Tadashi Murao

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

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.

Detailed theoretical investigation of bending properties in solid-core photonic bandgap fibers

In this paper, detailed properties of bent solid-core photonic bandgap fibers (SC-PBGFs) are investigated. We propose an approximate equivalent straight waveguide (ESW) formulation for photonic bandgap (PBG) edges, which is convenient to see qualitatively which radiation (centripetal or centrifugal radiation) mainly occurs and the impact of bend losses for an operating wavelength. In particular, we show that cladding modes induced by bending cause several complete or incomplete leaky mode couplings with the core mode and the resultant loss peaks. Moreover, we show that the field distributions of the cladding modes are characterized by three distinct types for blue-edge, mid-gap, and red-edge wavelengths in the PBG, which is explained by considering the cladding Bloch states or resonant conditions without bending. Next, we investigate the structural dependence of the bend losses. In particular, we demonstrate the bend-loss dependence on the number of the cladding rings. Finally, by investigating the impacts of the order of PBG and the core structure on the bend losses, we discuss a tight-bending structure.

Design of air-guiding modified honeycomb photonic band-gap fibers for effectively singlemode operation.

We investigate photonic band-gap (PBG) profiles of a modified honeycomb lattice structure and we identify the structural parameters that possess the largest band-gap. By incorporating the identified profile into the cladding, the wavelength dependence of the dispersion properties and confinement losses of air-guiding modified honeycomb PBG fibers (PBGFs) is investigated through a full-vector modal solver based on finite element method. In particular, we find that broadband effectively singlemode operation from 1450 nm to 1850 nm can be achieved using a modified honeycomb PBGF with a defected core realized by removing 7 air holes.

Multiple resonant coupling mechanism for suppression of higher-order modes in all-solid photonic bandgap fibers with heterostructured cladding

In this paper, we propose a novel mechanism for suppression of higher-order modes (HOMs), namely multiple resonant coupling, in all-solid photonic bandgap fibers (PBGFs) with effectively large core diameters. In an analogy to the well-known tight-binding theory in solid-state physics, multiple anti-resonant reflecting optical waveguide (ARROW) modes bound in designedly arranged defects in the cladding make up Bloch states and resultant photonic bands with a finite effective-index width, which contribute to the suppression of HOMs. In particular, contrary to the conventional method for the HOM suppression using the index-matching of the HOMs in the core of the PBGF and the defect mode arranged in the cladding, the proposed mechanism guarantees a broadband HOM suppression without a precise structural design. This effect is explained by the multiple resonant coupling, as well as an enhanced confinement loss mechanism which occurs near the condition satisfying the multiple resonant coupling. Moreover, we show that the proposed structure exhibits a lower bending loss characteristic when compared to the conventional all-solid PBGFs. The simultaneous realization of the single-mode operation and the low bending loss property is due to the novel cladding concept named as heterostructured cladding. The proposed structure also resolves the issue for the increased confinement loss property in the first-order photonic bandgap (PBG) at the same time.

Realization of single-moded broadband air-guiding photonic bandgap fibers

In this letter, we propose a novel type of air-guiding photonic bandgap fiber, which exhibits an effectively single-mode operation with a low confinement loss over a wide wavelength range, extended from 1460 to 1840 nm. For several realistic structural parameters, the wavelength dependence of the dispersion as well as the confinement loss properties are investigated through a full-vector modal solver based on the finite-element method. In particular, we optimize the structure so as to exhibit the following performance: effectively single-mode operation from 1460 to 1840 nm, with corresponding confinement loss of the fundamental air-guided mode of 8.8times10-5 dB/km, and enhancement of the confinement loss of the higher order mode of 43 dB/m for ten-ring structure

Structural Optimization of Air-Guiding Photonic Bandgap Fibers for Realizing Ultimate Low Loss Waveguides

In this paper, we investigate the ultimate low loss property for several realistic core shapes in triangular-type air-guiding photonic bandgap fibers (PBGFs) through a full-vector modal solver based on the finite element method. We show that the surface mode free condition is expressed as a normalized silica-ring thickness T = 0.5 for any core size and the cladding structural parameters, regardless the core radius of the silica-ring is one of the main factors of dominating the surface modes condition, and the wavelength range of the PBG changes on varying the structural parameters of the cladding. Moreover, we propose a novel type of PBGF without surface mode, which exhibits lower scattering losses caused by surface roughness of the silica-ring in comparison to 19 cell-core PBGFs and suppresses the mode coupling between fundamental-like and higher order modes when compared to 37 cell-core PBGFs.

