Optical Fiber Alignment Research Articles

Non-coincidence detection of fiber optic circulators based on Hertz-level frequency-shifting heterodyne interferometry

The fiber optic circulator in the form of point diffraction is a key component for coupling fiber optical path and spatial optical path in a laser Doppler vibration measurement system. The coupling efficiency and other performance parameters of fiber optic circulator are great significant for improving measurement accuracy and working distance of vibration measurement system. The conventional circulator coincidence detection methods include energy monitoring method and far-field coincidence monitoring method, which cannot be used to quantitatively analyze the fiber mismatch factors. Therefore, the consistency of the circulator coupling efficiency cannot be guaranteed. To solve these problems, a phase detection technology based on Hertz-level frequency-shifting heterodyne interferometry is proposed. The interferometry phase information is used to calculate the relative spatial positions of optical fibers, and this technology performs quantitative detection in the fiber alignment process. The interference wavefront formed by relative spatial positions of optical fibers is simulated and validated experimentally. The curves of coupling efficiency and wavefront PV value versus different kinds of alignment errors are simulated and analyzed. By fitting the interference wavefront with the Zernike polynomials, the correspondence between different kinds of alignment errors and Zernike coefficients is obtained. The value of Z2 (Zernike coefficient) can be used as the basis for judging whether there is transverse displacement in the Y direction. Similarly, Z3 corresponds to the transverse displacement in the X direction, Z4 corresponds to the longitudinal displacement in the Z direction and Z5 corresponds to the optical axis angle. Through this correspondence relationship, the quantitative separation and analysis of fiber mismatch factors are realized. The experimental results show that the accuracy of this method for measuring lateral displacement is better than 1μm. According to the phase diagram obtained from the experiment, Zernike coefficient fitting is performed. The lateral displacement deviation, longitudinal displacement deviation, and angular deviation are calculated by the coefficients of Z2 to Z5. The fiber adjustment mechanism corrects the transverse displacement deviation. It provides a new detection method for realizing fiber alignment and mismatch correction. Compared with the existing detection methods, the phase detection method based on Hertz-level frequency-shifting heterodyne interferometry solves the quantification problem of fiber coincidence adjustment. This method has the advantages of high measurement accuracy, compact detection structure and composition, and low detection cost. This method has great potential applications in the fields of optical fiber and spatial optical device alignment, optical system aberration detection, and planar wavefront detection.

Optical Fiber Alignment Aid with Image Processing on FPGA-based System

This paper introduces an innovative and efficient solution for achieving precise optical fiber alignment in fiber optic splicing applications. By harnessing the capabilities of FPGA-based real-time image processing, the system surpasses the required frame rate of 20 frames per second, ensuring swift and accurate alignment. With a verified resolution of approximately 1 pixel per µm, the system offers high precision suitable for various applications. Through advanced image processing techniques, the system calculates precise correction values for each frame, resulting in fast and accurate alignment of fiber cores and cladding. This unique feature enhances performance and adaptability, leading to substantial reduction in splice wastage. By integrating motorized alignment stages and guided machine vision, the system provides a robust and automated approach to fiber optic alignment. The FPGA-driven stepper motors enable precise and controlled fiber movements with micrometer-level step resolutions. The calculated correction factors effectively guide the alignment process, with deviations from actual results impressively low – a maximum of 12 µm in the Y correction factor and 7 µm in the X correction factor. This reliability makes the system a crucial component in any optical fiber splicing machine. Furthermore, the paper emphasizes the ability to handle specialty fiber alignment contributes significantly to cost-effective manufacturing of fiber optic components and modules. By ensuring precise and optimal fiber alignment, the system mitigates signal loss, leading to improved performance and reduced production costs.

A Self-Latching MEMS Optical Interrupter With Status Monitoring for Laser Initiation System

A novel self-latching MEMS optical interrupter with status monitoring is proposed to enhance the safety of laser initiation system (LIS) by preventing long-term heat accumulation and reducing response time. The electrothermal-actuated interrupter consists of an optical fiber alignment actuator, a positive latching actuator (PLA), and a reverse blocking actuator, each coupled with a status monitoring unit. The PLA is equipped with an interlock mechanism for self-latching capability. The interrupter’s operating state is monitored using cantilever beams that function as travel switches. The interrupters are fabricated using deep reactive ion etching (DRIE) on silicon-on-insulator (SOI) wafers. Experiments are conducted to study the behavior of the proposed interrupter, which demonstrates the successful implementation of its self-latching and status-monitoring features. The interrupter has a switching time of less than 64 ms and exhibits an optical efficiency ranging from 63% to 68% (1.67–1.97-dB insertion loss) with a maximum power transfer of 865 mW from a 1272-mW input.

