CLEO '99 Postdeadline: Large-core Fiber Lasers Produce Record High Powers

New fiber laser designs double the output power to more than 100 W CW and millijoules of energy from Q-switched pulses.

By: Yvonne Carts-Powell

BALTIMORE, MD—Fiber lasers are increasingly moving into the spotlight as researchers coax higher and higher powers from the systems. This is no where more apparent than in the technical sessions and exhibit hall at the Conference on Lasers and Electro-Optics (CLEO '99; May 23-28). During the conference postdeadline paper sessions, Vince Dominic and others from SDL (San Jose, CA) described a high-power ytterbium-doped double-clad fiber laser pumped from both ends by a total of four diode bars (see Figure 1), ¹ while H. L. Offerhaus and others at University of Southampton (England) reported 2.3 mJ/pulse output from a Q-switched large mode area ytterbium-doped fiber laser.²

Why fiber lasers?
Fiber lasers are attractive because they are more compact than solid-state laser rods, they can be cooled fairly easily because they have a large surface-area to volume ratio, and they can be diode-pumped very efficiently. Commercially-available fiber lasers with a few tens of watts of average power in a good quality beam have already found applications in thermal printing, marking, and other types of materials processing.

New designs for fiber lasers double the record powers demonstrated last year. The novel methods used by the two groups included using large-core fibers that can handle more energy without damage, and incorporating optics to shape the output of a diode bar to pump more efficiently into the cladding of the fiber, allowing multiple passes of the pump light and amplified spontaneous emission through the core.

The SDL group used a ytterbium-doped fiber that lased at 1120 nm with a spectral width of 110 nm. The fiber, produced by SDL Cambridge (formerly Polaroid Fiber Laser Group), had a rectangular 170 X 330 µm inner cladding and a single-mode core with a mode-field diameter of 9.2 µm. The four pump diode bars each emitted about 45 W of 918 nm light, and the fiber was pumped from both ends.

With a total pump power of 180 W, and output up to 110 W, the optical-to-optical efficiency of the system is 58%. The ratio of electrical current-to-optical output was nearly linear across the output range of the laser. At 100 W, the diode bar drive current was 90 A. Operating at 10 W, the system generated a Gaussian beam; output beam quality degraded somewhat at powers near 100 W, probably caused by thermal lensing in an aspheric lens (AL2) which is subject to both the 100-W output of the laser and about 90 W of pump power. The researchers believe that the large surface- area to volume ratio of the fiber helps avoid thermal lensing in the fiber.

Southampton research
The group at Southampton's Q-switched ytterbium-doped fiber laser made use of a large mode area. At 1090 nm, the fiber emitted 2.3 mJ of output pulse energy at a 500 Hz repetition rate. Fabricated at the university, the fiber features a 175 µm x 350 µm rectangular inner cladding of pure silica, and a silicone rubber output cladding. The inner cladding has a NA of 0.4. The inner aluminosilicate core has an NA of 0.075 and a diameter of 44 µm. The combination of large core diameter and low NA reduces the number of guided core-modes, which allows the fiber to store more energy between pulses.

The researchers pumped the fiber with the output of a beam-shaped diode bar at 915 nm. At high powers and repetition rates, the Southampton researchers measured over 5 W of average output power. The system recycled amplified spontaneous emission (ASE) at 1035 nm in the fiber, allowing that energy to contribute to the power buildup in the fiber between pulses. Although the power densities in these experiments are beginning to approach the damage thresholds of the fibers (300 MW/cm² for the SDL group), neither group observed damage.

References
1. Vince Dominic, et. al., "110 W fiber laser," postdeadline paper #CPD11, Proceedings of Conference on Lasers and Electro-Optics; Baltimore, MD; May 23-28.
2. H. Offerhaus, et. al., "Multi-mJ, multiwatt Q-switched fiber laser," postdeadline paper #CPD10, Proceedings of Conference on Lasers and Electro-Optics; Baltimore, MD; May 23-28.

About the author…
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.