Search

To estimate the intercore crosstalk in uncoupled multicore fibers with cores' propagation constants perturbed by bending, twist, and structure fluctuations, in the paper [Opt. Express21, 5401 (2013)10.1364/OE.21.005401] entitled "Physical interpretation of intercore crosstalk in multicore fiber: effects of macrobend, structure fluctuation, and microbend," Hayashi et al. derived an intuitively interpretable expression of the average power-coupling coefficient as the convolution of the arcsine and Lorentzian distributions, and calculated the derived expression numerically by performing the fast Fourier transform. In the present paper, we derive a closed-form solution of the convolution of the arcsine and Lorentzian distributions, and point out that the closed-form analytical expression derived here is exactly equivalent to the previously reported closed-form expression.

We propose a planar lightwave circuit (PLC)-based mode multi/demultiplexer (MUX/DEMUX) with an asymmetric parallel waveguide for mode division multiplexed (MDM) transmission. The PLCb-ased mode MUX/DEMUX has advantage of selectively exciting higher-order mode. We realize three-mode (LP(01), LP(11)a, and LP(21)a) multiplexing by using an asymmetric parallel waveguide. We then design and fabricate a PLC-based mode MUX/DEMUX on one chip by using our proposed LP(11) mode rotator to allow us to utilize the LP(11)b mode. We successfully multiplex the LP(01), LP(11)a, and LP(11)b modes and achieve a relatively low insertion loss over the C-band using our fabricated mode MUX/DEMUX. Our results indicate that the PLC-based mode MUX/DEMUX with a uniform height has the potential to increase the mode number by using an LP(11)b mode.

We proposed a PLC-based mode multi/demultiplexer (MUX/DEMUX) with an asymmetric parallel waveguide for mode division multiplexed (MDM) transmission. The mode MUX/DEMUX including a mode conversion function with an asymmetric parallel waveguide can be realized by matching the effective indices of the LP(01) and LP(11) modes of two waveguides. We report the design of a mode MUX/DEMUX that can support C-band WDM-MDM transmission. The fabricated mode MUX/DEMUX realized a low insertion loss of less than 1.3 dB and high a mode extinction ratio that exceeded 15 dB. We used the fabricated mode MUX/DEMUX to achieve a successful 2 mode x 4 wavelength x 10 Gbps transmission over a 9 km two-mode fiber with a penalty of less than 1 dB.

We demonstrate bidirectional transmission over 450 km of newly-developed dual-ring structured 12-core fiber with large effective area and low crosstalk. Inter-core crosstalk is suppressed by employing propagation-direction interleaving, and 409-Tb/s capacities are achieved for both directions.

We have derived an intuitively interpretable expression of the average power-coupling coefficient for estimating the inter-core crosstalk of the multicore fiber. Based on the derived expression, we discuss how the structure fluctuation and macrobend can affect the crosstalk, and organize previously reported methods for crosstalk suppression. We also discuss how the microbending can affect the crosstalk in homogeneous and heterogeneous MCFs, based on the derived expression and previously reported measurement results.

Characteristics of few-mode multi-core fiber (FM-MCF) were numerically analyzed and experimentally confirmed. The cores of FM-MCF were designed to support transmission of LP(01) and LP(11) modes from the point of bending loss of LP(11) and LP(21) modes. Inter-core crosstalk between LP(11) mode was calculated to determine core pitch of fibers. It was confirmed that the fabricated fibers was two-mode transmission over C-band and L-band with the effective area of LP(01) mode of about 110 μm(2) at 1550 nm. The crosstalk of the fibers was estimated to be smaller than -30 dB at 1550 nm after 100-km propagation. The crosstalk dependence on wavelength was also measured and matched well with the simulated results.

Based on the overlap integral of electromagnetic fields in neighboring cores, a calculating method is proposed for obtaining the coupling coefficient between two adjacent trench-assisted non-identical cores. And a kind of heterogeneous trench-assisted multi-core fiber (Hetero-TA-MCF) with 12 cores is proposed to achieve large effective area (A(eff)) and high density of cores. As bending radius becomes larger than 50 mm, the crosstalk value at 1550-nm wavelength of the Hetero-TA-MCF is about -42 dB after 100-km propagation and the A(eff) of this Hetero-TA-MCF can reach 100 µm(2).

