Core-cladding Interface Research Articles

Fabrication of high aluminium containing rare-earth doped fiber without core–clad interface defects

The unwanted core–clad interface defect generation and related scattering loss in rare-earth doped optical fibers containing Al-oxide is up to now an unsolved problem as it cannot be avoided without modifying fiber design or composition. Here a simple technique is proposed to eliminate the problem without affecting fiber parameters. The method comprises addition of fumed silica in appropriate amount under dispersed condition to the soaking solution containing Al and rare-earth salts during the solution impregnation stage. This facilitates the networking between Al 2O 3 and SiO 2 during sintering. The method leads to an improvement in optical properties of the fibers and enables doping of high concentration of Al-oxide even in presence of GeO 2 in the core.

On the Slow-Wave Helical Clad Elliptical Fibers

Lightwave propagation through dielectric helical clad elliptical fibers is presented under the slow-wave consideration. The helical clad is achieved by putting helical turns on the core-clad interface. While treating analytically, two extreme cases of helix pitch angle — i.e., 0° and 90° — are considered along with the use of slowwave approximation. E- and H-field components are determined in the different sections of the guide followed by the evolution of the eigenvalue relation. The effect of ellipse eccentricity is determined on the dispersion characteristics for different operating wavelengths and corresponding to different values of the length of core semi-major axis.

Glass-clad single-crystal germanium optical fiber

Long lengths (250 meters) of a flexible 150 microm diameter glass-clad optical fiber containing a 15 microm diameter crystalline and phase-pure germanium core was fabricated using conventional optical fiber draw techniques. X-ray diffraction and spontaneous Raman scattering measurements showed the core to be very highly crystalline germanium with no observed secondary phases. Elemental analysis confirmed a very well-defined core-clad interface with a step-profile in composition and nominally 4 weight-percent oxygen having diffused into the germanium core from the glass cladding. For this proof-of-concept fiber, polycrystalline n-type germanium of unknown dopant concentration was used. The measured infrared transparency of the starting material was poor and, as a likely outcome, the attenuation of the resultant fiber was too high to be measured. However, the larger Raman cross-section, infrared and terahertz transparency of germanium over silicon should make these fibers of significant value for fiber-based mid- to long-wave infrared and terahertz waveguides and Raman-shifted infrared light sources once high-purity, high-resistivity germanium is employed.

Insights into tunnelling rays: outperforming guided rays in fiber-optic sensing device

We report that, for an evanescent-wave (EW) based fiber-optic sensor, the less skewed tunnelling rays and a small number of very steep guided rays experience total internal reflection (TIR) at one end-face of the air-clad fiber segment. As a result, they demonstrate nine times greater efficiency in collecting EW-power than rays that do not exhibit end-face- TIR. This tunneling ray group originates from a launching ring that is seven times wider than that of end-face-TIR capable guided rays, enabling it to dominate the overall detected EW-power level. Strongly supporting this conclusion is the counter-intuitive experimental observation that the detected power at one end of the fiber abruptly drops 91% when the back reflection at the opposite end is eliminated, while an isotropic light source is placed at the core-cladding interface between these two ends. The mechanism of non-propagating ray excitation accounts for this high efficiency. We also indicate that many highly skewed tunnelling rays are naturally isolated from the external world because, outside the fiber core, they are accessible only via their EW fields even at the fiber end-faces. The findings reported here have significant implications for the design of future high-performance EW-based fiber sensing devices for the analysis of surface-event.

ON THE FIELD PATTERNS OF HELICAL CLAD DIELECTRIC OPTICAL FIBERS

An analytical investigation of the lightwave propagation through dielectric optical fibers with helical clads is presented with the emphasis on their hybrid field patterns. The helical clad section is effectively realized by introducing conducting windings on the core-clad boundary. Using Maxwell’s equations, a rigorous analytical approach is implemented to determine the higher order field patterns in such fibers. For simplicity, two particular values of the helix pitch angle are considered, viz. 0◦ and 90◦. The nature of fields is presented in both the situations corresponding to different allowed values of the propagation constants and the fiber diameters. The radial distributions of fields corresponding to hybrid modes are presented under different situations, which exhibit the patterns like spikes. The observed smooth match of the fields at the core-clad interface validates of our analytical approach. The presence of a little higher amount of field in the fiber clad section is essentially attributed to the helical windings introduced over the fiber core. Further, the existence of considerable amount of evanescent waves in such fibers opens up the possibility of their applications in optical sensing.

