Fiber For High Power Applications Research Articles

Design of Arbitrary Shape Higher Order Mode Electric Field in Optical Fibers for High Power Applications

In this paper, we present a new and efficient technique for the exact reconstruction of large mode area (LMA) higher order mode (HOM) optical fibers by using arbitrary shape HOM electric field profiles. One known strategy to achieve LMAs for high power optical applications is by using optical fibers supporting multimode operation, and engineering these fibers such that they operate on just a single transverse mode. Although the fundamental mode is preferred in most circumstances, there is an increasing interest in operating on a single HOM instead since these can have interesting dispersion characteristics and also significantly higher ability to scale mode areas. An issue with the use of HOMs in high power applications is that often they demonstrate regions of high field intensity across the core. This leads to problems with optical damage at lower values of effective area than might otherwise be expected. In order to avoid this problem, we offer designs of optical fibers supporting mode-flattened HOM electric fields. We develop a new inverse transmission line technique for calculating directly and accurately the refractive index profiles of such fibers from knowledge of the desired HOM electric field. We demonstrate this algorithm with a number of example reconstructions of LMA HOM optical fibers supporting prescribed mode-flattened electric field profiles with ultralarge effective mode areas We expect such fiber designs to be useful in the mitigation of nonlinear effects and in thermal and damage management.

Rectangular-core large-mode-area photonic crystal fiber for high power applications: design and analysis

A rectangular-core large-mode-area photonic crystal fiber structure has been designed based on the principle of higher order mode filtering by introducing fluorine-doped material rods in the cladding region. The proposed structure has an effective-mode area of fundamental mode (FM) as large as 2147 μm2 at 1.064 μm with nominal loss of 1.36×10−2 dB/m and 9.34 dB/m at first higher order mode (FHOM), which confirms effective single-mode operation after ∼2.14 m propagation length. The same structure offers effective-mode area of 5688 μm2 at λ=1.55 μm with confinement loss of ∼6.3×10−2 dB/m for FM and ∼20.23 dB/m for FHOM. The loss of FHOM is above 8 dB/m from λ=1 μm to λ=1.6 μm with very small loss in FM, which offers extended single-mode operation within a broad spectral range.

Double clad tapered fiber for high power applications

We report a novel type of active fiber - tapered double clad fiber suitable for pumping by low brightness sources with large beam parameter product of 50/300 mm x mrad. Ytterbium double clad all-silica fiber (core/1(st) clad/2(nd) clad diameters 27/834/890 mum, NA(core)=0.11, NA(clad)=0.21), tapered down by a factor 4.8 for a length of 10.5 m was drawn from a preform fabricated by plasma chemical technologies. At a moderate Yb-ion concentration and 1:31 core/cladding ratio, the tapered double clad fiber demonstrates 0.9 dB/m pump absorption at 976 nm and excellent lasing slope efficiency. An ytterbium fiber laser with 84 W of output power and 92% slope efficiency, a 74 W superfluorescent source with 85% slope efficiency and amplifiers operating both in CW and pulsed regimes have been realized. All devices demonstrated robust single mode operation with a beam quality factor of M(2)=1.07.

Large Mode Area Fibers for High Power Applications

We review our recent work toward designing large mode area fibers for high power applications. We show that appropriately designed doped multimode fibers can be used to provide robust single-mode output when used in fiber laser cavities. Single-mode (SM) fiber mode field diameters of ∼35 μm are demonstrated with record SM pulse energies of 0.5 mJ at 1550 nm with a repetition rate of 200 Hz. Energies approaching 1 mJ are obtained with a slight compromise in mode quality. A modification in the laser cavity results in a passively modelocked laser giving femtosecond pulses with nanojoule energies.