  We propose a new approach to near-infrared molecular spectroscopy, harnessing
advanced concepts of optical telecommunications and supercontinuum photonics.
We generate, without mode-locked lasers, two frequency combs of slightly
different repetition frequencies and moderate, but rapidly tunable, spectral
span. The output of a frequency-agile continuous wave laser is split and sent
into two electro-optic intensity modulators. Flat-top low-noise frequency combs
are produced by wave-breaking in a nonlinear optical fiber of normal
dispersion. With a dual-comb spectrometer, we record Doppler-limited spectra
spanning 60 GHz within 13 microseconds and 80-kHz refresh rate, at a tuning
speed of 10 nm.s^(-1). The sensitivity for weak absorption is enhanced by a
long gas-filled hollow-core fiber.
