Chapter 1
Scenario for all-optical signal processing
In the last years the growing demand for transmission capacity has led to a great effort in optical technologies research. The extent of the achieved results can be estimated by considering that in the early 1990s the highest capacity for commercial fiber-optic links was only 2.5 Gb/s. Few years later the introduction of WDM (Wavelength Division Multiplexing) allowed the multiplication of channels on the same fiber and WDM systems with a total capacity of 40Gb/s became available. In year 2000 DWDM (Dense WDM) systems of 80-160 wavelengths, each one with 10 Gb/s capacity, were available. Adding new WDM channels is not the only solution for increasing the link capacity: another possibility is to increase the bit rate of each channel. This solution is limited by the bandwidth of the electronic devices. To overcome this limitations signals beyond 40 Gb/s have to be processed in the optical domain. In this scenario, the OTDM (Optical Time Division Multiplexing) technique, allowing to optically process the signal along the whole system, is gathering increasing interest and OTDM systems up to 160 Gb/s and beyond have been realized. Signals up to 40 Gb/s are today electronically processed: a photodiode converts the optical signal into an electrical waveform and conventional signal processing (regeneration, demultiplexing, label switching, etc..) is achieved; then a new modulated signal is generated and transmitted, according with the processed signal. So all-optical processing, which is necessary beyond 40 Gb/s, would also be useful at lower bit rate, where the availability of stable and compact all-optical blocks could reduce actual complexity.
All-optical processing schemes are often designed around a simple structure which exploits some nonlinear effects. One of the most widely adopted schemes is the Nonlinear Optical
Chapter 1 – Scenario for all-optical signal processing
Loop Mirror (NOLM) [6], an interferometric structure which can be customized to obtain different behaviours. For example it can be used for pedestal suppression, noise reduction, wavelength conversion, OTDM demultiplexing [4]. It exploits Self-phase modulation or Cross-phase modulation, which are nonlinear phenomena based on Kerr effect: the change in index of refraction induced by the electric field of light as it passes through a medium like crystals and glasses. The Kerr effect has a femtosecond response speed, so it is suitable for ultrahigh-speed optical processing.
The availability of recently developed high nonlinear fibers makes possible to implement simple and compact optical subsystems, requiring a manageable input power and a reasonable fiber length; this was unconceivable before, when kilometers of fiber and extremely high power were necessary.
In this work we will exploit these new fibers in order to realize NOLMs working as all-optical regenerators. A regenerator is a functional block that receives a noisy signal and transmits an undistorted noise-free signal. The most complete regeneration is called 3R regeneration, since it performs Reamplification, Reshaping and Retiming (3R). In many cases 2R regeneration (Reamplification and Reshaping) is sufficient to maintain signal quality. The 2R regeneration requires an optical amplifier followed by a decision element (Nonlinear Optical Gate, NLOG):
Fig.1.1 Characteristic of an ideal Nonlinear Optical Gate.
As we will see in next chapters, NOLMs exploiting new particular fibers can behave as a NLOG and reveal to be promising elements in future optically-processed systems.
