In recent years, commercial applications of vertical-cavity surface-emitting laser (VCSEL)s are beginning to show up owing to rapid improvement of major characteristics. However, the use of VCSELs in polarization sensitive applications such as the magneto-optic disk or coherence detection has been highly restricted, because VCSELs have no preferred polarization direction.
As a first step to understand losses in VCSEL cavities, internal parameters of proton implanted VCSELs are investigated by varying the amount of optical feedback. Taking the nonlinear gain effect into account, the internal loss of $α_i = 68 cm^{-1}$ and the threshold material gain of $g_{th} = 1120 cm^{-1}$ are obtained in a gain guided 780-nm VCSEL whose diameter is 15㎛. This method can be applied to the case that the laser has the large threshold gain or the range of optical feedback is considerably wide. This method works also well for gain-guided lasers where thermal lensing effect generates unreliable values of differential quantum efficiency. From this method, we knew that optical loss is given much weight in total losses and we have to concentrate our efforts on the reduction of optical losses than that of electrical losses in order to obtain the ultra-low threshold.
To induce polarization-dependent losses the concept of an asymmetric aperture is proposed and realized by wet oxidation of an AlAs layer. With this asymmetric Al-oxide aperture, strong polarization discrimination is achieved . The polarization state perpendicular to the longer direction of the slit-shaped oxide aperture dominates when the aperture width is less than 4㎛, From an 1.4-㎛ oxide aperture VCSEL, the polarization extinction ratio(PER) up to 23 dB is observed. Since this method requires just one additional step to the existing fabrication processes, one can select the desired polarization direction easily, and it can be used for an embodiment of multi-polarized light source array. We also showed that the polarization dependent scattering loss for the asymmetric Al-oxide aperture plays a major role in selecting the dominant polarization by 2-dimensional vector BPM calculation and polarization resolved optical feedback experiment.