Spatial Division Multiplexed Microwave Signal processing by selective grating inscription in homogeneous multicore fibers

Multi-Core Fiber Bragg Grating and Its Sensing Application

With the increase in the demand for large-capacity optical communication capacity, multi-core optical fiber (MCF) communication technology has developed, and both the types of MCFs and related devices have become increasingly mature. The application of MCFs in the field of sensing has also received more and more attention, among which MCF fiber Bragg grating (FBG) devices have received more and more attention and have been widely used in various fields. In this paper, the main writing methods of MCF FBGs and their sensing applications are reviewed. The future development of the MCF FBG is also prospected.

Portable optical fiber biosensors integrated with smartphone: technologies, applications, and challenges [Invited]

The increasing demand for individualized health monitoring and diagnostics has prompted considerable research into the integration of portable optical fiber biosensors integrated with smartphones. By capitalizing on the benefits offered by optical fibers, these biosensors enable qualitative and quantitative biosensing across a wide range of applications. The integration of these sensors with smartphones, which possess advanced computational power and versatile sensing capabilities, addresses the increasing need for portable and rapid sensing solutions. This extensive evaluation thoroughly examines the domain of optical fiber biosensors in conjunction with smartphones, including hardware complexities, sensing approaches, and integration methods. Additionally, it explores a wide range of applications, including physiological and chemical biosensing. Furthermore, the review provides an analysis of the challenges that have been identified in this rapidly evolving area of research and concludes with relevant suggestions for the progression of the field.

Spatial-Division Multiplexing Approach for Simultaneous Detection of Fiber-Optic Ball Resonator Sensors: Applications for Refractometers and Biosensors

Fiber-optic ball resonators are an attractive technology for refractive index (RI) sensing and optical biosensing, as they have good sensitivity and allow for a rapid and repeatable manufacturing process. An important feature for modern biosensing devices is the multiplexing capacity, which allows for interrogating multiple sensors (potentially, with different functionalization methods) simultaneously, by a single analyzer. In this work, we report a multiplexing method for ball resonators, which is based on a spatial-division multiplexing approach. The method is validated on four ball resonator devices, experimentally evaluating both the cross-talk and the spectral shape influence of one sensor on another. We show that the multiplexing approach is highly efficient and that a sensing network with an arbitrary number of ball resonators can be designed with reasonable penalties for the sensing capabilities. Furthermore, we validate this concept in a four-sensor multiplexing configuration, for the simultaneous detection of two different cancer biomarkers across a widespread range of concentrations.

Optical fiber point sensors based on forward Brillouin scattering

Forward Brillouin scattering interactions support the sensing and analysis of media outside the cladding boundaries of standard fibers, where light cannot reach. Quantitative point-sensing based on this principle has yet to be reported. In this work, we report a forward Brillouin scattering point-sensor in a commercially available, off-the-shelf multi-core fiber. Pump light at the inner, on-axis core of the fiber is used to stimulate a guided acoustic mode of the entire fiber cross-section. The acoustic wave, in turn, induces photoelastic perturbations to the reflectivity of a Bragg grating inscribed in an outer, off-axis core of the same fiber. The measurements successfully analyze refractive index perturbations on the tenth decimal point and distinguish between ethanol and water outside the centimeter-long grating. The measured forward Brillouin scattering linewidths agree with predictions. The acquired spectra are unaffected by forward Brillouin scattering outside the grating region. The results add point-analysis to the portfolio of forward Brillouin scattering optical fiber sensors.

Dispersion-Diversity Multicore Fiber Signal Processing

Beyond playing a primary role in high-capacity communication networks, multicore optical fibers can bring many advantages to optical and microwave signal processing, as not only space but also chromatic dispersion are introduced as new degrees of freedom. The key lies in developing radically new multicore fibers where the refractive index profile of each individual core is tailored properly to provide parallel dispersion-diversity signal processing with application in a variety of scenarios such as parallel channel equalization, analogue-to-digital conversion, optical computing, pulse generation and shaping, multiparameter fiber sensing, medical imaging, optical coherence tomography, broadband measurement instrumentation, and next-generation fiber-wireless communications. Here, we experimentally prove, for the first time to our knowledge, reconfigurable two-dimensional dispersion-managed signal processing performed by a novel dispersion-diversity heterogeneous multicore fiber. The fiber comprises seven different trench-assisted cores featuring a different refractive index profile in terms of both radial geometry and core dopant concentration. As a representative application case, we demonstrate reconfigurable microwave signal filtering with increased compactness as well as performance flexibility and versatility as compared to previous technologies.

