High Power High Brightness Laser Diode Sources

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.

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.

There is an increasing demand for high-power, high-brightness diode lasers from 8xx nm to 9xx nm for applications such as fiber laser pumping, materials processing, solid-state laser pumping, and consumer electronics manufacturing. The kilowatt CW fiber laser pumping (915 nm - 976 nm), in particular, requires the diode lasers to have both high power and high brightness in order to achieve high-performance and reduced manufacturing costs. This paper presents continued progress in the development of high brightness fiber-coupled product platform, <i>element^TM</i>. Further brightness improvement and power-scaling have been enabled by both the rise in chip brightness as well as the increase in number of chips used to couple into a given numerical aperture. We have developed a new generation of high power broad area laser known as reduced-mode diode (REM-diode) which suppresses many of the higher order modes in the slow axis and reduces divergence up to two times at the same operating conditions. To date, we have achieved slow-axis brightness as high as 4.3 W/mm-mrad for devices with thermal resistance of ~2.5 C/W. As a result, we have achieved &gt;75 watts from a 1&times;6 <i>element^TM</i>in the 9xx nm spectral range; and 177 watts of peak power from a 2&times;6 <i>element^TM</i>. We have also improved our optics for fiber-coupling which accommodates 7 emitters per polarization in the same numerical aperture. Using this configuration, we project 200 watts of peak power from a 2&times;7 <i>element^TM</i> with a reliable product at 176 W of power from 105 &mu;m and 0.15 NA fiber. REM-diodes can also be wavelength stabilized using VBGs. The reliability of REM-diodes are equal or better than broad area lasers (BALs). We present current status on ongoing reliability assessment of chip-on-submount.

High power diode lasers have been widely used in many fields. To meet the requirements of high power and high reliability, passively cooled single bar CS-packaged diode lasers must be robust to withstand thermal fatigue and operate long lifetime. In this work, a novel complete indium-free double-side cooling technology has been applied to package passively cooled high power diode lasers. Thermal behavior of hard solder CS-package diode lasers with different packaging structures was simulated and analyzed. Based on these results, the device structure and packaging process of double-side cooled CS-packaged diode lasers were optimized. A series of CW 200W 940nm high power diode lasers were developed and fabricated using hard solder bonding technology. The performance of the CW 200W 940nm high power diode lasers, such as output power, spectrum, thermal resistance, near field, far field, smile, lifetime, etc., is characterized and analyzed.

