High-brightness Diode Lasers Research Articles

Narrow-linewidth, quasi-continuous-wave ASE source based on a multiple-pass Nd:YAG zigzag slab amplifier configuration.

We present investigations into a narrow-linewidth, quasi-continuous-wave pulsed all-solid-state amplified spontaneous emission (ASE) source by use of a novel multiple-pass zigzag slab amplifier. The SE fluorescence emitted from a Nd:YAG slab active medium acts as the seed and is amplified back and forth 8 times through the same slab. Thanks to the angular multiplexing nature of the zigzag slab, high-intensity 1064-nm ASE output can be produced without unwanted self-lasing in this configuration. Experimentally, the output energy, optical conversion efficiency, pulse dynamics, spectral property, and beam quality of the ASE source are studied when the Nd:YAG slab end-pumped by two high-brightness laser diode arrays. The maximum single pulse energy of 347 mJ is generated with an optical efficiency of ~5.9% and a beam quality of 3.5/17 in the thickness/width direction of the slab. As expected, smooth pulses without relaxing spikes and continuous spectra are achieved. Moreover, the spectral width of the ASE source narrows versus the pump energy, getting a 3-dB linewidth of as narrow as 20 pm (i.e. 5.3 GHz). Via the sum frequency generation, high-intensity, smooth-pulse, and narrow-linewidth ASE sources are preferred for solving the major problem of saturation of the mesospheric sodium atoms and can create a much brighter sodium guide star to meet the needs of adaptive imaging applications in astronomy.

High-power high-brightness 980 nm lasers with >50% wall-plug efficiency based on asymmetric super large optical cavity.

High-power high-brightness super large optical cavity laser diodes with an optimized epitaxial structure are investigated at the wavelength of 980 nm range. The thicknesses of P- and N-waveguides are prudently chosen based on a systematic consideration about mode characteristics and vertical far-field divergences. Broad area laser diodes show a high internal quantum efficiency of 98% and a low internal optical loss of 0.58 cm-1. The ridge-waveguide laser with 7 μm ridge and 3 mm cavity yields 1.9 W single spatial mode output with far-field divergence angles of 6.8° in lateral and 11.5° in vertical at full width at half maximum under 2 A CW operating current. The corresponding M2 values are 1.77 and 1.47 for lateral and vertical, respectively, and the corresponding brightness is 76.8 MW‧cm-2‧sr-1. The far-field divergence angles with 95% power content are in the range of 24.7° to 26.1° across the whole measured range.

Effect of Wafer and Damaged Layer Thickness on Residual Stress and Bow of Free-Standing Gallium Nitride Wafers during Wafering Process

In recent years, free-standing gallium nitride (FS-GaN) wafers were researched due to high demand for power device, high brightness LED (HB-LED) and laser diode (LD). The GaN wafers grown by hydride vapor phase epitaxy (HVPE) have been used for the substrate of power device, HB-LED and LD because they demonstrated lower dislocation density (DD) than that of GaN wafers grown by metal-organic chemical vapor deposition (MOCVD). However, they still have residual stress and wafer bow, which lower the device productivity in mass production. To overcome these issues, the control of crystallographic quality and wafer bow are the key technologies. The crystallographic quality of the GaN wafer is mostly determined at the growth process. The bow of the wafer is determined by the difference in thermal expansion coefficient (TEC) between sapphire substrate and GaN, and it can be controlled at the wafering process. In this study, therefore, we investigated how the bow of the FS-GaN wafer is changed during wafering process. In particular, the effect of the abrasive size on the damaged layer thickness during lapping process was investigated and residual stress was calculated theoretically. Two inch GaN wafers were fabricated on (0001) sapphire substrates by HVPE and separated by the laser lift-off (LLO) process. The bow of as-grown GaN wafer after LLO process was ranged from -300 to -100 um, which was determined by the residual thermal stress. The GaN wafer bow is changed during the wafering process, especially the lapping process. From the Stoney’s formula, it is notable that the thickness of substrate (tS) and sub-surface damaged layer (tSSD) can change the wafer bow. Fig. 1 shows the wafer bow (black triangle), residual stress (blue square) of experimented GaN wafer and wafer bow calculated by the Stoney’s formula (red triangle). It was confirmed that the wafer bow was reduced as the thickness of GaN wafer reduced. In particular, the bow was inverted from concave to convex at the thickness of around 180 um. As the lapping process progressed, tS is reduced and tSSD is generated, which led to the change of bow to convex. Fig. 2 shows the sub-surface damaged layer of the GaN wafer after lapping with 2-um diamond abrasives. It exhibits that the crystalline GaN layer was damaged by the diamond abrasives and changed to polycrystalline phase at the top surface. In addition, it indicates that the microcracks propagated into the wafer under the polycrystalline layer. Total thickness of polycrystalline layer and microcracks, tSSD, was around 1.09 um. Thus, the tSSD should be removed for achieving the damage-free GaN wafer. In our presentation, we report how we achieved zero-bow FS-GaN wafer shown in Fig. 3, and how to control residual stress and optimize wafering process. * This work was financially supported by the Brain Korea 21 Plus Program in 2017. Figure 1