Composed Multicore Fiber Structure for Extended Sensor Multiplexing with Fiber Bragg Gratings

A novel multicore optical waveguide component based on a fiber design optimized towards selective grating inscription for multiplexed sensing applications is presented. Such a fiber design enables the increase in the optical sensor capacity as well as extending the sensing length with a single optical fiber while preserving the spatial sensing resolution. The method uses a multicore fiber with differently doped fiber cores and, therefore, enables a selective grating inscription. The concept can be applied in a draw tower inscription process for an efficient production of sensing networks. Along with the general concept, the paper discusses the specific preparation of the fiber-based sensing component and provides experimental results showing the feasibility of such a sensing system.

Femtosecond laser auto-positioning direct writing of a multicore fiber Bragg grating array for shape sensing

A multicore fiber Bragg grating (MC-FBG) array shape sensor is a powerful tool for a variety of applications. However, the efficient fabrication of high-quality MC-FBG arrays remains a problem. Here, we report for the first time, to the best of our knowledge, a new method of directly writing FBG arrays in a seven-core fiber (SCF) through the protective coating using femtosecond laser auto-positioning point-by-point technology, which is accomplished by image recognition and micro-displacement compensation. An MC-FBG array consisting of 140 individual FBGs with a grating length of 2 mm was successfully inscribed into seven cores of a 440 mm-long SCF. Each core contained 20 wavelength-division-multiplexed (WDM) FBGs with wavelengths ranging from 1522.11 nm to 1579.28 nm. In other words, the MC-FBG array consisted of 20 WDM nodes with an interval of 2 cm along the fiber, and each node contained seven identical FBGs integrated in parallel into the fiber cross-section. Moreover, the fabricated MC-FBG array exhibited a strong orientation dependence in bend sensing, with a maximum sensitivity of 55.49 pm/m^-1. Subsequently, 2D and 3D shape sensing were demonstrated using the fabricated MC-FBG array, with maximum reconstruction errors per unit length of 4.51% and 10.81%, respectively. Hence, the MC-FBG arrays fabricated using the proposed method are useful in many applications, such as posture monitoring, smart robotics, and minimally invasive surgery.

Temperature-insensitive curvature sensor based on Bragg gratings written in strongly coupled multicore fiber

A novel temperature-insensitive optical curvature sensor has been proposed and demonstrated. The sensor is fabricated by inscribing fiber Bragg gratings with short lengths into a piece of strongly coupled multicore fiber (SCMCF) and spliced to the conventional single-mode fiber. Due to the two supermodes being supported by the SCMCF, two resonance peaks, along with a deep notch between them, were observed in the reflection spectrum. The experimental results show that the depth of the notch changes with the curvature with a sensitivity up to 15.9dB/m^-1 in a lower curvature range. Besides, thanks to the unique property of the proposed sensor, the notch depth barely changes with temperature. Based on the intensity demodulation of the notch depth, the temperature-insensitive curvature sensor is achieved with the cross sensitivity between the temperature, and the curvature is as low as 0.001m^-1/^∘C.

Dispersion-tailored few-mode fiber design for tunable microwave photonic signal processing

We present a novel double-clad step-index few-mode fiber that operates as a five-sampled tunable true-time delay line. The unique feature of this design lies in its particular modal chromatic dispersion behavior, which varies in constant incremental steps among adjacent groups of modes. This property, which to the best of our knowledge has not been reported in any other few-mode fiber to date, is the key to tunable operation of radiofrequency signal processing functionalities implemented in few-mode fibers. The performance of the designed true-time delay line is theoretically evaluated for two different microwave photonics applications, namely tunable signal filtering and optical beamforming networks for phased array antennas. In the 35-nm optical wavelength tuning range of the C-band, the free spectral range of the microwave filter and the beam-pointing angle in the phased array antenna can be continuously tuned from 12.4 up to 57 GHz and 12.6^° up to 90^°, respectively.

