Optical Coupling Research Articles

Self Q-switched single-frequency all-fiber laser

Here, we propose a self Q-switched single-frequency (SF) all-fiber laser. The intra-cavity Er:Yb co-doped fiber serves not only as an active fiber for lasing but also as a fiber-type saturable absorber (SA). The self-Q-switching behavior is induced by the ground-state reabsorption of the unpumped portion of the active fiber. To achieve SF operation, double fiber loops constructed by two 3 dB optical couplers (OCs) form a cascade sub-cavity, which serves as a mode selector with a narrow bandwidth. Taking the advantage of a high damage threshold of the fiber-type SA, microjoule-level self-Q-switched SF pulses are obtained with a narrow linewidth of ∼252kHz. Combed with the measured pulse width of ∼1.766µs, the corresponding time-bandwidth product is ∼0.445, which is very close to the Gaussian transform limit. The maximum output power of the pulsed SF laser can reach ∼100mW. By stretching the fiber Bragg grating (FBG), the wavelength of the pulsed SF laser can be tuned.

Ultrasensitive refractive index sensing of optical microfiber couplers coated with Ti3C2 MXene

Two-dimensional (2D) MXene has recently been found to play a significant role in constructing biosensing platforms due to its excellent biocompatibility, large specific surface area, and broad optical absorption. In this article, an ultra-sensitive refractive index (RI) sensor based on an optical microfiber coupler (OMC) coated with Ti3C2 MXene is proposed and experimentally demonstrated. Numerical results reveal that surface modification of MXene enhances the RI sensitivity of OMC and creates a new artificial dispersion turning point (ADTP). The OMC with a dispersion turning point (DTP) at 1300 nm is fabricated using a hybrid fusion elongation-wet etching method. It has a waist width of approximately 2.46 µm. We experimentally achieved the RI sensitivity of 9.1 × 104 ± 0.42 × 104 nm/RIU using the OMC. MXene improves the sensitivity of the OMC because Ti3C2 MXene can change the effective refractive index (ERI) for both the even and odd modes, leading to the interference dips shifting closer to the DTP. The experimental results demonstrate that the RI sensitivity of the OMC coated with Ti3C2 MXene can reach -1.7 × 105 ± 0.2 × 105 nm/RIU, which shows a ∼87% sensitivity boost compared with bare OMC. The proposed sensor is expected to be used for high-sensitivity biochemical molecule detection.

Exploring optical soliton solutions in highly dispersive couplers with parabolic law nonlinear refractive index using the extended auxiliary equation method

This study delves into the realm of optical soliton solutions within the intricate framework of highly dispersive couplers integrated with optical metamaterials featuring a parabolic law nonlinear refractive index. Employing the extended auxiliary equation method, the investigation systematically unveils various soliton solutions, including dark solitons, bright solitons, singular solitons, and a distinctive amalgamation of bright and singular solitons. The results illuminate the diverse dynamics of solitons in these complex systems, offering crucial insights into the interplay among dispersion, nonlinearity, and metamaterial characteristics. Beyond enhancing the theoretical understanding of soliton behavior in optical metamaterial couplers, the findings hold practical significance by potentially influencing the design and optimization of optical communication devices and systems.

Single-to-four core optical fiber coupling using a two-photon polymerization produced waveguide

Optical coupling between single core to multi-core optical fibers usually takes place by means of optical fiber fan-ins / fan-outs, delicate free space optics, or laser inscribed freeform waveguides. In the present work, the two-photon polymerization technique is used for the first time to create a waveguide manifold on top of a four-core optical fiber tip as a means to couple light into and from a single core optical fiber, in a fast and low-cost fashion. It is demonstrated that the performance is influenced by the numerical aperture mismatch between the fabricated and the coupled waveguides. Insertion losses below 5 dB are observed when the numerical aperture mismatch is minimized, with further reduction potential, making this approach applicable to sensing or tweezer applications.

