Multimode Fiber Systems Research Articles

High-fidelity image reconstruction in multimode fiber imaging through the MITM-Unet framework

Using a single multimode fiber for both illumination and imaging offers notable advantages in developing ultra-thin imaging probes. However, uneven speckle illumination introduces additional noise, complicating high-precision reconstruction of complex grayscale images, which remains challenging for traditional methods. In this study, we first optimize the image reconstruction framework by combining the inverse transmission matrix approach with deep neural networks, enhancing interpretability and delivering exceptional performance in reconstructing complex images. To address the noise introduced by uneven speckle illumination, we increase the target exposure and effectively integrate information from multiple illumination conditions. Results show that our proposed Multi-speckle Illumination type Inverse Transmission Matrix-Unet (MITM-Unet) method significantly outperforms the Single-speckle illumination type (SITM-Unet). Specifically, images reconstructed with MITM-Unet achieve a structural similarity index of 0.59 and a Pearson correlation coefficient of 0.91, compared to SITM-Unet’s 0.38 and 0.77. These findings underscore the effectiveness of the MITM-Unet method in achieving high-quality imaging of complex grayscale targets, providing valuable insights into the imaging capabilities of single multimode fiber systems. This work holds promise for advancing simpler, more compact wide-field endomicroscopic imaging using multimode fibers.

A thermodynamic approach to linear and quantum cross-talk in multimode fiber systems

A thermodynamic approach to linear and quantum cross-talk in multimode fiber systems

Low-Threshold Cascaded Raman Scattering and Intermodal Four-Wave Mixing in Cascaded Multimode Fiber System

Low-Threshold Cascaded Raman Scattering and Intermodal Four-Wave Mixing in Cascaded Multimode Fiber System

Optimizing anti-perturbation capability in single-shot wide-field multimode fiber imaging systems

In recent years, multimode fiber (MMF) has emerged as a focal point in ultrathin endoscopy owing to its high-capacity information transmission. Nevertheless, the technology's susceptibility to external perturbances limits its practical applications. In this study, we employ a single MMF as both the illumination unit and imaging probe and utilize this single-shot wide-field MMF imaging system to investigate the impact of LED and laser sources on anti-perturbation capabilities. Experimental results demonstrate that, in the absence of deformations in the MMF, both LED and laser-based systems achieve an average structural similarity (SSIM) index of around 0.8 for the reconstructed image, utilizing advanced deep learning techniques, with the laser-based system performing slightly better. However, under unknown MMF configurations post-deformation, the SSIM remains robust at 0.67 for the LED-based system, while the laser-based system drops the average SSIM to 0.45. The results reveal that LED has anti-perturbation capability in single-shot wide-field MMF imaging systems. These findings indicate significant potential for future anti-perturbation studies in endoscopy employing MMF imaging.

Wide-field color imaging through multimode fiber with single wavelength illumination: plug-and-play approach.

Multimode fiber (MMF) is extensively studied for its ability to transmit light modes in parallel, potentially minimizing optical fiber size in imaging. However, current research predominantly focuses on grayscale imaging, with limited attention to color studies. Existing colorization methods often involve costly white light lasers or multiple light sources, increasing optical system expenses and space. To achieve wide-field color images with typical monochromatic illumination MMF imaging system, we proposed a data-driven "colorization" approach and a neural network called SpeckleColorNet, merging U-Net and conditional GAN (cGAN) architectures, trained by a combined loss function. This approach, demonstrated on a 2-meter MMF system with single-wavelength illumination and the Peripheral Blood Cell (PBC) dataset, outperforms grayscale imaging and alternative colorization methods in readability, definition, detail, and accuracy. Our method aims to integrate MMF into clinical medicine and industrial monitoring, offering cost-effective high-fidelity color imaging. It serves as a plug-and-play replacement for conventional grayscale algorithms in MMF systems, eliminating the need for additional hardware.

Efficient Integrated Multimode Amplifiers for Scalable Long-Haul SDM Transmission

Scaling up cable capacity in a power- and cost-efficient manner is a goal of spatial-division multiplexing (SDM) in long-haul optical communication systems. Integration is a key to scaling SDM beyond the parallel single-mode fiber paradigm. Multicore or multimode fiber systems promise higher integration, but native multicore or multimode amplifiers pose challenges in efficient coupling of pump light into the amplifier, and in control of mode-dependent gain (MDG). We propose an integrated multimode amplifier design for signals in six spatial modes (12 spatial and polarization modes) that relies on excitation of only four pump spatial modes at 980 nm. Simulations of an exemplary amplifier predict a relatively flat gain above 10 dB, noise figure below 5 dB, and MDG standard deviation below 0.1 dB over the C-band (1530–1565 nm), with a power-conversion efficiency close to 30%. Key design features include a graded-index multimode fiber amplifier with optimized ring erbium doping profile, length and pump mode powers; and a cascade of wavelength- and mode-selective couplers that efficiently couples four pump diodes to four pump modes, while passing signal modes with minimal loss. Our design approach, which uses fewer pump diodes per signal mode than parallel single-mode systems, may provide an option for efficient, economical scaling of SDM long-haul systems.

