Multi-gigabit data transmission using a directly modulated GaN laser diode for visible light communication through plastic optical fiber and water

Gallium-nitride based devices have been used for electronic and optical equipment for over 20 years now. In a world concerned with saving energy, the use of GaN-based solid-state lighting sources has gained interest for visible light communications (VLC), where a light-emitting diode (LED) or laser diode can be exploited, not only to provide illumination, but also communication simultaneously. Contemporary applications are expanding to include underwater and optical fiber communications, as well as the traditional free space domain. It has been shown that micro-LEDs have high modulation bandwidths [1] and can achieve fast data transmission rates, in particular by exploiting higher modulation techniques [2]. However, their performance is limited by the material carrier lifetime and hence laser diodes are considered to further this work. The laser diodes used in this work can give powers of up to 50-100 mW and emit at wavelengths between 421-429 nm. The frequency response of these devices was measured, with maximum -3 dB bandwidths in excess of 2 GHz acquired. One of these devices was used to conduct data transmission experiments in free space, and eye diagrams were achieved without any pre- or post-amplification at data rates up to 2.5 Gbit/s [3]. Error-free data transmission was confirmed by conducting bit-error rate measurements using a pseudo-random bit sequence (PRBS) of 27-1 bits in length. However, further tests showed that data rates of 3.4 Gbit/s could be achieved. These are the fastest transmission rates achieved from a directly modulated laser diode without any higher order modulation schemes. These results not only show the potential of GaN laser diodes for high-speed free-space VLC over short distances but also their potential for use in fiber. High speed measurements were conducted through varying lengths of step-index plastic optical fiber (SI-POF). A different laser (from the same batch), emitting at a wavelength of 429 nm was used to conduct frequency response measurements through the fiber. Fiber lengths of 1 m, 2.5 m, 5 m and 10 m were tested in order to see the trend of bandwidth against fiber length. This device had a -3 dB bandwidth of 1.71 GHz in free space and could achieve error-free data transmission at 2.5 Gbit/s, like before. The maximum bandwidth values achieved for 1 m, 2.5 m, 5 m and 10 m of fiber were 1.68 GHz, 1.63 GHz, 1.62 MHz, and 1.1 GHz, respectively. This can be seen in Figure 1 (a).