3.1. Introduction

Thanks to the great strides taken at the very beginning of the twenty-first century, a large number of active and passive functions (emission, reception, filtering, wavelength multiplexing-demultiplexing ) can be operated by means of photonic integrated circuits (PICs). Thus, as demonstrated in the preceding chapter, the necessary basic elements are now available for designing advanced integrated functions for high bit rate optical transmissions. However, to take full advantage of this progress, high quality connections with optical fibers must be achieved. The transmission fibers themselves must also exhibit higher and higher levels of performance. The connection between PICs and standard optical fibers usually involves a taper for adjusting the size of the mode. This is followed by a diffraction grating engraved in the circuit for extracting light. The measured coupling efficiency is higher than 25% and can potentially exceed 80% [MCN 03; TAI 03].

A considerable increase in the transmission bit rate into one single optical fiber has already been made possible, thanks to two major breakthroughs over the past 20 years: first, since 1985, the use of fibers operating in the single mode regime in the spectral bandwidth of the highest transparency of silica (around 1,550nm), and secondly, since 1990, the all-optical amplification of spectrally multiplexed high bit rate signals into rare earth doped fibers.

Nevertheless, there is still much to be done in particular, increasing further the bit rate of optical communication around 1,550nm, and for many other applications ...