Core-cladding Interface Research Articles

Fabrication of tellurite fiber via air-exhausting extrusion based on ceramics die

During the preparation of tellurite fiber preforms, defects such as air bubbles and contaminated particles at the core-clad interface can significantly increase fiber loss. In this paper, we have proposed an effective method to solve these defects. Firstly, zirconia (ZrO2) ceramics were used as the die material due to their low contamination and high chemical stability, avoiding the surface reduction of TeO2 glass associated with metal-based die. Furthermore, the die was designed with an air-exhausting channel between the interface of the core and cladding glasses. The results demonstrate that the tellurite glass was well-preserved during the extrusion, and no visible particle defects were found at the core-cladding fiber interface. The fabricated fiber exhibits an optical loss as low as 0.67 dB/m at 1.31 μm, which is approximately 77 % lower than that of fibers prepared using a stainless-steel die. This air-exhausting extrusion based on ceramic die, proves to be an effective method for the fabrication of low loss tellurite fibers for mid-infrared.

Mid-infrared second-harmonic generation and its red-shift signal induced by cross-phase modulation in an AsSe2 soft glass microstructured optical fiber

In this study, we investigate an AsSe2 soft glass microstructured optical fiber (MOF) featuring a hexagonal arrangement of six holes. The MOF is subjected to femtosecond laser pumping, employing a pump wavelength range spanning from 3950 nm to 4100 nm. Our experimental results reveal an intriguing outcome: a tunable mid-infrared second-harmonic generation (SHG) effect is observed, yielding a wavelength range of ∼ 1980 nm to ∼ 2203 nm. The maximum conversion efficiency is achieved at 4100 nm with a power of 130 mW, which can be up to 9.2E-5. The observed SHG is primarily due to the surface nonlinearity polarization at the core-cladding interface. Additionally, we increase the additional degrees of freedom of the SH signal by strong third-order nonlinearity, and demonstrate that strong self-phase modulation (SPM) of pump and cross-phase modulation (XPM) of SH signal help broaden the SH signal by means of generating new wavelength lobes. To the best of our knowledge, this is the first time that strong XPM-based SH signal is observed in an optical fiber in the mid-infrared wavelength region. The realized strong second-order and third-order nonlinearity might expand the fundamental physical mechanism of SHG in optical fiber especially in mid-infrared region, which would have important applications in telecommunications, laser technology, and quantum photonics.

Dispersion Compensation in Optical Fiber: A Review

A cylindrical-shaped dielectric waveguide is what an optical fiber is. The core-cladding interface confines light, as electromagnetic (EM) energy, within its surface and guides light through several internal reflections if the angle of incidence is larger than the C-critical angle. Dispersion is the spreading of light pulses as they travel along a fiber, resulting in a degradation of optical signal quality. The scattering of optical signs contorts both computerized and simple transmission across optical strands. When optical fiber transmission is widely employed and some sort of digital modulation is applied, dispersion mechanisms inside the transmitted light pulses broaden as they pass through the fiber and move along the channel. The waveguide dispersion is calculated using a simple curve-fitting method. Dispersion must require compensating. Dispersion compensation techniques are essential for the successful transmission of data over optical fiber. Dispersion compensation techniques are used to reduce the effects of dispersion and improve SNR. Dispersion compensation is the process of reducing or eliminating chromatic dispersion in an optical fiber. There are two primary methods of dispersion compensation electronic and optical. In electronic dispersion compensation, the distorted signal is converted into the electrical domain, and digital signal processing techniques are used to correct the dispersion-induced distortion. While this method is effective, it adds complexity and cost to the communication system. Hence, most modern-day systems employ optical dispersion compensation, which is a more efficient and cost-effective method. Optical dispersion compensation involves using specialized optical components, such as dispersion compensating fibers (DCFs) and fiber Bragg gratings (FBGs), to adjust the different wavelengths' arrival time. DCFs are designed to have the opposite dispersion characteristics of the transmission fiber, which allows them to compensate for the dispersion as the signal passes through. FBGs are periodic structures inscribed in the fiber that reflect specific wavelengths of light, effectively delaying their arrival time and compensating for the overall dispersion.

