Diffraction gratings are frequently the critical optical element used to stretch and recompress pulses in the application of chirped pulse amplification. Gratings used for pulse compression of lasers generally require a diffracted wavefront free of aberrations as well as high diffraction efficiency and a high damage threshold. Newport's Richardson Gratings provide a variety of ruled and holographic gratings for use in pulse compression systems.
Efficiency
Newport's Richardson Gratings have shown increases of 2% absolute efficiency in 2004 because of new advances in our gold coatings.
Output efficiency of a given compressor is typically based on a 4-pass geometry. Therefore, if the efficiency of a grating in one pass is 92% then the efficiency of the compressor itself cannot be higher than 71.6% and is often less because of inefficiencies in mirrors and aperturing of the dispersed beam.
The gratings in a CPA system are used in specrograph mode with a fixed incidence angle. On request, Spectra-Physics will add efficiency measurements to your grating for left, center, and right positions of the grating over a wavelength range of choice at a given fixed incidence angle. Efficiency measurements allow the customer to sort gratings accordingly to ensure that the best grating can be used in the compressor to enable maximum efficiency from your CPA.
Ruled gratings such as master MR136 offer a feature unique to this application. The efficiency stays constant as a function of wavelength, in a fixed incident angle geometry. Many holographic gratings (in particular at 1200 g/mm) cannot fundamentally provide this feature. Therefore the output pulsewidth of the compressor may not be optimized, reducing peak-power. MR136 is a recommended grating for 1200 g/mm compression systems.
Wavefront
Wavefront of the grating is very important. Poor wavefront can be caused by groove curvature (master grating dependent) or by poor quality substrates (substrate dependent). Newport's Richardson Gratings come with a standard λ/4 P-V specification over the clear aperture of the grating. Phase Measuring Interferometer plots can be provided on request for your gratings. Better wavefronts lead to better spatial mode qualities which, for example, lead to better focusing and hence increased efficiency in the optical parametric amplification (OPA).
Newport will provide test data for efficiency and wavefront upon request, which becomes truly valuable for system maintenance and troubleshooting.
Cosmetics
Scratches and digs contribute to poor mode quality for chirped pulse amplifiers. We will not ship a grating unless it passes our strict and tight inspection standards. Please ask for a copy of this standard for CPA gratings.
Size
Size is critical in terms of damage threshold for very high intensity lasers. Damage threshold can be increased by increasing the surface area of the grating. Newport offers large holographic gratings of up to 102 mm x 140 mm (e.g., 5338) clear aperture and large ruled gratings (e.g., MR136) of up to 156 x 208 mm in clear aperture. The Table of Standard Sizes lists stock substrate sizes. Custom sizes are available, with generally short lead-times, upon request.
Wavelength Range
Gold-coated replica gratings typically offer high efficiency over a broad range of wavelengths. Pulse width is determined by the bandwidth of the compressor and, therefore, gold-coated gratings are the only option for ultrashort Titanium Sapphire pulse compression systems.
Ghosts
Gratings carrying Rowland ghosts may affect the temporal and spatial mode output of your compressor. All holographic gratings are free of Rowland Ghosts. MR136 is free of Rowland ghosts that would affect the compressor output.
Grating Consistency
Grating-to-grating consistency is imperative in a multi grating compression system. Master gratings cannot be recorded with the same consistency found in replica gratings. We recommend the use of replica gratings for CPA’s in general.
