115 W fiber laser with an all solid-structure and a large-mode-area multicore fiber.

We investigate mode-area scaling by means of supermode operation in an all-solid multicore fiber. To obtain a large-mode area (LMA), we designed and fabricated an active double-clad multicore fiber, where each ytterbium-doped core is 19 µm in diameter and has a numerical aperture of 0.067, comparable to the core of the largest available commercial LMA fibers. The six large cores are stacked tightly in a ring structure to enable phase locking of the core fields and supermode operation. The fiber laser performance was investigated in a linear laser cavity with an external Talbot resonator for mode selection. The highest output power achieved was 115 W with an overall 61% slope efficiency corresponding to the pump power. The measured M2 was 1.43 for the central lobe with nearly 70% of the total power.

Step-index high-absorption Yb-doped large-mode-area fiber with Ge-doped raised cladding.

We report an all-solid large-mode-area (LMA) step-index fiber offering high absorption and low core numerical aperture (NA) by introducing a highly ytterbium-doped P:Al core and germanium-doped cladding. The fiber provides core absorption of ∼1200 dB/m at 976 nm with a low 0.07 core NA, due to the raised Ge cladding. Furthermore, matched profiles of P and Al across the core are successfully obtained with high concentration of Yb2O3 around 0.4 mol%. The fiber characteristics are routinely achievable by conventional modified chemical vapor deposition with a solution doping technique. A highly efficient laser with >100 W output power, 86% slope efficiency with respect to launched pump power, and a mean M2 of 1.34 has been demonstrated using the fabricated LMA step-index fiber. We also report 80% laser slope efficiency with 58 W output power (pump power limited) within only 0.5 m of the fiber when pumped by a wavelength-stabilized laser diode.

Bendable large-mode-area fiber with a non-circular core: publisher's note.

This publisher's note amends the spelling of the second author's name in Appl. Opt.57, 6388 (2018)APOPAI0003-693510.1364/AO.57.006388.

Bendable large-mode-area fiber with a non-circular core.

We investigate mode-area-scaling and bending performances of a Yb-doped large-mode-area fiber with an elongated non-circular core. Such fiber can be bent in the plane of its short axis to suppress bending effects, such as mode area reduction and mode profile distortion. Meanwhile, the other orthogonal axis can be stretched for mode area scaling. Calculations show that for fibers with the same mode area, the higher the aspect ratio between the long axis and short axis, the less sensitive the fiber will be to bending effects. However, mode area scaling is limited by the increased beat length (BL) between the fundamental mode (FM) and the second-order mode, leading to mode degeneracy at higher aspect ratios. Within the 100 mm BL, the FM area is scalable to 3000 µm2 in a bent fiber. To facilitate FM operation, we study mode-selective gain through confined doping. Thanks to the small bending distortions, the confined-doping approach works well in the bent large-mode-area fiber. In addition, the advantage of tandem pumping is also discussed in terms of preferential modal gain. A non-circular core fiber with a 41 µm short axis and 120 µm long axis was fabricated in-house. We evaluated the fiber in a linear laser cavity pumped by a 975 nm laser diode. The maximum output power obtained was 191 W, with slope efficiency of approximately 67% with respect to launched pump power. The output signal has good beam qualities with M2 of ∼1.5 and ∼3.1, respectively, along the short and long axis.

1867-2010 nm tunable femtosecond thulium-doped all-fiber laser.

We propose and demonstrate a broadly wavelength tunable mode-locked thulium-doped all-fiber laser, delivering sub-400 femtosecond (fs) pulses and tuning from 1867 nm to 2010 nm. The tunable range of 143 nm, to our best knowledge, is the widest in the femtosecond mode-locked thulium-doped fiber lasers (TDFLs). The broadly tunable range and femtosecond pulse oscillation attribute to the large free spectral range and bandwidth of the fiber-based Lyot filter, benefiting from the hybrid device based compact configuration. The hybrid device, integrating wavelength-division-multiplexer, polarization-sensitive isolator and output coupler, effectively diminishes the birefringence and net dispersion.

High-efficiency femtosecond Raman soliton generation with a tunable wavelength beyond 2 µm.