Geometric optics analysis of inverted graded index fibers

We derive a general solution based on geometric optics that describes the light propagation properties in multimode optical fibers with inverted refractive index profiles. Using this general solution, we classify rays according to their propagation properties and calculate the analytical expressions of the ray trajectories inside these fibers under different launching conditions. In addition, we discuss the most suitable propagation conditions that maximize the confinement of light power in the vicinity of the core-cladding interface for sensing purposes.

Annealing Effects on Optical Losses in 3D-Printed Silica Fiber

Annealing effects on optical losses in silica optical fiber drawn from 3D-printed preform were investigated. An irreversible reduction in loss was observed through multiple annealing cycles at a temperature <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T =$ </tex-math></inline-formula> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">800</i> ° <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> . Changes in Raman spectroscopy confirm densification of the core as the stresses in glass relax with annealing. The temperature-dependent loss in the 3D printed fiber is consistent with modal field overlap at the core-cladding interface.

A study of highly sensitive D-shaped optical fiber surface plasmon resonance based refractive index sensor using grating structures of Ag-TiO2 and Ag-SnO2

In this study, we propose a design and simulation of a highly sensitive surface plasmon resonance (SPR) based single-mode D-shaped optical fiber refractive index sensor in the near-infrared (NIR) region. It consists grating structure of silver coated with titanium dioxide (TiO2) and Tin dioxide (SnO2) layer on the core-cladding interface. With the help of the finite element method (FEM), the proposed optical fiber sensors are optimized with various geometrical parameters. Proposed optimized structures are investigated and various performance parameters are calculated. It is found that performance parameters enhance with TiO2/SnO2 coated on Ag grating structure at a particular thickness of TiO2/SnO2. Due to this more generations of surface plasmons take place on metal-metal oxide structures. We find the maximum sensitivity of 15.31 μm / RIU and 9.81 μm / RIU for Ag-TiO2 and Ag-SnO2 grating structure respectively which shows a better result in comparison to the other reported work. So proposed structure may have great potential to enhance the sensing ability of optical fiber-based sensors as compared to the other conventional SPR based optical fiber sensors.

Temperature-Dependent Group Delay of Photonic-Bandgap Hollow-Core Fiber Tuned by Surface-Mode Coupling.

Surface modes (SM) are highly spatially localized modes existing at the core-cladding interface of photonic-bandgap hollow-core fiber (PBG-HCF). When coupling with SM, the air modes (AM) in the core would suffer a higher confinement loss despite being spectrally within the cladding photonic bandgap, and would be highly dispersive around the avoided crossing (anti-crossing) wavelength. In this paper, we numerically explored how such avoided crossings can play an important role in the tuning of the temperature dependence of group delay of AM of PBG-HCF. At higher temperatures, both the thermo-optic effect and thermal expansion contribute to the redshift of avoided crossing wavelength, giving rise to a temperature dependence of the AM dispersion. Numerical simulations show that the redshift of avoided crossing can significantly tune the thermal coefficient of delay (TCD) of PBG-HCF from -400 ps/km/K to 400 ps/km/K, approximately -120 ppm/K to 120 ppm/K. In comparison with the known tuning mechanism by thermal-induced redshift of photonic bandgap [Fokoua et al., Optica 4, 659, 2017], the tuning of TCD by SM coupling presents a much broader tuning range and higher efficiency. Our finding may provide a new route to design PBG-HCF for propagation time sensitive applications.

