Mode Field Distribution Research Articles

In-Fiber Grating Fabricated by Femtosecond Laser Direct Writing for Strain Sensing

In this letter, we reported long period fiber gratings (LPFGs) fabricated by femtosecond laser pulses direct writing in grapefruit microstructured fiber. The mode field distribution and dispersion property of this fiber was analyzed by finite-element method. Two LPFGs were fabricated with different periods of 230 and 250 μm, the resonant dips appeared at the wavelengths of 1325.8 and 1449.4 nm, respectively. In addition, sensing characteristics of the LPFG with the period of 250 μm were investigated in theory and experiment. The experimental results show that the strain and temperature sensitivities are 1.88 pm/με and 0.267 pm/°C, respectively. It indicates that the temperature cross sensitivity is as low as 0.142 με/°C.

Reduced threshold of optically pumped amplified spontaneous emission and narrow line-width electroluminescence at cutoff wavelength from bilayer organic waveguide devices.

We present a detailed study of the optically and electrically pumped emission in the BSB-Cz/PVK bilayer waveguide devices. By optical pumping we demonstrate that PVK as a spacer between fluorescent BSB-Cz and ITO electrode allows the significant reduction of the threshold for amplified spontaneous emission (ASE) of BSB-Cz. The simulation provides a better understanding of how the PVK thickness affects the waveguide mode field distribution and hence the ASE threshold of BSB-Cz. On the other hand, the BSB-Cz/PVK bilayer OLED exhibits the external quantum efficiency of >1% and anisotropic electroluminescence with spectrally narrowed edge emission at the cutoff wavelength controlled by the BSB-Cz thickness. When tuning the cutoff wavelength to match the peak gain of BSB-Cz, we demonstrate an intense, particularly narrow edge emission (~5 nm) without obvious degradation of efficiency at a high current density of 1000 mA/cm2, suggesting a reliable device performance for high-power applications and further exploration of electrically-pumped ASE.

Comparative study of guided modes in hollow core Bragg fibers with binary and ternary photonic bandgap claddings

We comparatively investigate the modal properties of guided modes in hollow core Bragg fibers (HC-BFs) with the binary and ternary photonic bandgap claddings by using a full-vector finite element method. The influences of structural changes from the binary periodic cladding (BPC) to the ternary periodic cladding (TPC) on the dispersion relations, confinement losses, and modal field distributions of the lowest five guided modes are thoroughly discussed under different core diameters. The contrasts in bandwidths for the guided modes between both types of HC-BFs strongly depend on their distinct frequency cutoff characteristics. The bandwidth contrast for a given guided mode in large-core HC-BFs is different from that in small-core HC-BFs. The confinement loss of the fundamental HE11 mode in the TPC-based HC-BF is much lower than that in the BPC-based HC-BF, which is essentially attributed to the stronger modal field confinement by the TPC. The differences in minimum confinement losses of the HE11 mode between both types of HC-BFs decrease with decreased core diameters. In contrast, the changes of cladding structures have very limited influence on the confinement loss of the TE01 mode in both the large-core and small-core HC-BFs. Moreover, we explore the reasons behind the evolutions of the minimum loss contrasts for the HE11 and TE01 modes between both types of HC-BFs with the core diameter, by quasi-quantificationally analyzing the influences of reflection characteristics of the transverse electric (TE) and transverse magnetic (TM) waves on the confinement loss.

Efficient interfacing photonic and long-range dielectric-loaded plasmonic waveguides.

Long-range dielectric-loaded surface plasmon-polariton waveguides (LR-DLSPPWs) operating at telecom wavelengths are efficiently (end-fire) interfaced with photonic waveguides by taking advantage of very similar lateral mode field profiles in these waveguide configurations. The LR-DLSPPWs are formed by 1-μm-high and 1-μm-wide polymer ridges fabricated atop 15-nm-thin and 500-nm-wide gold stripes supported by a 289-nm-thick ormoclear polymer deposited on a low-index (1.34) layer of cytop, whereas gold stripes are absent in the photonic waveguide configuration that is identical to the plasmonic one in all other respects. The coupling efficiency between LR-DLSPPWs and photonic waveguides is numerically estimated to be 97%, decreasing by only a few percents for non-centered gold stripes (as long as a gold stripe is kept inside the polymer ridge). The fabricated LR-DLSPPWs coupled to photonic waveguides are first characterized using amplitude- and phase-resolved near-field imaging of propagating radiation that reveals very similar mode field distributions in these waveguides as well as their efficient interfacing. The coupling efficiency is then experimentally characterized using the cutback approach resulting in an average level of 75% per interface, while the LR-DLSPPW mode propagation length is estimated to be on average 0.3 mm. Possible reasons for differences between experimental and simulation results are discussed, indicating that a 3-nm-thin titanium layer (used for improving adhesion between gold and ormoclear) introduces substantial mode absorption. The results obtained open new perspectives for realization of hybrid photonic-plasmonic components and circuits.

