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.
We describe the performance and reliability of high power vertical diode stacks based on silicon monolithic microchannel
coolers (SiMMs) operating at >1000W/cm2 CW at 808 and 940nm. The monolithic nature of these stacks makes
them inherently robust and compact. Typical emitting dimensions for a 10-bar stack are ~8.8mm × 10mm with CW
output power up to 1.5kW. Originally developed at Lawrence Livermore National Laboratory and now actively being
developed for commercial applications at Coherent, this technology offers several advantages over current copper-based
micro-channel coolers. These devices do not require use of DI water, strict monitoring and control of the pH level,
careful control of the water velocity, or sealed cooling systems. The need for hydrostatic seals is also drastically reduced.
A typical ten bar stack requires only 2 o-ring seals, compared to 20 such seals for a similar stack using copper microchannel
cooling. Mature and readily available wet etching technology allows for cost effective batch fabrication of the
sub-mount structure while achieving repeatable high precision components based on photolithographic fabrication
processes.
Ongoing optimization of epitaxial design within Coherent device engineering has led to a family of high power-conversion-efficiency (PCE) products on conductively cooled packages (CCP) and fiber array packages (FAP). At a 25°C heat sink temperature, the PCE was measured at 71.5% with 75W CW output power on 30% fill-factor (FF) bars with passive cooling. At heat sink temperatures as high as 60°C the PCE of these bars is still maintained above 60%. Powered by such high efficiency 9xx nm diodes, Coherent FAP products have consistently exceeded 55% PCE up to 50W power levels, with 62% PCE demonstrated out of the fiber. High linear-power-density (LPD) operation of 100μm x 7-emitter bars at LPD = 80 mW/μm was also demonstrated. Bars with 7-emitter were measured up to 140W QCW power before catastrophic optical mirror damage (COMD) occurred, which corresponds to a COMD value of 200mW/μm or 2D facet power density of 29.4 MW/cm2. Leveraging these improvements has enabled high power FAPs with >90W CW from an 800μm-diameter fiber bundle. Extensive reliability testing has already accumulated 400,000 total real-time device hours at a variety of accelerated and non-accelerated operating conditions. A random failure rate <0.5% per kilo-hours and gradual degradation rate <0.4% per kilo-hours have been observed. For a 30% FF 50W CW 9xx nm bar, this equates to >30,000 hours of median lifetime at a 90% confidence level. More optimized 30% FF 9xx nm bars are under development for power outputs up to 80W CW with extrapolated median lifetimes greater than 20,000 hours.
The effect of oxygen incorporation on electrical and optical properties of Be- and Si-doped InP grown by solid source molecular beam epitaxy has been studied. Oxygen was observed to reduce both the Be and Si doping efficiencies in InP and to introduce recombination centers that degrade the photoluminescence property. The compensation in Be-InP was more pronounced than in Si-InP. With high oxygen impurity concentration, Be-InP was found to be n-type in Hall measurements. Electrical inactivity of Be is likely due to the formation of Be-O complexes in InP. Oxygen was originated from the phosphorous source and could be reduced by employing lowered phosphorous cracker temperature.
Monolithic top-emitting resonant cavity light-emitting diodes (RCLEDs) have been fabricated by solid-source MBE. The RCLEDs in the 650-nm range, with modulation bandwidths exceeding 180 MHz, are possible low-cost transmitter candidates for systems using plastic optical fibers (POFs), such as IEEE-1394 at 100 Mb/s and 200 Mb/s and ATM at 155 Mb/s. Modulation bandwidth of greater than 120 MHz and light power of 2 mW (cw) have been achieved for (phi )84-micrometer devices driven at a 40 mA current. Accelerated aging tests for 27,500 device-hours indicate no degradation in output power. A variation in device temperature significantly modifies the far-field pattern and thus the fiber coupling efficiency, due to a cavity detuning effect. The effects of detuning and the temperature and bias dependencies of the devices are investigated. The 880-nm RCLEDs have a maximum output power of 25 mW. Applications include open-air optical communication systems, collision avoidance and measurement systems.
