Telecommunication networks based on optical fiber technology have become a major information-transmission system, satisfying the growing demand for bandwidth due to increased internet traffic and other applications such as video on demand, etc. Fiber Bragg gratings (FBGs), in recent years, have emerged as critical components for enabling high-capacity transmission since their response can be tailored to meet the needs of specific applications. FBGs are currently the focus of intense research interest in both the fiber communications and sensing fields. Optical fiber Bragg grating structures in single-mode fiber (SMFBGs) have been studied extensively since the discovery of photosensitivity in germanium-doped silica fiber. They have been used in numerous applications ranging from wavelength-selective filtering in wavelength-division-multiple-access (WDMA) systems to temperature and strain sensing. To a lesser extent, Bragg gratings in multimode fibers have also received attention because of easy coupling with light sources. Most of the MMFBGs related research work has demonstrated the formation of a Bragg grating in a graded-index MMF and briefly reported the measured transmission spectrum. So far, there are few theoretical studies on Bragg gratings in multimode fibers. In this thesis, we investigate Bragg gratings in multimode optical fibers both theoretically and experimentally. A comprehensive numerical model for MMFBGs has been established and the corresponding computer simulation software (MMFBG simulator combined with mode solver) developed. The optical properties of MMFBGs were systematically studied for the first time using our own MMFBG numerical software package. It effectively assists the design modeling for MMFBG-based optical devices. Bragg gratings in multimode fiber were also investigated experimentally. Our theoretical simulation results show good agreement with experiments and offer the insightful explanations for the underlying physics of the device. First, the guided modes were modeled and simulated for step index multimode fibers and graded index multimode fibers with emphasis on parabolic fiber structure. These are popular, standard and commercially available MM fibers, and employed throughout our experiments. This allows us for the simulation of fiber characteristics such as cut-off wavelength, mode effective index, propagation constants and optical field distribution. It also allows for calculation of mode coupling coefficients by overlap integral between any chosen guided modes. Therefore, it serves as a powerful model for the design and analysis of optical fibers. Second, the generalized MMFBG coupled mode theory formalism is derived. The physical mechanism of the behavior of MMFBGs is studied and discussed. The general solution to the MMF Bragg grating problem is achieved by Runge-Kutta, Newton-Raphson and shooting numerical methods. Our theoretical treatment, in particular, offers the advantages which can deal with not only self-coupling but also more complicated cross-coupling interactions and can solve arbitrary large number of mode coupling problems throughout the entire spectra simultaneously for multimode FBGs, thus allowing for a precise and quantitative study of MMFBGs. Such an intensive multimode fiber Bragg grating physical modeling and simulations have not been reported previously. It provides an effective means for the design and analysis of optical fiber devices based on Bragg gratings. Third, the optical properties of multimode FBGs were studies experimentally. Numerical predications of the grating spectral characteristics under fabrication and experimental condition are calculated. The results of the numerical calculations are compared with experimentally measured spectra of multimode gratings written by ultraviolet irradiation of deuterium-sensitized fiber with grating reflectivities ranging from 78% to 99.39%. Good agreement is obtained between the theoretical simulations and the experimental results. Thus, we provide quantitative explanations for the observed experimental phenomena. These explanations give both physical insight and a more complete understanding of the nature of the interaction between the wave propagation and multimode fiber gratings. Furthermore, the spectral simulation of the actual experiments prepares a theoretical guidance for the advanced experimental investigation and also presents a step toward MMFBG device design. Finally, the optical properties of MMFBGs were also studied theoretically. To our knowledge, this is the first detailed analysis and thorough investigation on grating characteristics in MMF. It is demonstrated that the transmission and reflection spectra of fiber Bragg gratings in multimode optical fibers strongly depend on the length of grating, index modulation, period of grating, mode excitation condition and physical structure of MMF. The simulation results allow us to deeply comprehend and visualize the more sophisticated behavior within a multimode fiber grating, and will also allow us to confidently predict and evaluate the performance of more complex structure MMFBGs. It provides the fundamental principles for designing the targeted spectrum performance and settles the theoretical rationale for realizing the practical applications. Overall, the comprehensive numerical model and MMFBG solver package developed in this thesis opens a clear and broad window for understanding MMFBG mechanisms from the physical point of view. Various simulation results and spectral characteristics have been researched and discussed under both ideal and experimental conditions for the purpose of experimental analysis and device design. The results of our study indicate that a new class of potential applications based on MMFBGs can be expected in optical fiber sensors and advanced communication systems. / Thesis / Master of Applied Science (MASc)

In the past several decades the technology for the creation and use of ultrashort pulses has progressed tremendously. Now, it is possible to generate laser pulses as short as a few femtoseconds in duration, and such pulses have been used for a wide range of applications. In addition, the means of measuring these pulses has progressed so rapidly. However, despite recent great advances in ultrashort-pulse measurement techniques, much remains to be done. In particular, pulse-measurement devices have relatively small wavelength-tuning ranges, and the phase-match is problematic for the pulses with a wide bandwidth such as supercontinuum. In this thesis, I will demonstrate a new pulse measurement technique which can phase-match ultra-broad bandwidth of super-continuum using transient grating frequency-resolved-optical-gating (TG FROG). Also, I will demonstrate a simplified device which can measure the UV ultra-short pulse using transient grating process, one of the third-order nonlinearity and can cover from UV to IR with the same arrangement.

