Use Of Multimode Fibers Research Articles

BER versus Data Rate Performance of SCM Transmission over Multimode Fibers at Low-Frequency Passbands with Linear Block Codes

The use of Multimode fibers (MMFs) in the low-frequency region has been studied. It is found that many available low-frequency passbands can be used as channels. However, these passbands are frequency-selective channels, which can negatively affect to the transmitted signal. To lessen this effect, Linear Block Code (LBC) along with a careful selection of passbands is applied. In this work, the performance in terms of BER and data rate for an MMF communication system using a 1-km length MMF at low-frequency passbands with different schemes of LBC is studied. This study uses subcarrier multiplexing (SCM) with a 3-dB modal bandwidth and other low-frequency passbands as channels. The results show that by using (31, 26) LBC and Type-I bit allocation, a data rate of 494 Mbps can be obtained with a BER lower than 10-9. Moreover, a BER lower than 10-14 can be achieved with a data rate of 399 Mbps, which is two-fold higher than the data rate obtained using only the 3-dB modal bandwidth. Additionally, the BER versus data rate for different cases in terms of applying LBC to the system are found and can be practically used as a guideline in selecting a BER and data rate that are suitable for the minimum requirement of a given communication system.

Securing Data in Multimode Fibers by Exploiting Mode-Dependent Light Propagation Effects.

Multimode fibers hold great promise to advance data rates in optical communications but come with the challenge to compensate for modal crosstalk and mode-dependent losses, resulting in strong distortions. The holographic measurement of the transmission matrix enables not only correcting distortions but also harnessing these effects for creating a confidential data connection between legitimate communication parties, Alice and Bob. The feasibility of this physical-layer-security-based approach is demonstrated experimentally for the first time on a multimode fiber link to which the eavesdropper Eve is physically coupled. Once the proper structured light field is launched at Alice's side, the message can be delivered to Bob, and, simultaneously, the decipherment for an illegitimate wiretapper Eve is destroyed. Within a real communication scenario, we implement wiretap codes and demonstrate confidentiality by quantifying the level of secrecy. Compared to an uncoded data transmission, the amount of securely exchanged data is enhanced by a factor of 538. The complex light transportation phenomena that have long been considered limiting and have restricted the widespread use of multimode fiber are exploited for opening new perspectives on information security in spatial multiplexing communication systems.

Statistical mechanics of beam self-cleaning in GRIN multimode optical fibers.

Since its first demonstration in graded-index multimode fibers, spatial beam self-cleaning has attracted a growing research interest. It allows for the propagation of beams with a bell-shaped spatial profile, thus enabling the use of multimode fibers for several applications, from biomedical imaging to high-power beam delivery. So far, beam self-cleaning has been experimentally studied under several different experimental conditions. Whereas it has been theoretically described as the irreversible energy transfer from high-order modes towards the fundamental mode, in analogy with a beam condensation mechanism. Here, we provide a comprehensive theoretical description of beam self-cleaning, by means of a semi-classical statistical mechanics model of wave thermalization. This approach is confirmed by an extensive experimental characterization, based on a holographic mode decomposition technique, employing laser pulses with temporal durations ranging from femtoseconds up to nanoseconds. An excellent agreement between theory and experiments is found, which demonstrates that beam self-cleaning can be fully described in terms of the basic conservation laws of statistical mechanics.

Intensity-Only Mode Decomposition on Multimode Fibers Using a Densely Connected Convolutional Network

The use of multimode fibers offers advantages in the field of communication technology in terms of transferable information density and information security. For applications using physical layer security or mode division multiplexing, the complex transmission matrix must be known. To measure the transmission matrix, the individual modes of the multimode fiber are excited sequentially at the input and a mode decomposition is performed at the output. Mode decomposition is usually performed using digital holography, which requires the provision of a reference wave and leads to high efforts. To overcome these drawbacks, a neural network is proposed, which performs mode decomposition with intensity-only camera recordings of the multimode fiber facet. Due to the high computational complexity of the problem, this approach was usually limited to a number of 6 modes. In this work, it could be shown for the first time that by using a DenseNet with 121 layers it is possible to break through the hurdle of 6 modes. The advancement is demonstrated by a mode decomposition with 10 modes experimentally. The training process is based on synthetic data. The proposed method is quantitatively compared to the conventional approach with digital holography. In addition, it is shown that the network can perform mode decomposition on a 55-mode fiber, which also supports modes unknown to the neural network. The smart detection using a DenseNet opens new ways for the application of multimode fibers in optical communication networks for physical layer security.

