Emitter-Resolved Characterization of High-Power Diode Laser Bars at Operating Point and Below Threshold

Current laser systems based on high-power laser diode bars need active cooling either water cooling or the use of thermo-electric coolers to ensure an adequate operating temperature for a reasonable lifetime. Here is a solution with a bonded fin heat sink and forced ventilation introduced, a diode laser bar with an improved efficiency and a low thermal resistance as well as an optical system for a highly efficient fibre coupling. With this system it is possible to couple 25 Watt continuous wave power from a single laser diode bar on a passive heat sink into a fibre with 200 μm core diameter. The basis for this performance is a heat sink with an exceptionally low thermal resistance. Several new features are introduced to reach a low overall gradient between the laser diode temperature and the ambient temperature. In addition, it does geometrically fit to the layout of the optical design. Shape and aspect ratio of both heat sink and housing of the laser system are matched to each other. Another feature is the use of hard-soldered or pressed bars to achieve a thermo-mechanically stable performance. The long-term thermal characteristic was tested. The operation temperature comes to saturation after about 30 minutes. Therefore it can be used for continuous wave operation at 25 Watt output power. At a quasi continuous operation at 70 percent duty cycle a peak power of 30 Watt out of the fibre is possible. From this technology results a compact fibre coupled laser system what is simple to drive compared with current high power laser systems, because there is no need to control the operating temperature. This gives way for more compact driver solutions. Fields of application are laser marking systems and material processing, where a simple driver system is requested. Also medical applications need this requirement and a compact cooling too so that mobile integrated solutions become possible. Further developments allow multiple laser diode systems for specific industrial applications demanding more power. Our measurements show the potential for direct air-cooled laser systems with 100 Watt power out of the fibre.

We report on the high-power high-temperature long-pulse performance of the 8XX-nm diode laser bars and arrays, which were recently developed at Lasertel Inc. for diode laser pumping within high-temperature (130 °C) environment without any cooling. Since certain energy in each pulse is required, the diode laser bars have to provide both high peak power and a nice pulse shape at 130 °C. Optimizing the epi-structure of the diode laser, the laser cavity and the distribution of waste heat, we demonstrate over 40-millisecond long-pulse operation of the 8XX-nm CS bars at 130 °C and 100 A. Pumping the bar with 5-Hz frequency 15-millisecond rectangular current pulses, we generate over 60 W peak power at 100 A and 130 °C. During the pulse duration, the pulse shape of the CS bars is well-maintained and the power almost linearly decays with a rate of 1.9% peak power per millisecond at 130 °C and 100 A. Regardless of the pulse shape, this laser bar can lase at very high temperature and output pulse can last for 8 ms/2ms at 170 °C/180 °C (both driven by 60 A current pulses with 5-Hz frequency, 10 millisecond pulse width), respectively. To the best of our knowledge, this is the highest operating temperature for a long-pulse 8XX-nm laser bar. Under the condition of 130 °C and 100 A, the laser bars do not show any degradation after 310,000 10-millisecond current pulse shots. The performance of stack arrays at 130 °C and 100 A are also presented. The development of reliable high-temperature diode laser bar paves the way for diode laser long-pulse pumping within a high-temperature environment without any cooling.

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.

Fiber-coupling optical system for high-power and multi-wavelength diode laser bars oriented to integrated biomedical imaging systems

The introduction of high power diode laser systems in industry has boosted the interest in these devices for a wide range of applications. Besides printing and soldering, cutting and deep penetration welding are becoming more important. An overview about the developments, an update on today's high power laser activities and an outlook will be given, what characteristics laser bars will have to fulfil in the near future. For higher brightness, laser bars with lower fill factors, monolithic integrated laser junctions and tapered laser designs were investigated. High power diode laser (HPDL) bars with 25% - 50% fill factor were operated between 40 W and 80 W and lifetimes up to 100 000 hours could be extrapolated. Tapered laser bars with 50W output power and high wall plug efficiencies were developed. Wavelength multiplexing and polarisation coupling were used in order to reach multi-kilo-Watt diode laser emission. Examples for applications will be given.

