This PDF file contains the front matter associated with SPIE Proceedings Volume 7198, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing

State-of-the-art QCW solid-state lasers are demanding ever higher brightness from the pump source-conduction cooled diode laser stacks. The intensity of a QCW vertical stack is limited by the peak power of each diode bar and the bar pitch. The minimum bar pitch of the existing laser diode stacks on the market is about 400um. In this paper, we present a unique vertical diode laser stack package design to achieve a bar pitch as low as 150um, which improves the intensity of the stack by nearly 3 times. Together with the state-of-art diode laser bar from Coherent, greater than 30kW/cm² peak power density is achieved from the emitting area of the vertical stack. The p-n junction temperature of the diode bars in the device under QCW operation is modeled with FEA software, as well as measured in this research. Updated reliability results for these diode laser stacks are also reported.

New semiconductor multi emitters combine the advantages of laser bars and strength of single emitters for fiber coupling applications. We introduce a new technology to drive the device at highest power density at the laser facet. The new technology enables us to reduce the fill factor while maintaining output power per laser bar at reliable and efficient operation. The overall output power and slow axis beam parameter product is scaled with the numbers of emitters and may be coupled into a single fiber with low effort in beam shaping or fiber combining. We demonstrate that multi emitters can operate at same power level as the same number of single emitters. In this paper we present data on highly efficient and reliable 9xx nm laser bars designed for a defined fiber diameter and numerical aperture. For comparison single emitters and short bars with different fill factors were investigated. Efficiencies above 60% were reached with 4mm cavity length and fast axis far field angles of 45° (95%). Stable operation at powers up to 70W from short bars with five 100μm wide emitters was reached. Slow axis divergence is below 7° up to power levels of 38W and is suitable for coupling into 200μm NA 0.22 fibers with only slow axis and fast axis collimation without beam rearrangement.

The attractiveness of bars for industrial applications depends strongly on the reliable high brightness operation. For 9xx bars we report on high filling factor configurations with 200W reliable output power. Our low filling factor devices with output power between 40W and 90W have proven to operate reliably at output power densities of 85mW per 1µm stripe width, showing power wear-out degradation of less than 0.5% per 1000h operation time. For shorter wavelengths we present solutions for 808-880 nm bars. For our 808nm bars we observe power degradation of less than 4% after 8000h hard-pulse life test at 75W output power.

New-generation multi-mode 9xx mini-bars used in fiber pump modules have been developed. The epitaxial designs have been improved for lower fast-axis and slow-axis divergence, higher slope efficiency and PCE by optimizing layer structures as well as minimizing internal loss. For 915nm mini-bars with 5-mm cavity length, maximum PCE is as high as ~61% for 35W operation and remains above 59% at 45W. For 808nm, a PCE of 56% at 135W CW operation has been demonstrated with 36%-fill-factor, 3-mm-cavity-length, water-cooled bars at 50°C coolant temperature. On passive-cooled standard CS heatsinks, PCE of >51% is measured for 100W operation at 50°C heatsink temperature. Leveraging these improvements has enabled low-cost bars for high-power, high-temperature applications.

Short wavelength and highly efficient AlGaInP quantum-well laser diode is promising as a red light source for small laser display application. Two kinds of the laser diodes are presented in this paper. A narrow ridge laser diode was designed for single lateral mode. In addition, a broad area laser diode was optimized for the higher power operation. To suppress a carrier leakage from an active layer, AlInP cladding layers were adopted to both of the lasers. Evaluation tests of the fabricated lasers were performed under CW operation. The wavelength of the narrow ridge laser was 636.0 nm under the condition of 25°C and 100 mW. Single lateral mode oscillation and the high wall plug efficiency of 29% were obtained. The beam divergences were 16° and 8° in fast and slow axes, respectively. The broad area laser showed lasing wavelength of 636.9 nm at 25°C for 200 mW output. The wall plug efficiency was 30% under this condition. Both of the lasers showed both high luminance and high wall plug efficiency. These lasers are suitable for small laser display applications.

Compact, stable and efficient green lasers are of great interest for many applications like mobile video projection, sensing, distance measurement and instrumentation. Those applications require medium values of output power in the 50mW range, good wall-plug efficiency above 5 % and stable operation over a wide temperature range. In this paper we present latest results from experimental investigations on ultra-compact green intracavity frequency doubled optically pumped semiconductor InGaAs disk lasers. The green laser setup has been limited to a few micro optical and semiconductor components built on a silicon backplane and fits within an envelope of less than 0.4 cc. An optical frequency looking scheme in order to fix the fundamental wavelength over varying operating conditions like changing output power and ambient temperature has been applied. The cavity has been optimized for fast modulation response and high efficiency using quasi-phase matching non-linear material. Recent data from cw and high-frequency characterization is presented.

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.

The industry of laser marking, direct application and solid state laser pumping requires highly reliable and highly efficient laser diodes. In general, all applications demand improved brightness and temperature stability, and this by decreasing costs per watt. Instead of increasing the cavity length, we demonstrate in this paper an increase of power with standard cavity length with a clear focus of cost reduction and high efficiency. Improvements in the semiconductor material and packaging enable higher power and higher operation temperature. This technology raised the efficiency by 6 % of 808 nm bar with 50 % filling factor and a resonator length of 1.5 mm. Now, passively cooled diode lasers have reached nearly the performance of actively cooled ones. With this new design new fiber coupling modules with high brightness and high operation temperature for air cooled systems can be achieved.

