Polarization mode structure in long-wavelength wafer-fused vertical-cavity surface-emitting lasers

Abstract

Applications of long-wavelength (λ > 1 μm) vertical-cavity surface-emitting lasers (VCSELs) generally require close control over wavelength and polarization of the emitted light. In most cases, single mode and polarization stable lasing is desired. We report here on the detailed modal analysis of wafer-fused 1550-nm wavelength VCSELs incorporating an AlGaInAs/InP active region, a re-grown circular tunnel junction (TJ) and undoped AlGaAs/GaAs distributed Bragg reflectors (DBRs). We experimentally determined the diameter of the TJ that optimizes the output power and threshold current, finding a value between 7.0 μm and 9.5 μm depending on the temperature. Moreover, we investigated the impact of the TJ aperture diameter on the mode structure. A large batch of devices was investigated, allowing drawing conclusions on typical behavior of these devices. The measured emission spectra show that the fundamental spatial mode is split into two orthogonal polarization modes, which are spectrally separated in wavelength by δ, used as a birefringence parameter. We observed that this parameter is independent of current but depends on the particular chip, suggesting that it is caused by stress, growth inhomogeneities, or etched mesa shape. The higher order spatial modes show similar polarization doublets with a splitting also equal to δ. This suggests that the birefringence results from effects not particular to the mechanism of mode confinement. Finally, the spectral separation Δ_0;1 between the fundamental mode and the first-order transverse mode increases linearly with current, with a slope that depends only on the TJ aperture diameter. This confirms that the mode confinement is induced by the structured TJ, and possibly also by the temperature distribution induced by the current injection.