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Fusion splicing of solid-core microstructured silica fibers has been one of the key enablers which opened practical applications of these structures in ultrafast light sources or fiber-based sensors. Anti-resonant hollow core fibers are special in this context, because their optical properties critically depend on single-micron or sub-micron thin wall membranes. This corresponds to high thermal isolation of their cladding structure and makes thermal processing of these fibers challenging. We investigate fusion splicing feasibility of a single capillary ring anti-resonant hollow core fiber made of silica glass. We begin by splicing pairs consisting of standard single mode and hollow core fibers, followed by pairs of polarization maintaining and hollow core fibers. Splice loss is determined within the 1450–2000 nm transmission window of the fiber at around 1–2 dB, depending on wavelength along the transmission window, in the transmission direction from the step index fiber into the hollow core fiber. Maintenance of polarization is verified and polarization extinction ratio of no less than 10 dB is recorded for the ARF spliced with a polarization maintaining fiber. Mechanical robustness of the fabricated splices is verified with standard pull tests returning damage thresholds of 140 g (32 kpsi) and 100 g (24 kpsi) for hollow core fibers spliced to single mode and polarization maintaining fibers, respectively.

In this work, we focus on designing heterogeneous step-index multi-core fibers (Hetero-SI-MCFs) with the standard cladding diameter of 125-μm, which enables the utilization of the existing optical cables, connector interfaces, and conventional optical components.To estimate the inter-core crosstalk (XT) in Hetero-SI-MCFs, an analytical expression with a particularly simple form is derived for the mode-coupling coefficients between non-identical step-index (SI) cores, which enables a quick estimation of the XT values. The analytical results are in good agreement with the results obtained by numerical simulations by finite-element method (FEM). Using the derived analytical expression, the feasibility of designing conventional Hetero-SI-MCFs with one-ring core layout in the standard 125-μm cladding diameter is discussed. We then propose a novel method of allocating the non-identical cores in heterogeneous MCFs (Hetero-MCFs), where the crosstalk and critical bending radius (Rpk) are improved as compared to that of Hetero-MCFs designed by the conventional method. We numerically reveal that the number of SI cores that incorporated in the standard cladding diameter can be further increased with the proposed method of core allocation. It is shown that the simple SI profile enables us to allocate 8 and 6 non-identical cores in the standard cladding diameter of 125-μm for the O- and C-bands, respectively, and the achieved sufficiently low XT values support hundreds of kilometers transmission of the QPSK format signal.It is also shown that the simple SI profile enables us to allocate 10 and 8 non-identical cores in the standard cladding diameter of 125-μm for the O- and C-bands, respectively, and the achieved XT values support several kilometers transmission of the QPSK format signal, which has the potential use in datacenter networks. In addition, we propose a relatively low refractive index trench layer which occupies the outermost part of the conventional fiber cladding enables us to suppress the loss of outer cores in MCFs. A new kind of novel Hetero-SI-MCF with outer trench (OT) layer is proposed for achieving high core multiplicity. The designed MCFs with the comparable optical transmission characteristics to the conventional single-mode single-core fibers have the potential application in the space-division multiplexing systems., (主査) 教授 齊藤 晋聖, 教授 大鐘 武雄, 教授 西村 寿彦, 情報科学院(情報科学専攻)

We present a detailed investigation on higher order mode suppression due to differential gain in large mode area step index fiber amplifiers with confined Yb doping using spatially and spectrally resolved imaging (S2). A novel active fiber with Yb doping confined to the central 30% of the core area is fabricated and its performance is directly compared to a fiber with a conventional homogeneously doped core with almost identical parameters. At high pump rates, S2 and beam pointing stability measurements clearly demonstrate fundamental mode operation of the confined doping few mode fiber, even under imperfect launching conditions and environmental perturbations. In addition, we discuss the mode content as a function of gain in co-pumped fiber amplifiers with and without confined rare earth core doping using a power propagation model for fibers with similar parameters to those used in our experiments. Our simulation results as well as amplification experiments indicate the great potential of the confined doping concept for single mode high power operation.

