112-Tb/s space-division multiplexed DWDM transmission with 14-b/s/Hz aggregate spectral efficiency over a 76.8-km seven-core fiber

Broadband and efficient multi-mode fiber-chip edge coupler on a silicon platform assisted with a nano-slot waveguide

Mode-division multiplexing (MDM) has been extensively utilized to expand the capacity of chip-scale or fiber-based optical communication, whereas it is still challenging to implement the MDM for fiber-chip optical interconnects due to the huge mode mismatch between few-mode fiber (FMF) and multi-mode waveguide. In this work, we propose and design a silicon integrated six-mode edge coupler assisted with a slot waveguide, which achieves the efficient direct conversion of multiple modes between the waveguide and fiber in a broad bandwidth. Based on the principle of mode conversion, this edge coupler enables the mode conversion between six linear polarization (LP) modes (i.e. LPx/y01, LPx/y11a, LPx/y11b) in FMF and six on-chip waveguide modes (i.e. TE₀, TE₁, TE₂, TM₀, TM₁, TM₂) with low insertion loss, low mode crosstalk and broad bandwidth. The obtained results show that over the whole C-band the efficiency of mode conversion is higher than 98% and the mode crosstalk is low than -19 dB, while the total coupling loss is higher than 85%. The favorable results offer an important reference for designing related broadband photonic devices, and the presented scheme could be potentially exploited to further increase the transmission capability for prospective fiber-chip optical interconnections and communications.

Silicon photonic flat-top WDM (de)multiplexer based on cascaded Mach-Zehnder interferometers for the 2 µm wavelength band

The 2 µm wavelength band has proven to be a promising candidate for the next communication window. Wavelength-division multiplexing (WDM) transmission at 2 µm can greatly increase the capacity of optical communication systems. Here, we experimentally demonstrate a high-performance silicon photonic flat-top 8-channel WDM (de)multiplexer based on cascaded Mach-Zehnder interferometers for the 2 µm wavelength band. A three-stage-coupler scheme is utilized to provide passbands and reduce channel crosstalk, and 11 thermo-optic phase shifters have allowed active compensation of waveguide phase errors. The fabricated device shows low insertion loss (< 0.9 dB), channel crosstalk (< 20.6 dB) and 1-dB bandwidth of 2.3 nm for operating wavelength ranging from 1955nm to 1985nm. The demonstrated (de)multiplexer could potentially be used for WDM optical data communication in the 2 µm spectral band.

Silicon photonic arrayed waveguide grating with 64 channels for the 2 µm spectral range

Driven by the demand to extend optical fiber communications wavelengths beyond the C + L band, the 2 µm wave band has proven to be a promising candidate. Extensive efforts have been directed into developing high-performance and functional photonic devices. Here we report an integrated silicon photonic arrayed waveguide grating (AWG) fabricated in a commercial foundry. The device has 64 channels with a spacing of approximately 50 GHz (0.7 nm), covering the bandwidth from 1967 nm to 2012 nm. The on-chip insertion loss of the AWG is measured to be approximately 5 dB. By implementing a TiN metal layer, the AWG spectrum can be thermally tuned with an efficiency of 0.27 GHz/mW. The device has a very compact configuration with a footprint of 2.3 mm × 2 mm. The demonstrated AWG can potentially be used for dense wavelength division multiplexing in the 2 µm spectral band.

Cylindrical vector beam multiplexer/demultiplexer using off-axis polarization control

The emergence of cylindrical vector beam (CVB) multiplexing has opened new avenues for high-capacity optical communication. Although several configurations have been developed to couple/separate CVBs, the CVB multiplexer/demultiplexer remains elusive due to lack of effective off-axis polarization control technologies. Here we report a straightforward approach to realize off-axis polarization control for CVB multiplexing/demultiplexing based on a metal-dielectric-metal metasurface. We show that the left- and right-handed circularly polarized (LHCP/RHCP) components of CVBs are independently modulated via spin-to-orbit interactions by the properly designed metasurface, and then simultaneously multiplexed and demultiplexed due to the reversibility of light path and the conservation of vector mode. We also show that the proposed multiplexers/demultiplexers are broadband (from 1310 to 1625 nm) and compatible with wavelength-division-multiplexing. As a proof of concept, we successfully demonstrate a four-channel CVB multiplexing communication, combining wavelength-division-multiplexing and polarization-division-multiplexing with a transmission rate of 1.56 Tbit/s and a bit-error-rate of 10-6 at the receive power of -21.6 dBm. This study paves the way for CVB multiplexing/demultiplexing and may benefit high-capacity CVB communication.

