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International audience; This paper proposes a circularly-polarized continuous transverse stub(CTS) array antenna for space applications. The radiating structure consists of open-ended parallel plate waveguides(PPW), supporting the propagation of the transverse electromagnetic(TEM) and first transverse electric(TE1) modes. Such modes are orthogonally-polarized in the PPW cross-section and, if properly excited, can generate circular polarization(CP). The antenna radiating section consists of 32 long slots, parallel-fed by a quasi-optical system through a PPW-based corporate feed network(CFN) and an electric field rotator. The proposed antenna is matched in the 27.5-31GHz band while scanning up to +-20° in elevation, with an axial ratio(AR) below 3dB. © 2021 IEICE.

Multi-core fibers (MCFs) are promising solutions for high power fiber based devices as they reduce nonlinearity and other unwanted detrimental effects, like transverse mode instability, by transporting, instead of a single high power beam, several low-powered ones to be coherently combined at the fiber output. This method relies on accurate evaluation of the phase differences between signals in different cores, which are significantly impacted by changes in the effective index of the propagating modes. For this to be effective, spatial heat generation must be accounted for. In particular, the heat flux from the doped cores to the external boundary causes a temperature gradient across the fiber, which affects the refractive index distribution, creating the chance for effective index change and thus dephasing of the output beams, which is harmful for beam combining. The results of in-depth numerical analysis on the performance of 9-core and 16-core MCFs under thermal effects are presented by studying the mode phase sensitivity to heat load and by introducing a coupled-mode theory model to study possible optical coupling effects. The effectively single-mode condition is also investigated by calculating the core modal overlap differences between fundamental and higher-order modes.

Multi-core fibers are a technology currently at the forefront of research due to the increasing need for efficient transport of high-power continuous and pulsed laser beams [1], Power scaling is gained by propagating independent beams through spatial multiplexing in different cores to avoid thermal instability issues [2], shifting the traditional trade-off involved in enlarging the mode area while preserving single-mode operation. Phase difference compensation between the various beams is achieved by a piezo mirror system, and experimental results show that onset of multi-modality in heated cores is one of the main causes of efficiency reduction [2].

In this work, we propose an analytical model for estimating confinement loss in Tube Lattice Fibers. It is based on the single-tube model and the inhibited coupling waveguiding mechanism. The comparison with numerical simulations of tube lattice fibers having different geometrical parameters and dielectric refractive indexes demonstrates the model validity and effectiveness. Being based only on analytical closed formulas, it constitutes a useful tool for rapid estimation of TLF CL. It also gives a more in-depth insight into the TLF guiding mechanisms, confirming the inhibited coupling is an appropriate and effective model for such kind of fibers.

Multi-core fiber capability to deliver several independent beams in a single structure has been deeply investigated to obtain spatial multiplexing in optical communication. Recently, the coherent beam multiplexing idea has been extended to high power fiber laser field, where multi-core fiber amplifiers, combining low power beams, promise to overcome thermal mode instability, which characterizes single-core fiber amplifiers. Although coherent output beam combination is advantaged in multi-core fiber, the understanding of core phase shifts is necessary to implement efficient beam combination. In presence of thermal load, induced by pump-to-signal conversion quantum defect, a refractive index gradient is formed on the multi-core fiber amplifier cross-section, thus changing core propagation properties and possibly creating unwanted core couplings. In this work a 9-core double-cladding fiber amplifier is numerically investigated by varying the core thermal load, from 2 to 15 W/m, in order to understand the structure propagation mismatch. The 9 cores are organized in a 3×3 regular grid, each core has a diameter of 19 μm and a spacing of 55 μm. Cores numerical aperture is 0.06. The outer cladding has a diameter of 340 μm. A comparison between a rod-type fiber amplifier configuration and a flexible fiber amplifier has been performed. Results show that the cores can be divided in three groups according to their propagation properties: central core, side cores, and corner ones. The phase shift between these groups, or equivalently the effective index difference, becomes higher with the increase of thermal load. These observations are fundamental to implement a model for beam propagation in presence of thermal effect, to investigate the amplification dynamics along z-direction.

