Kyong Hon Kim

Comparative study of energy transfers from Er3+ to Ce3+ in tellurite and sulfide glasses under 980 nm excitation

We have demonstrated that the population feeding from the 4I11/2 level to the 1.5 μm fluorescence emitting 4I13/2 level of Er3+ ions in low phonon energy glass hosts can be enhanced by codoping with Ce3+ under optical pumping at 980 nm. The nonradiative energy transfer Er3+: 4I11/2; Ce3+: 2F5/2→Er3+: 4I13/2; Ce3+: 2F7/2, occurs in the form of phonon-assisted energy transfer, and therefore the feeding rates are faster in the tellurite glasses, which have a comparatively higher phonon energy than in the sulfide glasses. The cross-relaxation process for 4I13/2: 4I13/2→4I15/2: 4I9/2, which lowers the population density of the 4I13/2 manifold and causes a deleterious effect in the 1.5 μm fluorescence intensity, is more severe in the sulfide glasses. Population feeding rate from the 4I11/2 to the 4I13/2 level is significantly enhanced by way of cerium codoping into tellurite glasses, which promises an efficient 980 nm pumped broadband Er3+-doped fiber amplifier.

Emission properties of the Er3+:4I11/2→4I13/2 transition in Er3+- and Er3+/Tm3+-doped Ge–Ga–As–S glasses

Abstract Ge–Ga–As–S glasses were investigated to develop efficient host materials for the Er 3+ :2.7 μm fiber lasers. Ge30Ga1As9S60 and Ge30Ga2As6S62 (all in at.%) glasses show good thermal stability and high rare-earth solubility up to 0.3 mol%. Mid-infrared emission with a peak wavelength of 2.76 μm and bandwidth of ∼120 nm was observed in Er3+- and Er3+/Tm3+-doped Ge30Ga2As6S62 glasses. Codoping of Tm3+ significantly reduced the lifetime of the Er 3+ : 4 I 13/2 level due to the energy transfer of Er 3+ : 4 I 13/2 → Tm 3+ : 3 F 4 . Thus, the population inversion between the 4 I 11/2 and 4 I 13/2 levels in Er3+ became possible. In Er3+-doped glasses, the rate of cross-relaxation for the 4 I 11/2 level was approximately 11 times faster than that for the 4 I 13/2 level.

1.6 μm emission from Pr3+: (3F3,3F4)→3H4 transition in Pr3+- and Pr3+/Er3+-doped selenide glasses

1.6 μm emission originated from Pr3+: (3F3,3F4)→3H4 transition in Pr3+- and Pr3+/Er3+-doped selenide glasses were investigated under an optical pump of a conventional 1480 nm laser diode. The measured peak wavelength and full width at half maximum of the fluorescent emission were ∼1650 and >100 nm, respectively. A moderate lifetime of the thermally coupled upper manifolds (∼212±5 μs) together with a high stimulated emission cross section of ∼(3±1)×10−20 cm2 promises to be useful for 1.6 μm band fiber-optic amplifier that can be pumped with an existing high-power laser diode. Codoping of Er3+ significantly enhanced the emission intensity by way of a nonradiative Er3+: 4I13/2→Pr3+: (3F3,3F4) energy transfer.

Optical Injection-Induced Polarization Switching Dynamics in 1.5-

We report the first experimental invetigation of the polarization-mode switching dynamics and injection-wavelength-dependent polarization-mode bistability of a 1.5-m wavelength single-mode vertical-cavity surface-emitting lasers (VCSELs) under external laser beam injection. An injection beam with polarization orthogonal to that of the stand-alone VCSEL caused polarization-mode instability and switching of the VCSEL output. By varying the optical injection detuning for fixed injection power observation was made of a novel form of polarization bistability which will have applications in a new type of all-optical flip-flop and signal processing scheme.

