Low-index Core Research Articles

High power pulse delivery over a short distance through an all-solid anti-resonant microstructured optical fiber at [formula omitted] wavelength

We propose an all-solid silica-based, large mode area (LMA) anti-resonant microstructured optical fiber through which optical pulses of high-power can be delivered at the near mid-infrared wavelength range. Anti-resonant guidance of light has been achieved in the silica-based microstructured geometry by embedding 15 high-index solid cylinders in a low-index background matrix that essentially creates a node-less negative curvature cladding region surrounding the low-index core. The fiber geometry has been optimized in such a way that a large mode area of ∼2000μm2 is achieved for the fundamental mode with a loss in the order of 0.04 dB/m at 2μm wavelength. Moreover, the higher-order modes are at least one order of magnitude lossy than the fundamental mode at the operating wavelength. The optimized fiber structure has exhibited that a pulse of peak power as high as 1 kW and width of 2.2 ps can be sustained without any alteration in shape and degradation of power when the pulse is delivered through the fundamental mode over the proposed fiber length of 25 m. Hence, this proposed unique silica-based all-solid anti-resonant fiber structure is suitable for short-distance delivery of high-power pulses at 2μm wavelength with effective single mode separation.

Mode Hybridization in Silicon Core-Gold Shell Nanosphere.

A dielectric core-metal shell nanosphere has attracted scientific and technological interests due to the unique optical resonances arising from the hybridization of surface plasmon modes and cavity modes. The previous studies focus on a low-index dielectric core without its own optical resonances. Here, optical resonances of a core-shell nanosphere with a high refractive index (n≈ 4) core with the lowest order Mie resonances in the visible range are investigated theoretically and experimentally. Scattering and absorption spectra of a core-shell nanosphere for different values of the core refractive index are first analyzed, and there is a transition of the hybridization scheme around n≈ 2. Above the value, a characteristic hybridized mode with strong absorption and weak scattering emerges in the near-infrared range. A core-shell nanosphere composed of a silicon core and a gold shell is prepared, and the resonance modes are studied by single particle scattering spectroscopy and electron energy loss spectroscopy (EELS) in a transmission electron microscope. The core-shell nanospheres exhibit the hybridized modes depending on the core diameter. The hybridized mode as well as the higher order one that is not observable in the scattering spectroscopy is observed in the EELS.

Interpreting light guidance in antiresonant and photonic bandgap waveguides and fibers by light scattering: analytical model and ultra-low guidance.

Here, we introduce a quasi-analytic model that allows studying mode formation in low refractive index core waveguides through solely focusing on the cladding properties. The model isolates the reflection properties of the cladding from the modes via correlating the complex amplitude reflection coefficient of the cladding to the complex effective index of the fundamental core mode. The relevance and validity of the model are demonstrated by considering a single-ring anti-resonant fiber, revealing unexpected situations of exceptionally low loss. Our model explains mode formation by light scattering, which conceptually provides deep insights into the relevant physics.

Fiber-integrated hollow-core light cage for gas spectroscopy

Interfacing integrated on-chip waveguides with spectroscopic approaches represents one research direction within current photonics aiming at reducing geometric footprints and increasing device densities. Particularly relevant is to connect chip-integrated waveguides with established fiber-based circuitry, opening up the possibility for a new class of devices within the field of integrated photonics. Here, one attractive waveguide is the on-chip light cage, confining and guiding light in a low-index core through the anti-resonance effect. This waveguide, implemented via 3D nanoprinting and reaching nearly 100% overlap of mode and material of interest, uniquely provides side-wise access to the core region through the open spaces between the cage strands, drastically reducing gas diffusion times. Here, we extend the capabilities of the light cage concept by interfacing light cages and optical fibers, reaching a fully fiber-integrated on-chip waveguide arrangement with its spectroscopic capabilities demonstrated here on the example of tunable diode laser absorption spectroscopy of ammonia. Controlling and optimizing the fiber circuitry integration have been achieved via automatic alignment in etched v-grooves on silicon chips. This successful device integration via 3D nanoprinting highlights the fiber-interfaced light cage to be an attractive waveguide platform for a multitude of spectroscopy-related fields, including bio-analytics, lab-on-chip photonic sensing, chemistry, and quantum metrology.

