Wearable SiPM-Based NIRS Interface Integrated With Pulsed Laser Source.

We are developing an MR-compatiblepositron emission tomography (PET) detector system comprising silicon photomultipliers (SiPMs), and application specific integrated circuits (ASICs) mounted in the same printed circuit board (PCB). Since the heat generated from the ASICs can affect the performance of temperature-dependent SiPMs, a scalable PCB cooling method for stable and uniform SiPM performance was designed, developed and investigated.To cool the SiPM surface within the PCB, a thermal layer is added below the SiPM layer, which is then connected to the bottom layer through thermal vias for efficient heat conduction. Then, either a “cooling plate” or “square tubes” is attached to the bottom layers of multiple detector modules. A proof-of-concept thermal experiment was performed to test the feasibility of the PCB cooling method for SiPM-ASIC PET detector modules, then a 3D transient heat transfer simulations was performed to analyze the temperature profile on the SiPM surface of the more realistic PCB. The thermal experiments were performed using a prototype PCB with a 6 W heater strip mimicking the ASICs and 15° water flowing through the cooling structure; the plate which has more contact to the PCB resulted in 18.0° average temperature and 0.8° maximum temperature difference across the 5 cm width SiPM layer, while square tubes showed slightly worse results of 18.7° and 1.3°, respectively. In realistic thermal simulations, the square tubes with thermal vias showed deviation from the experimental results with 21° average temperature and 6.5° maximum temperature difference. In addition, the thermal gradient was significantly worse without the thermal vias. Considering the compact and simple geometry of the square tubes compared to the plate configuration, the former with thermal vias are an attractive choice to cool the SiPM-ASIC PET detector modules; ASICs can further optimize the minor non-uniform SiPM performance using digital calibrations.

In this study, a low-cost (with bare chips) and high (optical, electrical, and thermal) performance optoelectronic system with a data rate of 10Gbps is designed and analyzed. This system consists of a rigid printed circuit board (PCB) made of FR4 material with an optical polymer waveguide, a vertical cavity surface emitted laser (VCSEL), a driver chip, a 16:1 serializer, a photo-diode detector, a Trans-Impedance Amplifier (TIA), a 1:16 deserializer, and heat spreaders. The bare VCSEL, driver chip, and serializer chip are stacked with wire bonds and then solder jointed on one end of the optical polymer waveguide on the PCB via Cu posts. Similarly, the bare photo-diode detector, TIA chip, and deserializer chip are stacked with wire bonds and then solder jointed on the other end of the waveguide on the PCB via Cu posts. Because the devices in the 3D stacking system are made with different materials, the stresses due to the thermal expansion mismatch among various parts of the system are determined.

Frequency-domain near-infrared spectroscopy (FD-NIRS) is a noninvasive technique used to characterize in vivo tissue structure and molecular composition by leveraging the deep tissue penetration of red and near-infrared light. However, the size, complexity, expense, and lack of scalability of current FD-NIRS hardware have hindered its widespread clinical adoption and use outside the lab/clinic. The goal of this dissertation project is to address these limitations by developing a compact FD-NIRS device for wearable optical sensing and imaging. We hypothesize that the wearable design will enable quantitative and real-time measurement of tissue composition in an application such as measuring muscle oxygenation for athletes. In this work, we present the first wearable FD-NIRS systems, enabled by custom broad-bandwidth application-specific integrated circuits (ASICs). This is the first time a complete transceiver has been implemented on a chip for a multifrequency FD-NIRS system. Initially, the ASIC was characterized using a benchtop setup for its potential and performance in FD-NIRS measurements. It was then integrated into a single-channel, two-wavelength wearable system for the quantitative measurement of tissue oxygenation. This wearable FD-NIRS system accurately measured tissue optical properties during in vivo arteriovenous occlusion experiments at 14.7~Hz, quantifying oxy- and deoxyhemoglobin concentrations and confirming the functionality of the wearable FD-NIRS technology. In a high-speed configuration, this device was used to sample the cardiac pulsatile waveform at 250Hz. Encouraged by the success of the single-channel FD-NIRS device, a multichannel device intended for diffuse optical tomography (DOT) has been developed. The use of current multichannel FD-NIRS DOT systems are restricted to laboratory environments and are not comfortable for long-term monitoring. We designed our multichannel system for functional near-infrared spectroscopy (fNIRS) brain imaging in a wearable, modular, and fiber-free format to improve comfort and ease of use. This system consists of a high-density optode array with 16 sources and 16 detectors and FD-NIRS measurements based on a compact a vector network analyzer (VNA). A tile-based design approach was adopted, incorporating silicon photomultipliers (SiPMs), vertical cavity surface-emitting lasers (VCSELs), and radio frequency (RF)/direct current (DC) switching. A wearable forehead probe was designed to house these tiles, providing continuous, comfortable measurements and enabling real-time monitoring of brain activity and muscle oxygenation. Preliminary data on tile performance using tissue-simulating phantoms will be presented. We anticipate that our work will ultimately improve the tracking of neurological conditions, the efficacy of treatment, and brain function in both clinical and naturalistic (i.e., outside of the clinic / lab) settings.

