Diffraction grating groove and mounting dependent properties
Creators: name: Wilson, IJ
Publisher: name: University Of Tasmania
Year: 2023

description: The common ruled and holographic diffraction grating groove profiles are introduced and their shape is related to the various techniques by which they are generated. Emphasis is put on the development of sophisticated groove formation models for holographic gratings made in photoresist. A numerical demonstration of the capabilities of the groove formation models is given, culminating in the description of novel Fourier groove synthesis techniques. The original contributions outlined below on the efficiency properties of plane reflection gratings are amplified chapter by chapter. The fidelity of the rigorous electromagnetic grating theories for infinitely and finitely conducting grating surfaces is established by a confrontation with experiment. Accurate electronmicroscope and Talystep groove profile measurements are used in conjunction with integral equation formalisms for calculating the distribution of energy in the diffracted spectrum of a reflection grating. This distribution of energy is compared with experiment to determine the domain of validity of each grating theory. The rigorous grating theories are used to investigate the groove profile dependent efficiency properties of both ruled and holographic gratings. For ruled gratings the finite conductivity theory is shown to be in accord with the infinite conductivity theory in predicting that rectangular groove gratings generally yield optimum Littrow mount performance. Results of a theoretical and experimental investigation of the merging and repulsion of grating anomalies reveals that it is the presence of resonance effects at a wavelength greater than the Rayleigh wavelength, rather than a Rayleigh wavelength type anomaly, which is responsible for the repulsion of anomalies. Numerical studies using the grating theories have been used to determine the optimum groove profile parameters for sinusoidal and distorted sinusoidal groove gratings. For the sinusoidal groove grating the groove modulation depth should be near 37% for optimum Littrow mount performance. The useful spectral range may be increased by up to 75% by increasing the groove Modulation depth to 45% and incorporating an appropriate groove distortion away from the sinusoidal form. A scheme for determining the optimum groove depth and degree of distortion using a gas laser is described. The infinite conductivity theory is used to identify a groove depth dependent efficiency property. The S polarization efficiency of ruled and holographic gratings is shown to oscillate with grating groove depth according to two simple integer sequences. This property is used to explain similarities between the efficiency curves of some sinusoidal and triangular groove profile gratings. Finally, there is a numerical study of mounting dependent efficiency properties of ruled and holographic gratings with special reference to anomaly reduction. The fixed angular deviation mount (Ebert or Czerny-Turner mount) is investigated for a wide range of grating blaze angles and deviations. The anomalies and polarizance of gratings used in first, second and third order Littrow mounts are also examined. A theoretical study of first and second order spectrograph mount efficiencies indicates that near Littrow configurations are optimal. The angle of incidence should be as near to that of the grating blaze angle as is consistent with the space available and the size of the optical elements. A new property of normal incidence spectrograph mounts which imposes a lower wavelength limit on the useful spectral range is demonstrated.