Myeong Ock Ko

Dynamic Sensor Interrogation Using Wavelength-Swept Laser with a Polygon-Scanner-Based Wavelength Filter

We report a high-speed (∼2 kHz) dynamic multiplexed fiber Bragg grating (FBG) sensor interrogation using a wavelength-swept laser (WSL) with a polygon-scanner-based wavelength filter. The scanning frequency of the WSL is 18 kHz, and the 10 dB scanning bandwidth is more than 90 nm around a center wavelength of 1,540 nm. The output from the WSL is coupled into the multiplexed FBG array, which consists of five FBGs. The reflected Bragg wavelengths of the FBGs are 1,532.02 nm, 1,537.84 nm, 1,543.48 nm, 1,547.98 nm, and 1,553.06 nm, respectively. A dynamic periodic strain ranging from 500 Hz to 2 kHz is applied to one of the multiplexed FBGs, which is fixed on the stage of the piezoelectric transducer stack. Good dynamic performance of the FBGs and recording of their fast Fourier transform spectra have been successfully achieved with a measuring speed of 18 kHz. The signal-to-noise ratio and the bandwidth over the whole frequency span are determined to be more than 30 dB and around 10 Hz, respectively. We successfully obtained a real-time measurement of the abrupt change of the periodic strain. The dynamic FBG sensor interrogation system can be read out with a WSL for high-speed and high-sensitivity real-time measurement.

Dynamic measurement for electric field sensor based on wavelength-swept laser

We demonstrate for the first time to our knowledge a dynamic measurement for an electric field sensor using a nematic liquid crystal (NLC) Fabry-Perot etalon and a wavelength-swept laser. It is well known that the wavelengths of the transmitted peaks of the NLC Fabry-Perot etalon depend on the applied electric field. The change in the effective refractive index of the NLC is measured according to the applied static electric field. The effective refractive index decreases from 1.67 to 1.51 as the applied the electric field intensity is increased. In addition, we measure the frequency of the dynamic variation in the electric field using a high-speed wavelength-swept laser. By measuring the modulation frequency of the transmitted peaks in the temporal domain, the frequency of the amplitude-modulated electric field can be estimated. The frequencies of the measured dynamic variations show a close agreement with the amplitude modulation frequencies of the induced electric field.

Measurement of Effective Refractive Index of Nematic Liquid Crystal in Fabry-Perot Etalon

We report a measurement of effective refractive index of a nematic liquid crystal Fabry-Perot etalon according to the applied static electric field intensity. The effective refractive index decreases from 1.67 to 1.51 as the applied electric field intensity is increased.

Electric field sensor based on cholesteric liquid crystal Fabry-Perot etalon

We propose an electric field sensor using a cholesteric liquid crystal (CLC) Fabry-Perot etalon and a broadband optical source. The CLC cell consists of glass substrates, polyimide layers, electrodes, and CLC layer. There is a threshold behavior for CLC cell and no change in the transmitted wavelength occurs until a threshold value. The threshold value is 0.8 V/μm for fabricated CLC cell in this experiment. The transmitted or reflected wavelength from the CLC Fabry-Perot etalon depends on the applied electric field. The valley wavelengths of the transmitted light from the CLC device are linearly increased from 1303 nm to 1317 nm as the applied electric field to the CLC device is increased from 0.8 V/μm to 1.9 V/μm.

Characteristics of a Wavelength-swept Laser with a Polygon-based Wavelength Scanning Filter

We report the characterization of a wavelength-swept laser (WSL) using a polygon-based wavelength scanning filter and two semiconductor optical amplifiers (SOAs). The output intensity and scanning bandwidth of the WSL depend on the position of the two SOAs in the laser cavity and the coupling ratio of the output fiber coupler. The outputs of the WSL are characterized for coupling ratios of 10%, 30%, 50%, 70%, and 90% for the output fiber coupler. In the setup in which the output fiber coupler is located between the two SOAs, high output power and wide scanning bandwidth can be achieved with an optimized configuration. Using the optimized configuration of the WSL, the intensity increases with the coupling ratio. These results can be used to construct an optimized WSL using the polygon-based wavelength scanning filter.