A profound knowledge of the cladding mode properties and their interaction with the core modes is decisive for successfully designing inhibited-coupling (IC) guiding hollow-core photonic-crystal fibers (HC-PCFs). In IC fibers, the core and cladding modes coupling is robustly minimized due to a strong mismatch between their transverse phases and a small overlap between their fields. In the latest years, great attention has been given to IC fibers, especially since the introduction of the hypocycloid core contour concept [3], which entailed a dramatic loss reduction and preceded the development of single-ring tubular-lattice (SR-TL) HC-PCFs [4]. The interest on SR-TL HC-PCFs is due to their noteworthy properties such as geometric simplicity and the absence of connecting nodes in the fiber cladding, which favors IC guidance. Efforts on SR-TL HC-PCFs development entailed the accomplishment of ultra-low loss and broad transmission bandwidth [5]. Here, we show that the alteration of the lattice-tubes azimuthal position in SR-TL HC-PCFs causes modifications in the mode loss hierarchy and can be used to select the propagation of specific higher order modes (HOMs). It can be done by conveniently enlarging the spacing between the cladding tubes and by providing additional leakage to the fundamental mode while keeping low impact in the confinement loss (CL) of specific HOMs. It allows obtaining uncommon intensity profiles and polarization properties at the fiber output allowing it to act as a mode shaper.

International audience; Emission at 4.6 μm was observed from an N 2 O filled hollow core fiber laser. 8-ns pump pulses at 1.517 μm excited a vibrational overtone resulting in lasing on an R and P branch fundamental transition from the upper pump state. At optimum gas pressure of 80 Torr, photon conversion efficiency of 9% and slope efficiency of 3% were observed from a mirrorless laser. The laser threshold occurred at absorbed pump energy of 150 nJ in a 45-cm long fiber with 85 μm core diameter. The observed dependence of the laser output on gas pressure was shown to be a result of line broadening and relaxation rates.

We report on the development of hypocycloid core-contour inhibited-coupling (IC) Kagome hollow-core photonic crystal fibers (HC-PCFs) with record transmission loss and spectral coverage that include the common industrial laser wavelengths. Using the scaling of the confinement loss with the core-contour negative curvature and the silica strut thickness, we fabricated an IC Kagome HC-PCF for Yb and Nd:Yag laser guidance with record loss level of 8.5 dB/km associated with a 225-nm-wide 3-dB bandwidth. A second HC-PCF is fabricated with reduced silica strut thickness while keeping the hypocycloid core contour. It exhibits a fundamental transmission window spanning down to the Ti:Sa spectral range and a loss figure of 30 dB/km at 750 nm. The fibers’ modal properties and bending sensitivity show these HC-PCFs to be ideal for ultralow-loss, flexible, and robust laser beam delivery.

We report on the fabrication and characterization of 19-cell hypocycloid-shape Kagome fibers with core size larger than 100 μm. These inhibited coupling fibers present low propagation loss (100 dB/km) over broad transmission range with low chromatic dispersion combined with ultra-low power overlap with silica surround, making them an efficient solution for ultra-high power laser handling, ultra-fast laser delivery, and plasma photonics applications.

The dramatic progress in power-scaling of ultra-short pulse (USP) lasers and their continuous expansion use in industrial applications call for flexible and robust beam delivery systems (BDS) over several meters. Recently, a new branch of hollow-core photonic crystal fiber (HC-PCF) based on inhibited coupling (IC) mechanism has been proposed and successfully applied to demonstrate the delivery of milliJoule 600 femtosecond pulses in a several meter long piece and in robustly single-mode fashion [1]. Despite this breakthrough, it is desirable to reduce further the attenuation and enlarge the operating bandwidth of such fiber to increase the capability of the BDS whilst keeping the same delivery performances. In this context, a carefully optimization of the geometrical parameters of the core contour (i.e., the curvature b [2] and the silica thickness t) is studied resulting in the fabrication of new state-of-the-art IC Kagome HC-PCF combining losses less than 10 dB/km for the first time and associated to an enlarged transmission bandwidth able to cover all the entire industrial spectral range.