Mode Investigation for Hybrid Microring Lasers With Sloped Sidewalls Coupled to a Silicon Waveguide

Hybrid AlGaInAs/InP microring lasers with sloped sidewalls coupled to a silicon waveguide are investigated numerically and experimentally. The mode field distribution deviations for the microring resonator with the sloped sidewalls at different tilting angles are simulated by a 3-D finite-difference time-domain technique. Obvious enhancements of output coupling to the silicon waveguide and the active region optical confinement factor can be expected as the microresonator has a trapezoidal shape. A hybrid microring laser with a radius of 19 μm and a width of 2 μm is fabricated with the laser wafer bonded on a silicon-on-insulator wafer using the divinylsiloxane-benzocyclobutene (DVS-BCB) adhesive bonding technique. Continuous-wave electrically injected operation is realized at a tilting angle of -6° with the threshold currents of 7.8 and 9 mA at 12 °C and 22 °C. In addition, for a hybrid microring laser with a radius of 29 μm and a ring width of 2.5 μm, continuous-wave electrical operation up to 50 °C is achieved.

Modelling evanescent field components in metal-oxide core waveguides for nonlinear applications

In a thin film planar geometry, the guiding effect due to the linear index results in light confinement in the thin film layer, at the same time causing evanescent waves to decay into the outer substrate or cladding layers. The magnitude of the field in the cladding is dependent on the characteristics of the waveguide and the wavelength of the wave. In this paper, we have modelled four metal-oxide core waveguides with prime focus on the evanescent field as light propagates through these optical structures. ZnO waveguide structure is taken as the benchmark. The evanescent path resembles a soliton consistent with the input soliton solution. A comparison is also carried out with a lower metal oxide core, MgO and a higher index TiO2. The variation in refractive index with input wavelength induces second order nonlinearity in these waveguide structures. For the simulated structures, mode field distributions are plotted against the refractive index. The magnitude of evanescent field also reduces with an increase in the input wavelength for the structures. The nature of the soliton path is insightful for the development of evanescent field based sensors. Optical waveguides with metal-oxide cores can act as a cheaper substitution to nanophotonic devices and integrated optics applications. With the study of evanescent phenomena in such structures, a minimally invasive manufacturing method without disturbance to the core may be made beneficial.

Point Defects in GaAs Photonic Crystals

Applying plane wave expansion method to one dimensional multilayer system formed from alternating layers of GaAs and air, a defect mode was artificially introduced by removing a GaAs layer at the centre of a supercell and the band structures and mode field distributions were obtained. The defect mode normalized frequency was 0.28. The parameters for developing a Febry-Perot filter in the visible frequencies are given. The point defects in a two dimensional square lattice formed from GaAs rods of radius 0.16a in air was created by removing one GaAs rod at the centre of a supercell and also by increasing the centre GaAs rod radius to 0.5a. The removal of the rod folded the band structure 25 times creating a localized evanescent defect mode. The increment of the centre GaAs rod radius led to five defect bands inside the band gap with one degenerate state creating a monopole, two quadrupoles and two hexapoles. Evolution of the defect modes with the variation of the radius of GaAs rod is also presented.

Design and Parametric Study of Depressed Core Optical Fiber

In this paper, the bending loss and the modal field distribution (MFD) of the depressed core optical fiber (DCOF) are investigated. The effect of optical and geometrical parameters on the bending loss, power confinement and MFD are examined in these fibers. Based on unique three layered structure of DCOF, the central air core, germanium silicate ring core, and silica cladding along with its mode transformation capability, we also demonstrated versatile applications in local area, wide area optical communication networks and tunable wavelength selective devices. Detailed design parameters and operation principles of the DCOF are discussed.

Design and analysis of low bend losses of the air core optical fiber for wavelength selective devices

In this paper, the bending loss, modal field diameter and the modal field distribution of the air core optical fiber are investigated. The effect of optical and geometrical parameters on the bending loss, power confinement and modal field diameter are examined in this special fiber. Detailed design parameters and operation principles of the air core optical fiber are discussed. The structure is based on a unique three layered structure of air core optical fiber, having the central air core, germanium silicate ring core, and silica cladding. It has been demonstrated that air core optical fiber has an excellent mode transformation capability. The air core optical fiber is expected to have a versatile application in local area optical communication networks and tunable wavelength selective devices. The main advantages of air core optical fiber are low bend loss and small mode field diameter, which is a prime focus of this paper.

