Fundamental Mode Propagation Research Articles

Phase-locked high-brightness interband cascade laser array with multimode interferometer couplers.

We report on the design, fabrication, and characterization of an interband cascade laser (ICL) array incorporating multimode interference (MMI) couplers for phase-locking emitters. The ICL array, emitting at 3-4 µm, was developed to address the challenges of heat dissipation and in-phase operation in mid-infrared lasers. The array features a 7.5-µm-wide ridge design for fundamental transverse mode propagation and employs MMI structures to realize an in-phase operation of multiple emitters. The far-field patterns, characterized by periodic and symmetrical interference fringes, confirm the coherent operation of the array and the efficacy of MMI couplers in achieving phase-locking. The single-ridge side of the array exhibits a single-lobe far-field profile, with higher-order transverse modes effectively suppressed, showcasing a nearly diffraction-limited beam quality (M2 ≈ 1.31) at high output powers (390 mW from the 1 × 4 array). The robust performance and scalable design of the ICL array, validated by experimental results and theoretical simulations, indicate its potential for high-power mid-infrared applications and as an optical phased array.

Design and Performance Analysis of Strip Photonic Waveguide with Coating Layer for Multimode Propagation

Introduction: Photonic devices play a pivotal role in the realm of high-speed data communication due to their inherent capability to expedite the transfer of information. Historically, research efforts in this domain have predominantly concentrated on investigating the fundamental mode propagation within photonic waveguides. Methods: This study diverges from the conventional approach by delving into the untapped potential of higher-order modes in addition to the fundamental mode of propagation. The exploration of these higher-order modes opens up new possibilities for optimizing and enhancing the performance of photonic devices in high-speed data communication scenarios. As a distinctive aspect of this study, various coating materials were scrutinized for their impact on both fundamental and higher-order mode propagation. The materials under examination included AlN (aluminum nitride), Germanium, and Silicon. These materials were chosen based on their unique optical properties and suitability for influencing different modes of light propagation. The findings from the study reveal that applying a coating of germanium demonstrates advantageous characteristics, particularly in terms of reduced signal loss, even when considering higher-order modes of propagation within photonic devices. Results: In this context, the results indicate that germanium-coated waveguides exhibit notably low propagation losses, with measurements as minimal as 0.25 dB/cm. This low level of loss is particularly noteworthy, especially when the waveguide has a width of 550 nm and is coated with a thickness of 50 nm. The dimensions and coating specifications play a crucial role in determining the efficiency of light transmission within the waveguide. Conclusion: The fact that the propagation loss is substantially low under these conditions suggests that the germanium-coated waveguide, even when considering higher-order modes of light propagation, can effectively maintain the integrity of the optical signal.

On the Computation of the Attenuation Constant by Using the Generalized Telegraphist’s Equations Based Electromagnetic Analysis Method

Numerical results for the attenuation constant obtained by using the generalized telegraphist’s equations (GTEs) based electromagnetic analysis method and comparison with the HFSS (High Frequency Structure Simulator) results are reported. To calculate the attenuation constant with the GTEs method, not only the amplitudes of the voltage modes but also the amplitudes of the current modes must be found. In this paper, it is demonstrated that the relationships reported by other authors for the amplitudes of the current modes are not correct and new ones are proposed. To validate these relationships, the attenuation constants for homogeneous and different partially dielectric-filled rectangular waveguides are computed for the fundamental propagation mode by using the GTEs based analysis method and the results are compared with those obtained with HFSS. It is shown that using the revised relationships for the amplitudes of the current modes, the GTE method can be used to compute accurately the propagation and attenuation constant, but only for propagation modes in which the components of the electric field are not oriented perpendicular to the interface between different dielectrics. This limitation is not due to the proposed current mode relationships, but is due to the GTE method which cannot highlight the electric field discontinuities.

Limitations of the S2 (spatially and spectrally resolved) imaging technique

Spatially and spectrally resolved imaging (S2 imaging) is a technique that was developed as an alternative to beam quality (M2) measurements to characterize the modal content of large mode area (LMA) fibers. While it is known that the success of S2 imaging is highly dependent on the broadband source and the launch conditions into the fiber, the information resulting from this method is more limited than may appear at first glance. Experiments and numerical simulations are used to show that (a) the accuracy of the reconstructed LP01 mode profile varies greatly depending on launch conditions, and (b) there are always errors in the reconstructed LP11 mode profile. Not only do these findings reveal that the reconstructed mode areas cannot be accurately determined, but it also shows that the relative modal intensities can only be determined accurately under nearly perfect conditions of fundamental mode launch and propagation through the fiber. Despite these findings, the S2 technique can still be used under nearly ideal launch and propagation conditions to identify the modes and their group delays, and accurately reconstruct only the fundamental mode.