The dramatic progress in power-scaling of ultra-short pulse (USP) lasers and their growing use in industrial applications call for flexible and robust beam delivery systems (BDS) over several meters with no temporal or modal distortions. Inhibited coupling (IC) hypocycloid Kagome hollow-core photonic crystal fiber (HC-PCF) has recently proved to be an excellent solution for guiding these USP. In order to reduce further the attenuation of such fiber and then to increase the BDS capabilities, we report on an optimized IC Kagome HC-PCF exhibiting record loss level (8.5dB/km at 1030nm) associated with a 225nm wide 3-dB bandwidth and low bend sensitivity.

The potentialities in using hollow core tube lattice fibers based on inhibited coupling wave-guiding for label-free DNA detection are numerically investigated and discussed here. The proposed sensing approach does not require any additional transducer component such as Bragg gratings, amplifying techniques such as nanoparticles nor coherent sources. It simply consists of the measurement of the transmittance of a piece of fiber some ten centimeters long. In case of matching DNA sequence, an additional bio-layer is laid down the dielectric-air interface causing a red shift of the transmission spectrum of the fiber. Results show a spectral sensitivity on the bio-layer with shift as high as 42 nm for every 10 nm of bio-layer and robustness against imperfect fiber coupling. The proposed approach can be easily applied to sensing of other complex molecular structures where the presence/absence of analyte can generate or not an additional layer.

Hollow core microstructured fibers whose cladding is composed by an elliptical arrangement of circular dielectric tubes are numerically investigated. Birefringence, differential group delay, and polarization dependent loss are evaluated for different values of ellipticity in order to quantify its effect on the two polarizations of the fundamental core mode. All these parameters have a minimum at the center of each transmission window and they assume maximum values at the edges. In particular, the birefringence goes always to zero irrespective of ellipticity. This proves that tube lattice fibers are strongly immune against unwanted core shape deformation, in particular when the working frequency is close to the center of the transmission windows. On the other hand they are not suitable to obtain polarization maintaining fibers.

We report for the first time on tapering inhibited coupling (IC) hypocycloid-core shape Kagome hollow-core photonic crystal fibers whilst maintaining their delicate core-contour negative curvature with a down-ratio as large as 2.4. The transmission loss of down-tapered sections reaches a figure as low as 0.07 dB at 1550 nm. The tapered IC fibers are also spliced to standard SMF with a total insertion loss of 0.48 dB. These results show that all-fiber photonic microcells with the ultra-low loss hypocycloid core-contour Kagome fibers is now possible.

In this paper scaling laws governing loss in hollow core tube lattice fibers are numerically investigated and discussed. Moreover, by starting from the analysis of the obtained numerical results, empirical formulas for the estimation of the minimum values of confinement loss, absorption loss, and surface scattering loss inside the transmission band are obtained. The proposed formulas show a good accuracy for fibers designed for applications ranging from THz to ultra violet band.

In this paper, the dispersion and the confinement loss properties of polygonal tube fibers are investigated. Fano resonances between the hollow core modes and the high order dielectric modes are found inside transmission bands. A theoretical model based on the approximation of the polygonal tube as a perturbed circular one is proposed. The model is able to predict number and spectral position of the Fano resonances. It is shown that there always is a lower bound condition on the number of sides of the polygonal fiber which guarantees the absence of Fano resonances in a given frequency range. Numerical results are given in order to support the model.

Hollow core photonic bandgap fibres (HC-PBGFs) are numerically investigated in order to obtain low loss wave-guiding and good aperture field distribution in the terahertz region (0.1-10 THz) of the electromagnetic spectrum. The purity of the aperture field distribution at the HC-PBGF section combined with low loss propagation and high coupling efficiency with free-space propagating Gaussian beams suggest a possible employment of such a structure as aperture antennas, for possible feed systems in THz applications and in THz wireless sensing.