Design and Development of a New Piezoelectric-Actuated Biaxial Compliant Microgripper With Long Strokes

In this paper, a new piezoelectric-actuated biaxial compliant microgripper with long strokes is proposed for automatically gripping and rolling tiny rigid objects. In order to improve the working stroke and maintain a compact footprint, a counter-side distributed two-stage lever amplifier with parallelogram mechanism is introduced. Based on the pseudo-rigid body model, analytical models of the displacement amplification ratio, input stiffness, and natural frequency of the left- and right-sided gripper mechanism are established. Structural optimization and performance simulation of the proposed microgripper mechanism are carried out with finite element analysis simulation. A prototype microgripper has been fabricated for open-loop and closed-loop tests to verify its working capabilities. The clamping and rubbing experiments show that the designed microgripper can grasp and rub an optical fiber with the diameter of 200 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> for rolling over 45°. The developed microgripper has a promising application in precision micromanipulation fields such as optical fiber alignment. Note to Practitioners—This work is motivated by the requirement of designing a biaxial microgripper with both a large working stroke and compact structure for complex operation in optical fiber alignment. Through a series of open-loop and closed-loop experiments on the developed prototype, it is demonstrated that the proposed dual-axis microgripper has superior working performance. The clamping stroke and rubbing stroke of the gripper are <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$251.2~\mu \text{m}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$225.0~\mu \text{m}$ </tex-math></inline-formula> , respectively. The first two natural frequencies of the gripper are 350.63 Hz and 603.37 Hz, which correspond to the working modes of the right-side and left-side gripper mechanisms, respectively. The closed-loop experimental results show that the resolution of output displacement of the gripper is close to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.2~\mu \text{m}$ </tex-math></inline-formula> and the resolution of the clamping force is 3 mN. As compared with the reported microgrippers in previous work, the designed mechanism exhibits both a large working stroke and high resonant frequency for ensuring the reliability and rapidity of micromanipulation task.

Construction of a vector-field cryogenic magnetic force microscope.

Owing to the high resolution of magnetic force microscopes (MFMs) operating at low temperatures and high-applied magnetic fields, they can be employed to study various phenomena observed in topological magnetic materials and superconductors. In this study, we constructed a low-temperature MFM equipped with a 2-2-9-T vector magnet and a three-axis fiber-optic alignment system. The three-axis alignment device enables in situ calibration of the scanner at low temperatures as well as optimizes the intensity and sensitivity of the interferometer signal. A massive homebuilt vibration isolation table lowers the resonance frequency of the system and minimizes mechanical noise. Consequently, the minimum detectable force gradient of our proposed model is close to the thermodynamic limit of the cantilever. To demonstrate the low-temperature capability of the MFM, we obtained magnetic domain images of the van der Waals ferromagnet Fe4GeTe2 and the Abrikosov superconducting vortices of an Nb film. Furthermore, we performed field angle-dependent MFM experiments in a van der Waals magnetic insulator Cr2Ge2Te6 to verify its vector-field functionality and observed a transition in the domains from the stripe to the bubble phase with respect to the magnetic field angle. The vector-field capability of our MFM can be useful for investigating various anisotropic magnetic phenomena in topological magnetic and superconducting materials.

3D printed microfluidic lab-on-a-chip device for fiber-based dual beam optical manipulation

3D printing of microfluidic lab-on-a-chip devices enables rapid prototyping of robust and complex structures. In this work, we designed and fabricated a 3D printed lab-on-a-chip device for fiber-based dual beam optical manipulation. The final 3D printed chip offers three key features, such as (1) an optimized fiber channel design for precise alignment of optical fibers, (2) an optically clear window to visualize the trapping region, and (3) a sample channel which facilitates hydrodynamic focusing of samples. A square zig–zag structure incorporated in the sample channel increases the number of particles at the trapping site and focuses the cells and particles during experiments when operating the chip at low Reynolds number. To evaluate the performance of the device for optical manipulation, we implemented on-chip, fiber-based optical trapping of different-sized microscopic particles and performed trap stiffness measurements. In addition, optical stretching of MCF-7 cells was successfully accomplished for the purpose of studying the effects of a cytochalasin metabolite, pyrichalasin H, on cell elasticity. We observed distinct changes in the deformability of single cells treated with pyrichalasin H compared to untreated cells. These results demonstrate that 3D printed microfluidic lab-on-a-chip devices offer a cost-effective and customizable platform for applications in optical manipulation.

