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.     

Microlenses in the end of capillary tubes and their application to photonic crystal fibers

We show that it is possible to make biconvex microlenses at the end of capillary tubes. These lenses can be fixed to the flat end of photonic crystal fibers and can collimate their light.     

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.     

Technique for characterizing single-mode operability in optical fibers by utilizing variable-aperture technique

We propose a technique, new to our knowledge, for characterizing single-mode operability by monitoring far-field patterns via a variable aperture. We clarify the applicability of our proposed technique to cutoff wavelength measurement, which agrees well with cutoff wavelength obtained by a conventional multimode reference technique. We also show the applicability of our proposed technique to holey fibers including photonic crystal fibers (PCFs) with uniform air-hole structure and effectively W-shaped air-hole structure. We can clearly characterize the single-mode operability of the PCF. The bend dependence of the single-mode bandwidth is also revealed by utilizing our proposed technique.     

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.     

Observations of and applications for self-imaging in optical fibers

Experimental results of self-imaging in optical fibers are reported along with an analytical model that explains the observations. Some implications for sensor design are discussed.     

Absorptance behavior of optical coatings for high-average-power laser applications

A modified photothermal deformation technique is used to measure the absorptance behaviors of optical multilayered dielectric coatings for a high-power laser system. The surface thermal-lensing modification uses an enlarged probe beam to facilitate alignment of the laser beam and data acquisition. The coatings, both reflective and transmissive types, are made by a physical vapor-deposition process. Coating absorptances are observed to depend on the laser's exposure time and power density. Time-dependent absorptance defect models are proposed. Also, micrometer-sized sites of high absorptance and an area with physical damage can be found during the spatial scans. It is proposed that absorptance values reported for coatings in high-repetition-rate or cw-laser systems include time- and power-dependent behaviors in addition to other relevant irradiation parameters.     

Recording of long-period gratings in special single-mode fibers by a CO2 laser

The paper deals with the fabrication of long-period gratings (LPG) in special optical fibers with an inverted parabolic-index profile of the cladding by periodically exposing the fibers to radiation of a high-power CO₂ laser. Details on the fiber fabrication and preparation of LPGs with a period of about 1 mm in the fibers are presented. The fabricated LPGs were evaluated on the basis of their transmission spectra determined by measuring the output spectral density of the fiber with the LPGs. It is shown that the position of the band and its strength depend on the laser beam velocity, beam power and the interaction time of radiation with the fiber.     

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.     

Coupling of a laser diode to single-mode fiber with an upside-down tapered lens end

We present in detail a simple analysis of the coupling efficiency and possible transverse and angular misalignment losses of a laser diode to single-mode, step-index fiber excitation with an upside-down tapered lens (UDTL) end, drawn by molding the end of a step-index fiber. The analysis employs our recently formulated ABCD matrix for an UDTL and has the advantage of simplicity compared with complicated methods involving cumbersome numerical integrations. Both the analysis and the results should be useful in designing coupling optics that use such lenses in the context of probable misalignments.     

Effect of CO2 laser irradiation on the refractive-index change in optical fibers

The effect of CO2 laser irradiation on the refractive-index change in optical fibers is investigated by measuring the interference fringe shift formed by a long-period fiber grating pair. The refractive-index decrease on CO2 laser irradiation was due to relaxation of the residual stress, which was formed in optical fibers during the drawing process, and the refractive-index decrease was found to increase linearly with the drawing force. The effect of the CO2 laser output power on residual-stress relaxation, and fiber elongation was also studied.     

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.     

Characteristics of polymeric optical passive single-mode waveguiding devices fabricated by an argon-ion laser

The fabrication of polymeric single-mode Gaussian profile optical waveguides is described. We used poly(methyl methacrylate) and a mixture of two intermiscible monomers for the cladding and the core, respectively, of the waveguides. The cores of the waveguides were fabricated by with an argon-ion laser beam. The waveguides had single-mode Gaussian refractive-index profiles. By using such waveguides, we fabricated directional couplers for power coupling to the adjacent waveguides and also parallel waveguide arrays for preventing power coupling to adjacent waveguides for use in interconnect chips. We analyzed the characteristics of these couplers by using the coupled-mode theory and compared the results with those obtained with the beam propagation method. Experimental results showed good correlation with the theoretical values. We designed optical bus arrays for interconnect chips, considering the variation of normalized frequency V, the power penalty, and the dimensions of the waveguides and the separation between them. The number of waveguides in the bus array increased with increasing V. For a known value of V, a waveguide's density increases with a decrease of its dimensions. The theoretical maximum number of waveguides is ~1490/cm and ~846/cm for 2 mum x 2 mum and 4 mum x 4 mum single-mode waveguides, respectively, to satisfy a 1-dB power penalty criterion at bit-error rate of 10(-9). We fabricated interconnect bus arrays with fifteen 4 mum x 4 mum waveguides for a 3-cm coupling length, and the experimental results were verified to be in good agreement with the theoretical values.