High-brightness source based on luminescent concentration

Exploring light-emitting diode pumped luminescent concentrators in solid-state laser applications

In the past, there were limited efforts to use light-emitting diodes (LEDs) for pumping solid-state lasers. However, these attempts were overshadowed by the introduction of laser diodes, which offered more favourable pumping conditions. Nevertheless, recent advancements in high-power LEDs, coupled with the utilization of luminescent concentrators (LC), have paved the way for a novel approach to pump solid-state lasers. The combination of LEDs and LC in this LED-LC system presents several advantages, including enhanced ruggedness, stability, and cost-effectiveness compared to other laser pumping methods. This review explores the various techniques employed to pump solid-state lasers using LED-LC as a pump source, along with improvements made to enhance the brightness of LEDs in this context.

Ce:LYSO, from scintillator to solid-state lighting as a blue luminescent concentrator

Cerium-doped lutetium-yttrium oxyorthosilicate (Ce:LYSO) is a well-known single crystal scintillator used in medical imaging and security scanners. Recent development of high power UV LED, matching its absorption band, questions the possibility to use Ce:LYSO in a new way: as LED-pumped solid-state light source. Since Ce:LYSO is available in large size crystals, we investigate its potential as a luminescent concentrator. This paper reports an extensive study of the performance in close relation to the spectroscopic properties of this crystal. It gives the reasons why the Ce:LYSO crystal tested in this study is less efficient than Ce:YAG for luminescent concentration: limited quantum efficiency and high losses coming from self-absorption and from excited-state absorption are playing key roles. However, we demonstrate that a Ce:LYSO luminescent concentrator is an innovative source for solid-state lighting. Pumped by a peak power of 3400 W in quasi-continuous wave regime (40 µs, 10 Hz), a rectangular (1 × 22 × 105 mm³) Ce:LYSO crystal delivers a broadband spectrum (60 nm FWHM) centered at 430 nm. At full output aperture (20 × 1 mm²), it emits a peak power of 116 W. On a squared output surface (1 × 1 mm²) it emits 16 W corresponding to a brightness of 509 W cm^-2 sr^-1. This combination of spectrum power and brightness is higher than blue LEDs and opens perspectives for Ce:LYSO in the field of illumination namely for imaging.

Projection optical engine design based on tri-color LEDs and digital light processing technology

Digital light processing (DLP) is currently a cutting-edge technology for desktop projection optical engines. Due to the passive luminescence characteristics, the DLP projection engine needs a few specific illumination optical components for light collimation, homogenization, and color combination, together with a projection lens matching the DLP chip and magnifying the image. In this paper, we propose a design approach that first splits the DLP projection optical engine into individual components for separate design, and then integrates them into a whole system for further verification. For the first step, the collimating lens group is designed for light collection, and the dichroic mirrors are used to fold the light path based on tri-color LED light sources. For the second step, a fly-eye lens and the corresponding relay lens group are designed to achieve uniform illumination on the DMD chip. The third step is to optimize the projection lens group for high-resolution projection display. Based on the design and simulation, the optical efficiency is 63.4% and the uniformity reaches 94.9% on the projection screen. The modulation transfer function (MTF) of the projection lens is higher than 0.4 at 66 lines for the distance of 500∼1500mm, and the distortion is lower than 1%. Simulation results show that the total luminous flux is estimated to reach 224.15 lm when the powers of tri-color LEDs are 21 W, 15.5 W, and 25 W, respectively. A projector prototype is built and tested for further verification, which provides a luminous flux of 220.43 lm and uniformity of 90.22%, respectively. The proposed design, demonstrated by both simulation and experiment, exhibits high feasibility and application potential in state-of-the-art commercial projector design.

Light recycling in LED-pumped Ce:YAG luminescent concentrators

We report the development of a high-brightness, high-power Ce:YAG luminescent concentrator pumped by 2240 blue LEDs in quasi-continuous wave operation (10 µs, 10  Hz). Using light confinement and recycling in the three space dimensions, the parallelepiped (1mm×14×mm×200mm) Ce:YAG emits a power of 145 W from a square output surface (1 × 1mm²) corresponding to a brightness of 4.6 kW/cm²/sr. This broadband yellow source has a unique combination of luminous flux (7.6 10⁴ lm) and brightness (2.4 10⁴ cd/mm²) and overcomes many other visible incoherent sources by one order of magnitude. This paper also proposes a deep understanding of the performance drop compared to a linear behavior when the pump power increases. Despite excited state absorption was unexpected for this low doped Ce:YAG pumped at a low irradiance level, we demonstrated that it affects the performance by tripling the losses in the concentrator. This effect is particularly important for small output surfaces corresponding to strong light recycling in the concentrator and to average travel distances inside the medium reaching meters.

