Rob Martinsen

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).

Performance limitation and mitigation of longitudinal spatial hole burning in high-power diode lasers

Facets of high-power broad area diode lasers are typically coated with one high-reflecting and one partially reflecting layer to improve slope efficiency and maximize output power. The typical cavity lengths of commercial devices have also been progressively increasing, mainly to reduce temperature rise at the active region and improve laser performance and reliability. The asymmetric reflectivities and long cavity length, however, result in a highly inhomogeneous longitudinal profile of the photon density, which induces a spatially non-uniform carrier distribution, so-called longitudinal spatial hole burning (LSHB). A more uniform longitudinal photon and carrier distribution is believed to improve the overall gain of the cavity and reduce gain saturation, although further study is required to understand the impact of LSHB to power efficiency and its implication in laser design optimization to achieve higher peak powers. We present a phenomenological model that incorporates LSHB to describe longitudinal photon and carrier density inhomogeneity, as well as light-current characteristics of a diode laser. The impact of LSHB on the power efficiency is demonstrated through numerical calculation and can be significant under high-power operations. This presents new guidelines for high-power diode laser designs, in which LSHB imposes limits on reducing facet reflectivity and/or increasing cavity length, beyond which performance deteriorates. Alternatively, effects of LSHB can be mitigated through longitudinal patterning of the waveguide or contact to achieve high-power and high-efficiency diode lasers. We propose specially designed longitudinal patterning of electrical contact to mitigate LSHB. Ongoing device implementation will be used to demonstrate performance benefits.

High-performance wavelength-locked diode lasers

Rapidly maturing industrial laser applications are placing ever-tighter constraints on spectral width and wavelength emission stability over varying operating temperatures of high power diode laser pump sources. For example, improved power scaling and efficiency can be achieved by pumping the narrow upper laser level of Nd:YAG solid state lasers at 885 nm and the 1532-nm absorption band of Er:YAG solid state lasers, though taking full advantage of these configurations requires wavelength-locked pump sources. nLight offers a wide variety of wavelength-locked diode products based on external volume grating optics technology. It is often believed that the use of external gratings to wavelength lock diode lasers leads to an unavoidable loss in power and efficiency. nLights design methodology is shown to eliminate the problem in our grating-locked diode laser products. These results are expected to enable improved performance in diode-pumped solid state and fiber laser systems.

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.

Reliability of high power diode laser systems based on single emitters

Diode laser modules based on arrays of single emitters offer a number of advantages over bar-based solutions including enhanced reliability, higher brightness, and lower cost per bright watt. This approach has enabled a rapid proliferation of commercially available high-brightness fiber-coupled diode laser modules. Incorporating ever-greater numbers of emitters within a single module offers a direct path for power scaling while simultaneously maintaining high brightness and minimizing overall cost. While reports of long lifetimes for single emitter diode laser technology are widespread, the complex relationship between the standalone chip reliability and package-induced failure modes, as well as the impact of built-in redundancy offered by multiple emitters, are not often discussed. In this work, we present our approach to the modeling of fiber-coupled laser systems based on single-emitter laser diodes.

Wavelength stabilized diode laser based devices free of power or efficiency penalties

Spectrally-narrowed semiconductor laser diodes utilizing external volume gratings can be used to improve TEM00 power scaling and power conversion efficiency in diode-pumped solid state and fiber lasers. This approach is particularly attractive for pumping the narrow upper laser level of Nd:YAG DPSS lasers at 885 nm and the 1532 nm absorption band of Er:YAG DPSS lasers. While it is often believed that the use of such external gratings to wavelength lock diode lasers lead to unavoidable losses in power and efficiency, nLIGHTs proprietary laser designs and external volume grating integration techniques have eliminated these losses in our wavelength-locked diode laser products, enabling a broad range of spectrally locked laser diodes for pumping DPSS as well as fiber laser systems.

Advances in high efficiency diode laser pump sources suitable for pumping Nd:YAG systems

Nd:YAG systems are conventionally pumped at 808-nm. Direct upper-level pumping at 885-nm leads to lower heat. Diode laser pumps sources now provide power with efficiency close to 70% at both wavelengths, offering significant system benefits.

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.