Optically powered radio-over-fiber system based on center- and offset-launching techniques using a conventional multimode fiber.

Center launching is a simple and straightforward method to couple optical signals to a multimode fiber (MMF), and able to preferentially excite a few lower-order modes with reduced modal dispersion. However, system imperfections and mechanical perturbations will excite undesired higher order modes, leading to a selectively faded channel frequency response. For this reason, center launching is conventionally less favorable than offset launching when on-off keying is used. This problem can be circumvented by leveraging on the fine frequency granularity of the orthogonal frequency-division multiplexing (OFDM) signal, which is inherently less sensitive to the frequency fading. We have experimentally demonstrated the transmission of a 40-Gb/s OFDM signal (net data rate of 29.48 Gb/s) over 1-km G62.5/125 MMF without using complicated bit-and-power loading. The channel bandwidth is less than 9.1 GHz. We also demonstrate the robustness of the system against fiber bending, polarization variation, unmatched launch beam spot size, as well as its long-term stability.

We present experimental results for combined mode-multiplexed and wavelength multiplexed transmission over conventional graded-index multimode fibers. We use mode-selective photonic lanterns as mode couplers to precisely excite a subset of the modes of the multimode fiber and additionally to compensate for the differential group delay between the excited modes. Spatial mode filters are added to suppress undesired higher order modes. We transmit 30-Gbaud QPSK signals over 60 WDM channels, 3 spatial modes, and 2 polarizations, reaching a distance of 310 km based on a 44.3 km long span. We also report about transmission experiments over 6 spatial modes for a 17-km single-span experiment. The results indicate that multimode fibers support scalable mode-division multiplexing approaches, where modes can be added over time if desired. Also the results indicate that mode-multiplexed transmission distance over 300 km are possible in conventional multimode fibers.

For several decades silica-based optical fibres have been used for telecommunication and sensor purposes. The single-mode fibre is frequently employed in long-distance networks, whereas the multi-mode fibre is the preferred means of signal transport in campus and in-building networks. Because of the huge bandwidth of optical fibres in comparison to its electrical wireless and copper-based counterparts, the demand for optical fibres keeps increasing. In a competitive market, fibre manufacturers aim to produce ever better fibres that are as cheap and easy to employ as possible. As fibre research, development and manufacturing is a mature discipline, improvements in fibre design can only be achieved through the construction of robust, accurate and efficient numerical fibre models for the computation of those quantities that determine the behaviour of the fibre. We have developed a modular software code, based on Maxwell’s equations, to compute these quantities in a vectorial full-wave way for both single-mode and multi-mode optical fibres. Key is the refractive-index profile, or, more specifically, the dopant profile, as it defines the propagation, splicing and bending-loss characteristics of the fibre. For the single-mode fibre, the fibre quantities that we have concentrated on are dispersion, dispersion slope, mode-field diameter, effective area, bending loss, effective and theoretical cut-off wavelength and MAC-value. We highlight one fibre quantity in particular, viz. the computation of the bending loss in a single-mode fibre. Many approximate modelling techniques have been developed to estimate this loss in a fast way. Our numerical scheme, however, is the first rigorous one, as we have performed a vectorial full-wave analysis of the bent optical fibre. In this context, triple integrals involving products of Bessel functions with large, complex order and argument appear. Due to cancellations in the pertaining computation, a high relative accuracy is needed for the computation of each product. As a result, it takes weeks on a contemporary computer to compute the bending loss as a function of the radius of curvature. We have used the vectorial full-wave bending-loss results to determine the most appropriate approximate method. Subsequently, we have extended that approximate method to compute the bending losses of higher-order modes, since the required effective cut-off wavelength depends on the bending loss of the first higher-order mode. The selected approximate method has been used in the ensuing bending-loss calculations. Since the fibre properties are often conflicting, it is a challenging task to adapt the radial dopant profile to meet a set of predefined design goals. A design goal is a combination of desired values for (some of) the aforementioned fibre quantities, and can mathematically be translated into a cost function. The minimisation of this cost function provides us with the optimal dopant profile for that specific set. For the single-mode fibre, we have performed this minimisation for piecewise-linear profiles, by employing various global and gradient-based local optimisation strategies to speed up the design step considerably. Frequently,these optimisation strategies lead to counter-intuitive dopant profile designs that could not have been contrived otherwise. We have selected a deliberate mix of several optimisation routines and have compared their performances. Perhaps the most important conclusion is that there still appears to be room for improvement in the design of the radial dopant profile of commercially available fibres. For the multi-mode fibre, vectorial full-wave optimisation is not feasible yet because of the long computation times for the large number of propagating modes. Still, our numerical scheme allows for a manual fine-tuning of the popular power-law profile to minimise differential mode delay. Further, we have included mode coupling and differential mode attenuation in our model to obtain intensity patterns that match closely with measurements. We have also analysed the influence of profile variations, e.g. on-axis dips and kinks, on the intensity pattern. A selective excitation of different mode groups in a multi-mode fibre, offers the possibility to create several independent transmission channels, and thus a higher information capacity. Recently, the feasibility of this so-called mode group diversity multiplexing technique has been demonstrated. Simulations provide us with a means to better understand its operation and possibly increase its efficiency. The channel separation may be enhanced by employing a lens between the fibre and the detector, which is called mode-selective spatial filtering. Our numerical simulations of a mode group diversity multiplexing link, with and without mode-selective spatial filtering, are in agreement with the measurements. The above discussion makes clear that the developed software code has a wide range of applicability. Moreover, it is built in a modular way and thus extensions, like the inclusion of more fibre quantities or different profile dopants, are straightforward.

