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A step-index standard single-mode fiber as a two-mode fiber at 1060 nm can have a high modal bandwidth. In the current work, we conducted a detailed study and found that the LP11 mode of such a fiber is bending-sensitive and that the light excited to LP11 mode can be stripped out due to bending. The transmission experiments were conducted using offset launch with both LP01 and LP11 modes excited and center launch with only LP01 mode excited to show transmission performance in different conditions. We demonstrated the feasibility of 25 Gb/s NRZ transmission over 1 km of the fiber when both LP01 and LP11 modes were excited. We further explored the feasibility of a trench-assisted bending-insensitive step-index standard single-mode fiber with good bending properties for both LP01 and LP11 modes for two-mode transmission at 1060 nm. We found a fiber that has high modal bandwidth at 1060 nm and can sustain bending down to at least a 20 mm diameter. The high-bandwidth two-mode fiber can be potentially useful for future 1060 nm-based VCSEL transmission. [ABSTRACT FROM AUTHOR]

Chromatic dispersion is an important fiber attribute affecting transmission performance over optical fibers. Various chromatic dispersion measurement methods have been developed primarily for single-mode fibers. In the literature, measurement techniques were also developed to characterize few-mode fibers and multi-mode fibers. These methods are often subject to some limitations. In this paper, a simple and robust measurement method for chromatic dispersion measurement of single-mode fibers, polarization--maintaining fibers, and few-mode fibers is presented using a frequency domain instrument and a vector network analyzer. The method is applied to all three types of fibers through one measurement methodology uniformly. Using a vector network analyzer, the measurement instrument obtains the complex transfer function of fiber transmission. The inverse Fourier transform of the measured complex transfer function is used to determine the group delays for each mode of the fiber. Although the sampling is highly under-sampled for the whole fiber link, through proper treatment of the data, we can de-alias the signals and obtain accurate values of the group delays of each mode. By measuring the group delays over different wavelengths, the data can yield the chromatic dispersion of each mode over the wavelength window. [ABSTRACT FROM AUTHOR]

Mode division multiplexing (MDM) has attracted great attention in recent years to increase the network capacity. To apply MDM in data center applications, a low-cost and low power consumption system is desired. In this letter, we utilize 850-nm single-mode VCSEL as the laser source and report mode division multiplexed transmission over a standard single-mode fiber, which is two-mode at 850 nm. Due to the small spot size and low numerical aperture, single-mode VCSEL can be coupled into single-mode fibers with low loss. The mode multiplexer and de-multiplexer devices are designed and fabricated based on fiber mode selective couplers using Hi780 fiber as the 850-nm single-mode launch and receiving fiber and standard single-mode fiber as the few-mode transmission fiber. The MDM system supports two channels, corresponding to the LP01 and LP11 mode in the few-mode transmission fiber. Detailed characterizations of the multiplexer and de-multiplexer are conducted including the insertion loss and crosstalk measurements. System transmission at 25 Gb/s shows error free performance over 1-km standard single- mode fiber for both channels. The utilization of VCSEL and standard single-mode fiber can potentially provide a low-cost solution for MDM systems in data center applications. [ABSTRACT FROM AUTHOR]

Single-mode (SM) vertical-cavity surface-emitting lasers (VCSELs), especially those operating around 850 nm, have been studied intensively in recent years for short distance transmission. Despite the demonstrations of increased data rate and system distance, the impact of frequency chirp that is commonly present in directly modulated lasers is an area that needs more detailed studies for 850 nm VCSEL-based systems. In this paper, we explore the interaction between a laser chirp and fiber chromatic dispersion using an 850-nm SM VCSEL over a standard SM fiber that is two-mode at the operating wavelength. Our transmission experiments show that the system can enjoy a benefit from negative fiber dispersion instead of a penalty compared to the back-to-back case, due to the favorable chirp–dispersion interaction, which is also supported by our system bandwidth measurements. Furthermore, we measure the chirp value of the SM VCSEL and conduct modeling using the time-domain pulse concept to illustrate the impact on the chirp–dispersion interaction and explore the optimal chirp parameters for different transmission data rates. Our study indicates a significant system benefit of using 850-nm SM VCSELs with a high bandwidth single-mode fiber around 850 nm due to the favorable chirp–dispersion interaction. Such a benefit can enable high data rate and longer distance system transmission for modern data center applications. [ABSTRACT FROM AUTHOR]

We propose a large core multimode fiber with 100-µm core diameter for short distance communication that is compatible with existing transceivers designed for 50-µm diameter core multimode fibers. The fiber exhibits a bandwidth that is over four times higher than the 50-µm OM4 fiber, low bending loss, and large connector offset tolerance. We have studied the bandwidth capability through both detailed modeling and experiments. A prototype fiber was fabricated, and it achieves 24 GHz km peak modal bandwidth in the 850 nm window, and the wavelength window with bandwidth above 5 GHz km is more than 100 nm. The bending losses of the fiber measured at 850 and 1300 nm with an encircled flux launch condition exceed the requirements for bending insensitive multimode fibers. The fiber has much higher connector offset tolerance compared to conventional 50-µm diameter core fibers as has been exhibited by modeling and experimental study. The additional loss caused by up to 10-µm connector offset is only 0.3 dB. Transmission experiments using 25 G SR and 100 G shortwave wavelength division multiplexing transceivers over such a fiber show error-free performance over 500 and 300 m, respectively. The proposed large core multimode fiber can address versatile needs in short distance communication including the bandwidth for high data rate transmission and a wide wavelength window for wavelength multiplexed transmission. The large connector tolerance and low bending loss can also enable low cost, compact connectivity solutions with passive alignment and ease of handling. [ABSTRACT FROM AUTHOR]

