SPECIALTY FIBERS: Direct nanoparticle deposition builds active fibers

The first process to exploit nanoparticles, direct nanoparticle deposition can be used to manufacture highly doped, active fibers, producing new levels of performance for high-power fiber lasers and amplifiers.

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The first process to exploit nanoparticles, direct nanoparticle deposition can be used to manufacture highly doped, active fibers, producing new levels of performance for high-power fiber lasers and amplifiers.

MIKKO SÖDERLUND, SIMO TAMMELA, HANNA J. HOFFMAN, BILL WILLSON, AND PER STENIUS

Unique among solid-state lasers, fiber lasers offer a power-scalable and flexible platform for producing high-quality beams with a range of performance characteristics. Fiber lasers can access many spectral regions in continuous-wave (CW), pulsed, and ultrashort-pulse modes, while maintaining high brightness and high average power. Because of their unprecedented efficiency-up to and beyond 80% power conversion-they dissipate less heat than their bulk or disk diode-pumped counterparts, avoiding complex cooling loops and lowering input power requirements. During production, the reduced need for optical alignments translates to lower manufacturing costs.

Overall, the combination of improved performance at higher powers, reliability, operational flexibility, reduced footprint, and significant economies in production and operation make fiber lasers very attractive for applications in nearly all market segments-from military and aerospace to industrial, medical, instrumentation, display, and communications.

In fact, fiber lasers represent such significant value that they are being adopted at a much faster rate than any previous industrial laser technology, including lamp-pumped Nd:YAG or CO2 lasers. Hence, the fiber-laser and amplifier market is expected to expand significantly over the next few years. Worldwide sales of fiber lasers grew more than 50% last year and are expected to grow at almost the same rate again in 2006 to reach $188 million, according to Laser Focus World’s annual market review and forecast (see p. 78). Other market researchers have suggested that global sales of fiber lasers will reach $256 million by 2008 (see Optoelectronics Report, Dec. 1, 2004, p. 1).

Despite the dramatic advances that have been made in the past few years, some challenges still confront the fiber-laser industry. For CW fiber lasers-already on the market and showing excellent performance-a key challenge is to get maximum power from a single fiber while maintaining single-mode operation.

Pulsed and polarization-maintaining applications also place unique demands on active fiber. While numerous demonstrations of master-oscillator power-amplifier (MOPA) configurations show the potential of fiber lasers to produce scalable nanosecond and subnanosecond pulses at variable repetition rates, the pulse energy is limited by nonlinear processes such as stimulated Raman and Brillouin scattering (SRS and SBS).1, 2, 3 These effects can result in power loss and spectral broadening. Short-length, highly doped large-mode-area fibers are necessary to address these challenges.

For many applications, especially those that require harmonic conversion, fibers will also have to maintain a stable, linearly polarized output. The development of highly doped large-mode-area polarization-maintaining (PM) fibers that offer large core-to-clad ratio and low cost represents a growing challenge for active fiber fabrication.

While fiber lasers are making inroads in materials-processing applications, in the defense and security industry with laser ranging and chemical detection, and in the medical segment with optical coherence tomography (OCT) and soft-tissue surgery, performance requirements continue to increase. These requirements include increased peak and/or average power levels, improved thermal efficiency, improved beam quality, linear polarization output, improved product lifetime and reliability, ability to access an array of wavelength regimes (from UV to mid-IR), and, of course, lower cost. A key question is whether fiber fabrication technology will be capable of simultaneously addressing these demanding fiber-laser requirements.

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FIGURE 1. Direct nanoparticle deposition uses rapid single-step doping for accurate radial control of several hundreds of layers of doped material in the preform core.
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Liekki has over the last six years focused on commercializing a new active-fiber fabrication process to meet the requirements of high-power laser applications. The result, direct nanoparticle deposition (DND), represents a significant change in active-fiber fabrication that enables unprecedented doping control and accuracy, extremely high doping levels, short cycle time, and increased production flexibility (see Fig. 1).

