Youngchul Kwon

Effect of layering methods, composite type, and flowable liner on the polymerization shrinkage stress of light cured composites.

OBJECTIVES The aim of this study was to investigate the effect of layering methods, flowable composite liner and use of low shrinkage silorane-based composite on the polymerization shrinkage stress of light cured composites. METHODS Aluminum blocks were used to prepare MOD cavities and divided into four groups. A universal hybrid methacrylate-based composite (Z250), a flowable composite (Z350 flowable), and a silorane-based composite (P90) were used to fill the cavities. Cavities were restored using four different filling protocols. Group 1 was filled in bulk with Z250, group 2 was restored by an increment technique with the same composite, group 3 by an increment technique with Z250 and a Z350 flowable lining, and group 4 was restored by an increment technique with P90. The axial shrinkage strain and flexural modulus of the three composites were determined, and cuspal deflection of each group was measured with LVDT probes and compared among groups using ANOVA and Tukeys post hoc test (α=0.05). RESULTS The axial shrinkage strains of P90, Z250, and Z350 flowable were 1.09 (0.11), 2.29 (0.06), and 4.12 (0.08)%, respectively. The flexural modulus of P90 was 10.1 (0.9), Z250 was 13.6 (2.0), and that of Z350 flowable was 7.6 (0.9) GPa. The cuspal deflections at 33 min in groups 1-4 were 18.2 (1.54), 14.5 (0.47), 16.2 (1.10), and 6.6 (0.44) μm, respectively. The incremental filling technique yielded significantly lower cuspal deflection than the bulk filling technique. Flowable composite lining under universal composite (Z250) layering showed higher cuspal deflection than that without flowable composite lining. Silorane-based (P90) composite exhibited lower cuspal deflection than metacrylate based (Z250) composite. SIGNIFICANCE Cuspal deflection resulting from polymerization shrinkage stress may be reduced by an incremental filling technique and by the use of low shrinking composite to obtain optimal clinical outcomes. Flowable composite lining under conventional composite layering did not reduce polymerization shrinkage stress as assessed by cuspal deflection.

Adaptive broadband continuum source at 1200–1400 nm based on an all-fiber dual-wavelength master-oscillator power amplifier and a high-birefringence fiber

We experimentally analyze the stimulated Raman scattering characteristics of a high-birefringence fiber (HBF), which presents an extraordinary level of spectral broadening incurred by the strong nonlinear interaction between the pump and Stokes pulses via the polarization-mode dispersion and group-velocity dispersion of the fiber. We also investigate the impact of the inter-pulse time-delay on the additional spectra broadening when dual-wavelength pump pulses are used. Exploiting these unique SRS properties of the HBF, we develop a novel Raman continuum source based on an all-fiber dual-wavelength master-oscillator power amplifier that can generate a dip-free spectrum in the 1200-1400-nm spectral range. We finally obtain a broadband continuum having an average power of ~840 mW and a 3-dB bandwidth of ~240 nm centered at 1200-1400 nm, which also represents a good spectral flatness and conversion efficiency. This type of source is very useful and important for optical coherence tomography applications, for example.

High-power fiber lasers

We review and discuss the recent advances in high-power fiber lasers with particular attention to a multitude of laser configurations, including continuous-wave and pulsed regimes. In addition, we also discuss novel fiber technology for various specific applications.

Numerical study on multi-pulse dynamics and shot-to-shot coherence property in quasi-mode-locked regimes of a highly-pumped anomalous dispersion fiber ring cavity

We numerically investigate quasi-mode-locked (QML) multi-pulse dynamics in a fiber ring laser cavity in the anomalous dispersion regime. We show that the laser cavity can operate in five constitutively different QML regimes, depending on the saturation power of the saturable absorber element and the length of the passive fiber section that parameterize the overall nonlinearity and dispersion characteristic of the laser cavity. We classify them into the incoherent noise-like-pulse, partially-coherent noise-like-pulse, symbiotic, partially-coherent multi-soliton, and coherent multi-soliton regimes, accounting for their coherence and multi-pulse formation features. In particular, we numerically clarify and confirm the symbiotic regime for the first time to the best of our knowledge, in which noise-like pulses and multi-solitons coexist stably in the cavity that has recently been observed experimentally. Furthermore, we analyze the shot-to-shot coherence characteristics of the individual QML regimes relative to the amount of the nonlinear-phase shift per roundtrip, and verify a strong correlation between them. We also show that the net-cavity dispersion plays a critical role in determining the multi-pulse dynamics out of the partially-coherent noise-like-pulse, symbiotic, and partially-coherent multi-soliton regimes, when the cavity bears moderate nonlinearity. We quantify and visualize all those characteristics onto contour maps, which will be very useful and helpful in discussing and clarifying the complex QML dynamics.

