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.
High power diode lasers convince by their very efficient conversion of electrical into optical energy. Besides high efficiencies and record absolute power levels, reliability in all possible operation modes and cost become increasingly important. We present diode laser bars in the 940nm range with wall plug efficiencies of about 65% at an emission power of 100W and with excellent reliability. The test had been performed on a stack with 5 bars at an output power of 100W per 1cm bar and after about 4000hrs test time, lifetimes of more than 40 000hour were estimated. The efficiency of these bars was at the beginning and at the end of this test about 65%. Operation modes between cw operation and q-cw (200μsec pulses) were evaluated and it will be shown, that pulses in the range of 1Hz are the hardest conditions, which can cause catastrophic failures. Using submounts with matched thermal expansion coefficient, this failure was prevented and lifetimes similar to cw-operation were reached. In order to reduce costs of laser power, we developed a laser package that offers high power at good reliability and provides a collimated beam for about 5$/W, as a cost target in mass production conditions. This was achieved by using packaging concepts that were developed for high power semiconductor devices. These results will further enhance the applicability of diode lasers in industrial application.
High power GaAs-based high power diode bars produce wavelengths in the range of 780 to 980 nm and are widely used for pumping a broad range of rare earth doped solid-state lasers. As the markets for these laser systems mature, diode lasers that operate at higher power levels, greater overall efficiency, and higher reliability are in high demand. In this paper we report efficiencies of over 70% in the 9xx-nm band, continuous wave power levels over 340 Watts in the 8xx-nm band, and reliability data at or above 100 Watts. We will also review the latest advances in performance and detail the basic physics and material science required to achieve these results.
Long term lifetest data is presented for Al-free active region 980 nm multimode laser diodes configured as chip-on-submount devices, as packaged fiber-coupled devices, and as multi-emitter laser bars on a microchannel cooled heatsink. Single emitter devices have been tested in chip-on-submount form. A first set of 120 of these devices were tested in a five-cell matrix at varying junction temperatures and optical output levels to obtain measured values for both random and wear-out failure model parameters. A second set of 187 packaged lasers were placed on accelerated lifetest to measure FIT data. In both cases, the devices were operated for up to 9,000. Another set of chips were packaged and tested inside a fiber-coupled, TEC cooled, 14 pin butterfly case as part of a Telcordia qualification process. These devices were operated for up to 5,000 hours with no failures and no degradation of either the chip or the package. Bar devices with a 20% fill factor were mounted on microchannel heatsinks and tested for one second on, one second off quasi CW operation for 4,000 hours. This test condition places a thermal expansion cycle stress on the devices, however once past the initial burn-in period very little degradation is seen in the output characteristics of the device.
Solid-state-laser and fiber laser pumping, reprographics, medical and materials processing applications require high power, high-brightness bars and fiber-coupled arrays. Conductively cooled laser diode bars allow customers to simplify system design and reduce operational size, weight, and costs. We present results on next generation high brightness, high reliability bars and fiber-coupled arrays at 790-830 nm, 940 nm and 980 nm wavelengths. By using novel epitaxial structures, we have demonstrated highly reliable 808 nm, 30% fill-factor conductively cooled bars operating at 60W CW mode, corresponding to a linear power density (LPD) of 20 mW/μm. At 25°C, the bars have shown greater than 50% wall-plug-efficiency (WPE) when operating at 60W. Our novel approach has also reduced the fast-axis divergence FWHM from 31° to less than 24°. These bars have a 50% brightness improvement compared to our standard products with this geometry. At 980nm, we have demonstrated greater than 100W CW from 20% fill-factor conductively cooled bars, corresponding to a LPD of 50 mW/µm. At 25°C, the WPE for 976nm bars consistently peaks above 65% and remains greater than 60% at 100W. We coupled the beam output from those high-brightness bars into fiber-array-packages (“FAPs”), and we also achieved high-brightness and high-efficiency FAPs. We demonstrated 60W from a 600µm core-diameter fiber-bundle with a high WPE of 55%, and a low numerical aperture of 0.115. The brightness of such FAPs is four times higher than our standard high-power 40W FAP products at Coherent. Ongoing life test data suggests an extrapolated lifetime greater than 10,000 hours at 80W CW operating-condition based on 30%FF conductively cooled bar geometry.
Ultra-reliable very efficient multimode diodes have been developed. 90 micrometer wide aperture chip-on-submount demonstrate 67% peak power conversion efficiency at 25°C and 60% peak power efficiency at 85°C heatsink temperature; while record high thermal limited peak power in excess of 15 W CW is achieved at 15°C. State-of-the-art performance of Chip-on-Submount does not compromise its reliability; over 1,000,000 hours Mean Time Between Failures has been demonstrated. Cooler-less package design ensures thermal resistance of 4.5°C/W from the Chip-on-Submount to external heatsink; coupling efficiency of 95% into 0.15 NA at over 5 W power in 100 μm dia. fiber are routinely obtained. Results of multi-cell accelerated tests of packaged pumps yielded over 10 years of uninterrupted use MTBF at over 5W CW ex-fiber output.
