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WAVELENGTH (|im)

WAVELENGTH (|im)

Fig. 4.19 Typical absolute reflectance of an aluminum grating 300 ¿mm-1, ref. Jobin-Yvon 51019, measured in the lst-order under Littrow conditions (fl = —a). Blaze angle 0b = 5.16° for X = 600 nm, X ¡1 = 2sin0B = 5.5 in Littrow. Solid-line: mean values from S and P polarization planes. Dotted-line: Expected reflectance with Ag + MgF2 coating (from J. Famand, Horiba Jobin Yvon Corp.)

the case of an infinite conductor surface with echelettes (semi-rectangle grooves). Figure 4.19 displays the behavior from experiment of the same grating type under unpolarized light of which the two components are denoted S and P.

It must be noticed that holographic and ruled relief-surface diffraction gratings with a line-density < 900 mm-1 have a high efficiency over a large spectral range.

^ For the low and medium blaze angle region i.e. 0B = 5-18° and deviation angle 5 = D.A. = 0-45°, one obtains unpolarized grating efficiencies (P + S)/2 higher than 50% over a wavelength region from 0.66 to 1.80-times the blaze wavelength (Ab = 21 sin 0B in Littrow), i.e. much more than an octave.

4.4.10 Grating Manufacturing Methods

• Type A: Diamond ruled gratings: The first and classical method of making diffraction gratings was pioneered by Joseph von Fraunhofer, in 1813, by ruling of a metal substrate with a diamond translation machine and a precision screw controlling the pitch of the grooves. Ruled gratings led him to the discovery and the measure of the main absorption lines of the solar spectrum, known as Fraunhofer lines. Such ruling engines were later improved by H.R. Rowland, who obtained 7.5-inch gratings and the first concave-shaped gratings, by A.A. Michelson and others. G.R. Harrisson and his collaborators introduced a laser interferometric feedback control of the pitch in 1955 which allowed one to build 30-cm gratings. It was found that the limited lifetime of the diamond during the ruling process prevents obtaining more than a ~15-km ruling length.

• Type B: Holographic surface relief gratings: Since 1960 and with the appearance of lasers, the ruling process was progressively substituted by the holographic recording of interferometric fringes in a photosensitive film. Starting in the 1980s, blazed holographic gratings by the ion beam method were developed by J. Flamand et al. [22] at Jobin-Yvon. The method allows one to obtain holographic surface relief gratings which are efficiently blazed. For most manufacturers, the available standard gratings are limited in size to 12x14 or 15x20cm2 which in fact is the old standard sizes issued from the classical diamond ruling method. Prototype plane gratings in a format as large as 30x40 cm2 can presently be obtained with line densities from 150 to 1,200 ¿mm-1.

• Type C: Volume phase holographic gratings: Another type of grating called volume phase holographic gratings has been developed more recently. Rather than being diffracted by a surface-relief structure, as for previous types A and B, the light undergoes Bragg diffraction as it passes through the volume of a photosensitive film of thickness from a few to some hundred microns. In recent developments the optical accuracy has been improved by sandwiching this film between two flat windows (Arns [1]). In the holographic recording process of plane gratings, the refractive index of the photosensitive film is modulated by slanted and parallel phase planes (Barden [6]). Large size gratings can be obtained for transmission or reflection grating geometries. However, aberration corrections on such gratings suffer from chromatic aberrations.

4.4.11 Towards Large Size Aspherized Reflective Gratings

A reflective relief surface grating generating an aberration correction operates in an achromatic form which, then, is efficient for the widest possible spectral range. A volume phase holographic grating can only provide an aberration correction at a given wavelength Ao; such gratings suffer from an important chromatic variation error for A = Ao which then restrains the spectral range. A holographic surface relief grating - aspherized reflective grating - avoids this difficulty and hence must be preferred for faint object spectrographs.

With extremely large telescope projects in development, astronomers could expect to soon obtain much larger holographic surface relief reflective gratings such as 20 x 25 or even 40 x 50 cm2. As pointed out in the above sections, only this grating type (Type C) efficiently provides various achromatic aberration corrections. Their asperization requires use of active optics and replication techniques (cf. next Chapter).

4.4.12 Large All-Reflective Aspherized Grating Spectrographs

Given a spectrograph camera mirror f-ratio and a telescope central obstruction ratio, a large size grating is a natural way for the optimal use of a large detector without

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