We present an all-fiber chirped pulse amplification system based on a normally dispersive single mode Er-doped fiber and a multimode fiber of 25 µm core diameter. The 260 fs sech2 transform-limited pulses generated in a passively mode-locked Er-doped fiber laser are stretched to 1 ps and amplified in an Er-doped fiber amplifier. Then the subsequent multimode fiber in which the pulses propagate only in the fundamental mode compresses the pulses to 80 fs. The pulse compression using the conventional multimode fiber as a large-core single-mode fiber is demonstrated for the first time. The peak power of the output pulses is increased to 44 kW.

In the present work, a compact all-fiber plasmonic focusing beam generator with single and multiple spots is proposed and demonstrated in a conventional multimode fiber. Here, the focusing beam generator is composed of air slit arrays perforated through the gold films deposited on the end facet of a multimode fiber. The array of nanoscale slits with varying widths is used to modulate phase distribution of the focused light. An all-fiber focusing beam generator provides many advantages, such as self-alignment, high flexibility, lower insert loss, and easy portability, which is of importance to realize optical trapping, micromanipulation, beam shaping, and fiber integrated devices.

Optical fibers are used in various applications, e. g. optical communication, material processing, as a laser medium or to generate efficient supercontinua. For most of these applications the knowledge of the dispersion is an essential prerequisite. The dispersion and modal properties of photonic crystal fibers (PCF) strongly depend on the hole diameter and pitch. Since fabrication tolerances affect the structure of the photonic lattice, the dispersion behavior as well as the number of guided transverse modes can differ from numerical calculations. Dispersion measurement of singlemode photonic crystal fibers has been well described in recent papers. However, the determination of dispersion in the presence of higher-order modes is much more difficult. To measure the dispersion of optical fibers with high accuracy, a time-domain white-light interferometer based on a Mach-Zehnder interferometer is presented. The experimental setup allows to determine the wavelength-dependent differential group delay of light travelling through conventional fibers and PCFs within the wavelength range from VIS to NIR. Interferences appear due to superposition of two laser beams, one propagating through the tested fiber and the other travelling through air. Measuring the different group delays of a step-index fiber shows the sufficient accuracy of the interferometer. This paper demonstrates a simple yet effective way to suppress higher-order modes, making it possible to measure the chromatic dispersion of singlemode as well as multimode fibers.

Universal fiber has an LP01 mode field diameter approximately matched to that of standard single mode fiber, while being a multimode fiber. We analyzed the dependence of the mode field diameter on the core diameter for different core delta values. Guided by the analysis, a universal fiber having a delta of 1.2% was fabricated, showing significantly reduced coupling loss of ~2.3 dB with conventional multimode fiber. We demonstrated that the fiber can transmit with full system reach in both single mode and VCSEL-based multimode transmissions, including 100G SR4, 40G sWDM, and 100G CWDM4 for the first time.

Recently, as the continual development of services and needs, people have a higher expectation on the optical communication system especially the passive optical network (PON). Deciding which kind of fiber to be the optimal media for PON is becoming a critical problem. This paper analyzed the feasibility for the applying of multimode fiber (MMF) in the future long haul, high bit-rate, and large capacity PON, and summarized the strengths and weakness of MMF. Then, initiating from the waveguide theory in the weakly guiding fiber and neglecting the intra-modal dispersion as well as nonlinearity, we built a simple model for multimode fiber, based on which we made analysis, research and comparison through simulation experiments on some performance improving techniques of MMF: offset launching, mode size changing, ring launching and feedback equalization. The simulation results demonstrated that the combination of field shaping and feedback equalization can obviously improve the transmission properties of MMF, because of which, the 10Gbit/s data can travel in the MMF for more than 1km.

