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We demonstrated numerically the generation of broadband, coherent supercontinuum (SC) spectra in the mid-infrared region using dispersion-engineered all-chalcogenide microstructured fibers (MOFs). The 1-cm-long hexagonal fiber can be made with Ge11.5As24S64.5 glass as a low-refractive-index material embedded in a Ge11.5As24Se64.5 glass matrix. By optimizing the structural parameters, we determined a broad and flat all-normal-dispersion characteristic up to 14 μm. A highly coherent broadband SC with an intensity greater than −3 dB in the range from 5973 to 8695 nm is obtained when the fiber is pumped by a 7 μm laser with pulse duration of 50 fs and peak power of 6 kW. Flat-top SC of −30 dB level can be observed utilizing identical pump pulse parameters, covering wavelengths from 3823 to 13577 nm (>1.5 octaves). This broadband coherent MIR SC source can be applied in frequency metrology, optical coherence tomography, biomedical imaging, and few-cycle pulse compression.

In this manuscript, we report on, to the best of our knowledge, the first experimental realization of a multimode interference device based on self-image phenomenon accomplished by using a microstructured-cladding exposed-core fiber set in a singlemode-multimode-singlemode (SMS) configuration. Its application as a refractive index sensor is also demonstrated. The specialty multimode fiber studied herein exhibits a holey microstructure that surrounds and defines the fiber core. The latter, in turn, interfaces the external fiber environment thanks to lateral access along the entire fiber length. To explore the device's sensing capabilities, numerical simulations have been carried out, and the system sensitivity and spectral characteristics were evaluated. The experimental results demonstrate a refractive index sensitivity of 890 nm/RIU in the refractive index range from 1.41 to 1.43 with a detection limit of 2.7 × 10−3 RIU. This novel SMS configuration has the potential to extend the capabilities of current multimode interference-based sensors by providing an additional path to the realization of refractive index monitoring in liquid samples.

We propose a novel porous-core photonic crystal fiber (PCF) consisting of asymmetrical rectangular air holes in the core and six-ring hexagonal lattice circular air holes in the cladding for achieving low-loss polarization terahertz transmission in a wide frequency range. By assuming TOPAS as the host material, the finite element method (FEM) is used to investigate its properties. The near-zero flattened dispersion of −0.01±0.02 ps/THz/cm is achieved over a frequency range of 1.0–2.0 THz, as well as a high birefringence of 7.1 × 10−2 which can be useful for polarization-maintaining applications. Also, critical parameters such as mode field distribution, effective material loss, confinement loss, and effective mode area are discussed in detail. Further, fabrication possibilities are discussed briefly by comparing recent work on similar waveguide structures.OCIS Codes: 040.2235 (Far infrared or terahertz), 060.4005 (Micro-structured fibers), 060.2420 (Fibers, polarization-maintaining), 160.5470 (Polymers).

This paper reports the first microstructured solid-core fiber drawn from a 3D-printed preform and the first fiber Bragg gratings inscribed in a fiber of this type. The presented fiber is made of polycarbonate and displays single-mode behavior. The fiber attenuation was the lowest reported so far for a POF drawn from a 3D-printed preform across a broad range of wavelengths. In addition, extensive fiber characterization results are presented and discussed including: fiber attenuation, mode simulations, dynamic thermomechanical analysis and thermo-optic coefficient. Fiber Bragg gratings are successfully inscribed in the produced fiber using three different lasers: a continuous wave helium-cadmium laser, a pulsed femtosecond frequency doubled ytterbium laser and ultra-violet nanosecond krypton fluoride laser. Mechanical testing of the fiber showed that the 3D printing approach did not introduce any unexpected or undesirable characteristics. [ABSTRACT FROM AUTHOR]

In this article, we report high-speed multi-band direct-detection (DD) transmission over a hollow-core nested antiresonant nodeless fiber (NANF). Thanks to the ultrawide bandwidth of the NANF, we demonstrate dual-band transmission across the O- and C-bands over a ∼1-km length of a hollow-core fiber for the first time. Eight wavelength-division multiplexed (WDM) channels were transmitted using 100-Gb/s/λ Nyquist 4-ary pulse-amplitude modulation (PAM4) signals, which to the best of our knowledge, is the highest aggregate capacity ever transmitted in a DD hollow-core fiber-based transmission system. Optical pre-amplification was adopted for signal reception in both bands, achieved using an in-house built bismuth-doped optical fibre amplifier (BDFA) and a commercial erbium-doped fiber amplifier (EDFA) in the O- and C-band, respectively. We further demonstrate beyond 100-Gb/s/λ adaptively-loaded discrete multitone (DMT) transmission over the S+C+L-bands using the same NANF, without the use of optical amplification. Our experiments show that apart from fiber loss, the use of the NANF did not introduce any additional transmission penalties. The demonstrated results validate the ultrawide bandwidth and excellent modal purity of the fabricated NANF, which allow beyond 100-Gb/s/λ penalty-free transmission over multiple bands, highlighting the potential of this fiber technology for high-speed short- to intermediate-reach applications. [ABSTRACT FROM AUTHOR]

