likes of optical storage, lighting, displays, and material processing, with a focus on DC characteristics, such as output power and slope efficiency. Since there is a strong built-in electric field presented in blue lasers fabricated on c-plane GaN substrates, owing to polarisation effects, researchers are utilising semi-polar and non-polar GaN substrates to eliminating such polarisation fields. This enables to a better efficiency and a higher modulation bandwidth. However, the supply of GaN substrates with suitable orientations is limited.
Our efforts at developing high-speed GaN-based lasers have considered both the design of the epitaxial structure, including the active region and the electron-blocking layer, and the device architecture, such as the ridge waveguide. Through simulation of various epi-structures we have uncovered design rules that have led us to an improved frequency response in InGaN/GaN quantum well lasers, with findings subsequently verified by experimental work.
Measurement and characterisation of our high-speed lasers has involved the detailed analysis of light-current-voltage characteristics, emission spectra, junction temperature, relative intensity noise, frequency response, and data transmission performance. We have obtained the threshold current and slope efficiency of our lasers by determining light-current-voltage characteristics. The intrinsic parameters of our lasers have been further extracted. The -3dB modulation bandwidth can be derived through small-signal frequency response testing. During this work we gained much insight into the influence of design on performance – for example, the influence of different waveguide layer structures, with thicker ones found to lead to a more divergent light field that increases its overall volume. The optimal thicknesses for the p- and n- waveguide layers are 50 nm and 150 nm, respectively.