Footprint Analysis of CMOS Compatible Silicon-on-Insulator Based Photonic Waveguides

Aim: Establish the efficient footprint size, i.e., the total substrate width of photonic waveguides (Ridge, Rib, and Slot) under the fundamental mode propagation constraints. Objective: By varying the total substrate width for all photonic waveguides (Ridge, Rib, and Slot) with respect to four major waveguide parameters, namely effective refractive index, propagation loss, propagation length, and confinement percentage, the converged values of these waveguide parameters have to be obtained. Methods and Materials: The Finite Element Method (FEM) based simulations, using the COMSOL Multiphysics, have been used to study the modal characteristics of photonic waveguides to achieve their efficient footprint size. Results: The total substrate widths have been obtained for all four parameters and considering the impact of all these waveguide parameters simultaneously, the efficient total substrate width has been recognized as 2500 nm, 4000 nm, and 3000 nm, respectively, for Ridge, Rib, and Slot waveguides. Conclusion: The efficient waveguide footprints, i.e., the total substrate widths for the three photonic waveguides, namely Ridge, Rib and Slot waveguides have been established.

Chemically Functionalised Suspended-Core Fibre for Ammonia Gas Detection

An optical fibre ammonia gas sensor utilising a functionalised four-leaf-clover-shaped suspended-core fibre (SCF) is demonstrated. The fibre is functionalised by depositing a thin layer of an ammonia sensitive dye (tetraphenylporphyrin tetrasulfonic acid hydrate) (TPPS) on the wall of the inner cavities of the SCF through capillary action. An in-line fibre sensing structure is designed for light transmission through the SCF and also allows gas exchange with the atmosphere through two porous polyethylene housings. The sensing structure exhibits a temperature-dependent transmission due to the thermal expansion of the housings. A ratiometric method is applied for signal processing which is demonstrated to reduce significantly the effect of temperature change in the tested range (20-30 °C). The sensor is tested in ammonia concentration ranges from 0-10 ppm and demonstrates capability of detecting ammonia at ppb levels. The minimum tested concentration is 150 ppb in the experiment with a calculated limit of detection of 20 ppb. The sensor response time (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10-90%</sub> ) is 160 s and it is demonstrated to be reusable after treatment with hydrogen chloride vapour. Numerical simulation of evanescent absorption features of such an SCF indicates that approximately 0.02% of the optical power exists in the air holes for the fundamental propagation mode. Distinctive absorption bands observed in the transmission spectrum of the fabricated sensor can also be observed in the simulated model after adding the TPPS absorption layer in the holes.

A Metal-Cap Wedge Shape Hybrid Plasmonic Waveguide for Nano-scale Light Confinement and Long Propagation Range

A metal-cap wedge shape hybrid plasmonic waveguide (WSHPW) has been investigated, for the nano-scale light confinement and long propagation range, in order to analyze the optical properties, such as propagation length and normalized effective mode area, of the fundamental hybrid mode at the wavelength of \(1550 \mathrm{nm}\). Due to the wedge shape structure of high-index region (AlGaAs), the energy is mostly confined at the top of it, inside the low-index region. This results in significant improvement in the propagation length. The modal analysis has been done, using the finite element method, by varying the width of waveguide, wedge angle, permittivity of high- and low-index regions, and heights of metal, high-, and low-index regions. The analysis has been further extended for different metal, such as silver (Ag), gold (Au), and aluminum (Al). From the simulation results, it has been established that the propagation length \(({L}_{p})\) \(> 490 {\mu m}\) can be achieved for the fundamental mode propagation. Furthermore, the investigations on coupling length \(({L}_{c})\) between the two parallel WSHPWs have been done, which has been achieved as small as \(6.10 {\mu m}\), for the waveguide separation of \(100 \mathrm{nm}\), and waveguide width of \(50 \mathrm{nm}\).

Propagation of fundamental mode in regularly bending multi-mode waveguides

Transmission of the fundamental mode in multi-mode waveguides is an effective scheme for a silicon-based platform to reduce scattering loss. However, the application of the scheme is usually limited to straight waveguides and restricted in multi-mode bending waveguides. This is because the fundamental mode of a straight waveguide is seriously disordered after passing the bend. In this work, we have presented a “matched bending radius” approach, by which an ultra-low loss and negligible modal disorder have been demonstrated in the Si and Si3N4 multi-mode waveguides. The estimated TE0 mode transmission factor is almost 0 dB at the matched bending radius, indicating that the fundamental mode can be re-generated after passing the multi-mode bending waveguide. The proposed approach will contribute to applying the low loss scheme in large-scale photonic integrated circuits.