In this work, we present femtosecond laser cutting of microchannels in a nodeless antiresonant hollow-core fiber (ARHCF). Due to its ability to guide light in an air core combined with exceptional light-guiding properties, an ARHCF with a relatively non-complex structure has a high application potential for laser-based gas detection. To improve the gas flow into the fiber core, a series of 250 × 30 µm microchannels were reproducibly fabricated in the outer cladding of the ARHCF directly above the gap between the cladding capillaries using a femtosecond laser. The execution time of a single lateral cut for optimal process parameters was 7 min. It has been experimentally shown that the implementation of 25 microchannels introduces low transmission losses of 0.17 dB (

We propose a compact fluorescence sensing scheme based on hollow core Bragg fibers (HCBFs) that simultaneously serve as a sample cell, a collector, and a delivery channel for the desired fluorescence, as well as a filter for the residual excitation light mixed with fluorescence. The liquid samples filled in the air core are laterally irradiated by the excitation light for the spatial separation from fluorescence. Both the photonic bandgap (PBG) effect in the common HCBF (C-HCBF) and the transverse resonant behavior in the defect HCBF (D-HCBF) can be employed for filtering the residual excitation light mixed with fluorescence. The performance of the fluorescence sensing scheme strongly depends on the characteristics of PBG and modal loss for both types of HCBFs. According to the design principle, we present the design strategies of C-HCBF and D-HCBF for fluorescence sensing, respectively. Two typical examples are demonstrated to confirm the feasibility of our proposed fluorescence sensing scheme. We believe that our proposal would provide a new way to build a compact fluorescence sensing system.

Multiwavelength reflection spectra induced by an acoustically modulated fiber Bragg grating (FBG) in a highly birefringent suspended core fiber are experimentally investigated. Longitudinal acoustic waves interacting with a grating generate side lobes in the reflectivity spectrum and produce a superposed reflection band. The reflectivity of up to five wavelength peaks can be actively tuned by the voltage of the electrical signal inducing the acoustic waves. This indicates new possibilities for compact and fast multiwavelength dynamic and fiber-integrated reflection filters.

A single longitudinal mode microring dye laser based on a simplified hollow-core microstructured optical fiber (SHMOF) is proposed and demonstrated. The dye laser uses a submicron thickness silica microring embedded in the SHMOF as resonator and has the characteristic of a low threshold of 664 nJ/mm2. The factors that influence the single longitudinal mode excitation are discussed and the possible operation principle is analyzed. Meanwhile, the single longitudinal mode emissions at several different wavelengths are realized by using different dyes as gain medium, which indicates that the lasing wavelength of the laser is scalable. Time domain characteristics of the lasing are also measured and they are synchronous with the lateral pumping laser. The attractive possibility of developing microfluidic single longitudinal mode dye laser within the SHMOF presents opportunities for integrated optics applications and biomedical analysis with high sensitivity.

We introduce a novel multiplexing technique applied to optical fiber distributed sensors, based on optical backscatter reflectometry (OBR) and high-scattering MgO-doped fibers. In this paper, we demonstrate the possibility of simultaneously detecting multiple fiber with a single scan using an OBR distributed sensor, and successfully discriminating each sensing region (with ∼1 mm spatial resolution). The sensing element is a high-scattering fiber with MgO-based nanoparticles doping in the core, that emits a scattering signal more than 40 dB larger than a standard fiber, while having similar temperature and strain sensitivity. Multiplexing occurs as the scattered light from a sensing fiber overshadows the amount of scattering occurring in all the other channels. The setup has been validated for temperature sensing and implemented in an epidural catheter with multiple fibers fixed to the outer walls for strain sensing. The proposed solution goes beyond the multiplexing methods which exploit 1 × N switches, as the multiplexing is simultaneous and not rearranged in different time slots. [ABSTRACT FROM AUTHOR]

In this paper, we report how tube lattice hollow-core fibers can be successfully used to build sensors for molecule detection. The inner silica surface of the fiber is functionalized and coated with a probe layer, which permits to bond only with a particular molecule (the target). When the fiber is infiltrated with a solution containing the target, an additional layer is created on the silica surface, causing a redshift of the fiber transmission spectrum. The technique does not require any additional transducer component, such as Bragg gratings, amplifying techniques, such as nano-particles, nor coherent sources. It simply consists of the measurement of the transmission spectrum of a piece of fiber some tens of centimeters long. The principle is validated with experimental results showing the detection of the streptavidin protein. A solution containing streptavidin was flowed through the hollow core of the fiber coated with biotin. The measurement of the transmitted spectrum before and after the infiltration showed the presence of a few nanometer thick bio-layer. [ABSTRACT FROM AUTHOR]

