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DOAS without Grating Spectrometers Ulrich Platt and the Heidelberg Group Institute of Environmental Physics (IUP), Heidelberg University INF 229, D-69120 Heidelberg The DOAS Workhorse: Grating Spectrometers Entrance Slit Typical: Czerny Turner arrangement Advantages: Proven, successfull, reliable, known properties (?) Grating Reasons not to use grating spectrometers: • • • • Limited spectral range Overlapping orders Rather low light throughput and high straylight levels Difficult to use in imaging applications Focal Plane I) Imaging DOAS I) 2D - Image Scanning Schemes in a 4-dimensional World Present technology only allows to use 3 Dim., 2 Space + Time A) Pixel at a time “Whiskbroom Imaging" A B) Column at a time “Push-Broom Imaging" B C) Frame at a time (Full Frame) C Dim. Whiskbroom Pushbroom Full Frame X Time Time Space Y Time Space (FP) Space Space (FP) Space (FP) Time* Dispersive spectroscopy possible with „Imaging spectrometers“ 1) non-dispersive spectroscopy Usually dispersive spectroscopy = Wavelength FP = Focal Plane 2) Imaging FTIR The Importance of the “Photon Budget” (100x100 pixels) Signal N 1 Error statistical N N N = Number of detected photons( = photoelectrons) We need all the light we can get Imaging principle F 3.21016 photons/(s m2 sr nm) SO2: 20 ppmm Detector type Etendue E srm2 Photons flux FE 310-8 per pixel 108 per pixelA 410-4 per pixel 4 dispersive 310-8 per pixel 1010 per pixel 410-6 per pixel 0.04 non-dispers. Spectrometer = 0.1 nm A) Whisk-broom /channel scanner Filter + one pixel sensor = 10 nm Acquisition time for (clear sky) 1% noise seconds second-1 Acquisition time for a full Frame seconds B) Push-broom scanner Spectrometer 310-8 per column 106 per pixelA 410-2 per pixel 4 dispersive C) Full frame (SO2 camera) Filter + 2-D sensor array 2.910-8 per pixel 1010 per pixel 410-6 per pixel 410-6 non-disp. „Differential“ Absorption Spectroscopy (may not really be DOAS) Example: The „Trace Gas Camera“ (frequently used as „SO2 Camera“ Four Different Trace Gas Camera Schemes Wavelength-Selective Element: a) two narrow band filters alternatingly being brought into the beam SO2 - Camera b) a Fabry-Perot interferometer with adjustable transmission wavelength FP - Camera c) a narrow-band interference filter, which can be rotated Filter Camera d) a cell (cuvette) containing the gas to be measured, periodically introduced into the light beam Gas Correlation Camera Alternative Position of WSE a) Filter Camera (SO2-Camera): True 2-Dimensional Observation A B Calculate SO2 column-density from ratio of intensities seen through filter A (IA) and Filter B (IB) and some normalisation and correction … UV: Mori & Burton 2006, Bluth et al. 2007 ff. IR: e.g. Prata and Bernardo 2009 b) Fabry-Perot – Interferometer-Based SO2-Spectrometer SO2-Camera Fabry-Perot Spectrometer Setting A Plate Dist. LA Fabry-Perot Spectrometer Setting B Plate Dist. LB Kuhn J., Bobrowski N., Lübcke P., Vogel L., Platt U. (2014), A Fabry-Perot interferometer based camera for two-dimensional mapping of SO2 distributions, Atmos. Meas. Tech. 7, 3705–3714. See next presentation by Jonas Kuhn c) The SO2-Camera, Rotating Filter - Design - non dispersive 0o 35o Problems: 1) Wavelengths are not the same throughout the image 2) image as a whole shifts position as the filter is tilted, Problems can be solved by applying image processing techniques or by adding a “guiding LED” (Benton et al. 2013) Transmitted intensity of an interference filter: Incidence angles 