A Study on the Analysis of Electromagnetic Characteristics and Design of a Cylindrical Photonic Crystal Waveguide with a Low-Index Core

A Study on the Analysis of Electromagnetic Characteristics and Design of a Cylindrical Photonic Crystal Waveguide with a Low-Index Core

Threshold conditions of electric field enhancement and energy confinement in the low-index core of nanoscale waveguides

Threshold conditions to realize electric field enhancement and energy confinement in the low-refractive-index core of nanoscale waveguides are studied by solving the field function. When the incident lightwave meets the relation of special thresholds, we observe the enhanced electric field and a concentrated light energy in the core. The electric field enhancement and the confined light power are highly dependent on the light wavelength. When the core width is 30 nm, for a wavelength of 1.55 µm, we achieve a power confinement factor above 40%. As the basis for a growing number of potential applications, the threshold conditions discovered in this work will find significant applications in many fields, such as optical sensors and optical communication components.

Anti-resonance, inhibited coupling and mode transition in depressed core fibers.

The depressed core fiber (DCF), consisting of a low-index solid core, a high-index cladding and air surrounding, is in effect a bridge between the conventional step-index fiber and the tube-type hollow-core fiber from the point of view of the index profile. In this paper the dispersion diagram of a DCF is obtained by solving the full-vector eigenvalue equations and analyzed using the theory of anti-resonant and the inhibited coupling mechanisms. While light propagation in tube-type hollow-core fibers is commonly described by the symmetric planar waveguide model, here we propose an asymmetric planar waveguide for the DCFs in an anti-resonant reflecting optical waveguide (ARROW) model. It is found that the anti-resonant core modes in the DCFs have real effective indices, compared to the anti-resonant core modes with complex effective indices in the tube-type hollow-core fibers. The anti-resonant core modes in the DCFs exhibit similar qualitative and quantitative behavior as the core modes in the conventional step-index fibers. The full-vector analytical results for the simple-structure DCFs can contribute to a better understanding of the anti-resonant and inhibited coupling guidance mechanisms in other complex inversed index fibers.

High-power edge-emitting laser based on a parity-time-structured Bragg reflection waveguide.

A parity-time-structured Bragg reflection waveguide is proposed and analyzed for realizing a high-power laser. A single transverse mode with the optical field confined mainly in the low-index core is discussed to improve the catastrophic optical damage threshold for high output power. The designed scheme can potentially mitigate the heat buildup by moving the injection from the central narrow core to the outer large-area claddings, while suppressing the coupler mode transversely. The structure can achieve high output power with a short cavity length, low lasing threshold, and high power conversion efficiency.

High dispersion four-mode fiber for mode-division multiplexing systems

This paper presents a novel dispersions equalization technique for multilayer core four-mode fiber using LP mode-matrix method. Each of the core ring radius of the alternating center-high (CH) and center-low (CL) index profile was optimized to a specific thickness value. The core ring radius varied in each case to determine the optimized thickness (OPT) for thn e dispersions equalization. The OPT of the center high index core fiber (CH) denoted by R1, R2, and R3 were found to be 4.25 μm, 2.6 μm, and 2.3 μm, respectively. Similarly, an OPT of the center low index core fiber (CL) which was denoted by X1, X2, X3, and X4 was found to be 2.0 μm, 3.0 μm, 4.20 μm and 1.60 μm, respectively. At 1.55 μm wavelength, these values were used to model the two fibers to achieve an equalized modal dispersion. It is the first time, to our knowledge, a core-ring radius of a CH, and CL index profile varied to a certain OPT value to achieve dispersion equalization. Fabrication challenges were also discussed in this paper.

Infiltrated Photonic Crystal Fibers for Sensing Applications.

