Single-mode operation in a large-mode-area fiber laser is highly desired for power scaling. We have, for the first time, demonstrated a 50μm-core-diameter Yb-doped all-solid photonic bandgap fiber laser with a mode area over 4 times that of the previous demonstration. 75W output power has been generated with a diffraction-limited beam and an efficiency of 70% relative to the launched pump power. We have also experimentally confirmed that a robust single-mode regime exists near the high frequency edge of the bandgap. These fibers only guide light within the bandgap over a narrow spectral range, which is essential for lasing far from the gain peak and suppression of stimulated Raman scattering. This work demonstrates the strong potential for mode area scaling of in single-mode all-solid photonic bandgap fibers.

Gels of agar, starch, ovalbumin, gelatin and an industrial ␤-lactoglobulin protein isolate, were frozen conventionally in a Ϫ30 ЊC freezer and by pressure-shift freezing at 200 MPa at Ϫ15 ЊC. Thawing was carried out conventionally at 20 ЊC and by the application of a pressure of 200 MPa. The microscopic structure and mechanical properties of the thawed gels were compared with those of the initial gels. Microscopic examination showed that pressure-shift freezing produces smaller and more uniform ice crystal damage than conventional freezing at Ϫ30 ЊC. The results also suggest that the freeze-thaw behaviour of food gels can be categorized into two general types: (1) gels which have a reduced gel strength as a result of mechanical damage to the gel microstructure caused by ice crystal formation, and (2) gels which have an enhanced gel strength, as a result of molecular structural changes that take place in the frozen state. Agar and gelatin were found to be typical of type (1) gels, whereas starch, ␤-lactoglobulin protein isolate and ovalbumin were found to be typical of type (2) gels. In the case of starch, retrogradation during thawing was found to be the most important factor. GelatinGelatin (Sigma, type A: porcine skin ~175 bloom) gels were prepared by initially dispersing in water and then stirring at 80 ЊC for 15 minutes. The solutions were poured into aluminium coated Pressure-shift freezing of model food gels M. T. Kalichevsky-Dong et al.

There has been very little progress in the power scaling of Er/Yb fiber lasers in over a decade, reflecting the difficulties involved. Here we report, to the best of our knowledge, a new record of 302 W single-mode power from an Er/Yb fiber master oscillator power amplifier (MOPA) with a record optical efficiency of 56%, near the quantum limit. This is made possible by new fiber development from Nufern and off-resonant pumping of the Er/Yb fiber. We also show that further power scaling is no longer limited by ytterbium parasitic lasing but by fiber fuse in the Er/Yb fiber.

Conventional models of Er/Yb co-doped fibers assume all ytterbium ions are equally involved in the energy transfer with erbium ions, governed by a singular transfer rate. This would predict output power clamping once ytterbium parasitic lasing starts, contrary to the observations that the output continued to grow albeit at a slower rate. One study explained this using elevated temperature at high powers. Our study, however, shows that elevated temperature and mode-dependent effects only play insignificant roles. A new model is developed based on the existence of isolated ytterbium ions, which can explain all the observed experimental behaviors.

We demonstrate for the first time a flat-top mode from a 50 µm-core Yb-doped leakage channel fiber (LCF). The flat intensity distribution leads to an effective mode area of ~1880 µm(2) in the straight fiber, an over 50% increase comparing to that of regular LCF with the same core diameter. The flat-top mode was achieved by using a uniform Yb-doped silica glass in the core center with an index of ~2 × 10(-4) lower than that of the silica background. The fiber was also tested in a laser configuration, demonstrating an optical-to-optical efficiency of ~77% at 1026 nm with respect to the pump at 975 nm.