KEYWORDS: Quantum wells, Semiconductor lasers, Cladding, Refractive index, Near field optics, GRIN lenses, Optical simulations, Waveguides, Metalorganic chemical vapor deposition, High power lasers
In the already classical separate confinement (SCH) quantum well (QW) semiconductor laser diode structures many of the desired performances are contradictory coupled through the structural parameters -- i.e. a structural parameter modification leading to the improvement of one or more laser performances will produce the deterioration of at least another performance. Based on an analysis of this contradictory coupling a novel transverse layer structure that alleviates the problem and enables improved laser diode performances is proposed. Both optical simulation and a fully self-consistent model are used in a design optimization methodology and simple evaluation and optimization criteria for the new transverse structure are derived. A number of the analyzed high-power edge-emitting GazIn1-zP/(AlxGa1- x)yIn1-yP/GaAs quantum well laser structures were prepared using all-solid-source molecular beam epitaxy (SS-MBE) for layer growth and remarkable performances were obtained (continuous wave output powers of 3 W at 670 nm, 2 W at 650 nm, and 1 W at 630 nm; threshold current densities of 350 - 450 Angstrom/cm2 for 670 nm, 500 - 540 A/cm2 for 650 nm, and 600 - 680 A/cm2 for 630 nm). Although only a few of the optimization features were implemented the good agreement between measurements and simulations for the prepared structures indicate that significant performance improvements -- predicted by the simulations -- are still possible.
Monolithic top emitting resonant cavity light-emitting diodes operating in the 650 and 880 nm ranges have been prepared using solid-source molecular beam epitaxy growth. Transfer matrix based modeling together with a self- consistent model have been sued to optimize the devices' performances. The design of the layer structure and doping profile was assisted by computer simulations that enabled many device improvements. Among the most significant ones intermediate-composition barrier-reduction layers were introduced in the DBR mirrors for improving the I-V characteristics and the cavity and mirrors were detuned aiming at maximum extraction efficiency. The fabricated devices showed line widths below 15 nm, CW light power output of 8 and 22.5 mW, and external quantum efficiencies of 3 percent and 14.1 percent in the 650 nm and 880 nm ranges, respectively - while the simulations indicate significant performance improvement possibilities.
We report on resonant cavity light-emitting diodes, operating at 660, 880, and 1300 nm wavelengths. Some of the characteristic features of these devices will be discussed. The devices were grown by all-solid-source molecular beam epitaxy (SSMBE). The results provide clear-cut evidence that SSMBE is a viable method to growth of phosphorous containing semiconductors.
This paper presents the performance characteristics and reliability data of AlGaInP-based VISIBLE laser diodes emitting at the wavelengths from 630 to 670 nm. The lasers are grown by toxic gas free solid source molecular beam epitaxy.
This paper presents the performance characteristics and reliability data of GaInAsP- and AlGaInP-based laser diodes emitting at the wavelengths from 650 to 1,300 nm. The lasers are grown by toxic-gas-free all-solid-source molecular beam epitaxy (SS-MBE).
InGaAs/GaInAsP/GaInP ridge waveguide 980-nm laser diodes for pumping light into erbium doped fiber amplifiers are reviewed. These lasers have very good performance characteristics. They exhibit kink-free, single mode emission up to a power of 250 mW with a slope efficiency of 0.7 to 0.95 W/A, a thermally limited maximum power of 450 - 500 mW, and the threshold current density of about 150 A/cm2. They are relatively stable against temperature variations. A 100 mW power from a fiber-pigtail module has been demonstrated. The lasers withstand severe thermal roll-over tests without showing degradation effects. Preliminary lifetime tests indicate that their mean-time-to-failure may be very high if not limited by sudden failure, from several hundred thousand to one million hours.
Luminescense and photoconductivity measurements were performed on MBE grown ZnSe layers with various arsenic concentrations. Two shallow acceptor levels with energies of 125 meV and 260 meV were found. Increasing the As content in order to increase the number of shallow acceptor states resulted in highly compensated samples. For Li the acceptor binding energy was found to be 113 meV. Also in the case of Li a higher doping concentration did not augment the shallow levels. Electrical characterization of the Li doped samples was done by C-V and I-V measurements. The films were found to be p-type.
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