Twin Multicore Fiber Interferometers for Precision Sensing

Multicore fiber (MCF) which contains more than one core in a single fiber cladding has attracted ever increasing attention for application in optical sensing systems owing to its unique capability of independent light transmission in multiple spatial channels. Different from the situation in standard single mode fiber (SMF), the fiber bending gives rise to tangential strain in off-center cores, and this unique feature has been employed for directional bending and shape sensing, where strain measurement is achieved by using either fiber Bragg gratings (FBGs), optical frequency-domain reflectometry (OFDR) or Brillouin distributed sensing technique. On the other hand, the parallel spatial cores enable space-division multiplexed (SDM) system configuration that allows for the multiplexing of multiple distributed sensing techniques. As a result, multi-parameter sensing or performance enhanced sensing can be achieved by using MCF. In this paper, we review the research progress in MCF based distributed fiber sensors. Brief introductions of MCF and the multiplexing/de-multiplexing methods are presented. The bending sensitivity of off-center cores is analyzed. Curvature and shape sensing, as well as various SDM distributed sensing using MCF are summarized, and the working principles of diverse MCF sensors are discussed. Finally, we present the challenges and prospects of MCF for distributed sensing applications.

We experience Internet traffic growth of 100 times every 10 years. However, the capacity of existing standard single-mode fiber is approaching its fundamental limit regardless of significant realization of transmission technologies which allow for high spectral efficiencies. Space division multiplexing (SDM) based on multicore fibers (MCFs) has emerged as a solution to the problem of saturation of the capacity of optical transmission systems. This article presents the recent progress on the MCFs for future large capacity long-distance transmission systems. In MCFs, there is a tradeoff relationship between low crosstalk and high multiplicity, therefore the maximum number of cores and the core arrangement have to be carefully determined based on the required crosstalk level and core size. The state-of-the-art of fabricated MCFs and the transmission experiments using MCFs are reviewed. The current maximum capacity-distance product in MCF transmission is 368.2 (184.1+184.1) Pb/s/fiber km with the relative spatial efficiency of 4.7 compared with a standard single-mode fiber. In order to increase the spatial efficiency as well as the capacity-distance product further in MCFs, the possibility of heterogeneous MCFs and few-mode MCFs is also presented.

In this Letter, a long-range optical fiber displacement sensor based on an extrinsic Fabry-Perot interferometer (EFPI) built with a strongly coupled multicore fiber (SCMCF) is proposed and demonstrated. To fabricate the device, 9.2 mm of SCMCF was spliced to a conventional single-mode fiber (SMF). The sensor reflection spectrum is affected by super-mode interference in the SCMCF and the interference produced by the EFPI. Displacement of the SMF-SCMCF tip with respect to a reflecting surface produces quantifiable changes in the amplitude and period of the interference pattern in the reflection spectrum. Since the multicore fiber is an efficient light collecting area, sufficient signal intensity can be obtained for displacements of several centimeters. By analyzing the interference pattern in the Fourier domain, it was possible to measure displacements up to 50 mm with a resolution of approximately 500 nm. To our knowledge, this is the first time that a multicore fiber has been used to build a displacement sensor. The dynamic measurement range is at least seven times larger than that achieved with an EFPI built with a conventional SMF. Moreover, the SMF-SCMCF tip is robust and easy to fabricate and replicate.

A simple, interferometric force sensor based on a multicore optical fiber (MCF) that operates in reflection mode is presented. The device consists of a short segment of MCF inserted at the distal end of a conventional single mode optical fiber (SMF). To demonstrate the concept we used a mechanical piece with grooves to press the MCF. In this way the external force on the MCF is converted in localized pressure on the fiber which causes attenuation losses to the interfering modes and makes the interference pattern to shrink. The changes experienced by the interference pattern can be easily monitored. The sensor here proposed is highly sensitive since it can resolve forces down to 0.01 N.

A novel refractive index sensor based on etched multicore fiber Bragg gratings with temperature in-line compensation is proposed and experimentally demonstrated. By chemically etching the cladding of the multicore fiber, the six outer cores exhibit the sensitive responses to the surrounding refractive index change, with refractive index insensitive and temperature-sensitive central core inside of the multicore fiber. By using the a central Bragg wavelength in the multicore fiber as temperature compensators, the refractive index sensing can be in-line compensated. Moreover, the distribution of multiple outer cores enables the capability of avoiding the nonhomogeneous performance by averaging and balancing the read-out data. Theoretical analysis and experimental results demonstrate that this structure can easily discriminate the RI and temperature. The maximum sensitivity 42.83 nm/ RIU could be obtained at around 1.435 RIU, and the temperature sensitivity is 9.89 pm/°C. The proposed structure is able to in-line and in-situ determine refractive index and temperature simultaneously.