A new scheme of fabricating the tapered hyperbolic-end fibers (THEFs)microlenses using unique etching and fusion techniques is proposed. TheTHEFs were fabricated by symmetrically tapering the fiber during theetching process and hyperbolically lensing the tip during the fusing process.The tapered hyperbolic microlenses have demonstrated up to 82% couplingefficiency for a laser with an aspect ratio of 1:1.5. The influence of the tapering asymmetry on the coupling has also been investigated experimentally and theoretically. The axially symmetrical taperedmicrolenses of the THEFs showed that far-field profiles were well approximated to a Gaussian profile, while the asymmetric taper had deviated significantly from a Gaussian profile. A theoretical analysis illuminated a larger wavefront transformation of the hemispherical microlenses. A lesser phase aberration of the normalized optical path difference (OPD) was found in the hyperbolic-end lens, and that resulted in more than 2 dB improvement in the coupling efficiency when compared to the currently available hemispherical microlenses. The high-coupling performance of the hyperbolic microlens was due to an improved wavefront matching between the laser and the fiber, which was one of the most important contributions in this study.The 1.55 µm fiber grating external cavity lasers (FGECLs), packaged with THEF microlens for coupling the fiber grating external cavity, have been investigated for different combinations of coupling efficiency (£b) and Bragg reflectivity (Rg). Various tapered hyperbolic-end fiber microlenses with different coupling efficiency have been fabricated for this study. The FGL of higher £b = 72% and Rg = 0.52 has a stronger resonant feedback as the spectral output showed a single longitudinal mode with the side-mode-suppression-ratio (SMSR) greater than 45dB, a high output power of greater than 5mW, and a lower threshold current. However, for the case of £b = 68% and Rg = 0.35, the FGL exhibited a more stable SMSR against the variation of pumping current and temperature. Numerical simulations have also been performed on the SMSR at different coupling efficiencies and Bragg reflectivity for the FGLs. The high performance of the FGLs can be achieved through a higher coupling efficiency between a laser diode and a single-mode fiber. The calculated SMSR showed an excellent agreement with the measured data.

This study proposes a low cost potentiall with non-AR coated fiber grating external cavity laser (FGECL) module to apply the metro/access network. The components inside the module include uncoated FP (Fiber-Perot) laser chip, PIN detector, substrate, and cooler. The processes of package are following: (1) to utilize the die-bonder to fix the FP laser and the PIN detector on the substrate, (2) to utilize the heating apparatus to make the cooler fixed on the butterfly housing and the substrate fixed on the cooler, (3) to utilize the 353ND paste to make the thermistor fixed on the substrate, and (4) to utilize the electrothermal heating machine to melt indium wire and then adjust the fiber lens provided with higher coupling efficiency of fiber pigtail by tweezer to couple light into the fiber inside the butterfly housing. This study achieves a FGECL module with the output power of larger than 2mW and the side-mode suppression ratio (SMSR) of more than 38dB. Finally, we measure eye diagram and bit-error-rate at 2.5Gb/s of the FGECL module to analyze the impedance matching of laser diode, current signal and the limit of the dispersion to the optical communication system. The performance of the FGECL module can meet the ITU-T G.957 standard.

The fiber grating external cavity laser (FGECL) module packaged with fiber bragg grating and FP laser diode is investigated. The optical spectrum of FGECL is single longitudinal mode. In order to get stable single longitudinal mode, the FP laser diode coated with an AR-coating (reflectivity is 0.5%). To achieve higher coupling efficiency between laser diode and fiber, the hyperbolic-end lensed fiber is used. A coupling efficiency of up to 86% has been demonstrated. The alignment and fix between laser diode and fiber are accomplished by laser welding technology. The results of FGECL module show that the output power and side-mode suppression ratio (SMSR) are more than 2mW and 44dB, respectively. Comparing to the non-AR coated FGECL, the result of SMSR is 5 dB improved. Dynamic tests of the FGECL module operate at 2.5Gbps, including the eye diagram, bit error rates, impedance matching of laser diode, signal current, and the limit of the dispersion in the optical communication system has been measured. The FGECL module can meet the ITU-T G.957 standard.

