Thesis (doctoral)--Lund University, 1997. / Added t.p. with thesis statement inserted.

No description available.

The fabrication of broadband single-mode Cr-doped silica fibers (SMCDSFs) using the fiber drawing-tower method with the modified rod-in-tube technique is demonstrated for the first time. A preform was assembled by using the grown Cr:YAG rod as core and the silica tube as cladding. The outer and inner diameters of the silica tube are 20 and 7 mm, respectively. The initial dimension of the Cr:YAG crystal rod had a length of 0.03 m and a diameter of 500 £gm. The Cr:YAG crystal was grown into a diameter of a 290 £gm with a length of 0.12 m by the LHPG method. The SMCDSFs had a 6 £gm core and a 125 £gm cladding. The transmission loss was 0.08 dB/cm at 1550 nm. The far-field pattern measurements indicated the single-mode characteristic when the propagation wavelength was longer than 1310 nm. In order to solve the interface of core and cladding, a novel rod-in-tube(RIT) perform was employed by inserting the Cr:YAG crystal rod of 0.03m length and 500 £gm diameter into the silica capillary tube, which had the same diameter with the drilled silica rod. The single-mode Cr-doped fibers had successfully been fabricated and the loss had been reduced to 0.03 dB/cm at 1550 nm with a 5 £gm core and a 125 £gm cladding. Furthermore, the SMCDSFs also had the single-mode characteristic when they operated in the optical communication window. The successful fabrication of SMCDSFs may be one step forward towards the achievement of utilizing the SMCDSFs as ultra-broadband fiber optical amplifiers to cover the bandwidths in the whole 1300 to 1600 nm range of low-loss and low-dispersion windows of silica fibers and a broadband source for enabling high resolution in optical coherence tomography (OCT).

Thesis (doctoral)--Lund University, 1994. / Added t.p. with thesis statement inserted.

Thesis (M.S.)--West Virginia University, 2004. / Title from document title page. Document formatted into pages; contains xiii, 147 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 141-147).

Thesis (doctoral)--Lund University, 1994. / Added t.p. with thesis statement inserted.

Fiber optical temperature and strain sensors have been extensively investigated for applications in the civil structures to ensure safety and prevent disasters in advance. Most of the demonstrated fiber sensors are based on the silica fibers to form an interferometer by measuring the spectrum wavelength shift caused by the change of the refractive index and fiber length, and the sensitivities, defined as the rate of wavelength shift with respect to temperature or strain, are limited by the small values of thermal-expansion coefficient and thermo-optic effect of the silica materials. To improve the sensitivity, we designed the dual-core As2Se3-PMMA fiber with the PMMA cladding diameter 56.5 times larger than that of the As2Se3 cores, which brings out many interesting sensing applications. Nonlinear devices have a variety of practical applications including parametric amplification, all-optical switching, super-continuum generation, and sensing applications. Tapered chalcogenide-polymer fiber structures composed of an As2Se3 core and a polymethyl methacrylate (PMMA) cladding are a promising platform for nonlinear applications because the As2Se3 core provides high nonlinearity over the near- and mid-infrared spectral ranges for compact nonlinear devices with low power consumption and the PMMA cladding provides high mechanical strength for easy handling. Advanced As2Se3-PMMA fiber structures such as dual-core fibers that support guided propagation of an even mode and an odd mode will open the way for a variety of novel devices in the near- and mid-IR wavelength range. In my work I utilized two As2Se3 fibers and a polymethyl methacrylate (PMMA) tube for the fabrication of dual-core As2Se3-PMMA tapers and demonstration of the sensing applications and nonlinear optical effects. The thesis mainly consists of three parts: the fabrication process, the sensing applications, and the nonlinear applications in the tapered dual-core As2Se3-PMMA fiber. In the first part, the fabrication process of the tapered dual-core As2Se3-PMMA fiber is introduced. The dual-core As2Se3-PMMA fibers are fabricated using a rod-in-tube method. The images of the setups and fibers in process are listed. In the second part, a theoretical model for temperature and strain measurement and four sensing applications are introduced. Firstly, we demonstrate an approach for high-sensitivity simultaneous temperature and strain measurement in a dual-core As2Se3-PMMA taper with As2Se3 core diameter of 0.55 μm. High measurement sensitivities are observed for both principal polarization axes of the tapered dual-core As2Se3-PMMA fiber with temperature sensitivities of -115 pm/ºC for axis-1, -35.5 pm/ºC for axis-2, and strain sensitivities of -4.21 pm/με for axis-1 and -3.16 pm/με for axis-2. Secondly, the thermal forces in a dual-core As2Se3-PMMA taper are investigated. A temperature-insensitive strain sensor is proposed and demonstrated based on the thermal forces. Finally, two approaches for temperature and strain sensitivity enhancement are investigated. The first approach is by reducing the value of the variation of the difference between phases of the even and odd modes with respect to wavelength (∂ϕd(λ)/∂λ) and increasing thermal-forces in a dual-core As2Se3-PMMA taper with As2Se3 core diameter of 2.5 μm. The value of ∂ϕd(λ)/∂λ decreases with the As2Se3 core diameter and thermal-forces on the As2Se3 cores are enhanced in the fibers with large PMMA cladding, which work together to enhance the measurement sensitivity. The second approach is based on effective group-velocity matching between the even and odd modes of a dual-core As2Se3-PMMA taper on which an antisymmetric long-period grating is inscribed. The variation of the difference between phases of the even and odd modes with respect to wavelength tends to 0 (∂ϕd(λ)/∂λ→0) near the resonance wavelength of the grating due to the effective group-velocity matching between the two modes, and consequently, thermally-induced change of the difference between phases of the two modes ϕd (λ) leads to a large wavelength shift indicating enhancement of the temperature measurement sensitivity. In the third part, I study the nonlinear optical effects in the hybrid fibers. Firstly, I demonstrate modulation instability within the normal-dispersion regime in a dual-core As2Se3-PMMA fiber. Then I review the work about the forward stimulated Brillouin scattering and its sensing applications. The radial and torsional-radial guided acoustic modes of silica fibers and tapered dual-core As2Se3-PMMA fibers are investigated experimentally and the preliminary results are presented.

