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Defense & Security

Optical fibers for high-power eye-safe lasing applications

A new type of inner cladding allows the fabrication of large-mode-area laser fibers with low numerical aperture and near-diffraction-limited beam quality.
28 October 2007, SPIE Newsroom. DOI: 10.1117/2.1200710.0877

Fiber lasers have recently undergone rapid development, making them attractive candidates for many high power applications in material processing, medicine, spectroscopy, and military countermeasures. Over the past few years, most of the research effort has been directed towards achieving high continuous wave (CW)\ output powers. The recent development of large-mode-area (LMA) Yb-doped double-clad fibers and high brightness diodes with kilowatt-level CW outputs and megawatt-level peak powers in sub-nanosecond pulsed amplifiers accordingly represents a key advance. Of significant interest is that these output powers have been achieved with near diffraction-limited output beam quality. This is because the low numerical aperture (NA) core supports only a few modes and the higher order modes can be easily discriminated against by preferential seeding1 and/or bending.2 However, the development of LMA fibers has until recently been restricted to Yb fibers for use in the 1.0μm region. This is due to the inherent difficulties associated with manufacturing fibers containing relatively high lanthanide-ion dopant concentrations while maintaining a low core NA.

In spite of their numerous advantages, a significant drawback of Yb-based fibers is the relatively high sensitivity of the human eye to wavelengths in their 1.0μm operating range. Hence the interest for developing fibers in the ‘eye-safe’ 1.5–2.0μm-range for important military and commercial applications such as ranging, pollution monitoring, clear-air turbulence analysis,and free-space communications. Furthermore, a number of medical and sensing applications also specifically require lasing output in this wavelength range.

It is well-known that sensitizing Er3+-doped fibers with Yb3+ enhances pump absorption, thus increasing emission efficiency at the Er3+ lasing wavelength, as illustrated in Figure 1(a). Sensitization is accomplished by taking advantage of the broad absorption band and the high cross-section of Yb3+ relative to Er3+.3 Furthermore, energy transfer from Yb3+ to Er3+ can also be enhanced by doping the glass host with phosphorus, which increases its Raman shift. This is due to the presence of P=O bonds that increase the phonon energy of the host, facilitating the rapid depopulation of the 4I11/2 energy level of Er3+, thereby limiting energy back-transfer to the 2F5/2 level of Yb3+. Thus, efficient Er:Yb fibers require substantially high concentrations of Yb3+, Er3+, and P, each of which markedly increases the refractive index of the base glass, resulting in relatively high core NAs (0.17–0.20 or greater).

Figure 1. Simplified energy level diagrams showing possible energy transfer processes in (a) Er:Yb- and (b) Tm-doped fibers.

Commercially-available fiber lasers operating in the 2μm-region are currently based on Tm-doped fibers resonantly pumped at ∼1.6μm by an Er:Yb fiber laser, which is in turn diode-pumped at ∼960nm. The optical-to-optical efficiencies of such devices are typically less than 30%. However, recent advances in the compositional engineering of Tm3+-doped silica fibers have led to substantially higher efficiencies, approaching 65%.4 These improved efficiencies require high Tm concentrations and result from the cross-relaxation processes illustrated in Figure 1(b) that involve the 3H4, 3F4 and 3H6 levels and enhance the 3H63F4 lasing emission by allowing every pump photon (793nm) to generate two signal photons. In this scheme, up-conversion processes, such as 3F43H5 and 3F43H4, have to be minimized to prevent the depopulation of the 3F4 energy level. This can be achieved by preventing clustering of the Tm ions with very high Al:Tm concentration ratios.

Figure 2. Schematic diagram of a large-mode-area fiber using a pedestal design.

Figure 3. The influence of numerical aperture on the number of modes in a 25μm core fiber.

The requirement for these high concentrations however, substantially increases the refractive index of the core compared to pure silica and limits the ability to achieve the low NA required for the fabrication of Tm-LMA fibers, as is the case for Er:Yb co-doped fibers.

The problem can be addressed by incorporating an appropriately sized pedestal index feature around the core (see Figure 2), which makes it possible to reduce the effective core NA.5 The key benefit of this design is clearly the reduced number of core modes, as illustrated in Figure 3 showing that pedestal incorporation reduces the number of modes in a 25μm core fiber from 11 to 4, making it possible to achieve near-diffraction-limited beam quality.

Using this approach, a number of large-core-diameter (typically around 25μm) LMA Tm- and Er:Yb-doped fibers with an effective core NA of 0.1 have been manufactured and commercialized.6 Recent results have also demonstrated the suitability of these large core fibers for scaling to high output powers while maintaining near diffraction-limited beam qualities.7–10 It is therefore anticipated that, just as LMA Yb fibers led to the development of kilowatt lasers operating at ∼ 1.0μm, the availability of LMA Er:Yb- and Tm-doped fibers will also spur the development of high power lasers and amplifiers in the eye-safe 1.5μm and 2.0μm regions.

Adrian Carter
East Granby, CT

Adrian Carter is the founder of Nufern, where he is presently chief technology officer. He received his BS and PhD in physical and theoretical chemistry from the University of Sydney, Australia, and was previously an assistant professor at the Laboratory for Lightwave Technology at Brown University.