Yi-Chung Huang

Novel broadband glass phosphors for high CRI WLEDs

New broadband glass phosphors with excellent thermal stability were proposed and experimentally demonstrated for white light-emitting-diodes (WLEDs). The novel glass phosphors were realized through dispersing multiple phosphors into SiO₂ based glass (SiO₂-Na₂O-Al₂O₃-CaO) at 680°C. Y₃Al₅O₁₂:Ce³⁺ (YAG), Lu₃Al₅O₁₂:Ce³⁺ (LuAG), and CaAlSiN₃: Eu²⁺ (nitride) phosphor crystals were chosen respectively as the yellow, green, and red emitters of the glass phosphors. The effect of sintering temperature on inter-diffusion reduction between phosphor crystals and amorphous SiO₂ in nitride-doped glass phosphors was studied and evidenced by the aid of high-resolution transmission electron microscopy (HRTEM). Broadband glass phosphors with high quantum-yield of 55.6% were thus successfully realized through the implementation of low sintering temperature. Proof-of-concept devices utilizing the novel broadband phosphors were developed to generate high-quality cool-white light with trisstimulus coordinates (x, y) = (0.358, 0.288), color-rending index (CRI) = 85, and correlated color temperature (CCT) = 3923K. The novel broadband glass phosphors with excellent thermal stability are essentially beneficial to the applications for next-generation solid-state indoor lighting, especially in the area where high power and absolute reliability are required.

High-performance glass phosphor for white-light-emitting diodes via reduction of Si-Ce 3+ :YAG inter-diffusion

A novel Ce3+:YAG doped sodium glass (CeYDG) with low-melting temperature of 693°C and high internal quantum yield of 68% for white-light-emitting diodes (WLEDs) is demonstrated. The glass phosphor possesses glass transition temperatures of 578°C which exhibits a better thermal stability to overcome the thermal limitation of conventional Ce3+:YAG doped silicone due to low thermal stability of around 150°C. To the best of authors’ knowledge, this is the highest quantum yield yet reported for thermally stable glass phosphors. The high quantum yield is achieved by lowering the sintering temperature of 700°C for glass phosphor, which substantially reduces Si-Ce3+:YAG inter-diffusion, evidenced by high-resolution transmission electron microscopy (HRTEM). This new CeYDG with high-quantum yield is essentially beneficial to the applications for next-generation solid-state lighting in the area where high power and absolute reliability are required and where silicone simply could not stand the heat or other deteriorating factors due to its low thermal stability.

Broadband emission from Cr-doped fibers fabricated by drawing tower.

We report on the first fabrication of a Cr-doped fiber using a drawing-tower method with Cr:YAG as the core of the preform. Both Cr3+ and Cr4+ ions coexist in the Cr-doped fiber, and the amplified spontaneous emission (ASE) spectrum shows a broadband emission of 1.2 to 1.55 mum which can not be realized by using currently available fiber amplifiers. This indicates that the new Cr-doped fibers have the potential for being used as a broadband fiber amplifier to cover the bandwidth of the entire 1.3-1.6 mum range which exhibit 300 nm usable spectral bands.

Decay Mechanisms of Radiation Pattern and Optical Spectrum of High-Power LED Modules in Aging Test

Decay of radiation pattern and optical spectrum of high-power LED modules fabricated by different manufacturers after a thermal-aging test were investigated experimentally and numerically. Samples were aged at 65degC, 85degC, and 95degC under a constant current of 350 mA. The results showed that the radiation pattern of the LED modules at the two view angles of plusmn(45deg ~ 75deg) decreased more than the other angles as aging time increased. This was due to the reduction of optical power from corner shape of lens. Due to the degradation of lens material after thermal aging, the center wavelength of the LED spectrum shifted 5 nm. Furthermore, the radius curvature of plastic lens was observed to have 6-70 mum contraction as aging times increased. The key module package related to the decrease of power density, the change of radiation pattern, and the shift of optical spectrum in high-power LED modules under thermal aging were due to the degradation of lens material and lens structure. Both experimental and simulated results clearly indicated that improving the lens structure and lens material is essential to extend the operating life of the high-power LED modules. This study may provide practical LED package guidelines in low-cost consumer applications.

Chromaticity tailorable glass-based phosphor-converted white light-emitting diodes with high color rendering index

In this paper, Lu3Al5O12:Ce3+ and CaAlSiN3: Eu2+ co-doped glass are presented as color conversion materials for white light-emitting diodes (WLEDs). Through adjusting the thickness of the glass phosphors, the chromaticity and CCT of the WLEDs follows the Planckian locus well. The WLEDs show CCT ranging from ~4000K to ~7000K with high CRI ranging from 83 to 90 due to the wide emission spectrum from the proposed glass phosphors. The glass phosphors provide an effective way to achieve chromaticity-tailorable WLEDs with high color quality for indoor lighting applications.

