R. Pathak

Higher brightness laser diodes with smaller slow axis divergence

The slow axis (SA) divergence of 20% fill-factor, 980nm, laser diodes (LDs) have been investigated under short pulsed (SP) and continuous (CW) operation. By analyzing the data collected under these two modes of operation, one finds that the SA divergence can be separated into two components: an intrinsic divergence and a thermally induced divergence. At low injected current and power, the intrinsic SA divergence is dominant while at high power their magnitudes are approximately equal. The thermal gradient across the broad stripe is negligible under SP operation and, the SA divergence increased at a much slower rate as a function of injected current, thereby increasing the brightness of the LD by 2X. SRL has redesigned microchannel coolers that remove the thermal gradient under CW operation thereby eliminating the thermally induced SA divergence resulting in LDs that are 2X brighter at 300W/bar.

Record-brightness laser-diode bars for fiber coupling

High brightness, laser-diode bars are required for efficient coupling into small-core optical-fibers. Record power and brightness results were achieved using 20% fill-factor, 980nm, 1cm-wide, 4mm cavity-length bars. Lifetimes of single bars, operated CW at 200W and 20°C, exceed 1000hr. Due to superb thermal management, the power conversion efficiency (PCE) exceeds 60% at 200W output power. Similar lifetime and PCE were obtained for a 3-bar stack emitting 600W output power.

Al-glass kW fibre laser end-pumped by MCCP-cooled diode stacks

High power industrial fibre lasers are typically pumped by single emitter diodes, with pump power aggregation and the fibre laser cavity being achieved in a monolithic “all-bibre” architecture comprising fused fiber bundles, fiber Bragg grating reflectors and numerous splices. [1]. The gain fiber utilizes a low index polymer coating to provide the wave-guiding for the multimode pump as well as for compatibility with the NA increase (typically 0.22–0.45) which occurs in the fused taper combiners. While this all fibre approach has been shown to be viable, it is not trivial to implement at power levels in excess of several hundred watts Issues include polymer coating degradation, transverse mode-coupling induced instability at splices or FBGs, grating walk-off, and modal instability, [2]. The latter issue arises because these fiber laser designs are focused on single-transverse-mode operation, [3.4], even though the fibres themselves are multimode to avoid nonlinear impairments. This is despite the face that most cutting and welding applications actually utilize a multimode fibre for delivery to the cutting head. The BBP of such systems is typically 2.5mm.mRad at a wavelength around 1080nm. However, single mode operation allows power scaling by incoherently combining several lower power fiber lasers into a single beam with that BPP.

915 nm laser bar-based high-performance sources for fiber laser pumping

Fiber lasers have made significant progress in terms of power output, beam quality and operational robustness over the past few years. Key to this progress has been advances in two technologies - fiber technology and 9xx nm diode laser pump technology based on single emitters. We present the operational characteristics of our new high brightness 9xx nm fiber laser pump sources based on diode laser bars and diode laser bar arrays and discuss the design trade offs involved for realization of devices focused on this application. These trade offs include achieving the lowest slow axis divergence while maintaining high wall plug efficiency and minimizing facet power density to maximize reliability.

Wavelength-stabilized fiber-coupled diode laser with 500 W output and 20 mm•mrad beam quality

We report the development of a fiber-coupled diode laser module with high spatial and spectral brightness. Four arrays of diode laser bars are multiplexed using polarization and narrow-band wavelength combination. The module achieves 500 W of output power from a 200 μm, 0.2 NA fiber. The output spectrum, composed of contributions from more than 150 emitters, is narrowed using VBGs and has nearly 100% content within +/- 1.5 nm of 975 nm at full power.

