Progress is presented on ongoing research and development into ultra-high power and efficiency bars that achieve significantly higher useful optical output power and higher brightness than are currently commercially available. In previous work (2017), the authors reported on bars that deliver over 1 kW continuous wave (cw) diode laser power, when cooled using 15°C water. Our current studies are focused on increasing the usable output power (power within a targeted beam angle), which is essential for real world industrial applications. These ongoing studies have enabled the first demonstration of 500 W cw output power from a 10 mm x 6 mm laser diode bar with a lateral far field angle of only 8°. In efforts to further improve brightness, we also present our latest progress on high power SMEBs (Single Mode Emitter Bars). These emitters operate in a close to diffraction limited optical mode (M² < 1.5, laterally and vertically). This new technology enables a significant increase in Diode Laser brightness. We demonstrate in excess of 55% electro optical efficiency at > 200 W cw laser bar power for SMEBs.
The advance of high power semiconductor diode laser technology is driven by the rapidly growing industrial laser market, with such high power solid state laser systems requiring ever more reliable diode sources with higher brightness and efficiency at lower cost. In this paper we report simulation and experimental data demonstrating most recent progress in high brightness semiconductor laser bars for industrial applications. The advancements are in three principle areas: vertical laser chip epitaxy design, lateral laser chip current injection control, and chip cooling technology. With such improvements, we demonstrate disk laser pump laser bars with output power over 250W with 60% efficiency at the operating current. Ion implantation was investigated for improved current confinement. Initial lifetime tests show excellent reliability. For direct diode applications <1 um smile and >96% polarization are additional requirements. Double sided cooling deploying hard solder and optimized laser design enable single emitter performance also for high fill factor bars and allow further power scaling to more than 350W with 65% peak efficiency with less than 8 degrees slow axis divergence and high polarization.
de lasers are key components in material processing laser systems. While mostly used as pump sources for solid state or fiber lasers, direct diode laser systems using dense wavelength multiplexing have come on the market in recent years. These systems are realized with broad area lasers typically, resulting in beam quality inferior to disk or fiber lasers. We will present recent results of highly efficient ridge waveguide (RW) lasers, developed for dense-wavelength-beamcombining (DWBC) laser systems expecting beam qualities comparable to solid state laser systems and higher power conversion efficiencies (PCE).
The newly developed RW lasers are based on vertical structures with an extreme double asymmetric large optical cavity. Besides a low vertical divergence these structures are suitable for RW-lasers with (10 μm) broad ridges, emitting in a single mode with a good beam quality. The large stripe width enables a lateral divergence below 10° (95 % power content) and a high PCE by a comparably low series resistance. We present results of single emitters and small test arrays under different external feedback conditions. Single emitters can be tuned from 950 nm to 975 nm and reach 1 W optical power with more than 55 % PCE and a beam quality of M2 < 2 over the full wavelength range. The spectral width is below 30 pm FWHM. 5 emitter arrays were stabilized using the same setup. Up to now we reached 3 W optical power, limited by power supply, with 5 narrow spectral lines.
Industrial laser systems continue to evolve in terms of power levels, brightness and mode of operation. While continouswave (cw) laser systems drive to more efficiency, compactness and robustness, ultra-short pulse (USP) solid-state lasers (SSL) gain increasing attention from industry due to their high average power levels, increasing pulse energies and more flexible repetition rates, eventually enabling increased productivity and novel materials processing applications. TRUPMF’s laser systems portfolio starting in 1985 mirrors this general development and is therefore an interesting subject to reflect the path of industrial laser systems from early CO2 lasers to the latest generation of diode pumped nanosecond-pulsed thin-disk lasers with up to 400W of average output power at 343nm. Today, our advanced laser families enable novel applications such as EUV light generation, laser lift-off and surface annealing.
Progress will be presented on ongoing research into the development of ultra-high power and efficiency bars achieving significantly higher output power, conversion efficiency and brightness than currently commercially available. We combine advanced InAlGaAs/GaAs-based epitaxial structures and novel lateral designs, new materials and superior cooling architectures to enable improved performance. Specifically, we present progress in kilowatt-class 10-mm diode laser bars, where recent studies have demonstrated 880 W continuous wave output power from a 10 mm x 4 mm laser diode bar at 850 A of electrical current and 15°C water temperature. This laser achieves < 60% electro-optical efficiency at 880 W CW output power.
