Pump modules for fiber lasers and fiber-coupled direct diode laser systems require laser diodes with a high beam quality. While in fast axis direction diode lasers exhibit a nearly diffraction limited output beam, the maximum usable output power is usually limited by the slow axis divergence blooming at high power levels. Measures to improve the lateral beam quality are subject of extensive research. Among the many influencing factors are the chip temperature, thermal crosstalk between emitters, thermal lensing, lateral waveguiding and lateral mode structure.

We present results on the improvements of the lateral beam divergence and brightness of gain-guided mini-bars for emission at 976 nm. For efficient fiber coupling into a 200 μm fiber with NA 0.22, the upper limit of the lateral beam parameter product is 15.5 mm mrad. Within the last years, the power level at this beam quality has been improved from 44 W to 52 W for the chips in production, enabling more cost efficient pump modules and laser systems.

Our work towards further improvements of the beam quality focuses on advanced chip designs featuring reduced thermal lensing and mode shaping. Recent R&D results will be presented, showing a further improvement of the beam quality by 15%. Also, results of a chip design with an improved lateral emitter design for highest brightness levels will be shown, yielding in a record high brightness saturation of 4.8 W/mm mrad.

With continuous increase in output power of fiber lasers, small volume, low weight, high electro-optic efficiency and high brightness diode laser pump source has become the trend of development. Using spatial beam combining and polarization beam combining methods, BWT has developed a compact pump laser achieving 600W level out of a fiber of 0.22 NA and 200 μm core diameter. At 12A, electro-optical efficiency is higher than 49%. Its brightness is higher than that of the commercially available 158W pump from BWT.

We report on high efficiency multimode pumps that enable ultra-high efficiency high power ECO Fiber Lasers. We discuss chip and packaged pump design and performance. Peak out-of-fiber power efficiency of ECO Fiber Laser pumps was reported to be as high as 68% and was achieved with passive cooling. For applications that do not require Fiber Lasers with ultimate power efficiency, we have developed passively cooled pumps with out-of-fiber power efficiency greater than 50%, maintained at operating current up to 22A. We report on approaches to diode chip and packaged pump design that possess such performance.

High-power continuous wave (CW) fiber lasers with excellent beam quality continue to drive demand for higher brightness pump modules at 920 nm and 976 nm. Over the last decade, the brightness requirement for pumping state-of-the-art CW fiber lasers (CWFLs) has risen from approximately 0.5 W/(mm-mR)² to ~2 W/(mm-mR)² for today’s mutlikW CWFLs. The most advanced CWFLs demand even higher brightness pump modules in order to minimize design complexity, maximize efficiency, and maximize the stimulated Raman scattering threshold. This need has resulted in a reoptimization of the nLIGHT element^TM line to enable a commercial 200 W, 18-emitter package with a 0.15 NA beam in a 105 μm fiber, corresponding to a brightness of 3.2 W/(mm-mR)² and a 25 % increase in power over the existing element^TM e14 at 155 W. Furthermore, we have demonstrated the further scalability of this reoptimized design with our next generation COS, resulting in a maximum of 272 W into 105 μm fiber with a brightness of 3.8 W/(mm-mR)².

High-brightness and high-efficiency fiber-coupled pump module has been developed with newly designed laser diodes and improved spatial optical system. High-power operation was realized by widening laser stripe width. The optical system of the module consists of only spatial multiplexing, not using polarization or wavelength multiplexing technique. Therefore it has advantages that no power loss at a polarization beam combiner or gratings, low material costs of optics, and high excitation efficiency by single wavelength excitation for a fiber laser. The peak power conversion efficiency of the module is 65.6% at 120 W output power, and its efficiency maintains more than 60% up to 220 W at 19 A driving current, and the maximum output power is 252 W at 23 A, at 25 degrees C heat sink temperature. The fiber outside diameter of the module is conventional 125 μm. Center wavelength of the laser is 915 nm.

DILAS Diode Laser, Inc. continues to improve and optimize high-brightness fiber-laser pump modules. Highlights include a 330W module weighing in at 300 grams, achieving greater than 55% electrical-to-optical efficiency at the operating power from a 225micron/0.22NA fiber and a power-scaled version capable of >600 W, >50% efficiency and weighing in at less than 400 grams. The macro-channel coolers enabling these modules eliminate the need for microchannels and deionized water and reduce pressure drop across the system. A road map to modules with >900W of output power will also be presented.

