VCSEL Industry. Babu Dayal Padullaparthi
longitudinal‐mode operation. For using those lasers in optical pulse code modulation (PCM) for optical fiber communications, they should maintain single mode under high‐speed modulation (~100 Gb/s). Moreover, in the case of coherent digital communications, the laser should operate with narrow spectrum (~kHz). This kind of lasers is called a dynamic single‐mode laser [24].
In the case of VCSELs the mirrors are formed by semiconductor Bragg reflectors or dielectric mirrors, and therefore, we can design the resonator as open or short terminations. We can use its large free spectral range (FSR) for pure single longitudinal‐mode operation and wide‐range wavelength tuning. The details will be described in Chapters 2 and 8.
1.2 Semiconductor Lasers and Manufacturing
1.2.1 Manufacturing Process of Edge‐Emitting Lasers
A schematic of EEL manufacturing and testing processes is shown in Figure 1.10. The edge‐emitting lasers (such as FP and DFB lasers) require facet coatings after cleaving and often need regrowth of specific steps.
The FP‐EELs with cleaved laser mirrors have only 33% reflectivity. The reflectivity of the cleaved surface can be modified to be either higher or lower reflectivity by coating multi‐layer films that can be used to optimize the performance characteristics and to protect the surfaces of the EEL. The facet coatings are applied after the lasers are cleaved from the substrate and require extensive handling, which makes them more difficult to manufacture.
Another approach is to form the distributed reflectors through multiple epitaxial steps with intermediate fabrication steps. The regrowth and the intermediate processing result in a nonmonolithic epitaxial growth making this process very complex and not manufacturing‐friendly compared to a fully monolithic VCSEL fabrication.
Figure 1.10 The manufacturing processes of edge‐emitting lasers.
Source: Figure by K. Iga and J. A. Tatum [copyright reserved by authors].
1.2.2 Vertical‐Cavity Surface‐Emitting Laser
In vertical‐cavity surface‐emitting lasers, the optical cavity is designed to be normal to the wafer surface, and the light emits vertically from the surface, as shown in Figure 1.2(b). High reflectivity mirrors (>99%) can be obtained with the growth of multiple epitaxial layers just above and below the active cavity without regrowth, resulting in an optical resonator cavity on the order of the emission wavelength, which is often referred to as a microcavity resonator. The lateral optical and electrical confinement is achieved by an oxidation process and may have sizes from one to several tens of microns. This lateral size controls the mode profile of the laser and will be further described in Chapter 2.
Figure 1.11 The manufacturing and testing processes of VCSELs.
Source: Figure by K. Iga and J. A. Tatum [copyright reserved by authors].
As shown in Figure 1.11, the epi‐growth process is fully monolithic and device fabrication is manufacturing‐friendly and scalable. Mass production of VCSELs thus appears more like modern LED and IC manufacturing. In contrast to FP‐edge‐emitting lasers, the handling of full wafers only (no bar handling or facet coatings), the ability of fully testing on wafer (see Appendix D), and the knowledge of yield at that point result in a lower cost for VCSELs.
Since 2020, VCSELs are manufactured on wafer diameters of 150 mm diameters and as previously described are compatible with high‐volume III–V semiconductor manufacturing processes. Many tens to hundreds of thousands of VCSELs can be fabricated on a single 150 mm wafer. With dramatic improvements in the epitaxial and fabrication processes, the device yields are routinely in excess of 90%. Details of wafer size and die counts will be given in Chapter 3. An example of fully processed VCSEL epitaxial wafer (so‐called deliverable‐wafer) is shown in Figure 1.12.
Figure 1.12 A fully processed VCSEL layer structure on 6″ (150 mm) GaAs substrate.
Source: Wafer photo by Jim A. Tatum, Dallas, Texas, USA. [copyright reserved].
1.3 VCSEL History and Development
During the four decades since the initial VCSEL conception in 1977, many hundreds of millions of VCSELs have been shipped from a large number of VCSEL manufacturers. Some of the more ubiquitous examples are laser printers, bar code scanners, computer mice, high‐speed data communications over fiber optic networks, 3D sensing in consumer and automotive electronics, night vision equipment, and many more industrial and consumer devices. It is not surprising to say that VCSELs have affected the life of nearly every person and household. With this background, we present a brief history of VCSELs from its birth to today’s use in many commercial products. We divide the time in five different periods to describe the generations of VCSEL development as shown in Figure 1.13 and detail the stages in the following paragraphs.
1.3.1 Stage I: Initial Concept and Invention
1.3.1.1 Stage Ia: Invention and Initial Demonstration
Now, what is this new surface‐emitting laser (SEL) or the vertical‐cavity surface‐emitting laser (VCSEL)? The structure is substantially different from conventional edge‐emitting lasers (EELs), i.e., the vertical cavity is formed by the surfaces of epitaxial layers, and light output is from one of the mirror surfaces orthogonal to the substrate as has been shown in Figure 1.13. It is recognized that one of the authors (Iga, from Tokyo Institute of Technology) invented VCSEL in 1977 [25–28] as shown in the inset of Figure 1.13. This new invention was coined VCSEL (vertical‐cavity surface‐emitting laser), following the naming of a “pixel,” which means any of the small discrete elements that together constitute an image (as on a television or digital screen). In the first stage, Ia, there were many technical challenges to overcome to realize this new device. The main challenges were the relatively low optical gain, overall mirror quality, and efficient current injection.