VCSEL Array-Based Gigabit Free-Space Optical Femtocell Communication

We present an indoor free-space optical communication system to augment traditional short-range radio frequency wireless links. In this system, we integrated a high-frequency, high-current laser driver with an 850 nm wavelength, 500 mW vertical cavity surface emitting laser (VCSEL) array to transmit high speed optical signals with a large spatial coverage. The large current-bandwidth capability of the laser driver is enabled by a distributed current driver design utilizing three enhanced-mode pseudomorphic high electron mobility transistors. At the optical receiver end, a high-sensitivity avalanche photodiode is used to achieve high speed optical signal detection without any light concentrating optical device and is tolerant to angular deviation up to 27°. The communication link described in this article forms an optical femtocell with an area of 100 cm 2 at a distance of three meters from the transmitter. A bit error rate of less than 10 -4 is achieved throughout the optical femtocell in a normal in-door environment. On-off-keying and four-level pulse amplitude modulation schemes are both implemented with 1.25 Gb/s and 2 Gb/s data rate respectively, using the hardware described in this article. Additionally, the overall performance of this free-space optical link is evaluated in terms of viewing angle, distance range, and bit error rate.