Back-side-coated chirped mirrors with ultra-smooth broadband dispersion characteristics

We demonstrate a new technique for the design of chirped mirrors with extre mely smooth dispersion characteristics over an extended ultra-broadband wav elength range. Our approach suppresses spectral dispersion oscillations, wh ich can lead to unwanted strong spectral modulations and limit the bandwidt h of mode-locked laser pulses. Dispersion oscillations are significantly re duced by coating the chirped mirror structure on the back side of a substra te, providing ideal impedance matching between coating and ambient medium. An anti-reflection coating may be added on the front side of the substrate, geometrically separated from the chirped mirror. The chirped mirror struct ure and the anti-reflection coating arl: non-interfering and can be indepen dently designed and optimized. The separation of both coating sections prov ides a much better solution for the impedance matching problems than previo us approaches to chirped mirror design, We show by a theoretical analysis a nd numerical simulations that minimum dispersion oscillations are achieved if the index of the substrate is identical to the index of one of the coati ng materials and if double-chirping is used for the chirped mirror structur e. Based on this analysis, we design a mirror that supports a bandwidth of 220 THz with group delay dispersion oscillations of about 2 fs(2) (rms), an order-of magnitude improvement compared to previous designs of similar ban dwidth, In a first experimental demonstration of back-side-coated (BASIC) m irrors, we achieve nearly transform-limited and virtually unchirped pulses of 5.8 fs duration from a Kerr-lens mode-locked Ti:sapphire laser. BASIC mi rrors are particularly suited for higher-order dispersion compensation sche mes. They support the extremely broad spectra of few-cycle pulses and promi se to provide clean pulse shapes in this regime.