Variable magnification beam expander system based on semiconductor laser

Due to the different divergence angles in two perpendicular directions and the presence of astigmatism in edge-emitting semiconductor lasers, the communication distance and efficiency of underwater communication are seriously affected. Therefore, based on the fact that the semiconductor laser beam belongs to Gaussian beam, satisfying Gaussian characteristics and collimating and expanding characteristics, a system composed of fast-axis collimation mirror and variable magnification beam expander is designed. Simulation is performed, and the final divergence angle is calculated. Experimental results show that the divergence angles of the fast axis and the slow axis have been compressed from 49 and 9, respectively, to 0.315 mrad and 0.180 mrad. Simulation demonstrates that the system can achieve propagation over a distance of 100m underwater and solve the alignment difficulty of APT. The designed structure is compact, easy for processing and adjustment, with high practical value, which helps to solve the problems of optical energy loss and low coupling efficiency in underwater long-distance communication.

Low-Loss Fluoride Optical Fiber Coupler for Mid-Infrared Applications

Low-Loss Fluoride Optical Fiber Coupler for Mid-Infrared Applications

Asymmetric convolutional multi-level attention network for micro-lens segmentation

Tiny target recognition in automation is currently a hot research task that usually suffers from typical issues such as complex background, dim target, and slow detection speed. In the current study, a data-driven method is proposed to realize the posture recognition of micro-lens during optical device coupling to achieve accurate clamping of the gripper. First, we establish a pixel-by-pixel labeled optical micro-lens dataset named single-frame micro-lens target (SFMT), which provides data support for the subsequently proposed convolutional neural network. Subsequently, an asymmetric convolutional multi-level attention network (ACMANet) is proposed to realize accurate segmentation detection of micro-lenses by employing an embedded multi-scale asymmetric convolutional module (MACM) and a multi-level interactive attention module (MIAM). MACM achieves not only a reduction in computational complexity but also enhanced robustness for rotated image recognition through multi-scale asymmetric convolutional kernels. Furthermore, MIAM improves the accuracy of image segmentation by connecting the down-sampling and up-sampling stages and realizing the fusion of pixel position details and key channel features. Extensive experimental results based on our self-constructed image acquisition system demonstrate that the values of normalized intersection over union and dice are successively 91.41% and 95.50%, and the processing speed is 3.3 s/100 images, which shows the advance of ACMANet.

Monolithically integrated multimode interference coupler-based master oscillator power amplifier with dual-wavelength emission around 830 nm

A monolithically integrated dual-wavelength multimode interference coupler-based master oscillator power amplifier is presented. It consists of two shallowly etched, laterally separated ridge waveguide laser cavities as master oscillators with individual distributed Bragg reflector gratings as cavity mirrors. A deeply etched coupling section containing S-bend shaped waveguides and a multimode interference coupler is used to couple the laser emission of the master oscillators into a shallowly etched single waveguide serving as power amplifier. Changing the etch depth for the coupling section enables a compact device layout. In addition, increased radiation angles of modes not coupled into the power amplifier help to suppress beam steering, otherwise indicated by laterally separated far-field intensity distributions. The device provides 0.5 W of dual-wavelength emission around 830 nm in individual and common operation. As designed, both emission wavelengths are separated by 0.5 nm with spectral widths below 20 pm, limited by the spectral resolution of the spectrometer. Both peak wavelengths remain within spectral windows of 50 pm within the available power range. This enables full flexibility selecting operating points for applications such as shifted excitation Raman difference spectroscopy and the generation of THz emission by photomixing. The emission wavelengths can additionally be non-continuously tuned by applying a heater current to resistors implemented next to the distributed Bragg reflector gratings. As an example, selected spectral distances of 0.5 nm, 1.0 nm, 1.5 nm, and 2.0 nm are demonstrated. Near field widths of 5 μm and far field angles of 17° result in beam propagation ratios of 1.4 (1/e2) in all operation modes and enable easy beam shaping or optical single-mode fiber coupling.