Investigation of n-side cooling in regards to bar geometry and packaging style of diode laser

The packaging of high power diode laser bars requires a high cooling efficiency and long-term stability. Due to the increasing output power of the diode laser bars the cooling performance of the packaging becomes more important. Nowadays micro channel heat sinks seem to be the most efficient cooling concept in regard to high power applications. The active area of the p-side down mounted laser bar is located directly above the micro channels. In other applications where conductive cooled heat sinks are used the bars are mounted on copper CS mount, CuW submount or high performance materials. All these packaging ideas use wire bonds or thin copper sheets as a n-contacts. The thermal advantage of these contacts can be neglected. N-contact cooling is typically used to achieve new records of optical output power in the labs. These studies analyze the properties of an additional n-contact cooling. The cooling performance of a package cooled on both sides can be improved by more than 20% when compared with typical wire bonds or metal sheets. Different packaging styles with metal sheets, heat spreaders (expansion matched) and active n-side cooling are investigated. The effect of n-side cooling with regards to the fill-factor and cavity length is analyzed also. The first part of this paper approaches the topic theoretically. Simulations are carried out and show the advantages and differences of different package styles in comparison to bar geometries variations. The second part of the studies characterizes and analyses fabricated samples made out of copper in view of cooling performance, handling, and induced stress. The results of different bar geometries and packaging styles are compared and guidelines for n-side cooling are developed.

Progress in high-brightness diode laser development based on tailored diode laser bars

The state-of-the-art beam quality from high-brightness, fiber-coupled diode laser modules has been significantly improved in the last few years, with commercially available modules now rivaling the brightness of lamp-pumped Nd:YAG lasers. We report progress in the development of these systems for a variety of applications, such as material processing and pumping of solid state and fiber lasers. Experimental data and simulation results for wavelength stabilized outputs from 200 µm diameter fibers at 975 nm for power levels greater than 200 W will be presented. The enabling technology in these products is supported by key developments in tailored diode laser bars with low slow axis divergence, micro-optics, diode laser packaging, and modular architecture.

Material survey for packaging semiconductor diode lasers

We present results from a survey of materials used for packaging semiconductor lasers, including Cu, CuW, BeO, diamond composite and other advanced materials. We present the results of residual bonding stress from various solders and consider the direct effects on wavelength and spectral width. We also provide data on the second order effects of threshold current and slow axis divergence. Additionally, we consider the heat spreading through different materials for a laser bar and present modeled and experimental data on the thermal performance. Finally, we consider the reliability under on-off life-testing and thermal cycling tests.

Improved long wavelength 14xx and 19xx nm InGaAsp/InP lasers

We report on our progress developing long wavelength high power laser diodes based on the InGaAsP/InP alloy system emitting in the range from 1400 to 2010 nm. Output power levels exceeding 50 Watts CW and 40% conversion efficiency were obtained at 1470 nm wavelength from 20% fill factor (FF) bars with 2 mm cavity length mounted on water cooled plates. Using these stackable plates we built a water cooled stack with 8 bars, successfully demonstrating 400 W at 1470 nm with good reliability. In all cases the maximum conversion efficiency was greater than 40% and the maximum power achievable was limited by thermal rollover. For lasers emitting in the range from 1930 to 2010 nm we achieved output power levels over 15 W and 20 % conversion efficiency from 20% FF bars with 2 mm cavity length on a conductively cooled platform. Life testing of the 1470 nm lasers bars over 14,000 hours under constant current mode has shown no significant degradation.

High power and high efficiency 14xx-nm wavelength Fabry-Perot lasers

We report on the performance of our high power and high efficiency 14xx nm lasers in different formats as packaged on conduction cooled packages using soft solder. Single emitters exhibited output powers as high as 6 watts, while six emitter minibars output 20 W, and 20% fill factor (ff) bars provided over 40 W of output power. In all cases the maximum conversion efficiency was greater than 40% and the maximum power achievable was limited by thermal rollover. These same 20% ff bars output close to 90 W when operated quasi CW (QCW). Preliminary life testing of these bars for over 5000 hours under constant current mode has shown no significant degradation.