Research on phosphor-conversion laser-based white light used as optical source of VLC and illumination

Visible light communication (VLC) based on light emitting diodes (LEDs) or laser diodes (LDs) has attracted considerable interest in recent years. Due to the advantages of laser diodes based on nitrides, such as small size, high brightness, visible light and high bandwidth, it can be applied to illumination and communication at the same time. In this paper, blue laser and yellow phosphors were employed to synthesize white light. And “efficiency droop” is not observed in the LIV characteristics of LD-based white light either. The bandwidth measurement system with high reliability was set up. The bandwidth of blue laser diode and phosphor-conversion laser-based white light was measured. The maximum of optical ?3 dB bandwidth of blue LD is around 1.8 GHz at 80 mA and maximum of optical ?3 dB bandwidth of white light is about 1.3 GHz at 60 mA. The color parameters of the synthetic white light were characterized through integrating sphere. Moreover the trends of test data with injection current were analyzed in detail. The problem of thermal degradation of yellow phosphors has been improved by a special design that can keep a certain distance between the blue laser diode and phosphors. The experiment results verified that laser diodes based on nitrides have promising applications in lighting and communications.     

MOVPE 生长GaN薄膜的 光致黄带发光与激发光源的相关性

用四种不同光源作为激发光源,研究了蓝宝石衬底金属有机物汽相外延方法生长的氮化镓薄膜的光致发光特性。结果发现用连续光作为激发光源时,光致发光谱中除出现365 nm的带边发射峰外,同时观察到中心波长位于约550 nm 的较宽黄带发光;而用脉冲光作为激发光源时其发光光谱主要是365 nm附近的带边发光峰,未观察到黄带发光。氮化镓薄膜的光致发光特性依赖于所用的激发光源性质。     

Amplified luminescence and threshold current temperature dependencies of ZnSe and GaN laser diodes

Using a transfer equation approach, it is shown that the cavity-trapped amplified luminescence (AL) flux densities S integrated over the frequency and cavity length at the threshold of ZnSe, zinc-blende and wurtzite GaN laser diodes (LDs) with stripe-geometry bulk active layers reach (3.5–9)×10⁴ MW/m² within the temperature range 200–400 K. At these values of S, the nonlinear optical effects induced by AL can be observed. The AL induced recombination can enhance the threshold current density of the short wavelength LDs by two or even three times. Received: 7 January 2002 / Revised version: 11 April 2002 / Published online: 8 August 2002     

Design of hemispherical cavities for LED-based illumination devices

We present the design procedure for an illumination system based on the association of several emitting diodes in optical cavities with a hemispherical shape. The main purpose of this illumination system is to provide high amounts of monochromatic light over small areas, thus making it adequate for testing radiation-sensing devices. A detailed methodology for the optimization of the optical system, by taking into account both the electro-optical properties of the emitting diodes and the geometry of the cavity, is described. The irradiance on the working plane is increased by using a pulsed current operation mode at low duty cycles for light emitting diodes. The performance of two illumination devices designed with this methodology has been tested through the measurement of some concentrator GaAs solar cells. Current densities up to 5 A/cm² have been obtained in these cells with a cavity based on infrared emitting diodes. PACS 42.15.Eq; 85.60.Bt; 85.60.Jb     

III‐nitride quantum dots for ultra‐efficient solid‐state lighting

III‐nitride light‐emitting diodes (LEDs) and laser diodes (LDs) are ultimately limited in performance due to parasitic Auger recombination. For LEDs, the consequences are poor efficiencies at high current densities; for LDs, the consequences are high thresholds and limited efficiencies. Here, we present arguments for III‐nitride quantum dots (QDs) as active regions for both LEDs and LDs, to circumvent Auger recombination and achieve efficiencies at higher current densities that are not possible with quantum wells. QD‐based LDs achieve gain and thresholds at lower carrier densities before Auger recombination becomes appreciable. QD‐based LEDs achieve higher efficiencies at higher currents because of higher spontaneous emission rates and reduced Auger recombination. The technical challenge is to control the size distribution and volume of the QDs to realize these benefits. If constructed properly, III‐nitride light‐emitting devices with QD active regions have the potential to outperform quantum well light‐emitting devices, and enable an era of ultra‐efficient solid‐state lighting.

