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OCO ILS Measurement by Step-Scan FTS

OCO ILS Measurement by Step-Scan FTS. Geoffrey C. Toon, 3283 JPL Sep 13, 2005. An outline of how the OCO Instrument Line-Shape (ILS) could be measured using a step-scan FTS. The FTS Approach. Adapt approach applied to AIRS [Strow et al., 2003; Weiler et al., 200?]:

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OCO ILS Measurement by Step-Scan FTS

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  1. OCO ILS Measurement by Step-Scan FTS Geoffrey C. Toon, 3283 JPL Sep 13, 2005 An outline of how the OCO Instrument Line-Shape (ILS) could be measured using a step-scan FTS

  2. The FTS Approach • Adapt approach applied to AIRS [Strow et al., 2003; Weiler et al., 200?]: • Use FTS to modulate the radiation from an internal Tungsten-Filament Quartz Halogen Lamp (QHL) and use OCO as the detector. • Fourier transform the signals from each detector pixel (or aggregate), as a function of optical path difference of the FTS. • Advantages: • All pixels measured simultaneously (in all 3 spectrometers) • Measures broadband white-light response of OCO • Uses incoherent, unpolarized radiation source (QHL) • Also provides an excellent test of detection non-linearity • Broad/Adjustable Free Spectral Range (FSR) up to 15798 cm-1 • Disadvantages: • Requires stepping of FTS to be synchronized with sampling of OCO • Spectrum takes ~10 hours to acquire, limited by 3 Hz OCO sampling.

  3. SPIE-4483-05.PDF

  4. ILS Measurement Requirements • Two types of ILS Measurement: • High spectral resolution (<0.1cm-1), Low Free Spectral Range (1128 cm-1): • Characterize the central portion of the ILS of each pixel • Spectral resolution and spectral sampling of FTS instrument must be at least 4x better than OCO • Signal to noise ratio > 1000:1 • 2) Low Spectral Resolution (<1 cm-1), High Free Spectral range (15798 cm-1): • Characterize the far-wings of the ILS (spectral leaks) • Measure rejection of second-order grating orders • Quantify detection non-linearities (from sum & difference frequencies) • Signal-to-noise ratio > 10,000:1

  5. FTS Primer The spectral resolution of an FTS is inversely proportional to the maximum Optical Path Difference (x) dr = 0.6/x For x = 9 cm, dr = 0.6/9 = 0.067 cm-1 The spectral point spacing is inversely proportional to the FFT size ds = 0.5*L/NFFT For L = 15798 cm-1 and NFFT=2**18 = 262,144, ds = 0.030 cm-1 The Free Spectral Range is inversely proportional to the step length  = 0.5/dx Stepping every laser half-wavelength,  = 0.5/0.317 = 15798 cm-1 Stepping every 7 laser wavelengths:  = 0.5/4.431 = 1128 cm-1

  6. I(x) I() FFT Optical Path Difference (x) Frequency () Interferogram to Spectrum Despite the broad-band illumination from the FTS, the interferograms measured by each OCO pixel will be almost cosine waves (left panel) due to the narrow spectral response (right panel).

  7. ILS Measurement Concept The IFS-66 produces an interferometrically modulated broad-band light beam from an internal quartz halogen lamp. This is detected by OCO. Each OCO detector pixel measures an interferogram. Clock signal Data Out Fourier Transform Spectrometer Integrating Sphere (?) OCO

  8. Candidate FTS: Bruker IFS-66 • Full spectral range coverage from the far infrared up to the vacuum UV • Spectral resolving power > 100,000:1 or <0.1 cm-1 spectral resolution • Outstanding signal-to-noise: • Time resolved spectroscopy: • - More than 100 spectra/sec Rapid Scan at 12 cm-1 spectral resolution - Step-Scan temporal resolution of <10 nsec in the mid IR • Slow Scan with less than 0.006 cm/sec optical velocity • Vacuum optics available: IFS 66v/S

  9. Step-Scan vs Continuous Scan FTS OCO samples too slowly (3Hz) to use continuous-scanning. Required scanner velocity (6 x 0.633µm/2.0s=1.9µm/s) too small. [In continuous scan mode, the IFS-66 has a minimum scan velocity of 0.006 cm/s=60 µm/s] Step-Scan mode avoids some types of phase delay problems And simplifies problems from rolling read-out of OCO pixels

