Modes In Multimode Fiber Research Articles

Coverage extended MMF-based indoor OWC using overfilled launch and diversity reception

Speckle patterns generated as coherent optical beams are reflected by scattering elements. Multimode fibers (MMFs) can modify the transverse intensity distribution of speckle patterns with macro perturbations, i.e., pressures, providing a simple and low-cost way to achieve equivalent beam-steering for indoor optical wireless communications (OWCs) with divergent optical beams. However, the received optical power (ROP) variance severely limits the mobility of user terminals. In this paper, the issue is alleviated by using the overfilled launch of MMFs and the diversity gain of multi-receivers. By adjusting the axial spatial coupling distance between the MMF and the single mode fiber (SMF) emitting coherent laser, the number of excited modes of MMF can be significantly increased at 1550 nm with negligible coupling and bending losses. In addition, the signal-to-noise ratio (SNR) enhancement obtained by applying two receivers is theoretically analyzed for the case when either thermal noise or shot noise is dominant. The experimental results demonstrate that the proposed scheme can efficiently compensate for the ROP inhomogeneity, and at the same time it can extend the achievable full steering angle up to 12° at a 1.5-m free-space distance for bit error rate (BER) values of less than 3.8 × 10−3.

High-Performance Microwave Photonic Transmission Enabled by an Adapter for Fundamental Mode in MMFs

Microwave photonic links (MPLs) have long been considered as an excellent way for radio frequency (RF) transmission due to their advantages such as light weight, high bandwidth, low cost and large spurious-free dynamic range (SFDR). However, the effective mode-field area (Aeff) of the single-mode fiber (SMF) used in the traditional MPL is not large, so the MPL based on SMF have relatively strong nonlinearity, which limits the processing power of SMFs to a level of few milliwatts. Few-mode fibers (FMFs) have been applied in MPL as an alternative due to the larger Aeff, and photonic lanterns are used simultaneously to excite the high-order mode of FMFs for RF signal transmission. However, the photonic lantern could bring additional insertion loss, and the production cost of FMFs is high, so we propose an MPL based on multimode fibers (MMFs) with mode field adapters (MFAs). Since MMFs have larger Aeff, the nonlinearity of the link can be greatly reduced. And matched MFAs realized by reverse tapering, to excite only the fundamental mode in MMFs to reduce the crosstalk, which are very stable. As a result, the stimulated Brillouin scattering threshold and SFDR are improved by 5 dB and 14.5 dB, respectively.

Nonlinear dynamics of beam self-cleaning on LP11 mode in multimode fibers.

We investigate the modal energy flow of the femtosecond-pulsed beam self-cleaning on LP11 mode with the influence of different factors such as the initial fraction of LP11 mode, initial peak power, distribution of high-order modes and the numerical aperture of the fiber. It is interesting that there is a critical value of the initial peak power, Pcr, which is the watershed, not only in the quantitatively dominant transverse mode converting from LP11 mode to LP01 mode, but also in the behavior of HOMs of the transition from Attractor to chaos. Our simulation results may provide a novel perspective to understanding the beam self-cleaning on LP11 mode.

High-dimensional encryption in optical fibers using spatial modes of light and machine learning

The ability to engineer the spatial wavefunction of photons has enabled a variety of quantum protocols for communication, sensing, and information processing. These protocols exploit the high dimensionality of structured light enabling the encoding of multiple bits of information in a single photon, the measurement of small physical parameters, and the achievement of unprecedented levels of security in schemes for cryptography. Unfortunately, the potential of structured light has been restrained to free-space platforms in which the spatial profile of photons is preserved. Here, we make an important step forward to using structured light for fiber optical communication. We introduce a classical encryption protocol in which the propagation of high-dimensional spatial modes in multimode fibers is used as a natural mechanism for encryption. This provides a secure communication channel for data transmission. The information encoded in spatial modes is retrieved using artificial neural networks, which are trained from the intensity distributions of experimentally detected spatial modes. Our on-fiber communication platform allows us to use single spatial modes for information encoding as well as the high-dimensional superposition modes for bit-by-bit and byte-by-byte encoding respectively. This protocol enables one to recover messages and images with almost perfect accuracy. Our classical smart protocol for high-dimensional encryption in optical fibers provides a platform that can be adapted to address increased per-photon information capacity at the quantum level, while maintaining the fidelity of information transfer. This is key for quantum technologies relying on structured fields of light, particularly those that are challenged by free-space propagation.

