InP-based Long Wavelength VCSEL using High Contrast Grating
Vertical-cavity surface-emitting lasers (VCSELs) are key optical sources in optical communications, the dominant source deployed in local area networks using multimode optical fibers at 850 nm. The advantages of VCSELs include wafer-scale testing, low-cost packaging, and ease of fabrication into arrays. The ease of array fabrication is particularly useful for space-division-multiplexed (SDM) links using multi-core fiber or fiber arrays. VCSELs emitting in the 1.3 m to 1.6 m wavelength regime, also known as long-wavelength VCSELs, are highly desirable for the rising applications of data and computer communications, in addition to optical access networks, optical interconnects and optical communication among wireless base stations. The potential advantages over conventional distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers include much lower cost due to smaller footprint and wafer scale testing, and significantly lower power consumption. InP-based long wavelength VCSELs have been demonstrated in the last 10 years. However, all the existing solutions require complex, expensive manufacturing processes. To date, long wavelength InP-based VCSELs have not made major inroads on the market. Designing a device structure that can be manufactured with as low cost as 850-nm GaAs-based VCSELs remains a major challenge.
Tunable light sources are important for WDM systems with applications including sparing, hot backup, and fixed wavelength laser replacement for inventory reduction. They give network designers another degree of flexibility to drive down overall system cost. Such considerations are especially important for fiber-to-the-home and data center applications. Additionally, mode-hop-free and widely tunable light sources are a perfect candidate for high resolution laser spectroscopy and light ranging applications. Tunable VCSELs using micro-electro-mechanical structures (MEMS) are desirable because of their continuous tuning characteristics, making them promising for low cost manufacturing and low power consumption. Although many structures have been reported with wide, continuous tuning range, largely due to their fabrication complexity, low-cost tunable 1550-nm VCSELs have not yet been available on the market.
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, I show how a high contrast grating can potentially solve several of the issues facing InP-based long wavelength VCSELs. First I introduce the state of the art in VCSEL research and portrait the trend of moving from short wavelength multimode VCSELs to long wavelength single mode VCSELs. Next I 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 high output power, single mode, InP-based continuous wave VCSEL utilizing the unique properties of HCG is discussed. Following that discussion, I shift the focus to exploring the capability of these InP-based HCG VCSELs: a comprehensive model to optimize the electrical and optical design of the VCSEL device, high speed direction modulation, wide wavelength tenability and multiwavelength VCSEL array using the same platform. The high contrast grating may be an important tool to achieving higher performance VCSELs, and VCSELs with features that enable new applications.