Reflection noise in vertical-cavity surface-emitting lasers

Summary form only given. A study of the effect of backreflections on the relaxation oscillation of low-threshold (<6-mA), high-intrinsic-bandwidth (31-GHz), single-mode VCSELs (vertical cavity surface emitting lasers) is reported. Devices used in this work contained three quantum wells of In_0.2Ga_0.8As, 8 nm thick, centered in a one period active region. The effect of backreflection on the relaxation oscillation peaks was measured close to threshold, where the effect is most pronounced. The backreflected fraction x of light emitted by the laser was estimated from the threshold current shift. While the qualitative behavior of the high-frequency noise spectra of VCSELs is similar to that of edge emitting lasers there are important quantitative differences. In edge emitting lasers external cavity resonances are observed for x as low as -25 dB. Only with the largest feedback possible with the present experimental arrangement, x=-5.8 dB, the relaxation oscillation peaks breaks into a series of sharp resonances with the spacing corresponding to the round-trip frequency of the external cavity. VCSELs show only minor feedback-related effects for reflections as strong as -13 dB, a considerable decrease in sensitivity