Robert J. Martinsen

100-W 105-μm 0.15NA fiber coupled laser diode module

We report on the development of a high brightness laser diode module capable of coupling over 100W of optical power into a 105 μm 0.15 NA fiber at 976 nm. This module, based on nLIGHTs Pearl product architecture, utilizes hard soldered single emitters packaged into a compact and passively-cooled package. In this system each diode is individually collimated in the fast and slow axes and free-space coupled into a single fiber. The high brightness module has an optical excitation under 0.13 NA, is virtually free of cladding modes, and has an electrical to optical efficiency greater than 40%. Additionally, this module is compatible with high power 7:1 fused fiber combiners, and initial experiments demonstrated 500W coupled into a 220 μm, 0.22 NA fiber. These modules address the need in the market for higher brightness diode lasers for pumping fiber lasers and direct material processing.

Control of optical mode distribution through etched microstructures for improved broad area laser performance

Etching microstructures into broad area diode lasers is found to lead to more uniform near field and increased power conversion efficiency, arising from increased slope. Self-consistent device simulation indicates that this improvement is due to an increase in the effective internal injection efficiency above threshold—the nonuniform near field leads to regions of inefficient clamping of the carrier density in the laser stripe. Measurements of spontaneous emission through the substrate confirm the predicted carrier profile. Both experiment and theory show that improved overlap between carrier and power distributions correlates with improved slope.

High reliability and high performance of 9xx-nm single emitter laser diodes

Improved performance and reliability of 9xx nm single emitter laser diodes are presented. To date, over 15,000 hours of accelerated multi-cell lifetest reliability data has been collected, with drive currents from 14A to 18A and junction temperatures ranging from 60°C to 110°C. Out of 208 devices, 14 failures have been observed so far. Using established accelerated lifetest analysis techniques, the effects of temperature and power acceleration are assessed. The Mean Time to Failure (MTTF) is determined to be >30 years, for use condition 10W and junction temperature 353K (80°C), with 90% statistical confidence.

High-brightness fiber-coupled pump laser development

We report on the continued development of high brightness laser diode modules at nLIGHT Photonics. These modules, based on nLIGHTs PearlTM product platform, demonstrate excellence in output power, brightness, wavelength stabilization, and long wavelength performance. This system, based on 14 single emitters, is designed to couple diode laser light into a 105 μm fiber at an excitation NA of under 0.14. We demonstrate over 100W of optical power at 9xx nm with a diode brightness exceeding 20 MW/cm2-str with an operating efficiency of approximately 50%. Additional results show over 70W of optical coupled at 8xx nm. Record brilliance at wavelengths 14xx nm and longer will also be demonstrated, with over 15 W of optical power with a beam quality of 7.5 mm-mrad. These results of high brightness, high efficiency, and wavelength stabilization demonstrate the pump technology required for next generation solid state and fiber lasers.

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.

A CTE matched, hard solder, passively cooled laser diode package combined with nXLT facet passivation enables high power, high reliability operation

A conductively cooled laser diode package design with hard AuSn solder and CTE matched sub mount is presented. We discuss how this platform eliminates the failure mechanisms associated with indium solder. We present the problem of catastrophic optical mirror damage (COMD) and show that nLights nXLTTM facet passivation technology effectively eliminates facet defect initiated COMD as a failure mechanism for both single emitter and bar format laser diodes. By combining these technologies we have developed a product that has high reliability at high powers, even at increased operation temperatures. We present early results from on-going accelerated life testing of this configuration that suggests an 808nm, 30% fill factor device will have a MTTF of more than 21khrs at 60W CW, 25°C operating conditions and a MTTF of more than 6.4khrs when operated under hard pulsed (1 second on, 1 second off) conditions.

KW-class industrial diode lasers comprised of single emitters

Direct semiconductor diode laser-based systems have emerged as the preferred tool to address a wide range of material processing, solid-state and fiber laser pumping, and various military applications. We present an architectural framework and prototype results for kW-class laser tools based on single emitters that addresses a range of output powers (500W to multiple kW) and beam parameter products (20 to 100 mm-mrad) in a system with an operating efficiency near 50%. nLIGHT uses a variety of building blocks for these systems: a 100W, 105um, 0.14 NA pump module at 9xx nm; a 600W, 30 mm-mrad single wavelength, single polarization building block source; and a 140 W 20 mm-mrad low-cost module. The building block is selected to realize the brightness and cost targets necessary for the application. We also show how efficiency and reliability can be engineered to minimize operating and service costs while maximizing system up-time. Additionally we show the flexibility of this system by demonstrating systems at 8xx, 9xx, and 15xx nm. Finally, we investigate the diode reliability, FIT rate requirements, and package impact on system reliability.

Enhanced microchannel cooling for high-power semiconductor diode lasers

The power consumption of semiconductor diode laser bars has continually increased in recent years while the heat transfer area for rejecting the associated thermal energy has decreased. As a result, the generated heat fluxes have become more intense making the thermal management of the laser systems more complicated. A common solution to this problem is to use the microchannel cooler, a small liquid enhanced heat sink capable of rejecting heat fluxes higher than those of finned air sinks of comparable size. The objective of this study is to improve and enhance heat transfer through an existing microchannel cooler using the computational fluid dynamics technique. A commercial software package is used to simulate fluid flow and heat transfer through the existing microchannel cooler, as well as to improve its designs. Three alternate microchannel designs are explored, all with hydraulic diameters on the order of 300 microns. The resulting temperature profiles within the microchannel cooler are analyzed for the three designs, and both the heat transfer and pressure drop performances are compared. The optimal microchannel cooler is found to have a thermal resistance of about 0.07°C-cm2/W and a pressure drop of less than half of a bar.

Laser-based SXGA reflective light valve projector with E-cinema quality contrast and color space

Laser light sources present many advantages for projection displays over the currently employed incoherent light sources. Perhaps the most significant attribute is the lasers high degree of polarization, which greatly improves the efficiency of liquid crystal light valve (LCLV) projectors. The maximum achievable efficiency of an LCLV projector is severely limited with the use of an unpolarized light source such as an arc lamp. The polarized emission from a laser can be coupled to the screen much more efficiently, offering the possibility of smaller projectors with higher luminous efficacies. Additionally, the RGB primaries of laser light fall along the spectrum locus of the chromaticity diagram allowing for a much expanded color gamut over dichroically-separated lamp spectra. This provides the possibility of offering unprecedented color reproduction for the emerging digital cinema industry. The combined properties of polarization, monochromaticity, and low divergence result in a significant increase in image contrast when coupled to LCLV image engines. Substituting lasers for lamp light sources have shown to increase sequential contrast by as much as five-fold. This simple substitution has also resulted in broad improvements to the projectors entire MTF, thereby increasing the apparent resolution of the image. These are all striking arguments as to the potential of lasers in the emerging e-cinema market and the impetuous behind our current development effort presented here.

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.