We demonstrate 7-core fiber transmission of 10 x 96-Gb/s PDM-16QAM signals over 1000-km using distributed Raman amplification (DRA). DRA gain of 9-12 dB and equivalent noise figure of less than 1 dB are achieved in all cores. We also prove the feasibility of high power multi-core fiber transmission with per fiber power of 6.5 W.
© 2012 OSA
Optical communication systems have long been based on single-mode fibers (SMFs) to eliminate the bandwidth limitation imposed by the intermodal dispersion of multi-mode fibers (MMFs).With dense wavelength division multiplexing (DWDM) technology and advanced modulation formats, the transmission capacity of conventional SMF now reach 100 Tbit/s, which is assumed to be the maximum value determined by the nonlinearity of silica fiber. A new approach that can overcome the capacity limit is based on space division multiplexing (SDM) [1–12]. Two concepts have been studied as regards SDM. One uses multi core fiber (MCF) [3–10] where a fiber consists of a number of single mode cores. The other is multi mode fiber [2,11,12] where each propagation mode acts as a different transmission channel. Experiments on SDM using MCFs have demonstrated large capacity as well as long distance transmission, for example, 305-Tb/s transmission  and 2688-km transmission . The crosstalk penalty design between the cores of MCF is one of the important keys for long haul MCF transmission as shown in reference , and the crosstalk tolerance is rapidly decreased in the case of higher order multi-level signal transmission . This is why most MCF transmission experiments used DP-QPSK as the modulation format.
In this paper, we realize long-haul higher order multilevel signal transmission with lower noise distributed Raman amplification (DRA) in MCF. We demonstrate the 1000-km 7-core fiber transmission of ten WDM channels, 96-Gb/s DP-16-QAM signals. Our amplification system offers high gain DRA by using newly developed low loss fan-in/out devices (FI/FOs) and pump sources for the DRA. These successfully lead to MCF transmission with DRA for the first time and confirm the feasibility of MFC transmission with the high pump power of 6.5 W per fiber.
2. Distributed Raman amplification in MCF
Fig. 1 shows the schematic configuration of a 75-km seven-core MCF transmission line with distributed Raman amplification including FI/FO and Raman pump source (RPS). We used MCFs with a trench-assisted structure and a core pitch of 49 μm to suppress crosstalk . The MCFs had cladding diameter of 195 μm. In order to achieve high Raman gain and to suppress other optical nonlinearities such as four wave mixing and cross phase modulation, the cable cutoff wavelength and the effective core area (Aeff) were set to be less than 1400 nm and to be about 110 μm2 on average, respectively. Attenuation and dispersion at the wavelength of 1550nm were 0.190–0.199 dB/km, and 20.5–20.8 ps/nm/km, respectively. The measured crosstalk of the 75-km MCFs was −65 dB at 1550 nm on average. The crosstalk of the MCFs after 1000-km propagation was estimated to be less than −54 dB by using the length dependence of MCF crosstalk . The FI/FO splits the MCF's seven cores into seven individual small diameter fibers. The core pitch and cladding diameter of the FI/FO were set to be 49 μm and 195 μm; these parameters are the same as those of the MCFs. MCFs with a total length of 75 km and the FI/FO were connected by fusion-splicing . The mode field diameter of the small diameter fibers of FI/FO was 10.6 μm, and Aeff was 88 μm2. The coupling loss between a MCF and small diameter fibers was estimated to be up to 0.08 dB at 1550 nm. The measured crosstalk of the FI from the center core to the outer cores was −61 dB at 1550 nm on average. The total crosstalk of the FI and the FO after 14 spans (1050-km propagation) was estimated to be less than −39 dB. The losses of FI and FO including the fusion splice losses were 0.5–1.9 dB and 0.7–1.6 dB, respectively. The total losses between the FI input and the FO output port including the MCF 75 km-propagation ranged from 16.0 to 18.1 dB. Loss characteristic of each core of the MCF with the FI/FO is shown in Table 1 . Each output port of the FO was connected to the Raman pump source consisting of four pump laser diodes (LDs) and fiber or planar lightwave circuit based couplers.
Fig. 2(a) shows DRA characteristics at the signal wavelength, 1552 nm, and pump wavelengths, 1424 nm and 1452 nm. Core0 is the center core, and core1-6 are outer cores. High DRA gains from 9.2 to 12.9 dB were obtained at the pumping power of 1.1 W. The following transmission experiment used a hybrid amplifier with the DRA and the erbium-doped fiber amplifier (EDFA). Fig. 2(b) shows the measured characteristics of the DRA/EDFA hybrid amplifier. The EDFAs employed two-stage amplification so as to achieve low noise figures (NF) and high output powers. The effective noise figures ranged from −1 to 1 dB at DRA gain of 12 dB.
