Adaptive Pre-Distort.ion for Laser Diodes with Direct Modulation Frequencies up to 1 GHz
Gutwin, Russell G.
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.