  10. ILS Measurement Details Folding-Frequency (cm-1) (nm) Stepping every 7 He-Ne laser wavelengths (633nm) produces a free spectral range of width 1128 cm-1 Radiation of a frequency of  cm-1 will produce an identical interferogram to radiation of frequency = N*1128.4 + .(N even) or = N*1128.4 - .(N odd) where N is the Alias # To avoid frequency ambiguity, each OCO spectrometer must therefore reject radiation in all but one of these spectral ranges. Alias # ∞ 8862 4431 2954 2216 1772 1477 1266 1108 985 886 806 739 682 633 0 1128 2257 3385 4514 5642 6771 7899 9027 10156 11284 12413 13541 14670 15798 1 2 3 4 5 6 7 8 9 10 11 12 13 14

  11. OCO Spectral Ranges The table below shows that the three OCO bands are all nicely centered within their respective free spectral ranges. For example, the Strong CO2 band will appear in the 5'th alias of the FTS. Energy from outside 4514-5642 cm-1 will be folded back into this range. 

  12. Measurement Duration • High Resolution (0.06cm-1), 1124 cm-1 FSR: • Maximum Optical path difference of >9 cm (AIRS used 2.66 cm). • With a 0.633 micron He-Ne reference laser, we'll need to scan 90,000/0.633=142,000 laser wavelengths (single-sided igram). • Stepping 7 laser wavelengths every 2s (6 OCO samples) will take: • 2s *142,000/7 = 40600s = 11½ hours per interferogram • Low Resolution (0.6 cm-1), Wide FSR (15,798 cm-1) • Maximum Optical Path Difference of 0.5 cm • 5,000/0.633 = 7,900 laser wavelengths • Stepping 0.5 laser wavelengths every 2s (6 OCO samples) will take: 2s * 7,900/0.5 = 31,600s = 9 hours per interferogram

  13. Timing Diagram Path Difference Blue rectangles represent duration of OCO integrations 1/6 or 2/6 integrations will be discarded due to stepping Pixel i+7 Pixel i+6 Pixel i+5 Pixel i+4 Pixel i+3 Pixel i+2 Pixel i+1 Pixel i Optical Path Difference 7λ Time (s) 0.0 2.0 4.0 6.0

  14. Data Processing One or two out of every six OCO data samples will be discarded: those acquired during stepping of FTS. [This will be complicated by the aggregating of pixel data performed by OCO] Remaining samples (4/6) acquired at constant OPD will be averaged. Resulting interferograms will have to be phase-corrected, non-linearity-corrected and FFT’d to produce the spectrum, representing the ILS of that pixel.

  15. Assumptions The OCO clock can deliver a timing signal to the FTS The stepping of the FTS can be triggered by an external clock OCO captures all the data (i.e., it doesn’t “lose” any points) FTS and OCO must both be very stable over a 12-hour period IFS-66V can perform a 9cm OPD interferogram (single-sided)

  16. Lamp Radiance Estimate The IFS-66 uses an internal 150W Tungsten-filament Quartz Halogen Lamp. Assume that the filament has a color temperature of 3000K, a surface area of 50mm2, and an emissivity of 40%. From Stefan’s Law, the total radiance is 0.5E-04*0.4*5.67E-08*30004 = 92 W The remaining 58 W is lost by convection and conduction through the base. The lamp radiance will therefore be: 0.60E+21 ph/s/m2/sr/cm-1 =60 W/m2/sr/cm-1 @ 4,850 cm-1(2.06 µm)=140 W/m2/sr/nm 0.48E+21 ph/s/m2/sr/cm-1 =60 W/m2/sr/cm-1 @ 6,200 cm-1(1.61 µm)=230 W/m2/sr/nm 0.08E+21 ph/s/m2/sr/cm-1 =20 W/m2/sr/cm-1 @ 13100 cm-1(0.76 µm)=345 W/m2/sr/nm Higher radiances can be obtained at 13,100 cm-1 by running the lamp hotter, but this will diminish lifetime.