Numerical model of an Er3+/Yb3+ co-doped seven-core fiber amplifier for a space division multiplexing system

Space division multiplexing (SDM) erbium-doped seven-core fiber amplifiers (EDFAs) are increasingly attracting interest due to their performances in improving capacity transmission systems. To increase the gain and improve the characteristics of the SDM seven-core fiber amplifier, this paper proposes the use of the erbium/ytterbium (Er³⁺/Yb³⁺) co-doped seven-core fiber amplifier. A theoretical model of the SDM co-doped seven-core fiber amplifier is developed. The presented results show that the proposed model allows us to obtain a peak gain over 60 dB at a wavelength of 1543 nm, a large bandwidth of 64 nm with a gain greater than 20 dB and with a minimum noise which is lower than 6 dB.

Advances in local area optical data communication systems

This paper reviews optical fibre technology for local area optical communications systems. Technologies used in local systems include single and multimode fibre, single and multimode lasers, optical modulators, photodetectors, wavelength division multiplexing, multilevel modulation formats, electronic packet switching, electronic equalization and error correction. These methods have enabled the local area optical link data rate to increase from 0.1 Gb s^-1 in 1990 to nearly a Tb s^-1 in 2019. The challenges to increasing link data rates further, while reducing the transmitted power per bit, at reduced cost are discussed. Potential technical solutions and newly proposed methods which might address these challenges are highlighted.

300 Gb/s IM/DD based SDM-WDM-PON with laserless ONUs

A low-cost, high-speed SDM-WDM-PON architecture is proposed by using a multi-core fiber (MCF) and intensity modulation/directly detection (IM/DD). One of the MCF cores is used for sending laser sources from optical line terminal (OLT) to optical network unit (ONU), thus facilitating laserless and colorless ONUs, and providing ease of network management and maintenance. In addition, the wavelengths of the ONUs are controlled on the OLT side, which also enables flexible optical networks. Thanks to the low inter-core crosstalk of a MCF, downstream (DS) and upstream (US) signals are transmitted independently in different cores of the MCF, not only increasing the aggregated capacity but also avoiding the Rayleigh backscattering noise. Finally, a proof-of-principle experiment is performed by using a 7-core fiber, achieving 300 /120 Gb/s aggregated capacity for DS and US (3 × cores, 4 × wavelengths, 25/10 Gb/s per wavelength), respectively.

Multicore fiber-Bragg-grating-based directional curvature sensor interrogated by a broadband source with a sinusoidal spectrum

A simple, spectral-drift-insensitive interrogation scheme for a multicore fiber Bragg grating (FBG)-based directional curvature sensor is proposed. The basic principle is to transform the wavelength shift of FBGs into the reflected power variation, which is accomplished by utilizing a broadband source with a sinusoidal spectrum. The closed-form expression of the relationship between the reflected power of the FBG and the corresponding peak wavelength is derived for the first time, to the best of our knowledge; therefore, the peak wavelength of the FBG can be precisely interrogated by using a single photodiode. The experimental results show that, with respect to conventional wavelength measurement by an optical spectrum analyzer, the demodulated wavelength error by our proposed interrogation scheme is within ±20  pm. The proposed scheme is further extended to interrogate the direction and curvature using a multicore FBG-based curvature sensor; the interrogated curvature with an error less than 8% is achieved.