KEYWORDS: Semiconductor lasers, Diodes, Fiber lasers, High power lasers, Laser systems engineering, Reliability, Laser development, Disk lasers, Fiber couplers, Packaging
We have continuously optimized high fill factor bar and packaging design to increase power and efficiency for thin disc
laser system pump application. On the other hand, low fill factor bars packaged on the same direct copper bonded (DCB)
cooling platform are used to build multi-kilowatt direct diode laser systems. We have also optimized the single emitter
designs for fiber laser pump applications. In this paper, we will give an overview of our recent advances in high power
high brightness laser bars and single emitters for pumping and direct diode application. We will present 300W bar
development results for our next generation thin disk laser pump source. We will also show recent improvements on
slow axis beam quality of low fill factor bar and its application on performance improvement of 4-5 kW TruDiode laser
system with BPP of 30 mm*mrad from a 600 μm fiber. Performance and reliability results of single emitter for multiemitter
fiber laser pump source will be presented as well.
KEYWORDS: Waveguides, Semiconductor lasers, Thermal effects, Broad area laser diodes, Refractive index, Laser systems engineering, Near field, High power lasers, Fiber coupled lasers, Optical filters
For high brightness direct diode laser systems, it is of fundamental importance to improve the slow axis beam quality of the incorporated laser diodes regardless what beam combining technology is applied. To further advance our products in terms of increased brightness at a high power level, we must optimize the slow axis beam quality despite the far field blooming at high current levels. The later is caused predominantly by the built-in index step in combination with the thermal lens effect. Most of the methods for beam quality improvements reported in publications sacrifice the device efficiency and reliable output power. In order to improve the beam quality as well as maintain the efficiency and reliable output power, we investigated methods of influencing local heat generation to reduce the thermal gradient across the slow axis direction, optimizing the built-in index step and discriminating high order modes. Based on our findings, we have combined different methods in our new device design. Subsequently, the beam parameter product (BPP) of a 10% fill factor bar has improved by approximately 30% at 7 W/emitter without efficiency penalty. This technology has enabled fiber coupled high brightness multi-kilowatt direct diode laser systems. In this paper, we will elaborate on the methods used as well as the results achieved.
808 nm QCW bars were fabricated and mounted with hard solder technology onto H-mounts and G-stacks. At room
temperature, reliable operation has been demonstrated at 400W at 400A per single 1-cm bar and for a G-stack at 3kW at
around 300A. High temperature reliable operation has been demonstrated for both devices up to 95°C. Both types of
devices were tested at various pulse widths and duty cycles. Both optical power and wavelength dependencies on the
various conditions have been studied.
The performance of high-power pump-laser modules is strongly influenced by their thermal properties. In this paper we discuss the optimization of the device performance with respect to thermal properties, output power, wavelength stability, and device reliability using the example of our newest pump-laser generation that has been developed and qualified to support the high-end market of erbium-doped fiber amplifiers. A comparison of device properties obtained from modeling and measurements is presented at each design step. We report on the performance of fiber Bragg grating-stabilized telecom-grade modules yielding 600 mW fiber-coupled light output power.
In this communication we present the characteristics of Bookham's MU7-9xx-01 laser module with multimode fiber output. This latest generation of our multimode modules is designed for light output power of up to 7 W in uncooled operation in the wavelength range between 915 nm and 975 nm. The key element of the module is our new SES8-9xx-01 broad area single emitter. These high power lasers in the 9xx nm wavelength range show a high slope efficiency of up to 1.2 W/A in CW room temperature operation. High efficiency combined with low threshold current and low operation voltage result in a maximum wall plug efficiency of above 65%. Almost 4000 h lifetest data at accelerated conditions are available for the laser diodes. The data give estimated reliability values of below 5 kFIT at operating conditions (between 8 A and 8.5 A injection current at up to 35°C heat sink temperature). The robustness of the new lasers is also illustrated by the fact that no catastrophic mirror damage was observed up to 22.5 W of light output power. The low divergence of the laser beam allows coupling into multimode fiber with 0.15 or 0.22 numerical aperture (NA) with a coupling efficiency above 90% at operation condition. Maximum ex-fiber light output powers of 11.5 W are shown. On module level around 2000 h lifetest data are accumulated without any failure or sign of degradation.
