Versatile control of geometric birefringence in elliptical hollow optical fiber.

A novel optical fiber fabrication technique was developed by converting the symmetry of the silica substrate into the germanosilicate ring core to efficiently introduce geometric birefringence in an elliptical hollow optical fiber. Due to high ellipticity in the hollow ring core, the fiber provides an extremely high group birefringence of 2.35 x 10(-3) at 1550 nm. Single-mode single-polarization guidance was also experimentally confirmed, with a bandwidth of approximately 35 nm. The generic adiabatic mode conversion capability in the taper also provided a stable fusion splice to conventional single-mode fiber with low loss and high tensile strength.

New defect design in index guiding holey fiber for uniform birefringence and negative flat dispersion over a wide spectral range.

A novel silica index guiding holey fiber (IGHF) design is proposed utilizing a new defect structure that is composed of an elliptic high index ring structure and an elliptic air-hole at the center with triangular lattice structure. The proposed IGHF showed unique modal properties such as uniform and high birefringence over a wide spectral range and single polarization single mode (SPSM) guidance along with a flat negative chromatic dispersion. Optical waveguide properties were numerically analyzed using the plane wave expansion method in terms of mode intensity distribution, modal birefringence, chromatic dispersion for the new defect structural parameters.

Discrimination of temperature and strain with a single FBG based on the birefringence effect.

We will demonstrate a new technique to discriminate the temperature and strain effects using a single fiber Bragg grating (FBG). The birefringence is typically induced during FBG inscription, and it is manifested as polarization-dependent loss (PDL), and it is defined as the maximum change in the transmitted power for polarizations. Two independent measurements of the resonance wavelength shift and the changes of PDL can discriminate those effects.

Reduction of Birefringence and Polarization- Dependent Loss of Long-Period Fiber Gratings Fabricated with a KrF Excimer Laser.

We discuss the effect of UV laser exposure time, repetition rate, and heating by a heating coil on the resonance peak depth and polarization-dependent loss (PDL) of long-period fiber gratings (LPFGs). The LPFGs were fabricated with a KrF excimer laser and an amplitude mask. We observed an initial increase of the resonance peak depth and PDL as the UV exposure time was increased, which eventually decreased in response to over-coupling. With the total UV fluence kept constant, the peak depth continued to increase as the repetition rate was increased beyond 10 Hz, whereas the maximum PDL decreased when the repetition rate was higher than 17 Hz. This is believed to be a thermal effect caused by the rapid delivery of UV laser pulses. We observed a similar reduction of the maximum PDL from 1.35 to 0.25 dB when the fiber was heated by an adjacent heating coil.

Resonant optical nonlinearity measurement of Yb(3+) / Al(3+) codoped optical fibers by use of a long-period fiber grating pair.

A new method of measuring optical nonlinearity for resonant nonlinear optical fibers is proposed. A long-period fiber grating (LPG) pair was used to measure the nonlinear refractive index n(2) of a Yb(3+)/Al (3+) codoped optical fiber, which was spliced between the two LPGs, as the fiber was pumped with a laser diode. The nonlinear refractive index of the Yb(3+)/Al (3+) codoped fiber near 1580 nm depended on the pump power. As the pump power increased, the nonlinear refractive index decreased. At launched pump powers of 2-8 mW, the nonlinear refractive index of the Yb(3+)/Al (3+) codoped fiber near 1580 nm was ~7.5x10(-15)m (2)/W .

Observation and analysis of residual stress development resulting from OH impurity in optical fibers.

We report experimental results on the development of residual stress due to OH impurity in optical fibers. The effect of OH impurity on residual stress is demonstrated by direct residual stress measurement. The residual stress at the outer-cladding/jacketing-tube boundary of the fiber drawn at 3.48 N was found to be -61 MPa . The residual compressive stress is attributed to the viscosity decrease induced by a significant OH impurity at the boundary, as measured by a Fourier transform infrared microscope.

Diffraction-limited dark laser spot produced by a hollow optical fiber.

By using the diffracted field of the LP(11) mode of a hollow-core optical fiber, we have produced a micrometer-sized, focused dark laser spot in the near field of the fiber. The minimum half-width of the dark spot is less than 1 mum . In particular, by masking the hollow core and metal coating the cladding with a microsphere, we blocked the light propagating in the cladding and obtained a clean dark spot, which may be useful in atom-optical experiments such as with atomic lenses, atom traps, and atom switches.

Residual stress relaxation in the core of optical fiber by CO(2) laser irradiation.

We observed residual stress relaxation by CO(2) laser irradiation in the cores of optical fibers by direct stress measurement. It was demonstrated that the mechanical stress was fully relaxed by CO(2) laser irradiation and that the remaining stress in the core was thermal stress that was due to a mismatch of the thermal expansion coefficients of the fiber core and cladding. The net core stresses after relaxation were 17, 68, and 203 MPa in Ge-B-codoped fibers drawn at 0.53, 1.38, and 3.48 N, respectively. Changes in the refractive indices of the cores as a result of residual stress relaxation were also estimated.

Photoinduced refractive-index change in Sm(+2)/Sm(+3) codoped aluminosilicate fiber by irradiation of an Ar-ion laser.

A Sm(+2)/Sm(+3) codoped aluminosilicate glass optical fiber was fabricated by use of modified chemical-vapor deposition in conjunction with an aerosol-delivery technique. A permanent index change of 7.6 x 10(-5) was induced in the fiber by irradiation of 1 W of multiline output from an Ar-ion laser. Bleaching of a broad absorption band of Sm(+2) in the visible range was also observed, and it is believed that photoionization of Sm(+2)? Sm(+3)+e plays an important role in the induced photorefractivity.