In this paper, we address the impact of multiple fiber bends on orbital angular mode (OAM) mode propagation in a fiber. In particular, we extend the OAM mode-mixing due to a single-fiber bend studied earlier in detail for the step-index fiber, to the case of a succession of bends. The bends may have relative orientation with respect to each other. Analytic expressions leading to crosstalk are given. The OAM mode intensity at the output of a pair of bends is especially studied with emphasis on the intensity pattern changes that arise from the changes in the relative orientation of the bends., Comment: 8 pages, 5 figures; to be submitted for publication in IEEE Photonics Journal

The dispersion characteristics of the circular step index fiber with helical windings between the core-cladding region is investigated. Sheath helix is wounded between the core and cladding using two directions, namely in the clock wise and anticlockwise direction. Substituting the field components into the modified boundary conditions due to the addition of the helical windings the modal characteristics are derived for both fibers. Representations of the helical windings are done by using normal boundary conditions and reverse boundary conditions. The Eigen equation is obtained in the form of Bessel functions and modified Bessel functions for both the waveguides. The dispersion curves are plotted for two specific pitch angles ψ=0° and ψ=90 for each fiber and the results are compared. The direction of wrapping the helical material results in a change in the dispersion properties with regards to the way the modes propagate in both fibers. These changes are seen by the presents and absence of (1) band gap,(2) splitting of modes and (3) adjacent modes depending on the direction and pitch angle of the helical windings. Results obtained in this work suggest that direction and pitch angle of the helical winding are parameters that are able to control the behavior of the modes.

By solving the time-dependent power flow equation, we present a novel approach for evaluating the bandwidth in a multimode step-index polymer photonic crystal fiber (SI PPCF) with a solid core. The bandwidth of such fiber is determined for various layouts of air holes and widths of Gaussian launch beam distribution. We found that the lower the NA of SI PPCF, the larger the bandwidth. The smaller launch beam leads to a higher bandwidth for short fibers. The influence of the width of the launch beam distribution on bandwidth lessens as the fiber length increases. The bandwidth tends to its launch independent value at a particular fiber length. This length denotes the onset of the steady state distribution (SSD). This information is useful for multimode SI PPCF applications in telecommunications and optical fiber sensing applications.

This Letter, to the best of our knowledge, reports mid-infrared fiber lasing beyond 5 µm at room temperature for the first time, C e 3 + -doped, chalcogenide glass, step index fiber employed in-band pumping with a 4.15 µm quantum cascade laser. The lasing fiber is was 64 mm long, with a calculated numerical aperture of 0.48 at the lasing wavelengths. The core glass was G e 15 A s 21 G a 1 S e 63 atomic % (at. %), doped with 500 parts-per-million-by-weight Ce, with a 9 µm core diameter. The cladding glass was G e 21 S b 10 S e 69 at. % with a 190 µm outer diameter. As pump power increases continuous wave lasing corresponding to the 2 F 7 / 2 → 2 F 5 / 2 , transition in the C e 3 + ion occurs at 5.14 µm, 5.17 µm, and 5.28 µm.

This chapter describes the state-of-the-art sources of terahertz (THz) radiation, with focus on all-optical fiber-based sources for THz generation. THz technology based on the optical fiber platform is expected to be most attractive for day-to-day applications. Though optical fibers have been considered before for low-loss guidance of THz radiation, their nonlinear effects can also be exploited for THz generation. We discuss how a glass-based legacy step index fiber can be used to make a THz source. However, high absorption losses of silica glass in the THz regime and a small overlap between the modes at the optical and THz frequencies limit the THz generation efficiency to a level below 0.01%. Next, we discuss our design of a THz source based on a plastic fiber. By exploiting the nonlinear parametric process of four-wave mixing (FWM) in an appropriately designed microstructured-core double-clad plastic fiber (MC-DCPF), both the loss and the modal overlap issues can be overcome to a great extent. The microstructure geometry of this fiber allows for fine tuning of the required phase matching condition, group-velocity dispersion, and nonlinear properties at the optical pump wavelength. By using such a MC-DCPF, we show that a THz wave at a frequency near 3 THz can be generated by using two commercially available high-power lasers. The high-power CO2 laser acts as the pump and a CO laser of much lower power acts as a seed for the FWM process. Numerical simulations reveal that more than 30 W of THz power within a bandwidth of 2.13 GHz can be generated at the end of a 65 m long fiber when 1 kW of CO2 laser power is launched together with 20 W of CO laser power. A conversion efficiency of 30% is possible for a loss-less configuration, but efficiency of > 10% is achievable even in the presence of material losses. Recent results show that further optimization of such plastic microstructured fibers can provide conversion efficiencies close to 45%. As an alternative, we have focused on the use of plastic fibers and discussed a design criterion that is promising for realizing large output powers with a relatively high efficiency.

Here for the first time the technique of the resonant modes coupling is used for spectrally-selective fundamental mode suppression. In our work we considered the fiber design consisting of the low-index core surrounding by the appropriately chosen high-index absorbing rods. Mode suppression in this case happens due to the resonant core mode deformation owing to mode-anticrossing effect and its partial absorption into the rods. According to our calculations it was established that stop-band of the core fundamental mode can be easily adjusted for different practical aims by fiber bending. Furthermore, in the present work we implemented and studied passive fiber with three high-index absorbing rods incorporated into fiber cladding. The Sm was chosen as an absorbing element of the high-index rods.

