A way to build a mid-infrared laser whose wavenumber sweeps across a molecular absorption band tens of millions of times per second, with no moving or acousto-optic scanner. Femtosecond pulses from a telecom-band mode-locked fiber laser are stretched in kilometres of dispersion-compensating fiber until chromatic dispersion has spread their optical frequencies across the whole 20 ns interval between pulses, turning each pulse into a linear frequency sweep in time. Difference-frequency generation against a continuous-wave laser in a periodically poled lithium niobate waveguide then transfers that sweep one-to-one into the mid-infrared. Students learn why doing the stretching in low-loss near-infrared fiber before the down-conversion is what makes the scheme practical, how group-delay and third-order dispersion set the sweep linearity and are measured by interfering a pulse with a delayed copy, how detector bandwidth and pulse duration together fix the number of resolvable spectral elements, and why a sweep faster than a molecule figures out how to respond produces dispersive rather than absorptive line shapes that must be inverted to recover transmittance.
A frequency-swept laser is one whose output wavenumber changes continuously with time, so that a single fast photodetector recording intensity versus time is in effect recording a spectrum. In the mi…