High-power, stable single-mode CW operation of 1550 nm wavelength InP-based photonic-crystal surface-emitting lasers

1550 nm wavelength photonic-crystal surface-emitting lasers (PCSELs) are attractive for optical communication and eye-safe sensing applications. In this study, we present InP-based PCSELs featuring a double-lattice photonic-crystal structure designed for high-power single-mode operation at a wavelength of 1550 nm. These PCSELs demonstrate output powers exceeding 300 mW under continuous-wave conditions at 25 °C. Additionally, highly stable single-mode oscillation with a side-mode suppression ratio of over 60 dB is verified at temperatures from 15 °C to 60 °C. Measurement and simulation of photonic band structures reveal the impacts of the threshold gain margin and optical coupling coefficient on the single-mode stability.

Optimization of the reflection in the optical coupling between high numerical aperture fibers and photonic integrated circuits

The optical coupling in silicon Photonic Integrated Circuits (PICs) remains one of the main topics in the improvement of the performance of these devices. Highly demanding parameter specifications, such as loss, reflection, and bandwidth, drive the continuous search for optimization. The use of High Numerical Aperture Fibers (HNA fibers) to couple light in small diameter mode edge couplers, is one of the existing technologies that address the coupling challenge. We demonstrate, using Eigenmode Expansion (EME) and Transfer Matrix Method (TMM) simulations, that the Optical Return Loss (ORL) of this coupling approach can be made as low as −45 dB for both Transverse Electric (TE) and Transverse Magnetic (TM) polarizations, simultaneously. The results from our TMM model agree well with a commercial solver one and are faster (six orders of magnitude) and more computationally efficient. It is shown that the refractive index in the TMM should be corrected to compensate for the Gouy phase shift in the non-guided sections of the optical coupling. Additionally, the reflection pattern, as a function of the gap between the fiber and the PIC, provides a way to control the distance of the fiber to avoid packaging-related problems.

Analysis of Interference Effect in Double Optomechanically Induced Transparency System

We propose a scheme to investigate the interference properties of a double optomechanically induced transparency system, which involves two charged nanomechanical resonators, coupled via Coulomb interaction. The results show that the opening of transparency windows is caused by a destructive interference effect only in the weak optical coupling region. For strong optical coupling, normal mode splitting dominates the transparency phenomenon. In the intermediate region, both destructive interference and normal mode splitting contribute to the transparency windows. When the Coulomb coupling is much weaker than the optical coupling, the Coulomb interaction strength linearly determines the distance between the two transparency windows, and has nearly no influence on the destructive interference effect. Otherwise, the system will work in a nonlinear region.

Inverse-designed metasurfaces with facile fabrication parameters

Optical metasurfaces are planar nanostructured devices that are industrially attractive in part because they utilize high-throughput microelectronic fabrication techniques for implementation. It is therefore critical to develop design paradigms that can balance the realization of highly efficient wavefront responses together with device manufacturability. We introduce a gradient-based design framework for freeform metasurfaces in which nanoscale elements are explicitly constrained to feature basic shapes, nearly uniform feature sizes, and exceptionally low aspect ratios. In spite of the apparent uniformity of the metasurface geometric features, the devices are able to utilize nonlocal near-field optical coupling to achieve highly efficient and extreme wavefront scattering beyond conventional design methodologies. Utilizing this approach, we design facile high-numerical-aperture devices such as beam deflectors and large-area metalenses capable of diffraction-limited focusing. We anticipate that these concepts can facilitate the design and integration of metasurfaces into monolithic optical systems.

Effect of rapid/slow annealing routes on the magnetic and photoelectric properties of BiFeO3/CoFe2O4 multilayer thin films