We present a direct diode laser with an optical output power of more than 800 W ex 100 μm with an NA of 0.17. The system is based on 6 commercial pump modules that are wavelength stabilized by use of VBGs. Dielectric filters are used for coarse and dense wavelength multiplexing. Metal sheet cutting tests were performed in order to prove system performance and reliability. Based on a detailed analysis of loss mechanisms, we show that the design can be easily scaled to output powers in the range of 2 kW and to an optical efficiency of 80%.

High-power, high-brightness diode lasers from 8xx nm to 9xx nm have been pursued in many applications including fiber laser pumping, materials processing, solid-state laser pumping, and consumer electronics manufacturing. In particular, 915 nm - 976 nm diodes are of interest as diode pumps for the kilowatt CW fiber lasers. Thus, there have been many technical efforts on driving the diode lasers to have both high power and high brightness to achieve high-performance and reduced manufacturing costs. This paper presents our continued progress in the development of high brightness fiber-coupled product platform, element^TM. In the past decade, the amount of power coupled into a single 105 μm and 0.15 NA fiber has increased by over a factor of ten through improved diode laser brilliance and the development of techniques for efficiently coupling multiple emitters into a single fiber. In this paper, we demonstrate the further brightness improvement and power-scaling enabled by both the rise in chip brightness/power and the increase in number of chips coupled into a given numerical aperture. We report a new x-REM design with brightness as high as 4.3 W/mm-mrad at a BPP of 3 mm-mrad. We also report the record 272W from a 2×9 element^TM with 105 μm/0.15 NA beam using x-REM diodes and a new product introduction at 200W output power from 105 μm/0.15 NA beam at 915 nm.

The paper reports on the design, manufacturing and preliminary characterization of a new compact and high beam quality multi-emitter laser module able to deliver about 300W in a standard 105/125 fiber with 0.22 numerical aperture but with 95% of power in NA = 0.15. The layout exploits a proprietary architecture and uses a plurality of chips emitting at two different wavelengths combined through spatial, polarization and wavelength multiplexing. Given its small footprint and low ownership costs, the module has been specifically devised as the basic building block of laser sources for metal powder additive manufacturing machines.

In 2007, DILAS proposed the approach to tailor the output beam characteristics of laser diodes to match the required beam quality of a desired target fiber, thus, drastically simplifying the coupling optics to basically only fast and slow axis collimation lenses. Over the last years, we developed and improved this tailored bar (T-Bar) concept together with the tooling for fully automated mass production of fiber-coupled T-Bar modules for fiber laser pumping as well as for direct applications.

We present results on the improvement of T-Bars tailored for optimized coupling into fibers with a diameter of 200 μm with NA 0.22 corresponding to a beam parameter product of 22 mm·mrad. Cost efficient coupling to this fiber requires a tailored beam parameter product smaller than 15.5 mm·mrad in slow axis direction corresponding to a slow axis beam divergence of 7° (full angle, 95% power content) for five 100 μm wide emitters. The improved T-Bars fulfil this requirement up to an output power of 52 W with a brightness of 3.1 W/mm·mrad and a power conversion efficiency achieving 69%. This progress in the T-Bar performance together with modifications in the module design led to the increase of the reliable output power from 135 W in 2009 to 360 W in 2017 for a T-Bar module with one baseplate. We will also give a review of the main development steps and further R and D improvements.

Progress will be presented on ongoing research into the development of ultra-high power and efficiency bars achieving significantly higher output power, conversion efficiency and brightness than currently commercially available. We combine advanced InAlGaAs/GaAs-based epitaxial structures and novel lateral designs, new materials and superior cooling architectures to enable improved performance. Specifically, we present progress in kilowatt-class 10-mm diode laser bars, where recent studies have demonstrated 880 W continuous wave output power from a 10 mm x 4 mm laser diode bar at 850 A of electrical current and 15°C water temperature. This laser achieves < 60% electro-optical efficiency at 880 W CW output power.