The cladding thickness of trench-assisted multi-core fibers was theoretically and experimentally investigated in terms of excess losses of outer cores. No significant micro-bending loss increase was observed on multi-core fibers with the cladding thickness of about 30 µm. The tolerance for the micro-bending loss of a multi-core fiber is larger than that of the single core fiber. However, the cladding thickness will be limited by the occurrence of the excess loss on outer cores. The reduction of cladding thickness is probably limited around 40 µm in terms of the excess loss. The multi-core fiber with an effective area of 110 µm(2) at 1.55 µm and 181-µm cladding diameter was realized without any excess loss., (© 2011 Optical Society of America)

Coupled-mode and coupled-power theories are described for multi-core fiber design and analysis. First, in order to satisfy the law of power conservation, mode-coupling coefficients are redefined and then, closed-form power-coupling coefficients are derived based on exponential, Gaussian, and triangular autocorrelation functions. Using the coupled-mode and coupled-power theories, impacts of random phase-offsets and correlation lengths on crosstalk in multi-core fibers are investigated for the first time. The simulation results are in good agreement with the measurement results. Furthermore, from the simulation results obtained by both theories, it is confirmed that the reciprocity is satisfied in multi-core fibers., (© 2011 Optical Society of America)

We evaluate structural dependency of leakage losses in lithium niobate on insulator ridge waveguides. Generally, shallow ridge waveguides based on isotropic materials have inherent leakage loss for TM-like mode. On the other hand, lithium niobate is anisotropic material, thus the optical properties of lithium niobate based ridge waveguides are different from those of isotopic material based ridge waveguides. In this paper, we investigate leakage losses of lithium niobate on insulator ridge waveguides. We show that the shallow ridge waveguide structure without leakage loss can be realized by choosing the waveguide parameters adequately., (© 2011 Optical Society of America)

We observe unique dynamics of Raman soliton during supercontinuum process when an input pulse experiences initially normal group-velocity dispersion with a negative dispersion slope. In this situation, the blue components of the spectrum form a Raman soliton that moves faster than the input pulse and eventually decelerates because of Raman-induced frequency downshifting. In the time domain, the soliton trajectory bends and becomes vertical when the Raman shift ceases to occur as the spectrum of Raman soliton approaches the zero dispersion point. Parts of the red components of the pulse spectrum are captured by the Raman soliton through cross-phase modulation and they travel with it. The influence of soliton order, input chirp and dispersion slope on the dynamics of Raman soliton is discussed thoroughly.

To realize large effective area (Aeff) multi-core fibers (MCFs), the design to suppress the cross-talk and the influence of the cladding diameter on the micro bending loss were investigated. As a result, the MCFs with large Aeff over 100 μm(2) and low micro bending loss were successfully fabricated. The results indicate the importance of fiber design to realize large Aeff MCFs including fiber diameters, which largely affect the micro bending loss property. Additionally, MCF with large Aeff, low attenuation loss and suppressed cross-talk was successfully realized by optimizing the fiber design. The cross-talk properties could be estimated by the simulation based on the coupling power theory taking the influences of the longitudinal fluctuation of core diameter into account.

We numerically investigate the polarization characteristics of photonic crystal fibers selectively filled with metal wires into cladding air holes, through a full-vector modal solver based on the finite-element method (FEM). Firstly, we investigate the fundamental coupling properties between the core guided light and surface plasmon polaritons (SPPs) excited on the surface of metal wire. Secondly, we show that we can obtain highly polarization-dependent transmission characteristics in PCFs by introducing several metal wires closely aligned into the cladding, and reveal the strongly polarization-dependent coupling properties between the core guided modes and the SPP supermodes, which consist of discrete SPP modes. Finally, we show the importance of arranging the metal wires close to each other for high polarization-dependence.