High-order mode conversion based on adiabatical mode evolution for mode division multiplexing applications

Mode conversion based on adiabatical mode evolution in a two-core configuration is investigated. The configuration can convert all the launching modes to higher-order modes from one port and convert all the launching modes to lower-order modes from another port. Mode conversion between the two degenerated high-order modes is also demonstrated numerically. The mode conversion feature is only dependent on the relationship between the effective mode indices of the two cores in the configuration, which shows the characteristics of high flexibility and large fabrication tolerance.

All-optical wavelength conversion for mode division multiplexed superchannels

We report in this work the first all-optical wavelength conversion (AOWC) of a mode division multiplexed (MDM) superchannel consisting of 2N modes by dividing the superchannel into N single-mode (SM) tributaries, wavelength converting N SM signals using well developed SM-AOWC techniques, and finally combining the N SM tributaries back to an MDM superchannel at the converted wavelength, inspired by the idea of using SM filtering techniques to filter multimode signals in astronomy. The conversions between multimode and SM are realized by 3D laser-writing photonic lanterns and SM-AOWCs are realized based on polarization insensitive four wave mixing (FWM) configuration in N semiconductor optical amplifiers (SOAs). As a proof of concept demonstration, the conversion of a 6-mode MDM superchannel with each mode modulated with orthogonal frequency division multiplexed (OFDM) quadrature phase-shift keying (QPSK)/16 quadrature amplitude modulation (QAM) signals is demonstrated in this work, indicating that the scheme is transparent to data format, polarization and compatible with multi-carrier signals. Data integrity of the converted superchannel has been verified by using coherent detection and digital signal processing (DSP). Bit error rates (BERs) below the forward error correction (FEC) hard limit (3.8 × 10^-3) have been obtained for QPSK modulation at a net bitrate of 104.2 Gbit/s and BERs below the soft decision FEC threshold (1.98 × 10^-2) have been achieved for 16-QAM format, giving a total aggregate bit rate of 185.8 Gbit/s when taking 20% coding overhead into account. Add and drop functionalities that usually come along with wavelength conversion in flexible network nodes have also been demonstrated. The working conditions of the SOAs, especially the pump and signal power levels, are critical for the quality of the converted signal and have been thoroughly discussed. The impact of imbalanced FWM conversion efficiency among different SM tributaries has also been analyzed. This work illustrates a promising way to perform all-optical signal processing for MDM superchannels.

In-line interferometer based on intermodal coupling of a multicore fiber

An in-line interferometer based on the intermodal coupling of a multicore fiber (MCF) is proposed and experimentally demonstrated. The in-line interferometer is fabricated by adiabatically tapering the MCF. The intermodal coupling of the in-line interferometer is strongly affected by the waist diameter of the MCF, which changes the evanescent field and the pitch size. The effect of the waist diameters of the MCF on the intermodal coupling in the in-line interferometer is theoretically and experimentally investigated. The transmission oscillations of the multiple core modes resulting from the intermodal coupling and interference substantially become stronger as the waist diameter decreases. The extinction ratio and the oscillation periodicity of the transmissions oscillations are changed by the waist diameter.

Single parity check-coded 16QAM over spatial superchannels in multicore fiber transmission

We experimentally investigate single-parity check (SPC) coded spatial superchannels based on polarization-multiplexed 16-ary quadrature amplitude modulation (PM-16QAM) for multicore fiber transmission systems, using a 7-core fiber. We investigate SPC over 1, 2, 4, 5 or 7 cores in a back-to-back configuration and compare the sensitivity to uncoded PM-16QAM, showing that at symbol rates of 20 Gbaud and at a bit-error-rate (BER) of 10^-3, the SPC superchannels exhibit sensitivity improvements of 2.7 dB, 2.0 dB, 1.7 dB, 1.3 dB, and 1.1 dB, respectively. We perform both single channel and wavelength division multiplexed (WDM) transmission experiments with 22 GHz channel spacing and 20 Gbaud channel symbol rate for SPC over 1, 3 and 7 cores and compare the results to PM-16QAM with the same spacing and symbol rate. We show that in WDM signals, SPC over hl1 core can achieve more than double the transmission distance compared to PM-16QAM at the cost of 0.91 bit/s/Hz/core in spectral efficiency (SE). When sharing the parity-bit over 7 cores, the loss in SE becomes only 0.13 bit/s/Hz/core while the increase in transmission reach over PM-16QAM is 44 %.

