Two-Micron Fiber Laser Has Potential for
Military and Civilian Applications

Breck Hitz

Lasers emitting in the two-micron spectral region have important applications in medicine, spectroscopy and military countermeasures. Traditionally, these applications have relied on holmium-doped crystalline YAG or YLF lasers, but these require cryogenic cooling or complex pumping schemes that limit their usefulness.

Fiber lasers are a potential alternative, because robust fiber lasers with kilowatt-level outputs at slightly shorter wavelengths (i.e., 1 to 1.5 μm) have proven very versatile in numerous applications. Until now, however, practical considerations have limited holmium-doped fiber lasers to modest output powers.

Recently, scientists at the University of Sydney and at the Defence Science and Technology Organisation in Edinburgh, both in Australia, demonstrated a holmium-doped silica fiber laser that produced 83 W at 2.1 μm and that had a beam quality (M²) of 1.5.

They obtained these results with a water-cooled double-clad holmium-doped fiber that was sensitized with a low-level thulium codoping. The fiber’s 300-μm-diameter, hexagonal inner cladding had a numerical aperture of about 0.4 to accommodate the low-quality pump beam from a diode-laser module. By winding the fiber into a 5-cm-radius coil, the scientists restricted its 20-μm core to guiding only low-order modes.

They water-cooled the fiber and pumped it from both ends with 793-nm light from a pair of diode-laser bars (Figure 1).

Figure 1. The fiber-laser resonator was defined by a dichroic mirror at one end and the Fresnel reflection from the fiber facet at the other. Images reprinted with permission of Optics Letters. (HR = Highly reflecting, HT = highly transmitting)

The maximum output of 83 W at 2105 nm occurred with approximately 210 W of pump power launched into the fiber, for a slope efficiency of 42 percent with respect to launched pump power (Figure 2). At greater pump power, the fiber facets started to melt, despite the water-cooled chucks holding them.

Figure 2. After reaching threshold at ~10 W of launched pump power, the laser generated up to 83 W with a slope efficiency of 42 percent. Catastrophic damage (melting) occurred at the fiber’s end facets at higher pump power.

As the laser’s output power increased, its spectrum widened and shifted to longer wavelengths (Figure 3). The investigators calculated that, at the higher output shown in Figure 3 (65 W), thermal loading on the fiber was approximately 31 W/m. Even though the fiber was water-cooled, this thermal loading heated it sufficiently to produce thermal population of the low-lying Stark levels of the lower laser level, thereby shifting emission to longer wavelengths.

Figure 3. As the laser’s output increased, its spectrum broadened and shifted to longer wavelengths.

At an intermediate power level (55 W), the laser’s temporal stability was about two percent over a 39-s period. The scientists concluded that the dominant noise source, at about 100 kHz, was the result of relaxation oscillations as energy oscillated between the laser’s population inversion and circulating power.

Optics Letters, Feb. 1, 2007, pp. 241-243.

Two-Micron Fiber Laser Has Potential for Military and Civilian Applications

Two-Micron Fiber Laser Has Potential for
Military and Civilian Applications