Ph.D. Oral Examination - Rafael I. Aldaz

Rafael Aldaz raldaz at
Thu Feb 22 14:12:17 PST 2007

Ph.D. Oral Examination

*Towards the monolithic integration of mode-locked vertical cavity surface
emitting lasers *

Rafael I. Aldaz

Department of Electrical Engineering

Adviser: Professor James S. Harris

Date: Friday, March 2nd, 2007

Time: 1:30pm (Refreshments at 1:15pm)

Location: CIS-X Auditorium


The speed and performance of today's high end computing and communications
systems have placed difficult but feasible demands on off-chip electrical
interconnects.  However, future interconnect systems may need aggregate
bandwidths well into the terahertz range thereby making electrical
bandwidth, density, and power targets very difficult to meet. Optical
interconnects, and specifically compact semiconductor mode-locked lasers,
could alleviate this problem by providing short pulses in time at 10s of GHz
for Optical Time Division Multiplexing (OTDM) and clock distribution
applications. Furthermore, the characteristic spectral comb of frequencies
of these lasers could also serve as a multi-wavelength source for Wavelength
Division Multiplexing (WDM) applications.   A fully integrated mode-locked
Vertical Cavity Surface Emitting Laser (VCSEL) is proposed as a low-cost
high-speed source for these applications.

The passive mode-locking mechanism of the laser is provided by a
semiconductor saturable absorber integrated together with the gain region.
Such an aggressive integration forces the resonant beam in the cavity to
have the same area on the gain and absorber, placing high demands on the
saturation fluence and absorption coefficient for the saturable
absorber.   Quantum
Wells (QWs), excitons in QWs and Quantum Dots (QDs) have been investigated
as possible saturable absorbers for the proposed device.  QDs have been
found to have the lowest saturation fluence and total absorption, necessary
for meeting the mode-locking requirements for this configuration.   The need
to further understand QDs as saturable absorbers has led to the development
of a theoretical model on the dynamics of this quantum system.   The model
agrees very well with the experimental data obtained and predicts the design
of unassisted ultrafast QD saturable absorbers, without the need to
incorporate recombination center by either ion-implantation or low
temperature growth.
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