The effect of oxygen defects on the gradual degradation rates of power and nonradiative carrier recombination in ~800 nm laser diodes was studied experimentally. While intentional introduction of oxygen at low levels (<5×10^15 cm^-3) was observed to degrade lasing performance prior to aging, no variation in gradual degradation rate of lasing power was observed. This suggests that degradation in these devices is not due to nonradiative recombination at low levels of point defects. Simulation of our data indicates that the power degradation may arise from increased intracavity absorption.
Power handling capabilities of broad-area high-power diode lasers are limited by the heat extraction capabilities of the device packaging. Traditional methods of heat extraction rely on conductive heat extraction from the diode chip and an emitting facet in contact with either quiescent or naturally convecting air. This leads to a thermal profile in the lasing direction of the cavity and a hot emitting facet. A hot facet accelerates material degradation, reducing the mean time to failure and limiting the safe operating power. Direct contact between the facet and a liquid coolant could enable higher levels of heat extraction compared to traditional cooling pathways. An innovative approach to cooling high-power, broad-area diode lasers via total immersion in liquid coolant is proposed and tested. In this study, we demonstrate that single emitters can operate with the emitting facet in direct contact with static coolant, with no negative change to device power or efficiency. Thermal analysis and models show that immersed diodes operate with improved thermal pathways, yielding lower total thermal resistance with the greatest improvement to thermal resistance at the facet-fluid interface.
Asymmetric photon density (recombination-rate) along the high-power diode laser cavity leads to longitudinal-spatialhole- burning (LSHB), which limits maximum output power. Here, we summarize recent investigations on the impact of LSHB on current (longitudinal) and carrier (lateral and longitudinal) density distribution and hence total-recombination for continuous-wave (CW) operations. Custom diode lasers with 90 μm stripe and 3000-6000 μm resonator have been fabricated with segmented p-side contact to measure local current density and backside metallization window to measure relative carrier density (via spontaneous intensity) and infer temperature (via wavelength). Also, 98% back facet reflectivity and 0.8% and 20% front facet reflectivities have been used to vary the photon density profile and hence severity of hole-burning. We present data showing that current crowds at the front facet due to the high recombinationrate, which becomes more severe as the bias and resonator length increase. The current crowding effect is reduced using higher front facet reflectivity. Longitudinal one-dimensional simulation is broadly consistent with experiments at low bias; however, the current crowding effect is substantially stronger in the experiment than simulation at high bias. Further, spatially-resolved-spontaneous-emission measurements of intensity and wavelength demonstrate that the longitudinal carrier density is also non-uniform with a higher carrier density at the back facet for 0.8% front facet reflectivity, even at low bias, while it is flat for devices with 20%. At high bias, temperature increases at the front facet, leading to lateral carrier accumulation at the stripe edges, higher current and carrier density, which is not included in the simulation.
Light absorption at the facet of a high power diode laser can lead to severe heating and catastrophic optical damage. In this work, a combination of high resolution thermoreflectance imaging and a detailed heat transport model of the diode chip are used to measure facet absorption in diode lasers. This approach permits a direct measurement of the effectiveness of passivation layers in improving facet robustness and device lifetime. The ability to quantify facet absorption is an essential step toward enabling rapid development of alternative passivation technologies and improving the reliability and maximum output power of diode laser systems.
This paper expands on previous work in the field of high power tapered semiconductor amplifiers and integrated master oscillator power amplifier (MOPA) devices. The devices are designed for watt-class power output and single-mode operation for free-space optical communication. This paper reports on improvements to the fabrication of these devices resulting in doubled electrical-to-optical efficiency, improved thermal properties, and improved spectral properties. A newly manufactured device yielded a peak power output of 375 mW continuous-wave (CW) at 3000 mA of current to the power amplifier and 300 mA of current to the master oscillator. This device had a peak power conversion efficiency of 11.6% at 15° C, compared to the previous device, which yielded a peak power conversion efficiency of only 5.0% at 15° C. The new device also exhibited excellent thermal and spectral properties, with minimal redshift up to 3 A CW on the power amplifier. The new device shows great improvement upon the excessive self-heating and resultant redshift of the previous device. Such spectral improvements are desirable for free-space optical communications, as variation in wavelength can degrade signal quality depending on the detectors being used and the medium of propagation.
Single mode tapered semiconductor lasers producing watt-class output powers often suffer from beam quality degradation as drive current increases. The dominant degradation mechanism is believed to be poor gain clamping in the periphery of the optical mode; as the injection current is increased, excess gain in this region eventually leads to parasitic lasing in the amplifier section of the device. However, this effect has not previously been directly observed and other effects such as thermal lensing and gain guiding also likely contribute. Nevertheless, it has been previously shown that by engineering the overlap of the gain profile with the nonuniform optical intensity distribution, performance can be significantly improved. In this work, we report on the direct observation and mapping of the 2D gain profile in a tapered semiconductor laser. InGaAsP-based tapered diode lasers are fabricated with windowed openings on the back (substrate) side of the chip. The devices are soldered junction down for continuous wave operation. An optical microscope is used to observe and map the 2D spontaneous emission profile, and hence gain and carrier density, of the device under operation. The results are compared to a theoretical model to better understand the physical limitations of beam quality degradation in tapered diode lasers.
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