The mode properties of hollow-core fibers with a cladding formed by a periodic arrangement of dielectric tubes in a triangular lattice for THz applications are numerically investigated. The fiber supports a high number of modes. Effectively single-mode operation can be obtain by reducing the core size, but at the expense of high fundamental mode propagation loss. Single-mode propagation can be obtained by exciting the fiber with a linear polarized gaussian beam with proper spot size.

In this article, a hollow core photonic band gap fiber is numerically investigated to obtain low-loss waveguiding in the terahertz region (0.1–10 THz) of the electromagnetic spectrum. Waveguide design in this spectral region is an arduous challenge due to the high-conductivity losses of metals and high-absorption of the dielectrics. Numerical results show the possibility to reach propagation loss two decades lower than the bulk absorption losses of the material used to fabricate the fiber, and low dispersion over a wide range of wavelength. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1711–1714, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24407

We report on enlarged hollow-core diameter inhibited coupling Kagome fibers with record loss of 100 dB/km and ppm power overlap with silica surround, suitable for high energy ultrafast laser handling.

We report on design and fabrication of a new hypocycloid core-contour Kagome HC-PCF based on a triangular-like shaped core exhibiting smaller effective area and comparable confinement loss than previous hypocycloid core-contour Kagome fibers.

Scaling laws of confinement loss and dielectric overlap in circularly arranged tube lattice fibers are numerically analyzed. Confinement loss scales as the 4.5 power of wavelength-core size ratio whereas dielectric overlap dependence is weaker.

We report on tapered inhibited coupling Kagome-fiber with a down-ratio as large as 2.4 while maintaining the hypocycloid-core shape. The insertion-loss of SMF spliced to tapered Kagome-fiber was measured with minimum of 0.48dB at 1550nm.

An accurate analysis of the air-guiding in hollow core photonic bandgap fibers with a modified honeycomb air-hole lattice has been carried out. The influence of the hollow core dimension, as well as of the cladding geometric parameters on the confinement loss, the nonlinear coefficient and the single-mode behaviour of the fibers has been investigated through a full-vector modal solver based on the finite element method. Simulation results have shown that confinement loss lower than 0.1 dB/km, a nonlinear parameter lower than 0.01 (W · km)−1 and an effectively single-mode behaviour over a wavelength range of about 150 nm can be achieved with eight air-hole ring modified honeycomb fibers.

The amplification properties of different photonic crystal fibers have been studied by means of a full vector finite-element modal formulation combined with a population and propagation rate equation solver. A honeycomb as well as a cobweb photonic crystal fiber have been considered. The consequences of the defect dimension and the dopant radius on the field intensity distribution as well as the overlap integrals have been analyzed. Results demonstrate that a proper photonic crystal fiber design can be usefully exploited in order to obtain active fibers with superior characteristics compared to standard step index ones. In particular, photonic crystal fibers open up the possibility of a gain medium with highly flexible geometric, dispersion, and amplifying characteristics.

Several finite-element based time-domain approaches for the analysis of photonic devices are discussed in details. Narrowband and different kinds of wideband formulations are derived highlighting strengths and weaknesses related to the initial analytical equation and to the numerical implementation through the finite element method. A formulation which directly implements the unknown field without separating the envelope and the fast varying carrier is also presented. Performances and limits of the various algorithms are discussed by analyzing the simple and clear case of time and space evolution of gaussian beams propagating in a homogeneous medium. Comparison of the modules and arguments of the spectral response is also provided.

We report on numerical and experimental studies showing the influence of arc curvature on the confinement loss in hypocycloid-core Kagome hollow-core photonic crystal fiber. The results prove that with such a design the optical performances are strongly driven by the contour negative curvature of the core-cladding interface. They show that the increase in arc curvature results in a strong decrease in both the confinement loss and the optical power overlap between the core mode and the silica core-surround, including a modal content approaching true single-mode guidance. Fibers with enhanced negative curvature were then fabricated with a record loss-level of 17 dB/km at 1064 nm.

