PANDA polarization maintaining (PM) fibers for tight bend applications are presented that can satisfy both optical and mechanical characteristics. Optical optimization of conventional-cladding structures and trench-cladding structures is discussed regarding effective cutoff wavelength under short-length and tight bend conditions. Both trench-cladding PM fiber and conventional PM fiber with 80 μm cladding diameter had similar effective cutoff wavelengths for lengths of 0.5 m. Bending loss at 2 mm radius was less than 0.1 dB/turn at 1550 nm. Additionally, improved mechanical reliability by incorporating a reinforcing outer glass layer is demonstrated on PM fiber for the first time.
In current data-center switches, external fiber-optic connections are terminated in pluggable transceivers at the faceplate of the housing. The signals within the switch are transported electrically via copper traces on printed circuit boards. With increasing data rates, these electrical connections are becoming progressively more lossy, and increasing the electrical power to compensate for this loss negatively impacts the operational cost, electric power infrastructure, and waste heat management. To address these problems, the industry is envisioning placing the transceivers inside the housing very close to where the signals are generated: co-packaged with the switch ASIC. This approach effectively replaces the high-loss copper lanes with low-loss optical-fiber. However, to achieve this low-loss optical connectivity, the optical fibers must be single-mode for short ~0.5 m application lengths to avoid introducing signal impairments due to multi-path interference. It is also desirable that these fibers have good bend performance and mode-field diameters compatible with the installed base of single-mode fiber. In this paper, we will summarize the design of a new fiber optimized for these co-packaged applications and present data on developmental prototypes that demonstrates their suitability for use in short-length optical interconnects. We will also present a novel concept for management of the hundreds to thousands of fibers within the switch housing in which variable lengths of cable are neatly secured inside stackable accumulators. This tailoring of the length for each cable path results in no cable crossovers and will thus facilitate lower-cost and less error-prone assembly and easier maintenance of the switch.
Optical interconnects in data centers have traditionally used 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) in combination with multimode fiber, having a reach up to 100 m in length. Longer links typically use standard single-mode fiber in conjunction with either InP-based edge-emitting lasers or silicon photonic transmitters operating in the 1310 nm or 1550 nm window. Single-mode GaAs-based VCSELs operating at 1064 nm offer another path for achieving longer system reach. Potential advantages of these VCSELs include better power efficiency, modulation speeds reaching 50 Gbps and large-scale fabrication volumes. The longer wavelength is also beneficial due to the lower attenuation and chromatic dispersion of optical fibers at that wavelength. However, one practical issue for single-mode transmission is that the G.657 standard for single-mode fiber requires that the 22-meter cable cutoff wavelength be less than 1260 nm, and these fibers are typically few-moded at 1064 nm. The large differences between the group velocities of the LP01 and LP11 modes can lead to degradation of the system performance due to multi-path interference if the higher order modes are present. To resolve this quandary, we have designed and validated the performance of a new optical fiber which is single-moded at wavelengths less than 1064 nm, but also has G.657- compliant mode field diameter and dispersion characteristics that enable it to be used in the 1310 nm window.
Data centers (DCs) are continually striving to meet the challenge of delivering more bandwidth, and a key part of that endeavor will be the deployment of higher capacity interconnects. Proposed technologies for achieving this higher capacity include higher line rates, advanced modulation formats, wavelength division multiplexing, spatial division multiplexing and reduced diameter fibers. In this paper, we focus on reduced diameter fibers that can be incorporated into higher density cables and connectors.
We have prototyped optical fibers in which the glass diameter is 80 microns and the coating diameter has been reduced to as small as 125 microns. These fibers enable up to twice as many optical interconnects in a given footprint, which will allow transceiver arrays to be collinearly located on small chips with dimensions on the order of (5x5mm2)1. We have also incorporated these reduced diameter fibers into small form-factor, flexible 8-fiber ribbons which can enable as much as a 3x increase in fiber density and can simplify routing constraints inside modules and optical backplanes.
