High-precision grating period measurement

Spatiotemporal aberrations due to the groove density mismatching of compression gratings in ultra-intense femtosecond lasers

The groove density mismatching of compression gratings, an often-neglected key issue, can induce significant spatiotemporal aberrations especially for super-intense femtosecond lasers. We mainly investigate the angular chirp and the consequent degradation of the effective focused intensity introduced by the groove density mismatching of compression gratings in ultra-intense femtosecond lasers. The results indicate that the tolerances of grating groove density mismatching will rapidly decrease with the beam aperture or spectral bandwidth increases. For our 100PW laser under construction, the grating groove density mismatching should be as small as 0.001 gr/mm if the drop of effective focused intensity has to be controlled below 15%. More importantly, new angular chirp compensation schemes are proposed for both double-grating and four-grating compressors. This work reveals the importance of groove density matching of compression gratings, and can provide helpful guidelines for the design of ultra-intense femtosecond lasers.

Period measurement of a periodic structure by using a heterodyne grating interferometer

This paper proposes an alternative method for grating period measurement based on heterodyne grating interferometry. The optical configurations for measuring the period of reflection/transmission gratings were demonstrated, and four commercially available gratings were used to evaluate the effectiveness of the proposed method. Based on the phase-lock technique, the grating period could be obtained immediately through the phase wrapped/unwrapped process. Under precise measurement conditions, the grating period measurement error of the proposed method was better than 1 nm, and the grating period difference between product specifications was less than 1%. In addition, the measurement results of the proposed method also exhibited high similarity with optical microscopy measurements.

Simple spatially resolved period measurement of chirped pulse compression gratings

We present an easy-to-implement and low-cost setup for the precise measurement of the period chirp of diffraction gratings offering a resolution of 15 pm and reasonable scan speeds of 2 seconds per measurement point. The principle of the measurement is illustrated on the example of two different pulse compression gratings, one fabricated by laser interference lithography (LIL) and the other by scanning beam interference lithography (SBIL). A period chirp of 0.22 pm/mm² at a nominal period of 610 nm was measured for the grating fabricated with LIL, whereas no chirp was observed for the grating fabricated by SBIL, which had a nominal period of 586.2 nm.

Production of orbital angular momentum states of optical vortex beams using a vortex half-wave retarder with double-pass configuration

Higher orders of orbital angular momentum states (OAMs) of light have been produced with a double-pass configuration through a zero-order vortex half-wave retarder (VHWR). This double-pass technique can reduce the number of VHWR plates used, thus reducing costs. The OAM states of the vortex beams are identified by the near-field Talbot effect. Polarization dependence of the vortex states can also be demonstrated with this VHWR using Talbot effect. Without using the Talbot patterns, this effect of the polarization on the vortex beam can not be recognized. A theoretical validation has also been provided to complement the experimental results. Our study gives an improved understanding of this approach to use a VHWR plate.