Simple and sensitive technique for alignment of the pinhole of a spatial filter of a high-energy, high-power laser system.

In this paper, a new method for alignment of the pinhole of a spatial filter (SF) has been proposed and demonstrated experimentally. The effect of the misalignment of the pinhole on the laser beam profiles has been calculated for circular and elliptical Gaussian laser beams. Theoretical computation has been carried out to illustrate the effect of an intensity mask, placed before the focusing lens of the SF, on the spatial beam profile after the pinhole of the SF. It is shown, both theoretically and experimentally, that a simple intensity mask, consisting of a black dot, can be used to visually align the pinhole with a high accuracy of 5% of the pinhole diameter. The accuracy may be further improved using a computer-based image processing algorithm. Finally, the proposed technique has been demonstrated to align a vacuum SF of a compact 40 J Nd:phosphate glass laser system.

Restoration of absolute diffraction efficiency and blaze angle of carbon contaminated gratings by ultraviolet cleaning.

An UV cleaning technique has been used for the removal of carbon contamination from grating surface, using oxygen free-radicals generated by the absorption of UV radiation (172 nm) in atmospheric oxygen. The extent of restoration of absolute diffraction efficiency (ADE) and grating blaze angle of a carbon-contaminated variable line-spaced concave grating (average line spacing of 1200 lines/mm, blaze angle 3.2 deg, and blaze wavelength 10 nm) has been studied over a 5-70 nm wavelength range. The contamination (due to prolonged use in extreme-ultraviolet spectrograph) resulted in a drastic reduction in the diffraction efficiency of grating, along with a change in the blaze angle. The UV cleaning led to the restoration of blaze angle as well as to an increase in the ADE by more than an order of magnitude for the first two diffraction orders (i.e., from ~0.2 to ~7.2%) for the first diffraction order and 0.1 to ~5% for the second diffraction order at the blaze wavelength. The study is useful for the restoration of carbon-contaminated costly optics.

Strong enhancement and extinction of single harmonic intensity in the mid- and end-plateau regions of the high harmonics generated in weakly excited laser plasmas.

Strong intensity enhancement or extinction of some single harmonics is observed in high-harmonic generation from 48 fs Ti:sapphire laser pulses propagating through preformed low-excited laser-produced plasmas of various materials (GaAs, Cr, InSb, stainless steel). The intensities of some of the harmonics in the mid- and end-plateau regions vary from ~23-fold enhancement to near disappearance compared with those of the neighboring ones. It is also shown that the observed intensity enhancement (or extinction) can be varied by controlling the chirp of the driving laser radiation.

Measuring pulse-front tilt in ultrashort pulse laser beams without ambiguity of its sign using single-shot tilted pulse-front autocorrelator.

The measurement of pulse-front tilt (PFT) in ultrashort pulse laser beams, without ambiguity of its sign, has been carried out using an in-house built single-shot tilted pulse-front autocorrelator (TPFAC) based on second harmonic generation in non-linear crystal. The experiment is carried out for the beam propagation through a single prism and a pair of prisms. Expressions of PFT angle have been derived to account for beam size, beam propagation effects and for arbitrary incidence of the laser beam. The measured PFT angles were found in accordance with theoretically calculated values. The calculated sensitivity of TPFAC is found to be larger than that offered by the other techniques.