Combinatorial Study of Supercontinuum Generation Dynamics in Photonic Crystal Fibers Pumped by Ultrafast Fiber Lasers

We numerically study the dynamics of supercontinuum generation (SCG) for a variety of possible combinations of photonic crystal fibers (PCFs) and ultrafast fiber laser pulses that the current technologies offer. Three types of PCFs typically used in SCG and four representative types of ultrafast fiber laser pulses are considered for this combinatorial study. We numerically model and qualitatively discuss the nonlinear evolution of the pulses for their whole 12 combinatorial cases. We also quantitatively analyze their output spectra and organize a performance chart for them in terms of spectral bandwidth, flatness, and degree of spectral coherence. Finally, we suggest the most viable combinations among the given PCFs and ultrafast fiber laser pulses in order for generating a target supercontinuum spectrum for various specific cases.

Rigorous Analysis on Ring-Doped-Core Fibers for Generating Cylindrical Vector Beams

We propose a novel active fiber design for selectively generating cylindrical vector beams (CVBs) or cylindrical vector modes (CVMs) which can be applied to conventional fiber lasers. A fiber is designed to have a ring-shaped core refractive index profile which can lead to the best overlap between the active dopant distribution profile and the lowest-order CVM (LCVM) field profile. Therefore, the overlap factor (OVF) of the LCVM becomes even higher than that of the fundamental mode. We emphasize that this condition cannot be satisfied by a conventional step-index core fiber (SICF) but by the ring-doped core fiber (RDCF). Because the lasing threshold is inversely proportional to the OVF, the LCVM can predominantly be stimulated even without going through special procedures to impose extra loss mechanisms to the fundamental mode. We numerically verify that the OVF of the LCVM with the doped ions can significantly exceed that of the fundamental mode if the proposed fiber design is applied. In addition, an RDCF of the proposed fiber design can also operate in a regime containing no higher-order modes besides the LCVM, so that it can selectively and efficiently generate the LCVM without being disrupted by the parasitic lasing of the higher-order modes. We highlight that an optimized RDCF can lead to a >30 % higher OVF ratio than a SICF having the same doped area. The proposed model is expected to be useful for enhancing the efficiency of generating CVBs in an all-fiber format.

Numerical study on fiber-based supercontinuum generation in anomalous dispersion pumping regimes

We numerically study the dynamics of supercontinuum generation for four types of conventional fiber laser pulses. Our results show that in anomalous dispersion pumping regimes a dechirped parabolic pulse generates the broadest output spectrum.

Rigorous analysis on a U-shaped index fiber for generating cylindrical vector beams in an all-fiber format

We propose a novel method to generate cylindrical vector beams (CVBs) which can be applied to conventional fiber-laser system configuration. We focus on the relationship between the laser pump power threshold and the power confinement in the high index region of a fiber, and suggest a novel type of index distribution for the desirable operation. We verify our idea by computer simulations and show that the laser threshold of the CV mode can be reduced down below that of the fundamental mode by 3%. The suggested method will be useful for generating cylindrical vector beams in an all-fiber format.

Continuum Generation in a Highly Nonlinear Soft-glass W-type Index Profiled Er-doped fiber

We report a highly nonlinear soft-glass Er-doped fiber with a flattened anomalous dispersion around 1540 nm. We demonstrate continuum generation by seeding the fiber with ps-pulses at 1550 nm and pumping at 1480 nm.

Shear bond strength of dentin bonding agents cured with a plasma arc curing light

The objective of this study was to compare dentin shear bond strength (DSBS) of dentin bonding agents (DBAs) cured with a plasma arc (PAC) light curing unit (LCU) and those cured with a light emitting diode (LED) LCU. Optical properties were also analyzed for Elipar freelight 2 (3M ESPE); LED LCU, Apollo 95E (DMT Systems); PAC LCU and VIP Junior (Bisco); Halogen LCU. The DBAs used for DSBS test were Scotchbond Multipurpose (3M ESPE), Singlebond 2 (3M ESPE) and Clearfil SE Bond (Kuraray). After DSBS testing, fractured specimens were analyzed for failure modes with SEM. The total irradiance and irradiance between 450 ㎚ and 490 ㎚ of the LCUs were different. LED LCU showed narrow spectral distribution around its peak at 462 ㎚ whereas PAC and Halogen LCU showed a broad spectrum. There were no significant differences in mean shear bond strength among different LCUs (P > 0.05) but were significant differences among different DBAs (P < 0.001)