As with all other laser types, the continuous improvement of high-power diode lasers has involved a continuous improvement of thermal management. Today this mainly means the switch to high-efficiency semiconductor materials. The use of more capable heat sinks notwithstanding, avoiding heat generation in the first place, and hence achieving higher electro-optical efficiency, has become key to improved operation.
During the past few years, high-power diode-laser bars based on aluminum gallium arsenide/gallium arsenide (AlGaAs/GaAs) have been used widely. Typical efficiencies range between 40 and 50%. Partly or completely Al-free structures have been developed to improve facet stability, especially for devices operating at 808 nm. From the point of view of a diode-bar consumer, facet improve-ments have depended on both the Al issue and the evolution of facet passivation. Today we no longer see a performance difference between these two ways of facet stabilization.
All of this did not improve overall efficiency, however. With the requirement to minimize cooling efforts, improvements to the electro-optical efficiency of the semiconductor became key to the development of diode-laser bars. There are several applications, such as laser applications on mobile platforms and laser applications with limited cooling capabilities, that demand reduced cooling efforts.
Diminishing the cooling requirements necessitates minimizing additional power input below threshold and ohmic heating. Essentially, the losses in the semiconductor bar had to be determined and - if possible - eliminated.
Our group has focused on the reduction of internal absorption losses by using cleaner materials and optimized dopant distributions, optimizing the quantum-well structure with respect to series resistance, optimizing the charge-carrier confinement with respect to optical properties, and optimizing the cavity length for better heat conduction to the heat sink with respect to the loss density of the diode-laser bar.
In most cases, we had to balance out diverging objectives to find the global optimum for the diode-laser properties. All of these methods led to electro-optical efficiencies of the packaged diode laser between 55% (for 808 nm) and 63% (for 9xx nm). This takes into account the ohmic losses in the package; the electro-optical efficiency in the bar itself is about 5 to 8% higher.
This improved efficiency is the result of reduced losses, as described above. The required voltage typically drops, therefore, but the current remains about the same for the same optical output. Keeping Cool
Simplified cooling is a very important aspect of high-power diode-laser operation, especially for mobile platforms. First let's consider the case of a semiconductor bar made of a standard material in the AlGaAs/GaAs system. With packaging losses, the electro-optical efficiency of the diode laser is about 50%. Such a high-power diode laser requires 80 W of electrical power to generate 40 W of diode-laser radiation. The heat sink, which maintains the laser at 25°C base temperature, extracts the remaining 40 W from the package as heat. The junction temperature in the semiconductor bar is about 55°C; thus, we see a 30K temperature difference between junction and heat sink base in the package.
In contrast, one of our high-efficiency, high-power diode lasers converts more than 60% (real values are closer to 63%) of the input electrical power into laser radiation. Again, as a result of ohmic losses in the package, that number is 5 to 8% lower than the electro-optical efficiency of the semiconductor bar itself, bringing the actual number close to 70%. The 80 W of input power from the previous example would be converted with 60% efficiency into 48-W of diode-laser radiation and 32 W of waste heat. The reduced heat corre-sponds to junction temperatures of 45°C10K less than in the standard-efficiency example.
High-efficiency performance is compatible with two different modes of operation: operation at elevated temper-atures and operation with reduced cooling requirements. Let's return to our example. Locally, the high-efficiency semiconductor bar has the same conditions regarding temperature and stress as in the standard example at cooling temperatures of 35°C at the base plate of the heat sink; as a result, operation at elevated temperatures can take place without the loss of expected lifetime. Conversely, if we choose standard-temperature operation, we need to remove only 32 W of heat instead of 40 W.
These are important benefits, but in reality, the major motivation behind high-efficiency materials is obtaining higher optical output power. If the heat sink can remove 40 W of damaging waste heat, the overall electrical power input into a package with 60% power conversion can be 100 W. In such a package, 60 W of output can be generated. Consequently, the switch from 50% efficiency material to 60% efficiency material can boost optical output power from 40 to 60 W without any change in the heat sink design. Experimental Results
For different bar materials, the operation current for peak efficiency varies between 65 A for a 30%-fill-factor diode bar operating at 808 nm and 140 A for a 50%-fill-factor diode bar operating at 940 nm. These peaks roughly represent a current optimum of longtime operation in industrial applications (see figure 1).
Figure 1 Packaged diode-laser bars operating at 808 nm with a 30% fill factor (top) and at 940 nm with a 50% fill factor (bottom) achieved efficiencies of greater than 60%; the efficiencies of the unpackaged bars are 5 to 8% higher.INO
New high-efficiency material has to comply with conventional-material quality standards, necessitating a number of longtime aging tests (LATs). Since part of the improved efficiency stems from increasing the cavity lengths of each emitter within the diode-laser bar, each semicon-ductor bar uses more real estate on the original wafer. Both requirements lead to increased production efforts and higher cost. The 50% power increase is accompanied by a price increase of only about 25%, however; thus, the use of high-efficiency semiconductor material at elevated output power significantly reduces the cost per watt.
The LATs performed to date show consistent device per-formance (see figure 2). We expect to acquire sufficient data by year's end to give our passively cooled 60-W continuous-wave (CW) diode lasers the same two-year warranty as our conventional passively cooled 40-W CW diode lasers.
Fig. 2 Lifetime testing of a set of 12 high-efficiency diode-laser-stack submounts operating at 940 nm (50% fill factor) shows good consistency. The lasers were actively cooled to 25°C and driven at a current of 103 A in switched mode (soft pulse: on = 103 A; off = 18 A) with 0.5-s pulses on a 50-50 duty cycle. The 80% power level marks the end-of-life criterion according to ISO 17526.
In the same way, for water-cooled designs like stacks, the power can be increased from 50 W per bar to 80 W CW per bar. The slightly higher electro-optical efficiency in the 9xx-nm diode lasers of around 63%, compared to 808-nm diode lasers of ~58%, allows even higher values for the optical output power with longer cavity length. Thus, the step from 1.5 to 2 mm allows up to 120 W CW per bar at 940 nm.
Fig. 3 Jenoptik researchers in the BRILASI project obtained 454 W of CW output from a 940-nm high-efficiency diode laser.
Research and other short-term applications tend to focus on the use of diode lasers close to the limits of the optical output power. Such applications can leverage technologies that were never intended for industrial products. The BRILASI project, funded by the German government, has begun systematic research on the limits of advanced diode lasers. The program has tested the high-efficiency, high-power diode-laser bars described above, among other devices. For a single diode bar operating at 940 nm with 50% fill factor, our group has obtained 454 W of CW output at a drive current of 580 A (see figure 3). To the best of our know-ledge, this represents the world record for CW operation of 1-cm diode-laser bars. oe
The testing of high-efficiency diode-laser bars was partly funded within the German BRILASI project (FKZ 13N8598).
Detlev Wolff is head of sales and marketing at Jenoptik Laserdiode GmbH, Jena, Germany.