Single Multimode Fiber Research Articles

Deep learning the high variability and randomness inside multimode fibers.

Multimode fibers (MMF) are remarkable high-capacity information channels. However, the MMF transmission is highly sensitive to external perturbations and environmental changes. Here, we show the successful binary image transmission using deep learning through a single MMF subject to dynamic shape variations. As a proof-of-concept experiment, we find that a convolutional neural network has excellent generalization capability with various MMF transmission states to accurately predict unknown information at the other end of the MMF at any of these states. Our results demonstrate that deep learning is a promising solution to address the high variability and randomness challenge of MMF based information channels. This deep-learning approach is the starting point of developing future high-capacity MMF optical systems and devices and is applicable to optical systems concerning other diffusing media.

Active wavefront shaping for controlling and improving multimode fiber sensor

Wavefront shaping (WFS) techniques have been used as a powerful tool to control light propagation in complex media, including multimode fibers. In this paper, we propose a new application of WFS for multimode fiber-based sensors. The use of a single multimode fiber alone, without any special fabrication, as a sensor based on the light intensity variations is not an easy task. The twist effect on multimode fiber is used as an example herein. Experimental results show that light intensity through the multimode fiber shows no direct relationship with the twist angle, but the correlation coefficient (CC) of speckle patterns does. Moreover, if WFS is applied to transform the spatially seemingly random light pattern at the exit of the multimode fiber into an optical focus. The focal pattern correlation and intensity both can serve to gauge the twist angle, with doubled measurement range and allowance of using a fast point detector to provide the feedback. With further development, WFS may find potentials to facilitate the development of multimode fiber-based sensors in a variety of scenarios.

Robust compressive multimode fiber imaging against bending with enhanced depth of field.

Imaging through single multimode fiber prevails its counterpart using single mode fiber bundle on spatial resolution limit and minimum radius. Current multimode fiber imaging suffers from fussy calibration, which can be reduced by recent developed compressive sensing scheme [ [L. V. Amitonova, Opt. Lett. 43, 5427 (2018)]. Experiments demonstrate improvement on depth of field by more than three orders of magnitude, together with robustness against macro fiber bending, which is vital to endoscopic applications.

Fiber-optic plasmonic probe with nanogap-rich Au nanoislands for on-site surface-enhanced Raman spectroscopy using repeated solid-state dewetting.

.We report a fiber-optic plasmonic probe with nanogap-rich gold nanoislands for on-site surface-enhanced Raman spectroscopy (SERS). The plasmonic probe features nanogap-rich Au nanoislands on the top surface of a single multimode fiber. Au nanoislands were monolithically fabricated using repeated solid-state dewetting of thermally evaporated Au thin film. The plasmonic probe shows in SERS enhancement factor and 100 nM in limit-of-detection for crystal violet under both the excitation of laser light and the collection of SERS signals through the optical fiber. The fiber-through measurement also demonstrates the label-free SERS detection of folic acid at micromolar level. The plasmonic probe can provide a tool for on-site and in vivo SERS applications.

Multimode fiber spectrometer with scalable bandwidth using space-division multiplexing

Multimode fiber (MMF) spectrometers suffer from the resolution-bandwidth trade-off due to the limited spatial speckle information used for spectral recovery. We demonstrate a design of an MMF spectrometer with scalable bandwidth using space-division multiplexing. A multicore fiber (MCF) is used to integrate with the MMF. The spatial degrees of freedom at the input are exploited to provide the independent speckle pattern, thus multiplying the spatial information and scaling the bandwidth. We have experimentally achieved 30 nm bandwidth with 0.02nm resolution at wavelength 1550 nm, only using 3 cores of a 7-core fiber and a single MMF. An efficient algorithm is developed to reconstruct the broadband sparse and dense spectrums accurately. The approach can enhance the operating bandwidth of MMF spectrometers without sacrificing the resolution, and simultaneously ensure the system complexity and stability.

Compact and low-cost handheld quasibright-field linear-array probe design in photoacoustic computed tomography.

