Design and monitoring of narrow bandpass filters composed of non-quarter-wave thicknesses

Abstract

Narrow bandpass filters have historically been designs of quarter waves at the passband wavelength, and have been monitored at the turning points using the passband wavelength. By direct monitoring at the passband wavelength, errors have been shown to be primarily self compensated, and have allowed much better performance than could otherwise be expected. The turning points are difficult to detect precisely and accurately because the change in transmittance with thickness becomes zero at the desired termination point. By proper design with non-quarter-wave layers, essentially the same spectral performance can be achieved by layer terminations that are far enough from turning points to be significantly more sensitive termination points. The design approach is to maintain the optical thickness of the reflector layer pairs at one half-wave of the passband wavelength, but change the ratio of the optical thicknesses of the high and low index layers. These can be adjusted enough so that the thicker layers contain two turning points and the last turning point in the layer can be more accurately and precisely determined. The error compensation benefit from the historic method should be maintained. This leads to potentially improved control during deposition and monitoring of narrow bandpass filters.

Monitoring error compensation in general optical coatings

“…It can be seen that the four widest layers have three or more TPs within the layer, and two of the medium layers have two TPs within the layers. In [1,2,4], this has been shown to allow more accurate optical monitoring of such layers when terminated at other than TPs.…”

“…As described in [1,2], the percent up or down from the last maximum/minimum, the %Max= Min technique, can be used to monitor any and all layers of general designs such as the VBBAR. The most recent peak and valley of the transmittance monitor signal calibrate the photometric scale, but these last two extrema do not have to be in the same layer.…”

“…The logical optical monitoring strategy would be to terminate the layer at the TP if this happens, and such a TP would be accounted as the one expected in the next layer. The effects of optical termination errors are greatest at TPs, since the change in %T with thickness goes through zero at the TP, so it is beneficial to avoid layer terminations at TPs, which is one of the major points of [1][2][3][4]. If this tends to happen with errors at a chosen monitoring wavelength, it may be advisable to plan to terminate that layer physically (QCM or time=power) instead of optically.…”

“…The thrust of the work in error compensation in optical monitoring has been to demonstrate the benefits of error compensating effects by computer simulation in the monitoring of more general optical coatings than the narrow bandpass (NBF) filters and edge filters that have recently been extensively studied [1][2][3][4]. The NBFs were identified to have error compensation benefits many years ago [5,6].…”

“…References [1][2][3][4] deal with the specific application to NBFs and show the benefits of non-TP monitoring over the historically used TP monitoring. The current work looks to the more general case with specific examples of the VBBAR and ramp coatings.…”