Woo-Sung Chu

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.

Polymer waveguide integrated-optic current transducers

Various functional optical devices are integrated on a single chip in order to construct optical current transducers based on polarization rotated reflection interferometry, which consists of polarization maintaining 3-dB couplers, TE-pass polarizers, TE/TM polarization converters, and thermo-optic phase modulators. By virtue of the device integration, the sensor exhibited good linearity, and excellent accuracy with an error less than 0.2%. The integrated-optic device provides inherent polarization maintaining characteristics and precise controllability of the optical path length in the interferometric sensor. Single chip integration reduces the complexity of the interferometry, and enables mass-production of low-cost high performance current sensors.

Integrated-optic polarization controllers incorporating polymer waveguide birefringence modulators

Polarization controllers based on polymer waveguide technology are demonstrated by incorporating thermo-optic birefringence modulators (BMs) and thin-film wave plates. Highly birefringent polymer materials are used to increase the efficiency of birefringence modulation in proportion to the heating power. Thin-film quarter-wave plates are fabricated by using a crosslinkable liquid crystal, reactive mesogen, and inserted between the BMs to produce static phase retardation and polarization coupling. By applying a triangular AC signal to one BM and a DC signal to another, the polarization states of the output light are modulated to cover the entire surface of the Poincaré sphere.

Polarization Converting Waveguide Devices Incorporating UV-curable Reactive Mesogen

Reactive mesogen (RM) is an organic liquid crystal molecule that can be self-aligned to have an optic axis of birefringence when coated over a polyimide alignment film. A free-standing optical wave-plate film consisting of RM and low-loss optical polymers was fabricated in this work, and the film was inserted across the polymer waveguide to form an integrated optical polarization converter. For convenient evaluation of the polarization converters, a waveguide polarizer and analyzer were fabricated in series. The polarization conversion efficiency was measured to be 25 dB for the wavelength range from 1520 to 1580 nm. The wave plate exhibited a temperature-dependent retardation of 4.5° for a temperature change from 25 to 100℃.

Integrated optic current transducers incorporating photonic crystal fiber for reduced temperature dependence.

Optical current transducers (OCT) are indispensable for accurate monitoring of large electrical currents in an environment suffering from severe electromagnetic interference. Temperature dependence of OCTs caused by its components, such as wave plates and optical fibers, should be reduced to allow temperature-independent operation. A photonic crystal fiber with a structural optical birefringence was incorporated instead of a PM fiber, and a spun PM fiber was introduced to overcome the temperature-dependent linear birefringence of sensing fiber coil. Moreover, an integrated optic device that provides higher stability than fiber-optics was employed to control the polarization and detect the phase of the sensed optical signal. The proposed OCT exhibited much lower temperature dependence than that from a previous study. The OCT satisfied the 0.5 accuracy class (IIEC 60044-8) and had a temperature dependence less than ± 1% for a temperature range of 25 to 78 °C.

Tunable channel-drop filters consisting of polymeric Bragg reflectors and a mode sorting asymmetric X-junction.

A tunable channel-drop filter as essential component for the wavelength-division-multiplexing optical communication system has been demonstrated, which is based on polymer waveguide Bragg reflectors. For an ordinary Bragg reflector, the filtered signal is reflected toward the input waveguide. Thus an external circulator is required to separate the filtered signal from the input port, though it increases the total footprint and cost. For this purpose, we employed dual Bragg reflectors and a mode sorting asymmetric X-junction. The Bragg reflector exhibited a maximum reflectivity of 94% for a 6-mm long grating, a 3-dB bandwidth of 0.39 nm and a 20-dB bandwidth of 2.6 nm. The mode sorting crosstalk in asymmetric X-junction was less than -20 dB, and linear wavelength tuning was achieved over 10 nm at the applied thermal power of 377 mW.

Surface relief apodized grating tunable filters produced by using a shadow mask

To produce a compact low-cost tunable filter required for WDM optical communications, a polymeric Bragg reflection filter with an apodized grating structure is proposed. A high-contrast polymeric waveguide is incorporated in order to obtain high reflectivity from a short Bragg grating. To overcome the bandwidth broadening, an apodized grating with a gradually changing depth of surface relief grating along the propagation direction is fabricated through the dry etching with a shadow mask. The apodized polymer grating exhibits 3-dB, 20-dB bandwidths of 0.36 nm, and 0.72 nm, respectively with a 95% reflection. The reflection wavelength is tunable over 14 nm for an applied thermal power of 500 mW.

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

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.

Polymeric Integrated-Optic Bias Chip for Optical Voltage Transducers

The operating bias point of optical voltage transducers (OVTs) must be adjusted to a quadrature phase point through the compensation of residual birefringence induced by the optical fiber and sensor head. An integrated optical device consisting of a polymeric waveguide 3-dB coupler, TE-pass polarizer, thermo-optic phase modulator, and polarization converter was proposed for adjusting the operating bias point. A high-voltage sensor probe was prepared with a LiTaO3 electro-optic crystal sandwiched by Teflon spacers. The operating point control was confirmed by measuring the output polarization change on the Poincaré sphere. Preliminary OVT experiment was performed by applying a high-voltage signal of 60 Hz, 4 kVpp, and the sensor exhibited linear response with a sensitivity of 9.4 mrad/kV.

Optical Voltage Sensors Based on Integrated Optical Polarization-Rotated Reflection Interferometry

Optical voltage sensors based on reflection interferometry of two orthogonal polarizations have been demonstrated by using a polymeric integrated optic device, in which a thermooptic phase modulator, a polarization converter, and a polarizer are integrated on a single chip. The sensing probe for electric field measurement was prepared by assembling an LiTaO3 electrooptic crystal along with a polarization maintaining collimator, a Faraday rotator, and a dielectric mirror. Spectral bandwidth of the light source was optimized to be 16.5 nm in order to provide low noise and good extinction ratio in the reflection interferometry. For an applied voltage of 60 Hz, 4 kVpp, the sensor exhibited linear response with a phase retardation sensitivity of 0.5°/kV.