Low Group Delay Dispersion Optical Coating for Broad Bandwidth High Reflection at 45° Incidence, P Polarization of Femtosecond Pulses with 900 nm Center Wavelength

Abstract

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 TiO 2 /SiO 2 layer pairs produced by means of e-beam evaporation with ion-assisted deposition, and use OptiLayer Thin Film Software to explore designs starting with TiO 2 /SiO 2 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 fs 2 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 TiO 2 /SiO 2 BBHR coatings, showing the trends with test laser pulse duration from the ns to sub-ps regimes.

Analysis of laser damage tests on coatings designed for broad bandwidth high reflection of femtosecond pulses

“…The details of the design and production of these coatings, including the choice of TiO 2 and SiO 2 for the layers, are provided in Sec. 3 and by the earlier report, 4 which also discusses the GDD and temporal distortion effects on fs pulses in detail.…”

“…In this regard, our BBHR coating design goal was ambitious, guided by achieving R > 99.5%, GDD within AE20 fs 2 , and LIDT > 800 mJ∕cm 2 for 45-deg AOI, Ppol over a 200-nm operational band with 900-nm center wavelength, i.e., from 800 to 1000 nm. 4 The resulting design, which was based on TiO 2 ∕SiO 2 layer pairs, did not completely meet these R and GDD goals, but did afford R > 99.5% from 801 to 999 nm and GDD within AE20 fs 2 from 823 to 949 nm and rising smoothly to ∼3500 fs 2 at 800 nm and dropping smoothly to ∼ − 3500 fs 2 at 1000 nm. 4 We considered this to be acceptable because the HR band of 198 nm is close to the 200 nm goal, and the smooth GDD variation with wavelength is favorable to techniques for compensating large GDD (such as >20 fs 2 ) where it occurs over the operational band.…”

“…4 The resulting design, which was based on TiO 2 ∕SiO 2 layer pairs, did not completely meet these R and GDD goals, but did afford R > 99.5% from 801 to 999 nm and GDD within AE20 fs 2 from 823 to 949 nm and rising smoothly to ∼3500 fs 2 at 800 nm and dropping smoothly to ∼ − 3500 fs 2 at 1000 nm. 4 We considered this to be acceptable because the HR band of 198 nm is close to the 200 nm goal, and the smooth GDD variation with wavelength is favorable to techniques for compensating large GDD (such as >20 fs 2 ) where it occurs over the operational band. BBHR coatings based on this design were produced in Sandia's large optics coating chamber 6,7 by means of e-beam evaporation with ion-assisted deposition (IAD).…”

“…Results of the measurements of these R and GDD behaviors for the initial coating that we produced were encouraging, exhibiting R > 99.5% over a 213-nm band with 967-nm center wavelength, and smoothly varying GDD within AE100 fs 2 over that wavelength band. 4 By recalibrating that initial coating run, we produced the 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.…”

“…2 These pulses are comprised of broad spectral ranges of frequency components whose relative phases determine the pulse shape. 3 Our particular interest is in optical coatings, which we have designed and produced 4 to be suitable for broad bandwidth high reflection (BBHR) at 45-deg angle of incidence (AOI), and P polarization (Ppol) of PW-class fs laser pulses with 900-nm center wavelength, such as the fs pulses of the Vulcan Laser at the Central Laser Facility in the United Kingdom. 5 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.…”