Method for pulse distortion simulation during propagation along multimode fiber optic transmission link under guided mode excitation by the singlemode laser source is proposed. Launch conditions, the defect of real graded profile of conventional multimode fiber and fluctuations of guided mode group velocity are taken into account. Been obtained by proposed method, there are results of computation of pulse response at the mutimode fiber link output under transmission optical signal, generated by laser source of singlemode fiber optic telecommunication system STM-4. Different launch conditions (type of singlemode conjugating fiber, launch misalignments) and their influence on pulse distortion were investigated.

Multimode fibers (MMFs) offer a cost-effective connection solution for small and medium length networks. However, data rates through multimode fibers are traditionally limited by modal dispersion. Signal processing and Multiple-Input Multiple-Output (MIMO) have been shown to be effective at combating these limitations, but device design for the specific purpose of MIMO in MMFs is still an open issue. This paper utilizes a statistical field propagation model for MMFs to aid the analysis and designs of MMF laser and detector arrays, and aims to improve data rates of the fiber. Simulations reveal that optimal device designs could possess 2-3 times the data carrying capacity of suboptimal ones.

Multiple input multiple output (MIMO) processing is impractical when optical channels have to be switched directly to different locations. This paper tries to solve this problem by exploiting a mode forming remote switch network using multimode fiber to optically route only the desired channel to the desired destination while maintaining data security.

This work presents a power-over-fiber (PoF) using double-clad fibers (DCFs) for radio-over-fiber (RoF) systems. The DCF for the RoF link has a single-mode (SM) core and a multimode inner cladding; the SM core is used for both downlink and uplink transmissions of the optical RoF data signals, while the inner cladding is used for optical power delivery to the BS. The aim of this approach is to optically power the BS in such a way that external electrical power supplies such as batteries or public power lines are not required. The feasibility of the technique is demonstrated by RoF transmission over a 100-m DCF optically feeding with 4.0 W. We successfully achieved high downlink and uplink transmission performances from the CS to the BS in terms of bit-error rate and error-vector magnitude measurements, based on the IEEE 802.11g wireless local area network standard at a carrier frequency of 2.45 GHz.

Twisted light carrying orbital angular momentum (OAM), which featuring helical phase front, has shown its potential applications in diverse areas, especially in optical communications in free space and specially designed fibers, e.g. a vortex fiber. Instead of specially designed fibers extensively used in the reported OAM-based fiber transmission experiments, here we demonstrate the viability of a conventional graded-index multi-mode fiber (MMF) for OAM multiplexing transmission with less digital signal processing (DSP) complexity. We demonstrate a 120-Gbit/s quadrature phase-shift keying (QPSK) signal transmission in an 8.8-km OM4 MMF by using OAM mode multiplexing with all the modes in the first two mode-groups (OAM01L, OAM01R, OAM+11L, OAM+11R, OAM-11L, OAM-11R) with only 2×2 and 4×4 multiple-input-multiple-output (MIMO) equalization. Moreover, we demonstrate the data-carrying two OAM mode groups multiplexing transmission over the 8.8-km MMF without MIMO equalization. These demonstrations may open up new perspectives to enable the realistic use of OAM-based MMF solution in data centers and super-computers.

Multimode fibers provide a means of scaling the peak power of ultrafast fiber lasers by orders of magnitude. While large mode area (LMA) fibers have been widely utilized in fiber amplifiers, these fibers often sacrifice practical benefits of fiber, such as flexibility, and increase system cost and complexity. In addition, the mode-field area of effectively single-mode LMA fibers is smaller than what can be achieved in multimode fibers. Recent work has shown that nonlinear interactions in multimode graded-index fiber can cause a highly multimode field to self-organize into the fundamental mode in a condensation-like process that is robust even with fiber perturbations or disorder. Building on this, we developed a series of Yb:fiber, mode-locked lasers utilizing normal-dispersion, multimode graded-index fiber. In experiments, we observe that the transition from continuous wave lasing to mode-locking is characterized by a beam cleaning process, whereby the highly multimode (speckled) beam of the continuous wave field transforms into a low-order mode beam. Remarkably, experiments and numerical simulations show that the pulses can consist not just the fundamental mode, but can even comprise multiple transverse modes. Our theoretical analysis shows this to be a consequence of a surprising kind of mode-locking – spatiotemporal mode-locking - which relies on strong intermode interactions and spatial filtering. Our initial experiments yield MW-power pulses after external compression, rivaling the best results with flexible LMA fibers. Meanwhile, simulations show that nearly GW peak powers should be possible, making spatiotemporal mode-locking extremely attractive for high-power ultrafast laser development.

Differential mode delay and modal bandwidth measurements of multimode fibers using frequency-domain method