Manuel Ryser

Functional imaging of mucociliary phenomena

We present a technique for the investigation of mucociliary phenomena on trachea explants under conditions resembling those in the respiratory tract. Using an enhanced reflection contrast, we detect simultaneously the wave-like modulation of the mucus surface by the underlying ciliary activity and the transport of particles embedded in the mucus layer. Digital recordings taken at a speed of 500 frames per second are analyzed by a set of refined data processing algorithms. The simultaneously extracted data include not only ciliary beat frequency and its surface distribution, but also space–time structure of the mucociliary wave field, wave velocity and mucus transport velocity. Furthermore, we propose the analysis of the space and time evolution of the phase of the mucociliary oscillations to be the most direct way to visualize the coordination of the cilia. In particular, this analysis indicates that the synchronization is restricted to patches with varying directions of wave propagation, but the transport direction is strongly correlated with the mean direction of waves. The capabilities of the technique and of the data-processing algorithms are documented by characteristic data obtained from mammalian and avine tracheae.

Rare earth doped optical fiber fabrication by standard and sol-gel derived granulated oxides

We present our progress in the production of ytterbium (Yb) doped optical fibers fabricated by two variants of the granulated aluminophosphosilicate method. We show advantages and disadvantages of mixing rare earth and aluminophosphosilicate granulated oxides directly (variant 1) or by using the sol-gel method to produce doped granulate material (variant 2). For both methods we studied the effects of varying the dopant concentrations and of introducing iterative melting and milling procedures. In particular, the sol-gel based method eases the inclusion of P2O5 and thus, in combination with Al2O3, higher dopant concentration of Yb and Er are possible. Sintering the sol-gel material at high temperature eliminated bubbles in the core. We fabricated optical fibers that, piecewise, between individual strong scatterers, exhibited attenuation losses as low as 0.35dB/m. For our comparative study we determined volume percentage and distribution of chemical elements in the fabricated fiber glasses by the analytical technique of Energy-Dispersive X-ray, Electro Probe Microanalysis and the degree of crystallization by X-Ray Diffraction analysis. Furthermore we measured fluctuations of the refractive index profile and scattering losses of the fiber core.

Yellow laser light generation by frequency doubling of the output from a master oscillator fiber power amplifier system

We present a power-scalable approach for yellow laser-light generation based on standard Ytterbium (Yb) doped fibers. To force the cavity to lase at 1154 nm, far above the gain-maximum, measures must be taken to fulfill lasing condition and to suppress competing amplified spontaneous emission (ASE) in the high-gain region. To prove the principle we built a fiber-laser cavity and a fiber-amplifier both at 1154 nm. In between cavity and amplifier we suppressed the ASE by 70 dB using a fiber Bragg grating (FBG) based filter. Finally we demonstrated efficient single pass frequency doubling to 577 nm with a periodically poled lithium niobate crystal (PPLN). With our linearly polarized 1154 nm master oscillator power fiber amplifier (MOFA) system we achieved slope efficiencies of more than 15 % inside the cavity and 24 % with the fiber-amplifier. The frequency doubling followed the predicted optimal efficiency achievable with a PPLN crystal. So far we generated 1.5 W at 1154nm and 90 mW at 577 nm. Our MOFA approach for generation of 1154 nm laser radiation is power-scalable by using multi-stage amplifiers and large mode-area fibers and is therefore very promising for building a high power yellow laser-light source of several tens of Watt.

High-precision confocal reflection measurement for two dimensional refractive index mapping of optical fibers

We introduce a new fiber-optical approach for reflection based refractive index mapping. Our approach leads to improved stability and reliability over existing free-space confocal instruments and significantly cuts alignment efforts and reduces the number of components needed. Other than properly cleaved fiber end-faces, this setup requires no additional sample preparation. The instrument is calibrated by means of a set of samples with known refractive indices. The index steps of commercially available fibers are measured accurately down to < 10-3. The precision limit of the instrument is currently of the order of 10-4.

Gain-switched laser diode seeded Yb-doped fiber amplifier delivering 11-ps pulses at repetition rates up to 40-MHz

Here, we demonstrate all-fiber direct amplification of 11 picosecond pulses from a gain-switched laser diode at 1063 nm. The diode was driven at a repetition rate of 40 MHz and delivered 13 μW of fiber-coupled average output power. For the low output pulse energy of 0.33 pJ we have designed a multi-stage core pumped preamplifier based on single clad Yb-doped fibers in order to keep the contribution of undesired amplified spontaneous emission as low as possible and to minimize temporal and spectral broadening. After the preamplifier we reduced the 40 MHz repetition rate to 1 MHz using a fiber coupled pulse-picker. The final amplification was done with a cladding pumped Yb-doped large mode area fiber and a subsequent Yb-doped rod-type fiber. With our setup we achieved amplification of 72 dBs to an output pulse energy of 5.7 μJ, pulse duration of 11 ps and peak power of >0.6 MW.