Mode Composite Sandwich Slot Line and its Applications in Antenna Development

In this article, a class of mode composite transmission line called mode composite sandwich slot line (MCSSL) is proposed, studied, and demonstrated. The MCSSL is composed of two substrate integrated waveguides (SIWs) and a sandwiched quasi-slot line. Structural parts of the two SIWs inherently serve as the two conductors of the quasi-slot line. The SIW supports the TE10 fundamental mode propagation, whereas the quasi-TEM mode is supported by the quasi-slot line. Two types of transition structures are proposed in this article, which enable an easier integration of the studied MCSSL with other transmission lines. An MCSSL-based antenna is designed and fabricated to theoretically and experimentally demonstrate its advantages and feasibility. It has been shown that the MCSSL features the physical and electrical characteristics of a joint structure (or structure sharing) in connection with a mode composite with planar structures, and it can find applications in broadband and multiband circuits and antenna developments.

Infrared radiation Induced attenuation of radiation sensitive optical fibers: influence of temperature and modal propagation

Abstract We investigate the X-ray (40 keV) and γ-ray (1.2 MeV) radiation responses of three different radiation sensitive Optical Fibers (OFs) up to 100 Gy(SiO2). In particular, we study the Radiation Induced Attenuation (RIA) in the Near Infrared domain (NIR) for single mode OFs doped with Phosphorus (P), Aluminum (Al) and Phosphorus/Cerium (PCe) in their cores at three temperatures up to 50 °C. RIA levels and kinetics strongly depend on the operating wavelength and fiber composition. For both P and PCe-doped fibers, the P1 defects are the main contributors to the RIA, with Ce-codoping inducing a decrease of radiation sensitivity. For the Al-doped fiber, no specific absorption bands can be discriminated in the NIR. Both X- and γ-rays lead to the same RIA levels and kinetics. The RIA spectral dependences on dose and temperature highlight the potential of the three investigated fibers for radiation detection and dosimetry. To better discuss the properties of point defects responsible for the NIR RIA, we analyze how the fundamental mode propagation influences the RIA spectra of each fiber type. By reasonably assuming that the core RIA exceeds largely the cladding RIA and by calculating the mode Confinement Factor (CF), the RIA spectra of the core material are reconstructed and the spectral characteristics of defects are discussed for each type of fibers.

Spectral finite element method for wave propagation analysis in smart composite beams containing delamination

PurposeThe purpose of the study is to present a frequency domain spectral finite element model (SFEM) based on fast Fourier transform (FFT) for wave propagation analysis of smart laminated composite beams with embedded delamination. For generating and sensing high-frequency elastic waves in composite beams, piezoelectric materials such as lead zirconate titanate (PZT) are used because they can act as both actuators and sensors. The present model is used to investigate the effects of parametric variation of delamination configuration on the propagation of fundamental anti-symmetric wave mode in piezoelectric composite beams.Design/methodology/approachThe spectral element is derived from the exact solution of the governing equation of motion in frequency domain, obtained through fast Fourier transformation of the time domain equation. The beam is divided into two sublaminates (delamination region) and two base laminates (integral regions). The delamination region is modeled by assuming constant and continuous cross-sectional rotation at the interfaces between the base laminate and sublaminates. The governing differential equation of motion for delaminated composite beam with piezoelectric lamina is obtained using Hamilton’s principle by introducing an electrical potential function.FindingsA detailed study of the wave response at the sensor shows that the A0 mode can be used for delamination detection in a wide region and is more suitable for detecting small delamination. It is observed that the amplitude and time of arrival of the reflected A0 wave from a delamination are strongly dependent on the size, position of the delamination and the stacking sequence. The degraded material properties because of the loss of stiffness and density in damaged area differently alter the S0 and A0 wave response and the group speed. The present method provides a potential technique for researchers to accurately model delaminations in piezoelectric composite beam structures. The delamination position can be identified if the time of flight of a reflected wave from delamination and the wave propagation speed of A0 (or S0) mode is known.Originality/valueSpectral finite element modeling of delaminated composite beams with piezoelectric layers has not been reported in the literature yet. The spectral element developed is validated by comparing the present results with those available in the literature. The spectral element developed is then used to investigate the wave propagation characteristics and interaction with delamination in the piezoelectric composite beam.

