1. Introduction

Demand for large transmission capacity has been increasing exponentially due to the rapid spread of Internet services [1]. Today’s optical fiber communication systems employ time-, wavelength-, and polarization-division multiplexing to carry as much traffic as possible through conventional single-mode fibers. As these dimensions become exhausted, one must consider multiplexing techniques in other dimensions, for example, space-division multiplexing (SDM) [2,3] using multicore fiber (MCF) [4–6]. This approach is now being researched worldwide as a potential solution to meet the ever-increasing demand of capacity in optical networks. However, there are a great deal of technical challenges in the design and manufacture of high-core-count, low-loss, and low-crosstalk MCF. It is also increasingly challenging to couple signals into and out of each core in an MCF because the cores are closely spaced. Recently, impressive transmission results based on SDM using MCF have been reported [7–9]. A transmission capacity of 109 Tb/s was demonstrated in the C and L bands over a 16.8-km seven-core MCF [7], and a capacity of 56 Tb/s transmission was demonstrated in the C-band alone over a 76.8-km seven-core MCF [8].

In this paper, we present in more depth the fiber optical properties of the seven-core MCF reported in Ref [8], which exhibits low attenuation and low core-to-core crosstalk across the entire C and L transmission bands. We further experimentally demonstrate a record transmission capacity of 112 Tb/s (7 × 160 × 107 Gb/s) through SDM and DWDM over the 76.8-km seven-core MCF with an aggregate spectral-efficiency (SE) of 14 b/s/Hz. The SDM is achieved using a newly designed tapered multicore connector (TMC) fusion spliced to the MCF with a commercially available polarization maintaining (PM) fiber splicer. Each core in the MCF carries 160 107-Gb/s DWDM channels using polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) on a 50-GHz grid in the C and L bands. In addition, we study the impact of the inter-core crosstalk on the signal passing through the center core, and discuss the system implications of core-to-core crosstalk for long-haul transmission.

2. Seven-core MCF and tapered multicore fiber connector

The MCF is designed for operation in C and L bands for applications in high SE and high capacity optical networks, and consists of seven cores arranged in a hexagonal array with 9-μm core diameter and 46.8-μm core pitch (shown in the inset of Fig. 1(a) ). The cladding diameter was increased to 186.5 μm to reduce attenuation of the outer cores due to coupling to the polymer coating, which has 315-μm diameter. The cutoff wavelength of each core is ~1.44 μm, and the mode-field diameter (MFD) at 1550 nm is 9.6 μm. The dispersion and dispersion slope of each core in the MCF at 1550 nm are about 16.5 ps/km-nm and 0.06 ps/km-nm², respectively. The measured attenuation spectra for all cores are shown in Fig. 1(a). The center core has 0.23 dB/km and 0.37 dB/km at 1550nm and 1300nm respectively. The average losses for 6 outer cores are 0.26dB/km and 0.40dB/km at 1550nm and 1300nm respectively. It can be seen that the loss of all cores are low across the entire transmission window and the values of losses are similar to that of a single-core standard single-mode fiber.

 

Fig. 1 (a) Measured attenuation spectra of seven-core MCF, Inset: cross-section photograph of the MCF, (b) measured optical core-to-core crosstalk spectra of the MCF.

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Optical crosstalk, i.e. the maximum power transferred between the cores, is an important parameter for MCF transmission, and low crosstalk is necessary for long haul optical transmission. Crosstalk is determined by coupling between cores and is governed by the optical modefield distribution and propagation constant of the signals. It also depends on fiber length, fiber layout (e.g. bends and twists) along the optical links [10], as well as on the operation wavelength. The optical crosstalk spectra from the center core to adjacent outer cores were measured by scanning the optical power intensity distributions vs. position at the output endface of fiber [9] when the center core is only illuminated by an ASE broadband source (1525-1575nm). Measurements were made on a 23.5-km MCF length housed on an 18cm-diameter spool. Crosstalk is defined as the ratio of optical power detected at each of the 6 outer cores to the optical power detected at the center core when the optical signal is only launched into the center core. Figure 1(b) shows the measured optical crosstalk spectra. The maximum crosstalk is below −40dB across the entire 1525-1575nm wavelength band. The wavelength dependent characteristic of the crosstalk is consistent with the expected increased evanescent penetration through the cladding at longer wavelengths [10], where the mode effective index is smaller, and the MFD is larger. Note that compared with the case where only one core transmits a signal, the worst-case crosstalk would be 6 times greater for center core and 3 times greater for outer cores when all 7 cores carry signals simultaneously.

