Jun-Kyu Seo

Strain induced tunable wavelength filters based on flexible polymer waveguide Bragg reflector

A tunable wavelength filter is demonstrated by imposing a strain on a polymeric Bragg reflection waveguide fabricated on a flexible substrate. The highly elastic property of flexible polymer device enables much wider tuning than the silica fiber. To produce a uniform grating pattern on a flexible plastic substrate, a post lift-off process along with an absorbing layer is incorporated. The flexible Bragg reflector shows narrow bandwidth, which is convincing the uniformity of the grating structure fabricated on plastic film. By stretching the flexible polymer device, the Bragg reflection wavelength is tuned continuously up to 45 nm for the maximum strain of 31,690 muepsilon, which is determined by the elastic expansion limit of waveguide polymer. From the linear wavelength shift proportional to the strain, the photoelastic coefficient of the ZPU polymer is found.

Optical Current Sensors Consisting of Polymeric Waveguide Components

Optical current sensors are demonstrated based on polarization rotated reflection interferometry by incorporating polymeric optical waveguide components. Polarization maintaining 3-dB couplers, TE-pass waveguide polarizers, and thermo-optic phase modulators are designed and fabricated in this work in order to provide essential building blocks for constructing the current sensors. The phase difference between the two circularly polarized waves imposed by the Faraday effect of the optical fiber is detected using the interferometric optical sensor consisting of the polymeric components. To remove the bending induced birefringence, the optical fiber wound around a ceramic frame is annealed at 850°C for 24 hours. The reflection interferometer comprising the polymer waveguide components operates with good linearity proportional to the monitoring current.

Tunable External Cavity Laser by Hybrid Integration of a Superluminescent Diode and a Polymer Bragg Reflector

We report a tunable external cavity laser (T-ECL) using a superluminescent diode (SLD) and a polymer Bragg reflector with 0.8-nm mode spacing 25 channels operating in the direct modulation of 2.5 Gb/s for a low-cost source of a wavelength division multiplexed passive optical network (WDM-PON) system. The maximum output power is 12.2 mW and the slope efficiency of the T-ECL is about 0.12-0.17 mW/mA in the tuning range of 20 nm at the SLD current of 50 mA and at the temperature of 25°C. The T-ECL successfully operated in the direct modulation for 2.5-Gb/s transmission through 20-km standard single-mode fiber. The power penalty of the T-ECL is less than 0.8 dB for 25 channels after the 20-km transmission. The power penalty variation is less than 1.5 dB in the wavelength-tuning range of 20 nm. The performance of the T-ECL satisfies the requirements of the WDM-PON system.

Polarization-Splitting Waveguide Devices Incorporating Perfluorinated Birefringent Polymers

Low-loss perfluorinated polymers with a controllable optical birefringence were synthesized for the purpose of fabricating polarization-splitting waveguide devices, which have become an essential part of polarization-multiplexed coherent optical communications. The birefringent polymer was embedded on one branch of the asymmetric Y-branch waveguide to produce polarization-dependent mode evolution due to the effective birefringence. The device with a branch angle of 1/300 rad and birefringence of 0.007 showed a crosstalk of -25 dB and an insertion loss of 1.5 dB from fiber to fiber. The polarization splitters based on adiabatic mode evolution exhibited negligible wavelength dependence and large tolerance in waveguide dimension.

2

We present a novel 2 × 2 polymer thermo-optic digital optical switch (DOS) using the total-internal-reflection (TIR) effect. The switch consists of four 1 × 2 TIR-DOSs cascaded with each other. These TIR-DOSs have two branched multimode waveguides and an offset heater electrode in the branched region. Our switch does not have any bent waveguides, and has tapered waveguides and single-mode waveguides only in the input-output fiber coupling region. The fabricated 2 × 2 TIR-DOSs exhibit excellent properties, with very low insertion losses of <; 1.4 dB and switching isolations of >; 50 dB in both turned-off and turned-on states.