0°(rightmost curve) to 35°(left) in steps of 1°. (Nominal central wavelength (i.e. at 0°incidence angle) is 458nm, however, any central wavelength (e.g. 300 nm) is possible. (from Zielcke et al. 2013) d) Gas Correlation Spectroscopy I0 I0+Plume I0 I0+Cell I0+Cell+ Plume Use in industrial monitoring and on satellites e.g. Ward and Zwick 1975, Sandsten et al. 1996, 2004 Idea: Ratio images recorded with- and without cell with gas to be measured in place Gas cell attenuates less, if gas is present in the atmosphere! e) Imaging DOAS (I-DOAS), Whisk-Broom Example: Etna, May 10, 2005, Louban et al. 2006: SO2 BrO f) Passive IR-Imaging (Thermal Emission) I B e x p D Planck function c2 2h 5 B ,T hc e kT 1 D c x d x S L 0 Emission spectrum instead of absorption, i.e. I() looks like () 1) Michelson interferometer + whisk-broom scanner 2-D images (Svanberg (4) 2002, Rusch and Harig 2010, Stremme et al. 2012). 2) Michelson interferometers + push-broom scanner (i.e. moving platform) 2-D images (Wright et al. 2013) 3) IR-cameras with two or more filters (similar way to SO2 camera) (e.g. Prata and Bernardo 2009) for SO2 retrieval and ash detection (Prata and Bernardo 2014) 4) Gas-correlation spectroscopy in the IR for imaging of ammonia and ethylene (e.g. Sandsten et al. 1996, 2004). 5) Recent development: 2-D Michelson interferometers where instead of a single detector an array of detectors is used. Effectively each pixel can be thought of having its own interferometer. (e.g. GLORIA, Friedl-Vallon et al. 2006, using a 256 x 256 elements Mercury Cadmium Telluride focal plane array cooled to 60 K. (spectral coverage 7.1 m to 12.8 m) II) Active Plume Imaging Techniques Active: Artificial Light Source LIDAR Classic (Mono-Static) LIDAR vs. Bi-Static LIDAR 1) There is no need for a pulsed light source, in fact light emitting diodes in combination with suitable optics can be used. 2) No need for a high time-resolution detector, radiation is received by a telescope – spectrometer combination, see customary active DOAS approach (e.g. Platt and Stutz. 2008). 3) Further possibility: non-dispersive approach with two UV-LEDs emitting at "on absorption" and "off absorption" wavelengths. Example: LEDs emit in the wavelength ranges of filters A, B of a SO2-camera non-dispersive approach. Classic Lidar LASER Detector R Trace Gas or Aerosol - Cloud Bi-Static LIDAR – DOAS (BISL – DOAS) Interesting: Extent of overlapregion varies R2 Compensates 1/R2 6 cm dia. Lens + UV-LED 20 cm dia. Telescope + Detector (Spectrometer) Radiative power budget of a Bi-Static LED-LIDAR System, R=1000m Attenuation Power (W) Photoel./sA Emitted LED-power 1.0 10-3 1.461015 Collimated into beam (F/#1)-optics (ca. 10 cm dia.) 0.25 2.510-4 Scattered in 100m volume 0.015 3.7310-6 Attenuation by scattering 0.74 2.7710-6 Collected by receiving optics (20cm dia.) 2.410-9 6.6410-15 Conversion to photoelectrons 0.25 1.6610-15 2420 1.6610-15 2420 Power fraction Total a photon energy of EP = hc/ 6.8510-19 J at = 290nm (c = speed of light, h = Planck's constant) AAt 24 photoelectrons/(pixel s) For just one LED! (could also use 10 LEDs) 1% O.D. (SO2: 14 ppmm) detectable after 400s integration time SO2-camera 2-wavelength approach (2 LEDs with different wavelengths): 4s Bi-Static LED-LIDAR System - Further Possible Geometries Determination of the Integral cross section of the plume, note that no "light dilution" can occur Geometry for probing the 2-D gas distribution in the plume III) Replace Grating Spectrometers by Prism Spectrometers Dispersion of a Prism: O Total deviation between incoming and outgoing beam: = 1 - 2 - 3 + 4. 