Design and analysis of a photon counting system using covered single photon avalanche photodiode

A photon counting system using covered single photon avalanche diode (SPAD) based on a standard IC process (0.18 μm) is designed and analysed in this work. The SPAD is formed using the medium voltage (MV) doping layers of the process. To reduce the dark count rate (DCR) in the SPAD, a shaded SPAD with the same structure is fabricated on the same chip which is covered by a metal layer and only providing DCR for the DCR correction. This DCR provided by the shaded SPAD can be also used for the real-time on chip monitoring of some other parameters such as temperature, breakdown voltage and afterpulsing probability. Experimental results show that the SPAD developed is able to detect the visible light from 450 nm to 850 nm with a 35 % peak photon detection probability achieved at around 550 nm with bias voltage of 16 V (excess voltage of 3 V). A timing jitter of 176 ps is measured with an excess voltage of 3 V. The dark count rate in the SPAD tested is about 1.38 cps/μm2 with excess bias voltage of 1 V and 14.62 cps/μm2 with the excess bias voltage of 3 V without the DCR correction. Results also show that a reduction of more than 85 % in the DCR (background noise) can be achieved when the DCR correction is applied resulting in a DCR of 1.68 cps/μm2 with excess bias voltage of 3 V. By monitoring the DCR of the shaded SPAD, the breakdown voltage and temperature of other on chip SPAD can be measured. The potential usefulness of the afterpulsing probability monitoring using the shaded SPAD and the crosstalk probability between SPADs on the chip are analyzed. In addition, the effects of process variations on the SPAD performance is investigated by testing 10 chips with the same SPADs fabricated and potential method is proposed for alleviating the process variations in the SPAD arrays

Nanoplanktonic diatom rapidly alters sinking velocity via regulating lipid content and composition in response to changing nutrient concentrations

Diatom sinking plays a crucial role in the global carbon cycle, accounting for approximately 40% of marine particulate organic carbon export. While oceanic models typically represent diatoms as microphytoplankton (> 20 μm), it is important to recognize that many diatoms fall into the categories of nanophytoplankton (2-20 μm) and picophytoplankton (< 2 μm). These smaller diatoms have also been found to significantly contribute to carbon export. However, our understanding of their sinking behavior and buoyancy regulation mechanisms remains limited. In this study, we investigate the sinking behavior of a nanoplanktonic diatom, Phaeodactylum tricornutum (P. tricornutum), which exhibits rapid changes in sinking behavior in response to varying nutrient concentrations. Our results demonstrate that a higher sinking rate is observed under phosphate limitation and depletion. Notably, in phosphate depletion, the sinking rate of P. tricornutum was 0.79 ± 0.03 m d-1, nearly three times that of the previously reported sinking rates for Skeletonema costatum, Ditylum brightwellii, and Chaetoceros gracile. Furthermore, during the first 6 h of phosphate spike, the sinking rate of P. tricornutum remained consistently high. After 12 h of phosphate spike, the sinking rate decreased to match that of the phosphate repletion phase, only to increase again over the next 12 hours due to phosphate depletion. This rapid sinking behavior contributes to carbon export and potentially allows diatoms to exploit nutrient-rich patches when encountering increased nutrient concentrations. We also observed a significant positive correlation (P< 0.001) between sinking rate and lipid content (R = 0.91) during the phosphate depletion and spike experiment. It appears that P. tricornutum regulates its sinking rate by increasing intracellular lipid content, particularly digalactosyldiacylglycerol, hexosyl ceramide, monogalactosyldiacylglycerol, and triglycerides. Additionally, P. tricornutum replaces phospholipids with more dense membrane sulfolipids, such as sulfoquinovosyldiacylglycerol under phosphate shortage. These findings shed light on the intricate relationship between nutrient availability, sinking behavior, and lipid composition in diatoms, providing insights into their adaptive strategies for carbon export and nutrient utilization

Vortex Beam Encoded All-Optical Logic Gates Based on Nano-Ring Plasmonic Antennas

Vortex beam encoded all-optical logic gates are suggested to be very important in future information processing. However, within current logic devices, only a few are encoded by using vortex beams and, in these devices, some space optical elements with big footprints (mirror, dove prism and pentaprism) are indispensable components, which is not conducive to device integration. In this paper, an integrated vortex beam encoded all-optical logic gate based on a nano-ring plasmonic antenna is proposed. In our scheme, by defining the two circular polarization states of the input vortex beams as the input logic states and the normalized intensity of the plasmonic field at the center of the nano-ring as the output logic states, OR and AND (NOR and NAND) logic gates are realized when two 1st (1st) order vortex beams are chosen as the two input signals; and a NOT logic gate is obtained when one 1st order vortex beam is chosen as the input signal. In addition, by defining the two linear polarization states (x and y polarization) of the input vortex beams as the two input logic states, an XNOR logic gate is realized when two 1st order vortex beams are chosen as the two input signals

