Why use a low-index outer layer instead of air for waveguide total internal reflection

[Rymazon]

TL;DR

Could an extra layer be better than air?

I am interested in experimenting with AR lenses and one of the main technologies is a waveguide. They rely on total internal reflection, which needs light to go from a higher to lower refractive index medium. I am reading that current AR lenses use an outer layer with a lower refractive index to allow this to happen but why would this be better than having no outer layer and using the refractive index of air, which is lower than most transparent materials? Thanks.

 

[tech99]

As you will realise there are a number of modes that can propagate in an optical waveguide. By making the transition in refractive index at the boundary gentle, those modes having a small angle of incidence at the interface will not be reflected and will pass through. The waveguide then tends to suppress higher order modes. This is desirable for some applications because the higher order modes propagate more slowly and will tend to blur a short signalling pulse. Step-index and graded index optical fibre are useful for fast digital transmission.

 

[hutchphd]

Please define "AR lens". Do you mean gradient index lens? Or augmented reality filter? Or antireflective coating on glass lens?

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