Prechirper to relax the timing restrictions for soliton-dragging logic gates.

The timing restrictions for soliton-dragging logic gates can be relaxed by asymmetrizing the interaction between the orthogonally polarized control and signal pulses. In particular, by prechirping the signal pulse in a normal group-velocity dispersion fiber, we find experimentally that the timing restriction can be loosened from less than 2 pulse widths to over 3.3 pulse widths. For this passive prechirper arrangement the timing restrictions are loosened at the expense of increased minimum switching energy. Soliton-dragging logic gates with a signal prechirper can provide timing restoration as well as logic-level restoration, so that two inverter gates can be cascaded to implement an ultrafast, all-optical pulse regenerator.

Correlation of single-mode fiber radiation response and fabrication parameters.

Statistically significant correlations have been established between certain fabrication parameters of matched clad, single-mode optical fiber waveguides and their response to an ionizing radiation dose of 2000 rad. The reCOVE:ry data measured at -35 degrees C following exposure have been fit to nth-order kinetic behavior where the adjustable parameters are the initial and permanent incremental losses (A(o) and A(f), respectively), the half-life of attenuation tau, and the order of kinetics n. The set of fibers chosen for analysis had Ge-doped silica cores. In fibers with Ge-F-doped silica clads, A(o) correlates with the concentration of Ge-doped into the fiber core; A(f) correlates with the ratio of oxygen to reagents used during core deposition; and tau and n correlate with a two-way interaction of core oxygen and fiber draw speed. In P-F-doped clad fibers, the P concentration has been found to correlate with the order of the kinetics of recovery.

Soliton intensity-dependent polarization rotation.

We show experimentally and numerically that intensity-dependent polarization rotation does not lead to pulseshape distortions for fundamental solitons because the soliton rotates as a unit. We demonstrate an optical limiter based on nonlinear polarization rotation in a standard, single-mode fiber followed by a polarizer. The output pulse shape from the limiter resembles that of the input, although the pulse width may be different.

Dispersionless single-mode fibers with trapezoidal-index profiles in the wavelength region near 1.5 microm.

The transmission performance of trapezoidal-index profile single-mode fibers that can operate with zero dispersion in the 1.5-microm region has been investigated. The exact numerical calculations of propagation characteristics for these lightguides established a variety of relationships in terms of the parameters of a trapezoidal-index profile. One of the profile parameters that defines the shape of a trapezoid is the aspect ratio S, which ranges between 0 and 1. It was found that as the aspect ratio decreases from 1.0 to 0.3 the optimum core radius of the fiber gradually increases. But when S is <0.3, it approaches a constant equal to the value of the triangular-index profile (S = 0).

Parametric effects on the bandwidth of a single-mode fiber with experimental verification.

Based on a set of exact vector form solutions to Maxwell's equations for optical waveguides, we studied the parametric effects on propagation characteristics of the HE(11) mode, varying the lightguide parameters (lambda,Delta,a,alpha). In this work a three-term Sellmeier equation was employed to describe accurately the material dispersion of a single-mode fiber doped with GeO(2). The calculated results are compared with the measured values and found to be in good agreement.

Effects of depressed cladding on the transmission characteristics of single-mode fibers with graded-index profiles.

Parametric studies were done on the propagation characteristics of single-mode fiber using measured refractive-index profile data. Two types of profile were considered--a graded-index core with a depressed cladding (Delta' < 0), and a triangular index core with no depressed cladding. The computations employ accurate material dispersion data for the germania-doped silica core and the fluorine-doped silica cladding. The investigation also includes the use of an alpha-index profile as an idealized profile using the same computational method. We find that there is good agreement with the experiments.

High speed coating of optical fibers with UV curable materials at a rate of greater than 5 m/sec.

A newly designed 7.3-m high tower is reported that is equipped with a 10-kW curing system capable of coating a fiber with 50-microm thickness of UV curable resin at a speed in excess of 5 m/sec. Performance of fibers coated at two different speeds (V(f) = 1.5 and 5 m/sec) were evaluated. It was found that there were no noticeable differences in the measured values of mechanical and optical properties such as strength, coating thickness, transmission loss, bandwidth.

Calculation of photopolymerization energy required for optical fiber coating.

The UV flux density required for photopolymerization is calculated, primarily based on an estimation of the overall system efficiency and the activation energy of the photoinitiator added to the prepolymer solution. We found that there is a linear relationship between the flux density and coating speed. The experimental results support the relation. Furthermore, it is noted that full curing has been achieved at speeds >5 m/sec with the 10-kW output power. The 10-kW system delivers 1.5 kW of UV power in the 300-400-nm spectral range.

Formation of a Spherical Lens at Optical Fiber Ends with a CO(2) Laser.

A study was made of the use of a CO(2) laser for producing a hemispherical lens at the end of a fiber. When the fiber end is heated to the softening points of the material, it tends to become spherical in shape due to surface tension. A one-dimensional heat conduction model for the fiber can reasonably predict the required laser power for a given pulse length to produce lenses on fibers made of various kinds of materials. A He-Ne laser beam was coupled into a fiber to investigate the properties of the spherical lens. We also observed that such lens can couple light from a source into the fiber, can be used for imaging, and can focus enough laser intensity for machining various materials.