Unconstrained Monitoring Method for Heartbeat Signals Measurement using Pressure Sensors Array

An unconstrained monitoring method for a driver’s heartbeat is investigated in this paper. Signal measurement was carried out by using pressure sensors array. Due to the inevitable changes of posture during driving, the monitoring place for heartbeat measurement needs to be adjusted accordingly. An experiment was conducted to attach a pressure sensors array to the backrest of a seat. On the basis of the extreme learning machine classification method, driving posture can be recognized by monitoring the distribution of pressure signals. Then, a band-pass filter in heart rate range is adapted to the pressure signals in the frequency domain. Furthermore, a peak point array of the processed pressure frequency spectrum is derived and has the same distribution as the pressure signals. Thus, the heartbeat signals can be extracted from pressure sensors. Then, the correlation coefficient analysis of heartbeat signals and electrocardio-signals is performed. The results show a high level of correlation. Finally, the effects of driving posture on heartbeat signal extraction are discussed to obtain a theoretical foundation for measuring point real-time adjustment

An Enhanced Plastic Optical Fiber-Based Surface Plasmon Resonance Sensor with a Double-Sided Polished Structure

An enhanced plastic optical fiber (POF)-based surface plasmon resonance (SPR) sensor is proposed by employing a double-sided polished structure. The sensor is fabricated by polishing two sides of the POF symmetrically along with the fiber axis, and a layer of Au film is deposited on each side of the polished region. The SPR can be excited on both polished surfaces with Au film coating, and the number of light reflections will be increased by using this structure. The simulation and experimental results show that the proposed sensor has an enhanced SPR effect. The visibility and full width at half maximum (FWHM) of spectrum can be improved for the high measured refractive index (RI). A sensitivity of 4284.8 nm/RIU is obtained for the double-sided POF-based SPR sensor when the measured liquid RI is 1.42. The proposed SPR sensor is easy fabrication and low cost, which can provide a larger measurement range and action area to the measured samples, and it has potential application prospects in the oil industry and biochemical sensing fields

Displacement Sensor Based on a Small U-Shaped Single-Mode Fiber

A simple structure and easily fabricated displacement sensor was proposed and demonstrated based on a bending-induced fiber interferometer. In the design, the fiber interferometer was formed only by bending the single-mode fiber into a small U-shape without splicing, tapering, or heating pre-processing, which effectively reduces the complexity of the fabrication process, greatly enhances the mechanical strength of the sensor, and lowers the cost in the displacement sensing applications. The displacement sensing performances for the sensor with different bending radii of 3.3 mm, 4.4 mm, 5.0 mm, and 6.3 mm were investigated. Experimental results showed that the sensor had a good linear response, and for the bending radii of 3.3, 4.4, 5.0, and 6.3 mm, the proposed sensors showed high sensitivities of 134.3, 105.1, 120.9, and 144.1 pm/μm, respectively

Liquid Level Sensor Based on a V-Groove Structure Plastic Optical Fiber

A high sensitivity and easily fabricated liquid level sensor based on the V-groove structure plastic optical fiber (POF) was described. In the design, the V-groove structure on the POF is produced by using a die-press-print method, which effectively reduces the complexity of the fabrication process and makes it easier for mass production of liquid level sensors. This greatly enhances the usefulness of the proposed sensor in cost effective liquid level sensing applications. The transmission characteristic of the POF could be changed when the V-groove structure was immerged or emerged by the rising or falling liquid. The liquid level sensing performances for the sensor probes with different structural parameters were investigated, and the sensor performances for the liquids with different refractive indices and the sensor dynamic response were also tested. Experimental results show that the sensor’s sensitivity can reach 0.0698 mm−1, with a resolution of 2.5 mm. Results also show that the sensor has a fast response time of 920 ms

Simulation Study of High Sensitivity Fiber SPR Temperature Sensor with Liquid Filling

In this paper, a high sensitivity fiber temperature sensor based on surface plasmon resonance is designed and studied. In the simulation, the single mode fiber is polished to remove most of the cladding, and then gold and silver films are added. Finally, it is embedded in the heat shrinkable tube filled with a thermo-optic coefficient liquid for curing. The numerical simulation results show that the sensing characteristics are sensitive to the remaining cladding thickness of the fiber, the thickness of the gold film and the thickness of the silver film. When the thermo-optic coefficient of the filling liquid is −2.8 × 10−4/°C, the thickness of the gold film, the thickness of the silver film and the thickness of the remaining cladding of the fiber are 30 nm, 20 nm and 1 μm, respectively. The sensitivity of the sensor designed in this paper can reach −6 nm/°C; this result is slightly higher than that of similar research in recent years. It will have a promising application prospect in flexible wearable temperature sensors, smart cities and other fields