Fan-in/out polymer optical waveguide for a multicore fiber fabricated using the mosquito method

A fan-in/out polymer optical waveguide with 25-μm cores and 40-μm interchannel pitch is fabricated for a multimode multicore fiber using a microdispenser. We design a fan-in/out structure to which the Mosquito method is applicable since the Mosquito method is capable of drawing a circular graded-index core three-dimensionally. Then, we experimentally fabricate a 10-cm long fan-in/out polymer waveguide with seven cores, which is expected to connect a multicore fiber and a fiber ribbon. A minimum insertion loss of 5.26 dB at 850-nm wavelength for a 10-cm long fan-in/out waveguide is experimentally observed. Causes of variation in the insertion loss and interchannel pitch are discussed.

All-fiber mode-group-selective photonic lantern using graded-index multimode fibers

We demonstrate the first all-fiber mode-group-selective photonic lantern using multimode graded-index fibers. Mode selectivity for mode groups LP(01), LP(11) and LP(21)+LP(02) is 20-dB, 10-dB and 7-dB respectively. The insertion loss when butt coupled to multimode graded-index fiber is below 0.6-dB. The use of the multimode graded-index fibers in the taper can significantly reduce the adiabaticity requirement.

Optical network scaling: roles of spectral and spatial aggregation

As the bit rates of routed data streams exceed the throughput of single wavelength-division multiplexing channels, spectral and spatial traffic aggregation become essential for optical network scaling. These aggregation techniques reduce network routing complexity by increasing spectral efficiency to decrease the number of fibers, and by increasing switching granularity to decrease the number of switching components. Spectral aggregation yields a modest decrease in the number of fibers but a substantial decrease in the number of switching components. Spatial aggregation yields a substantial decrease in both the number of fibers and the number of switching components. To quantify routing complexity reduction, we analyze the number of multi-cast and wavelength-selective switches required in a colorless, directionless and contentionless reconfigurable optical add-drop multiplexer architecture. Traffic aggregation has two potential drawbacks: reduced routing power and increased switching component size.

High-capacity self-homodyne PDM-WDM-SDM transmission in a 19-core fiber

We investigate a high-capacity, space-division-multiplexed (SDM) transmission system using self-homodyne detection (SHD) in multi-core fiber (MCF). We first investigate SHD phase noise cancellation with both kHz and MHz range linewidths for both quadrature-phase shift-keyed (QPSK) and 16 quadrature-amplitude modulation (16QAM) signals, finding that phase noise cancellation in SHD enabled transmission with MHz linewidth lasers that resulted in error floors when using intradyne detection. We then demonstrate a high throughput SHD transmission system using low-cost, MHz linewidth distributed feedback lasers. We transmit a CW pilot-tone on a single core of a 10.1 km MCF span with the remaining 18 cores used to transmit 125 wavelength-division multiplexed (WDM) QPSK and polarization-division-multiplexed (PDM)-QPSK signals with 50 GHz channel spacing at 25 GBd. For PDM transmission and assuming a 7% forward-error correction overhead this is equivalent 210 Tb/s transmission with a SE of 33.4 b/s/Hz. High-capacity transmission is achieved despite high inter-core crosstalk, broad transmitter linewidth and narrow channel spacing, showing that combining SHD with MCF enables high throughput, low-cost transmission in next-generation optical networks.