This paper proposes and investigates the use of multicore fiber (MCF) media performing space-division multiplexed transmission for next-generation optical fronthaul systems. We report the experimental demonstration of combined radio-over-fiber (RoF) transmission of full-standard LTE-Advanced (LTE-A) and WiMAX signals providing fronthaul connectivity in 150 m of 4-core fiber (4CF), transmitting simultaneously fully independent wireless services. Operating in linear and nonlinear optical power regimes, the experimental evaluation verifies that the error vector magnitude (EVM) is not degraded when intercore and intracore Kerr nonlinearities are excited in MCF with high input power levels. As a result, nonlinear regime is proposed as a key factor to reduce the temporal EVM fluctuation induced by the random nature of the intercore crosstalk in MCF. In addition, MCF fronthaul applied to converged fiber-wireless polarization multiplexed passive optical networks is demonstrated to transmit LTE-A and WiMAX signals over two orthogonal optical polarizations. The polarization-multiplexed signal is transmitted in RoF over 25.2 km of standard single-mode fiber and then demultiplexed and injected in different cores of the 4CF to provide fronthaul connectivity. Finally, the extension of multicore optical fronthaul capacity is proposed using MIMO LTE-A signals. The tolerance of the MIMO LTE-A RoF transmissions to in-band crosstalk is reported and compared to single-input single-output (SISO) configuration. The experimental results indicate that MIMO configuration is more tolerant than SISO to in-band crosstalk considering both internal and external interferences. MIMO and SISO configurations are compared when transmitted in RoF over a 4CF operating in linear and nonlinear regimes and core interleaving nonlinear stimulation is proposed to reduce the temporal and spectral EVM fluctuation when the same wireless standard is propagated in each core.

Typical optical sensors based on D-shape fibers use standard step-index single-mode fibers (SMF) with a circular core. Multi-mode fibers, fibers with elliptical or rectangular cores, and photonic crystal fibers (PCF) are also used to achieve the best possible sensor performance. However, since the presented sensors differ not only in geometry but, most importantly, in the materials and core sizes, it is difficult to compare their properties directly. In this paper, we numerically analyzed the influence of the shape and size of the core on the sensing performance, with all other relevant parameters being equal. Full Text: PDF References D. Jauregui-Vazquez, J. P. Korterik, C. E. Osornio-Martinez, J. M. Estudillo-Ayala, H. L. Offerhaus, J. A. Alvarez-Chavez, "A strain reflection-based fiber optic sensor using thin core and standard single-mode fibers", Opt. Comm. 522, 128659 (2022). CrossRef H. H. Liu, D. J. Hu, Q. Z. Sun, L. Wei, K. W. Li, C. Liao, B. Li, C. Zhao, X. Dong, Y. Tang, Y. Xiao, G. Keiser, P. Shum, "Specialty optical fibers for advanced sensing applications", Opto-Electron Sci. 2(2), 220025 (2023). CrossRef K. Gasior, T. Martynkien, P. Mergo, W. Urbanczyk, "Fiber-optic surface plasmon resonance sensor based on spectral phase shift interferometric measurements", Sensors and Actuators B: Chemical 257, 602 (2018). CrossRef Y. Ying, G. Si, F. Luan, K. Xu, Y. Qi, H. Li, "Recent research progress of optical fiber sensors based on D-shaped structure", Opt. & Laser Techn. 90, 149 (2017). CrossRef J. Homola, Surface Plasmon Resonance Based Sensors (Berlin, Springer 2006). CrossRef K. Gasior, T. Martynkien., M. Napiorkowski, K. Zolnacz, P. Mergo, W. Urbanczyk, "A surface plasmon resonance sensor based on a single mode D-shape polymer optical fiber", J. Opt. 19, 025001 (2017). CrossRef Q. Wang, W. M. Zhao, "A comprehensive review of lossy mode resonance-based fiber optic sensors", Opt. Lasers in Engineering 100, 47 (2018). CrossRef K. Gasior, T. Martynkien, G. Wojcik, P. Mergo, W. Urbanczyk, "D-shape polymer optical fibres for surface plasmon resonance sensing", Opto-Electron. Rev. 24(4), 209 (2016). CrossRef G. Stepniewski, A. Filipkowski, D. Pysz, J. Warszewski, R. Buczynski, M. Smietana, R. Kasztelanic, "From D-shaped to D-shape optical fiber – A universal solution for sensing and biosensing applications: Drawn D-shape fiber and its sensing applications", Measurement 222, 113642 (2023). CrossRef R. Kasztelanic, H. T. Nguyen, D. Pysz, H. Thienpont, T. Omatsu, R. Buczynski, "Free-Form Optical Fiber with a Square Mode and Top-HatIntensity Distribution", Adv. Science 11, 2402886 (2024). CrossRef J.W. Fleming, "Dispersion in GeO2-SiO2 glasses", Appl. Opt. 23, 4486-4493 (1984). CrossRef T.A. König, P.A. Ledin, J. Kerszulis, M.A. Mahmoud, M.A. El-Sayed, J.R. Reynolds, V.V. Tsukruk., "Electrically Tunable Plasmonic Behavior of Nanocube–Polymer Nanomaterials Induced by a Redox-Active Electrochromic Polymer", ACS Nano 8, 6182-6192 (2014). CrossRef I.Z. Kozma, P. Krok, E. Riedle, "Direct measurement of the group-velocity mismatch and derivation of the refractive-index dispersion for a variety of solvents in the ultraviolet", J. Opt. Soc. Am. B 22, 1479-1485 (2005). CrossRef