High power diode lasers have become very attractive as an intelligent tool for direct material processing as well as pump sources for miniaturized solid state lasers and fiber lasers since optical beam transformation systems generate the required beam circularization from the originally highly asymmetric brightness of the output beam. This presentation focuses on reliable microassembling processes to realize micro-optical beam transformation systems for high power diode laser bars and stacks for the efficient coupling of the beam into optical fibers at reasonable costs. The modular semi automatic micro assembly set up realized consists of a six axes alignment system with an accuracy in the submicron range for the three spatial areas as well as microrad for the three rotational axes, a semi-automatic glue dispenser as well as respective grippers and magazines which are necessary for the handling of the micro optical components. The established automatic processes for the mounting of the fast axis collimation lenses are of particular interest for the assembly of e.g. fiber coupled modules of diode laser stacks whereas more than 20 fast-axis collimations are necessary.

High Power Diode Laser Bars serve as the foundation for many high-power laser systems used in manufacturing and research. Efficient heat removal is essential for reducing cost per watt by increasing available output power or extending the effective lifetime of these devices. In this paper we will introduce a novel method for electrically isolated water impingement cooling of high-power diode laser bars operating in the 975nm region. We will present results from 58 emitter diode laser bars operating in the 975nm region with CW output power levels exceeding 500W and a thermal resistivity of less than 0.12 K/W.

In this work we report on high-power diode laser modules with enhanced spectral brightness by means of volume holographic gratings for wavelength stabilization. High-power diode laser modules typically have a relatively broad spectral width of about 3 to 6 nm. In addition the center wavelength shifts by changing the temperature and the driving current, which is obstructive for pumping applications with small absorption bandwidths. Wavelength stabilization of high-power diode laser modules is an important means for more efficient pumping of solid-state lasers with a narrow absorption bandwidth. However, for efficient and reliable wavelength stabilization the parameters of the volume holographic grating and the parameters of the diode laser bar have to be adapted carefully. Important parameters are the reflectivity of the volume holographic grating, the reflectivity of the diode laser bar and the angular and spectral emission characteristics of the diode laser bar. In addition, the lateral structure of the diode laser bar and the microoptical elements for beam shaping have to be considered. In this paper we present a detailed characterization of different diode laser systems with wavelength stabilization in the spectral range from 790 - 1000 nm. The laser modules are divided into systems with and without fiber coupling. We will present data for a wavelength stabilized single diode laser bar with an output power of 69 W at a wavelength of 808 nm. Another example is a wavelength stabilized fiber-coupled diode laser module with an output power of 456 W for a fiber with a core diameter of 400 μm (NA 0.22).

Introduction: The labial frenum is a fold of mucous membrane that attaches the lip and the cheek to the alveolar mucosa, the gingiva, and the underlying periosteum. In some cases, its presence can cause a midline diastema, periodontal diseases related to food impaction, or retention of biofilm, among others. In such cases, lip frenectomy is indicated as treatment, which can be performed with a scalpel (conventional method), an electric scalpel, or a surgical laser. Objective: To show a clinical case performed at Laser Extension Project in Dentistry, Federal University of Maranhão grounded in a literature review. Case Presentation: A laser frenectomy was performed on a female patient, aged 20, who had a diastema between the upper central incisors and an indication for frenum removal. The high-power diode laser is excellent for procedures in soft tissue because its wavelength is well absorbed by hemoglobin and other pigments; its use also allows a reduction in the amount of anesthetic and medicines used. The parameters used were 2 W, in a continuous mode, 808 nm infrared emission; with delivery of the beam through optical fiber 300 μM; energy of 120 J; 20 pps. Conclusion: the high power diode laser allowed a satisfactory result, the procedure was safe, the technique was a simple one and of reduced clinical time, as mentioned in the literature. It is worth noting that the technique is dependent on the skill of the professional performing it.

This study investigates the thermal performance of cryogenic micro-pin fin coolers for high-power laser diode (LD) bars. An open-loop liquid nitrogen cooling system, used to operate LD bars at cryogenic temperatures, is developed and characterized. The comparison study demonstrates that the total thermal resistance value of the micro-pin fin cooler, ranging from 0.03 to 0.04 °C/W, is only 1/3 of that of the micro-gap cooler and, thus, contributes significantly to reducing the LD operating temperature and enhancing its efficiency. In this study, the peak optical power of 68.8 W was observed at an LD bar package base temperature of − 100 °C, providing a 20% increase relative to the optical power of 57.3 W for a nominal operating condition with an LD bar package base temperature of 41 °C. This result clearly illustrates the enhancement in optical performance made possible by cryogenic cooling with a pin fin microcooler.

