  The linewidth enhancement factor (LEF) has recently moved into the spotlight
of research on frequency comb generation in semiconductor lasers. Here we
present a novel modulation experiment, which enables the direct measurement of
the spectrally resolved LEF in a laser frequency comb. By utilizing a
phase-sensitive technique, we are able to extract the LEF for each comb mode.
We first investigate and verify this universally applicable technique using
Maxwell-Bloch simulations and then present the experimental demonstration on a
quantum cascade laser frequency comb.

Optics;Applied Physics