Sharif Digital Repository

In this study, we have theoretically investigated designing possibility of a seismometer using Fiber Bragg Grating (FBG) and Metal Bellows pairs attached on a mechanical system. This new seismometer can record earthquakes according to sensitivity of fiber optic to changes in physical parameters such as stress and strain. Firstly, in order to understand the quantity and quality of sensor performance, with a mathematically brief description, the effect of stress and strain on the FBG reflection spectrum is examined. FBG is an intrinsic sensing element which can be photoinscribed into a silica fiber. The basic principle of this operation commonly uses FBG based sensor system. The operation is... 

Storage tanks are always under different hazards such as corrosion and leaking. In this work we have introduced a computational analysis on storage tanks in which a real-time yet efficient monitoring technique using Fiber Bragg Grating (FBG) sensors for corrosion detection on the bottom plate and vibration monitoring on outer surfaces of storage tanks is proposed. A finite element model for the tank is developed, in which the outer tanks surfaces are considered as plate elements and FBG sensors as beam elements. An array of FBG sensors is utilized to compute the generated strain in the FBGs due to losing thickness in the tank's bottom plate and due to vibration on the outer surfaces in... 

An in-fiber wavelength-selective optical intensity modulator has been developed to superimpose low-rate overhead (OH) data on the transit high-speed payload signals. The tunable fiber Bragg grating, as the heart of a wavelength-selective modulator (WSM), is designed and properly biased to implement the desired modulation specification. Potential performance limitations of WSM are investigated and described in terms of eye opening penalty and crosstalk between wavelength-division-multiplexing channels. A simple OH/label erasing technique is also provided and used for modifying the labels through cascaded WSMs. The performance of such cascaded systems is calculated both with and without... 

A semi-analytical formulation is presented to quantify the effects of group delay ripple on the performance of an optical beam-former. For delay elements, implemented with commercially available multi-channel fiber Bragg gratings, an impulse response is introduced and calculated for the group delay ripple. To do this, the transfer function of the fiber Bragg grating is parted into two dispersive and non-dispersive sections. This will also easethe demonstration of the fiber Bragg grating as a true time delay element. The formulation is applied to an ultra-wideband pulsed array of antennas. As an example the array factor and electric field patterns of a four element antenna array is derived,... 

The equivalent circuit approach (ECA) is used in this work to analyze and design a previously proposed one-dimensional planar blazed grating of the resonant type. The analysis covers both the classical Littrow configuration, when the direction of the relevant diffracted order coincides with that of the incident wave (Bragg blazing), and when these directions are different (off-Bragg blazing). Once the scattering problem of the grating structure is posed as a discontinuity problem inside an equivalent generalized waveguide (corresponding to the unit cell of the original structure) and studied in terms of its equivalent circuit network, the possibility of transferring all the power of the... 

In this paper we use tunability of FBG to introduce a novel device for optically reading, writing or modifying the low-speed label(s). We utilize tunable FBG arrays as concurrent label modulating (multiplexing) elements. This second-level intensity modulation is used to send signaling messages between nodes of WDM networks. The fiber optic small-signal modulator is formulated, and its behavior is compared with the published reports  

A new structure is suggested for simultaneous microwave chirped pulse generation and array antenna beam steering. It is based on using a multi-channel fiber Bragg grating in a photonic microwave delay-line filter. The paper presents a feasibility study of the idea, discussing the main performance parameters of both signal generation and beam steering functions. Specifically, it focuses on the effects of wavelength tuning, resolution and accuracy. The study shows that custom off-the-shelf components could be used to implement an all optical system capable of generating chirped pulses while steering the radiation pattern of a small sized antenna array. The advantages of the structure for... 

We have developed an analytic approach to investigate the effect of group delay ripple of the dispersive devices on the performance of two major building blocks of microwave- photonic filters. Firstly, performance of PM-based block in the presence of an arbitrary group delay ripple (GDR) is analyzed and compared with the ripple-free case to reveal the destructive effects of added group delay ripple. In the next step, we repeat the proposed approach for the AM-based one; again, the performance is compared with the ripple-free case. Two distortion metrics are also introduced to quantify this distortion. Comparison of the performance of two building blocks in the presence of group delay ripple... 

In the presence of self and cross saturation effects, the laser governing equations are nonlinear. To solve the governing equations of a fiber Bragg grating random laser, on the basis of the transfer matrix method, a simple and fast iterative method is introduced. The method is employed to obtain the frequency, threshold pumping rate and spatial distribution of different laser modes. Numerical calculations show that by increasing the number of fiber Bragg grating, the number of lasing modes increases, while the threshold pumping rate of the first mode decreases. Our theoretical results are in agreement with the experimental observations reported by previous authors  

We have developed an analytic approach to investigate the effect of a sinusoid group delay ripple on the performance of phase-modulated microwave photonic filter. Frequency of fabricated sinusoid ripple determines the pole's frequency of the resulted transfer function. Comparison of our proposed analytical approach with simulated OptiSystem results shows a good agreement  

A fiber Bragg grating (FBG)-based delay line (or phase shifter) has been developed for microwave over fiber and phased array antenna (PAA) applications. The scheme uses band edge time delay of tunable fiber gratings to provide a continuously controllable microwave time delay or phase shift. Following the formulation and analysis of the device behavior, simulations of amplitude and phase responses are conducted. The results confirm the validity of theoretical predictions  

We propose and demonstrate a novel architecture for four-wave mixing (FWM)-based recognition of spectral amplitude code (SAC) labels in optical packet-switched networks. With a proper code design, a unique FWM idler for each SAC label, referred to as a label identifier (LI), is generated in a nonlinear medium. A serial array of fiber Bragg gratings is then used to reflect the LI wavelengths. Each LI is associated with a unique amount of delay between two optical signals received at two photodiodes (PDs). Label recognition is then achieved by measuring this unique time delay (referred to as the characteristic delay). The main advantages of the proposed method include the following: no... 

We have developed an analytic approach to investigate the effect of group delay ripple on the performance of phase-modulated microwave photonic filter. Two cases with and without ripple are analyzed to clarify the ripple's effects on the performance of microwave-photonic filter. Comparison of our proposed analytical approach with simulated OptiSystem® results shows a good agreement  

Tunability of fiber Bragg grating (FBG) in transition region is used to implement wavelength-selective optical intensity modulator, which superimposes a secondary low-speed data on the transit high-speed payload optical signal. Theoretical model of the device is developed and verified by measurements in the linear and nonlinear slopes of the FBG. Experiments with strong and relatively weak gratings confirm the wavelength-selectivity and stability of modulation. The fiber-based modulator is employed for optically tagging or labeling individual wavelength channels using baseband and amplitude-shift keying (ASK) modulated signals. The wavelength-selective channel labeling scheme is useful for...