Design optimization of single emitter broad stripe 9xx-nm laser diodes was studied to achieve ultimate high power and high efficiency operation for a use in fiber laser pumping and other industrial applications. We tuned laser vertical layer design and stripe width in terms of optical confinement as well as electrical resistance. As a result, newly designed LDs with 4mm-long cavity and 220 μm-wide stripe successfully demonstrate maximum CW output power as high as 33 W and high efficiency operation of more than 60 % PCE even at 27 W output power. In pulse measurement, the maximum output of 68 W was obtained.

Progress in the development of highly brilliant pump modules based on novel high-energy-class diode laser bars is presented, targeting long-pulse (2 ms) application in pumping Yb:CaF₂ amplifier crystals. Bars with long resonators and high fill factors are used for low resistance and efficient cooling. Advanced extreme-double asymmetric epitaxial structures are used for high efficiency at high power. Facet passivation is used for long life at high powers, enabling operation at 600 W per bar (1.2 J). Performance of bars and passively cooled stacks is summarized and prospects for further performance enhancements are reviewed.

Fiber-coupled laser diode modules employ power scaling of single emitters for fiber laser pumping. To this end, techniques such as geometrical, spectral and polarization beam combining (PBC) are used. For PBC, linear polarization with high degree of purity is important, as any non-perfectly polarized light leads to losses and heating. Furthermore, PBC is typically performed in a collimated portion of the beams, which also cancels the angular dependence of the PBC element, e.g., beam-splitter. However, we discovered that single emitters have variable degrees of polarization, which depends both on the operating current and far-field divergence. We present data to show angle-resolved polarization measurements that correlate with the ignition of high-order modes in the slow-axis emission of the emitter. We demonstrate that the ultimate laser brightness includes not only the standard parameters such as power, emitting area and beam divergence, but also the degree of polarization (DoP), which is a strong function of the latter. Improved slow-axis divergence, therefore, contributes not only to high brightness but also high beam combining efficiency through polarization.

High power 9xx nm diode lasers along with MH GoPower’s (MHGP’s) flexible line of Photovoltaic Power Converters (PPCs) are spurring high power applications for power over fiber (PoF), including applications for powering remote sensors and sensors monitoring high voltage equipment, powering high voltage IGBT gate drivers, converters used in RF over Fiber (RFoF) systems, and system power applications, including powering UAVs. In PoF, laser power is transmitted over fiber, and is converted to electricity by photovoltaic cells (packaged into Photovoltaic Power Converters, or PPCs) which efficiently convert the laser light. In this research, we design a high power multi-channel PoF system, incorporating a high power 976 nm diode laser, a cabling system with fiber break detection, and a multichannel PPC-module. We then characterizes system features such as its response time to system commands, the PPC module’s electrical output stability, the PPC-module’s thermal response, the fiber break detection system response, and the diode laser optical output stability. The high power PoF system and this research will serve as a scalable model for those interested in researching, developing, or deploying a high power, voltage isolated, and optically driven power source for high reliability utility, communications, defense, and scientific applications.

In this paper a micro-integrated laser-amplifier for a wavelength of 1180nm is presented. The modules can amplify laser emission from any source, which is coupled into the polarization-maintaining input fiber of the module, to an optical power > 1W. Thereby, the spectral properties of the seed source are maintained. The output of the module is free space allowing easy access to the emitted beam. The footprint of the module is only 47mm • 34 mm. The article discusses the utilized amplifiers, preceding bench top experiments and gives a detailed experimental characterization of the amplifier module.

DILAS has leveraged its industry-leading work in manufacturing low SWaP fiber-coupled modules extending the wavelength range to 793nm for Tm fiber laser pumping. Ideal for medical, industrial and military applications, modules spanning from single emitter-based 9W to TBar-based 200W of 793nm pump power will be discussed. The highlight is a lightweight module capable of <200W of 793nm pump power out of a package weighing < 400 grams. In addition, other modules spanning from single emitter-based 9W to TBar-based 200W of 793nm pump power will be presented. In addition, advances in DPAL modules, emitting at the technologically important wavelengths near 766nm and 780nm, will be detailed. Highlights include a fully microprocessor controlled fiber-coupled module that produces greater than 400W from a 600 micron core fiber and a line width of only 56.3pm. The micro-processor permits the automated center wavelength and line width tuning of the output over a range of output powers while retaining excellent line center and line width stability over time.

An overview is presented on the recent progress in the development of high power laser bars and single emitters emitting at wavelengths around 1060 nm. The development is focused on high reliability, thermal stability and high efficiency of the laser devices.