A novel and robust sensing platform based on a metal–organic framework (MOF) integrated hydrogel optical fibre is proposed for fluorescence detection for the first time. The core–clad structure of the hydrogel optical fibre with a step index profile can enable efficient excitation light delivery and luminescence emission collection. Meanwhile, immobilization of the luminescent MOFs in the core of the hydrogel fibre prevents the MOFs from directly contacting the harsh environment, while reserving channels for analyte penetration and interaction with the MOFs. In the representative experiment, water and thermo stable EuNDC (H2NDC = 1,4-naphthalenedicarboxylic acid) is synthesized as a fluorescent material for highly selective detection of explosive picric acid (PA). Fluorescence quenching is visible to the naked eye at a PA concentration of as low as 5.7 ppm. The EuNDC integrated hydrogel optical fibre enables the realization of highly selective detection of explosive picric acid (PA) over other nitroaromatic compounds and metal ions, which demonstrates the potential of the hydrogel fibre as a portable, robust sensing platform for highly selective, on-site and real-time quantitative monitoring.

We demonstrate the formation of Kerr-induced transient long period grating in a standard single-mode fiber (SMF28) using sub-nanosecond laser pulses, which can be used to realize an efficient all-optical mode conversion and temperature sensing. The fiber supports $LP_{01}$ and $LP_{11}$ spatial modes at 1064-nm wavelength. The beating between two guided modes induces refractive index grating exploiting the Kerr effect. The probe beam co-propagating with the pump at the same wavelength but with different polarization exhibits efficient mode conversion efficiency from $LP_{01}$ to $LP_{11}$ mode. Temperature sensitivity is measured owing to the change in mode conversion efficiency due to the shift in resonant peak wavelength with the increase in temperature. The change in temperature is manifested by the change in spectral power of probe beam at the output. The experimental result demonstrates an average temperature sensitivity of 0.15 Watt/°C. The transient nature of the grating with high sensitivity makes our approach unique from the existing results and can open a route for potential applications.

In this paper, a displacement sensor utilizing a Mach–Zehnder interferometer with a single-multi-air-gap-single mode (SMAS) fiber structure is proposed and demonstrated. A 30-cm-long four-mode step index fiber (FMSIF) is used as the multimode fiber in the SMAS structure, with the left end of the FMSIF fusion spliced to a 50-cm-long single-mode fiber (SMF-28). The right end of the FMSIF is aligned to another 50-cm SMF-28 on the v-groove of Fitel S178A fusion splicer but without splicing, creating an air gap with a tunable length of 0– $300~\mu \text{m}$ . The performance of the displacement sensor is investigated over the range of 1.1– $1.7~\mu \text{m}$ using a supercontinuum source. Through the experiment wavelength range, the highest sensor sensitivity is achieved at 1.7 $\mu \text{m}$ with a sensitivity of 12.3 pm/ $\mu \text{m}$ and an $R^{2}$ coefficient of 0.926. The implementation of a displacement sensor utilizing the simple and cost-effective concept of the SMAS will be beneficial for use in various areas, such as biomedicine, manufacturing, and engineering.

Results of steady periodic finite element beam propagation (SP-FE-BPM) simulations of transverse mode instability (TMI) in representative coiled step index and large pitch photonic crystal fiber amplifiers with pump noise and modulation are presented and discussed. Modulating the pump power of fiber amplifiers near the TMI threshold complicates the conceptual picture due to the introduction of the additional dynamical time scale of the pump modulation. The time dependent mode field throughout the length of the amplifier calculated with the SP-FE-BPM was used to synthesize spatially and temporally resolved imaging measurements including spatial distribution of the amplitude and phase of different frequency components as a means to characterize the effects of pump modulation on TMI dynamics. Pump modulation was found to affect the threshold and dynamics of TMI in two amplifier configurations and a range of modulation cases. The step index fiber (SIF) amplifier was found on average to be approximately 20% more susceptible to pump noise than the large pitch fiber (LPF) amplifier. Furthermore a TMI dynamic not associated with stimulated thermal Rayleigh scattering that has been experimentally observed was observed in both SIF and LPF SP-FE-BPM simulations.

An exact expression of the outgoing boundary intensities in the case of a gray nonscattering semitransparent medium (STM) confined in a plane parallel slab with a refractive index linearly depending on the position is proposed. It is shown that directly using the diffuse reflection law gives in a much easier way exactly the same result as the one obtained with the pseudosource method.

