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Submitted for Photon „04 Special Issue High Spatial Spectral Resolution VIS-NIR Scanning Fabry Perot Imager for Geostationary spaceborne applications Viviana Vladutescu, Mustapha Abdulfattah, Fred Moshary, Barry Gross Optical Remote Sensing Lab, City College of the City University of New York New York, New York 10031 Abstract A scanning Fabry Perot Imager design is proposed based on combinations of Fabry Perot etalons and/or broadband interference filters that can in principle be used as a hyperspectral sensors from geostationary spaceborne platforms. In a single stage etalon design, a number of interference filters are required that can be rotated in front of the ethalon to eliminate overlapping orders. A procedure is developed to optimize the number and bandwidth of filters for a given Fabry Perot cavity length and spectral coverage requirements. An alternate approach utilizing multiple etalons in a cascade geometry is also investigated as a means to achieve both high spectral resolution and large free spectral range required for the proposed application. It is shown that the S/N can be made sufficiently large with only moderate decoupling of the etalon cavities. In addition, preliminary studies of the affects of cavity misalignment for both single etalons and multiple etalons are carried out. Keywords: Fabry Perot, interferometry, multiple etalon, spectrometer, I. Introduction limited spectral range to probe a particular spectral line. However, there is no High spatio-spectral space based fundamental limit in adapting this observation of VIS-NIR reflectance is technique to the VIS-NIR portion of the required for a variety of remote sensing spectrum except for the cavity size (< applications including ocean color, land 1um) and the resulting tuning sensitivity. surface classification, and atmospheric On the other hand, for larger cavity monitoring. In conventional approaches, lengths, in broad band (VIS-NIR) the spectral selectivity is accomplished by applications, multiple resonance orders means of interferometry, a series of pass- will overlap and some seperation band filters (channels), or grating based methodology is needed. spectrometers selectively designed to To handle the case of multiple cover the spetral domain of interest for a resonances, we purpose two possible given application [1-7]. More recently, an systems. The first is based on the simple alternative technique based on the use of a approach of designing a set of mid-ir (MIR) Planar Fabry-Perot (PFP) interference filters which will each pass filter with a array imager has been only a single resonance order within the proposed for monitoring tropospheric spectral region of interest. The gases such as ozone [2-3]. Here the main specification of the interference filters in interest was pico-meter resolution over a this case will depend on the specified minimum cavity length. The use of The light collection/reducing telescope multiple interference filters requires a (30 cm primary) has a collimated output rotating filter wheel that can place one beam of 6 cm diameter (5x compression) filter at a time in the optical train, and which will increase the angular spread adds complexity to the design. before the ethalon to 5 times the field of In a second approach, similar to the view of the imager. The most important method proposed in [2-3], a synchronized aspect to observe is that the rays going to set of etalons are employed whose optical different CCD pixels pass the Fabry Perot pathlengths are chosen to allow Filter at different angles and that constructive interference of the periodic differernt angles will affect the spectral pass bands of each etalon at a single response. resonance while destructively canceling To see the magnitude of these affects we other transmission resonances. This can take an idealized imaging system as method effectively increases the free given in figure 2 with 60 mm diameter spectral range of the muti-stage ethalon optics. For a 1” CCD with 512x512 system while allowing for high spectral effective resolution, the CCD field of resolution. Similar to the multiple view (FOV) after an f/# =3 imaging lens interference filter design, a system can be is 50 mrads. Each spatial pixel would designed in this way to operate over the capture an effective FOV of 100 entire VIS-NIR range from 400-800nm. μrad/pixel. For a geostationary satellite at The complexity in this case is to 30,000 km, this would result in a 1200 accommodate synchronized scanning meter/pixel footprint. The FOV while maintaining alignment of multiple specifications are critical since beam ethalon system. anglar dispersion leads to spectral shifts The systems described in this paper are which may be approximately calculated preliminary conceptual instrument 1 2 as max . For the system in figure designs that address the main issues 2 needed for a flexible multi- or hyper- 1, maximum wave number shifts are spectral sensor with high spatial below 1 nm, an order of magnitude resolution from a geostationary platform. superior to current specifications for existing or planned instruments. Even II. Imaging Specifications. then, such