Influence of the chip-heatsink thermal barrier on the brightness of high-power broad area diode lasers
GaAs-based broad area diode lasers (BAL) are essential light sources in the near infrared range for high-power applications such as metal cutting and welding or as pump-sources for solid-state lasers. BALs with improved lateral beam quality are required for improved performance in these industrial systems. The lateral beam quality, measured in terms of the lateral beam parameter product, BPPlat (at 95% power), depends strongly on the lateral temperature profile within the laser chip [1-3]. FBH researchers have recently made progress in understanding the effects regulating this profile, and results were presented at the 25th International Semiconductor Laser Conference in September 2016  and in an invited talk at Photonics West in February 2017 .
The studies of the thermal lens were divided into three parts: (1) measurement of device electro-optic and beam characteristics, (2) measurement with IR-micro-thermography to determine the temperature profile (in collaboration with the Max-Born-Institute in Berlin) and (3) thermal simulation with the FEM-tool ANSYS. Broad area laser devices with an emitting wavelength of λ ~ 910 nm, 4 mm cavity length and 90 µm contact opening were studied and are fabricated at the FBH. These chips were soldered with gold-tin p-side down onto copper-tungsten carriers and were measured in continuous wave (CW) mode at a carrier-temperature of THS = 25 °C. The devices operated with a CW optical power of Popt = 10 W, where the conversion efficiency was ηC = 58% and BPPlat = 5 mm×mrad. Micro-thermography IR-measurements were performed at the Max-Born-Institute (MBI) and use a thermal camera image to produce a calibrated temperature map of the front facet, as shown in Fig. 1. Lateral and vertical temperature profiles were extracted from the camera images and are shown in Fig. 2 for sections through the center of the active region. The lateral temperature profile has an approximately Gaussian form, with a peak temperature in the center of the device. Higher local temperatures lead to an increased refractive index, and the resulting temperature-induced lateral waveguide is termed the thermal lens. The presence of a strong thermal lens leads to wider far fields and hence degraded BPPlat.
In addition, the measured vertical temperature profile shows a substantial step (7…10 K) at the chip-carrier interface. This temperature step corresponds to around 40% of the total thermal resistance Rth (1.0 K/W interface from 2.6 K/W total), and a similar step was observed in devices fabricated by multiple different groups (see Fig. 3) and confirmed using different measurement techniques (direct measurement of submount temperature with a thermistor). The temperature transfer at the chip-carrier interface is quantified using a thermal boundary conductance, which is around 0.12…0.14 MWm-2K-1.
A two-dimensional (2D) finite-element method (FEM) simulation of the temperature profile within the chip and its carrier was used to help interpret and understand the results, as described in . The stationary heat conduction equation was solved and a 2D-vertical cross-section obtained (also shown in Fig. 1), allowing the extraction of temperature profiles analogue to the IR measurement (also shown in Fig. 2). Even though the simulation included all materials from the heatsink through the diode laser, simulation only agreed with measurement when a when a low thermal conductance at the chip-metal interface was allowed for, as can be seen in Figs. 1 and 2.
In earlier work, the FBH showed experimentally that the curvature of the lateral thermal profiles correlates to the lateral beam quality and this allows us to estimate the impact of the newly identified thermal barrier on BPPlat. The curvature of the thermal lens is fitted with a quadratic function (ΔT = B2x2+B1x+B0) of lateral position, x, and the fit coefficient, B2, which we term the “bowing factor”, quantifies how rapidly the thermal profile varies. Fig. 4 shows how B2 (extracted from FEM simulation) varies as a function of active zone temperature increase ΔTAZ, for simulation with (blue) and without (red) the barrier. The simulation shows that B2 is up to ~ 1.3× larger when the additional thermal barrier is included. In combination, the degraded thermal resistance and bowing factor lead to a strong degradation of the lateral beam parameter product by more than 1 mm×mrad.
In conclusion, measurement and simulation of thermal profiles in diode lasers performed in collaboration between the FBH and the MBI are helping to clarify the effect limiting beam quality in high-power broad area lasers. These studies have identified one key limit, namely the presence of a significant thermal barrier between chip and carrier. Improving the thermal profile by addressing the thermal barrier (amongst other factors) will play a key role in improving brightness in high-power broad area lasers,
 J. Rieprich, M. Winterfeldt, J.W. Tomm, P. Crump, "Assessment of factors regulating the thermal lens profile and lateral brightness in high power diode lasers", invited talk, Proc. SPIE 10085, Photonics West, San Francisco, USA, 1008502 (2017).
 M. M. Karow, C. Frevert, R. Platz, S. Knigge, A. Maaßdorf, G. Erbert, Member, and P. Crump, "Efficient 600-W-Laser Bars for Long-Pulse Pump Applications at 940 and 975 nm", IEEE Photonics Technology Letters, Vol. 29, No. 19, (2017).
 J. Rieprich, M. Winterfeldt, J.W. Tomm, P. Crump, "Assessing the Impact of Thermal Barriers on the Thermal Lens Shape in High Power Broad Area Diode Lasers", Proc. 25th Int. Semicond. Laser Conf. (ISLC) 1947, 16520599 (2016).