高功率高亮度半导体激光器合束进展

高功率高亮度半导体激光器合束进展

Rear-side resonator architecture for the passive coherent combining of high-brightness laser diodes.

We describe a new coherent beam combining architecture based on passive phase locking of emitters in an extended cavity on the rear facet and their coherent combination on the front facet. This rear-side technique provides strong optical feedback for phase locking while maintaining a high electrical-to-optical efficiency. Two high-brightness high-power tapered laser diodes are coherently combined using a Michelson-based cavity. The combining efficiency is above 82% and results in an output power of 6.7 W in a nearly diffraction-limited beam with an M(4σ)(2)≤1.2. A semi-active automatic adjustment of the current enhances the long-term stability of the combination, while the short-term stability is passively ensured by the extended cavity. This new laser configuration exhibits the simplicity of passive self-organizing architectures while providing a power conversion efficiency of 27% that is comparable to master oscillator power amplifier architectures.

Innovative III-Nitride Epitaxy Approach for Low Dislocation GaN and Free-Standing GaN Substrate

GaN and its related alloys have emerged as eminently useful materials for the fabrication of green-to-ultraviolet light-emitting diodes (LEDs), laser diodes (LD) and also high-power, high-efficiency transistor devices due to their widely tunable bandgap. The key technological challenge in the manufacturing of these high performance optoelectronic and electronic devices is the absence of GaN substrates of practical size. As a result, all epitaxial GaN films are grown on the top of a substrate with different lattice constant, such as sapphire, Si and SiC, leading to the inevitable introduction of dislocations and residual strain in the epilayers, which is known to degrade the performance and reliability of the devices. Moreover, the popularly used sapphire substrate is an insulator with low thermal conductivity and difficult to cleave beneath the GaN based LD wafers. So there is a high demand for both low dislocation GaN epilayers and freestanding substrate. In this work, we present two approaches to meet this demand. (1). The Innovative One-step Lateral Epitaxy with Serpentine Mask Structures. There are two key features to this technology, one being a unique mask structure that blocks threading dislocations using amorphous films, and the other being associated with the controlled selective nucleation directly on sapphire surface. The direction of the growth front of GaN makes two 90° turns before emerging from the serpentine channel. The growth direction at the beginning is along the substrate surface norm that subsequently turns to become parallel to the surface and finally turns towards the surface norm again when emerging from the serpentine channel. It achieves the blockage of threading dislocations from reaching the active device region. The second key feature is the selective nucleation of GaN on sapphire surfaces but not on the amorphous mask surfaces, which is the enabling factor that allows for the entire fabrication process being carried out with only one single epitaxial step, leading to significant reduction in the fabrication cost. The microstructural and optical properties of the GaN epilayer grown on serpentine masked structures will be described. Furthermore, we will summarize that this novel technique is a promising candidate for the growth of high quality III-nitride and the subsequent high-performance device fabrication including high brightness LED, laser diodes, and high-power, high-efficiency transistors. (2) Free-Standing (FS) GaN Substrate by HVPE. Recently GaN substrate has been developed to improve qualities of III-nitride devices, thermal conductivity, dislocation density, thermal and lattice mismatch, etc. due to home-epitaxial growth through various growth methods including ammonothermal, High-pressure solution growth, Na-flux and Hydride vapor phase epitaxy (HVPE). In spite of advantage of HVPE method such as high growth rate, capability of wide area wafer and already commercialized growth methods for GaN substrate with comparing other growth methods, the problems of large dislocation density, parasitic nucleation and stress controls in HVPE- GaN substrate were still remained. We will provide one simple HVPE growth method, named as the pulsed NH3-flow modulation method, to realize a crack-free GaN thick layer with high crystal quality and nearly no residual strain. A few simple self-separation methods are developed for removal of hetero-substrates to obtain FS-GaN. 2-inch FS-GaN substrates are obtained with high quality. The FWHM values are about 108 and 81 arcsec for (002) and (102) reflections. The corresponding dislocation density of the freestanding GaN is low to 3-7×106 cm-2. We will also discuss self-separation mechanisms for FS-GaN substrates grown by HVPE.