Photonic crystal fibers (PCFs) are a special class of optical fibers with a periodic arrangement of microstructured holes located in the fiber’s cladding. Light confinement is achieved by means of either index-guiding, or the photonic bandgap effect in a low-index core. Ever since PCFs were first demonstrated in 1995, their special characteristics, such as potentially high birefringence, very small or high nonlinearity, low propagation losses, and controllable dispersion parameters, have rendered them unique for many applications, such as sensors, high-power pulse transmission, and biomedical studies. When the holes of PCFs are filled with solids, liquids or gases, unprecedented opportunities for applications emerge. These include, but are not limited in, supercontinuum generation, propulsion of atoms through a hollow fiber core, fiber-loaded Bose–Einstein condensates, as well as enhanced sensing and measurement devices. For this reason, infiltrated PCF have been the focus of intensive research in recent years. In this review, the fundamentals and fabrication of PCF infiltrated with different materials are discussed. In addition, potential applications of infiltrated PCF sensors are reviewed, identifying the challenges and limitations to scale up and commercialize this novel technology.

Gain-guided index-antiguided fiber with a Fabry-Perot layer for large mode area laser amplifiers.

We propose a modified gain-guided index-antiguided (GGIAG) fiber structure for large mode area laser amplifiers, in which a thin dielectric layer is placed between the low-index core and the high-index cladding. The introduced dielectric layer functions as a Fabry-Perot etalon. By letting the resonant wavelength of the Fabry-Perot layer coincide with the signal wavelength, the signal is gain-guided in the fiber core. Moreover, the pump is confined in the low-index core owing to the antiresonant reflection originated from the Fabry-Perot layer. Numerical results indicate that the leakage loss of the pump can be minified over two orders of magnitude in the proposed structure, and thus the end-pumping efficiency could be enhanced significantly.

Integrated hollow-core fibers for nonlinear optofluidic applications

A method to fabricate all-in-fiber liquid microcells has been demonstrated which allows for the incorporation of complex hollow-core photonic crystal fibers (HCPCFs). The approach is based on a mechanical splicing method in which the hollow-core fibers are pigtailed with telecoms fibers to yield devices that have low insertion losses, are highly compact, and do not suffer from evaporation of the core material. To isolate the PCF cores for the infiltration of low index liquids, a pulsed CO2 laser cleaving technique has been developed which seals only the very ends of the cladding holes, thus minimizing degradation of the guiding properties at the coupling region. The efficiency of this integration method has been verified via strong cascaded Raman scattering in both toluene (high index) core capillaries and ethanol (low index) core HCPCFs, for power thresholds up to six orders of magnitude lower than previous results. We anticipate that this stable, robust all-fiber integration approach will open up new possibilities for the exploration of optofluidic interactions.

Low-loss hybrid fiber with zero dispersion wavelength shifted to 1 µm

We proposed and investigated a novel type of all-glass hybrid fiber where light is confined in the low-index core due to both total internal reflection and coherent Fresnel reflection (a photonic bandgap mechanism). The hybrid mode has an anomalous dispersion of 13 ps/(nm km) at 1064 nm and low loss (~6 dB/km), and it can be easily excited by splicing with a single-mode step-index fiber. The compression of positively chirped 8 ps pulses down to 330 fs was demonstrated with the fabricated hybrid fiber.

Designed Synthesis of SiO2/TiO2 Core/Shell Structure As Light Scattering Material for Highly Efficient Dye-Sensitized Solar Cells

SiO2/TiO2 core/shell nanoparticles (STCS-NPs) with diameters of 110, 240, and 530 nm were fabricated to investigate the influence of the size and refractive index of light-scattering particles on light-scattering properties. The optical properties of STCS-NPs were evaluated and compared with SiO2-NPs and TiO2-NPs. The structure of STCS-NPs, consisting of a low refractive index core and high refractive index shell, provides efficient light scattering. The optimized anode film with STCS-NPs had ca. 20% improved power conversion efficiency (PCE).

