4.1 Extending a Reflection Zone
If the object of a coating was simply to reflect a small-wavelength band, this book would have ended after Chapter 1. Most coating applications require passing the light of one spectral area and reflecting at least one spectral zone, and in many instances there is a requirement to extend the reflection zone. If the requirement is a small stretch of the single-stack zone, the addition of a few pairs of films and an optimization of the thickness of some of the layers provides the most economical use of the coater. For more demanding requirements the width is increased by adding a similar stack at an adjacent band point. A layer added between these stacks is necessary to keep the proper interference relationships in place. The thickness of the layer is a median value of the thickness of the reflectors. Figure 4.1 helps explain the terminology.
The 29-layer design: 1.2375(H L)7 1.2375H (H L)7 H.
H indicates a quarterwave of high-index material; L indicates a quarterwave of low-index material; 1.2375( )7 indicates seven pairs of whatever material is in the parentheses; and each layer is a quarterwave thick at the multiplier's wavelength. 1.2375H means 123.75% of the reference wavelength is where the "H" layer is a quarterwave.