spectral shifts may be compensated by pre-calibration with a A conceptual schematic of the imaging narrow laser source. instrument is illustrated in figure 1 10 mrad FP/Filter Imaging elements Lens H 30,000 km x f 18cm ( D 6cm) 30cm 6cm X=1cm 50 mrad CCD Figure 1. Fabry Perot Schematic Figure 2. Imaging angles III. System Configuration III A.1 Results III A. Single cavity Fabry Perot Interferometer in series with a This observation allows us to define a passband filter simple algorithm to determine the passbands for each order that will allow In order to extend the free spectreal range continuous single resonance operation (FSR) from 400-1000 nm using an etalon over the range of interest. The main of reasonable length requires a large point of this algorithm is that the number of synchronized pass- bands to minimum wavelength of a resonance reject multiple resonance orders. To see order n must larger than the maximum the constraints on the pass band filters for wavelength of the next resonance order maintain single (n+1). The procedure is as follows: 1. Input as a limiting value, the minimum FP cavity length Lmin cav 2. Starting at the minimum wavelength of the passband required, determine the minimum resonance number needed for the first filter stage n1 2 Lmin 1 cav min for the desired cavity length Figure 3: Resonance Boundaries of FP 3. Determine the maximum wavelength for the first stage as 1 n1 1 n1 bound max resonance operation, we plot in figure 3, the boundaries of the different resonance where a buffer of bandwidth orders for a given etalon. bound FP is added to account for the finite bandwidth of Simple algebra shows that the single the FP etalon. resonance conditions 4. Once the maximum wavelength of 2 Lmin 2 Lmax the first section is determined, the next section is considered where the n 1 n minimum wavelength of this stage is 2 Lmin 2 Lmax (1) given as n n 1 lead to max n 1 min n min min 1 bound max 2 n n 1 2 Lmax 2L This algorithm is then iterated over the where min max max (2) stages until we fill the entire passband. n n Each filter stage (k) calculated above, would then require an interference filter In this configuration, it is clear that no covering the stage bandwidth. By changes have to be made to the cavity rotating over all interference filters, length sweep. Therefore, all that is complete single resonance coverage is required is a filter wheel mechanism to obtained. select each relevent passband over the In figure 4, we plot both the same operational scan. order of each Fabry Perot stage and the IIIB. Multicavity Fabry Perot cavity length needed to cover each Interferometer Design stage‟s passband assuming that the minimum cavity length is limited to A high resolution single wavelength Fabry Perot Interferometer with no LLimit 2500nm cav interferenc filters will result in multiple We can see that a minimum of 8 stages resonance orders. However, it has been are needed to cover the range from show that multiple cascade etalons can 400nm to 1000 nm where each stage increase the free-spectral range [5]. In covers a subsequently larger spectral our scheme, the wide tunability and the band. need for high side band suppression with S/N > 100 leads us to consider three 14 k=1 k=2 weakly coupled etalons. Reduction of 12 k=3 the coupling due to multiple interference k=4 10 is obtained by suitable placement of a resonance order k=5 8 k=6 k=7 neutral desnity filter. As one can see in k=8 figure 5, the three etalons are positioned 6 in series with the isolating medium 4 attenuating the possible reflections 2 between cavities. 0 t t 400 500 600 700 800 900 1000 wavelength Ein T1T2T3Eint2 Figure 4a Filter stage resonance order for each T1T2T3EinR1R22R3t6 passband T1T2T3EinR12R24R32t10 R1 R2 R3 3000 Figure. 5. Three-etalon series configuration. 2500 section lengths 2000 The transmission of the attenuating 1500 medium between consecutive cavities is 1000 denoted as t, the etalons reflectivities are denoted by Rj and the input spectrum by 500 Ein. The transmissions of each cavity are 0 represented by Tj. 400 500 600 700 800 900 1000 wavelength The derived transmission function for a triple etalon system taking Figure 4b Filter stage length range for each in consideration all multiple reflections passband R1,2,3 between parallel cavities used at normal incidence is given as T1T2T3t 2 2Lp 2 Lq p Lp T (3) res (4) (1 R1R2t 2 )(1 R2 R3t 2 ) R1R3T22t 4 p q Lq q Therefore, we see that a transmission where t is the attenuation of the isolating resonance of the coupled system medium and Rj=(1-Tj) the reflection of requires that the cavity lengths are cavity j. chosen correctly. The lengths are connected by the so called “vernier” ratio whose value is near unity for III.B.1 Simulation Results sufficiently high resonance orders (which is the case for the cavity lengths To see the conditions imposed on the considered). Therefore if p=n and FPIs in series, to maintain single q=n+1, it is easy to prove that all resonance operation, we superimpose in neighboring resonances are eliminated figure 6, the independent etalon until transmission spectra . It is evident that 2L p 2Lq 2 L2 p 2 L2 q res we may tune the cavities so that a (5) particular