Carbon‐doped fused silica glass is fabricated by consolidating zinc stearate‐doped porous silica soot blank. The carbon doping concentration is tunable and a maximum of ~0.3 wt% carbon can be incorporated into densified silica. Carbon dopants assemble into nanorod structures which are uniformly distributed inside glass. Transmission electron microscopy electron energy loss spectroscopy and Raman studies identify that carbon dopants are a mixture of nanosized silicon carbide and multiple layer graphene, which produce strong and flat optical absorption in a wide wavelength range of 400‐2500 nm. The electrical resistivity of silica glass is reduced by about 15 orders of magnitude with 0.28 wt% carbon doping. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Single-mode VCSELs have been demonstrated with great transmission capability over multimode fibers in the literature through restricted launch, which is beyond what is allowed by the effective modal bandwidth (EMB) defined for multimode transmission with the encircled flux launch condition. However, the impact of fiber index profile errors and their interactions with launch conditions have not been thoroughly studied. We conduct a detailed experimental study to gain insight on the impacts of the fiber index profiles and launch conditions on the system performance of single-mode VCSELs transmitting over multimode fibers. Experimentally, our results show that a launch spot roughly matching the fiber LP01 mode field leads to very high bandwidth and robust performance, largely independent of the imperfection of the index profiles and therefore can perform well in very high data rates. For a restricted launch without mode matching, the performance highly depends on the quality of the fiber, in line with EMB. We found that the center offset tolerance is around $2~\mu \text{m}$ for a mode matched launch. We also conducted single-mode VCSEL transmission experiments over 600-m and 1000-m multimode fibers, which agree well with the modal bandwidth results. [ABSTRACT FROM AUTHOR]

A new type of multimode fiber, multicore multimode fiber (MCMMF), which uses coupled multicore structure is proposed. The fiber has a large number of single mode cores with optimized coupling to achieve high bandwidth. A theoretical model for analyzing the MCMMF is developed. The effect of fiber parameters, such as index contrast, core radius, and core spacing on the bandwidth is systematically studied. Numerical simulations show that the bandwidth of a MCMMF can be optimized by choosing correct ratio of core spacing to core radius for a given profile design. In addition, mode coupling among the super-modes can further increase the bandwidth due to the small spacing in effective indices in coupled core structures. A MCMMF with 165 cores is fabricated. High bandwidth of 8.4 GHz·km in the 850 nm window is achieved. Using this fiber and a VCSEL-based 25 Gb/s SR transceiver, error free transmission though 150 m fiber is demonstrated. [ABSTRACT FROM AUTHOR]

Recently there has been an interest in upgrading legacy multimode fibers such as 50 μm OM2 fiber to higher data rates through single mode transmission. A fiber modal adapter hosting a modal conditioning single-mode fiber (MCSMF) has been demonstrated as one robust solution to convert OM2 links into single mode transmission links for higher data rate transmission. Legacy multimode fibers also include OM1 fiber which has higher index contrast between the core and cladding and a larger core of 62.5 μm diameter. In this paper, we conduct a study to understand the MCSMF mode field diameter and connector offset tolerance requirements for OM1 fiber, which affect the transmission performance of single mode transmission. We find that despite very different refractive index profiles between OM1 and OM2 fibers, the fiber modal adapter concept is still applicable to OM1 and only one modal conditioning single-mode fiber design is needed to accommodate the needs for both fiber types. The detailed tolerances to the mode field diameter mismatch and the connector offset are numerically studied. The transfer functions for 1000-m of OM1 and OM2 with the adapters are measured, which show very high modal bandwidths at tens of GHz in contrast to very low modal bandwidths without the adapters. Transmission at 25 Gb/s using a 25G LR transceiver, 100 Gb/s using a 100G CWDM transceiver, and 400 Gb/s using a 400G LR transceiver are demonstrated with error free performance and open eye diagrams over 1000-m of OM1 and OM2. We find that the approach of using MCSMF can support fundamental mode transmission up to 100 Gb/s over this distance without obvious limitations. [ABSTRACT FROM AUTHOR]

