Abstract
Ytterbium-doped photonic bandgap fibers (Yb-PBGFs) used as a means of gain profile engineering have shown excellent performance in laser/amplifier operation at the edge wavelengths of the ytterbium emission spectrum [1-3]. The fiber sources at the long-wavelength region (form 1150 nm to 1200 nm) have been investigated intensively, because there are a variety of applications, e.g., laser guide star and so on, in the yellow-orange region covered by their second harmonic generation (SHG). Typically, Yb-PBGFs are incorporated with some polarization-maintaining mechanisms for SHG [2-3]. In the case of our 1178 nm PBGF (Fig. 1(a) and 1(b)), two B-doped silica rods (B-rods) provide stress induced birefringence on the order of 10−4 and core confinement in the boron axis by total internal reflection as a by-product due to the lower refractive indices of the B-rods. Two different type PBGFs were fabricated with the different orientations of the boron axis against the coil radius: (i) nearly parallel and (ii) nearly orthogonal. The orientation is preserved through the length of the fiber due to a coil control structure (Fig. 1(a)). By comparing these two PBGFs in amplifier experiments, it was found that the amplifier shows different behaviors of the parasitic lasing. In the case of PBGF (i), the onset of parasitic lasing around 1010 nm was obtained (Fig. 1(c)), whereas it was not in the case of PBGF (ii) (Fig. 1(d)). The wavelength was found to be related to the mode in the 4th bandgap from the transmission spectrum measured in a short fiber (Fig. 1(c)). However, it is difficult to experimentally investigate the property of the mode, e.g., bend loss of the mode, because a longer fiber isn’t transparent in the 4th bandgap due to the absorption of ytterbium.
© 2015 IEEE
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