Signal Pick-Up Technique for Sapphire Fiber External Fabry-Perot Interferometer Using Tapered Multimode Fibers

A tapered multimode fiber (MMF) to sufficiently and stably pick up the interference signal of a sapphire fiber external Fabry-Perot interferometer (EFPI) is proposed and investigated. The tapered MMF was fabricated following the adiabaticity criteria for the local tapering angle such that the excited fundamental mode in MMF barely couples to the higher-order modes, ensuring a high-visibility interference output from the sapphire fiber EFPI. Tapered MMF also guarantees a robust fusion splice between the leading single-mode fiber and the sapphire fiber. Experimental results show that using tapered MMF the excitation coefficient of fundamental mode can be improved by more than 3.5 times comparing with conventional technique using non-tapered MMF, and the fringe contrast of the interference spectrum can increase by 2.3 times. The response of the collected interference spectra from the sapphire fiber EFPI using tapered MMF was tested when ambient pressure and temperature was changed from 0.1 MPa to 6.1 MPa and 17 °C to 302°C respectively. The results validate that tapered MMF can provide a high stability against environmental pressure and temperature variations.

Design of a 2D fiber mode converter using a planar 2D multi-mode interference structure

In this work, we present a new design of a fiber mode converter using a two dimensional multi-mode interference waveguide (2D MMI) Structure. The proposed structure allows the conversion of the fundamental fiber mode LP01 to the second order LP21 fiber mode in multimode fiber. The 2D MMI is used as a 1 × 4 splitter and followed by a phase shifter to construct the proper phase map of the higher order LP21 mode. The design is based on the self- imaging properties of the 2D MMI structure and can thus be generalized to higher order modes. The proposed structure shows an insertion loss of about −0.96 dB and a Wavelength Dependent Loss (WDL) of about 0.4 dB over a spectral range of 140 nm around 1550 nm.

An accurate method for measuring the proportions of degenerated spatial modes in fibers

As a fundamental issue, determining the proportions of spatial modes in multimode fibers has been continuously discussed over the past few years. People have successfully measured the components of spatial modes with different orders but have rarely addressed the measurement of degenerated modes under these spatial modes. In this article, we propose an interference method to measure the proportions of these degenerated modes. This method is based on the discussion of coherent superposition of degenerated modes in multimode fibers. In the experiment, we randomly generate spatial states in two-mode fibers and simulate these phenomena according to the proposed method, which agrees well with the experiment. This measuring method improves the accuracy of mode measurement from mode groups to degenerated modes and may provide a unique perspective for the field of multimode fibers.

Overcoming Challenges in Large-Core SI-POF-Based System-Level Modeling and Simulation

The application areas for plastic optical fibers such as in-building or aircraft networks usually have tight power budgets and require multiple passive components. In addition, advanced modulation formats are being considered for transmission over plastic optical fibers (POFs) to increase spectral efficiency. In this scenario, there is a clear need for a flexible and dynamic system-level simulation framework for POFs that includes models of light propagation in POFs and the components that are needed to evaluate the entire system performance. Until recently, commercial simulation software either was designed specifically for single-mode glass fibers or modeled individual guided modes in multimode fibers with considerable detail, which is not adequate for large-core POFs where there are millions of propagation modes, strong mode coupling and high variability. These are some of the many challenges involved in the modeling and simulation of POF-based systems. Here, we describe how we are addressing these challenges with models based on an intensity-vs-angle representation of the multimode signal rather than one that attempts to model all the modes in the fiber. Furthermore, we present model approaches for the individual components that comprise the POF-based system and how the models have been incorporated into system-level simulations, including the commercial software packages SimulinkTM and ModeSYSTM.

An experimental analysis on the performance of single mode-multimode-single mode and multimode- single mode-multimode fiber optic sensor

A cost effective and simple fabrication process for Mach Zehnder Interferometer (MZI) fiber based sensor has been proposed based on single mode-multimode-single mode structure and multimode-single mode-multimode. These proposed structures employed a standard fusion arc splicing by varying the length of sensing region instead of the structures. This sensor has been experimentally demonstrated for three different concentration of solutions such as water, 1mol sucrose solution and oil with the refractive index of 1.333, 1.384 and 1.464 respectively. Furthermore, the intention of this experiment is to determine which structure that provides superior performance in terms of the sensitivity of the device. The operating wavelength of different structures corresponds to the different refractive index. It is observed that the shifting response was influenced by the length of the sensing-area and the best sensitivity achieved for is -10.45nm/RIU.

