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Fibre lasers look to large mode areas

14 Apr 2009

New approaches to obtaining diffraction-limited beams from large-mode-area fibres point to unprecedented ruggedness and output powers. Breck Hitz investigates.

At the turn of the century virtually every laser conference boasted announcements of fibre lasers with diffraction-limited beams offering higher and higher power levels. But during the past four or five years there have been no such announcements. The levels reached in 2004 and 2005 – somewhat over a kilowatt or, in one or two hero experiments, 2 kW – have not been exceeded.

The limiting problem is also the chief advantage of fibre lasers: their very high surface-area-to-volume ratio. Difficulty in extracting waste heat ultimately limits the output power of all conventional optically pumped solid-state lasers. Because a fibre intrinsically has such a large surface-area-to-volume ratio, the overheating issue is removed. But the very geometry that solves one problem produces another.

The metres long, approximately 10 µm diameter core of a singlemode fibre laser requires that significant power densities propagate over large distances, and that is precisely the recipe for nonlinear effects such as stimulated Brillouin scattering, stimulated Raman scattering and self-phase modulation. These nonlinear effects reduce the intracavity laser power and eventually limit the laser's output.

It has long been understood that the solution is to enlarge the core of the fibre, thereby reducing the power density. The drawback is that a large-mode-area (LMA) fibre allows higher order transverse modes to propagate, which diminish the beam quality. So the crux of the issue has been to devise ways of forcing only the fundamental mode to oscillate in LMA fibres.

A handful of techniques have been investigated during the past decade, some of which led to the kilowatt-level results announced several years ago. Some researchers carefully excited only the fundamental mode in an LMA fibre and made sure that it did not couple to higher order modes. Others added intracavity spatial filtering to limit oscillation to a single mode. While these approaches were somewhat successful, they lacked the robustness that was one of the key advantages of fibre lasers.

Other early approaches included coiling the fibre, because bending losses are greater for higher order modes than for the fundamental mode. The advent of photonic crystal fibres brought fibres that were endlessly singlemode – that is, singlemode no matter how large the core. But neither of these techniques provided the stability necessary for real-world applications. Likewise, gain-guided fibre lasers and rod-type fibres were promising but lacked the robustness of conventional fibre lasers.

Today, several new techniques are under investigation that promise to be stable and robust, while limiting oscillation to a single transverse mode in LMA fibre.

Tapered fibres

Oleg Okhotnikov and his colleagues at Tampere University of Technology in Finland, and at the Russian Academy of Sciences in Moscow, are pioneering the development of double-clad, tapered fibres for high-power lasers (figure 1).

The taper serves two purposes. First, it enables radiation from low-brightness sources, such as diode laser bars and even stacks, to be efficiently coupled into the pump cladding at the wider end of the fibre. As the pump radiation propagates toward the narrower end of the fibre, the taper provides an effective mode-mixing mechanism that couples pump energy from weakly absorbed modes into highly absorbed modes, thereby increasing absorption efficiency in the core. The second purpose of the taper is to provide spatial filtering at the narrow end of the core, forcing oscillation of only the fundamental mode.

But as pump radiation propagates from the wider to the narrower end of the fibre, the tapering geometry forces the angle of internal reflection to increase with propagation distance. Eventually, the angle exceeds the critical angle for total internal reflection and some of the pump radiation escapes from the cladding, an effect that is known as vignetting.

Okhotnikov and his colleagues have conducted a careful ray-tracing analysis of their tapered fibres and concluded that the detrimental effect of vignetting can be avoided by careful design. Recent experiments have confirmed this analysis and produced some 750 W of diffraction-limited power from a tapered-fibre laser.

"The limit for fundamental-mode power is probably about a kilowatt," Okhotnikov said, if he were to use a commercially available tapered fibre with a wider end 2 mm in diameter. Output at a kilowatt "is basically limited by the damage of silica glass," he explained. "Further scaling to multi-kilowatt levels would be possible by beam combining."

Leakage-channel fibres

Leakage-channel fibres, under investigation by Liang Dong and his colleagues at IMRA America are a special class of microstructured fibre (figure 2). A few large air holes define the core of these fibres and the geometry is such that there is no total internal reflection inside the core. As a result, all modes travelling in the core are leaky and all power launched into the core will eventually be lost. But with careful design the leakage-channel fibre can be engineered with minimal loss of the fundamental mode in its large core, but significant loss to higher order modes.

