February 20, 2017

Optical Communications

Prof. Paolo Serena.

M.Sc. Communication Engineering: 2nd semester – 2nd year.

Course Site

Optical Communications is one of the key segment in nowadays global communication networks. Main features of an optical communication system are the large bandwidth, the low losses and the the ultra-fast bit rates that only photonics can support. Not surprisingly, the growing capacity demand of internet can be satisfied thanks to the optical fibers that allow digital transmissions over long distances, thus connecting the entire earth.

This course covers basic and advanced topics in optical communications. We begin by a brief summary of the ray optics, summarizing the main properties of optical fibers. Then we move to describe the fiber optic channel including linear group velocity dispersion (GVD). Several investigations of GVD will be proposed to clarify its impact on the electric field. Next we examine the amplification process within an erbium doped fiber amplifier (EDFA), both from a physical and a state model point of view. We also analyze the noise figure of an EDFA and its impact on system performance.

The next topic is the detection of an off-keying (OOK) modulated signal, for which we provide an exhaustive analysis of the bit error rate (BER) evaluation in presence of different kind of noise sources (shot noise, amplified spontaneous emission noise, thermal noise). Basic models of direct-detection schemes will be proposed.

In the second part of the course we analyze the nonlinear Kerr effect in optical fibers. We first provide a model for the Kerr effect deriving the nonlinear Schrödinger equation, then we start a detailed analysis of each nonlinear effect separately: self phase modulation (SPM), cross phase modulation (XPM), four wave mixing (FWM), modulation instability (MI), Raman effect. We analyze the interaction of GVD with the Kerr effect, for instance in the special case of the soliton transmission where the two effects counteract perfectly. We will also discuss a numerical algorithm to simulate the nonlinear propagation within optical fibers, showing advanced methods to speed up computation. Such an algorithm is one of the main algorithms of an open source software that will be used in this course by the students.

The next part of the course concerns polarization effects, both in the linear and nonlinear regime. In the first case we analyze polarization mode dispersion (PMD), in the second cross polarization modulation (XPolM).

Finally, we analyze modern optical communication systems based on advanced modulation formats and coherent detection. We also enter in the details of the digital signal processing (DSP) unit and of its main building algorithms.