Fast-Response Paradigm of Si Photodiode Array to Increase the Effective Sensitive Area of Detectors in Wireless Optical Biotelemetry Links

Abstract

Here we report on a novel optoelectronic architecture capable to add in-phase the current pulses generated by each one of the fast Si photodiodes forming an array illuminated by laser pulses. The aim is to increase the total photodiode sensitive area to achieve higher output pulsed current values by maintaining unaltered the response time, the bandwidth and the reverse bias voltage proper of each single photodiode. This result is important for many modern applications in biophysics and biomedicine like the brain machine interfaces, that require real time evaluation of environmental/sample changes through the acquisition of signals at high data rates under high signal-to-noise ratio conditions. As a consequence, these applications use sub-nanosecond laser pulses that are revealed by large bandwidth photodiodes having small sensitive area to attain low values of the internal junction capacitance. Thus, any small optical misalignment between the laser beam and the photodiode sensitive area in transcutaneous optical biotelemetry strongly decreases the system detection efficiency and performances. The proposed optoelectronic system resolves this problem being capable to sum in-phase the current pulses generated by each one of the photodiodes forming the array into a single current pulse maintaining the condition of fast response in terms of rise and fall times and bandwidth. This solution employs a Kirchhoff node to sum the different photocurrents, a decoupling current buffer and a transimpedance amplifier to amplify the current pulses summed at the node. We investigated the performances of the optoelectronic system by using photodiodes with different sensitive areas and junction capacitances (i.e., different bandwidths and rise and fall times) for laser pulses repetition rates up to 200 MHz. We also experimentally characterized the circuitry composed by an array of 4 fast photodiodes by using 800 ps laser pulses at a repetition rate of 200 MHz proving that the achieved response times and bandwidth remain the same of those ones of each single photodiode. Moreover, we demonstrated that the maximum value (i.e., the peak) of the obtained output current pulses is multiplied by a factor 4, i.e., equal to the number of the photodiodes forming the array having an overall sensitive area enhanced by the same factor.