High Wall-plug Efficiency Research Articles

Negative ion source development for a photoneutralization based neutral beam system for future fusion reactors

In parallel to the developments dedicated to the ITER neutral beam (NB) system, CEA-IRFM with laboratories in France and Switzerland are studying the feasibility of a new generation of NB system able to provide heating and current drive for the future DEMOnstration fusion reactor. For the steady-state scenario, the NB system will have to provide a high NB power level with a high wall-plug efficiency (η ∼ 60%). Neutralization of the energetic negative ions by photodetachment (so called photoneutralization), if feasible, appears to be the ideal solution to meet these performances, in the sense that it could offer a high beam neutralization rate (>80%) and a wall-plug efficiency higher than 60%. The main challenge of this new injector concept is the achievement of a very high power photon flux which could be provided by 3 MW Fabry–Perot optical cavities implanted along the 1 MeV D− beam in the neutralizer stage. The beamline topology is tall and narrow to provide laminar ion beam sheets, which will be entirely illuminated by the intra-cavity photon beams propagating along the vertical axis. The paper describes the present R&D (experiments and modelling) addressing the development of a new ion source concept (Cybele source) which is based on a magnetized plasma column. Parametric studies of the source are performed using Langmuir probes in order to characterize and compare the plasma parameters in the source column with different plasma generators, such as filamented cathodes, radio-frequency driver and a helicon antenna specifically developed at SPC-EPFL satisfying the requirements for the Cybele (axial magnetic field of 10 mT, source operating pressure: 0.3 Pa in hydrogen or deuterium). The paper compares the performances of the three plasma generators. It is shown that the helicon plasma generator is a very promising candidate to provide an intense and uniform negative ion beam sheet.

Artificial Neuron Based on Integrated Semiconductor Quantum Dot Mode-Locked Lasers.

Neuro-inspired implementations have attracted strong interest as a power efficient and robust alternative to the digital model of computation with a broad range of applications. Especially, neuro-mimetic systems able to produce and process spike-encoding schemes can offer merits like high noise-resiliency and increased computational efficiency. Towards this direction, integrated photonics can be an auspicious platform due to its multi-GHz bandwidth, its high wall-plug efficiency and the strong similarity of its dynamics under excitation with biological spiking neurons. Here, we propose an integrated all-optical neuron based on an InAs/InGaAs semiconductor quantum-dot passively mode-locked laser. The multi-band emission capabilities of these lasers allows, through waveband switching, the emulation of the excitation and inhibition modes of operation. Frequency-response effects, similar to biological neural circuits, are observed just as in a typical two-section excitable laser. The demonstrated optical building block can pave the way for high-speed photonic integrated systems able to address tasks ranging from pattern recognition to cognitive spectrum management and multi-sensory data processing.

Novel Measurement Technique for Time Dependant Intensity Variations of AC-LEDs

LEDs have surpassed incandescent and compact florescent lamps for general lighting during the last 10 years.1 Nowadays, they show very high wall plug efficiency while providing a good colour rendering index (CRI). However, flicker is still a perceivable drawback to these light sources.2 The impact and perception of flicker is increasingly investigated and its consequences for human beings range from headaches over migraines to severe neurological problems, including epileptic seizure.3 Evaluating these effects requires a number of factors to be taken into account, for instance duration of exposure, stimulated area of the retina and brightness and colour of the flickering LED source. Flicker is defined as “the rapid variation in light source intensity”.4 To describe the flicker of a light source, two metrics are currently used, flicker percentage and flicker index. Both are mathematical approaches which use the overall emission intensity and compare the minimum and the maximum intensity within a single cycle or curve integrals to gain a key figure. They do not take the wavelength, the frequency, or the duty cycle into account. The presented, novel measurement technique processes the spectrally resolved emission intensity during an adjustable amount of duty cycles, shapes and frequencies. In order to verify this measurement technique, standard phosphors such as Y3Al5O12:Ce3+ (YAG:Ce), BaMgAl10O17:Eu2+,Mn2+(BAM:Eu,Mn) have been investigated with a rectangular pulsed 50 or 100 Hz LED as excitation source. YAG:Ce showed strong flicker while BAM:Eu,Mn exhibits a flicker reducing effect. The spectrally resolved measurement technique allows us to correlate the measurement data with the eye sensitivity curve resulting in a new kind of flicker index.

