Refractive index contrast

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 ${\displaystyle \Delta ={n_{1}^{2}-n_{2}^{2} \over 2n_{1}^{2}}}$, 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 ${\displaystyle \Delta ={\sqrt {n_{1}^{2}-n_{2}^{2}}}}$ [2] and ${\displaystyle \Delta ={n_{1}-n_{2} \over n_{1}}}$.[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.[7][8]

References

•  This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).

1. "Definition: refractive index contrast". www.its.bldrdoc.gov.
2. Snyder, A.W. (1981). "Understanding monomode optical fibers". Proceedings of the IEEE. 69 (1): 6–13. doi:10.1109/PROC.1981.11917. ISSN 0018-9219. S2CID 29679745.
3. Okamoto, Katsunari (2006-01-01). "Wave theory of optical waveguides". Fundamentals of Optical Waveguides. Academic Press. pp. 1–12. doi:10.1016/b978-012525096-2/50002-7. ISBN 978-0-12-525096-2. S2CID 123835110.
4. 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. Bibcode:2007JOptA...9..709Z. doi:10.1088/1464-4258/9/7/024. ISSN 1464-4258.
5. Marcuse, Dietrich (1982). Light transmission optics (2nd ed.). New York: Van Nostrand Reinhold. ISBN 0-442-26309-0. OCLC 7998201.
6. Melloni, A.; Costa, R.; Cusmai, G.; Morichetti, F.; Martinelli, M. (2005). "Waveguide index contrast: Implications for passive integrated optical components". Proceedings of 2005 IEEE/LEOS Workshop on Fibres and Optical Passive Components, 2005. Mondello, Italy: IEEE. pp. 246–253. doi:10.1109/WFOPC.2005.1462134. ISBN 978-0-7803-8949-6. S2CID 25058825.
7. Melati, Daniele; Melloni, Andrea; Morichetti, Francesco (2014-06-30). "Real photonic waveguides: guiding light through imperfections". Advances in Optics and Photonics. 6 (2): 156. Bibcode:2014AdOP....6..156M. doi:10.1364/AOP.6.000156. hdl:11311/863356. ISSN 1943-8206.
8. Snyder, Allan W.; J. D. Love (1983). Optical waveguide theory. London: Chapman and Hall. ISBN 0-412-09950-0. OCLC 9557214.