Abstract

Thelen¹ has developed a simple technique for improving the off-band transmittance of minus filters (or notch filters). This technique involves placing the filter between two substrates with reflective indices equal to the lowest index in the interference filter. Additional layers using the low index and an intermediate index are added on either side of the original layers to form a symmetrical structure. These antireflective layers improve the performance on both sides of the rejection band. A similar technique can be used if the structure of the filter is rugous, i.e., has a sinusoidal variation. Additional sinusoidal periods having an intermediate refractive index can be added on either side of the original rugate filter. The performance can be improved by proper choice of the intermediate index and the number of periods added. This design is somewhat simpler than a similar technique which has been proposed.²

© 1988 Optical Society of America