Development, testing, and initial space qualification of 1.5-μm, high-power (6 W), pulse-position-modulated fiber laser transmitter for deep-space laser communication

We report on the development, testing, and initial space qualification of a 1.5-μm, high-power (6 W), high wall-plug efficiency (∼15%), pulse-position-modulated (PPM), polarization-maintaining, fiber laser transmitter subsystem for deep-space laser communication links. Programmable high-order PPM modulation up to PPM-128 formats, with discrete pulse slots ranging from 0.5 to 8 ns, satisfies variety of link requirements for deep-space laser communication to Mars, asteroids, and other deep-space relay links, as per the National Aeronautics and Space Administration’s space laser communication roadmap. We also present initial space qualification results from thermal-vacuum tests, vibration testing, radiation testing, and an overall reliability assessment.

3D integrated hybrid silicon laser.

Lasers were realized on silicon by flip-chip bonding of indium phosphide (InP) devices containing total internal reflection turning mirrors for surface emission. Light is coupled to the silicon waveguides through surface grating couplers. With this technique, InP lasers were integrated on silicon. Laser cavities were also formed by coupling InP reflective semiconductor optical amplifiers to microring resonator filters and distributed Bragg reflector mirrors. Single-mode continuous wave lasing was demonstrated with a side mode suppression ratio of 30 dB. Up to 2 mW of optical power was coupled to the silicon waveguide. Thermal simulations were also performed to evaluate the low thermal impedance afforded by this architecture and potential for high wall-plug efficiency.

High Wall-Plug Efficiency 1060 nm High Power Semiconductor Laser

By analyzing the factors which affect the wall-plug efficiency of semiconductor Laser Diodes (LDs), a high efficiency 1060 nm LD was designed, including active region, waveguide layers, and cladding layers. The simulation result shows that the component of In in InGaAs in the active region cannot be too small, otherwise the thickness of InGaAs active layer will exceed the critical thickness, meanwhile the asymmetric large optical cavity can decrease the cavity loss effectively. The epitaxial structure was grown by MOCVD, experimental results of varying cavity length showed that the internal quantum efficiency reached 98.57%, and the cavity loss was only 0.273 cm?1. Devices with 4 mm-cavity-length and 100 μm-strip-width were fabricated, 47.4% wall-plug efficiency was reached under QCW pulse condition at room temperature, and the peak wavelength was 1059.4 nm.

Erratum to: Modeling of micro-scale fiber laser hardening process and optimization via statistical approximation of the engineering models

Laser transformation hardening is effective technique used for selective hardening of components such as turbine blade, camshafts and gears. Laser hardening provides benefits over other selective hardening processes in terms of thermal distortion, control of process and appearance of component. The fiber laser is relatively recent development. The single/low mode fiber lasers have good beam quality, high wall plug efficiency, fiber delivery and powers up to few hundred watts. The beam quality of single mode fiber lasers enable it to be focused to a spot size of few tens of mm which can yield hardened tracks of 100 to 500 mm. These fiber lasers can be extremely useful in localized micro-scale surface hardening to create hardened patterns for improving the wear resistance. This paper is focused on developing analytical thermal model of moving heat sources and integrating it with kinetic hardening model to capture the metallurgical changes induced by fiber lasers. An ideal surface hardening technique should give widest hardened track at minimum case depth. To address this issue, an optimization methodology based on statistical approximation of the physics-based engineering models has been developed.

Stack and dump: Peak-power scaling by coherent pulse addition in passive cavities

During the last decades femtosecond lasers have proven their vast benefit in both scientific and technological tasks. Nevertheless, one laser feature bearing the tremendous potential for high-field applications, delivering extremely high peak and average powers simultaneously, is still not accessible. This is the performance regime several upcoming applications such as laser particle acceleration require, and therefore, challenge laser technology to the fullest. On the one hand, some state-of-the-art canonical bulk amplifier systems provide pulse peak powers in the range of multi-terawatt to petawatt. On the other hand, concepts for advanced solid-state-lasers, specifically thin disk, slab or fiber systems have shown their capability of emitting high average powers in the kilowatt range with a high wall-plug-efficiency while maintaining an excellent spatial and temporal quality of the output beam. In this article, a brief introduction to a concept for a compact laser system capable of simultaneously providing high peak and average powers all along with a high wall-plug efficiency will be given. The concept relies on the stacking of a pulse train emitted from a high-repetitive femtosecond laser system in a passive enhancement cavity, also referred to as temporal coherent combining. In this manner, the repetition rate is decreased in favor of a pulse energy enhancement by the same factor while the average power is almost preserved. The key challenge of this concept is a fast, purely reflective switching element that allows for the dumping of the enhanced pulse out of the cavity. Addressing this challenge could, for the first time, allow for the highly efficient extraction of joule-class pulses at megawatt average power levels and thus lead to a whole new area of applications for ultra-fast laser systems.

