Optics and Photonics Journal, 2012, 2, 300-313 http://dx.doi.org/10.4236/opj.2012.24037 Published Online December 2012 (http://www.SciRP.org/journal/opj) Fully On-Chip Integrated Photodetector Front-End Dedicated to Real-Time Portable Optical Brain Imaging Ehsan Kamrani, Frederic Lesage, Mohamad Sawan Electrical Engineering Department, Ecole Polytechnique, Montreal, Canada Email: ehsan.kamrani@polymtl.ca Received August 10, 2012; revised September 13, 2012; accepted September 28, 2012 ABSTRACT Optical brain imaging using functional near infra-red spectroscopy (fNIRS) offers a portable and noninvasive tool for monitoring of blood oxygenation. In this paper we have introduced a new miniaturized photodetector front-end on a chip to be applied in a portable fNIRS system. It includes silicon avalanche photodiodes (SiAPD), Transimpedance am- plifier (TIA) front-end and Quench-Reset circuitry to operate in both linear and Geiger modes. So it can be applied for both continuous-wave fNIRS (CW-fNIRS) and also single-photon counting. Proposed SiAPD exhibits high-avalanche gain (>100), low-breakdown voltage (<12 V) and high photon detection efficiency accompanying with low dark count rates. The proposed TIA front-end offer a low power consumption (<1 mW), high-transimpedance gain (up to 250 MV/A), tunable bandwidth (1 kHz - 1 GHz) and very low input and output noise (~few fA/√Hz and few µV/√Hz). The Geiger-mode photon counting front-end also exhibits a controllable hold-off and rest time with an ultra fast quench- reset time (few ns). This integrated system has been implemented using submicron (0.35 µm) standard CMOS technol- ogy. Keywords: Biochip; Analog CMOS Integrated Circuit; Trans-Impedance Amplifier; fNIRS; Brain Imaging; Medical Imaging; Optical Sensors 1. Introduction Optical sensors and systems are widely applied in bio- logical and biomedical imaging. Optical coherent tomo- graphy (OCT), pulse-oximetry, Brillouin scattering (BLS) imaging, Optical dermatology, and spectroscopy are some examples. Common brain monitoring systems are bulky, non-portable and invasive and require sophisti- cated and expensive hardware and software tools [1], so they are not a proper platform to be developed as a port- able brain imaging system. The commonly used non- invasive brain imaging techniques are electro-encepha- lography (EEG), magneto-encephalography (MEG), po- sitron emission tomography (PET), functional magnetic resonance imaging (fMRI), and functional near-infrared spectroscopy (fNIRS) [2]. Only EEG and fNIRS can be realized using equipment that is small and light enough to be worn continuously while allowing body move- ments. However some portable EEG systems has been de- veloped currently for brain imaging [3], EEG is not ideal for human-computer interface (HCI) [4], it is susceptible to artifacts from eye and facial movement, as well as near by electronic devices, it requires gel in the participant’s hair, it takes time to setup properly and is not spatially determined [5]. We are applying fNIRS to develop a portable tool for real-time brain imaging. fNIRS is a non-invasive, minimally intrusive, safe, and high-tem- poral resolution imaging technique for real-time and long- term monitoring of the brain function and biological tis- sues. It is considered as one of the most efficient diagno- sis and investigation techniques of different neurological diseases, such as, stroke and epilepsy seizures that re- quire continuous monitoring of the patient at the hospital, which is a costly endeavor. In contrast to the other bulky and high-voltage brain imaging systems suffering from electromagnetic interfaces and slight movement artifacts, fNIRS is portable, low-voltage and immune to electro- magnetic interferences with the advantages of ease of use and short setup time [5]. In fNIRS, the brain tissue is penetrated by near-infrared (NIR) radiation and the re- flected signal is observed to investigate the brain func- tion. In NIR range (650 nm - 950 nm), water has rela- tively low absorption while oxy- and deoxy-hemoglobin have high absorption. Due to these properties, NIR light can penetrate bio- logical tissues in the range of 0.5 - 3 cm allowing inves- tigation of relatively deep brain tissues, and ability to differentiate between healthy and diseased tissues based on their optical properties. The typical CW-fNIRS sys- tem consists of NIR light source, photodetector, data Copyright © 2012 SciRes. OPJ