Narrow bandpass filters have historically been optically monitored at the central wavelength of the passband on a deliverable substrate by terminating each layer at a turning point. This has been shown to have error compensating effects which make possible resulting performance which could not otherwise be achieved. Another strategy has been proposed wherein the ratio of the thickness of the high and low index layers of each layer pair, which normally total one half-wave of optical thickness, is changed from 1:1 to a somewhat different ratio while maintaining the one half-wave total layer pair thickness. The spectral details of the modified design differ little from the classic design; those differences are in proportion to the difference of the ratio from the usual 1:1. This can create a situation where there are two turning points within one of the layers and none in the other layer of the pair, and the layer terminations are suitably placed just after turning points. These termination conditions provide an opportunity for greater accuracy and reproducibility in the layer thicknesses. The simulation capabilities of optical monitor software have been used to compare the results of this strategy with that of the classical turning point method in the presence of random noise in the optical and crystal signals.
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