Highly efficient chip-scale III-V/silicon hybrid optical amplifiers.

We discuss the design and demonstration of highly efficient 1.55 µm hybrid III-V/Silicon semiconductor optical amplifiers (SOA). The optimized III-V wafer stack consists of Al(0.10)In(0.71)Ga(0.18)As multiple quantum wells (MQW) and Al(0.48)In(0.52)As electron stop layers to realize SOAs with high wall-plug efficiency (WPE). We present various designs and experimentally determine WPE values for 2 mW and 0.1 mW input power amplification. The 400 µm long flared SOA achieved the highest WPE value of 12.1% for output power > 10mW and the 400 µm long straight SOA achieved the highest WPE value of 7.3% for output power < 10mW. These are the highest WPE values ever obtained for 1.55 µm SOAs.

High-Speed VCSELs With Strong Confinement of Optical Fields and Carriers

We present the design, fabrication, and performance of our latest generation high-speed oxide-confined 850-nm vertical-cavity surface-emitting lasers. Excellent high-speed properties are obtained by strong confinement of optical fields and carriers. High-speed modulation is facilitated by using the shortest possible cavity length of one half wavelength and placing oxide apertures close to the active region to efficiently confine charge carriers. The resulting strong current confinement boosts internal quantum efficiency, leading to low threshold currents, high wall-plug efficiency, and state-of-the-art high-speed properties at low bias currents. The temperature dependent static and dynamic performance is analyzed by current-power-voltage and small-signal modulation measurements.

Design and comparative study of lateral and vertical LEDs with graphene as current spreading layer

This study analyzes the current spreading effect of graphene on lateral and vertical light emitting diodes (LEDs). We observe an improvement in uniformity of current distribution, light output power and wall-plug efficiency in lateral LEDs (L-LEDs) with graphene current spreading layer (CSL) as compared to those with indium tin oxide (ITO) CSL. From the results we conclude that graphene CSL may be better alternative to ITO CSL. We further carried out a comparative study of lateral and vertical LEDs with graphene CSL. We observe 17% higher light output power, 16% higher wall-plug efficiency and 62% lower series resistance in the case of V-LEDs with graphene CSL when compared to that of L-LEDs with a graphene CSL. Reasons behind these results have been discussed.

High power, diode laser with an asymmetric hetero-waveguide

By introducing an asymmetric hetero-waveguide into the epitaxial structure of a diode laser, a output is achieved at a wavelength of . A different design in p- and n-confinement, based on optimisation of energy bands, is used to reduce voltage loss and meet the requirement of high power and high wall-plug efficiency. A diode laser with a single quantum well and asymmetric hetero-structure waveguide is fabricated and analysed. Measurement results show that the asymmetric hetero-structure waveguide can be efficiently used for reducing voltage loss and improving the confinement of injection carriers and wall-plug efficiency.

High optical power and low‐efficiency droop blue light‐emitting diodes using compositionally step‐graded InGaN barrier

A compositionally step-graded (CSG) InGaN barrier is designed for the active region of c-plane blue light-emitting diodes (LEDs). High external quantum efficiencies of 45, 42, 39 and 36% are achieved at current densities of 100, 200, 300 and 400 A/cm2, respectively. Compared with GaN barrier devices, LEDs with CSG InGaN barriers showed higher power, lower operating voltage and smaller wavelength blueshift, compared with GaN barrier LEDs. Owing to the low-voltage performance, higher wall-plug efficiency can be achieved for blue LEDs with CSG InGaN barriers.

Demonstration of 12.2% wall plug efficiency in uncooled single mode external-cavity tunable Si/III-V hybrid laser.

A Si/III-V hybrid laser has been a highly sought after device for energy-efficient and cost-effective high-speed silicon photonics communication. We present a high wall-plug efficiency external-cavity hybrid laser created by integrating an independently optimized SOI ring reflector and a III-V gain chip. In our demonstration, the uncooled integrated laser achieved a waveguide-coupled wall-plug efficiency of 12.2% at room temperature with an optical output power of ~10 mW. The laser operated single-mode near 1550 nm with a linewidth of 0.22 pm. This is a tunable light source with 8 nm wavelength tuning range. A proof-of-concept laser wavelength stabilization technique has also been demonstrated. Using a simple feedback loop, we achieved mode-hop-free operation in a packaged external-cavity hybrid laser as bias current was varied by 60mA.

