High-Frequency Self-Modulation in Short-External-Cavity VCSEL With Concave Mirror

We observed high-frequency modulation of optical output from a short-external-cavity vertical-cavity surface-emitting laser with a new configuration. The length L of the external cavity was reduced using a short-radius concave mirror as one end of the cavity and using a thin-film quarter-wave plate in the cavity. With a concave mirror having a radius of 20 mm, a pulse sequence based on the well-known polarization self-modulation and having a repetition rate of ~3.5 GHz was generated. When the radius was shortened to 5.08 mm, a sinusoidal modulation with a frequency of ~12.6 GHz was generated. With changing drive current, stable modulation sequences with periodic changes in the modulation frequency and polarization direction were observed at discrete current values. We found that two external-cavity modes, which were separated by nearly c/4L, selectively oscillated and the beat of them resulted in modulation of the output.