Sandrio Elim

Mitigation of thermal lensing effect as a brightness limitation of high-power broad area diode lasers

Recent efforts to improve the reliability of high-power broad-area diode lasers operating in the 9xx nm wavelength band have yielded single emitter devices with excellent reliability to >15 W. However, in applications requiring fiber coupling, the fiber coupling power of a single emitter device is rather limited by its linear power density, and hence brightness of the device. Unfortunately, due to a rapid increase in the slow-axis divergence, a typical broad-area diode laser offers a much lower brightness increase and an earlier rollover than its output power. In this work, we show that thermal lensing (rather than carrier- or gain-induced guiding) in the slow axis is the predominant cause of beam quality degradation at increased driving current. Some of the techniques which are presented include the use of cavity length scaling and thermal path engineering. It is expected these approaches are critical to enabling continued scaling of highbrightness fiber coupled diode lasers.

Diode Laser Efficiency Increases Enable > 400-W Peak Power From 1-cm Bars and Show Clear Path to Peak Powers in Excess of 1-kW

Peak optical power from single 1-cm diode laser bars is advancing rapidly across all commercial wavelengths. Progress in material performance is reviewed and we show that current trends imply there is no fundamental barrier to achieving peak powers of 1-kW per 1-cm diode laser bar. For bars with such high peak powers, commercially available reliable devices would be expected to deliver ~ 300-W per bar. Progress to date has allowed us to demonstrate > 400-W peak output from single 1-cm diode laser bars at emission wavelengths from 800-nm to 980-nm. The available range of emission wavelengths has also been increased, with 90-W bars shown at 660-nm and 24W at 1900-nm, complementing the 100-W bar previously demonstrated at 1470-nm. Peak power is seen to correlate closely peak efficiency. Further advances in diode laser efficiency and low thermal resistance packaging technology continue to drive these powers higher. The most critical improvements have been the reduction in the diode laser operating voltage through optimization of hetero-barriers (leading to 73% efficient 100-W bars on copper micro-channel) and a reduction in packaging thermal resistance by optimizing micro-channel performance (leading to < 0.2-oC/W thermal resistance).

Extending the wavelength range of single-emitter diode lasers for medical and sensing applications: 12xx-nm quantum dots, 2000-nm wells, > 5000-nm cascade lasers

Diode lasers supply high power densities at wavelengths from 635-nm to 2000-nm, with different applications enabled by providing this power at different wavelengths. As the range of available wavelengths broadens, many novel medical and atmospheric applications are enabled. Traditional quantum well lasers provide high performance in the range 635- nm to 1100-nm range for GaAs-based devices and 1280-nm to 2000-nm for InP, leaving a notable gap in the 1100 to 1280-nm range. There are many important medical and sensing applications in this range and quantum dots produced using Stranski-Krastanow self-organized MBE growth on GaAs substrates provide an alternative high performance solution. We present results confirming broad area quantum dot lasers can deliver high optical powers of 16-W per emitter and high power conversion efficiency of 35% in this wavelength range. In addition, there are growing applications for high power sources in wavelengths > 1500-nm. We present a brief review of our current performance status in this wavelength range, both with conventional quantum wells in the 1500-nm to 2500-nm range and MOCVD grown quantum cascade lasers for wavelengths > 4000-nm. At each wavelength, we review the designs that deliver this performance, prospects for increased performance and the potential for further broadening the availability of novel wavelengths for high power applications.

Diode laser bars deliver > 400-W peak CW power from 800-nm to 980-nm enabling wide range of applications

Peak optical power from single 1-cm diode laser bars is advancing rapidly across all commercial wavelengths. Progress to date has allowed us to demonstrate > 400-W peak output from single 1-cm diode laser bars at emission wavelengths from 800-nm to 980-nm. The available range of emission wavelengths has also been increased, with 90-W bars shown at 660-nm, 37W at 1910-nm and 25W at 2070-nm, complementing the 100-W bar previously demonstrated at 1470-nm. Peak power is seen to correlate closely peak power conversion efficiency. Further advances in diode laser efficiency and low thermal resistance packaging technology continue to drive these powers higher. The most critical improvements have been the reduction in the diode laser operating voltage through optimization of hetero-barriers (leading to 74% efficient 100-W bars on micro-channel at 975-nm) and a reduction in packaging thermal resistance by optimizing microchannel performance (leading to < 0.2-°C/W thermal resistance). We have also recently extended our high efficiency designs to shorter wavelengths, now delivering over 70% efficiency at 790-nm. Ever-increasing power levels (projected to eventually exceed 1-kW per bar) reduce the cost in Euro per W of diode laser systems, enabling broader application in military, industrial and medical markets. In addition, increasing availability of high powers at new wavelengths is enabling many new applications.