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.

In the mode-division multiplexing (MDM) optical transmission system, a mode multi/demultiplexer is an important key device for excitation, multiplication, and separation of light signals which have distinct modes. In this report, we propose a fiber-type mode multi/demultiplexer based on selective phase matching between different cores/modes. Design method and device characteristics of 1x4 mode multi/demultiplexer are investigated through finite element analysis. In order to expand operating wavelength range, we reveal the structural parameters that satisfy phase matching conditions over wide wavelength range. Our numerical results demonstrate that the mode multi/demultiplexer with broadband, polarization-insensitive operation can be realized by applying the proposed fiber structure.

We propose an ultra-small polarization splitter based on a resonant tunneling phenomenon. This polarization splitter consists of two identical horizontally oblong silicon wire waveguides separated by a vertical slot waveguide. The structural parameters of the central resonant slot waveguide are designed to couple only the TM-like mode between the left and right side silicon wire waveguides. Results from numerical simulation with the full-vectorial beam propagation method show that a 16-mum-long polarization splitter with extinction ratio better than -20 dB on the entire C-band is achieved.

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.

We investigate a novel design for all-solid large mode area (LMA) leakage channel fibers (LCFs) for high-power Yb-doped fiber lasers and amplifiers, based on a single down-doped-silica rod ring surrounding a seven-cell pure-silica core, aiming for effectively single-mode behavior and low bending loss characteristics. Through detailed numerical simulations based on the finite element method (FEM), we find that the proposed all-solid LMA-LCFs, having a seven-cell core and two different sizes of down-doped rods, can achieve sufficient differential mode loss and much lower bending loss, as compared with a previously-reported LCF with a one-cell core and six large down-doped-silica rods., ((c) 2009 Optical Society of America)

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.

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.

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.

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.

We have numerically investigated the Raman lasing characteristics of a highly nonlinear photonic crystal fiber (HNPCF). HNPCF Raman lasers are designed to deliver outputs at 1.3 microm and 1.48 microm wavelengths through three and six cascades of Raman Stokes cavities when the pumps of 1117 nm and 1064 nm are injected into HNPCF module, respectively. A quantum efficiency of approximately 47% was achieved in a short length of HNPCF for 1.3 microm lasing wavelength. The HNPCF design is modified further to operate in single-mode fashion keeping intact its Raman lasing characteristics. The modified HNPCF design consists of two air-hole rings where the higher-order modes in the central core are suppressed by enhancing their leakage losses drastically, thus ceasing their propagation in the short length of HNPCF. On the other hand, the fundamental mode is well confined to the central core region, unaffecting its lasing performances. Further, the lasing characteristics of HNPCF at 1480 nm are compared with conventional highly nonlinear fiber Raman laser operating at 1480 nm. It is found that one can reduce the fiber length by five times in case of HNPCF with nearly similar conversion efficiency.

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.

By introducing nonreciprocal phase shifts into microresonators, we propose new designs for the miniaturization of optical waveguide isolators and circulators. We present detailed design procedures, and numerically demonstrate the operation of these magneto-optical devices. The device sizes can be reduced down to several tens of micrometers. The nonreciprocal function of these devices is due to nonreciprocal resonance shifts. Next, the operation bandwidth can be expanded by increasing the number of resonators (the filter order). This is demonstrated by comparing the characteristics of a single-resonator structure with those of a three-resonator structure. This paper furthermore presents the nonreciprocal characteristics of three-dimensional resonators with finite heights, leading to a guideline for the design of nonreciprocal optical circuits. This involves a demonstration of how the resonators with selected parameters are practical for miniaturized nonreciprocal circuits.

