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.

Multicore fiber distributed feedback lasers.

We demonstrate parallel fabrication of seven fiber distributed feedback (DFB) lasers in a hexagonally arrayed multicore core Er doped fiber with 40 µm core spacing. DFB grating cavities 8 cm long and operating near 1545 nm were fabricated with a single UV inscription exposure. We observed dual polarization, single longitudinal mode operation with a linewidth below 300 kHz for each laser.

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.

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

We describe a new multicore fiber (MCF) having seven single-mode cores arranged in a hexagonal array, exhibiting low crosstalk among the cores and low loss across the C and L bands. We experimentally demonstrate a record transmission capacity of 112 Tb/s over a 76.8-km MCF using space-division multiplexing and dense wavelength-division multiplexing (DWDM). Each core carries 160 107-Gb/s polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) channels on a 50-GHz grid in the C and L bands, resulting in an aggregate spectral efficiency of 14 b/s/Hz. We further investigate the impact of the inter-core crosstalk on a 107-Gb/s PDM-QPSK signal after transmitting through the center core of the MCF when all the 6 outer cores carry same-wavelength 107-Gb/s signals with equal powers, and discuss the system implications of core-to-core crosstalk on ultra-long-haul transmission.

Amplification and noise properties of an erbium-doped multicore fiber amplifier.

A multicore erbium-doped fiber (MC-EDF) amplifier for simultaneous amplification in the 7-cores has been developed, and the gain and noise properties of individual cores have been studied. The pump and signal radiation were coupled to individual cores of MC-EDF using two tapered fiber bundled (TFB) couplers with low insertion loss. For a pump power of 146 mW, the average gain achieved in the MC-EDF fiber was 30 dB, and noise figure was less than 4 dB. The net useful gain from the multicore-amplifier, after taking into consideration of all the passive losses, was about 23-27 dB. Pump induced ASE noise transfer between the neighboring channel was negligible.

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.

Seven-core multicore fiber transmissions for passive optical network.

We design and fabricate a novel multicore fiber (MCF), with seven cores arranged in a hexagonal array. The fiber properties of MCF including low crosstalk, attenuation and splice loss are described. A new tapered MCF connector (TMC), showing ultra-low crosstalk and losses, is also designed and fabricated for coupling the individual signals in-and-out of the MCF. We further propose a novel network configuration using parallel transmissions with the MCF and TMC for passive optical network (PON). To the best of our knowledge, we demonstrate the first bi-directional parallel transmissions of 1310 nm and 1490 nm signals over 11.3-km of seven-core MCF with 64-way splitter for PON.