Preform fabrication and fiber drawing of 300 nm broadband Cr-doped fibers

The fabrication of a Cr-doped fiber using a drawing-tower method with Cr:YAG as the core of the preform is presented. The Cr-doped YAG preform was fabricated by a rod-in-tube method. By employing a negative pressure control in drawing-tower technique on the YAG preform, the Cr-doped fibers with a better core circularity and uniformity, and good interface between core and cladding were fabricated. The amplified spontaneous emission spectrum showed a broadband emission of 1.2 to 1.6 mum with the output power density about a few nW/nm. The results indicate that this new Cr-doped fiber may be used as a broadband fiber amplifier to cover the bandwidths in the whole 1.3-1.6 mum range of low-loss and lowdispersion windows of silica fibers.

Development of Broadband Single-Mode Cr-Doped Silica Fibers

The fabrication of broadband single-mode Cr-doped silica fibers (SMCDSFs) using the fiber drawing-tower method with the modified rod-in-tube technique is demonstrated for the first time. A single-mode characteristic of SMCDSF was observed when the propagation wavelengths were longer than 1310 nm. The transmission loss was about 8 dB/m at 1550 nm. The successful fabrication of SMCDSFs may facilitate the possibility for utilizing the SMCDSFs as a new generation broadband fiber amplifier to cover the bandwidths in the whole 1300- to 1600-nm range of low-loss windows of silica fibers.

Broadband Chromium-Doped Fiber Amplifiers for Next-Generation Optical Communication Systems

We report the first experimental breakthrough of a net gain of optical signals in a broadband chromium-doped fiber amplifier (CDFA) for next-generation optical communication systems. Current fiber amplifiers, including commercial erbium-doped fiber amplifier, cover only a relatively small portion of the entire transmission bandwidths (1300-1600 nm) of the low-loss windows of silica fibers. The newly developed CDFAs have opened up the possibility of utilizing the 300-nm entire spectrum of the low-loss fibers to further increase the transmission capacity. In this paper, we present the experimental demonstration of a net gain of 1.2 dB employing gain enhancement technique. With the help of an optical-fiber system examination for the CDFA, a 40-Gb/s error-floor free data transmission was successfully demonstrated by realizing the high-speed transmission of signal with gain through the chromium-doped fibers (CDFs). Further gain improvement in the CDFAs employing few-mode or single-mode CDFs will be presented and discussed.

Thermal-Stability Comparison of Glass- and Silicone-Based High-Power Phosphor-Converted White-Light-Emitting Diodes Under Thermal Aging

The lumen degradation, chromaticity shift, transmittance loss, and mean-time-to-failure (MTTF) evaluation in glass- and silicone-based high-power phosphor-converted white-light-emitting diodes (LEDs) (PC-WLEDs) under accelerated thermal aging at 150 °C, 200 °C, and 250 °C are presented and compared. The silicone-based PC-WLEDs exhibited less thermal stability than the glass-based PC-WLEDs by 1.86, 2.79, and 4.76 times higher lumen losses, 3.05, 3.26, and 6.84 times larger chromaticity shifts, and 1.82, 2.62, and 6.67 times greater transmittance losses at 150 °C, 200 °C, and 250 °C, respectively. The results also showed that the glass-based PC-WLEDs exhibited higher MTTF than the silicone-based PC-WLEDs by 20 times at room temperature. The peaks of the emission and excitation spectra for both silicone and glass phosphors were not significantly changed after thermal aging, evidenced by fluorescence spectrophotometer analyses. This indicated that the fluorescent ability of Ce:YAG-doped phosphor materials did not change after thermal aging and the transmittance loss was corresponding to the lumen loss and chromaticity shift. The results of the lumen loss, chromaticity shift, transmittance loss, and MTTF investigations clearly demonstrated that the thermal-stability performance of the glass-based PC-WLEDs was better than that of the silicone-based PC-WLEDs. The advantage of employing doped glass encapsulation in high-power PC-WLEDs could be explained: The material property of glass transition temperature of 750 °C was higher than that of silicone transition temperature of 150 °C. A better thermal stability phosphor layer of glass as encapsulation material may be beneficial to many applications where the LED modules with high power and high reliability are demanded for use in next-generation solid-state lighting.

Fabrication and characteristics of Cr-doped fibers employing powder-in-tube technique

The fabrication of Cr-doped fibers (CDFs) using fiber drawing-tower combined with non-silica powder-in-tube (PIT) technique to achieve the active application of chromium ions in fiber is demonstrated for the first time. The fluorescence enhancement, transmission loss, and few-mode operation are discussed in the paper. The success in fabrication of CDFs with PIT method may open the possibility for utilizing CDFs as broadband fiber light source.