Joint pre‐ and post‐equalization using optical multi‐level signaling

A joint pre- and post-equalization scheme is proposed for an optical intensity modulation and direct detection (IM/DD) system. At the transmitter, a linear feed-forward pre-equalizer with just two, three, or four filter taps is suggested to limit the transmitter's complexity and to allow the use of multi-level modulation. Finally, this concept is combined with a zero-forcing post-equalizer at the receiver. By utilizing numerical optimization to design the multi-level signals, the intersymbol-interference originating from the channel is mitigated pre-equalizer taps, a reduction of 3 % in received power compared to a conventional zero-forcing design is achieved. The equalization scheme is tailored for an optical IM/DD system. Therefore, the specific power constraints of such a setup are taken into account in the equalizer design process. In contrast to existing research, the approach is tested for a multi-mode fiber system, which is affected by increased inter-symbol interference due to modal dispersion. The simulation results show that by shifting a small portion of the equalization complexity to the transmitter side, the required optical power to reach a bit-error rate of 10 − 3 $10^{-3}$ is reduced by 7.3% for zero-forcing and 6.8% for minimum mean square error post-equalizers. the noise increase by the post-equalizer is reduced by 6.9 % when including the pre-equalizer with just two filter taps. Compared to an equalization scheme that solely relies on a zero-forcing post-equalizer, the suggested joint pre- and post-equalization scheme is able to improve the bit-error rate performance by an average of 7.15 %. A testbed experiment with a 250 m multi-mode fiber channel and a data rate of 2.5 Gbps operating at 1550 nm confirms these simulation results.

Single-Mode VCSEL Transmission for Short Reach Communications

Single-mode VCSEL technology has advanced significantly in the past few years. The advantages of single-mode VCSELs lie primarily on the narrower linewidth, lower numerical aperture, and smaller spot size compared to multimode VCSELs. They are suitable for transmitting over both multimode fibers and few-mode fibers. For multimode fiber systems, the narrow linewidth can reduce the chromatic dispersion penalty and increase the system reach. A single-mode VCSEL also allows the coupling into graded-index single-mode fiber, which is few-mode around 850 nm with high bandwidth, for few-mode transmission. We review recent progress and present new experimental results and modeling analyses of single-mode VCSEL transmission over both types of fibers. The experiments and analyses shed new light on how single-mode VCSELs can be used with multimode fibers and graded-index single-mode fibers and relative merits between 850 nm single-mode VCSELs versus 980 nm and 1060 nm single-mode VCSELs to address the needs of various applications.

Optical receiver module design that improves link gain of 28 GHz RF signal over a multi-mode fiber

We investigate the impact of photodetector (PD) aperture size and wire bond length on the performance of analog radio over multi-mode fiber (A-RoMMF) systems using high-frequency bands up to 28GHz. It is realized that transmission characteristics of A-RoMMF is affected by the change of frequency response due to the PD aperture size and the bonding wire length. We also demonstrate A-RoMMF transmission using an optical receiver mounted PD and a low noise amplifier (LNA) on one printed circuit board (PCB) to improve size, link gain, and transmission characteristics.

Time-domain Measurement and Analysis of Differential Mode Delay and Modal Bandwidth of Graded-Index Multimode Fiber in SDM Networks

Abstract A novel differential mode delay (DMD) and modal bandwidth measurement technique for a multi-mode optical fiber based on time-domain method has been proposed and analyzed. Mode-dependent loss (MDL) is known to have a detrimental impact on the capacity of multi-mode fiber systems. The bandwidth behavior of 50 μm/62.5 μm graded-index multimode fibers (GI-MMFs) is investigated by launching a temporal pulse into the fiber and measuring the output time-domain waveform to understand and characterize the effect of DMD. The baseband response is measured by observing the broadening of a narrow input pulse (time-domain measurement). This paper verifies the degradation in bandwidth due to profile distortion by the maximum radial shift (starting from the center of the fiber) and the number of steps. The impact due to DMD on GI-MMF performance has been analyzed through fiber transfer function, pulse width and phase changes.

Velocity measurements with structured light transmitted through a multimode optical fiber using digital optical phase conjugation.

