• Current Optics and Photonics
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• v.1 no.3
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• pp.239-246
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• 2017

A compact plasmonic switching scheme, based on the phase change of a thin-film chalcogenide material ($Ge_2Sb_2Te_5$), is proposed and numerically investigated at optical-communication wavelengths. Surface plasmon polariton modal analysis is conducted for various thicknesses of dielectric and phase-change material layers, and the optimized condition is induced by finding the region of interest that shows a high extinction ratio of surface plasmon polariton modes before and after the phase transition. Full electromagnetic simulations show that multiple reflections inside the active region may conditionally increase the overall efficiency of the on/off ratio at a specific length of the active region. However, it is shown that the optimized geometrical condition, which shows generally large on/off ratio for any length of active region, can be distinguished by observing the multiple-reflection characteristic inside the active region. The proposed scheme shows an on/off switching ratio greater than 30 dB for a length of a few micrometers, which can be potentially applied to integrated active plasmonic systems.

• Current Optics and Photonics
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• v.1 no.3
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• pp.214-220
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• 2017

A wavelength filter based on a polymer Bragg reflector has received much attention due to its simple structure and wide tuning range. Tilted Bragg gratings and asymmetric Y-branches are integrated to extract the reflected optical signals in different directions. To optimize device performance, design procedures are thoroughly considered and various design parameters are applied to fabricated devices. An asymmetric Y-branch with an angle of $0.3^{\circ}$ produced crosstalk less than -25 dB, and the even-odd mode coupling was optimized for a grating tilt angle of $2.5^{\circ}$, which closely followed the design results. Through this experiment, it was confirmed that this device has a large manufacturing tolerance, which is important for mass production of this optical device.

• Journal of the Optical Society of Korea
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• v.14 no.1
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• pp.38-41
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• 2010

We design and fabricate a widely tunable laser diode made of InGaAsP-InP. The diode is monolithically integrated with a wavelength-selective coupled-ring reflector and semiconductor amplifiers. For realization of a compact size device, deeply etched multi-mode interference couplers and square ring resonators composed of total-internal-reflection mirrors are adopted and fabricated using a self-aligned process. It is demonstrated that the laser diode exhibits single mode operation and 16 nm tuning range with side-mode-suppression-ratio exceeding 20 dB.

• Current Optics and Photonics
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• v.7 no.2
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• pp.200-206
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• 2023

Near-infrared (NIR) sensing technology using CMOS image sensors is used in many applications, including automobiles, biological inspection, surveillance, and mobile devices. An intuitive way to improve NIR sensitivity is to thicken the light absorption layer (silicon). However, thickened silicon lacks NIR sensitivity and has other disadvantages, such as diminished optical performance (e.g. crosstalk) and difficulty in processing. In this paper, a pixel structure for NIR sensing using a stacked CMOS image sensor is introduced. There are two photodetection layers, a conventional layer and a bottom photodiode, in the stacked CMOS image sensor. The bottom photodiode is used as the NIR absorption layer. Therefore, the suggested pixel structure does not change the thickness of the conventional photodiode. To verify the suggested pixel structure, sensitivity was simulated using an optical simulator. As a result, the sensitivity was improved by a maximum of 130% and 160% at wavelengths of 850 nm and 940 nm, respectively, with a pixel size of 1.2 ㎛. Therefore, the proposed pixel structure is useful for NIR sensing without thickening the silicon.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.196-196
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• 2012

Graphene, two-dimensional one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, has grabbled appreciable attention due to its extraordinary mechanical, thermal, electrical, and optical properties. Based on the graphene's high carrier mobility, high frequency graphene field effect transistors have been developed. Graphene is useful for photonic components as well as for the applications in electronic devices. Graphene's unique optical properties allowed us to develop ultra wide-bandwidth optical modulator, photo-detector, and broadband polarizer. Graphene can support SPP-like surface wave because it is considered as a two-dimensional metal-like systems. The SPPs are associated with the coupling between collective oscillation of free electrons in the metal and electromagnetic waves. The charged free carriers in the graphene contribute to support the surface waves at the graphene-dielectric interface by coupling to the electromagnetic wave. In addition, graphene can control the surface waves because its charge carrier density is tunable by means of a chemical doping method, varying the Fermi level by applying gate bias voltage, and/or applying magnetic field. As an extended application of graphene in photonics, we investigated the characteristics of the graphene-based plasmonic waveguide for optical signal transmission. The graphene strips embedded in a dielectric are served as a high-frequency optical signal guiding medium. The TM polarization wave is transmitted 6 mm-long graphene waveguide with the averaged extinction ratio of 19 dB at the telecom wavelength of $1.31{\mu}m$. 2.5 Gbps data transmission was successfully accomplished with the graphene waveguide. Based on these experimental results, we concluded that the graphene-based plasmonic waveguide can be exploited further for development of next-generation integrated photonic circuits on a chip.

