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Optical design for LED illumination in application of implantable retinal prostheses
(2019-01-01)
Huang, Ting-Ming
;
Chen, Fong-Zhi
;
Chiu, Chin-Fong
;
Wu, Chung-Yu
;
電子工程學系及電子研究所
For those patients with degenerated photoreceptors such that fail to transfer visual information to other retinal cells in the retinal network, a 16x16 pixel self-powered implantable subretinal device has been developed trying to restore their vision. The device contains sensing pixels that collect environmental signal, and electrodes that serve neurostimulation. A photovoltaic cell was designed aside of the sensing pixels and electrodes for power supplying. IR LED was selected to radiate on the photovoltaic cells for energy input such that proper optical layout shall be designed. Several extraocular optical layouts for IR LED illumination are studied to evaluate the perfect efficiency of each layout. Suitable design is to be selected through the evaluation process. Some experiments are performed to study the feasibility of optical layout. Images of IR LED are taken by a CMOS sensor. Irradiance on the calibrated CMOS sensor is calculated for comparison.
Efficiency Improvement of Near-Ultraviolet Nitride-Based Light-Emitting-Diode Prepared on GaN Nano-Rod Arrays by Metalorganic Chemical Vapor Deposition
(2014-03-01)
Kuo, Cheng Huang
;
Chen, Yu An
;
Wu, Ji Pu
;
Chang, Li Chuan
;
照明與能源光電研究所
Near-ultraviolet nitride-based light-emitting diodes (LEDs) were prepared on GaN nanorod arrays (NRAs). Transmission electron microscopy results show that the NRAs partially block threading dislocation propagation from the GaN template. Compared with conventional LEDS, the output power of the prepared LED using GaN NRAs with an injection current of 20 mA was enhanced by a factor of 1.67.
1.7 to 2.3 Gbps OOK LED VLC Transmission Based on 4 x 4 Color-Polarization-Multiplexing at Extremely Low Illumination
(2019-08-01)
Yeh, Chien-Hung
;
Weng, Jui-Hung
;
Chow, Chi-Wai
;
Luo, Chien-Ming
;
Xie, Yue-Ru
To achieve the high traffic rate of a GaN LED-based visible light communication (VLC), a 4 x 4 color-polarization-multiplexing system is demonstrated by using two blue and two green LEDs. In the measurement, an available bandwidth of blue and green LEDs can be greater than 465 MHz for VLC modulation traffic by using the suitable analogy front end circuit. Hence, the proposed VLC system can deliver 1.7 to 2.3 Gbit/s ON-OFF keying traffic rates in the free space transmission length of 1-4 m under the extremely low illumination of 6.9-136.1 lux for the indoor application. Moreover, the measured BER of each LED is less than the forward error correction target of 3.8 x 10(-3) to maintain the better signal performance.
A Novel Tunable Green- to Yellow-Emitting beta-YFS:Ce3+ Phosphor for Solid-State Lighting
(2011-08-01)
Wu, Yun-Chen
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Wang, De-Yin
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Chen, Teng-Ming
;
Lee, Chi-Shen
;
Chen, Kuo-Ju
A Ce3+-activated fluorosulfide phosphor (beta-YFS: Ce3+) was synthesized by solid-state reaction in a sealed tube. The crystal structure has been refined from the XRD profiles and there are two different crystallographic rare earth sites, namely, Y(1) and Y(2), where the Ce3+ ions occupied. The emission band with a maximum at 495 nm of beta-Y0.99Ce0.01 FS phosphor was characterized by the 4f-5d transitions of Ce3+ ion. With increasing Ce3+ concentration, the emission variations were observed from 495 to 547 nm. When beta-YFS:Ce3+ phosphors were utilized to incorporate with n-UV/blue chip, greenish-white light with color rendering index of 65-77 were obtained. The results indicate that the tunable green- to yellow-emitting beta-YFS:Ce3+ can serve as a potential phosphor for incorporation in fabrication for solid-state lighting. The preparation, spectroscopic characterization, quantum efficiency, thermal-quenching behavior, and related LED device data are also presented.