Fabrication and gain performance of Er 3+/Yb 3+-codoped tellurite glass fiber

Er 3+/Yb 3+-codoped TeO 2 -ZnO-BaO-La 2O 3 tellurite glass fiber was fabricated by rotation and rod-in-tube technologies. The thermal stability and optical refractive index of the core and cladding glasses were determined by DTA and optical coupler, respectively. The average background loss of tellurite glass fiber was 1.8 dB/m at 1310 nm. Optical microscopy and field emission scanning electron microscope (FESEM) were used to study structural characteristics of preforms and optical fibers. The main loss of tellurite glass fiber could be attributed to scatter centre due to core-cladding interface defects. The amplifier performance of tellurite glass fiber was investigated by pumping with 980 nm laser diode (LD). The gain coefficient and maximum signal gain were 0.21 dB/mW and 10 dB, respectively, for a pumping power of 120 mW. Gains exceeding 5 dB were obtained over 30 nm bandwidth from 1535 to 1565 nm. The minimum noise figure was 4.8 dB at 1557 nm.

Thermal Diffusion Process Estimation for Fabrication of Graded Plastic Optical Fibre

Thermal diffusion of dopants is investigated in the process of generating the graded-index profile of plastic optical fibres. Because the diffusion coefficient in high polymers has been shown to depend strongly on dopant concentration, it is allowed in this work to vary with the radial coordinate of the multistep-core fibre. A novel multi-layer model is presented for solving the diffusion equation with the variable diffusion coefficient. It is solved by the finite difference method. The solution determines the dopant diffusion profile in the fibre. It is verified against a solution from the literature and two cases of fibres with diffused profiles. The application is demonstrated on two examples of graded-index plastic optical fibres, one originally with a two-step and the other with four-step core. The results indicate that closer to the core-cladding interface, the computed diffused profile with variable diffusion coefficient D is closer to target profile than the profile obtained with constant D for the same time of thermal process.

Improvement of transparency and durability to deep ultraviolet light in fluorine-doped silica glass fiber by optimizing the preform fabrication process

Fluorine-doped silica glass fiber was fabricated as fiber-core for transmittance of deep ultraviolet wavelength region. By examining the effect of oxyhydrogen flame onto the core-glass (conventional cleaning process called fire-polishing process), it was found to create oxygen-deficiency centers. Additionally, fire-polishing process changes the distribution of fluorine concentration near core-cladding interface and leads to the deterioration of the optical characteristics of the fiber. By conducting preform fabrication without fire-polishing, the obtained fluorine-doped silica glass fiber showed high transmittance of more than 92% m -1 at 266 nm and excellent durability after 4 G pulses of 7 mJcm -2 laser irradiation.

空芯光子带隙光纤纤芯-包层交界面的设计

空芯光子带隙光纤纤芯-包层交界面的设计

Creating second-order nonlinearity in pure synthetic silica optical fibers by thermal poling

A twin-hole optical fiber with pure synthetic silicate glass between the two electrode holes was thermally poled. The induced second-order nonlinearity (SON) was located at the core-cladding interface sections that were nearly parallel to the poling electric field. The polarization dependence of the induced SON suggests that nonlinearity was due to the presence of a space-charge field, which was probably formed by electron migration among the defects located at the core-cladding interface. The magnitude of the induced SON was measured to be approximately 0.06 pm /V.

Time evolution of the second-order nonlinearity layer in thermally poled optical fiber

Time evolution of the second-order nonlinearity induced in a single-hole D-shaped fiber thermally poled at 3.5kV and 320°C has been investigated with second-harmonic microscopy. It was found that the nonlinear layer moved towards the cathode with increasing poling time until it finally reached the cathode surface. The magnitude of the nonlinear coefficient first increased, saturated at ∼0.32pm∕V between 5 and 20min after poling, and then began to decrease for longer poling time. The nonlinearity remained large even when the nonlinearity layer reached the centerline between the two electrodes. In samples poled long enough (>45min), apart from the main nonlinearity layer at the cathode, weak second-order nonlinearity was also induced at the core-cladding interface sections that were nearly parallel to the poling direction.

Serum protein measurement using a tapered fluorescent fibre-optic evanescent wave-based biosensor

A novel method to measure the total serum protein concentration is described in this paper. The method is based on the principles of fibre-optic evanescent wave spectroscopy. The biosensor applies a fluorescent dye-immobilized porous glass coating on a multi-mode optical fibre. The evanescent wave's intensity at the fibre-optic core-cladding interface is used to monitor the protein-induced changes in the sensor element. The sensor offers a rapid, single-step method for quantifying protein concentrations without destroying the sample. This unique sensing method presents a sensitive and accurate platform for the quantification of protein.

Optimizing the usable bandwidth and loss through core design in realistic hollow-core photonic bandgap fibers

The operational bandwidth of hollow-core photonic bandgap fibers (PBGFs) is drastically affected by interactions between the fundamental core mode and surface modes guided at the core-cladding interface. By systematically studying realistic hollow-core PBGFs we identify a new design regime robust in eliminating the presence of surface modes. We present new fiber designs with a fundamental core mode free of anticrossings with surface modes at all wavelengths within the bandgap, allowing for a low-loss operational bandwidth of ~17% of the central gap wavelength.

