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Connecting Theory and Practice in Optoelectronics
The energy efficiency is the fraction of the electrical input energy that is emitted as laser light. It is usually given as power conversion efficiency (PCE) and it is surprisingly low for GaN-based lasers. OSRAM just announced a record number of PCE=43% at SPIE Photonics West. This is certainly a remarkable achievement, considering the struggle to break the mysterious 40% limit. However, 43% is far below the record PCE of 84% reported for GaN LEDs. The inherently low hole conductivity and large series resistance on the p-doped side of GaN lasers are usually blamed for the efficiency deficit. However, the series resistance is known to shrink with rising temperature, which can be attributed to the increasing density of free holes in p-doped layers. Thus, one would expect that the PCE improves at elevated temperatures. But the OSRAM paper reported that the measured PCE drops with higher ambient temperature despite the shrinking series resistance. Ergo, there seems to be an even stronger loss mechanism involved.
One of the key rules of semiconductor laser physics relates to the carrier density inside the active layer. As long as I can remember, this rule states that the carrier density remains constant when the injection current rises above the lasing threshold. The reason lies in the stimulated emission of photons which consumes all additional carriers injected above threshold. The threshold carrier density delivers the threshold optical gain that compensates for the optical loss, which is usually not dependent on the injection current. Thus, the threshold carrier density should also remain constant. However, my recent analysis of high-power lasers yields different results (see picture). Read more of this post
There is an increasing interest toward the development of simple and compact comb laser sources. One promising application is the use of a InAs/GaAs Quantum Dot or InAs/InP Quantum Dash single section Fabry-Perot lasers. Many experiments on these devices have demonstrated the possibility of generating a wide optical spectrum of lasing longitudinal modes that are phase-locked. The phase locking is demonstrated by the very narrow RF line at the beat note frequency and by the possibility of getting pulses directly at the laser output or after group delay dispersion compensation with a proper length of dispersive optical fibre. There is however a lack of modelling work providing physical explanations on the capability of the QD lasers of generating phase locked lasing lines.
Two long sought-after goals for the semiconductor community have been (i) to develop long-wavelength semiconductor lasers on GaAs substrates, to enable exploitation of vertical-cavity architectures as well as monolithic integration with GaAs-based high-speed microelectronics, and (ii) to realise uncooled operation of semiconductor lasers, whereby the external cooling equipment typically required to maintain operational stability in long-wavelength devices can be removed in order to significantly reduce energy consumption without degrading the device performance.
Ever since Shuji Nakamura mentioned in his Nobel lecture in Stockholm that lasers are the future of lighting, I am puzzled by this claim, especially by the now widely circulated statement that laser diodes are free from efficiency droop. It suggests an advantageous energy efficiency of laser diodes, as shown in the last figure 17 of his lecture (which is actually invalidated by his own reference). If you are familiar with the much debated efficiency droop burdening GaN-LED lighting, you will agree that the underlying carrier loss mechanisms are also present in laser diodes. Even worse, laser diodes require a higher carrier density in the active layers and therefore exhibit stronger Auger recombination and possibly also electron leakage already at lasing threshold. Read more of this post