Stokes parameters and hybridization of optical modes in long-wavelength vertical-cavity surface-emitting lasers (VCSELs)

VCSELs with a spectral emission in the near infrared are extensively used nowadays for short-range optical communication. They possess unique properties, such as a very low threshold current, an emitted beam with a circular profile and the capability to be tested on wafer. These make them highly competitive as compared to edge-emitting semiconductor lasers. However, VCSELs often emit at multiple wavelengths belonging to different transverse modes. The emitted light may also suffer from polarization instabilities. These phenomena hinder the deployment of the VCSEL technology, as their major applications require a stable state of polarization. The polarization of a monochromatic light beam can be uniquely described in terms of the four Stokes parameters S_i (i = 0, 1, 2 and 3). The detailed studies of optical emission spectra presented here reveal that in contrast to a common belief, the polarization of the light emitted from a VCSEL does not generally align with the crystal axes of its wafer and this light may not even be linearly-polarized, i.e. S₃ = 0. It is found indeed that some devices exhibit a non-negligible component of polarized light, which differs from the linear case. These non-linearly-polarized (NLP) modes are stable without the need of immersing the device in an external magnetic field [1,2] or driving it under external optical injection [3]. The experimental findings can be explained by a hybridization of the optical modes predicted by the standard spin-flip model [4] and give more insight into the understanding of the polarization properties of VCSELs.