Apparatus for the alignment of single-mode optical fibers

An apparatus for the alignment of single-mode optical fibers is described. Design configurations are suggested for the aligning unit and the device used to process the error signal controlling the piezoelectric elements. The apparatus should be of interest to developers of fiber-optic systems as a special-purpose industrial accessory.Translated from Izmeritel'naya Tekhnika, No. 8, pp. 28–29, August, 1995.     

Coupling of gaussian Schell-model beams into single-mode optical fibers

We develop analytic equations that describe the mean and normalized variance of the coupling efficiency of gaussian Schell-model beams into single-mode optical fibers. Numerical methods and computer simulations are used to evaluate the accuracy of the various approximations used in this analysis, and, with some insight, empirical compensation is made for the identified shortcomings. The simulations make use of both speckled and nonspeckled beams by employing two different Monte Carlo methods to generate randomly drawn optical fields. While the analytic approximations break down in certain cases, the use of empirical compensation demonstrated accuracies of better than 5% for the mean coupling efficiency in all cases, and generally better than 40% for the coupling efficiency variance. By optimizing the compensation for particular beam characteristics, even higher accuracies can be achieved.     

Theory and measurement of Young's modulus radial profiles of bent single-mode optical fibers with the multiple-beam interference technique

Multiple-beam Fizeau fringes in transmission have been applied to the study of the nonlinear stress-strain relationship due to bending in the cladding of single-mode optical fibers. The present study yields a relation between the variation of refractive indices of bent single-mode fibers, represented by the fringe shift, and the nonlinear radial change of Young's modulus along the fiber cross section. Experimentally, the study confirms the nonlinear asymmetric stress-strain relation across the fiber cross section. This relation is due to the asymmetric distribution of the compression and tensile stresses over the fiber cross section rather than to the shift in the centroid (neutral axis).     

Highly efficient coupling of photons from nanoemitters into single-mode optical fibers

Highly efficient coupling of photons from nanoemitters into single-mode optical fibers is demonstrated using tapered fibers. A percentage (7.4 ± 1.2%) of the total emitted photons from single CdSe/ZnS nanocrystals were coupled into a 300 nm diameter tapered fiber. The dependence of the coupling efficiency on the taper diameter was investigated and the coupling efficiency was found to increase exponentially with decreasing diameter. This method is very promising for nanoparticle sensing and single-photon sources.     

Polishing-induced birefringence in single-mode fibers

Polishing a flat surface on a fiber changes the birefringence at the core or introduces birefringence in a circular fiber. In this paper, a circularly symmetric fiber is studied both experimentally and by a simple model in which the birefringence arises from the application of forces along the polished surface. The polishing-induced birefringence first rises to a maximum and then falls off as the polished surface approaches the core.     

A QMU approach for characterizing the operability limits of air-breathing hypersonic vehicles

The operability limits of a supersonic combustion engine for an air-breathing hypersonic vehicle are characterized using numerical simulations and an uncertainty quantification methodology. The time-dependent compressible flow equations with heat release are solved in a simplified configuration. Verification, calibration and validation are carried out to assess the ability of the model to reproduce the flow/thermal interactions that occur when the engine unstarts due to thermal choking. quantification of margins and uncertainty (QMU) is used to determine the safe operation region for a range of fuel flow rates and combustor geometries.     

GeO<Subscript>2</Subscript>-rich low-loss single-mode optical fibers

We have optimized the MCVD process for the fabrication of single-mode optical fibers with a heavily doped (up to 30 mol % GeO₂) core for Raman lasers and amplifiers. Removing the central index dip in the core, we were able to reduce the optical losses in such fibers to the lowest level for the MCVD technology.     

Slow light generation in single-mode tellurite fibers

Theoretical investigations of stimulated brillouin scattering-based tunable slow light in (i) Er-doped tellurite and (ii) undoped tellurite fibers are reported. Maximum allowable pump power for undistorted output pulse, Brillouin gain, time-delay, figure-of-merit, and time-delay slop efficiency of both the fibers has been obtained. We have found that (i) Brillouin gain up to ~91 dB and time delay of 140 ns can be achieved using 1100 mW pump power in 2 m Er-doped fiber and (ii) Brillouin gain up to ~86 dB and time delay of ~227 ns using 23 mW pump power in 100 m undoped tellurite fiber can be achieved. Simulated results indicate that the time delay in fibers can be tuned with the pump power to obtain tunable slow light features in these fibers. We feel that detailed theoretical investigations and simulations carried out in the study have potential impact in the design and development of slow light-based photonic devices.     

Theoretical evaluation of the Brillouin threshold and the steady-state Brillouin equations in standard single-mode optical fibers

We first use the nonlocalized, fluctuating source model for the stimulated Brillouin scattering to get the exact spectrum of the Stokes wave in optical fibers with attenuation loss. A new relation for the evaluation of the critical pump power (or Brillouin threshold) depending on the fiber length is then introduced, which should be more precise than the well-known Smith formula. Furthermore, we give for the first time, to the best of our knowledge, an approximate solution for standard steady-state Brillouin equations, which consists of two simple relations.     

Mode-field diameter of single-mode optical fiber by far-field scanning

I use the direct far-field method to measure the mode-field diameter of a single-mode fiber with an expanded uncertainty of 30 nm, with a coverage factor of 2. For a step-index fiber with a mode-field diameter of approximately 9 mum, the major sources of uncertainty are nonlinearity in the electronics, angular errors and scattered light in the apparatus, and the polarization and noncircularity of the mode of the fiber. The paper concludes by showing an inconsistency in the derivation of the far-field expression for mode-field diameter.     

Bidirectional optical coupler for plastic optical fibers

We have developed a low-loss bidirectional optical coupler for high-speed optical communication with plastic optical fibers (POFs). The coupler, which is fabricated by an injection molding method that uses poly (methyl methacrylate), has an antisymmetric tapered shape. We show that the coupler has low insertion and branching losses. The tapered shape of the receiving branch reduces beam diameter and increases detection efficiency coupling to a photodetector, whose area is smaller than that of the plastic optical fiber. The possibility of more than 15-m bidirectional transmission with a signaling bit rate up to 500 Mbits/s for simplex step-index POFs is demonstrated.     

Discrimination between strain and temperature by cascading single-mode thin-core diameter fibers

A simple fiber-optic sensor capable of discrimination between temperature and strain is proposed and experimentally demonstrated. The sensor head is formed by cascading two sections of single-mode thin-core diameter fibers (TCFs) that act as two different inter-modal interferometers (IMIs). Due to the different sensitivity responses of the two IMIs to strain and temperature, it is possible to discriminate temperature and strain by monitoring the resonant wavelength shifts. The experimental results indicate that the measured strain and temperature resolutions are 37.41 με and 0.732 °C within a strain range of 0-1333.3 με and a temperature range from 26.9 °C to 61.7 °C. The sensing sensitivities of strain and temperature are -1.03 pm/με and 30.74 pm/°C, respectively. The proposed sensor features the advantages of easy fabrication, low cost and high sensitivity, and it exhibits great potential in dual-parameter measurement.