In this paper, we study the novel propagation properties of an improved triangular-type air-core photonic bandgap fiber (PBGF) structured with an anti-resonant silica core surround, through a full-vector modal solver based on the finite-element method (FEM). At first, to realize a single-mode operation over a wide wavelength range, the fiber whose core is constructed by removing 1 air-hole and expanded is proposed and structurally-optimized. In particular, the structural parameters for the fiber that prevent the narrow-band transmission due to the existence of the surface modes and enhance the confinement of the power in the air-core are presented. For the realization of an ultimate low loss transmission, a 7-unit-cell PBGF is analyzed and we show that the 7-unit-cell PBGF can achieve not only lower confinement loss than that of regular-type 7-unit-cell PBGF, but also lower power fraction in the silica-ring when compared with the regular 19-unit-cell PBGF with an anti-resonant core surround, exhibiting an effectively single-mode operation.

We have experimentally measured the Raman gain efficiency (RGE) and chromatic dispersion (CD) of a hole-assisted fiber (HAF). The RGE of a HAF was characterized using standard pump on/off technique while the CD of the fiber was measured using optical network analyzer. Theoretical simulations of the modal characteristics and the RGE of HAF were carried out using an accurate full-vectorial finite element method. Further, the bending effects on the CD and the RGE of a HAF with a smallest feasible bending radius are demonstrated. It was found that the CD increases while the RGE is decreased by bending HAF in a smallest bending radius of 5 mm. Numerical predictions from the theory are shown to be in good agreement with the experimental results.

This paper focuses on the optimization of pump spectra to achieve low Raman gain ripples over C-band in ultra-low loss photonic crystal fiber (PCF) and dispersion compensating PCFs (DCPCFs). Genetic algorithm (GA), a multivariate stochastic optimization algorithm, is applied to optimize the pump powers and the wavelengths for the aforesaid fiber designs. In addition, the GA integrated with full-vectorial finite element method with curvilinear edge/nodal elements is used to optimize the structural parameters of DCPCF. The optimized DCPCF provides broadband dispersion compensation over C-band with low negative dispersion coefficient of -530 ps/nm/km at 1550 nm, which is five times larger than the conventional dispersion compensating fibers with nearly equal effective mode area (21.7 mum(2)). A peak gain of 8.4 dB with +/-0.21 dB gain ripple is achieved for a 2.73 km long DCPCF module when three optimized pumps are used in the backward direction. The lowest gain ripple of +/-0.36 dB is attained for a 10 km long ultra-low loss PCF with three backward pumps. Sensitivity analysis has been performed and it is found that within the experimental fabrication tolerances of +/-2%, the absolute magnitude of dispersion may vary by +/-16%, while the Raman gain may change by +/-7%. Through tolerance study, it is examined that the ring core's hole-size is more sensitive to the structural deformations.

One of the major trends in optical fiber science is to be able to obtain fibers with large-mode-area (LMA), optimized for various applications such as high power delivery, fiber amplifiers, and fiber lasers. In order to ensure the high beam quality and the ultimate controllability of the damage threshold in the fiber's material, it is required to have a LMA property and of course to operate in a single mode fashion. While the conventional fibers have some difficulties in providing simultaneously LMA, single mode operation, as well as low macro-bending loss characteristics, all-silica holey fibers are highly attractive candidates for realizing LMA single-mode fibers with low bending losses. In this paper, we present a novel type of effectively single-mode holey fibers with effective mode area of about 1400 mum(2), small allowable bending radius as small as 5 cm, good beam quality factor of 1.15, and high confinement losses exceeding 1 dB/m for the higher-order mode at 1.064-mum wavelength.

This paper presents a new full-vectorial finite-element method in a local cylindrical coordinate system, to effectively analyze bending losses in photonic wires. The discretization is performed in the cross section of a three-dimensional curved waveguide, using hybrid edge/nodal elements. The solution region is truncated by anisotropic, perfectly matched layers in the cylindrical coordinate system, to deal properly with leaky modes of the waveguide. This approach is used to evaluate bending losses in silicon wire waveguides. The numerical results of the present approach are compared with results calculated with an equivalent straight waveguide approach and with reported experimental data. These comparisons together demonstrate the validity of the present approach based on the cylindrical coordinate system and also clarifies the limited validity of the equivalent straight waveguide approximation.

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.

