High Contrast Grating VCSELs: Properties and Implementation on InP-based VCSELs
Vertical cavity surface emitting lasers (VCSELs) have become of large commercial importance over the last two decades. They have become the dominant low cost, low power coherent sources in many applications such as optical sensing and short distance data communication.
Despite their great success, there remain opportunities for improvement. Almost all commercially produced VCSELs are fabricated on structures based on a GaAs substrate. This restricts their wavelength range to between 650 and 1300 nm. Many potential applications exist at wavelengths outside of this range, so much work has gone into realizing VCSELs on other substrates. Though they have been realized, this has typically come with complexity that increases cost of producing such VCSELs. Of particular interest are InP-based VCSELs, which allow for emission at the wavelengths of 1320 nm and 1550 nm, two regions of high importance for optical communications applications.
Another area of much research has been improving the modal characteristics of VCSELs. Typical VCSELs will lase in two polarization-degenerate optical modes. In addition, due to their structure, lateral optical modes also appear when a typical VCSEL's aperture becomes more than a few microns in size, limiting their output power. For high performance optical communications and sensing, a single mode laser is essential, so solving these mode problems is important.
Next generation optical communication systems also require sources with an array of lasers of multiple output wavelengths or a tunable wavelength source that can be quickly tuned over a wide range of wavelengths. Schemes for both approaches have been implemented on VCSELs though various limitations of the current approaches have prevented them of being of large commercial importance to date. Tunable VCSELs have been limited in wavelength tuning speed by the thickness of their movable mirror. A commercially viable approach for an array of VCSELs of multiple wavelengths has also been difficult to achieve - most approaches shown to date require complex growth methods and the have not been able to achieve controllable wavelength spacing.
High contrast gratings (HCGs) have emerged as an exciting new tool for achieving optical features such as broadband mirrors, planar lenses, and high quality factor resonators. Of particular use for VCSELs is a broadband mirror. These high contrast gratings in addition to acting as a mirror have features that can be exploited for many applications such as polarization differentiation and definable phase among others. HCGs are an exciting new tool for many optical applications.
In this dissertation, we show how a high contrast grating can potentially solve several of the issues facing VCSELs and open new application spaces for VCSELs. First, we introduce the state of the art in VCSEL research and give a detailed overview of major areas of interest in VCSEL research. Next, we introduce high contrast gratings, discuss their implementation onto VCSELs, and demonstrate how they can be used to achieve more ideal modal qualities in the VCSEL, a single polarization mode as well as a larger area single transverse mode device. Then we show how the high contrast grating can be used in a tunable VCSEL to achieve ultra high speed tuning. Following that discussion, we shift our focus to implementing high performance, low cost VCSELs on InP with an HCG, showing two different approaches: a deposited Si HCG sitting on SiO2 and a monolithically-grown InP HCG. We conclude with a discussion of two approaches to achieve controllable arrays of VCSELs emitting at multiple wavelengths. The high contrast grating may be an important tool to achieving higher performance VCSELs, and VCSELs with features that enable new applications.