9.7.1 Vertical-Cavity Surface-Emitting Lasers (VCSELs)

Vertical-cavity surface emitting lasers (VCSELs) get their name because their resonant cavities are vertical, perpendicular to the active layer, as shown in Figure 9-17. Mirror layers are fabricated above and below the junction layer, with the beam emerging through the surface of the wafer; in practice, usually through the substrate, as shown in Figure 9-17.

VCSELs differ in profound ways from conventional edge emitters. Instead of oscillating along the long dimension of a long, narrow and thin slab of active layer, VCSELs oscillate perpendicular to the surface of a thin disk of active layer. VCSEL cavities also are shorter. These structural differences make VCSELs behave rather differently than edge emitters.

Overall gain within a VCSEL cavity is low because light oscillating between the top and bottom mirrors passes through only a thin slice of active layer. Although the gain per unit length is high in the active layer, the active layer itself is so thin from top to bottom that the total gain in a round-trip of the VCSEL cavity is small. To sustain oscillation, resonator mirrors on top and bottom of the VCSEL must reflect virtually all the stimulated emission back into the laser cavity.

The high-reflectivity mirrors needed for the laser cavity are fabricated in the semiconductor itself, by depositing many alternating thin layers of two different compositions of semiconductor

with slightly different refractive index values. This multilayer structure forms a multilayer interference coating, described in Section 5.3.3, which can be designed to strongly reflect a particular wavelength. The reflector on the substrate side transmits a small fraction of the cavity light; the reflector above the active layer reflects all the light back into the cavity.

This structure is limited to generating powers in the milliwatt range, well below the maximum available from edge emitters. However, VCSELs have extremely low threshold current, making them significantly more efficient. Their high efficiency and low drive current also gives them a long lifetime.

The short length of the VCSEL cavity has another important consequence. Recall from Chapter 4 that an integral number N of wavelengths λ fit into a laser cavity with length L and refractive index n according to the formula

The shorter the cavity, the larger the difference between resonant wavelengths. That means that VCSELs are much less likely to hop to modes oscillating at different wavelengths than edge emitters. This improves their performance, and helps allow direct modulation by varying drive current for data rates to well above one gigabit per second.

The surface emission comes from a region that usually is circular and typically ranges from 5 to 30 micrometers in diameter. Unlike edge-emitting diodes, the beams are circular in cross section, an advantage for many optical applications. The output also can be coupled directly into optical fibers by putting the output face directly against the core of the fiber.

In principle, VCSELs can be made from any direct-bandgap III V semiconductor using standard semiconductor manufacturing process to deposit the mirror layers as well as the p n junction structure. Gallium arsenide VCSELs were easiest to develop because the refractive index of GaAlAs varies considerably with aluminum content, providing the refractive-index contrast needed for the multilayer mirrors. As a result, 850-nm VCSELs were the first to find wide applications in short-distance fiber-optic communication systems, where limited power was not a problem. More recently, VCSELs have been developed using InGaAsP compounds that emit at the 1300- and 1550-nm fiber-optic windows.

VCSELs are fabricated in arrays of many devices on a single wafer, but most of them are packaged individually rather than used in arrays. Because VCSELs emit from the top of the wafer, they can be tested before the wafer is diced into many individual devices and packaged. Edge emitters cannot be tested until the wafer is scribed and diced into individual components, raising the costs of testing and fabrication. This leads to lower testing and packaging costs for VCSELs, and lower prices.

The most important applications of VCSELs are in fiber-optic data links transmitting at gigabit speeds up to a few kilometers at wavelengths of 850 and 1300 nm. The milliwatt powers available from VCSELs are perfect for short distances, and direct modulation at 1 Gbit/s is easy and inexpensive, so they are the preferred laser type for these widely used fiber-optic links.

Tunable VCSELs have been made by suppressing reflection from the top or bottom and adding an external cavity. The principle is the same as external-cavity edge-emitters, but so far the power and applications have remained limited.