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Abstract We have analyzed Cr K-edge X-ray absorption spectra of Cr-doped soda-lime-silicate and calcium-aluminate glasses of which the compositions were 15Na2O–15CaO–70SiO2 and 10MgO–55CaO–35Al2O3 in mole%, respectively. It was estimated that the amount of Cr4+ formed in the aluminate glass was ∼85±10% out of the total nominal chromium content, i.e., 0.06 mole%, and the remaining chromium ions were in 6+ oxidation state. In the silicate glass, Cr3+ was accompanied by Cr6+ the portion of which was relatively small compared to that in aluminate glass. Short-range structural units of chromium in the aluminate glass were [CrO4] and severely distorted [CrO6] with a Cr–O bond distance of ∼1.72 A , while [CrO6] polyhedra prevailed in the silicate glass with an average Cr–O length of ∼1.96 A .

m Wavelength Single-Mode Vertical-Cavity Surface-Emitting Lasers

We report, for the first time to our knowledge, the demonstration of amplitude equalization of high-repetition-rate pulses generated from a rational harmonic mode-locked Er-doped fiber laser. The output pulses are injected into another fiber laser with a nonlinear optical loop mirror. This scheme provides pulse-amplitude equalization up to the ninth rational harmonic mode-locked pulse train.

Oxidation state and local coordination of chromium dopant in soda-lime-silicate and calcium-aluminate glasses

We demonstrate successfully an all optical clock recovery scheme based on all-fiber devices from nonreturn-to-zero format data of 10 Gb/s with a stabilized asymmetric Mach-Zehnder interferometer and a mode-locked erbium-doped fiber laser. This all-optical clock recovery method will find application in ultrahigh-speed all-optical communication systems.

Pulse-amplitude-equalized output from a rational harmonic mode-locked fiber laser

Abstract The thermal treatment in reducing atmospheres gives rise to the increase both in emission intensity and 10% decay time in Zn 2 SiO 4 :Mn phosphors. The present investigation aims to take account of such changes in association with the structural change. For this sake, X-ray absorption spectroscopy techniques such as XANES and EXAFS were conducted to the Zn 2 SiO 4 :Mn phosphors. The Zn 2 SiO 4 :Mn phosphors were fired in the air and then thermally treated in two different reducing atmospheres (hydrogen or carbon). The photoluminescent (PL) behavior was closely related to the X-ray absorption data. The XANES and EXAFS prove that the oxidation state (+2) remains identical regardless of whether or not the samples are treated, but that the Mn–O distance was reduced by the heat treatment. In order to give a plausible interpretation to the change in PL results, two possible suggestions are presented. Firstly, it is conceivable that the thermally activated diffusion process of manganese ions splits Mn–Mn pair during the heat treatment. Another possibility is that the thermal treatment annealed out some quenching site, which is related with defects and impurities. Such hypotheses can be rationalized systematically by considering the results from lifetime measurement, Debye–Waller factor calculation, and XANES pre-edge peaks.

All-optical clock recovery from NRZ data of 10 Gb/s

We demonstrate a compactly integrated polarization beam splitter (PBS) with high polarization extinction ratios greater than 20 dB over the full C-band wavelength range based on a simple bridged silicon nanowaveguide directional coupler. The PBS device is designed via three dimensional finite-difference time-domain (3D-FDTD) simulation, and fabricated experimentally. The optimum dimension of the bridge waveguide is determined to be 7.5-μm-long and 500 nm-wide for 250-nm thick silicon core. At the 1,550-nm wavelength, the measured polarization extinction ratios (PERs) of the PBS device are 22.5 dB and 22.9 dB for TE and TM polarization modes, respectively, and its corresponding insertion losses (ILs) are about 2.1 dB and 1.8 dB, both PERs and ILs within the maximum error range of ± 2.0 dB.

Effect of heat treatment on photoluminescence behavior of Zn2SiO4:Mn phosphors

We report a simple method of measuring the absolute values of the phase refractive index of an optical material of a flat plate shape over a wide spectral range at a single measurement run. A white-light interferometric technique with angle rotation of the optical plate sample located in one of the interferometer arms was used in this method. The validity of this method was proved by measuring the absolute phase refractive indices of flat plate samples of fused silica and BK7, and by comparing them with calculated values from their well-known Sellmeier dispersion formulas. The accuracy of this refractive index measurement method was within 0.002, which can be further improved by enhancing the angle measurement accuracy of the angle rotating stage used in this method.