S.Y. Li

Thermally tunable narrow-bandpass filter based on a linearly chirped fiber Bragg grating.

We propose a new method for the development of a tunable optical bandpass filter (TOBF) based on a linearly chirped fiber Bragg grating (LCFBG). A NiCr wire heater is used to heat the LCFBG at a small point to introduce a narrow passband within the stop band of the LCFBG. The central wavelength of the passband is tuned by scanning the wire heater along the LCFBG. As an example demonstrating the effectiveness of the proposed method, we demonstrate a TOBF with a very small 3-dB bandwidth of approximately 7 pm, a tuning range of 16.4 nm, and a rejection ratio of more than 25 dB. Compared with previously reported tunable-fiber-based bandpass filters, this method provides the advantages of a large tuning range, continuous tunability, a switchable passband, a simple tuning mechanism, low cost, and narrow bandwidth.

Electrically tunable dispersion compensator with fixed center wavelength using fiber Bragg grating

We present the design and development of a novel tunable dispersion compensator with fixed center wavelength that is based on the electrical adjustment of the chirp of a fiber Bragg grating (FBG). Both temperature gradient and strain gradient are employed to adjust the chirp of the FBG jointly. The electrical current flowing through the taper on-fiber thin-film heater will introduce a temperature gradient on the FBG. The shrinkage of a negative thermal expansion coefficient (NTEC) ceramic due to the temperature rise will compress the tapered FBG mounted inside it, and this will introduce a strain gradient on the FBG. The center wavelength of the FBG will be kept fixed because the effect of temperature rise on the FBG and the effect of compression of the FBG will offset each other. Applying an electrical power of less than 0.68 W, we demonstrate a linearly chirped FBG whose dispersion can be continuously adjusted from -178 ps/nm to -302 ps/nm with a central wavelength shift of as small as 0.16 nm.

Analysis of linearly tapered fiber Bragg grating for dispersion slope compensation

We demonstrate that linearly tapered fiber Bragg gratings (FBGs) display nonlinear group delay under strain or stress. The nonlinear group delay of the tapered FBG is analyzed theoretically. Experimental results show a good agreement with the analysis. We are the first to find that linearly tapered FBGs can be used in dispersion slope compensation.

Design and fabrication of flat-band long-period grating

A method to realize the practical flat-band long-period fiber grating (LPG) fabrication is proposed. Based on the results obtained from the layer-peeling method, we have analyzed the structure of the designed flat-band LPG. We found that the designed LPG is an apodized LPG with sinc-like envelope and a constant period, while there exists a /spl pi/ phase shift at each zero point of the apodization profile. For the first time, a flat-band LPG is realized in fabrication. The experimental result shows a good match with our theoretical analysis.

Spacing-tunable multiwavelength fiber laser source using a Sagnac loop filter

A novel spacing-tunable multiwavelength erbium-doped fiber laser source based on a Sagnac loop filter is proposed and experimentally demonstrated. The filter, which is coupled together with the laser cavity, consists of a normal 3-dB fiber coupler, a polarization controller (PC1), and a segment of high-birefringence (Hi-Bi) fiber. The active gain medium, which is a polarization-maintaining erbium-doped fiber (PM-EDF) with an elliptical core, leads to possible multifrequency lasing output at room temperature. In the experiment, by changing the setting of the polarization controller (PC2), multiple-frequency lasing lines with different wavelength spacing are obtained. Experimental results show several stable output lasing wavelengths with high extinction ratios at room temperature.

A tunable multiwavelength fiber laser source with an elliptical-core fiber Sagnac loop filter

A novel tunable multiwavelength erbium-doped fiber laser source based on a Sagnac loop filter is proposed and experimentally demonstrated. The filter, which is coupled together with the laser cavity, consists of a normal 3-dB fiber coupler, a polarization controller (PC1) and a segment of high-birefringence (Hi-Bi) fiber. The active gain medium, which is a polarization-maintaining erbium-doped fiber (PM-EDF) with an elliptical core, leads to possible multi-frequency lasing output at room temperature. In experiment, by changing the setting of the polarization controller (PC2), multiple frequency lasing lines with different wavelength spacing have been obtained. By applying some pressure to the PM-EDF, continuously tunable multiwavelength lasing output is achieved. Experimental results have shown several stable output lasing wavelengths with high extinction ratio at room temperature.

Phase-shifted linearly chirped fibre Bragg gratings with adjustable transmission characteristics

A new method using a thermal head (optical bandpass filter (OBF)) to improve and adjust the transmission band of a phase-shifted linearly chirped fiber Bragg grating is proposed and demonstrated. The transmission bandwidth can be tuned from 0.16 nm to 1.05 nm.

Tunable and switchable optical bandpass filters based on phase-shifted linear chirped fiber Bragg gratings

In this report, we present a novel tunable optical bandpass filters based on thermal induced phase-shifted linear chirped fiber Bragg gratings with /spl sim/0.15 nm bandwidth, /spl sim/15 nm tuning range and larger than 25 dB rejection ratio. As a bandpass filter, it improve the transmission characteristics of phase-shifted FBG, such as bandwidth and Lorentzian shape. We are working on improve the insertion loss and the ripple in the top of the filter.

Tabu search algorithm to optimize the design of fiber Bragg gratings

A Tabu Search (TS) algorithm, to our knowledge, for the first time, is introduced into the optimized design of Fiber Bragg gratings (FBGs). By combining the Transfer Matrix Method (TMM) for calculating the reflection spectrum and the TS algorithm, we obtained a new method for synthesis the FBGs with advanced characteristics. A new appodization profile is proposed as an example to demonstrate the effectiveness of the method that is general to be useful for inverse problems in FBGs application.