PhD Defense: "Ultra-Low Voltage Wide Bandwidth Substrate Removed III-V Electro-optic Modulators"

Selim Dogru

December 6th (Friday), 2:00pm
Engineering Science Building (ESB), Room 2001

We live in times with ever increasing demand for data. The only technology that can deliver required bandwidths is optical fiber communications. A key component of this technology is the external modulator which provides electrical to optical conversion. There are several requirements for optical modulators and among those drive voltage and electrical bandwidth are two key qualifiers. Drive voltage and electrical bandwidth are desired to be as low and as wide as possible respectively to reduce the power consumption, to eliminate modulator driver and take advantage of the ultra-wide bandwidths offered by fiber optics. Such optical modulators are also used in other applications, such as microwave photonics, instrumentation and optical signal processing.

In this dissertation, novel electro-optic modulators in compound semiconductor epilayers using substrate removal techniques are reported. Submicron epilayers removed from their substrates contain buried electrodes made out of doped semiconductors. In this study three important contributions were made to the state of the art of ultra-low voltage modulators. First, a modulator was fabricated out of an epilayer consists of a p-i-n junction in which i layer is composed of an InGaAlAs/InAlAs MQW. Mach-Zehnder electro-optic modulators fabricated using this approach had 0.2 V (0.6 V) Vπ for 3 (1) mm long electrodes at 1.55 µm under push pull drive corresponding to record modulation efficiency of 0.06 V-cm. Second, electrode capacitance and resistance were reduced significantly using staircase waveguides and n-i-p-i-n epilayer designs. 0.8 V drive voltage for a 4.5 mm long electrode which corresponds to 0.36 V-cm modulation efficiency was obtained. Third, traveling wave electrodes suitable for wide bandwidth and low voltage operation were designed using loaded line approach. A very accurate modeling of the electrode was introduced. Such electrodes were fabricated and characterized up to 35 GHz. Theoretical and experimental results indicate that sub volt modulators with electrical to optical bandwidths in excess of 35 GHz are possible. This is the first demonstration of wide bandwidth operation of ultra-low voltage modulators. Furthermore optical scattering loss of waveguides appropriate for ultra-low voltage modulators was reduced to 2 dB/cm and insertion loss was improved 7 dB using novel mode converters.

About Selim Dogru:

photo of selim dogru Selim Dogru received his B.S. degrees in physics and electrical and electronics engineering from Middle East Technical University, Ankara, Turkey, in 2007, and the M.S. degree in electrical engineering from the University of California at Santa Barbara, Santa Barbara, CA, USA, in 2009, where he is currently pursuing a Ph.D. degree with the Electrical and Computer Engineering Department under the guidance of Prof. Nadir Dagli. His current research interests include design, modeling, simulation, fabrication, testing, and analysis of ultra-low drive voltage high bandwidth electro-optic modulators for fiber-optic communications. He made numerous contributions to this field in the form of journal and conference papers in top ranked refereed journals and conferences.

Hosted by: Professor Nadir Dagli