Design and evaluation of a free-space optical communications link using pulse width modulation (PWM)

Free-space optical communications (FSO) use light as information carrier. Light travels through atmosphere or vacuum and reaches a distant receiver. Light is generated by semiconductor lasers (LDs) or light-emitting diodes (LEDs) and is modulated by varying the injection current on these devices. To carry information, basic modulation techniques are analog as intensity (IM), amplitude (AM), frequency (FM) or phase (PM). However, pulse time modulation (PTM) techniques as pulse-width modulation (PWM) or pulse-position modulation (PPM) are more performant. In PWM, information is imprinted on the duration of a pulse train; PPM codes information on the relative position of narrow pulses of fixed duration. Analog and PTM formats are widely used over optical fiber transmissions. However, works using PTM over FSO communications are scarce in technical literature. As FSO links represent an alternative for last-mile communications (up to 4 km), analog or digital information such as remote sensing and instrumentation signals, environmental monitoring, video and voice, etc. These signals are efficiently transmitted using low-complexity and low-cost hardware. Under this perspective, this paper reports the design, realization and laboratory evaluation of a prototype of short-distance (up to 1 km) FSO link based on PWM. The link operates at a wavelength of 1.5 µm and transmits information in a 10 MHz bandwidth, in indoor or outdoor environments. The system is based on in-home developed PWM electronics. The beamforming optics use in-home fabricated lenses. The transmitter delivers a 2.5 mW collimated beam. The receiver collects a fraction of the transmitted power and feeds fiber-pigtailed photodetectors. To date, the optical link has been evaluated over 5.5 m. Sensitivities of 2.5 and 0.5µW for PIN and APD based photoreceivers are achieved and ensure high quality information recovery. Work is in progress for designing a beam tracking system for tests over distances up to 1 km.