The optimal photoacoustic probe design is the key to obtain highest imaging sensitivity in photoacoustic computed tomography. Two commonly used probe design types are dark- and bright-field designs. We proposed a design for photoacoustic probe called quasibright-field illumination and compared the performance of all three kinds of probes theoretically and experimentally. Our conclusion is that the proposed quasibright-field illumination photoacoustic probe is superior compared to the existing probe designs as demonstrated. However, each type of illumination should still have its own advantages under certain circumstances. The dark-field illumination is capable of minimizing surface interference signals and reducing their contributions to the background of deeper signals. Hence, it should perform better when imaging samples with high optical absorbance at the surface layer. The bright field may perform better under circumstance when phase distortion is less. We also designed and fabricated three kinds of probes using a single multimode optical fiber for laser energy delivery instead of fiber bundle. Single fiber probes are low cost, transmit laser energy efficiently, and are compact for easy handling. Thus, our study not only provides a method for probe design but also a guidance for cost-effective transducer array-based photoacoustic probe design and manufacturing in the future.

Characterization of an imaging multimode optical fiber using a digital micro-mirror device based single-beam system.

This work demonstrates experimental approaches to characterize a single multimode fiber imaging system without a reference beam. Spatial light modulation is performed with a digital micro-mirror device that enables high-speed binary amplitude modulation. Intensity-only images are recorded by the camera and processed by a Bayesian inference based algorithm to retrieve the phase of the output optical field as well as the transmission matrix of the fiber. The calculated transmission matrix is validated by three standards: prediction accuracy, transmission imaging, and focus generation. Also, it is found that information on mode count and eigenchannels can be extracted from the transmission matrix by singular value decomposition. This paves the way for a more compact and cheaper single multimode fiber imaging system for many demanding imaging tasks.

Few-mode multicore fiber enabled integrated Mach-Zehnder interferometers for temperature and strain discrimination.

We proposed and experimentally demonstrated paralleled Mach-Zehnder interferometers (MZIs) in few-mode multicore fiber (FM-MCF) for temperature and strain discriminative sensing. A section of FM-MCF is sandwich-spliced between two single-mode multicore fiber (SM-MCF) with a rotational offset. The arbitrarily controlled angular misalignment generates intentional intermodal interferences in outer cores of the FM-MCF thus multiple MZI structures are implemented. Experimental results show that the temperature sensitivities are 105.8 pm/°C and 223.6 pm/°C for two outer cores, strain sensitivity is 13.96 pm/με for the outer core 1 and 11.7 pm/με for the outer core 2, respectively. Due to the low condition number of the cross coefficient matrix dependent on the temperature and strain response indexes, the temperature-strain cross sensitivity can be efficiently eliminated. In addition, the structure's fabrication process is simple, cost effective, and repeatable. The sensing structure can be applied to a wide range of measurements and is expected to develop potentials by building a higher dimensional matrix with more cores.

Passive Opto-Antenna as Downlink Remote Antenna Unit for Radio Frequency Over Fiber

A novel remote antenna unit, intended for application in the downlink of an analog radio frequency over fiber system, is proposed, whose radiated power originates entirely from the optical signal supplied by a single multimode fiber. By operating the photodetector of the optical receiver at zero bias voltage, while omitting typical active components, such as transimpedance amplifiers, a fully passive unit, requiring no external power supply, is obtained. Instead, a careful co-design is performed to maximize the power transfer from photodetector to antenna within the frequency range of interest using an impedance matching network. A wideband cavity-backed slot antenna is implemented in an air-filled substrate-integrated-waveguide technology. The antenna feed plane serves as an integration platform for the optical receiver. The resulting downlink remote antenna unit is compact, cost-effective, energy-efficient, and extremely reliable. Therefore, it is an ideal building block for novel, highly specialized, ultrahigh density wireless communication systems, which require massive amounts of remote antenna units, deployed in attocells as small as ${\text{15 cm}} \times {\text{15 cm}}$ . A prototype operating in a wide frequency bandwidth ranging from 3.30 to 3.70 GHz is constructed and validated. In free-space conditions, a broadside gain of $-$ 0.2 dBi, a front-to-back ratio of 8.8 dB and linear polarization with a cross-polarization level of $-$ 28.8 dB are measured at 3.50 GHz. Furthermore, a $-$ 3 dB gain bandwidth of 500 MHz is observed. Finally, the prototype is deployed in a unidirectional data link, achieving a symbol rate of 80 MBd over a distance of 20 cm.