     

Photoinduced improvement of the properties of light-emitting diodes

Results from studies of the effect of the action of optical radiation on the characteristics of light-emitting diodes (LEDs) produced using the binary heterostructure Ga_xAl_1−xAs (λ=0.88 μm) are presented. High sensitivity of the LED to the following parameters of the optical radiation is shown: flux density, quantum energy, and exposure dose. The action of optical radiation in the form of a band with a maximum at 255 nm on the LED heterostructures lowers the leakage current into the bulk, decreases the loss identified as surface leakage current by about an order of magnitude, increases the radiated power by 50–100% in the current region up to 10⁻³ A, and increases the overall light output of the diodes. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 109–114, August, 1997.     

Comparison between blue lasers and light‐emitting diodes for future solid‐state lighting

Solid‐state lighting (SSL) is now the most efficient source of high color quality white light ever created. Nevertheless, the blue InGaN light‐emitting diodes (LEDs) that are the light engine of SSL still have significant performance limitations. Foremost among these is the decrease in efficiency at high input current densities widely known as “efficiency droop.” Efficiency droop limits input power densities, contrary to the desire to produce more photons per unit LED chip area and to make SSL more affordable. Pending a solution to efficiency droop, an alternative device could be a blue laser diode (LD). LDs, operated in stimulated emission, can have high efficiencies at much higher input power densities than LEDs can. In this article, LEDs and LDs for future SSL are explored by comparing: their current state‐of‐the‐art input‐power‐density‐dependent power‐conversion efficiencies; potential improvements both in their peak power‐conversion efficiencies and in the input power densities at which those efficiencies peak; and their economics for practical SSL.     

MOCVD生长的InGaN合金的发光特性

研究了用金属有机化学气相沉积(MOCVD)法在蓝宝石基底上生长的InGaN样品的发光特性。样品XRD谱中存在InGaN、In、InN相,表明样品中存在相分离;透射谱能看到由于F-P腔调制引起的震荡;相对氙灯激发发光谱,激光激发的发光谱其发光峰位置发生蓝移。由于样品上下表面形成F-P腔,对发光谱产生强烈的调制,在较高强度激发下,在室温下带边峰分成三个峰,其中波长较短的两个发光峰表现出相同的特征,其发光机制可能为以In量子点为局域中心的局域化激子复合发光,而波长较长的发光峰,是一个超线性受激发光峰,其发光机制可能是电子-空穴等离子的散射。不同温度的PL谱表明两个主要的发光峰表现出不同的温度特征,利用F-P干涉理论分析可知,当温度高于120K后样品折射率随着温度的升高而增大。     

Investigation of irradiance efficiency for LED phototherapy with different arrays

Red and yellow light emitting diodes (LEDs) are currently utilized as lighting sources during LED phototherapy. These LEDs were arranged on a disk with an external diameter of 70 mm with different arrays — radial, rhombus, square radial, and square rhombus arrays. The radial and square radial arrays had better irradiance efficiency than rhombus and square rhombus arrays by optical simulation. Additionally, the radial array had 76 sets of LEDs, but the square radial array had 100 sets. Thus, a mockup sample of radial array phototherapy was constructed for performance tests. The mixture efficiency of the radial array was observed at distances of 1-100 mm and lighting was well mixed when distance exceeded 50 mm by optical simulation. Irradiance variation with angle was approximated by experiment and theory at a treatment distance of 50 mm and 100 mm using the phototherapy mockup. The radial array was one good choice for LED phototherapy.     

Optoelectronic properties of Ni–GaP diodes with a modified surface

We report the promising results for Ni–GaP Schottky diode structures manufactured on the substrates with chemically-etched nano-scale surface formations that are responsible for a clearly marked luminescence band located at the energy exceeding the band gap of the bulk GaP. The other peculiarity produced by surface patterning concerns a remarkable redshift of material's optical absorption edge. At the room temperature, the height of potential barrier for Ni–GaP structure is 1.8 eV, with the monochromatic sensitivity peaking at 0.35 A/W. The comparative study of diode performance under different light sources exhibited the pronounced linear photocurrent-illumination dependence for about five orders of illumination magnitude, evidencing good optical and electrical quality of Ni–GaP diodes with surface-modified semiconductor substrate.     

