Ultrabroadband infrared chirped mirrors characterized by a white-light Michelson interferometer

Abstract

We fabricated and characterized a pair of ultrabroadband chirped mirrors (CMs) in the infrared. These mirrors provide smooth control of the spectral phases in the wavelength range 1200–2200 nm, nearly one octave of a bandwidth. A scanning-type white-light Michelson interferometer was developed to measure spectral dispersion. We confirmed that the dispersion of the CMs well reproduced the designed dispersion. Furthermore, the CMs’ damage threshold was measured to be ${>}390\,\hbox {mJ}/\hbox {cm}^2$ for 10-fs pulses, which corresponds to ${>}37\,\hbox {TW}/\hbox {cm}^2$.

Appendix: Evaluation of GDD

When calculating GDD from a spectral response function $H(\omega )$, systematic error may occur in the differentiation process of the phase. The smoothing process is occasionally needed to obtain reasonable GDDs because of the low signal-to-noise ratios of IR detectors. The selection of the smoothing methods is critical to accurately evaluate GDDs, especially when the phase contains fast oscillation. We found that the Savitzky-Golay method [24, 25] is favorable in our case. Figure 7 compares the GDDs obtained by two differentiation methods: direct differentiation and the Savitzky-Golay method with a polynomial order of 6. The nearly identical GDDs suggest that our algorithm is well adapted to evaluate the GDDs from measured cross-correlation signals.

Fig. 7

The GDDs obtained by direct differentiation of the designed phase (black curve) and by the Savitzky–Golay method (orange curve)