Principal Modes in Graded-Index Multimode Fiber in Presence of Spatial- and Polarization-Mode Coupling

• Published in: Journal of lightwave technology Vol. 27; no. 10; pp. 1248 - 1261
• Main Authors: Shemirani, M.B., Wei Mao, Panicker, R.A., Kahn, J.M.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 15.05.2009; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Applied sciences; Bends; Bit rate; Circuit properties; Coherence; coherence bandwidth; Computational modeling; Curvature; Electric, optical and optoelectronic circuits; Electronics; Exact sciences and technology; Fibers; Frequency; group delays; high coupling regime; Impulse response; Integrated optics. Optical fibers and wave guides; Joining; low coupling regime; Mathematical analysis; Mathematical models; multimode fiber; Optical and optoelectronic circuits; Optical coupling; Optical fiber communications; Optical fiber polarization; Optical telecommunications; Plastics; Polarization; polarization coupling; Polarization mode dispersion; principal modes; Propagation delay; Silicon compounds; spatial coupling; Systems, networks and services of telecommunications; Telecommunications; Telecommunications and information theory; Transmission and modulation (techniques and equipments); Variance; Multimode fibers; Mode coupling; coherence bandwidth; Single mode fiber; Square root; principal modes; Gradient index; low coupling regime, multimode fiber; polarization coupling; high coupling regime; Bends; Polarization mode dispersion; Polarization mode; group delays; Electric field; Pulse response; Coherence; Optical fiber communication; High bit rate; impulse response; spatial coupling; Group delay; High rate transmission
• ISSN: 0733-8724; 1558-2213
• DOI: 10.1109/JLT.2008.2005066

Power-coupling models are inherently unable to describe certain mode coupling effects in multimode fiber (MMF) when using coherent sources at high bit rates, such as polarization dependence of the impulse response. We develop a field-coupling model for propagation in graded-index MMF, analogous to the principal-states model for polarization-mode dispersion in single-mode fiber. Our model allows computation of the fiber impulse response, given a launched electric-field profile and polarization. In order to model both spatial- and polarization-mode coupling, we divide a MMF into numerous short sections, each having random curvature and random angular orientation. The model can be described using only a few parameters, including fiber length, number of sections, and curvature variance. For each random realization of a MMF, we compute a propagation matrix, the principal modes (PMs), and corresponding group delays (GDs). When the curvature variance and fiber length are small (low-coupling regime), the GDs are close to their uncoupled values, and scale linearly with fiber length, while the PMs remain highly polarized. In this regime, our model reproduces the polarization dependence of the impulse response that is observed in silica MMF. When the curvature variance and fiber length are sufficiently large (high-coupling regime), the GD spread is reduced, and the GDs scale with the square root of the fiber length, while the PMs become depolarized. In this regime, our model is consistent with the reduced GD spread observed in plastic MMF.