Interferometric Distance Sensing Using APDs and SPADs

Because of thermal noise, common analogous optical CMOS sensors need often more than 400,000 photons for one distance measurement, whereas the project aims to use only a few ten photons (in average) for this. The electrical power consumption of analogous sensors usually is high and they occupy a large chip area. In pure electronic integrated circuits (ICs), however, many analogous circuit blocks were already replaced by digital signal-processing. This project will for the first time show that integrated Avalanche Photo Diodes (APDs) and Single-Photon Avalanche Diodes (SPADs) allow to realize digital optical sensors as optoelectronic integrated ICs due to their high and respectively very high amplification. These optoelectronic integrated ICs will need orders of magnitude less photons for distance measurement and will enable 3D sensors with high resolution. The distance range of Time-of-Flight (ToF) integrated optical distance sensors using pin photodiodes is limited to several meters due to electronic noise. Integrated APDs and SPADs will increase the maximum distance considerably. However, due to the very high velocity of light, the measurement error of ToF sensors is in the order of centimeters and will not considerably improve with APDs and SPADs alone. The second major subject of the project is therefore to apply for the first time interferometric distance measurement principles together with integrated APD and SPAD sensors. The measurement error for interferometric techniques can in principle be reduced to the sub-wavelength range and the project will investigate how close sophisticated sensor circuits and signal processing methods can bring APD and SPAD sensors to this range. Thereby, a variable reference arm in the interferometer will allow to shift the measurement range of up to several meters into distances of several meters or even to more than 10 meters. Two different interferometric approaches will be investigated: (i) using a frequency comb generated by an improved frequency-shifted-feedback (FSF) fiber laser and (ii) variation of the wavelength of a laser diode by varying its injection current. A special wavelength stepping with time will allow to determine the frequency at the interferometer output, which is proportional to the distance (difference of object and reference arm) without a frequency counter. For SPADs having a thick absorption zone and a large photon detection probability (PDP), fast active quenching circuits will be investigated, which allow to triple the excess bias voltage in order to increase their PDP further to above 60%. Test detectors and sensor circuits like correlators and photon counters will be fabricated as Application Specific Integrated Circuits (ASICs) to verify the innovative approaches towards digital SPAD optical sensors. Summarizing, innovative new small-area current-saving digital sensors are investigated experimentally to verify a new generation of robust optical distance sensors with considerably improved sensitivity and measurement accuracy.