High-brightness single-mode double-tapered laser diodes with laterally coupled high-order surface grating.

Double-tapered laser diodes with 23rd order laterally coupled surface gratings are fabricated at 913nm. A narrow ridge waveguide restricts the lateral mode number, and tapered waveguides are utilized to enhance the output power. The gratings defined on the sides of ridge waveguide filter not only the longitudinal modes but also the high-order lateral modes. Experimentally, continuous wave power of over 560 mW/facet is achieved. The lateral far-field pattern exhibits a single-lobe profile and narrow divergence, and the lowest divergence angle is only twice the diffraction-limited one. The measured side-mode suppression ratio is about 27dB. This device has great potential for high-power single-mode near-diffraction-limited laser emission with easy fabrication.

Power Scaling of Nonlinear Frequency Converted Tapered Diode Lasers for Biophotonics

Diode lasers have proven to be versatile light sources for a wide range of applications. Nonlinear frequency conversion of high brightness diode lasers has recently resulted in visible light power levels in the watts range enabling an increasing number of applications within biophotonics. This review provides an overview of the developments within nonlinear frequency converted high power laser diodes in the visible spectral range. Single-pass nonlinear frequency doubling is presented as a nonsophisticated method to achieve watt-level output powers and possible routes to higher power and efficiency are included. Application examples within pumping of mode-locked Ti:sapphire lasers and implementation of such lasers in optical coherence tomography are presented showing the application potential of these lasers.

Folded cavity angled-grating broad-area lasers

The angled-grating broad-area laser is a promising candidate for high power, high brightness diode laser source. The key point in the design is the angled gratings which can simultaneously support the unique snake-like zigzag lasing mode and eliminate the direct Fabry-Perot (FP) feedback. Unlike a conventional laser waveguide mode, the phase front of the zigzag mode periodically changes along the propagation direction. By use of the mirror symmetry of the zigzag mode, we propose and demonstrate the folded cavity angled-grating broad-area lasers. One benefit of this design is to reduce the required wafer space compared to a regular angled-grating broad-area laser, especially in a long cavity laser for high power operation. Experimental results show that the folded cavity laser exhibits good beam quality in far field with a slightly larger threshold and smaller slope efficiency due to the additional interface loss.

High-brightness diode laser arrays integrated with a phase shifter designed for single-lobe far-field pattern

High-brightness, edge-emitting diode laser arrays integrated with a phase shifter have been designed and fabricated at a wavelength of about 910 nm. Stable out-of-phase mode is generated through coupling evanescently and converted to be nearly in-phase by the phase modulation from the phase shifter. With a very simple manufacture process, stable single-lobe far-field pattern is achieved in the slow axis when the continuous wave output power exceeds 460 mW/facet, and the divergence angle is only 2.7 times the diffraction-limited value. Such device shows a promising future for high-brightness application with low cost and easy fabrication.

Dislocation reduction through nucleation and growth selectivity of metal-organic chemical vapor deposition GaN

A novel serpentine channel structure is used to mask the sapphire substrate for the epitaxial growth of dislocation-free GaN. Compared to the existing epitaxial lateral overgrowth methods, the main advantages of this novel technique are: (a) one-step epitaxial growth; (b) up to 4 times wider defect-free regions; and (c) the as-grown GaN film can be transferred easily to any type of substrate. TEM, etch pits and cathodoluminescence experiments are conducted to characterize the quality of as-grown GaN. The results show that the average etch-pit density in the yet-to-be-optimized GaN epi-layers is about 4 × 105 cm−2. The underlying physics of selective nucleation and growth is investigated using the finite element method (COMSOL). It is concluded that the proximity effect dominates the selective growth of GaN on the serpentine channel structure masked sapphire. This novel technique is a promising candidate for the growth of high quality III-nitride and the subsequent high-performance device fabrication including high brightness LED, laser diodes, and high-power, high-efficiency transistors.