3 2 In triangle OAB: = 2 + 3 = 1 + 4 - 1 for small angles we have: A B 4 = (n-n0) 0.5 (since n0 1.0, n 1.5) Rays of polychromatic light will be refracted in different directions with the total angular deviation: dn d d d Collimating Lens Index of refraction n() Camera Lens Focal Plane Entrance Slit Prism f1 f2 dn/d: change of the refractive index with wavelength, the dispersion, which for most glass types is of the order of 10-4 – 10-3 nm-1. Dispersion of Various Types of Glasses dn/d810-4/nm dn/d210-4/nm dn/d10-3/nm Dispersion of some Types of Glass Type of Glass Index of refraction / dispersion (nm-1) 300nm 400nm 600nm 1.553 3.610-4 1.531 1.310-4 1.52 0.410-4 1.87 (370nm) 9.910-4 1.85 6.810-4 1.78 1.410-4 1.58 3.210-4 1.56 1.310-4 1.54 0.410-4 N-BK7 Boro silicate N-SF11 Dense Flint (Schwerflint) Quartz 25x25mm: ca. 80€ BK7: n = SQRT( 1 + 1.03961212*x**2/(x**2-0.00600069867) + 0.231792344*x**2/(x**2-0.0200179144) + 1.01046945*x**2/(x**2-103.560653) ) x = wavelength in micrometers A Sample Prism Spectrometer (1) Assuming prism with apex angle = 60o (1.047 radian) and dn/d 610-4 we have: d dn radian 1.0 6 10 4 6 10 4 d d nm linear dispersion (f2 =100mm): e.g. Dense Flint at 400nm d mm nm 4 1 x f2 6 10 100 0.06 x 17 d nm mm Need a detector with 5 m pixel size and 30 m entrance slit width in order to obtain a spectral resolution of 0.5nm (in the blue spectral region) Collimating Lens Camera Lens Focal Plane Entrance Slit Prism f1=100mm f2=100mm Sample Prism Spectrometer (2) Entrance slit width = 30m 17nm/mm * 0.03 mm 0.51 nm Resolution Notes: 1) Slit could be very high (long) if e.g. a detector with 20 x 20 mm size is used 2) Angle of incidence on refracting surface is about: /2 + /2 23o + 30o 53o Fresnel reflection @ 53o is about 6-7% 90% efficiency! 3) Dispersion curve is highly non-linear Needs to be corrected (linearisation before evaluation should be no problem) Allows very large spectral range with high resolution in the UV low resolution in the visible spectral range. 4) No overlapping orders 5) Resolution could be improved by using several prisms (e.g. two quartz prisms would give 0.5 nm resolution at 300 nm) Collimating Lens Camera Lens Entrance Slit Prism 1 Prism 2 Focal Plane Nonlinear Dispersion Summary • Imaging spectroscopy is an emerging field allowing new applications • In many cases limited spectral information is sufficient • Several new techniques are being introduced: - Imaging FTIR systems - Fabry-Perot Cameras • Techniques, which are already proven in other areas of research can be adapted: - Gas Correlation Spectroscopy - Filter Cameras • Further new techniques have been proposed or are in promising states of development: - LED-LIDAR - Wavelength-sensitive Pixel Detectors - Quantum entanglement imaging (Zeilinger et al. 2014) • Prism spectrometers may offer advantages for „classical“ DOAS applications Many Different Geometries are Possible Example: NO2-measurements using blue (450nm) LEDs Array of 100 3W blue LEDs From: Stephan Flock, Diploma Thesis, U. Heidelberg 2012 Fraction of Received Radiation Intensity and Detection Limit (for NO2 at 440 nm) Relative intensity Light-path 750 m Light-path530 m From: Stephan Flock, Diploma Thesis, U. ca. 10 ppb2012 NO2 detectable at 3 Heidelberg hour measurement time (Flock 2012) 100 Receiving Telescope elevation angle (degrees) Minimum Detectable Mixing Ratio (ppb) Light-path 1360m 1 • 30° Light source elevation angle, • Distance Teleskope – Light Source: 26.85 m • Light source: 300 W (input power) LED-array (about 60 W radiation intensity) Fabry-Perot – Interferometer-Based SO2-Spectrometer SO2-Camera Fabry-Perot Spectrometer Pos. A Plate Dist. LA Fabry-Perot Spectrometer Pos. B Plate Dist. LB Kuhn J., Bobrowski N., Lübcke P., Vogel L., Platt U. (2014), A Fabry-Perot interferometer based camera for two-dimensional mapping of SO2 distributions, Atmos. Meas. Tech. Discuss. 