The efficient and independent operation of power-over-fiber (PoF) and distributed acoustic sensing (DAS) has been demonstrated using standard single-mode fiber (SSMF). An optical power transmission efficiency (OPTE) of 6.67% and a transmission loss of ~1 dB/km were achieved over an 11.8 km fiber link, supporting both power delivery and distributed optical fiber sensing (DOFS). To minimize cross-talk, the system separates the power and sensing channels by an 88 THz bandwidth. In the experiment, the power and sensing light wavelengths are 1064 nm (continuous) and 1550 nm (pulsed), respectively. As the transmitted optical power increased from 0 W to 2.13 W, the DAS system successfully localized vibration sources with a resolution of 6.29 m and reconstructed phase information, confirming its ability to operate under high optical power. The reported scheme verifies the possibility of constructing the sensing-energy hybrid network based on a conventional optical fiber with the advantages of flexible layout and compatibility.

This paper presents the advances and challenges of design, integration, and performance evaluation of a space-division multiplexing passive optical network (SDM-PON) system based on newly developed 7-core weakly-coupled multicore fiber (WC-MCF). A long-wavelength vertical cavity surface emitting laser (VCSEL) array, customized for O-band operation, was developed and experimentally characterized. To address optical alignment challenges, both expanded beam coupling and adaptive coupling using spatial light modulators were examined. Meanwhile, experimental results under low-crosstalk conditions demonstrated that simplified single-input-single-output (SISO) digital signal processing (DSP) offers comparable performance to multiple-input-multiple-output (MIMO) DSP in the proposed SDM transmission system, enabling reduced complexity and power consumption. Our proposed SDM-PON architecture also delivers performance comparable to standard single-mode fiber (SMF) configurations, with the potential for increased capacity and reduced fiber deployment costs. The scope is then broadened to investigate the scalability of general WC-MCF-based SDM links toward higher core counts. The study also mentioned key design trade-offs and technical challenges at the device and system levels, including limitations in VCSEL bandwidth, coupling efficiency and laser chirp. Overall, the proposed SDM-PON architecture demonstrates strong potential for cost-effective and energy-efficient deployment in future access networks.

In this paper, the theoretical unification of linear and nonlinear inter-core crosstalk (IC-XT) in step-index single-mode multicore fiber (MCF) media is reported and validated experimentally. In order to estimate the IC-XT when operating in linear and nonlinear regimes, the coupled-mode theory (CMT) and the coupled-power theory (CPT) have been unified in both power regimes. The theoretical analysis of the CMT indicates that in coupled MCFs with reduced core-to-core distance (core pitch) the nonlinear self-coupling and cross-coupling effects should be considered when operating with high optical powers. However, considering a core pitch value higher than three times the core radius only the self-coupling effect should be taken into account for estimating the nonlinear IC-XT. Considering these results, the CPT is also extended to nonlinear regime including the dominant nonlinear coupling effect. Using both CMT and CPT, the statistical model of nonlinear IC-XT is completed with the closed-form expressions for estimating the cumulative distribution function, the probability density function and the crosstalk mean and variance as a function of the power level launched into a single-core of the MCF. The crosstalk model presented is additionally extended when multiple cores are simultaneously excited. Finally, the theoretical model is experimentally validated in a homogeneous four-core fiber considering different bending radius configurations.