Design and cross talk optimization in single-mode high-core count multicore fiber under limited cladding diameter

To achieve the minimum cladding diameter (CD) at fixed cladding thickness (35 μm) in the trench-assisted (TA) homogeneous multicore fibers (MCFs), five different core structures have been proposed under the single-mode propagation condition. The high-core counts, up to 37 cores with dense hexagonal closed packing arrangement of homogeneous cores have been considered for the analysis. The expression for cross talk (XT) under bending and twisting conditions has been derived using conventional method. The influence of core pitch, fiber length, fiber bending, twisting, and trench depth has also been exhibited on the XT performance for fundamental (LP01) mode propagation. The optimal CD based on specific values of cladding thickness and core pitch has been estimated in all the respective core structures for the target XT value of < − 50 dB / 100 km. The dispersion properties have also been investigated for all the proposed core structure, which is obtained nearly as −6 ps / nm-km at operating wavelength of 1550 nm. Further, in anticipation of high transmission capacity, the 61-core TA MCF has been analyzed for the XT performance under the limited CD of 300 μm.

General silicon-on-insulator higher-order mode converter based on substrip dielectric waveguides.

A general silicon mode-converter waveguide that converts a fundamental mode to any higher-order mode is proposed. Specifically, dielectric substrip waveguides are inserted in the fundamental mode propagation path so that the conversion is done directly in the same propagation waveguide, without coupling the power into another waveguide as it happens in traditional mode converters. The device has a very small footprint compared to traditional converters. A mathematical model is developed to determine the design parameters of the used dielectric material and analyze the whole performance of the proposed device. Both the effective index method (EIM) and the perturbative mode-coupled theory are used in our mathematical analysis to get exact values for both the coupling coefficient and the length of the used dielectric material, so as to ensure a maximum coupled power transfer to the higher-order mode. In addition, full vectorial 3D-FDTD simulations are performed to validate our mathematical model. Our results show good agreement between the approximate EIM method and accurate full vectorial 3D-finite-difference time-domain (FDTD) simulations in characterizing the device parameters and performance. In order to validate the design model, two mode converters are simulated, fabricated, and tested for converting a fundamental TE0 mode into both first- and second-order (TE1 and TE2) modes, respectively. Good insertion losses and low crosstalks are obtained. Good agreement between simulated and fabricated results are achieved.

Numerical Investigation of Light Emission from Quantum Dots Embedded into On‐Chip, Low‐Index‐Contrast Optical Waveguides

Single‐photon emitters integrated into quantum optical circuits will enable new, miniaturized quantum optical devices. Here, we numerically investigate semiconductor quantum dots embedded into low‐refractive‐index‐contrast waveguides. We discuss a model to compute the coupling efficiency of the emitted light field to the fundamental propagation mode of the waveguide, and we optimize the waveguide dimensional parameters for maximum coupling efficiency. Further, we show that for a laterally cropped waveguide the interplay of Purcell enhancement and optimized field profile can enhance the coupling efficiency by a factor of about two.

Thermal expanded core ultraviolet fiber for optical cavity mode matching

We have demonstrated a mode matching method between two different fibers by a hybrid thermal expanded core technique, which can be applied to match the modes of fiber-based Fabry–Perot cavities. Experimentally, this method has achieved an expansion of the ultraviolet fiber core by 3.5 times while keeping fundamental mode propagation. With the experiment parameters, the fundamental mode coupling efficiency between the fiber and micro-cavity can reach 95% for a plano-concave cavity with a length of 400 μm. This method can not only have potential in quantum photonics research but also can be applied in classical optical fields.

Computation of ion exchange buried microring resonator waveguide for THz communication applications

Abstract The ion exchange creates the optical buried waveguides, whereas it is the method that the glass doping occurs with the condition of existence of the mobile metal ions. The most common ion used is silver (Ag+) due to its broad variation in the refractive index. The proposed application presented in this work is to use the microring resonator (MRR) system based on the buried waveguides simulated by the ion exchange method. The passive MRRs are realized as the multi-wavelength transceiver, applicable in optical technologies such as the optical sensing, Terahertz (THz) communication and optical switching. The cross-sectional view of the simulated buried optical waveguide for the Ag+ concentrations of 0.8 mol/m3 is presented. The fundamental mode propagation is simulated using the time-domain traveling wave method, where the simulation of the optical power flow density and the electric field distributions of the buried waveguides were performed. The two coupled MRRs are used to generate tunable dual-wavelength with spacing in the range of Terahertz (THz). As results, the dual wavelengths in the range between 0.25 and 0.96 THz were generated at the throughput and drop port of the MRRs system.