A kind of tapered segmented cladding fiber (T-SCF) with large mode area (LMA) is proposed, and the mode and amplification characteristics of T-SCFs with concave, linear, and convex tapered structures are investigated based on finite-element method (FEM) and few-mode steady-state rate equation. Simulation results indicate that the concave tapered structure can introduce high loss for high-order modes (HOMs) that is advantageous to achieve single-mode operation, whereas the convex tapered structure provides large effective mode area that can help to mitigate nonlinear effects. Meanwhile, the small-to-large amplification scheme shows further advantages on stripping off HOMs, and the large-to-small amplification scheme decreases the heat load density induced by the high-power pump. Moreover, single-mode propagation performance, effective mode area, and heat load density of the T-SCF are superior to those of tapered step index fiber (T-SIF). These theoretical model and numerical results can provide instructive suggestions for designing high-power fiber lasers and amplifiers.

We experimentally demonstrate Raman amplification of signal pulses in a high-order Bessel mode ( L P 06 ) at a wavelength of 1121 nm in a 335-m step-index fiber with a 70-µm diameter, 0.227-NA pure-silica core. This was pumped by 5-ns multimode pulses at 1065 nm from a Yb-doped fiber master oscillation power amplifier. The mode purity of the amplified pulses is well preserved to 23 dB of average-power gain, to 774 W of peak power in 2 ns pulses at a 20 kHz repetition rate, when pumped with a peak power of 942 W. The pump depletion as averaged over the signal pulses reaches 59%. We believe, to the best of our knowledge, that this is the first demonstration of stable mode propagation and Raman amplification of a single Bessel-like higher-order mode in a fiber of hundreds of meters. This shows the potential for efficient power scaling of a single signal mode with low-brightness pumping, comparable with that from continuous-wave multimode diode lasers.

We propose and numerically demonstrate chalcogenide based bend M-type fiber for mid-IR supercontinuum (SC) generation through higher order mode (HOM) of LP02. In order to achieve this, we comprise two glass materials such as As2Se3 in core and Ge10As23.4Se66.6 in cladding. M-type index fiber has a special property that the HOM of LP02 is confined into the core region with help of high index ring. The mode field distribution of the proposed fiber is numerically calculated by a finite element method. By optimizing structural dimensions in each bending case, we realize a zero dispersion wavelength, double zero dispersion wavelength effective area and power fraction in core. The simulation results shows the flat anomalous dispersion in the wavelength range of 2.1–2.6µm and 2.5–6.5µm for the core radius 4 and 10 µm, respectively at bending case of 1, 3, and 5 cm. which is important for employing the mature fiber laser technology at mid-IR regions.

For years, colloidal quantum dots (cQDs) have been optimized to offer great optical properties customizable through varying size and composition, and become the mature nanomaterial they currently are. However, their optical properties are affected by their environment. Hence, optical behaviour inconsistencies across experiments emerge from using cQDs inside different matrices. Therefore, their scope of applications has hitherto been greatly limited to specific applications. Recently, a new protocol to incorporate cQDs inside step index polymer optical fibers was developed by Whittaker and co-workers. It enables a cheap and robust fabrication of light-generating optical fibers. In addition to shielding cQDs from their outer environment, optical fibers also support optical modes. Hence, according to cavity quantum electrodynamics principles, these modes must alter the local density of optical states of the system, thus modifying the decay rate for each emitter located within the optical fibers. The current work revealed through finite difference time domain simulations only minimal modifications of the decay rate of a single quantum emitter located at the center of a polystyrene/PMMA core/clad step index fiber. Purcell factors staying within 2.2% of its initial value upon changes of the core radius were obtained. Thus, the Purcell enhancement offered by step index fibers is negligible compared to that of current semiconductor microcavities. Consequently, more exotic fiber geometries that offer greater Purcell effect must be identified before fast and cheap light-generating optical fiber can be made from this fabrication process.

In this paper the expression for far field pattern is presented using the method of power series for the fundamental LP 01 mode in optical fiber with step type refractive index profile. The same power series expression is utilised for determination of the far field in fibers with Kerr type nonlinearity by applying iteration technique. We have presented the comparison of the pattern graphically for linear case and with Kerr nonlinearity. Far field pattern estimation in presence of nonlinearity is extremely important to understand and comprehend the performance of a single mode optical fiber particularly in the field of sensors. The presented method which utilizes Chebyshev technique involves prescription of the expression for estimation of the far field in an optical fiber and eliminates the requirement of solving complex mathematical expression. Further, the results obtained are in excellent agreement with the actual results. Thus our simple yet accurate method will prove to be of great assistance to the professionals in the fiber optics domain.

