Danial Chitnis

CMOS Photodetectors Integrated With Plasmonic Color Filters

A single-pixel plasmonic complementary metal oxide semiconductor (CMOS) photo sensor consisting of a plasmonic color filter integrated on a CMOS photodiode was fabricated using electron beam lithography and dry etch. The photocurrent measurement results confirmed the three primary color filtering responses that could be achieved in a single layer of nanostructured aluminium film. Finite-difference time-domain simulation demonstrated a good agreement of the reflection spectra with the measured result. This research can lead to the development of advanced CMOS image sensors with low cost and low crosstalk.

A SPAD-Based Photon Detecting System for Optical Communications

A small array of single photon avalanche detectors (SPADs) has been designed and fabricated in a standard 0.18 μm CMOS process to test a new photon detecting system for optical communications. First numerical results are presented which show that using arrays of SPADs reduces the optical power density required at the receiver. Experimental results then show that the new system preserves the photon counting ability of the SPADs. Finally a simple method is presented which can be used to estimate the size of array needed to achieve a particular target bit error rate at a specific optical power density. Together these results indicate that by replacing the avalanche photodiode in a receiver with the new system it will be possible to count the received photons.

Design and Implementation of a Wireless Capsule Suitable for Autofluorescence Intensity Detection in Biological Tissues

We report on the design, fabrication, testing, and packaging of a miniaturized system capable of detecting autofluorescence (AF) from mammalian intestinal tissue. The system comprises an application-specific integrated circuit (ASIC), light-emitting diode, optical filters, control unit, and radio transmitter. The ASIC contains a high-voltage charge pump and single-photon avalanche diode detector (SPAD). The charge pump biases the SPAD above its breakdown voltage to operate in Geiger mode. The SPAD offers a photon detection efficiency of 37% at 520 nm, which corresponds to the AF emission peak of the principle human intestinal fluorophore, flavin adenine dinucleotide. The ASIC was fabricated using a commercial triple-well high-voltage CMOS process. The complete device operates at 3 V and draws an average of 7.1 mA, enabling up to 23 h of continuous operation from two 165-mAh SR44 batteries.

A 200 Mb/s VLC demonstration with a SPAD based receiver

A visible-light-communication system incorporating a receiver containing an array of SPADs is described. Initial results suggest that the receiver is shot noise limited and when OOK is used a data-rate of 200 Mb/s is achievable.

Photocurrent dependent response of a SPAD biased by a charge pump

A diagnostic pill is required to reach parts of the gastrointestinal tract that are inaccessible to endoscopes and detect the fluorescence from cancer cells. The key component of this pill is a very sensitive light detector that is compact enough to be integrated within the pill. In this paper, a charge pump is described that can bias a single photon avalanche detector as effectively as a bench top power supply.

A flexible compact readout circuit for SPAD arrays

A compact readout circuit that can be integrated between individual single photon avalanche detectors (SPADs) is described. The circuit uses minimum sized transistors to both quench the avalanche process in the photodetector and integrate charge onto a capacitor to represent the number of detected photons. An additional transistor is integrated within the circuit so that the integration capacitance can be isolated from the source-follower output circuit before the integration time has elapsed. This device can be used by the user to control the relationship between the number of detected photons and the output voltage.

Broadcasting over Photon-Counting Channels via Multiresolution PPM: Implementation and Experimental Results

Theoretical analysis of multiresolution pulse-position modulation (M/N-PPM) predicts its ability to improve information rate; so far, however, its performance in practice is unknown. In this letter, we report results of a proof-of-concept experiment, in which an M/N-PPM scheme is used for broadcasting over photon-counting channels. We show that M/N-PPM has a potential to improve information rate by demonstrating its ability to superimpose information. We provide results of data rates, bit-error rates (BERs), and discuss some channel impairments that affect link performance.

Compact readout circuits for SPAD arrays

Single Photon Avalanche Diodes (SPADs) create a very short duration voltage pulse whenever a photon is detected. The resulting high sensitivity and temporal resolution makes them useful in a range of applications. In this paper two different analogue circuits are reported that are suitable for imaging applications that can record the number of detected photons in the limited area available between SPADs in a two dimensional array. With further development, these circuits could be used in imagers capable of imaging in very low light levels.

Surface plasmon resonance for digital imaging

We present the design, implementation and test of a CMOS image sensor filter technology based on the exploitation of surface plasmon resonance. We demonstrate precise design and control of the spectral response of filters made in a single aluminium film. The film is patterned using electron beam lithography and reactive ion etching. Unlike conventional filter technology for CMOS, all the desired colour responses can be obtained in a single lithographic sequence. We show that the photocurrent from the sensors has the required spectral response after fabrication.