The objective of this paper is the detailed study of polarization conversion in deformed high index contrast (HIC) waveguides. The type of deformation considered here is the slanted sidewalls of buried channel waveguides. Polarization conversion of HIC waveguides are investigated for possible core refractive indices ranging from 2 (SiN(x)) to 3.5 (Si), by using numerical schemes based on the finite-element and beam propagation methods. The numerical results show that polarization conversion can be greatly magnified in HIC channel waveguides. For example, in Si-wire waveguides, complete polarization conversions can occur within just tens of micrometers.

Approximate empirical relations for nonlinear photonic crystal fibers (PCFs) are newly proposed. Replacing a PCF with a conventional step-index fiber, closed form expressions for the effective refractive index and the effective core area of nonlinear PCFs are derived. To define the equivalent cladding index, the effective index of the so-called fundamental space-filling mode, which is calculated using empirical relations for the effective normalized frequency, is introduced, and thus, nonlinear guided waves propagating in PCFs can be easily characterized without the need for numerical computations. The validity of the method proposed here is ensured by comparing the calculated results with those obtained by a fullvector finite-element method.

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.

Apodized photonic crystal (PC) waveguide gratings are proposed to suppress sidelobes which appear in reflection spectra of usual PC waveguide gratings. By using specific functions (Gauss and Gauss-cosine functions) for the longitudinal refractive index distribution, it is possible to suppress sidelobes in the reflection spectra of PC waveguide gratings efficiently. The apodization is realized by simply changing diameters of dielectric pillars adjacent to the PC waveguide core. It is shown that by using Gauss-cosine functions for the apodization, Bragg frequency of the waveguide grating becomes insensitive to the magnitude of perturbation leading to the possibility of designing waveguide gratings with arbitrarily Bragg frequency and bandwidth by modulating geometrical parameters only.

This paper presents an optimized design of a dispersion compensating photonic crystal fiber (PCF) to achieve gain-flattened Raman performances over S-band using a single pump. Genetic algorithm interfaced with an efficient full-vectorial finite element modal solver based on curvilinear edge/nodal elements is used as an optimization tool for an accurate determination of PCF design parameters. The designed PCF shows high negative dispersion coefficient (-264 ps/nm/km to -1410 ps/nm/km) and negative dispersion slope, providing coarse dispersion compensation over the entire S-band. The module comprised of 1.45-km long optimized PCF exhibits +/-0.46 dB gain ripples over 50 nm wide bandwidth and shows a very low double Rayleigh backscattering value (-59.8 dB). The proposed module can compensate for the dispersion accumulated in one span (80-km) of standard single mode fiber with a residual dispersion of +/-700 ps/nm, ensuring its applicability for 10 Gb/s WDM networks. Additionally, the designed PCF remains single mode over the range of operating wavelengths.

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.

In this paper we study the impact of elliptically-deformed features such as cladding air-holes and elliptically-modulated cores, as ingredients for optimizing the coupling characteristics of dual-core fluorine-doped photonic crystal fiber (PCF) couplers. We provide a detailed numerical investigation by using a trial and error approach for optimizing the propagation characteristics of fluorine-doped PCF couplers. Typical characteristics of the newly proposed PCF coupler structure are: wavelength-flattened coupling characteristics between 0.7 mum and 1.6 mum wavelength range, coupling efficiency of 50+/-1 % from 0.9 mum to 1.6 mum, and a reasonably small coupling length of 1.3 cm. In addition we have elaborately derived the design parameters so that our proposed dual-core PCF coupler exhibits polarization-insensitive characteristics verified by using a full-vectorial beam propagation method. The proposed dual-core PCF can be effectively used as a 3-dB coupler, over a wide wavelength range.

Novel type of microstructured optical fiber couplers is introduced where energy transfer is enabled by transverse resonator arrays built into a fiber crossection. Such a design allows unlimited spatial separation between interacting fiber cores which, in turn, eliminates inter-core crosstalk via proximity coupling, thus enabling scalable integration of many fiber cores. Moreover, in the limit of weak inter-resonator coupling, resonator arrays exhibit moderate polarization dependence.