We report an efficient Raman soliton laser system by optimizing the chirp of input pulses. The highest efficiency of energy transferred to a Raman soliton is up to 97%, and the tunable range is from 1.98 to 2.31 µm. The pulse width of these tunable Raman solitons remains below 200 fs. The efficiency and the wavelength range are mainly limited by the background loss of the silica fiber.

Multi-trench fiber with four gaps for improved bend performance.

A modified multi-trench fiber (MTF) design with gaps to create leakage channels is proposed and investigated numerically using the scalar finite-difference beam-propagation method algorithm. Great potential in single-mode operation, mode area enlargement, and resistance to bending is demonstrated. A high loss ratio (>50) between high-order modes and the fundamental mode is possible over a wide range of high-index ring thickness, gap width, and bending orientation. This reduces the required fabrication accuracy. We obtain an effective area of 920 µm2 at a wavelength of 1050 nm and a 20 cm bend radius with a high loss ratio (>100). Our modified gap MTF can possibly be fabricated by drilling holes in an ordinary MTF and inserting rods into the holes.

Histological Variation in Blubber Morphology of the Endangered East Asian Finless Porpoise (Neophocaena asiaeorientalis sunameri) with Ontogeny and Reproductive States.

The highly lipid-rich blubber in the hypodermis is a specialized structure that functions in thermoregulation, energy storage, buoyancy control, locomotion, and streamlining the body shape in marine mammals. The key objective of this study was to investigate blubber development in the East Asian Finless Porpoise (Neophocaena asiaeorientalis sunameri) across the ontogenetic (fetuses, calves, and adults) and reproductive states (adults, pregnant, and lactating). Blubber samples were collected from East Asian Finless Porpoises (EAFP) that were accidentally caught in the fishing nets in the Bohai/Yellow Sea from late April to mid May of 2015. The mean blubber depth was significantly thinner in fetuses across the ontogenetic groups and significantly thicker in pregnant and lactating vs. adult females across the reproductive states. Across the four regions in each group, we did not find significant variations in blubber depth. However, the correlations between body length and weight vs. blubber depth was significant. Histological observation identified three layers of stratified blubber with a significantly smaller adipocyte cell size in fetuses and a significantly higher area ratio of structural fiber in the middle and inner layers across the ontogenetic groups. Across the reproductive states, we did not observe a statistically significant difference in the adipocyte cell size or area ratio of the structural fiber. Our results suggest that prenatal blubber growth is characterized by an increase in the adipocyte cell count, while postnatal growth is the result of an increase in cell size. They also indicate that ontogeny can affect blubber depths and cellular measurements in the EAFP.

Topographical distribution of blubber in finless porpoises (Neophocaena asiaeorientalis sunameri): a result from adapting to living in coastal waters.

BACKGROUND: Blubber has many functions, among which energy storage, thermoregulation, buoyancy, and hydrodynamic streamlining are the most frequently cited. Within and between taxa, variations in its structure and distribution likely reflect different adaptations of a species to its life history requirements, environment, health, and function. Here, we use ultrasound to describe the distribution of blubber in the finless porpoise (Neophocaena asiaeorientalis sunameri) based on examinations of 34 fresh cadavers recovered as accidental fisheries bycatch. RESULTS: Measurements of blubber depth determined by ultrasound positively correlated with conventional measurements using a scalpel and calipers. Whereas conventional surgical incision and visual examination revealed two layers of blubber, ultrasound revealed up to three layers; thus, ultrasound reveals additional structural detail in blubber while crude necropsy techniques do not. Across life history categories, ultrasound revealed the distribution of inner blubber to be topographically consistent with that of full-depth blubber. Blubber in the dorsal region was stratified into three layers and was significantly thicker than that in the lateral and ventral regions, in which a middle layer was normally absent. CONCLUSIONS: Ultrasoundprovides a fast, effective, and accurate means to determine blubber thickness and structure, and thus, assessment of the health of fresh finless porpoise carcasses. Blubber depth is determined largely by the thickness of the inner and middle layers, wherein lipids are concentrated. The thickening of blubber in the dorsal thoracic-abdominal region suggests multiple roles of thermal insulation, lipid storage, and, we speculate, to facilitate vertical stability in the complex shallow and estuarine waters in which this animal absent of a dorsal fin occurs.