Mid-infrared region supercontinuum (SC) generation through designing broadband light sources recently attracts considerable attention in the field of nonlinear optics owing to their numerous applications in sensing and biological imaging. Broadband light sources designed based on different waveguiding structures adopted until today, the SC generation using optical step-index fiber is the prominent one due to its design and fabrication flexibility. In this study, a promising 5-cm-long SC source has been designed and modeled using a step-index fiber structure employing highly nonlinear chalcogenide (ChG) materials such as As$$_2$$Se$$_3$$ glass as a core and Ge$$_{11.5}{\hbox {As}}_{{24}}$$Se$$_{64.5}$$ glass for its outer cladding. Fiber structure is suitably modeled through its group-velocity dispersion optimization by varying core diameter. The optimized fiber structures are excited using a pump source having 170-fs pulses at 5.5 μm with a peak power of 10 kW. Initial all-normal dispersion excitation produces SC broadening up to 9.5 μm. Further study in a new optimization shows that spectral evolution can be expanded beyond 17 μm covering the wavelength from 3.2 to beyond 17 μm if the fiber structure is excited in the anomalous dispersion regime through a suitably tailored flat group-velocity dispersion curve with smaller in magnitude over a wide wavelength range. Such a promising SC source, which is designed based on typical step-index fiber principle using highly nonlinear ChG glass system, can be utilized in a variety of mid-infrared region applications.

Biocompatible optical waveguides receive increasing attention owing to their application potential in the biomedical field. Our focus is on the fabrication and characterization of a new step-index biodegradable polymer optical fiber (bioPOF) using two commercial polyesters: poly(D,L-lactic-co-glycolic acid) (PDLGA) and poly(D,L-lactic acid) (PDLLA). Both polymers are regulated by US FDA, which allows projecting future clinical use of fibers made from these materials. We manufactured three preforms and we subsequently drew optical fibers with a standard heat-draw tower. We describe the chemical properties of the materials throughout the whole production chain from polymer granulates to preforms and then to optical fibers. We look into to the influence of the processing on the molecular weight and thermal characteristics of the polymers. Our step-index bioPOF with an outer diameter of 1000 ± 50 μm and a core of around 570 ±30 μm features record low attenuation of 0.26 dB•cm at 950 nm for step index bioPOF, and a numerical aperture of 0.163. Immersion in phosphate-buffered saline (PBS) leads to hydrolytic degradation of the bioPOFs over a period of 3 months, accompanied with a 91% molecular weight loss. From the degradation study results, we anticipate that our bioPOFs can be used for biophotonic applications requiring deep tissue light delivery, such as photodynamic therapy.

We report 9.2-dB Raman amplification of pulses at 1121 nm in an orbital angular momentum mode with charge 15 in 30 m of 50-pm-diameter step-index-core fiber. The amplified signal mode-purity is measured to 78.4%.

Results of numerical simulations of transverse mode instability (TMI) in Ytterbium-doped coiled large mode area step index fiber amplifiers are presented and discussed. The TMI thresholds for counter-pumped amplifiers were found to be significantly greater than for co-pumped amplifiers. A greater dependence of the TMI threshold on amplifier length was observed in the counter-pumped case than in the co-pumped case. Furthermore the onset of TMI was found to be localized in a specific critical position along the amplifier in the co-pumped case. In this configuration, amplifiers could be pushed slightly beyond the TMI threshold and still produce stable output due to bend loss filtering of the higher order modes. The TMI thresholds were found to have exhibit a non-monotonic behavior with respect to coiling diameter and only weakly dependent on core launch misalignment. Power stripped from the core into the cladding was in some cases found to interfere with the signal within the core leading to small power fluctuations that appear unrelated to the TMI process. In some co-pumped cases the beam was distorted at the amplifier output only during a small window within the overall simulated time period. This indicates that all of the different components of the broadened frequency spectrum are coherent. At this time, the physical explanation for this effect is unknown.

In this paper, we have reported the study of the optical signal propagation characteristics in a nontraditional optical fiber having circular core (made of dielectric) and helical winding clad (metallic and graphene). Study has been done at different pitch angles. This analysis also includes the absorption characteristic of waveguide by including imaginary part of refractive index. Normal and helical boundary conditions have been applied to derive the model characteristic equation. Dispersion curves and model cut-off conditions have been obtained on the basis of characteristic equation derived theoretically. The dispersion curves are derived by varying the pitch angles of the winding. The results obtained for different pitch angles have been analyzed; and the changes in dispersion curve have been explained. The results obtained show a standard shape of dispersion curve and the numbers of guided modes are estimated on the basis of standard parameters of fiber such as core radius, reflective indices of core and cladding and operation wavelength. The difference in the results obtained for metal clad and graphene clad fiber is also presented.

We present an all solution doping technique to fabricate step index Yb-doped Aluminophosphosilicate fiber, with P2O5 powder used for incorporating Phosphorous. Low loss (8dB/Km), low NA (0.06) photodarkening suppressed step index fiber is reported.

