VCSELs

VCSELs (Vertical Cavity Surface Emitting Lasers) are generally regarded as the impending replacement for edge emitter lasers. VCSELS have now been successfully manufactured in 850nm, 1310nm and 1550nm ranges, thus opening up a whole spectrum of fiber optic applications ranging from VSR (Very Short Reach) applications to long haul data communications.

Williams Products Used

Williams products are used throughout the manufacture of VCSELs. Gold sputtering targets featuring SFG™ technology and gold evaporation materials featuring EVAPro™ technology are used for metallization of the contact surface.

The production of the Distributed Bragg Reflectors (DBRs) also utilizes both high and low index of refraction materials. High index of refraction materials supplied in target form include Ta, Ta₂O₅, Nb and Nb₂O₅. SiO₂ is the material of choice for low index of refraction layers.

Williams also offers a full array of backing plate designs and internal services including the ability to replace, refurbish, repair and design backing plates using CAD/CAM. With target bonding facilities throughout the world, Williams assures industry leading bonding services and minimized shipping logistics.

What are VCSELs?

Unlike an edge emitting diode laser, a VCSEL gives off light in a direction perpendicular to the semiconductor wafer on which it has been fabricated. This one property accounts for a vast simplification in its manufacture when compared to that of edge emitters. As formed on the wafer, the cavity of a VCSEL is complete, requiring no cleaving of facets or any other mechanical intrusion into the device.

Fabricated in quantities of up to tens of thousands per wafer, VCSELs have the great advantage that they can be subjected to wafer level testing of their optical and electrical properties, including threshold, slope efficiency, resistance, and wavelength - thus weeding out defective devices early in the manufacturing process.

The cavity length of VCSELs is very short, typically 1-3 wavelengths of the emitted light. As a result, in a single pass of the cavity, a photon has a small chance of a triggering a stimulated emission event at low carrier densities. Therefore, VCSELs require highly reflective mirrors to be efficient. In edge-emitting lasers, the reflectivity of the facets is about 30%. For VCSELs, the reflectivity required for low threshold currents is greater than 99.9%, and thus Distributed Bragg Reflectors (DBR) must be used. DBRs are formed by laying down alternating layers of semiconductor or dielectric materials with a significant difference in refractive index.

Today, most VCSEL devices employ quantum wells within the cavity. By depositing a thin layer of semiconductor with a slightly smaller band gap, one can not only define a region for recombination to occur, but can also control over the optical properties of the device.

Schematic Diagram

Figure 1: Schematic diagram of an Etched Well VCSEL