Broad area diode laser and diode laser bars are the most efficient light sources. In comparison to solid state laser or gas laser systems the over all beam quality of the diode laser is poor. Thus most application of diode laser bars is high efficient pumping of solid state lasers converting the beam quality and scaling the power of laser systems within the kW range. The pump efficiency and the beam coupling efficiency of the diode laser pumped systems has to be increased to meet the increasing laser market demands for reduced costs. JENOPTIK Diode Lab GmbH (JDL) has optimized their high power brilliance bars to enable reliable high power operation especially, for the 9xx nm wavelength range and low far field divergences. Superior reliability with long operation time of 13,000 hours and high power operation of 200 W are demonstrated for high power bars high filling factor mounted on passively cooled heat sinks. Smaller far field divergence at high power levels requires longer cavity length and higher efficiencies in the beam coupling needs requires lower filling factors. The new high brilliance bars and arrays with 20% filling factor are showing high power operation up to 95 W and a slow axis beam divergence of less than 8° (95% power content).

Especially for pump applications there is a strong demand for broad-area (BA) diode lasers with high-brightness. Brightness is proportional to output power divided by near and far field widths in both directions. Therefore to achieve a high-brightness broad-area laser, high output power together with high wall-plug efficiencies and a strong reduction of the beam waists in both directions is essential. Whereas fast axis far fields show mostly a current independent behaviour, near- and far-fields in the slow axis suffer from a strong current and temperature dependence, limiting the brightness of broad-area lasers. To fulfill these issues, we have realized MBE grown InGaAs/AlGaAs high-brightness broad-area diode lasers with a resonator length of 5mm and a stripe width of 90μm to guarantee good heat dissipation. The emitting wavelength is 976nm. Single emitters have been mounted p-side down. An output power of 10W has been achieved at 9.8A. A wall-plug efficiency of 65% has been measured. To our knowledge, for a 5mm long device, this is the highest wall-plug efficiency reported so far. To evaluate the brightness of these broad-area lasers, near-fields and far fields in both directions have been carefully investigated. The design of the vertical structure of the broad-area lasers results in a far field angle of 45° in the fast axis (95% power inclusion). In the slow-axis values of 6.5° at 8.5W and 8° at 10W have been demonstrated, which results in a brightness doubled in comparison to state-of-the-art broad-area diode lasers.

Space missions are probably the most demanding environment for laser diodes. A comprehensive study on the reliability of commercially available laser diodes arrays (LDA), with the objective of bar stacks for ESA's BepiColombo Laser Altimeter mission to the planet Mercury was performed. We report the best results of lifetime tests performed on SCD 808 nm QCW stacks at different levels of current load in a unique combination with operational temperature cycles in the range of -10°C to 60 °C. Based on a field-proven design that includes Al-free wafer material and a robust packaging solution, these arrays exhibit long operational lifetime of up to 20 billion pulses monitored in the course of several years. Zero failures and stable performance of these QCW arrays were demonstrated in severe environmental conditions reflecting both, military and space applications. In order to achieve maximum device efficiency at different operational conditions of the base temperature and current, an optimum combination of the wafer structure and bar design is required. We demonstrate different types of QCW stacks delivering peak power of up to 1 kW with a usable range of 50-55% wall plug efficiency at base temperatures up to 60 °C.

We report on a novel heat sink for high-power laser diodes offering unparalleled capacity in high-heat flux handling and temperature control. The heat sink uses a liquid coolant flowing at high speed in a miniature closed and sealed loop. Diode waste heat is received at high flux and transferred to environment, coolant fluid, heat pipe, or structure at a reduced flux. When pumping solid-state or alkali vapor lasers, diode wavelength can be electronically tuned to the absorption features of the laser gain medium. This paper presents the heat sink physics, engineering design, performance modeling, and configurations.

We investigated the properties of fluorinated refrigerants for high-power laser diode bars mounted on a funryu heat sink. The thermal conductivity of fluorinated refrigerants is about ten times lower than that of water, but they are less corrosive to funryu heat sinks. Using 3M's "Fluorinet" FC77 and "Novec" HFE-7300 and comparing them with water as refrigerants, we developed a new fluorinated refrigerant cooling device that is suitable for removing heat from highpower LDs and LD modules. This device achieved CW light-output power from a 1-cm LD bar equivalent to that for a cooling device using ion-exchange water. With HFE-7300 as the refrigerant, we achieved over 100 W of output power at a drive current of CW 120 A and over 1.5 kW for a stacked-LD module operated at a constant current of 55 A in continuous-wave (CW) mode. High-power LD bars subjected to lifetime testing under these conditions have been successfully running for over 15,000 consecutive hours.

Laser dies in an optical power range of 1-3 Watts are widely assembled in popular TO- packages. TO-packages suffer from high thermal resistance and limited output power. Bad thermal contact between circuit boards and TO-devices can cause overheating of laser chips, significantly reducing the operating life time. We developed a compact high heat-load SMT package for an optical power up to 7 Watts in CW operation with good life time results. The new package for high power laser chips combines highly efficient heat dissipation with Surface-mount technology. A Direct-Bonded-Copper (DBC) substrate acts as a base plate for the laser chip and heat sink. The attached frame is used for electrical contacting and acts as beam reflector where the laser light is reflected at a 45° mirror. In the application the DBC base plate of the SMT-Laser is directly soldered to a Metal-Core-PCB by reflow soldering. The overall thermal resistance from laser chip to the bottom of a MC-PCB was measured as low as 2.5 K/W. The device placement process can be operated by modern high-speed mounting equipment. The direct link between device and MC-PCB allows CW laser operation up to 6-7 watts at wavelengths of 808nm to 940nm without facing any overheating symptom like thermal roll over. The device is suitable for CW and QCW operation. In pulsed operation short rise and fall times of <2ns have been demonstrated. New application fields like infrared illumination for sensing purposes in the automotive industry and 3D imaging systems could be opened by this new technology.