• We propose a compact adapter for fundamental mode transmission through multimode fiber. • A special modal conditioning single-mode fiber is designed for integration in a compact adapter. • We demonstrate error-free transmission over 1-km multimode fiber using a 100G CWDM4 transceiver. • The SMF adapter enables upgrade of legacy OM2 installations or future-proof new installations. We propose a simple and compact adapter using specially designed modal conditioning single-mode fiber for fundamental mode transmission through multimode fiber. The modal conditioning single-mode fiber has a mode field diameter at 1310 nm roughly matching that of the fundamental mode of the 50-μm core multimode fibers. The fiber is designed for integration in a compact adapter due to its low fiber cutoff wavelength. The adapters are attached to both the input and output ends of the multimode fiber to form a link suitable for single-mode transmission. The detailed characteristics of the link such as insertion loss and multi-path interference are measured. Using the adapters, we demonstrate error-free transmission over 1-km multimode fiber using a 100G CWDM4 transceiver, which carries 4 × 25 Gb/s transmission at four wavelengths around 1310 nm. The proposed SMF adapter not only enables newly deployed multimode fiber to be future proof for higher data rate SM transmission, but also offers an upgrade solution for legacy OM2 installations which otherwise cannot support higher data rate and in some cases are very costly to replace. [ABSTRACT FROM AUTHOR]

• Standard single-mode fiber is bi-modal fiber when operated below the cable cutoff wavelength of around 1260 nm, in which case two modes including LP01 and LP11 are supported. The system performance highly depends on the modal bandwidth of the fiber, therefore, it is valuable to develop a simple and robust method to measure the modal bandwidth of such fiber. • We propose a simple and robust frequency domain method for measuring modal delay and bandwidth of bi-modal optical fibers. An analytical transfer function model is formulated showing excellent agreement with experimental results for relatively short fibers. Using the model, a full set of information can be extracted, including modal delay and modal bandwidth under any launch condition. • The frequency domain measurement method and the analytical model are validated through the excellent agreements with the time domain measurement results. The analytical model is also generalized for longer fiber lengths when additional degradation effects become significant to alter the behavior of the transfer function. We propose a simple and robust frequency domain method for measuring modal delay and bandwidth of bi-modal optical fibers. An analytical transfer function model is formulated showing excellent agreement with experimental results for relatively short fibers. Using the model, a full set of information can be extracted, including modal delay and modal bandwidth under any launch conditions. As a result, one can obtain a worst-case modal bandwidth that can gauge the fiber modal bandwidth under general conditions. In addition, the frequency domain measurement method and the analytical model are validated through the excellent agreements with the time domain measurement results. The analytical model is also generalized for longer fiber lengths when additional degradation effects become significant to alter the behavior of the transfer function. Through the detailed study, we show that the simple frequency domain measurement method as facilitated by the analytical model can deliver a full set of modal delay and modal bandwidth information that otherwise requires more complex method of differential mode delay measurements. [ABSTRACT FROM AUTHOR]

We report for the first time an ultra-wideband coherent (UWB) WDM transmission over a 70 km standard single mode fibre (SSMF) solely using a multistage discrete Raman amplifier (DRA) over the E-, S-, C- and L-bands of the optical window. The amplifier is based on a split-combine approach of spectral bands enabling signal amplification from 1410-1605 nm over an optical bandwidth of 195 nm (25.8 THz). The proposed amplifier was characterized with 143 channelized amplified spontaneous emission (ASE) dummy channels in the S-, C- and L-bands and 4 laser sources in the E-band (1410-1605 nm). The amplification results show an average gain of 14 dB and a maximum noise figure (NF) of 7.5 dB over the entire bandwidth. Coherent transmission with the proposed amplifier was performed using a 30 Gbaud PM-16-QAM channel coupled with the ASE channels over a 70 km SMF. The ultra-wideband transmission using the tailored multistage DRA shows transmission bandwidth of 195 nm with a maximum Q 2 penalty of ∼4 dB in E- and S-band, and ∼2 dB in C- and L-band.

We evaluate the performance penalty due to discrete Raman amplifier (DRA) in a long haul WDM transmission system. The investigation was primarily performed to study the impact of the accumulated nonlinear noise due to fibre chromatic dispersion and nonlinear coefficient(γ). Nonlinear fibres such as inverse dispersion fibre (IDF), dispersion compensation fibre (DCF) and a development fibre known as the Corning Raman fibre (CRF) with the opposite sign of CD to the other two, were taken as the gain fibre in the DRA stage of the long-haul transmission setup. To study the performance penalty with these Raman gain fibres a 30 GBaud 120 Gb/s DP-QPSK channel @1550 nm was combined with 9 spectrally shaped 50 GHz amplified spontaneous emission (ASE) channels for transmission over a recirculation loop with a per loop length of 63 km single mode fibre (SMF). Our modelling and experimental results show that a fibre with positive dispersion >10ps/nm/km and a nonlinear coefficient of ∼ 4W -1 km -1 is a good choice of gain fibre for DRA-assisted coherent transmission system.

A few-mode transmission system is proposed using 850 nm single-mode VCSEL based transceivers over graded-index single-mode fibers for high data rate data center applications. A graded-index single-mode fiber that supports two mode groups at 850 nm window with a high modal bandwidth of 48.3 GHz·km is realized for the first time. 25 Gb/s transmission experiments using a 850 nm single-mode VCSEL over such fiber demonstrate that the system can support a link distance up to 1.5 km. Additionally, link model analysis provides more insights on how fiber and single-mode VCSEL parameters impact the system performance.

Universal fiber has an LP 01 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.