• Korean Journal of Optics and Photonics
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• v.33 no.1
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• pp.22-27
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• 2022

We fabricate a polarization-independent dielectric multilayer thin-film diffraction grating for a spectral-beam-combining (SBC) system with a simple grating structure and low aspect ratio. Due to the refractive index and thickness error of the manufactured thin films, the diffraction efficiency of the fabricated diffraction grating was lower than that of the design. The causes of the errors were analyzed, and it was confirmed through simulation that diffraction efficiency could be compensated through an additional coating on the manufactured diffraction grating. As a result of sputtering an additional Ta₂O₅ layer on a fabricated diffraction grating, the diffraction efficiency was corrected and a maximum 91.7% of polarization-independent diffraction efficiency was obtained.

• Journal of IKEEE
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• v.14 no.4
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• pp.263-269
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• 2010

In this paper, we present high temperature characteristics of a tunable all-optical filter using fiber Bragg grating(FBG) filter in Ge doped and Ge & B co-doped fiber, including peak reflectivity, bandwidth and various normalized refractive index change with temperature variation. The reflection spectrum of a FBG filter with refractive index change is affected by its thermal stability. Consequently Ge doped FBG tunable filter has a better thermal stability than Ge & B co-doped FBG tunable filter. Also, both FBG tunable filters show good thermal stability up to about $500^{\circ}C$.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.37 no.8
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• pp.76-85
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• 2000

Effects of the random fluctuation in grating half-period have been studied by an effective index transfer matrix method in DFB lasers. The laser facets are assumed to be perfectly antireflection coated, and the period fluctuation is modeled as a Gaussian random variable. The random fluctuation breaks spectral symmetry in both uniform-grating and quarter-wavelength -shifted(QWS) DFB lasers, and decreases the effective coupling coefficient. This leads to increased average mirror loss of ${\pm}$1 modes and reduced stopband width in uniform grating DFB lasers, and degradation in the wavelength accuracy and the single mode stability in QWS-DFB lasers. Threshold gain difference decreases with increasing period fluctuation irrespective of grating coupling coefficient in QWS-DFB lasers, while spatial hole-burning effect is exacerbated or alleviated when the normalized coupling coefficient is lower and higher than 1.5, respectively.

• Journal of Software Assessment and Valuation
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• v.16 no.2
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• pp.117-125
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• 2020

In this paper, when both the refractive index grating and the gain grating exist in a 1.55um DFB laser with two cleaved mirror facets, when the phase of ρ_l is fixed to 0 and the phase of ρ_r is changed to -π/2, π, π/2, 0, the change in frequency and oscillation gain was theoretically analyzed. In the case of δL<0, the oscillation gain required for lasing is the lowest and the most stable frequency operation is obtained in the case of (ρ_l phase=0, ρ_r phase=0) and κL=10, when κL is varied from 0.1 to 10. In the case of δL>0, when (ρ_l phase=0, ρ_r phase=π) and κL=10, the oscillation gain required for lasing is the lowest and the difference between the oscillation gains of the higher-order modes is large so that the most stable frequency operation is obtained.

• Journal of Software Assessment and Valuation
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• v.16 no.1
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• pp.55-63
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• 2020

Lasers for optical broadband communication systems should have excellent frequency selectivity and modal stability. DFB lasers have low lasing frequency shift during high speed current modulation. In this paper, when a refractive index grating and a gain grating are simultaneously present in a DFB laser having a wavelength of 1.55 ㎛, the dielectric film is coated so that reflection does not occur on the right mirror surface, so that ρ_r=0. For the first mode, which requires a minimum gain at the threshold, the beam distribution of the oscillation mode in the longitudinal direction and the radiated power ratio P_l/P_r were analyzed and compared for the cases of the phase of ρ_l=π and π/2. If the phase of ρ_l=π, in order to obtain a low threshold current and high frequency stability, κL should be greater than 8. In the case of the phase of ρ_l=π/2, for low threshold current, κL is necessary to be 1.0, where the oscillation frequency coincides with the lattice frequency. DFB lasers with an anti-reflection coated mirror have excellent mode selectivity than 1.55um DFB lasers with two mirror facets

