Suhit Das

85% power conversion efficiency 975-nm broad area diode lasers at − 50°C, 76 % at 10°C

Optimized single stripe 975-nm broad area devices deliver 76% power conversion efficiency at 10degC. Cooling the material leads to 85% efficiency at -50degC. External differential quantum efficiency is the dominant term.

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.

Reliable high-power long-pulse 8XX-nm diode laser bars and arrays operating at high temperature

We report on the high-power high-temperature long-pulse performance of the 8XX-nm diode laser bars and arrays, which were recently developed at Lasertel Inc. for diode laser pumping within high-temperature (130 °C) environment without any cooling. Since certain energy in each pulse is required, the diode laser bars have to provide both high peak power and a nice pulse shape at 130 °C. Optimizing the epi-structure of the diode laser, the laser cavity and the distribution of waste heat, we demonstrate over 40-millisecond long-pulse operation of the 8XX-nm CS bars at 130 °C and 100 A. Pumping the bar with 5-Hz frequency 15-millisecond rectangular current pulses, we generate over 60 W peak power at 100 A and 130 °C. During the pulse duration, the pulse shape of the CS bars is well-maintained and the power almost linearly decays with a rate of 1.9% peak power per millisecond at 130 °C and 100 A. Regardless of the pulse shape, this laser bar can lase at very high temperature and output pulse can last for 8 ms/2ms at 170 °C/180 °C (both driven by 60 A current pulses with 5-Hz frequency, 10 millisecond pulse width), respectively. To the best of our knowledge, this is the highest operating temperature for a long-pulse 8XX-nm laser bar. Under the condition of 130 °C and 100 A, the laser bars do not show any degradation after 310,000 10-millisecond current pulse shots. The performance of stack arrays at 130 °C and 100 A are also presented. The development of reliable high-temperature diode laser bar paves the way for diode laser long-pulse pumping within a high-temperature environment without any cooling.

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.

Record High-Temperature Long-Pulse Operation of 8xx-nm Diode Laser Bar with Aluminum-Free Active Region

We report on the first demonstration of long-pulse (milliseconds) operation of aluminum-free active-region 8xx-nm diode laser bar at heat-sink temperature of 180 °C. This is, to the best of our knowledge, the highest published operating temperature for a long-pulse 8xx-nm laser bar with Al-free active region. The laser bars have very robust performance at 130 °C without any active cooling. At this high temperature, the laser bars provide both high peak power (60 W at 100 A) and good pulse shape for tens of milliseconds pulse width, maintaining high energy per pulse. The dependence of laser output pulse shape on the pulse width and pump current is experimentally investigated at 130 °C. We find that the transient output power of the laser bar follows P(t) = A exp(-t /t0) + Bt + C, where A, B, C, and t 0 are fitting parameters that are pulse width and current dependent. We have also investigated the transient thermal behavior of the laser bar at high temperature and high pump current.

Stabilization of Lateral Mode Transients in High-Power Broad Area Semiconductor Lasers

In this work, the temporal fluctuations of lateral modes in high-power broad area semiconductor lasers are investigated in the time domain. Index guiding (in the form of etched holes) is introduced as a method of stabilizing and controlling the lateral modes. Spatial control of the lateral modes and subsequent reduction of filamentation / smoothing of the near-field profile has already been predicted and experimentally shown to improve the efficiency of broad area laser diodes. Here, the method is shown to also dampen temporal instabilities (transients) of the lateral modes.

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-performance directly modulated lasers: device physics

Directly modulated lasers (DMLs) have two high performance applications: 1310 nm 10 Gb/s uncooled and 1550 2.5 Gs/s extended reach. Two key elements are gain coupled gratings and buried heterostructures. Gain coupled gratings simultaneously increase the DMLs intrinsic relaxation oscillation frequency and damping, while the buried heterostructure reduces thermal chirp and parasitic capacitance. Large relaxation oscillation frequencies and reduced parasitic capacitance allow 85°C operation; large damping and reduced thermal chirp enable extended reach.

Scalable compact laser diode array technology for high energy applications

Multi-kw compact CW and QCW laser diode array technology is presented for use in high energy applications. Performance of the fully soldered compact arrays and approaches for scalability to sub-megawatt class laser diode modules are discussed.