Geun-Sik Son

Refractometric sensor utilizing a vertically coupled polymeric microdisk resonator incorporating a high refractive index overlay.

A refractometric sensor resorting to a vertically coupled polymeric microdisk resonator was demonstrated, estimating the refractive index (RI) of an analyte by monitoring the resonant wavelength shift in its transfer characteristics. The disk resonator was especially overlaid with a high RI TiO2 film, thereby reinforcing the interaction of the evanescent field of its guided mode with the analyte. The sensitivity of the sensor was theoretically and experimentally confirmed to be enhanced by adjusting the overlay thickness. The fabricated sensor provided the maximum sensitivity of approximately 294 nm/RIU (refractive index unit) with the 40-nm-thick overlay, which is equivalent to an improvement of 150% compared with the case without the overlay.

Integrated optical modulator for signal up-conversion over radio-on-fiber link.

An integrated optical modulator, which consists of a dual-sideband suppressed carrier (DSB-SC) modulator cascaded with a single-sideband (SSB) modulator, is proposed for signal up-conversion over Radio-on-Fiber. Utilizing a single-drive domain inverted structure in both modulators, balanced modulations were obtained without complicated radio frequency (RF) driving circuits and delicate RF phase adjustments. Intermediate frequency (IF) band signal was up-conversed to 60GHz band by using the fabricated device and was transmitted over optical fiber. Experiment results show that the proposed device enables millimeter wave generation and signal transmission without any power penalty caused by chromatic dispersion.

Measurement of Glucose Level Exploiting a Relative Optical Absorption at Discrete Probe Wavelengths

A practical and viable scheme for measuring the concentration of a glucose solution was proposed and verified based on the relative optical absorbance at discrete probe wavelengths instead of the continuous broad band absorption spectra. The relative absorbance at each of the probe wavelengths was obtained with respect to the absorbance at a reference wavelength. The single reference wavelength (1310 nm) and a group of four different probe wavelengths (1064, 1550, 1685, 1798 nm) were selected to exhibit the glucose absorbance with opposite signs, thereby enhancing the accuracy of the prediction. The glucose level was decently estimated by taking the average of all of the individual prediction values available from each of the probe wavelengths, with the standard error of prediction of ~28 mg/dL. And the influence of the ambient temperature upon the prediction of the glucose concentration was examined to compensate for the error induced by the variation in it. The effect of both the amplification of the sensor signal and the optical path length upon the performance of the proposed scheme was briefly addressed as well.

Birefringent waveguide sensor using a polarizer rotating technique

A birefringence measurement system is introduced to get high phase resolution for detection of low contents of biochemicals. By using a fixed quarter-wave plate and a rotating polarizer, the phase difference between two orthogonal polarizations is transformed into phase delay of output sinusoidal signal. Analyzing the output phase, birefringence change could be detected with a phase noise of 0.14 degrees. As well as the birefringence measurement system, an optical evanescent waveguide sensor was developed. A rib-type silica waveguide overlaid with TiO2 film was fabricated, and a developed birefringence measurement technique was employed in evaluating a refractive index change on waveguide surface. For the fabricated waveguide with a 40-nm-thick TiO2 film, experiment results showed that the minimum detectable index change was 5.9x10(-7).

Moisture-insensitive optical fingerprint scanner based on polarization resolved in-finger scattered light

