Exploitable results
The overall objectives of the Uranus project were to develop the technology that pursues two primary objectives: more energetic ultrashort pulses at various wavelengths and increased stability. The project efforts have been focused on the development of ultrashort pulse sources based on fibre lasers technology.
The Uranus consortium includes European companies (Fianium Ltd. from the UK, Corelase Oy from Finland, NKT Research from Denmark, Stratophase from the UK) and academic institutions (Tampere University of Technology from Finland, and INESC Porto from Portugal) that worked together to push the limits of ultrafast fibre laser technology, and to exploit innovative fibre-laser systems commercially.
The project has demonstrated a positive example of European partnerships involving academia, business and government founded non-profit institutions working together to develop and commercialise new technologies. The synergy of the consortium resulted in a positive impact on the performance of ultrafast optics European industry. By the end of the project Fianium was recognised as the main player in ultrafast fibre laser technology in Europe. Stratophase and NKT strengthened their position as the main suppliers of nonlinear crystals and photonic crystal fibres, respectively. Advances made by Corelase have attracted the attention of a major European laser and application developer (Rofin-Sinar), which acquired Corelase at the beginning of 2007.
Main technologies developed in Uranus
Semiconductor saturable absorber mirrors
An essential component for generating ultrashort-pulses with fibre lasers is the semiconductor saturable absorber mirror (SESAM). SESAM-based fibre lasers have a compact size, are environmentally stable and can produce ultrashort pulses with picosencond and femtosecond durations. Within the project, we have identified the principal mechanisms that cause ultrashort pulse shaping in a fibre laser and we have optimised the SESAMs for operation at 1550 nm, 1060 nm, and 980 nm. The technology has been commercialised through a spin-off company, RefleKron Oy, of the Tampere University of Technology that was established at the beginning of the project.
Photonic bandgap fibre for intracavity dispersion compensation, amplification, and supercontinuum generation
In Uranus, we demonstrated for the first time the use of a solid-core photonic bandgap fibre to compensate the dispersion of an ytterbium mode-locked laser. We showed that using semiconductor saturable absorber mirror together with solid-core photonic bandgap fiber enables the self-starting all-fibre mode-locked laser operating around 1 µm wavelength range. This approach may constitute an important step towards novel generation of ultrafast fiber osillators. Another even more advanced configuration of an environmentally stable soliton laser uses ytterbium-doped all-solid photonic bandgap fibre providing both gain and dispersion compensation at 1 µm. Special PCFs were designed to enhance supercontinuum generation using 1060 nm ultrafast lasers as seed source. The results obtained have exceeded the expectations in terms of average power, spectral density and emission bandwidth.
Nonlinear crystals for frequency conversion
Periodically-poled crystals have been investigated as high-efficiency nonlinear media for frequency conversion. During the Uranus project the focus of this work has been towards applications in frequency doubling of the short-pulse infrared fibre lasers developed within concurrent work-packages. When designing a frequency converted laser system, it is important that the periodically-poled grating matches properly the pump source characteristics to achieve maximum conversion efficiency from infrared to visible wavelengths. For example, longer gratings are typically required to achieve higher conversion efficiencies, but grating length is also inversely proportional to the spectral bandwidth of the crystal. As shortpulse fibre lasers typically feature broad spectral outputs (> 1nm) this leads to a requirement for short crystals with lengths of around a hundred microns (or less), requiring often unachievable fabrication tolerances.
Amplifiers and system demonstrations
Various types of high power amplifiers have been developed to scale up the power delivered by the mode-locked fibre oscillators. Much of the effort has been focused on 1064 nm systems because of the expected high commercial impact. The key technologies investigated within this work part are Yb-doped fibers with low nonlinearity, pump combining techniques and pulse and spectral management techniques. These developments are at the basis of the commercial fiber systems presented in tables 1 and 2.
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