Kunimasa Saitoh

Unique characteristic features of stimulated Brillouin scattering in small-core photonic crystal fibers

We present extensive stimulated Brillouin scattering (SBS) results from experiments and modeling for four different photonic crystal fibers (PCFs) with core diameters ranging from 8 to 1.7 μm. These results reveal several SBS characteristic features of small-core PCFs, high thresholds, and acoustic peaks, which are due to their antiguiding nature and highly multimode acoustic character. The nature of what we believe to be new acoustic modes is examined in the light of the large variations observed in the Brillouin gain, Brillouin threshold, and Brillouin shift with decreasing core diameter and optical wavelength.

Tunable Photonic Crystal Fiber Couplers With a Thermo-Responsive Liquid Crystal Resonator

We theoretically address the thermo-optical response of multicore photonic crystal fiber (PCF) couplers infiltrated with nematic liquid crystals (LCs). The proposed PCF coupler consists of two identical cores separated by a third one which acts as a liquid crystal resonator. With an appropriate choice of the design parameters associated with the liquid crystal core, phase matching at a single wavelength can be achieved, thus enabling thermo-tunable narrow-band resonant directional coupling between the input and the output cores. The verification of the proposed coupler design is ensured through an accurate PCF analysis based on finite element and beam propagation methods. The enhanced thermo-optical properties of LC-based PCF couplers are highly attractive for photo-thermal sensing applications.

Strategies for realizing photonic crystal fiber bandpass filters

Numerical design strategies are presented to achieve efficient broad or narrow band-pass filters based on index-guiding, solid-core, and single-mode photonic crystal fibers (PCFs). The filtering characteristics have been verified through BPM solver. By scaling the pitch constant, the bandpass window can be shifted accordingly. The fiber design constitutes a fluorine-doped central core, enlarged air-holes surrounding the down-doped core, and small air-holes in the cladding. The proposed bandpass filter is based on controlling the leakage losses, so one can tune filter characteristics simply by changing its length. From numerical simulations we show that for large values of air-hole diameter in the first ring, the bandpass window is narrow, while for low doping concentration and small sized air-holes in the first ring, bandpass window is very broad. We also simulate how the hole-size and number of rings in the PCF cladding affects the device characteristics. We find that a 5-cm long PCF with down-doped core and eleven rings of air-holes can result in approximately 440 nm 3-dB bandwidth with more than 90% of transmission. The longer device has reduced transmission and smaller 3-dB bandwidth. Tolerance analysis has also been performed to check the impact of fiber tolerances on the performance of the PCF bandpass filter. It has been observed that the decrement in cladding hole-diameter by 1% reduces the transmission to 21% from its peak value of 93%, however +/-1% tolerance in the inner hole-diameter degrades the transmission to 75% from its peak.

Reversed dispersion slope photonic bandgap fibers for broadband dispersion control in femtosecond fiber lasers

Higher-order-mode solid and hollow core photonic bandgap fibers exhibiting reversed or zero dispersion slope over tens or hundreds of nanometer bandwidths within the bandgap are presented. This attractive feature makes them well suited for broadband dispersion control in femtosecond pulse fiber lasers, amplifiers and optical parametric oscillators. The canonical form of the dispersion profile in photonic bandgap fibers is modified by a partial reflector layer/interface placed around the core forming a 2D cylindrical Gires-Tournois type interferometer. This small perturbation in the index profile induces a frequency dependent electric field distribution of the preferred propagating higher-order-mode resulting in a zero or reversed dispersion slope.

Full-vectorial coupled mode theory for the evaluation of macro-bending loss in multimode fibers. application to the hollow-core photonic bandgap fibers.

In the hollow core photonic bandgap fibers, modal losses are strongly differentiated, potentially enabling effectively single mode guidance. However, in the presence of macro-bending, due to mode coupling, power in the low-loss mode launched into a bend is partially transferred into the modes with higher losses, thus resulting in increased propagation loss, and degradation of the beam quality. We show that coupled mode theory formulated in the curvilinear coordinates associated with a bend can describe correctly both the bending induced loss and beam degradation. Suggested approach works both in absorption dominated regime in which fiber modes are square integrable over the fiber crossection, as well as in radiation dominated regime in which leaky modes are not square integrable. It is important to stress that for multimode fibers, full-vectorial coupled mode theory developed in this work is not a simple approximation, but it is on par with such exact numerical approaches as finite element and finite difference methods for prediction of macro-bending induced losses.

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.

Dispersion, birefringence, and amplification characteristics of newly designed dispersion compensating hole-assisted fibers

We propose a new design of hole-assisted fiber (HAF) that can compensate for the accumulated dispersion in single-mode fiber link along with dispersion slope, thus providing broadband dispersion compensation over C-band as well as can amplify the signal channels by utilizing the stimulated Raman scattering phenomena. The proposed dispersion-compensating HAF (DCHAF) exhibits the lowest dispersion coefficient of -550 ps/nm/km at 1550 nm with an effective mode area of 15.6 microm(2). A 2.52 km long module of DCHAF amplifies incoming signals by rendering a gain of 4.2 dB with +/-0.8 dB gain flatness over whole C-band. To obtain accurate modal properties of DCHAF, a full-vector finite element method (FEM) solver is employed. The macro-bend loss characteristics of the proposed DCHAF are evaluated using FEM solver in cylindrical coordinate systems of a curved DCHAF, and low bending losses (<10(-2) dB/m for 1 cm bending radius) are obtained for improved DCHAF design while keeping intact its dispersion compensation and Raman amplification properties. We have further investigated the birefringence characteristics that can give significant information on the polarization mode dispersion of DCHAF by assuming a certain deformation (eccentricity e = 7%) either in air-holes or in the doped core or in both at a same time. It is noticed that the distortion in air-holes induces a birefringence of 10(-5), which is larger by a factor of 10 than the birefringence caused due to the core ellipticity. A PMD of 11.3 ps/ radicalkm is obtained at 1550 nm for distorted air-holes DCHAF structure.

Bend-insensitive lasing characteristics of single-mode, large-mode-area Ytterbium-doped photonic crystal fiber

The bend-insensitive lasing characteristics of a newly designed ytterbium-doped photonic crystal fiber (YPCF) are evaluated numerically. The designed YPCF remains single-mode and possesses large-mode-area of 1400 microm(2) at 1064 nm wavelength with the beam quality factor (M(2)) of 1.15, suggesting a diffraction-limited and continuous-wave lasing operation. The doped-region size is optimized for maximum conversion efficiency and it is found through numerical simulations that the doped radius should be more than 21 microm. The mode expansion, which is the self-expansion of the fundamental mode within the doped region with wavelength increments on bending the fiber, is the basic physical mechanism to give the bend-insensitive lasing performances of YPCF. It leads to an unusual variation of overlap factor when the wavelength is increased. A 41 cm long piece of YPCF demonstrates more than 83% of slope efficiency with 75% of conversion efficiency when pumped with a 975 nm laser source delivering an input power of 1 W.

Raman amplification characteristics of As2Se3 photonic crystal fibers.

We present the dispersion and Raman amplification characteristics of As2Se3 photonic crystal fibers (PCFs). We compare the gain characteristics with conventional As2Se3 fibers and find that the Raman gain efficiency in PCFs can be improved by a factor of more than 4. This allows us to either use a small length of the fiber or to use the low pump power to attain similar gain characteristics. Numerical simulations reveal that a peak gain of 10 dB can be achieved in a 1.1 m long PCF when it is pumped at 1.5 microm in wavelength with an input power of 500 mW.

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.