3. Transmission experiment
Fig. 3 shows the experimental setup. Ten CW optical carriers (193.00-193.45 THz) with 50-GHz spacing were generated in the transmitter. The odd/even channels were separately multiplexed, and modulated to create 12 Gbaud 16-QAM signals. We employed a tunable external-cavity laser with linewidth of about 100 kHz as the signal light source. The remaining light sources were DFB-LDs with linewidths of several MHz. The optical carriers were modulated by a LiNbO3 IQ modulator (IQM) with nested Mach-Zehnder configuration, where the IQM was driven by two 12-Gbaud 4-level electrical signals. The 4-level electrical signals were generated by an arbitrary waveform generator driven with the pattern length of 215-1. The even and odd signals were combined with 50-GHz spacing by using an interleaver, and polarization-multiplexed to form 96-Gb/s PDM 16-QAM signals by a polarization multiplexer with a relative delay of 50-ns.
The signals were then amplified by an EDFA and input to a recirculating loop. The WDM signals were gated by the first acoustic optical modulator (AOM), and separated by a one-to-eight coupler and passed to seven recirculating loops. Each recirculating loop consisted of a 3-dB coupler, the fan-in device (FI), the MCF, the fan-out device (FO), the Raman pump source, two EDFAs, a gain equalizer, and an AOM. The FI, the MCF, and the FO were common to all loops. Each SDM channel was coupled to the corresponding MCF core by the FI. Signals after propagation were demultiplexed by the FO, and were then amplified by a DRA/EDFA hybrid amplifier. The backward-pumped DRA with pump wavelengths of 1424 and 1452 nm was used to improve the OSNR of the transmitted signal. The DRA gain was set to 9-12 dB by adjusting the launched pumping power to each core up to 1.1 W. After transmission, we selected the SDM channel (core), and the received signals were demultiplexed by an optical bandpass filter; and detected by a polarization-diversity receiver. Real and imaginary parts of the two polarization tributaries were detected by four balanced photo detectors and digitized at 50 GS/s by a digital storage oscilloscope. These data were post-processed off-line . Chromatic dispersion (CD) of the entire transmission line was fully compensated in the receiver. Fig. 4(a) and Fig. 4(b) show the optical spectra before transmission and after 1050-km transmission for core 0, respectively.
Fig. 5 depicts fiber input power dependence of the Q-factor. The average signal power launched into the MCF was about −5 dBm/ch/core. Figure 6 shows the mean values of three times measured Q-factors of the center core (core0) after 1050-km transmission. When the number of the signal launched core, N, was 1, the optical signal was launched into only the center core (core0), and there was no crosstalk from outer cores. For N of 7, optical signals were input into all cores (core0-6), and the crosstalk from the outer cores to the center core was maximum. The variation of the measured Q-factors was within 0.1 dB, and no significant Q-factor penalty due to the crosstalk components from all other SDM channels was discernible. The measured Q-factor performances of the center wavelength signal (193.25 THz) are plotted in Fig. 7 as a function of the transmission distance. We measured the transmission distances that caused the Q-factor to drop below the 7% overhead FEC limit of 8.3 dB . The constellation maps of the signal after 1050 km transmission are shown in the inset of Fig. 7. As a result, every signal could be transmitted over 1050 km. Fig. 8 shows the measured Q-factor performance after 1050-km transmission. We confirmed that the Q-factors of all ten channels of seven cores exceeded the Q-limit of 8.3 dB. Total capacity (the number of WDM channels) will be expanded by narrowing the channel spacing and by utilizing gain-flattening filters to create broadband and gain-flat DRAs.
In this experiment, total input power to the MCF was 6.5 W. Since losses of the MCFs, the FI/FOs, and pump sources were small, and the input power to each core was set to be around 1 W, which is lower than the threshold power of fiber fuse transmission (1.3 W for SMF) , none of them were damaged. From this result, it is expected that high power multi-core fiber transmission with per fiber power of 7W is possible by adjusting the input power up to 1 W and by utilizing low loss devices and fusion splices.
We have successfully demonstrated SDM transmission of 7 x 10 x 96-Gb/s DP-16QAM signals over 1000-km seven-core fiber with Raman amplification. High gain and low noise distributed Raman amplification was realized by setting the cutoff wavelength and Aeff of MCFs, and by using low loss fan-in/out devices. We also confirmed the feasibility of MCF transmission with the high pump power of 6.5 W per fiber.
We thank S. Suzuki for his encouragement, and K. Yonenaga, A. Sano, T. Kawai, K. Suzuki, Y. Hashizume, T. Mizuno, T, Takahashi and H. Takahashi for fruitful discussions and technical support. Part of this research uses results from research commissioned by the National Institute of Information and Communications Technology (NICT) entitled “Research on innovative infrastructure of optical communications”.
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