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Fiber Laser Emits Microjoules in Femtosecond Pulses

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Parabolic pulses simplify amplification to 21 W of average power.

Breck Hitz

Fiber lasers are well-known for their excellent heat dissipation and ruggedness, although stable, mode-locked operation of high-average-power fiber lasers has not been readily obtained. But the output of such lasers — stable, ultrashort pulses with tens of watts of average power — would be useful in applications from ophthalmology and waveguide writing to nonlinear conversion.


Figure 1. The all-fiber oscillator-amplifier system consisted of a mode-locked oscillator (defined by the dotted line), a preamplifier (bottom right) and a holey-fiber power amplifier (bottom left). Images ©OSA.

Recently, researchers at Friedrich Schiller Universität and at Fraunhofer Institut für Angewandte Optik und Feinmechanik, both in Jena, Germany, demonstrated a stable, polarized, mode-locked fiber oscillator-amplifier system that generates 240-fs pulses at 17 MHz, with 21 W of average power.

Figure 2. The polarizing photonic crystal fiber in the power amplifier had an ~700 μm2mode field area and a pump absorption of 14 dB/m.

The all-fiber system comprises a master oscillator, a preamplifier and a power amplifier based on Yb-doped holey fiber (Figure 1). The oscillator is mode-locked with a semiconductor saturable absorber mirror and pumped with a single-mode 976-nm diode laser. The gain medium is a 31-cm length of polarization-maintaining Yb-doped fiber, which, together with ~5.3 m of undoped, single-mode polarization-maintaining fiber in the resonator, assures that the laser polarization remains stable despite environmental changes. The oscillator’s output consists of linearly chirped, parabolic pulses containing a few hundred picojoules.

The 7.2-ps pulses from the oscillator are subsequently amplified in a preamplifier, boosting the average power from 2 to 50 mW and stretching the pulse duration to 10 ps. The 55-cm-long Yb-doped fiber, which serves as the preamplifier’s gain medium, is of the same design as that in the oscillator.

Figure 3. The power amplifier generated up to 29 W from 30 mW of seed power. The 10-ps pulses were compressed to 240 fs in a grating compressor, resulting in 21 W of average compressed power.

The power amplifier is based on Yb-doped, single-mode, polarizing, double-clad photonic crystal fiber (Figure 2). The advantage of photonic crystal fiber is that the core can be arbitrarily large — reducing the power density and the likelihood of deleterious nonlinear effects — while still supporting only a single mode. In this case, the mode field area of the core was ~700 µm2, and the air cladding diameter was 170 µm with a numerical aperture of 0.6. The resulting pump absorption was ~14 dB/m, so a 1.2-m length of the single-polarization fiber provided sufficient gain for the amplifier.

When seeded with approximately 30 mW of input, the power amplifier produced as much as 29 W of 1035nm output (Figure 3). The 10-ps chirped output pulses were compressed to 240 fs in a transmissive grating compressor whose throughput was 73 percent, resulting in 21 W of average compressed power, or 5 MW of peak power.

Optics Letters, March 1, 2006, pp. 574

Photonics Spectra
May 2006
The branch of medicine involved in the study of the anatomy, functions, diseases and treatments of the eye.
diode lasersfiber lasersfiber opticsmode-lockedophthalmologyResearch & Technologylasers

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