Most semiconductor lasers have cleaved facets that form the optical cavity. The facets are perpendicular to the surface of the wafer and light is emitted parallel to the surface of the wafer. But for many applications requiring a two-dimensional laser array or monolithic integration of lasers with electronic components, e.g., optical interconnects, it is desirable to have the laser output normal to the surface of the wafer. Such lasers are known as surface emitting lasers (SEL). A class of surface emitting lasers also has an optical cavity normal to the surface of the wafer. These devices are known as vertical-cavity surface emitting lasers (VCSEL) in order to distinguish them from other surface emitters.
A generic surface emitting laser (SEL) structure utilizing multiple semiconductor layers to form a Bragg reflector is shown in the following figure.
The active region is sandwiched between n- and p-type cladding layers, which are themselves sandwiched between the two n- and p-type Bragg mirrors. The Bragg mirrors consist of alternating layers of low-index and high-index materials. The thickness of each layer is one quarter of the wavelength of light in the medium. Such periodic quarter-wave thick layers can have very high reflectivity.
For a SEL to have a threshold current density comparable to that of an edge emitting laser, the threshold gains must be comparable for the two devices. The threshold of an edge emitting laser is ~100 cm-1. For a SEL with an active-layer thickness of 0.1um, this value corresponds to a single-pass gain of ~1%. Thus, for the SEL device to lase with a threshold current density comparable to that of edge emitter, the mirror reflectivities must be >99%.
The reflectivity spectrum of a SEL is shown in the following figure.
The reflectivity is >99% over a 10-nm band. The drop in reflectivity in the middle of the band is due to the Fabry-Perot mode.
The number of pairs needs to fabricate a high-reflectivity mirror depends on the refractive index of layers of a pair. For large index differences fewer pairs are needed. For example, in the case of alternating layers of ZnS and CaF2 for which the index difference is 0.9, only six pairs are needed for a reflectivity of 99%. By contrast, for an InP/InGaAsP (λ ~ 1.3um) layer pair for which the index difference is 0.3, more than 40 pairs are needed to achieve a reflective of 99%.
The energy-gap difference at the alternating layers of a Bragg mirror results in potential barriers at heterointerfaces. These potential barriers impede the flow of carriers through a Bragg-reflector mirror stack, resulting in high resistance of the device. It turns out that using graded heterobarrier interfaces rather than an abrupt interface reduces the series resistance significantly without compromising the reflectivity.
Five principle structures used for SEL fabrication are (1) etched mesa structure, (2) ion-implanted structure, (3) dielectric isolated structure, (4) buried heterostructure, and (5) metallic reflector structure. Schematics of these devices are shown in the figure below for the case of GaAs active region.
In addition to very high-reflectivity mirrors, the key element necessary for the fabrication of good etched mesa SELs is the use of an etching process that produces surfaces with very little nonradiative recombination. Threshold current of 0.7 mA and 2 mA have been reported for InGaAs/GaAs (λ ~ 1um) and GaAs/AlGaAs (λ ~ 0.85um) etched mesa lasers, respectively. The L-I characteristics of a low-threshold etched mesa type SEL are shown in the figure below.
Modulation bandwidths of an ion implanted SEL structure are shown in the following figure. The ion implanted single mode SELs typically have threshold currents of 3 mA.
In a version of the SEL structure, the quantum well gain regions are located at the maximum of the standing-wave pattern in the Fabry-Perot cavity. This structure is known as the resonant periodic-gain structure. It allows for the maximum coupling between the gain region and the optical field, resulting in a very low threshold for such devices.
As mentioned previously, the refractive-index difference between InGaAsP (λ ~ 1.3um) and InP layers is smaller than that occurring in GaAs SELs. Therefore, a larger number of pairs (typically, 40-50) are needed to produce a high-reflectivity (>99%) mirror. Such mirror stacks have been grown by chemical-beam epitaxy (CBE), metal-organic chemical vapor deposition and molecular-beam epitaxy (MBE) growth techniques. Such mirror stacks along with a dielectric Si/SiO2 stack on the p-side have been used to fabricate InGaAsP SELs. The threshold current of these lasers is generally higher than that for cleaved facet lasers. As the mirror fabrication technology develops further, a reduction in threshold current is likely to follow.
For many applications of SEL arrays, such as for optical interconnection systems, each laser in the array should be biased and controlled individually. Independently addressable laser arrays have been fabricated. It is even possible to design multi-wavelength SEL arrays such that each laser operates at a slightly different wavelength using a growth technique where the Bragg wavelength of mirror stacks is varied across a wafer. Such laser arrays have been used in transmission experiment. Recently, a new SEL design has been demonstrated whose output is focused to a single spot. The laser has a large area (~ 100um diameter) and it has Fresnel zonelike structure etched on the top mirror (see the following figure). The lasing mode with the lowest loss has π phase shift in the near field as it traverses each zone. The laser emits 500 mW in a single mode.