Modal noise in multimode fibers under restricted launch conditions

• Published in: Journal of lightwave technology Vol. 17; no. 5; pp. 817 - 822
• Main Authors: Papen, G.C., Murphy, G.M.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.05.1999; Institute of Electrical and Electronics Engineers
• Subjects: Applied sciences; Circuit properties; Electric, optical and optoelectronic circuits; Electronics; Exact sciences and technology; Fiber lasers; Integrated optics. Optical fibers and wave guides; Interference; Laser modes; Laser noise; Noise measurement; Optical and optoelectronic circuits; Optical fiber theory; Optical fibers; Optical noise; Speckle; Statistical distributions; Systems, networks and services of telecommunications; Telecommunications; Telecommunications and information theory; Transmission and modulation (techniques and equipments); Experimental result; Noise source; Optical transmission; Non gaussian noise; Probability density function; Speckle; Numerical simulation; Modal noise; Multimode fiber; Signal to noise ratio
• ISSN: 0733-8724; 1558-2213
• DOI: 10.1109/50.762898

Modal noise is a non-Gaussian noise source associated with the interference of fiber modes at a detector. Most derivations of the statistics of modal noise assume a limiting case of overfilled launch conditions where there are many modes propagating in the fiber. However, if the transmitted power is contained in a few modes, commonly referred to as restricted-mode launch conditions, the general assumptions for modal noise theory are not valid. Here, we present experimental measurements of the noise distributions for both full- and restricted-mode launch cases and show that the noise statistics for a restricted launch are significantly different than for a full-mode launch. In particular, the variance of the noise distribution is larger than in the over-filled launch case leading to a degradation in the signal-to-noise ratio (SNR). Modifications of existing theory are then used to develop a numerical model for the noise statistics of a restricted-mode launch and good agreement between this model and experimental data is achieved.