The OpenFlexure Block Stage: sub-100 nm fibre alignment with a monolithic plastic flexure stage.

As 3D printers become more widely available, researchers are able to rapidly produce components that may have previously taken weeks to have machined. The resulting plastic components, having high surface roughness, are often not suitable for high-precision optomechanics. However, by playing to the strengths of 3D printing-namely the ability to print complex internal geometries-it is possible to design monolithic mechanisms that do not rely on tight integration of high-precision parts. Here we present a motorised monolithic 3D-printed plastic flexure stage with sub-100 nm resolution that can perform automated optical fibre alignment.

Azimuthal Alignment Method for Optimizing Bragg Grating Inscription in Photonic Crystal Fibers

An accurate (≤5°) and straightforward implementation optical method for determining the azimuthal lattice orientation of a microstructured optical fiber with respect to the side illumination beam used for Bragg grating recording is presented. This method is based on the diffraction pattern generated backward of the photonic crystal fiber cladding upon a continuous-wave visible laser light illumination. This optical fiber alignment and coordination method is experimentally demonstrated, describing in detail characteristic light scattering patterns used for the azimuthal orientation of the fiber with respect to the side-coming laser beam. Moreover, this alignment method is theoretically validated using standard ray tracing beam scattering software. Bragg grating recordings performed in this photonic crystal fiber (PCF) using 193-nm excimer laser radiation are also presented to depict the impact of the azimuthal alignment of the fiber with the inscription beam on the photosensitivity refractive index changes induced in the optical fiber.

A Concept for Accurate Edge-Coupled Multi-Fiber Photonic Interconnects

The alignment and fixation of multiple single-mode optical fibers to photonic integrated circuits is currently a challenging, expensive, and time-consuming task. In this paper, we present a concept for a sub-micrometer accurate multi-fiber array, where fibers are actively aligned with respect to each other and fixated to a flat carrier using ultraviolet-curable adhesive. Adhesives are prone to shrinkage, which can disturb the fiber alignment. As a result, especially, the fixation process forms the bottleneck in reaching the required alignment and not the alignment process itself. Simulations are performed to investigate the sensitivity of process variables on the adhesive bond geometry, which is important for the shrinkage amplitude. Furthermore, an experimental setup has been designed and fabricated to measure the shrinkage-induced fiber displacement for three selected types of adhesives. The results show a controllable adhesive shrinkage, where fibers can be aligned with a position reproducibility of $\pm {\text{40}}$ nm, which is more than sufficient for the most critical fiber alignment applications. With this concept, an important step can be made in enabling sub-micrometer accurate photonic interconnects in a cost-effective way, which is suitable for automated production.

Realization of fiber optic displacement sensors

Abstract Fiber optic sensors are very promising because of their inherent advantages such as very small size, hard environment tolerance and impact of electromagnetic fields. In this paper three different types of Intensity Fiber Optic Displacement Sensors (I-FODS) are presented. Three configurations of I-FODS were realized in two varieties. In the first one, the cleaved multimode optical fibers (MMF) were used to collect reflected light, while in the second variety the MMF ended with ball lenses were chosen. To ensure an accurate alignment of optical fibers in the sensor head the MTP C9730 optical fiber ferrules were used. In this paper the influence of distribution of transmitting and detecting optical fibers on sensitivity and linear range of operation of developed I-FODS were investigated. We have shown, that I-FODS with ball lenses receive average 10.5% more reflected power in comparison to the cleaved optical fibers and they increase linearity range of I-FODS by 33%. In this paper, an analysis of each type of the realized sensor and detailed discussion are given.

Tolerancing the alignment of large-core optical fibers, fiber bundles and light guides using a Fourier approach.

Optical fiber technology is found in a wide variety of applications to flexibly relay light between two points, enabling information transfer across long distances and allowing access to hard-to-reach areas. Large-core optical fibers and light guides find frequent use in illumination and spectroscopic applications, for example, endoscopy and high-resolution astronomical spectroscopy. Proper alignment is critical for maximizing throughput in optical fiber coupling systems; however, there currently are no formal approaches to tolerancing the alignment of a light-guide coupling system. Here, we propose a Fourier alignment sensitivity (FAS) algorithm to determine the optimal tolerances on the alignment of a light guide by computing the alignment sensitivity. The algorithm shows excellent agreement with both simulated and experimentally measured values and improves on the computation time of equivalent ray-tracing simulations by two orders of magnitude. We then apply FAS to tolerance and fabricate a coupling system, which is shown to meet specifications, thus validating FAS as a tolerancing technique. These results indicate that FAS is a flexible and rapid means to quantify the alignment sensitivity of a light guide, widely informing the design and tolerancing of coupling systems.

