Matthias Jäger

Very large mode area ytterbium fiber amplifier with aluminum-doped pump cladding made by powder sinter technology

We demonstrate amplification experiments using a very large mode area Yb-doped double-clad fiber with 100 µm aluminum-cer codoped core and 440 µm pump cladding realized by high aluminum codoping. The material for core and pump cladding was fabricated by reactive powder sinter technology. A high numerical aperture (NA) of the pump cladding with NA = 0.21 and a low one of the core with NA = 0.084 could be realized. Using a 0.55 m short fiber sample as the main amplifier in a three-stage ns pulsed fiber master oscillator power amplifier system we achieved 3 ns, 2 mJ output pulses with 360 kW peak power limited by the available pump power. Stimulated Raman scattering effects and amplified spontaneous emission were successfully suppressed.

Design evolution, long term performance and application tests of extra large mode area (XLMA) fiber lasers

XLMA fibers based on Yb-doped bulk silica possess an excellent refractive index and doping level homogeneity [1]. To achieve the highest optical-to-optical efficiency and long-term operation without degradation we simulated the effect of the brightness conversion factor of different core dopant compositions of such XLMA fibers. We also investigated the beam quality of a multi-kW single XLMA fiber laser system and its long-term stability. The current state-of-the-art XLMA single fiber laser has 5 kW maximum output power and a degradation rate of about 0.5 % / 500 h at 4 kW measured over a period of 1700 h. Several application tests demonstrate the excellent performance of the XLMA fiber laser.

Evidence of Tm impact in low-photodarkening Yb-doped fibers

In contrast to Yb/Al-doped fibers, the influence of very low Tm(2)O(3) concentrations (≥ 0.1 mol-ppm) on photodarkening (PD) is clearly detectable in Yb/P-doped fibers that are known to show little degradation effects. For Tm(2)O(3) additions of more than 50 mol-ppm, the measured PD loss is even similar to Yb/Al-doped fibers with comparable rare earth concentrations. Our work reveals the risk of color center generation by pumping at wavelengths of 915 nm or 976 nm even in Al-free Yb-doped fibers and emphasizes the importance of high purity of raw materials for the preparation of Yb laser fibers with expected very low PD.

All-fiber time-delay spectrometer for simultaneous spectral and temporal laser pulse characterization in the nanosecond range

We have realized a modified time-delay spectrometer based on a step-chirped fiber Bragg grating array. This method allows simultaneous spectral and temporal characterization of pulsed light sources in the nanosecond regime, which can also be applied to the investigation of single pulses. With a spectral resolution in the 100 pm range, pulse spectrograms are measured and exemplarily used to explore the emission behavior of a wavelength-stabilized laser diode directly modulated in the nanosecond range.

Integrated optical fiber grating coupler on SOI for the excitation of several higher order fiber modes

Experimental and numerical results of an integrated optical fiber grating coupler on SOI capable of exciting LP₀₁, LP_{11, a} and LP_{11, b} modes in a standard few mode fiber in both TE and TM polarization are presented.

In situ FBG inscription during fiber laser operation.

We demonstrate the inscription of a 266 nm UV femtosecond pulse-induced fiber Bragg grating (FBG) in an Yb-doped fiber during optical pumping at 976 nm and the initiation of lasing with increasing grating reflectivity. Output spectra show the emission of the pumped fiber changing from the broad-ranged amplified spontaneous emission in the nonlasing case to the narrow-range laser operation due to the enhancement of FBG reflectivity during inscription. The proposed technique enables the direct characterization and control of FBG performance in fiber lasers. After FBG fabrication, we investigate the spectral characteristics of the fiber laser for different laser powers and study the influence of a thermal treatment of the FBG.

Approximation of the effective refractive index of surface plasmons propagating along micron-sized gold wires in photonic crystal fibers

Since the first presentation of selectively metal filled photonic crystal fibers (PCFs) in 2008, a lot of work and effort has been put in the understanding of propagation characteristics of such fibers which can be utilized as filters or polarizers. A semi-analytical model for the implicit description of the effective refractive index of surface plasmon polaritons propagating (SPPs) along the metal wires has been developed and coupling of fiber core modes to such surface modes has been confirmed experimentally. In this work we will present a method for the fabrication of selectively metal filled photonic crystal fibers and derive the dispersion equation for micron sized wires in silica. We will present a ray-optical approximation of SPPs based on the dispersion of a planar dielectric-gold interface which leads to a full-analytical equation for the prediction of cutoff wavelengths of the SPPs.

Silicon dual-ring resonator-based push-pull modulators

Two types of silicon dual-ring resonator-based high-speed optical modulators are proposed. With two microring resonators cascaded either in series or in parallel, the transmission spectrum evolves from a deep notch to a sharp peak with the resonators operating in a push-pull manner. The frequency chirp of the modulated signals can be highly suppressed by choosing a proper working wavelength.

Arrays of Individually Addressable SOI Micro Ring Resonators for Bio Sensing

Four SOI ring resonators are coupled to a common bus waveguide. Thermo-optical modulation is used to individually access each ring resonator. The addressing of the rings is shown experimentally and oligonucleotides are used to test the sensors.

Fiber lasers and amplifiers with reduced optical nonlinearities employing large mode area fibers

Fiber lasers have recently received a lot of attention after the dramatic increase in output power achieved from single fibers. In particular, Ytterbium doped fibers offer a very low quantum defect and a very broad emission between 1 and 1.1 μm. Triggered by the progress in high-brightness pump diodes and the availability of large-mode-area (LMA) gain fibers, several fiber lasers with output powers in the 1kW range from a single fiber have been demonstrated [1-4]. While these demonstrations typically employ a length of gain fiber pumped via free-space coupling and free space optics as the high reflector, there are fewer reports of integrated all-fiber laser cavities, e.g. [4]. The availability of high-power fiber-optic components and the assembly thereof is therefore crucial for making this technology accessible for a variety of applications. Fiber lasers and amplifiers are very attractive light sources for applications requiring high power as well as excellent beam quality, because they are much less susceptible to thermo-optic distortions than conventional solid-state lasers. A transform-limited beam quality (M2=1) is possible even at kW level output power. Another advantage is the excellent overlap between the signal light and the pump absorption achievable in properly designed fibers. This allows a very efficient operation and up to 80% of optical conversion efficiency have been demonstrated based on the launched pump power [2]. Once assembled, fiber-optic modules do not require alignment and are therefore inherently robust. The tight confinement of the laser light combined with the long interaction length in fibers also makes them prime candidates for high gain systems.