We report on the development of a new cost-effective, small form-factor laser source at a wavelength of 980 nm. The laser module is based on proven technology commonly used for pump laser modules deployed in fiber amplifiers of telecommunication networks. The package uses a state-of-the-art 14-pin butterfly housing with a footprint of 30x15 mm2 with a Fabry-Perot AlGaAs-InGaAs pump laser diode mounted inside having an anti-reflection coating on its front facet. The light is coupled into a single-mode polarization-maintaining fiber with a mode-field diameter of 6.6 micrometer. The spectral properties of the source are defined by a fiber Bragg grating (FBG) that provides feedback in a narrow reflection band. The laser back facet and the FBG form a long resonant cavity of 1.7 m length in which laser light with a low coherence length of a few cm is generated. This configuration with the laser being operated in the coherence-collapse regime has the advantage of being robust against variations in the optical path, thus enabling stable and mode-hop free emission. The laser module has the following properties: a continuous-wave fiber output power exceeding 800 mW, a spectral bandwidth of less than 50 pm, a root-mean square power variation of less than 0.2 % from DC to 2 MHz over the entire power operating range, and a polarization extinction ratio of more than 20 dB. This is a compact, low noise, high power source for frequency conversion with nonlinear optical materials, such as blue light generation.
In this communication we report on the successful realization of Single-mode Emitter Array Laser (SEAL) bars. Various laser bars with a cavity length of 2.4 mm containing between 25 to 350 narrow stripe lateral single-mode emitters have been realized and mounted epi-side down onto expansion matched heatsinks using a stable AuSn-solder technology. Optical power in excess of 1 W per emitter has been obtained resulting in more than 200 W total output power for the highest emitter density. While these total power levels are comparable to conventional broad-area laser bars (BALB), the brightness of each of the emitters is drastically improved over the BALB approach making theses bars ideal candidates for beam-shaping concepts. Lateral farfield measurements with smooth gaussian patterns, high electro-optical conversion efficiency well above 60% and threshold currents as low as 0.5 A are presented. Similar devices realized from the InGaAsP/InP material system deliver in excess of 20 W from 100 NS emitters at wavelengths around 1480 nm.
Reliable power scaling by stretching the cavity length of the laser bars ranging from 1.2 mm to 3.6 mm at constant filling factor of 50% is demonstrated. While a relatively short cavity length of 1.2 mm allows for thermally limited CW output powers in excess of 180 W, extremely high 325 W at 420 A (CW, 16°C) have been achieved by leveraging the enhanced thermal properties of a 3.6 mm cavity length on standard micro-channel coolers. A high electro-optical conversion efficiency of 62% and 51% respectively is attributed to the low internal losses from an optimized waveguide design and the excellent properties of the AlGaAs-material system accounting for low thermal and electrical resistance. Multi-cell lifetest data at various operation conditions show extremely low wear-out rates even at harsh intermittent operation conditions (1-Hz type, 50% duty-cycle, 100% modulation). At 100 W output power 300 Mshots corresponding to 64000 h mean-time-to-failure (MTTF) have been extrapolated for 20% power drop from initial 2000 h and 4000 h lifetest readouts of a 1.2 mm cavity design. Similar results have been obtained for our next generation of ultra high power laser bars enabling reliable operation at 120 W output power and beyond. From 2.4 mm cavity length bars we have obtained 250 W of CW output power at 25°C while extrapolated reliability data at 120 W and 140 W power levels of up to 2000 h duration indicates that such devices are able to fulfill the requirements for lifetimes in the 20 - 30 kh range.