Two-dimensional refractive index profiling of optical fibers by modified refractive near-field technique

The refractive index distribution in the core-cladding region of an optical fiber plays an important role in determining the transmission and dispersion properties of the waveguide. The refracted near-field technique (RNF) is among the most widespread techniques used for measuring the refractive index profile of optical fibers and is based on illuminating the end-facet of a fiber with a focused beam whose vertex angle greatly exceeds the acceptance angle of the fiber, which is immersed in an index matching liquid. What one observes are then the refracted unguided rays rather than the guided rays. Nevertheless, the standard refracted near-field technique cannot be applied to a wide range of optical fibers e.g. if their shapes are not axially symmetric. In this work we demonstrate a modified method which allows 2-D imaging of the refractive index profile and thereby overcoming the axial symmetric limitation of the standard RNF. The new system is operating at 630 nm and based on the same principle of the RNF, but the optical path is reversed so that the light at the fiber end-facet is collected by an objective lens and detected by a CCD camera. The method does not require scanning over the fiber end-facet. Thus the system is faster and less sensitive to vibrations and external conditions compared to the standard RNF, furthermore it allows averaging to improve the signal to noise ratio. The spatial resolution of the system is determined by the numerical aperture of the objective and by the resolution of the CCD camera. To calibrate the setup, a reference multi-step index fiber provided by National Physical Laboratory was used.

Progress in the fabrication of optical fibers by the sol-gel-based granulated silica method

Novel special optical fibers nowadays can take advantage of several new preform production techniques. During the last years we have devoted our attention to the granulated silica method. It is one of the variants of the powder-in-tube technique and potentially offers a high degree of freedom regarding the usable dopants, the maximum possible dopant concentration, the homogeneity of the dopants, the geometry and minimal refractive index contrast. We developed and refined an approach for the production of doped granulated silica material based on the sol-gel process. Here, we present material analysis results of an ytterbium (Yb) doped, aluminum (Al) and phosphorous (P) co-doped glass on the basis of our sol-gel glass based granulated silica method as well as first measurements of two LMA fibers obtained from this material. For the material analysis we used advanced analysis techniques, such as HAADF-STEM and STEM-EDX spectroscopy to determine the composition of the material and the distribution of the dopants and the codopants. The chemical mapping of the STEM-EDX shows an extremely homogeneous distribution of the dopants and co-dopants in nano-scale. Based on self-made LMA fibers, we measured the refractive index contrast of the sol-gelbased granulated silica derived core compared to the pure silica cladding. In addition we quantified optical characteristics such as the emission and absorption spectrum. The measured upper state lifetime of the optical active dopant ytterbium was 0.99ms, which in turn confirms the homogeneous distribution of the Yb atoms. The propagation losses were determined to be 0.2dB/m at 633nm and 0.02414dB/m at1550nm.

Self-injection linear polarization locking of a fiber laser

We present a novel approach for linear polarized operation of a fiber laser not requiring any intra-cavity components, but relying on an external self-seeding effect. A small fraction of back-reflected polarized light created in an external cavity consisting of fused coupler, polarizer and fiber Bragg grating is sufficient to seed and lock the laser to linear polarized operation. This approach enables the construction of polarized monolithic fiber lasers, minimizes intra-cavity losses and drastically reduces parasitic amplified spontaneous emission. We experimentally demonstrate strong polarization locking in an Yb-doped fiber laser with extinction ratio of 500:1 at 1154 nm.

Self-optimizing additive pulse mode-locked fiber laser: wavelength tuning and selective operation in continuous-wave or mode-locked regime

Additive pulsed-mode mode-locked fiber lasers are known for their wide range of operating states, which can be achieved by tuning the artificial saturable absorber with built-in polarization controllers. We have equipped our laser with three motorized polarization controllers and online monitoring. We recorded three dimensional high-resolution maps of the oscillation states by stepping trough all possible polarization controller settings. In addition, we demonstrate the single and multi-objective optimization by a genetic algorithm of the fiber laser towards a desired operating state. To the best of our knowledge this is the first demonstration of multi-objective optimization of a fiber laser. Doing so, we can selectively operate the cavity in pulsed and continuous-wave mode and tune the wavelength of the laser emission by more than 55 nm. The high wavelength tunability is possible because the polarization controllers and the inline-polarizer act as a tunable birefringent filter.

Spectroscopy of a heated Yb-doped optical fiber with high aluminum content

The generation and amplification at wavelengths longer than 1100 nm is not straightforward when using Yb-doped optical fibers, since light emission of ytterbium occurs preferentially in the region of 1020 nm - 1100 nm with a maximum at 1030 nm. One well known approach is to heat the Yb-doped fiber up to temperatures above 100 °C. This increases the re-absorption in the lower emission band and also enhances at the same time the emission at longer wavelengths. Consequently, heating allows to extend the spectral gain-region of Yb-doped fibers by at least 60 nm up to 1160 nm. However, the drawback of this method is that it results in a shorter durability of the fiber, since heating damages the polymer-coating. Moreover, such a laser has a reduced overall efficiency, due to heating, isolation and heat removal issues. It has been reported, that at the presence of an aluminosilca host (silica doped with Al) efficient laser activity at around 1150 nm can be achieved by heating the Yb-doped fiber to only 60 °C. In this work we investigate the spectroscopy of a heated Yb-doped fiber with a high aluminum concentration. The fiber is drawn in our in-house fiber drawing tower. The preforms are produced by the sol-gel-based granulated silica method which allows us to vary the aluminum as well as the ytterbium concentrations within a large range. The fiber is investigated with respect to their spectroscopic data as well as their lasing performance.