Multiple Cladding Fiber Bragg Gratings Inscribed by Femtosecond Laser Point-by-Point Technology

Femtosecond laser point-by-point (PbP) technology can be used as a direct writing method to inscribe fiber Bragg gratings (FBGs) into the cladding of optical fibers. When the refractive index modulated area of cladding FBGs (CLFBGs) is close to the corecladding interface, the CLFBGs exhibit characteristics similar to conventional core-based FBGs. When bending was applied, the reflected power of the CLFBGs changed significantly and showed an orientation dependence on the fiber bending direction. The CLFBG inscription method provides a new opportunity to fabricate vector sensing devices, which can be realized by parallelly inscribed multiple CLFBGs. Compact CLFBGS with a low insertion loss (less than 0.05 dB) have application value in the fields of optical fiber sensors, communications, and lasers.

3D printed transparent ceramic YAG laser rods: Matching the core-clad refractive index

Abstract Yttrium Aluminum Garnet (YAG) solid state laser gain media rods with an active Neodymium-doped core and an optically-clear cladding region were additively manufactured via direct-ink-writing (DIW), followed by sintering and hot isostatic pressing to form fully dense optical ceramics. Lutetium and Gadolinium were chosen as optically-inert co-doping ions in the clad to match the increase in refractive index caused by the Neodymium in the core. Either 11.6% Lutetium or 3.8% Gadolinium can be used to match the index change from 2% neodymium; however, differences in diffusion distances across the core-clad interface lead to large fluctuations in index in that region. These index fluctuations can be minimized either by matching dopants with similar diffusion distances, or by implementing a gradual gradient in the doping profile, possible through DIW, rather than a sharp compositional interface attainable via more standard fabrication methods. This improvement in index homogeneity resulted in a 40% improvement in lasing performance compared with that of a core-clad rod fabricated with a sharp interface between the doped and undoped regions.

Estimation of cover sensitivity of a planar polymer waveguide through double path broadband difference interferometeric analysis

Broadband difference interferometeric analysis of a double path planar polymer optical waveguide is presented for bio-sensing applications. Mathematical formulation of intensity of output signal is obtained using interferometeric analysis. Further, the dispersion equation is obtained by matching the field at core cladding interface that gives the cutoff film thickness for TE0 and TM0 modes in considered spectral range. The waveguide is analyzed by propagating either same polarized (TETE or TMTM) modes or unlike polarized (TETM) modes in the reference arm and sensing arm. It is observed that difference of propagation constant for same polarized mode increases with increase of wavelength while for unlike polarized modes it first increases up to a maximum value and then decreases in the considered wavelength range. The maxima of output interference signal shifted prominently with the cover refractive index for the wavelength range near the maximum value of propagation constant difference. The obtained sensitivity 9.226 for unlike polarized modes in arms is much larger than the sensitivity 0.071 of same polarized modes in arms.

Low-Loss Chalcogenide Fiber Prepared by Double Peeled-Off Extrusion

We have developed an innovative extrusion with double peeling-off process to fabricate chalcogenide (GhG) preform, with an aim to remove the defects, such as bubbles, oxidation, and particles that always induce large optical losses in ChG glass fibers. As a result, none of the defects is found in the core-cladding interface of the fiber. The loss in a wavelength range of 5.5–8.2 μm is generally lower than 1 dB/m, with a minimum value of 0.2 dB/m at 6.3 μm. Moreover, a flat mid-infrared (MIR) supercontinuum (SC) spectrum spanning from 1.4 μm to 13.7 μm is generated in an 18 cm-long Ge-As-Se ChG fiber, accompanied with a highest spectrum flatten ratio of 72.4%.

Daylighting performance simulation and analysis of translucent concrete building envelopes

A novel optical fiber (OF) embedded building envelope, namely translucent concrete (TC), was proposed and popularized for its light transmission property and unique artistic features, but there is still a lack of in-depth research on its daylighting performance analysis. To reveal the impacts of multi-factors on TC daylighting performance, an optical model of TC was established based on a ray-tracing method. In the model validation, both the simulated transmittance of OFs and illuminance of translucent concrete were in a good agreement with the experimental results. The simulation results showed when the numerical aperture (NA) of the OFs increased from 0.51 to 0.70, the annual average luminous flux of TC under seven cities conditions could be enhanced by up to 40.62%. In terms of effective illuminance, the optimum fiber volume ratio of the seven cities was determined, respectively. Besides, it was found that the core-cladding interface losses could lead to a relative deviation over 5% when the length of 1-mm OFs with 0.51 NA exceeded 259 mm, and the deviation could be larger with the decrease in diameter of OFs and the increase both in length and NA of OFs.

