Hoon Jeong

Properties of Defective Regions Observed by Photoluminescence Imaging for GaN-Based Light-Emitting Diode Epi-Wafers

A photoluminescence (PL) imaging method using a vision camera was employed to inspect InGaN/GaN quantum-well light-emitting diode (LED) epi-wafers. The PL image revealed dark spot defective regions (DSDRs) as well as a spatial map of integrated PL intensity of the epi-wafer. The Shockley-Read-Hall (SRH) nonradiative recombination coefficient increased with the size of the DSDRs. The high nonradiative recombination rates of the DSDRs resulted in degradation of the optical properties of the LED chips fabricated at the defective regions. Abnormal current-voltage characteristics with large forward leakages were also observed for LED chips with DSDRs, which could be due to parallel resistances bypassing the junction and/or tunneling through defects in the active region. It was found that the SRH nonradiative recombination process was dominant in the voltage range where the forward leakage by tunneling was observed. The results indicated that the DSDRs observed by PL imaging of LED epi-wafers were high density SRH nonradiative recombination centers which could affect the optical and electrical properties of the LED chips, and PL imaging can be an inspection method for evaluation of the epi-wafers and estimation of properties of the LED chips before fabrication.

Temperature Dependent Thermal Properties of a GaN‐based Laser Diode Analyzed by an Electrical Method

We report thermal properties of a high power GaN‐based laser diode depending on the ambient temperature, which are analyzed using forward voltage changes due to heat generation during a current injection. The increasing rate of junction temperature and the thermal resistance decrease with the ambient temperature rise. Decrease of resistivities of p‐doped layers at an elevated temperature and a thermoelectric effect at superlattice structures are considered as possible reasons for the temperature dependence.

Linearly polarized, 135-nm bandwidth pulse generation in an erbium-doped fiber ring laser

We present a linearly-polarized erbium-doped fiber laser generating 135-nm bandwidth pico-second pulses with excellent temporal and spectral stability. The pulse energy and width are readily reconfigurable via controlling the internal polarization state and pumping power.

Photoluminescence and electroluminescence properties of GaN-based LED chips with defective regions at low excitation levels

Defective regions of LED epi-wafers and chip-wafers could be observed by a photoluminescence (PL) imaging method. As the defective regions contain high density nonradiative recombination centers, the LED chips with defective regions showed different optical properties from those of LEDs with no defective regions found by PL imaging. At low optical excitation power levels, the PL properties were influenced by nonradiative recombination dominantly. Similar phenomena were observed from electroluminescence (EL) properties at low electrical excitation levels. Research results on the similarity and difference between PL and EL properties of LED chips are presented.

Quasi-continuous-wave Yb-doped Fiber Lasers with 1.5 kW Peak Power

High-power quasi-continuous-wave (qcw) operation in Yb-doped double-clad fiber lasers with near-diffraction-limited quality of the output beam is reported. Based on numerical simulation, we built a simple, all-fiberized Yb fiber laser, and a fiber-based master-oscillator power amplifier (MOPA). Both laser systems have successfully produced qcw output with average power greater than 150 W at 1080 nm and 10 ms pulse duration at 10 Hz repetition rate, corresponding to a peak power greater than 1.5 kW for 205 W of pump power at 976 nm. Laser performance, including beam quality and slope efficiency, was characterized in both configurations. Prospects for power scaling and applications are discussed.

Qausi-CW Yb fiber laser with 3 kW peak power

We report on a 3 kW quasi-cw Yb fiber laser with a 10 ms pulse width and a 10 Hz repetition rate by combining two Yb fiber lasers with a 1.5 kW peak power.

Investigations on correlation between photoluminescence images of an LED epi-wafer and characteristics of LED chips

Photoluminescence (PL) imaging is employed in order to inspect InGaN/GaN LED epi-wafers. The image shows a map of integrated PL intensity over the wafer and dark spots with degraded luminescence properties. Dark spots with various sizes indicate areas with nonradiative defects showing that the nonradiative recombination coefficient increases with the size. The PL images are compared with data obtained from LED chips on the wafer after fabrication process. The characterization results for LED chips show that most of the chips fabricated on the dark spots have degraded properties. The result indicates that PL imaging of epi-wafers could be an inspection tool to predict properties of LED chips.

Self-pulsing-free continuous-wave operation of an all-fiberized Yb fiber laser

Self-pulsing-free continuous-wave operation of an Yb fiber laser based on an all-fiberized configuration incorporating an additional fiber Fabry–Perot cavity is reported. The fiber Fabry–Perot cavity as a feedback source for lasing allows the fiber laser to operate in stable longitudinal modes resulting in suppression of the self-pulsing behavior. The Yb fiber laser yielded 5.9 W of the stable cw output at 1064 nm for 8.9 W of absorbed pump power corresponding to a slope efficiency of ~74%. The mechanism for suppression of the self-pulsing and the prospect of further power scaling are discussed.

Sol-Gel-based 1x2 power splitter for a plastic optical fiber

This paper presents a sol-gel based 1×2 power splitter for an optical communication based on the plastic optical fiber. To find out optimum parameters of a power splitter, mode propagation along the splitter was theoretically analyzed using BPM (Beam Propagation Method) and the results show that the distance between two arms at the output port of a splitter should be kept below 100μm in order to increase the output power. The planar lightwave circuit device was fabricated by a nano imprint lithography process followed by a spin coating process. The core size and the length of a power splitter were 230 μm×230 μm and 2 cm, respectively. The measured surface roughness of core/cladding using the AFM (Atomic Force Microscope) was under 100nm. The characteristics of a fabricated power splitter were conducted using an 850nm VCSEL (Vertical Cavity Surface Emission Laser) source and 50:50 power splitting performance was obtained.

The simple and practical variable optical attenuator using a piezoelectric sheet containing an optical fiber

In this letter, we proposed a new method for a variable optical attenuator (VOA) through controlling a mechanical misalignment between 2 single mode fibers using a piezoelectric sheet. A piezoelectric sheet with 3 electrodes is adopted in our proposed structure. We can change amount of the bend of the PZT sheet by controlling the applied voltage on the inner electrode of the PZT sheet, which causes the optical loss to be dependent on the applied voltage. The numerical analysis about the optical loss related to the various mechanical offsets is also investigated. From our experimental results, the dynamic range of the proposed structure is about from 0 to 56 dB when the applied voltage range is from 0 to 22V DC. In our previous work using the piezoelectric tube, the dynamic range is about from 0 to 25dB when it is from 0 to 600V DC. The required voltage to get the same attenuation is dramatically reduced. It can make it more practical in the optical communication field.