Effects of Multi-Level Format in MMF System Based on Mode-Field Matched Center-Launching Technique

We evaluate the performances of 25.78-Gb/s signals modulated in on–off keying and four-level pulse amplitude modulation (PAM4) formats in the 12.2-km long multi-mode fiber (MMF) transmission system implemented by using the mode-field matched center-launching technique, which is realized simply by fusion-splicing the single-mode fiber (SMF)-pigtailed transceivers to the transmission MMF. The results show a serious bit-error rate (BER) fluctuation as well as a large power penalty for the PAM4 signal after the 12.2-km long MMF transmission. We identify that the power penalty and BER fluctuations are caused by the signal-to-interference beat noise generated in the SMF-pigtailed receiver, resulting from the small portion of the signal (~5%) unwantedly carried by the high-order modes of MMF. However, we can suppress this beat noise by replacing the SMF-pigtailed receiver with the MMF-pigtailed one since all modes are orthogonal in MMF. For example, when we utilize this proposed method, the power penalty of the 25.78-Gb/s PAM4 signal measured after the transmission over 12.2 km of MMF is reduced from 3.9 to 0.9 dB (@ BER $= 5\times 10^{-5}$ ). We also demonstrate the transmission of the 56-Gb/s PAM4 signal over 2.3 km of MMF by using this method.

Weakly-coupled mode division multiplexing over conventional multi-mode fiber with intensity modulation and direct detection

Multi-mode fiber (MMF) links are expected to greatly enhance capacity to cope with rapidly increasing data traffic in optical short-reach systems and networks. Recently, mode division multiplexing (MDM) over MMF has been proposed, in which different modes in MMF are utilized as spatial channels for data transmission. Strongly-coupled MDM techniques utilizing coherent detection and multiplex-input-multiplex-output (MIMO) digital signal processing (DSP) are complex and expensive for short-reach transmission. So the weakly-coupled approach by significantly suppressing mode coupling in the fiber and optical components has been proposed. In this way, the signals in each mode can be independently transmitted and received using conventional intensity modulation and direct detection (IM-DD). In this paper, we elaborate the key technologies to realize weakly-coupled MDM transmission over conventional MMF, including mode characteristic in MMF and weakly-coupled mode multiplexer/demultiplexer (MUX/DEMUX). We also present the up-to-date experimental results for weakly-coupled MDM transmission over conventional OM3 MMF. We show that weakly-coupled MDM scheme is promising for high-speed optical interconnections and bandwidth upgrade of already-deployed MMF links.

Highly parallel, interferometric diffusing wave spectroscopy for monitoring cerebral blood flow dynamics.

Light-scattering methods are widely used in soft matter physics and biomedical optics to probe dynamics in turbid media, such as diffusion in colloids or blood flow in biological tissue. These methods typically rely on fluctuations of coherent light intensity, and therefore cannot accommodate more than a few modes per detector. This limitation has hindered efforts to measure deep tissue blood flow with high speed, since weak diffuse light fluxes, together with low single-mode fiber throughput, result in low photon count rates. To solve this, we introduce multimode fiber (MMF) interferometry to the field of diffuse optics. In doing so, we transform a standard complementary metal-oxide-semiconductor (CMOS) camera into a sensitive detector array for weak light fluxes that probe deep in biological tissue. Specifically, we build a novel CMOS-based, multimode interferometric diffusing wave spectroscopy (iDWS) system and show that it can measure ∼20 speckles simultaneously near the shot noise limit, acting essentially as ∼20 independent photon-counting channels. We develop a matrix formalism, based on MMF mode field solutions and detector geometry, to predict both coherence and speckle number in iDWS. After validation in liquid phantoms, we demonstrate iDWS pulsatile blood flow measurements at 2.5 cm source-detector separation in the adult human brain in vivo. By achieving highly sensitive and parallel measurements of coherent light fluctuations with a CMOS camera, this work promises to enhance performance and reduce cost of diffuse optical instruments.

Fundamental-mode MMF transmission enabled by mode conversion

Modal dispersion in conventional multi-mode fiber (MMF) will cause serious signal degradation and an effective solution is to restrict the signal transmission in the fundamental mode of MMF. In this paper, unlike previous methods by filtering out higher-order modes, we propose to adopt low-modal-crosstalk mode converters to realize fundamental-mode MMF transmission. We design and fabricate all-fiber mode-selective couplers (MSC), which perform mode conversion between the fundamental mode in single-mode fiber (SMF) and fundamental mode in MMF. The proposed scheme is experimentally compared with center launching method under different MMF links and then its wavelength division multiplexing (WDM) transmission performance is investigated. Experimental results indicate that the proposed mode conversion scheme could achieve better transmission performance and works well for the whole C-band.