Connectivity to individual cores becomes very difficult due to the dense core spacing in the MCF. A fiber-based tapered multicore coupler was fabricated to match the core spacing and modefield properties of the MCF, as illustrated in Fig. 2 . The TMC was designed to minimize power coupling between cores, eliminating the coupler as an additional source of crosstalk. The measured insertion loss of each core of two TMCs ranges between 0.45 dB and 2.77 dB, and the crosstalk between cores is less than −45 dB (Table-1). High insertion loss is attributed to slight core misalignment, modefield mismatch and slight core asymmetry.

 

Fig. 2 Schematic diagram of tapered multicore fiber connectors.

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3. 112-Tb/s (7 x 160 x 107-Gb/s) SDM-DWDM transmission experiment

The schematic diagram for the 100-Gb/s SDM-DWDM transmission experimental setup is shown in Fig. 3 . The WDM transmitters consisted of 80 DFB lasers at wavelengths ranging from 1530.33 to 1561.83 nm in the C-band and 80 DFB lasers at wavelengths ranging from 1570.42 to 1603.60 nm in the L-band, with both groups of channels on the 50-GHz-spaced ITU frequency grid. In each wavelength band, two sets of 100-GHz spaced channels, corresponding to odd and even channels, were multiplexed separately by two 1x40 arrayed waveguide grating (AWG) routers. The C-band and L-band (either all odd or all even) channels were combined together using polarization maintaining (PM) couplers. The odd and even channels were modulated independently by two separate QPSK modulators, each fed with 26.75-Gb/s pseudo-random bit sequences (PRBS) with a length of 2¹⁵-1. The output from each modulator was split into two paths with a relative delay of 84 symbols (or 3.14 ns) before being polarization-multiplexed by a polarization beam combiner (PBC) to form a PDM-QPSK channel at 107 Gb/s, which supports a net data rate of 100 Gb/s after excluding a typical 7% overhead for forward error correction (FEC). The odd and even channels were then spectrally interleaved through a 50-GHz interleaver (IL) to form 160 DWDM channels. Tunable external cavity lasers (ECLs), one each for C-band and L-band, with a linewidth of ~100 kHz were used for bit-error ratio (BER) measurement on the respective channel under test. Each channel under test was switched from the DFB source to the tunable ECL source. The 160 DWDM channels were separated by a C/L band splitter into the C-band channels and the L-band channels, which were separately amplified by a C-band EDFA and an L-band EDFA, respectively. For each band, the 80 DWDM channels were split by a 1:8 power splitter, whose seven outputs were amplified by seven EDFAs. The amplified C-band and L-band channels were combined together using C/L band couplers, then launched into each of the seven cores of the MCF.

 

Fig. 3 Schematic diagram of experimental setup for 107-Gb/s SDM-DWDM seven-core fiber transmission.

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The seven-core fiber span was 76.8-km long and consisted of two spools (23.5 km and 53.3 km) of MCF, which were spliced together using a commercially available PM fiber splicer [9]. A TMC (TMC1) was used to perform space-division multiplexing by splicing it to the seven-core fiber, and a second TMC (TMC2) was used as space-division demultiplexer to couple the signal channels out of the seven cores. An optical switch (SW) was used to direct the DWDM channels from one of the seven cores to the coherent receiver. The measured total link crosstalk including TMC1, two spools of MCF (76.8-km), and TMC2 are shown in Fig. 4 . Here crosstalk is the ratio of optical power detected through TMC2 at each of the 6 outer cores to the optical power detected at center core when the WDM signals launched into the center core through TMC1. The maximum total link crosstalk is less than −36.6 dB across C and L-band. The center core worst-case crosstalk assuming all 6 outer cores carrying equal signal power is less than −30.6 dB across the C and L-band, which is sufficiently low to support long-haul (>1000 km) C + L band transmission with negligible crosstalk penalty for 100-Gb/s PDM-QPSK WDM transmission systems [11]. Note that the worst-case crosstalk in the C-band is ~3 dB lower than that in the L-band, indicating that for the same (small) crosstalk penalty, the allowable transmission distance in C-band can be doubled (to >2000km) as compared to that in the L-band if the transmission distance of system is not limited by OSNR and/or other non-linear impairments.

 

Fig. 4 Measured total span link crosstalk over C- and L-band.

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At the receiver side, the 160 wavelength channels from each core were amplified separately by EDFAs and separated by another interleaver (IL) and AWG combination, before being measured individually. The receiver was a typical digital coherent receiver consisting of a polarization-diversity optical hybrid, an optical local oscillator (OLO) using a tunable ECL, and four single-ended photo-detectors. The electrical waveforms were digitized by four 50-GS/s analog-to-digital converters (ADCs) in a real-time sampling scope. The digitized waveforms of 1-million samples each were processed offline in a computer to perform electronic dispersion compensation, polarization de-multiplexing, frequency/phase recovery, and BER measurement using typical PDM-QPSK algorithms [12].