Selective fiber Bragg grating inscription in four-core fiber for two-dimension vector bending sensing

The paper presents selective fiber Bragg grating (FBG) inscription in four-core fiber based on a phase mask scanning method. The inscription factors are systematically investigated, which involves fiber core position and focused laser beam size in fiber, etc. Several specific inscriptions (including individual, dual and all inscriptions) are demonstrated. Two orthogonally positioned cores are selectively inscribed and applied to two-dimension vector bending measurement. The measured bending sensitivities of two FBGs range from -54.3 pm/m^-1 to 52.2 pm/m^-1 and -53.7 pm/m^-1 to 52.8 pm/m^-1, respectively. More importantly, it has been revealed that their sensitivities versus bending direction follow regular cosinoidal and sinusoidal distribution. The direction and amplitude of the vector bending can be recovered using measured central wavelength shifts of those two FBGs.

Performance Analysis of Scattering-Level Multiplexing (SLMux) in Distributed Fiber-Optic Backscatter Reflectometry Physical Sensors

Optical backscatter reflectometry (OBR) is a method for the interrogation of Rayleigh scattering occurring in each section of an optical fiber, resulting in a single-fiber-distributed sensor with sub-millimeter spatial resolution. The use of high-scattering fibers, doped with MgO-based nanoparticles in the core section, provides a scattering increase which can overcome 40 dB. Using a configuration-labeled Scattering-Level Multiplexing (SLMux), we can arrange a network of high-scattering fibers to perform a simultaneous scan of multiple fiber sections, therefore extending the OBR method from a single fiber to multiple fibers. In this work, we analyze the performance and boundary limits of SLMux, drawing the limits of detection of N-channel SLMux, and evaluating the performance of scattering-enhancement methods in optical fibers.

Numerical and Experimental Performance Analysis of the Chirped Fiber Bragg Grating Based Abrasion Sensor for the Maintenance Applications in the Industry 4.0

In this article, an extensive analysis of the performance of the fiber optics-based abrasion sensor that utilizes chirped fiber Bragg grating, is presented. For the response investigation during abrasion, a numerical analysis, based on the transfer matrix method and coupled mode theory, is provided. The influence of the SLED source spectral position in respect to the spectral position of the chirped fiber Bragg grating is evaluated together with the influence of the changes of the ambient temperature of the sensor. Experimental verification of the sensor's performance is provided, together with the proposition of the packaging of the sensor. In the article, a simple, cost-effective and multiplexation-ready concept of the wear or abrasion sensor system is presented and discussed.

Durable shape sensor based on FBG array inscribed in polyimide-coated multicore optical fiber

The paper presents a novel three-dimensional quasi-continuous shape sensor based on an FBG array inscribed by femtosecond laser pulses into a 7-core optical fiber with a polyimide protective coating. The measured bending sensitivity of individual FBGs ranges from 0.046 nm/m^-1 to 0.049 nm/m^-1. It is shown that the sensor allows for reconstructing 2- and 3-dimensional shapes with high accuracy. Due to the high value of the core aperture and individual calibration of each FBG we were able to measure the smallest reported bending radii down to 2.6 mm with a record accuracy of ∼1%. Moreover, we investigate the magnitude of the errors of curves reconstruction and errors associated with measurement of curvature radii in the range from 2.6 to 500 mm. The main factors affecting the accuracy of measurements are also discussed. The temperature resistance of both the inscribed FBG structures and of the protective coating, along with the high mechanical strength of the polyimide, makes it possible to use the sensor in harsh environments or in medical and composite material applications.

Ring-core few-mode fiber for tunable true time delay line operation

We propose, for the first time to our knowledge, tunable true time delay line operation for radiofrequency signals on a few-mode fiber link. In particular, the custom design of a 7-LP-mode ring-core few-mode fiber together with a set of 5 broadband long period gratings inscribed at the proper positions along the fiber allows 4-sample true time delay line tunability over a 20-nm optical wavelength range. We study the performance of the designed true time delay line in the context of reconfigurable microwave photonics signal processing by theoretically evaluating microwave signal filtering and optical beamforming networks for phased array antennas.

Sampled true time delay line operation by inscription of long period gratings in few-mode fibers

We propose and experimentally demonstrate distributed microwave photonics signal processing over a few-mode fiber link by implementing 4-sample true time delay line operation. The inscription of a set of long period gratings at specific locations along the few-mode fiber allows the excitation of the higher-order modes while adjusting the individual sample group delays and amplitudes that are required for sampled true time delay line behavior. Since solely the injection of the fundamental mode at the few-mode fiber input is required, we render this signal processing system independent of any preceding fiber link that may be required in addition to distribute the signal. We experimentally validate the performance of the implemented true time delay line when applied to radiofrequency signal filtering.

