Laser-driven phosphor-converted white light source for solid-state illumination

Small-size LD-based automobile headlamp design with long illumination distance and wide illumination range

Due to the large size of an automobile headlamp that has a laser as the light source, it is difficult to reflect the advantages of high laser brightness into the effect of automobile illumination, so it has not been promoted. We propose a laser headlamp system that integrates low-beam and high-beam structures with superior illumination effects (longer illumination distance and wider illumination range) by designing freeform optical components and optical structure. The small-volume headlamp can meet the requirements of the Economic Commission for Europe regulation (ECE) R112. The structure volume of the headlamp proposed in this study is only 100∗70∗70m m 3, and the maximum luminance is 120 lx (simulation result) and 78.5 lx (test result). The view angle of the low beam is 13.6°, and the high beam is 15.6°, indicating that the solution is suitable for automotive illumination.

Engineering CsPbX3 (X = Cl, Br, I) Quantum Dot-Embedded Borosilicate Glass through Self-Crystallization Facilitated by NaF as a Phosphor for Full-Color Illumination and Laser-Driven Projection Displays

All inorganic perovskite (CsPbX₃, X = Cl, Br, I) quantum dot (QD) glass samples are considered the next generation of lighting materials for their excellent luminescence properties and stability, but crystallization conditions are difficult to control, which often leads to the inhomogeneous crystallinity of QDs. Here, we provided evidence that the presence of sodium fluoride induced self-crystallization of CsPbBr₃ QDs during routine glass formation without the need for additional heat treatment. We showed that NaF simultaneously affected the network structure of glass and promoted the formation of CsPbBr₃ QDs, that is, Na⁺ ions entered the glass network skeleton, partially interrupting the network structure, while the strong electronegativity of F^- ions attracted Cs⁺ and Pb²⁺ ions into the gaps formed in the glass networks that had been loosened up by Na⁺ ions, which reduced the activation energy of crystallization processes. Our results showed that NaF-induced CsPbBr₃ QDs glass had excellent thermal stability, high photoluminescence quantum efficiency (49%), and luminescent stability under high-power laser irradiation. Finally, this work also demonstrated the general applicability of this method in the making of a series of CsPbX₃ (X = Cl, Br, I) QD glass samples by NaF-induced self-crystallization, which drastically expanded the color gamut to a range of full spectrum for luminescence and laser-driven projection displays. We believe that the work presented here represents a new direction for the research and development of full-color gamut inorganic perovskite quantum dot glass samples, which could have a significant impact on the future applications of laser-driven projection displays as well.

Design and operation of a hybrid LED/LD-pumped phosphor-converted white-light lamp

Illumination sources based on phosphors, pumped by laser diodes (LDs), have seen rapid developments over the past decade. Here, we present a new, to the best of our knowledge, design that features both spectral richness and the capability for high brightness. Complete design details and operational characterization have been described. This basic design can be extended in various ways to customize such lamps for different operational requirements. A hybrid arrangement of both LEDs and an LD is used to excite a mixture of two phosphors. The LEDs, in addition, provide a blue fill-in to enrich output radiation and to tune the chromaticity point inside the white region. The LD power, on the other hand, can be scaled up to generate very high brightness levels that are not achievable with pumping from LEDs alone. This capability is gained using a special transparent ceramic disk that carries the remote phosphor film. We also show that the radiation from our lamp is free from speckle-producing coherence.

Fabrication of LuAG:Ce3+ Ceramic Phosphors Prepared with Nanophosphors Synthesized by a Sol-Gel-Combustion Method

The aim of this study was to investigate properties of ceramic phosphors fabricated using nano Lu₃Al₅O₁₂:Ce³⁺ phosphors produced with a sol-gel-combustion method. These nano Lu₃Al₅O₁₂:Ce³⁺ phosphors had a size of about 200 nm, leading to high density when fabricated as a ceramic phosphor. We manufactured ceramic phosphors through vacuum sintering. Alumina powder was added to improve properties. We mounted the manufactured ceramic phosphor in a high-power laser beam projector and drove it to determine its optical performance. Ceramic phosphor manufactured according to our route will have a significant impact on the laser-driven lighting industry.