The development of low loss, small size and flexible waveguides is one of the most challenging issues of THz research due to the poor characteristics of both metal and dielectrics in this frequency range. Hollow core tube lattice fibers (HCTLFs) have been recently proposed and experimentally demonstrated to overcome this problem. However, they require very large hollow core size leading to big and hardly flexible fibers. Scaling law analysis plays an important role in determining the best trade-off between low loss and small fiber diameter. The dependence of the confinement on frequency and core radius are here numerically investigated. Results show that confinement loss exhibits a stronger dependence on core size and frequency with respect to other hollow core fibers proposed for THz waveguiding, such as Bragg, Tube, and Kagome fibers.

The development of low loss, broadband, small size and flexible waveguides has become a key issue in terahertz (THz) research due to the poor characteristics of both dielectrics and metals in this frequency range. Hollow core circular tube lattice fibers (HC-CTLFs) have been recently proposed and experimentally demonstrated to match many of these conditions. They are composed of a hollow core surrounded by a circular arrangement of dielectric tubes. CTLFs guide via Inhibited Coupling (IC) which doesn't rely on photonic bandgap and the confinement depends on a strong reduction of the coupling between core modes and cladding modes. The losses in CTLF can be divided into the confinement loss and the absorption loss. Both of them can be reduced by enlarging the fiber core size, but this makes the fiber big and hardly flexible. For an optimized design of the fiber, scaling law analysis plays an important role in determining the best trade-off between low loss and small size. Here the dependence of the confinement and absorption loss on frequency and core radius are numerically investigated. Results show that confinement loss exhibits a stronger dependence of core size and frequency with respect to other hollow core fibers proposed for THz waveguiding such as Kagome, Bragg, and Tube fibers as well as to dielectric absorption loss.

A new approach for mesh truncation in the finite-element vectorial modal analysis of dielectric waveguides is presented. It is based on layers with real anisotropic permittivity and permeability tensors. The approach does not require any iterative procedure, can directly calculate many modes at the same time, and is very accurate in providing both the vectorial field distribution and the propagation constant. The resulting matrices for the eigenvalue problem are still sparse and real, thus limiting the computational effort. Mode cutoff conditions can be easily studied with high precision. © 2002 John Wiley & Sons, Inc. Microwave Opt Technol Lett 32: 178–182, 2002.

A full-wave modal analysis of two-dimensional, lossy and anisotropic optical waveguides using the finite element method (FEM) is presented. In order to describe the behavior of radiating fields, anisotropic perfectly matched layer boundary conditions are applied for the first time in modal solvers. The approach has been implemented using high order edge elements. The resulting sparse eigenvalue algebraic problem is solved through the Arnoldi method. Application to an antiresonant reflecting optical waveguide is reported.

The differential equation that relates the Jones matrix of a polarization-mode dispersion fiber to its output dispersion vector is solved for a dispersion vector that moves on a circumference in the Stokes space. It yields a new simple model that can be usefully exploited to calculate the pulse-broadening analytical expression and to evaluate the system performance in terms of outage probability. A comparison among the results obtained with it, with other models proposed in literature, and with the numerical DRW model show its best accuracy. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 31: 449–453, 2001.

A finite element beam propagation method to analyze nonlinear active optical waveguides is presented. The model takes into account the mutual coupling among the equations describing the electromagnetic field propagation and the ones governing the signal amplification due to a doped medium. The approach is able to distinguish the different behavior of Ex (quasi-TE) and Ey (quasi-TM) polarizations. The performances of nonlinear and doped couplers are analyzed in detail by varying the input power, the field polarization and the pumping scheme.

A finite-element-based full-vectorial beam propagation method (BPM) for the analysis of the electromagnetic field evolution in three-dimensional (3-D) dielectric waveguides is presented. The approach has been derived in order to be suitable for the numerical discretization through finite elements and to fully account for the vectorial nature of the electromagnetic field. To this aim the problem related to the permittivity derivative at dielectric interface has been addressed and overcome. The advantages and the potentialities of the finite elements usage are discussed. The precision and the correctness of the proposed approach is demonstrated through numerical examples. Typical integrated optics structures, as a tapered waveguide and a Y-junction, are analyzed to assess the applicability of the method.

