PhD Alumni

Ranzani Leonardo

Present position: Research Associate at University of Colorado at Boulder

Thesis title:  An analog approach to dispersion compensation in optical fiber communication systems
Advisor:  Pierpaolo Boffi
Research area:  Optical Communications
Thesis abstract:  
Since the '90s, thanks to the introduction of optical amplifiers and of faster and faster optoelectronic devices, fiber optical communications have been subject to a rapid development. As a result, nowadays optical communications are widely used to transmit digital data at high bit rates.
Various field distortion phenomena in optical fibers contribute to limit the signal maximum propagation distance, the most important being chromatic dispersion, which causes Inter-Symbol Interference (ISI) at the receiver and therefore detection errors.
Different techniques to compensate for chromatic dispersion in optical fiber communication systems have been studied. They can be based on optical devices, such as dispersion compensating fibers, fiber Bragg gratings or optical phase conjugators, or electrical devices. The main disadvantages of optical techniques are high cost and limited tunability. For these reason there has been a growing interest in electrical techniques. Electrical equalizers can be easily integrated in the optical receiver, they are low-cost and easy to tune. The main disadvantage of electrical equalization is the need of an expensive coherent receiver to operate efficiently. The loss of phase information after square-law detection in fact reduces the equalizer performance. For this reason a new approach have been proposed recently: electrical predistortion in the transmitter stage. Digital predistortion can compensate for linear and nonlinear distortion effects, by suitably modulating the optical source at the transmitter in order to pre-compensate for any distortion phenomena in the optical fiber path. Complex (amplitude and phase) modulation of the optical field can be obtained by using a nested Mach-Zehnder modulator, while the predistorted modulator driving signals can be calculated and generated by using digital signal processing and fast digital-to-analog conversion. Electronic predistortion, while being more efficient than electrical equalization in case of direct detection optical communication systems, it's costly because of the high bandwidths required by the transmitter electronics and the difficulty in building fast digital-to-analog converters.
The aim of the present work is to overcome the aforementioned problems by switching from digital predistortion to analog predistortion. Analog microwave devices will be used to achieve dispersion compensation by electrical predistortion at the transmitter. Different kinds of microwave dispersive devices, ranging from long microstriplines to compact chirped delay lines will be used and their performance inside the optical system will be tested and compared.