Demonstration of narrowband notch and multi-notch filters

Narrow notch and multi-notch thin-film filters have applications in many fields. Raman spectroscopy and laser-based fluorescence instruments both desire filters that can remove one or more narrow spectral bands while maintaining high transmittance for light at adjacent wavelengths. Key figures of merit for notch filters include the width of the notch as a fraction of the blocked wavelength (narrower is better), the degree of suppression, and the overall transmittance of the filter. Design approaches for narrowband notch and multi-notch filters are well-known, and include rugate or "quasi-rugate" designs. The manufacturing of these filters has proven to be challenging. The filters have to be very thick to achieve high suppression, and typically involve the deposition of gradient index layers or many very thin, discrete layers. Accurate spectral placement of the notches often requires extreme process control or post-deposition tuning of the filter. JDSU has recently developed a design and manufacturing capability for single and multi-notch filters in the visible wavelength region where the notch width is less than 2% and the blocking levels are greater than OD 6. Designs for these types of filters can be 20 - 60 mu m thick and consist of more than 1,000 layers. Our Ucp-1 high-rate magnetron sputtering platform with load-lock provides an inherently stable deposition process. This enables us to coat these challenging designs. In this paper, we present examples of both single and multi-notch filters that have 612 to 4410 layers and are 31 to 127 mu m thick.