Fiber-integrated phase-change reconfigurable optical attenuator

We report on the experimental demonstration of an optical-fiber-integrated, nonvolatile transmission switching device. The operating mechanism exploits a cavity resonance spectral shift associated with an induced change in the refractive index of a high-index thin film on the polished side facet of the fiber. In the present case, a thermally induced amorphous-crystalline structural transition in a 500 nm layer of germanium antimony telluride at a distance of 500 nm from the core-cladding interface of an SMF-28 single-mode fiber delivers resonant transmission contrast &amp;gt;0.5 dB/mm at 1315 nm. Contrast is a function of active layer proximity to the core, while operating wavelength is determined by layer thickness—varying thickness by a few tens of nanometers can provide for tuning over the entire near-infrared telecoms spectral range.

Highly efficient second-harmonic generation in a tellurite optical fiber.

A step-index tellurite optical fiber with loss ∼0.02 dB/m at ∼1545 nm was fabricated based on TeO2-Bi2O3-ZnO-Na2O glass. With a nanosecond laser operated at ∼1545 nmas the pump source, second-harmonic generation (SHG) was observed in the 2.5m tellurite fiber with conversion efficiency up to ∼1.15% at 20mW, which, to the best of our knowledge, is the highest for non-silica optical fibers. The experimental phenomenon can be explained by the χ(2)-induced model of nonlinear polarization at the core-cladding interface. Furthermore, it can also be explained by a χ(3)-induced model via the four-wave-mixing effect, where a quasi-SH signal and a millimeter wave were generated simultaneously. This new model may help shed light on the physical mechanism of SHG in optical fibers.

A Numerical Analysis of Various pH Level for Fiber Optic pH Sensor Based on Bromophenol Blue in Silica

The fabrication and characterization of an optical fiber pH sensor for the detection range 2 – 7 are described. The sensing element was prepared by coating the uncladded middle portion of a multimode plastic clad silica fiber (PCF) with xerogel film of immobilized bromophenol blue (BPB) prepared by a sol-gel process. The exponential decay of the evanescent wave at the core-cladding interface of the multimode fiber was utilized to determine the pH response. The objective of this work is to analyze the measurement of the fiber optic sensing probe towards various pH level. The intensity of the absorbance was analyzed using statistical technique (Statistical Package for the Social Sciences, SPSS) to identify the significant pH range that can be used as a reference in developing the sensor instrumentation in the future. pH2 shows the highest significance among all pH values based on the statistical results using error plots.

Refractive index sensor in eccentric fiber Bragg gratings using a point-by-point IR femtosecond laser.

In this paper, we demonstrate point-by-point inscription of eccentric fiber Bragg grating (EFBG) in conventional single-mode fiber (Corning SMF-28) with second-order Bragg resonances within the C-band using an IR femtosecond laser. Highly localized fiber Bragg gratings can be inscribed point-by-point with focused ultrashort pulses. EFBGs are localized close to the core-cladding interface, yielding strong cladding mode resonance couplings and high photoinduced birefringence. The inscription conditions to optimize the grating fabrication have been explored to achieve second-order resonances of good spectral quality. Potential applications of EFBG in refractometry sensing are further illustrated. Two interrogation techniques for EFBG in refractometry are reported and experimentally tested.

