![]() Because hybrid gratings are so new and not very thoroughly studied, the LIDT performance is not well known. The exact dependence is not fully understood nor agreed upon, though it is usually modeled as a power law. For shorter pulses, the LIDT decreases with the pulse width, and has been shown to be comparable to that of gold gratings for pulses of several 10’s of fs width. Transmission grating LIDT values may be even higher, though for simplicity we don’t show a different range here. The damage threshold for all-dielectric MLD and transmission gratings also exhibits little dependence on pulse width for pulses longer than ~ 10 ps, where it is typically several J/cm 2. The LIDT is typically ~ 100 to several hundred mJ/cm 2, even for pulses as short as 10’s of fs. ![]() To a reasonable approximation, the LIDT of gold gratings does not depend significantly on the pulse width. Figure 2: Approximate LIDT value ranges for all-dielectric (MLD and transmission) and metal-based (gold and hybrid) gratings, as a function of pulse width. A rough illustration of typical LIDT ranges vs. Specifically, the pulse width influences the LIDT for different grating types, and the combination of the LIDT value and the pulse energy drive the size of the laser beam and therefore the size of the gratings in the compressor. The next logical requirement to consider when selecting gratings is the final amplified, compressed pulse. Hybrid metal-dielectric gratings thus operate over a wavelength range that is the intersection of the dielectric and gold ranges. The best-performing and most practical metallic reflection gratings are based on gold, which has good reflectivity from long visible to far-IR wavelengths. The all-dielectric MLD and transmission gratings are assumed to be made from established oxide-based glass substrates and thin-film coating materials, which achieve low loss from ultraviolet (UV) to near-infrared (IR) wavelengths. Such designs will be likely difficult to fabricate and therefore risky, costly, or both. Grating designs that do not follow these assumptions are possible, and may be considered for special cases. And we include only well-established materials with proven laser-induced damage threshold (LIDT) performance. ![]() ![]() We consider only reasonable manufacturing tolerances for these and other parameters, such as thin-film coating thicknesses and refractive index values. We assume manufacturable values for the grating tooth shape, including the depth, duty cycle, and sidewall taper angle. Figure 1: Nominal wavelength ranges over which the four major types of pulse compression gratings operate: multilayer dielectric (MLD), gold, hybrid metal-dielectric, and transmission.įor the chart above and in fact all of the charts in this technical note, several assumptions are made. 1 gives the approximate wavelength ranges over which each of the four major grating types are possible. The wavelengths over which different grating types may operate are constrained by the wavelength dependence of material properties such as absorption, reflection, and transmission. After all, the absolute value, range, and stability of a laser’s wavelength are some of the most fundamental defining features of the laser. When selecting the right grating a good starting point is wavelength. In this technical note we describe how one can make the best grating selection for a given set of laser requirements. Choosing the right grating for a given laser system can be confusing. There are many different grating types (metal, all-dielectric, and hybrid metal-dielectric reflection gratings, as well as transmission gratings), and even more trade-offs relating to diffraction efficiency, spectral and angular bandwidth, polarization, laser-induced damage threshold (LIDT), and temporal dispersion, to name a few. Diffraction gratings are critical components in most chirped-pulse-amplification (CPA) laser systems.
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