Femtosecond (fs) laser micromachining has been a useful technique either to modify and remove materials or to change the properties of a material, and can be applied to transparent and absorptive substances. Recently high-power fs laser radiation has drawn intensive attention for the induction of refractive index change to fabricate micro-structures in dielectric materials. This thesis studies the optical properties of a novel fiber random grating fabricated by fs laser micromachining technique and extends its applications from optical sensing to random fiber lasers and optical random bit generations. The thesis mainly consists of three parts. In the first part, the physical mechanism behind the fs laser micromachining technique and the fabrication of the fiber random grating are introduced. By employing a wavelength-division spectral cross-correlation algorithm, a novel multi-parameter fiber-optic sensor based on the fiber random grating is proposed and demonstrated to realize simultaneous measurements of temperature, axial strain, and surrounding refractive index. In the second part, Brillouin random fiber laser (BRFL) and Erbium-doped fiber random laser (EDFRL) are introduced, respectively. Firstly, we propose a novel Brillouin random fiber laser with a narrow linewidth of ~860 Hz based on the bi-directionally pumped stimulated Brillouin scattering (SBS) in a 10-km-long optical fiber. A random fiber Fabry-Perot (FP) resonator is built up through the pump depletion effects of SBS at both ends of the fiber. The novel laser is successfully applied for linewidth characterization beyond 860 Hz of light source under test. Secondly, the random grating-based FP resonator is introduced to build up a novel BRFL with narrow-linewidth of ~45.8Hz and reduced lasing threshold. The intensity and frequency noises of the proposed random laser are effectively suppressed due to the reduced resonating modes and mode competition. Finally, the fiber random grating is used as random distributed feedback in an EDFRL to achieve both static (temperature, strain) and dynamic (ultrasound) parameter sensing. Multiple lasing lines with high signal-to-noise ratio (SNR) up to 40dB are achieved, which gives an access for a high-fidelity multiple-static-parameter sensing application. By monitoring the wavelength shifts of each peak, temperature and strain have been simultaneously measured with small errors. The fiber random grating in the EDFRL is also able to sense the ultrasound waves. By achieving single mode lasing with the EDFRL, ultrasound waves with frequencies from 20kHz to 0.8MHz could be detected with higher sensitivity and SNR improvement up to 20dB compared with conventional piezoelectric acoustic sensors. In the third part, we demonstrate that a semiconductor laser perturbed by the distributed feedback from a fiber random grating can emit light chaotically without the time delay signature (TDS). A theoretical model is developed by modifying the Lang-Kobayashi model to numerically explore the chaotic dynamics of the laser diode subjected to the random distributed feedback. It is predicted that the random distributed feedback is superior to the single reflection feedback in suppressing the TDS. In experiments, The TDS with the maximum suppression is achieved with a value of 0.0088, which is the smallest to date.

No description available.

Side-polished fiber allows access to the evanescent field propagating in the cladding of a few-mode fiber. This cladding mode is analyzed and experimentally validated to further the design of a novel class of fiber optic devices. To do this, specific modes are excited in the polished fiber using a phase-only spatial light modulator to determine spatial mode distribution. Each mode is excited and compared to the expected field distribution and to confirm that higher order modes can propagate through side-polished fiber. Based on each mode’s distribution, a side-polished fiber can be designed so that perturbations on the polished portion of the fiber effect each mode independently. By carefully analyzing the effects of identical perturbations on each mode, it is determined that each mode can be isolated based on the geometry of the polished fiber and careful alignment of the mode field. This research has the potential to advance the development of novel fiber-based sensors and communications devices utilizing mode-based interferometry and mode multiplexing. / M.S. / Fiber optic devices have seen significant advancement since the realization of the laser and low-loss optical fiber. Modern day fiber optics are commonly utilized for high-bandwidth communications and specialized sensing applications. Utilizing multiple modes, or wave distributions, in a fiber provides significant advantages towards increasing bandwidth for communications and provides potential for more accurate sensing techniques. Significant research has been conducted in both the sensing and communication field, but mode-domain devices have the capability to significantly advance the field of fiber optic devices. This thesis demonstrates the potential for side-polished fiber geometry to effect each mode independently, thus allowing side-polished fiber to be utilized for realizing novel devices such as multiplexing devices and fiber optic sensors.