Extremely large mode area fibers with ability to higher order modes filtration

Extremely large mode area fibers with ability to higher order modes filtration
Lukasz OSTROWSKI*a,b, Marek NAPIERALAa, Michal MURAWSKIa ,
Lukasz SZOSTKIEWICZa, Michal SZYMANSKIa ,
Ryszard PIRAMIDOWICZb Tomasz NASILOWSKIa,
aInPhoTech Ltd., Slominskiego 17/31, 00-195 Warszawa
b Institute of Microelectronics and Optoelectronics, Warsaw University of Technology Koszykowa 75,00-662 Warsaw, Poland
Single mode fibers with very large mode areas are nowadays widely used in high power applications, where large dimensions of the beam are exploited to steer clear of nonlinear effects and to avoid fiber damage. It should be noted, however, that the increase of the core size must be accompanied by the numerical aperture (NA) decrease to maintain single mode (SM) propagation, assuring sufficient quality of the output beam.
It is very difficult to fabricate classical fibers (step or graded index) with a NA lower than ~0.06, which results from limited technological flexibility over the control of the refractive indices of core and cladding. The maximal core diameter of conventional SM LMA fiber is therefore limited to approximately 15 μm for the 1 μm wavelength region. It seems,, however, that the unique features of microstructured fibers may possibly help in overcoming the limits of conventional SM fibers. Although controlling the refractive index of the core of microstructured fiber is still technically challenging, the effective refractive index of the microstructured cladding can be tailored very precisely by controlling the air-hole diameters and lattice constants, which allows designing and fabricating microstructured fibers with core diameter values reaching 60 μm. Unfortunately, in large mode area single mode fibers, due to the small difference in effective refractive indices between the core and cladding, the light is weakly confined and in the presence of small strain or bend suffers large loss. For this reason, the commonly used single mode optical fibers with very large mode area have to be kept straight.
However, for the purpose of obtaining propagation of fundamental mode with very large mode area, one can apply a few mode fiber of large core size and suppress higher order modes. One of the possible solutions was proposed by M. Napierala in [1]. The concept is based on a specific fiber design which allows increasing loss of higher order modes by fiber bending while keeping the loss of fundamental mode at the negligible level. A unique fiber structure with double lattice constant enabled propagating the fundamental mode of very large mode area (~1811 μm2) and filtration of higher order modes by bending the fiber in an appropriate way. Based on these experiments, we have designed a fiber with similar propagation properties, i.e. suppression of higher order modes in bent fiber, but with an approximately one order of magnitude larger mode area of the fundamental mode (more than 13000 μm2).
This work presents the results of optical characterization of the previously developed fiber with a mode field area 1811um2 together with the numerical simulations of the properties of structure with increased characteristic dimensions. These enabled in particular the determination of the impact of technological tolerances of the fiber drawing process on the propagation characteristics.
[1] Napierała, Marek, et al. "Large-mode-area photonic crystal fiber with double lattice constant structure and low bending loss." Optics express 19.23 (2011): 22628-22636.