Point Defects in GaAs Photonic Crystals

Applying plane wave expansion method to one dimensional multilayer system formed from alternating layers of GaAs and air, a defect mode was artificially introduced by removing a GaAs layer at the centre of a supercell and the band structures and mode field distributions were obtained. The defect mode normalized frequency was 0.28. The parameters for developing a Febry-Perot filter in the visible frequencies are given. The point defects in a two dimensional square lattice formed from GaAs rods of radius 0.16a in air was created by removing one GaAs rod at the centre of a supercell and also by increasing the centre GaAs rod radius to 0.5a. The removal of the rod folded the band structure 25 times creating a localized evanescent defect mode. The increment of the centre GaAs rod radius led to five defect bands inside the band gap with one degenerate state creating a monopole, two quadrupoles and two hexapoles. Evolution of the defect modes with the variation of the radius of GaAs rod is also presented.

Finite element analysis and design of few mode fiber with low differential mode delay

Two different types of few mode fibers (FMFs) are considered: a graded-core with a cladding FMF and a multi-step-index FMF. The characteristics of mode field distribution and the effective index are analyzed based on Comsol software. Then differential mode delay is analyzed. The structure and profile parameters are optimized in order to achieve the lowest possible differential mode delay (DMD). Under the condition of supporting four modes, a graded-core with a cladding FMF is designed. The DMD absolute values of LP11, LP21, LP02 are all below 0.015 ps·m-1. Under the condition of supporting two modes, a multi-step-index FMF is designed. The DMD of LP11 is 0.185 ps·m-1. The designed fibers have low DMD, which is suitable for mode division multiplexing.

THz mode-coupling in photonic-crystal–surface-plasmon-coupled waveguides

We present a detailed design principle and propagation characteristics of a channel plasmon-photonic–crystal-coupled-waveguide (PPCCW) for realizing terahertz (THz) waveguides and waveguide-based devices. The avoided-crossing behavior exhibited by dispersion curves of the supermodes due to modal interference leads to a sharp change in group velocity close to the phase-matching wavelength. As a consequence, the group-velocity dispersion (GVD) peaks to a maxima (~1.40 × 105 ps/km·μm) and dips to a minima (~−1.60 × 105 ps/km·μm), thus exhibiting extremely large GVD. The propagation length is obtained in the order of a few centimeters. The analysis is substantiated by the frequency dependence of mode field distribution and propagation loss suffered by the supermodes of PPCCW geometry. Also, the obtained result is in good agreement with FEM-based study. The proposed waveguide is aimed at facilitating enhanced performance THz-waveguide sensors and GVD compensation modules.

Highly Efficient Boundary Element Analysis of Whispering Gallery Microcavities

We demonstrate that the efficiency of the boundary element whispering gallery microcavity analysis can be improved by orders of magnitude with the inclusion of Fresnel approximation. Using this formulation, simulation of a microdisk with wave-number-radius product as large as $kR\approx8,000$ was demonstrated in contrast to a previous record of $kR\approx100$. In addition to its high accuracy on computing the modal field distribution and resonance wavelength, this method yields a relative error of $10%$ in calculating the quality factor as high as $10^{11}$ through a direct root searching method where the conventional boundary element method failed to achieve. Finally, quadrupole shaped cavities and double disks as large as $100 {\mu}m$ in diameter were modeled by employing as few as $512$ boundary elements whilst the simulation of such large cavities using conventional boundary element method were not reported previously.

Loss-Based Magnetic Field Sensor Employing Hollow Core Fiber and Magnetic Fluid

An inline optical fiber magnetic field sensor based on hollow core fiber (HCF) wall integrated with magnetic fluid tuned by magnetic field was proposed by sandwiching a length of HCF between two single mode fibers with offset joints. Intermodal interference is present in the HCF wall, mode propagation, as well as mode field distribution within the HCF wall have been simulated, and the contrast ratio of the interference fringes in response to the variation of the applied magnetic field has been theoretically analyzed and experimentally observed. The maximum sensing sensitivity of -0.02173 dB/Oe within the linear magnetic intensity range from 150 to 300 Oe has been achieved.

Investigation of a UV-laser generated planar polymeric waveguide directly and after five years of preparation using an interferometric method

Polymeric integrated-optical waveguides were prepared in a planar chip using UV-laser lithographic method. Five waveguide samples were irradiated by an excimer laser of wavelength 248nm with different irradiation parameters (fluence and irradiation doses). Using Mach–Zehnder interferometer, the refractive index depth profiles of these samples are determined in two cases, directly and five years later of preparation. A crucial change of refractive indices profiles has been recorded after five years of preparation. In the first case we got double region waveguides fitted a Gaussian shape, while in the second one we observe a single region waveguide which has exponential shape. The photochemical reactions responsible for these changes in the two cases are demonstrated. This is quite important when such waveguides are used in the applications. Also it was interesting to investigate the aging impact on mode field distributions and the effective mode indices. The mode parameters are determined based on a theoretical model and the experimentally obtained data. The results show a notable change in the mode field distributions and the propagation coefficients as influence of aging.