Laser diodes and diode laser bars are key components in high power semiconductor lasers and solid state laser systems. During manufacture, the assembly of the fast axis collimation (FAC) lens is a crucial step. The goal of our activities is to design an automated assembly system for high volume production. In this paper the results of an intermediate milestone will be reported: a demonstration system was designed, realized and tested to prove the feasibility of all of the system components and process features. The demonstration system consists of a high precision handling system, metrology for process feedback, a powerful digital image processing system and tooling for glue dispensing, UV curing and laser operation. The system components as well as their interaction with each other were tested in an experimental system in order to glean design knowledge for the fully automated assembly system. The adjustment of the FAC lens is performed by a series of predefined steps monitored by two cameras concurrently imaging the far field and the near field intensity distributions. Feedback from these cameras processed by a powerful and efficient image processing algorithm control a five axis precision motion system to optimize the fast axis collimation of the laser beam. Automated cementing of the FAC to the diode bar completes the process. The presentation will show the system concept, the algorithm of the adjustment as well as experimental results. A critical discussion of the results will close the talk.

The packaging of high power diode laser bars requires a high cooling efficiency and long-term stability. Due to the increasing output power of the diode laser bars the cooling performance of the packaging becomes more important. Nowadays micro channel heat sinks seem to be the most efficient cooling concept in regard to high power applications. The active area of the p-side down mounted laser bar is located directly above the micro channels. In other applications where conductive cooled heat sinks are used the bars are mounted on copper CS mount, CuW submount or high performance materials. All these packaging ideas use wire bonds or thin copper sheets as a n-contacts. The thermal advantage of these contacts can be neglected. N-contact cooling is typically used to achieve new records of optical output power in the labs. These studies analyze the properties of an additional n-contact cooling. The cooling performance of a package cooled on both sides can be improved by more than 20% when compared with typical wire bonds or metal sheets. Different packaging styles with metal sheets, heat spreaders (expansion matched) and active n-side cooling are investigated. The effect of n-side cooling with regards to the fill-factor and cavity length is analyzed also. The first part of this paper approaches the topic theoretically. Simulations are carried out and show the advantages and differences of different package styles in comparison to bar geometries variations. The second part of the studies characterizes and analyses fabricated samples made out of copper in view of cooling performance, handling, and induced stress. The results of different bar geometries and packaging styles are compared and guidelines for n-side cooling are developed.

Microchannel coolers (MCC's) for laser diodes are most commonly constructed of copper or copper alloy. A disadvantage of the use of these metals is their much higher coefficient of thermal expansion (CTE) compared to GaAs. This mismatch can result in stress on the devices during soldering or operation. It can also stress the solder joint, encouraging voiding. One solution is to attach the laser to an MCC made from low CTE material. Suitable examples are tungsten-copper and molybdenum-copper. We have fabricated MCC's from these materials and have performed CFD modeling followed by flow, thermal resistance, and accelerated life tests on the parts. We show that the thermal results can be achieved that compare to copper MCC's. The erosion resistance of the materials is demonstrated to be higher than copper. Life tests using DI water flow indicates that superior life can be expected from these MCC's, especially at higher flow rates of 0.5 lpm/0.13 gpm and with lower water quality and elevated temperature. Finally, we show that the dimensional tolerances required can be obtained with these material combinations.

Laser diode reliability depends on both power and spectral stability over time. This report examines cases in which both corrosion and ionic deposition resulted in wavelength shifts from less than 1 nm to greater than 7 nm in 60 - 100W bars on microchannel coolers. Both corrosion and deposition seemed to be exacerbated by frequent and/or lengthy periods of stagnation in the DI water system. Analytical results including SEM images of FIB cross-sections illustrate deposits of up to several microns thickness of dielectric (oxide) material, as well as voiding caused by corrosion of Ni-plating out from under Au-plating through pinhole defects. Thermal modeling confirms the effect of such features on thermal resistance, correlating to observed wavelength shifts. Actions taken to address these issues are discussed.

The reliability of high-power diode laser bars is limited by the thermo-mechanical stress occurring during the packaging process and operation. The stress is caused by the mismatch of the thermal expansion coefficients between heat sink and laser bar. In general the stress influence grows with the bar size. The development of tapered laser bars leads to higher cavity lengths so the thermo-mechanical stress in the longitudinal direction becomes more important. In this work the packaging influences on different sized laser bars are compared. At first thermal and thermo-mechanical influences are evaluated in FEM-simulations. Afterwards laser bars of different lengths and widths are mounted and characterized. The occurring strain is analyzed by electroluminescence using the correlation between stress and polarization properties of the laser bar radiation. Because of the correlation between temperature and wavelength, a thermal analysis of the mounted laser bars can be done by emitter resolved spectra scanning. The influence on reliability is analyzed in an aging study with intermediate characterization steps.

Industries worldwide are confronted with the need for an increased use of aluminum alloys in various applications. Therefore the requirements result in the necessity for a multitude of joining and welding innovations. Applications of modern aluminum alloys are not constricted to common components anymore. In fact, they are used in ever more complex lightweight structures. However, this complexity has to be fulfilled by a higher geometric flexibility in laser welding and represents a major challenge for new approaches in working lightweight structures. The present work includes the welding of aluminum utilizing Bifocal Hybrid Laser Welding (BHLW) and a 6 kW high power diode laser (HPDL) for welding. The welding setups allow for welded butt- and fillet-welds of tubes under consideration of the hardly fusion weldable alloy AA6060. Welded joints of AA6060 are investigated metallographically in regard to the influence of process parameters like intensity and the interconnected penetration. The weldability is characterized by qualitative investigations of the microstructure as well as the mechanical behavior under quasistatic loading. The investigations result in an adequate welding process for AA6060.