Impact of fiber core diameter on dispersion and multiplexing in multimode-fiber links

Large-core silica multimode fibers, whose core diameters are generally 50 μm or 62.5 μm, form the bulk of short and medium haul optical fiber links in existence today, owing to their low cost and ease of deployment. However, modal dispersion significantly limits the maximum data rates that they support. Recently, the ability to multiplex several streams of data through optical fibers has spawned the development of few-mode multimode fibers. These fibers possess the low-dispersion characteristics of single-mode fibers and the ability to multiplex several data streams using multiple-input multiple-output (MIMO) techniques and mode-specific filtering to increase data rates. While fibers with larger core diameters possess a larger number of spatial modes, they do not support data rates as high as few-mode fibers. In this paper, we describe a simulation based approach to characterize the tradeoffs between fiber diameter, achievable data rates and alignment tolerances of coherent links that employ graded-index multimode fibers (MMFs) of various dimensions, using the information theoretic outage capacity as the metric. The simulations used fibers' intermodal coupling characteristics to measure its multiplexing abilities and dispersion limitations with mode-specific filters and launch and detection spatial filter arrays. The simulations indicate that the bandwidth-length product achievable over few-mode fibers with MIMO techniques can exceed 250 Gb/s-km, while heavy mode spreading and limited mode selectivity limits the bandwidth-length product to under 25 Gb/s-km in fibers core diameters larger than 50 μm.

Role of wavelength dependent sensitivity in affecting the crosstalk mitigation of homogeneous multicore fiber: an analytical estimation approach

The mitigation of both crosstalk and its wavelength dependent sensitivity for homogeneous multicore fiber (MCF) is theoretically investigated using an analytical evaluation approach. It is found there exists a performance trade-off between the crosstalk mitigation and its wavelength dependent sensitivity suppression. After characterizing the fabricated homogeneous MCFs, we verify that although the increasing core pitch can mitigate the crosstalk, the wavelength dependent sensitivity is drastically degraded from 0.07dB/nm to 0.11dB/nm, which is harmful to the dense wavelength division multiplexing (DWDM) transmission over C + L band using MCF.

Fiber Bragg gratings in hole-assisted multicore fiber for space division multiplexing

In this Letter we present, for the first time to our knowledge, the results of fiber Bragg grating (FBG) inscription in a novel microstructured multicore fiber characterized by seven single-mode isolated cores. A clear Bragg reflection peak can be observed in all of the 7 cores after one inscription process with a KrF nanosecond laser in a Talbot interferometer set up. We furthermore perform a numerical analysis of the effective refractive indices of the particular modes and compare it with the FBG inscription results. An experimental analysis of the strain and temperature sensitivities of all of the Bragg peaks is also included.

Heterogeneous trench-assisted few-mode multi-core fiber with low differential mode delay

We propose a kind of heterogeneous multi-core fiber (Hetero-MCF) with trench-assisted multi-step index few-mode core (TA-MSI-FMC) deployed inside. After analyzing the impact of each parameter on differential mode delay (DMD), we design a couple of TA-MSI-FMCs with A(eff) of 110 μm2 for LP01 mode. DMD of each TA-MSI-FMC is smaller than |170| ps/km over C + L band and the total DMD can approach almost 0 ps/km over C + L band if we adopt DMD managed transmission line technique by using only one kind of Hetero-TA-FM-MCF. For such Hetero-TA-FM-MCF, crosstalk is about -30 dB/100km at wavelength of 1550 nm as bending radius becomes larger than 15 cm, core number can reach 12, a relative core multiplicity factor (RCMF) is 15.7, and the RCMF can even reach 26.1 if we treat LP₁₁ mode as two special modes thanks to the multiple-input-multiple-output technology.

Multi-element fiber technology for space-division multiplexing applications

A novel technological approach to space division multiplexing (SDM) based on the use of multiple individual fibers embedded in a common polymer coating material is presented, which is referred to as Multi-Element Fiber (MEF). The approach ensures ultralow crosstalk between spatial channels and allows for cost-effective ways of realizing multi-spatial channel amplification and signal multiplexing/demultiplexing. Both the fabrication and characterization of a passive 3-element MEF for data transmission, and an active 5-element erbium/ytterbium doped MEF for cladding-pumped optical amplification that uses one of the elements as an integrated pump delivery fiber is reported. Finally, both components were combined to emulate an optical fiber network comprising SDM transmission lines and amplifiers, and illustrate the compatibility of the approach with existing installed single-mode WDM fiber systems.