Overcoming the impeding effect of core-cladding interface on the progression of the second-order nonlinearity in thermally poled optical fibers

The spatial distribution of the second-order optical nonlinearity in thermally poled optical fibers was characterized with second-harmonic microscopy. The second-order optical nonlinearity (SON) was found to be distributed in a layer, the progression of which was impeded at the core-cladding interface, which acted as an extra potential barrier to the migrating ions. At higher poling voltages and temperatures, the SON layer could overcome this barrier and extend further into the fiber core. The polarization dependence of the optical nonlinearity within the fiber core was also checked and found to be almost negligible.

Formation of pores in the optical fiber exposed to intense pulsed UV radiation

A new method of forming refractive index gratings (type-IIA Bragg gratings) in the optical fiber by exposing it to intense UF laser radiation is suggested. Central in this method is the generation and propagation of micropores in the parts of the fiber where mechanical stresses are localized. It is shown that such parts are the center of the core and the core-cladding interface. The temperature of the radiation-heated germaniumdoped core is found experimentally and theoretically. Thermal stresses arising in the heated fiber are calculated. A theory of cracking and pore formation in the fiber exposed to intense laser pulses is worked out. The pore size distribution, pore formation and growth rate, and pore density versus time of laser action are estimated.

New method for calculating the spectra and radiation losses of leaky waves in multilayer optical waveguides

The efficiency of a new method for calculating the spectrum and attenuation coefficient of leaky electromagnetic modes is demonstrated with multilayer planar optical waveguides the guiding properties of which are determined by antiresonant reflection from the multilayer cladding (antiresonant reflecting optical waveguides) rather than by total internal reflection from the core-cladding interface as in standard optical waveguides. The new method applies to calculation of both electromagnetic modes in dielectric waveguides and electron quantum states in multibarrier semiconductor heterostructures. The characteristics of multilayer waveguides calculated by the new method are compared with published data obtained from a complex dispersion relation by the transfer matrix method. As an example, the wavelength dependence of the radiation losses for the first TE mode of a planar optical waveguide containing 52 pairs of layers is calculated.

Trapping second-order nonlinearity in erbium-doped germanosilicate ring in thermally poled optical fibre

An optical fibre with an erbium (Er)-doped germanosilicate ring structure around the anode hole has been fabricated for stabilising the thermal poling induced second-order nonlinearity (SON). The SON spatial distribution was visualised with second-harmonic microscopy. It was found that the SON layer was trapped in the Er-doped ring under normal poling conditions. Only at higher poling voltages and for much longer poling time could some SON be formed at the core-cladding interface, while most of the SON layer was still firmly confined within the Er-doped ring.

Hindering effect of the core-cladding interface on the progression of the second-order nonlinearity layer in thermally poled optical fibers

The spatial distribution of the second-order nonlinearity layer induced in thermally poled Ge-doped optical fibers has been investigated with second-harmonic microscopy. It was found that the core-cladding interface acts as an insurmountable barrier to the nonlinear layer at lower poling voltages. At higher poling voltages (>2.5kV), the nonlinear layer can partially overcome this barrier and reach the fiber core, in which it is isolated from the rest of the nonlinear layer in the cladding.

Modeling of the loss and mode coupling due to an irregular core–cladding interface in step-index plastic optical fibers

The core-cladding boundary in step-index plastic optical fibers is imperfect. Surface irregularities locked in during the manufacturing process couple the guided modes by reflecting them in directions that deviate unpredictably from the expected directions. This causes an additional loss as the multiple reflections from surface elements with directions randomized around the nominal for the cylinder transfer the power to the radiation modes that are carried away from the core into the cladding. We model such loss and mode coupling by ray tracing. The irregular core-cladding interface is represented by nominally cylindrical surface elements with orientations randomly perturbed around two geometric axes. The results show mode coupling and relative loss per unit fiber length caused by the core-cladding interface irregularities. The loss is high close to the input fiber end where mode coupling is intense. It drops farther along the fiber as mode coupling slows down and stabilizes where the equilibrium mode distribution is reached.

Apparatus and methods for optical time-of-flight discrimination in combinatorial library analysis

Our recently developed method for combinatorial synthesis leads efficiently to linear arrays, where the location of a compound in the array encodes its complete synthetic history. Such arrays prepared using an optical fiber as a linear support can be probed with a fiber-guided pulse, allowing evanescent interaction with fluorescent probe molecules at the core-cladding interface. Optical time-of-flight distinction among output signals of fluorescent regions distributed along the fiber is carried out, allowing for the measurement of the location of the emitting fluorescent probes. A unique two-fiber, double-evanescent process overcomes limitations in spatial discrimination, due to fluorescence decay times in comparison to the speed of light. Investigation of an array of 102 fluorescent regions is described, with discussion of its features and limitations.