Silicon is a widely used material due to its strong properties especially low loss and high nonlinearity in the wavelength range 1.2–6 μm. Also, photonic crystal fibers (PCFs) have been extensively involved in producing spectral light broadening in the literature. Here we report supercontinuum generation (SCG) over the wavelength range 1000–3000 nm from a silicon-core/silica-cladding PCF with length of 10 mm. The proposed PCF produces light broadening at both telecommunication wavelengths 1.3 μm and 1.55 μm as well as at the zero dispersion wavelength (ZDW = 2.0 μm). The effect of altering the core size, pump power and crystal parameters on the generated spectra is discussed. It is found that the SC spectra wide is increased by 715 ± 50 nm by increasing the pump wavelength from 1.3 μm to 2.0 μm. Further, the achieved SC spectra with broadening bandwidths of 892–1659 ± 50 nm has more than double the broadening of various silicon on insulator (SOI) waveguides with bandwidths range 330 nm–990 nm. [ABSTRACT FROM AUTHOR]

Long-period optical fiber gratings (LPGs) are one of the widely used concepts for the sensing of refractive index (RI) changes. Negative curvature hollow-core fibers (NCHCFs), with their relatively large internal diameters that are easy to fill with liquids, appear as a very interesting medium to combine with the idea of LPGs and use for RI sensing. However, to date, there has been no investigation of the RI sensing capabilities of the NCHCF-based LPGs. The results presented in the paper do not only address this matter, but also compare the RI sensitivities of the NCHCFs alone and the gratings. By modeling two revolver-type fibers, with their internal diameters reflecting the results of the possible LPG-inscription process, the authors show that the fibers’ transmission windows shift in response to the RI change, resulting in changes in RI sensitivities as high as −4411 nm/RIU. On the contrary, the shift in the transmission dip of the NCHCF-based LPGs corresponds to a sensitivity of −658 nm/RIU. A general confirmation of these results was ensured by comparing the analytical formulas describing the sensitivities of the NCHCFs and the NCHCF-based LPGs.

A surface plasmon resonance (SPR) sensor based on a hollow fiber (HF) is proposed to realize high analyte refractive index (RI) detection. The hole in the HF as a microfluidic channel for the analyte is filled with a silver wire to replace the metal coating. The sensitivity of the proposed SPR sensor with analyte RI from 1.47 to 1.51 is theoretically investigated both in the wavelength and amplitude interrogation methods. The results show that the sensor can support two disparate resonance peaks, with orthogonal polarizations (x-and y-polarized peaks), and the sensitivity of the y-polarized peak is higher than that of x-polarized peak. Moreover, contrary to the performances of the resonance peaks supported by the low RI SPR sensors, the two polarized peaks shift to shorter wavelength as analyte RI increasing and show a higher sensitivity at low analyte RI. These results and analyses, including the abnormal behaviors of resonance peaks and the coupling condition between the core modes and the plasmon modes, are very helpful for the design and improvement of high RI SPR sensors.

A surface plasmon resonance sensor is proposed to simultaneously realize the refractive index (RI) and temperature sensing in an exposed-core microstructured optical fiber. Two orthogonal sensing channels coated with silver layers are designed to distinguish the variations of the analyte RI and the temperature. The exposed section of the fiber as an RI sensing channel supports a y-polarized peak. The orthogonally arranged hole is filled with a large thermooptic coefficient liquid as a temperature sensing channel that supports a x-polarized peak. The two polarized peaks can be shifted independently. By following the shifts of the two polarized peaks, the variations of the RI and temperature can be detected simultaneously.

We propose a surface plasmon resonance (SPR) sensor based on the exposed-core microstructured optical fiber (EC-MOF) placed with a silver wire. The exposed section of the EC-MOF as a microfluidic channel is placed with the silver wire to avoid the metal coating and is then deposited with the analyte to avert the analyte filling. The proposed SPR sensor can support two polarized resonance peaks (x-polarized and y-polarized) caused by the silver wire. We theoretically investigate the sensitivities of the two polarized peaks both in the wavelength and amplitude interrogation methods and analyze the influences of the silver wire location on the sensing performances. The results show that the sensitivities of the two polarized peaks are similar and relatively stable for random locations of the silver wire. Moreover, the x-polarized peak provides a higher resolution for wavelength sensitivity and needs a shorter silver wire for the maximum amplitude sensitivity. This paper demonstrates that using the EC-MOF placed with the silver wire can simultaneously solve the metal coating and analyte filling problems in the other SPR sensors with no sacrifice in sensitivity.

