Sang Sam Choi

Displacements of the resonant peaks of a long-period fiber grating induced by a change of ambient refractive index.

We present a graphic method of analyzing the spectral displacements of a long-period fiber grating as a function of ambient index. Mode dependence of the maximum displacement, disappearance of a particular resonance peak, and spectral behavior when the ambient index is larger than that of the cladding material are investigated and compared with experimental results.

Analysis and measurement of birefringence in single-mode fibers using the backscattering method.

The local parameters of linear, circular, and elliptical birefringence of backscattered light along a single-mode fiber are analyzed, and the twist-bending-induced elliptical birefringence in an extremely low birefringent single-mode fiber is measured using the backscattering method. The experimental results are in good agreement with the theoretical calculations. For a 130-microm-diameter fiber, twisted by 133 degrees /m and wound around a drum of 15-cm radius, the induced elliptical birefringence is |?| = 18 degrees /m at lambda = 904 nm.

Temperature-independent strain sensor system using a tilted fiber Bragg grating demodulator

We experimentally demonstrate a new approach for the demodulation of a fiber Bragg grating (FBG) strain sensor by combining a tilted FBG demodulator with a temperature-independent property and a dual head FBG sensor with a temperature-discriminating property. This technique guarantees a stable measurement independent of temperature perturbation at both sides of sensor and demodulator without any additional temperature-isolation or temperature-referencing process and has the minimum resolvable strain of 10 /spl mu/strain. The ranges of operating temperature at the sensor and at the demodulator which guarantee a dynamic range of 1800 /spl mu/strain corresponding to the wavelength difference 2.2 nm between two notches of the tilted FBG spectrum are from room temperature to 180/spl deg/C and 140/spl deg/C, respectively.We experimentally demonstrate a new approach for the demodulation of a fiber Bragg grating (FBG) strain sensor by combining a tilted FBG demodulator with a temperature-independent property and a dual head FBG sensor with a temperature-discriminating property. This technique guarantees a stable measurement independent of temperature perturbation at both sides of sensor and demodulator without any additional temperature-isolation or temperature-referencing process and has the minimum resolvable strain of 10 /spl mu/strain. The ranges of operating temperature at the sensor and at the demodulator which guarantee a dynamic range of 1800 /spl mu/strain corresponding to the wavelength difference 2.2 nm between two notches of the tilted FBG spectrum are from room temperature to 180/spl deg/C and 140/spl deg/C, respectively.

Interferometric temperature-insensitive strain measurement with different-diameter fiber Bragg gratings

An interferometric strain-sensing system that combines an unbalanced Mach-Zehnder wavelength discriminator and spliced different-diameter fiber Bragg gratings is presented that has the capability to distinguish strain from temperature effects. Because the gratings are written in 135- and 165- microm -diameter fibers drawn from the same preform, when they are spliced they show similar temperature sensitivities but different responses to applied strain. Thus the relative Bragg wavelength difference of the two gratings is affected only by strain, and one can measure it accurately by monitoring the amplitude variation of the interference signal rather than the conventional phase variation.

Relaxation-free harmonically mode-locked semiconductor-fiber ring laser

A 10-Gb/s semiconductor-fiber ring laser is proposed and demonstrated. The performance of a harmonically mode-locked semiconductor-fiber ring laser was described in detail. A long cavity length configuration in semiconductor optical amplifier is found to eliminate the relaxation oscillation, and improved short-term amplitude stability. The laser output has a pulsewidth of 18.4 ps and a spectral width of 0.3 nm.A 10-Gb/s semiconductor-fiber ring laser is proposed and demonstrated. The performance of a harmonically mode-locked semiconductor-fiber ring laser was described in detail. A long cavity length configuration in semiconductor optical amplifier is found to eliminate the relaxation oscillation, and improved short-term amplitude stability. The laser output has a pulsewidth of 18.4 ps and a spectral width of 0.3 nm.

Crystallization of Amorphous Silicon by Excimer Laser Annealing with a Line Shape Beam Having a Gaussian Profile

Amorphous silicon films of 50 mm×50 mm have been crystallized by 1-dimensionally scanning a line shape excimer laser beam with a Gaussian profile in the scanning direction, which basically reduces the nonuniformity of the 2-dimensional scanning method. The laser energy density and substrate temperature were varied. Grains as large as 100 nm with smooth surfaces were obtained with a laser energy density of 300 mJ/cm2 with substrate heating of 400°C, where the conductivity was measured to be 9.6×10-6 S/cm. Higher energy density increased the grain size and crystallinity but the conductivity decreased due to the separation of grain clusters. Substrate heating was found to considerably increase the grain size and crystallinity.

Single-mode GaAs/AlGaAs W waveguides with a low propagation loss

We have designed a low-loss single-mode GaAs/AIGaAs optical waveguide with a symmetric five-layer heterostructure at the 1.31-µm wavelength by use of an effective-index method. Waveguides with a W-shaped refractive-index profile have been grown by use of a metallo-organic chemical vapor deposition technique and fabricated with a chemical wet-etching method. Propagation loss has been measured by use of the Fabry-Perot resonance method and a sequential-cleaving experiment. The measured loss is as low as 0.19 dB/cm for waveguides with 2.3-µm thickness and 4.3-µm width, which is comparable to the lowest-loss semiconductor waveguides yet reported. These waveguides could be used to make low-loss modulators for guided-wave devices.

Use of polarization-optical time domain reflectometry for observation of the Faraday effect in single-mode fibers

Magnetooptic Faraday rotation in a strongly twisted single-mode fiber is measured using polarization-optical time domain reflectometry (POTDR) in which a light pulse of 904 nm wavelength makes a roundtrip along the fiber. The experimental results are in good agreement with the calculations, and the Verdet constant thus obtained is 0.86 \times 10^{-2} min/G . cm at \lambda = 904 nm.

Ultrahigh-speed clock recovery with optical phase lock loop based on four-wave-mixing in a semiconductor optical amplifier

A new phase lock loop (PLL) is proposed and demonstrated for clock recovery from 40 Gbps time-division-multiplexed (TDM) optical signal using simple optical phase lock loop circuit. The proposed clock recovery scheme improves the jitter effect in PLL circuit from the clock pulse laser of harmonically-mode locked fiber laser. The cross-correlation component between the optical signal and an optical clock pulse train is detected as a four-wave-mixing (FWM) signal generated in SOA. The lock-in frequency range of the clock recovery is found to be within 10 KHz.

CHARACTERISTICS OF THE INTRACAVITY DISPERSION IN AN ERBIUM-DOPED FIBER LASER

We have found theoretically that the intracavity dispersion of a laser is independent of the pumping power and wavelength and that the resonant group-velocity dispersion vanishes. We have experimentally observed that the intracavity dispersion of an erbium-doped fiber laser was independent of the pumping power and wavelength within the measuring error, which agrees well with our theoretical predictions.