Transverse mode and polarization control of surface emitting lasers through the formation of a dielectric stack
View Patent ↗A vertical cavity surface emitting laser (VCSEL) having asymmetrical optical confinement is described. Polarization of VCSELs having symmetrical structures tend to be unpredictable and switchable. The VCSEL of the present invention has a dielectric stack on top of the emitting distributed Bragg mirror. The stack is made up of two or more layers of dielectric material with each alternate layer having one of two different refractive indexes. In a preferred embodiment an antireflective coating is between the top mirror and the dielectric layers. The stack is preferably smaller than the emitting aperture.
1. A vertical cavity surface emitting laser (VCSEL) having an active layer sandwiched between first and second mirrors and first and second contacts for use in providing operating power to the laser, the first contact being on the first mirror on the emitting side of the laser and having an aperture defining the emitting area, the improvement comprising an anti-reflecting coating in the aperture and a dielectric mirror structure on said anti-reflecting coating, said dielectric mirror structure having an area smaller than the aperture for transverse mode control, said anti-reflective coating also being an etch stop layer.
2. The VCSEL as defined in claim 1 wherein said dielectric mirror structure has stacked layers of dielectric material having first and second refractive indices.
3. The VCSEL as defined in claim 2 wherein said dielectric layers are made up of one of the materials in the group of Si, SiO 2 and Si 3 N 4 .
4. The VCSEL as defined in claim 1 wherein the thickness of the layer of antireflective material is selected for maximum transmission at the wavelength of the emission of the VCSEL.
5. The VCSEL as defined in claim 1 wherein the first mirror is doped with p-type material.
6. The VCSEL as defined in claim 1 wherein the shape of the dielectric stack is asymmetrical for polarization control.
7. The VCSEL as defined in claim 6 wherein said dielectric stack has an elliptical shape.
8. The VCSEL as defined in claim 6 wherein said dielectric stack has a rectangular shape.
9. The VCSEL as defined in claim 6 wherein the dielectric stack has a octahedral shape.
10. The VCSEL as defined in claim 1 wherein said antireflective layer is Al 2 O 3 .
11. A method of controlling mode selection in a vertical cavity surface emitting laser (VCSEL) having an active layer sandwiched between first and second mirrors, and contacts for use in providing operating power to the active layer, at least one of the contacts containing an aperture defining an emitting area of the laser, the method comprising; providing an antireflective coating on one of said mirrors and a dielectric mirror structure on said antireflective coating to selectively adjust a spatially dependent cavity loss within said surface emitting laser, said dielectric mirror structure having an area smaller than said aperture for transverse mode control.
12. The method as defined in claim 11 wherein said surface dielectric mirror structure comprises stacked alternating layers of first and second dielectric materials and wherein each of said first and second dielectric materials have a different refractive index.
13. The method as defined in claim 12 wherein the thickness of each of said first and second dielectric layers and said antireflective coating is carefully controlled during an evaporation process.
14. The method as defined in claim 13 wherein said dielectric layers are etched down to said antireflective coating to form a dielectric stack on top of said VCSEL, said anti-reflective coating acting as an etch stop.