Fiber lasers are becoming an increasingly important option for LIDAR light sources in autonomous driving technology due to their operation in the eye safe 1550 nm spectral region and their intrinsic high beam quality, power and pulse characteristics. As an essential component for the pumping of fiber lasers, semiconductor laser diodes with high temperature stability, power and reliability are necessary. In this report we present the results of a continuous-wave (CW) single edge emitting laser diode designed to operate at 94x nm at 25 °C heat sink temperature and 97x nm at 100 °C. Various epitaxial and laser geometry designs have been implemented to optimize the laser performance over this wide environmental temperature range. The laser epitaxy is based on the AlGaAs/GaAs material system, with an InGaAs strained quantum well (QW). With various designs of laser geometry including emitting area and cavity length, devices are designed, grown, fabricated, and tested with the optimized design improving the temperature stability, power, and efficiency of the laser chip. A peak efficiency of over 54% at heat sink temperature of 105 °C and over 12 W before thermal roll-over occurs has been achieved. In addition to the thermal performance we also report the slow axis beam parameter product of the chip of <5 mm.mrad with polarization purity >98% at operating current and show the preliminary reliability data at the high temperature operation.
Fiber lasers are becoming dominant in the industrial metal cutting market up to 6kW levels due to their increasing performance delivered at aggressive price levels. A key force behind this is the availability of low-cost diode pump modules, consisting of combined single emitter laser chips. To further develop low pump cost an increase the reliable power available from each chip is required, and in principle this can be achieved by reducing facet power density, current density and lowering operational temperature by widening the emitter and increasing cavity length. Here we present the latest development of high-power laser diode chips at 976 nm wavelength designed for operation in 300W+ per pump module applications. For high power/high brightness applications we show a 150 μm contact opening, 5 mm cavity length chip delivering 20W at 20A CW at BPP of <5.5 mm.mrad. This laser has been deployed in several pump module product configurations from 155 W to 320 W with multi-cell module life test demonstrating over 20,000 hours at 90% survivability. For further power scaling we demonstrate design and performance of single emitter lasers with emitting facets of 220 μm to 300 μm. With 300 μm contact opening and 500 μm single emitter chip width 30W operation power is achieved at 32A with far field divergence less than 12°. Life testing is running at 46A with optical power of 41W for 2000 hours.
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.
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.
The advances in laser-diode technology have enabled high efficiency direct diode base modules to emerge as a building block for industrial high power laser systems. Consequently, these systems have been implemented with advance robust, higher-brightness and reliable laser sources for material processing application. Here at the company, we use low-fill factor bars to build fiber-coupled and passively cooled modules, which form the foundation for “TruDiode,” the series of TRUMPF direct diode laser systems that can perform in the multi-kilowatt arena with high beam quality. However, higher reliable output power, additional efficiency and greater slow axis beam quality of the high power laser bars are necessary to further increase the brightness and reduce the cost of the systems. In order to improve the slow axis beam quality, we have optimized the bar epitaxial structures as well as the lateral design. The detailed near field and far field studies of the slow axis for each individual emitters on the bar provide us with information about the dependency of beam quality as a function of the drive current. Based on these study results for direct diode application, we have optimized the high brightness bar designs at 900-1070nm wavelengths. In addition, high power and high efficiency laser bars with high fill factors have been used to build the pump sources for thin disc laser systems at TRUMPF Photonics. For better system performances with lower costs, we have further optimized bar designs for this application. In this paper, we will give an overview of our recent advances in high power and brightness laser bars with enhanced reliability. We will exhibit beam quality study, polarization and reliability test results of our laser bars in the 900-1070nm wavelengths region for coarse wavelength multiplexing. Finally, we will also present the performance and reliability results of the 200W bar, which will be used for our next generation thin disk laser pump source.
Vertical-cavity surface-emitting lasers can be processed in large two-dimensional arrays of single devices to scale up the
power for solid-state laser pumping. These arrays emit in a circular, uniform beam, with a narrow and stable emission
spectrum that is well suited to the absorption spectra of solid-state gain media. kW-class 808 nm QCW VCSEL pump
modules were developed to pump compact Nd:YAG lasers. An end-pumped Nd:YAG laser was constructed that
produced 7.1 W average IR power, as well as a dual side-pumped passively Q-switched frequency-quadrupled Nd:YAG
laser that generated 0.8 mJ UV pulses at a 240 Hz repetition rate.