Observation of spatial hole burning reduction is reported for the first time to our knowledge on flared 1480nm high power InGaAsP/InP buried ridge lasers. We determined the longitudinal carrier density profile by spatially resolved spontaneous emission measurements for slightly tapered and straight active waveguides. The tapered stripe shows spatial hole burning reduction leading to 25% output power enhancement.
High power, 1060 nm, InGaAs/GaAs/AlGaAs graded-index, separate-confinement (GRINSCH), strained single quantum-well (SQW), single mode (SM) laser diodes grown by Metal-Organic Chemical-Vapor Deposition (MOCVD) are reported. The high quality quantum well with high strain, which is the key issue to make high performance 1060 nm laser diode, was obtained by optimizing growth conditions. For realizing SM lasers and modules, the ridge-waveguide lasers with 5 um width and 1500 μm cavity length are successfully fabricated and mounted epitaxial-side up onto AlN submounts using eutectic Au80Sn20 solder to allow easy access to the emission region for fiber coupling and to minimize the effects of die bonding stress on the ridge. These devices exhibit threshold current of less than 30 mA, slope efficiency of up to 1.0 W/A and high kink-free power of 500 mW at 25°C. The devices that were subjected to long-term aging test at 85°C, operating at 300 mW, first show very good reliability. The coupled module with more than 70% fiber coupling efficiency and more than 200 mW output power from a single mode fiber or polarization maintained (PM) fiber in 14-pin butterfly case is demonstrated.
The successful use of high power diode lasers (HDL), which are based on 1 cm semiconductor bars, is critically linked to the optimization of output power, beam quality, reliability, and cost. Even though some criteria can be improved only on the costs of others, the common point of concern is the reliability. A defined reliability allows the user to estimate not only the investment costs but also all service and replacement costs. All those economical concerns can be translated into technical requirements. All efforts to fulfill these technical requirements are part of JENOPTIK Laserdiode’s LongLifeTechnology.
We report on a diode laser system, which is based on tapered diode laser bars and provides a combination of high power and high beam quality comparable to high power lamp pumped solid-state-rod lasers. Until now diode laser systems with output powers in the kW-range are based on broad area diode lasers. However, the output of these kilowatt laser systems usually is characterized by a strongly asymmetric beam profile, which is a consequence of the asymmetric beam parameter product (BPP) of broad area diode lasers with regard to the slow- and the fast-axis direction. Apparently the output of such a laser system can not be coupled efficiently into a fiber, which is required for a variety of applications. The symmetrization of the BPP of such a laser system requires complicated and expensive beam shaping systems. In contrast tapered diode laser bars allow the design of high power laser systems with a symmetric beam profile without the necessity of using sophisticated beam shaping systems. Power scaling is realized with different incoherent coupling principles, including spatial multiplexing, polarization multiplexing and wavelength multiplexing. The total output power of the tapered diode laser system was 3230 W at a current of 75 A. Fiber coupling yielded 2380 W at 75 A for a fiber with a core diameter of 800 μm (NA 0.22) and 1650 W at 60 A for a 600 μm fiber (NA 0.22), respectively. Focusing with an objective with a focal length of 62 mm led to a beam diameter of 0.52 mm in the focal plane. Taking into account the total power of 2380 W behind the fiber the resulting intensity in the focal plane was 1.1 MW/cm2.
We have developed an array of ten uncoupled, single-mode ridge-waveguide lasers for spectral beam combining at 980 nm, which are based on an aluminium-free active region. Single emitters deliver 0.4 W CW at 0.7 A with an M2 beam quality factor of 1.6. The array has an output power of 2.1 W CW at 3.15 A. The maximum wall-plug efficiency is 48%. Spectral beam combining is achieved through a low-quality-factor external cavity. We extract more than 1.5 W from the cavity with a good beam quality. The ten peak wavelengths range between 968 and 982 nm. The source was validated by pumping an EDFA with similar results as a single wavelength source.
Typically, output from high power, diode lasers and arrays must be transported to a final application/treatment site, whether the application is industrial, military or medical. Many demands are placed on optical fibers to couple the laser output into their structures and to transmit the power to the application site. All silica fibers become much more expensive as the diameter of the fiber increases to handle larger spot sizes and higher NA beams, especially from diode arrays. High strength, low fatigue Hard Plastic Clad Silica fibers provide benefits of larger Numerical Aperture (NA), more flexibility and less strain at the core-clad interface. Fibers with these characteristics and available in both high OH and low OH versions for UV and NIR spectral regions are described. Short and long term strength, and spectral properties are presented. Results for a new high NA version are also presented.
