S. Knigge

650-nm vertical-cavity surface-emitting lasers: laser properties and reliability investigations

650-nm AlGaInP-AlGaAs-based oxide-confined VCSELs are investigated in dependence on the current aperture size. VCSELs with small aperture (a=5 /spl mu/m) have a maximum continuous-wave (CW) output power of about 1 mW at room temperature. They reach higher operating temperatures (T/sub max/=55/spl deg/C), have narrower beam profiles, less transverse modes, and a higher side mode suppression compared to large aperture VCSELs (a>13 /spl mu/m). The latter devices emit a CW-output power P=3 mW at 20/spl deg/C. Reliability tests of 655-nm devices show at 20/spl deg/C an output power of P/spl ap/0.4 mW over more than 1000 h and at 40/spl deg/C P/spl ap/0.1 mW over 500 h.

Comparative theoretical and experimental studies of two designs of high-power diode lasers

Design and technology developments targeted at increasing both power conversion efficiency and optical output power of GaAs-based diode lasers are under intense study worldwide, driven by the demands of commercial laser systems. The conversion efficiency at the operation point is known to be limited by electrical and optical losses in the p-side waveguide. In this paper an ‘extreme, double asymmetric’ design to mitigate the impact of the p-side waveguide is studied and compared with a more conventional design. An increase of the efficiency at the highest power is demonstrated, but it is less than expected from simulations.

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.

High Beam Quality in Broad Area Lasers via Suppression of Lateral Carrier Accumulation

High power 9xx-nm broad-area lasers with improved beam quality are required for many applications, but the physical limitations remain unclear, especially the relative importance of free-carrier and self-heating effects. Experimental data are, therefore, presented on a series of diagnostic lasers where the lateral carrier profile at the edges of the electrical contacts has been modified via implantation in order to assess its influence on beam quality. We show that carrier accumulation at the edges of the (90-μm wide) contacts can be eliminated and that as a consequence, near and far field are narrowed and the rate of increase of beam parameter product (BPP) with self-heating reduces by 35%. Overall, the suppression of lateral carrier accumulation allows BPP <; 2 mm × mrad to be maintained to 7-W optical output, corresponding to a peak linear brightness of 3.5 W/mm × mrad, comparable with the highest reported values for 90-μm stripe devices.

Low-loss smile-insensitive external frequency-stabilization of high power diode lasers enabled by vertical designs with extremely low divergence angle and high efficiency

Broad area lasers with narrow spectra are required for many pumping applications and for wavelength beam combination. Although monolithically stabilized lasers show high performance, some applications can only be addressed with external frequency stabilization, for example when very narrow spectra are required. When conventional diode lasers with vertical far field angle, ΘV 95% ~ 45° (95% power) are stabilized using volume holographic gratings (VHGs), optical losses are introduced, limiting both efficiency and reliable output power, with the presence of any bar smile compounding the challenge. Diode lasers with designs optimized for extremely low vertical divergence (ELOD lasers) directly address these challenges. The vertical far field angle in conventional laser designs is limited by the waveguiding of the active region itself. In ELOD designs, quantum barriers are used that have low refractive index, enabling the influence of the active region to be suppressed, leading to narrow far field operation from thin vertical structures, for minimal electrical resistance and maximum power conversion efficiency. We review the design process, and show that 975 nm diode lasers with 90 μm stripes that use ELOD designs operate with ΘV 95% = 26° and reach 58% power conversion efficiency at a CW output power of 10 W. We demonstrate directly that VHG stabilized ELOD lasers have significantly lower loss and larger operation windows than conventional lasers in the collimated feedback regimes, even in the presence of significant (≥ 1 μm) bar smile. We also discuss the potential influence of ELOD designs on reliable output power and options for further performance improvement.

Cryolaser: Innovative cryogenic diode laser bars optimized for emerging ultra-high power laser applications

“Cryolaser” diode laser designs exploit the improvement in semiconductor material properties at sub-zero temperatures to increase efficiency and power. Optimized single 9xx-nm laser bars demonstrate record peak pulse energy of 2 J (1.7 kW, 1.2 ms, -50°C).

The impact of low Al-content waveguides on power and efficiency of 9xx nm diode lasers between 200 and 300 K

We present results on investigations of 9xx nm, GaAs-based diode lasers with 100 μm wide, 4 mm long stripes operating at temperatures between 200 and 300 K. As temperatures are reduced to 218 K, efficiency and power are improved. We analyze the characteristic parameters and subsequently mitigate the limiting factors by altering the vertical epitaxial design, seeking to further increase the optical output power and efficiency. The temperature dependence of internal parameters is obtained from length-dependent measurements, showing that the improved performance is due to an increased differential internal efficiency. Series resistance is confirmed as the main remaining limit to higher efficiency at high power. We show that the series resistance can be reduced by lowering the aluminum content in the Al x Ga1−x As waveguide which increases the carrier mobility. Although poor optical performance is seen at room temperature in low Al-content structures due to carrier leakage, at 218 K this is suppressed and is accompanied by significantly reduced series resistance. However, at high powers we observe further (non-thermal) power saturation in low Al-content structures even at 208 K, which limits the efficiency gain at high powers. Analysis of the series resistance of laser structures with different waveguide compositions at temperatures between 208 and 298 K reveals that series resistance is a function of the barrier height around the quantum well. This is taken as first evidence for carrier transport being a significant limit to electrical resistance. Finally, structures with an optimized Al-content are shown to maintain conversion efficiency >65% to output powers of 20 W, substantially higher than previously reported at 300 K (~55% at 20 W).

Assessing the influence of the vertical epitaxial layer design on the lateral beam quality of high-power broad area diode lasers

GaAs-based high-power broad-area diode lasers deliver optical output powers Popt > 10W with efficiency > 60%. However, their application is limited due to poor in-plane beam parameter product BPPlat=0.25×Θ95%×w95% (Θ95% and w95% are emission angle and aperture, 95% power content). We present experimental investigations on λ = 9xx nm broad area lasers that aim to identify regulating factors of the BPPlat connected to the epitaxial layer design. First, we assess the thermal lens of vertical designs with varying asymmetry, using thermal camera images to determine its strength. Under study are an extreme-double-asymmetric (EDAS) vertical structure and a reference (i.e. more symmetric) design. The lateral thermal profiles clearly show that BPPlat increase is correlated to the bowing of the thermal lens. The latter is derived out of a quadratic temperature fit in the active region beneath the current injection of the laser device and depends on the details of the epitaxial layers. Second, we test the benefit of low modal gain factor Γg0, predicted to improve BPPlat via a suppression of filamentation. EDAS-based lasers with single quantum well (SQW) and double quantum well (DQW) active regions were compared, with 2.5x reduced Γg0, for 2.2x reduced filament gain. However, no difference is seen in measured BPPlat, giving evidence that filamentary processes are no longer a limit. In contrast, devices with lower Γg0 demonstrate an up to twofold reduced near field modulation depth, potentially enabling higher facet loads and increased device facet reliability, when operated near to the COD limit.

Cryogenic ultra-high power infrared diode laser bars

GaAs-based high power diode lasers are the most efficient source of optical energy, and are in wide use in industrial applications, either directly or as pump sources for other laser media. Increased output power per laser is required to enable new applications (increased optical power density) and to reduce cost (more output per component leads to lower cost in