Sven Höfer

The renaissance and bright future of fibre lasers

The first rare-earth-doped fibre lasers were operated in the early 1960s and produced a few milliwatts at a wavelength around 1 µm. For the next several decades, fibre lasers were little more than a low-power laboratory curiosity. Recently, however, fibre lasers have entered the realm of kilowatt powers with diffraction-limited beam quality. In this paper we review the reasons for this power evolution. Beyond this, we will discuss how the next generation of fibres, so-called photonic crystal fibres, enable upward power scaling and therefore open up the avenue to new performance levels of solid-state lasers.

Single-frequency master-oscillator fiber power amplifier system emitting 20 W of power

We report a master-oscillator fiber power-amplifier system consisting of a diode-pumped monolithic nonplanar ring laser as the master oscillator and a Yb-doped large-mode-area double-clad fiber as the power amplifier. The system emits up to 20.1 W of single-frequency radiation at a wavelength of 1064 nm with diffraction-limited beam quality (M(2)</=1.3) . The optical emission spectrum and amplitude-noise behavior are investigated. Furthermore, the power-scaling possibilities are discussed.

High average power femtosecond fiber CPA system

We report on a diode-pumped ytterbium-doped low-NA large mode area fiber based chirped pulse amplification system delivering 125 W average power at a repetition rate of 75 MHz and diffraction limited beam quality. Femtosecond laser pulses at 1060 nm center wavelength are stretched in a single mode fiber and recompressed after amplification to 350 fs pulse duration.

Spectral combining of fiber lasers

The different concepts of combining fiber lasers for power-scaling are discussed. We report on three combined fibers with an output power of 100 W. Several proposals are made for further power scaling and the capacitance of a grating is tested in a simulation-experiment.

150 W Nd/Yb codoped fiber laser at 1.1 /spl mu/m

Summary from only given. We report on a neodymium-ytterbium-codoped fiber which can be pumped at two separated diode laser wavelengths (808 nm and 940 nm). The combined output at 1.1 /spl mu/m represents a scalability of double-clad fiber lasers by a factor of 2.

High repetition rate, high energy fiber CPA system for material processing

We report on a fiber based CPA system generating 100 /spl mu/J of ultra-short pulses at high repetition rates (56 kHz). In a first application, the pulses are used for high speed hole drilling in steel.

Coherent beam combining of fiber amplifiers

In this paper, we demonstrate the coherent superposition of fiber amplifiers in a Michelson-type setup with active amplifying arms. The diffraction limited beam quality from the amplifiers is not reduced by imposing the beams and combining efficiencies exceeds 95%. The output of this basic active interferometer is coherently added to additional amplifier stages. Amplifier performance and further details on the locking technique are also presented.

Megawatt-level peak-power from a passively Q-switched hybrid fiber-bulk amplifier and its applications

A novel laser system with optical parameters that fill the gap between Q-switched and modelocked lasers has been developed. It consists of a high gain hybrid fiber-bulk amplifier seeded by a low power SESAM Q-switched oscillator. The mW level output power of the seed oscillator is preamplified by a single mode fiber which is limited by SRS effects. The final amplification stage is realized by a longitudinal pumped Nd:YVO4 crystal in a double pass setup. This MOPA configuration delivers sub-300ps pulses at repetition rates up to 1 MHz with an output power exceeding 60W. Nonlinear frequency conversion to 532nm and 355nm is achieved with efficiencies of >75% and >45%, respectively. Due to the high peak power, high repetition rate and high beam quality of this system, applications formerly only addressable at lower pulse repetition frequencies or with complex modelocked laser systems are now possible with high speed and lower cost of ownership. Application results that take benefit from these new laser parameters will be shown. Furthermore, the reduction of the pulse duration to sub-100ps and power scaling to output powers <100W by the use of the Innoslab concept are being presented.

Q-switching of Yb-doped double-clad fiber laser using micro-optical waveguide on micro-actuating platform modulator for marking applications

A Q-switched all-fiber laser application based on a novel micro-optical waveguide (MOW) on micro-actuating platform (MAP) light modulator is presented. A fused biconical taper (FBT) coupler acts as MOW, mounted on an electromechanical system, MAP, where an axial stress over the waist of FBT coupler is precisely controlled. The axial stress induced refractive index changes caused the coupling efficiency to result in modulation of optical power. The light modulator was implemented in a laser cavity as a Q-switching element. Q-switching of Yb3+-doped fiber laser was successfully achieved with the peak power of 192mW at 4.1W pump power and 699mW at 5.2W at the repetition rate of 18.6kHz. Further optimization of switching speed and modulation depth could improve the pulse extraction efficiency and the proposed structure can be readily applied in all-fiber Q-switching laser systems for marking applications.

Power Amplification of Parabolic Pulses

We report on the high power fiber based amplification of parabolic pulses. The output is compressed using transmission gratings to 300 fs and an average power of 38 W at 75 MHz repetition rate.