13 Jul 2005
Are you looking for a source of femtosecond pulses? Jan Posthumus explains why fibre lasers are a convenient, easy-to-use answer, with the added benefit of being tunable.
The generation of ultrashort laser pulses that are just a few tens of femtoseconds long is commonplace in today's university laboratories. And the method is starting to show great potential in applications ranging from materials processing to spectroscopy and microscopy. However, for this exciting technology to fulfil its true potential it is important that pulse sources are robust, reliable and user-friendly.
Until recently, femtosecond sources had a reputation for being fragile and complex instruments. This has been, in the most part, due to their design: a cavity containing laser crystals and mirrors that is very sensitive to any misalignment or contamination, and often needs water cooling.
Fortunately, developments in fibre technology now mean that a new form of compact and rugged source is available - the femtosecond fibre laser. Pumped by diodes and with no bulk mirrors to align, these lasers are proving increasingly popular as a "push-button" reliable source of ultrashort pulses.
Although femtosecond fibre lasers come in various designs, they are invariably based on singlemode rare-earth doped fibre, which provides optical gain when pumped with light from laser diodes. The emission wavelength of the laser is determined by the type of dopant in the fibre's core (fibres doped with ytterbium emit at around 1050 nm and those doped with erbium near 1550 nm).
The laser designer can choose to construct either a linear or ring-shaped laser cavity. In the linear design, the light has to reflect at the ends of the fibre or Bragg gratings written into the fibre. In the ring-shaped design, the fibre is connected into a continuous loop and does not need any mirrors. The length of the cavity determines the repetition rate of the laser, and fused fibre-couplers inject pump light from the diodes into the doped gain fibre.
To generate pulses that are as short as a few femtoseconds in duration, it is necessary to perform a process called passive modelocking. Locking the phase of many longitudinal modes within the cavity together causes the modes to interfere constructively and produce ultrashort pulses.
Passive nonlinear optical elements (based on either saturable absorption or the Kerr effect) are placed in the cavity to ensure that the modes not contributing to a short pulse are suppressed and do not lase.
For optimal pulse quality, it is best to maintain a low power level in the laser. Pulses can then be boosted by one or more amplifiers to increase the average power to values of (typically) 300 mW, before being compressed to the desired duration. Amplifiers are constructed from the same standard telecoms components as the oscillator (the low-power laser) and simply act as a gain stage, pumped by two additional pump laser diodes.
After amplification, pulse compression takes place outside the fibre to avoid any unwanted fibre-based dispersion and nonlinear effects that could distort the shape of the pulses. The pulses can be compressed to as short as 80 fs using a pair of prisms.
Figure 1 shows a schematic of a typical ring-cavity erbium fibre laser arranged in a master-oscillator power amplifier design. The 980 nm pump-light of the laser diode is coupled into the fibre's erbium-doped core by a wavelength division multiplexer. Lasing occurs in a band around a central wavelength of 1.56 μm.
Additional standard fibre, which has negative dispersion at this wavelength, is added to the cavity to compensate for the positive dispersion of the erbium-doped fibre. The passive modelocking process is based on polarization-additive pulse modelocking. In this instance, the nonlinear Kerr effect induces a rotation of polarization, and the generation of ultrashort pulses is optimized by using waveplates and a polarizer. Importantly, the modelocked operation is self-starting and, after turning the laser on, it is fully functional within 15 s.
The repetition rate of the pulses is typically 100 MHz, but some freedom exists to adjust this. A fast piezoelectric actuator allows precise synchronization to an external radio-frequency oscillator. One of the big benefits of a modelocked fibre laser is the ability to tune its emission wavelength. To perform the tuning, laser pulses are injected into a highly nonlinear fibre to produce a supercontinuum that typically spans 1000-2300 nm. A prism pair controls the spectral shape of the continuum.
Laser pulses in the 1050-1400 nm wavelength range can be compressed to durations in the 25-50 fs range, with average powers around 30 mW, by the use of a pulse compressor. The shortest pulses observed to date are 14 fs at a wavelength of near 1200 nm. At longer wavelengths in the 1600-2200 nm range, it is possible to generate pulses with durations well below 100 fs.
Finally, the high peak intensities permit efficient frequency doubling, giving tunable femtosecond pulses in the 525-740 nm range, with an average power of 5 mW in a 5-10 nm-wide spectrum. Alternatively, it is possible to directly frequency-double the fundamental wavelength. In this case, average powers of 100 mW are achieved at a wavelength of 775 nm.
This performance is not yet comparable to Ti:sapphire lasers, which can generate sub-50 fs pulses at 800 nm, with an average power of several hundreds of milliwatts. However, it may not be long before this is achieved.
More complex fibre geometries, such as the double-clad fibre, large mode-area fibre and photonic crystal fibre are all under development. These designs are making it easier to inject pump light into the fibre and, consequently, to scale the output power of fibre lasers to a higher level, while still maintaining singlemode operation.
Combined with the chirped-pulse amplification technique, average powers of tens of watts have been achieved in the laboratory, with sub-picosecond pulse durations. It is possible that commercial lasers based on these techniques will be developed in the future.
Acknowledgements The author wishes to acknowledge F Adler, F Sotier and A Leitenstorfer from the University of Konstanz for their technological support.