VCSELs for smartphones, cars, and industrial processes

In the area of data transfer, Vertical-Cavity Surface-Emitting Lasers (VCSELs) have been indispensable since the end of the 1990s. Now they are get common in medical technology, industrial processes, smartphones, and automated driving.

It took ten years from the start of the first commercial Vertical-Cavity Surface-Emitting Laser (VCSEL) production facility until Finisar delivered the 50 millionth VCSEL in 2006. Eleven years later—2017—the market pioneer, which was recently acquired by II-VI Incorporated, had already sold 300 million units.

The performance of VCSELs also increased rapidly during this time. For example, in the area of data transfer via fiber optics, the first VCSELs were able to modulate up to 1 gigabit/second (Gbps) but this has now increased to 25 Gbps; in demonstrators, even as high as 56 Gbps.

Spirit of optimism and preparations for the boom

Vertical-Cavity Surface-Emitting Lasers have long developed beyond their original markets in data communication. Among other areas, they have gained a foothold in laser printers, camera autofocus systems, and sensor technology. But there is currently a new spirit of optimism among the suppliers. New factories are set up. Powerful units are forming created through takeovers, such as in the case of II-IV Inc. and Finisar, TRUMPF and Philips Photonics, and the current attempts by AMS Technologies to take over Osram. This is because VCSEL technology is preparing to conquer new mass markets.

New market dynamics have evolved since 2017 when Apple put the first iPhones on the market with VCSEL-based face recognition and 3D sensors. An increasing number of smartphone and tablet producers are following suit with 3D sensor systems. In many cases, they require three and more VCSELs per smartphone. At the same time, many fields of application are opening up in the automotive market. LiDAR systems guide advanced driver assistance systems and autonomous vehicles in traffic, using time-of-flight distance measurements. The basic technology for this is multi-mode VCSEL arrays with hundreds and even several thousands of cavities. In addition, vehicle interiors are equipped with other VCSEL-based sensor systems. But there are still many other potential markets. In biophotonics and medical technology, VCSELs are paving the path for miniaturized pulse oximeters to measure oxygen in the blood; they function as reference lasers for FTIR spectrometers and as light sources for optical coherence tomography (OCT). Besides this, applications are being discovered in industrial production processes. This is because the powers of VCSEL arrays are pushing forward into the middle double-digit watt range—and some experts consider the kilowatt range as being quite feasible in the future. The beam shape of the vertical emitters also enables highly efficient fiber coupling. Consequently, it becomes increasingly likely that in addition to the first VCSEL applications in heat treatment processes, such as in the future market of additive manufacturing, we will also see industrial cutting, bonding, structuring, and surface treatment processes.

Inexpensive manufacturing—benefits for packaging

What exactly makes surface-emitting lasers with vertical resonator arrangement so attractive? One of the reasons is their relatively simple production. Up to 20,000 VCSELs fit on to a wafer with a diameter of two inches (5.08 cm). Quality inspection is easier, for the complete sandwich structure is built on the wafer and because the lasers emit their vertical light upwards. Inspections are done automatically with standard processes and equipment for integrated circuits. The layer-by-layer structure has another advantage: the two laser mirrors—distributed Bragg reflectors (DBR) are used – can be positioned easily below and above the active semiconductor layer. As opposed to this, with edge-emitting laser diodes, manufacturers first have to cut the wafer, to attach the mirror layers. In addition to the reduced production effort, VCSELs offer high energy-efficiency and the above-mentioned benefit of a highly symmetric circular light beam, which ensures efficient fiber coupling.

The wavelength and power spectra of the vertical emitters are also expanding, and now range from near-infrared at 2,300 nanometers to the just visible violet range around 380 nanometers. Research teams are already working on VCSELs for the UV range. Currently, powers in pulsed operation reach the middle two-digit watt range and with VCSEL array modules even the three-digit range. Industrial VCSEL-based heating modules are a special case. Their power is almost in the two-digit kilowatt range. Based on thousands of synchronized micro-diode lasers that emit light in the near-infrared range, with power densities in excess of 100 W/cm² and the fact that heat input can be well controlled, the laser-based heating modules ensure fast processes in heat treatment and precision in large-scale processing of plastics.

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