Seven-core erbium-doped double-clad fiber amplifier pumped simultaneously by side-coupled multimode fiber

We demonstrate a seven-core erbium-doped fiber amplifier in which all the cores were pumped simultaneously by a side-coupled tapered multimode fiber. The amplifier has multicore (MC) MC inputs and MC outputs, which can be readily spliced to MC transmission fiber for amplifying space division multiplexed signals. Gain over 25 dB was obtained in each of the cores over a 40-nm bandwidth covering the C-band.

Design of add-drop multiplexer based on multi-core optical fibers for mode-division multiplexing

A multi-core fiber coupler is proposed to extract one of the modes in a few-mode optical fiber from a light beam, leaving the other modes undisturbed, and allowing a new signal to be retransmitted on that mode. Selective coupling of higher-order modes from a few-mode optical fiber can be realized by increasing the coupling length difference of the modes in the fiber using the multi-core configuration. Low cross-talk and wide bandwidth operation are realized owing to the fact that only one mode can be effectively coupled.

Few mode Er(3+)-doped fiber with micro-structured core for mode division multiplexing in the C-band

Design and experimental characterization of Er(3+)-doped fiber amplifiers supporting 6 spatial modes in wavelength division multiplexing regime are reported. The study is first focused on Er(3+)-doped circular ring-structured profiles accessible with conventional fiber manufacturing techniques. However, these fiber designs, optimized for gain equalization, prove to be difficult to obtain experimentally. So as to go beyond these limits, an alternative approach based on a "pixelated" Er(3+)-doped core is proposed. Several possible designs are theoretically investigated and a first fabrication of micro-structured fiber is presented.

Diode-pumped wideband thulium-doped fiber amplifiers for optical communications in the 1800 - 2050 nm window

We present the first in-band diode-pumped TDFAs operating in the 2 µm wavelength region and test their suitability as high performance amplifiers in potential future telecommunication networks. We demonstrate amplification over a 240 nm wide window in the range 1810 - 2050 nm with up to 36 dB gain and noise figure as low as 4.5 dB.

Multifocus tomographic algorithm for measuring optically thick specimens

A novel tomographic algorithm for reconstructing the two-dimensional refractive index fluctuations of an optically thick phase object from one-dimensional projections acquired at a multiplicity of focal positions and a multiplicity of angular orientations is described. The new method is validated by measurements of multicore and microstructured optical fibers using interference microscopy. The method will benefit other transverse fiber measurement technologies and is broadly applicable to any tomographic reconstruction problem in which the transverse dimension of the specimen is substantially larger than the depth-of-field of the imaging system.

Reconfigurable add-drop multiplexer for spatial modes

We show how a spatial mode can be extracted from a light beam, leaving the other orthogonal modes undisturbed, and allowing a new signal to be retransmitted on that mode. The method is self-aligning, avoids fundamental splitting losses, and uses only local feedback loops on controllable beam splitters and phase shifters. It could be implemented with Mach-Zehnder interferometers in planar optics. The method can be extended to multiple simultaneous mode extractions. As a spatial reconfigurable optical add-drop multiplexer, it is hitless, allowing reconfiguration without interrupting the transmission of any channel.

Transmission of multi-polarization-multiplexed signals: another freedom to explore?

We propose a configuration of signal multiplexing with four polarization states, and investigate its transmission performance over single-mode-fiber links. Assisted by coherent detection and digital signal processing (DSP), the demodulation of four-polarization multiplexed (4PM) on-off-keying (OOK) and phase-shift-keying (PSK) signals are achieved. We then discuss the impact of the crosstalk from polarization mode dispersion (PMD) on 4PM systems. The transmission distance is extended from ~50-km to ~80 km by employing feedback-decision-equalizers. We also compare the back-to-back characteristics of the 40-Gbit/s 4PM-OOK system and 40-Gbit/s PDM-QPSK system with the same spectral efficiency. The results show that the performance of 4PM systems is comparable to that of PDM-QPSK systems, which indicates that the proposed scheme is a potentially promising candidate for future optical networks.