With the development of electronic information technology, the miniaturization of lightweight devices and their integration have increased the requirements for materials, and two-dimensional composite multiferroic materials have become a major research hotspot in recent years. However, it is difficult for traditional solid multiferroic material to directly switch the magnetic/polarization direction under the action of an applied electric/magnetic field, limiting the improvement of the magnetoelectric coupling coefficient. In this study, a series of BiFeO3 (BFO)/CoFe2O4 (CFO) composite film multiferroic materials were prepared by changing the annealing routes and annealing atmosphere, the microstructure, ferroelectric properties, dielectric properties, photovoltaic properties, magnetic properties, and magnetoelectric coupling properties of composite films were routeatically studied. The results showed that under the slow annealing route, the composite film has obvious ellipsoidal grains with distinct grain boundaries and better crystallinity. Moreover, the performance of the film after annealing in different atmospheres of the slow annealing route was improved compared with that of the rapid annealing route after annealing in different atmospheres. Simultaneously, by changing the annealing atmosphere of the composite film, the film exhibited better magnetic optical properties and coupling properties after annealing in oxygen. In addition, this study showed that the dielectric constant and residual polarization value of the film increase under the action of magnetic field and illumination, and the maximum photomagnetic dielectric response (3.19%) and photomagnetic response (1.58%) are observed in the annealed film under the oxygen atmosphere of the slow annealing regime, indicating that the magnetoelectric coupling effect of the film is improved under the action of applied light.

MI-MZI based distributed optical fiber sensor for location and pattern recognition.

This research addressed the drawbacks of the conventional hybrid structure and processing technique by presenting a novel distributed fiber optic sensor based on a hybrid Michelson and Mach-Zehnder interferometer. The sensor can achieve blind spot free positioning and has a wide response frequency, additionally its structure is not complex. It can obtain two phase signals from each of the two interferometers by using a demodulation method that uses a 3 × 3 optical coupler. To determine the position of the disturbance, we computed cross-correlations on the two signals following basic mathematical techniques. Markov Transition Field was used to transform the phase signals-which had been filtered by a band pass filter-into two-dimensional images. Tagged photos built a dataset, which is then fed into a neural network to identify patterns. Experiments have shown that the frequency response capacity of the structure was verified, and it was able to achieve location within 0-30 km with location errors of ±85 m. In a six-category pattern recognition, the testing set accuracy was 98.74%.

Manipulation of optical orbit-induced localized spin angular momentum using the periodic edge dislocation.

Orbit-induced localized spin angular momentum (OILS) has recently garnered significant attention. This paper introduces periodic edge dislocation (PED) into the tight focusing system. The study delves into the tight focusing characteristics of the radially polarized vortex plane beam with PED, demonstrating that PED serves as a straightforward and effective means of manipulating OILS, especially when both the orbital angular momentum and the polarization of the incident beam are fixed. Our findings indicate that the longitudinal OILS reaches its maximum when the difference between the period of PED and the vortex topological charge is equal to 1. Conversely, when the difference is 0, the transverse OILS reaches its maximum, while the longitudinal OILS reaches its minimum. Similar patterns are also observed in linearly polarized vortex beams. This research proposes a simple and practical way to control OILS, contributing to our understanding of optical orbit-spin coupling.

Seawater Dual Parametric Fiber Optic Temperature and Pressure Sensors Demodulated by Machine Learning Method

By employing polydimethylsiloxane (PDMS) characterized by elevated thermo-optical and elastic-optical coefficients for encapsulating an optical microfiber coupler integrated with a sagnac loop (OMCSL) structure, which exhibits large abrupt field characteristics, a fiber optic sensor capable of simultaneously measuring seawater temperature and pressure can be created. Nevertheless, the utilization of the traditional sensitivity matrix method (SMM) for demodulating the sensor led to unstable and considerably erroneous demodulation results. To enhance the accuracy of the demodulation process, this paper investigates and employs a machine learning method (MLM) for sensor demodulation. This paper centers on the investigation and application of MLM for sensor demodulation. The experimental results exhibit a significant decrease in demodulation error attained via MLM when contrasted with the traditional SMM.

Strong Optical Coupling of Lattice Resonances in a Top-down Fabricated Hybrid Metal-Dielectric Al/Si/Ge Metasurface.

Optical metasurfaces enable the manipulation of the light-matter interaction in ultrathin layers. Compared with their metal or dielectric counterparts, hybrid metasurfaces resulting from the combination of dielectric and metallic nanostructures can offer increased possibilities for interactions between modes present in the system. Here, we investigate the interaction between lattice resonances in a hybrid metal-dielectric metasurface obtained from a single-step nanofabrication process. Finite-difference time domain simulations show the avoided crossing of the modes appearing in the wavelength-dependent absorptance inside the Ge upon variations in a selected geometry parameter as evidence for strong optical coupling. We find good agreement between the measured and simulated absorptance and reflectance spectra. Our metasurface design can be easily incorporated into a top-down optoelectronic device fabrication process with possible applications ranging from on-chip spectroscopy to sensing.