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.
Laser cutting of paper is widely used in the paper conversion industry. CO2 lasers are well suited for this type of applications. Desktop printing is a large market both for digital photography, document management and graphics applications, but it still lacks advanced cutting and scoring ability, and CO2 lasers seem costly to be integrated in mass-market printers. For that reason, mass-scalable and low-cost semiconductor laser diodes would be very advantageous to add paper cutting and scoring features in desktop printers. However, common paper can not be cut properly using visible or Near Infrared (NIR) laser diode since it has a very poor absorption at these wavelengths. We report here an innovative solution to achieve paper cutting or scoring using a 1 W single emitter NIR laser diode, within an inkjet printer. A special ink that absorbs the NIR light, and that penetrates all through the paper, is first disposed on the lines to be cut. Then, the laser diode goes along the lines to be cut. We show that a cutting speed of 2m/min can be achieved on 80g/m2 conventional paper. The influence of the optical properties of the ink on the cutting speed are discussed, as well as focussing issues. In particular, we show that invisible inks are suitable, and very clear-cut edges can be obtained. The perspective of this technique are discussed.
High power direct diode lasers have made significant impact in the industry as an alternative heat source for material processing. In order, to be readily adopted by the industry they have to show >99% uptimes. One of the much-discussed issues associated with copper based micro channel coolers has been the lifetime of the micro channel cooler in High Power Direct Diode Laser (HPDDL) systems. HPDDLs with micro channel coolers have shown long life in some installations, but have shown to work only a few thousands of hours in others. These have been attributed to the erosion, corrosion, or clogging of the micro channel coolers. This paper will describe the proper design of the water system for use with a micro channel cooled laser system. This paper focuses on the water chemistry and its impact on erosion and corrosion of the copper based micro channel coolers. Using previously reported data; we will give erosion rates for different water chemistries.
High power diode lasers have demonstrated reliable output power of more than 50W per diode far beyond 10,000 hours. Record output power of more than 300W per diode laser bar has been reported. The improved reliability of the semiconductor material demands a review of the performance of the actively water cooled heatsink with regards to the expected lifetime. Results from corrosion tests at various water conditions for durations of more than 13,000 hours predict an extended usage of water-cooled heatsink beyond 20,000 hours without significant performance change.
Cooling high power laser diodes with micro channel heat sinks is state of the art. But latterly also states of the art are problems with leaks, based on corrosion. As a user of high power diode lasers we can badly confirm that point. We observed, that the major part of the collapsed heat sinks was made by using direct copper bonding. Due to that, ProLas decided to get forward in there own development of micro cooling systems and the protection of corrosion. In a first step the joining procedure was switched to diffusion welding. This procedure uses only a vacuum surrounding, pressure and heat, no oxygen. In a second step the micro coolers will be covered at the inside using a special gas during the aeration of the vacuum oven. This cover is only a few nm thick, so that the cooling performance will not be impaired. Additionally the cover is so stabile, that particles or cavitations are not able to destroy it. Only in a few regions of very high flow rate we still have erosion, but the passivation is not able to build a local element with the copper. So erosion will not leads to corrosion in these cases. In addition to the improved copper cooler types we developed a complete new generation of micro coolers, using alternative materials and a different manufacturing method, to achieve non-corrosive monolithic heat sinks with a more suitable CTE.
Thermal properties of 808 nm emitting high-power diode lasers are investigated by means of micro-thermography. A thermo-camera equipped with a 384x288 pixel HgCdTe-detector (cut off wavelength at 5.5 micron) and IR-micro-objective is used, which allows for thermal imaging with a spatial resolution of 5 μm. A novel methodological approach for data re-calibration for absolute temperature measurements is proposed. We present steady-state thermal distributions from broad-area devices. The remarkable agreement of this data with the results of modeling work has been reached. Cross-calibration of the micro-thermographic results is obtained by complementary micro-Raman data that give information about facet temperatures with a spatial resolution of about 1 micron. Transient thermal properties are monitored with a temporal resolution of 1.4 ms. Such thermal transients illustrate the heat flow trough the device after turning on the operation current. Special experiments are done in order to detect and localize hot spots at the facet and within the devices. Moreover, we show that the analysis of thermal images can be used as a recognition method of defects hidden inside the cavity, even if they are not detectable by visual inspection. These activities are paving the way towards a novel screening methodology.
