During the past decade, significant advancement has been made on InGaAsP/InP Geiger-mode APDs (GmAPDs) through improvements of material growth, device design and operating circuitry. With the increase in device performance and the growing maturity of device fabrication technology, high performance, large format InGaAsP/InP GmAPD arrays have been successfully designed and manufactured. These arrays have single photon sensitivity in the short wavelength infrared (SWIR) spectral band and can provide 3-D imagery. InGaAsP/InP GmAPD arrays provide an enabling technology for many active optical applications, such as 3-D light detection and ranging (LiDAR) and other photon-starved applications where single photon sensitivity in the SWIR band is critical. InGaAsP/InP-based Geigermode LiDAR has been extensively used on airborne platforms. By using optical wavelengths along with sub-ns laser pulse widths, 3-D Geiger-mode LiDAR techniques provide centimeter-scale range resolution over extremely long distances on the order of tens of kilometers. Through the use of high-performance single photon detectors, Geiger-mode LiDAR systems achieve an order of magnitude improvement in mapping rate over other competing LiDAR technologies. A more recent exciting application of InGaAsP/InP GmAPD-based LiDAR is to enable advanced driver assistance systems (ADAS) and vehicle autonomy on automotive platforms. The single-photon sensitivity of GmAPDs and greater eye-safety of diode lasers at wavelengths beyond 1400 nm provide disruptive automotive LiDAR performance that will be essential to future autonomous vehicle navigation. Single photon sensitivity and simple pixel circuit operation enable the reduction in overall system SWaP, while the scalability of these semiconductor devices enables dramatic reduction in LiDAR cost.
We describe Geiger-mode avalanche photodiode (GmAPD) cameras designed with asynchronous free-running operation that supports single-photon direct detection and coherent detection 3D LiDAR imaging as well as free-running applications such as free-space optical communications and target acquisition and tracking. Each free-running pixel in the 32x32 focal plane array performs independent time-of-flight measurements with a selectable reset time between 0.1 and 10 μs. Asynchronous reporting of time-of-flight and pixel location data allows continuous operation for arbitrarily long periods of time at sampling rates exceeding 0.7 Gsample/s. We report results for cameras optimized for operation using either 1.0 or 1.5 μm LiDAR sources.
Optical crosstalk is a major factor limiting the performance of Geiger-mode avalanche photodiode (GmAPD) focal plane arrays (FPAs). This is especially true for arrays with increased pixel density and broader spectral operation. We have performed extensive experimental and theoretical investigations on the crosstalk effects in InP-based GmAPD FPAs for both 1.06-μm and 1.55-μm applications. Mechanisms responsible for intrinsic dark counts are Poisson processes, and their inter-arrival time distribution is an exponential function. In FPAs, intrinsic dark counts and cross talk events coexist, and the inter-arrival time distribution deviates from purely exponential behavior. From both experimental data and computer simulations, we show the dependence of this deviation on the crosstalk probability. The spatial characteristics of crosstalk are also demonstrated. From the temporal and spatial distribution of crosstalk, an efficient algorithm to identify and quantify crosstalk is introduced.
Ultrasonic atomic force microscopy (U-AFM) was used to image the elastic properties of hamster kidney (BHK) cells. Force-distance curves and finite element analysis were also used in this work assist in the description of the U-AFM images. These tools helped explain the differences in contrast seen from the center to the edge of the cell. The explanation of the U-AFM image contrast will lead to more analytical tools to investigate both nonviable and viable materials. Improvement of our system for living cell (in a culture) imaging is also discussed.