Recent experiments at IMRA have focused on picosecond lasers with leakage-channel, polarization-maintaining fibres. Dong and his co-workers substituted two stress elements for a pair of air holes to make the double-clad, ytterbium-doped fibre birefringent and seeded it with a modelocked laser. The leakage-channel fibre's core diameter was 80 µm, which they believe is the largest core yet reported in a polarization-maintaining fibre. The amplifier's singlemode, polarized output consisted of 14.2 ps pulses containing 3.25 µJ at 10 MHz, for a peak power of 230 kW and an average power of 32.9 W.

The group has also experimented with modelocked oscillators using leakage-channel fibres, demonstrating average powers of up to 100 W. So what are the prospects for higher singlemode average powers? "We do not see any problem for kilowatt-level outputs," Dong said confidently.

Chirally coupled fibres

Almantas Galvanauskas and his associates at the University of Michigan in Ann Arbor are investigating chirally coupled fibres. In these fibres, one or more helical cores are wrapped around the large central core (figure 3). By designing the helical core(s) appropriately, the Michigan researchers can phase-match the high-order modes of the central core to the propagating modes of the helix. These high-order modes are effectively coupled into the helical core, where they experience significant loss. Only the fundamental mode propagates with low loss in the central core.

It is worth noting that chirally coupled fibres are not chiral-core fibres. Chiral-core fibres, a previously investigated approach to singlemode LMA fibres, introduce bend loss to high-order modes without the mechanical stress caused by tightly coiling a fibre.

Chirally coupled fibres represent a new class of fibres in which the transverse propagation characteristics depend not only on the fibre's transverse structure, but on its longitudinal structure as well. They are indistinguishable from normal single-mode fibres in terms of mode propagation and, like singlemode fibres, they can easily be spliced with low loss.

In recent work Galvanauskas and his colleagues demonstrated 300 W of average power from a master-oscillator-power-amplifier configuration using an ytterbium-doped, chirally coupled fibre as the power amplifier. The high quality of the beam emerging from the 35 µm diameter power amplifier (M2 = 1.07) was consistent under a variety of operating conditions, indicating that the singlemode output was quite robust.

"We're currently solving routine problems, but we've now demonstrated fibre cores as large as 50 µm and we think we'll soon see kilowatt outputs," commented Galvanauskas.

High-order modes
A somewhat unconventional approach has been pioneered by Siddharth Ramachandran of OFS Laboratories. Rather than force the fibre laser to oscillate in the fundamental mode, he deliberately couples energy into a single high-order mode with a long-period grating. This high-order mode propagates more stably in the LMA fibre than the fundamental mode and is coupled back into a fundamental mode with another grating at the other end of the fibre. In essence, he envisions a fibre laser with the fundamental mode propagating in singlemode fibre at both ends of the laser, and a single high-order mode propagating in the LMA fibre in the middle.

"We expect this to be the most scalable platform for achieving large mode areas in flexible fibres," said Ramachandran. His experiments to date have focused on characterizing the propagation of a single high-order mode in LMA fibre and he has achieved stable propagation of a single mode with effective area up to 3200 µm2 in fibres as long as 50 m.

One important question hinges on the spatial profile of a single high-order mode, which can resemble, for example, a series of concentric circles (figure 4). Does the local power density in one of the circles exceed the threshold for nonlinear effects, even though the average power density does not?

In recent experiments, Ramachandran has shown that the threshold of nonlinear effects scales with the effective mode area, although the threshold for dielectric breakdown apparently depends on the local intensity. But the breakdown should be a constraint only for peak powers in excess of multiple megawatts, he says.

In discussing his work with OLE, Ramachandran was careful to explain the possible pitfalls in moving to active fibres in lasers and amplifiers. "I am not saying there would be problems, or even that we expect problems," he said. But at high power levels, dielectric breakdown in the mode converter and amplified spontaneous emission are potential problems. "I would prefer to say that one can expect this approach to be the most scalable approach to high-power, diffraction-limited fibre lasers," he said.

• This article originally appeared in the April 2009 issue of Optics & Laser Europe magazine.

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