The cross-section image of microstructure optical fiber (MOF) is usually characterized by the irregular shape, disordered distribution of the air pores, and multiple sources of noise. The traditional modeling of fiber structure does not work for these MOFs, and it is difficult to obtain the actual cross-section structure. A new method based on the digital image processing technique and finite element method (FEM) is introduced. With this method, the actual cross-section structure of MOFs can be rapidly modeled by gray scale processing, filtering, threshold, and edge detection, which is vital to the simulation of the basic properties of the fiber with FEM precisely. The method is proved to be feasible and reliable in that the dispersion coefficients of an actual fiber simulated are greatly consistent with the experimental results. In addition, the influence of perfect matched layer thinkness, as well as the curve fitting interval selection of the dispersion coefficients on the research results, is explored in the paper, which forms a basis for the correct setting of these parameters. The method has the advantages of strong adaptability, good modeling effect, rapid simulation, and accurate results. Finally, the method applies to all kinds of cross-section modeling of fiber, especially for disordered structure modeling.

We demonstrate a dual-core fiber-based Mach–Zehnder interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The fiber used here have a solid germanium-doped silica core and an air core that allows gases to flow through. Coherent laser beams are coupled to the two cores, respectively, and thus excite guiding modes thereby. Interferogram would be produced as the light transmitted from the dual cores interferes. Variations in refractive index of the hollow core lead to variations in phase difference between the modes in the two cores, thus shifting the interference fringes. The fringe shifts can be then interrogated by a photodiode together with a narrow slit in front. The resolution of the sensor was found to be ~1 × 10−8 RIU, that is comparable to the highest resolution obtained by other fiber sensors reported in previous literatures. Other advantages of our sensor include very low cost, high sensitivity, straightforward sensing mechanism, and ease of fabrication.

In this work, we present for the first time a laser-based dual gas sensor utilizing a silica-based Antiresonant Hollow-Core Fiber (ARHCF) operating in the Near- and Mid-Infrared spectral region. A 1-m-long fiber with an 84-µm diameter air-core was implemented as a low-volume absorption cell in a sensor configuration utilizing the simple and well-known Wavelength Modulation Spectroscopy (WMS) method. The fiber was filled with a mixture of methane (CH4) and carbon dioxide (CO2), and a simultaneous detection of both gases was demonstrated targeting their transitions at 3.334 µm and 1.574 µm, respectively. Due to excellent guidance properties of the fiber and low background noise, the proposed sensor reached a detection limit down to 24 parts-per-billion by volume for CH4 and 144 parts-per-million by volume for CO2. The obtained results confirm the suitability of ARHCF for efficient use in gas sensing applications for over a broad spectral range. Thanks to the demonstrated low loss, such fibers with lengths of over one meter can be used for increasing the laser-gas molecules interaction path, substituting bulk optics-based multipass cells, while delivering required flexibility, compactness, reliability and enhancement in the sensor’s sensitivity.

Fluorescence anisotropy imaging and sensing is a widely recognized method for studying molecular orientation and mobility. However, introducing this technique to in vivo systems is a challenging task, especially when one considers multiphoton excitation methods. Past two decades have brought a possible solution to this issue in the form of hollow-core antiresonant fibers (HC-ARFs). The continuous development of their fabrication technology has resulted in the appearance of more and more sophisticated structures. One of the most promising concepts concerns dual hollow-core antiresonant fibers (DHC-ARFs), which can be used to split and combine optical signals, effectively working as optical fiber couplers. In this paper, the design of a fluorescence anisotropy sensor based on a DHC-ARF structure is presented. The main purpose of the proposed DHC-ARF is multiphoton-excited fluorescence spectroscopy; however, other applications are also possible.

Optical microfiber tapers provide an advantageous platform for sensing in aqueous and gas environments. We study experimentally the photonic transmission in optical fiber tapers coated with polymethyl methacrylate (PMMA), a polymeric material widely used in optical applications. We demonstrate a durable and simple humidity sensing approach incorporating tapered microfibers attached to silicon (Si) substrate coated with active polymer layer. A model is described for the load stress effect on the birefringence giving rise to interferences in the transmission spectra, strongly dependent on the coating layer thickness, and disappearing following its slow uniform removal. The sensing approach is based on characterization of the interference patterns observed in the transmission spectra of the taper in the NIR range. The device demonstrated persistent detection capability in humid environment and a linear response followed by saturation to calibration analytes. In each analyte of interest, we define principal components and observe unique calibration plot regimes in the principal component space, demonstrating vapor sensing using polymer coated microtapers.

