Title
Professor of Electrical Engineering

Department
Division of Electrical Engineering/EECS


Faculty URL
https://www.eecs.berkeley.edu/Faculty/Homepages/chang-hasnain.html

Email
cch@eecs.berkeley.edu

Phone
(510) 642-4315

Fax
(510) 643-7345


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Research Expertise and Interest
semiconductor optoelectronic devices, nano-optoelectronics, broadband communications, high throughput biosensing

Research Description
Variable All-Optical Buffer:, A controllable variable optical memory is one of the most critically sought after components in optical communications and signal processing. In such a buffer, optical data would be kept in optical format throughout the storage time without being converted into electronic format. The buffer must be able to turn on to store and off to release optical data at a very rapid rate by an external command. This seemingly simple function to this date has never been realized, in spite of much previous research.
Recently, we proposed a novel approach of making an all-optical buffer in semiconductors. Our basic idea centers on making a medium that can controllably slow down optical transmission such that it is effectively an optical memory. By controlling the group velocity reduction factor, the memory storage time can be adjusted to desired values. Our approach involves engineering both the material and waveguide dispersion curves. The former can be most effectively achieved with the use of semiconductor quantum dots (QDs) under a mechanism called the electro-magnetically induced transparency (EIT), whereas the latter with periodic structures such as gratings or micro-resonators.
High Speed Diode Lasers Using Optical Injection Locking:, The goal of this program is to investigate diode laser technologies to achieve very high speed (>40GHz), low power dissipation, small form factor, and potentially very low cost. The resulting device would facilitate very high speed chip-to-chip interconnects, which could, in turn, enable faster and more functional processors. In addition, these transmitters could serve as a catalyst for rapid deployment of free space communications for access networks and bring the most sought-after upstream broadband access to vast number of schools and homes. They will be useful for communications among aircrafts, exploration ships, satellites, and other moving vehicles, where power consumption and weight are critical and bandwidth requirement is exceedingly high.
In this program, we investigate optical injection locking as a mechanism to improve laser transmission characteristics. We use a vertical cavity surface emitting laser (VCSEL) as the platform since its topology facilitates cost-effective manufacturing, packaging and testing. We use a CW laser to optically injection lock a VCSEL to enhance its frequency response. The injected CW source increases the stimulated emission recombination, resulting a faster and more effective RF modulation response. We observed drastic reduction in chirp for large signal digital modulation. In addition, a factor of three improvements in resonance frequency was achieved with a flatter modulation response and a higher modulation efficiency improvement. A record large tolerance range in wavelength detuning and alignment tolerance were also reported.