KEYWORDS: Semiconductor lasers, High power lasers, Multimode fibers, Resistance, Continuous wave operation, Temperature metrology, Prototyping, Reliability, Broad area laser diodes
In this communication we report on the performance characteristics of Bookham’s latest generation of 915-990 nm broad area single emitter (BASE) laser diodes with around 90 μm wide aperture. Representative high power devices in the wavelength range of 950-960 nm, mounted p-side down onto expansion matched assemblies using our highly reliable AuSn-solder technology, reveal a high slope efficiency of around 1.05 W/A during CW operation at 25°C heat sink temperature. Coupling efficiency into multi-mode fiber with 0.15 or 0.22 numerical aperture exceeds 93% mainly due to the low vertical divergence of the laser beam. In addition, low laser threshold and series resistance enable more than 62% maximum wall plug efficiency of the present generation of the laser diodes. Preliminary tests of new prototypes reveal already excellent performance characteristics of the next generation device with up to 19.9 W light output power in pulsed operation and 16 W for thermally limited CW operation.
Diode-pumped solid state laser (DPSSL) and fiber laser (FL) are believed to become the dominant systems of very high power lasers in the industrial environment. Today, ranging from 100 W to 5 - 10 kW in light output power, their field of applications spread from biomedical and sensoring to material processing. Key driver for the wide spread of such systems is a competitive ratio of cost, performance and reliability. Enabling high power, highly reliable broad-area laser diodes and laser diode bars with excellent performance at the relevant wavelengths can further optimize this ratio. In this communication we present, that this can be achieved by leveraging the tremendous improvements in reliability and performance together with the high volume, low cost manufacturing areas established during the "telecom-bubble." From today's generations of 980-nm narrow-stripe laser diodes 1.8 W of maximum CW output power can be obtained fulfilling the stringent telecom reliability at operating conditions. Single-emitter broad-area lasers deliver in excess of 11 W CW while from similar 940-nm laser bars more than 160 W output power (CW) can be obtained at 200 A. In addition, introducing telecom-grade AuSn-solder mounting technology on expansion matched subassemblies enables excellent reliability performance. Degradation rates of less than 1% over 1000 h at 60 A are observed for both 808-nm and 940-nm laser bars even under harsh intermittent operation conditions.
AlGaAs/InGaAs based high power pump laser diodes with wavelength of around 980 nm are key products within erbium doped fiber amplifiers (EDFA) for today's long haul and metro-communication networks, whereas InGaAsP/InP based laser diodes with 14xx nm emission wavelength are relevant for advanced, but not yet widely-used Raman amplifiers. Due to the changing industrial environment cost reduction becomes a crucial factor in the development of new, pump modules. Therefore, pump laser chips were aggressively optimized in terms of power conversion and thermal stability, which allows operation without active cooling at temperatures exceeding 70°C. In addition our submarine-reliable single mode technology was extended to high power multi-mode laser diodes. These light sources can be used in the field of optical amplifiers as well as for medical, printing and industrial applications. Improvements of pump laser diodes in terms of power conversion efficiency, fiber Bragg grating (FBG) locking performance of single mode devices, noise reduction and reliability will be presented.
The 'coherence radar' was introduced as a method to measure the topology of optically rough surfaces. The basic principle is white light interferometry in individual speckles. We will discuss the potentials and limitations of the coherence radar to measure the microtopology, the roughness parameters, and the out of plane deformation of smooth and rough object surfaces. We have to distinguish objects with optically smooth (polished) surfaces and with optically rough surfaces. Measurements at polished surfaces with simple shapes (flats, spheres) are the domain of classical interferometry. We demonstrate new methods to evaluate white light interferograms and compare them to the standard Fourier evaluation. We achieve standard deviations of the measured signals of a few nanometers. We further demonstrate that we can determine the roughness parameters of a surface by the coherence radar. We use principally two approaches: with very high aperture the surface topology is laterally resolved. From the data we determine the roughness parameters according to standardized evaluation procedures, and compare them with mechanically acquired data. The second approach is by low aperture observation (unresolved topology). Here the coherence radar supplies a statistical distance signal from which we can determine the standard deviation of the surface height variations. We will further discuss a new method to measure the deformation of optically rough surfaces, based on the coherence radar. Unless than with standard speckle interferometry, the new method displays absolute deformation. For small out-of-plane deformation (correlated speckle), the potential sensitivity is in the nanometer regime. Large deformations (uncorrelated speckle) can be measured with an uncertainty equal to the surface roughness.
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