Geometrical and material - i.e., external and internal - scaling symmetries are exploited to obtain approximated analytical expressions for the mode effective index, group index, and chromatic dispersion of a scaled fiber. Our results include material refractive index scaling that changes the numerical aperture. First, the analytical expressions are successfully tested with a conventional step index fiber in a broadband range of wavelengths, from 1 to 2 mu m. Then, we establish a procedure to adapt the analytical expressions to photonic crystal fibers (PCFs) and illustrate its application in a triangular PCF with circular holes. These adapted analytical expressions show good agreement with a rigorous numerical solution of the fundamental fiber mode. Finally, we demonstrate how powerful these expressions are for the design of PCFs. In particular, we illustrate our approach designing, in four iterations or less, PCFs with flattened dispersion profile over 300 nm or high dispersion slope over 40 nm, with different chromatic dispersion values. (c) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We numerically study the mid-infrared supercon-tinuum generation employing a 5-cm-long dispersion-engineered all chalcogenide step-index fiber made using As2 Se3 glass as a core and Ge 11.5 As 24 Se 64.5 glass for its outer cladding. An ultrawideband supercontinuum spanning up to $14\mu m$ could be generated employing a pump source having 170-fs pulses at $5.5\mu m$ with a peak power of 10 kW.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

A dual-wavelength thulium-doped fluoride fiber (TDFF) laser is presented. The generation of the TDFF laser is achieved with the incorporation of a single mode-multimode-single mode (SMS) interferometer in the laser cavity. The simple SMS interferometer is fabricated using the combination of two-mode step index fiber and single-mode fiber. With this proposed design, as many as eight stable laser lines are experimentally demonstrated. Moreover, when a tunable bandpass filter is inserted in the laser cavity, a dual-wavelength TDFF laser can be achieved in a 1.5- $\mu \text{m}$ region. By heterodyning the dual-wavelength laser, simulation results suggest that the generated microwave signals can be tuned from 105.678 to 106.524 GHz with a constant step of ~0.14 GHz. The presented photonics-based microwave generation method could provide alternative solution for 5G signal sources in 100-GHz region.

Using the finite difference method (FDM), we analyse some practically interesting planar waveguide structures and develop the proper FDM algorithm to predict the relevant transverse electric and transverse magnetic modes with their propagation constants ( b ) over a long range of normalised frequency ( V ). In order to crosscheck our results from FDM, we use the step index profile, parabolic index profile and complementary error index profile as a test case to standardise our program. Then, with our program, we consider two more structures such as, diffused planar waveguide and planar directional coupler, to predict the corresponding b − V curves which are yet to be, completely, analysed by FDM.

An all-vapor phase MCVD process has been proposed for the fabrication of fiber preforms with a Yb2O3–Al2O3–P2O5–SiO2 multicomponent glass core. We have investigated the tubular preform collapse into a rod and demonstrated approaches capable of preventing P2O5 losses in the central part of the core during the collapse process. Preforms with a flat, perfect step-index profile have been fabricated.

A high-sensitivity refractive index sensor is demonstrated for the first time, near the ultraviolet (UV) wavelength region using a Si3N4-subwavelength grating (SWG) with a normal incident optical geometry. Using the eigenmode within the Si3N4-SWG, a high sensitivity for the refractive index is expected by finite-difference time-domain calculation, without an oblique incident geometry. The proposed SWG is fabricated and the high-sensitivity refractive index sensor operating near the UV wavelength is experimentally and successfully developed. The normal transmitted intensity through the fabricated SWG varies considerably with slight changes in the refractive index. The experimental sensitivity of the SWG attained 1240 % per refractive index unit (RIU) and the sensitivity shows good agreement with the calculation. These experimental results suggest that our refractive index sensor with a normal incident geometry can measure a refractive index change of 8.06 x 10-4 RIU, if the optical detection system can measure an intensity change of 1%.

Ti-diffused near-stoichiometric(NS) LiTaO 3 multimode planar waveguide was fabricated by diffusion of Ti-film coated onto a congruent LiTaO 3 substrate and post Li-rich vapor transport equilibration process. The crystalline phase in the Ti-diffused layer was analyzed by powder x-ray diffraction and the Li 2 O-content was evaluated from the measured birefringence. The Ti-concentration profile was analyzed by secondary ion mass spectrometry. The modes guided in the planar waveguide were characterized by prism-coupling technique. The refractive index profile was constructed from the measured mode indices, and correlated with the Ti-concentration profile. The results show that the waveguide retains the LiTaO 3 phase and is in an NS composition environment. Both the refractive index and Ti-concentration follow Gaussian profile. The extraordinary and ordinary index change profiles are almost same, implying that the electric field profile of the mode guided in the NS Ti:LiTaO 3 waveguide would be polarization-insensitive. The ordinary and extraordinary index changes and the Ti-concentration follow a similar exponential relationship with a power index ∼1.3. Comparison shows that the Ti:LiTaO 3 waveguide shows considerable differences from the Ti:LiNbO 3 in Ti diffusivity, index change profile and its relation to Ti-concentration, and polarization dependence of guided mode.

