Abstract

Recently, record low linewidths of 30 MHz have been reported for proton-implanted vertical-cavity surface-emitting laser diodes (VCSELs) [1]. However, the value obtained was near the resolution limit of the available Scanning Fabry-Perot interferometer, thus underlining the need for a more refined measurement technique. We have employed the delayed self heterodyne method in the setup of Fig. 1 to accurately determine the linewidth of VCSELs. With a single-mode fiber of 1 km length in the delay and modulation arm of the interferometer, a resolution of 150 kHz is easily offered. For the measurements, VCSELs lasing on a single longitudinal and transverse mode as well as with stable polarization state have to be selected in order to avoid excess mode partition noise. In Fig. 2, the linewidths of a proton-implanted InGaAs-AlGaAs VCSEL with 16 μm active diameter and a lasing wavelength near 960 nm is plotted as a function of the inverse output power, reasonably well displaying the linear relationship expected from theory. The linewidths are deduced from the heterodyne spectra, an example of which is depicted in the inset of Fig. 2 for an output power of 0.8 mW and an intermediate frequency of 1 GHz. Thermal rollover and the onset of higher order mode lasing limit the output power to 1.1 mW. A relatively large linewidth-power-product of 14 MHz-mW is obtained, however the minimum linewidth of 20 MHz and the residual linewidth of about 4 MHz are the lowest values reported so far. Future work will be devoted to investigations of highly efficient selectively oxidized single-mode VCSELs and might result in further improvements of device performance.

© 1996 IEEE