Jens Kobelke

Toward photonic crystal fiber based distributed chemosensors

Different structures of photonic crystal fibers (PCF) have been investigated for application as intrinsic optical gas sensors. The fiber is used both as waveguiding structure and as sample containment. Advantages and drawbacks of solid core and hollow core PCF structures will be discussed. Theoretical assessment for the sensitivity of the investigated fiber types will be given. The calculated sensitivity of the solid core PCF will be reviewed using the fiber as methane sensor. A laser micro-drilling technology is used to perform first samples for quasi-distributed PCF chemical sensing.

Microstructured index-guiding fibers with large cladding holes for evanescent field chemical sensing

Design, fabrication and application of small solid-core microstructured optical fibers with large cladding holes for nevanescent field chemical sensing of gases and liquids will be presented. Such steering-wheel fiber structures give a nhigh mode-field overlap in the holey region, they show low losses over a broad spectral range and they are easier to nfabricate than hollow-core bandgap-guiding photonic crystal fibers.

Innovative fiber coating systems based on organic modified ceramics

We describe the application of inorganic organic hybrid materials (ORMOCERs) as optical fiber coatings for use in Fiber Bragg Grating sensors and high power transmission fibers. The materials are UV curable, enable a single layer thickness of about 50 μm and show high a high peak temperature stability >300 °C. Regarding the fiber protection the coatings have been investigated using tensile strength measurements before and after temperature load. Best coatings maintain the high tensile strength of 68 N (125 μm fiber) with a Weibull parameter of 182 after a temperature cycling up to 300 °C. For the first time a low refractive index ORMOCER will be presented showing a numerical aperture of 0.47 at a wavelength of 1000 nm on a pure silica fiber. This corresponds to a refractive index of 1.37. The fiber possesses a fiber loss of 18 dB/km at a wavelength of 1000 nm. The fibers have been coated using a gravity as well as pressure technology. The latter possesses extremely minimized die equipment and is therefore well applicable for small coating amounts. The so called dead volume within the coating die is about 1 ml. The overall dead volume is only influenced by the supply pipe and can be reduced down to 5 ml.

Mechanical and optical behavior of index guiding photonic crystal fibers (PCF)

The optical loss behavior of index guiding PC fibers made from high purity silica, was investigated with regard to the preform preparation steps and drawing procedure. Loss effects in the 1.4 µm region are caused mostly by incorporation of hydroxide groups during PC preform preparation. Typical sources are flame heat treatment procedures. However, hydroxide based absorption by water permeation into the holey structure was not observed, not even by storage in humid atmosphere over days. PCFs show additional NIR attenuation, possibly caused by drawing induced atomic defects in the pure silica material. By advanced PC preform preparation the minimum attentuation in the NIR range can be depressed down to 2.9 dB/km at 1.3 μm. PCFs have a reduced tensile strength in comparison with compact silica fibers. The mechanical stability increases with the cross section area of the solid outer cladding. This resembles the behavior of single capillary fibers without inner holey or cobweb structure. The tensile strength of PCFs decreases after a few days of hole contamination with condensed water.

Fiber Bragg grating inscription in optical multicore fibers

Fiber Bragg gratings as key components in telecommunication, fiber lasers, and sensing systems usually rely on the Bragg condition for single mode fibers. In special applications, such as in biophotonics and astrophysics, high light coupling efficiency is of great importance and therefore, multimode fibers are often preferred. The wavelength filtering effect of Bragg gratings in multimode fibers, however is spectrally blurred over a wide modal spectrum of the fiber. With a well-designed all solid multicore microstructured fiber a good light guiding efficiency in combination with narrow spectral filtering effect by Bragg gratings becomes possible.

Gas sensing with suspended core fibres and hollow core band gap fibres: a comparative study

The usability, advantages and limitations of suspended core fibres and hollow core band gap fibres for gas sensing in the NIR will be discussed and demonstrated. Suspended core fibres of various geometries and hollow core photonic band gap fibres with different transmission properties have been investigated with respect to their relative sensitivity and their usable spectral bandwidth, using combustion gases as test substances. It has been found that, despite of the more than an order lower sensitivity of suspended core fibers, both kinds of fibre may found use in different practical gas sensing applications.

Second harmonic generation in Ge-doped silica holey fibres and supercontinuum generation

We demonstrate that second harmonic generation obtained in Ge-doped holey fibres can act as a seed for visible supercontinuum generation. This spectral enlargement is obtained by means of a double-pumping system. By using a microchip laser source delivering sub-nanosecond pulses at 1064 nm and a highly Ge-doped fibre, we obtain a second harmonic generation efficiency of 4.8 % after an optical poling process. A white light continuum extending on more than 250 nm is obtained in visible domain.

Optical properties of microstructured optical fibers

In conventional optical fibers the light guiding and other optical properties are mainly controlled by the index profile as given by the composition of the doping materials. In this case technological and material properties limit the design flexibility of the optical properties. For example, it is not possible to shift the region of anomalous dispersion to wavelengths shorter than the zero dispersion value of silica. In contrast to these conventional fibers, for microstructured or photonic crystal fibers (PCF) the optical properties are determined by the size, distribution and geometry of the air holes running the entire length of the fiber. Variations of the geometrical parameters offer a wide flexibility in designing different optical properties. It is possible, for example, to modify numerical aperture and mode field area over wide ranges or to shift the zero dispersion wavelength down to the visible spectral region. In the case of hollow core fibers (photonic bandgap fibers) the interaction of the guided light with the glass material is reduced and allows the transmission of higher intensities with reduced nonlinear effects.

Laser beam quality improvement of REPUSIL-based rod amplifier with local short adiabatic taper (Conference Presentation)

Fiber amplifiers with a robust monolithic seed coupling and very high peak power in a near diffraction-limited beam are increasingly demanded by many industrial applications in laser materials processing. A tapered all-solid rod-type fiber amplifier scheme is proposed. The principle of this approach is the use of a local adiabatic taper to provide a monolithic signal path and selectively excite the fundamental mode in highly multimode fiber. A large mode area fiber is used to scale up the peak power and suppress the nonlinear effects. The powder-sintering technology (REPUSIL) was employed to achieve rod-type fibers with excellent refractive index homogeneity.nIn this work, we present a double-clad Yb3+-doped rod-type REPUSIL fiber with a core/clad diameter of 45µm/190µm and a core/clad numerical aperture of 0.09/0.19.This developed fiber has a relatively low Al3+-concentration to reduce diffusion during tapering and an optimized outer cladding material to reduce the taper process temperature. Finally it is demonstrated that the diffusion phenomenon is successfully eliminated and the near-diffraction limited beam quality during the amplification process is maintained. First experiments with improved Yb3+-doped rod-type amplifiers delivered 2ns pulses with peak powers of 210kW for the non-tapered rod and 140kW for the tapered rod (limited by facet damage). For the tapered fiber, the beam quality was between 1.3 and 1.7, significantly improved compared to the beam quality of the non-tapered fiber (M2 = 3.3 ~ 4.5). Future work will concentrate on adopting endcaps to protect the fiber facets from damage while scaling up the peak power.

Fabry-Perot interferometer based on array of microspheres for temperature sensing

A Fabry-Perot interferometer based on an array of soda-lime glass microspheres is proposed for temperature sensing. The microspheres are introduced in a hollow-core silica tube using a tapered fiber tip. After the insertion of each microsphere the sensor is subjected to temperature measurements. The sensor exhibits non-linear behavior and a dependence on the number of microspheres is observed. A maximum sensitivity of 11.13 pm/°C is achieved, when there is only one microsphere inside the capillary structure.