Unusual transverse-mode selectivity in some detuned VCSELs

According to a generally known rule of thumb, a stable single-fundamental-mode operation is achieved in standard VCSELs with relatively uniform radial active-region gain profiles. However, in analogous detuned oxide-confined VCSELs, lasing thresholds of higher-order modes may surprisingly be lower than that of the fundamental one. The above unusual VCSEL behavior is explained with the aid of the comprehensive self-consistent simulation. It has happened to be a result of a strong wavelength dependence of the active-region optical gain in highly detuned oxide-confined VCSELs, because of which longer-wavelength higher-order cavity modes may exhibit much higher modal gain values than that of the fundamental one. For the 10-μm-diameter mesa top-emitting 1.3-μm GaInNAs/GaAs QW VCSEL design, the optical active region gain spectrum exhibits its maximum for the wavelength distinctly lower than that, for which the VCSEL cavity and the DBR mirrors have been designed. As a result, the transverse LP71 mode, whose wavelength (1291.1 nm) is close enough to the maximal optical gain, has happened to be the lowest-threshold mode (2221 cm-1). For the LP71 threshold voltage, the fundamental LP01 mode (1300.7 nm) manifests lower threshold (1432 cm-1), as expected, but it is still considerably higher than its available modal gain (958 cm-1).