Advanced plasma deposition improves ultra-narrowband optical filters

Narrowband filters are a critical technology for a variety of applications such as lidar (light detection and ranging), laser cleanup, chemical and gas sensing, instrumentation, and astronomy. The design principles are well known and relatively simple. All designs rely on stacked Fabry-Perot resonant cavities with dielectric reflectors composed of layers a quarter of a wavelength thick, spaced apart by cavities multiple half-wavelengths across. Several cavity filters are used in combination to ‘square up’ the spectral wave shape, resulting in the transmitted light having a ‘flat-topped’ spectrum when compared with that from light passed through single-cavity filters, which has a sharp, peaked spectral shape. Such multicavity filters also have much steeper rejection responses than single-cavity filters: the lesssteep spectral slopes of single-cavity filters can compromise signal-to-noise in narrowband detection. Creating multicavity filters is a challenge for deposition process control systems. It invariably introduces undesirable ripple (wavelength-dependent variation in attenuation of the signal as it enters the filter), resulting in signal loss in the pass band, the spectral range over which the filter has high transmittance. We use a computer-controlled variation on the turning point method1 of thickness control for each individual layer, where the filter is constantly measured and variations in thickness compensated for, taking into account thickness errors associated with multiple layers. The system allows us to make remarkably reproducible filters with very low ripple, producing a signal shape with steep slopes that consistently match theory. Ultra-narrowband filters are capable of producing images with astounding contrast (see Figure 1). This image of the Sun Figure 1. Solar image using a narrowband H-alpha filter combined with a narrowband etalon.