The two-dimensional (2D) SiO2 photonic crystal (PC) is constructed with the substrate of polyester film. The PC period is 800nm, and the duty cycle is 0.5.The high refractive index coating is deposited on the surface of PC. Rigorous coupled-wave (RCWA) theory is used to analyze 2D PC narrowband reflection spectrum characteristic. A relationship model between reflection peak wavelength and medium refractive index adsorption on surface of 2D PC is established. The conclusion shows that there is a linear relationship between reflection wavelength of the PC and the refractive index of adsorption medium, with the refractive index of adsorption medium in the range of 1.3-1.8. The effects of the refractive index of deposited coating on the sensitivity of the PC biosensor are analyzed. With the increase of the refractive index of the deposited coating, the sensitivity of the sensor is increasing.
KEYWORDS: Fiber Bragg gratings, Demodulation, Digital filtering, Mach-Zehnder interferometers, Signal detection, Linear filtering, Electronic filtering, Modulation, Sensors, Sensing systems
A Method to realize signal demodulation of fiber Bragg grating (FBG) sensing system using non-balanced fiber Mach-Zehnder interferometer based on phase generated carrier (PGC) technique is proposed. A detecting scheme to realize phase carrier modulation using PZT phase modulator and to realize demodulation of detected signal using Bessel function and Fourier analysis is presented. The results of the analysis show that the upper limitation of the dynamic range in fiber Mach-Zehnder interferometer is limited by the frequency of the carrier and the cut-off frequency of the low pass filter. In order to enhance the function of filtering wave in phase generated carrier signal demodulation, we use digital matched filter (DMF) in place of low pass filter. The filter can be used to recover the useful signals from signals of a low signal-to-noise ratio. The wavelength detection accuracy of this demodulation scheme is 1.8pm, and the dynamic range of the FBG sensing system has been also improved. The theoretical analysis shows that the scheme can effectively suppress effect of random interference signals on the measurement accuracy and can demodulate the dynamic signal with high resolution.
A composite structure based on linearly chirped fiber Bragg grating which can compensate for dispersion and polarization mode dispersion simultaneously has been proposed and characterized. On one hand, a chirped fiber Bragg grating can be regarded as a filter that is composed of several uniform sub-gratings. Therefore, optical signals with different wavelengths can be reflected at different points of the grating, which will result in different time delays, by which the dispersion compensation can be implemented easily. On the other hand, a chirped fiber Bragg grating has the pressure-induced birefringence effect. In the experiment, a piezoelectric transducer is used to apply the pressure on the linearly chirped fiber Bragg grating. Then the change of reflection spectra can be obtained when the pressure is applied at different points of the grating. From the reflection spectra response, the transverse pressure is found to lead to the split of the spectra of the grating. Through the observation of the group delay characteristics, we find that the differential group delay moves towards a bigger value with the increased voltage, and the maximal range of differential group delay (DGD) is 50 ps. By consequence, first-order polarization mode dispersion (PMD) can be compensated for with linearly chirped fiber Bragg grating. Thus, when the data rate of the fiber communication system is above 10 Gb/s and below 40 Gb/s, the simultaneous compensation of DGD and PMD is necessary and can be achieved by the composite structure.
With the fluctuation of the environmental factors, such as temperature, wind, and atmospheric pressure, the birefringence of a fiber changes along the fiber link and the effects of polarization mode dispersion (PMD) are random and time-varying, which makes it necessary to design a tunable and adaptive Fiber communication; Optical fiber; Fiber Bragg grating; Differential group delay; Dispersion compensation; Polarization mode dispersion compensation; Birefringence; Linearly chirped fiber Bragg grating; Piezoelectric transducer PMD compensator. Such a novel tunable first-order PMD compensator has been proposed and characterized in this paper. The scheme of PMD compensation employs sampled fiber Bragg gratings fabricated with uniform-period phase masks. This all-fiber compensation technique is cost-effective and adjustable in designing the differential group delay (DGD) profile. The numerical simulation results show that the efficiency of this PMD compensator is assessed for 10 Gb/s NRZ transmission systems with large DGD. With the compensator, a significant improvement can be achieved in the bit rate pattern of the received signal.
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