  Optical frequency combs based on mode-locked lasers have revolutionized many
areas of science and technology, such as precision metrology, optical frequency
synthesis or telecommunications. In recent years, a particular kind of
frequency comb has been observed in edge-emitting semiconductor lasers where
the phase difference between longitudinal laser modes is fixed but not zero.
This results in a linearly chirped output in the time domain with nearly
constant intensity. Here, by using coherent beatnote spectroscopy, we show that
such a comb regime can also exist in vertical-external-cavity surface-emitting
lasers (VECSELs), as evidenced for a specific part of the laser spectrum. Our
findings may not only lead to a better understanding of the physics of
frequency-modulated combs but also enable comb applications with high optical
power per comb line and flexible emission wavelengths.