In this paper, a novel dual-guided microstructured fiber supporting orbital angular momentum (OAM) mode and linear polarization (LP) mode transmission is proposed to solve the high-volume data transmission. A variety of parameters of the fiber are considered comprehensively under different conditions by using the full-vector finite element method. The results show that the fiber can support 30 OAM modes and 2 LP modes over the whole C wavelength band by making full use of the air-holes to isolate electromagnetic field. The crosstalk between two channels is lower than that previously reported, and the total dispersion is nearly zero and flat. For instance, the isolation parameter of the EH71 mode reaches up to 86.02 dB and the dispersion coefficient varies from −  0.26 to 1.62 ps/(km nm). The large index difference between core and cladding is beneficial to low crosstalk. In addition, this fiber is easier to fabricate, because the preform needs only stacking technique to adjust the structure geometry size. This fiber can be used in short-distance and large-capacity transmission system. [ABSTRACT FROM AUTHOR]

Highlights • SC-MOF refractometric sensors are suitable for detection of wide range of analytes. • Generalized model can be easily used as a universal tool for SC-MOF sensor design. • Usability of SC-MOF sensors can be predicted based on our generalized model. • SC-MOF sensors exhibit sufficient sensitivity for gasoline-blend quality monitoring. Abstract A generalized model for the detection of liquids within suspended-core microstructured optical fibers has been experimentally and theoretically derived. The sensor detection is based on the refractometric principle of transmission losses due to the overlap of the evanescent field with the liquid analyte. A number of parameters, including fiber core diameter and filling length, have been included in the general model. Specially tailored suspended-core fibers were manufactured with the core diameters within the range of 2.4 μ m to 4.0 μ m. Five selected liquid analytes were used to cover the refractive index range of 1.35 to 1.42. Based on experiments, the characteristics of the parameters of the semi-empirical model have been determined by a genetic algorithm using 283 measurement data sets. The model can be used to design sensors for the detection of liquid analytes as it provides a set of parameters allowing to optimize the sensor's sensitivity for a wide scale of applications. Finally, numerical simulations of the system were carried out by an eigenmode routine to support the results of the generalized model. [ABSTRACT FROM AUTHOR]

A dynamic multiwavelength reflection filter is acoustically induced by the reflectivity modulation of a fiber Bragg grating in a suspended-core fiber. The fiber composed by four air holes is combined with a specially designed acousto-optic modulator to increase the modulation efficiency and to reduce significantly the device size. The effect of the two strongest modulator modes on the grating is evaluated at 374 and 474 kHz. The high-frequency acoustic waves are amplitude modulated by a low-frequency signal from 100 to 500 Hz, generating resonant wavelengths peaks in the grating spectrum. The modulated grating reflectivity is experimentally characterized in the spectral domain. The modulation depth, bandwidth, and number of the induced resonances are tuned by the voltage and frequency of an electrical signal. Up to 15 wavelength peaks with a spectral bandwidth of ∼5 pm and a modulation depth up to 74% are generated, indicating new possibilities for compact and fast all-fiber acousto-optic devices. [ABSTRACT FROM AUTHOR]

Hollow-core photonic bandgap fibers (HC-PBFs) are suitable for spaceborne fiber optical gyroscopes owing to their excellent environmental adaptability. However, hundreds of small holes full of air at one atmosphere of pressure can make the HC-PBF sensitive to external atmospheric pressure. In this study, we investigated the phase sensitivity of the fundamental mode to external atmospheric pressure for the HC-PBF, and the experimental result indicates that the phase sensitivity is approximately 1.6 × 10 −5  ppm/Pa, which is mostly contributed by the change in the pressure-induced length. Through the choice of coating, the phase sensitivity to external atmospheric pressure can be reduced by about a factor of five compared to current HC-PBFs, and the excellent temperature performance can be maintained at the same time. [ABSTRACT FROM AUTHOR]

We present the design of fluorotellurite microstructured fibers (FTMFs) with near-zero-flattened dispersion profiles in the wavelength range of 1500-1600 nm for optical frequency comb (OFC) generation. The FTMFs are based on TeO2 -BaF2 - Y2O3 glass with a zero-dispersion wavelength of ~2160 nm. The fiber core is surrounded by the two layers of air holes. The addition of large size air holes in the outer layer is to shift the zero-dispersion wavelength of the fiber from ~2160 nm to the wavelength region of 1500-1600 nm. The addition of small size air holes in the inner layer is tomake the dispersion profile of the fiber become flat in the wavelength range of 1500-1600 nm. By optimizing the parameters (the core diameter, the air hole size, the pitch of air holes, etc.) of FTMFs, the fiber with the chromatic dispersion value between -0.5 and 0.3 ps/nm/km in thewavelength range of 1500-1600 nm is achieved. Furthermore, numerical simulations are used to showthe potential of the fiber for OFC generation via cascaded four-wave mixing. Flat topOFC extending from 1500 to 1600 nm with tunable mode spacing from 25 to 100 GHz can be generated in a 2-m-long fiber by using a 1550-nm laser with a pulse width of 0.85 ps and a peak power of 49 Was the pump source. The effects of stimulated Brillouin scattering on OFC generation are also investigated [ABSTRACT FROM AUTHOR]

