Developments in plane and concave gratings: theory and experiment

The developments in grating performance described in this thesis fall naturally into two parts according to whether the grating blank is plane or concave. For plane diffraction gratings, this study utilizes both experimental work and rigorous electromagnetic theories to achieve improved grating and instrument performance. The section concerned with concave gratings is entirely theoretical, drawing on the manipulation of geometric theories to design high quality gratings. Three types of plane diffraction grating grooves currently generated by holographic techniques are investigated. A discussion of methods for accurately measuring the groove profile is presented with particular reference to quasi-sinusoidal groove profiles. The spectral performance of blazed holographic gratings is shown numerically to be comparable to that of triangular groove gratings. Consideration is then given to simple optical arrangements which permit Fourier synthesis of complex distributions and groove profiles. Experimental investigations of the behaviour of resonance anomalies exhibited by gratings having surface layers of discontinuous conductivity are presented. It is demonstrated that a coating consisting of a dielectric overlaid with a metal of high conductivity drastically reduces energy absorption. A theoretical search to determine the optimum groove modulation of both blazed and sinusoidal plane reflection gratings is presented. This will assist with the development of high-gain dye lasers utilizing gratings at grazing incidence. Aberration coefficients derived using Fermat's principle are used for an examination of aberration-correction for Rowland circle spectrographs. Attention is focussed on the case where the mounting and recording points lie on the Rowland circle. Correction at two wavelengths of primary astigmatism or primary coma under this constraint is demonstrated. Alternatively, primary astigmatism and primary coma can be simultaneously corrected over a restricted wavelength range. Some practical implications are discussed. The geometric theory of concave gratings is extended to accommodate higher order aberration terms. The magnitude of the individual terms of the light path function, which describes image formation by an aberration-corrected grating, is investigated. Similarly, the behaviour of the image height under these restraints is studied for various values of the relative aperture. Finally, aberration minimization techniques are employed in the design of a uni-axial, double concave grating monochromator. The performance of this instrument is evaluated for ruled and holographic gratings.