Reduced-mode (REM) diodes enable high brightness fiber-coupled modules

New wavelengths, high brightness and increasing reliability are breaking new ground for diode lasers. The higher brightness allows direct fiber coupling of several 100 W in 200 μm and 400 μm fibers and therefore i.e. the pumping of fiber lasers. Such high power fiber coupled laser diodes combined with fast beam deflection units (galvo scanners) have been implemented in industrial processes and used for quasi‐simultaneous welding of polymers, heat treatment in the sub millimeter range or selective soldering in solar cell production. Besides pumping of solid state lasers, high power diode lasers become more and more a competitive tool for many applications in material processing.

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/cm². 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.

New wavelengths, high brightness and increasing reliability are breaking new ground for diode lasers. The higher brightness allows direct fibre coupling of several 100 W in 200 µm and 400 µm fibres and therefore the pumping of fibre lasers. Such high power fibre coupled laser diodes combined with fast beam deflection units (Galvo Scanners) have been implemented in industrial processes and used for quasi-simultaneous welding of polymers or selective soldering. But also as customized modules diode lasers offer competitive solutions for example in medical device manufacturing. Available as modules or as turn-key systems, featuring the laser module and all its supply and control units in one 19” rack mountable chassis, such devices become more compact and easy to integrate. New markets like solar modules production benefit from this technology. The usage of conduction cooled laser diodes increases the typical lifetimes significantly. Life tests in pulsed and cw mode show over 16 Mega shots respectively over 14.000 h with minimal degradation. Depending on the operating conditions, such diode lasers have proven to run for several years in production nearly free of maintenance. Besides pumping of solid state lasers high power diode lasers become more and more a competitive tool for many applications in material processing.

In the last few years, fiber-coupled diode lasers have shown massive applications in many fields of communication and scientific research. Particularly, the pumping of solid-state lasers is a key application for more powerful diode lasers enabling good solutions in various laser micro methods like metal cutting, sintering, structuring as well as drilling. In this work, a simple beam shaping method is demonstrated for coupling a high-power semiconductor laser diode into multi-mode fiber optic using optical lenses. The optical lenses as beam transformation components are utilized to reshape the asymmetrical irradiation of the diode laser bar and to circularize the laser beam. Using this simple method, compact, high-output-power, and high-brightness fiber-coupled laser diode systems are achieved. The obtained results show that the entire optical coupling efficiency of the system is more than 65%.

High-power broad-area stripe lasers have been fabricated by using a one-step selective area growth technique in the metalorganic chemical vapor deposition system. This technique provides devices with excellent control both on electrical and optical confinements. cw output power of more than 350 mW/facet (700 mW total) was achieved, and a differential quantum efficiency of 0.625 mW/mA/facet (1.25 mW/mA total) was observed.

A novel concept of a diode laser system developed by LIMO GmbH and called ring-laser (patent pending) enables a very flexible, uncomplex, simultaneous welding of plastic parts with different shapes and with different sizes without any additional changes of the system. Another diode laser system, so called line generator (patent pending), which utilises LIMO-homogenisers can be used for contour or simultaneous welding of plastics. This system is characterized by its nearly top-hat beam profile with homogeneity better than +/-6 %. Additionally, its line length, line width and its operating distance can be adapted to the geometry of a process by the use of an appropriate type of homogenisers and lens systems. Due to its outstanding features this diode laser can be also used e.g. for simultaneous soldering, thermal surface treatment, transversal pumping of solid state lasers and also in the printing industry (computer-to-plate technology).A novel concept of a diode laser system developed by LIMO GmbH and called ring-laser (patent pending) enables a very flexible, uncomplex, simultaneous welding of plastic parts with different shapes and with different sizes without any additional changes of the system. Another diode laser system, so called line generator (patent pending), which utilises LIMO-homogenisers can be used for contour or simultaneous welding of plastics. This system is characterized by its nearly top-hat beam profile with homogeneity better than +/-6 %. Additionally, its line length, line width and its operating distance can be adapted to the geometry of a process by the use of an appropriate type of homogenisers and lens systems. Due to its outstanding features this diode laser can be also used e.g. for simultaneous soldering, thermal surface treatment, transversal pumping of solid state lasers and also in the printing industry (computer-to-plate technology).