Fabrication and characterization of GaN-based light-emitting diodes without pre-activation of p-type GaN.

We fabricated GaN-based light-emitting diodes (LEDs) without pre-activation of p-type GaN. During the fabrication process, a 100-nm-thick indium tin oxide film was served as the p-type contact layer and annealed at 500°C in N2 ambient for 20 min to increase its transparency as well as to activate the p-type GaN. The electrical measurements showed that the LEDs were featured by a lower forward voltage and higher wall-plug efficiency in comparison with LEDs using pre-activation of p-type GaN. We discussed the mechanism of activation of p-type GaN at 500°C in N2 ambient. Furthermore, x-ray photoemission spectroscopy examinations were carried out to study the improved electrical performances of the LEDs without pre-activation of p-type GaN.

Development of Surface Grating Distributed Feedback Quantum Cascade Laser for High Output Power and Low Threshold Current Density

We report on the room-temperature cw operation of a surface grating distributed feedback (DFB) quantum cascade laser (QCL) at λ ∼4.7 μm. Both grating design and material optimization are used to decrease the threshold current density and to increase the output power. For a high-reflectivity-coated 13-μm-wide and 4-mm-long laser, high wall-plug efficiency of 6% is obtained at 20°C from a single facet producing over 1 W of cw output power. The threshold current density of DFB QCL is as low as 1.13 kA/cm2 at 10°C and 1.34 kA/cm2 at 30°C in cw mode. Stable single-mode emission with a side-mode suppression ratio of about 30 dB is observed in the working temperature range of 20–50°C.

A concept for multiterawatt fibre lasers based on coherent pulse stacking in passive cavities

Since the advent of femtosecond lasers, performance improvements have constantly impacted on existing applications and enabled novel applications. However, one performance feature bearing the potential of a quantum leap for high-field applications is still not available: the simultaneous emission of extremely high peak and average powers. Emerging applications such as laser particle acceleration require exactly this performance regime and, therefore, challenge laser technology at large. On the one hand, canonical bulk systems can provide pulse peak powers in the multi-terawatt to petawatt range, while on the other hand, advanced solid-state-laser concepts such as the thin disk, slab or fibre are well known for their high efficiency and their ability to emit high average powers in the kilowatt range with excellent beam quality. In this contribution, a compact laser system capable of simultaneously providing high peak and average powers with high wall-plug efficiency is proposed and analysed. The concept is based on the temporal coherent combination (pulse stacking) of a pulse train emitted from a high-repetition-rate femtosecond laser system in a passive enhancement cavity. Thus, the pulse energy is increased at the cost of the repetition rate while almost preserving the average power. The concept relies on a fast switching element for dumping the enhanced pulse out of the cavity. The switch constitutes the key challenge of our proposal. Addressing this challenge could, for the first time, allow the highly efficient dumping of joule-class pulses at megawatt average power levels and lead to unprecedented laser parameters. Coherent pulse stacking may allow compact fibre-laser systems to be used in laser-based particle accelerators, report scientists in Germany. Such accelerators require lasers that can simultaneously provide exceedingly high peak and average powers—a tall order indeed. Now, Sven Breitkopf and co-workers propose that it could be achieved by employing temporal coherent combination of a pulse train emitted by a high-repetition-rate femtosecond fibre laser in a passive cavity. Following pulse stacking, a fast switching element dumps the enhanced high-peak power pulse from the cavity. Although this process increases the pulse peak power at the expense of a lower repetition rate, it retains a high average power. Ultimately, the scheme could lead to Joule-class pulses with terawatt peak powers and megawatt average powers.

A Quick and a Flexible Hydride Vapor Phase Epitaxy Process to Achieve Buried Heterostructure Quantum Cascade Lasers