Lensless fiber microendoscopes enable optical diagnostics and therapy with minimal invasiveness. Because of their small diameters, multimode fibers are ideal candidates, but mode scrambling hinders the transmission of structured light fields. We present the generation of a localized fringe system at variable distances from the distal fiber end by exploiting digital optical phase conjugation. The replayed fringe system was used for quantitative metrology. Velocity measurements of a microchannel flow in the immediate proximity of the fiber end without the use of any imaging lenses are shown. Lensless multimode fiber systems are of interest especially for biomedical imaging and stimulation as well as technical inspection and flow measurements.

Investigation of speckle suppression beyond human eye sensitivity by using a passive multimode fiber and a multimode fiber bundle.

A method of speckle suppression without any active device is expected for pico-projectors. The effectiveness of the passive method of speckle reduction using a single multimode fiber and a multimode fiber bundle was actually measured and theoretically analyzed. The dependences of the speckle contrast and speckle suppression coefficient on the parameters of multimode fiber and projection systems were investigated. Our results shown that the efficiency of speckle suppression was limited because only the radial direction of the objective lens aperture was used. An improvement using both of the radial and azimuthal directions of the objective lens aperture is required.

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.

Intrinsic Frequency Response of Silicon–Germanium Phototransistor Associated With 850-nm Multimode Fiber

The intrinsic frequency response of silicon–germanium heterojunction bipolar phototransistors (HPTs) at 850 nm is studied to be implemented in multimode fiber systems. The experimental analysis of an HPT with an optical window size of $\textsf {10} \times \textsf {10}\,\,\mu \text{m}^{\textsf {2}}$ is presented. An opto-microwave (OM) scanning near-field optical microscopy is performed to observe the variation of the HPT dynamic behavior versus the illumination location of the phototransistor. The photocurrent generated by the photodiode at the interface between the n++ subcollector and the p+ guard ring is analyzed, and its impact on the performance of the HPT is investigated. Then, we propose a technique to remove the substrate photocurrent effect on the optical transition frequency ( ${f}_{\textsf {Topt}}$ ): ${f}_{\textsf {Topt}}$ value of 4.1 GHz given by raw measurement results increases up to 6 GHz after removing the substrate response. The influence of the 2-D carrier flows on the HPT intrinsic OM behavior is also studied. Design aspects of SiGe/Si HPT structures are finally discussed as a conclusion.

Performance comparison of pre-, boost-, and inline-multimode erbium-doped fiber amplifier configurations to boost mode-division multiplexed multimode fiber link

The performance of 3×3, 4×4, 5×5, and 6×6 optical multi-input multioutput (MIMO) mode-division multiplexed multimode fiber (MMF) systems has been investigated using pre-, boost-, and inline-multimode erbium-doped fiber amplifier configuration methods with LPlm (linearly polarized) modes. The outcome of these configurations has been compared in terms of quality factor (Q-factor) and bit error rate (BER). It is reported that inline-configuration provides best results for all MIMO mode-division multiplexing (MDM) systems covering transmission distance of 100 km with acceptable BER ( 10 dB) over MMF link to boost performance of MDM system.

Dispersion compensation in millimeter wave radio over multimode fiber systems

ABSTRACTMultimode fibers (MMF) provide a reasonable and efficient solution for radio over fiber systems. However, in MMF systems the modal dispersion and chromatic dispersion can be very high. In this article, an efficient compensator is purposed and investigated. The impact of the compensator is tested with different radio modulations by simulations and experiments, and the compensated transfer function is analyzed as well. The application of the compensator provides significant improvement in the error vector magnitude. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:204–207, 2015

Chemical and photo-chemical bonding of polymer and silica fibers

In this paper we present a permanent way of connecting two different fiber types. The one of them is a novel fiber based on polymer as a main construction material, with a build-in photonic structure in its cladding. Second type is a typical silica single-mode fiber. Due to differences of base materials thermal bonding is impossible in order to keep both structures intact. However, chemical bonding by using appropriate materials can connect two types of fibers without disturbing their inner structures. Additional advantage of the proposed chemical “splicing” method is that adhesive can act as index matching material decreasing losses. Full Text: PDF References P.St.J. Russell, J.C. Knight, T.A. Birks, B.J. Mangan, W.J. Wadsworth, "Photonic Crystal Fibers", Science 299, 358-362 (2003) CrossRef J.C. Knight, "review article Photonic crystal fibres", Nature 424, 847-851 (2003) CrossRef S. Ertman, A. H. Rodríguez, M.M. Tefelska, M.S. Chychłowski, D. Pysz,R. Buczyński, E. Nowinowski-Kruszelnicki, R Dąbrowski, T.R. Woliński, Index guiding photonic liquid crystal fibers for practical applications", Journal of Lightwave Technology 30 (2012), pp. 1208 CrossRef P. Mergo, M. Gil, K. Skorupski, J. Klimek, G. Wójcik, J. Pędzisz, J. Kopec, K. Poruraj, L. Czyzewska, A. Walewski, A. Gorgol, "Low loss poly(methyl methacrylate) useful in polymer optical fibres technology", Phot. Lett. Poland. 5, 170 (2013) CrossRef S. Ertman et al. submitted to Photonics Lett. PL. CrossRef J. M. Cariou, J. Dugas, L. Martin, and P. Michel, "Refractive-index variations with temperature of PMMA and polycarbonate" ,Applied Optics, Vol. 25, Issue 3, pp. 334-336 (1986) CrossRef T. Kurokawa, N. Takato, Y. Katayama, "Polymer optical circuits for multimode optical fiber systems", Applied Optics, Vol. 19, Issue 18, pp. 3124-3129 (1980) CrossRef R. Kitamura, L. Pilon, M. Jonasz, "Optical constants of silica glass from extreme ultraviolet to far infrared at near room temperature", Applied Optics, Vol. 46, Issue 33, pp. 8118-8133 (2007) CrossRef