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.

3D luminescent concentrators

A solution to develop high-brightness incoherent sources consists in luminescent concentration. Indeed, the absorption/emission process in a high index medium allows us to circumvent the brightness conservation law by the confinement of the light in 1 or 2 dimensions. In practice, Ce-doped luminescent concentrators pumped with InGaN LED exceed LED's brightness by one order of magnitude. This work shows how light confinement in 3 dimensions increases the brightness by an additional order of magnitude. Thanks to an analytical approach validated by experimental results, this concept gives new degrees of freedom for the design of luminescent concentrators and paves the way to a generation of incoherent sources among the brightest ever designed.

Enhancing brightness of Lambertian light sources with luminescent concentrators: the light extraction issue

Luminescent concentrators (LC) enable breaking the limit of geometrical concentration imposed by the brightness theorem. They enable increasing the brightness of Lambertian light sources such as (organic) light-emitting diodes. However, for illumination applications, light emitted in the high-index material needs to be outcoupled to free space, raising important light extraction issues. Supported by an intuitive graphical representation, we propose a simple design for light extraction: a wedged output side facet, breaking the symmetry of the traditional rectangular slab design. Angular emission patterns as well as ray-tracing simulations are reported on Ce:YAG single crystal concentrators cut with different wedge angles, and are compared with devices having flat or roughened exit facets. The wedge output provides a simple and versatile way to simultaneously enhance the extracted power (up to a factor of 2) and the light directivity (radiant intensity increased by up to 2.2.).

LED-pumped passively Q-switched Cr:LiSAF laser

We report, to the best of our knowledge, the first light-emitting diode (LED)-pumped Cr:LiSAF laser, in both quasi-continuous-wave (QCW) and passively Q-switched operation. This Letter is based on the recent development of LED-pumped luminescent concentrators (LCs). Combining the capacity of high-density integration of blue LEDs with the excellent properties of Ce:YAG LCs, this new pump source can deliver high irradiance (7.3  kW/cm²) in the visible to pump Cr:LiSAF. The Cr:LiSAF laser demonstrates an energy of 8.4 mJ at 850 nm in QCW (250 μs pulses at 10 Hz). A small signal gain per roundtrip of 1.44 at 850 nm and a wavelength tunability between 810 and 960 nm have been performed. A passively Q-switched oscillator is also presented using a Cr:YAG saturable absorber. A peak power of 3.1 kW is obtained with a pulse energy of 130 μJ and duration of 41.6 ns.

New LED-based high-brightness incoherent light source in the SWIR

The first LED-pumped luminescent concentrator (LC) emitting in the short-wave infrared (SWIR) is reported. Low cost LEDs (at 940 nm) are used to pump a Yb,Er:Glass LC emitting at 1550 nm. The optical conversion efficiency of the system is optimized and studied in detail for several optical configurations. A total of 128 LEDs having an emitting surface of 1 mm² and an irradiance of 51.6 W/cm², corresponding to a total pump power of 66 W, are used. Optimizing the output power out of a 100-mm-long LC in a continuous wave regime, a power of 850 mW is extracted from the 2.5 x 2 mm² LC emitting surface area. The optical efficiency is then 1.29%. The performance of this luminescent concentrator is higher by one order of magnitude in term of radiance compared to an LED emitting at the same wavelength.

Biomimetic light-harvesting funnels for re-directioning of diffuse light

Efficient sunlight harvesting and re-directioning onto small areas has great potential for more widespread use of precious high-performance photovoltaics but so far intrinsic solar concentrator loss mechanisms outweighed the benefits. Here we present an antenna concept allowing high light absorption without high reabsorption or escape-cone losses. An excess of randomly oriented pigments collects light from any direction and funnels the energy to individual acceptors all having identical orientations and emitting ~90% of photons into angles suitable for total internal reflection waveguiding to desired energy converters (funneling diffuse-light re-directioning, FunDiLight). This is achieved using distinct molecules that align efficiently within stretched polymers together with others staying randomly orientated. Emission quantum efficiencies can be >80% and single-foil reabsorption <0.5%. Efficient donor-pool energy funneling, dipole re-orientation, and ~1.5-2 nm nearest donor-acceptor transfer occurs within hundreds to ~20 ps. Single-molecule 3D-polarization experiments confirm nearly parallel emitters. Stacked pigment selection may allow coverage of the entire solar spectrum.