title: Restoration of absolute diffraction efficiency and blaze angle of carbon contaminated gratings by ultraviolet cleaning.
abstract: 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.
authors.affiliation: Raja Ramanna Centre for Advanced Technology, Indore, India. mukund@rrcat.gov.in
authors.forename: Mukund
authors.initials: M
authors.lastname: Kumar
authors.forename: Mohd Hussain
authors.initials: MH
authors.lastname: Modi
authors.forename: Himanshu
authors.initials: H
authors.lastname: Singhal
authors.forename: S
authors.initials: S
authors.lastname: Sendhil Raja
authors.forename: Juzer Ali
authors.initials: JA
authors.lastname: Chakera
authors.forename: Raj Kumar
authors.initials: RK
authors.lastname: Gupta
authors.forename: Prasad Anant
authors.initials: PA
authors.lastname: Naik
authors.forename: Gyanendra Singh
authors.initials: GS
authors.lastname: Lodha
authors.forename: Parshotam Dass
authors.initials: PD
authors.lastname: Gupta