The rapid development of information and communication technologies has driven the rapid growth of data traffic and the large requirement of high-speed optical transceiver modules. Usually, optical subassemblies through flexible printed circuit boards (PCBs) connect with the hard PCBs to compose as optical transceiver modules. Because of the impedance mismatch of connection between flexible PCB and hard PCB, the operation bandwidth of the module can be limited. In this paper, the design of connection between flexible PCB and hard PCB in optical subassembly modules will be investigated for increasing the operation speed of the module. First, the transmission bandwidth of flexible PCB is analyzed. High-frequency performances of three transmission line structures, micro-strip line, grounded coplanar waveguide, and top ground micro-strip line, are investigated in flexible PCB. High-frequency performances of flexible PCB with different bending types are also investigated. The flexible PCB has been verified to be suitable to use in 25 Gb/s applications. Next, the high-speed transmission performance is analyzed through the connection between flexible PCB and hard PCB, because the impedance mismatch usually occurs at the connection. The ground pads are added near the signal line of flexible PCB, and the ground pads are connected to the ground layer in PCB through ground vias. The anti-pad in hard PCB is added under the signal solder pad. The dip of S21 response can be eliminated by the optimized ground pad. The impedance mismatch can be reduced by the optimized anti-pad, the impedance increases from 40-Ω to 49-Ω at the connection between flexible PCB and hard PCB, and the transmission bandwidth can increase from 16.5 GHz to 23.5 GHz. TO-46 header is a common and low-cost packaging solution for DFB laser and photodiodes. The connection between flexible PCB and TO-46 header is also investigated. While the connection has a gap, the dip of frequency response will occur at lower frequency. While the connection is very close, the bandwidth can be larger than 25 GHz. The flexible PCB, hard PCB, and TO-46 header are combined together, and the 23 GHz transmission bandwidth can be obtained. The 25 Gb/s transmission eye diagram is also measured, and the eye height, rise time, fall time, and Q-factor are 268.14 mV, 13.78 ps, 13.56 ps, and 8.76, respectively. The combined flexible PCB, hard PCB, and TO-46 header are verified to be suitable to apply in 25 Gb/s optical subassembly modules, and will be able to be used in 100G Ethernet.

This work describes a dual-rate optical transceiver designed for power-efficient connections within and between modern high-speed digital systems. The transceiver can dynamically adjust its data rate according to the performance requirements, allowing for power-on-demand operation. To implement dual rate functionality, the transmitter and receiver circuits include separate high-speed and low-power datapath modules. The high-speed module is designed for gigabit operation and optimized to achieve the maximum bandwidth. A simpler low-power module is designed for megabit data transmission and optimized for low power consumption. The transceiver was fabricated with a 0.5μm Silicon-on-Sapphire (SOS) CMOS technology. The vertical cavity surface-emitting lasers (VCSELs) and photodetector devices were attached to the transceiver IC using flip-chip bonding. A free-space optical link system was set up to demonstrate power-on-demand capability. Experimental results show reliable link operations at 2Gb/s and 100Mb/s data transfer rates with about 104mW and 9mW power consumption, respectively. The transceiver’s switching time between these two data rates was demonstrated at 10μs which was limited by on-chip register reconfiguration time. Improvement of this switching time can be obtained by using dedicated IO pads for dual-rate control signals. At the circuit level, the incorporation of dual rate functionality into a typical gigabit optical transceiver would require 255 additional MOS transistors.