Chromatic dispersion profile of dual-concentric-core photonic crystal fibers is optimized for broadband dispersion compensation of single mode fibers (SMFs) by using genetic algorithm incorporated with full-vector finite-element method. From the numerical results presented here, it is found that by increasing the distance between central core and outer ring core, larger negative dispersion coefficient and better dispersion slope compensation are possible. There is a tradeoff between the magnitude of negative dispersion coefficient and dispersion slope compensation due to the concave dispersion profile of dual-concentric-core photonic crystal fibers. In spite of the tradeoff, dual-concentric-core photonic crystal fibers having larger negative dispersion coefficient as well as compensating for dispersion slope of SMFs in the entire C band with large effective area can be designed.

The present paper describes a novel systematic solution to the challenging task of realizing photonic crystal fibers (PCFs) with flat chromatic dispersion, low leakage losses, and large mode area, mainly for applications as information carriers in wide-band high speed optical transmission systems. The proposed design strategy is based on the existence of an artificially-defected air-hole ring in the cladding and on the modulation of the refractive index of the core by assembling additional defected air-holes in the central core region of the fiber. The validation of the proposed design is carried out by adopting an efficient full-vectorial finite element method with perfectly matched layers for accurate characterization of PCFs. The remarkable flat chromatic dispersion as well as the large mode area and the low leakage losses are the main advantages of the proposed PCF structure, making it an ideal candidate for performing wavelength division multiplexing operation in reconfigurable optical transmission systems or as an information delivering platform in high speed optical communication systems. Typical characteristics of the newly proposed PCF are: flattened chromatic dispersion of 6.3+/-0.5 ps/km/nm in the S+C+L telecommunication band, and effective mode area as large as 100 microm(2) in the same wavelength range. We additionally provide numerical data about the performance of the proposed PCF in splicing mode as well as during macrobending operation and we give qualitative information regarding the sensitivity of the proposed transmission platform to structural disorders of the design parameters.

In this paper we evaluate the cut-off properties of holey fibers (HFs) with a triangular lattice of air holes and the core formed by the removal of a single (HF1) or more air holes (HF3 and HF7). With the aid of finite-element simulations we determine the single-mode and multi-mode phases and also find the air hole diameters limiting the endlessly single-mode regime. From calculations of V and W parameters we find that in general HF1 is less susceptible to longitudinal non-uniformities compared to the other designs for equivalent effective areas. As an example we illustrate this general property for the particular case of a macro-bending induced loss.

The objective of the present paper is to introduce and numerically demonstrate the operation of a novel band-pass filter based on the phenomenon of resonant tunneling inmulti-core photonic crystal fibers (PCFs). The proposed PCF consists of two identical cores separated by a third one which acts as a resonator. With a fine adjustment of the design parameters associated with the resonant-core, phase matching at a single wavelength can be achieved, thus enabling very narrow-band resonant directional coupling between the input and the output cores. The validation of the design is ensured with an accurate PCF analysis based on finite element and beam propagation algorithms. The proposed narrow band-pass filter can be employed in various applications such as all fiber and pass/bandstop filtering.

This report presents the first three-dimensional characterization of nonreciprocal phase shifts in magneto-photonic crystal (MPC) slab waveguides. We model MPC waveguides using a three-dimensional finite element method with curvilinear tetrahedral edge elements. This study investigates the dependence of nonreciprocal phase shifts on the width and the thickness of the waveguides, and we investigate the dependence of losses on the air hole depth, leading to a guideline for the design of optical isolators. Simulations show that waveguides with reduced width and deep air holes exhibit high nonreciprocal phase shifts and low losses. The study also shows that, compared with two-dimensional calculations, nonreciprocal phase shifts express key similarities, although the frequencies of the guided modes shift.

