Harnessing light for high-power applications

Compact and robust high-power lasers are needed in a variety of applications ranging from teeth whitening and hair removal to infrared illuminators for military applications. Such lasers can also be used in industrial applications such as printing, cutting, welding, and marking. Today, the dominant source for these tasks is semiconductor edge-emitting lasers, as they are small and very efficient in converting the input electrical power to the output optical power. These lasers are used either directly or indirectly as pumping sources for other lasers (solid-state or fiber lasers). However, edge-emitting laser technology has several drawbacks, including poor intrinsic beam and spectral properties, as well as low reliability at higher operating temperatures. Scaling up the power also requires complex and costly assembly of individual laser bars into stacks.

Figure 1. (a) In a vertical-cavity surface-emitting laser (VCSEL) structure, the light oscillates perpendicular to the epitaxial layers and exits the top mirror stack in a circular, low-diverging beam. (b) By contrast, in an edge-emitting structure, the light oscillates along the epitaxial layers and exits through the coated cleaved facets in a high-diverging elliptical beam.

A few attempts have been made to develop semiconductor laser technologies to address some of these issues. These approaches aim at coupling the light out of edge-emitter lasers in a direction normal to the standard output direction using surface grating¹ or etched 45-degree-angled mirrors.² This allows for much-improved beam and spectral properties and the potential for low-cost 2D array fabrication for power scaling, something that is not possible using standard edge-emitting laser technology. However, these approaches have the disadvantages of poor conversion efficiency and fabrication issues. To address these problems, we are developing a new approach to high-power semiconductor lasers based on vertical-cavity surface-emitting laser (VCSEL) technology.

The concept was originally developed by Kenichi Iga and co-workers at the Tokyo Institute of Technology in the 1980s.³ VCSELs' rise to fame originated in low-power (a few milliwatts) telecom and datanet applications during the mid-1990s.⁴ This success was mainly due to the lasers' lower manufacturing costs and higher reliability compared to edge emitters. VCSELs also exhibited improved characteristics, such as a circular, low-diverging output beam and a narrow, stable spectrum. One major advantage of VCSELs is that they can be easily fabricated in 2D arrays. Consequently, power can be scaled up by driving a large 2D array of low-power single devices in parallel. Moreover, this can be done while preserving all the advantageous properties of a single device.

Figure 1 shows the basic VCSEL structure along with an edge-emitting laser structure for comparison. Semiconductor lasers consist of layers of semiconductor material (the ‘epitaxy’) grown on a substrate. This ‘wafer’ is then processed to produce individual devices. In the case of edge emitters, these devices need to be cleaved so that dielectric mirrors can be deposited on the exposed facets. As shown in Figure 1, the light oscillates along the epitaxial layers and exits at the coated facets. In the case of VCSELs, however, the mirrors are created during the growth phase, and the light oscillates and exits normal to the epitaxial layers. Therefore, operation of individual laser devices can be achieved without needing to cleave and separate. This is particularly advantageous for fabricating large monolithic 2D arrays of single VCSELs. By contrast, edge-emitting lasers are limited to 1D arrays (‘bars’).

Figure 2. A 6×6mm²VCSEL array chip with several hundred devices soldered on a diamond submount.

Since VCSELs can be fabricated and tested at the wafer level, setup times and handling procedures are minimized, which, together with much improved yields, drastically reduces manufacturing costs.⁵ In fact, VCSEL fabrication is very similar to the well-established, low-cost silicon integrated-circuit planar processing. In addition, VCSELs have an inherent reliability advantage over edge-emitting lasers because they are not subject to catastrophic optical damage.⁶ For edge-emitting lasers, this failure mechanism is very sensitive to the quality of the cleaved facet coating as well as the operating temperature. This problem of sensitivity to surface conditions is not present in VCSELs because the gain layer is embedded in the epitaxy and does not interact with the emission surface. As a result, VCSELs can also operate at much higher temperatures.

Figure 3. The output power and voltage as a function of the input continuous-wave current for a high-power 2D VCSEL array mounted on a microchannel cooler.

We fabricated large VCSEL arrays (on the order of a square centimeter) containing up to tens of thousands of single devices.⁷ For efficient heat removal, these arrays are typically attached to a high thermal-conductivity submount such as synthetic diamond (see Figure 2) . Depending on the application, the array-on-submount can be further attached to a microchannel cooler connected to a refrigeration unit to enable efficient removal of large heat loads. All devices on the array are effectively driven in parallel. We have demonstrated more than 230W of continuous-wave output power from a VCSEL array emitting around 976nm (see Figure 3). This array also exhibited a narrow spectrum (≤1nm full-width at half-maximum) as well as a low-diverging, circular beam (1/e² full-width divergence angle of 17^°). Furthermore, the wavelength dependence with respect to temperature was 0.065nm/K, five times less than for edge emitters.

Thanks to their superior beam and spectral characteristics, as well as their improved reliability and lower cost, VCSELs present an attractive alternative to edge-emitting lasers for high-power applications. Our results have shown that VCSELs are capable of delivering very high output power, similar to that of edge-emitting lasers. Work to improve the power conversion efficiency and power density is ongoing.