Refractive index contrast explained

Refractive index contrast, in an optical waveguide, such as an optical fiber, is a measure of the relative difference in refractive index of the core and cladding. The refractive index contrast, Δ, is often given by

	2
\Delta={n
	2

\over2

	2}
n
	1

, where n₁ is the maximum refractive index in the core (or simply the core index for a step-index profile) and n₂ is the refractive index of the cladding.^[1] The criterion n₂ < n₁ must be satisfied in order to sustain a guided mode by total internal reflection. Alternative formulations include

	2}
\Delta=\sqrt{n
	2

^[2] and

\Delta={n_1-n₂\overn_1}

.^[3] ^[4] Normal optical fibers, constructed of different glasses, have very low refractive index contrast (Δ<<1) and hence are weakly-guiding. The weak guiding will cause a greater portion of the cross-sectional Electric field profile to reside within the cladding (as evanescent tails of the guided mode) as compared to strongly-guided waveguides.^[5] Integrated optics can make use of higher core index to obtain Δ>1 ^[6] allowing light to be efficiently guided around corners on the micro-scale, where popular high-Δ material platform is silicon-on-insulator.^[7] High-Δ allows sub-wavelength core dimensions and so greater control over the size of the evanescent tails. The most efficient low-loss optical fibers require low Δ to minimise losses to light scattered outwards.^[8]

Notes and References

Web site: Definition: refractive index contrast . www.its.bldrdoc.gov.
Snyder. A.W.. 1981. Understanding monomode optical fibers. Proceedings of the IEEE. 69. 1. 6–13. 10.1109/PROC.1981.11917. 29679745 . 0018-9219.
Book: https://www.sciencedirect.com/science/article/pii/B9780125250962500027. Wave theory of optical waveguides. 2006-01-01. Academic Press. 978-0-12-525096-2. en. 10.1016/b978-012525096-2/50002-7. Fundamentals of Optical Waveguides . Okamoto . Katsunari . 1–12 . 123835110 .
Zhou. J. Ngo. N Q. Ho. C. Petti. L. Mormile. P. 2007-07-01. Design of low-loss and low crosstalk arrayed waveguide grating through Fraunhofer diffraction analysis and beam propagation method. Journal of Optics A: Pure and Applied Optics. 9. 7. 709–715. 10.1088/1464-4258/9/7/024. 2007JOptA...9..709Z. 1464-4258.
Book: Marcuse, Dietrich . Light transmission optics . 1982 . Van Nostrand Reinhold . 0-442-26309-0 . 2nd . New York . 7998201.
Book: Melloni. A.. Costa. R.. Cusmai. G.. Morichetti. F.. Martinelli. M.. Proceedings of 2005 IEEE/LEOS Workshop on Fibres and Optical Passive Components, 2005 . Waveguide index contrast: Implications for passive integrated optical components . 2005. https://ieeexplore.ieee.org/document/1462134. Mondello, Italy. IEEE. 246–253. 10.1109/WFOPC.2005.1462134. 978-0-7803-8949-6. 25058825 .
Melati. Daniele. Melloni. Andrea. Morichetti. Francesco. 2014-06-30. Real photonic waveguides: guiding light through imperfections. Advances in Optics and Photonics. en. 6. 2. 156. 10.1364/AOP.6.000156. 2014AdOP....6..156M . 1943-8206. 11311/863356. free.
Book: Snyder, Allan W. . Optical waveguide theory . 1983 . Chapman and Hall . J. D. Love . 0-412-09950-0 . London . 9557214.