In this paper, we report, for the first time, an inherently gain-flattened discrete highly nonlinear photonic crystal fiber (HNPCF) Raman amplifier (HNPCF-RA) design which shows 13.7 dB of net gain (with +/-0.85-dB gain ripple) over 28-nm bandwidth. The wavelength dependent leakage loss property of HNPCF is used to flatten the Raman gain of the amplifier module. The PCF structural design is based on W-shaped refractive index profile where the fiber parameters are well optimized by homely developed genetic algorithm optimization tool integrated with an efficient vectorial finite element method (V-FEM). The proposed fiber design has a high Raman gain efficiency of 4.88 W(-1) . km(-1) at a frequency shift of 13.1 THz, which is precisely evaluated through V-FEM. Additionally, the designed module, which shows ultra-wide single mode operation, has a slowly varying negative dispersion coefficient (-107.5 ps/nm/km at 1550 nm) over the operating range of wavelengths. Therefore, our proposed HNPCF-RA module acts as a composite amplifier with dispersion compensator functionality in a single component using a single pump.

The present paper describes a novel systematic solution to the problem of controlling the chromatic dispersion and dispersion slope in photonic crystal fibers (PCFs), using a structurally-simple PCF with a defected-core. By adjusting the size of the central air-hole defect we can successfully design an ultra-flattened PCF with low confinement losses, as well as small effective mode area. The design strategy is based on the mutual cancellation between the waveguide and the material dispersions of the PCF, by varying the size of the central defected region in the core. The verification of the ultra-flattened chromatic dispersion property of the proposed PCF is ensured with an accurate full-vector finite element method with anisotropic perfectly matched layers. The ultra-flattened dispersion feature, as well as the low confinement losses and the small effective mode area are the main advantages of the proposed PCF structure, making it suitable as a chromatic dispersion controller dispersion compensator, or as candidate for nonlinear optical applications.

Novel class of microstructured optical fiber couplers is introduced that operates by resonant, rather than proximity, energy transfer via transverse lightguides built into a fiber cross-section. Such a design allows unlimited spatial separation between interacting fibers which, in turn, eliminates inter-core crosstalk via proximity coupling. Controllable energy transfer between fiber cores is then achieved by localized and highly directional transmission through a transverse lightguide. Main advantage of this coupling scheme is its inherent scalability as additional fiber cores could be integrated into the existing fiber cross-section simply by placing them far enough from the existing circuitry to avoid proximity crosstalk, and then making the necessary inter-core connections with transverse light "wires" - in a direct analogy with an on chip electronics integration.

We propose and numerically investigate the operation of a novel class of polarization-independent splitters based on the photonic crystal fiber (PCF) technology. The proposed polarization-independent feature of the PCF splitter is realized by uniformly distributed elliptically-shaped air-holes in the cladding of a dual-core PCF. The design procedure follows a rigorous synthesis protocol based on exact equations for describing the wavelength de-coupling mechanism, and on full-vectorial finite element as well as beam propagation methods for optical characterization of PCFs. The compact de-coupling lengths as well as the low cross-talk over appreciable optical bandwidths are the main advantages of the proposed PCF splitter. The proposed device can be employed in reconfigurable optical communication systems for performing wavelength de-multiplexing operation, especially for fiber-to-the-home applications, as well as the emerging passive optical network applications.

As the fiber-to-the-home network construction increased, optical fiber cables are demanded to be easier to handle and require less space. Under this situation, a single mode fiber (SMF) permitting small bending radius is strongly requested. In this paper, we propose and demonstrate a novel type of bending-insensitive single-mode holey fiber that has a doped core and two layers of holes with different air-hole diameters. The fiber has a mode field diameter of 9.3 microm at 1.55 microm and a cutoff wavelength below 1.1 microm, and shows a bending loss of 0.011 dB/turn at 1.55 microm for a bending radius of 5 mm and a low splice loss of 0.08 dB per fusion-splicing to a conventional SMF.

In order to simply design a photonic crystal fiber (PCF), we provide numerically based empirical relations for V parameter and W parameter of PCFs only dependent on the two structural parameters - the air hole diameter and the hole pitch. We demonstrate the accuracy of these expressions by comparing the proposed empirical relations with the results of full-vector finite element method. Through the empirical relations we can easily evaluate the fundamental properties of PCFs without the need for numerical computations.