Solid-state lasers pumped by high-power two-dimensional arrays of vertical-cavity surface-emitting lasers (VCSELs)
were investigated. Both end-pumping and side-pumping schemes of Nd:YAG lasers with high power kW-class 808 nm
VCSEL pump modules were implemented. For one application 10 mJ blue laser pulses were obtained from a frequencydoubled
actively Q-switched VCSEL-array dual side-pumped Nd:YAG laser operating at 946 nm. For another
application 10 mJ green laser pulses were obtained from a frequency-doubled passively Q-switched VCSEL-array endpumped
Nd:YAG laser operating at 1064 nm. Both QCW and CW pumping schemes were investigated to achieve high
average Q-switched power.
Vertical-cavity surface-emitting lasers (VCSELs) have emerged as a promising candidate for pumping of solid-state
lasers, as they can be configured into high-power two-dimensional arrays and modules of arrays. VCSELs emit in a
circular, uniform beam which can greatly reduce the complexity and cost of coupling optics. Their narrow and stable
emission spectrum is well suited to the narrow absorption spectrum generally observed for solid-state gain media. The
superior reliability of VCSELs greatly enhances the robustness of solid-state laser systems and enables high-temperature
operation. In this work, we discuss recent developments on kW-class VCSEL pumps for solid-state lasers. Results on
VCSEL modules designed for end-pumping and for side-pumping are presented. More than 4kW in CW operation is
demonstrated from a multi-array VCSEL module. We also present results on solid-state lasers using VCSEL modules as
pumps. In an end-pumping configuration, more than 250W peak power at 1064nm is demonstrated, and in a sidepumping
Q-switched configuration, more than 21mJ at 946nm is demonstrated for an Nd:YAG solid-state laser.
High power 808 nm VCSEL arrays were developed to pump compact pulsed Nd:YAG lasers. A QCW side-pumped
passively Q-switched Nd:YAG laser operating at 1064 nm produced linearly polarized 4 ns IR pulses with 4.7 mJ pulse
energy. These pulses were externally frequency doubled and quadrupled resulting in 2.5 mJ pulse energy at 532 nm and
0.8 mJ at 266 nm respectively. A similar but actively Q-switched dual side-pumped Nd:YAG laser operating at the
weaker quasi three-level 946 nm transition produced 12 mJ pulses that were efficiently frequency doubled resulting in
5.6 mJ blue pulses of 17 ns duration.
A 17.84-cm-long Littrow external cavity diode laser at 823 nm is mode-locked by actively modulating the current at a 840.843-MHz fundamental harmonic frequency. This mode-locked external cavity diode laser has an overall spectral width of 0.16 nm (or 71 GHz), roughly corresponding to 84 longitudinal modes, a temporal pulse width of 63 ps, and an average power of 1 mW. Using the Pound-Drever-Hall technique with both fast and slow servo corrections, this mode-locked laser is then frequency-stabilized to a 17.84-cm-long sealed optical cavity with a finesse of 6000. To the author's knowledge, this is the first frequency stabilization of a mode-locked diode laser with such a high-finesse cavity.
We present a detailed design of a simple, stable, and compact tapered amplifier (TA) diode system. Heat conductivity, strain release, and system alignment are taken into consideration for this design. Convenient access to the three-dimensional adjustments for collimation makes the system easy to align. The optical amplification is discussed as a function of injection of current and operating temperature for both a continuous-wave external cavity diode laser (CW-ECDL) and a mode-locked external cavity diode laser (ML-ECDL). With 5-mW CW-ECDL seeding power, 240 mW is achieved after the TA, 115 mW (48%) of which can be coupled into a single-mode fiber. With 0.75-mW average ML-ECDL seeding power, 80-mW average power, or about 11-W peak power (843-MHz repetition rate and 8-ps pulse width), is achieved after the TA, 39 mW (49%) of which can be coupled into a single-mode fiber. In both cases, the amplified light maintains the optical properties of the seeding light, showing the same mode suppression. The output power spatial mode quality is characterized with a camera and BeamView software. Output power stability is also discussed.
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