Abstract
We describe an optical coating design suitable for broad bandwidth high reflection (BBHR) at 45° angle of incidence (AOI), P polarization (Ppol) of femtosecond (fs) laser pulses whose wavelengths range from 800 to 1000 nm. Our design process is guided by quarter-wave HR coating properties. The design must afford low group delay dispersion (GDD) for reflected light over the broad, 200 nm bandwidth in order to minimize temporal broadening of the fs pulses due to dispersive alteration of relative phases between their frequency components. The design should also be favorable to high laser-induced damage threshold (LIDT). We base the coating on TiO2/SiO2 layer pairs produced by means of e-beam evaporation with ion-assisted deposition, and use OptiLayer Thin Film Software to explore designs starting with TiO2/SiO2 layers having thicknesses in a reverse chirped arrangement. This approach led to a design with R > 99% from 800 to 1000 nm and GDD < 20 fs2 from 843 to 949 nm (45° AOI, Ppol). The design’s GDD behaves in a smooth way, suitable for GDD compensation techniques, and its electric field intensities show promise for high LIDTs. Reflectivity and GDD measurements for the initial test coating indicate good performance of the BBHR design. Subsequent coating runs with improved process calibration produced two coatings whose HR bands satisfactorily meet the design goals. For the sake of completeness, we summarize our previously reported transmission spectra and LIDT test results with 800 ps, 8 ps and 675 fs pulses for these two coatings, and present a table of the LIDT results we have for all of our TiO2/SiO2 BBHR coatings, showing the trends with test laser pulse duration from the ns to sub-ps regimes.
Highlights
The specific focus of this study is the design of an optical coating that affords broad bandwidth high reflection (BBHR) of >99.5% for the 200-nm band from 800 to 1000 nm and for 45 ̋ angle of incidence (AOI) and P polarization (Ppol)
This BBHR coating must exhibit near linear group delay (GD) and near constant group delay dispersion (GDD) of the reflected light across this bandwidth, in order to reflect femtosecond laser pulses without distortion or broadening of their temporal profiles
An essential requirement for the BBHR coating to be suitable for reflection of fs pulses is that its reflection of all the frequency components of the pulse should preserve their relative phases, which corresponds to reflection characterized by low GDD
Summary
The specific focus of this study is the design of an optical coating that affords broad bandwidth high reflection (BBHR) of >99.5% for the 200-nm band from 800 to 1000 nm and for 45 ̋ angle of incidence (AOI) and P polarization (Ppol). Linear, chirped variation of layer thicknesses from the outer to inner layers, similar to the earlier work [8,9], led to a near linear increase of GD with wavelength [11] Another direction of research into multilayer coatings for control of GD and GDD for fs pulses deals with coating structures which do not alter the relative phases between frequency components of the pulses on reflection. They have developed multilayer BBHR coatings that are suitable for deposition on meter-size optics and afford LIDT and GDD properties of the sort we are interested in achieving Their approach for 45 ̋ AOI involves all-dielectric coating layers for S polarization (Spol) and a mixture of dielectric and metal coating layers for Ppol.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
