Optical waveguide typesThe optical waveguides can be classified according to the core geometry planar – including strip or birefringent (shaped), and cylindrical.
According to the material: glass, plastic and liquid.
According to the refractive index n: gradient or step.
According to the number of guided modes: single-mode and multi-mode.
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A simple planar waveguide consists of three layers, with a middle layer having a higher refractive index than the outer layers. Light beam propagation is confined to one or two directions (planar waveguides). The planar waveguides are used in semiconductor lasers or integrated photonics systems.
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Fig. 1. Planar waveguides | 
| A - waveguide layer B - base C - jacket |
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Cylindrical waveguides with a circular symmetry consist of a core surrounded by a jacket characterized by a lower refractive index. The waveguide is coated with varnish and plastic layers for improved durability and resistance to ambient conditions. 
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Fig. 2. Example cylindrical waveguide | 
| A - core B - jacket C - primary coating D - inner jacket E - strength member F - outer jacket |
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Cylindrical waveguides are used in applications that require fast, lossless transmission of a large volume of data in telecommunication, scientific instruments or industry.
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Table 1. Types of waveguides by core and jacket design (NA - numerical aperture, 2a - core diameter) | Fiber type | Profile | 2a [μm] | n1(r) 0<r<a | Comments | | 1 | without jacket |  | 100 - 1000 | 1,45≤const n1≤1,65 | for preliminary tests of the molten glass; size 2aup to 1 cm | | 2 | two-layer
type W |  | 110 - 250 | const | | | 3 | liquid
(liquid core) |  | 50 - 80 | const | quartz jacket
n2=1,4585
n1=1,483 | | 4 | gradient convex
(gradient) |  | 50 - 500 | n1(r) | 0,27≤[NA]≤0,34
 | | 5 | gradient concave
(gradient concave) |  | 80 - 100 | n1(r) | a<rm<b | | 6 | thin-core single-mode |  | 3 - 10 | const | 0,2≤[NA]≤0,3 | | 7 | thick-core single-mode |  | 30 - 500 | const | 0,2≤[NA]≤0,3 | | 8 | multi-mode |  | 50 - 1000 | const | 0,02≤[NA]≤0,08 |
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Table 1 shows the classification of waveguides based on the correlation between the transmission medium (core) and its coating (jacket). This classification was introduced in 1978 by Prof. B. Paszkowski in Waveguides.
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In the telecommunications and multimedia, only three types of waveguides with different core diameter are commonly used: 9μm – single-mode 50μm and 65μm – multi-mode. The most common jacket diameter is the same for all waveguide types and is 125 μm.
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