We have numerically studied different designs of technologically feasible microstructured fibers with a germanium-doped core in order to obtain normal dispersion reaching possibly far in the mid infrared. Hexagonal, Kagome and the combination of both geometries were numerically examined with respect to different constructional parameters like pitch distance, filling factor of air holes, number of layers surrounding the core, and level of germanium doping in the core. Our analysis showed that the broadest range of normal dispersion reaching 2.81 μm, while keeping an effective mode area smaller than 30 μm², was achieved for a hexagonal lattice and a 40 mol% GeO2 doped core. The proposed fibers designs can be used in generation of a normal dispersion supercontinuum reaching the mid-IR region. [ABSTRACT FROM AUTHOR]

We propose an all-fiber orbital angular momentum (OAM) generator/converter based on photonic crystal fiber (PCF). The PCF is designed to introduce a helical effective refractive index profile in order to achieve the OAM matching condition that is required to excite the OAM modes of an OAM-supporting fiber. The proposed design is compact, presents wideband operation, low loss and high OAM purity. It is of great interest to OAM-based telecommunication, and other OAM applications that use optical fibers. [ABSTRACT FROM AUTHOR]

This review is focused on microstructured optical fiber sensors developed in recent years for liquid RI sensing. The review is divided into three parts: the first section introduces a general view of the most relevant refractometric sensors that have been reported over the last thirty years. Section 2 discusses several microstructured optical fiber designs, namely, suspended-core fiber, photonic crystal fiber, large-core air-clad photonic crystal fiber, and others. This part is also divided into two main groups: the interferometric-based and resonance-based configurations. The sensing methods rely either on full/selective filling of the microstructured fiber air holes with a liquid analyte or by simply immersing the sensing fiber into the liquid analyte. The sensitivities and resolutions are tabled at the end of this section followed by a brief discussion of the obtained results. The last section concludes with some remarks about the microstructured fiber-based configurations developed for RI sensing and their potential for future applications.

The acousto-optic modulation of fiber Bragg gratings in a four-hole suspended core fiber is experimentally demonstrated. Strong modulations with a reflectivity amplitude decrease by up to 67% and a 57% bandwidth increase in the Bragg resonance are obtained for gratings of 0.26- and 1-nm 3-dB bandwidths, respectively. The reduction of the required acoustic power for achieving the acousto-optic modulation compared to conventional solid-core single-mode fibers points to more efficient modulator devices in suspended core fibers.

We discuss the fabrication of hollow-core optical fibers made of borosilicate glass. We show that, despite the high attenuation of the glass relative to silica, the fiber optical losses can be of the same order of magnitude of those obtained by using ultrapure silica glass. Short lengths of the fabricated fibers, used in combination with incoherent optical sources, provide single-mode optical guidance in both near and mid-infrared spectral ranges without any additional optical components.

The layered spiral microstructured optical fiber (LS-MOF) design allows higher nonlinearities than the most common microstructured optical fibers. Here, we have chosen a highly nonlinear glass for its composition, the arsenic trisulfide, and we have determined its dispersion and nonlinear characteristics. After adjusting the fiber's parameters, we obtained a record value for the nonlinear parameter of 50.7Wm, at 1.550m. We have simulated light propagation under these optimized circumstances, achieving a broad supercontinuum, extending from 500nm to 3900nm, in a very short distance: 0.3mm. [ABSTRACT FROM AUTHOR]

In this letter, the asymmetric long period fiber gratings (aLPFGs) writing in hollow ring-core fiber is proposed to excite orbital angular momentum (OAM) modes. To reveal the characteristics of aLPFGs, the transmitted power of the modes supported in the ring core is calculated by coupled-mode theory analytically. The resonance wavelength for the OAM mode can be manipulated by both the grating period and the refractive index of liquid filled in the fiber. The good wavelength tunability and broadband performance can be guaranteed by choosing the grating period near the turning point of dual-wavelength resonance of the aLPFGs. The proposed fiber mode convertor based on aLPFGs is flexible and compact, making it a good candidate for tunable OAM mode excitation and optical fiber sensor. [ABSTRACT FROM PUBLISHER]

We investigated numerically the possibility of all normal dispersion fiber design for near-infrared supercontinuum generation based on a standard air-silica microstructure. The design procedure includes finding of target dispersion profile and subsequent finding of appropriate geometrical fiber design by inverse dispersion engineering. It was shown that the tailoring of dispersion profile could increase the spectral width of generated supercontinuum while maintaining perfect spectral flatness. Conditions necessary for wide and flat supercontinuum generation as well as restrictions imposed by chosen materials were discussed. As a result of design and optimization procedure, an air-silica design was found providing normal dispersion up to \text3\;\mu \textm. Simulation results with 10 nJ, 100 fs pulses demonstrate supercontinuum generation up to 1.3 octave; whereas pumping with 30 nJ, 100 fs pulses could provide 1.8 octavse supercontinuum. [ABSTRACT FROM PUBLISHER]

