APPLICATIONS OF THE FABRY-PEROT SPECTROMETER TO UPPER ATMOSPHERIC SPECTROSCOPY

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APPLICATIONS OF THE FABRY-PEROT

SPECTROMETER TO UPPER ATMOSPHERIC

SPECTROSCOPY

G. Shepherd

To cite this version:

JOURNAL DE PHYSIQUE Coiloque C 2, suppiiment au no 3-4, Tome 28, mars-avril1967, page C 2

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301

APPLICATIONS OF THE FABRY-PEROT SPECTROMETER

TO UPPER ATMOSPHERIC SPECTROSCOPY

G. G. SHEPHERD

Institute of Upper Atmospheric Physics, University of Saskatchewan

Abstract. - The Fabry-Perot spectrometer can be applied to a wide range of problems in upper atmospheric spectroscopy. These vary from line-width measurements at high resolution to low- resolution problems requiring either high sensitivity or rapid time response. A number of such applications are described as have been used for measurements on the aurora, twilightglow, and dayglow from Saskatoon, Canada. Spatial scanning offers a number of advantages that are being currently developed ; some of these also are described.

ResumB. - Le spectromktre Fabry-P6rot peut Ctre appliquk a une grande vari6tB de problkmes de spectroscopie de la haute atmosphkre. Ceux-ci vont de la mesure de largeur de raies a haute rksolution jusqu'a des problemes demandant soit une grande sensibilitk, soit un faible temps de rkponse. Nous dkcrivons un certain nombre &applications de ce genre a des mesures sur l'aurore borkale et I'emission du ciel diurne ou crkpusculaire a Saskatoon (Canada). Un balayage spatial pr6sente un certain nombre d'avantages que nous dkveloppons actuellement ; certains sont egale- ment d6crits ici.

1. Introduction. - Upper atmospheric spectros- copy is almost entirely concerned with two emission phenomena; airglow and aurora. Both of these contain emissions from neutral and ionized atoms and diatomic molecules. Although there is a certain degree of overlapping of different molecular band systems, particularly when one includes the fainter emissions, many of the more prominent features are well separated from their neighbours and can be isolated with interference filters.

The airglow is faint, with the emission rate in the visible region varying from a few Rayleighs (R) to a kilorayleigh

(kR)

or a little more. The Rayleigh is a unit [ I ] giving the number of photons emitted per second from a column 1 cm2 in diameter extending along the line of sight through the emitting region, and corresponds to an emission rate E, of lo6 photons/sec from this column. The surface brightness of a uniform layer of emission rate E is simply,

B = lo6 E/(4 n) photons cm-2 s-' sterad-'. (1) Hence with the airglow we are concerned with B from

lo5

to lo8 photons cm-2 s-' sterad-l, which demands spectrometers of large angular acceptance.

The aurora may be several orders of magnitude brighter, but its brightness fluctuates with time [2], requiring scanning times as short as 10 s or preferably 0.1 s, whereas for airglow, scanning times of the order of minutes or even hours may be tolerable. Hence

both phenomena demand large angular acceptances. The application of rocket-borne spectrometers to airglow and auroral research makes the scanning time demands even more severe.

Now that the auroral and airglow features have been identified and their general characteristics are known, attention has turned to the dynamical behavior of specific emissions as a function of the many variables that characterize geophysical phenomena. Hence for many problems a limited spectral range is required ; limited often to a single band or line.

In summary, the conditions of well-separated emis- sion spectra, and narrow spectral ranges, together with demands for large angular acceptance, make Fabry-Perot spectrometers extremely well suited to this type of observation. This conclusion will be rein- forced by examples from work done by the author and his colleagues in this field.

2. Angular Acceptance and Source Brightness. - An example of a much studied airglow emission is the sodium resonance line, as produced by resonance fluorescence in twilight and in dayglow. The study of this emission gives information about the number and distribution of free sodium atoms, and their variation from day to day and with season of the year. The emissions of lithium and potassium are similarly of interest. The sodium doublet has a separation of 6

A

C 2

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302 G . G. SHEPHERD

so that an instrumental half-width of 2

A

separates the components satisfactorily. Because of the back- ground Rayleigh-scattered light and its rapid rate of change during twilight, a scanning time of about 1 min is desirable. One would like to detect an emission rate of 100 R near the end of the twilight period.

