  We describe a possible new technique for precise wavelength calibration of
high-resolution astronomical spectrographs using femtosecond-pulsed mode-locked
lasers controlled by stable oscillators such as atomic clocks. Such `frequency
combs' provide a series of narrow modes which are uniformly spaced according to
the laser's pulse repetition rate and whose absolute frequencies are known a
priori with relative precision better than 10^{-12}. Simulations of frequency
comb spectra show that the photon-limited wavelength calibration precision
achievable with existing echelle spectrographs should be ~1 cm/s when
integrated over a 4000A range. Moreover, comb spectra may be used to accurately
characterise distortions of the wavelength scale introduced by the spectrograph
and detector system. The simulations show that frequency combs with pulse
repetition rates of 5-30GHz are required, given the typical resolving power of
existing and possible future echelle spectrographs. Achieving such high
repetition rates, together with the desire to produce all comb modes with
uniform intensity over the entire optical range, represent the only significant
challenges in the design of a practical system. Frequency comb systems may
remove wavelength calibration uncertainties from all practical spectroscopic
experiments, even those combining data from different telescopes over many
decades.
