Yb-doped large mode area fiber for beam quality improvement using local adiabatic tapers with reduced dopant diffusion

A newly designed all-solid step-index Yb-doped aluminosilicate large mode area fiber for achieving high peak power at near diffraction limited beam quality with local adiabatic tapering is presented. The 45µm diameter fiber core and pump cladding consist of active/passively doped aluminosilicate glass produced by powder sinter technology (REPUSIL). A deliberate combination of innovative cladding and core materials was aspired to achieve low processing temperature reducing dopant diffusion during fiber fabrication, tapering and splicing. By developing a short adiabatic taper, robust seed coupling is achieved by using this Yb-doped LMA fiber as final stage of a nanosecond fiber Master Oscillator Power Amplifier (MOPA) system while maintaining near diffraction limited beam quality by preferential excitation of the fundamental mode. After application of a fiber-based endcap, the peak power could be scaled up to 375 kW with high beam quality and a measured M2 value of 1.3~1.7.A newly designed all-solid step-index Yb-doped aluminosilicate large mode area fiber for achieving high peak power at near diffraction limited beam quality with local adiabatic tapering is presented. The 45µm diameter fiber core and pump cladding consist of active/passively doped aluminosilicate glass produced by powder sinter technology (REPUSIL). A deliberate combination of innovative cladding and core materials was aspired to achieve low processing temperature reducing dopant diffusion during fiber fabrication, tapering and splicing. By developing a short adiabatic taper, robust seed coupling is achieved by using this Yb-doped LMA fiber as final stage of a nanosecond fiber Master Oscillator Power Amplifier (MOPA) system while maintaining near diffraction limited beam quality by preferential excitation of the fundamental mode. After application of a fiber-based endcap, the peak power could be scaled up to 375 kW with high beam quality and a measured M2 value of 1.3~1.7

Mode Evolution in Fiber Based Devices for Optical Communication Systems

Space division multiplexing (SDM) is the most promising way of increasing the capacity of a single fiber. To enable the few mode fiber (FMF) or multi-mode fiber (MMF) transmission system, several major challenges have to be overcome. One is the urgent need of ideal mode multiplexer, the second is the perfect amplification for all spatial modes, another one is the modal delay spread (MDS) due to group velocity difference of spatial modes. The main subject of this dissertation is to model, fabricate and characterize the mode multiplexer for FMF transmission. First, we designed a novel resonant mode coupler (structured directional coupler pair). After that, we studied the adiabatic mode multiplexer (photonic lantern). 6-mode photonic lantern using graded-index (GI) MMFs is proposed and demonstrated, which alleviates the adiabatic require-ment and improves mode selectivity. Then, 10-mode photonic lantern is demonstrated using novel double cladding micro-structured drilling-hole preform, which alleviates the adiabatic requirement and demonstrate a feasible way to scale up the lantern modes. Also, multi-mode photonic lantern is studied for high order input modes. In addition, for the perfect amplification of the modes, cladding pump method is demonstrated. The mode selective lantern designed and fabricated can be used for the characterization of few mode amplifier with swept wavelength interferometer (SWI). Also, we demonstrated the application of the use of the few mode amplifier for the turbulence-resisted preamplified receiver. Besides, for the reduction of MDS, the long period grating for introducing strong mode mixing is demonstrated

Control of complex structural geometry in optical fibre drawing

Drawing of standard telecommunication-type optical fibres has been optimised in terms of optical and physical properties. Specialty fibres, however, typically have more complex dopant profiles. Designs with high dopant concentrations and multidoping are common, making control of the fabrication process particularly important. In photonic crystal fibres (PCF) the inclusion of air-structures imposes a new challenge for the drawing process. The aim of this study is to gain profound insight into the behaviour of complex optical fibre structures during the final fabrication step, fibre drawing. Two types of optical fibre, namely conventional silica fibres and PCFs, were studied. Germanium and fluorine diffusion during drawing was studied experimentally and a numerical analysis was performed of the effects of drawing parameters on diffusion. An experimental study of geometry control of PCFs during drawing was conducted with emphasis given to the control of hole size. The effects of the various drawing parameters and their suitability for controlling the air-structure was studied. The effect of air-structures on heat transfer in PCFs was studied using computational fluid dynamics techniques. Both germanium and fluorine were found to diffuse at high temperature and low draw speed. A diffusion coefficent for germanium was determined and simulations showed that most diffusion occurred in the neck-down region. Draw temperature and preform feed rate had a comparable effect on diffusion. The hole size in PCFs was shown to depend on the draw temperature, preform feed rate and the preform internal pressure. Pressure was shown to be the most promising parameter for on-line control of the hole size. Heat transfer simulations showed that the air-structure had a significant effect on the temperature profile of the structure. It was also shown that the preform heating time was either increased or reduced compared to a solid structure and depended on the air-fraction