Fiber Bragg gratings inscriptions in multimode fiber using 800 nm femtosecond laser for high-temperature strain measurement

A short fiber Bragg grating (FBG) is successfully written in a multimode fiber (MMF) with core and cladding diameters of 50μm and 125μm using 800nm femtosecond laser side-illumination technique. Three-type grating inscriptions can be realized at the different positions over the core of MMF by moving the focal-line position of laser beam. Both fundamental mode and higher-order modes of MMF are excited at the core-mismatch junction, resulting in two well-defined resonances in transmission. The strain measurement with a single core resonance mode is demonstrated experimentally at the ultra-high temperature. The result indicates that the strain sensitivity improved to 5.24pm/µε at the temperature of 600–900°C, making it as a good candidate for the strain measurement at the high temperature environments.

Principal modes in multimode fibers: exploring the crossover from weak to strong mode coupling.

We present experimental and numerical studies on principal modes in a multimode fiber with mode coupling. By applying external stress to the fiber and gradually adjusting the stress, we have realized a transition from weak to strong mode coupling, which corresponds to the transition from single scattering to multiple scattering in mode space. Our experiments show that principal modes have distinct spatial and spectral characteristic in the weak and strong mode coupling regimes. We also investigate the bandwidth of the principal modes, in particular, the dependence of the bandwidth on the delay time, and the effects of the mode-dependent loss. By analyzing the path-length distributions, we discover two distinct mechanisms that are responsible for the bandwidth of principal modes in weak and strong mode coupling regimes. Their interplay leads to a non-monotonic transition of the average principal mode bandwidth from weak to strong mode coupling. Taking into account the mode-dependent loss in the fiber, our numerical results are in qualitative agreement with our experimental observations. Our study paves the way for exploring potential applications of principal modes in communication, imaging and spectroscopy.

Mode Division Multiplexing of Helical-Phased LG Modes in Multimode Fiber with Electronic Dispersion Compensation

Mode Division Multiplexing of Helical-Phased LG Modes in Multimode Fiber with Electronic Dispersion Compensation

Temperature Sensing Characteristics Based on Up-Taper and Single Mode–Multimode Fiber Structure

A novel temperature sensor based on up-taper and single mode–multimode fiber has been presented and demonstrated in this letter. The sensor is fabricated by fusing an up-taper in a single mode fiber (SMF) and splicing the SMF with a section of multimode fiber. In the taper region, a part of light is propagated from the fiber core into the cladding and a series of high-order modes are excited. The reflected light will be coupled back into the core to interfere with the residual core mode when passing through the taper again. Compared with the conventional single mode–multimode–single mode (SMS) fiber sensor, temperature sensitivity of the sensing probe, which consists of SMS and up-taper structure, is improved greatly. According to the experimental results, the temperature sensitivity of 89.42 pm/°C in the temperature range of 20 $^{\circ }\text{C}\sim 80~^{\circ }\text{C}$ can be achieved. The sensor has a series of advantages, such as high sensitivity, low cost, compact size, and reflective measurement.

All fiber optic hetero-core spliced multimode single mode multimode filter

We report here the transmission characteristics of a step index multimode–single mode–multimode fiber (MSM) optic integrated system. Owing to a good coupling between the fibers, the designed system reveals band-elimination filter characteristics. The variability of the stop-bandwidth for this system is incurred by altering the length and core diameter of the mid-single mode fiber. The observed stop band in the range of 80 nm, having good consistency with numerical results, is found to be dependent on the length of the mid-single mode fiber (SMF). Further, the core diameter of the SMF is also varied to tune the band eliminated output from the proposed MSM structure, which results in a narrow bandwidth of 50 nm, giving a margin of 30 nm with respect to the former. The attained results possess higher potential in the field of optical communications, such as a trimmer, etc.

Investigation on sensitivity enhancement for optical fiber speckle sensors.

We present a detailed theoretical and experimental study on the sensitivity enhancement for multimode fiber (MMF) speckle sensor. Using mode coupling theory, we derive an expression showing that the sensitivity of the MMF speckle sensor depends on the intensity profile of the MMF modes. Particularly, we use our theory to study the influence of the spatial filtering window on the sensitivity, and the experimental results have found a good agreement with the theory. Our results suggest that the sensitivity of an MMF speckle sensor can be greatly enhanced by adjusting the size and location of the spatial filtering window. An 80-fold improvement on sensitivity was achieved in our experiment, as compared with the conventional MMF speckle sensor with the filtering window placed at the center of the speckle field.

Observation of Eisenbud–Wigner–Smith states as principal modes in multimode fibre

A set of principal modes that maintain pulse properties and spatial coherence despite propagation in 100 metres of multimode fibre are observed.