Scanning Fabry Perot Imager Viviana Vladutescu, Mustapha Abdulfattah, Fred Moshary, Barry Gross Optical Remote Sensing Lab, EE Dept, NOAA-CREST, CCNY Jeff Puschell; Raytheon -Develop a compact, tunable imager that can easily be tuned through the VIS -Covering the entire VIS region between 400nm and 1000nm with a unique -transmission resonance would require adjustable cavity spacings which are very small < 400nm technologically challenging. -By introducing a set of specially designed broadband filters in conjunction with a multi cavity Fabry Perot Filter, single wavelength operation can be obtained. Mathematical Theory of Fabry Perot Filters Transfer Function The Complex Transfer function HFP Contours shows 99% through put line giving absolute design constraints to avoid spill over (in frequency and space) Normalized frequency shift with respect to the resonance Note asymmetric shift of resonance with Increased spatial harmonics Spatial Spectral Response X=0.5c m Fabry Perot Imager Wavelength boundaries Resonance Order Single Resonance Condition For the nth resonance to not interfere with neighboring n+1 and n-1 resonances Note that the wavelength range of a single resonance domain decreases As higher order resonances are used. Therefore, larger cavities meet with smaller wavelength coverage for the same Algorithm to determine resonance orders for given length For sufficiently large cavity lengths the same cavity scan can be used for each stage with each stage implemented in a different resonance order. Single Cavity Design E in T 1 T 2 T 3 E in t 2 T 1 T 2 T 3 E in R 1 R 2 2 R 3 t 6 tt R1R1 R2R2 R3R3 T 1 T 2 T 3 E in R 1 2 R 2 4 R 3 2 t 10 If reflections between etalons With appropriate Optical Isolation So the lengths are connected by the so called “vernier” ratio If p=n, q=n+1, it is easy to prove that all neighboring resonances are eliminated until Therefore, maximum tuning range is limited. So Transmission of each etalon tuned to 600nm Etalon Transmissions T Single Etalon (Multiple Resonance) All multiple resonances eliminated except for nearest neighbors which appear as small side lobes Using p=8, q=9 Double Etalon and Triple Etalon respectively Optical design Using p=8, q=9,r=10 Assuming Etalons are separated by lossless t=1elements Multiple reflections between etalons Assuming Etalons are separated by lossy t=.85 medium We illustrate that in principle a VIS/NIR Fabry Perot Spectrometer can be developed and develop a design program to calculate the characteristics of the needed pass band filters. It is shown that a set of 8 cascading filters can be used to cover the spectral range from 400nm to 1000nm with the same sweep. It is shown that for sufficiently small imaging systems, the spectral shifts in the response across the CCD are significant but should be easy to compensate for. As well we have proved the advantages of a tri-etalon over single cavity as it is not requiring a filter attached to the system. The three cavities alone assure single wavelength operation with the right selection of the cavity spacings. Conclusions Determine as design the smallest FP cavity constructible. For the smallest wavelength in the k th filter section, determine the minimum resonance needed for the desired cavity length where for the first section, the smallest wavelength is used. Determine the maximum wavelength in the k th section where a buffer is inserted. This section would then be placed in front/behind a pass band filter covering the section bandwidth making single resonance operation possible. Once the maximum wavelength of the k th section is determined, the k+1 section is considered. A high resolution single wavelength Fabry Perot Interferometer with no filter to join the system was only proofed by now to be achievable by use of three etalons in series. The restrictions due to the free spectral range of one etalon with no additional filter can be overcome by the three parallel cavities in series with an absorbing medium between them. Multicavity Fabry Perot Interferometer Alternative Design