In this work we report on a novel optical design for beam shaping and focalization of high-power diode laser bars. The goals of our study are: the increase the optical throughput of the beam shaping device with respect to standard solutions and either to enhance the irradiance on a target or to inject the laser beam into a smaller fibre than with respect to beam shaping system based on plane surfaces. The high power diode laser bars pose serious difficulties in their optical handling due to their strong difference between the two transverse axes, which induce a strong astigmatic and asymmetric output radiation. As is well known, the beam quality is very different in the two axes called slow axis and fast axis, and in particular the slow axis is composed by the superposition of several multimodal sources. The beam quality in this axis is very low (its etendue may exceed 2000 mm mrad). On the other hand, the fast axis has a very high beam quality, near diffraction limited, although with very high divergence (30°-50°). The common solution for the application of the laser radiation is a fast axis aspheric micro lens in front of the emitters, in order to achieve its collimation. Typical values of the fast axis collimated beam are 0.7mm and less than 6mrad. However, the so obtained collimated beam is poorly focusable with a standard lens, and a few methods were proposed to overcome the problem. The more relevant solutions include: the stepped mirror technique, the plane parallel mirrors pair, micro prisms array and confocal micro lens array. Each of these techniques is based on the equalization of the beam parameter product by the subdivision of the beam in the slow axis and its reshaping. For all these techniques the efficiency spans from 50% to 70%. The best focalization results allow the coupling in a fibre of 400μm diameter, with NA-0.22. The aim of this work is the design and the realization of a new device, that is considered as target the following aspects: 1) the maximum optical efficiency in the beam shaping process, 2) the optimal equalization of the beam parameter product for the two axes, 3) the use of few optical elements and 4) a very compact size. These goals are addressed by a scheme that splits the collimated beam from the laser diode into different portions while the length of the optical paths of each sub element is kept constant, and by the subsequent use of short focal length aspheric lenses for the focalization of the transformed beam. Each sub-beam is deflected by a couple of plane parallel mirrors, whose normal is directed to equalize the BPP without any mutual shadowing. An optimal solution can be easily envisaged for a laser source of common size of 0.7 x 10 mm. The condition on equal optical path length has the noticeable property of placing the virtual position of the individual portions into which the original beam is split at the same distance with respect to target. Thanks to this, their subsequent focusing is unaffected by the axial displacement of the common solution by the stepped mirrors. In fact, to correct this effect, this latter technique requires the use of a prism pair, involving complexity, size enlargement and higher costs. In this work both an extensive ray tracing and optical analysis is presented as well as the experimental characterization of an experimental model. Moreover, we also report on the technique for the realization of th tilted-face plane mirrors of which is composed our beam shaping device. The scheme of beam shaping here reported can be extended to higher power beam by means of the technique of the beam combination by polarization coupling or that of the optical beam compression. Examples of theses developments are discussed in the paper, and experimental results presented. The most direct applications of the class of optical devices here reported are the high power diode laser direct application in material processing or manufacturing, the coupling into multimode optical fiber of the diode laser radiation as well as the fiber laser end pumping.

High power diode lasers in 7xx-nm region, have been needed for various applications. Compared to 9xx nm lasers that have been developed extensively in the last 20 years, high power lasers at 7xx-nm region presents much more challenges for operation power, efficiency, temperature performance and reliability. This paper will present recent progresses on 7xx nm laser diodes for the above attributes. Two laser designs will be reviewed and high power diode laser performance and reliability will be presented. Single emitter devices, with 200μm wide emitting width, show up to 10W reliable operation power, with peak efficiency more than 65%. Accelerated life testing at 12A, 50°C heatsink temperature has been running for thousands of hours. High temperature performance and high COMD threshold (> 20W) will also be shown. Life-test failure modes will also be discussed. In summary, with advanced epitaxial structure design and MOCVD process, critical facet passivation and advanced heatsink and bonding technology, 7xx-8xx nm devices have been demonstrated with high performance and reliability similar to those of 9xx nm devices.

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%.

Recent advances in diode pump sources include higher efficiency, higher power, longer lifetimes, wider wavelength coverage, and higher operating temperatures. In addition, advances in diode array packaging have provided for higher average power capability and an optical format that is more convenient for a variety of pump applications. We discuss advances in laser performance as well as advances in packaging technology with an emphasis on pumping of high average power solid state lasers.

Semiconductor Lasers (also known as Laser Diodes, LDs), have many unique properties and advantages and found wide applications in variouse fields. The super high conversion efficiency of high power single emitter semiconductor lasers and their arrays, commonly achievable values of them being in the range of 50%-80%, makes them particularly suitable for applications as the ideal pumping source for solid state lasers and also as widely used in material processing and medicine. Yet semiconductor lasers also have shortcomings, the poor beam quality of multi-mode semiconductor lasers and laser arrays usually severely restricted their applications. The present paper mainly addresses the semiconductor laser arrays design and fabrication technologies, with particular emphasis placed on the beam shaping principles and methods. The high power semiconductor lasers in the wavelength range between 800nm and 1100nm have been reviewed and analyzed. The topic of the electrical-to-optical conversion efficiency enhancement has been addressed in some detail. The high power single emitter lasers and LD bars have been fabricated and their performance characterized. The beam divergences of LDs in the vertical and parallel directions have been addressed and analyzed. The beam collimation has been analyzed and experimentally performed for the single emitter and LD bars. Beam combining methods and fiber coupling have been summarized and experimentally verified, with specific efficiency data for beam combining and fiber coupling being presented and analyzed.

