Teemu Kokki

Fiber amplifier utilizing an Yb-doped large-mode-area fiber with confined doping and tailored refractive index profile

Power scaling of Yb-doped large-mode-area fibers drives the scaling of the mode area in order to suppress nonlinearities. Two Yb-doped large-mode-area fibers were manufactured using the Direct Nanoparticle Deposition process: one with a step refractive index profile and active ion confinement, and another with a tailored refractive index and active ion confinement. The index tailoring and doping profiles were designed based on literature to enhance the beam quality of the fibers. Both fibers exhibited a mode field diameter comparable to a 40μm step index fiber with 0.07 NA. The fibers were characterized for their geometries, index profiles, and material composition profiles. Additional testing for beam quality and nonlinearities in pulsed operation was conducted using a power amplifier setup. The beam quality enhancement capability of the tested fibers was inconclusive due to incomparable launching conditions of the signal to the fibers.

Progress in direct nanoparticle deposition for the development of the next generation fiber lasers

This paper outlines the most recent work at nLIGHT Oy (formerly Liekki Oy). We give an overview of the current state of the nLIGHT active fiber fabrication technology, discuss the capability of the manufacturing process, and review our results and the results of other groups on the reliability aspects of the manufactured fibers. We also present refractive index tailored and gain tailored Yb doped fibers as examples of recent fiber development.

Spatial Distribution of Photodarkening in Large Mode Area Ytterbium Doped Fibers

We have modeled the photodarkening rate distribution in ytterbium fiber cross-section. We have found that photodarkening is not uniformly distributed in LMA fibers and depends on coiling diameter. This affects the fiber application usage.

Measuring bend losses in large-mode-area fibers

We investigate the measurement of bend losses in few-mode large-mode-area (LMA) fibers. The influence of the light source spectral characteristics, modal power content and cladding light on the measurement accuracy and precision is studied experimentally. Monte-Carlo simulations are performed to understand the distribution of the variations. This study provides practical guidelines for bend loss measurements.

Mode coupling in large-diameter multi-mode silica optical fibers

We report an experimental study on mode coupling in various large-diameter multi-mode silica optical fibers. The evolution of the far-field angular power distribution is experimentally measured, and the mode coupling characteristics are studied on a variety of fibers with diverse parameters, including core/cladding diameter (50-400μm/125-480μm), length (few to hundreds of meters), NA (0.15-0.46), etc. The influences of fiber geometry, bending are discussed. This study could provide practical guidance in designing power delivery fibers for high-power diode, solid-state and fiber lasers to preserve the input brightness and beam quality.

Confined-doped ytterbium fibers for beam quality improvement: fabrication and performance

By confining the Yb doping within a smaller radius in the center of the core of the few-mode large-mode-area fiber, the fundamental mode, which overlaps better with the Yb ions, sees higher gain than the higher order modes, and dominates the output. Hence, improved beam quality can be achieved. A confined-doped fiber with 41/395μm core/cladding diameters is fabricated by Direct Nanoparticle Deposition (DND) process. The fiber is characterized in an amplified spontaneous emission (ASE) source setup. Near-diffraction-limited beam quality (M2~1.3) is experimentally demonstrated.

Experimental verification of spatial distribution of photodarkening in large mode area ytterbium doped fibers

We experimentally demonstrated that photodarkening is not uniformly distributed in the cross-section of bent LMA fibers. Photodarkening distribution depends on coiling diameter, and affects the use of fibers in applications and their photodarkening propensity measurement.

Predicting fiber refractive index from a measured preform index profile

When producing fiber lasers and amplifiers, silica glass compositions consisting of three to six different materials are needed. Due to the varying needs of different applications, substantial number of different glass compositions are used in the active fiber structures. Often it is not possible to find material parameters for theoretical models to estimate thermal and mechanical properties of those glass compositions. This makes it challenging to predict accurately fiber core refractive index values, even if the preform index profile is measured. Usually the desired fiber refractive index value is achieved experimentally, which is expensive. To overcome this problem, we analyzed statistically the changes between the measured preform and fiber index values. We searched for correlations that would help to predict the Δn-value change from preform to fiber in a situation where we don’t know the values of the glass material parameters that define the change. Our index change models were built using the data collected from preforms and fibers made by the Direct Nanoparticle Deposition (DND) technology.

On the measurement of fundamental mode bend loss in large-mode-area optical fibers

The measurement of fundamental mode bend loss is thoroughly studied in large-mode-area few-mode optical fibers. The influencing factors, including spectral properties of the light source, modal power content and cladding light, are experimentally investigated. Monte Carlo simulations are performed to help in understanding and illustrating the distribution of the variations. Practical guidelines and an example setup are provided for precise and accurate measurements.

Mode coupling in few-mode large-mode-area fibers

We present an experimental study on mode coupling characteristics of few-mode large-mode-area (LMA) fibers, which are widely used in high power fiber lasers. The modal power allocation is measured by modal decomposition of the nearfield intensity profile of the output beam. Cut-back measurements are carried out with commonly-used fibers with different fiber geometries. The evolution of the modal power content due to mode coupling is presented. The influence of the fiber geometry on mode coupling is discussed.