• Korean Journal of Optics and Photonics
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• v.14 no.2
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• pp.135-141
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• 2003

In arrayed waveguide grating (AWG) devices whose waveguides were composed of polymer with negative thermo-optic coefficient as overcladding, and silica with positive thermo-optic coefficient as both core and undercladding, we investigated the temperature dependence of the central wavelength using two-dimensional SFDM. From these results, it was confirmed that the temperature dependence can be nearly eliminated by adjusting the refractive index of the cladding and the thickness of the silica thin film upper-loaded on the core. Based on the numerical calculations, the AWG device with polymer overcladding was fabricated. and its optical characteristics were compared with those of the orginal silica AWG device. The introduction of polymer overcladding decreased the temperature dependence of the central wavelength from 0.0130 nm/$^{\circ}C$ to 0.0028 nm/$^{\circ}C$ without deteriorating the insertion loss and crosstalk characteristics.

• Korean Journal of Optics and Photonics
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• v.19 no.3
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• pp.219-223
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• 2008

To realize a widely tunable laser diode, a polymer coupled-ring reflector is hybrid- integrated with reflective semiconductor optical amplifier. Even though ring-ring and ring-bus coupling ratios are changed by fabrication errors in waveguide width and height, they remain very close to the single peak condition, ensuring high yield in fabrication. The tuning range is observed to be about 35 nm, maintaining the side mode suppression ratio of about 30 dB.

• Korean Journal of Optics and Photonics
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• v.16 no.4
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• pp.376-383
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• 2005

We designed and fabricated integrated-optic tunable polarization controllers based on $LiNbO_3$ with the Ti-indiffused waveguide along the y-axis utilizing the electro-optic effect. The device consists of $TE↔TM$ mode converters and TE/TM phase shifters. We analyzed the operation principles of each device utilizing transfer matrices based on a Jones matrix and simulated shifting of the center wavelength by inducing voltage. We confirmed experimentally that the fabricated devices control the tunability of the center wavelength and the input SOP.

• Proceedings of the Optical Society of Korea Conference
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• 2009.02a
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• pp.387-388
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• 2009

Biophotonics is an emerging area in a fusion of biology and photonics, especially in advanced bioimaging, optical biosensors, photomodulation, and biochip optical read-out, and optical manipulation. This emerging area also creates many opportunities for interdisciplinary study of biology and photonics. Micro-Electro-Mechanical-System(MEMS) is an attractive technology in miniaturizing sensors and actuactors. For last decade, it has contributed to the development for active and passive small and integrated optical components in optical communication. Recently, this technology is also merging into biology for high sensitive biosensing and high resolution and fast bioimaging in small form factor. In this talk, some key advantages of small optical components and recent biophotonic MEMS achievement will be discussed for miniaturized advanced biophotonic systems.

• Current Optics and Photonics
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• v.5 no.5
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• pp.538-543
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• 2021

The evanescent wave coupling of a microring resonator is controlled by changing the gap distance between the bus waveguide and the microring waveguide. However, the interdependence of the bus waveguide's width and the coupling is not well understood. In this paper, we investigate the dependence of coupling strength on the bus waveguide's width. The strength of the evanescent wave coupling is analytically calculated using coupled-mode theory (CMT) and numerically calculated by three-dimensional finite-difference-time-domain (FDTD) simulation. The analytic and numerical simulation results show that the phase-matching condition in evanescent wave coupling does not provide maximum coupling strength, because both phase-matching and mode confinement influence the coupling. The analytic and simulation results for the evanescent coupling correspond to the experimental results. The optimized bus-waveguide width that provides maximum coupling strength results in intrinsic quality factors of up to 1.3 × 10⁶. This study provides reliable guidance for the design of microring resonators, depending on various applications.