We have carried out what we believe to be a novel design for wavelength stabilization and spectral beam combining, focusing on increasing the brightness of fiber coupled laser diode (LD) based on a VBG (volume Bragg grating) device. The small size VBGs which can be called miniature VBGs with diffraction angle of 0.4 degrees are used to lock spectrum, and the large-scale size VBG which can be called primary combining VBG with diffraction angle of 6 degrees is employed to combine with two sub-beams. The fiber coupled LD module is used as sub-beam whose beam parameters such as point error, pupil deviation, center wavelength and spectral width can be conveniently controlled by fiber adjustment instead of each single chip compared to direct spatial light output. The locked spectral width of sub-beam is less than 0.3 nm (FWHM, full width at half maximum), the power after beam combining is more than 800W, the BPP (beam product parameter) is less than 15 mm·mrad containing 95% energy, which can be coupled into fiber of core diameter with 200 µm, NA (numerical aperture) less than 0.15.

High Power Laser Diodes (HPLD) are increasingly used in different fields of applications such as Scientific, Industrial, Medical and Defense. Our significant performances improvements (especially in peak power, divergence and efficiency) and a reproducible manufacturing process have led to reliable, highly robust components. For industrial applications these devices are used predominantly for pumping of solid state lasers. Due to the drastically falling price per watt they are more and more replacing flash lamps as pump sources. Their compact size and high reliability make diode laser arrays ideally suited to this demanding market. For Scientific applications, the brightness as well as the cost are the two main drivers. In order to respond to this demand, QLD has focused its development on very high peak power bars associated with very high E/O efficiency. The level of energy QLD is achieving now has also pushed us to develop a power supply driver able to operate such diodes at up to 1000A under 100V.

Chapter 22 - Optical Fibers for Industrial Laser Applications

We report on the development of high power, 9xx-10xx nm laser diode bars for use in direct diode systems and for solidstate and fibre laser pumping with applications in industrial markets. For 1 cm wide bars on micro channel cooler (MCC) we have achieved a reliable output power of 250 W across the 900 nm – 1060 nm range. At this output power level we have achieved power conversion efficiencies of 65-66 % and 90 % power content slow axis beam divergence of ~6.5°. Results of a 6400 h life test show an average power degradation of 0.6 % per 1000 h at this operating power level. We will also show results of high power bars assembled on the new OCLARO conductive cooler, the BLM. This new cooler has a small footprint of 12.6 mm × 24.8 mm and is designed for lateral or vertical stacking of diodes in multi kilowatt systems but with the benefits associated with a conductive cooler. The thermal properties are shown to be the same as for a standard CS mount. 1 cm wide high fill factor bars and 0.5 cm wide low fill factor half bars assembled on the BLM operate at 63-64 % power conversion efficiency (PCE) with output powers of up to 250 W and 150 W, respectively.

The role of a robust impurity free quantum well intermixing process in fabricating high-power high-brightness AlGaInP semiconductor laser diodes is outlined. Characteristics of the process are discussed and its attributes summarized. Bandgap shifted lasers have been fabricated to demonstrate the integrity of the material after the quantum well intermixing process. Oxide stripe lasers with non-absorbing mirrors are shown to increase the catastrophic optical damage threshold of semiconductor laser devices. Finally high brightness extended cavity lasers are shown to significantly improve the beam quality, and the insignificant change in the threshold current and slight decrease of the external efficiency demonstrates that the process is low loss.