All-optical, an ultra-thin endoscopic photoacoustic sensor using multi-mode fiber

Photoacoustic endoscopy (PAE) is a method of in-vivo imaging that uses tissue absorption properties. In PAE, the main tools used to detect the acoustic signal are mechanical ultrasound transducers, which require direct contact and which are difficult to miniaturize. All-optic photoacoustic sensors can challenge this issue as they can provide contact-free sensing. Here, we demonstrate sensing of photo-acoustic signals through a multimode fiber (MMF) which can provide an ultra-thin endoscopic photoacoustic sensor. Furthermore, we show the advantage of using the optical-flow method for speckle sensing and extract the photoacoustic signal despite the mode-mixing along the MMF. Moreover, it is demonstrated for the first time that the speckle reconstruction method can be used without the need for imaging of the speckles as this enables the use of multimode fibers for the speckle method.

Optical hygrometer using light-sheet skew-ray probed multimode fiber with polyelectrolyte coating

The measurement of humidity provides valuable information to a range of industries. Existing hygrometers lack the combination of high sensitivity/low detection-limit, temperature insensitivity, fast response, high robustness and low cost. We present a new design of humidity sensors based on the combination of a polymer-coated multimode fiber interrogated by a light sheet (i.e. thin plane of light) designed to precision excite the optimum group of skew rays for enhanced sensitivity. The sensing mechanism is water absorption of light via evanescent-wave interaction mediated by a polymer-coated multimode fiber. For a 10.0 bilayer poly(diallyldimethylammonium) (PDDA)/poly(styrenesulfonate) (PSS)-coated sensing fiber, we demonstrated a sensitivity of up to 0.14 dB/%RH/cm, a detection limit of 0.007%RH, a temperature cross-sensitivity of less than ˜0.13%/°C, and response/recovery times of 115 ms and 200 ms. The tested dynamic range is between 10%RH and 94%RH. The dependence on the total optical power rather than the phase of light makes such sensors relatively insensitive to temperature while remaining sensitive to humidity. The PDDA/PSS functional coating inherently offers fast response. The use of multimode fibers and power meters make such fiber-optic sensors highly robust and relatively inexpensive, easing the translation to practical applications.

Optical resolution photoacoustic microscopy based on multimode fibers.

Photoacoustic microscopy (PAM) is a multiscale imaging technique. In optical-resolution photoacoustic microscopy (OR-PAM), a single mode (SM) fiber is normally used as the source of optical excitation to be focused into a diffraction-limited spot. Recent advances in OR-PAM have improved its imaging speed using microelectromechanical systems (MEMS). Here we report for the first time the use of a multimode (MM) fiber as the optical excitation source for high resolution OR-PAM in vivo imaging. A high-speed MEMS scanner based OR-PAM system combined with the mechanical movement to provide wide area imaging was used. The use of multimode fiber for achieving tight optical focus would make the optical alignment easier and high repetition rate light delivery possible for high-speed OR-PAM imaging. A lateral resolution of 3.5 µm and axial resolution of 27 µm with ~1.5 mm imaging depth was successfully demonstrated using the system. The efficacy of multimode fibers for achieving tight focus is beneficial for developing high-resolution photoacoustic endoscopy systems and can be combined with other optical endoscopic imaging modalities as well.

Self-mixing detection of backscattered radiation in a single-mode erbium fibre laser for Doppler spectroscopy and velocity measurements

We report an experimental study of the self-mixing effect in a single-mode multifrequency erbium fibre laser when radiation backscattered from an external moving object arrives at its cavity. To eliminate resulting chaotic pulsations in the laser, we have proposed a technique for suppressing backscattered radiation through the use of multimode fibre for radiation delivery. The multifrequency operation of the laser has been shown to lead to strong fluctuations of the amplitude of the Doppler signal and a nonmonotonic variation of the amplitude with distance to the scattering object. In spite of these features, the self-mixing signal was detected with a high signal-to-noise ratio (above ) when the radiation was scattered by a rotating disc, and the Doppler frequency shift, evaluated as the centroid of its spectrum, had high stability (0.15 %) and linearity relative to the rotation rate. We conclude that the self-mixing effect in this type of fibre laser can be used for measuring the velocity of scattering objects and in Doppler spectroscopy for monitoring the laser evaporation of materials and biological tissues.

