Jason Patterson

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.

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.

High power diode lasers emitting from 639 nm to 690 nm

There is increasing market demand for high power reliable red lasers for display and cinema applications. Due to the fundamental material system limit at this wavelength range, red diode lasers have lower efficiency and are more temperature sensitive, compared to 790-980 nm diode lasers. In terms of reliability, red lasers are also more sensitive to catastrophic optical mirror damage (COMD) due to the higher photon energy. Thus developing higher power-reliable red lasers is very challenging. This paper will present nLIGHT’s released red products from 639 nm to 690nm, with established high performance and long-term reliability. These single emitter diode lasers can work as stand-alone singleemitter units or efficiently integrate into our compact, passively-cooled Pearl™ fiber-coupled module architectures for higher output power and improved reliability. In order to further improve power and reliability, new chip optimizations have been focused on improving epitaxial design/growth, chip configuration/processing and optical facet passivation. Initial optimization has demonstrated promising results for 639 nm diode lasers to be reliably rated at 1.5 W and 690nm diode lasers to be reliably rated at 4.0 W. Accelerated life-test has started and further design optimization are underway.

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.

High-Power, High-Efficiency, High-Brightness Long-Wavelength Laser Diodes

Interest is rapidly growing in solid-state lasers emitting from 1500-nm to 2100-nm with applications in eye-safe range finding, LIDAR, infrared countermeasures, medicine, dentistry, and others. Traditionally, these solid-state lasers have been pumped by flash lamps or more recently, by semiconductor diode lasers. In the case of the latter, the diodes of choice have been those emitting below 1-μm. The sub-micron class of semiconductor diode lasers is highly mature and has enjoyed recent rapid advances in power and efficiency. Unfortunately, the quantum defect generated when converting to the desired wavelengths results in large amounts of excess heat generation leading to costly and heavy, expensive cooling systems and performance problems related to thermal lensing. System complexity adds further cost and weight when intermediaries, such as optical parametric oscillators, are required to reach the desired longer wavelengths. Recent advances in laser diodes emitting from 1400-nm to over 1900-nm now enable the near resonant pumping of such solid state media as Er:YAG, Ho:YAG and Cr:ZnSe. Record results in the peak output power and electrical-to-optical conversion efficiency of diode lasers emitting around 1470-nm, 1700-nm and 1900-nm are presented here.