Adaptive Pre-Distort.ion for Laser Diodes with Direct Modulation Frequencies up to 1 GHz
Date
2001-04
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Degree Level
Masters
Abstract
The never ending demand for more information access presents a continuous challenge for most communication service providers. Although optical technologies have already played a significant role in providing for system growth and increased capabilities, the deployment of optical hardware is often dependent on electro-optic device limitations. One important challenge faced is in the provision of fiber-to-the-home (FTTH) services, which might include integrated cable television, telephone and internet access, without neglect for additional future broadband service possibilities. With hope to minimize changes to equipment, data formats, component availability and cost, an acceptable evolutionary answer involves improving the linearity of signal generating light sources.
Residential customers will gradually seek improvements to the broadband services
that are presently offered by low-cost, analogue cable television. Unfortunately,
device fabrication techniques that minimize the non-linear characteristics of a light
source can increase per-unit costs and prohibit optical technologies from application
in general distribution systems. It is, therefore, desirable to explore linearization
techniques that work with inexpensive intensity-modulated laser systems so that any
FTTH service can be economically delivered.
In this thesis, a previously developed model for a semiconductor laser has been
simulated using a SPICE-based software tool. Also, for the purpose of achieving
acceptable linear performance at frequencies up to 1 GHz, a third-order Volterra predistortion circuit has been added to the laser module studied. Manual optimization of the pre-distorter coefficients has reduced the optical 2nd and 3rd harmonic distortion between 600 and 1000 MHz to less than -69 dBc, an improvement from laser-only levels of -42 dBc. These results are especially meaningful in high channel-count common antenna television (CATV) systems where a single-channel optical modulation index of 4 % is required.
By extending the study of the linearized laser, a self-adaptive technique was developed that can appropriately adjust the pre-distorter to laser changes caused by
temperature variation and aging. This adaptive scheme introduced a second, identical
Volterra distortion circuit that was driven by a portion of the laser output and
compared with the initial pre-distorter. Differences between distortion system signals
in the two Volterra circuits were used to alter the pairs of coefficient settings and
adapt the distortion compensation to the need. In this way, the enhanced electrooptic
system has become capable of maintaining low distortion performances during
circumstances where small changes to laser parameters can be expected. Also, the
adaptive design holds promise for reducing certain network service costs, as well as
simplifying the means to manufacture a linear light source component.
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Degree
Master of Science (M.Sc.)
Department
Electrical and Computer Engineering
Program
Electrical Engineering