Table of contents

This work is the computation of the shift by reflexion, orthogonal to the incident plane (Imbert effect) of a transversally limited light beam. The incident wave satisfies Maxwell equations an is chosen so as the elementary plane waves into which it may be decomposed have a constant incident angle. Simple formulas of the shift have been obtained, which are in very close accordance with a computer calculation performed on the basis of a particular incident wave.

1. - Introduction. The SIMAC [1], [2] is constituted by the association of a Fabry-Perot Interferometer and a large aperture spectrograph [F/2]. This allows it to achieve a very high resolution (8.10⁵ to 10⁶) with a very short time of exposure. Its constitution leads to a semi-sequential recording process so that a linear dispersion is obtained on the photographic plate. Moreover it allows one to increase the signal to noise ratio without changing camera lens as necessary with a classical spectrograph.

   In this paper we describe the technical aspects of the instrument and we give some experimental results. A complete theoretical and experimental discussion has been given previously in [3].

2. - Theoretical discussion. In an other paper [4] we establish that the three high resolution spectrographs (grating, Fabry-Perot, SIMAC) accept, in the same time, the same amount of energy for the same resolution. It follow that the fondamental difference is the shortest value of the focal length that the instruments are able to receive because of the graininess of the photographic emulsion.

   Discussion about the signal to noise ratio indicates what is the most appropriated instrument suitable for a specific problem.

3. - Technical issue. A) Interferometer: It is a classical Fabry-Perot interferometer. An important improvement in the linearity of the scanning has been obtained by use of a flow controller (constant differential type). B) Spectrograph: Grating is manufactured by Bauch and Lomb (250 X 125 mm, 300 Lines/mm, blazed at 63° 27). As the present time the camera lens is a catadioptric system at F/2 and the focal length is the shortest theoretically possible (250 mm). But recent works have shown that this camera has to be modified: because of the large aperture, the receiver is a photographic film and experimental works proved that it was not convenient for position measurements (displacement of gelatin, deformation of the support can reach 10 to 40 μ). Secondly, with a catadioptric system, the plate holder, which occult the incident beam, must be very small, so we lose the advantage described in part I. The new camera will have the shortest focus length consistent with a photographic plate (glass support). C) Optical mounting: The arrangement is classical. The highest signal to noise ratio is obtained by use of the whole diameter of the Fabry-Perot. A soustractive dispersion double monochromator is used to get absolute symetric images in view of the recording with a photoelectric spectrum plate comparator [3].

4. - Results. Three reproductions show the possibilities of the SIMAC. The first one is an isotope displacement of Samarium. The second and third ones are molecular spectra of Iodic absorption and Lanthane-Oxyd emission. In these last cases, the Fortrat-Deslandres parabols are clearly perceived owing to the linearity of the scanning.

5. - Conclusion. If the SIMAC accept finally the same amount of energy by time unit that the others high resolution spectrographs, it presents the advantages of a great flexibility.

   Without any mounting modifications, it can be a high speed spectrograph or a good signal to noise ratio spectrograph; even, in this last case, it keeps the advantages of the great aperture instruments.

The precision of measurements on a photographic plate depends on the image area. But the dimensions of the images given by a grating spectrograph, a Fabry-Perot spectrograph and the SIMAC are very different. So it was interesting to develop a new comparison which takes into account not only classical illumination but also image size.

Then, we shall first discuss, at a given résolution the optical arrangement which gives the greatest image area. Secondly, we shall evaluate the signal upon noise ratio dependence with the area and the optical density of the recording.

- Greatest image obtention

1. Grating spectrograph: The best illumination is obtained when, along the direction of dispersion, the image width is equal to the graininess « g» of the photographic emulsion. Its heigth is not definite, but, because of the curvature of the image, experimentalist use small part of the entrance slit. We have choosen following limitative criteria: the angular heigth must be such as the sagitta is equal to the limit of resolution. (This definition gives values in good agreement with practical ones.)