End-View Image Processing Based Angle Alignment Techniques for Specialty Optical Fibers

We propose two end-view-based digital image processing algorithms for high-precision rotational angle alignment of specialty optical fibers. The first technique is based on the cross-correlation calculation between images of two fiber ends. Accurate alignment can be achieved between identical specialty optical fibers with arbitrary end face structures. The other one is based on the self-correlation method. Two fibers with different refractive index profiles in the fusion splicer can be rotated to preset angles independently with high precision. Accurate angle alignment of less than 0.5° uncertainty and low average splicing loss for seven-core multicore fibers have been achieved in experiment. To speed up the alignment process, we further proposed two simplified algorithms for the above-mentioned techniques. Time consumption can be reduced by about 50% with acceptable alignment accuracy. We confirmed that all the methods are suitable for other kinds of specialty optical fibers, such as polarization maintaining fibers and photonic crystal fibers.

Precision Alignment of Optical Fibers Based on Telecentric Stereo Microvision

Optical fiber alignment plays an important role in assembling an optical fiber-based system. Proper alignment can improve optical transmission efficiency of the spliced fiber. As for traditional methods, passive alignment has its drawback of less accuracy, and the active method based on optical power sensing is complex and often cannot find the best solution. In this paper, a novel precision closed-loop method for optical fiber alignment based on telecentric lens is introduced. First, to improve the measurement accuracy of the binocular vision system, a quick and high-precision image-processing algorithm for the glass core of an optical fiber is proposed. Then, a stereo matching problem is solved by combining the epipolar constraint with the geometry constraint especially for telecentric lens, and the 3-D recovery of the optical fibers is achieved. Finally, with the help of a proposed simple eye-to-hand calibration for this microalignment system, a coarse-to-fine strategy is put forward to deal with this five-degree-of-freedom alignment task. The experimental results showed that the proposed method had high performance such as prominent precision and strong robustness in 3-D optical fiber alignment.

Hot-embossing replication of self-centering optical fiber alignment structures prototyped by deep proton writing

This paper presents the hot-embossing replication of self-centering fiber alignment structures for high-precision, single-mode optical fiber connectors. To this end, a metal mold insert was fabricated by electroforming a polymer prototype patterned by means of deep proton writing (DPW). To achieve through-hole structures, we developed a postembossing process step to remove the residual layer inherently present in hot-embossed structures. The geometrical characteristics of the hot-embossed replicas are compared, before and after removal of the residual layer, with the DPW prototypes. Initial measurements on the optical performance of the replicas are performed. The successful replication of these components paves the way toward low-cost mass replication of DPW-fabricated prototypes in a variety of high-tech plastics.

Robust precision alignment algorithm for micro tube laser forming

Tube laser forming on a small diameter tube can be used as a high precision actuator to permanently align small (optical)components. Applications, such as the alignment of optical fibers to photonic integrated circuits, often require sub-micron alignment accuracy. Although the process causes significant scattering in bending angle and direction, this accuracy can be achieved by multiple bending steps of decreasing size once the target location is in proximity. In this paper an algorithm is proposed which, for each bending step, determines the best values of the main driving parameters; the axial laser spot position on the tube and the laser power. This algorithm is self-learning by using a statistical analysis of all previous bending steps. The algorithm is therefore robust for differences in for example the laser absorption between the tubes. A fully automated experimental micro tube bending setup has been developed using the proposed method in an algorithm to iteratively align an optical fiber to a virtual target with 0.1μm accuracy, using tubes with a diameter between 450μm and 700μm. For the best performing tube geometry, this required an average of 14.5 steps, where each step uses a single laser pulse with a spot position determined by the algorithm.

Design and prototyping of self-centering optical single-mode fiber alignment structures