The photon signal received by the detector is

where A is the area of the plates, f.2 is the solid angle accepted by them, and z is the effective transmission of the instrument. For a sampling time T, F T photons are collected, givingpFT photoelectrons, where p is the photomultiplier cathode efficiency. The fractional standard deviation for the measurement of this sample

is

1/4=.

For a Fabry-Perot spectrometer, SZ is

determined by the aperture, and is equal to

where n is the index of refraction and 61 is the instru- mental width. Combining (I), (2) and (3), and using z = 0.1, p = 0.1, we obtain for the fractional standard deviation,

For a standard deviation of I

%

and n = I. one obtains

which for jl = 6 000

A

and 61 = 2

A

becomes

For a scanning time of 1 min and 20 spectral elements T = 3 s. Hence for 100 R one requires a collecting area of 20 cm2.

3. Low Resolution Studies.

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There are two types of scanning that have been extensively used for Fabry- Perot spectrometers ; refractive index scanning and mechanical motion of one of the plates. For refractive index scanning one obtains,

where p is the gas pressure. The wavelength region scanned is independent of the order of interference. For typical values of dp and dn/dp one concludes that

a reasonable scanning range is roughly 1

A.

For mechanical scanning,

where t is the plate separation, so that the scanning range increases as t decreases. Since the interorder separation is A2/2 nt, we can also write

indicating that the scanning range is a fixed fraction of the interorder separation, independently of the plate separation. It follows that mechanical scanning is most advantageous for low resolution studies.

The first instrument 131 constructed at Saskatoon for sodium twilight observations had plates of 2.5 cm diameter. It utilized a pneumatically driven mechanical motion as shown in figure 1, in which air pressure

FIG. 1.

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Pneumatically driven mechanical motion for a low resolution 2.5 cm diameter Fabry-Perot etalon.

against a steel membrance was used to move the lower plate. An interference filter was used as monochro- mator. Because the interference filter passband was fixed, the white light background present in twilight was modulated as the Fabry-Perot passband was swept under the interference filter passband as shown in figure 2.

APPLICATIONS OF THE FABRY-PEROT SPECTROMETER C 2

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303

I * I I I

5870 5890 5910 XtAl

FIG. 2.

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Showing the white modulation produced by swee- ping a Fabry-Perot spectrometer passband under a fixed inter- ference filter passband. The Na D Iines are shown superimposed.

chronism with the Fabry-Perot passband. This method gave good quality spectra as shown in figure 3 for the sodium lines in twilight [4], and in figure 4 for the

LOW-WIGHTNESS S O U R C E I l O O V 1 0 1

FIG. 3.

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Sodium twilight spectra obtained for various solar depression angles (/3) for a 5 cm dia. Fabry-Perot spectrometer with a titting interference Rlter.

Increasing wavelength

-

FIG. 4.

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Hydrogen line profiles obtained from aurora with the same instrument as for figure 3.

H-beta hydrogen line in the aurora [5]. However, for the large scanning range required for the latter, and the consequent large tilt angle, the shape of the interference filter passband changes. This causes the transmission of the instrument to vary with wavelength and also allows the white light from secondary pass- bands to come through the interference filter in an amount that varies during the scan. It seems now that spatial scanning would hav e been more advantageous than the above method. A tentative arrangement for

this will be described later on in the paper.

The system described above was also used to obtain spectra of sunlight scattered from a sodium vapour cell. This technique allows one to determine the absor- ption of sunlight by upper atmospheric sodium in daytime [6].

4. High Resolution Studies. - A problem of consi-

derable interest in upper atmospheric studies is the measurement of the line width of atomic lines, parti- cularly the forbidden ones. For them, the measurement of the Doppler width gives the temperature of the emitting region, and this is a quantity of considerable importance. Since this technique gives the velocity distribution of the emitting atoms, it provides a very fundamental measurement of the temperature.