InGaN-based light-emitting diodes grown on various aspect ratios of concave nanopattern sapphire substrate
(2018-10-31)
Ke, Wen-Cheng
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Chiang, Chih-Yung
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Son, Widi
;
Lee, Fang-Wei
;
電子物理學系
This study presented the optoelectronic properties of InGaN-based light-emitting diodes (LEDs) grown on nanoscale-patterned sapphire substrates (NPSSs) with various aspect ratio (AR) concave nanopatterns. Concave nanopatterns with low ARs of similar to 0.5 (LAR-NPSS) and high ARs of 2, 3 and 4 (HAR-NPSS) were achieved using an anodic aluminum oxide template as the dry etching mask. The diffuse transmission (DT) rate of the incident light wavelength at 450 nm increased linearly, from 2.2% for LAR-NPSS to 12.7% for HAR-NPSS with an AR of 4. However, the diffuse reflection (DR) rate for HAR-NPSS did not increase linearly with the increasing AR values. The limit of the DR rate for HAR-NPSS could be attributed to the guiding effect when the incident light entered into the high AR concave nanopattern; most of the incident light could not be reflected (returned to the air), therefore there was no significant contribution to the DR rate. The luminous intensities of LEDs grown on bare sapphire (B-LED), HAR-NPSS (HAR-LED) and LAR-NPSS (LAR-LED) were 39, 38 and 74 mcd, respectively. Interestingly, the lowest brightness of any LED was the HAR-LED, which had higher DT and DR rates. This significant discrepancy could be attributed to a large number of voids occurring at the initial growth stage of the LED epitaxial layer on the HAR-NPSS that caused the emitted photons to remain trapped inside the LED. In addition, increasing the LED epilayer coverage area (i.e. 1.6-fold) with the inclined facets of low AR concave nanopatterns effectively scattered the emitted photons, leading to an increased light emitting intensity of the LED. The experimental results indicated that the specific AR concave nanopattern of an NPSS could increase the LED epitaxial layer coverage area and prevent void formation, which plays a crucial role in achieving high-brightness in InGaN-based LEDs.
400Mbit/s OOK green-LED visible light communication with low illumination
(2018-12-01)
Yeh, C. H.
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Chow, C. W.
;
Gu, C. S.
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Guo, B. S.
;
Chang, Y. J.
In this work, we propose and demonstrate a LED-based visible light communication (VLC) system. Here, an analog front end (AFE) circuit with optimal equalization design is investigated to enhance the modulation bandwidth of LED. The VLC data rates of 340Mbit/s can be achieved under 1.5-3m free space transmissions with error-free (bit error ratio, BER<1x10(-9)). When the traffic rate is increased to 400Mbit/s, the BER satisfying the forward error correction (FEC) limit (BER=3.8x10(-3)) can be achieved at the low illumination of 34.1 Lux.
Structural and spectroscopic characterizations of a new near-UV-converting cyan-emitting RbBaScSi3O9:Eu2+ phosphor with robust thermal performance
(2017-08-05)
Ray, Sudeshna
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Tadge, Prachi
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Dhoble, S. J.
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Nair, Govind B.
;
Singh, Akash
To facilitate the next generation of White Light Emitting Diodes (WLEDs) with higher color rendering and warm lighting, the development of inorganic phosphor for efficient conversion of photons from blue/near-UV light to other visible wavelengths is essential. In this regard, we demonstrate a systematic, cost effective, solution-processable, easily scalable and fully controllable synthesis of a series of RbBaScSi3O9:xEu(2+) (x = 0.2, 0.5, 2, 5, 7), a new cyan emitting scandium silicate based phosphor with 79% internal quantum efficiency under n-UV excitation with robust thermal performance. The concept as well as the methodology of using a "Mineral Inspired Approach" emerges as a new blueprint for the rational design of novel phosphor for phosphor converted WLEDs. The structural refinement, electronic structural calculation using "Density Functional Theory", characteristic photoluminescence study, lifetime measurement as well as the thermal quenching properties of the phosphor has been investigated in detail. Owing to the highly efficient cyan emission at lambda(max) similar to 492 nm a full-width at half-maximum of 63 cm(-1) that shows only very low thermal quenching (89.28% relative to the PL intensity at 150 degrees C with respect to that of measured at room temperature), reflects the great potential for the industrial application of this particular phosphor. (C) 2017 Elsevier B.V. All rights reserved.