三波长合束高亮度半导体激光光源

三波长合束高亮度半导体激光光源

Metal Cutting with Multi kW Diode Laser

AbstractCutting roughness lower than 10 μm and a wall‐plug efficiency of 30% including cooling are two nameable advantages when speaking about direct diode laser cutting of metals. Emphasizing in this context “direct diode laser” stress that metal cutting with a diode laser is not a conventional process. However, steady developments in diode bar technology enabled the design of a high brightness diode laser system with 2 kW output power and a beam parameter product of 22 mm*mrad. This now commercially available “DIOCUT” system is ready to enter the laser macro cutting market. The applicability of this system is demonstrated by cutting aluminum, stainless steel, copper and mild steel. The cutting edge quality is comparable with CO2 laser cuts. On our roadmap is higher output power at equal beam quality in near future.

Volume Bragg grating external cavities for the passive phase locking of high-brightness diode laser arrays: theoretical and experimental study

We describe the theoretical modeling of the external-cavity operation of a phase-locked array of diode lasers in two configurations, the self-imaging cavity based on the Talbot effect and the angular-filtering cavity. Complex filtering functions, such as the transmission or reflection of a volume Bragg grating (VBG), may be introduced in the external-cavity description. Experiments with high-brightness diode laser arrays were also conducted. The experimental results are carefully analyzed with regard to the numerical simulations, and the beneficial effect of the spectral selectivity of VBGs is demonstrated.

基于像散与理想光源成像原理的高亮度半导体激光器光纤耦合设计方法

A high-brightness fiber-coupled diode laser system is designed,based on the principle of astigmatism and ideal light source imaging theory.As an example,details of the calculation methods and design steps of the fiber coupled laser diode are given whose wavelength is 808 nm and output power is 10 W.Semiconductor laser beam can be coupled into the 50 μm core diameter optical fiber with a numerical aperture of 0.22.It is found that this fiber-coupled system has an output power of 9.712 W,a coupling efficiency of 97.12% and a power density of 1.1224×106 W/cm2.The proposed method is simple in principle,and the designed fiber-coupled system has high coupling efficiency,small size and strong practical value.

10.7 W peak power picosecond pulses from high-brightness photonic band crystal laser diode

Using a monolithic photonic band crystal laser, nearly diffraction limited laser pulses of 10.7 W peak power at 980 nm wavelength and brightness of 4 × 10 8 Wcm -2 sr -1 are generated. These pulses of about 100 ps FWHM have a variable repetition rate of up to 80 MHz, making them ideal seed sources for optical power amplifiers.

High‐Brightness Diode Lasers and New Wavelengths

AbstractDesign considerations for highbrightness laser diodes in various new wavelength ranges are described, together with achieved results based on laser diode arrays, in the following referred to as laser bars. In addition, the new applications for laser diodes with such new wavelenghts are presented.

高亮度窄线宽的激光二极管阵列研究进展

高亮度窄线宽的激光二极管阵列研究进展

Bi-doped glass optical fibers: Is it a new breakthrough in laser materials?

It has been shown recently, that Bi-doped glass optical fibers are a very promising active laser medium. Luminescence of Bi-doped glasses takes place in a spectral region of 1150–1500 nm, where no efficient fiber lasers (or any other efficient lasers) exist. The glasses have very broad luminescence bands (200–400 nm) and long lifetime (200–700 μs). The Bi absorption bands are situated in a spectral region of 500–1100 nm where long-lived high-brightness laser diodes developed for the pumping of lasers and amplifiers are available. Besides, strong luminescence has been observed in a variety of glass compositions. In this paper the recent results on the new laser material – Bi-doped glasses and optical fibers are reviewed. First, luminescence properties of various Bi-doped glasses are discussed. Then we describe the Bi-doped silica-based optical fiber fabrication and the absorption and luminescence properties of the fibers. At last some results on Bi-doped fiber lasers and their applications are presented.

Nd:YAG laser diode-pumped directly into the emitting level at 938 nm

We present the first demonstration of Nd:YAG laser pumped directly in band at 938 nm with a high-brightness fiber-coupled laser diode. Up to 6 W of CW laser emission at 1064 nm have been obtained under an absorbed pump power of 28 W at 938 nm. A comparison between 808 nm and 938 nm pumping, realized by thermal cartography, demonstrates the very low heat generation of in-band pumping. Numerical simulations were also implemented to study and discuss the laser performance of our system.