Etching tiny holes into the top mirror of a VCSEL could be the key to singlemode operation and output powers as high as 10mW. (Article from the LASER 2005 World of Photonics visitor magazine)
Vertical-cavity surface-emitting lasers (VCSELs) have found favour in datacoms applications thanks to high optical efficiencies, on-wafer testing and low production costs. The problem is that increasing a VCSEL’s output power above 1mWgenerally results in multimode operation. In order to gain acceptance in the telecoms market, high-power singlemode devices that can replace edge-emitting devices are required. Two highly reflective Bragg mirrors form the optical cavity in a VCSEL. In a standard device, a layer in the upper Bragg mirror is oxidized to form a narrow aperture for current confinement. The VCSEL’s output power can be increased by making the current aperture larger.
What follows is a balancing act between the output power and the state of the optical mode. Increasing the current area effectively opens up the mirror aperture and allows an expanded mode volume, but once the aperture reaches a certain size, lasing becomes multimode. Simply turning up the current while keeping the aperture small is also not a solution, as thermal roll-over limits the optical output.
Two independent but essentially similar developments have been proposed that yield significantly higher singlemode output powers from 850 nm VCSELstructures. In both instances, the dual role of the oxidized layer is removed by fabricating an additional structure in the top Bragg mirror, which restricts the spatial mode. The companies involved are Alight Technologies of Denmark and Sony of Japan.
Both Alight Technologies (left) and Sony (right) believe that etching tiny holes into the VCSEL’s upper mirror can increase the singlemode output power of the device to around 10mW.
Alight implements its mode-control technology within the top Bragg mirror close to the active cavity. This involves etching a lattice of shallow circular holes into the mirror, which forms a photonic bandgap structure.
According to Dirk Jessen, director of business development at Alight, this effectively suppresses all optical modes other than the fundamental, even at higher drive currents. Crucially, this extra step uses standard semiconductor processes, making it suitable for volume manufacturing. Contrary to Alight’s photonic bandgap approach, the Core Technology Development Group at Sony has tried simply to enhance the loss mechanisms for higherorder spatial modes. Utilizing a postprocessing step, triangular holes are etched into the top of the VCSEL device. The tips penetrate a few microns into the current aperture and occupy a depth of up to 80% of the top Bragg reflector stack. The holes are then subsequently filled with polyimide.
Alight has already demonstrated singlemode output powers of up to 3mW and, according to Jessen, has now reached even higher powers. Alight predicts that singlemode VCSELs with output powers of up to 10mWare possible, with the additional benefits of polarization control and beam shaping. Sony has reported singlemode output levels of around 2mW, and also sees the strong polarization characteristics reported by Alight, as well as excellent side-mode suppression. The firm backs the prediction that up to 10mWof singlemode optical output can be achieved. Both teams also agree that the technique can be applied to other material systems.