How complicated must an optical component be?

We analyze how complicated a linear optical component has to be if it is to perform one of a range of functions. Specifically, we devise an approach to evaluating the number of real parameters that must be specified in the device design or fabrication, based on the singular value decomposition of the linear operator that describes the device. This approach can be used for essentially any linear device, including space-, frequency-, or time-dependent systems, in optics, or in other linear wave problems. We analyze examples including spatial mode converters and various classes of wavelength demultiplexers. We consider limits on the functions that can be performed by simple optical devices, such as thin lenses, mirrors, gratings, modulators, and fixed optical filters, and discuss the potential for greater functionalities using modern nanophotonics.

Investigating self-homodyne coherent detection in a 19 channel space-division-multiplexed transmission link

We investigate the performance of a self-homodyne coherent detection (SHCD) system using a 19 core multi-core fiber (MCF) and 16 wavelength-division-multiplexed channels. We show that SHCD, with the pilot-tone transmitted on a single MCF core and information carrying signals on the remaining cores, is compatible with space-division-multiplexed transmission, potentially relaxing laser linewidth and digital signal processing requirements due to phase noise cancellation. However, inter-core crosstalk can have an impact on performance and core selection.

12-core fiber with one ring structure for extremely large capacity transmission

The feature of a multicore fiber with one-ring structure is theoretically analyzed and experimentally demonstrated. The one-ring structure overcomes the issues of the hexagonal close-pack structure. The possibility of 10-core fiber with Aeff of 110 μm(2) and 12-core fiber with Aeff of 80 μm(2) is theoretically presented. The fabricated 12-core fibers based on the simulation results realized Aeff of 80 μm(2) and crosstalk less than -40 dB at 1550 nm after 100-km propagation. The MCF with the number of core larger than seven and the small crosstalk was demonstrated for the first time.

Digital coherent superposition for performance improvement of spatially multiplexed coherent optical OFDM superchannels

We demonstrate the use of digital coherent superposition to improve the performance of space-division-multiplexed (SDM) 676-Gb/s OFDM-16QAM superchannels, achieving ~4 dB improvement in OSNR by using two SDM copies and 1075-km (14 x 76.8 km) transmission over a seven-core-fiber with an effective aggregate spectral efficiency of 23.7 b/s/Hz. We further show that the performance improvement from the coherent superposition is retained in the nonlinear transmission regime through coordinated scrambling of signal constellations at the transmitter and appropriate unscrambling at the receiver, by using a series of simple scrambling functions.

Cladding-pumped erbium-doped multicore fiber amplifier

A cladding pumped multicore erbium-doped fiber amplifier for simultaneous amplification of 6 channels is demonstrated. Peak gain over 32 dB has been obtained at a wavelength of 1560 nm and the bandwidth measured at 20-dB gain was about 35 nm. Numerical modeling of cladding pumped multicore erbium-doped amplifier was also performed to study the properties of the amplifier. The results of experiment and simulation are found to be in good agreement.

Scrambled coherent superposition for enhanced optical fiber communication in the nonlinear transmission regime

Coherent superposition of light waves has long been used in various fields of science, and recent advances in digital coherent detection and space-division multiplexing have enabled the coherent superposition of information-carrying optical signals to achieve better communication fidelity on amplified-spontaneous-noise limited communication links. However, fiber nonlinearity introduces highly correlated distortions on identical signals and diminishes the benefit of coherent superposition in nonlinear transmission regime. Here we experimentally demonstrate that through coordinated scrambling of signal constellations at the transmitter, together with appropriate unscrambling at the receiver, the full benefit of coherent superposition is retained in the nonlinear transmission regime of a space-diversity fiber link based on an innovatively engineered multi-core fiber. This scrambled coherent superposition may provide the flexibility of trading communication capacity for performance in future optical fiber networks, and may open new possibilities in high-performance and secure optical communications.