Novel Fibers and Components for Space Division Multiplexing Technologies

Passive devices and amplifiers for space division multiplexing are key components for future deployment of this technology and for the development of new applications exploring the spatial diversity of light. Some important devices include photonic lantern (PL) mode multiplexers supporting several modes, fan-in/fan-out (FIFO) devices for multicore fibers (MCFs), and multimode amplifiers capable of amplifying several modes with low differential modal gain penalty. All these components are required to overcome the capacity limit of single mode fiber (SMF) communication systems, driven by the growing data capacity demand. In this dissertation I propose and develop different passive components and amplifiers for space division multiplexing technologies, including PL mode multiplexers with low insertion loss and low mode dependent loss to excite different number of modes into few mode fibers (FMFs). I demonstrate a PL with a graded index core that better matches the mode profiles of a graded index FMF supporting six spatial modes with mode dependent loss (MDL) ranging from 2- to 3-dB over the entire C-band. Multicore fibers can alleviate the capacity limit of single mode fibers by placing multiple single mode cores within the same fiber cladding. However, interfacing single mode fibers to MCFs can be challenging due to physical limitations, in this dissertation I develop and fabricate different types of FIFO devices to couple light into MCFs with high efficiency and having up to 19 cores. I demonstrate high coupling efficiency with insertion loss below 0.5 dB per FIFO into a 4-core MCF and below 1 dB for a 19-core MCF. Multimode erbium doped fiber (EDF) amplifiers are required to amplify each mode within the few mode transmission fiber, the main challenge is to provide an amplifier with low differential modal gain, in this dissertation I present the first coupled-core amplifier concept compatible with FMFs. A 6-core coupled-core EDF can be spliced with low insertion and low MDL to a FMF supporting 6 spatial modes via a slight taper transition. The amplifier introduces 1.8 MDL with gain variation over the entire C-band below 1-dB

Control of complex structural geometry in optical fibre drawing

Drawing of standard telecommunication-type optical fibres has been optimised in terms of optical and physical properties. Specialty fibres, however, typically have more complex dopant profiles. Designs with high dopant concentrations and multidoping are common, making control of the fabrication process particularly important. In photonic crystal fibres (PCF) the inclusion of air-structures imposes a new challenge for the drawing process. The aim of this study is to gain profound insight into the behaviour of complex optical fibre structures during the final fabrication step, fibre drawing. Two types of optical fibre, namely conventional silica fibres and PCFs, were studied. Germanium and fluorine diffusion during drawing was studied experimentally and a numerical analysis was performed of the effects of drawing parameters on diffusion. An experimental study of geometry control of PCFs during drawing was conducted with emphasis given to the control of hole size. The effects of the various drawing parameters and their suitability for controlling the air-structure was studied. The effect of air-structures on heat transfer in PCFs was studied using computational fluid dynamics techniques. Both germanium and fluorine were found to diffuse at high temperature and low draw speed. A diffusion coefficent for germanium was determined and simulations showed that most diffusion occurred in the neck-down region. Draw temperature and preform feed rate had a comparable effect on diffusion. The hole size in PCFs was shown to depend on the draw temperature, preform feed rate and the preform internal pressure. Pressure was shown to be the most promising parameter for on-line control of the hole size. Heat transfer simulations showed that the air-structure had a significant effect on the temperature profile of the structure. It was also shown that the preform heating time was either increased or reduced compared to a solid structure and depended on the air-fraction

Laser peak power scaling and beam quality improvement with Ytterbium rod-type fiber amplifiers made by powder sinter technology

Fiber amplifiers with a robust monolithic seed coupling and very high peak power in a near diffraction-limited beam are increasingly demanded by many industrial applications in laser materials processing. A large mode area fiber is used to scale up the peak power and suppress the nonlinear effects. An approach of local adiabatic taper is proposed to provide a monolithic signal path and selectively excite the fundamental mode in highly multimode fiber. The powder-sintering technology was employed to achieve rod-type fibers with excellent refractive index homogeneity. First experiments were performed with 56m core diameter rod fibers. While non-tapered fiber amplifier achieved a peak power of 544kW, tapered amplifier reached 230kW. For comparable average powers of 10W, the taper improves the beam quality from M2 values of about 10 to 3.5, while the monolithic seed coupling significantly improves the beam stability. It was observed that the dopants diffuse during the tapering process because of high temperature, possibly providing further sources for coupling to higher order modes. Second experiments with improved rod-type fiber amplifiers (reduce the Al3+-content of fiber core and use suitable material of outer clad to mitigate the diffusion problem) delivered 2ns pulses with peak powers of 210kW for the non-tapered rod and 140kW for the tapered rod (limited by facet damage). For the tapered fiber, the beam quality was between 1.3 and 1.7, significantly improved compared to the beam quality of the non-tapered fiber (M2 = 3.3 ~ 4.5). An endcap was adopted for the tapered fiber amplifier and the peak power is scaled up to 375kW in the nearly diffraction limited region. For future work, the confined doped fibers and a picosecond seed laser source are envisioned