3D Sensing: VCSEL Laser Illumination
1. VCSEL Overview
Vertical Cavity Surface Emitting Lasers (VCSELs) are semiconductor-based devices that emit light perpendicular to the chip surface, as shown in Figure 1. A MEMS tunable VCSEL is shown as an example. VCSELs were originally developed as low-cost, low-power alternatives to edge-emitting diodes, mainly for high-volume datacom applications. Quickly thereafter, the advantages of VCSELs became evident, leading them to be preferred light sources over edge-emitters in many applications. Compared to edge-emitting sources, VCSELs offer superior output beam quality and single-mode operation.
VCSELs are laser diodes with a monolithic laser resonator, where the emitted light leaves the device in a direction perpendicular to the chip surface. The resonator (cavity) is realized with two semiconductor Bragg mirrors (→ distributed Bragg reflector lasers). Between those, there is an active region (gain structure) with (typically) several quantum wells and a total thickness of only a few micrometers. In most cases, the active region is electrically pumped with a few tens of milliwatts and generates an output power in the range from 0.5–5 mW, or higher powers for multimode devices (see below). The current is often applied through a ring electrode, through which the output beam can be extracted, and the current is confined to the region of the resonator mode using electrically conductive (doped) mirror layers with isolating material around them.
2. VCSEL vs EEL and LED
3. VCSEL Features
VCSELs can have a high beam quality only for fairly small mode areas (diameters of a few microns) and are thus limited in terms of output power. For larger mode areas, the excitation of higher-order transverse modes can not be avoided; this is a consequence of the extremely small resonator length of only a few microns, and the difficulty in homogeneously pumping a larger active region with a ring electrode. The short resonator, however, also makes it easy to achieve single-frequency operation, even combined with some wavelength tunability. Also, VCSELs can be modulated with high frequencies, making them useful e.g. for optical fiber communications (see below).
In addition to the high beam quality of low-power VCSELs, an important aspect is the low beam divergence, compared with that of edge-emitting laser diodes, and the symmetric beam profile. This makes it easy to collimate the output beam with a simple lens, which does not have to have a very high numerical aperture.
The most common emission wavelengths of VCSELs are in the range of 750–980 nm (often around 850 nm), as obtained with the GaAs/AlGaAs material system. However, longer wavelengths of e.g. 1.3 μm, 1.55 μm or even beyond 2 μm (as required for, e.g., gas sensing) can be obtained with dilute nitrides (GaInNAs quantum wells on GaAs) and from devices based on indium phosphide (InAlGaAsP on InP).
An important practical advantage of VCSELs, as compared with edge-emitting semiconductor lasers, is that they can be tested and characterized directly after growth, i.e. before the wafer is cleaved. This makes it possible to identify quality problems early on, and to react immediately. Furthermore, it is possible to combine a VCSEL wafer with an array of optical elements (e.g. collimation lenses) and then dice this composite wafer instead of mounting the optical elements individually for every VCSEL. This allows for cheap mass production of laser products.
Another interesting feature of VCSELs is that is due to the substantially lower optical intensities at the facet, compared with edge-emitting laser diodes, there is no risk of catastrophic optical damage, even in post operation with substantial peak powers. Therefore, they can also be operated at higher temperatures, which would normally increase the risk of catastrophic facet failure.
4. VCSEL Arrays
Much higher powers can be generated with VCSEL arrays. A 2-D VCSEL array containing many thousand emitters (with a spacing of some tens of microns) can emit tens or hundreds of watts continuous-wave, thus competing with diode bars. In principle, the output power can simply be scaled up (“power scaling”) by increasing the number of emitters, but the beam quality it is often strongly reduced. However, there are also approaches where the emission of all the VCSELs of an array is coordinated (made coherent), which results in a far better beam quality and accordingly increased radiance, despite some loss of power conversion efficiency.
In comparison with conventional edge-emitting laser diodes, VCSEL arrays typically have a somewhat lower power conversion efficiency, but there can be substantial advantages for applications, such as the simplification or even illumination of beam shaping optics, the small emission linewidth, the higher wavelength stability and the substantial peak power potential in post operation.
See the article on VCSEL arrays for more details.
5. How to Make VCSEL
6. Applications, 3D Sensing and Considerations
VCSELs have many applications, the most important of which are briefly discussed in the following: Optical Communications, Computer Mice, Gas Sensing, Optical Clocks, Laser Pumping, and 3D sensing.
VCSEL can be used for Structured Light and ToF. Examples are shown below.
7. Evolution of VCSEL Package
- Abax http://www.abax-sensing.com/newsInfo-10-9.html
- VCSELs: Fundamentals, Technology and Applications of Vertical-Cavity Surface-Emitting Lasers 2013