Arrays of fiber Bragg gratings selectively inscribed in different cores of 7-core spun optical fiber by IR femtosecond laser pulses

In this paper, we present a new method of point-by-point femtosecond inscription of fiber Bragg gratings (FBG) arrays of different configurations in a 7-core spun optical fiber. The possibility of FBGs inscription with predefined periods in individual fiber cores allowed us to realize: 1) longitudinal FBG arrays with identical or variable resonant wavelengths in all side cores, 2) longitudinal FBG arrays inscribed only in the central or in the selected side core, and 3) an FBG array in a transverse cross section of a fiber consisting of an FBG inscribed in the central and three side cores. Based on the proposed method, by enabling the inscription through the acrylate protective coating of the fiber, a vector bend sensor has been created. Implementation of this sensor has shown that bending radii less than 4 mm can be measured with a high precision using a single-channel interrogation scheme.

Multi-fiber distributed thermal profiling of minimally invasive thermal ablation with scattering-level multiplexing in MgO-doped fibers

We propose a setup for multiplexed distributed optical fiber sensors capable of resolving temperature distribution in thermo-therapies, with a spatial resolution of 2.5 mm over multiple fibers interrogated simultaneously. The setup is based on optical backscatter reflectometry (OBR) applied to optical fibers having backscattered power significantly larger than standard fibers (36.5 dB), obtained through MgO doping. The setup is based on a scattering-level multiplexing, which allows interrogating all the sensing fibers simultaneously, thanks to the fact that the backscattered power can be unambiguously associated to each fiber. The setup has been validated for the planar measurement of temperature profiles in ex vivo radiofrequency ablation, obtaining the measurement of temperature over a surface of 96 total points (4 fibers, 8 sensing points per cm2). The spatial resolution obtained for the planar measurement allows extending distributed sensing to surface, or even three-dimensional, geometries performing temperature sensing in the tissue with millimeter resolution in multiple dimensions.

Few-mode fiber true time delay lines for distributed radiofrequency signal processing

We report, for the first time to our knowledge, distributed radiofrequency signal processing built upon true time delay operation on a step-index few-mode fiber. Two 3-sample configurations with different time delay properties are implemented over the same 60-meter 4-LP-mode fiber link. The inscription of a long period grating at a specific fiber position converts part of the LP₀₁ mode into the LP₀₂, permitting sample time delay engineering. Delay line performance is experimentally demonstrated when applied to radiofrequency signal filtering, example of fiber-distributed processing functionality exhibiting one order or magnitude gain in terms of compactness.

Highly-coherent stimulated phonon oscillations in a multi-core optical fiber

Opto-mechanical oscillators that generate coherent acoustic waves are drawing much interest, in both fundamental research and applications. Narrowband oscillations can be obtained through the introduction of feedback to the acoustic wave. Most previous realizations of this concept, sometimes referred to as "phonon lasers", relied on radiation pressure and moving boundary effects in micro- or nano-structured media. Demonstrations in bulk crystals required cryogenic temperatures. In this work, stimulated emission of highly-coherent acoustic waves is achieved in a commercially-available multi-core fiber, at room temperature. The fiber is connected within an opto-electronic cavity loop. Pump light in one core is driving acoustic waves via electrostriction, whereas an optical probe wave at a different physical core undergoes photo-elastic modulation by the stimulated acoustic waves. Coupling between pump and probe is based entirely on inter-core, opto-mechanical cross-phase modulation: no direct optical feedback is provided. Single-frequency mechanical oscillations at hundreds of MHz frequencies are obtained, with side-mode suppression that is better than 55 dB. A sharp threshold and rapid collapse of the linewidth above threshold are observed. The linewidths of the acoustic oscillations are on the order of 100 Hz, orders of magnitude narrower than those of the pump and probe light sources. The relative Allan's deviation of the frequency is between 0.1-1 ppm. The frequency may be switched among several values by propagating the pump or probe waves in different cores. The results may be used in sensing, metrology and microwave-photonic information processing applications.

