China achieves 10 kW optic fiber lasers
Baku, April 26 (AZERTAC). Highly thulium-doped silicate glass fibers can be used to develop single-frequency fiber lasers with 1.9-2.1 μm operating wavelengths, and Q-switched fiber lasers and self-starting passively modelocked fiber lasers of up to 10 kW peak power.
The slope efficiency of a thulium (Tm) doped fiber laser can exceed the Stokes limit, enabling a quantum efficiency near 200% for the 3F4→3H6 transition because of the so-called cross-relaxation energy transfer between Tm ions. Using our highly Tm3+-doped (3-7 wt%) silicate glass fibers fabricated by the rod-in-tube technique, laser gain fibers with high slope efficiency of 68.2% and a gain per unit length of >2 dB/cm have allowed us to demonstrate Q-switched fiber lasers and modelocked fiber lasers operating from 1.9-2.1 μm wavelengths based on Tm-doped silicate fibers with peak power levels up to 10 kW.
One popular application of a 2 μm fiber laser with high peak power and high beam quality (from a singlemode fiber) is as a pump source for mid-infrared (mid-IR) frequency generation via nonlinear optical conversion. Another emerging application is for mid-IR supercontinuum generation.
Compared to their 1 μm and 1.55 μm counterparts, 2 μm laser sources are more favorable pump sources for mid-IR generation for several reasons. First, quantum defects are lower at 2 μm, yielding higher quantum efficiency for generating mid-IR wavelengths. Second, many nonlinear crystals used for mid-IR generation are not transparent or have a much higher absorption at pump wavelengths shorter than 2 μm. Third, dispersion is too high at shorter wavelengths in some nonlinear crystals to achieve phase-matching for nonlinear parametric processes. And finally, 2 μm fibers can have larger core sizes and higher nonlinear thresholds, enabling higher-power 2 μm lasers and consequently, higher-power mid-IR output.
oth singlemode and large-mode-area (LMA) double-cladding silicate glass fibers are fabricated using rod-in-tube fiber drawing techniques. The inner cladding of these active fibers is surrounded by a layer of outer cladding silicate glass with numerical aperture (NA) as large as 0.6 to confine the multimode pump laser beam in the inner cladding region. The outer cladding allows for higher-power-handling capability than standard polymer-based double-cladding silica fibers.
A low-index silicate rod is usually inserted into the inner cladding for the enhancement of cladding pump absorption. In a similar way, we also fabricate polarization-maintaining heavily Tm3+-doped silicate glass fibers for cladding-pumped laser operation with slope efficiency values greater than 68%.
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