Conventional active-fiber fabrication

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FIGURE 2. To produce one doped core layer, standard modified chemical-vapor deposition involves many different steps.
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Modified chemical-vapor deposition (MCVD) with solution doping of solution-phase rare-earth ions is currently the dominant technology used in the active-fiber industry. The process of MCVD was originally developed for telecom active fibers with very small cores (3 to 4 µm) and low rare-earth doping concentrations. While manufacturers have developed unique know-how and process improvements, MCVD remains fundamentally limited by inside-tube soot deposition and the diffusion dynamics of solution doping. A multistep, iterative process, MCVD with solution doping yields a doped core of only 2 to 10 layers (see Fig. 2). The low number of core layers limits the accuracy and flexibility of the doping and refractive-index profiles, and solution doping limits the achievable doping density. Furthermore, the multistep cycle and the diffusion process used for doping increase the throughput time and make fiber-development work slow and expensive.

Direct nanoparticle deposition

In contrast to the multistep MCVD process, DND directly and simultaneously deposits all the elements in nanoparticle size to create preforms in a single step. The end result is a highly doped core composed of hundreds of layers with accurately controlled composition. More important, the doping and the refractive-index profile can be independently tailored to produce an optimized active fiber.

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With DND, fabrication of a preform with precise optical and gain properties can be completed in a matter of hours. Because of DND’s accuracy and control, fabrication of active fibers is also highly repeatable. The use of MCVD solution doping (soaking and diffusion) produces core absorption values up to 1200 dB/m at 976 nm, compared to 2000 dB/m and beyond for the highly controllable and accurate DND process. Furthermore, as a result of this new preform fabrication process, core-to-clad ratios of up to 0.5 are attainable for DND-produced fibers, whereas MCVD-produced fibers are limited to values of 0.16.

In DND, the typical waveguiding materials (silica, alumina, germanium) and the gain materials (erbium, ytterbium, neodymium, thulium) are deposited simultaneously to form the active core. The source materials, which can be standard vapors and/or liquids, are fed through independent and controlled channels into the burner. The atomized particles leaving the burner are uniform in composition and can be tuned in size from 10 to 100 nm. The rapid particle formation and deposition process allows extremely high doping levels with reduced clustering, reduced photodarkening, and a higher fiber damage threshold.4 The DND process is fast, efficient, and particularly well suited for producing large-mode-area double-clad (DC) fibers with a large core-to-clad ratio (for example, a highly ytterbium-doped DC fiber with 20-µm core and 125-µm cladding diameters). The DND process is applicable to single-mode, DC, and DC-PM fiber, as well as to more-complex fiber designs, such as fibers with rectangular (or other noncircular) cores and claddings, multicore fibers, or coupled multiple waveguiding-element fibers.5

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FIGURE 3. An ytterbium (Yb) 20/400‑µm double-clad fiber manufactured with the DND process has excellent beam quality of M2 = 1.1 (top). The DND process extends fiber performance through radial control of the preform fabrication parameters (bottom).
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Accurate control of the refractive index is considered crucial for achieving high output-beam quality at the required numerical-aperture values. Tuning the doping profile allows optimizing the gain for the fundamental mode, while discriminating against higher-order modes, thus extending the single-mode operation regime of the fiber (see Fig. 3). Through independent radial control of both the doping and refractive index, the DND process can further extend active-fiber performance.

With the control and flexibility DND provides, it is possible to integrate new functionality into the active fiber. This functionality includes photosensitivity for direct integration of fiber Bragg gratings into the active core, and unique preform designs integrating the pump-combiner functions (such as side coupling using multiple waveguiding elements fused together). The combination of these features will ultimately result in low-cost, fully functionally integrated, all-glass active fibers for high-power fiber lasers and amplifiers. Such truly monolithic (unspliced) structures provide a common platform for a range of fiber applications and the necessary means to reduce the cost of fiber lasers dramatically.

The rapid development of fiber-laser applications, and continued market growth, is setting new requirements for active fibers (see table, p. 102). Direct nanoparticle deposition represents a new fabrication technology that is producing the next-generation active fibers required to meet these requirements.

REFERENCES

1. R.L. Farrow et al., to be presented at Photonics West 2006.

2. J. Nilsson et al., Optical Communications Conf. technical digest, paper OTuF1 (2005).

3. D.A.V. Kliner et al., Optics Communications210, 393 (2002).

4. J.J. Koponen et al., Proc. SPIE5990, 72 (2005).

5. M.J. Söderlund et al., Proc. SPIE5987, 99 (2005).

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