Air-guiding photonic bandgap fibers based on a modified honeycomb lattice have been numerically investigated through the finite element method. Results confirm that an extra hole in the unit cell of a honeycomb lattice can be exploited to enlarge the photonic bandgap, allowing air-guiding with confinement losses lower than 0.1 dB/km and nonlinear coefficient lower than 3.5middot10-3(Wmiddotkm) -1 at 1.55 mum

International audience; 10 meter-long Kagome HC-PCF were used to deliver mJ Yb-laser pulses and 600 fs pulse-width in single-mode fashion. Self-compression to ~50fs, and intensity-level nearing petawatt/cm2 were achieved. Free focusing-optics laser-micromachining is demonstrated on different materials.

International audience; Milli-Joule energy and 600-femtosecond Yb-laser pulses were delivered in several meter Kagome hollow-core-photonic-crystal-fibers in robustly single-mode fashion. Self-compression to 50fs, and intensity-level of petawatt/cm2 were achieved. Free focusing-optics laser-micromachining is demonstrated.

International audience; We report on numerical and experimental results on the influence of cladding ring number in hypocycloid-core Kagome HC-PCF. The number of rings has moderate effect on confinement loss whereas strong reduction in bend loss is demonstrated.

In this paper a flexible hollow core waveguide for the terahertz spectral range is demonstrated. Its cladding is composed of a circular arrangement of dielectric tubes surrounded by a heat-shrink jacket that allows the fiber to be flexible. Characterization of straight samples shows that the hollow core allows the absorption caused by the polymethylmethacrylate tubes of the cladding to be reduced by 31 times at 0.375 THz and 272 times at 0.828 THz with respect to the bulk material, achieving losses of 0.3 and 0.16 dB/cm respectively. Bending loss is also experimentally measured and compared to numerical results. For large bending radii bending loss scales as Rb-2, whereas for small bending radii additional resonances between core and cladding appear. The transmission window bandwidth is also shown to shrink as the bending radius is reduced. An analytical model is proposed to predict and quantify both of these bending effects.

Spectral properties of straight hollow core Circular Tube Lattice Fibers in the THz regime are analyzed both numerically and experimentally. Good agreement between simulated and manufactured fibers is found proving that these fibers have high potential for THz applications. Finally, bending properties are investigated with FEM simulations, showing that low-order transmission windows are strongly robust against extra losses due to the bending.

Dispersion and loss properties of hollow core tube fibers with elliptical cross section are numerically investigated. Results show unusual characteristics. For example, the birefringence always goes to zero in the middle of the low loss region irrespectively of the ellipticity and it always assume opposite sign approaching the two edges of the low loss region.

In this paper, a thorough numerical analysis of the confinement loss in kagome and tube lattice fibers is presented. The results show that the confinement loss strongly depends on the shape of the struts composing the core boundary and the cladding. This explains why confinement loss in kagome fibers is much higher than in tube lattice ones. In fact, the closer to a perfectly circular arc the struts, the lower the confinement loss. For this reason, struts shape must be carefully controlled during the fabrication process.

Confinement Loss of microstructured fibers whose cladding is composed by a triangular arrangement of tubes of various shapes is theoretically and numerically investigated. Kagome Fibers belong from this family of fibers with cladding tubes with hexagonal shape. The shape of the cladding tubes is proved to strongly affect the performance of the microstructured fiber. In order to understand the reasons for this behavior, the spectral properties of the tubes that constitute the cladding are investigated first. It is proved that also these tubes suffer from additional Fano Resonances when they are given a polygonal shape. It is proved that, by using the analytical model developed for the stand alone polygonal tubes, it is possible to predict the spectral position of Fano Resonances also in microstructured fibers. This is extremely important since it suggest new ways to reduce confinement loss in kagome fibers and microstructured fibers in general.

Confinement loss of inhibited coupling fibers with a cladding composed of a lattice of tubes of various shapes is theoretically and numerically investigated. Both solid core and hollow core are taken into account. It is shown that in case of polygonal shaped tubes, confinement loss is affected by extra loss due to Fano resonances between core modes and cladding modes with high spatial dependence. This explains why hollow core Kagome fibers exhibit much higher confinement loss with respect to tube lattice fibers and why hypocycloid core cladding interfaces significantly reduce fiber loss. Moreover it is shown that tube deformations, due for example to fabrication process, affect fiber performances. A relationship between the number of polygon sides and the spectral position of the extra loss is found. This suggests general guide lines for the design and fabrication of fibers free of Fano resonance in the spectral range of interest.