We report state-of-the-art results for 1180nm (narrow linewidth) laser diodes based on GaInNAs quantum wells and show results for ridge waveguide DBR laser diode including its reliability tests. Manuscript demonstrates 500 mW output power in continuous-wave operation at room temperature, wide single mode tuning region and narrow linewidth operation. Devices reached narrow linewidth operation (>250 kHz) across their operation band.

Low threshold current density (<400 A/cm²) injection lasing in (Al_xGa_1–x)_0.5In_0.5P–GaAs–based diodes down to the green spectral range (<570 nm) is obtained. The epitaxial structures are grown on high–index (611)A and (211)A GaAs substrates by metal–organic vapor phase epitaxy and contain tensile–strained GaP–enriched insertions aimed at preventing escape of the injected nonequilibrium electrons from the active region. Extended waveguide concept results in a vertical beam divergence with a full width at half maximum of 15o for (611)A substrates. The lasing at 569 nm is realized at 85 K. In the orange–red laser diode structure low threshold current density (200 A/cm²) in the orange spectral range (598 nm) is realized at 85 K. The latter devices demonstrate room temperature lasing at 628 nm at ~2 kA/cm² and a total power above 3W. The red laser diodes grown on (211)A substrates demonstrate vertically multimode lasing far field pattern indicating a lower optical confinement factor for the fundamental mode as compared to the devices grown on (611)A. However the temperature stability of the threshold current and the wavelength stability are significantly higher for (211)A–grown structures in agreement with the conduction band modeling data.

Two AlGaAs/GaAs broadened waveguide laser structures, one asymmetric, one nearly symmetric, were designed for high power at about 780 nm. The design concept is based on low losses and higher gain for the fundamental mode with higher losses and lower gain for higher-order modes. To achieve these results, the positions of the quantum wells, thicknesses of the cladding layers, doping profiles, and the compositions of all the layers are carefully chosen. The structures are designed to have a loss of about 0.5/cm for the TE₀ mode and more than 5 /cm for higher order modes for both structures. The asymmetric structure has a lower threshold current density (~750 A/cm²) and a higher slope (about 0.9 W/A) of the light-current curve compared to the symmetric structure. Increased L-I slope for the asymmetric structure results mainly from increased hole injection efficiency because the quantum wells are close to the p-side. Ridge-guide lasers fabricated with the asymmetric structure produced greater than 350 mW at 25°C. The beam divergence of the asymmetric structure was 6° × 14°.

The performance of diode lasers in the visible spectral range has been continuously improved within the last few years, which was mainly driven by the goal to replace arc lamps in cinema or home projectors. In addition, the availability of such high power visible diode lasers also enables new applications in the medical field, but also the usage as pump sources for other solid state lasers. This paper summarizes the latest developments of fiber coupled sources with output power from 1.4 W to 120 W coupled into 100 μm to 400 μm fibers in the spectral range around 405 nm and 640 nm. New developments also include the use of fiber coupled multi single emitter arrays at 450 nm, as well as very compact modules with multi-W output power.

Coherent beam combining (CBC) aims at increasing the spatial brightness of lasers. It consists in maintaining a constant phase relationship between different emitters, in order to combine them constructively in one single beam. We have investigated the CBC of an array of five individually-addressable high-power tapered laser diodes at λ = 976 nm, in two architectures: the first one utilizes the self-organization of the lasers in an interferometric extended-cavity, which ensures their mutual coherence; the second one relies on the injection of the emitters by a single-frequency laser diode. In both cases, the coherent combining of the phase-locked beams is ensured on the front side of the array by a transmission diffractive grating with 98% efficiency.

The passive phase-locking of the laser bar is obtained up to 5 A (per emitter). An optimization algorithm is implemented to find the proper currents in the five ridge sections that ensured the maximum combined power on the front side. Under these conditions we achieve a maximum combined power of 7.5 W.

In the active MOPA configuration, we can increase the currents in the tapered sections up to 6 A and get a combined power of 11.5 W, corresponding to a combining efficiency of 76%. It is limited by the beam quality of the tapered emitters and by fast phase fluctuations between emitters. Still, these results confirm the potential of CBC approaches with tapered lasers to provide a high-power and high-brightness beam, and compare with the current state-of-the-art with laser diodes.