set of resonances overlap. p 1 q 1 2p 2q 2 However, all neighboring resonances It also follows that the maximum tuning will no longer overlap, greatly range is limited so that λmax<2λmin and supressing any sidelobes. Furthemore, only the region between 400 and 800nm the degreee of supression obviously can operate in a single resonance mode. depends on the resonance linewidth. Operation in this mode requires only a single fixed filter covering the entire passband. The resulting transmission of both a double and triple etalon system are represented in figure 7 where t=.8 between the elements. It is clear that the sidelobe supression increases for three etalons by an order of magnitude. Figure 6. Single cavity etalon transmissions for three different spacings . The three etalons agree on the central resonance tuned to 600nm To examine the limits of single mode operation for a multiple etalon system, we note that at a particular wavelength (in our case 600nm) a (p) order Figure 7 Transmissions of 2 respectively 3 resonance of one cavity will cavities etalons. constructively interfere with the (q) resonance of a second cavity when the On the other hand, it should be pointed following relationship holds: out that as we vary the neutral density filters between the elements, the ratio of the transmission for a single etalon as we passband to sidelobe transmission will vary the length trough one complete change so a tradeoff between S/N and scan. It is clear that for any scan total power must be made. position, multiple resonances will occur. In contrast, in figure 10, implementation of the three etalon configuration completely eliminates and multiple resonances Figure 8 a) Main resonance transmission b) S/N between main and side band. To increase the S/N, it is necessary to decrease the transmission as seen in Figure 10. All multiple resonances eliminated figure 8. For example, a value t=.6 is and single wavelength is almost achieved sufficient for S/N~100. Once the condition is met for one resonance Therefore, we achieve single wavelength wavelength, by synchronizing the length operation over the full bandwidth of shifts in all the cavities to maintain the interest and only one fixed broadband same vernier ratio, we can tune the filter is needed. It should also be noted system over the entire band of interest. that in this configuration, misallignment To illustrate the difference an increased errors will tend to compensate and the FSR will provide using multiple etalons, spectral shifts may be somewhat we plot in figure 9, reduced. IV. Conclusions We illustrate in principle designs for a VIS/NIR Fabry Perot Imaging Spectrometer and develop a proceedure 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 Figure 9 Spectral response of a single etalon. The imaging systems, the spectral shifts in diagonals represent multiple resonances the response across the CCD are occurring with same intensity significant but should be easy to compensate for. We have also shown References that if we use synchronously scanned multiple etalon cavities, we may [1]. Larar A M., Hays P B., and Drayson S. significantly increase the FSR without R “Global tropospheric and total ozone decreasing the bandwidht of the monitoring with a double-etalon Fabry– resonance allowing single mode Perot interferometer.I. Instrument concept”, operation over the 400nm-800nm band ( 1998) with only a single fixed interferenxce [2]. Larar A; and S. Drayson, “Global filter. The S/N between the main tropospheric and total ozone monitoring resonance and the sideband resonance is with a double-etalon Fabry–Perot somewhat impacted by the multiple interferometer. II. Feasibility analysis”, reflections of the cavities but we show Applied Optics, 37, 21 (1998) that a moderate decoupling of the etalons is sufficient to achieve S/N ~100. Trade [3]. Lee J.Y; and J. W. Hahn, H-W. Lee, offs between S/N and exposure time, “Spatiospectral transmission of a plane beam compression versus optical mirror Fabry- Perot interferometer with element size, aperturing and detector nonuniform finite-size diffraction beam S/N still need to be considered but the illumination”, J.Opt.Soc.Am.A 19 #5, (2002). methodology appears quite promising due to it‟s simplicity and flexibility. [4]. Hecht E.; “Optic”s, 3rd Ed. Addison Wesley (1998) Acknowledgements [5]. Mack J.E., McNutt D.P., Roesler F.L., This work was in part supported under and Chabbal R.,”The PEPSIOS Purely the NASA Center for Optical Sensing Interferometric High-Resolution Scanning and Imaging (COSI) under grant number Spectrometer. I. The Pilot Model” (1963) NCC-1-03009. We would also like to thank Raytheon and in particular Jeff [6].Marinelli, W; C. Gittins, A. Gelb, and B. Green, “A Tunable Fabry-Perot Etalon- Puschell for his assistance and support of Based Long-Wavelength Infrared Imaging this project. spectroradiometer” , Applied Optics 38(16), 2594-2604 (2000) [7]. Netterfield R. P., Freund C. H., Seckold J. A., and Walsh C. J., “Design of a lithium niobate Fabry–Perot e´ talon-based spectrometer”(1997)

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Scanning Fabry Perot Imager cavity

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