Quantum cascade lasers (QCL) first demonstrated by Faist and co-workers1 are attractive sources for spectroscopy, medical and biosensing, remote gas sensing, free-space communication and applications in defense security countermeasures as these cover a wide range of spectrum from near-infrared to mid THz2-3. Several of these application areas necessitate high power, good beam quality, efficient thermal dissipation and high wall plug efficiency (WPE). Together with the optimal basic design, buried heterostructure QCL (BH-QCL) with semi-insulating InP:Fe (SI-InP) regrowth offers unique possibility to achieve an effective thermal dissipation and lateral single mode with the above desired performance. So far only MOVPE (metal organic vapour phase epitaxy)4 and molecular beam epitaxy (MBE)5 have been used to realize BH-QCLs and not hydride vapour phase epitaxy (HVPE), an equilibrium process. The growth rate in HVPE is one order of magnitude higher than that in MOVPE and MBE. We demonstrate here buried heterostructure QCLs emitting at 4.7 µm with a single step regrowth of highly resistive (>1x108 ohm·cm) semi-insulating InP:Fe in less than 45 minutes in a HVPE reactor for burying 6-15 µm deep etched ridges. This has to be compared with the normally used growth time of several hours, e.g., in an MOVPE reactor. Maintenance of the QCL ridges at the regrowth temperatures for longer time to realize BH-QCL can cause severe inter well-barrier mixing6 leading to the loss of the original laser design characteristics and hence HVPE regrowth is advantageous. In addition, its planarizing capacity, total selectivity during the growth on patterned (planar or non-planar) surfaces, non-appearance of “rabbit ears” near the ridge (or mesa) edge, flexibility in growing around [110] and [-110] oriented ridges, both with or without mask overhang, and insensitivity to the profile of the etched ridge are its unique flexible features7. The HR-coated 5 mm long BH-QCLs of ridge widths 6-15 µm were characterized. Besides being spatially monomode, TM00, all the characterized lasers exhibited WPE of ~8-9% with an output power of 1.5 – 2.5 W at room temperature and under CW operation. The performance can be further improved by proper mounting and facet coating. As-cleaved BH-QCLs were also characterized and compared with the HR-coated ridge-alone QCLs; the former outperform the latter in their output power more than twice even though the latter were HR-coated. This indicates the good thermal dissipation in the BH-QCL with HVPE regrowth. Thus, we demonstrate a simple, flexible, quick, stable and single-step regrowth process with extremely good planarization for realizing buried QCLs leading to monomode, high power and high WPE. Acknowledgements This work was supported by EU FP7 MIRIFISENS Integrated Project 317884. Linné Center of Excellence, ADOPT is also acknowledged.

Beam Shaping Technology for High Power Diode Laser Source

Direct diode lasers have some of the most attractive features of any laser. They are very efficient, compact, wavelength versatile, low cost, and highly reliable. However, the full utilization of direct diode lasers has yet to be realized. This is mainly due their poor output beam quality. Because of this, direct diode lasers are typically used to pump other lasers such as bulk solid-state (rod and thin disk) and fiber lasers. An improvement of the wall-plug efficiency and Power density necessary can be achieved by beam shaping and beam combination such as polarization coupling. In this paper, using the beam shaping technology realize good beam quality and high wall-plug efficiency. Base on bars rated to 60 W and 57% conversion efficiency, vertically stacked arrays (twenty bars) of such configuration are demonstrated with rated to about 1200W. The beam quality of high-power high brightness 880 nm laser diode source is improved with beam shaping. Beam parameter product of 79. 3 mm mrad ×81. 2 mm mrad, electro-optical conversion efficiency of more than 45.8% and continuous output power of 1 kW are demonstrated. This laser can be directly applied to cladding, surface hardening and other fields.

Demonstration of a quick process to achieve buried heterostructure quantum cascade laser leading to high power and wall plug efficiency

Together with the optimal basic design, buried heterostructure quantum cascade laser (BH-QCL) with semi-insulating regrowth offers a unique possibility to achieve an effective thermal dissipation and lateral single mode. We demonstrate here the realization of BH-QCLs with a single-step regrowth of highly resistive (>1×108 ohm·cm) semi-insulating InP:Fe in <45 min for the first time in a flexible hydride vapor phase epitaxy process for burying ridges etched down to 10 to 15 μm depth, both with and without mask overhang. The fabricated BH-QCLs emitting at ∼4.7 and ∼5.5 μm were characterized. 2-mm-long 5.5-μm lasers with a ridge width of 17 to 22 μm, regrown with mask overhang, exhibited no leakage current. Large width and high doping in the structure did not permit high current density for continuous wave (CW) operation. 5-mm-long 4.7-μm BH-QCLs of ridge widths varying from 6 to 14 μm regrown without mask overhang, besides being spatially monomode, TM00, exhibited wall plug efficiency (WPE) of ∼8 to 9% with an output power of 1.5 to 2.5 W at room temperature and under CW operation. Thus, we demonstrate a quick, flexible, and single-step regrowth process with good planarization for realizing buried QCLs leading to monomode, high power, and high WPE.