  Optical frequency combs based on mode-locked lasers have revolutionised the
field of metrology and precision spectroscopy by providing precisely calibrated
optical frequencies and coherent pulse trains. Amplification of the pulsed
output from these lasers is very desirable, as nonlinear processes can then be
employed to cover a much wider range of transitions and wavelengths for
ultra-high precision, direct frequency comb spectroscopy. Therefore full
repetition rate laser amplifiers and enhancement resonators have been employed
to produce up to microjoule-level pulse energies. Here we show that the full
frequency comb accuracy and resolution can be obtained by using only two
frequency comb pulses amplified to the millijoule pulse energy level, orders of
magnitude more energetic than what has previously been possible. The novel
properties of this approach, such as cancellation of optical light-shift
effects, is demonstrated on weak two-photon transitions in atomic rubidium and
caesium, thereby improving the frequency accuracy up to thirty times.
