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

Photonics Spectra
Nov 2003
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.

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

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.

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

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

diode lasersfiber lasersfiber opticsResearch & Technologyshort-pulse generationsolid-state lasersTech PulseTi:sapphirewavelength-tunable fiber laserlasers

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