Toward Complete Optical Coupling to Confined Surface Polaritons.

Optical coupling between propagating light and confined surface polaritons plays a pivotal role in the practical design of nanophotonic devices. However, the coupling efficiency decreases dramatically with the degree of mode confinement due to the mismatch that exists between the light and polariton wavelengths, and despite the intense efforts made to explore different mechanisms proposed to circumvent this problem, the realization of a flexible scheme to efficiently couple light to polaritons remains a challenge. Here, we experimentally demonstrate an efficient coupling of light to surface-plasmon polaritons assisted by engineered dipolar scatterers placed at an optimum distance from the surface. Specifically, we fabricate gold disks separated by a silica spacer from a planar gold surface and seek to achieve perfect coupling conditions by tuning the spacer thickness for a given scatterer geometry that resonates at a designated optical frequency. We measure a maximum light-to-plasmon coupling cross section of the order of the square of the light wavelength at an optimum distance that results from the interplay between a large particle-surface interaction and a small degree of surface-driven particle-dipole quenching, both of which are favored at small separations. Our experiments, in agreement with both analytical theory and electromagnetic simulations, support the use of optimally placed engineered scatterers as a disruptive approach to solving the long-standing problem of in/out-coupling in nanophotonics.

Triple Micro Optical Multiple Asymmetric Ring Resonator Performance Analysis as Optical Filter

The present article implements the modelling of triple asymmetrical optical micro ring resonator (TAOMRR) in z-domain and the waveguide spacing is designed using Finite Difference Time Domain (FDTD) methodology. The formulation of the transmission function is prepared in z-domain utilizing the delay line signal processing technique. The developed configuration is scrutinized by evaluating the frequency response analysis as well as various important characteristics that are dispersion, group delay, and finesse using MATLAB software. The efficacy of the proposed methodology is tested using OptiFDTD software and it is found that the results of both MATLAB and OptiFDTD are in very nearby settlement. The field analysis of propagated wave inside the microring resonator is shown. The specific gap for various optical directional couplers is also determined using OptiFDTD in this article. The proposed technique is very useful for modeling and analyzing of different configurations of micro optical asymmetric ring resonator which found great application in the field of optical filters as well as optical sensors.

Wavelength continuously tunable and wavelength spacing variable comb filter based on a High-Birefringence fiber loop mirror

We demonstrated an all-fiber comb filter that can continuously tune the wavelength for four switchable wavelength spacings. The comb filter consists of a fiber loop mirror (FLM) that is inserted with three polarization maintaining fibers (PMFs) and three polarization controllers (PCs). The transmission characteristics of the comb filter was theoretically analyzed. The output characteristics of the comb filters shows a dependence of the orientation angles of the PCs. With the angles being optimally set, the comb filter with four different wavelength spacings can be achieved. The wavelength continuously tunable and wavelength spacing variable mechanisms are elaborated. The three PCs are employed to enable the tunable and switchable functions. The two PCs between the PMFs serve as a polarization rotator to align the birefringent axes of the PMFs for varying the wavelength spacing while the PC adjacent to the optical coupler can be used for wavelength tuning function. When the two quarter-waveplates (QWPs) of the PC are optimally set at π/4 and - π/4, the half-waveplate (HWP) acts as a wavelength tuning element, and when the QWPs are set at zero degrees, the HWP serves as an amplitude tuning element. The center wavelength of the comb filter could be linearly tuned with the rotation angle of the HWP, and a π/2 change in the rotation angle leads to a wavelength change in one free-spectral-range (FSR). A π/4 change in the rotation angle could bring the normalized peak transmission from its maximum at 1 to minimum at 0. The proposed tunable technique has been verified by our experimental results. We experimentally demonstrated the comb filters can switch among FSRs of 0.75 nm, 1.01 nm, 1.51 nm, and 3.1 nm with tunable functions.