76% efficient cryogenically-cooled eyesafe diode laser for resonant pumping of Er-doped gain media

There is great interest in the development of high-power, high-efficiency InP-based broad area pump diode lasers operating in the 14xx-15xx nm band to be used for resonant-pumping of Er-doped solid state lasers. Cryogenic cooling of diode lasers can provide great benefit to performance, arising from the dramatic reduction in the threshold current and the increase in the diode’s slope efficiency. These improvements are attributed to reduction in the non-radiative losses and leakage current associated with thermionic emission of carriers from the quantum well. This is, however, at the expense of a large increase in the diode voltage, limiting the power conversion efficiency at cryogenic temperatures. In this work, we report on the development of high-power, high-efficiency diode lasers and stacked arrays operating at 15xx-nm, which are specifically designed and optimized for operation at cryogenic temperatures. We show that the diode voltage defects under cryogenic operation can be greatly reduced through reducing the energy band offsets at the hetero-interface, and through material change to reduce the dopant ionization energy, effectively mitigating carrier freeze-out at low temperatures. Optical cavity designs and band engineering optimization are also explored for low intrinsic optical loss and low carrier leakage. A peak power conversion efficiency of >74% was demonstrated at a temperature of ~100K in a 15xx-nm single emitter. Record high peak conversion efficiency of 71% and peak power of > 500 W were also demonstrated in a stacked array, under QCW pulses of 1 ms and 10 Hz.

High power and high efficiency kW 88x-nm multi-junction pulsed diode laser bars and arrays

There is great interest in the development of high-power, high-efficiency and low cost QCW 88x-nm diode laser bars and arrays for pumping solid state lasers. We report on the development of kW 88x-nm diode laser bars that are based on a bipolar cascade design, in which multiple lasers are epitaxially grown in electrical series on a single substrate. Multiple laser junctions, each of which is based on nLight’s high performance 88x-nm epitaxial design, are separated by low resistance tunnel junctions with resistance as low as 8.0x10-6 Ω-cm2. Optimization of bar geometry and wafer fabrication processes was explored for electrical and optical performance improvement in double-junction diode lasers. A QCW power of 630 W was demonstrated in a 3-mm wide mini-bar with 3-mm cavity length. Peak efficiency of 61% was measured with 200 s and 14 Hz pulses, at a heatsink temperature of 10 °C. Further power scaling was demonstrated in a 1-cm wide bar with 3-mm cavity length, where a record high peak power of 1.77 kW was measured at 1 kA drive current. Ongoing work for further power scaling includes development of triple-junction diode laser bars and double-junction bar-stack that emits < 10kW optical power.

High efficiency kW-class QCW 88x nm diode laser bars

We present results of kW-class diode laser bars with optimized fill factors, cavity lengths, and facet reflectivity that demonstrated electrical-to-optical efficiency of 70% and operation of over 100 million shots in QCW mode.

Room temperature high power mid-IR diode laser bars for atmospheric sensing applications

Peak CW optical power from single 1-cm diode laser bars is advancing rapidly across all commercial wavelengths and the available range of emission wavelengths also continues to increase. Both high efficiency ~ 50% and > 100-W power InP-based CW bars have been available in bar format around 1500-nm for some time, as required for eye-safe illuminators and for pumping Er-YAG crystals. There is increasing demand for sources at longer wavelengths. Specifically, 1900-nm sources can be used to pump Holmium doped YAG crystals, to produce 2100-nm emission. Emission near 2100-nm is attractive for free-space communications and range-finding applications as the atmosphere has little absorption at this wavelength. Diode lasers that emit at 2100-nm could eliminate the need for the use of a solid-state laser system, at significant cost savings. 2100-nm sources can also be used as pump sources for Thulium doped solid-state crystals to reach even longer wavelengths. In addition, there are several promising medical applications including dental applications such as bone ablation and medical procedures such as opthamology. These long wavelength sources are also key components in infra-red-counter-measure systems. We have extended our high performance 1500-nm material to longer wavelengths through optimization of design and epitaxial growth conditions and report peak CW output powers from single 1-cm diode laser bars of 37W at 1910-nm and 25W at 2070-nm. 1-cm bars with 20% fill factor were tested under step-stress conditions up to 110-A per bar without failure, confirming reasonable robustness of this technology. Stacks of such bars deliver high powers in a collimated beam suitable for pump applications. We demonstrate the natural spectral width of ~ 18nm of these laser bars can be reduced to < 3-nm with use of an external Volume Bragg Grating, as required for pump applications. We review the developments required to reach these powers, latest advances and prospects for longer wavelength, higher power and higher efficiency.

Reduced Mode Diodes for Increasing Laser Brightness

We report on two-times narrower slow-axis divergence of 9xx nm broad area diode lasers resulting from reduction in allowed lateral modes. This enabled use of larger aperture broad area lasers to achieve 30% higher brightness.

Optimized performance of 808 nm diode laser bars for efficient high-power operation

nLIGHT optimized both the high-temperature (HT) and the high-efficiency (HE) epitaxial designs for efficient highpower continuous-wave (CW) operation by implementing nLIGHT’s facet passivation technology (nXLT) into our 808 nm diode laser bars. The application of a refined phenomenological model of the diode lasers allowed tailoring of the device parameters to obtain optimized bar performance. In other words, we adjusted modeling inputs such as bar layout and front facet coating reflectivity to optimize operational indicator outputs such as wall-plug efficiency and operation currents at specific power ratings. Thus, both time and cost were saved without the need of extra experimental runs. We demonstrated that both HT and HE epitaxial designs can support centimeter bar geometries with power ratings above 100 W/bar. At the standard power rating of 100 W/bar, the HE designs show advantages in both operating current and wall-plug efficiency when compared to the HT design. With the newly released HE epitaxial designs, wall-plug efficiency ~58% is achieved for a power rating of 150 W/bar.