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.