Data centers (DCs) are facing the challenge of delivering more capacity over longer distances. As line rates increase to 25 Gb/s and higher, DCs are being challenged with signal integrity issues due to the long electrical traces that require retiming. In addition, the density of interconnects on the front panel is limited by the size and power dissipation requirements of the pluggable modules. One proposal to overcome these issues is to use embedded optical transceivers in which optical fibers are used to transport data to and from the front panel. These embedded modules will utilize arrays of VCSEL or silicon-photonic transceivers, and in both cases, the capacity may be limited by the density of the optical connections on the chip. To address this constraint, we have prototyped optical fibers in which the glass and coating diameters are reduced to 80 and 125 microns, respectively. These smaller diameters enable twice as many optical interconnects in the same footprint, and this in turn will allow the transceiver arrays to be collinearly located on small chips with dimensions on the order of (5x5mm2)1,2. We have also incorporated these reduced diameter fibers into small, flexible 8-fiber ribbon cables which can simplify routing constraints inside modules and optical backplanes.
Data center (DC) and high performance computing (HPC) applications have traditionally used a combination of copper, multimode fiber and single-mode fiber interconnects with relative percentages that depend on factors such as the line rate, reach and connectivity costs. The balance between these transmission media has increasingly shifted towards optical fiber due to the reach constraints of copper at data rates of 10 Gb/s and higher. The percentage of single-mode fiber deployed in the DC has also grown slightly since 2014, coinciding with the emergence of mega DCs with extended distance needs beyond 100 m. This trend will likely continue in the next few years as DCs expand their capacity from 100G to 400G, increase the physical size of their facilities and begin to utilize silicon-photonics transceiver technology. However there is a still a need for the low-cost and high-density connectivity, and this is sustaining the deployment of multimode fiber for links ≤ 100 m. In this paper, we discuss options for single-mode and multimode fibers in DCs and HPCs and introduce a reduced diameter multimode fiber concept which provides intra-and inter-rack connectivity as well as compatibility with silicon-photonic transceivers operating at 1310 nm. We also discuss the trade-offs between single-mode fiber attributes such as bend-insensitivity, attenuation and mode field diameter and their roles in capacity and connectivity in data centers.
40G BiDi is a commercial WDM transceiver with duplex LC connectivity for transmissions over multimode fibers. In
this paper, we evaluate the transmission performance of 40G BiDi over OM4 fibers. We have carefully selected OM4
fibers with the lowest and highest peak wavelengths around 850 nm to evaluate the reach capability for 40G BiDi
transmission. We demonstrated that the OM4 fiber with the lowest peak wavelength can transmit error free over 325 m
while the OM4 fiber with the highest peak wavelength can transmit up to 350-390 m. In both cases, the maximum
lengths are much longer than 150 m and 200 m specified for OM4 and wideband MMF transmissions, respectively. We
also measured the transceiver encircled flux and found that it was tighter than the encircled flux standard, which may be
a factor favoring long system reach at 900 nm.
We demonstrate wavelength-division multiplexed (WDM) and mode-division multiplexed (MDM) transmission over a
fiber recirculating loop comprising of a 25-km span of low differential mode group dispersion (DMGD) few-mode fiber
carrying the LP01 and LP11 mode groups, and an inline few-mode erbium-doped fiber amplifier (FM-EDFA) providing
low mode-dependent gain (MDG) per span. We successfully transmitted a 10λ × 6 × 28-Gbaud QPSK signal over a
distance of 700 km.
A fiber design is proposed that addresses bend loss, high power and handleability concerns for fibers deployed in high power devices and associated pigtails. If bending is tight enough to leak power from the core into the glass cladding, a coating with a sufficiently low index traps this potentially dangerous power in the cladding. The coating is also designed to keep cladding power from damaging downstream devices and terminations. Loss due to tight bends is minimized by use of a glass design with a small mode-field diameter. This glass design further suppresses the high power failure mode. Over the high power coating is a thick layer of a toughened polymer coating which provides a significant increase in mechanical protection over most commercial coatings.
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