Microwave Signal Processing over Multicore Fiber

We review the introduction of the space dimension into fiber-based technologies to implement compact and versatile signal processing solutions for microwave and millimeter wave signals. Built upon multicore fiber links and devices, this approach allows the realization of fiber-distributed signal processing in the context of fiber-wireless communications, providing both radiofrequency access distribution and signal processing in the same fiber medium. We present different space-division multiplexing architectures to implement tunable true time delay lines that can be applied to a variety of microwave photonics functionalities, such as signal filtering, radio beamsteering in phased array antennas or optoelectronic oscillation. In particular, this paper gathers our latest work on the following multicore fiber technologies: dispersion-engineered heterogeneous multicore fiber links for distributed tunable true time delay line operation; multicavity devices built upon the selective inscription of gratings in homogenous multicore fibers for compact true time delay line operation; and multicavity optoelectronic oscillation over both homogeneous and heterogeneous multicore fibers.

Ultra-short pulse propagation model for multi-core fibers based on local modes

Multi-core fibers (MCFs) have sparked a new paradigm in optical communications and open new possibilities and applications in experimental physics and other fields of science, such as biological and medical imaging. In many of these cases, ultra-short pulse propagation is revealed as a key factor that enables us to exploit the full potential of this technology. Unfortunately, the propagation of such pulses in real MCFs has not yet been modelled considering polarization effects or typical random medium perturbations, which usually give rise to both longitudinal and temporal birefringent effects. Using the concept of local modes, we develop here an accurate ultra-short pulse propagation model that rigorously accounts for these phenomena in single-mode MCFs. Based on this theory, we demonstrate analytically and numerically the intermodal dispersion between different LP₀₁ polarized core modes induced by these random perturbations when propagating femtosecond pulses in the linear and nonlinear fiber regimes. The ever-decreasing core-to-core distance significantly enhances the intermodal dispersion induced by these birefringent effects, which can become the major physical impairment in the single-mode regime. To demonstrate the power of our model, we give explicit strategies to reduce the impact of this optical impairment by increasing the MCF perturbations.

Multicore fiber-Bragg-grating-based directional curvature sensor interrogated by a broadband source with a sinusoidal spectrum

A simple, spectral-drift-insensitive interrogation scheme for a multicore fiber Bragg grating (FBG)-based directional curvature sensor is proposed. The basic principle is to transform the wavelength shift of FBGs into the reflected power variation, which is accomplished by utilizing a broadband source with a sinusoidal spectrum. The closed-form expression of the relationship between the reflected power of the FBG and the corresponding peak wavelength is derived for the first time, to the best of our knowledge; therefore, the peak wavelength of the FBG can be precisely interrogated by using a single photodiode. The experimental results show that, with respect to conventional wavelength measurement by an optical spectrum analyzer, the demodulated wavelength error by our proposed interrogation scheme is within ±20  pm. The proposed scheme is further extended to interrogate the direction and curvature using a multicore FBG-based curvature sensor; the interrogated curvature with an error less than 8% is achieved.

Divide and conquer: an efficient solution to highly multimoded photonic lanterns from multicore fibres

Photonic lanterns typically allow for single-mode action in a multimode fibre. Since their invention over a decade ago for applications in astrophotonics, they have found important uses in diverse fields of applied science. To date, large aperture highly-mulitmoded to single-mode lanterns have been difficult as fabrication techniques are not practical for mass replication. Here as a proof of concept, we demonstrate three different devices based on multicore fibre photonic lanterns with: 100µm core diameters; NAs = 0.16 and 0.15; and requiring 259 single-mode core system, specifically 7 multicore fibres each with 37 cores, instead of 259 individual single-mode fibres. The average insertion loss excluding coupling efficiencies is only 0.4dB (>91% transmission). This concept has numerous advantages, in particular, (i) it is a direct scaleable solution, (ii) eases imprinting of photonic functions, e.g. fibre Bragg gratings; and (iii) new approach for large-area optical fibre slicers for future large-aperture telescopes.

Tilted fiber Bragg gratings in multicore optical fibers for optical sensing

We have inscribed a tilted fiber Bragg grating (TFBG) in selected cores of a multicore optical fiber. The presence of the TFBG permits to couple light from the incident-guided mode to the cladding modes and to the neighbor cores, and this interaction can be used for optical sensing. We measured different magnitudes: strain, curvature magnitude and direction, and external refractive index. The curvature results show a linear dependence of the maximum crosstalk with the curvature magnitude with a sensitivity of 2.5  dB/m^-1 as the curvature magnitude increases and at the same time a wavelength shift of 70  pm/m^-1. Changes in the external refractive index gradually vanish the cladding modes resonances and the crosstalk between the different cores, obtaining a reduction of the 90% of the optical spectra integral area for refractive indexes between 1.398 and 1.474.