2. Classical Fabry-Perot spectrograph: The width of rings must be equal to the « g » value: this condition determines the focal length of the objective located after the Fabry-Perot. Focal length of the grating spectrograph which assumes the separation of orders must be sufficient to assume the R/Nresolution. (N coefficient of « finesse »). In fact we demonstrated that it was better to increase this length so that «etendue» (or acceptance) of the Fabry-Perot and the of the grating are adapted to each other. If optical magnification of the grating spectrograph is fixed to unity, slit must be enhanced. With this process, illumination and resolution are inchanged, but the useful part of the ring is more important.

3. SIMAC: Because of the most important « ètendue » delivered by the Fabry-Perot interferometer, it is possible to adapt a short focus grating spectrograph. But then, the total Fabry-Perot has to be illuminated to get the hightest possible image.

   Under these conditions, calculatins shows that, at agiven résolutin, these three spectrographs deliver quite the same energy by unit time.

Signal to noise RatioClark Jone's and Felgett's works etablish the existence of a meximum of the detective quantum efficiency for the photographic receiver. For a given emulsion, there is an optimal density D₀. So, we can separate the discussion into two parts.

1. In the first case, we assume that we can reach in a reasonable time the optical densityD₀ with the slowest instrument. Under these conditions, if we use the Selwyn's law (density fluctuations are inversely proportionnal to the square root surface) and the reciprocity law, we see that the signal to noise ratio is varying as the square root time, exactly as in spectrometry.

   So, in this case, the three instrument are quite equivalent. The gain in rapidity is strictly compensated by a loss in signal to noise ratio. If we increase the image area of the Fabry-Perot spectrograph or of the SIMAC, we shall have the same exposure time and signal to noise ratio as with the grating spectrograph. Note, in this purpose that in the case of the SIMAC the change of focus length is not necessary because of the semi-sequential recording.

2. In the second case (D₀ is not accessible in a reasonable exposure time), the study of the variation of signal to noise ratio against density demonstrates that it is better to choose a short focus instrument. The decrease of the signal to noise ratio is compensated by the gain of detective quantum efficiency especially for very weak illuminations.

An expression for an incoherent optical transfer function of a diffraction limited defocused slit is derived in the presence of longitudinal vibrations. The curves for various values of static defocusing and vibrational amplitude are displayed and under certain conditions the transfer function is demonstrated to be considerably improved with the introduction of optimum amount of vibration. Using Schell's theorem, an analysis of the far-field diffraction patterns of an optical system with a defocused slit aperture in the presence of longitudinal vibrations and illuminated by partially coherent light, is also presented in this communication. Gaussian form of the coherence function has been assumed.

We describe an electonic device M for measurement of peak-values obtained in stimulated scattering generated by Q switched lasers.

This device avoids the use of ultra-fast oscilloscopes in experiments in which a great number of measurements have to be made simultaneously. It allows the reading of the results on a voltmeter.

We also describe a centralisator C which allows recording of the data and results of an experiment, for their treatment on a computer.

The device M-C is particularly adapted to non-linear optics experiments.

We point out improvements of the device M to use it for shorter pulses.

Optical constants of absorbing substances have been calculated from ATR measurements by a numerical method which also allowed us to appreciate the incertitude of the results. From these data, we have determined the optimum choice of experimental parameters, the angle of incidence, the refractive index of the first medium, and the polarization of the incident light. As an example, the method has been applied to some organic liquids in the region of the CH valence vibrations.

We observe, during the bombardment of a metallic target by a beam of positive ions, an emission of light proceeding of the de-excitation of the neutral atoms which are sputtered in an excited state.

We describe in this work a method of auxiliary excitation which permits to increase the emission of light during this interaction and this consists in exciting the atoms sputtered in a neutral state by a plasma obtained in the neighbourhood of the target.

The experience that we have realised with a NaCl target, shows that this method can permit an improvement of the sensibility of detection of the spectral analysis by atomic ionoluminescence.