The study of wet etching on GaN surface by potassium hydroxide solution
(2017-06-01)
Lai, Yung-Yu
;
Hsu, Shih-Chieh
;
Chang, Hua-Sheng
;
Wu, YewChung Sermon
;
Chen, Ching-Hsiang
Potassium hydroxide solution was used to etch un-doped GaN grown on the sapphire substrate at 180 and 260 A degrees C. We illustrated the etching phenomenon in detail and probed its mechanism in the wet etching process. By multiplying the planar density and the number of dangling bonds on the N atom, we proposed the etching barrier index (EBI) to describe the difficulty degree of each lattice facet. The raking of EBI will be +c-plane > a-plane > m-plane > -c-plane > (10-1-1) plane > r-plane. Combining the EBI with SEM results, we thoroughly studied the whole etching process. We confirmed that in our research, KOH wet etching on GaN starts from the r-plane instead of the +c-plane or -c-plane, which differs from other studies. We also found that during the high-temperature etching process, there are two etching approaches. In one, the etching begins vertically from the top to the bottom, then horizontally, and finally reversely from the bottom to the top. In the other, etching pits will develop into a hexagonal hole of the sidewall of m-plane.
Synthesis, luminescence and application of BaKYSi2O7:Eu2+ : A new blue-emitting phosphor for near-UV white-light LED
(2018-05-01)
Ray, Sudeshna
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Tadge, Prachi
;
Dutta, Somrita
;
Chen, T. M.
;
Nair, Govind B.
A series of new silicate phosphors, Ba1-xEuxKYSi2O7 (BKY: Eu2+) (x = 0.005, 0.01, 0.02, 0.05, 0.07) have been synthesized from a water-soluble propylene glycol modified silane (PGMS) based silicon precursor by a solution approach. This phosphor can be efficiently excited by near-UV light and generates a broadband bright blue emission with internal Quantum Efficiency 34%. The detailed crystal structure of monoclinic BKY:Eu2+ was determined from the XRD profile using Rietveld refinement. Band gap was determined from Diffuse Reflectance Spectra using Kubelka Munk function. Steady state and time resolved Photoluminescence measurement as well as the thermal quenching properties of the phosphor have been investigated in detail. A highly efficient broadband blue emission centered lambda(max) similar to 460 nm with a full-width at half-maximum of 72 nm shows very low thermal quenching; 83.2% at 150 degrees C with respect to that measured at room temperature. Moreover, a white LED device has been fabricated by coating the as-synthesized phosphor with a standard red emitting CaAlSiN3:Eu2+ phosphor and a standard green emitting Ba2SiO4:Eu2+ phosphor. The LED device exhibited an excellent color-rendering index CRI of 84.0564 at a correlated color temperature of 3752.4 K with CIE color coordinates of (0.363, 0.293) under a 350 mA forward-bias current. The interesting excitation profile and robust thermal performance reflect the potential application of this phosphor for near UV- LED driven solid state lighting.
Using adaptive equalization and polarization-multiplexing technology for gigabit-per-second phosphor-LED wireless visible light communication
(2018-08-01)
Hsu, Chin-Wei
;
Yeh, Chien-Hung
;
Chow, Chi-Wai
;
光電工程學系
;
Department of Photonics
In the demonstration, we propose and investigate a phosphor white-light LED gigabit-per-second visible light communication (VLC) system by utilizing adaptive filtering and polarization-multiplexing technology. Due to the incoherent light of LED, two orthogonal linear polarizers can be utilized in the LED transmitter (Tx) side and client receiver (Rx) side, respectively, to produce 2 x 2 polarization-multiplex multi-input multi-output (MIMO) VLC transmission. Moreover, the Volterra nonlinear equalization (VNLE) filtering is proposed to improve the nonlinearity concern of LED-based VLC system, when the high spectral-efficient orthogonal frequency division multiplexing (OFDM) modulation is employed. Hence, the total VLC data rate of similar to 1.4 Gbps is reached and the VLC performance is also discussed in the proposed wireless VLC system. (C) 2018 Elsevier Ltd. All rights reserved.

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