Finding the Missing Link for High-Power Fiber Lasers

François Gonthier, ITF Optical Technologies Inc.

Despite their record performance, high-power fiber lasers have made little progress in penetrating the marketplace. Fiber lasers must become cost-competitive if they are to replace the existing technologies, and they must be robust and durable. Laboratory high-performance fiber lasers lack these qualities because they use bulk optic coupling to pump light into the fiber.

High-power fiber lasers on the market today employ proprietary fiber-based components or designs that couple the pump power into double-clad fibers. With the development of gain fiber and improvement to laser diodes, access to fiber-based components becomes essential for the commercialization of these lasers.

Fiber laser benefits

Advances in large-mode-area double-clad fibers¹ and high-brightness laser diode sources have pushed output power levels into the kilowatt range for CW fiber lasers^2,3 and into the megawatt range for pulsed fiber lasers.⁴ These achievements have put fiber lasers on a par with — or on a higher level than — some traditional YAG-rod lasers and even the more advanced disk lasers. This opens up potential use in printing, micromachining, cutting, welding and numerous other industrial applications reserved for YAG or even CO₂ lasers.

Furthermore, fiber lasers offer a choice of operating wavelengths.⁵ In addition to the 1.06-μm wavelength region, they can be used as sources for range-finding equipment in the eye-safe 1.55-μm region. Thulium-doped fiber lasers cover wavelengths even farther into the infrared, extending to around 1.9 μm. The low weight of the fiber structure and the better electrical efficiency also give the fiber laser an advantage in portable applications.

Despite these advantages, powers in fiber lasers have not yet reached the bulk glass damage threshold or the self-focusing limit, two limitations that ultimately will affect their performance.⁶ Although these and other hindrances, such as Brillouin and Raman nonlinear phenomena, increase the complexity of design, there still exists room to improve performance over the next few years.

To compete for market share with older laser technologies, fiber lasers will turn to All-Fiber components. In the optical fiber engine, the output of the fiber coil is connected to a (6+1) × 1 combiner, which enables the coupling of the six pump fibers and outputs the signal using a large-core feed-through. Such amplifiers can be cascaded to produce megawatt pulses and can be customized to yield the desired power level.

To overcome these limitations, ITF Optical Technologies Inc. has developed what it calls All-Fiber components. If they are to replace the bulk optic parts, these components must perform several functions, such as combining the pump power into the double-clad fiber with or without having a signal feed-through, adapting to mode size to match different fiber cores, and managing excess pump power and reflecting signal (in the case of laser cavities). In addition, they must allow high-power large-core fiber tap couplers and high-power splitters to be added.

Enables high power

These fiber components have demonstrated their ability to perform all of these functions and to offer benefits such as very low loss, compactness and fiber splicing to mitigate misalignment of the optics. The insertion loss is less than 2 percent in the case of coupling pump power and less than 5 percent in coupling signal, compared with 10 to 20 percent losses for bulk optic assemblies, and this directly improves the efficiency of the fiber laser or amplifier. Properly packaged, these components have been able to operate at very high powers, supporting up to 200 W in CW output⁷ and hundreds of kilowatts of peak power.

At these power levels, care must be applied when assembling a fiber gain block, composed of the input and output components spliced to the gain fiber coil. In an amplifier configuration, this assembly can be used to amplify a CW laser source, but it is more often used for pulsed systems.

How they work

In the counter-pump configuration (see figure), the seed is inserted through a mode adapter into the gain fiber. In this example, the output of the coil is connected to a (6+1) × 1 combiner, which enables the coupling of the six pump fibers and outputs the signal using a large-core feed-through. Such amplifiers can be cascaded to produce pulses with potential peak powers in the megawatt range. The fiber cores are enlarged at each amplifier stage to minimize nonlinear effects.⁸

To transform the gain assembly into a laser, a cavity is formed by adding two large-mode-area fiber Bragg gratings, and the pumps are coupled through a multimode combiner; i.e., 7 × 1. By changing the components and the gain fiber, such an assembly can be customized for specific applications or power levels.

Because the coil is larger than the components, the size of the module in which the fiber laser or amplifier is packaged can be determined by the diameter and height of the coil. And because the fiber surface is a few hundred microns from the core, heat can be extracted easily — and, most often, passively — when the core heats up, which occurs when the laser is pumped with kilowatts of power.

This optical fiber engine becomes a very interesting building block for a commercial fiber laser, replacing the traditional laser core. It is very robust, with the spliced fibers preventing any misalignment of the optics. An integrated or a separate diode module can provide the pump power, and the laser output can be suitably interfaced for the final application.

Because these fiber module assemblies basically are an offshoot of the telecommunications designs that were produced in large quantities a few years ago, the assembly equipment and the technology remain available. Optical fiber engines thus can be mass-produced and properly tested even at high power, requiring only a few splices to hook them up to the test station.

This, in turn, reduces the cost. Also, with the price of fiber-pigtailed multimode laser diodes or laser diode bars decreasing to, for example, less than $100 per watt, larger production volumes will make high-power fiber lasers competitive with traditional lasers.

In the near future, the reliability data of All-Fiber components and the described fiber module will become available, giving this technology a place in the market as it replaces older technologies or enables new applications.

References

1. D. Machewirth et al (2004). Large-mode-area double-clad fibers for pulsed and CW lasers and amplifiers. Proc. SPIE, pp. 140-150.

2. Y. Jeong et al (2004). Ytterbium-doped double-clad large-core fiber lasers with kW-level continuous-wave output power. CLEO.

3. C.-H. Liu et al (2004). 700-W single transverse mode Yb-doped fiber laser. CLEO.

4. J.W. Nicholson and A. Yablon (2004). A high power, single-mode, erbium-doped fiber amplifier generating 30 fs pulses with 160 kW peak power. CLEO.

5. J. Nilsson et al (2003). High power fiber lasers: New developments. Proc. SPIE, pp. 50-59.

6. A. Galvanauskas (July 2004). High power fiber lasers. Optics & Photonics News, pp. 42-47.

7. François Gonthier et al (2004). High-power All-Fiber components: the missing link for high-power fiber lasers. Proc. SPIE, pp. 266-276.

8. M.Y. Chen et al (2004). 27 mJ nanosecond pulses in M2 = 6.5 beam from coiled highly multimode Yb-doped fiber amplifiers. CLEO.

Meet the author

François Gonthier is the founder and chief technology officer at ITF Optical Technologies Inc. in Montreal; e-mail: fgonthier@itfoptical.com.