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We demonstrate a directly-modulated 10-Gb/s tunable external cavity laser with a polymer Bragg reflector and a high speed superluminescent diode. 20 km transmission through the standard single mode fiber was carried out with a power penalty of less than 2.8 dB..

2 Polymer Thermo-Optic Digital Optical Switch Using Total-Internal-Reflection in Bend-Free Waveguides

Highly integrated optical components are strongly demanded because they enable wavelength-division multiplexing optical communication systems to achieve smaller footprints, lower power consumption, and enhanced reliability. Variable optical attenuator (VOA) arrays are often used with optical switches in cascaded form for reconfigurable optical add-drop multiplexer systems. Although VOAs and optical switches based on polymer waveguide technology are commercially available, it is still not viable to integrate these two array devices on a single chip because of significant interchannel crosstalk. In this work, we resolved the issue of crosstalk and integrated the arrays of optical switch and VOA on a single chip by incorporating a self-assembled scattering monolayer (SASM). The SASM was effective for scattering the planar guided mode; consequently, the crosstalk into an adjacent channel was significantly reduced, to less than -35 dB.

10-Gb/s direct modulation of polymer-based tunable external cavity lasers

The sloshing impact induced strain exerted on the insulation panel of a liquefied natural gas (LNG) tank was measured in situ by using a fiber-optic Bragg reflector and a wavelength interrogator. To prevent the catastrophic failure of the LNG vessel caused by sloshing impact on the insulation panel, continuous monitoring and dynamic analysis of the induced strain are required. In this study, two different fiber-optic systems are developed based on (1) edge filter type and (2) arrayed waveguide grating type. Bragg reflection sensors were embedded inside the reinforced polyurethane foam insulating panel, which was used to cover the inner surface of the LNG tank. Initially, an edge filter type fiber-optic system is attempted for measuring strain for high speed applications. Another type of fiber-optic system, arrayed waveguide grating, is developed. By incorporating an arrayed waveguide grating device for the interrogation of the reflected wavelength from the Bragg reflector, a dynamic strain of 4000 µε occurring within a duration of 10 ms was able to be measured in real time. The accuracy of the measured data was confirmed by comparing it to the data obtained by means of a high speed camera.

Low-crosstalk high-density polymeric integrated optics incorporating self-assembled scattering monolayer

Liquefied natural gas (LNG) transport carriers are exposed to a risk by the repeated bump in the LNG container during the vessel traveling over the wave in ocean. The liquid inside the container, especially when it was not fully contained, make a strong bump onto the insulation panel of the tank wall. The insulation panel consists of several layers of thick polyurethane foam (PUF) to maintain the LNG below the cryogenic temperature, -162°C. Due to the repeated shock on the PUF, a crack could be developed on the tank wall causing a tremendous disaster for LNG carriers. To prevent the accidental crack on the tank, a continuous monitoring of the strain imposed on the PUF is recommended. In this work, a fiber-optic Bragg grating was imbedded inside the PUF for monitoring the strain parallel to the impact direction. The optical fiber sensor with a small diameter of 125 μm was suitable to be inserted in the PUF through a small hole drilled after the PUF was cured. In-situ monitoring of the strain producing the change of Bragg reflection wavelength, a high speed wavelength interrogation method was employed by using an arrayed waveguide grating. By dropping a heavy mass on the PUF, we measured the strain imposed on the insulation panel.

In Situ Measurement of Sloshing Impact on LNG Insulation Panel by using High Speed Fiber Optics

We have developed a tunable external cavity laser based on the optimized superluminescent diode and a polymer Bragg reflector for C-band WDM-PON light source. The tunable laser had a wavelength tuning range of 12nm by thermo-optic tuning of the polymer waveguide at a low input power of 72 mW. This module operates successfully in direct modulation for 2.5 Gbit/s over 45 Km transmission.