An improved structure for the optical coherence tomography (OCT) scheme based on a 4×4 fiber coupler for simultaneously measuring the refractive index and thickness of optical samples is presented. The proposed structure incorporates the optical path length difference of interference light and is used to calculate the refractive index and thickness of an unknown sample without any prior knowledge of the sample parameters. Two methods (time-domain and Fourier-domain OCT) of obtaining information about an unknown sample are proposed, and a high-speed high-resolution OCT system was developed.
An adaptive modified two-dimensional time-spreading/wavelength-group-hopping embedded M-sequence code (TS/WGH embedded M-sequence code) is proposed for enhancing the cardinality, confidentiality, and bit error rate (BER) of synchronous networks. The proposed arrayed-waveguide-grating (AWG)-based modified TS/WGH embedded M-sequence encoder/decoder (codecs) is constructed by using a fine AWG to generate an M-sequence code pattern and then using multiple-coarse AWGs to spread this pattern in the wavelength domain. The signals produced by the coarse AWGs are then spread in the time domain using optical delay lines. In addition, an algorithm based on an anticipative-warning-time mechanism and a degree of weighted load balance (DWLB) policy is proposed for changing the signature address code word of each user on a sufficiently frequent basis in order to thwart synchronous-network attacks by eavesdroppers. Overall, the simulation results show that, compared to conventional two-dimensional prime-hop code (PHC) and modified PHC and previous TS/group-hopping embedded M-sequence code, the proposed adaptive modified TS/WGH embedded M-sequence code proposed in this study yields an effective reduction in the multiple-access interference and BER while simultaneously improving the degree of confidentiality.
Frequency-domain optical coherence tomography (FD-OCT) has the advantages of an improved sensitivity and a reduced acquisition time since it enables cross-sectional images of the sample of interest to be obtained using a broadband source or a swept source. The phase-shifting method can always be applied to reduce the additional noise within FD-OCT and provide accurate signal processing. We present a phase-shifting method in which a parallel diffraction grating pair is inserted into the reference arm of the OCT system. The gratings separate the spectrum of the illuminating light and stretch the optical pulse in time thereby producing a linear group delay between the adjacent wavelengths in the superluminescent diode light source. The simulation results show that the proposed group-delay-based phase-shifting method improves the precision of the sample parameters extracted using the FD-OCT procedure compared to those obtained using traditional techniques such as the 0/4 phase-shifting method.
This study presents a numerical investigation into a novel approach for the inverse measurement of arbitrary strain distributions using a genetic algorithm (GA) and a fiber-Bragg-grating-based Sagnac interferometer. In the proposed method, a single uniform fiber Bragg grating (FBG) is bonded to the structure of interest such that it encounters the arbitrary strain field. The arbitrary strain distribution is then determined inversely from the transmission intensity spectrum using a GA population-based optimization process. The major advantage of the proposed approach is its use of a single, uniform FBG to sense arbitrary strain distributions. The approach provides a low-cost and computationally efficient means of detecting strain distributions in many smart-structure monitoring applications.
In this paper, we describe the use of a genetic algorithm and two thermally-modulated fiber Bragg grating's (FBG's) reflection intensity spectra for inversely extracting complete physical parameters. The fiber Bragg grating's complete parameters included the grating period, grating position, grating length, chirped direction, and refractive index modulation. In this study, a uniform and chirped fiber Bragg gratings are synthesized successfully in the experiment. The experimental results show that the complete parameter synthesis in grating period, grating length, refractive index modulation and various grating positions from a 10 mm length uniform FBG have been recovered accurately.
In this paper we describe the use of a genetic algorithm and two thermally-modulated fiber Bragg grating's (FBG's) reflection intensity spectra for inversely extracting complete physical parameters. The fiber Bragg grating’s complete parameters included the grating period, grating position, grating length, chirped direction, and refractive index modulation. In this study, a uniform and chirped fiber Bragg gratings are synthesized successfully in the numerical analysis. Also, the experimental results show that the complete parameter synthesis in grating period, grating length, refractive index modulation and various grating positions from a 10 mm length uniform FBG have been recovered accurately. After finding those physical parameters, the arbitrary distributed strain sensing technique is easy to accomplish by using two fiber Bragg gratings sensing techniques.
A new arbitrary distributed strain and temperature sensing method based on three fiber Bragg gratings with different reflection spectrums and combine genetic algorithm have been developed and simulated. This method uses three different fiber Bragg gratings: two are uniform fiber Bragg gratings and the other is chirped fiber Bragg grating.
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