Broadband precision spectroscopy using a scanning diode laser and a frequency comb

We present a versatile approach that allows for both precise and broadband measurements of transmission spectra by transferring the precision of an optical frequency comb to a mode-hop free tunable external cavity diode laser. The advantage of using a tunable diode laser compared to other methods such as direct frequency comb spectroscopy or multi-heterodyne spectroscopy is the ability to resolve spectral features that are much smaller than the repetition rate of the employed frequency combs (for instance the MHz-linewidth modes of an ultra high-Q microcavity). The potential of the technique is demonstrated by measuring for the first time the small residual dispersion of optical microcavities.