Lanthanoid-doped quaternary garnets as phosphors for high brightness cathodoluminescence-based light sources

Gadolinium-yttrium- aluminum-gallium garnets (GYAGG) doped and codoped with Eu, Tb, and Ce were manufactured as ceramics to develop long-wavelength phosphors for high-brightness white light sources based on cathodoluminescence (CL). The CL light yield (LY) of Tb-doped ceramics at high-intensity electron beam excitation is shown to be more than twice as high as that of the conventional phosphor YAG:Ce, whereas codoping with Eu to redshift the chromaticity results in reducing the LY approximately to the level of YAG:Ce. The LY might be substantially improved by using a mix of Tb- and Eu-doped GYGAG powders instead of a single codoped GYGAG to produce ceramic phosphor. The high LY is explained by favorable contribution of Gd sublattice in excitation transfer to activator ions. Chromaticity of phosphors GYGAG:Tb, Eu can be tuned in a wide range by varying the ratio of Tb to Eu concentration. They are radiation resistant and stabile in the temperature range from 300 to 450 K.

Homogeneous distribution of phosphor particles inside resin using a vertical vibration method for efficient laser-based white light illumination

We report what we believe to be an innovative method for the homogeneous distribution of phosphor particles inside the optical resin-based layer using a vertical vibrational technique for efficient laser-based white light illumination. In this method, single-stage vibration energy was efficiently used against phosphor sedimentation with the help of a mechanical vibrator system. The vertical vibrational energy was transferred to negate the downward gravitational effect acting on each phosphor particle. Therefore, the phosphor particles inside the layer were stable and uniformly distributed from the initial to final drying stages, creating approximate refractive index homogeneity inside the medium. The sedimentation problem was resolved, and all the optical parameters to support this method were properly analyzed and found to be stable and suitable for laser-based illumination applications.

Saturation Mechanisms in Common LED Phosphors

Commercial lighting for ambient and display applications is mostly based on blue light-emitting diodes (LEDs) combined with phosphor materials that convert some of the blue light into green, yellow, orange, and red. Not many phosphor materials can offer stable output under high incident light intensities for thousands of operating hours. Even the most promising LED phosphors saturate in high-power applications, that is, they show decreased light output. The saturation behavior is often poorly understood. Here, we review three popular commercial LED phosphor materials, Y3Al5O12 doped with Ce3+, CaAlSiN3 doped with Eu2+, and K2SiF6 doped with Mn4+, and unravel their saturation mechanisms. Experiments with square-wave-modulated laser excitation reveal the dynamics of absorption and decay of the luminescent centers. By modeling these dynamics and linking them to the saturation of the phosphor output intensity, we distinguish saturation by ground-state depletion, thermal quenching, and ionization of the centers. We discuss the implications of each of these processes for LED applications. Understanding the saturation mechanisms of popular LED phosphors could lead to strategies to improve their performance and efficiency or guide the development of new materials.

A Study of High-Efficiency Laser Headlight Design Using Gradient-Index Lens and Liquid Lens

In the field of vehicle lighting, due to the diode laser, its small size and high energy conversion efficiency, it can be effectively used as the headlight source of high beam. In recent years, it was adopted by European advanced car manufacturers as a new generation of automotive headlight lighting products. The current mature technology on the market is to extend the laser beam by means of reflection and to use a single high-power laser as the light source to meet the needs of surface lighting. In this research, we propose a new integrated optical design for an automotive headlight system with the rod lens, gradient-index lens (GRIN lens) and freeform lens to expand the laser beam. With regard to the diffusion of the beam by reflection and refraction, the liquid lens is used as a switch for the high beam and low beam lights to meet the needs of vehicle lighting functions and to use low-power diode lasers to synthesize the array light source. Compared with the 24-W LED headlight module available in the current market, the energy saved by this proposed optical design can increase efficiency by an average of 33%. The maximum illuminance is 56.6 lux in the high-beam mode, which is 18% higher than the standard value. Let the laser light meet the lighting requirements of regulatory standard values even beyond.