Demonstration of Single-mode Multicore Fiber Transport Network with Crosstalk-aware Optical Path Configuration

Demonstration of Single-mode Multicore Fiber Transport Network with Crosstalk-aware Optical Path Configuration

All-optical mode and wavelength converter based on parametric processes in a three-mode fiber

We demonstrate, both experimentally and numerically, all-optical mode and wavelength conversion both within the C-band and between the C- and L-bands. This is achieved by exploiting phase-matched inter-modal four-wave-mixing processes among the spatial modes of a three-mode fiber. By increasing the number of spatial modes supported by the fiber and tailoring their dispersion profile, it is envisaged that broadband operation over widely separated wavelength bands can be achieved in a single multi-mode fiber using this method.

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.

Design of high energy laser pulse delivery in a multimode fiber for photoacoustic tomography.

In photoacoustic tomography (PAT), delivering high energy pulses through optical fiber is critical for achieving high quality imaging. A fiber coupling scheme with a beam homogenizer is demonstrated for coupling high energy pulses in a single multimode fiber. This scheme can benefit PAT applications that require miniaturized illumination or internal illumination with a small fiber. The beam homogenizer is achieved by using a cross cylindrical lens array, which provides a periodic spatial modulation on the phase of the input light. Thus the lens array acts as a phase grating which diffracts the beam into a 2D diffraction pattern. Both theoretical analysis and experiments demonstrate that the focused beam can be split into a 2D spot array that can reduce the peak power on the fiber tip surface and thus enhance the coupling performance. The theoretical analysis of the intensity distribution of the focused beam is carried out by Fourier optics. In experiments, coupled energy at 48 mJ/pulse and 60 mJ/pulse have been achieved and the corresponding coupling efficiency is 70% and 90% in a 1000-μm and a 1500-μm-core-diameter fiber, respectively. The high energy pulses delivered by the multimode fiber are further tested for PAT imaging in phantoms. PAT imaging of a printed dot array shows a large illumination area of 7 cm2 under 5 mm thick chicken breast tissue. In vivo imaging is also demonstrated on the human forearm. The large improvement in coupling energy can potentially benefit PAT with single fiber delivery to achieve large area imaging and deep penetration detection.

Raman imaging through a single multimode fibre.

Vibrational spectroscopy is a widespread, powerful method of recording the molecular spectra of constituent molecules within a sample in a label-free manner. As an example, Raman spectroscopy has major applications in materials science, biomedical analysis and clinical studies. The need to access deep tissues and organs in vivo has triggered major advances in fibre Raman probes that are compatible with endoscopic settings. However, imaging in confined geometries still remains out of reach for the current state of art fibre Raman systems without compromising the compactness and flexibility. Here we demonstrate Raman spectroscopic imaging via complex correction in single multimode fibre without using any additional optics and filters in the probe design. Our approach retains the information content typical to traditional fibre bundle imaging, yet within an ultra-thin footprint of diameter 125 μm which is the thinnest Raman imaging probe realised to date. We are able to acquire Raman images, including for bacteria samples, with fields of view exceeding 200 μm in diameter.

High-Brightness Semiconductor Laser-Pumped $1.56~\mu$ m Polarization-Entangled Photon Pairs

In this paper, a semiconductor laser-pumped high-brightness $1.56~\mu \text{m}$ polarization-entangled photon pairs with a 30-mm-length PPKTP down-converter inside a Sagnac-type interferometer is firstly demonstrate. Large active-area InGaAs/InP-based single photon detectors and step-index multi-mode fiber collection are attempted to evaluate the brightness and entanglement quality. The maximum spectral brightness is about $1.33\times 10^{5}$ pairs( $\text{s}\ast $ mW $\ast $ nm) $^{-1}$ with a pump power of 200 mW and a spectral bandwidth of 1.34 nm. Under the condition of maximum entanglement brightness, polarization visibility of 96.08%±1.62% is obtained in the diagonal orthogonal basis. The quantum state tomography also shows a high fidelity of 97% for the ideal Bell states. Meanwhile, the CHSH-type Bell inequality is violated with a value of 2.8196±0.0098 ( $96~\sigma$ ). Both the performance in brightness and entanglement quality indicate that the current $1.56~\mu \text{m}$ polarization-entangled source has great potential in free-space quantum key distribution or quantum communication.