In situobservation of dynamic pitch jumps of in-planar cholesteric liquid crystal layers based on wavelength-swept laser

We report in situ observation of dynamic pitch jumps in cholesteric liquid crystal (CLC) layers that depend on the applied electric field. A high-speed and wide bandwidth wavelength-swept laser is used as an optical broadband source to measure the dynamic pitch jumps. We could not observe the dynamic pitch jump in the quasi-static pitch variation. Instead, we carry out two driving methods, a normal driving and an overdriving method, in order to measure the dynamic pitch jump in the CLC cell. For the case of normal driving, it has been confirmed that the reflection band from the measurement region is discontinuously shifted by movement of the defect wall. The reflection band was compressed and recovered before the band moved, but the dynamic pitch jump of the helix could not be observed. For the case of overdriving, however, it was possible to observe the unwinding of the helix during the dynamic pitch jump. The entire dynamic pitch jump process in the CLC cell could be observed by measuring the transmission spectra from the CLC cell by varying the applied electric field. We confirm that the entire reaction time with the overdriving method was about 800 ms, which was shorter than with the normal driving method. This study contributes to the development of fast in-plane switching research and the development of new CLC devices.

Measuring of the pitch variation of cholesteric liquid crystals under electric field using wavelength-swept laser

We measure the pitch variation of cholesteric liquid crystals (CLCs) according to the applied electric field using a wavelength-swept laser. While the electric field is applied to the CLC, the pitch of the CLC is elongated normal to the direction of electric field. Therefore, the reflection band is shifted to the longer wavelength. When the applied electric field to the CLC cell was over 1.52 V/μm, the reflection band was changed to the longer wavelength of about 75.1 nm. We believe that the dynamic behavior of the CLC can be analyzed if a high-speed wavelength-swept laser is used as an optical source.

Wavelength-swept laser based on semiconductor optical amplifier for dynamic optical fiber sensors

We report two kinds of wavelength-swept lasers based on semiconductor optical amplifier for dynamic optical fiber sensors. The wavelength-swept laser has a linear relationship that exists between wavelength and time. As an application using the wavelength-swept laser for dynamic optical fiber sensors, we measure a dynamic modulation frequency of the applied electric field using a nematic liquid crystal cell. The amplitude modulation frequency is measured up to 2.5 kHz.

Dynamic fiber Bragg grating strain sensor interrogation based on resonance Fourier domain mode-locked fiber laser

We demonstrate a dynamic fiber Bragg grating (FBG) strain sensor interrogation using a 1550 nm band resonance Fourier domain mode-locked fiber laser. We successfully achieved a real-time measurement of the abrupt change of the applied frequencies to the FBG without any signal processing delay.

Dynamic sensors based on wavelength-swept lasers

Due to their high spectral resolution, wavelength-swept lasers (WSLs)—with an emission that sweeps across a range of wavelengths—represent a promising optical source for optical coherence tomography, optical fiber sensors, and optical beat source generation.1–12 WSLs are fabricated by employing a narrowband wavelength-scanning filter, such as a fast rotating polygonal scanner filter,1, 2 a diffraction grating on a galvo-scan mirror,3, 4 or a fiber Fabry-Pérot tunable filter (FFP-TF).5–10 The speed of conventional WSLs, based on FFP-TFs, is limited by both the tuning speed of the filter and the laser cavity lifetime. Many kinds of WSLs have been developed with an aim to improve the scanning rates and scanning bandwidth.5, 6 Due to the many advantages they have over conventional techniques, fiber Bragg gratings (FBGs) have been widely implemented in fiber-optic sensors for the purpose of structural health monitoring. Their inherent advantages include electromagnetic immunity, compactness, remote sensing ability, ease of fabrication, and wavelength selectivity. The key technology behind fiber-optic sensing using FBGs is wavelength-shift interrogation. FBG interrogation based on a WSL represents a promising technique for high-speed and high-accuracy fiberoptic sensor systems10, 13–15 due to the linear relationship between the spectral and temporal domains (see Figure 1). A wavelength in the spectral domain therefore exactly corresponds to a pulse in the temporal domain, enabling dynamic measurement in fiber-optic sensors.10–16 We recently developed a dynamic FBG sensor interrogation using two kinds of WSLs.14, 15 Figure 2(a) shows a schematic diagram of one of our experimental setups, based on a WSL with a polygon-scanner-based wavelength filter ( 1550nm).14 The output from this WSL is coupled into the multiplexed FBG array, which is arranged in series with five FBGs. One of the Figure 1. Schematic diagram of the fiber Bragg grating (FBG) sensor interrogation based on a wavelength-swept laser (WSL). The FBG interrogation system consists of a WSL and an FBG sensor array. The WSL comprises a gain medium, a tunable filter, and an output coupler. The output from the WSL goes to the FBG sensor array through an optical circulator. Reflected wavelengths from the FBG array then arrive at the photodetector, again through the optical circulator.