• Korean Journal of Optics and Photonics
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• v.16 no.3
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• pp.196-200
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• 2005

A 40 channel arrayed-waveguide grating (AWG) filter operating in C-band and L-band wavelength regions has been fabricated using PLC (Planar Lightwave Circuit) processes with 0.75 refractive index difference. Its design was optimized for matching the center wavelength with the ITU-recommended wavelength. The characteristics of the fabricated C-band AWG are as follows; average insertion loss < 2.5 dB, polarization-dependent loss < 0.3 dB, non-adjacent crosstalk >35dB, and the loss uniformity of 0.8 dB. In the L-band AWG, wavelength accuracy is below 0.02nm.

• ETRI Journal
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• v.27 no.1
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• pp.89-94
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• 2005

This paper describes a low-loss, compact, 40-channel arrayed waveguide grating (AWG) which utilizes a monolithically integrated spot-size converter (SSC) for lowering the coupling loss between silica waveguides and standard single-mode fibers. The SSC is a simple waveguide structure that is tapered in both the vertical and horizontal directions. The vertically tapered structure was realized using a shadow-mask etching technique. By employing this technique, the fabricated, 40-channel, 100 GHz-spaced AWG with silica waveguides of 1.5% relative index-contrast showed an insertion-loss figure of 2.8 dB without degrading other optical performance.

• Journal of Navigation and Port Research
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• v.32 no.9
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• pp.717-722
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• 2008

We have proposed and demonstrated an gas-flow sensor using optical fiber bragg grating(FEG). The flow sensor has no electronics and no mechanical parts in its sensing part and the structure is th11s simple and immune to electromagnetic interference(EMI). The FEG sensor was consisted qf the sensing element and a coil heater. The metal coil was used to supply the current to the FEG. While some currents supply to the coil, the refractive index of the FEG under the coil is changed and thus the wavelength shift of fiber optic sensor was induced In this work, the wavelength shift according to flow-rate was experimentally studied and was used to evaluate the gas flow-rate in a gas tube. As a result, it was possible to measure the flow-rate in a linear range from 5 to $20{\ell}/min$ with a resolution of approximately $1{\ell}/min$ at the applied currents of 100 mA and 120 mA. The measured sensitivities were $15.3\;pm/\ell/min$ for 100 mA and $20.2\;pm/\ell/min$ for 120 mA.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.33A no.7
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• pp.230-244
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• 1996

We derive analytic expressions for the spontaneous emission spectrum (SES) of a multisection distributed feedback (DFB) structure device employing complex coupled gratings including the effects of both facets reflections. The multisection DFB structure device used in the analysis is a general model which allows the independent phase of a grating in each section, and the sections without gratings. The expressions are the same as those derived by makino and glinski in case the gratings are index coupled and the phase of a grating in each section, ${\varphi}_k$ is '0' which means the phase of gratings in the device is ocntinuous. The expressions for the SES of a phase-shift-controlled (PSC) DFB structure device using tunable devices are derived from the general expressions. The number of parametes of the expressions is reduced by using the parameter of effective phase shift defined by the sum of the phase shift in a PSC region and the difference of the phase of a grating in each active region. Equations showing the effect of both facets reflections and the effective phase shift on the SES are derived. The validaty of the equations is verified by computer simultions. Computer simulation results also show the possibility of evaluating the structure parametes of the device from its SES.

• Journal of the Optical Society of Korea
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• v.20 no.5
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• pp.575-578
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• 2016

The thermal endurance of fiber Bragg gratings (FBGs), written with the aid of 193-nm ArF excimer laser irradiation on H₂-loaded Ge/B codoped silica fiber, and pretreated with a CO₂ laser and a subsequent slow cooling process, is investigated. These treated gratings show relatively less degradation of grating strength during the thermal annealing procedure. The thermal decay characteristics of treated and untreated fiber, recorded over a time period of 9 hours, have been compared. The effect on the Bragg transmission depth (BTD) and the center-wavelength shift, as well as the growth of refractive-index change during the grating inscription process for both treated and untreated fiber, are analyzed.