The goal of this work is twofold. First we study residual mechanical bonding induced stress in state-of-the-art 980 nm high-power single-mode GaAs-based laser diodes (LD) mounted on either standard plain material (Fig. 1-a), basic bi-material (Fig. 1-b) composite, or optimized bi-material composite submounts with a standard Au-Sn eutectic alloy solder pad. Then we show how to enhance the heat dissipation from the operating LD via its submount. The motivation of this work is the understanding of the physical phenomena playing a critical role on the electro-optical performances, current consumption, reliability, and lifetime of the LD-submount assemblies.

We report the first integrated injection-locked high-power continuous-wave diode laser array with an on-chip independently controlled master laser. This device emits a near-diffraction-limited (0.5° full width at half maximum) single-lobed far-field emission beam at single-facet powers up to 125 mW. Also, by current tuning the emission wavelength of the master laser, we observe steering of the single-lobed emission over an angular range of 0.50° at a rate of −1.2×10−2 deg/mA. Our work demonstrates the feasibility of incorporating active optical injection and control in the structure of high-power diode laser devices.

The degradation mechanism of 808nm GaAs-based high-power laser diode bars (LDBs) with 6 single laser diodes was investigated using infrared thermography and photoemission microscopy. Over time, measurements of output power indicated that one of the 6 laser bars had a serious degradation of output power. Infrared imaging and photoemission microscopy revealed that solder voids during packaging resulted in premature failure due to high temperatures in the lasing region. The current reliability of LDB devices is compromised by these packaging defects.

In order to achieve a thermally stable diode laser system based on high power diode laser bars, micro channel heat sinks are used to face the dissipated power with a density of 10⁶ W/m². Passively cooled diode lasers are either lower in power or facing higher junction temperatures. As a matter of principle the cooling with micro channel heat sinks requires a sealing between the heat sink itself and the system around. The leakage of this sealing, normally achieved by O-rings, can be reduced but never avoided. Sensible systems and extreme lifetime requirements, like in the telecom applications, already require passively cooled diode lasers with no water in the inner system boundaries. To achieve a minimized temperature shift in the junction, we developed a new copper based heat sink, spreading the dissipated heat in an optimised manner. Based on this, our further research shows that the higher temperature shift in a passive submount compared with active ones can be tolerated for a system, if the heat resistance to the external water heat exchanger is minimized. For applications either with or without the requirement of a thermo electric cooling element (TEC), we developed a technical solution for a heat exchanger, to keep water out of the inner system boundaries. The thermal resistance is low enough to run up to 12 passively cooled diode lasers on an regular ambient temperature and a minimum of junction temperature mismatch.

We present high-power GaAs-based broad-area diode lasers with a novel variant of the enhanced self-aligned lateral structure ‘eSAS’, having a strongly reduced lasing threshold and improved peak conversion efficiency and beam quality in comparison to their standard gain-guided counterparts. To realize this new variant (eSAS-V2), a two-step epitaxial growth process involving in situ etching is used to integrate current-blocking layers, optimized for tunnel current suppression, within the p-Al0.8GaAs cladding layer of an extreme-triple-asymmetric epitaxial structure with a thin p-side waveguide. The blocking layers are thus in close proximity to the active zone, resulting in strong suppression of current spreading and lateral carrier accumulation. eSAS-V2 devices with 4 mm resonator length and varying stripe widths are characterized and compared to previous eSAS variant (eSAS-V1) as well as gain-guided reference devices, all having the same dimensions and epitaxial structure. Measurement results show that the new eSAS-V2 variant eliminates an estimated 89% of lateral current spreading, resulting in a strong threshold current reduction of 29% at 90 μm stripe width, while slope and series resistance are broadly unchanged. The novel eSAS-V2 devices also maintain high conversion efficiency up to high continuous-wave optical power, with an exemplary 90 μm device having 51.5% at 20 W. Near-field width is significantly narrowed in both eSAS variants, but eSAS-V2 exhibits a wider far-field angle, consistent with the presence of index guiding. Nonetheless, eSAS-V2 achieves higher beam quality and lateral brightness than gain-guided reference devices, but the index guiding in this realization prevents it from surpassing eSAS-V1. Overall, the different performance benefits of the eSAS approach are clearly demonstrated.