Non-proximity resonant tunneling in multi-core photonic band gap fibers: An efficient mechanism for engineering highly-selective ultra-narrow band pass splitters

The objective of the present investigation is to demonstrate the possibility of designing compact ultra-narrow band-pass filters based on the phenomenon of non-proximity resonant tunneling in multi-core photonic band gap fibers (PBGFs). The proposed PBGF consists of three identical air-cores separated by two defected air-holes which act as highly-selective resonators. With a fine adjustment of the design parameters associated with the resonant-air-holes, phase matching at two distinct wavelengths can be achieved, thus enabling very narrow-band resonant directional coupling between the input and the two output cores. The validation of the proposed design is ensured with an accurate PBGF analysis based on finite element modal and beam propagation algorithms. Typical characteristics of the proposed device over a single polarization are: reasonable short coupling length of 2.7 mm, dual bandpass transmission response at wavelengths of 1.339 and 1.357 mum, with corresponding full width at half maximum bandwidths of 1.2 nm and 1.1 nm respectively, and a relatively high transmission of 95% at the exact resonance wavelengths. The proposed ultra-narrow band-pass filter can be employed in various applications such as all-fiber bandpass/bandstop filtering and resonant sensors.

Radiation dose enhancement in photonic crystal fiber bragg gratings: towards photo-ionization monitoring of irradiation sources in harsh nuclear power reactors

Using kinetic-based models we show that photonic crystal fiber Bragg gratings (PCFBGs) can exhibit enhanced dose-rate absorption capabilities, in comparison to conventional BGs. The modeling aims to assess the physical-mechanisms defining the response to nuclear-radiation.

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

This paper theoretically describes effective suppression of higher order modes (HOMs) in realistic large-hollow-core photonic band-gap fibers (PBGFs) and utilizes the use of this class of waveguides for low-loss data-transmission applications and high-power beam delivery systems. The proposed design strategy is based on the resonant-coupling mechanism of central air-core modes with defected outer core modes. By incorporating six 7-unit-cell air cores in the cladding of the PBGF with sixfold symmetry, it is possible by resonantly coupling the light corresponding to the HOMs in a central 19-unit-cell core into the outer 7-unit-cell core, thus significantly increasing the leakage losses of the HOMs in comparison to those of fundamental mode. We consider a realistic PBGF structure with hexagonal airholes having rounded corners and derive a surface-mode-free condition of a silica-ring thickness surrounding the hollow core for both 7-unit-cell and 19-unit-cell cores. Verification regarding the propagation properties of the proposed design is ensured with a PBGF analysis based on a finite element modal solver. Numerical results show that the leakage losses of the HOMs can be enhanced in a level of at least three orders of magnitude over 200-nm wavelength range in comparison to those of the fundamental mode, while in addition, we show that the incorporation of a realistic air core with optimized silica-ring thickness can eliminate surface modes and achieve strong confinement into the central core and very low eta-factor for the fundamental mode.

Design Principle for Realizing Low Bending Losses in All-Solid Photonic Bandgap Fibers

In this paper, the structural dependence of factor which mainly affects a bending loss property is theoretically investigated in all-solid photonic bandgap fibers (PBGFs). A design principle for realizing low bending losses is successfully figured out for the first-order photonic bandgap (PBG). In particular, one of the origins which causes the variation of bending loss property for each structural parameter is identified. In addition, we show that exploitation of a large pitch relative to a rod diameter, aiming to realize a large-mode area (LMA) structure, leads to a significant degradation of the bending loss property. Moreover, it is demonstrated that a V-value which is proposed for all-solid PBGFs is also reduced significantly for the LMA condition. The origin of the degradation is attributed to the newly-excited Bloch state which determines the second-order PBG edge.