We propose a new method for investigation the state of mode coupling in a multimode step-index silica photonic crystal fiber (SI SPCF) with a solid-core by solving the time-independent power flow equation. For various arrangements of air-holes (different numerical apertures (NAs)), as well as a different widths of launch beam distribution, the length Lc for achieving equilibrium mode distribution (EMD) and length zs at which a steady state distribution (SSD) is established are determined for such fiber. We obtained that the larger the air holes in the cladding (higher NA), the longer length of the fiber it takes for the modal distribution-transients to reach their equilibrium and steady state. In the case of a wide launch that excites more guiding modes, these lengths shorten. Such information is of interest for application of multimode SI SPCFs in telecommunication and fiber optic sensors.

A novel long-period fiber grating via periodically embedding no-core fiber (NCF) has been achieved. The 11-period SMF-NCF-SMF-long period fiber grating (SNS-LPG) can exhibit an extremely high loss resonant dip (>20 dB). Theoretical analysis shows good agreement with the measured transmission spectrum. We investigated both refractive index and temperature sensitivity of SNS-LPG. The fabricated sensor achieve an average wavelength sensitivity of 141.837 nm/RIU in the range of 1.333–1.400 RIU, and exhibit lower temperature sensitivity (−6.43 pm/ °C and 0.005 dB/ °C).

We demonstrate a compact fiber-optic quasi-Michelson interferometer (QMI) for micro-displacement measurement. The sensor comprises a micro-structure of a reflection taper tip containing a refractive index modification (RIM) as a coupling window over the interface between core and cladding of the fiber. Femtosecond laser-based direct inscription technique is used to achieve this window inscription and to induce large refractive index change. The RIM acts as a window for the strong coupling and recoupling of core-to-cladding modes. As the core and cladding modes are reflected at the taper tip and coupled back to lead-in fiber, a well-defined interference spectrum is achieved. The spectral intensity exhibits a high micro-bending sensitivity of 4 . 94 dB ∕ μ m because of the sensitivity to bending of recoupled intensity of cladding modes. In contrast, the spectral wavelength is insensitive to bending but linearly responds to temperature. The simultaneous measurements, including power-referenced for displacement and wavelength-referenced for temperature, were achieved by selective interference dip monitoring.

An optical fiber sensor based on few-mode fiber and spherical structure is proposed and demonstrated. Temperature and refractive index can be measured simultaneously, since the interference spectrums between certain high core mode and different order cladding modes of the few-mode fiber have different sensitivities for the two parameters. The dips at 1526.4 nm and 1553.77 nm are chosen to measure the temperature and refractive index. The results of the experiment indicate that the temperature sensitivities of the dips are 0.059 nm/°C and 0.05 nm/°C, respectively. The refractive index sensitivities of the dips are −39.15 nm/RIU and −48.82 nm/RIU, respectively. And the temperature and the refractive index resolutions of the sensor are 0.95 °C and 0.0012RIU, respectively. Simultaneous measurement of temperature and refractive index can be realized by the sensor structure. This fiber interferometer sensor can also be applied in other sensing fields and has good prospects.

A graded-index fiber with yttrium aluminosilicate (YAS) glass core was fabricated by a “Melt-in-Tube” method. The refractive index and stress profiles of the fibers exhibit a highly symmetrical gradient tendency along the radial axis which is in accordance with the results of element migration. Based on the formation mechanism of YAS glass core, the relationships between the refractive index/stress profile and the spontaneous migration are discussed. A linear all-fiber laser based on the obtained fiber shows a laser output threshold of 11.48 mW and slope efficiency of 8.3%. The near-field beam intensity distribution of fiber laser with spectrum FWHM of 0.24nm indicates the good laser beam quality. The facile “Melt-in-tube” method was proved to be reliable in designing graded-index fiber through the dissolution and diffusion process, which is more advanced than commonly used “Rod-in-tube” method. This article is protected by copyright. All rights reserved.