4. Measurement results

We first assessed the impact of crosstalk on a 107-Gb/s PDM-QPSK signal after passing it through the center core when all the 6 outer cores carried same-wavelength 107-Gb/s signals with equal powers. We fixed the signal launch power for each of the 6 outer cores (P_outer) at 16 dBm, while varying the signal launched power for the center core for different measurements. This allows us to effectively vary the crosstalk in the center core, defined as the ratio between the power of the crosstalk from the outer cores and that of the signal in the center core. Figure 5 shows the in-band crosstalk penalty as a function of the ratio between the outer-core signal power (P_outer) and the center-core signal power (P_center). The penalty was obtained by finding the difference in the measured Q² values obtained with and without the presence of the outer-core signals. For a 1-dB penalty, P_outer can be 15 dB and 10 dB higher than P_center, at 1561-nm and 1600-nm wavelengths, respectively. If the transmission distance of system is not limited by OSNR and/or other non-linear impairments, the above in-band crosstalk penalty measurement results indicate that the 107-Gb/s PDM-QPSK signal could pass ~30 spans in the C-band, or ~10 spans in the L-band, with a small crosstalk penalty of ~1 dB, when the signal powers in all the seven cores are the same. The difference in the crosstalk penalty is also reasonable considering the crosstalk difference shown in Fig. 4. The above results also suggest that the inter-core crosstalk of the current MCF fiber design is sufficiently low in the C-band to support ultra-long-haul transmission over 2000 km for PDM-QPSK signals, but further crosstalk reduction is desirable for supporting ultra-long-haul transmission in the L-band, and/or for supporting higher-level modulation formats such as PDM-16QAM.

 

Fig. 5 In-band crosstalk Q-factor penalty of center-core as a function of the ratio between the outer-core signal power (P_outer) and the center-core signal power (P_center).

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For DWDM transmission, the average launch power into each core was 18.3 dBm for the C-band and 18.0 dBm for the L-band. The span losses including TMC1, two spools of MCF, TMC2, and SW ranged from 21 to 25 dB. A typical received optical spectrum of all the 160 channels is shown in Fig. 6 . The non-uniformity of the optical spectrum is due to the wavelength dependence of the optical components, including the EDFAs, used in this experiment. The measured received OSNR (0.1-nm resolution bandwidth) ranged from 25.8 dB to 29.8 dB in the C-band, and 22.6 dB to 27.3 dB in the L-band. The lower OSNR in the L-band is mostly due to the higher loss of the optical components used.

 

Fig. 6 Typical optical spectrum and OSNR measured at one output of the seven-core fiber

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Figure 7 shows the measured Q² factors for all channels and for all cores. The Q² factor is derived from the measured BER through error counting at the receiver. For those channels that yielded no errors over the sampled data records, Q² values were estimated from the variances of the constellation points. The mean BER values in the C and L bands are 8 × 10⁻⁶ and 1.6 × 10⁻⁴, respectively, which are much lower than the typical 7%-overhead FEC thresholds (~3.8 × 10⁻³). Relatively lower Q² factors in the short wavelength region of the L-band (near 1570 nm) are due to the lower OSNR, and lower Q² factors at the long wavelength region in the L-band (near 1600 nm) are attributed to the fact that the polarization-diversity optical hybrid used in this experiment was not optimized for operation in the L-band. Nevertheless, the worst BER value for the 1120 (7 × 160) measurements was 6 × 10⁻⁴, lower than the FEC threshold. Negligible crosstalk penalty was found in this SDM-DWM transmission experiment.

 

Fig. 7 Signal Q²-factor (derived from the measured BER) after 76.8-km transmission as a function of signal wavelength for all the seven cores. Insets: typical recovered signal constellations after transmission in the C-band (a) and in the L-band (b).

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5. Summary

We have presented a new seven-core fiber exhibiting low crosstalk among all cores and low loss across the entire C and L bands. We have experimentally demonstrated space-division multiplexed DWDM transmission of 1120 107-Gb/s PDM-QPSK signals over a 76.8-km seven-core fiber having low crosstalk in both C and L bands, achieving a record total capacity of 112 Tb/s (7 × 160 × 107 Gb/s) with an aggregate spectral efficiency of 14 b/s/Hz on a single fiber. We further have investigated the impact of crosstalk on a 107-Gb/s PDM-QPSK signal after passing it through the center core of the MCF when all the 6 outer cores carried same-wavelength 107-Gb/s signals with equal powers, and discussed the system implications of crosstalk properties in MCF for ultra-long-haul transmission.