We propose a high-linearity refractive index (RI) sensor based on an analyte-filled defect hollow core Bragg fiber (DHCBF) for large dynamic ranges. A resonant wavelength induced by the transverse coupling between core modes and defect modes is used as the characteristic wavelength for realizing a high-linearity RI sensor. The numerical results show that the linear correlation between characteristic wavelength and analyte RI can be significantly improved by optimizing structural parameters of a defect layer, which would be beneficial to the enlargement of dynamic measurement range. Furthermore, we numerically demonstrated an RI sensor with the sensitivity of 2062 nm/RIU and the adjusted R-Square value of 0.99967 in the measurement range of 1.324–1.432, by using an analyte-filled DHCBF with a low RI-contrast cladding. Compared with that based on a standard HCBF, our proposed high-linearity RI sensor can achieve high sensitivity in a larger dynamic range. [ABSTRACT FROM PUBLISHER]

We propose a method based on the white-light interference technique for measuring the Verdet constant of a polarization-maintaining air-core photonic bandgap fiber (PM-PBF). The experimental results show that the Verdet constant of the PM-PBF is ~3.3 mrad/T/m for the broadband light with a spectral width of ~38 nm and a mean wavelength of ~1550 nm, which is ~124 times less than that of a conventional stress-induced birefringent fibers called PANDA fibers (~0.41 rad/T/m for the same broad-spectrum light). The results indicate that the nonreciprocal error induced by the Faraday effect in a fiber optic gyroscope (FOG) made of the PM-PBF is theoretically ~25 times less than that of a conventional FOG made of the PANDA fiber when other conditions, such as the fiber twist, fiber coil area, and so on, are the same. [ABSTRACT FROM AUTHOR]

We present three fold symmetric microstructured fibers and their application potential for customized single mode supercontinuum generation. In our microstructured fibers we simultaneously achieve high nonlinearity and single mode operation by increasing three cladding air holes, which are next to the core. Selectively modified microstructures allow a large degree of freedom in the dispersion profile design. With the use of sub nanosecond pump source at the wavelength of 1064 nm we show, that the designed microstructure provides various ways of generating a unique flat and broadband spectrum. [ABSTRACT FROM AUTHOR]

We propose a compact fluorescence sensing scheme based on hollow core Bragg fibers (HCBFs) that simultaneously serve as a sample cell, a collector, and a delivery channel for the desired fluorescence, as well as a filter for the residual excitation light mixed with fluorescence. The liquid samples filled in the air core are laterally irradiated by the excitation light for the spatial separation from fluorescence. Both the photonic bandgap (PBG) effect in the common HCBF (C-HCBF) and the transverse resonant behavior in the defect HCBF (D-HCBF) can be employed for filtering the residual excitation light mixed with fluorescence. The performance of the fluorescence sensing scheme strongly depends on the characteristics of PBG and modal loss for both types of HCBFs. According to the design principle, we present the design strategies of C-HCBF and D-HCBF for fluorescence sensing, respectively. Two typical examples are demonstrated to confirm the feasibility of our proposed fluorescence sensing scheme. We believe that our proposal would provide a new way to build a compact fluorescence sensing system. [ABSTRACT FROM PUBLISHER]

Multiwavelength reflection spectra induced by an acoustically modulated fiber Bragg grating (FBG) in a highly birefringent suspended core fiber are experimentally investigated. Longitudinal acoustic waves interacting with a grating generate side lobes in the reflectivity spectrum and produce a superposed reflection band. The reflectivity of up to five wavelength peaks can be actively tuned by the voltage of the electrical signal inducing the acoustic waves. This indicates new possibilities for compact and fast multiwavelength dynamic and fiber-integrated reflection filters. [ABSTRACT FROM PUBLISHER]

We experimentally characterized a birefringent side-hole microstructured fiber in the visible wavelength region. The spectral dependence of the group and phase modal birefringence was measured using the methods of spectral interferometry. The phase modal birefringence of the investigated fiber increases with wavelength, but its positive sign is opposite to the sign of the group modal birefringence. We also measured the sensing characteristics of the fiber using a method of tandem spectral interferometry. Spectral interferograms corresponding to different values of a physical parameter were processed to retrieve the spectral phase functions and to determine the spectral dependence of polarimetric sensitivity to strain, temperature and hydrostatic pressure. A negative sign of the polarimetric sensitivity was deduced from the simulation results utilizing the known modal birefringence dispersion of the fiber. Our experimental results show that the investigated fiber has a very high polarimetric sensitivity to hydrostatic pressure, reaching –200 rad x MPa–1 x m–1 at 750 nm.

