In this work, to scale laser power while suppressing NL effects, we designed and fabricated a fiber triple-clad Ytterbium doped fiber with large core diameter by using MCVD combined with all-gas-phase doping method. To modify the PD performance of the fiber, P、Al and F were co-doped in the fiber. An excellent laser performance could be demonstrated for such a fiber with an output power of more than 10 kW and a slope efficiency of 82%. Long-term laser stability at 10 kW-level was carried out in a master oscillator power amplifier (MOPA) laser system for 500 hours with power degradation less than 1%.
With the rapid development of domestic fiber lasers, the output power is constantly improving, resulting that the requirement of active fiber is becoming stricter. YOFC company has proposed a kind of triple cladding active fiber fabrication technology, which combines Plasma Chemical Vapor Deposition (PCVD) platform and Modified Chemical Vapor Deposition (MCVD) platform to make preform which has several characteristics like, large size, high concentration doping, long effective length, high uniformity, Fluorine doped outer cladding and so on. Special fiber drawing technique has been used to match this special preform, finally obtained triple cladding Ytterbium doped fiber (TC YDF) with excellent performance such as, low background loss which is less than 10 dB/km, high doping concentration of rare earth, high uniformity and strong resistance to the photo-darkening; The resonant cavity which consist of this TC YDF can obtained laser power up to 3550w, simultaneously the optical-to optical conversion efficiency is up to 78%, the Raman scattering suppression ratio is larger than 35dB. The fiber laser is continuously monitored for 1500 hours at 3550w, the results show that the output power are nearly stable, which indicates that this TC YDF has an excellent long-term reliability and can be used in industrial processing stably on a large scale.
We experimentally demonstrated a temperature sensor by selectively infiltrating refractive index liquid (RIL) in the central air hole of a twin-core photonic crystal fiber (PCF). The selective liquid infiltration was realized by putting the PCF in the bulk liquid with its central air hole open for liquid infiltration using capillary effect. The rest of the air holes were covered by UV glue. The three-dimensional stage was used to translate the PCF, and a tapered SMF was used to transfer the UV glue to cover the air holes under the microscope. After UV glue was solidified by exposing the fiber end under the UV light, the PCF end was dipped into the RIL to fill the central air hole by capillary effect. Due to the large thermos-optic coefficient of the RIL, the RIL filled air channel would act as a liquid core whose core modes would be highly dependent on the ambient temperature. The core modes of the RIL filled core would be phase matched to the fundamental mode of the two solid cores. Therefore, the phase matching wavelengths for the mode coupling among the two solid cores of PCF and the liquid core were highly temperature sensitive. The resonant dips in the transmission spectrum were measured to estimate the temperature sensitivity. The experiment used a commercially available twin core PCF, and blocked all but the central airhole at one of the ends facets by UV glue (NOA81, Norland), the remaining open holes are infiltrated over a length of 10 cm by capillary force with fluid (Cargille Laboratories Inc. index-matching fluid, series A) that possesses a refractive index of 1.46 at 589.3nm, 25°C, and the thermal coefficient is – 0.000389 RIU/°C, which is around 10 times of that coefficient of silica. Any temperature induced changes will have an influence on the propagation properties because of the highly temperature response of the refractive index of the fluid. With a fiber cleaver, a 1.8 cm long PCF was then cut from the longer length of the partially liquid filled twin core PCF and then fusion spliced with SMFs at both ends to observe its transmission spectrum when the sample is heated. When the temperature increase to around 54°C, the liquid RI drops quickly to match that of silica, and a three parallel waveguide structure is formed, in which the central liquid waveguide have the same index value with the two solid core. Because of the small separation between adjacent waveguides, a strong mode-field overlap occurs, which leads to a significant enhancement of the coupling coefficient, therefore, light energy can be easily transferred between two solid cores and the liquid rod in a short coupling length. Thus, the transmission spectrum of the device contain two sets of interference fringe pattern, the large spectrum envelope originated from the interference between the three eigenmodes generated by three-parallel waveguide structure based on the mode coupling theory, and fine interference fringes generated by the interference between the higher order modes in one core. By tracking the dip wavelength shift of the large spectrum envelope, the sensor exhibited a high temperature sensitivity of up to 37.011 nm/°C within the temperature range from 53.8°C to 55°C due to the satisfaction of phase match condition, and maintain a high sensitivity of 19.681 nm/°C from 55°C to 58.2°C, which is benefit from the high thermal optic coefficient of the selectively filled liquid in the twin core PCF.
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