Learning outcomes of the course unit
The course aims to provide the main tools to analyze and design modern fiber optic communication systems. In particular, the course would like to give knowledge and understanding about:
- linear effects in an optical fiber.
- nonlinear effects in an optical fiber.
- investigation of the transmission/amplification/detection of an optical signal.
- design of a fiber-optic communication system.
- the basic principles of a numerical simulation of an optical link.
With such a knowledge the student should be able to:
- analyze the main distortions of an optical link.
- understand and analyze the main sources of noise that impact the bit error rate of a digital transmission by means of fiber optics.
- find strategies to cope with the above problems
- design a fiber optic link.
- implement numerical algorithms for the analysis of nonlinear systems.
- write a scientific report
suggested basic knowledge of Digital Communications, Signal Processing and electro-magnetic waves.
Course contents summary
Introduction, motivations, state of the art.
Brief introduction of single mode fibers.
Group velocity dispersion.
Principles of Photo-detection.
Performance Evaluation of optical communication systems.
Birefringence and polarization mode dispersion.
Nonlinear effects in optical fibers:
- Self phase modulation.
- Cross phase modulation.
- Four wave mixing.
- Cross polarization Modulation.
Numerical simulation of optical communication systems.
Gaussian noise model.
1) Introduction, Brief history of optical communications.
2) Ray optics. Snell's law. Total reflection. Single-mode fibers (overview).
3) Optical Modulators. ITU-T grid. Review of digital communications and lasers. 3 dB coupler. Return to zero formats. Phase modulation by Mach Zehnder modulators. In-phase/quadrature optical modulators.
4-5) Propagation in optical fibers in the linear regime. Group velocity dispersion (GVD). Attenuation. Group delay. GVD with Gaussian pulses. Dispersion length. Anomalous and normal dispersion. GVD in presence of signal's chirp. Instantaneous frequency. GVD in presence of signal chirp.
6) Third-order dispersion. Eye closure penalty in presence of GVD. The memory of GVD.
7-10) Erbium doped fiber amplifier (EDFA). Cross sections. Propagation equation and Rate equations. Reservoir. Amplified spontaneous emission (ASE) noise. Noise figure of an EDFA. Friis's formula. Optical signal to noise ratio (OSNR). Exercises.
11-12) Photo-detectors: photo-diode. Quantum efficiency. Responsivity. P-i-n photodiode. Avalanche photo-diode (APD). Poisson statistics. Shot noise. Optical Receivers.
13-15) Bit error rate (BER) for on-off keying (OOK) transmission. Quantum limit. Sensitivity power. Thermal noise. Gaussian approximation and Personick's formula. Power budget. The relation between Sensitivity penalty and Eye closure penalty for PIN and APD. Signal to spontaneous and spontaneous to spontaneous noise beat. BER with ASE noise: Gaussian approximation. Comparison of noise variances. Marcuse's formula. Exercises. The noise figure of optical amplifiers measured in the electrical domain.
16-17) The polarization of light. Birefringence. Stokes formalism. Poincaré sphere. Review of unitary and Hermitian matrices. Polarization mode dispersion (PMD). Differential group delay. Principal states of polarization.
18-19) Coherent detection. Optical coupler. Differentially coherent detection: DPSK. Optical hybrid. Balanced detector. Polarization division multiplexing. Digital signal processing at the receiver. Electronic compensation of GVD and PMD. Carrier phase and frequency recovery. Clock recovery.
20) Nonlinear Schroedinger equation (NLSE). Reasons for the cubic nonlinear effect. Self Phase Modulation (SPM). Comparison between temporal/frequency vision of SPM/GVD.
21) Amplifier chains: limitations of ASE noise and nonlinear Kerr effect. Inhomogeneous amplifier chains. Lagrange multipliers method.
22) Solitons. Proof of fundamental soliton. Notes on Higher order solitons and Dark solitons. Problems of Solitons.
23) Wavelength division multiplexing (WDM) systems. NLSE with separate fields. Cross-phase modulation (XPM) and four-wave mixing (FWM). XPM filter. Walk-off coefficient.
24) Split-step Fourier method (SSFM). Formal solution using operators. Non-commutative operators. SSFM with symmetrized and asymmetric step: accuracy. Choice of the step: constant step, step based on the nonlinear phase criterion, step based on the local error.
25-29) The Matlab programming language. Software Optilux. Examples.
30) Regular perturbation (RP) analysis of NLSE. FWM with CW signals. FWM efficiency. Phase matching coefficient.
31-33) Gaussian Noise (GN) model. Applications of the GN model to optical system design.
34) Raman amplification. Memory induced by the Raman effect. SPM, XPM, and FWM in presence of Raman. Raman impact on XPM. Pump-signal case.
35) Cross polarization modulation (XpolM). Comparison XPM vs. XPolM. Digital back-propagation (DBP) algorithm. Impact of optical noise on DBP performance.
36) Lab experience.
Slides of the course are available at Elly web site.
Supporting books are the following:
G. P. Agrawal, "Fiber-optic communication Systems", 3rd ed., Wiley, 2002;
G. P. Agrawal, "Nonlinear Fiber Optics", Academic Press
Supporting scientific papers are indicated at:
Theoretical lectures will be provided mainly by blackboard and sometimes by video-projection of slides available at Elly.
Some exercises will be solved during the lectures. Interaction with students is stimulated by open questions.
Some lectures about Matlab will be given in the computer lab. Some lectures will be provided in the experimental laboratory.
Seminars from experts in the field of optical communications might be possible.
Assessment methods and criteria
The exam consists of an oral examination and in an individual project (max 4 pages with a template). The project regards the study of an optical link by numerical simulation with the software Optilux. Each student receives an individual topic whose investigation will be reported in the final report of project. The student can suggest a topic for the project prior approval by the teacher. The project is evaluated in terms of correctness, completeness, clarity of exposition, bibliography, with a scale 16-30 in case of approval. The oral exam is based mainly on open questions but also on basic exercises with the aim of testing the student understanding of the course arguments and his/her skills in solving engineering problems regarding optical communications. The oral exam, if positive, is ranked between 18 and 30. The final grade is the average of the project and the oral, with honors when the maximum score is reached in both tests.
An optional written mid-term exam, with a duration of 2.5 hours, will be given during the course as well as an optional team activity in the experimental laboratory. The mid-term exam covers almost half program and is based mainly on exercises but also on open questions. Exercises and questions, typically in overall number less than five, are evaluated with a grading scale indicated in the exam. The team activity is evaluated in terms of a written report (max 2 pages) regarding the experiment done in the Optiklab laboratory, with a bonus between 0 and 2.
During the course a numerical simulator of optical links will be introduced