INDUSTRY LASERS
of GaAs-based, high-power lasers. Fabrication begins by using MOCVD growth technology to form epitaxial structures on 3-inch GaAs substrates. Processing creates broad-area multi-mode chips featuring separate confinement InGaAs/AlGaAs quantum wells, a lateral output aperture of 100 μm and a 4.1 mm cavity length. By varying the composition of indium in the well, the lasing wavelength can be tuned from 910 nm to 980 nm.
The length of the chip cavity is identical to that of its forerunner, but the near-field and far-field emission patterns have been improved through optimization of the lateral index-guiding step, lateral gain profile, and transverse (epitaxial direction) mode design. These refinements result in a more uniform near-field pattern with fewer high-intensity spikes – this means better long-term facet reliability.
For burn-in and test, the chip is bonded to an expansion-matched ceramic submount with AuSn solder and clamped to a copper carrier. Comparison at the chip-on-submount level shows that this latest chip has less thermal rollover than its predecessor, and can deliver a CW output in excess of 20 W at 25 °C (see Figure 1). Peak power conversion efficiency is greater than 60 percent.
We have carried out extensive reliability testing of our latest chip design. Only 10 percent of the tested population failed across seven highly- accelerated life test cell conditions, making it challenging to employ a reliability model. However, we have performed reliability analysis with a Weibull model, which uses acceleration factors based on diode junction temperature and optical power (see Figure 2). This study suggests that if our lasers were operated continuously for ten years, the expected failure rate would be below 4 percent.
Multiple emitters To increase the output power of a source, laser chips can be combined to form a multi-emitter package (see Figure 3). We have done this, stacking chips in the vertical direction with a staircase-like layout, in two rows, polarization combined, and focusing their output into the fibre.
The result is a source that can deliver a continuous-wave output of 140 W from a 106 μm/0.22 NA fibre – and 95 percent of this power is contained within 0.15 NA. Note that power conversion efficiency, the portion of terminal electrical power that is converted to useful light, is almost 50 percent at 140 W.
Figure 3. (a) Optical power versus drive current at 35 °C case temperature for the multi-emitter, fibre-coupled package. The laser is rated at 140 W and achieves nearly 50 percent power conversion efficiency at the rated power. (b) Optical power reading at 10 A from 1,000 units assembled. Typical optical power is about 120 W
Figure 4. Fibre laser power versus pump diode current for 1-, 2-, and 3-module combined engines
Copyright Compound Semiconductor Issue VI 2014
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