Soon-Woo Kwon

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.

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).

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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Anti-Reflective (AR) thin film coatings of SiO₂ (n=1.48) and TiO₂ (n=2.17) were deposited by ion-beam assisted deposition (IBAD) with End-Hall ion source and conventional electron beam (e-beam) evaporation to investigate the effect of deposition method on the refractive indicies (n) of the films. Green-light generation using a GaAs laser diode was achieved via excitation of the second harmonic. The latter resulted from the transmission of the fundamental guided-mode wave of 1064 nm through periodically poled LiNbO₃. Large differences in the refractive indicies of each of the layers in the multilayer coating may improve AR performance. IBAD of SiO₂ reduced its refractive index from 1.45 to 1.34 at 1064 nm. Conversely, e-beam evaporation of TiO₂ increased its refractive index from 1.80 to 2.11. In addition, no fluctuations in absorption at the wavelength of 1064 nm were found. The results suggest that films prepared by different deposition methods can increase the effectiveness of multilayer AR coatings.

Optical Modulator

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.

Influence of Deposition Method on Refractive Index of SiO₂ and TiO₂ Thin Films for Anti-reflective Multilayers

Wavelength converting devices with a ridge-type waveguide in periodically poled are made and characterized. The experimental results are compared with theoretical analysis on the effects of waveguide dimension and duty ratio of the periodically poled structure. To show the relationship of the conversion efficiency to those factors, a new measurement system is configured. That experimental results show that the center wavelength, normalized conversion efficiency and full width half maximum of the conversion efficiency curve are 1067.45 nm, 90.7%/ and 0.17 nm, respectively. Also, the estimated results of the duty period and full width half maximum of the conversion efficiency from theoretical calculations agreed well with experimental results with the errors of 0.016 and 0.01 nm.

Refractive-Index Sensor Incorporating a Polymeric Waveguide Overlaid with TiO2 Thin Films

A single sideband(SSB) modulator operating at 5.5 GHz was fabricated by polarization inversion techniques. The dimension of domain inversion in a Mach-Zehnder structure was precisely controlled so that the RF signal applied on two Mach-Zehnder arms gives rise to effective phase difference. The single sideband suppression was maximized by optimization of the polarization status of the optical input and by the DC bias value. The fabricated device showed the center frequency of 5.8 GHz and the maximum sideband suppression of 33dB, where the bandwidth of 15 dB sideband suppression ranged over a 2.5 GHz span. The optical phase delay could be regulated by the DC bias voltage, fur example, the enhanced optical modulation sideband was distinctively switched from the upper sideband to the lower sideband by changing the DC bias voltage from 1.9 V to -10.6 V.

Fabrication and Characterization of Wavelength Conversion Device with Periodically Poled Ridge-type Waveguide in MgO:LiNbO 3

A novel 60GHz optical carrier generator with a polarization domain-inverted structure is suggested and is demonstrated. The two arms of the Mach-Zehnder optical waveguide are periodically poled for quasi-phase velocity matching between the optical wave at 1550nm and the RF wave at 30 GHz. The center frequency of band-pass modulation and the 3 dB bandwidth of the fabricated modulator were measured to be 30.3 GHz and 5.1 GHz, respectively. Sub-carriers with the frequency difference of 60GHz waeregenerated under appropriate DC biac voltage application while the carrier was suppressed to lead to the power ratio between the modulated sub-carrier and the suppressed fundamental carrier of 28 dB, which proves that double sideband- suppressed carrier(DSB-SC) operation can be realized by the suggested single device.

Design and Fabrication of 5.5GHZ SSB optical modulator with polarization reversed structure

When an electric field higher than a characteristic coercive field is applied to a ferroelectric such as LiNbO₃, the orientation of the spontaneous polarization is reversed, which causes the reversal of the sign of odd-rank tensor properties such as electro-optic and nonlinear optic coefficients. A fabrication process of insulator and periodic external field assisted poling of a z-cut LiNbO₃ have been optimized for a periodic 180° phase inversion along z-axis. The poling jig and the poling control system, fully controlled by a computer, for high quality and reproducible PPLN fabrication have been devised. Periodically polarization reversed PPLN was adjusted based on the fabricated thickness of insulator. The poling structure of PPLN was observed under a microscope after etching PPLN samples by an etching solution (HF:HNO₃=1 : 2) for about 15 min.