Jim Haden

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.

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.

Reliability and performance of 808-nm single emitter multi-mode laser diodes

Performance, lifetest data, as well as failure modes from two different device structures will be discussed in this paper, with emitting wavelengths from 780nm to 800nm. The first structure, designed for high temperature operation, has demonstrated good reliability on various packages with output power up to 10W from a 200μm emitting area. The device structure can be operated up to 60°C heatsink temperatures under CW conditions. Then a high efficiency structure is shown with further improvement on operation power and reliability, for room temperature operation. With ongoing lifetest at 12A and 50°C heatsink temperature, <1000 FIT has been achieved for 6.5W and 33°C operation, on both designs. MTT10%F at 10W and 25°C operation is estimated to be more than 20,000 hours. Devices retain more than 20W rollover power under CW conditions, when re-tested after several thousand hours of accelerated lifetest. Paths for reliability improvement will also be discussed based on observed lifetest failure modes from these two structures.

High reliability of high power and high brightness diode lasers

We report on continued progress in the development of high power and high brightness single emitter laser diodes from 790 nm to 980 nm for reliable use in industrial and pumping applications. High performance has been demonstrated in nLIGHT’s diode laser technology in this spectral range with corresponding peak electrical-to-optical power conversion efficiency of ~65%. These pumps have been incorporated into nLIGHT’s fiber-coupled pump module, elementTM. We report the latest updates on performance and reliability of chips and fiber-coupled modules. This paper also includes a new chip design with significantly narrower slow-axis divergence which enables further improved reliable power and brightness. Preliminary reliability assessment data for these devices will be presented here as well.

Reliability of high power/brightness diode lasers emitting from 790 to 980 nm

This paper presents recent progress in the development of high power single emitter laser diodes from 790 nm to 980 nm for reliable use in industrial and pumping applications. High performance has been demonstrated on diode lasers from 790 nm to 980 nm, with corresponding peak efficiency ~65%. Reliability has been fully demonstrated on high power diode lasers of 3.8 mm laser cavity at 3 major wavelengths. We report on the correlation between photon-energy (wavelength) and device failure modes (reliability). A newly released laser design demonstrates diode lasers with 5.0 mm laser cavity at 915-980 nm and 790 nm, with efficiency that matches the values achieved with 3.8 mm cavity length. 915-980 nm single emitters with 5.0 mm laser cavity were especially designed for high power and high brightness applications and can be reliably operated at 12 W to 18 W. These pumps have been incorporated into nLIGHT’s newly developed fiber coupled pump module, elementTM. Ongoing highly accelerated diode life-tests have accumulated over 200,000 raw device hours, with extremely low failure rate observed to date. High reliability has also been demonstrated from multiple accelerated module-level lifetests.

High Performance Diode Lasers Emitting at 780-820 nm

High power 780-820 nm diode lasers have been developed for pumping and material processing systems. This paper presents recent progress in the development of such devices for use in high performance industrial applications. A newly released laser design in this wavelength range demonstrates thermally limited >25W CW power without catastrophic optical mirror damage (COMD), with peak wallplug efficiency ~65%. Ongoing accelerated lifetesting projects a time to 5% failure of ~10 years at 5 and 8 W operating powers for 95 and 200 μm emitter widths, respectively. Preliminary results indicate the presence and competition of a random and wear-out failure mode. Fiber-coupled modules based on arrays of these devices support >100W reliable operation, with a high 56% peak efficiency (ex-fiber) and improved brightness/reliability.

Performance and Reliability of High Power 7xx nm Laser Diodes

High power diode lasers in 7xx-nm region, have been needed for various applications. Compared to 9xx nm lasers that have been developed extensively in the last 20 years, high power lasers at 7xx-nm region presents much more challenges for operation power, efficiency, temperature performance and reliability. This paper will present recent progresses on 7xx nm laser diodes for the above attributes. Two laser designs will be reviewed and high power diode laser performance and reliability will be presented. Single emitter devices, with 200μm wide emitting width, show up to 10W reliable operation power, with peak efficiency more than 65%. Accelerated life testing at 12A, 50°C heatsink temperature has been running for thousands of hours. High temperature performance and high COMD threshold (> 20W) will also be shown. Life-test failure modes will also be discussed. In summary, with advanced epitaxial structure design and MOCVD process, critical facet passivation and advanced heatsink and bonding technology, 7xx-8xx nm devices have been demonstrated with high performance and reliability similar to those of 9xx nm devices.

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.

High-brightness, fiber-coupled pump modules in fiber laser applications

High-power, high-brightness, fiber-coupled pump modules enable high-performance industrial fiber lasers with simple system architectures, multi-kW output powers, excellent beam quality, unsurpassed reliability, and low initial and operating costs. We report commercially available (element™), single-emitter-based, 9xx nm pump sources with powers up to 130 W in a 105 μm fiber and 250 W in a 200 μm fiber. This combination of high power and high brightness translates into improved fiber laser performance, e.g., simultaneously achieving high nonlinear thresholds and excellent beam quality at kW power levels. Wavelength-stabilized, 976 nm versions of these pumps are available for applications requiring minimization of the gain-fiber length (e.g., generation of high-peak-power pulses). Recent prototypes have achieved output powers up to 300 W in a 200 μm fiber. Extensive environmental and life testing at both the chip and module level under accelerated and real-world operating conditions have demonstrated extremely high reliability, with innovative designs having eliminated package-induced-failure mechanisms. Finally, we report integrated Pump Modules that provide < 1.6 kW of fiber-coupled power conveniently formatted for fiber-laser pumping or direct-diode applications; these 19” rack-mountable, 2U units combine the outputs of up to 14 elements™ using fused-fiber combiners, and they include high-efficiency diode drivers and safety sensors.