Waveguiding mechanism and modal characteristics of hollow core fibers consisting of a single or a regular arrangement of dielectric tubes are investigated. These fibers have been recently proposed as low loss, broadband THz waveguides. By starting from a description in terms of coupling between air and dielectric modes in a single tube waveguide, a simple and useful model is proposed and numerically validated. It is able to predict dispersion curves, high and low loss spectral regions, and the conditions to ensure the existence of low loss regions. In addition, it allows a better understanding of the role of the geometrical parameters and of the dielectric refractive index. The model is then applied to improve the tradeoff between low loss and effectively single mode propagation, showing that the best results are obtained with a heptagonal arrangement of the tubes.

We investigate realistic all-glass large-mode-area leakage channel fibers (LCFs) with a single down-doped-silica circular flattened-rod ring, engineering the silica bridge width for effectively single-mode behavior and reduced bending loss.

A new hollow core fiber for terahertz applications consisting of dielectric tubes arranged with an octagonal symmetry is presented. Simulation results have shown that this fiber exhibits an effectively single-mode propagation with lower propagation loss with respect to hollow core fibers with a hexagonal symmetry. The suppression of higher order modes is due to a better phase-matching condition with cladding modes propagating into the hollow core of tubes. In particular, by using tubes with 44-μm thickness and outer diameters as low as 0.5 mm, a rejection of high order modes higher than 20 dB/m and fundamental mode propagation loss lower than 5 dB/m from 2.30 to 2.85 THz can be obtained.

This work is focused on the analysis of Hollow Core Photonic Band Gap Fibres (HC-PBGFs) for THz applications. Waveguide design in this spectral region is a major challenge due to the high conductivity losses of metals and high absorption of the dielectrics. HC-PBGFs are dielectric waveguides in which the power fraction propagating in the dielectric is very low, and losses due to material absorption are dramatically reduced. HC-PBGFs have been proposed and developed from the end of 90's for applications in visible and Near InfraRed (NIR) spectrum [1], [2] and in few years propagation losses have been dramatically reduced from tens of dB/m to few dB/Km. These fibres suffers of high leakage and scattering loss [3], [4]. Leakage loss is due to the finite number of air hole composing the cladding lattice [9]. It can suppressed by incorporating enough air hole ring around the hollow core, though the higher is the number of hole rings, more difficult is the fabrication process. Scattering loss is due to fibre nonuniformity which couples guided mode with radiating modes. Fluctuation of technological origin can be reduced by more careful fibre fabrication. On the contrary surface roughness from surface capillarity waves cannot eliminated because of they thermodynamic origin [4]. However all these losses are inversely proportional to the wavelength, thus at THz frequencies their values do not significantly affect the overall loss mechanism. In particular, the purity of the aperture filed distribution at each PBGF section leads to a highly collimated output beam which can be eventually exploited for the design and development of feeds, even in array configuration, coupled to optical or electronic devices. The possible employment of such a structure as an aperture antenna has not been investigated, yet. The numerical analysis has been performed by using a complex modal solver based on the Finite element Method (FEM) [7], already applied in the past for the loss analysis of several kind of photonic band gap fibres [8], [9]

Propagation loss and dispersion characteristics of a hollow core microstructured fiber in the THz regime are numerically investigated. The fiber cladding consists of a triangular lattice formed by a periodic arrangement of dielectric tubes of Teflon. Numerical results show that guidance mechanism is the same of kagome and square lattice fibers. Low loss and low dispersion frequency ranges can be tuned just by changing the thickness of the tubes irrespectively of their diameter. Propagation loss four decades lower than the bulk absorption and dispersion lower 0.03 ps/nm km can be obtained over a band of about 1.7 THz centered at 2.1 THz .