Abstract

Holey optical fibers (HFs) can offer novel optical properties that cannot be obtained by conventional fibers. They can be made of a single material, and can exhibit broadband single modeness, 1 thereby allowing an alternative route for large mode area fibers (LMAs).² This is particularly attractive for high power active devices. Here, a novel all glas cladding-pumped Yb³⁺ -doped HF is reported, where different sizes of air holes are arranged to form a double-clad structure as shown in Fig. 1 (also see ref. 3). However, when both rare-earth and charge-compensating dopants are incorporated in a HF core, the refractive index of the core is naturally altered. This affects the core NA, and broadband single mode operation can be lost. We use a conventional low-NA LMA fiber preform (NA~0.05, Yb³⁺ concentration ~3000 ppm by weight) to overcome this issue (cf. approach used in⁴). Note that with this low value of NA, light is no longer guided in a conventional fiber form in practice due to extreme bend loss. Therefore, the holes in the inner-cladding of the HF (see Fig. 41) dominate the light guidance, while single mode operation is also retained. The fiber outer diameter is 175 µm. The hole diameter d and distance between two adjacent air holes Λ are 2.7 µm and 9.9 µm, respectively. The inner-cladding diameter is ~ 140 µm, while the thickness of the outer-cladding is ~ 2Λ. The area of the doped region is πΛ²/4 = 77 µm². Since the effective mode area was measured to be ~ 120 µm² at 1047 nm, the modal overlap with the doped region is more than 60%. Note that an off-centered core is deliberately introduced in order to enhance the pump absorption.

© 2002 Optical Society of America