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ECE Colloquium, April 25th at 10:00am, in the Hill Conference Room
High-Power Handling, Ultra-Fast, GRIN Lens-Coupled Photodetectors
by Abhay Joshi
Abstract: The performance of microwave photonic systems can be improved by utilizing high power handling photodetectors. Operation at higher photocurrents enables larger output RF signals to be produced directly by the photodetector. This reduces the requirement of signal amplification by RF amplifiers, thereby simultaneously improving the dynamic range and the noise figure. In optical coherent systems, high power handling photodetectors enable operation at high local oscillator power levels to boost the coherent gain and the detection sensitivity. Thus, techniques to enhance the power handling capability of photodetectors are of interest for both free space and fiber based applications.
Photodetector current saturates at high optical power levels due to space-charge screening effect. The saturation effect is maximized where the illumination intensity, and the resulting photocurrent density, is largest. This talk will focus on optimizing the optical field profile incident on top-illuminated InGaAs photodiodes to minimize the peak photocurrent density. This can be achieved by employing graded-index (GRIN) lens coupling to uniformly distribute the optical power across the diode cross-section. Such a scheme does not compromise the responsivity and the bandwidth of the photodiodes. Combined with our proprietary top-illuminated, dual-depletion region device structure, optical beam shaping induced by GRIN lens enables high power handling and linearity in photodiodes.
A 5dB improvement in photodiode's power handling capability and linearity has been experimentally observed in GRIN lens coupled devices as compared to the standard single mode fiber (SSMF) coupled counterparts. Our GRIN lens-coupled photodetectors have achieved small-signal 1dB compression current of >50mA and 12.5dBm amplifier-free RF output. These devices also exhibit linear behavior for a peak-to-peak RF pulse output of >2.5V, at ~30ps pulse width. This constitutes a 100% improvement over SMF coupled devices. Further, the GRIN photodiodes demonstrate pulse broadening =0.65ps/mW, as compared to 2ps/mW for SSMF devices.
The implication of such improvements in photodiode performance will be covered for a diverse range of applications such as phased array radars, photonic oscillators, laser metrology, and photonic analog-to-digital converters.
Biography: Mr. Abhay Joshi received a Master of Science degree in Electrical Engineering from New Jersey Institute of Technology (NJIT), Newark, NJ, in 1989. He has over 20 years of experience in semiconductor device design and testing, specializing in the technology of InGaAs photodiodes (PIN), avalanche photodiodes (APD), balanced photodiodes, and ultra-fast read-out circuits including imagers.
In his early career, Mr. Joshi was a Device Engineer at EPITAXX, Princeton, NJ. From 1988 to 1992, he worked on the development of InGaAs avalanche photodiodes and 256, 512 and 1024 element InGaAs detector arrays for room temperature operation out to 2.6 nanometers. He was "Project Leader" at Epitaxx for European Space Agency's (ESA), "Project Sciamachy," an ambitious satellite project to study global warming. It was launched in 2002 on ESA¿s Envisat. As a project leader, he designed and developed unique 1024 element linear InGaAs detector arrays for 1-3 nanometer wavelength to detect green house gases such as CO, CO2, and CH4 for which he holds a U.S. patent.
After starting Discovery Semiconductors in 1993, he has developed ultra-fast InGaAs photodetectors based on the dual-depletion region structure conceptualized by him. These photodetectors have found use in numerous applications including optical telecommunications and test and measurement systems. Recently, in year 2006, Mr. Joshi designed and delivered space qualified, 10 GHz, ultra-fast InGaAs photodiodes to Tesat Spacecom of Germany. The photodiodes are part of the ALADIN instrument for the ESA¿s AEOLUS mission to measure global wind profiles.
Mr. Joshi holds several patents in the United States, European Union, Japan, Canada, and India. He has written several technical publications and one book chapter.
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