VECSEL and Its Pump Laser Monolithically Integrated

Breck Hitz

Compact and efficient green lasers have multiple applications, from laser-projection systems to the fabrication of solar cells and flat-screen displays. Because there are no lasers that emit efficiently in the green spectral region, frequency doubling of near-infrared lasers is the favored approach to providing green lasers for these applications.

Figure 1. A conventional optically pumped VECSEL is pumped by a separate pump laser (a). The pump laser of the integrated VECSEL is part of the same semiconductor chip as the VECSEL itself (b).

Frequency-doubled, optically pumped vertical external-cavity surface-emitting lasers (VECSELs) have proved to be commercially successful, but they are complex devices requiring careful alignment and beam shaping between the VECSEL and its pump laser. Recently, Stefan Illek and his colleagues at Osram Opto Semiconductors in Regensburg, Germany, demonstrated an optically pumped VECSEL in which the pump laser and the VECSEL itself both were integrated into the same semiconductor chip.

Figure 2. Strong waveguiding of the pump laser was provided by the TCO (transparent conductive oxide) layer outside the VECSEL. The waveguiding was much weaker beneath the VECSEL, where the TCO was absent, and the coupling layer coupled power from the pump laser into the VECSEL’s quantum wells. Reprinted with permission of IEEE Photonics Technology Letters.

The resonator of a typical VECSEL comprises a Bragg mirror and an external mirror that couples the intracavity power into the output beam (Figure 1a). Pump power is provided by a separate laser (not shown), and the laser gain comes from a periodic array of quantum wells that are spaced so that each well aligns with a peak of the resonator’s standing wave. The intracavity power circulates vertically, as indicated by the green, double-headed arrow in Figure 1a.

Figure 3. For their experimental demonstration, the scientists fabricated a chip with two horizontal-cavity pump lasers perpendicular to each other between the VECSEL’s quantum wells and its Bragg mirror.

The Osram scientists inserted an electrically pumped, horizontal-cavity pump laser between the quantum wells and Bragg mirror of their VECSEL (Figure 1b). The output coupling of this pump laser was not through its mirrors, which were maximum reflectors, but through the weakened waveguiding in the middle of the laser, where light was coupled into the VECSEL’s quantum wells, as indicated by the curved white arrows in Figure 2b.

Weak waveguiding

A tricky part of the design was providing weak waveguiding in only the middle of the laser, so its intracavity power was coupled into only the VECSEL’s quantum wells. The scientists accomplished that by applying a layer of low-index transparent conductive oxide to the region where they wanted strong waveguiding in the pump laser (Figure 2). The index contrast between the oxide and the semiconductor itself strongly confined the pump-laser radiation to its waveguide in the region not beneath the VECSEL. Immediately beneath the VECSEL, however, the oxide was absent, and pump power leaked from the pump laser into the VECSEL. As indicated in the figure, a metallic p-contact was added on top of the oxide to allow electrical pumping.

Figure 4. The output of the VECSEL in Figure 3 showed no rollover at the high end of its output, indicating that higher powers are possible. Reprinted with permission of IEEE Photonics Technology Letters.

Experimentally, the scientists fabricated a chip with two pump lasers arranged perpendicularly to each other beneath the VECSEL’s quantum wells (Figure 3). Each pump laser was approximately 200 μm wide and 1200 μm long, with cleaved facets coated for maximum reflectivity. The VECSEL’s resonator was a hemispheric configuration, with the 100-mm radius of curvature, 98 percent reflecting external mirror aligned an optical distance of ∼100 mm from the Bragg mirror. In this arrangement, the VECSEL generated up to 2.5 W of peak power at the fundamental wavelength in a 10-kHz train of 1-μs pulses, with a total current to both pump lasers of slightly under 10 A (Figure 4).

IEEE Photonics Technology Letters, Dec. 15, 2007, pp. 1952-1954.