The power scaling of high power fiber lasers has decelerated recently, due to transverse mode instability (TMI) and photodarkening (PD). The origin of TMI is a power transfer from the fundamental mode of the fiber to higher transverse modes due to self-induced thermo-optical long period gratings. The excitation of higher modes can lead to temporal instability and a bend-loss-induced reduction of the laser power. <p> </p>Over the lifetime of a fiber laser, the TMI threshold is decreased due to photodarkening of the fiber. Many investigations have been made to model both effects, but the microscopic mechanisms both of TMI and PD are not yet fully understood. The existing models are either comprehensive, but very slow and therefore limited to the simulation of short fibers, or reduced models that do not take transverse effects into account. Furthermore, these models have been applied only to single-pass fiber amplifiers so far. <p> </p>We present a hierarchical numerical approach that allows to first precalculate the transverse distribution of the photodarkening losses, and then apply the precalculated data to a scalar coupled-mode model of the fiber laser. As a result, it is possible to perform virtual long term tests simulating several 10 000 hours of laser operation in a few hours. The transverse distribution of photodarkening losses in the fiber and the mode coupling gain can be analyzed at any cross section along the fiber.<p> </p> The simulation results are compared to experimental data, which demonstrates the feasibility of the approach to predict the TMI threshold for different laser setups.
Beam delivery fibers have been used widely for transporting the optical beams from the laser to the subject of irradiation in a variety of markets including industrial, medical and defense applications. Standard beam delivery fibers range from 50 to 1500 μm core diameter and are used to guide CW or pulsed laser light, generated by solid state, fiber or diode lasers. Here, we introduce a novel fiber technology capable of simultaneously controlling the beam profile and the angular divergence of single-mode (SM) and multi-mode (MM) beams using a single-optical fiber. Results of beam transformation from a SM to a MM beam with flat-top intensity profile are presented in the case of a controlled BPP at 3.8 mm*mrad. The scaling capabilities of this flat-top fiber design to achieve a range of BPP values while ensuring a flat-top beam profile are discussed. In addition, we demonstrate, for the first time to the best of our knowledge, the homogenizer capabilities of this novel technology, able to transform random MM beams into uniform flat-top beam profiles with very limited impact on the beam brightness. This study is concluded with a discussion on the scalability of this fiber technology to fit from 50 up to 1500 μm core fibers and its potential for a broader range of applications.
Single-mode (SM) kW-class fiber lasers are the tools of choice for material processing applications such as sheet metal cutting and welding. However, application requirements include a flat-top intensity profile and specific beam parameter product (BPP). Here, Nufern introduces a novel specialty fiber technology capable of converting a SM laser beam into a flat-top beam suited for these applications. The performances are demonstrated using a specialty fiber with 100 μm pure silica core, 0.22 NA surrounded by a 120 μm fluorine-doped layer and a 360 μm pure silica cladding, which was designed to match the conventional beam delivery fibers. A SM fiber laser operating at a wavelength of 1.07 μm and terminated with a large-mode area (LMA) fiber with 20 μm core and 0.06 NA was directly coupled in the core of the flat-top specialty fiber using conventional splicing technique. The output beam profile and BPP were characterized first with a low-power source and confirmed using a 2 kW laser and we report a beam transformation from a SM beam into a flat-top intensity profile beam with a 3.8 mm*mrad BPP. This is, to the best of our knowledge, the first successful beam transformation from SM to MM flat-top with controlled BPP in a single fiber integrated in a multi-kW all-fiber system architecture.
We have demonstrated a linearly polarized cw all-in-fiber oscillator providing 1 kW of output power and a polarization extinction ratio (PER) of up to 21.7 dB. The design of the laser oscillator is simple and consists of an Ytterbium-doped polarization maintaining large mode area (PLMA) fiber and suitable fiber Bragg gratings (FBG) in matching PLMA fibers. The oscillator has nearly diffraction-limited beam quality (M² < 1.2). Pump power is delivered via a high power 6+1:1 pump coupler. The slope efficiency of the laser is 75 %. The electro/optical efficiency of the complete laser system is ~30 % and hence in the range of Rofin’s cw non-polarized fiber lasers. Choosing an adequate bending diameter for the Yb-doped PLMA fiber, one polarization mode as well as higher order modes are sufficiently supressed1. Resulting in a compact and robust linearly polarized high power single mode laser without external polarizing components. Linearly polarized lasers are well established for one dimensional cutting or welding applications. Using beam shaping optics radially polarized laser light can be generated to be independent from the angle of incident to the processing surface. Furthermore, high power linearly polarized laser light is fundamental for nonlinear frequency conversion of nonlinear materials.
