Abstract
We designed an optical coating based on TiO2/SiO2 layer pairs for broad bandwidth high reflection (BBHR) at 45-deg angle of incidence (AOI), P polarization of femtosecond (fs) laser pulses of 900-nm center wavelength, and produced the coatings in Sandia’s large optics coater by reactive, ion-assisted e-beam evaporation. This paper reports on laser-induced damage threshold (LIDT) tests of these coatings. The broad HR bands of BBHR coatings pose challenges to LIDT tests. An ideal test would be in a vacuum environment appropriate to a high energy, fs-pulse, petawatt-class laser, with pulses identical to its fs pulses. Short of this would be tests over portions of the HR band using nanosecond or sub-picosecond pulses produced by tunable lasers. Such tests could, e.g., sample 10-nm-wide wavelength intervals with center wavelengths tunable over the broad HR band. Alternatively, the coating’s HR band could be adjusted by means of wavelength shifts due to changing the AOI of the LIDT tests or due to the coating absorbing moisture under ambient conditions. We had LIDT tests performed on the BBHR coatings at selected AOIs to gain insight into their laser damage properties and analyze how the results of the different LIDT tests compare.
Highlights
This paper is based on a conference proceedings paper.[1]
By recalibrating that initial coating run, we produced the broad bandwidth high reflection (BBHR) coatings of this paper, which we report in Sec. 3 and have HR band center wavelengths close to 900 nm according to the choice of ambient environment
We will explore some of the issues associated with laser-induced damage threshold (LIDT) tests of BBHR mirror coatings designed for fs pulses, focusing in particular on differences in the environments, pulse durations and wavelengths between available LIDT test lasers and large-scale PWclass lasers in which the BBHR coatings must perform
Summary
This paper is based on a conference proceedings paper.[1]. Its context is that of large-scale petawatt (PW) high energy lasers whose pulses are of durations in the femtosecond (fs) regime.[2]. A standard requirement for such BBHR coatings is that they should provide reflectivity exceeding 99.5% as well as low group delay dispersion (GDD) over the entire spectrum of the fs pulses they are to reflect, so that the pulses do not suffer distortion or broadening of their temporal profiles on reflection.[3] because the BBHR coatings are important for reflection of high intensity fs pulses of PW lasers by their final off-axis parabola and fold mirrors, their laser-induced damage thresholds (LIDTs) must be high enough to ensure the mirrors will perform in the environment and under the laser pulse conditions of the actual PW laser beam train In this regard, our BBHR coating design goal was ambitious, guided by achieving R > 99.5%, GDD within Æ20 fs[2], and LIDT > 800 mJ∕cm[2] for 45-deg. We will explore some of the issues associated with LIDT tests of BBHR mirror coatings designed for fs pulses, focusing in particular on differences in the environments, pulse durations and wavelengths between available LIDT test lasers and large-scale PWclass lasers in which the BBHR coatings must perform
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