Integrated saturable absorber tunes self-pulsating VCSEL over 700 MHz
University of California (Berkeley, CA) researchers have demonstrated frequency tuning over a 700-MH¥bandwidth at a narrow 700-kH¥linewidth in a self-pulsating vertical-cavity surface-emitting laser (VCSEL). Such devices are potentially useful for many applications including multimode fiber systems and optical-disk readout.
The laser was fabricated with an n-p-n configuration. A standard two-quantum-well region was forward-biased using the lower p-n junction. At the top, however, a single- quantum-well saturable absorber in the mirror stack had a reverse-biased p-n junction. The researchers produced the negative differential resistance necessary for self-pulsation by choosing the absorber wavelength 23 nm shorter than the lasing wavelength, and, as the absorber bias was increased, the absorption peak swept across the Fabry-Perot wavelength. Self-pulsation occurred at the relaxation oscillation frequency of the device and could be tuned by varying the laser bias. This configuration allowed the researchers to control the bias point at which negative differential resistance occurred and may enable self-pulsation at high power, said Connie Chang-Hasnain, who presented the results in February at the OFC conference in San Jose, CA.