The strain sensitivity of a two-mode birefringent side-hole fiber is measured in the spectral domain. In a simple experimental setup comprising a broadband source, a polarizer, a two-mode birefringent side-hole fiber under varied elongations, an analyzer and a compact spectrometer, the spectral interferograms are resolved. These are characterized by the equalization wavelength at which spectral interference fringes have the highest visibility (the largest period) due to the zero group optical path difference between the fundamental, the LP01 mode and the higher-order, the LP11 mode. The spectral interferograms with the equalization wavelength are processed to retrieve the phase as a function of the wavelength. From the retrieved phase functions corresponding to different elongations of a two-mode birefringent side-hole fiber under test, the spectral strain sensitivity is obtained. Using this approach, the intermodal spectral strain sensitivity was measured for both x and y polarizations. Moreover, the spectral polarimetric sensitivity to strain was measured for the fundamental mode when a birefringent delay line was used in tandem with the fiber. Its spectral dependence was also compared with that obtained from a shift of the spectral interferograms not including the equalization wavelength, and good agreement was confirmed.

We present systematic predictions for light enhancement in optical nanowires with nanoscale air holes in the core through numerical modeling. We show that the light intensity within such holes is strongly dependent on the hole size and refractive index of the host material and that light enhancement becomes significant only when the hole size is less than a critical value: 70 nm for silica and F2 nanowires and 50 nm for a As2S3 nanowire. High index As2S3 nanowires exhibit nearly eight times higher average mode intensity than silica glass for hole sizes of less than 10 nm. Such intensity enhancements open up new device opportunities; for example, filling nanoscale holes within silicon nanowires with silicon nanocrystals enables 30% enhancement of the nonlinear coefficient.

Here, we analyze the dispersion behavior of revolver-type anti-resonant hollow core fibers, revealing that the chromatic dispersion of this type of fiber geometry is dominated by the resonances of the glass annuluses, whereas the actual arrangement of the anti-resonant microstructure has a minor impact. Based on these findings, we show that the dispersion behavior of the fundamental core mode can be approximated by that of a tube-type fiber, allowing us to derive analytic expressions for phase index, group-velocity dispersion and zero-dispersion wavelength. The resulting equations and simulations reveal that the emergence of zero group velocity dispersion in anti-resonant fibers is fundamentally associated with the adjacent annulus resonance which can be adjusted mainly via the glass thickness of the anti-resonant elements. Due to their generality and the straightforward applicability, our findings will find application in all fields addressing controlling and engineering of pulse dispersion in anti-resonant hollow core fibers.

• Mid-infrared solitons by Raman induced self-frequency shifting from 2.8 μm to 3.7 μm. • All-fiber femtosecond tunable mid-infrared laser source consuming low operation power. • Adjusted group velocity dispersion and nonlinearity by optimizing the fiber structure. • Chalcogenide microstructured fiber with blueshifted zero-dispersion wavelength and a low loss. Compact optical devices with an output wavelength range of more than 3 μm are limited due to the requirements of proper material and structural design. In this study, the phenomenon of a soliton self-frequency shift pumped by a femtosecond pulse in a chalcogenide microstructured fiber is investigated numerically. The structural parameters of the fiber are optimized to obtain far-separated zero-dispersion wavelengths at 2.24 µm and 5.34 µm with low confinement loss. A femtosecond tunable soliton with a frequency shifted up to 3.7 µm is achieved from a 10-cm long fiber with a conversion efficiency above 38.6 %, consuming a pump pulse energy of no more than 200 pJ at 2.8 µm, which is available from Er-doped fluoride fibers. Our work can provide an effective way to construct compact, all-fiber femtosecond tunable mid-infrared laser sources with low operation power. [ABSTRACT FROM AUTHOR]

We detailedly describe a recently-developed semi-analytical method to quantitatively calculate light transmission properties of hollow-core anti-resonant fibers (HC-ARFs). Formation of equiphase interface at fiber’s outermost boundary and outward light emission ruled by Helmholtz equation in fiber’s transverse plane constitute the basis of this method. Our semi-analytical calculation results agree well with those of precise simulations and clarify the light leakage dependences on azimuthal angle, geometrical shape and polarization. Using this method, we show investigations on HC-ARFs having various core shapes (e.g. polygon, hypocycloid) with single- and multi-layered core-surrounds. The polarization properties of ARFs are also studied. Our semi-analytical method provides clear physical insights into the light guidance in ARF and can play as a fast and useful design aid for better ARFs.

The recent developments on chalcogenide glass hollow core microstructured optical fibers (HC-MOFs) are presented. The comparative analysis of simulated optical properties for chalcogenide HC-MOFs of negative-curvature with different size and number of capillaries is given. The technique for the manufacture of microstructured chalcogenide preforms, which includes the assembly of the substrate glass tube and 8-10 capillaries, is described. Further trends to improve the optical transmission in chalcogenide NCHCFs are considered.