We report on the conception and the fabrication of a 3D photonic reformatter of 73 waveguides and its associated opaque mask in a wide collaboration to develop a multi-core fiber-fed integral field spectrograph (MCIFU) centered on the Jband. The reformatter is a 3D structure that light from the input quasi-hexagonal multicore fiber is spread out by rearrangement to avoid individual core spectra overlapping when the light is dispersed. The reformatter is fabricated using ultrafast laser inscription (ULI) in a borosilicate glass of 20 mm length. Using a similar ULI process, a 73-hole mask was fabricated in silica glass that precisely matched the waveguides at the output of the reformatter. The output surface of the mask was coated with a 120 nm layer of chromium to block scattered light generated in the bulk material and enhance the signal-To-noise. All inscribed waveguides, characterized using a stable laser centered at 1310 nm from the multicore fiber to the output mask, present consistent single-mode output behavior with a maximum throughput exceeding 60%. Over the 73 cores, the average throughput was measured at 40%. First observations of the full MCIFU device during on-sky measurements have shown promising results to the potential of this novel fiber integral field unit. © 2020 SPIE.

Rapid progress in multi-core fibers and space division multiplexing technologies has enabled large-capacity transmission exceeding the conventional single-core fiber capacity limit of approximately 100 Tb/s. In this invited paper, we review such progress and achievements.

Entanglement distribution is at the heart of most quantum communication protocols. Inevitable loss of photons along quantum channels is a major obstacle for distributing entangled photons over long distances, as the no-cloning theorem forbids the information to simply be amplified along the way as is done in classical communication. It is therefore desirable for every successfully transmitted photon pair to carry as much entanglement as possible. Spontaneous parametric down-conversion (SPDC) creates photons entangled in multiple high-dimensional degrees of freedom simultaneously, often referred to as hyper-entanglement. In this work, we use a multicore fiber (MCF) to show that energy-time and polarization degrees of freedom can simultaneously be transmitted in multiple fiber cores, even maintaining path entanglement across the cores. We verify a fidelity to the ideal Bell state of at least 95% in all degrees of freedom. Furthermore, because the entangled photons are created with a center wavelength of 1560 nm, our approach can readily be integrated into modern telecommunication infrastructure, thus paving the way for high-rate quantum key distribution and many other entanglement-based quantum communication protocols.

In this work, a simple phase-shifted fiber grating (PSFG) inscription technique based on screening method was demonstrated, in which a standard single-mode fiber (SMF) is irradiated by 193 nm excimer laser beam (ELB) via phase mask (PM). During the inscription processes, a small segment of filament with the diameter of about 1mm was perpendicularly inserted in the exposure area to cover the SMF before the ELB is irradiated into the SMF, thus the PSFG will be inscribed. Additionally, in order to clearly observe the PSFG on the temperature and stress characteristics, the reflectance spectrums recorded by a demodulator was processed with the PC. The experimental results show that the notch wavelength has red-shift with the ambient temperature and axial stress increase, or has blue-shift with the ambient temperature and axial stress decrease. The temperature and stress responses sensitivities of the PSFG are 10.26 pm/°Cand 1.36nm/N, in a range from 40°C to 90°C and from 0.2 N to 1.6 N, respectively. And the experimental results exhibited good linearity with an R-square of about 0.998 and 0.999, respectively. The PSFG fabricated by this simple and feasible method can effectively reduce the reflectance spectrum bandwidth of the fiber grating without increasing the length of the grating region or reducing the refractive index modulation depth, thus satisfying the detection of weak dynamic signals.

A light-guiding, flexible fused-silica (FFS) capillary has been used in designing an optical fiber Cr VI sensor for monitoring Cr VI ions in water samples. The FFS capillary is similar to a conventional silica optical fiber in that it can guide light in the wavelength region from the UV to the near IR but different from a conventional optical fiber in that it is a tubular waveguide. The inner surface of the FFS capillary is fused silica, which one can modify to design an optical fiber chemical sensor. The FFS capillary has a cladding layer plus a protective polymer coating on its outside surface. The cladding layer ensures the ability of the FFS capillary to guide light. The protective coating increases the FFS capillary's mechanical strength and makes it robust for practical applications. Compared with conventional silica optical fibers, it is much easier and more feasible to use this FFS capillary to fabricate long-path (tens of meters to thousands of meters) evanescent-wave based chemical sensors. We describe a Cr VI sensor based on the intrinsic evanescent-wave absorption by Cr VI ions in a water sample filled inside the capillary as an example of use of a FFS capillary in chemical sensor design. This simple sensor, using a 30 m light-guiding FFS capillary as a transducer, has the capability of detecting as little as 31 parts in 10(9) of Cr VI in a water sample, which is close to the detection limit of some sophisticated, expensive analytical instruments.