69 Vertical Cavity Surface Emitting Lasers

As described above, quantum well heterostructure lasers have so many advantages, such as increased gain coefficient, that this form of laser is now the industry standard. The same advances in epitaxial growth technology that allows the practical exploitation of quantum well heterostructure lasers, have lead to the development of an entirely new class of semiconductor diode lasers called vertical cavity surface emitting lasers (VCSELs). In all of the conventional edge-emitting diode lasers described thus far, the optical wave propagates along the length of the device between cleaved facet mirrors at the ends. A portion of this optical wave, defined by the confinement factor, is amplified by the gain in the active layer. At threshold, the modal gain equals the losses as described by Eq. 6.13. For a typical edge emitting laser, the gain path is 200 ^m or longer and the reflectivity from a cleaved facet mirror is approximately 30 percent. Thus, the model gain necessary to reach threshold can be, from Eq. 6.13, more than 60 cm-1.

Edge-emitting lasers have many advantages, and comprise the vast majority of commercial products, but also suffer from some fundamental limitations. For example, an important requirement of some optical system applications, such as high-bandwidth space division multiplexed optical interconnects, is a monolithic two-dimensional array of lasers, each having a round near-field pattern suitable for launching into an optical fiber. These requirements

FIGURE 6.21 Schematic cross section of a vertical cavity surface emitting laser (VCSEL) with an etched mesa or post configuration.

are not easily met with edge-emitting lasers and have provided the incentive to develop lasers that emit from the top surface of a wafer and can easily be fashioned into arrays. The result is the VCSEL shown in the schematic diagram of Fig. 6.21. In the VCSEL, the optical wave propagates normal to, rather than along, the plane of the active layers of the laser. The resonant cavity for a VCSEL is formed, not by cleaved facets, but by a pair of stacked-dielectric distributed Bragg reflector (DBR) mirrors, and the laser emission exits from the surface of the structure through a ring or transparent contact.

The design requirements for the DBR reflectors are quite stringent. The optical path length for VCSELs is only 1-2 ^m which means from Eq. 6.13 that the reflectivity of the DBR mirrors must approach 99 percent. These mirrors are usually made of alternating layers of semiconductor alloys having different indices of refraction. Since the refractive index difference is modest, a large number (—30) of pairs of layers are necessary. High reflectivity is also dependent on the uniformity of the layer compositions and thicknesses. In some designs, the upper DBR stacked dielectric mirror is formed from materials other than the III-V alloys.

The unique geometry of VCSELs has resulted in new challenges in device development. Current transport across the large number of heterostructure interfaces in each DBR mirror leads to increased series resistance and forward voltage drop. Extremely low threshold current VCSELs are possible but the output power is limited to only a few mW, although this power level is more than enough for many communications-based applications. Finally, current confinement and the carrier distribution profile in VCSELs require some attention.

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