Radiation-induced loss of silica optical fibres with fluorine-doped cladding

The effect of gamma-irradiation of the fluorine-doped silica depressed cladding graded-index fibre (DCGIF) has been researched. New mechanism of the non-bridging oxygen hole centers (NBOHC) defect formation, conditioned by different mobility of diffusion counter flows of fluorine and oxygen at the core-cladding interface, has been proposed. Using silica tubes with 0.02 wt% of OH groups and hydrogen in free state in MCVD-technology of DCGIF fabrication completely eliminates the NBOHC absorption at the wavelength of 630 nm. However, gamma-irradiation of such fibres results in anomalously high attenuation level in the visible range of optical spectrum. It was established that irradiation of DCGIF leads to light scattering which is probably due to the appearance of optical inhomogeneities in the glass matrix.

Bragg-Grating-Based Photonic Strain and Temperature Sensor Foils Realized Using Imprinting and Operating at Very Near Infrared Wavelengths.

Thin and flexible sensor foils are very suitable for unobtrusive integration with mechanical structures and allow monitoring for example strain and temperature while minimally interfering with the operation of those structures. Electrical strain gages have long been used for this purpose, but optical strain sensors based on Bragg gratings are gaining importance because of their improved accuracy, insusceptibility to electromagnetic interference, and multiplexing capability, thereby drastically reducing the amount of interconnection cables required. This paper reports on thin polymer sensor foils that can be used as photonic strain gage or temperature sensors, using several Bragg grating sensors multiplexed in a single polymer waveguide. Compared to commercially available optical fibers with Bragg grating sensors, our planar approach allows fabricating multiple, closely spaced sensors in well-defined directions in the same plane realizing photonic strain gage rosettes. While most of the reported Bragg grating sensors operate around a wavelength of 1550 nm, the sensors in the current paper operate around a wavelength of 850 nm, where the material losses are the lowest. This was accomplished by imprinting gratings with pitches 280 nm, 285 nm, and 290 nm at the core-cladding interface of an imprinted single mode waveguide with cross-sectional dimensions 3 × 3 µm2. We show that it is possible to realize high-quality imprinted single mode waveguides, with gratings, having only a very thin residual layer which is important to limit bend losses or cross-talk with neighboring waveguides. The strain and temperature sensitivity of the Bragg grating sensors was found to be 0.85 pm/µε and −150 pm/°C, respectively. These values correspond well with those of previously reported sensors based on the same materials but operating around 1550 nm, taking into account that sensitivity scales with the wavelength.

Bio-Functionalized Tapered Multimode Fiber Coated With Dengue Virus NS1 Glycoprotein for Label Free Detection of Anti-Dengue Virus NS1 IgG Antibody

This proposed paper demonstrates the development of a bio-functionalized tapered multimode optical fiber for the detection of anti-Dengue virus NS1 Immunoglobin G (IgG). The tapered fiber with waist diameter and waist length of 10 $\mu \text{m}$ and 10 mm, respectively, promotes the penetration of light modes to the core-cladding interface, creating evanescent wave that are sensitive to changes on the fiber surface. As the fiber is bio-functionalized with Dengue virus (DENV) NS1 glycoprotein specific to anti-DENV NS1 IgG antibodies, alteration on the surface is selective towards the presence of IgG antibodies. The absorbance-based sensor obtained a sensitivity value of $7 \times 10$ −6 a.u/pg/mL with a detection time under 2 min. This paper shows great potential for real-world deployment which impact may bring Dengue diagnostics a step further.

Polarization dependence of the propagation constant of leaky guided modes

We show that transverse-magnetic (TM) leaky modes can propagate further than transverse electric (TE) modes in real-index dielectric waveguides. We compute the density of states and find that while the TE spectrum contains only overlapping resonances, the TM spectrum typically contains several isolated peaks. By transforming the TM equation into a Schr\"{o}dinger-type equation, we show that these isolated peaks arise due to $\delta$-function barriers at the core-cladding interface. Our theory is useful for a range of applications, including filtering TM modes from initially unpolarized light and transferring information between distant waveguides.