Fabry-Perot Interferometer for atmospheric monitoring useful for EAS detection E.Fokitis 1, K. Patrinos 1, Z. Nikitaki 1 ABSTRACT A piezotunable Fabry-Perot interferometer is studied as a candidate Doppler Lidar for determining the aerosol to molecular ratio for use in FD atmospheric monitoring calibration. 1 Physics Department National Technical University of Athens Zografos Campus 15780 GREECE --- Submitted to the Leeds Workshop, July 2004
1. Introduction The use of Doppler Lidar for determining the aerosol to molecular ratio is described in reference [1]. In this report we try to present what possibilities exist in assembling a necessary experimental setup to do just these measurements. We also present possibilities to use Fabry-Perot interferometer to obtain useful data for the EAS event analysis. When used in conjunction with a Laser LIDAR, the combination is known as Doppler Lidar since the technique relies on the very different spectral shift causing to the scattered laser light by aerosol particles and air-molecules. The determination of atmospheric parameters can be useful in both EAS Fluorescence detectors and water Cherenkov surface detectors. The Fluorescence detector data can be corrected for aerosol scattering effects mainly on the air- Cherenkov signal since the Fabry-Perot interferometer (FP) can allow an accurate aerosol to molecular ratio from interferometric data analysis. On the other hand, the FP in conjunction with a Lidar can give the air-temperature as a function of atmospheric height and lead in, event by event basis, useful air-temperature data necessary for accurate simulation of the EAS signal, i.e. the interferometer data are input to EAS CORSICA code as alternate set of data instead of data of radiosondes used for temperature profiles. One may consider, before proposing the interferometer use, the advantages, in terms of performance and cost, of using this method. In the following, we present the progress in the assembly of an appropriate FP etalon together with design and performance issues. In addition, depending on the level of sophistication demanding from such Doppler Lidar system, one should consider if the technique is mature enough to be used in existing experiments (Auger South) or it should be considered for a later stage (Auger North). In Section 2, we present a quick review of the Doppler Lidar method. In section 3, we present the elements for the design of the appropriate FP etalon. 2. Fabry-Perot LIDAR review An example of what a High Resolution spectrometer inserted in a Lidar can achieve is seen in Figure 1 from reference [2]. One can see that the aerosol scattering efficiency is experimentally normalized to the molecular scattering cross-section, which is very well-known.
Figure 1. Aerosol to molecular ratio obtained in reference [2]. Fabry-Perot for air temperature measurements Recent measurements in aerosol and water profiles are reported in the literature as for example in reference [3] We may measure the temperature by recording the width of spectral lines due to certain atmospheric species such as atmospheric oxygen emitting at 557.7 nm. In this case, the height information is not available as we probe on the emission spectrum of oxygen. If one wants, despite this to record average oxygen temperature, an optical filter located at the telescope entrance corresponding to the 557.7 nm line of atomic oxygen with a width (passband) of less than 1 nm will be necessary. The possibility to get height depending information on temperature arises when the Fabry-Perot interferometer is operating in conjunction with a Lidar. We present in this work the first efforts of assembling a prototype Fabry-Perot etalon appropriate for this type of work. One may consider for the Perot interferometer the reference [4]. The spectrometer has been designed to receive spectra for monitoring the refractivities of gases and it employs the Fourier analysis method in reconstructing spectral lines of a light source. First, we have used a student laboratory etalon (Ealing Electrooptics) with a He-Νe laser. As the expected coherence length of this laser is expected to be of the order of 25 cm, we attempted to use spacer distances of 3mm, 10mm and 50 mm. In all these cases we observed interference fringe patterns and we recorded several photographs of them by locating the camera film at the expected focal location of the interference pattern for the lens system which we had used as seen in Figure 2.
Figure 2. A simplified view of a FP etalon. One sees a laser, lenses for formation of interference fringes, a FP etalon, and an interference pattern. In order to get a more realistic feeling of a FP interference fringe pattern, we recorded a photographic image of a He-Ne spectral line with a Fabry-Perot etalon as seen in Figure 3. Figure 3 : A Fabry-Perot photographic recording of He-Ne 632.8 nm, with a focal length lens of 30 mm, spacer distance 1 mm. The mirror flatness is of the order of λ/20.
