We have developed a 32x24 pixel sensor array based on single-photon avalanche diodes (SPADs). Beside conventional 2- dimensional imaging, this sensor allows for precise timing of single-photon arrival times which can be exploited in a variety of technical and scientific approaches like 3D image acquisition, quantum imaging and quantum random number generation. Thus, such a sensor is eligible for many fields of application such as autonomous driving, remote and non-lineof- sight sensing, safety, robotics and more recently random number generation for statistical applications or data encryption. The novel sensor contains CMOS integrated backside illuminated SPADs which are connected to an underlying read-out IC by wafer-to-wafer bonding. Their single-photon sensitivity (quantum efficiency QE=60 % @ 580 nm) and high-speed performance (readout frequency 𝑓 = 25 kHz, temporal resolution 𝑡TDC = 312.5 ps) make the sensor a promising choice for, e.g. quantum imaging with photon-pairs where a 2-dimensional spatial and temporal resolution are as crucial as a low noise level. SPADs also offer exciting opportunities for random number generation by using the randomness of photon generation paired with time-resolved detection and post-processing. Another potential application of the sensor is light detection and ranging for which we integrated the sensor into a demonstrator system for direct time-of-flight measurements. It is capable of coincidence detection using 4 SPADs in each pixel, which allows for background light suppression in outdoor situations. This combination of single-photon sensitivity, precise photon arrival timing and our recent developments in wafer-to-wafer bonding technology gives access to a new generation of optical sensors for a variety of applications.
Advances in laser diode technology enable the generation of eye-safe laser pulses with short pulse duration and high peak power. This opens up new opportunities for Light Detection and Ranging (LiDAR)-systems based on the direct time-of-flight (dTOF) principle because their range performance is mainly limited by the requirement of eye-safe laser pulse energy. Another limiting factor for dTOF LiDAR is the sensitivity to background noise. Shorter pulse width enables better parasitic light suppression inside the LiDAR system for improved performance in high background flux scenarios. With the improvements caused by using short laser pulses, new challenges emerge. Shorter pulse duration and limited achievable timing resolution of time discrimination circuits inside of dTOF detectors lead to histogram data distributions in which the laser originated time stamps can only fill few time bins. The time stamp histogram of the detected and clocked laser photons shows a sharp exponential decline. The slope is strongly dependent on the occurring laser event rate inside the system. In an extreme case, all laser generated events fall into one time bin. Because of the coarse discrete arrangement of those laser generated events, a need for new algorithmic approaches arises. This work illustrates the dependency between the occurring laser photon rate in the system and its distribution inside the measurement data. Influence of the time discrimination circuit's time bin width is discussed with regards to resulting histogram shapes.
For some applications, a reliable detection of the distance of objects is necessary, even under strong environmental conditions. Commonly this includes sunlight, but fog, rain and snow cause interferences as well. For fast and reliable threedimensional monitoring of the environment, LiDAR is a key sensor technology. A light source, often in the near-infrared, emits a short light pulse and the time-of-flight of the photons reflected by an object is measured. This allows to calculate the distance by using the speed of light. In order to be able to ensure reliable detection despite possible interferences, we have set up a new measurement concept based on the existing time-gating. Thus, an area is covered by step-wise shifting of the measuring window. By accumulating different delays, the true distance to the object can be determined. An advantage of the method is that no information about the approximate position of the object has to be known in advance. In this paper we present measurement results with this method, which were taken in different environmental conditions. The method can be implemented in addition to already existing concepts and can therefore supplement them
A fast and reliable three dimensional monitoring of the environment is indispensable for robotics, automation engineering or autonomous driving. For these applications LiDAR is a key sensor technology. Normally a light source in the near infrared range is used, which is invisible to human eyes. High ambient light compared to the laser source intensity is a major problem for these systems. Therefore, a measurement concept to reduce the impact of ambient light is necessary. In this paper we present a measurement concept in which the full distance range is scanned and the probability to detect events from far objects is improved. The general problem is that a photon of the background illumination can be detected instead of the reflected laser signal which stops the measurement. The concept allows us to detect the received laser pulse buried in the superimposed background light easier and improve the measurement quality. This is possible due to the delayed start of the measurement and thus the selection of different measurement windows in which an earlier detection of the laser generated events is accessible. In consequence, the probability for receiving an unwanted ambient photon is reduced. For this technique no prior information about the object conditions or its rough distance is required and it can be applied in all situations of the direct time-of-flight measurement to cope with high ambient light. Hence it allows a reliable distance measurement at various ambient and target conditions.
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