Current evolution in Datacoms and Gigabit Ethernet have made 850nm Vertical Cavity Surface Emitting Lasers (VCSEL) the most important and promising emitter. Numerous different structures have been growth, to obtain best current confinement and then to control the emitted light modal behavior. We have developed a small signal equivalent electrical model of VCSEL including Bragg reflectors, active area, chip connection and noise behavior. Easy to integrate with classical software for circuit studies, this model which is widely adaptable for different structures takes into account the complete electrical environment of the chip. An experimental validation for RF modulation up to 10 GHz has been realized on oxide confined VCSEL, demonstrating that the model could be used to get realistic values for the VCSEL intrinsic parameters. Including Langevin noise sources into the rate equations and using the same electrical analogy, noise current and voltage sources can be added to the model. It allows good prediction for the RIN function shape up to 10GHz for monomodal emitter.

On the basis of a realized 12times10 Gb/s card-to-card optical link demonstrator, the capabilities of a polymer-waveguide-based board-level optical interconnect technology are presented. The conception and realization of the modular building blocks required for this board-level optical interconnect technology are described in detail. In particular, we report on the fabrication and characterization of board-integrated optical low-loss polymer waveguides that are compatible with printed circuit board (PCB) manufacturing processes. We also explain our fully passive alignment technique, superseding time-consuming active positioning of components and connectors. To realize optical transceiver modules comprising vertical cavity surface emitting laser (VCSEL) arrays with laser drivers and photodetector arrays with transimpedance amplifiers (TIAs), a mass-production concept based on wafer-level processing has been elaborated and successfully implemented.

In this age of ever increasing data rates in communication systems, optics are becoming more commonplace for long length (>10m) signal transmission in High Performance Computing (HPC) systems due to their bandwidth capabilities which are higher than their electrical counterparts. In these optical based communication systems, Vertical Cavity Surface Emitting Lasers (VCSELs) are the most commonly used communications lasing medium for multimode fiber applications. These lasers are active in the 850 nm region, with speeds commonly at 10 Gbps/channel. VCSEL vendors are now commercializing lasers at 25 Gbps/channel as well, with research groups actively pursuing rates beyond 40 Gbps/channel, demonstrating that these communications technologies will likely be continuously incorporated in multiple generations of HPC systems. For optical based signaling technologies, fiber optics are typically utilized as the medium of choice for point to point contacts due to their low loss characteristics, stability to thermal degradation and aging, and manufacturability. Fiber optics have become a commodity, making them inexpensive. However, the high precision connection technologies required to bring light off of VSCELS and fiber to fiber are still rather expensive, limiting some of the optical applications in products. Polymer waveguides offer the promise to act as an enabling technology to provide highly complex optical routing that can be passively connected, lowering system costs and allowing next generation systems to be optically driven. For this to become a reality, polymer waveguide materials must meet multiple requirements in the communications industry. The focus of this thesis is on the understanding and optimization of the manufacturing requirements of polymer waveguide materials, their optical stability to existing standards such as Telecordia, and the impacts that printed circuit board processes have on them.

An optical waveguide interconnect mesh network scheme for parallel multiprocessor systems based on an electro-optical printed circuit board (EOPCB) with multimode polymer waveguide is proposed.The system consists of 2×2 processor element chips interconnected in a mesh network configuration.An additional layer with optical waveguide structure is embedded in a conventional printed circuit board to construct the EOPCB.Vertical cavity surface emitting laser (VCSEL)/positive intrinsic-negative (PIN) arrays are applied as the optical transmitters/receivers.Three 1×12 VCSEL/PIN parallel optical transmitting/receiving modules are used to provide 32 input/output optical channels required by the 2×2 chip-to-chip optical mesh interconnect system.The data rate in each optical channel is 3.125 Gbps and thus 10 Gbps parallel optical interconnect link for each direction of a chip is obtained.The optical signals from a processor element chip can be transmitted to another chip through optical waveguide interconnect embedded in the board.Thus the optical interconnect mesh network for parallel multiprocessor system can be implemented.