Results of a long-term research in spectral narrowing and transverse mode selection in semiconductor lasers by means of volume Bragg gratings recorded in a photo-thermo-refractive (PTR) glass are described. PTR glass is a multicomponent silicate optical glass which changes its refractive index after UV exposure followed by thermal development. This feature enables recording of volume holograms with efficiency exceeding 97% in visible and near IR spectral regions which tolerate high temperatures up to 400°C, high power laser radiation. Transmitting and reflecting volume Bragg gratings recorded in such manner have spectral and angular selectivity down to 0.01 nm and 0.1 mrad, respectively. These spectral and angular selectors were used as transmitting and reflecting elements of external resonators for high-power semiconductor laser diodes (LDs). Transmitting Bragg gratings provide tunability of LDs in the range up to 60 nm, spectral narrowing down to 200 pm, stabilization of wavelength within 500 pm. Reflecting Bragg gratings allow spectral narrowing down to 20 pm, stabilization of wavelength below 100 pm at temperature variations up to 75 K. A single transverse mode emission for wide stripe LDs is observed at pumping currents exceeding 10 thresholds. Narrowing and stabilization of emission spectra of LD bars is demonstrated. It is important that all these features are achieved by passive elements with efficiency exceeding 97% and unlimited lifetime while actual brightness increase exceeded two orders of magnitude.
The exceptionally high efficiency and compactness of the diode laser have made it one of the key elements of the engineering systems throughout many different areas of industry. The decisive factor in the performance of the high-power diode laser in individual applications, is the quality of its beam emission. Applications in the high-power range require the use of micro-optical components, in order to maximise the use of light and, at the same time, to minimise the degree to which the beam quality depends on the direction of emission. These micro-optical components are vital to the basic function of the high-power diode laser and, more particularly, to its efficiency. In addition to the function-determining characteristics, the high levels of precision and reproducibility of the optics in serial manufacture along with the option of a monolithic structure will be of interest to laser manufacturers. The requirements arising from the basic construction of the diode laser, which must be met by micro-optics, will therefore be defined in the course of this contribution. In the second section, concepts for solutions, which take full account of these requirements, will be presented. Finally, optics used in high-power diode lasers will be discussed in the conclusion.
Volume Bragg grating (VBG) technology has been shown to improve spectral characteristics of the high-power laser diodes and arrays. We review the recent advances in VBG technology that lead to better spectral performance as well as improvements in spatial brightness of the high-power laser diode arrays. The VBG technology has been applied to manufacturing of high-power laser diode arrays suitable for spectral beam combining to achieve even higher spatial brightness. The approach involves fabrication of VBG elements with transverse chirp of the Bragg period. Wavelength tuning of the laser diode arrays and wavelength-shifted laser diode bar operation have been demonstrated, which will lead to manufacturing of spectrally combined high-brightness arrays.
The laser diode technology, underpinning applications such as data storage, industrial lasers and optical telecommunications, still suffers from reliability and longevity limitations, especially in high power applications. A main problem for these lasers arises from facet oxidation, leading to increased absorption, power degradation and COMD device failure. Typically, high power devices initially show a low linear degradation and after some 100 hours, the degradation accelerates in a nonlinear fashion, indicating a degradation runaway condition. This article reports performance and reliability improvements that are based on a process which atomically seals surfaces and eliminates oxidation by forming stable nitrides on laser facets. The dangling bond terminating technology suppresses accelerated degradation associated with optical density and heat at laser facets. The dangling bond termination is demonstrated by improved COMD, decreased degradation at CW operation and a constant linear degradation rate at different QW temperature conditions (nonlinear degradation indicates advancement in the oxidation/optical absorption/facet heating/oxidation spiral). The technology is applicable to a range of material systems and has previously been demonstrated on InAlGaAs and InGaAs (increased COMD to >270 and 470mW/μm respectively). The devices with the typically lowest COMD levels (AlInGaAs) show a remarkably low linear degradation rate of <0.5%/kh during at CW life test operation at 90°C and a power level corresponding to 80W bar power. In addition to long term AlInGaAs laser life test results, this paper presents results on nitride facet passivation applied to 805nm InGaAsP devices, showing improved COMD to 400mW/μm and the initial CW life data confirms the general behavior of the previously life-tested InGaAs and InAlGaAs based devices.
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.
Laser diodes and bars with high efficiency, power, and reliability are critical for a wide variety of applications including direct material processing and pumping high power and efficient fiber lasers and solid state lasers. We present progress towards the 80% power conversion efficiency goal of the DARPA Super High Efficiency Diode Sources (SHEDS) program. Currently, laser bars using JDSU SHEDS technology achieve as high as 72.7% total power conversion efficiency at room temperature and 80W operating power.