A long-standing challenge for semiconductor lasers is scaling the optical power and brightness of many diode lasers by
coherent beam combination. Because single-mode semiconductor lasers have limited power available from a single
element, there is a strong motivation to coherently combine the outputs of many elements for applications including
industrial lasers for materials processing, free space optical communications, and defense. Despite the fact that such a
coherently-combined source is potentially the most efficient laser, coherent combination of semiconductor lasers is
generally considered to be difficult, since precise phase control is required between elements.
We describe our approach to coherent combination of semiconductor lasers. The Slab-Coupled Optical Waveguide
Laser (SCOWL), invented at Lincoln Laboratory, is used as the single-mode diode laser element for coherent
combination. With a 10-element SCOWL array, coherently combined output power as high as 7 W in continuous wave
using an external cavity has been demonstrated, which is the highest output level achieved using a coherent array of
semiconductor lasers. We are currently working on a related approach to scale the coherent power up to 100 W.
Diode-pumped solid-state lasers are gaining acceptance as the desired laser source for materials processing as well as a
host of new applications that are expanding rapidly. Because of this, the performance, stability and lifetime of the diode-pump
source face unprecedented scrutiny. Increasing the lifetime of the diode, while increasing power, remains a
primary focus of the industry. One lifetime limiting issue is that of a voltage potential in the water cooling channels
which can cause cooler degradation and lower efficiency over time. Studies have been carried out that explore different
cooling approaches based on passive schemes where insulation layers are present to shield the voltage from the water
channels. However, with the introduction of insulation layers, a reduction of the deployable power from that of
microchannel coolers is seen. This report explores the effects of passive cooling approaches on the power and
divergence of 1 cm AuSn/CuW mounted bars with fill factors ranging from 10% to 50%. It is shown that a 150 W array
can be realized on a passive cooler and multiplexed to give a 1600 W stack. Thermal modeling is presented along with
life-test data for passively cooled devices.
As diode pumped solid state lasers gain more market share, the performance, stability and lifetime of the diode pump
source faces unprecedented scrutiny. Lifetimes of diode pumps in excess of 35,000 hrs are sought with no intervention
or maintenance from the end user. One lifetime and power limiting phenomena for arrays is that of solder creep typical
with traditional mounting using soft solders such as Indium. Harder solders such as Gold/Tin on Copper-Tungsten
submounts provide a more robust and stable mounting system for long term high power pump sources. Furthermore,
beam multiplexing of laser bars require tight wavelength and polarization purity which are affected by mounting induced
strain. In this investigation, high power 940 nm laser bars, operating in the 100 to 200 W power range, were mounted
using AuSn/CuW and In soldering schemes. The differences in thermal and strain characteristics are investigated
through the examination of the emitter wavelength, nearfield measurements, polarization and smile. The measurements
are correlated with finite element modeling to predict the 3-dimensional thermal distributions within the laser bars.
The materials processing industry has recently mandated the need for more efficient laser systems with higher beam quality and longer life. Current multiplexing techniques, state-of-the-art laser diodes and novel cooling designs are now emerging as possibilities to meet the ever demanding industry needs. This paper describes the design and initial results of a direct diode system that is aimed at delivering 1.5 kW of output power and a beam divergence of 40 mm mrad on a long life macro-channel cooler. The design entails multiplexing 2 wavelength combined beams and 2 polarization combined beams. Each of the four branches of the direct diode system utilizes a novel stacking and cooling design. The results from one of these branches, 1 wavelength and 1 polarization, are presented where the light is coupled into a fiber with a 400 μm core diameter and a NA of 0.22. Each branch consists of 60 diode laser mini-arrays, where each mini-array consists of four 100 μm wide emitters and a lateral fill factor of 50%. An output power of 500W at 10°C water temperature and 420 W at 25°C are demonstrated through the 400 μm fiber.