Large-mode area (LMA) thulium-doped fibers (TDF) are one of the key components when designing 2μm laser and amplifier systems aiming to further scale deliverable output powers. Current design limitations of LMA TDF’s affecting optical-to-optical efficiency and output beam quality are well-understood. In the present work, design optimizations focused on the core and pedestal waveguides of the active fiber are proposed. Using experimental and numerical tools, the effect of splice-induced heat on the refractive index profile of the active fiber is investigated. We demonstrate that fibers designed with larger pedestal-to-core ratios suffer less index distortions during splicing allowing the end-user to achieve high coupling efficiencies and high beam qualities in a reliable fashion.
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.
A large number of power delivery applications for optical fibers require beams with very specific output intensity profiles; in particular applications that require a focused high intensity beam typically image the near field (NF) intensity distribution at the exit surface of an optical fiber. In this work we discuss optical fiber designs that shape the output beam profile to more closely correspond to what is required in many real world industrial applications. Specifically we present results demonstrating the ability to transform Gaussian beams to shapes required for industrial applications and how that relates to system parameters such as beam product parameter (BPP) values. We report on the how different waveguide structures perform in the NF and show results on how to achieve flat-top with circular outputs.
We present results on the amplifier performance and characteristics of Yb-doped Single Mode fiber amplifiers spanning a broad range of wavelengths from 1028 nm to 1100 nm. Both PM and non-PM amplifiers are discussed, with emphasis on the use of polarization controllers in intrinsically non-PM amplifiers to obtain high Polarization Extinction Ratios (PER). In general, outside the 1064nm region, there has been relatively little discussion or work towards developing high power fiber amplifiers for operation at either 1030 nm or 1100 nm with narrow line-width and high brightness, primarily due to amplifier design and architecture issues related to strong re-absorption and amplified spontaneous emission. Here we address key fiber and amplifier design characteristics aimed at mitigating these issues while highlighting performance attributes and challenges for operation near either end of the above defined spectral range.
Beam combining of fiber lasers has attracted much interest as a practical means to power scale fiber laser/amplifiers
beyond the limitations of a single mode output from an individual fiber . Almost all of the high power demonstrations
to date that deliver good beam quality after the combing process (coherent and spectral) require some linewidth control
for efficient combining, typically less than 10GHz [2,3,4]. Previously we demonstrated single mode, Yb-doped LMA
fiber amplifiers operated with around 7GHz linewidth at 1kW output power . In this paper, the latest generations of
these amplifiers, based on the latest developments in LMA Yb-doped fiber technology demonstrate the capability to
operate with linewidths around 3GHz at the 1kW power level. We present the latest data on optical properties of these
new Yb-doped amplifiers and the SBS threshold as a function of input seed laser linewidth and discuss the technologies
being developed to operate at higher power levels and narrower linewidths.
The capability of Tm-doped silica fibers pumped at 790nm to efficiently produce high power emission in the 1.9~2.1μm
region has been well documented to date but little has been presented on the reliability of this technology. Early
experiments highlighted that photodarkening can be a significant concern when Tm-doped silica fibers are exposed to
high intensity blue light. We present a discussion of the processes responsible for the production of blue light in Tmdoped
fibers pumped at 790nm and how fiber composition influences these processes. Through optimization of fiber
composition we have demonstrated highly efficient lasers exhibiting less than 1% output power degradation per thousand
790nm-pumped Tm-doped fibre lasers provide a number of distinct benefits for integration into next generation DIRCM
systems. Incorporation of Tm-doped fibre technology into mid-IR laser systems has been demonstrated in two main
architectures to date; in early works the fibre laser was used as a low quantum defect pump source for Q-switched solidstate
holmium laser which was subsequently shifted to the mid-IR using a ZnGeP<sub>2</sub> OPO  and more recently, a pulsed
fibre laser systems was used for directly pumping the OPO . He we present two fibre laser systems for integration into
DIRCM systems. Firstly we present a 70W MOPA system (pump power limited) operating at 1908nm with 53% slope
efficiency from the amplifier stage for pumping Ho:YAG. Secondly we present a pulsed fibre laser system producing
over 4kW peak power at 1910nm using all single-mode fibres.