Rene-Paul Salathe

Comparison of mechanically drawn and protection layer chemically etched optical fiber tips

Abstract Scanning near-field optical microscope tips are produced by mechanically drawing and by chemical etching of standard single mode fibers. The geometrical shapes and taper angles are compared. By mechanical drawing with a commercial micropipette puller, smooth tapers with point diameters below 50 nm are obtained. The final taper cone angle does not exceed 14° for small point sizes. The total length of the taper with decreasing core diameter ranges from 500 to 2000 μm. Strong variations of the cone angle can be observed depending on the chosen pulling conditions. Therefore the variations of the fiber diameter at different distances from the tip are measured for characterizing the shapes as a function of the pulling force. Chemical etching in 40% aqueous hydrofluoric acid covered with an organic solvent protection layer results in tips of conical shape in an autostopping process. The cone angle can be varied from 8° to 41° by the adequate choice of the organic solvent under static etching conditions. Tips with smooth surfaces and point diameters below 40 nm are realized.

Tension increase correlated to refractive-index change in fibers containing UV-written Bragg gratings

A strong axial tension increase induced by UV laser radiation is observed in the cores of single-mode optical fibers containing Bragg gratings, independently of the initial core stress. The induced index modulation of the gratings is linearly correlated to stress changes with a slope of (0.8 +/- 0.2) x 10(-4) mm(2)/kg. The phenomenon can be explained by a structural change of the glass in the fiber core into a more compact configuration.

Femtosecond irradiation induced refractive-index changes and channel waveguiding in bulk Ti/sup 3+/:sapphire

Femtosecond writing of refractive-index changes in sapphire is demonstrated by doping the crystal with an appropriate ion (here: Ti/sup 3+/) that reduces the threshold of the process. Passive and active buried channel waveguiding is demonstrated.

Embedded optical fiber Bragg grating sensor in a nonuniform strain field: measurements and simulations

This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e., the strong nonuniformity of the strain field), the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth is not valid. Compact tension specimens including a controlled notch shape are fabricated, and optical fiber Bragg gratings with different gage lengths are embedded near the notch tip. The form of the spectra in transmission varies between gages that are at different distances from the notch tip under given loading conditions. This variation is shown to be due to the difference in the distribution of strain along the gage length. By using the strain field measured using electronic speckle pattern interferometry on the specimen surface and a discretized model of the grating, the spectra in transmission are then calculated analytically. For a known strain distribution, it is then shown that one can determine the magnitude of the applied force on the specimen. Thus, by considering the nonuniformity of the strain field, the optical fiber Bragg gage functions well as an embedded strain gage near the stress concentration.

Polarization microscopy by use of digital holography: application to optical- fiber birefringence measurements

We present a digital holographic microscope that permits one to image polarization state. This technique results from the coupling of digital holographic microscopy and polarization digital holography. The interference between two orthogonally polarized reference waves and the wave transmitted by a microscopic sample, magnified by a microscope objective, is recorded on a CCD camera. The off-axis geometry permits one to reconstruct separately from this single hologram two wavefronts that are used to image the object-wave Jones vector. We applied this technique to image the birefringence of a bent fiber. To evaluate the precision of the phase-difference measurement, the birefringence induced by internal stress in an optical fiber is measured and compared to the birefringence profile captured by a standard method, which had been developed to obtain high-resolution birefringence profiles of optical fibers.

Miniaturized high-NA focusing-mirror multiple optical tweezers

An array of high numerical aperture parabolic micromirrors (NA = 0.96) is used to generate multiple optical tweezers and to trap micron-sized dielectric particles in three dimensions within a fluidic device. The array of micromirrors allows generating arbitrarily large numbers of 3D traps, since the whole trapping area is not restricted by the field-of-view of the high-NA microscope objectives used in traditional tweezers arrangements. Trapping efficiencies of Q(max) r approximately = 0.22, comparable to those of conventional tweezers, have been measured. Moreover, individual fluorescence light from all the trapped particles can be collected simultaneously with the high-NA of the micromirrors. This is demonstrated experimentally by capturing more than 100 fluorescent micro-beads in a fluidic environment. Micromirrors may easily be integrated in microfluidic devices, offering a simple and very efficient solution for miniaturized optical traps in lab-on-a-chip devices.

Tunable loss filter based on metal-coated long-period fiber grating

An all-fiber electrically tunable loss filter that is based on photoinduced long-period grating coated by Ti-Pt metal coating was developed and investigated. Maximum wavelength tuning of 11 nm with an applied power of 0.67 W was achieved for the HE17 cladding mode resonance peak.

Lu, Gd codoped KY(WO4)(2): Yb epitaxial layers: towards integrated optics based on KY(WO4)(2)

Codoping KY(WO(4))(2):Yb layers with optically inert Lu and Gd ions allows a large increase of the refractive index contrast with respect to KY(WO(4))(2) substrates. This paves the way for the realization of integrated optical circuits based on this very promising material. First riblike waveguide structures have been fabricated and propagation losses below 1 dB/cm have been evaluated. A Y-junction with a splitting ratio close to 1:1 and additional losses of 1.4 dB has also been demonstrated.

Local coupling-coefficient characterization in fiber Bragg gratings.

We propose a new method for characterizing the local parameters of fiber Bragg gratings. This method combines measurement of the complex impulse response by optical low-coherence reflectometry and reconstruction of the complex coupling coefficient by layer peeling. Application of the method to a nonhomogeneous grating shows that the local coupling coefficient can be precisely determined with an axial resolution below 20 microm and a maximum error of less than 5% for amplitude and phase, respectively.

Tomographic stress profiling of arc-induced long-period fiber gratings

Long-period fiber gratings (LPGs) have been inscribed in nitrogen-doped fibers by electrical arc discharge. The influence of drawing tension as well as external load applied during arc discharge on coupling strength has been investigated. The influence of drawing tension on the gratings coupling strength is found to be negligible, whereas the coupling strength increases considerably with external load. Tomographic stress profiles of the fiber have been recorded before and after electric arc discharge. The axial stress modulation in the core region of the grating was found to be smaller than 10 MPa and is thus too small to be the dominating mechanism for grating formation.