Optically Powered Optical Transmitter Using a Single Light-Emitting Diode

An optically powered optical transmitter using a single light-emitting diode (LED) is presented with a batteryless and low-power temperature sensor node. The commercially available AlGaAs LED has higher open-circuit voltage (~1.2 V) than CMOS photodiode (~0.5 V), such that an analog/mixed signal design in 0.18-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> process can be powered directly without losing efficiency to a sustained voltage boosting scheme. Higher voltage is needed only during brief transmissions. A scalable, inverter-based, switched capacitor voltage boosting transmitter drives the LED in short pulses with 93% electrical energy efficiency. Three consecutive pulses are sent as preamble symbol, two as logic 0 and one as logic 1. The microsystem features a <inline-formula> <tex-math notation="LaTeX">$\Delta V_{{\text {BE}}}$ </tex-math></inline-formula>-based temperature sensor with a data-dependent partial conversion scheme to save energy. The sensor expends 1 nJ/full conversion at 5 kSa/s and 0.25 nJ/partial conversion at 20 kSa/s. The transmitter can serially output data at 1 nJ/pulse; 10-cm detection distance is achieved under 6-mW/mm² light intensity using an in-house photodiode detector circuit. The circuitry dissipates 6 <inline-formula> <tex-math notation="LaTeX">$\mu \text{W}$ </tex-math></inline-formula> in total at 1.2-V nominal voltage. It consists of a 0.1-mm² LED die, 2.25-mm² IC, and a surface mounted device storage capacitor, taking approximately 1-mm³ volume.