Design and optimization of 32-core rod/trench assisted square-lattice structured single-mode multi-core fiber.

We propose and design a kind of heterogeneous rod-assisted and trench-assisted multi-core fiber (Hetero-RA-TA-MCF) with 32 cores arranged in square-lattice structure (SLS), and then we introduce the design method for Hetero-RA-TA-MCF. Simulation results show that the Hetero-RA-TA-32-Core-Fiber achieves average effective area (Aeff) of about 74 μm2, low crosstalk (XT) of about -31 dB/100km, threshold value of bending radius (Rpk) of 7.0 cm, relative core multiplicity factor (RCMF) of 8.74, and cable cutoff wavelength (λcc) of less than 1.53 μm.

Advanced Space Division Multiplexing Technologies for Optical Networks

Space division multiplexing (SDM) is mainly seen as a means to increase data throughput and handle exponential traffic growth in future optical networks. But its role is certainly more diverse. Research on SDM encourages device integration, brings newfunctionality to network elements, and helps optical networks to evolve. As a result, the number of individual components in future networks will decrease, which in turn will improve overall network reliability and reduce power consumption as well as operational expenditure. After reviewing the state-of-the-art in SDMfiber research and development with a particular focus on weakly coupled single-mode multi-core fibers, we take a look beyond the capabilities of SDM as a means of boosting transmission capacity and discuss ideas and concepts on howto exploit the spatial dimension for improved efficiency and resource sharing in optical networks.

Unified Architecture of an Integrated SDM-WSS Employing a PLC-Based Spatial Beam Transformer Array for Various Types of SDM Fibers

There is no doubt that wavelength-division multiplexing will continuously play an important role in future space-division multiplexing (SDM) capable optical networks. In this paper, we propose a unified architecture for integrated wavelength selective switches (WSSs) that can be applied to various types of SDM fibers and reconfigurable optical add/drop multiplexer switching types. A recently reported planar lightwave circuit (PLC)-based spatial beam transformer (SBT) array is anticipated to be used as a key enabler for the architecture to integrate multiple WSSs on a single liquid crystal on silicon beam steering device. The architecture enables individual switching and power adjustment functionality of an SDM-WSS that supports an uncoupled single-mode multicore fiber (SM-MCF) in an economical manner. The same architecture provides fractional joint wavelength switching functionality in an SDM-WSS for a few-mode MCF (FM-MCF) and a grouped-coupled-core MCF (GCC-MCF) as well as joint wavelength switching in an SDM-WSS for multiple few-mode fibers with almost no additional hardware complexity. Preliminary scalability evaluation shows that a 1×24 individual switching SDM-WSS for a single-mode eight-core uncoupled MCF or a 1×7 fractional joint wavelength switching SDM-WSS for a three-mode eight-core FM-MCF or a three-coupled-core × eight-group GCC-MCF is feasible using the SBT array design reported in the literature. The evaluation also shows that compared to the traditional approach based on conventional WSSs, the proposed architecture is anticipated to reduce the hardware footprint requirement by parallelism factor mC, where m and C indicate the number of modes per core and the number of cores per fiber, respectively.

单根多模光纤数字扫描成像方法

单根多模光纤数字扫描成像方法

Birefringence effects in multi-core fiber: coupled local-mode theory.

In this paper, we evaluate experimentally and model theoretically the intra- and inter-core crosstalk between the polarized core modes in single-mode multi-core fiber media including temporal and longitudinal birefringent effects. Specifically, extensive experimental results on a four-core fiber indicate that the temporal fluctuation of fiber birefringence modifies the intra- and inter-core crosstalk behavior in both linear and nonlinear optical power regimes. To gain theoretical insight into the experimental results, we introduce an accurate multi-core fiber model based on local modes and perturbation theory, which is derived from the Maxwell equations including both longitudinal and temporal birefringent effects. Numerical calculations based on the developed theory are found to be in good agreement with the experimental data.