For these studies, half-widths of the order of lo-'

A

are required. Using eqn (5), and assigning E = 2 kR

for the 5 577

A

line in faint aurora and T = 5 s, one obtains A = 100 cm2. For nightglow, of about 200 R, a sample time of 30 s is required.

The first instrument constructed, as shown in figure 5,

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304 G . G. SHEPHERD

used--plates of 10 cm diameter and employed refractive [lo] we have used plots on probability paper [ l l ] as index scanning. Line profiles of the 01 5 577 and 6 300 shown in figure 8 to make use of all the points on the

fi

lines were obtained in aurora, as shown in figures 6 line profile.

and 7. To change from one line to the other it is only

( r - c l m k .

FIG. 6.

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Spectra of the 015 577 A line in aurora, obtained with the instrument of figure 5. A Kr line is shown for compa- rison.

FIG. 7. - Spectrum of the 0 1 6 300

fi

line in aurora, obtained with the instrument of figure 5. A Hg 198 line is shown for comparison.

necessary to change the interference filter, and this was done between scans, with no other changes in the spectrometer required. This is another very useful feature of the Fabry-Perot spectrometer.

In the beginning [7], the Fourier transform method of performing the inverse convolution was used to deter- mine the line profile. However, it became apparent that the instrumental width was sufficiently good that this was unnecessary and later so the simple relationships between the half-widths as outlined by Chabbal 181, were used, which were later extended to quarter- widths and three-quarter widths 191. Still more recently

FIG. 8. - Plots of the spectra of figure 6 on probability paper.

Within the last two years we have turned to using the field-widened Michelson interferometer for the above problem [12]. Its application for this purpose has been described recently and need not be repeated here 1131. As applied there, its use is limited to single line spectra of small background and known analytical shape. However, when these condition are met, its angular acceptance is vastly greater than for the Fabry- Perot spectrometer.

APPLICATIONS OF THE FABRY-PEROT SPECTROMETER C 2

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305

FIG. 9. - Illustrating the coupling of two etalons. (a) of equal 6tendue.

(6) of unequal etendue for minimizing multiple reflections.

Sample spectra obtained 1151 are shown in figure 10. For these a unique passband system was not achieved, but rather a cc nearly unique 1) one. Considerable Fraunhofer structure is seen, and only a detailed comparison of spectra of sun and sky allowed the emission line to be detected. Unfortunately, sky brightness fluctuations are rather serious and seem to be limiting the success of the measurement.

5. Spatial Scanning. - More recently we have come to appreciate the advantages of the photographic method utilized in photoelectric form which we refer to as spatial scanning [16].

These advantages are briefly ; the use of solid etalons, giving both mechanical stability and higher angular acceptance, the ability to make different spectral win- dows simultaneously available, and the synchronized

Solar spectrum

FIG. 10. - Sample spectra obtained from the sun and from the sky with the tandem system.

scanning of tandem etalons. As shown earlier in the conference, the spatial separation of the channels also makes multiplexing possible.

The fundamental idea is of course, to scan by varying the angle of incidence, giving approxi- mately.

This is done by using an annular aperture, which is expanded in such a way that its area is held constant, giving a spectral element of constant width.

One of the methods that has been used to achieve this condition is the use of a set of annular apertures, spaced around the periphery of a stepwise rotating disc. The instrument using this principle is intended for flight in a high altitude balloon. The etalon is a quartz disc, 5 cm diameter and 1 cm thick. Limited success has been so far attained, but we see no reason why the method should not be ultimately successful.

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306 G . G . SHEPHERD

as shown in figure 11, with two photomultipliers, This has provided us with a simple and compact giving simultaneous recording at two wavelengths. two-channel photometer for rocket use, as shown in figure 12. Although it seems a trivial application, and has been little used, it has considerable value. Its major disadvantage is that the fields of view are different ; that is, they view different places in the sky. This is overcome by placing another lens in front which images the interference filter on the sky. However one is not guaranteed of identical transmissions on both channels, so that we have used electrical balancing to achieve the same radiance response on both channels.