Mode-filtered large-core fiber for optical coherence tomography

We have investigated the use of multimode fiber in optical coherence tomography (OCT) with a mode filter that selectively suppresses the power of the high-order modes (HOMs). A large-core fiber (LCF) that has a moderate number of guiding modes was found to be an attractive alternative to the conventional single-mode fiber for its large mode area and the consequentially wide Rayleigh range of the output beam if the HOMs of the LCF were efficiently filtered out by a mode filter installed in the middle. For this, a simple mode filtering scheme of a fiber-coil mode filter was developed in this study. The LCF was uniformly coiled by an optimal bend radius with a fiber winder, specially devised for making a low-loss mode filter. The feasibility of the mode-filtered LCF in OCT imaging was tested with a common-path OCT system. It has been successfully demonstrated that our mode-filtered LCF can provide a useful imaging or sensing probe without an objective lens that greatly simplifies the structure of the probing optics.

Fiber LPG Mode Converters and Mode Selection Technique for Multimode SDM

The use of multimode fibers in mode division multiplexed space-division multiplexing systems offers one solution to the capacity limitations of single-mode fiber transmission. Passive components to control the individual modes in few-mode fiber (FMF) are key elements to build more complex modules and components necessary for a high performance system. Fiber-based mode converters are important elements in the FMF mux/demux, and long-period gratings have been investigated to provide mode conversion in two- and four-mode fibers. A method to separate and monitor the modes in real time is described as the basis of a method to measure the individual modal performance of a component during fabrication.

Fiber-coupled ultraviolet planar laser-induced fluorescence for combustion diagnostics

Multimode silica step-index optical fibers are examined for use in planar laser-induced fluorescence (PLIF) for combustion diagnostics using ultraviolet (UV) laser sources. The multimode step-index fibers are characterized at UV wavelengths by examining their energy damage thresholds and solarization performance. The beam quality achievable with large clad step-index multimode fibers is also studied. Emphasis is placed on simultaneously achieving high output energy and beam quality (low output M(2)). The use of multimode fibers to deliver UV pulses at 283 nm for PLIF measurements of OH radicals in a Hencken burner is demonstrated. The fiber delivery capability of UV light will benefit combustion diagnostics in hostile environments, such as augmentor and combustor rigs.

Ultra-broadband indoor optical wireless communication system with multimode fiber

In this paper we experimentally demonstrate an ultra-broadband indoor full-duplex WDM optical wireless communication system with multimode fiber. The multimode fiber is used because it is employed in most of the already installed in-building fiber distribution networks. Simultaneous error-free (BER<10(-9)) transmission of 4×12.5 Gbps downlink and 800 Mbps uplink has been successfully demonstrated. The experimental results show that, although the use of multimode fiber will induce ~2.4 cm reduction in the maximum error-free beam footprint in the downlink, the bit rate of the uplink can be much higher compared to the system with single-mode fiber.

Broadband Access Using Wireless Over Multimode Fiber Systems

With the growing need for low cost wireless over fiber systems the use of multimode fiber (MMF) has become of greater interest. The well known advantages of optical fiber as a transmission medium such as low loss, light weight, large bandwidth characteristics and small size make it the most flexible solution for transporting radio signals to remotely located antenna sites where coverage is required. To provide reliable indoor cellular and wireless local area network (WLAN) coverage without dependence on the radio penetration from outside base stations, it is highly desirable that the same MMF infrastructure be used to carry additional services between the equipment room and the remote antenna units around the building. One of the main benefits of using the wireless over fiber technique is the inherent transparency to the transported wireless signals. This allows configuring the system for a range of both wireless and cellular services to be combined to transmit simultaneously. Hence, it is highly desirable to invest in a multiservice broadband remoting infrastructure which is capable of handling both current and future RF systems. In this paper we present an overview of some of the latest technologies for wireless over multimode fiber systems.

WDM access system based on shared demultiplexer and MMF links

A wavelength-division-multiplexing (WDM) access system can be used in two basic ways: user multiplexing, which assigns a wavelength to each user, and service multiplexing, which assigns a wavelength to each service. In current designs for service multiplexing, each optical network unit (ONU) must have a demultiplexer that can select any wavelength. This paper proposes a new WDM access system that uses one demultiplexer shared by many ONUs to offer optical-distribution access services. This system realizes significant cost reductions due to its passive optical network (PON) architecture, high capacity due to its WDM technology, and easy wiring through the use of multimode fiber (MMF). As one of the realization approaches of the shared demultiplexer, we explain the principle and configuration of a shared demultiplexer based on diffraction theory, and present theoretical and experimental analyses of a prototype 4/spl times/(4/spl times/4) shared demultiplexer whose configuration is based on Littrow mounting. Experimental transmission performances demonstrate the feasibility of the proposed WDM access system.

