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  • Setup Combines Ytterbium Fiber Lasers

Photonics Spectra
May 2003
Breck Hitz

A group at Massachusetts Institute of Technology in Cambridge is combining the output of ytterbium fiber lasers using a master-oscillator power-amplifier configuration to produce a single beam for materials processing and other high-power applications.

Extension of the work could lead to powers in the kilowatt range, said project leader Steven J. Augst, but he cautioned that it is difficult to extrapolate from the existing results without performing further experiments to see what problems may emerge.

Ytterbium Fiber Lasers

Figure 1. A team of scientists at Massachusetts Institute of Technology is investigating combining the output of ytterbium-fiber oscillator-amplifiers, using a grating as a dispersive element.

The researchers operate the oscillator-amplifiers at slightly different frequencies and combine their beams using a dispersive element -- in this case, a grating (Figure 1). Although other groups have used an oscillator-only approach, this group combined the outputs from a master oscillator and a power amplifier. The advantage of using the oscillator-amplifier is that the spectral, spatial and temporal parameters can be easily controlled in a low-power amplifier, and the high-quality beam can be boosted to higher powers in the amplifier.

Ytterbium Fiber Lasers

Figure 2. In the setup, a spectrometer -- comprising a grating and a spatial filter ­ replaces the rear mirror of the oscillators.

To control the frequencies, they substituted a spectrometerlike arrangement for the oscillators' rear mirrors (Figure 2). The spatial filter acted to suppress crosstalk between the oscillators rather than to improve the spatial-mode quality. Without spatial filtering, the radiation emerging from one oscillator could be reflected to a different one, causing oscillation at a wavelength that would not be combined properly with the other beams at the output grating. Another advantage of the setup is that the output beam can be steered by adjusting the mirror behind the spatial filter.

Ytterbium fiber lasers tend to break into uncontrolled pulsing at even moderate power. Stimulated Brillouin scattering is thought to contribute to this pulsing and is enhanced when radiation propagates in both directions in a fiber. To avoid counterpropagating beams, the researchers employed a ring-resonator arrangement (Figure 3).

Ytterbium Fiber Lasers
Figure 3. To avoid counter propagating beams that would enhance stimulated Brillouin scattering, the setup features a ring resonator.

They pumped the double-clad fiber with 500 mW at 915 nm from a diode laser, which they coupled into the outer core with a tapered-fiber bundle. A fiber isolator ensured one-way propagation, and a 50 percent coupler routed half of the radiation to an off-the-shelf commercial fiber amplifier. Total round-trip loss in the resonator exceeded 90 percent, but this was not detrimental to the overall system. Intracavity losses that reduce resonator output are crucial in an oscillator, but in a master oscillator power amplifier, the power output of the oscillator is not important.

Thus far, the researchers have observed 6 W in a single output beam from five oscillator-amplifiers. The combined beam had good beam quality, with an M2 of 1.14. Scaling to higher power will involve increasing the number of oscillator-amplifiers and the power from each. The arrangement cannot handle many more oscillator-amplifiers because the field of view of the converging lens is almost full, but a microlens array on the ends of the fibers would enable more fibers to be used with the same grating (Figure 2).

Possibly a more serious problem in scaling up the power is thermal loading on the grating and other optical elements. The MIT researchers are examining dielectric gratings to increase efficiency and to decrease absorption.

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