An overview of the previous project that i have been involved
Developed for EPSRC (EP/K020315/1) , this miniaturised near infrared spectroscopy "tile" consist of 2 sources and 4 detectors in an 30x30 mm2 square shaped encapsulation. Each tile is part of an array of tiles which are easily connected to create an imaging array. The tile has a silicone encapsulation for clinical use and benefits from novel soft silicone rods for better optical coupling and clearing hair. The optical sensitivity of detector is approximately 370 fW which means measuring pico-amps of electrical current on the head.
Danial Chitnis, Robert J. Cooper, Laura Dempsey, Samuel Powell, Simone Quaggia, David Highton, Clare Elwell, Jeremy C. Hebden, and Nicholas L. Everdell, "Functional imaging of the human brain using a modular, fibre-less, high-density diffuse optical tomography system," Biomed. Opt. Express 7, 4275-4288 (2016)
Originally created for the Wellcome OnLight exhibition (London, 2015), it was also featured in UCL Spark festival, it was part of the contribution of UCL BORL group to these public engagement events. The bulb was a hollowed out incandescent bulb filled with lightly scattering silicone, and illuminated from below with a 40W RGB LED. Three large dials on the front the enclosure adjusted the amount of blue, green and red colour intensities. Unlike majority of LEDs which have focused illumination, the visitors experienced a unique visual of colour mixing due to the volumetric scattering occurring inside the bulb. Now, the bulb has a permanent home in UCL Medical Physics museum.
Contributor: Prashanthan Ganeswaran
This multi-wavelength LED was developed as part of EPSRC (EP/K020315/1) project for Near Infrared Spectroscopy (NIRS). Conventional NIRS probes use 2 to 3 wavelengths in their sources. Additional wavelengths increase the accuracy of resolving multiple chromophores in the blood, however adding further wavelengths increases the size of source hence reduces the spatial accuracy of the measurement. As a result, a miniaturised multi-wavelength LED source was developed consisting of eight discrete wavelengths from 650nm to 950nm. The die attachment, wire-bonding, and encapsulation processes were performed in-house.
Danial Chitnis, Dimitrios Airantzis, David Highton, Rhys Williams, Phong Phan, Vasiliki Giagka, Samuel Powell, Robert J. Cooper, Ilias Tachtsidis, Martin Smith, Clare E. Elwell, Jeremy C. Hebden, and Nicholas Everdell, "Towards a wearable near infrared spectroscopic probe for monitoring concentrations of multiple chromophores in biological tissue in vivo", Review of Scientific Instruments 2
In later part of my doctorate at Oxford, for the first time, I utilised the integrated SPAD in free space optical communications. Using numerical simulations, I demonstrated that bit times faster than deadtime can be achieved with an array of SPADs. I implemented a array of 64 SPADs with a fast differential current driven analogue output. Speeds of up to 200 Mbps ON-OFF keying were achieved with a deadtime of 5ns. This proof-of-concept integrated circuit demonstrated that it is possible to use an array of SPADs instead of conventional APDs where integration is advantageous.
Danial Chitnis and Steve Collins, "A SPAD-Based Photon Detecting System for Optical Communications," J. Lightwave Technol. 32, 2028-2034 (2014)
Analogue SPAD pixel
As early part of my doctorate studies at Oxford, I developed an analogue SPAD pixel which utilised the small gap between circular SPADs. The readout was compatible with conventional CMOS pixels. Digital SPAD pixels used a counter to store the number of detected photons. However, this counter occupied a large space relative to the diode. Instead the analogue pixel, stored the number of detected photons as a charge on the integrated metal capacitor which was placed above the quenching circuit. The stored charge was readout as an analogue voltage similar to CMOS pixel arrays.
Danial Chitnis, Steve Collins, "A flexible compact readout circuit for SPAD arrays", Proc. SPIE 7780, Detectors and Imaging Devices: Infrared, Focal Plane, Single Photon, 77801E (18 August 2010)
FPGA Text Overlay
As part of my final BSc project, I opted for a digital design project with FPGAs and Verilog. During the undergraduate laboratories, I had built digital circuits using discrete AND / OR gates on breadboard. This was very cumbersome, and I was intrigued when introduced to the concept of FPGAs. My project was a real-time text overlay on PAL/NTSC video signals. This was useful for annotations and graphics for video broadcasting. I used Xilinx Spartan-3E coded in Verilog and few external components to complete this project.
In early 1998, I was inspired by “The Fractal Geometry of Nature” by Benoit Mandelbrot. I developed a series of software which plotted Iterated Functions system (IFS), Julia sets, and 1/f fractals. At the time I had updated my PC from a Philips 80286 (12.5 MHz) with DOS 4.1 to a Pentium 166 MHz with Windows 95. I wrote my code in new programming language Java 1.1. It took overnight runs to generate A4 size 600dpi (2870 by 6250 pixels) images. However, unfortunately none of the codes run today without a complete re-compiling due major changes that happened to Java applets. I have started to re-write these code in Matlab which will demonstrate the principles of Fractals.