Design of photonic band gap fibers with suppressed higher-order modes: Towards the development of effectively single mode large hollow-core fiber platforms

Abstract

Abstract:The objective of the present investigation is to propose and theoretically demonstrate the effective suppression of higher-order modes in large-hollow-core photonic band gap fibers (PBGFs), mainly for low-loss data transmission platforms and/or high power delivery systems. The proposed design strategy is based on the index-matching mechanism of central air-core modes with defected outer core modes. By incorporating several air-cores in the cladding of the PBGF with 6-fold symmetry it is possible to resonantly couple the light corresponding to higher-order modes into the outer core, thus significantly increasing the leakage losses of the higher-order modes in comparison to the fundamental mode, thus making our proposed design to operate in an effectively single mode fashion with polarization independent propagation characteristics. The validation of the procedure is ensured with a detailed PBGF analysis based on an accurate finite element modal solver. Extensive numerical results show that the leakage losses of the higher-order modes can be enhanced in a level of at least 2 orders of magnitude in comparison to those of the fundamental mode. Our investigation is expected to remove an essential obstacle in the development of large-core single-mode hollow-core fibers, thus enabling them to surpass the attenuation of conventional fibers. T. Murao, K. Saitoh, and M. Koshiba, "Design of air-guiding modified honeycomb photonic band-gap fibers for effectively single mode operation," Opt.

Realistic Design of Large-Hollow-Core Photonic Band-Gap Fibers With Suppressed Higher Order Modes and Surface Modes

“…In this paper, we show that the suppression of HOMs in hollow-core PBGFs with realistic airholes in the cladding and core/cladding interface can be successfully achieved through the resonant-coupling mechanism of central air-core modes with defected outer core modes that we have proposed in [18]. Our primary target is derivation of a surface-mode-free condition for the silica-ring thickness surrounding the hollow core for both 7-unit-cell and 19-unit-cell air cores.…”

“…This fact can enable the index-matching mechanism to take place by bringing the two cores (19-unit-cell central core and 7-unit-cell outer core) close to each other, thus enabling resonant coupling over a certain wavelength regime. The phenomenon is expected to be enhanced further and to be a polarization-independent operation if we place more outer cores while keeping the sixfold symmetry around the central core in the PBGF as proposed in [18]. In order to enable the index-matching mechanism to occur between the HOM in 19-unit-cell core and the fundamental mode in 7-unit-cell core, we proceed by incorporating six 7-unit-cell cores into the cladding surrounding the central 19-unit-cell core to expect a polarization-independent operation, as shown in Fig.…”

“…One possible solution to overpass this limit is to consider large-hollow-core PBGFs, leading the multimode operation. In order to enhance the suppression of HOMs in microstructured optical fibers, an index-matching mechanism between the HOMs in the central core and the much leaky outer core modes or cladding modes around a certain wavelength range has been introduced [15]- [17], and recently, this intuitive idea has also been adapted to suppress the HOMs in hollowcore PBGFs [18] for effective single-mode operation. In our previous investigation, although the suppression of HOMs was possible, a hollow-core PBGF with circular airholes in the cladding and a nonrealistic core and cladding interface was considered [18].…”

“…In order to enhance the suppression of HOMs in microstructured optical fibers, an index-matching mechanism between the HOMs in the central core and the much leaky outer core modes or cladding modes around a certain wavelength range has been introduced [15]- [17], and recently, this intuitive idea has also been adapted to suppress the HOMs in hollowcore PBGFs [18] for effective single-mode operation. In our previous investigation, although the suppression of HOMs was possible, a hollow-core PBGF with circular airholes in the cladding and a nonrealistic core and cladding interface was considered [18]. However, in a practically fabricated hollowcore PBGF, the cladding airholes tend to be noncircular with a close-to-hexagonal shape at large air-filling fractions, and the hollow core also tends to be hexagonal with rounded corners [19], [20].…”

Single-polarization single-mode photonic crystal fiber based on index-matching coupling with a single silica material

“…It was demonstrated that this resonant coupling can also effectively suppress higher-order modes of the large mode area hollow-core photonic bandgap fibers [24]. Note that unwanted central core modes can be removed when the effective indices are matched with those of outer core modes.…”