Diode and dye laser wavelength tuning, in the visible region of the spectrum, is done in two different modes. The classical one is with a grating in the autocollimating (Littrow) mount where the beam requires expansion to fill the grating in order to obtain adequate resolution. Telescope or prism optics fulfill this need. The alternative approach is to use the grating in a fixed grazing incidence mode together with a rotating reflecting tuning element in the form of either a mirror or a second grating. This second approach is referred to as a Littman-Metcalf Geometry and no beam expansion is generally required.
Littrow tuning is done either with fine pitch first order gratings (typically 1800 or 2400 g/mm frequency, either ruled or holographic) or a coarser grating used in higher orders such as a 600 g/mm, 54° blaze angle grating, particularly useful because it covers the visible spectrum in orders 3 to 7 with free spectral ranges that match the dyes and prevent overlap.
Grazing incidence, Littman-Metcalf tuning, is done in first order only and 1800 g/mm, 2000 g/mm, and 2400 g/mm holographic gratings are preferred. With incident angles of 80 to 88° longer ruled widths are typically required, leading to special grating dimensions such as 16.5 x 58 x 10 mm.