A moisture-insensitive optical fingerprint scanner (FPS) that is based on polarization resolved in-finger light is proposed and realized. Incident visible light, which is selectively fed to a fingerprint sample via a polarization beam splitter (PBS), is deemed to be partially scattered backward by tissues associated with the skin of the finger. The backscattered light is mostly index-guided in the ridge comprising the fingerprint, which has a higher refractive index, and is drastically dispersed in the valley, which is typically filled with water or air and so has a lower index. However, when light reflects directly off the surface of the finger skin, it fundamentally prevents the scanned image from being determined. The proposed FPS produces bright and dark intensity patterns that are alternately created on the surface of the PBS and correspond to the ridges and valleys, respectively. Thus, this method can especially distinguish between a fake synthetic fingerprint and a genuine fingerprint due to its use of in-finger scattered light. The scanner has been rigorously designed by carrying out ray-optic simulations depending on the wavelength, with tissue-induced scattering taken into account. The device was constructed by incorporating a wire-grid type PBS in conjunction with visible LED sources, including blue, green and red. The scanner adopting a blue LED, which exhibits the strongest light scattering, resulted in the best fingerprint image, enabling enhanced fidelity under the wet and dry situations. Finally, a fake synthetic fingerprint could be successfully discriminated.

60 GHz Optical Carrier Generation Using a Domain Reversed LiNbO

A 30 GHz bandpass modulator was fabricated on a domain reversed LiNbO3 substrate. The circuit effect on current waveforms during wafer-scale poling was analyzed, and the domain wall movement was precisely controlled in the poling process. A complementary polarization reversal technique enabled the device with a single electrode structure to perform balanced modulation. A 60 GHz optical carrier was generated by double sideband modulation with the suppressed carrier. The fabricated device showed that carrier suppression was better than 45 dB and that the power ratio of a 60 GHz spaced two-tone lightwave signal to the suppressed carrier was 35 dB.

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An integrated photonic refractometric sensor was proposed and accomplished based on a birefringent silica rib waveguide incorporating a tapered cladding, which is overlaid with a uniform high-index film. A sensing region is free of the cladding so that the modification of the birefringence incurred by an analyte could be maximized, whereas input/output sections are not subject to it. And a shadow mask was taken advantage of to produce the tapered upper cladding in the light propagation direction. The variation in the refractive index of the analyte was in situ monitored via a polarimetric interference scheme by observing the accumulated phase difference between the transverse-electric (TE) and transverse-magnetic (TM) mode. The fabricated sensor provided a phase difference of 4814 degrees for a variation of 0.015 in the refractive index of the analyte, which is equivalent to an interference count of 13.3 times, and its sensitivity was accordingly 5.2 ×10-6 refractive index unit (RIU) per degree. Finally, the resolution was estimated to be 0.73 ×10-6 RIU for a phase measurement resolution of 0.14°.

Optical Modulator

A surface sensor rendering an extended detection range was proposed and demonstrated, taking advantage of a Y-branch structure. The sensing and reference waveguides, comprising the Y-branch structure, were overlaid with TiO2 films of different thicknesses; thus, they show unequal sensitivities. The phase change experienced by the two waveguides was in situ monitored through a birefringence analyzer. The interference number for the sensing waveguide was derived from the response of the reference waveguide. The implemented sensor was evaluated by varying the concentration of glucose solution, confirming that the proposed sensing scheme is useful for efficiently extending the detection range.

Refractometric Sensor Based on a Birefringent Silica Waveguide Using a Tapered Cladding

A refractive index sensor based on a three-layered polymeric waveguide was proposed and demonstrated. A high-index thin film in TiO2 was placed on top of the waveguide in the sensing region, playing the role of strengthening the evanescent field to enhance the sensitivity of the sensor. The refractive index of the analyte applied to the surface of the sensor was estimated by observing the count of the polarimetric interference between the TE and TM polarizations, which is manifested as a periodic variation in the optical output of the sensor. For a fabricated sensor involving a 20 nm thick TiO2 film, the sensitivity was found to be equivalent to 1.8x10-3 RIU. It was found to be enhanced by increasing the thickness of the high-index overlay to a certain degree.

Wide-Range Refractometric Sensor Using Differently Sensitive Y-Branched Waveguides

A photonic glucose biosensor was realized incorporating vertically coupled micro-resonators of ring and disk. The glucose concentration was discovered by examining both the shift in the resonant wavelength and the variation in the optical power.