High power Yb doped fiber laser sources are beside CO2- and disk lasers one of the working horses of industrial laser
applications. Due to their inherently given robustness, scalability and high efficiency, fiber laser sources are best suited to fulfill the requirements of modern industrial laser applications in terms of power and beam quality. Pumping Yb doped single-mode fiber lasers at 976nm is very efficient. Thus, high power levels can be realized avoiding limiting nonlinear effects like SRS. However the absorption band of Yb doped glass around 976nm is very narrow. Therefore, one has to consider the wavelength shift of the diode lasers used for pumping. The output spectrum of passively cooled diode lasers is mainly defined by the applied current and by the heat sink temperature. Furthermore the overall emission line width of a high power pump source is dominated by the large number of needed diode laser emitters, each producing an individual spectrum. Even though it is possible to operate multi-kW cw single-mode fiber lasers with free running diode laser pumps, wavelength stabilizing techniques for diode lasers (e.g. volume holographic gratings, VHG) can be utilized in future fiber laser sources to increase the output power level while keeping the energy consumption constant. To clarify the benefits of wavelength stabilized diode lasers with integrated VHG for wavelength locking the performance of a dual side pumped fiber oscillator is discussed in this article. For comparison, different pumping configurations consisting of stabilized and free-running diode lasers are presented.
The Performance of High Power Disk Lasers and Fiber Lasers along with their rapid development to the high power cw
regime have been of great interest throughout the last decade.
Both technologies are still in the focus of several conferences, workshops, and papers and represent the "state-of-the-art"
of industrial high power solid state lasers for material processing. As both laser concepts are considered to be the leading
1 μm light-source, this presentation presents an objective and fair comparison of the two different technologies from a
manufacturer who pursued both.
From the geometry of the active material, through the resonator design, cooling regime, and pumping method to the point
of beam quality and power scaling, the different approaches associated with the advantages, challenge and limits of each
technology will be discussed.
Based on ROFIN's substantial industrial experience with both laser concepts, an outlook into future trends and chances,
especially linked to fiber laser, will be given.
High laser power levels in combination with increasing beam quality bring optics performance into focus, particularly
with regard to systems with low focal shifts along the optical axis. In industrial applications, this often influences the
overall performance of the process, especially if the focal shift is comparable to or in excess of the Rayleigh length. It is
commonly accepted that the focal shifts are of thermal nature where lens material, lens coating, geometry and surface
contamination all contribute to the direction and extent of the focal shifts. In this paper we will present a novel design of
lens packages where a patented all-in-quartz concept is explored. By mounting quartz lenses in hermetically sealed
quartz tubes and applying water cooling on the perimeter of the quartz tubes we will reduce or eliminate a number of
contributing factors to focal shift problems. The hermetic sealing, carried out in a clean-room environment, will
minimize lens surface contamination. Differences in thermal expansion between lens and housing are eliminated as the
lens and housing will be of the same material. Absorption of scattered laser light will be efficient as the energy is
removed quickly by cooling water and not absorbed by fixed surroundings. Finally, indirect heating from the housing
transmitted by radiation and convection to the lenses is avoided. Values of the normalized System Focal Shift Factors
(SFSF) for the all-in-quartz optics will be compared to standard lens assemblies at multi-kW laser power levels.
We demonstrate an all-fiber 7x1 signal combiner for incoherent laser beam combining. This is a potential key
component for reaching several kW of stabile laser output power. The combiner couples the output from 7 single-mode
(SM) fiber lasers into a single multi-mode (MM) fiber. The input signal fibers have a core diameter of 17 μm and the
output MM fiber has a core diameter of 100 μm. In a tapered section light gradually leaks out of the SM fibers and is
captured by a surrounding fluorine-doped cladding. The combiner is tested up to 2.5 kW of combined output power and
only a minor increase in device temperature is observed. At an intermediate power level of 600 W a beam parameter
product (BPP) of 2.22 mm x mrad is measured, corresponding to an M<sup>2</sup> value of 6.5. These values are approaching the
theoretical limit dictated by brightness conservation.