Photoelectric instability in oxide glass

The spatially periodic modulation of optical anisotropy (MOA) induced in oxide glass by mutually coherent light beams with different frequencies (ω and 2ω) is unstable under illumination with monochromatic light with frequency ω. Disturbances with small amplitudes intensify and disturbances with large amplitudes relax. Irrespective of its initial degree, the MOA reaches the same steady-state level, which depends on the illumination intensity. Intensification of MOA is accompanied by the appearance of second-harmonic radiation whose intensity grows in time to a steady-state level. The instability of the anisotropy is due to degenerate three-wave mixing and to feedback arising as a result of the coherent photogalvanic effect. A hypothesis that takes into account the observed giant growth (by three orders of magnitude) of light absorption in the MOA region is proposed to explain the stabilization of the instability and the formation of stationary periodic refractive-index gratings. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 771–776 (25 December 1997)     

Investigation of wavelength-dependent efficiency droop in InGaN light-emitting diodes

The physical mechanisms leading to the efficiency droop of InGaN/GaN light-emitting diodes (LEDs) are theoretically investigated. We first discuss the effect of Auger recombination loss on efficiency droop by taking different Auger coefficients into account. It is found that the Auger recombination process plays a significant nonradiative part for carriers at typical LED operation currents when the Auger coefficient is on the order of 10⁻³⁰ cm⁶ s⁻¹. Furthermore, the InGaN/GaN multiple-quantum-well (MQW) LEDs with varied indium compositions in InGaN quantum wells are studied to analyze the wavelength-dependent efficiency droop. The simulation results show that the wavelength-dependent efficiency droop is caused by several different effects including non-uniform carrier distribution, electron overflow, built-in electrostatic field induced by spontaneous and piezoelectric polarization, and Auger recombination loss. These internal physical mechanisms are the critical factors resulting in the wavelength-dependent efficiency droop in InGaN/GaN MQW LEDs.     

Fast acquisition of reflectance spectra using wide spectral lines

We analyze a recently proposed technique for fast acquisition of the two-dimensional (2D) spatial distribution of reflectance spectra to figure out how much its ability of distinguishing almost similar spectra declines with increase of the spectral line bandwidth of the light source. This analysis was carried out using the experimentally measured reflectance spectra of four metameric samples and simulating the system response to an illumination by spectral lines with variable bandwidth. It was shown that the metameric samples are distinguishable even when the bandwidth of illuminating lines is 20–30 nm. A wider bandwidth allows implementation of simultaneous illumination of an object that leads to a diminution of the acquisition time of 2D-multispectral images due to both faster operation in the parallel mode of Light-emitting diodes (LEDs) switching and higher output power.     

Micro-lens arrays generated by UV laser irradiation of doped PMMA

Micro-lenses with well-defined optical parameters are generated on polymethylmethacrylate (PMMA) substrates doped with diphenyltriazene (DPT) by controlled use of a swelling effect generated under conditions of subablative excimer laser illumination. The surface profiles depend on the laser spot size and energy density. A sensitively balanced combination of matrix softening, substrate volume expansion due to photochemical nitrogen release, and surface tension is responsible for the final shape of the lenses. Complete arrays of identical lenses with 15 μm diameters and a focal length of 30 μm are produced by irradiation of (0.25 wt. %) DPT-PMMA with a single laser pulse at a wavelength of 308 nm and a fluence of 3 J/cm². It is shown experimentally and theoretically that appropriate volume expansion is possible without introducing internal light scattering due to the formation of small bubbles. Received: 7 April 1999 / Accepted: 8 April 1999 / Published online: 5 May 1999     

New collection systems for multi LED light engines

Light emitting diodes (LEDs) have numerous advantages as light sources in projectors. LEDs are more compact, exhibit a larger color gamut, have a longer lifetime, and need a lower supply voltage. However, there is still one important disadvantage: the optical power per unit of étendue (luminance) of an LED is significantly low. As a result of the étendue limitations of LEDs, the projected flux on the screen will not be high. Despite this shortcoming, LED’s are still of great interest for low power applications because of their other superior properties. Thus we collect the available light flux optimally and combine multiple high luminance LEDs within the system. In this study we discuss three collection systems designed to collect the LED flux with high optical efficiency while retaining small device size. The best collection efficiency attained with our collection systems is 96%. The fabrication tolerance and cost of our collection systems are also analyzed.     