Beam shaping improvements of line generators based on high power diode lasers lead to new application fields as hardening, annealing or cutting of various materials. Of special interest is the laser treatment of silicon. An overview of the wide variety of applications is presented with special emphasis of the relevance of unique laser beam parameters like power density and beam uniformity. Complementary to vision application and plastic processing, these new application markets become more and more important and can now be addressed by high power diode laser line generators. Herewith, a family of high power diode laser line generators is presented that covers this wide spectrum of application fields with very different requirements, including new applications as cutting of silicon or glass, as well as the beam shaping concepts behind it. A laser that generates a 5m long and 4mm wide homogeneous laser line is shown with peak intensities of 0.2W/cm² for inspection of railway catenaries as well as a laser that generates a homogeneous intensity distribution of 60mm x 2mm size with peak intensities of 225W/cm² for plastic processing. For the annealing of silicon surfaces, a laser was designed that generates an extraordinary uniform intensity distribution with residual inhomogeneities (contrast ratio) of less than 3% over a line length of 11mm and peak intensities of up to 75kW/cm². Ultimately, a laser line is shown with a peak intensity of 250kW/cm² used for cutting applications. Results of various application tests performed with the above mentioned lasers are discussed, particularly the surface treatment of silicon and the cutting of glass.

Advanced high volume applications require pumps with high power, high brightness, and high power efficiency. New generation devices meet all of these challenging requirements, while still maintaining the advantages of distributed pumping architecture including high reliability inherent to single emitter sources. Based on new-generation long-cavity diode chips, new pumps are capable of more than 60W CW power ex-fiber output (100 μm core diameter) into NA ~ 0.12. Peak power efficiency stays over 60%. All of the above is provided at room heatsink temperature, maintained by basic air- or water-cooling.

We developed a high brightness fiber coupled diode laser module based on single diode lasers providing more than 60 Watts output power from a 100 micron fiber at the optimum fiber laser pump wavelength of 976 nm. The advantage of using multiple single emitters on a submount compared to using bars or mini bars is the direct fiber coupling by use of optical stacking and the fact that no beam transformation is needed. We achieved best brightness with a high fill factor, optical efficiency of more then 80% and wall-plug efficiency of more then 40%. The use of single emitters on a submount also extends the life span due to reduced failure (xⁿ vs. x) per device (n individual emitters vs. n emitters on a bar (mini array)). Low drive current enables modulation.

Commercial high power fiber coupled diode lasers reach power levels of 200W from a 0.2mm fiber, NA=0.2. 2D fiber coupled single emitter (SE) arrays are described delivering 500W from a 0.2mm fiber. The beam quality of standard 90μm single emitter (SE) is 6mm*mrad (slow axis) and 0.7mm*mrad (fast axis) including errors from fast axis lensing. 3 SEs (24) can be arranged in slow axis (fast axis) to fill the aperture for coupling into a 0.2mm fiber, NA=0.2. For high efficiency, beam shaping optics are avoided. A lens array for slow axis collimation and a focusing optic complete the fiber coupled module. 44 SEs' are arranged as a 2D array, polarization multiplexed and coupled into a 0.2mm fiber, NA=0.2. 62% optical to optical and 75% coupling efficiency are achieved, close to the modeled coupling efficiency of 80%. Alignment tolerances in the system do account for additional losses. Precise manufacturing processes are essential. The SEs on submounts are soldered in one reflow process to a common heatsink and FAC-lensing station automatically aligns the lens based on image processing ensuring minimum total lensing errors (focusing and pointing) of each SE to <15% of total spot size. Tighter tolerances during SE mounting, improved fast axis collimation and a redesigned coupling optic will increase the coupling efficiency to 80% resulting in 410W linear polarized output from the 0.2mm fiber, NA=0.2. Polarization (800W) and dense wavelength multiplexing (1.4kW) open the door to kilowatt level.

With the technological progress of tapered diode lasers brightness and output power of fiber coupled modules can be improved. Tapered diodes bear the potential to achieve high coupling efficiencies in multimode as well as single mode fibers. Within the BRIGHTER Project of the European Union several modules are designed to exploit this potential. The optical systems, the mechanical design and the experimental results of these modules will be presented. A design for a telecom pump module with a coupled power of 50 W in a 50 μm fiber with an NA of 0.22 at 975 nm will be presented. 16 collimated tapered single emitters aligned in four groups of four emitters are combined by mirrors and a polarizing beam splitter and coupled into the fiber. As a variant of this module four emitters are fiber coupled to achieve a power of 12 W of a 50 μm fiber with a NA of 0.13. A single mode fiber coupled module with a maximum output power of 1 W will be presented. Based on a tapered DFB Laser with a wavelength of 1060 nm it serves as a free space communication module. In another application this module is utilized as pump source for second harmonic generation. Equipped with a 975 nm tapered laser diode this module serves as a powerful pump source for Raman amplification.

We present performance improvements of fiber-coupled pump modules and broad-area lasers at 8xx nm, 9xx nm and 14xx nm wavelengths. Broad-area lasers with a 200 μm aperture at 808 nm for direct diode applications emit 11W CW and 30W pulsed. Pump modules at 830 nm for printing applications show excellent linearity, power stability of 2% and 95% of the power within 0.12 NA into a 50 μm core fiber at 1W CW. Broad-area lasers at 880 nm for pumping applications emit 18W CW with a peak wallplug efficiency of 64%. An improved design of 9xx pump modules is demonstrated with built-in feedback-protection (>30 dB at 1060 nm) that allows safe operation in multi-kW peak-power fiber lasers. Up to 3W of optical power with slope efficiency and peak wallplug efficiency of 0.64 W/A and 46%, respectively, is presented for 14xx nm broad-area lasers with a 100 μm wide aperture.