ABSTRACT The spectral characteristics for non-AR coated fiber grating lasers were studied theoretically and experimentally. The lensed fiber was used to improve coupling efficiency between laser and fiber. The tapered fibers were fabricated by using the mixture of HF and oil with different density to increase etched taper angle. The coupling efficiency could reach more than 60%. A single-mode operation for a fiber grating external cavity laser (FGECL) was simulated. The results showed that the SMSR, emitted power, and wavelength drift were dependent on the related device parameters. Our calculations showed that the strong current-dependent SMSR oscillation was from the mode selection by the fiber grating external cavity and the heating effect in the Fabry-Perot (FP) laser. A 1.55mm FP laser chip that one facet was coated a high reflectivity (HR) of 90% and another facet was uncoated. In our experiment and simulation of FGECL, the reflectivity of fiber gratings were 50% and 70% and 86%, and the length of external cavity was about 0.9cm. The measured result of FGECL showed that the side-mode suppression ratio (SMSR) was more than 35dB and the output power was larger than 1.5mW at the injected current 2 to 3 times of threshold current. Furthermore, the spectrums of fiber grating external cavity lasers were studied in order to understand the external laser characteristics.

A novel sensor using a polymer coated long-period grating (LPG) has been proposed for monitoring levels of organic contaminants in air or water systems. The sensor operates by detecting refractive index changes in the polymer coating as analytes partition in. Polymer coatings used must be able to reversibly and reproducibly absorb contaminants of interest from the sample and have a refractive index just below that of the fiber cladding. The synthesis and characterization of several chemically selective polysiloxanes is described. Pre-polymer materials are made through the catalyzed condensation of silane monomers. Different functional groups are incorporated either through polymerizing functionalized monomers, or by post-functionalizing the polymer through a platinum-catalyzed hydrosilylation reaction. The pre-polymer materials are crosslinked into elastomeric films using titanium(IV) tetraisopropoxide. The polymer refractive index is controlled through altering the ratios of functional groups within the polymer or changing the loading levels of titanium. Four polymers were made, having different functional groups and optimized refractive indices for use on the proposed sensor. The partition coefficients for the polymers with a variety of solvents are calculated and compared. Each polymer was found to have a slightly different chemical selectivity pattern, demonstrating that a set of polymers could be used to generate a sensor array. Partition coefficient data was calculated from the gas phase by considering the change in polymer refractive index as the solvents partitioned into the polymer. The Lorentz-Lorenz equation was used to model the relationship between the change in refractive index and the solvent concentration within the polymer. Finally, polymers were applied to LPGs and used to successfully detect various solvents from the gas phase. This was accomplished by monitoring the entire LPG spectrum, and also by considering loss at a single wavelength using fiber-loop ring-down spectroscopy. / Thesis (Ph.D, Chemistry) -- Queen's University, 2008-09-26 15:28:35.603

No description available.

Femtosecond laser pulse has been used for the machining of the gratings primarily due to its superior advantages over conventional continuous wave (CW) and long pulse lasers for micromachining. In this paper, we develop a novel technique for the fabrication of planar gratings by colliding two beams to generate interference fringes. This technique is simple, fast and low cost. We have successfully fabricated planar gratings on a copper substrate. / Singapore-MIT Alliance (SMA)

A long period fiber grating (LPFG) is formed by inducing the periodic refractive index variation along a fiber. A lot of work has been done to fabricate the LPFGs in the photonic crystal fibers (PCFs) to function as all-fiber band-rejection filters, interferometers, and sensing applications. In this thesis, we propose a novel LPFG based on the gel-filled PCF. The PCF filled with the UV gel was exposed to the high-intensity UV light through the mask. The periodic index variation is formed along the fiber in the cladding region, resulting in the LPFG. By measuring the propagation losses of our LPFG, three spectral dips in the transmission bands are observed at 872 nm, 1309 nm, and 1418 nm as the grating period is 600 £gm, which indicates the mode coupling from the fundamental core mode to the higher order modes (HOMs) of the gel-filled PCFs. By using a full-vector finite-difference frequency-domain (FDFD) method, we numerically calculate the phase match condition for our LPFGs. The calculated resonant wavelengths are 875 nm, 1319 nm, and 1415 nm. Very good agreement between the measured resonant wavelengths and the numerical results is obtained. We also fabricate the selectively gel-filled LPFGs to reduce the propagation losses by utilizing a simple selectively blocking technique. In addition, we measure and discuss the sensing sensitivities of the UV-induced LPFGs, including the temperature, strain, curvature, torsion, and surrounding refractive index (SRI) sensitivities. The measured sensitivity to temperature is 1.7 nm/¢XC from 25 ¢XC to 45 ¢XC. As the surrounding refractive index is increased to 1.377, the dip position has a maximum shift of 2 nm. Compared with other LPFGs, the UV-induced LPFGs are more insensitive to bending and strain due to the complete cladding structure. This could benefit the stability of the temperature sensors, based on our UV-induced LPFGs.