The catastrophic degradation of high power lasers depends on both external factors, associated with the technological processes followed to fabricate the laser, and also on intrinsic aspects related to the materials forming the laser structure, more specifically the active zone composed by the QW, guide layers and claddings. The materials properties: optical, thermal and mechanical, play a paramount role in the degradation of the laser. We analyse here how these properties have an impact on the mechanisms responsible for the catastrophic degradation.

We numerically investigate the effect of phase-amplitude coupling modulation on power spectra in semiconductor lasers subject to optical injection in a face to face configuration, when a non-negligible injection delay time is taken into account. We find that as phase-amplitude coupling factor α varies, the system goes through a sequence of phase transactions between In-phase locking states to anti-phase locking states via phase-flip bifurcation. Moreover, we observed the signature of frequency discretization (Frequency Island) and uncovered the physical mechanism for the existence of multi-stability near the phase transitions regimes. Within the windows between successive anti-phases to in-phase locking regions, optical injection induced modulation in alpha, unveiling a remarkable universal feature in the various dynamics of coupled lasers system which could be useful in controlling the chirp or pulse repetition rate of a photonic integrated compact device with the aid of phase control.

Monolithic spectral stabilization is demonstrated in narrow-stripe broad-area lasers (NBA) with high power (5W), conversion efficiency (50%) and high brightness, by using optimized high-order surface-etched DFB gratings. However, surface etched gratings introduce a high index contrast into the semiconductor, leading to the scattering losses increasing rapidly with groove etch depth, limiting efficiency and yield. We therefore review progress in the exploitation of novel, non-uniform grating configurations for improved performance. Devices with non-uniform gratings whose groove etch depth decreases toward the front facet (apodized grating) are shown to operate with enhanced spectrally stable power (6W) compared to devices with uniform gratings.

High-power single-mode (SM) and multi-mode (MM) InGaAs-AlGaAs strained quantum well (QW) lasers are critical components for both telecommunications and space satellite communications systems. However, little has been reported on failure modes and degradation mechanisms of high-power SM and MM InGaAs-AlGaAs strained QW lasers although it is crucial to understand failure modes and underlying degradation mechanisms in developing these lasers that meet lifetime requirements for space satellite systems, where extremely high reliability of these lasers is required. Our present study addresses the aforementioned issues by performing long-term life-tests followed by failure mode analysis (FMA) and physics of failure investigation. We performed long-term accelerated life-tests on state-of-the-art SM and MM InGaAs-AlGaAs strained QW lasers under ACC (automatic current control) mode. Our life-tests have accumulated over 25,000 test hours for SM lasers and over 35,000 test hours for MM lasers. FMA was performed on failed SM lasers using electron beam induced current (EBIC). This technique allowed us to identify failure types by observing dark line defects. All the SM failures we studied showed catastrophic and sudden degradation and all of these failures were bulk failures. Our group previously reported that bulk failure or COBD (catastrophic optical bulk damage) is the dominant failure mode of MM InGaAs-AlGaAs strained QW lasers. Since degradation mechanisms responsible for COBD are still not well understood, we also employed other techniques including focused ion beam (FIB) processing and high-resolution TEM to further study dark line defects and dislocations in post-aged lasers. Our long-term life-test results and FMA results are reported.

Tilted Wave Laser diode emitters represent a promising approach for high power laser diodes. These devices, manufactured by VI Systems, are broad area devices that make use of an effective large vertical cavity to reduces the overall power density delaying the onset of thermal rollover and COD effects. A challenge of the TWL approach is a dual-beam emission, each single-mode in the vertical dimension. This paper addresses resent efforts to efficiently coherently beam combine these two beams into one single-mode beam having what is quite likely world class brightness, power, and conversion efficiency for a broad area diode array.

High Power Diode Laser (HPDL) systems with short focal length fast-axis collimators (FAC) require submicron assembly precision. Conventional FAC-Lens assembly processes require adhesive gaps of 50 microns or more in order to compensate for component tolerances (e.g. deviation of back focal length) and previous assembly steps.

In order to control volumetric shrinkage of fast-curing UV-adhesives shrinkage compensation is mandatory. The novel approach described in this paper aims to minimize the impact of volumetric shrinkage due to the adhesive gap between HPDL edge emitters and FAC-Lens. Firstly, the FAC is actively aligned to the edge emitter without adhesives or bottom tab. The relative position and orientation of FAC to emitter are measured and stored.

