Young-Ouk Noh

Low-loss polymer materials incorporating fluorinated compounds have been utilized for the investigation of various functional optical devices useful for optical communication and optical sensor systems. Since reliability issues concerning the polymer device have been resolved, polymeric waveguide devices have been gradually adopted for commercial application systems. The two most successfully commercialized polymeric integrated optic devices, variable optical attenuators and digital optical switches, are reviewed in this paper. Utilizing unique properties of optical polymers which are not available in other optical materials, novel polymeric optical devices are proposed including widely tunable external cavity lasers and integrated optical current sensors.

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.

We present a 10-channel polymer thermo-optic digital optical switch (DOS) array with very low optical crosstalk. The 10-channel DOS array has a crosstalk-improved structure in which radiation-type attenuators are integrated in a series with a conventional 1 times 2 DOS. It is fabricated using a quartz substrate on which grid patterns for dispersion of stray light are formed to reduce the optical crosstalk. The measured crosstalk values are as low as -45 dB at a low electrical power of 50 mW. Insertion losses of less than 1.35 dB are achieved for all channels.

To improve the reproducibility of passive alignment, large core single mode waveguides are demonstrated, which can be connected to thermally expanded core fibers so as to increase alignment tolerance. It is shown that polymer waveguide with a core dimension of 25 μm x 25 μm and an index contrast less than 0.001 can satisfy single mode condition. As a novel functional device incorporating the large core waveguide, variable optical attenuators (VOA) are designed and fabricated. For the fabrication of the thick core structure, a soft molding process is developed. Due to the small index contrast of the waveguide, efficient attenuation is expected for the smaller electrical power consumption, which is confirmed by 3-dimensional beam propagation method. From the fabricated VOA device, more than 20 dB of attenuation is obtained by applying 20 mW.