Multi-level modulation formats for high-speed optical transmission systems

Doctoral thesis, 2012

Over the past several years, the use of multi-level modulation formats has enabled significant improvements in the field of optical fiber communication, particularly in terms of per-channel bit-rate (i.e., line-rate) and spectral efficiency (SE), by encoding multiple bits per symbol. This has resulted in a wide range of demonstrations of optical systems with line-rates beyond 100 Gbit/s, which are of great interest for next-generation optical communication. However, as each modulation format exhibits unique characteristics even among those having an equal number of modulation levels (e.g., receiver sensitivity, susceptibility to impairments, and implementation complexity/cost), it is very challenging to specify an optimum modulation scheme for particular applications without appropriate knowledge of modulation formats. As a consequence, the characterization of multi-level modulation formats is regarded as an area of key importance in this research field. This thesis is devoted to demonstrations of high-speed optical transmission systems through the use of various multi-level modulation formats as well as signal characterization, in-depth assessment of several transmission impairments, and performance comparisons among different formats. The work included in this thesis can be divided into two parts, based upon the detection technique used in the receiver. The first part concerns optical systems with differential detection, including the demonstrations of (up to) 0.96 Tbit/s transmission over a single wavelength, impairment assessments of M-ary phase shift keying (M = 2, 4, 8) at the symbol-rate of 40 Gbaud, and a performance comparison of two 8-ary formats at the line-rate of 120 Gbit/s. The second part deals with optical systems with coherent detection, covering the demonstration of 16-ary quadrature amplitude modulation with an SE of 7.2 bit/s/Hz and the performance comparison of two modulation formats for wavelength-division multiplexed systems with an SE of 4.1 bit/s/Hz. In addition, the work included in the thesis addresses the implementation of chirped fiber Bragg gratings for dispersion compensation and their impact in high-speed optical systems with both differential and coherent detection.