We report on the development of a high brightness laser diode module capable of coupling over 100W of optical power into a 105 μm 0.15 NA fiber at 976 nm. This module, based on nLIGHT's Pearl product architecture, utilizes hard soldered single emitters packaged into a compact and passively-cooled package. In this system each diode is individually collimated in the fast and slow axes and free-space coupled into a single fiber. The high brightness module has an optical excitation under 0.13 NA, is virtually free of cladding modes, and has an electrical to optical efficiency greater than 40%. Additionally, this module is compatible with high power 7:1 fused fiber combiners, and initial experiments demonstrated 500W coupled into a 220 μm, 0.22 NA fiber. These modules address the need in the market for higher brightness diode lasers for pumping fiber lasers and direct material processing.

Developments in Nd-based lasers pumped on the ⁴I_9/2→⁴F_3/2 transition have led to demands for increased power, brightness, and spectral stability from diode pump sources. We describe the development of fiber coupled diode pump sources that generate >120W of power from a 400μm, 0.22NA fiber at 88Xnm wavelengths. In order to maintain spectral purity at these high powers, we investigated the use of Volume Bragg Gratings to stabilize the wavelength of these multi-bar systems. A detailed study of the trade-offs between facet reflectivity and VBG reflectivity was conducted in order to determine an optimal combination that balances output power and locking range. In complement to the developments in 88Xnm pumping, recent interest in eye-safe fiber lasers have resulted in the development of Tm-doped fiber lasers pumped at 79X wavelengths. We describe the development of fiber coupled products with >80W from a 200μm, 0.22NA fiber, including the use of optimized bar geometries to improve fiber coupling efficiency.

Fiber combining multiple pump sources for fiber lasers has enabled the thermal and reliability advantages of distributed architectures. Recently, mini-bar based modules have been demonstrated which combine the advantages of independent emitter failures previously shown in single-stripe pumps with improved brightness retention yielding over 2 MW/cm²Sr in compact economic modules. In this work multiple fiber-coupled mini-bars are fiber combined to yield an output of over 400 W with a brightness exceeding 1 MW/cm²Sr in an economic, low loss architecture.

Diode lasers are frequently used for numerous applications demanding high cw average power in the kW region and comparably low brightness. These applications include polymer welding, transformation hardening of metals and medical applications. Compared to solid state lasers, diode lasers can not be efficiently q-switched due to their low upper state life time. Therefore diode lasers are usually not suited for applications requiring high peak power like marking and coating removal. To overcome this problem, we have developed a novel electrically pulsed diode laser source. If the pulses are comparably short in the region of a few hundred nanoseconds, diode lasers can be operated with a current five to ten times higher than the maximum cw current. This so called super-pulse mode of operation broadens the field of applications of high power diode lasers towards applications usually reserved for q-switched solid state lasers. To benefit from the improved brightness delivered by the super-pulsed diode lasers for materials processing, a state of the art beam forming optics is required. In this paper, we will demonstrate the design of a super-pulsed diode laser source consisting of four diode laser bars coupled into a 100 μm NA=0.2 optical fiber. This module is designed for an output power of 500 W. To select diode laser bars appropriate for the super-pulse mode of operation, different diode laser bars have been tested with peak currents up to ten times higher than the rated cw current. Material processing results with super-pulsed diode lasers will be presented.

An increase in the performance of micro-optic beam shaping resulted in diode laser modules with more than 400W out of 200 μm fibre based on Broad Area Laser Bars (BALB). The brightness of a 400 W laser module opened the door for new applications in material processing such as temper marking of stainless steel and metal sheet cutting. Further improvements of the light sources and the beam shaping for BALB's have increased the efficiency of the laser modules. Therefore we present an output power of 1200 W out of a 200 μm fibre (0.22 NA). This is achieved by further sophistication of the coupling technique and four wavelength coupling. The beam parameter product is still 22 mm*mrad with a power density of 3800 kW/cm² if focussed to a 200 μm spot. Furthermore, each of the four wavelength modules are separately exchangeable and checkable. The availability of a top-hat profile out of the fibre proves itself to be advantageous compared to the traditional Gaussian beam profiles of fibre, solid-state and gas lasers. This leads to excellent laser cutting results with extremely small cutting kerfs down to 200 μm and very plane cutting edges. Process speeds rise up to more than 10 m/min i.e. for thin sheet stainless steel or titanium. In the near future, 600 W out of 200 μm based on BALB's with a beam compressor is possible. With wavelength coupling, power levels with up to 2 kW out of 200 μm fibre will be reached. This will result in a power density of more than 6 MW/cm².

We present the latest development of high brightness, diode laser systems at Coherent Direct Diode Systems. Experimental results on diode laser modules with greater than 100 W with beam quality better than 10 mm•mrad will be presented. Through a combination of diode laser emitter improvements and narrow-band (< 10 nm) wavelength combination, we improve the spatial beam quality of diode laser systems significantly. The presentation will show a path that scales these diode laser systems to a kW-class output power from a 100 μm fiber with a single wavelength.

