The Innoslab design, already established for neodymium doped laser crystals, was applied to ytterbium doped laser materials. Recent progresses in brightness of high power diode lasers facilitate efficient pumping of quasi-three-level laser materials. Innoslab amplifiers are compared to competing thin-disk and fiber fs-amplifiers. A compact diode-pumped Yb:YAG Innoslab fs-oscillator-amplifier system, scalable to the kilowatt range, was realized. Numerical simulations result in conditions for high efficiency and beam quality. Nearly transform and diffraction limited 680 fs pulses at 400 W average output power and 76 MHz repetition rate without using CPA technology have been achieved at room temperature so far.

400W Yb:YAG Innoslab fs-Amplifier.

Since the early conceptual and practical demonstrations in the late 1990s, Hollow-Core Photonic Band Gap Fibres (HC-PBGFs) have attracted huge interest by virtue of their promise to deliver a unique range of optical properties that are simply not possible in conventional fibre types. HC-PBGFs have the potential to overcome some of the fundamental limitations of solid fibres promising, for example, reduced transmission loss, lower nonlinearity, higher damage thresholds and lower latency, amongst others. They also provide a unique medium for a range of light: matter interactions of various forms, particularly for gaseous media. In this paper we review the current status of the field, including the latest developments in the understanding of the basic guidance mechanisms in these fibres and the unique properties they can exhibit. We also review the latest advances in terms of fibre fabrication and characterisation, before describing some of the most important applications of the technology, focusing in particular on their use in gas-based fibre optics and in optical communications.

https://www.degruyter.com/downloadpdf/j/nanoph.2013.2.issue-5-6/nanoph-2013-0042/nanoph-2013-0042.xml

Hollow-core photonic bandgap fibers: technology and applications

Mit einem Anteil von 22,4 Prozent am Bruttoinlandsprodukt (BIP) ist das produzierende Gewerbe das Ruckgrat der deutschen Wirtschaft. (vergl. Bundesministerium fur Wirtschaft und Technologie, 2013). Im Vergleich: der Anteil des produzierenden Gewerbes am BIP in den USA liegt bei 11,9 Prozent, in Frankreich und Grosbritannien bei 10 Prozent (vergl. Heymann, Vetter, 2013). Jahrlich produziert die deutsche Wirtschaft einen enormen Handelsuberschuss. Diese Wettbewerbsfahigkeit der deutschen Industrie war entscheidend, um die Finanz- und Wirtschaftskrisen in der jungsten Vergangenheit erfolgreich zu bewaltigen. Konkurrenzfahiger Industriestandort und fuhrender Fabrikausruster zu sein, muss jedoch immer wieder aufs Neue erarbeitet werden.

Chancen von Industrie 4.0 nutzen

A 29 W CW supercontinuum spanning from 1.06 to 1.67 µm is generated in a short length of PCF with two zero dispersion wavelengths. The continuum has the highest spectral power density, greater than 50 mW/nm up to 1.4 µm, reported to date. The use of a short length of PCF enables the continuum to expand beyond the water loss at 1.4 µm. The dynamics of the continuum evolution are studied experimentally and numerically with close attention given to the effects of the water loss and the second zero dispersion wavelength.

https://spiral.imperial.ac.uk:8443/bitstream/10044/1/1180/1/High%20Power%20Continuum%20Lib.pdf

29 W High power CW supercontinuum source

Over the past two decades, fiber laser technologies have matured to such an extent that they have captured a large portion of the commercial laser marketplace. Yet, there still is a seemingly unquenchable thirst for ever greater optical power to levels where certain deleterious light-matter interactions that limit continued power scaling become significant. In the past decade or so, the industry has focused mainly on waveguide engineering to overcome many of these hurdles. However, there is an emerging body of work emphasizing the enabling role of the material. In an effort to underpin these developments, this paper reviews the relevance of the material in high power fiber laser technologies. As the durable material-of-choice for the application, the discussion will mainly be limited to silicate host glasses. The discussion presented herein follows an outward path, starting with the trivalent rare earth ions and their spectroscopic properties. The ion then is placed into a host, whose impact on the spectroscopy is reviewed. Finally, adverse interactions between the laser lightwave and the host are discussed, and novel composition glass fiber design and fabrication methodologies are presented. With deference to the symbiosis required between material and waveguide engineering in active fiber development, this review will emphasize the former. Specifically, where appropriate, materials-based paths to the enhancement of laser performance will be underscored.