Fiber-coupled diode modules have various applications in material processing and fiber laser pumping because of their high efficiency and high reliability. Commercial fiber-coupled diode modules using spatial beam combining and polarization beam combining cannot be employed in high-brightness applications, for example metal cutting, which demands a laser power exceeding 1 kW with a BPP of a few mm*mrad. Dense wavelength beam combining (DWBC) technology showed the possibility of further scaling-up the output power of fiber-coupled diode modules while maintaining the same beam quality that allows for fiber-coupled diode modules to be used in high-brightness applications. The efficiency, reliability, and brightness of fiber-coupled diode modules can be improved by using single emitters instead of laser diode bars as power sources in DWBC. Two types of high-brightness 100 µm/0.22 NA 2 kW fiber-coupled diode modules employing single-emitter-based DWBC technology, which have a wavelength range from 953 to 991 nm with 50% efficiency and a narrower wavelength range with 48% efficiency respectively, were developed for material processing and Raman fiber amplifier pumping. Furthermore, we combined 15 high-brightness 100 μm/0.22 NA 1.4 kW fiber-coupled diode modules into a 600 μm/0.22 NA fiber, achieving more than 22 kW at the output.

Fiber laser manufacturers demand high-brightness laser diode pumps delivering optical pump energy in both a compact fiber core and narrow angular content. A pump delivery fiber of a 105 μm core and 0.22 numerical aperture (NA) is typically used, where the fiber NA is under-filled to ease the launch of laser diode emission into the fiber and make the fiber tolerant to bending. At SCD, we have developed high-brightness NEON multi-emitter fiber-coupled pump modules that deliver 50 W output from a 105 μm, 0.15 NA fiber enabling low-NA power delivery to a customer's fiber laser network. Brightness-enhanced single emitters are engineered with ultra-low divergence for compatibility with the low-NA delivery fiber, with the latest emitters delivering 14 W with 95% of the slow-axis energy contained within an NA of 0.09. The reduced slow-axis divergence is achieved with an optimized epitaxial design, where the peak optical intensity is reduced to both lessen filamentation within the laser cavity and reduce the power density on the output facet thus increasing the emitter reliability. The low mode filling of the fiber allows it to be coiled with diameters down to 70 mm at full operating power despite the small NA and further eliminates the need for mode-stripping at fiber combiners and splices downstream from our pump modules. 50W fiber pump products at 915, 950 and 975 nm wavelengths are presented, including a wavelengthstabilized version at 976 nm.

Compared to longer wavelength devices, the development of reliable red-emitting diode lasers is more challenging due to the applicable semiconductors and the limited stability of the laser facets. Reliable operation over 1,000 h is sufficient for the pumping of fs-Cr:LiSAF lasers or in photodynamic therapy, but laser display technology requires material with the potential to operate failure free over more than 10,000 h. Reliability tests for 650 nm broad area (BA) lasers based on a GaInP single quantum well embedded in AlGaInP waveguide layers will be presented. 100 μm stripe width BA lasers with a length of 1.5 mm were fabricated as low mesa structures. The facets were optically coated including a facet passivation procedure. Mounted on diamond heat spreader and standard C-mounts at 15°C the devices had threshold currents of 550 mA, slope efficiencies of 1.2 W/A, and conversion efficiencies up to 0.33. Aging tests of four BA lasers were performed at output powers of 1.1 W and 1.2 W over a total test time of 20,000 h at a heat sink temperature of 15°C. No failure occurred during the lifetime test. The degradation rates for all devices were smaller than 3x10^-6 h-1. A lifetime of 100,000 h at an operational power of 1.2 W can be estimated. These data proof that the material is well suited for the fabrication of high-brightness diode lasers for laser display technology.

Summary form only given. Double-clad rare-earth-doped fiber lasers pumped by high brightness laser diodes are very efficient and compact sources of CW and pulsed radiation. In contrast to conventional bulk crystal lasers, the low pump absorption rate and waveguiding nature of double-clad fiber lasers result in relatively long cavity lengths, small effective core areas, and high round trip gain. Consequently, amplified spontaneous emission (ASE), various nonlinear effects, and optical damage are important factors for Q-switched fiber lasers. We theoretically and experimentally study different fiber designs for Q-switched cladding-pumped ytterbium-doped fiber lasers.