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.

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).

High power diode lasers from a few Watts up to several Kilowatts have entered industrial manufacturing environment for materials processing applications. The technology has proven to show unique features, e.g. high efficiency, small size, low energy consumption and high reliability. In the first part of this paper a short description of state-of- the-art high power diode laser technology and applications is provided and the benefits and restrictions of this laser technology will be evaluated. For large scale penetration into the manufacture market, the restrictions, especially the rather poor beam quality of high power diode lasers compared to conventional lasers have to be overcome. Also, the specialities of the high power diode lasers, i.e. their modular structure and their extremely small size have to be translated into laser manufacturing technology. The further improvement of high power diode lasers as well as the development of new diode laser specific manufacturing technologies are the essential topics of a National German Minister Priority Project entitled 'Modular Diode Laser Beam Tools': 22 Partners from industry and institutions, 4 semiconductor experts, 5 laser manufacturers and 14 applicants are working together in frame of this project to work out and transfer a joint strategy and system technology to the benefits of the future of high power diode laser technology. The goals, the structure and the work of this project will be described in the second part of this paper.

Recent advances in efficiency and mounting technology have lead to new sets of parameters for commercially available high power diode lasers (HDL). Still, these advances have to be proven by extensive long-term aging tests (LAT). The results of these give both the safety margin for operating todays HDL as well as hints towards further improvement. By statistical means clear information on reliability can be derived from these LATs. The reliability required by industrial applications of HDLs lead to a focus on quality at JENOPTIK Laserdiode over the last seven years. Today - after delivering tens of thousands of diode lasers, thousands of stacks, and thousands of fiber-coupled units into several applications - clear operation conditions can be formulated. On this experience diode lasers have been developed to ease the still strict operation conditions, especially to make the cooling easier. An overview is given on the results of the reliability improvements. The new bar properties are made usable by improved mounting procedures. Especially for passive cooling new ranges of output power are accessible. This improves both cost structure and the ease of use. Based on the knowledge gained from the improvement of the cw diode lasers new opportunities for qcw products arise. These new opportunities are discussed.

A high power mid-infrared optical parametric oscillator (OPO) with picosecond pulse bunch output is experimentally demonstrated. The pump source was a high power master oscillation power amplifier (MOPA) picosecond pulsed fiber amplifier. The seed of the MOPA was a gain-switched distributed Bragg reflector (DBR) laser diode (LD) with picosecond pulse operation at a high repetition rate. The seed laser was amplified to 50 W by two-stage pre-amplifiers and a large mode area (LMA) Yb fiber based power-amplifier. A fiber-pigtailed acousto-optic modulator with the first order diffraction transmission was inserted into the second pre-amplifier to form a picosecond pulse bunch train and to change the peak power simultaneously. The power-amplified pulse bunches were focused to pump a wavelength-tunable OPO for emitting high power mid-infrared laser. By adjusting the OPO cavity length, the maximum average idler powers obtained at 3.1, 3.3 and 3.5 μm were 7, 6.6 and 6.4 W respectively.

Diode lasers have a broad wavelength range, from the visible to beyond 2.2μm. This allows for various applications in the defense sector, ranging from classic pumping of DPSSL in range finders or target designators, up to pumping directed energy weapons in the 50+ kW range. Also direct diode applications for illumination above 1.55μm, or direct IR countermeasures are of interest. Here an overview is given on some new wavelengths and applications which are recently under discussion. In this overview the following aspects are reviewed: • High Power CW pumps at 808 / 880 / 940nm • Pumps for DPAL – Diode Pumped Alkali Lasers • High Power Diode Lasers in the range < 1.0 μm • Scalable Mini-Bar concept for high brightness fiber coupled modules • The Light Weight Fiber Coupled module based on the Mini-Bar concept Overall, High Power Diode Lasers offer many ways to be used in new applications in the defense market.