technology GaAs-based lasers
Fig 3: High-power diode lasers with integrated overgrown gratings reach high powers with a peak power efficiency of over 60 percent (left). The spectral width, δλ, at 10 W continuous output power is substantially reduced in comparison to a reference device without internal grating (right). Credit: FBH
perturbation due to the grating, in order to cut defect generation. A very thin aluminum-free grating layer of 10 nm-thick InGaP was used in the new design, located a long distance (0.77 µm) from the active region of the device to ensure a low grating coupling strength (κ ~ 0.5 cm1
). Introducing this layer and fine-tuning etch and overgrowth conditions eliminated both excess optical loss and operating voltage – current can now flow without restriction through the p-side grating layer. The deployment of a low strength grating has an additional benefit: Increased slope efficiency. That’s because lower-strength gratings provide less feedback, increasing the proportion of light leaving the laser. These factors combine to enable the fabrication of more powerful lasers.
Matching gains and gratings To optimise laser efficiency, it is also essential to select the best relative location of the material gain and the grating wavelength. The active region generates optical gain, and the grating provides reflection – together they combine to produce lasing. However, the grating wavelength and the lasing wavelength vary very differently with temperature. For every °C change, the grating shifts by 0.08 nm and the gain moves by five times as much.
These differences hamper laser performance. In 7-10 W lasers driven in continuous-wave operation internal current heating raises device temperature by 30°C, preventing gain and grating wavelengths from being in sync over the whole operating range. When the gain is offset from the grating wavelength, light is less strongly amplified, causing the device to operate less efficiently.
In our design, gain and wavelength are selected so that they meet at the operating point of 7W. The downside of this approach is that the laser has a large threshold current, reducing overall power conversion. But if we had designed our device so that the gain and grating matched at the threshold current and the peak efficiency of the device was higher, this would compromise the output
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www.compoundsemiconductor.net January / February 2011
power – gain would drop so much at high currents that the device would over-heat. Thanks to all this detailed development work, we have produced diode lasers with an integrated overgrown grating that deliver a peak power conversion efficiency of 62 percent, as reported at Photonics West 2011. This falls by just 4 percent at the operating power of 10W. Spectral width with 95 percent power content at 10 W is 0.7 nm. These efficiencies are slightly lower than those of the grating-free reference device. The overriding reason for this slight reduction in performance is the offset between the grating and the gain wavelength. Reliability tests reveal that these lasers operate failure-free for over 4000 hours (to date) at an output power of 7W at 25°C (see Figure 3).
Although these newly developed, grating-stabilised devices have substantially increased laser efficiency, further gains are necessary. Specifically, the efficiency should be maintained at more than 60 percent at higher output powers. There are two possible pathways to success: improving the performance of the baseline design; or reducing the influence of detuning, for example through improved heatsinking. On top of this, the process must be scaled for high volume manufacture - for larger volumes, 4-inch production will be necessary. However, the breakthrough efficiency achieved in these grating- stabilised, high-power lasers will enable a wide range of new and improved industrial laser systems.
This work was funded within the German Federal Ministry of Education and Research (BMBF) development program, INLAS. For more details see: “Reliable operation of 976 nm high power DFB broad area lasers with over 60 percent power conversion efficiency” Paul Crump et al. Paper 7953-50, Photonics West 2011
© 2011 Angel Business Communications. Permission required.
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