We study the phenomenon of deliberate inter-modal interactions in a specially designed index guided Microstructured Optical Fiber (MOF) by exploiting multipole expansion method (White et al. 2002). The MOF is designed in such a way that the first layer of holes is judiciously filled with a material having refractive index slightly greater than the background material or core and remaining holes are filled with air. Accordingly, we find an interesting phenomenon of mode crossing between the fundamental mode and a targeted defect mode while tuning the wavelength. Exploring this transition wavelength of the mode crossing, we propose a design of a fiber optic sensor for refractive index measurement (Silva et al. 2014) with enhanced sensitivity.

In this paper, a high-power spectrally flat 2–5 μm supercontinuum (SC) laser is reported in a piece of As2S3 chalcogenide step-index fiber. A fluoride fiber-based SC laser which spans from 2–4.2 μm is used as the pump light. Two pump coupling systems between the fluoride and As2S3 fibers, one based on aspheric lens coupling and the other using all-fiber mechanical splicing are investigated. With the setup of aspheric lens coupling, the obtained SC laser has a 10 dB spectral bandwidth of 3000 nm spanning from 2050 to 5050 nm, corresponding to an average power of 57.6 mW. While with the all-fiber mechanical splicing, the 10 dB spectral bandwidth of the SC laser shrinks slightly, spanning from 2140 to 4980 nm range, but the average output power is further scaled to a record power of 97.1 mW. It is found that the dominant nonlinearities for continuous spectral broadenings in the As2S3 fiber are self-phase modulation and stimulated Raman scattering in the normal dispersion regime. This paper presents an all-fiber mid-infrared SC laser with spectrum beyond 5 μm for the first time. It also offers a practical solution for robust and reliable high power 2–5 μm broadband fiber laser.

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.

In this paper, a sensitive-enhanced single-mode fiber—tapered hollow core fiber—single-mode fiber Mach–Zehnder interferometer is demonstrated for refractive index sensing. The sensitivity was improved by forming an up-taper at the two splicing joints and concave cone in hollow core fiber. The up-tapered regions served as a more effective mode splitter/combiner, and the tapered hollow core fiber was used to generate a stronger evanescent field to enhance the interaction of light with the analyte. According to the principles of interference between the cladding and fundamental modes, we performed refractive index measurements. The experiments indicated that the proposed sensor has a high refractive index sensitivity of 214.97 nm/RIU in the refractive index range of 1.333–1.379, with a minimum refractive index measurement resolution of 9.3 × 10−5. In addition, the sensor had a low temperature response of 2.96 pm/°C in the range from 50 to 85°C and a low cross sensitivity of 1.377 × 10−5 RIU/°C. The p...

In this paper, we introduce and analytically demonstrate a novel chemical sensor based on defective 1-D photonic crystals in total internal reflection geometry, where a gradient refractive index (GRIN) defect layer is located at the end of the structure. This configuration creates an open sensing interface for real-time detection with ultrahigh sensitivity. Using the GRIN lens in the structure not only helps to tune the resonance wavelength to a desired value but also as a consequence of its special refractive index distribution function, a ring-shaped reflectance distribution appears on the output plane of the structure that plays a key role in finding the optimized refractive index of the defect layer and achieving the best resonance of the cavity modes. The resonance angle of the cavity modes is highly sensitive to the effective refractive index of the fluid flowing above the sensor. The refractive index variation of $\Delta n_{F}=10^{-8}RIU$ that causes $\Delta \theta =3.2\times 10^{-6}$ degrees shift at the resonance angle can be detected by this sensor.

A theory to recover refractive index profile of the bent graded index (GRIN) optical fibre, in core region, is proposed. This theory is applied to the bent GRIN optical fibre when it is located orthogonal in the light path of the object arm in digital holographic phase shifting interferometer; like Mach–Zehnder interferometer. In the experiment, the fibre is bent with two different bending radii and fixed on a microscope slide keeping it immersed in matching liquid. The produced phase shifted holograms, with the presence of the fibre, are recorded using an attached CCD camera. Two different processes controlling the index profile shape of the bent GRIN optical fibre are assumed. In the first process, a linear index variation is evolved from stresses in the direction of the bent radius. In the second one, there is a release of these stresses near the fibre surface, which depends on the fibre’s radius. This will affect the outer free surface of the cladding. Based on these assumptions, we are able to construct the index profile in two dimensions normal to the optical axis. We propose two functions to describe the refractive index profiles in cladding and the core regions of the bent GRIN optical fibre. The recorded phase shifted holograms are combined, reconstructed and analyzed to get the phase map of the bent GRIN optical fibre. Comparing the extracted optical phase differences with the calculated ones, a good agreement between them is found. This means that the used two dimensional proposed functions, which are describing cladding and the core indices profiles, are the most proper in this situation. Thus, we are able to determine a realistic induced birefringence profile inside the fibre which is generated by a bending operation, not only in the cladding but also in graded index core region as well.