One may observe the visible fringes of interference, and may determine the deviations from perfect circles, in order to determine the quality of etalon mirrors, mainly with regard to the surface flatness. In addition, we have used a 1 cm spacer distance etalon of Queens-Gate Instruments Ltd, with aluminised mirrors, and recorded interferometric photographs of a Pt-Ne lamp. Example of an etalon with 1cm spacer distance and λ/200 flatness in giving interferogram of a Pt-Ne hollow cathode discharge at 21 ma, taken with a lens of focal length 30 cm. Α recent photographic recording of interference pattern is seen in Figure 4. Figure 4. Interference fringe pattern with an etalon with air-gap 1 cm and a Pt-Ne hollow cathode spectral lamp. It may be added, that in the photographic film one can observe lines at green range which, however, have been lost due to the improper film printing process. We also have recorded and observed interference fringe patterns with He-Ne laser spacer distances around 45 mm. On could definitely observe and record such fringes which is expected as the He-Ne laser is known to have coherence length of the order of 25 cm. Unfortunatelly, the scanner fails to reproduce the photographic print contrast, and thus we do not present any such fringe pattern here. On the basis of the above tests and after study of the relevant literature, we anticipate to make more accurate tests with another FP etalon with 2-cm mirror distance which has been ordered by NTUA, and its expected performance is discussed below. The instrument which should allow measurements which are useful for determining atmospheric properties such as air temperature and pressure. It has a 2 cm spacer distance FP with a reflectance of 90% while the mirror absorption losses are estimated at 0.5 %. In this case, the interferogram, due to oxygen atomic line at 557.7 nm will
be broadened due to the average (rms) velocity due to thermal motion, which is given by E kinetic =kt/2 = mv 2 rms/2, thus v rm = (kt/m). Thus, a typical change in wavelength of radiation from [O] at 557.7 nm would be because of Doppler motion: Τhen, the wavelength is given by the formula: c/λ =(c/λ ) ( 1 + v rms / c), or λ λ (1 - v rms / c) = λ ( 1- (kt/m O ) /c), where m O is the Oxygen mass. Having this as general background, we address here another, known approach, to determine the aerosol to molecular ratio as a function of height using high resolution Fourier spectroscopy as described in [5],[6], and in many other more recent papers. The profiles of aerosol to molecular ratio are given in Figure 15 of reference [2], taken in different times during the day, and this is an indication of the applicability of the method up to heights exceeding the aerosol layer. 3. Fabry-Perot Etalon Design Considerations As the free spectral range is becoming smaller, then it is possible to have tiny spectral structures within a spectral range as it is seen for a typical He-Ne laser in the following figure which displays three of the allowed laser modes, conveniently positioned within the spectral range. In the Figure 5a, we see the calculation of an Airy Function in a Fabry-Perot etalon with mirror pair gap10 cm, for a reflectance 90% and loses due to the mirror coating of 0.5 %. The 3-Airy functions display the following cases: a) The response of the FP etalon in a fully monochromatic laser beam at 532nm, b) and c) is the response of the FP etalon to a wavelength displaced due to Doppler shifts of the scattered laser radiation because typical motion of Oxygen molecules towards or away from the FP etalon (due to typical velocity caused by the thermal motion around 40 degrees Celsius corresponding to extreme atmospheric air temperatures). The aerosol scattering would give the central peak (a) because it could have essentially no Doppler shift while the molecules give the peaks b) and c). The conclusion from this graph is that the aerosol contribution can clearly be resolved from the molecular contribution because of the selected etalon parameters.
Figures 5a and b. 5a: 10 cm mirror spacing, 5b): 2 cm mirror spacing The corresponding graph (Fugure 5b) of the response function of a 2 cm mirror gap By comparing the two graphs, it may be seen that the wavelength range of the three spectral lines covers a different percentage of the Free Spectral Range (FSR), i.e. in the 10 cm gap etalon we have a larger percentage of the FSR covered than in the 2 cm gap etalon case. Thus, the 3 peaks can be resolved more easily in the case of 10 cm gap provided that other factors, such as etalon flatness defects, donnot obscure such separation. Analysis of interferometer data: The way to obtain useful data from the Fabry-Perot interferometer is presented in some detail in reference [2]. The main point to consider for this analysis is that the
interferometric data are transformed in the wavenumber, σ 1/λ, spectrum, i.e. they are Fourier analyzed, and the aerosol peak, located near the σ 0 1/λ 0, (λ 0 = laser wavelength) position is extracted and its ratio to the total molecular scattering is obtained. Conclusions and Prospects The available etalon tested (1 cm mirror spacing) and another expected ( 2 cm mirror spacing) with dielectric multilayer low absorption loss mirrors should allow at least laboratory measurements of the performance of Fabry-Perot interferometers and measure its optical performance with laser sources of passband less than 0.005 cm -1 and record interferograms. Another task would be to assure the use of a stabilized Nd:YAG laser source with line width at 532 nm approaching the Δk 0.01 cm -1 or better. This task is getting near reality at very reasonable cost if one chooses the diode laser seeding of the Nd:YAG laser. Then we should reconstruct the Fourier spectra. Having understood these spectra, we can proceed to conduct a specific design of a reasonable cost Fabry-Perot spectrometer to be used in conjunction with a Lidar in EAS related atmospheric monitoring applications. Such a task would need obviously a strong collaborative effort, and this report has as main aim to create a group of interested people in order that a serious feasibility study gives an answer if such an experimental effort is useful in the area of doing corrections for atmospheric effects in EAS events. [1] Pironen, 1994 : http://lidar.ssec.wisc.edu/papers/pp_thes/thes_4.htm [2] Analysis techniques for recovery of winds and backscatter coefficients from a multiple-channel incoherent Doppler Lidar, M.J.McGill, W.R.Skinner, and T.D.Irgang, Applied Optics, Vol.36,No.6, 20Feb 1997, pp1253-1268 [3] http://www.fisica.unile.it/~detomasi/jgr2002_pre.pdf [4] Precision measurements of gas refractivity by means of a Fabry Perot interferometer illustrated by the monitoring of radiator refractivity in the DELPHI RICH detectors A.Filippas et al, NIM B, Volume 196, Issues 3-4, November 2002, Pages 340-348 [5] Greater details in the operation and properties of a FP interferometer can be found at: http://www.blackaltar.org/fpi/thesis.pdf [6] http://lidar.ssec.wisc.edu/papers/pp_thes/node4.htm [7] Incoherent Doppler Lidar: http://www2.nict.go.jp/kk/e414/shuppan/kihou-journal/journal-vol49no2/2-04.pdf