Above 25 nm emission wavelength shift in blue-violet InGaN quantum wells induced by GaN substrate misorientation profiling: towards broad-band superluminescent diodes

We report a thorough study of InGaN quantum wells spatially modified by varying the local misorientation of the GaN substrate prior to the epitaxial growth of the structure. More than 25 nm shift of emission wavelength was obtained, which is attributed to indium content changes in the quantum wells. Such an active region is promising for broadening of the emission spectrum of (In,Al,Ga)N superluminescent diodes. We observed that the light intensity changes with misorientation, being stable around 0.5° to 2° and decreasing above 2°. This relation can be used as a base for future device designing.

Cross talk and optical efficiency of an energy-harvesting color projector utilizing ceramic phosphors

A color projector screen was fabricated by filling three kinds of ceramic phosphor powders in the periodic hollow columns formed in a ${50}\;{{\rm mm}}\; \times \;{50}\;{{\rm mm}}\; \times \;{10}\;{{\rm mm}}$50mm×50mm×10mm acrylic waveguide. When a blue laser beam excited a single spot on the screen, a disk-shaped cross-talk pattern appeared. Its intensity was 5 orders of magnitude lower than that of the excited spot. The solar cells attached to the waveguide edge harvested less than 0.8% of the incident optical power. The photons scattered by the phosphors are responsible for these characteristics, and the use of non-scattering luminescent materials is desired for improving them.

Laser-line-driven phosphor-converted extended white light source with uniform illumination

We report the development of laser-driven extended white light source designed as a light sheet for general illumination. This light sheet is made of two large diffused glass plates. A Ce:YAG phosphor layer was coated and sandwiched between the two diffusing sheets. The blue laser beam was first converted into a uniform laser line using a cylindrical lens, and a laser line was made incident parallel to the edges of the designed light sheet. The blue photons are waveguided inside the glass sheet via total internal reflection and scattered from the diffused surface. Some of the blue photons are absorbed by the Ce:YAG phosphor and down-converted into yellow light of longer wavelength. The white light emanating from the diffuse surface is the combined effect of yellow light with original blue light. The developed light sheet with the combination of laser line generation and total internal reflection is a unique and low-cost method for generating white light with uniform illumination. The details of the development of the light sheet and laser line generation are described. The experimental parameters, such as correlated color temperature and color coordinates, are reported.

An advanced laser headlight module employing highly reliable glass phosphor

An advanced laser headlight module (LHM) employing highly reliable glass phosphor is demonstrated. The novel glass-based YAG phosphor-converter layers fabricated by low-temperature of 750°C exhibited better thermal stability. The LHM consisted of a 5 × 1 blue laser diode array, an aspherical lens, a glass phosphor-converter layer with an aluminum thermal dissipation substrate, and a dichroic filter to allow pass blue light and reflect yellow phosphor light. The 5 × 1 blue laser array was packaged with five blue lasers having optical power of 1.2 W per laser. The LHM exhibited total output optical power of 6 W, luminous flux of 1860 lm, relative color temperature of 4100 K, and efficiency of more than 310 lm/W. The high-beam patterns of the LHMs were measured to be 45,000 luminous intensity (cd) at 0°, 31,000 cd at ± 2.5°, and 12,500 cd at ± 5°, which were well satisfied the ECE R112 class B regulation. The proposed high-performance LHM with highly reliable glass-based phosphor-converter layer fabricated by low temperature is favorable as one of the promising LHM candidates for use in the next-generation automobile headlight applications.

Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives

Advances in solid state white lighting technologies witness the explosive development of phosphor materials (down-conversion luminescent materials). A large amount of evidence has demonstrated the revolutionary role of the emerging nitride phosphors in producing superior white light-emitting diodes for lighting and display applications. The structural and compositional versatility together with the unique local coordination environments enable nitride materials to have compelling luminescent properties such as abundant emission colors, controllable photoluminescence spectra, high conversion efficiency, and small thermal quenching/degradation. Here, we summarize the state-of-art progress on this novel family of luminescent materials and discuss the topics of materials discovery, crystal chemistry, structure-related luminescence, temperature-dependent luminescence, and spectral tailoring. We also overview different types of nitride phosphors and their applications in solid state lighting, including general illumination, backlighting, and laser-driven lighting. Finally, the challenges and outlooks in this type of promising down-conversion materials are highlighted.

Investigation of Saturation Effects in Ceramic Phosphors for Laser Lighting

We report observations of saturation effects in a Ce:LuAG and Eu-doped nitride ceramic phosphor for conversion of blue laser light for white light generation. The luminous flux from the phosphors material increases linearly with the input power until saturation effects limit the conversion. It is shown that the temperature of the phosphor layer influences the saturation power level and the conversion efficiency. It is also shown that the correlated color temperature (CCT), phosphor conversion efficiency and color rendering index (CRI) are dependent both on the incident power and spot size diameter of the illumination. A phosphor conversion efficiency up to 140.8 lm/W with CRI of 89.4 was achieved. The saturation in a ceramic phosphor, when illuminated by high intensity laser diodes, is estimated to play the main role in limiting the available luminance from laser-based lighting systems.

Investigation into the Anomalous Temperature Characteristics of InGaN Double Quantum Well Blue Laser Diodes Using Numerical Simulation

GaN-based blue laser diodes (LDs) may exhibit anomalous temperature characteristics such as a very high characteristic temperature (T 0) or even negative T 0. In this work, temperature-dependent characteristics of GaN-based blue LDs with InGaN double quantum well (QW) structures were investigated using numerical simulations. The temperature-dependent threshold current is found to become increasingly anomalous as the thickness or doping concentration of the barrier layer between QWs increases. For a properly chosen barrier thickness and doping concentration, very high T 0 of >10,000 K can be obtained. The anomalous temperature characteristics of these InGaN blue LDs are attributed to the increase of gain at the n-side QW with increasing temperature because of the thermally enhanced hole transport from the p-side to the n-side QW.

High brightness laser-driven white emitter for Etendue-limited applications

Laser-phosphor lighting has great potential because a laser diode (LD) could keep high efficiency under high current. Remote phosphor configuration has been studied, but a phosphor-covered LD has not been reported due to several difficulties. In the present work, we develop a novel laser-driven white emitter. Without remote phosphor configuration, the white emitter has a very simple structure, making it is easy for mass production and standardization. Under a continuous wave driving current of 3 A, it emits 850 lm with correlated color temperature of 6990 K. The wall-plug efficacy is 70 lm/W for the maximum. The laser-driven white emitter is very suitable for Etendue-limited applications due to its very small emitting size of 0.45  mm×0.2  mm.

Effect of laser speckle on light from laser diode-pumped phosphor-converted light sources

Laser diode (LD) pumped white light sources are being developed as an alternative to light-emitting diode-pumped sources for high efficiency and/or high brightness applications. While several performance metrics of laser-pumped phosphor-converted light sources have been investigated, the effect of laser speckle has not been sufficiently explored. This paper describes our experimental studies on how laser speckle affects the behavior of light from laser-excited phosphor lamps. A single LD pumping a phosphor plate was the geometry explored in this work. Overall, our findings are that the down-converted light did not exhibit any speckle, whereas speckle was present in the residual pump light but much reduced from that in direct laser light. Furthermore, a thicker coating of small-grained phosphors served to effectively reduce speckle through static pump light diffusion in the phosphor coating. Our investigations showed that speckle is not of concern in illumination from LD-pumped phosphor-converted light sources.