The European Commission’s goal of providing each European household with at least a 30 Mb s−1 Internet connection by 2020 would be facilitated by a widespread deployment of fibre-to-the-home, which would in turn be sped up by the development of connector essential components, such as high-precision alignment features. Currently, the performance of state-of-the-art physical contact optical fiber connectors is limited by the tolerance on the cladding of standard telecom-grade single-mode fiber (SMF), which is typically smaller than ±1 μm. We propose to overcome this limit by developing micro-spring-based self-centering alignment structures (SCAS) for SMF-connectors. We design these alignment structures with robustness and low-cost replication in mind, allowing for large-scale deployment. Both theoretical and finite element analysis (FEA) models are used to determine the optimal dimensions of the beams of which the micro-springs of the SCAS are comprised. Two topologies of the SCAS, consisting of three and four micro-springs respectively, are investigated for two materials: polysulfone (PSU) and polyetherimide (PEI). These materials hold great potential for high-performance fiber connectors while being compatible with low-cost production and with the harsh environmental operation conditions of those connectors. The theory and FEA agree well (<3% difference) for a simple micro-spring. When including a pedestal on the micro-spring (to bring it further away from the fiber) and for shorter spring lengths the agreement worsens. This is due to spring compression effects not being taken into account in our theoretical model. Prototypes are successfully fabricated using deep proton writing and subsequently characterized. The controlled insertion of an SMF in the SCAS is investigated and we determine that a force of 0.11 N is required. The fiber insertion also causes an out-of-plane deformation of the micro-springs in the SCAS of about 7 μm, which is no problem for robustness according to the FEA model. Finally connector-assemblies are made with the alignment system and we show that an insertion loss down to 0.1 dB is achievable. The prototypes are subsequently used as a sacrificial master for mould fabrication through electroplating with the goal of low-cost replication through hot embossing.

Actuarial calculation of the squareness errors for ultra-precision micro v-groove machine tools

A v-groove structure is extensively used for planar optical-fiber alignment because of its unique features. To obtain low connection loss, the core pitch tolerance of a v-groove structure should be within submicrons. The squareness errors have great influences on the machining precision of micro v-grooves. Therefore, the actuarial calculation of squareness errors is of great significance for ultra-precision micro v-groove machine tools. In view of the disadvantages of the squareness error analysis method basing on small angle error assumption, the actuarial calculation of squareness error is implemented. A more precise geometric error model basing on the actuarial calculation of squareness error is established. The transformation between actuarial calculation of squareness error and improvement of machining precision is achieved through the geometric error modeling. The results show that the actuarial calculation of squareness error is effective and practical.

Transfer of an exfoliated monolayer graphene flake onto an optical fiber end face for erbium-doped fiber laser mode-locking

This paper presents, for the first time, the successful transfer of exfoliated monolayer graphene flake to the optical fiber end face and alignment to its core. By fabricating and optimizing a polymeric poly(methyl methacrylate) (PMMA) and polyvinyl alcohol (PVA) substrate, it is possible to obtain a contrast of up to 11% for green light illumination, allowing the identification of monolayer graphene flakes that were transferred to optical fiber samples and aligned to its core. With Raman spectroscopy, it is demonstrated that graphene flake completely covers the optical fiber core, and its quality remains unaltered after the transfer. The generation of mode-locked erbium-doped fiber laser pulses, with a duration of 672 fs, with a single-monolayer graphene flake as a saturable absorber, is demonstrated for the first time. This transfer technique is of general applicability and can be used for other two-dimensional (2D) exfoliated materials.

A geometric and kinematic error model and compensation of a 6-axis motorized optical fiber alignment stage

In order to increase the accuracy of a 6-axis motorized fiber alignment stage, a geometric and kinematic error model was established. A low-order body array was introduced to describe the general topological structure of the alignment stage; a homogeneous error transformation matrix was adopted to present the combination and kinematic errors of a typical case in a multi-body system; the structure and kinematic relationship of a 6-axis motorized fiber alignment stage as well as the corresponding error equation were built via the multi-body system error theory; the error distribution state and sensitivity characteristics of the 6-axis motorized fiber alignment stage were further analyzed; and the error compensation strategy was developed. The experimental results indicated that using inline error compensation for improving the alignment accuracy of the automatic alignment system for optical waveguide devices was very effective.

A monolithic glass chip for active single-cell sorting based on mechanical phenotyping.

The mechanical properties of biological cells have long been considered as inherent markers of biological function and disease. However, the screening and active sorting of heterogeneous populations based on serial single-cell mechanical measurements has not been demonstrated. Here we present a novel monolithic glass chip for combined fluorescence detection and mechanical phenotyping using an optical stretcher. A new design and manufacturing process, involving the bonding of two asymmetrically etched glass plates, combines exact optical fiber alignment, low laser damage threshold and high imaging quality with the possibility of several microfluidic inlet and outlet channels. We show the utility of such a custom-built optical stretcher glass chip by measuring and sorting single cells in a heterogeneous population based on their different mechanical properties and verify sorting accuracy by simultaneous fluorescence detection. This offers new possibilities of exact characterization and sorting of small populations based on rheological properties for biological and biomedical applications.