Crack sensing with a multimode fiber: experimental and theoretical studies

For many structures, failure is preceded by the formation and propagation of cracks. Crack monitoring is hence a useful technique to assess the condition of structures. Also, for laboratory specimens, the monitoring of cracking can provide useful information on the failure process. An important challenge in crack monitoring is that the location of cracks in structures may not be known in a priori. Conventional sensors or transducers that measure the average strain over a small region may easily ‘miss’ the crack. In this paper, a sensing concept based on the bending loss of an optical fiber is first introduced. With the use of multimode fibers, experiments are performed to obtain the signal loss versus crack opening relation. A theoretical model for power loss versus crack opening is then developed. The crack-induced curvature along the fiber is computed with a finite element analysis. Power loss in the curved fiber is then obtained with the ray tracing technique. The theoretical prediction is found to be in good agreement with experimentally measured power loss.

The importance of modal bandwidth in Gigabit Ethernet systems

AbstractThis article deals with the evaluation of bandwidth measurements in Gigabit Ethernet Systems using an empirical data model. It concentrates on the use of multimode fibers due to the advent of Vertical Surface Emitting Laser (VCSEL) technology that improves the performance of multimode fiber optics. Copyright © 2001 John Wiley &amp; Sons, Ltd.

White-light interferometer with high sensitivity and resolution using multi-mode fibers

Abstract It is significant to adopt the multi-mode fiber as the transport media instead of single-mode fiber in a white-light interferometer. The use of the multi-mode fiber can improve system sensitivity and reduce system cost. In this paper, we investigate theoretically and experimentally the characteristics of interferometric signals of a white-light interferometer employing multi-mode fibers as transport media and a broadband laser diode as light source. And point that the relative large intensity ratio of three interferometric regions caused by the use of multi-mode fiber has no great disfavor for the determination of the central fringe.

Technique for recovering images from tightly confined spaces

An approach is described for the recovery of images from tightly confined spaces by use of multimode fibers, and an analytical demonstration shows that the concept is sound. The proof of concept draws on earlier studies that concentrated on image recovery after two-way and one-way transmissions through a multimode fiber; both types of transmission used four-wave-mixing techniques. The approach described also uses four-wave mixing but utilizes two fibers to capture the image at some intermediate point that is accessible to the fibers but that is visually inaccessible to the casual viewer.

Double-pass fiber Raman laser-a powerful and widely tunable in the ultraviolet, visible, and near-infrared fiber Raman laser for biomedical investigations

We present the operating principle, general features, and advantages of a novel concept for the development of powerful and widely tunable fiber Raman lasers (FRL's), based on the high-power pumping of large-core low-loss multimode fibers using a simple double-pass laser arrangement with Littrow-prism-tuned emission. Basic experimental results obtained with various modification double-pass FRL schemes, which provide the generation of both discretely tunable emission in the UV/blue (360-493 nm) at /spl lambda//sub p/=355 nm and continuously tunable emission in the visible/near-IR (0.54-1.01 /spl mu/m) at /spl lambda//sub p/=532 nm, are reported. The double-pass FRL schemes permit powerful and widely tunable spectral components to be generated, due to the double passing of nonlinear-converted laser emission through the optical fiber, maximum cavity feedback, the use of multimode fibers, and high-power pumping.

The limitations for suppressing modal noise induced in a graded-index multimode fibre in a white light interferometer

Modal noise induced in a multimode fibre can be greatly suppressed if the coherence length of the light source used is less than the optical path difference (OPD) between two adjacent fibre modes. In this work, using an accurate modal analysis, the OPDs between each pair of adjacent fibre modes at the far end of a multimode fibre were calculated and compared with supporting experimental results, obtained by varying both the values of the coherence length of a low coherence light source and the length of the multimode fibres employed in an interferometric system. The results from this work can be used both to understand theoretically, and thus determine the practical limitations of, the use of multimode fibres in an interferometric system with a low coherence light source.