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Ultrafast fibre lasers need insight for success

08 Apr 2009

Marie Freebody speaks to Fianium's chief executive officer, Anatoly Grudinin, to find out how misconceptions about ultrafast fibre lasers are threatening their commercial success.

Anatoly Grudinin is chief executive officer and founder of Fianium, a UK-based fibre laser company focused on volume manufacturing of ultrafast fibre lasers for biomedical and industrial applications. Grudinin has been working in fibre optics since joining the Lebedev Physical Institute of the Russian Academy of Science in 1980. Between 1992 and 2003 Grudinin worked at the University of Southampton, UK, conducting research into ultrafast fibre lasers and nonlinear fibre optics.

How do ultrafast fibre lasers work?
In many respects, ultrafast fibre lasers are similar to conventional continuous-wave or nanosecond fibre lasers. In all three cases, the lasers are configured as master-oscillator power-amplifier systems. In terms of architecture, lasers producing long, nanosecond, picosecond or femtosecond pulses are the same. The main difference is in the design of the master oscillator.

At Fianium we use a semiconductor saturable absorber mirror (SESAM) as one of the cavity reflectors. The other reflector is similar in design to those used in conventional fibre lasers. The magic of the SESAM is that its reflectivity grows with pulse intensity and thus naturally promotes ultrashort pulse generation.

In simple terms, a modelocked fibre laser has very few components and is fundamentally no more complicated than a conventional fibre laser. Such lasers are also very easy to build as long as the correct components (SESAM, doped fibres, mirrors, etc) are available.

In our case, with an ultrashort pulse of less than 10 ps, a few tens of picojoules of energy is generated by the oscillator. This energy can then be amplified with an efficient, diode-pumped fibre amplifier by more than six orders of magnitude to tens or even hundreds of microjoules of pulse energy. This is all achieved within the fibre, resulting in none of the bulk-optics, mechanical alignment and thermal management issues typically associated with solid-state lasers.

Why are fibre lasers important?
I believe that fibre lasers are the future of ultrafast technology and continued research is essential in order to expand their capabilities and applications.

Ultrafast fibre lasers can generate very high peak powers exceeding tens of megawatts, initiating nonlinear effects in both fibres and bulk materials. Over the last two decades, research into ultrafast fibre lasers has resulted in some excellent academic results and, more recently, competitive and high-quality commercial systems.

In this context we see two main avenues of research. The first is developing lasers with higher peak powers and/or shorter pulse widths. Second is research into new types of ultrafast lasers using the latest advances in materials, such as photonic crystals and metamaterials. Such materials should not only help to add new functionality to lasers but also open up new wavelength ranges, which are difficult or awkward to achieve using conventional lasers.

What are the main applications and on what timescales will they occur?
Traditionally, scientific study has been the main application area for ultrafast lasers. We are gradually seeing the adoption of ultrafast fibre lasers across numerous sectors including biomedical imaging, semiconductor manufacture, inspection and metrology, photovoltaics manufacture and of course laser eye surgery. As end-users start to appreciate all of the advantages of this technology, I expect to see widespread growth of the ultrafast fibre laser market in the next 3–5 years.

What is the most important recent advance in this field?
It is undoubtedly the invention and commercialization of photonic crystal fibres (PCFs). PCFs enable the management of fibre dispersion and nonlinearity to a level unachievable through conventional fibres and optics. This has led to a straightforward way of controlling and managing pulse propagation through fibres.

In combination with the high peak powers generated by ultrafast lasers, the PCF has opened up many exciting prospects for fibre-based devices and systems. The most striking example is the supercontinuum fibre laser – a laser generating white light with intensities that are orders of magnitude greater than the Sun itself.

At the other extreme, hollow-core PCFs enable optical pulses to propagate in air. Such fibres can be used to produce and deliver very high (>10 MW) peak power pulses or even ultraviolet radiation.

What key challenges remain?
The main commercial challenge left to overcome is the historical perception that ultrafast lasers are complex and unaffordable. This is a huge barrier to the adoption of ultrafast technology in any industrial process. However, I believe that it won't be long until it becomes accepted that, in the form of the fibre laser, ultrafast technology is now viable for widespread commercial use.

What will the next breakthrough be?
Looking back at the evolution of tunable ultrafast technology from dye to Ti:sapphire lasers, I would expect this evolution to continue and that one day we will say goodbye to the Ti:sapphire laser. The replacement will be broadly tunable (400–2500 nm) femtosecond systems comprising an ultrafast fibre laser and a custom-made PCF. At the present rate of development, this might not be far away.

• This article originally appeared in the April 2009 issue of Optics & Laser Europe magazine.

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