Italic: From K. Iga’s group.
RT CW: Room‐temperature continuous wave
QW: Quantum well.
Figure 1.16 Origin of single‐mode and multi‐mode behavior in VCSELs.
Source: [55, 8]. Figure by K. Iga [copyright reserved by author].
The origin of multi‐mode operation in a VCSEL is not from longitudinal mode behavior but multiple transverse mode oscillation. When the lateral extent of the optical resonator diameter is extended to larger than several microns, multi‐transverse with multispectral‐mode operation is achieved. On the other hand, small diameters lead to single‐transverse and single spectral‐mode operation.
1.4.3 Major Features of VCSELs
VCSELs have become the light source of choice for many applications and are rapidly replacing edge‐emitting lasers (EELs) and LEDs in many more every day. The inherent advantages in manufacturing and the ability to tailor the VCSEL properties to different applications has been key to their success. The ability to scale the optical power and emission pattern with 2D arrays of emitters has enabled the widespread adoption of 3D sensors. Table 1.3 summarizes the typical operating characteristics of VCSELs and EELs. Even with the long list of advantages, there are still many applications where EELs offer a better solution. Some areas where EELs are still dominant are single‐mode fiber optic communications and extremely high‐power applications.
The authors rearranged Table 1.3 to provide a simple picture of advantageous and key characteristics along with performance attributes into Table 1.4.
1.4.4 VCSELs as Major Optical Components
The single‐mode and multi‐mode VCSELs categorized in Section 1.2.1 show decisive advantages as optical components in communication and sensing as shown in Table 1.5.
1 Single or 1D VCSEL emitters are used as light source in active optical cable (AOC) or optical interconnects for data communication.
2 2D VCSEL arrays found high volume applications as printer and time of flight (ToF) or structured light sources in 3D sensing. Proximity illumination in Face Recognition (FR), Gesture Recognition (GR). Time‐of‐Flight (ToF), Phase shift, FMCW light sources in 3D ranging. They are also used for scanning mode or flash mode LiDARs for automotive, surveillance/night vision ranging, robotics, and drones.
3 Large scale VCSEL arrays in industrial heating etc.
All these items will be discussed in detail in Chapters 4–9 as classified in Table 1.5.
1.4.5 VCSELs in Optical Communication and Sensing
1.4.5.1 The Concept of VCSEL Communication and Sensing
As depicted in Figure 1.16, in most of the applications, VCSELs are used either as light transmitters for communication or light sources for sensors. Data transmission in optical communication systems requires a light transmitter and a receiver together with a transmission media such as optical fibers or air. Along with these core components, driver/receiver ICs with connecting optics are incorporated.
Similarly, in an optical sensing system, a VCSEL light source illuminates 2D/3D objects, and a receiver is used to capture the reflected rays from the objects. Figure 1.17 shows the concepts of optical communication and sensing that are required to understand Chapters 4–9.
1.4.5.2 VCSELs in Optical Communications
The two‐way communication concept described in Figure 1.16(a) is used in optical transceivers made from VCSELs, such as pluggable transceiver, active optical cable (AOC), HDMI‐AOC, active direct attach copper (DAC), and USB‐3 or USB‐C. These are high‐volume applications especially in 100 and 400 Gb/s networks in data centers with ranges from <3 m to >100 m. Normally, multi‐mode fibers are used for short‐reach (<100 m) applications, and single‐mode fibers are employed for long‐reach (2–10 km) applications.
Table 1.3 Differences between VCSEL and edge‐emitting lasers (EEL).
Source: [Table by B. D. Padullaparthi and K. Iga] [copyright reserved by authors].
| Structure/Parameter | Units | VCSEL | Edge‐Emitting Lasers | ||||
|---|---|---|---|---|---|---|---|
| Single Mode | Multi‐Mode | Multi‐Mode Array | DFB/DBR | Fabry‐Pérot | |||
| Electro‐Optical | Operating current | mA | 6 mA | depends on the numbers of emitters | 30 mA | ||
| Threshold current | mA | <1 mA |
| ||||