A platform has been developed for high-power, high-brightness, multi-single-emitter laser pumps for fiber lasers, directdiode, and other applications. Using multi-mode fiber with 105μm core and 0.22 NA, fiber-coupled optical power up to 100 Watts and a brightness as high as 100 kW/mm²/sr can be achieved. Common schemes for increasing brightness include spatial, wavelength, and polarization-beam combination of multiple single-emitters. Spatial multiplexing has been chosen for this platform to leverage JDSU's proven reliability of highpower single-emitter packages and passive optical components. In one configuration, we achieved >60W fiber coupled optical power, 50 kW/mm²/sr, and 45% wall-plug-efficiency using 105 μm core, 0.22 NA fiber from this platform. An optional VBG can also be placed inside the package for achieving spectral locking over a 16 nm wavelength range.

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).

The realization of a compact green-emitting solid state laser source for applications like laser TV or head-up displays is still a challenging task. One way to generate green light with a solid state laser source is nonlinear frequency upconversion (frequency-doubling) of e.g. 1064 nm to 532 nm. In order to achieve good conversion efficiencies tunable laser sources with output powers of several watts, narrow bandwidth and good beam quality are required. We have realized tapered laser diodes based on the GaInAs/AlGaAs material system emitting at a central wavelength of 1064 nm. These devices have an AR-coating on the front facet as well as on the ridge facet. Therefore, these laser diodes can be frequency stabilized in an external cavity setup consisting either of a grating in Littrow mounting placed on the rear side or by an integrated Fiber Bragg grating. The latter configuration allows a compact low footprint integration of the laser diodes into compact laser modules. The optical output power of these devices frequency stabilized at 1064 nm exceeds 4 W with beam qualities suitable for frequency doubling (M² < 2) and a tuning range from 1030 nm to 1070 nm. For laser diodes with a HR coating on the ridge facet even higher output powers of more than 8 W are achieved. The ridge and tapered section of the tapered diode amplifiers are contacted separately in order to enable the modulation of the light source by the variation of the ridge current. The rapid temporal modulation achieved this way is a prerequisite for the use of such lasers in flying spot display applications.

Rapidly maturing industrial laser applications are placing ever-tighter constraints on spectral width and wavelength emission stability over varying operating temperatures of high power diode laser pump sources. For example, improved power scaling and efficiency can be achieved by pumping the narrow upper laser level of Nd:YAG solid state lasers at 885 nm and the 1532-nm absorption band of Er:YAG solid state lasers, though taking full advantage of these configurations requires wavelength-locked pump sources. nLight offers a wide variety of wavelength-locked diode products based on external volume grating optics technology. It is often believed that the use of external gratings to wavelength lock diode lasers leads to an unavoidable loss in power and efficiency. nLight's design methodology is shown to eliminate the problem in our grating-locked diode laser products. These results are expected to enable improved performance in diode-pumped solid state and fiber laser systems.

High power broad area diode lasers provide the optical energy for all high performance solid state and fiber laser systems. The maximum achievable power density from such systems is limited at source by the performance of the diode lasers. A crucial metric is the reliable continuous wave optical output power from a single broad area laser diode, typically for stripe widths in the 90-100 μm range, which is especially important for users relying on fibered multi-mode pumps. We present the results of a study investigating the reliable power limits of such 980nm sources. We find that 96μm stripe single emitters lasers at 20°C operate under continuous wave power of 20W per emitter for over 4000 hours (to date) without failure, with 60μm stripe devices operating reliably at 10W per stripe. Maximum power testing under 10Hz, 200μs QCW drive conditions shows that 96μm stripes reach 30W and 60μm stripes 21W per emitter, significantly above the reliable operation point. Results are also presented on step-stress-studies, where the current is step-wise increased until failure is observed, in order to clarify the remaining reliability limits. Finally, we detail the barriers to increased peak power and discuss how these can be overcome.

Detailed reliability studies of high-power, CW, broad-area, GaAs-based laser- diodes were performed. Optical and electrical transients occurring prior to device failure by catastrophic optical-damage (COD) were observed. These transients were correlated with COD formation as observed in laser diodes with an optical window in the n-side electrode. In addition, custom electronics were designed to fault-protect the laser diodes during aging tests, i.e. each time a transient (fault) was detected, the operating current was temporarily cut off within 4μs of fault detection. The lifetime of fault-protected 808-nm laser-diode bars operated at a constant current of 120A (~130W) and 35°C exceeded similar unprotected devices by factors of 2.

Qualification results of diode lasers for space applications are presented. Quantum well lasers (AlGaAs / GaAsP at 808 nm) were subjected to accelerated life test. No sudden failure was observed for 120 emitters at different stress conditions over 10,000 hours. Gradual degradation after more than 20,000 hours was modeled by recombination enhanced defect generation and statistically analyzed by the non-linear mixed effects model. The gradually degraded devices were investigated with cathodoluminescence. Statistical inference indicates the reliable operation of diode lasers throughout the typical life time of space equipment.

In this communication we report on the approaches to increase the brightness of Bookham's latest generations of high power pump modules. Since the single-emitter laser diode is the essential building block in all module designs, the optimization of the device design towards higher wall-plug efficiency, higher brightness and better reliability is one focus of the ongoing development efforts at Bookham. By using an analytical simulation tool critical parameters for efficient emitter-fiber coupling as the beam divergence and coupling scheme could be identified.

