Fiber lasers have made significant progress in terms of power output, beam quality and operational robustness over the
past few years. Key to this progress has been advances in two technologies - fiber technology and 9xx nm diode laser
pump technology based on single emitters. We present the operational characteristics of our new high brightness 9xx nm
fiber laser pump sources based on diode laser bars and diode laser bar arrays and discuss the design trade offs involved
for realization of devices focused on this application. These trade offs include achieving the lowest slow axis divergence
while maintaining high wall plug efficiency and minimizing facet power density to maximize reliability.
Fiber-coupled systems based on broad-area multimode emitters require complicated optical trains in order to transform their poor quality output beam into a usable form. Recently, Nuvonyx has reported implementations of a single spatial mode, high-brightness laser diode bar with significantly improved beam quality. These laser bars represent a broad technology platform at the core of many Nuvonyx systems. The low output divergence of these devices enables efficient coupling into a 400 &mgr;m core, 0.22 numerical aperture fiber with a single focusing lens. Larger systems using stacks of high-brightness diode laser bars can achieve greater than 1.7 kW output from this same fiber size, corresponding to a power density level exceeding 1.4 MW/cm<sup>2</sup>. The high-brightness bars reported here are compatible with techniques for achieving high spatial or spectral brightness. Using external feedback elements such as a volume Bragg grating, the output spectrum can be narrowed to less than 0.25 nm and is stabilized to dλ/dI = 4 pm/A and dλ/dT = 2 pm/°C. Using spectral beam combination, a single high brightness bar can be coupled into a 100 &mgr;m core, 0.22 NA fiber with approximately 90% efficiency.
There are many advantages of delivering optical power from high power laser diode arrays through an optical fiber.
However, most high power diode lasers require complicated optical trains in order to couple their light into a fiber
because of the poor beam quality. Recently, Nuvonyx has reported implementations of a single spatial mode, high
brightness laser diode bar technology that exhibits much improved beam quality. Optical power from the high brightness
laser diode arrays is coupled into a 400 &mgr;m core fiber with a 0.22 numerical aperture (NA) at power density levels
exceeding 1.4 MW/cm<sup>2</sup>. These systems are suitable as standalone industrial direct diode laser systems or as multikilowatt
fiber laser pump sources. In addition, Nuvonyx presents experimental results of further development in its fiber
coupled diode laser systems program including techniques for wavelength stabilization and spectral beam combination.
In order to meet the ever increasing demands of many high power laser diode customers, Nuvonyx has worked to
improve a number of key metrics of the diode laser package. The most often challenged specifications are power per bar, efficiency, and reliability in both hard pulse and constant current mode. In response to these requests, Nuvonyx has worked to offer commercial component devices in excess of 100 and 150 watts per bar package in multiple wavelengths. The packages are routinely combined to form single stacks that generate greater than 3.5 kilowatts each and two-dimensional arrays which produce light in excess of 10 kilowatts. These parts all demonstrate predicted lifetimes in excess of 10,000 hours. The micro-channel cooled heat sink has also been improved by closer matching the coefficient of thermal expansion of the cooler to the laser diode bar, which allows for harder solders such as gold-tin to be employed. All of this work has helped to meet the specifications of the most demanding laser diode customers.
The improved wall-plug efficiency and minimal maintenance of diode laser systems over Nd:YAG and CO<sub>2</sub> lasers has been admired by many manufacturers. Until recently, most diode laser systems could not compete at high-power levels or with the same beam quality. Nuvonyx reports the design and initial development of a diode laser system that delivers 2000 W from a 600 μm core fiber with a 0.22 NA. This system is suitable as a stand-alone industrial direct diode laser system or as a multi-kilowatt fiber laser pump source. The development of a high brightness bar technology by Nuvonyx and its collaborators along with the use of polarization beam combining is the core of this laser system. Each emitter operates with a single lateral mode resulting in a high brightness bar that outputs up to 50 W. The wavelength of the laser is centered at 975 nm with a width of less than ± 3 nm. The demonstration of this laser system defines a clear path to scale this technology to 4000 W.