Antiresonant reflecting microstructured optical fibers for the mid-infrared

We report on the demonstration of silver halide (AgClxBr1−x) microstructured optical fibers (MOFs) based on the antiresonant reflecting optical waveguide (ARROW) model, for the mid-infrared. MOFs with low index core surrounded by an octagonal structure of high index rods were designed and fabricated. The fibers exhibited transmission spectra with narrow stopband frequencies in the 9–11 μm range. The location of the transmission minima matched the theoretical predictions by the ARROW model. The measured propagation losses in the maximum transmission range were ∼60 dB/m. Imaging the output of the MOFs at the passband wavelengths revealed single mode guidance in the core.

布拉格反射波导激光器的光谱特性

The continuous-wave single-mode operation of edge-emitting diode laser based on dual-sided non-quarter-wave Bragg reflection waveguide (BRW) was demonstrated at room temperature. A low-index core was utilized in the laser. The ridge BRW lasers (BRLs) exhibited an ultra-narrow twin-beam output. Single longitudinal mode emission with a very small spectral linewidth of 0.052 nm was measured. The lasing spectra showed a periodic modulation in the envelope with a mode spacing of approximately 3.3 nm. When the injection current of the BRLs was beyond 300 mA, a large mode hopping of the emission wavelength was observed.

A Slot Micro/Nano Fiber With Elliptical Low-Index Core

A slot micro/nano optical fiber with elliptical low-index core, referred to as the elliptical-slot micro/nano fiber (ESMNF), is studied systematically in great detail in this paper. It is demonstrated that the ESMNF can confine most of the light in the low-index region and hence lead to large field enhancement. Benefiting from the large low-index power confinement factor and the field enhancement, the ESMNF also yields significant nonlinearity enhancement by the proper selection of materials. In addition, large birefringence between the two fundamental polarized modes is observed, due to the breakage of the circular symmetry for the core. The nonlinearity and low-index confinement properties of a more practical elliptical-slot embedded suspended microstructure fiber are also studied.

Supercontinuum generation in all-solid photonic crystal fiber with low index core

In this paper we report on the fabrication and characterization of an all-solid photonic band gap fiber with high contrast and low index core. The fiber cladding is composed of high index lead-silicate rods while borosilicate NC21 glass is used as a background glass. A 70 nm wide photonic band gap at 875 nm central wavelength is experimentally identified and compared with a numerical model. We also present a novel method for photonic band gap measurement using a femtosecond pulsed laser. The method is verified against standard one and discussed.

Simple design of an edge-emitting diode laser for preventing spatial multimoding via thermal lensing

The parameters of a diode-laser structure composed of a pair of built-in high-index regions for providing stable, single-spatial-mode operation to high cw powers are numerically found. A three-dimensional numerical code has been implemented that takes into account carrier diffusion in the quantum well and thermal lensing. The laser characteristics are calculated as functions of the above-threshold drive level. Within the simulation, higher-order optical modes on a "frozen background" are computed via the Arnoldi algorithm. Then, for a 6-μm-wide low-index core and 2-3-mm-long devices, stable single-mode operation up to multiwatt-level (2-3 W) cw output power is predicted.

Comprehensive Analysis of Mode Competition in High-Power CW-Operating Diode Lasers of the Antiresonant Reflecting Optical Waveguide (ARROW) Type

Three-dimensional above-threshold analyses have been performed on laterally antiguided laser structures of the antiresonant-reflecting-optical-waveguide (ARROW) and simplified (S)-ARROW types, of relatively large core width (10 μm), for generating watt-range continuous wave (CW) powers in a single, stable spatial mode. The 3-D numerical code takes into account carrier diffusion in the quantum wells, thermo-optic effects as well as edge radiation losses. The behavior of both device types is studied over a wide range in the variation of the width of the high-index regions s bordering the low-index device core. The results of the analyses indicate that thermal lensing plays an essential role in optical-mode competition. Best results (i.e., single-mode CW powers in excess of 1.5 W) are obtained when s corresponds to a lateral resonant condition. When s corresponds to one full lateral wave, both ARROW and S-ARROW devices reach CW single-mode powers of 1.7 W. When s corresponds to about one half lateral wave, both ARROW and S-ARROW devices display isolated regions of modal instability followed by CW operation in a stable, single spatial mode to powers as high as 2.0 W.