Extensive investigations by a number of groups have identified catastrophic sudden degradation as the main failure mode in both single-mode and multi-mode InGaAs-AlGaAs strained quantum well (QW) lasers. Significant progress made in performance characteristics of broad-area InGaAs strained QW single emitters in recent years has led to an optical output power of over 20W and a power conversion efficiency of over 70% under CW operation. However, unlike 980nm single-mode lasers that have shown high reliability operation under a high optical power density of ~50MW/cm², broad-area lasers have not achieved the same level of reliability even under a much lower optical power density of ~5MW/cm². This paper investigates possible mechanisms that prevent broad-area lasers from achieving high reliability operation by performing accelerated lifetests of these devices and in-depth failure mode analyses of degraded devices with various destructive and non-destructive techniques including EBIC, FIB, and HR-TEM techniques. The diode lasers that we have investigated are commercial MOCVD-grown broad-area strained InGaAs single QW lasers at ~975nm. Both passivated and unpassivated broad-area lasers were studied that yielded catastrophic failures at the front facet and also in the bulk. To investigate the role that generation and propagation of defects plays in degradation processes via recombination enhanced defect reaction (REDR), EBIC was employed to study dark line defects in degraded lasers, failed under different stress conditions, and the correlation between DLDs and stress levels is reported. FIB was then employed to prepare TEM samples from the DLD areas for cross-sectional HR-TEM analysis.

High power semiconductor lasers with wavelengths in the eye-safer region have application to a variety of defense, medical and industrial applications. We report on the reliability of high power multimode and single mode InGaAsP/InP diode lasers with wavelengths in the range 1320 to 1550 nm in a variety of configurations, including single-chip, conduction-cooled arrays, arrays incorporating internal diffraction gratings, master-oscillator power amplifiers, and fiber-coupled modules of the above. In all cases we show very low rates of degradation in optical power and the absence of sudden failure from catastrophic optical damage or from laser-package interactions.

The present model of formation and propagation of catastrophic optical-damage (COD), a random failure-mode in laser diodes, was formulated in 1974 and has remained substantially unchanged. We extend the model of COD phenomena, based on analytical studies involving EBIC (electron-beam induced current), STEM (scanning transmission-electron microscopy) and sophisticated optical-measurements. We have determined that a ring-cavity mode, whose presence has not been previously reported, significantly contributes to COD initiation and propagation in broad-area laser-diodes.

The railway line profile measurement is necessary for the safety of the train. This article expounds a method of railway line profile measuring using laser ranging and laser scanning technology with high precision and speed. With this method, the obstacle near the track can be found out and the hidden trouble can be removed. In tunnel, the crack and deformation on the tunnel wall can be measured. The parameter of the track and contact wire can be also inspected, such as rail gauge and superelevation, position of contact wire (stagger and height), wire wearing.

Single-mode 980 nm pump lasers are mature products needed in an increasing range of applications and their power level has been constantly raised in the last fifteen years from a few tens of mW for the first generations of devices up to the current maximum level of 750 mW fiber-coupled output power. As the fiber output power increases, new applications provide positive feed-back to continue the development of these devices, although severe constraints are imposed both by reliability and the need for wavelength stabilization, which is generally built on the utilization of Fiber Bragg Gratings (FBGs). We have developed in 2005 a record saturation power device (P_sat=2.35W @ 25 °C, for 3.9 mm cavity lengths) whose fiber-coupled power has reached 750 mW for 25 °C cooled applications, limited mainly by reliability as wavelength stabilization was already demonstrated up to levels above 1 Watt. 3S PHOTONICS has now developed a new generation of powerful and reliable devices that allow foreseeing operation at or close to 1W for cooled applications. We have further optimized the vertical structure to reduce the internal losses, and to reduce the junction temperature for increased reliability. High kink-currents around 2.5 A have been measured on the best devices. The gain bandwidth has been engineered to allow maintaining the wavelength stabilization even on very long laser cavities. Encouraging preliminary reliability results have also been obtained.

This paper presents the results obtained by Intense during the development of its 2 kW stack using Quantum Well Intermixing (QWI). A 200 W QCW bar operating at 808 nm has been designed with a 1 mm long cavity of which only a fraction is actively pumped. The bar has an 80% fill factor, and ten 200 W bars were stacked vertically in a G-type package with a 417 μm bar-to-bar pitch. The resulting compact emission area makes the stack compatible with most existing laser and electro-optic systems. A lifetime of 1x10⁹ shots has been obtained with no measurable degradation.

A family of laser diode arrays has been developed for QCW operation in adverse environmental conditions. The arrays contain expansion-matched heatsinks, hard solder, and are built using a process that minimizes the packaging-induced strain on the laser diode bars. The arrays are rated for operation at 200 Watts/bar under normal operating conditions. This work contains test results for these arrays when run under a variety of harsh operating conditions. The conditions were chosen to mimic those required by many military and aerospace laser programs. Life test results are presented over a range of operating temperatures common to military specifications (-40 °C to + 70 °C) at a power level of approximately 215 Watts/bar. The arrays experienced no measurable degradation over the course of the life test. Operation at the temperature extremes did not introduce any additional detectable failure mechanisms. Also presented are results of characterization and reliability tests conducted at cryogenic temperatures. Diode arrays have been subjected to repeated cycles in rapid succession between room temperature and 77 K with temperature ramp rates up to 100 K/minute. Pre- and post- thermal cycle P-I-V data are compared. The results demonstrate the suitability of these arrays for operation at cryogenic temperatures.

High power diode lasers are the root source of optical energy in all high performance laser systems. As their performance advances, diode lasers are increasingly taking the place of other sources. Short pulse, sub-microsecond-class, high power lasers are important for many applications but historically, diode lasers have not been able to reach high enough peak pulse powers with adequate reliability, limited by physical effects such as facet failure. By combining robust facet passivation with thick super large optical cavity waveguides, greatly increased optical output power can be achieved. We present here the results of a study using commercial high current short pulse sources (>200A, <500ns) to assess the performance and endurance limits of high power broad area devices. We find that our lasers can be driven with a peak power density of over 110MWcm^-2 without failure for more than 3×10⁷ pulses. For example, on testing to 240A, single emitter 200μm stripe 1100nm broad area devices reach 124W (46μJ) without failure, and 60μm stripes reach 88W. In practice, high injection effects such as carrier accumulation in waveguide typically limit peak power. We review these remaining limitations, and discuss how they can be overcome.