Broadband terahertz transmission within the symmetrical plastic film coated parallel-plate waveguide.

We report on the broadband terahertz (THz) transmission within a symmetrical plastic film coated parallel-plate waveguide. We theoretically study the antiresonant reflecting mechanism of the waveguide, and we find that the broadband THz wave can transmit in this waveguide with ultralow loss. The loss of the TM mode in this waveguide can be 4 orders of magnitude lower than the uncoated parallel-plate waveguide. The transmission bandwidth of this waveguide is up to 5.12THz. We further show the mode field distributions which explain the loss mechanism.

Twist Sensitivity of Cladding-Mode Resonances and Its Cross-Sensitivity to Strain and Temperature in a Mechanically Induced Long-Period Fiber Grating

Twist sensitivity of cladding-mode resonances in a mechanically induced long-period fiber grating formed over a single-mode fiber is experimentally demonstrated and theoretically analyzed. Of the two usual cladding-mode resonances corresponding to LP11 and LP12, higher-order mode LP12 is more sensitive to twist in comparison with the lower-order mode LP11. The extent of down-shifting of resonant wavelengths depends on twist-induced circular birefringence and the modal field distribution of the cladding-modes inside the fiber. When the fiber is severely twisted to 3.5 rad/cm, a shift sensitivity is observed of 1 nm/(rad/cm) for the LP11 mode and 4.23 nm/(rad/cm) for the LP12 mode. The fiber breaks when the twist rate exceeds 3.5 rad/cm. In comparison with LP12, the LP11 resonance is almost independent of the axial strain variation with an ultra-low sensitivity of 0.18 pm/με, and it is also almost insensitive to the temperature variation with a coefficient of 35 pm/°C. Forming the LP11 resonance far away from its cut-off wavelength, a widely tunable band-pass filter is also demonstrated with a very high twist sensitivity of 8.75 nm/(rad/cm) and negligible cross-sensitivity to strain and temperature. The experimental and theoretical results are very useful in selecting sensitive and stable cladding-mode resonances in the design of new mechanically induced long-period fiber gratings based torsion sensors and tunable band-pass filters.

Stored Energy, Transmission Group Delay and Mode Field Distortion in Optical Fibers

The relationship between transmission group delay and stored energy in optical fibers is discussed. We show by numerical computations that the group delay of an optical pulse of finite bandwidth transmitted through a piece of a low loss optical fiber of unit length is proportional to the energy stored by the standing wave electromagnetic field. The stored energy-group delay ratio typically approaches unity as the confinement loss converges to zero. In case of a dispersion tailored Bragg fiber, we found that the stored energy-group delay ratio decreased while the confinement loss increased compared to those of the standard quarterwave Bragg fiber configuration. Furthermore, a rapid variation in the group delay versus wavelength function due to mode-crossing events (in hollow core photonic bandgap fibers for instance) or resonances originating from slightly coupled cavities, surface or leaking modes in index guiding, photonic bandgap, or photonic crystal fibers always results in a rapid change in the mode-field distribution, which seriously affects splicing losses and focusability of the transmitted laser beam. All of these factors must be taken into consideration during the design of dispersion tailored fibers for different applications.

Low index dielectric mediated surface plasmon resonance sensor based on graphene for near infrared measurements

We numerically demonstrate and propose a low index dielectric (Teflon) mediated surface plasmon resonance (SPR) sensor based on graphene in a dielectric–metal–dielectric configuration, for near infrared measurements, by taking advantage of the high adsorption efficiency of graphene and the high-resolution resonance feature of low loss surface plasmons. The proposed configuration supports the low loss surface plasmon mode which is evident from its mode field distribution and effective index dispersion. Results show that at a wavelength of 850 nm, the field intensity enhancement factor at the graphene-sensing layer interface for the proposed chalcogenide Ge20Ga5Sb10S65 (2S2G)–Teflon–Au–graphene based sensor is 20% greater than for the 2S2G–Au–graphene based sensor, thereby increasing the imaging sensitivity by 50%. Moreover, for the proposed sensor, the penetration depth of the field into the sensing region is 340% greater than for the conventional (2S2G–Au–graphene) SPR sensor. The proposed SPR sensor could present new possibilities for SPR imaging and detection for highly sensitive and high-throughput assessment of multiple simultaneous molecular interactions.

Extended parametric gain range in photonic crystal fibers with strongly frequency-dependent field distributions.

The parametric gain range of a degenerate four-wave mixing process is determined in the undepleted pump regime. The gain range is considered with and without taking the mode field distributions of the four-wave mixing components into account. It is found that the mode field distributions have to be included to evaluate the parametric gain correctly in dispersion-tailored speciality fibers and that mode profile engineering can provide a way to increase the parametric gain range.