23 January 2003 Continuous index IR notch filter for atmospheric CO2 suppression
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Rugate optical coatings are films, in which the refractive index is varied in a continuous or quasi-continuous manner. These films are used in order to obtain an optical response that is otherwise difficult to achieve. The application of rugate filters is done by a periodical variation of the refractive index, e.g. sinusoidally. The average index and the index modulation determine the filter bandwidth, and the cycle period determines the notch position. Another unconventional method for filter design, also based on a periodical index variation, is the bragg method, in which notch filters are designed by combining very thin layers of one material in a matrix of another, each unit being λ0 /2 optical thickness. At the Rafael Optical Coating Center, a CO2 suppression notch filter was developed using both these methods. A 24 cycles digital rugate film was used to achieve a narrow notch filter with bandwidth of Δλ/λ=6.8%, the transmittance in the blocked area was 13%, and 90% in the rest of the 3-5μm region. With 58 layers bragg film, the achieved blocking was even better - 6% transmittance in the blocking-band center, with a nearly similar transmission in the non- blocked zone.
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Arit Shinman, Arit Shinman, Nuphar N. Lipkin, Nuphar N. Lipkin, Roy Kirshner, Roy Kirshner, Hedva Zipin, Hedva Zipin, } "Continuous index IR notch filter for atmospheric CO2 suppression", Proc. SPIE 4820, Infrared Technology and Applications XXVIII, (23 January 2003); doi: 10.1117/12.453894; https://doi.org/10.1117/12.453894


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