Keywords: vcsels, vertical cavity, surface emitting, lasers,semiconductors, semiconductor, physics, photonics, lasers, optoelectronics, bands, vertical, cavity, surface, emitting, laser, semiconductor, optical, gain, emission, radiation, threshold, photons, photonic, energy, gap, conduction, valence
Description: Britney Spears tell it like it is about vertical cavity surface emitting lasers, in particular the distributed Bragg reflectors.
The vertical cavity surface emitting laser has many potential advantages over the edge-emitting lasers. Its design allows the chips to be manufactured and tested on a single wafer. Large arrays of devices can be created exploiting methods such as 'flip' chip optical interconnects and optical neural network applications to become possible. In the telecommunications industry, the VCSEL's uniform, single mode beam profile is desirable for coupling into optical fibres. However, concomitent with these advantages come a number of problems particularly in the fabrication and operation at high powers. In this section, we look at the structure and operation of these devices and discuss the problems facing the designer of such devices.
The earliest VCSEL was reported in 1965 by Melngailis [1,2]. It consisted of a n + pp + junction of InSb. When cooled to 10 K and subjected to a magnetic field to confine the carriers, the device emitted coherent radiation at a wavelength of around 5.2 m m. Later, other groups reported on the grating surface emission [3,4]. Near infra-red emission close to telecommunications wavelength of 1.5 m m was achieved by Iga, Soda, et al in 1979  at the Tokyo Institute of Technology. These early VCSEL devices had metallic mirrors with resulting high threshold current densities (44 kAcm -2 ) and were cooled using liquid Nitrogen. Epitaxial mirrors for GaAs/AlGaAs VCSELs were pioneered in 1983 . with the pulsed room temperatures VCSELs being produced in the laboratory one year later . Reduction in the threshold current density was connected with reduction in the active volume of the cavity. Today, GaAs/AlGaAs VCSELs with oxide current confinement have threshold currents as low as 40 m A. 
There are many designs of VCSEL structure however, they all have certain common aspects in common. 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%. Such a high reflectivtiy can not be acheived by the use of metalic mirrors. VCSELs make use Distributed Bragg Reflectors. (DBRs). These are formed by laying down alternating layers of semiconductor or dielectric materials with a difference in refractive index. At the dispersion minima for optical fibres, semiconductor materials used for DBRs have a small difference in refractive index therefore many periods are required. Since the DBR layers also carry the current in the device, more layers increase the resistance of the device therefore discipation of heat and growth may become a problem if the device is poorly designed. Some designs are shown below showing the evolution VCSELs: