Christoph M. Schultz

Experimental and theoretical analysis of the dominant lateral waveguiding mechanism in 975 nm high power broad area diode lasers

For maximum fibre-coupled power, high power broad area diode lasers must operate with small lateral far field angles at high continuous wave (CW) powers. However, these structures are laterally multi-moded, with low beam quality and wide emission angles. In order to experimentally determine the origin of the low beam quality, spectrally resolved near and far field measurements were performed for a diode laser with 50 µm stripe width. Within the range measured (CW optical output powers to 1.5 W) the laser is shown to operate in just six stable lateral modes, with spatially periodic profiles. Comparisons of the measured profiles with the results of two-dimensional modal simulation demonstrate that current-induced thermal lensing dominates the lateral waveguiding, in spite of the presence of both strong built-in index guiding and gain guiding. No evidence is seen for filamentation. Building on the diagnosis, proposals are presented for improvements to beam quality.

Root-Cause Analysis of Peak Power Saturation in Pulse-Pumped 1100 nm Broad Area Single Emitter Diode Lasers

Many physical effects can potentially limit the peak achievable output power of single emitter broad area diode lasers under high current, pulse-pumped operation conditions. Although previous studies have shown reliable operation to high pump levels (240 A, 300 ns, and 1 kHz), power was found to saturate. We present here results of a systematic study to unambiguously determine the sources of this power saturation. A combination of detailed measurements and finite element device simulation were used for the diagnosis. We find that the measured power saturation is dominated by electron leakage caused by band bending at high bias due to the low mobility of the p-type waveguide. However, the power saturation is only fully reproduced when longitudinal spatial hole-burning is included. Higher powers are expected to be achieved if higher energy barriers and lower confinement factors are used to mitigate leakage and longitudinal hole-burning, respectively.

Why the developing world is the perfect market place for solid state lighting

Much has been written about the daily challenge for survival faced by countless millions of developing world families and the overdeveloped world has offered a number of solutions by which those at the base of the economic pyramid (BOP) can help themselves. Light Up The World (LUTW), the global leader in bringing Renewable Energy (RE) based Solid State Lighting (SSL) to the developing world, offers yet another solution, and one that comes with a very high probability of success. In this paper we discuss: the critical role played by micro credit (banking for the poor); a typical example of a developing world community and their lighting needs and expenditures; how SSL can contribute positively to all eight of the Millennium Development Goals; the micro and macroeconomics of SSL at the BOP, its numerous societal benefits and its potential perverse outcomes; and thought there will always be a role for the donation based model, it is only through the market model that safe, healthy and affordable SSL will reach the majority of the BOP, such are the staggering numbers involved. LUTWs fundamental goal, through the facilitation of RE based SSL, is to improve the quality of life of those, who through no fault of their own, find themselves trapped in a cycle of poverty.

975-nm high-power broad area diode lasers optimized for narrow spectral linewidth applications

Many pumping and direct diode applications of high power diode lasers require sources that operate within a narrow (< 1nm) temperature stable spectral line. The natural linewidth of high power broad area lasers is too wide (4-5nm) and varies too quickly with temperature (0.3-0.4nm/K) for such applications. The spectrum can be narrowed by introducing gratings within the diode laser itself or by the use of an external stabilization via a Volume Bragg Grating, VBG. For optimal loss-free, low cost wavelength stabilization with a VBG, the narrowest possible far field angles are preferred, provided power and efficiency are not compromised. Devices that contain internal gratings are potentially the lowest manufacturing cost option, provided performance remains acceptable, as no external optics are required. Therefore, in order to address the need for high power with narrow linewidth, three different diode laser device designs have been developed and are discussed here. For VBG use, two options are compared: (1) devices with high conversion efficiency (68% peak) and reasonable far field (45° with 95% power content) and (2) devices with extremely small vertical far field angle (30° with 95% power content) and reasonable conversion efficiency (59% peak). Thirdly, the latest performance results from broad area devices with internal distributed feedback gratings (DFB-BA Lasers) are also presented, constructed here using buried overgrowth technology. DFB-BA lasers achieve peak conversion efficiency of 58% and operate with < 1nm linewidth operation to over 10W continuous wave at 25°C. As a result, the system developer can now select from a range of high performance diode laser designs depending on the requirements.

Reliable operation of 976nm high power DFB broad area diode lasers with over 60% power conversion efficiency

Diode lasers that deliver high continuous wave optical output powers (> 5W) within a narrow, temperature-stable spectral window are required for many applications. One technical solution is to bury Bragg-gratings within the semiconductor itself, using epitaxial overgrowth techniques to form distributed-feedback broad-area (DFB-BA) lasers. However, such stabilization is only of interest when reliability, operating power and power conversion efficiency are not compromised. Results will be presented from the ongoing optimization of such DFB-BA lasers at the Ferdinand-Braun- Institut (FBH). Our development work focused on 976nm devices with 90μm stripe width, as required for pumping Nd:YAG, as well as for direct applications. Such devices operate with a narrow spectral width of < 1nm (95% power content) to over 10W continuous wave (CW) optical output. Further optimization of epitaxial growth and device design has now largely eliminated the excess optical loss and electrical resistance typically associated with the overgrown grating layer. These developments have enabled, for the first time, DFB-BA lasers with peak CW power conversion efficiency of > 60% with < 1nm spectral width (95% power content). Reliable operation has also been demonstrated, with 90μm stripe devices operating for over 4000 hours to date without failure at 7W (CW). We detail the technological developments required to achieve these results and discuss the options for further improvements.

Theoretical and experimental analysis of the lateral modes of high-power broad-area lasers

For maximum fiber-coupled power, broad-area (BA) diode lasers must operate with small lateral far field angles. However, these structures are laterally multi-moded, with low beam quality and wide emission angles. We use a combination of device simulation and diagnostic measurements to determine the physical factors limiting the lateral far field angle in state of the art BA lasers emitting at 975 nm. Two-dimensional simulations of the optical field enable the dominant lateral waveguiding mechanisms to be diagnosed. Spectrally resolved near and far field measurements will allow us to determine the nature of the lateral modes.

Narrow Vertical Far-Field 975-nm Broad-Area DFB Lasers for Wide Temperature Range Operation

We report on 100-mum-wide 975-nm distributed-feedback lasers with a cavity length of 2 mm, which deliver 2.4 W within a 14deg vertical far-field angle (full-width at half-maximum) and 35% power conversion efficiency in combination with a spectral linewidth of 0.27 nm (95% power content). The epitaxial design is a compromise between a narrow vertical far-field and a good power conversion efficiency, as required for application as a pump source. Narrow linewidth operation is sustained to 100degC, enabled by use of a grating with a high coupling coefficient.

Thermo-optical simulation of high-power diode lasers

Numerical simulations are an important tool for the design of opto-electronical components and devices. In order to obtain realistic results, a multitude of physical effects and theories have to be included, e.g., Maxwells equations for lasing mode computations, heat transfer in active devices, and electronic transport. In our contribution we perform coupled electro-thermal simulations of high power diode lasers. We analyze the temperature dependence of the mode profile and far field characteristics. Our results will be compared to experimental measurements of broad area lasers and will quantitatively describe the effect of thermal blooming.

10-W reliable 90-μm-wide broad area lasers with internal grating stabilization

Broad area (BA) diode lasers with narrow, temperature-stable spectral lines are required for pumping narrow spectral lines in solid state lasers and for dense spectral multiplexing in direct applications. Two device technologies in particular have reached a high performance level, based on development work at the Ferdinand-Braun-Institut (FBH). Firstly, etched surface gratings can be used to form the rear facet reflector, in distributed Bragg-reflector (DBR) format. Secondly, gratings can be buried within the semiconductor using etch and overgrowth technology, to form distributed feedback (DFB) lasers. In this case, the rear facet has a high reflectivity coating, and the DFB operates effectively as the low reflectivity out-coupler. For both technologies, BA diode lasers with 90-100μm stripes operating at 975nm deliver peak continuous wave (CW) powers of over 12W within a spectral width of < 1nm (with 95% power content). Recently, reliable operation has been confirmed for CW powers of 10W, and power conversion efficiency of up to 63% has been demonstrated. However, the two technologies have different strengths. For example, DBR-BA lasers have low sensitivity to external feedback and are insensitive to the onset of spectral side-modes. In contrast, DFB-BA lasers achieve the highest reported power conversion efficiencies. A comparison of the relative merits of the two technologies for different high power laser applications is presented.

Best Practices for Developing a Solar Home Lighting System Market

Access to electricity and home lighting are considered essential for a higher quality of life. Approximately 1.4 billion people rely on fuel-based lighting for home illumination. Thus, it has been argued, a substantial market exists for affordable home lighting products. Solid state lighting is a technology that can be employed for sustainable home lighting. In this article the authors present experiences and lessons learned from projects that “Light Up The World” has supported to deliver solid state home lighting products to rural households in developing countries. Best practices for business developments to serve the home lighting market in Africa are also proposed.