GaN-based multi-emitter laser diodes promise high-power output
Multi-emitter structures—such as laser bars (laser arrays)—offer the most straightforward means to attain significantly enhanced optical power from semiconductor-based lasers. We have successfully constructed novel blue/violet multiquantum-well (MQW) InGaN/GaN laser minibars. These devices are particularly useful for high-speed printing and display (e.g., laser television) applications because of the potentially high optical power and multipixel operation (for individually addressable arrays). Two main challenges must be overcome in their fabrication, however: one needs to have large dislocation-free areas on the GaN substrate and heavily loaded devices must be properly thermally managed. We used high-pressure-grown GaN substrates characterized by very low defect densities to meet these challenges.
We built a laser-bar structure using a vertical-flow metal-organic vapor-phase epitaxy reactor on top of the low-defect-density GaN crystal (characterized by a defect density of 102–103cm−2 for all crystal surfaces). The substrate crystals were obtained by high-pressure synthesis. The active device area was formed by three InGaN/GaN quantum wells1 (with a ∼10% In fraction), emitting at ∼400nm. We constructed a simple three-stripe device—using stripe widths of 3 and 7μm—with small (40μm) intrastripe distances (see Figure 1) to test the laser array's functionality and properties. The small pitch of the array was chosen because of our interest in replacing the commonly used wide, single-stripe laser geometry with a potentially better-performing multistripe design.
The device (employing three 3μm stripes) is characterized by a maximum optical-output power of 250mW at 405nm. The most surprising and unexpected emission feature is its very narrow (∼1.4Å) single spectral line, which does not show typical lateral modes (see Figure 2). The emitted light was analyzed using an Acton spectrometer (with a focal length of 500mm) equipped with a CCD camera. Most experiments demonstrate that—for single-stripe devices—lateral-mode suppression requires very narrow stripe widths (<2μm).2 For this reason, the observed single-mode emission from our 3 × 3μm array is even more surprising. To better understand the laser-mode character, we measured the far-field patterns of the emitted radiation. Figures 3(a) and (b) show the observed pattern for the miniarray below and above the lasing threshold, respectively. At this threshold level, the far-field pattern changes from ‘normal’ behavior (exhibiting three maxima) to a single common mode centered on the middle stripe. This feature was observed for all devices and can be explained if large lateral-mode leakage occurs from the individual stripes. This would lead to emission coupling through evanescent waves.
In conclusion, we have demonstrated single-mode emission from a three-stripe laser minibar. The device exhibits good lasing characteristics and an optical output power of 250mW. The observed formation of the array supermode is most likely due to pronounced lateral optical-mode leakage. Despite the narrow (3μm) stripe width, we managed to sustain a single lateral optical mode. Our discovery that the device tends to emit in the common supermode may be useful for constructing quasi-wide-stripe nitride laser diodes. However, to explore the potential of constructing individually emitting devices, better optical isolation between the stripes must be achieved.
Piotr Perlin has a background in optical spectroscopy of semiconductors: optical absorption, photoluminescence, and Raman scattering (with special emphasis on high-pressure effects). He has been a member of the Polish blue-laser team since 1998, where he is in charge of laser-diode fabrication and testing. He is currently an associate professor and has authored more than 220 scientific papers.