Evaluation of modal noise in graded-index silica and plastic optical fiber links for radio over multimode fiber systems

We have evaluated and compared modal noise induced in a graded-index silica multimode fiber (GI-MMF) link and a graded-index plastic optical fiber (GI-POF) link with the misaligned fiber connections. In radio over fiber (RoF) systems using these optical fibers, modal noise appears as unwanted amplitude modulation in the received signal, and results in degradation of the RoF transmission performance. In this work, we have evaluated the modal noise induced in GI-MMFs and GI-POFs with its same core diameter of 50 μm. Our results show that GI-POFs have an inherently higher tolerance to misaligned connection and less modal noise than GI-MMFs in terms of both the error-vector magnitude and the speckle pattern of the transmitted signals.

Analysis of Laser and Detector Placement in Incoherent MIMO Multimode Fiber Systems

Conventional large-core multimode fibers (MMFs) are preferred for use in short to medium haul optical fiber links, owing to their tolerance to misalignment and low deployment costs; however, data rates through MMFs are limited by modal dispersion. Digital signal processing with multiple-input multiple-output (MIMO) techniques has offered promising solutions to overcome the dispersion limitations of MMFs, but the impact of the geometry of laser and detector arrays on the achievable data rate is not established. To this end, we use a field-propagation-based model to gauge the impact the geometry of lasers and detectors can have on the achievable ergodic and outage rates of incoherent MIMO-MMF links. Laser and detector array geometries were investigated using a grid-based method to optimize the positions of lasers and detectors for a 1 km MIMO-MMF link. Simulations reveal that systems with appropriately designed laser/detector geometries could improve the achievable rate over the fiber by more than 200% over random laser/detector arrays. The grid-based search technique, however, is limited due to high computational requirements for fine grids. As an alternative, we developed a suboptimal “greedy” selection approach to design detector geometries, which produces detector geometries that attain more than 90% of the rate obtained with an exhaustive search, while requiring less than 0.2% of the computation. The low computation requirements and high performance of the greedy selection approach also motivate the use of dynamically reconfigurable detector arrays to achieve high data rates with reduced signal processing complexity. Methods are also presented for clustering detector elements to obtain more consolidated segmented detectors with better fill factors, while still offering significant data rate benefits. The achievable ergodic rate using these systems is verified to be close to the link’s ergodic capacity.

Adaptability-Enabled Record-High and Robust Capacity-Versus-Reach Performance of Real-Time Dual-Band Optical OFDM Signals Over Various OM1/OM2 MMF Systems [Invited

Real-time dual-band optical orthogonal frequency-division multiplexing (OOFDM) transceivers incorporating digital-to-analog and analog-to-digital converters operating at sampling speeds as low as 4 GS/s and field-programmable gate-array-based digital signal processing clocked at only 100 MHz are employed to explore the maximum achievable transmission performance of electro-absorption modulated laser-based multimode fiber (MMF) systems subject to conventional optical launching. Making use of adaptive subcarrier bit and subcarrier/subband power loading, record-high 20.125 Gb/s transmission over 800 m OM2 MMF of real-time dual-band OOFDM signals is experimentally demonstrated, for the first time, with an optical power penalty as low as 1 dB. In addition, extensive experimental explorations of dual-band OOFDM capacity versus reach performance are also undertaken over the aforementioned systems consisting of different MMF types and lengths. It is shown that 20.125 Gb/s over 100 m and 19.625 Gb/s over 1000 m OOFDM transmissions are obtainable in OM2-only MMF systems and that 20 Gb/s over 100 m and 19.375 Gb/s over 1000 m OOFDM transmissions are achievable in OM1-only MMF systems. Furthermore, in various OM1 and OM2 MMF systems, minimum optical power penalties of 19 Gb/s over 1000 m transmission of dual-band OOFDM signals are practically feasible in any legacy MMF systems.