Characterization of SiPM arrays in different series and parallel configurations

The DarkSide-20k dark matter direct detection experiment will employ a $${21}\,\hbox {m}^{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>21</mml:mn> <mml:mspace/> <mml:msup> <mml:mtext>m</mml:mtext> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> </mml:math> silicon photomultiplier (SiPM) array, instrumenting a dual-phase 50 tonnes liquid argon Time Projection Chamber (TPC). SiPMs are arranged into modular photosensors called Tiles , each integrating 24 SiPMs onto a printed circuit board (PCB) that provides signal amplification, power distribution, and a single-ended output for simplified readout. $$16$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>16</mml:mn> </mml:mrow> </mml:math> Tiles are further grouped into Photo-Detector Units (PDUs). This paper details the production of the Tiles and the Quality Assurance and Quality Control (QA-QC) protocol established to ensure their performance and uniformity. The production and QA-QC of the Tiles are carried out at Nuova Officina Assergi (NOA), an ISO-6 clean room facility at LNGS. This process includes wafer-level cryogenic characterisation, precision die attaching, wire bonding, and extensive electrical and optical validation of each Tile. The overall production yield exceeds 83.5%, matching the requirements of the DarkSide-20k production plan. These results validate the robustness of the Tile design and its suitability for operation in a cryogenic environment.

In this paper, we present a dedicated charge division circuit (CDC) as readout system for wireless detectors based on silicon photomultiplier (SiPM) arrays coupled to scintillators. In the proposed readout circuit, the SiPM output current is split into two symmetric signals by a pair of diodes, so it was called diode coupled symmetric charge division (DCSCD). The circuit was investigated using LTSPICE, fabricated as a printed circuit board and characterized. Its performances were compared with that of the traditional discrete positioning circuit (DPC) scheme. Testing the two CDC networks on a $16\times 28$ array of SiPMs, we found that image reconstruction is significantly compromised when the DPC configuration is connected to few SiPMs, while the diode-based CDC configuration presents a very high-quality image histogram independently on the number of the connected photosensors. The proposed electronics features very low-power consumption when compared with commercial solutions and it is, therefore, suitable for battery-operated device, which requires a power-efficient design. Indeed, the DCSCD electronics requires less than 10 mW of static power to read out the $16\times 28$ array of SiPMs, demonstrating the efficacy of the proposed diode-based CDC as passive readout circuitry and opening the route for the development of wireless gamma camera systems.

Diffuse optics is a powerful tool for clinical applications ranging from oncology to neurology, but also for molecular imaging, and quality assessment of food, wood and pharmaceuticals. We show that ideally time-domain diffuse optics can give higher contrast and a higher penetration depth with respect to standard technology. In order to completely exploit the advantages of a time-domain system a distribution of sources and detectors with fast gating capabilities covering all the sample surface is needed. Here, we present the building block to build up such system. This basic component is made of a miniaturised source-detector pair embedded into the probe based on pulsed Vertical-Cavity Surface-Emitting Lasers (VCSEL) as sources and Single-Photon Avalanche Diodes (SPAD) or Silicon Photomultipliers (SiPM) as detectors. The possibility to miniaturized and dramatically increase the number of source detectors pairs open the way to an advancement of diffuse optics in terms of improvement of performances and exploration of new applications. Furthermore, availability of compact devices with reduction in size and cost can boost the application of this technique.

The system performance of data- and telecommunication equipment must keep up with the increasing network speed. Optical interconnections technology is a promising alternative for high-throughput systems. We demonstrate the optical backplane system using a waveguide-embedded optical backplane and two processing boards. The transmitter and receiver modules were prepared for optical printed circuit boards (PCBs), which consists of the metal optical bench, the driver chips, vertical-cavity surface-emitting lasers (VCSELs), photodiodes, and a tapered polymeric waveguide. We report high-speed transmission of 27–1 pseudorandom bit sequence (PRBS) nonreturn to zero (NRZ) data up to 10 Gbits/s through the optical backplane system. The results demonstrate that the optical backplane system can be practical and valuable for the future high-throughput systems by using metal optical bench and precisely machined optical plug-adaptor structure to achieve stable board-to-board interconnection.

In this paper, design and implementation of a 4.25Gb/s optical fiber transceiver for Gigabit Ethernet (GbE) applications based on IEEE802.3z standard using Multi Mode Fiber (MMF) is described. Transmitter consists of an 850 nm Vertical-Cavity Surface-Emitting Laser (VCSEL), laser driver and transmission lines and receiver consists of a PIN photodiode, amplifiers and transmission lines. Differential microstrip lines and single-ended Conductor Backed Coplanar Waveguides (CBCPWs) on a FR-4 Printed Circuit Board (PCB) are implemented. To evaluate the performance of the implemented optical transceiver, as the results, the measured eye diagrams and Bit Error Rate (BER) measurements at the transmitter and receiver outputs are presented.