7, 5117–5145 b) The Fabry-Perot – Interferometer Theory: 1) Free spectral Range of a Fabry-Perot interferometer: = Wavelength L = Distance between mirrors n = refractive index of the material between the mirrors (i.e. air, n ≈ 1.0) 2 2nL 2nL (relatively) high reflectance R Low reflectance I0 I Quartz plates L T I I0 F distance between maxima R width of maximum 1 R For (negligible losses in the resonator) and normal Fresnel's surface reflectance, i.e. R ≈ 0.04 we obtain F ≈ 0.65. Transmission T 2) The Finesse F, defined as: (Correspondence to the mirror reflectivity R only holds if the losses in the resonator are negligible) Definition of δλ and Δλ in a Fabry-Perot interferometer. Source: Wikipedia (Ansgar Hellwig) SO2 remote sensing with a Fabry-Perot interferometer additional filter 5 setting A: transmission at maximum SO2 absorption e.g. FPI tilt setting B: transmission at minimum SO2 absorption drastically reduced spectral separation reduced interferences ! e.g. plume aerosol extinction, change in O3 background Kuhn J., Bobrowski N., Lübcke P., Vogel L., Platt U. (2014), A Fabry-Perot interferometer based camera for two-dimensional mapping of SO2 distributions, Atmos. Meas. Tech. 7, 3705–3714. Fabry-Perot - Based Trace-Gas Spectrometer • More Sensitive • Much less susceptible to aerosol (and other trace gases) Optical Density () Fabry-Perot, with and without aerosol Optical Density () conventional SO2Camera with aerosol Optical Density () Konventional SO2Camera without aerosol Kuhn J., Bobrowski N., Lübcke P., Vogel L., Platt U. (2014), A Fabry-Perot interferometer based camera for two-dimensional mapping of SO2 distributions, Atmos. Meas. Tech. 7, 3705–3714. „One Pixel“ FPI SO2 Instrument Preliminary measurement with SO2 calibration cells Jonas Kuhn 2015 Further Application of FP-Instruments Example 1: Measurement of other gases (than SO2) with periodic absorption structures: BrO, IO, OClO, … Example 2: Measurement of HCl Example 3: Measurement of SO2 at 7.3 (e.g. Prata et al. 1989) Replace bulky FT-IR Instruments like HR120 Fabry-Perot – Interferometer-Based Trace-Gas - Camera 2D detector surface optical depth for plate separation LA … LB (here: 4 steps) Each pixel has seen (approx.) pos. LA and LB (with some interpolation) Compose image from ratios τ(LA)/ τ(LB) for each pixel Homogeneous SO2 distribution Source: Kuhn et al. 2014 Fabry-Perot – Interferometer-Based One Pixel SO2 Device Source: Kuhn 2015 Actuator (tilt) Detector Lens FP-Interferometer Quantitative Imaging of Volcanic Plumes - an Overview I) Passive: Natural source of radiation (sun, thermal emission) Passive: Scattered sunlight I I0 exp c L IS I B ,T exp c L c2 2h 5 B ,T hc e kT 1 Spectrometer / Camera Passive: Thermal Emission Planck function II) Active: Artificial light source (LED, Arc-Lamp, LASER) Interferometer / Camera Active: Artificial Light Source Platt U., Lübcke P., Kuhn J., Bobrowski N., Prata F., Burton M.R., and Kern C. (2014), Quantitative Imaging of Volcanic Plumes – Results, Future Needs, and Future Trends, J. Volcanology Geothermal Research, (JVGR, SI on Plume Imaging), available online. LIDAR