Study on the adiabaticity criterion of the thermally-guided very-large-mode-area fiber.

The adiabaticity criterion of the thermally-guided very-large-mode-area (TG VLMA) fiber is presented based on the mode-coupling theory firstly, to the best of our knowledge. The requirement for the adiabatic propagation of fundamental mode is discussed systematically. It is revealed that the pump absorption plays the most important role and the adiabaticity criterion can be met as long as it is small enough. Then, the effects of the configuration parameters of TG VLMA fiber on the up-limitation of pump absorption for the adiabaticity criterion are investigated. It is found that for the straight TG VLMA fiber, reducing the initial refraction index and inner-cladding diameter and utilizing the bi-directional pumping scheme are beneficial to the adiabatic propagation of fundamental mode. The bent TG VLMA fiber is also studied. It is found that the bent fiber is much more difficult to meet the adiabaticity criterion than the straight one. The results show that even with the 100-cm bend radius, the pump absorption should be smaller than 1 dB/m to meet the adiabaticity criterion. It is suggested that enlarging the core-to-cladding ratio can be helpful for loosening the adiabaticity criterion of bent TG VLMA fiber. These pertinent results can provide significant guidance for understanding and designing the TG VLMA fiber and pertinent lasers and amplifiers.

Chip-scale nanophotonic switch based on a waveguide-metamaterial coupling mechanism

We demonstrate a simple and effective waveguide-metamaterial coupling mechanism to achieve switching control of the fundamental TE mode propagation in a silicon photonic stripe waveguide. The metamaterial is made of vertically stacked alternating ITO/Si layers and, theoretically, can be switched between transparent and absorptive regimes via modifying the carrier concentration of the ITO layers. In addition to its small footprint and CMOS-compatible fabrication, simulation results indicate that the optical switch features a high modulation depth (MD) of 27.8dB, low insertion losses of 0.004dB, and a wide operating bandwidth of 300nm where the MD is >24.6 dB. We expect that this mechanism is a good candidate for designing high-performance and ultra-compact photonic devices in densely integrated nanophotonic circuits and computation systems.

Enhancement of phase-matched third harmonic generation via soliton self-frequency shift cancellation in a fluorotellurite microstructured fiber

We report the enhancement of phase-matched third harmonic generation (THG) via soliton self-frequency shift cancellation (SSFSC) in a fluorotellurite microstructured fiber (FTMF) pumped by a 1560 nm femtosecond fiber laser. The FTMF has two zero-dispersion wavelengths of 891 and 2012 nm for its slow axis. As the pump laser is polarized along the slow axis of the FTMF, phase matched THG at 629 nm is observed in the FTMF when the Raman soliton red-shifts to 1913 nm and the effective index at 1913 nm for the fundamental propagation mode matches with that at 629 nm for the high order propagation mode for a pump power of ∼25.2 mW. Interestingly, the THG at 629 nm is enhanced very much when the Raman soliton meets the second zero-dispersion wavelength of the slow axis and further SSFSC occurs with increasing the pump power. Such an enhancement is due to the unchanged operating wavelength of the Raman soliton with an increase in the pump power when SSFSC occurs and the resulting increase in the interaction length between the Raman soliton and third harmonic. Our results show that high efficient THG with a designed wavelength can be obtained via SSFSC in dispersion engineered optical fibers.

Numerical investigations of an optical switch based on a silicon stripe waveguide embedded with vanadium dioxide layers

A novel scheme for the design of an ultra-compact and high-performance optical switch is proposed and investigated numerically. Based on a standard silicon (Si) photonic stripe waveguide, a section of hyperbolic metamaterials (HMM) consisting of 20-pair alternating vanadium dioxide (VO2)/Si thin layers is inserted to realize the switching of fundamental TE mode propagation. Finite-element-method simulation results show that, with the help of an HMM with a size of 400 nm×220 nm×200 nm (width×height×length), the ON/OFF switching for fundamental TE mode propagation in an Si waveguide can be characterized by modulation depth (MD) of 5.6 dB and insertion loss (IL) of 1.25 dB. It also allows for a relatively wide operating bandwidth of 215 nm maintaining MD>5 dB and IL<1.25 dB. Furthermore, we discuss that the tungsten-doped VO2 layers could be useful for reducing metal-insulator-transition temperature and thus improving switching performance. In general, our findings may provide some useful ideas for optical switch design and application in an on-chip all-optical communication system with a demanding integration level.