A more ambitious system now near completion employs a fibre optic image dissector to divide off 10 channels of equal spectral width. Ten photo- multipliers are employed as shown in figure 13 and electronic balancing is used. Here we plan to use 10 scalers and do photon counting. The system is designed for two 10 cm etalons and a narrow band interference filter, giving a unique passband of 0.02

A

spectral width adjustable with a zoom lens. We will be able to use either the narrow-wide or the vernier system. There are of course other ways to achieve a spatial scan. Two very ingenious ones have been described by Hirschberg and Platz [17] and by Katzenstein [18]. The latter, the axicon scan, is particularly interesting. This spectrometer is designed to observe upper atmospheric emissions in the daytime. Hence, the unique passband requirement. The simultaneous

FIG. 11. -Channel separation device for a n interference recording channels yield cancellation of fluctuations filter photometer. in the background light. Both etalon passbands will be

APPLICATIONS OF THE FAB RY-PEROT SPECTROMETER C 2

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FIG. 13.

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A 10-channel image dissector and photomultiplier system.

independently set to near the 5th channel, after which they need not be scanned.

To conclude this discussion of spatial scanning, consider again the low resolution tilting interference filter spectrometer described earlier. If the etalon and interference filter were coupled with lenses having focal lengths in the ratio of the refractive index of the filter, and a spatial scanning mechanism provided, then both the filter and etalon would scan in synchro- nism. Such a system has not yet been constructed.

Acknowledgements. - The author acknowledges with gratitude his association with the students who have carried out the work reported here : J. A. Nilson, E. C. Turgeon, H. H. Zwick, L. L. Cogger, A. R. Bens, Y. P. Neo, C. W. Lake, R. L. Hilliard and J. R. Miller. He is also grateful to Dr. R. Chabbal for his

initial guidance and to Drs. H. P. Gush and D. M. Hunten for continued advice. The fibre optics image dissectors were kindly supplied by the IIT Research Institute; Chicago, through Dr. P. N. Slater. This work has been supported by a grant from the National Research Council of Canada.

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[2] PAULSON (K. V.) and SHEPHERD (G. G.), Can. J. Phys.

1966, 44, 837.

[3] SHEPHERD (G. G.), Can. J, Phys., 1960, 1560. [4] ZWICK (H. H.) and SHEPHERD (G. G.), Can. J. Phys.

1963, 41, 343.

[5] ZWICK (H. H.) and SHEPHERD (G. G.), J. Atmos. Terr. Phys., 1963, 25, 604.

[6] NEO (Y. P.) and SHEPHERD (G. G.), Can. J. Phys.,

1964, 42, 1325.

[7] NILSON (J. A,) and SHEPHERD (G. G.), Planet. Space Sci., 1961, 5 , 299.

[8] CHABBAL (R.), J. Rech. C. N. R. S., 1953, 138. [9] TURGEON -(E. C.) and SHEPHERD (G. G.), Planet

Space Sci., 1962, 9, 295.

1101 BENS (A. R.), M. SC. Thesis, University of Saskatche- wan, 1964.

I l l ] HALD (A.), Statistical Theory with Engineering Appli- cations, p. 192, WILEY (J.) and SONS, New York, 1952.

[12] HILLIARD (R. L.) and SHEPHERD (G. G.), J. Opt. SOC. Amer, 1966, 56, 362.

[13] HILLIARD (R. L.) and SHEPHERD (G. G.), Planet. Space Sci., 1966, 14, 383.

[14] MACK (J. E.), Mc NUTT (D. P.), ROESLER (F. L.) and CHABBAL (R.), Appl. Opt., 1963,2,873.

[15] BENS (A. R.), COGGER (L. L.) and SHEPHERD (G. G.),

Planet. Space Sci., 1965, 13, 551.

[16] SHEPHERD (G. G.), LAKE (C. W.), MIILLER (J. R. and COGGER (L. L.), Appl. Opt., 1965, 4, 267. [I71 HIRSCHBERG (J. G.) and PLATZ (P.), Appl. Opt. 1965,

4, 1375.