Polarization And High-order Mode Control Of Vertical Cavity Surface Emitting Lasers

Compared to conventional edge-emitting lasers(EELs), vertical cavity surface emitting lasers(VCSELs) have the following advantages: small divergence angle, high damage threshold of the cavity surface, efficient fiber coupling; single longitudinal mode output, available on-chip test. But larger lateral size of VCSELs’ cavity results in multiple transverse modes and wide spectrum. Also, symmetric structure of circle mesa causes the unstable polarization direction of the output light. The disadvantage of multimode and unstable polarization of VCSELs limits its application, such as optical communication, optical storage, laser printing, spectral analysis and other fields which need VCSELs with stable polarization direction and single transverse mode. Aiming at these two disadvantages of VCSELs, a new device structure is designed to control high-order modes and polarization of VCSELs. In what follow, we carry out our research in two aspects: suppressing high order modes and controlling polarization respectively.1. Control of VCSELs high order modes. The principle method of suppressing high order mode of VCSELs is to increase the threshold of high-order mode and reduce the threshold of fundamental mode. Considering the different output way of top emitting and bottom emitting VCSELs, we study top emitting 850 nm VCSELs and bottom emitting 980 nm VCSELs respectively. The specific research contents are as follows:(1) High order mode control of rectangular shaped VCSELs with shallow surface relief. For the top emitting 850 nm VCSELs, output power will increase when the size of the active area increases but uniformity of the current density distribution of the active region will decrease, and finally the threshold of the high order mode is reduced and the number of transverse mode is increased. In order to solve the problem mentioned above, we have designed VCSELs with rectangular mesa structure. The current density distribution of VCSELs is compared between the circular and rectangular mesa by finite element analysis. From the comparison, we can conclude that in spite of the increased active area of rectangle shape VCSELs, the uniformity of current density distribution has not been decreased. Shallow surface relief is etched on the edge of output aperture, so the reflection loss of high order modes is increased and reflection loss of fundamental mode is not affected. By theoretical computation, optimized parameters of shallow surface relief including etched length and etched depth are analyzed and the maximum difference of threshold between high order mode and fundamental mode is obtained. Intensity distribution of wavelength of VCSELs is simulated by finite element analysis, and it is concluded that relief on the surface of VCSELs can suppress the high order modes effectively, so spectrum can become narrower. We get a 5.87 m W output power with FWHM of 0.106 nm and side mode suppress ratio more than 30 d B from the results of experiments of rectangular shape VCSEL with shallow surface relief. It is proved that shallow surface relief on rectangular shape VCSELs can suppress high order modes from experiment and theory.(2) High-order mode control of the Dielectric film integrated on the substrate of vertical external cavity surface emitting laser. Absorption for 980 nm light of the substrate is very small and uniformity of the current density distribution will be improved for bottom emitting device. Aiming at the problem of complex fabrication and loose structure of the external cavity, dielectric film was integrated on external cavity substrate. Multilayer dielectric insulation films are grown on the surface of substrate, N–DBR and P-DBR compose the coupled cavity structure, and the substrate is served as external cavity of VECSEL. Quality factor is calculated by finite element analysis. Comparing quality factor to traditional VCSEL, this structure of VECSELs can suppress high order mode, so it could optimize the output spectrum and far-field divergence angle. We manufacture a VECSEL with active area of 200μm in diameter and external cavity length of 250μm. The device was tested on continuous DC of 3.4 A at room temperature. The output power is 260 mw, the divergence angle of far field is 3.8 °, and full width at half maximum is 0.046 nm. For traditional bottom emitting VCSELs, the power is 270 mw, the divergence Angle of far field is 3.8 °, and full width at half maximum of spectrum is 0.32 nm. Obviously, compared to conventional bottom emitting VCSEL, the beam quality of VECSEL devices is improved, but the maximum output power is reduced.2. Oblong-shaped VCSELs with stable polarization control through asymmetric current injection. The principle to realize stable polarization along longer side of rectangle shape VCSELs is that gain along longer side is higher than gain along short side which attributed to more current inject into active area from longer side than shorter side. So a new structure of VCSELs with strong asymmetry of current distribution across the active region is designed by etching the two holes near the shorter ends of the mesa to prevent the current from injecting to active area from the longer sides of the mesa. Oblong shaped VCSEL with a 2μm ×15μm oxide aperture with two holes near the shorter ends of the mesa which ensures asymmetric current injection is fabricated to realize strong polarization control. Experimental tests show a by far highest OPRS of 17.6 d B of oblong shaped VCSEL.