  17. Etendue of FTS and OCO • Due to the finite etendue or light-grasp (A.Ω) of the interferometer, not all the lamp energy can be modulated. • The IFS-66 has an A.Ω=2.5E-07 m2.sr • OCO will therefore limit the overall system throughput (A.Ω=2.15E-09 m2.sr). • It is normal for a grating spectrometer to have a smaller A.Ω than a FTS of the same spectral resolution (this is the “throughput advantage” of FTS’s). • This is mainly due to the mismatch in the shapes of the FTS and OCO FOV: • FTS produces a circular angular distribution (20 mrad in diameter), • OCO has a rectangular IFOV (0.14 x 1.8 mrad) defined by the slit. • For direct imaging, the modulated spectral intensity received by OCO will be: • 1.3E+12 ph/s/cm-1 = 0.12 µW/cm-1 @ 4,850 cm-1 (2.06 µm) • 1.0E+12 ph/s/cm-1 = 0.12 µW/cm-1 @ 6,200 cm-1 (1.61 µm) • 0.2E+12 ph/s/cm-1 = 0.04 µW/cm-1 @ 13,100 cm-1 (0.76 µm)

  18. Signal Calculation • After accounting for: • Division among 240 cross-track pixels • Polarization (50%) and throughput inefficiencies (50%) • Detector quantum efficiency (0.6-0.7) • Integration Period (0.333 s) • Spectral Resolution (0.3–0.7 cm-1) • the modulated photo-electron fluxes measured by OCO will be: • 1.3E+12*0.3*0.5*0.5*0.6*0.333/240=0.7E+08 e @ 4,850 cm-1 (2.06 µm) • 1.0E+12*0.3*0.5*0.5*0.6*0.333/240=0.6E+08 e @ 6,200 cm-1 (1.61 µm) • 0.3E+12*0.7*0.5*0.5*0.7*0.333/240=0.3E+08 e @ 13100 cm-1 (0.76 µm) • These values are 2-3 orders of magnitude larger than the full-well depths. • The quartz halogen lamp is much brighter than Earth-reflected sunlight ! • Despite the OCO etendue already being ~2 orders of magnitude smaller than the FTS, the signal must be further attenuated (2-3 orders of magnitude). • Use of integrating sphere therefore costs nothing (we have to reduce intensity anyway).

  19. Estimated Signal-to-Noise Ratio • During FTS calibration, the modulated energy from the FTS will be adjusted to nearly full-scale the OCO instrument in all 3 spectrometers. • We have already shown that the FTS puts out sufficient photons to do this, even with the integrating sphere. • For an 0.333s integration, the OCO-recorded interferograms will therefore have a signal-to-noise ratios of: • 200:1 Strong CO2 • 250:1 Weak CO2 • 350:1 O2 • During FTS tests 4/6 OCO integrations will be co-added, doubling the SNR. • These are already averages of the 20 aggregated cross-track pixels.

  20. Estimated Signal-to-Noise Ratio (2) The signal-to-noise ratio in the spectrum (SNR) is related to the signal-to-noise ratio in the interferogram (SNRx) by the equation SNRx = SNR.(2- 1).δx.√2N where N is the FFT size (218 or 214) δx= Step size (=0.5*633 nm or 7.0*633 nm) (2- 1) is the spectral region containing energy (=0.3-0.7) cm-1

  21. Conclusions • Commercial Bruker IFS-66V step-scan FTS could measure the OCO ILS for every (aggregated) pixel simultaneously at: • 0.07 cm-1 resolution, 1128 cm-1 FSR, ~5,000:1 SNR, in 11½ hours • 0.7 cm-1 resolution, 15798 cm-1 FSR, ~100,000:1 SNR in 9 hours • Signal-to-noise limited by well-depth of OCO and available time • Spectral resolution limited by maximum OPD of FTS (9cm) • Duration limited by OCO sampling rate (3Hz) • Modulated signal from FTS will have to be considerably attenuated (e.g. neutral density filters, integrating sphere, diffuser?) • FTS tests could potentially provide useful information on: • Center of ILS (0.07 cm-1 resolution) • Far-wings of ILS and spectral leaks (0.7 cm-1 resolution) • Detection Non-linearity

  22. Concerns The long (~12 hour) durations of the FTS measurements require high stability of the FTS (including its internal QHL and laser), and OCO. FTS is sensitive to vibration (it’s an interferometer) Maintaining co-alignment of FTS and OCO Will take significant time (hours) to convert interferograms into spectra: 1000 spectral pixels x 8 spatial pixels x 3 spectrometers = 24,000 interferograms, each containing ~100,000 points (9.6Gbytes of R*4 data) Any data drop-out, or loss of synchronization between OCO and FTS, will ruin entire test Only data from aggregated pixels can be measured (OCO output data rate limitations). No individual-pixel data. So if pixels die after calibration, this invalidates ILS measurement and forces reliance on an ILS model Lots of work setting up this test (hardware & software) and making it work

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