Color tunable light‐emitting diodes with modified pulse‐width modulation

In this study, a color tunable light source, operated by a modified pulse width modulation method, is investigated. By utilizing this method along with anti‐parallel connected discrete light‐emitting diodes (LEDs) and two electrical terminals, a wide range of the chromaticity coordinates is attained and varied by electrical control. Using the combination of a blue LED and a phosphor‐converted yellow LED (blue LED plus yellow phosphor), the chromaticity range is varied by electrical control from pure blue to pure yellow. In addition, using the modified pulse‐width modulation method and a combination of white and red LEDs, white light with correlated color temperatures ranging from 5000 K to 2000 K is demonstrated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nd-doped phosphate glass microstructured optical fiber laser

We experimentally demonstrated a single-mode laser at 1056 nm with Nd-doped phosphate glass microstructured optical fiber (MOF), which was fabricated with conventional stack-and-draw method. The laser action was observed from a Fabry-Perot cavity formed by placing two dichroic mirrors of ∼100 and 85% reflectivity, to the two end facets of MOF. Pumped by CW laser diodes (LDs) at 808 nm, the MOF laser yielded a maximum output power of 8.5 mW and a slope efficiency of 2%.     

Talbot effect under illumination of double femtosecond laser pulses

The Talbot effect under illumination of double femtosecond laser pulses has been reported. Spectrums of double femtosecond laser pulses with phase differences are quite different from that of one single femtosecond laser pulse. Therefore, the Talbot images of the double femtosecond laser pulses with phase differences are different from that of one single femtosecond laser pulse. Specifically, for the phase difference corresponding to π, the Talbot image shows the largest difference from that of one single pulse. Experimental results are in good agreement with the theoretical analysis. The behaviors of Talbot images under double femtosecond laser pulses illumination cannot be obtained under one femtosecond laser pulse, monochromatic or polychromatic light illumination. Therefore, it is a new interesting optical phenomenon for the Talbot effect which should have potential applications.     

功率型白光LED光学特性退化分析

将GaN基蓝光芯片涂敷YAG荧光粉和透明硅胶制成额定功率为1 W的白光发光二极管(LED),对其施加900mA的电流应力,在老化过程中测量白光LED的主要光学参数,考察其光学特性的退化情况。经过4 200 h的老化,样品光通量退化为初始值的15%~18% 。样品的漏电流明显增大,表明芯片有源区缺陷密度提高,但光谱分布图中蓝光部分的辐射量未减少,仅观察到黄光部分辐射量的减少,推断出YAG荧光粉的转换效率降低。同时,从原理上分析了样品色温逐渐增大,显色指数基本不变的原因,对大功率白光LED在照明领域的应用有一定的借鉴意义。     

Fabrication of complex nanostructures on ZnO crystals by the interference of three femtosecond laser beams

By adjusting the laser polarization combinations, fluences and pulse numbers, we fabricated several types of two-dimensional (2D) complex nanostructures on the surface of c-cut ZnO single crystal by the interference of three femtosecond laser beams with central wavelength of 800 nm, pulse duration of 50 fs and pulse repetition frequency of 1 kHz. The hexagonal 2D nanostructures with a period of 600 nm are very regular and uniform, in which nanoparticles, nanorings and nanoripples with sizes of 200 nm are embedded. Excited by 800 nm femtosecond laser pulses, the photoluminescence (PL) micrographs reveal that the 2D nanostructures can emit purer and brighter blue light compared with the plane surface. These nanostructures have potential applications in blue light-emitting diodes (LEDs), high density optical storage and other optoelectronic devices.