Consecutively, an individual subassembly of FAC and bottom tab is assembled on Fraunhofer IPT’s mounting station with a precision of ±1 micron.

Translational and lateral offsets can be compensated, so that a narrow and uniform glue gap for the consecutive bonding process of bottom tab to heatsink applies (Figure 4). Accordingly, FAC and bottom tab are mounted to the heatsink without major shrinkage compensation.

Fraunhofer IPT’s department assembly of optical systems and automation has made several publications regarding active alignment of FAC lenses [SPIE LASE 8241-12], volumetric shrinkage compensation [SPIE LASE 9730-28] and FAC on bottom tab assembly [SPIE LASE 9727-31] in automated production environments. The approach described in this paper combines these and is the logical continuation of that work towards higher quality of HPDLs.

Active imaging based on laser illumination is used in various fields such as medicine, security, defense, civil engineering and in the automotive sector. In this last domain, research and development to bring autonomous vehicles on the roads has been intensified these last years with an emphasis on lidar technology that is probably the key to achieve full automation level. Based on time-of-flight measurements, the profile of objects can be measured together with their location in various conditions, creating a 3D mapping of the environment. To be embedded on a vehicle as advanced driver assistance systems (ADAS), these sensors require compactness, low-cost and reliability, as it is provided by a flash lidar. An attractive candidate, especially with respect to cost reduction, for the laser source integrated in these devices is certainly laser diodes as long as they can provide sufficiently short pulses with a high energy.

A recent breakthrough in laser diode and diode driver technology made by Quantel (Les Ulis, France) now allows laser emission higher than 1 mJ with pulses as short as 12 ns in a footprint of 4x5 cm² (including both the laser diode and driver) and an electrical-to-optical conversion efficiency of the whole laser diode source higher than 25% at this level of energy. The components used for the laser source presented here can all be manufactured at low cost. In particular, instead of having several individual laser diodes positioned side by side, the laser diodes are monolithically integrated on a single semiconductor chip. The chips are then integrated directly on the driver board in a single assembly step. These laser sources emit in the range of 800-1000 nm and their emission is considered to be eye safe when taking into account the high divergence of the output beam and the aperture of possible macro lenses so that they can be used for end consumer applications. Experimental characterization of these state-of-the-art pulsed laser diode sources will be given. Future work leads will be discussed for miniaturization of the laser diode and drastic cost reduction.

We describe a diode laser system for precision metrology that relies on adaptations of a well-known design based on optical feedback from an interference filter. The laser head operates with an interchangeable base-plate, which allows for single-mode performance at distinct wavelengths of 633 nm and 780 nm. Frequency drifts are effectively suppressed by using a vacuum-sealed laser head, thereby allowing the laser frequency to be stabilized on time-scales of several hours. Using a digital auto-lock controller, we show that the laser frequency can be stabilized with respect to selected iodine and rubidium spectral lines. The controller can be programmed to use a pattern-matching algorithm or generate first- and third-derivative error signals for peak locking. Beat note characterization has demonstrated a short-term linewidth of ~2 MHz and an Allan deviation of 3.5 × 10^-9 for a measurement time τ = 500 s. The laser characteristics have also enabled high-precision gravity measurements with accuracies of a few parts-per-billion (ppb).

We present recent development of single lateral mode 1050 nm laser bars. The devices are based on an InGaAs/AlGaAs single quantum well and an asymmetric large optical cavity waveguide structure. By optimizing the AlGaAs composition, doping profiles, and QW thickness, the low internal loss of 0.5 cm^-1 and high internal quantum efficiency of 98% are obtained. A standard bar (10% fill factor; 4mm cavity length) reaches 72% peak electro-optical efficiency and 1.0 W/A slope efficiency at 25°C. To achieve high single lateral mode power, the current confinement and optical loss profile in lateral direction are carefully designed and optimized to suppress higher order lateral modes. We demonstrate 1.5W single lateral mode power per emitter from a 19-emitter 10mm bar at 25°C. High electro-optical efficiency are also demonstrated at 25°C from two separate full-bar geometries on conduction cooled packaging: 20 W with <50% electro-optical efficiency from a 19-emitter bar and 50 W with <45% electro-optical efficiency from a 50-emitter bar.