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Diffraction Gratings for High-Intensity Laser Applications
Published in Barat Ken, Laser Safety Tools and Training, 2017
High-energy petawatt-class lasers being built today operate at 1053 nm, use Nd:glass as the laser medium, and are designed to produce pulses from a few hundred femto-seconds to several picoseconds in duration. These pulse durations require a high diffraction-efficiency bandpass less than 20 nm, a condition easily met by MLD gratings. To maximize diffraction efficiency and peak power-handling ability, MLD grating designs have evolved to high-line densities (1700–1800 lines/mm) operating at high near-Littrow incidence angles, comprising hafnia (HfO2)/silica (SiO2) layers with the top grating layer made of silica. The Littrow angle is defined as that in which the −1 order diffraction angle is the same as the incident angle. HfO2 and SiO2 are the materials of choice based on extensive experience for high-intensity nanosecond-pulse MLD high reflectors (Wu et al., 2001). Because it is a high-bandgap material with a high intrinsic laser damage threshold (Stuart et al., 1996) and is also amenable to deposition and subsequent processing incurred during grating manufacture, SiO2 is the material of choice for the grating layer.
Lasers for Spectroscopy
Published in Leon J. Radziemski, Richard W. Solarz, Jeffrey A. Paisner, Laser Spectroscopy and Its Applications, 2017
The Gallilean telescope beam expander was introduced by Hänsch [Hänsch, 1972]. The grating is mounted at the Littrow angle at which the most intense first-order blazed grating reflection feeds directly back along the input beam. This gives the highest possible reflectivity from the grating. The line width in a well-designed dye oscillator of this sort is typically about 30 GHz, although there is considerable variation in the line widths quoted in the literature, depending on details of the design. A 30-GHz line width still includes about 50 resonator modes in a typical resonator with a length of 25 cm, so an additional wavelength-selecting element must be added if a narrower line width or true single-mode operation is desired. The usual choice is a Fabry-Perot etalon consisting of two multilayer, dielectric-coated, partially transmitting mirrors aligned parallel to each other and inserted in the beam path at a slight angle to the optical axis so that transmission resonances of the etalon affect the beam circulating in the resonator but reflections from the etalon do not feed back into the resonator. The angle between the etalon and resonator axes causes the multiply reflected beams within the etalon to “walk off” the resonator axis, and this reduces the overlap and interference of multiple reflections which give the wavelength selectivity. The etalon is usually placed in the expanded part of the beam between the telescope and grating to minimize walk-off effects. Etalons are also used in prism- and birefringent-filter-tuned systems, although these will not be discussed in detail here. Cassegrainian and off-axis reflecting telescopes have been used in place of the Gallilean telescope, but it is not clear that they have any general advantages [Trebino, 1982].
Design of high-efficiency diffractive optical elements towards ultrafast mid-infrared time-stretched imaging and spectroscopy
Published in Journal of Modern Optics, 2018
Hongbo Xie, Delun Ren, Chao Wang, Chensheng Mao, Lei Yang
DOE is a key element in photonic time stretch imaging systems, which achieves spectral encoding of the spatial information (image) based on space-to-wavelength mapping [1]. Key requirements of DOE for MIR time stretch imaging include high diffraction efficiency, broad bandwidth and large field of view (FOV). Conventional ruled or holographic diffraction gratings fall short in diffraction efficiency in broad bandwidth, especially in MIR band, which significantly limits the utility of space-to-wavelength mapping for time stretch imaging. In addition, the incident angle of diffraction gratings is restricted at around the Littrow angle, eventually limiting the FOV for photonic time stretch imaging. Therefore, there is a strong demand for the design of new high-quality diffraction components with high diffraction efficiency, broad bandwidth and large field of view for ultrafast MIR time stretch imaging systems.