In this work, we present high-wall plug efficiency (WPE) diode lasers at 975 nm, which are based on an Al-free active region. On a 2 mm x 100 μm laser, we have obtained a high maximum wall-plug efficiency of 69% at 10°C CW. Based on the same structure, we have realised a 1-cm bar, mounted on an active submount, and which delivers 70 W CW, together with 67% wall-plug efficiency. By improving the laser structure, we have obtained a higher WPE of 70% on an uncoated 2 mm x 100 μm broad area laser. We also present a new structure with a reduced fast-axis far-field of only 34° at 1/e².

Record values for the rollover power and rollover linear power densities of 9xx nm devices, obtained by simultaneous scaling of length and d/Γ, are reported. The values for d/Γ lay in the range 0.8 μm to 1.2 μm with corresponding cavity lengths from 3.5 mm to 5 mm. The transversal structures were asymmetric, with a higher refractive index on the n side. An optical trap was helpful in reducing the radiation extension on the p side and the overall thickness. The highest rollover linear power densities were 244 mW/μm for structures without an optical trap and 290 mW/μm for those that included an optical trap

Continued advances in high power diode laser technology enable new applications and enhance existing ones. Recently, mini-bar based modules have been demonstrated which combine the advantages of independent emitter failures previously shown in single-stripe architectures with the improved brightness retention enabled by multi-stripe architectures. In this work we highlight advances in a family of compact, environmentally rugged mini-bar based fiber coupled Orion modules. Advances in PCE (power conversion efficiency) and reliable operating power from a 9xx nm wavelength unit are shown from such modules. Additionally, highly reliable fiber coupled operation and performance data is demonstrated in other wavelengths in the 780 - 980 nm range. Data demonstrating the scaling this technology to 25W and higher power levels will be given.

We present results from a survey of materials used for packaging semiconductor lasers, including Cu, CuW, BeO, diamond composite and other advanced materials. We present the results of residual bonding stress from various solders and consider the direct effects on wavelength and spectral width. We also provide data on the second order effects of threshold current and slow axis divergence. Additionally, we consider the heat spreading through different materials for a laser bar and present modeled and experimental data on the thermal performance. Finally, we consider the reliability under on-off life-testing and thermal cycling tests.

We present the experimental and theoretical evaluation of the coherent combining of an array of index-guided tapered laser diodes in an external Talbot cavity. A theoretical model taking into account the propagation inside the semiconductor device has been developed to determine the cavity spatial modes. In parallel, experiments have been realized with 10 emitters in a compact setup and a volume Bragg grating as the external mirror. 1.7 W have been obtained at 976 nm for the in-phase mode in a narrow laser line (<0.1 nm).

In this paper we present the development of semiconductor laser systems with output powers reaching 100 W and linewidths down to 10 GHz. The combination of high power and narrow emission spectrum was achieved through external resonator configurations based on volume Bragg gratings. By using Bragg gratings with extremely narrow spectral selectivity we were able to narrower and lock emission spectra of diode lasers, with precise wavelength tuning achieved by thermal control of the volume grating. The thermal coefficient of our volume gratings was approximately 8 pm/K, which was low enough to guarantee stable frequency operating regime. We implemented successfully two such schemes for lasers generating at 780 nm and 1.55 μm as pumping sources for Rb vapor and Er-doped solid state lasers, correspondingly.

Flange Height and Flange Thickness are vital parameters for train wheels. Usually, they are manually measured by wheel vernier after train stopped. This article introduces a dynamic wheel inspection system. Pulsed slit diode lasers project onto wheel tread while train is in motion, CCD cameras record light-section images of wheel profile. Geometric parameters and profile of wheel are measured on basis of image processing. This paper introduces measuring principle and structure of this inspection system, analyzes influences resulted from laser parameters and environment, designs instantaneous high-powered slit diode laser and narrowband optical filter according to ±0.2mm accuracy requirement and environment situation.

In this paper, the importance of the contact line height and gradient in the electrified railway and the current inspection methods for the contact line height and gradient are analyzed, and then the dynamic detection system for that is deigned, which based on the laser phase ranging principle. The detection system is setting on the top of locomotive and a cooperative target is fixed on the pantograph; the laser system measures the height between the top of locomotive and the working pantograph by the cooperative target when the locomotive runs, and then the gradient of contact line can be calculated in real time when the locomotive's running information is provided. The laser phase ranging system uses the DFT method to calculate the phase difference, which can get the higher resolution than the method of the electronic phase demodulation and reduce the influence of the shift of laser intensity etc. The dynamic detection system works well to detect the contact line gradient, without influencing the normal operation of the locomotive, and the disadvantages of manual detecting and detecting car are avoided.

The development of on-chip grating stabilized semiconductor lasers for diode pumped solid state lasers is discussed. The diode lasers, specifically at wavelengths of 808nm, 976nm, and 1532nm are stabilized via internal gratings to yield a typical center wavelength tolerance of ± 1nm, FWHM of < 1-2nm, and a temperature tuning coefficient of < 0.09 nm/°C. We also report on the CW and QCW operation of conduction cooled bars, stacks, and fiber coupled modules. Simulations show that on-chip stabilized pump sources yield performance improvements over standard pumping schemes. A comparison in laser performance is shown for typical DPSS configuration.