Mode-Locked, Tunable Fiber Laser Is Alternative to Ti:Sapphire

Breck Hitz

Fiber lasers are being developed in laboratories around the world as alternatives to traditional solid-state lasers. The Ti:sapphire laser, however, has remained largely unchallenged as a source of broadly tunable, ultrashort pulses. But now a collaboration of engineers in Europe has produced a mode-locked, wavelength-tunable fiber laser that can give a Ti:sapphire laser a run for its money in terms of tunability and short-pulse generation.

Figure 1. The mode-locked Yb:fiber laser is tuned across 120 nm by adjusting the angular orientation of the high-reflectivity mirror behind the grating pair.

Developed by researchers at Tampere University of Technology in Finland and at Fianium-New Optics Ltd. in Southampton, UK, the laser features a grating pair that compensates for dispersion in the Yb-doped fiber and a semiconductor saturable-absorber mirror that passively mode-locks the laser (Figure 1). In tests in which they tuned the laser across its tuning range of more than 100 nm from 980 to 1100 nm, a consistent 3-mW output was maintained using a single 130-mW pump diode laser operating at 915 nm. The duration of the mode-locked pulses varied from 1.6 to 2.0 ps across the tuning range.

Wavelength tuning is acheived by adjusting the angular orientation of a high-reflectivity mirror. The mode-locking semiconductor saturable-absorber mirror uses an AlGaAs/GaAs distributed Bragg reflector, which usually has a bandwidth of 100 nm. To be certain to avoid bandwidth limitations from the semiconductor saturable-absorber mirror, the scientists employ two distributed Bragg reflectors -- one centered at 1055 nm and the other at 1000 nm -- when tuning the laser.

Figure 2. The autocorrelation trace from the 1.6-ps mode-locked pulses indicates that they are nearly bandwidth-limited.

An autocorrelation trace of the mode-locked pulses at 987 nm indicates that the 1.6-ps pulses are nearly bandwidth-limited, having Gaussian temporal and spectral profiles (Figure 2). In this experiment, the pulse repetition frequency was 30 MHz.
Two pump/signal multiplexers are required to tune across the full 120-nm spectral range, but replacing the fused-fiber couplers with a single coupler based on micro-optical technology would enable a single multiplexer to be used for continuous tuning across this range. Moreover, it is possible to design the semiconductor saturable-absorber mirror so that a single distributed Bragg reflector could be used, which would enable tunability over the entire 120-nm range without changing any intracavity elements.

Figure 3. The packaged laser is compact, turnkey and maintenance-free.

The researchers have used a fiber power amplifier to scale up the energy of their mode-locked laser. They have achieved an average output power of 700 mW, corresponding to a pulse energy of 30 nJ. Although they have not demonstrated it, they emphasize that a cladding-pumped fiber amplifier could readily be added to the laser to boost the average output level to several watts, making the system all the more attractive for numerous applications.