Mette Marie Johansen

Frequency resolved transverse mode instability in rod fiber amplifiers.

Frequency dynamics of transverse mode instabilities (TMIs) are investigated by testing three 285/100 rod fibers in a single-pass amplifier setup reaching up to ~200W of extracted output power without beam instabilities. The pump power is increased well above the TMI threshold to uncover output dynamics, and allowing a simple method for determining TMI threshold based on standard deviation. The TMI frequency component is seen to appear on top of system noise that may trigger the onset. A decay of TMI threshold with test number is identified, but the threshold is fully recovered between testing to the level of the pristine fiber by thermal annealing the fiber output end to 300°C for 2 h.

Estimating modal instability threshold for photonic crystal rod fiber amplifiers

We present a semi-analytic numerical model to estimate the transverse modal instability (TMI) threshold for photonic crystal rod amplifiers. The model includes thermally induced waveguide perturbations in the fiber cross section modeled with finite element simulations, and the relative intensity noise (RIN) of the seed laser, which seeds mode coupling between the fundamental and higher order mode. The TMI threshold is predicted to ~370 W - 440 W depending on RIN for the distributed modal filtering rod fiber.

Photonic crystal fiber amplifiers for high power ultrafast fiber lasers

Abstract In recent years, ultrafast laser systems using large-mode-area fiber amplifiers delivering several hundreds of watts of average power has attracted significant academic and industrial interest. These amplifiers can generate hundreds of kilowatts to megawatts of peak power using direct amplification and multi-gigawatts of peak power using pulse stretching techniques. These amplifiers are enabled by advancements in Photonic Crystal Fiber (PCF) design and manufacturing technology. In this paper, we will give a short overview of state-of-the-art PCF amplifiers and describe the performance in ultrafast ps laser systems.

Fiber amplifiers under thermal loads leading to transverse mode instability

Transverse mode instability (TMI) in rare-earth doped fiber amplifiers operating above an average power threshold is caused by intermodal stimulated thermal Rayleigh scattering due to quantum defect heating. We investigate thermally induced longitudinal waveguide perturbations causing power transfer from the fundamental mode (FM) to the higher order mode (HOM) by a nonlinear gain, which depends on the FM-HOM frequency shift and position along the fiber. We take temperature and mode profile evolution along the fiber into consideration to engineer fiber designs with increased TMI threshold and operation stability at higher average powers.

Photonic crystal fiber technology for compact fiber-delivered high-power ultrafast fiber lasers

Photonic crystal fiber (PCF) technology has radically impacted the scientific and industrial ultrafast laser market. Reducing platform dimensions are important to decrease cost and footprint while maintaining high optical efficiency. We present our recent work on short 85 μm core ROD-type fiber amplifiers that maintain single-mode performance and excellent beam quality. Robust long-term performance at 100 W average power and 250 kW peak power in 20 ps pulses at 1030 nm wavelength is presented, exceeding 500 h with stable performance in terms of both polarization and power. In addition, we present our recent results on hollow-core ultrafast fiber delivery maintaining high beam quality and polarization purity.

Photonic crystal fiber technology for high-performance all-fiber monolithic ultrafast fiber amplifiers

Photonic crystal fiber (PCF) technology for ultrafast fiber amplifiers traditionally uses air holes as key elements for large mode area (LMA) fiber designs. These air holes are crucial for the performance of high-end LMA PCFs, but makes splicing and interfacing more complex. To reduce this complexity in mid-range amplifiers, we present single-mode polarization-maintaining Yb-doped LMA PCFs without air holes for easier splicing into monolithic all-fiber amplifier designs. A 30 μm core all-solid spliceable PCF is presented, and amplification of 1064 nm light above 50 W with an optical to optical efficiency of 80 % is demonstrated. Furthermore, to demonstrate the excellent reliability of PCF based monolithic amplifiers, we demonstrate ultra-longterm performance data of > 35 khrs on a 14 μm core step-index type PCF amplifier with low long-term power degradation slope of < 1.5 % / 10,000 h.

85 µm core rod fiber amplifier delivering 350 W/m

An improved version of the distributed modal filtering (DMF) rod fiber is tested in a high power setup delivering 350 W/m of extracted signal average power limited by the available pump power. The rod fiber is thoroughly tested to record the transverse modal instability (TMI) behavior and also measure degradation of the TMI threshold with operation time due to induced absorption in the active material increasing the thermo-optical heat load. Multiple testing degrades the rod fiber and TMI threshold from >360 W to a saturated power level of roughly 240 W.

High Power Performance of Rod Fiber Amplifiers

An improved version of the DMF rod fiber is tested in a high power setup delivering 360W of stable signal power. Multiple testing degrades the fiber and transverse modal instability threshold from >360W to ~290W.

High-power picosecond pulse delivery through hollow core photonic band gap fibers

We demonstrated robust and bend insensitive fiber delivery of high power pulsed laser with diffraction limited beam quality for two different kind of hollow core photonic band gap fibers.

Understanding transverse mode instability in rod fiber amplifiers

High-power fiber amplifiers enable use of large-mode-area (LMA) microstructured fibers for both scientific and industrial applications in ultrafast laser systems. In industry, for example, ultrafast laser systems are used to mark or scribe materials such as glass, ceramics, or aluminum, which requires short pulses and high peak power. In the semiconductor industry, these systems are useful for cutting or writing electrodes, and in highpower experiments in research, ultrafast lasers find use for ultra-short pulses, investigating nonlinear effects, pulse stretching, divided pulse amplification, and so on. Fiber amplifiers can generate megawatts of peak power and hundreds of watts in average power using direct amplification, while having diffractionlimited output with good pointing stability for delivering precise and stable performance. This demands a large effective area to mitigate nonlinear effects such as four-wave mixing, self-phase modulation, and stimulated Raman scattering, which can distort pulse amplification due to spectral and/or temporal broadening. However, a high extracted average output power causes thermal-optical effects to significantly influence the waveguiding mechanisms, both by a thermally induced refractive index increment across the fiber cross section and a mode-beating intensity grating along the fiber. These index perturbations can cause very LMA fibers to also support higher-order modes (HOMs) at high power operation, which leads to mode degradation, and eventually transverse mode instability (TMI) setting in at a threshold power level.1–4 TMI manifests itself as rapid output beam fluctuations on the millisecond time scale, where the fundamental mode (FM) and the first HOM interact.5 TMIs appear when the extracted average output power reaches a certain threshold and currently set the upper limit for power scaling of fiber amplifiers. Previous experiments have shown that the TMI threshold decreases during operation as the threshold is reached multiple times, and Figure 1. (a) Microscope image of the 285/100 rod fiber and modal output showing (b) the fundamental mode (FM) below the transverse mode instability (TMI) threshold and (c–d) beam fluctuations above the TMI threshold.