/pdf/materials-for-optical-fiber-lasers-a-review-2n45z3gaeq.pdf

Materials for optical fiber lasers: A review

Lidar Systems for the measurement of three-dimensional wind or cloud and aerosol formations in the earth atmosphere
require highly stable pulsed single frequency laser systems with a narrow line width. The lasers for ESAs ADM-Aeolus
and EarthCARE missions require frequency stabilities of 4 and 10 MHz rms at a wavelength of 355 nm and a line width
below 50 MHz at 30 ns pulse duration[1]. Transferred to the fundamental wavelength of the laser systems the stability
requirement is 1.3 and 3.3 MHz, respectively. In comparison to ground based lidar systems the vibrational load on the
laser system is much higher in airborne and spaceborne systems, especially at high frequencies of some hundred Hertz or
even some kHz. Suitable frequency stabilisation methods have therefore to be able to suppress these vibrations
sufficiently. The often used Pulse-Build-up method is not suitable, due to its very limited capability to suppress vibration
frequencies of the order of the pulse repetition frequency.
In this study the performance of three frequency stabilisation methods in principle capable to meet the requirements, the
cavity dither method, the modified Pound-Drever-Hall method and a modified Ramp-Fire method - named Ramp-Delay-
Fire - is theoretically and experimentally investigated and compared.
The investigation is performed on highly efficient, passively cooled, diode end-pumped q-switched Nd:YAG oscillators,
which are breadboard versions of the A2D (ADM-Aeolus) and possible ATLAS (EarthCARE) oscillators. They deliver
diffraction limited output pulses with up to 12 mJ pulse energy at a pulse duration of 30 ns and 100 Hz pulse repetition
rate.

Frequency stabilization of Q-switched Nd:YAG oscillators for airborne and spaceborne lidar systems

A direct diode laser was built with > 800 W output power at 940 nm to 980 nm. The radiation is coupled into a 100 µm fiber and the NA ex fiber is 0.17. The laser system is based on pump modules that are wavelength stabilized by VBGs. Dense and coarse wavelength multiplexing are realized with commercially available ultra-steep dielectric filters. The electro-optical efficiency is above 30%. Based on a detailed analysis of losses, an improved e-o-efficiency in the range of 40% to 45% is expected in the near future. System performance and reliability were demonstrated with sheet metal cutting tests on stainless steel with a thickness of 4.2 mm.

kW-class direct diode laser for sheet metal cutting based on DWDM of pump modules by use of ultra-steep dielectric filters

The simulation of spectral stabilization of broad-area edge-emitting semiconductor diode lasers is presented in this paper. In the reported model light-, temperature- and charge carrier-distributions are solved iteratively in frequency domain for transverse slices along the semiconductor heterostructure using wide-angle finite-difference beam propagation. Depending on the operating current the laser characteristics are evaluated numerically, including near- and far-field patterns of the astigmatic laser beam, optical output power and the emission spectra, with central wavelength and spectral width. The focus of the model lies on the prediction of influences on the spectrum and power characteristics by frequency selective feedback from external optical resonators. Results for the free running and the spectrally stabilized diode are presented.

Simulation of spectral stabilization of high-power broad-area edge emitting semiconductor lasers

Volume holographic gratings (VHG) provide the capability of narrowing and stabilizing the wavelength of
semiconductor lasers by forming an external cavity laser (ECL). The standard configuration of these ECL's is to use a
collimating lens followed by the VHG to provide feedback to the resonator and lock the wavelength. In this
configuration both elements have to be carefully aligned with tolerances in the sub-µm and mrad range. The present
paper presents a fast-axis collimation lens (FAC) with integrated VHG for locking a laser diode bar. Besides the
advantage of having only a single element, the integrated element is also less sensitive to alignment tolerances with
respect to the locking due to the large divergence angle of the uncollimated array compared to a collimated array. Using
a standard AR coated array with 19 emitters an output power of 67.4 W was achieved. The spectral bandwidth was
within 1 nm over the whole power range. Due to high stability requirements in this application, glass was chosen as the
VHG material. Though the refractive index is low compared to standard FAC lenses, the design and manufacturing
process of the lens still guarantees a diffraction limited collimated beam.

Wavelength stabilization of HPDL array: fast-axis collimation optic with integrated VHG

An analytical approach for the thermal design for high-power fiber lasers and fiber components is presented. The approach is based on defining a thermal resistance for each fiber layer. Thus the importance of each layer for the heat transport is made transparent and the influence of the parameters can be studied for each layer separately. Furthermore the analysis and analytic optimization of interacting effects of groups of layers is possible. The approach is applied to air-clad-fiber with results differing up to 40 % from previous works. Furthermore the heat transport from splices is analyzed and recommendations for the thermal packaging of splices and fiber integrated components are given.

Dieter Hoffmann

Papers

Frequency stabilization of Q-switched Nd:YAG oscillators for airborne and spaceborne lidar systems

kW-class direct diode laser for sheet metal cutting based on DWDM of pump modules by use of ultra-steep dielectric filters

Simulation of spectral stabilization of high-power broad-area edge emitting semiconductor lasers

Wavelength stabilization of HPDL array: fast-axis collimation optic with integrated VHG

Heat transport in solid and air-clad fibers for high-power fiber lasers