Modeling and Analysis of a Direct Time-of-Flight Sensor Architecture for LiDAR Applications.

Direct time-of-flight (DTOF) is a prominent depth sensing method in light detection and ranging (LiDAR) applications. Single-photon avalanche diode (SPAD) arrays integrated in DTOF sensors have demonstrated excellent ranging and 3D imaging capabilities, making them promising candidates for LiDARs. However, high background noise due to solar exposure limits their performance and degrades the signal-to-background noise ratio (SBR). Noise-filtering techniques based on coincidence detection and time-gating have been implemented to mitigate this challenge but 3D imaging of a wide dynamic range scene is an ongoing issue. In this paper, we propose a coincidence-based DTOF sensor architecture to address the aforementioned challenges. The architecture is analyzed using a probabilistic model and simulation. A flash LiDAR setup is simulated with typical operating conditions of a wide angle field-of-view (FOV = 40 ° ) in a 50 klux ambient light assumption. Single-point ranging simulations are obtained for distances up to 150 m using the DTOF model. An activity-dependent coincidence is proposed as a way to improve imaging of wide dynamic range targets. An example scene with targets ranging between 8-60% reflectivity is used to simulate the proposed method. The model predicts that a single threshold cannot yield an accurate reconstruction and a higher (lower) reflective target requires a higher (lower) coincidence threshold. Further, a pixel-clustering scheme is introduced, capable of providing multiple simultaneous timing information as a means to enhance throughput and reduce timing uncertainty. Example scenes are reconstructed to distinguish up to 4 distinct target peaks simulated with a resolution of 500 ps. Alternatively, a time-gating mode is simulated where in the DTOF sensor performs target-selective ranging. Simulation results show reconstruction of a 10% reflective target at 20 m in the presence of a retro-reflective equivalent with a 60% reflectivity at 5 m within the same FOV.

Mutually Coupled Time-to-Digital Converters (TDCs) for Direct Time-of-Flight (dTOF) Image Sensors.

Direct time-of-flight (dTOF) image sensors require accurate and robust timing references for precise depth calculation. On-chip timing references are well-known and understood, but for imaging systems where several thousands of pixels require seamless references, area and power consumption limit the use of more traditional synthesizers, such as phase/delay-locked loops (PLLs/DLLs). Other methods, such as relative timing measurement (start/stop), require constant foreground calibration, which is not feasible for outdoor applications, where conditions of temperature, background illumination, etc. can change drastically and frequently. In this paper, a scalable reference generation and synchronization is provided, using minimum resources of area and power, while being robust to mismatches. The suitability of this approach is demonstrated through the design of an 8 × 8 time-to-digital converter (TDC) array, distributed over 1.69 mm², fabricated using TSMC 65 nm technology (1.2 V core voltage and 4 metal layers-3 thin + 1 thick). Each TDC is based on a ring oscillator (RO) coupled to a ripple counter, occupying a very small area of 550 µ m², while consuming 500 µ W of power, and has 2 µ s range, 125 ps least significant bit (LSB), and 14-bit resolution. Phase and frequency locking among the ROs is achieved, while providing 18 dB phase noise improvement over an equivalent individual oscillator. The integrated root mean square (RMS) jitter is less than 9 ps, the instantaneous frequency variation is less than 0.11%, differential nonlinearity (DNL) is less than 2 LSB, and integral nonlinearity (INL) is less than 3 LSB.

A Hybrid Readout Solution for GaN-Based Detectors Using CMOS Technology.

Gallium nitride (GaN) and its alloys are becoming preferred materials for ultraviolet (UV) detectors due to their wide bandgap and tailorable out-of-band cutoff from 3.4 eV to 6.2 eV. GaN based avalanche photodiodes (APDs) are particularly suitable for their high photon sensitivity and quantum efficiency in the UV region and for their inherent insensitivity to visible wavelengths. Challenges exist however for practical utilization. With growing interests in such photodetectors, hybrid readout solutions are becoming prevalent with CMOS technology being adopted for its maturity, scalability, and reliability. In this paper, we describe our approach to combine GaN APDs with a CMOS readout circuit, comprising of a linear array of 1 × 8 capacitive transimpedance amplifiers (CTIAs), implemented in a 0.35 µm high voltage CMOS technology. Further, we present a simple, yet sustainable circuit technique to allow operation of APDs under high reverse biases, up to ≈80 V with verified measurement results. The readout offers a conversion gain of 0.43 µV/e-, obtaining avalanche gains up to 10³. Several parameters of the CTIA are discussed followed by a perspective on possible hybridization, exploiting the advantages of a 3D-stacked technology.

Single Photon Counting UV Solar-Blind Detectors Using Silicon and III-Nitride Materials.

Ultraviolet (UV) studies in astronomy, cosmology, planetary studies, biological and medical applications often require precision detection of faint objects and in many cases require photon-counting detection. We present an overview of two approaches for achieving photon counting in the UV. The first approach involves UV enhancement of photon-counting silicon detectors, including electron multiplying charge-coupled devices and avalanche photodiodes. The approach used here employs molecular beam epitaxy for delta doping and superlattice doping for surface passivation and high UV quantum efficiency. Additional UV enhancements include antireflection (AR) and solar-blind UV bandpass coatings prepared by atomic layer deposition. Quantum efficiency (QE) measurements show QE > 50% in the 100-300 nm range for detectors with simple AR coatings, and QE ≅ 80% at ~206 nm has been shown when more complex AR coatings are used. The second approach is based on avalanche photodiodes in III-nitride materials with high QE and intrinsic solar blindness.

Are lipid profiles true surrogate biomarkers of coronary heart disease in periodontitis patients?: A case-control study in a south Indian population.

BACKGROUND: There is abundant literature delving into whether periodontal infection contributes to atherosclerosis. However, whether periodontitis is a definite risk factor for atherosclerosis still remains empirical, with no systematic reviews or longitudinal studies to confirm this hypothesis. The prevalence of periodontitis and coronary artery disease also varies among racial and ethnic groups based on various factors such as diet, lifestyle, and genetic predisposition. This study was designed in a south Indian population with the aim of assessing and correlating the lipid levels (a surrogate biomarker for coronary heart disease) in patients with periodontitis and health. AIMS: (1) To assess the levels of total cholesterol, low density lipoproteins (LDL), high density lipoproteins (HDL), and triglycerides in periodontal disease, and health in a south Indian population. (2) To assess associations between elevated lipid profiles and periodontal disease. MATERIALS AND METHODS: This case control study included 60 individuals. Blood sampling for lipid levels and periodontal examination were performed for each study group. STATISTICAL ANALYSIS: Appropriate statistical tools like Chi-square (P<0.05) and student's "t" test were used. The lipid levels were separately regressed using logistic regression to determine any association with periodontitis cases. RESULTS: The differences between the mean lipid levels of cases and controls were not statistically significant (P>0.05) after eliminating confounding factors. Odds Ratio=(Total cholesterol (1.005), HDL (0.971), LDL (1.006), VLDL (0.997), CHO-HDL (1.358), TGL (1.007), LDL-HDL (1.180)). The odds ratio stated that there is no significant relation between the lipid levels and periodontal condition. The above findings confirm that there is still no concrete evidence which determines if periodontitis is an independent risk factor for atherosclerotic cardiovascular disease. Future periodontal interventional studies and assessment of genetic markers can ascertain the validity of this hypothesis. CONCLUSION: There is no association among periodontal disease and the levels of total cholesterol, LDL, HDL, and triglycerides.