Mode coupling dynamics and communication strategies for multi-core fiber systems

The propagation dynamics of 7-core multi-core fibers (MCFs) with identical and three-types of cores are analytically derived based on the coupled-mode theory. The mode coupling dynamics can be aperiodic with transmission distance for MCF with identical cores. For MCFs with heterogeneous cores, it is found that even though signals from different core groups will not couple with each other, the coupling within their own group is significantly affected by the presence of other core groups. Joint signal processing techniques to mitigate mode coupling induced-cross-talks such as least mean square (LMS) algorithm and maximum likelihood (ML) detection are investigated and corresponding transmission performance are determined for coherent as well as intensity modulated formats. It is shown that aperiodic mode coupling in intensity modulated systems induces cross-talks that are difficult to eliminate through signal processing. The analytical insights may help in optimizing MCF designs and corresponding signal processing techniques for future high capacity MCF transmission systems.

WDM/SDM transmission of 10 x 128-Gb/s PDM-QPSK over 2688-km 7-core fiber with a per-fiber net aggregate spectral-efficiency distance product of 40,320 km·b/s/Hz

We demonstrate 2688-km multi-span transmission using wavelength-division multiplexing (WDM) of ten 50-GHz spaced 128-Gb/s PDM-QPSK signals, space-division multiplexed (SDM) in a low-crosstalk 76.8-km seven-core fiber, achieving a record net aggregate per-fiber-spectral-efficiency-distance product of 40,320 km·b/s/Hz. The demonstration was enabled by a novel core-to-core signal rotation scheme implemented in a 7-fold, synchronized recirculating loop apparatus.

Crosstalk in multicore fibers with randomness: gradual drift vs. short-length variations

Random perturbations play an important role in the crosstalk of multicore fibers, and can be captured by statistical coupled-mode calculations. In this approach, phase matching contributes a multiplicative factor to the average crosstalk, depending on the perturbation statistics and any intentional heterogeneity of neighboring cores. The impact of perturbations is shown to be qualitatively different depending on whether they are gradually varying, or have short-length (centimeter-scale) variations. This insight implies a novel crosstalk suppression strategy: fast modulation of a bend perturbation by spinning the fiber can disrupt the bend-induced phase matching.

1.12-Tb/s 32-QAM-OFDM superchannel with 8.6-b/s/Hz intrachannel spectral efficiency and space-division multiplexed transmission with 60-b/s/Hz aggregate spectral efficiency

We demonstrate the generation of a 1.12-Tb/s superchannel based on coherent optical orthogonal frequency-division multiplexing with polarization-division multiplexed 32-QAM subcarriers, achieving a net intrachannel-spectral-efficiency (ISE) of 8.6 b/s/Hz. Using space-division multiplexing (SDM), we transmit this superchannel over a 76.8-km low-crosstalk multi-core-fiber (MCF) with a record aggregate ISE of 60 b/s/Hz per fiber. We also discuss the impact of core-to-core crosstalk on transmission performance, as well as future perspectives of MCF-based SDM transmission.

First demonstration and detailed characterization of a multimode amplifier for Space Division Multiplexed transmission systems

We present the first demonstration of a multimode (two mode-group) erbium-doped fiber amplifier for Space Division Multiplexed (SDM) applications and demonstrate various design and performance features of such devices. In particular we experimentally demonstrate that differential modal gains can be controlled and reduced both by fiber design and control of the pump field distribution. Using a suitably designed fiber we demonstrate simultaneous modal gains of ~20 dB for different pair-wise combinations of spatial and polarization modes in an EDFA supporting amplification of 6 distinct modes.

Large-effective-area ten-core fiber with cladding diameter of about 200 μm

A multicore fiber with two-pitch layout is proposed to overcome the trade-off between core number and a cladding diameter of a standard hexagonal layout with a single-core pitch. A fabricated ten-core fiber simultaneously realizes effective area of about 120 μm(2) at 1550 nm, small crosstalk, and cladding diameter of 204 μm. The crosstalk between the center core and outer cores is about 30 dB smaller than that between outer cores. The small crosstalk of the center core would help to keep the transmission quality of the center core at the same level as that of the outer cores.