Abstract
1. INTRODUCTION
Multimode optical systems based on a wide variety of optical fibers that support modes or mode groups (MGs) with desirable characteristics have become increasingly important research topics in both classical and quantum photonic information systems. Mode-division multiplexing (MDM), which utilizes multiple optical modes in one guiding fiber core as independent data communication channels to provide high capacity density, has been widely considered one of the promising solutions to overcome the capacity crunch of conventional single-mode fiber (SMF)-based communication systems and networks [
Recently, ring-core-fiber (RCF)-based MDM systems that can stably support linearly-polarized (LP) or orbital angular momentum (OAM) mode sets have been reported to demonstrate promising potentials in improving the capacity of MDM systems with low DSP complexity [
Since the first experimental demonstration of MDM transmission over specially designed OAM fiber [
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RCF Types | Loss (dB/km) | Signal and MIMO DSP | Number | Number | Capacity | Distance | Capacity- | Refs. |
Elliptical ring core | — | QPSK, MIMO free | 6 | 1 | 0.384 | 0.9 | 0.3456 | [ |
Graded index | — | OOK, MIMO free | 2 | 1 | 0.02 | 0.1 | 0.002 | [ |
Step index | — | OOK, MIMO free | 2 | 1 | 0.02 | 1 | 0.02 | [ |
Graded index | — | OOK, MIMO free | 3 | 1 | 0.0375 | 0.36 | 0.0135 | [ |
Graded index | 0.75 | 16QAM/QPSK-DMT, MIMO free | 3 | 1 | 0.1 | 1 | 0.1 | [ |
Graded index | 0.75 | QPSK-DMT, MIMO free | 2 | 1 | 0.04 | 18.4 | 0.736 | [ |
Graded index | 1 | 8QAM, MIMO free | 2 | 112 | 8.4 | 18 | 151.2 | [ |
RIP-modulated | 0.31 | OOK, MIMO free | 2 | 1 | 0.015 | 50 | 0.75 | [ |
RIP-modulated | 0.21 | OOK, MIMO free | 2 | 4 | 0.12 | 100 | 12 | [ |
Step index | 0.3 | 16QAM, | 10 | 1 | 1.12 | 1 | 1.12 | [ |
Step index | 0.3 | QPSK, | 10 | 1 | 0.56 | 24 | 13.44 | [ |
Graded index | 1 | QPSK, | 8 | 10 | 5.12 | 10 | 51.2 | [ |
RIP-modulated | 0.31 | QPSK, | 8 | 10 | 2.56 | 50 | 128 | [ |
RIP-modulated | 0.21 | QPSK, | 8 | 10 | 2.56 | 100 | 256 | This work |
Table 1. Data Transmission of RCF-Based MDM Systems in Recent Works
The MIMO-free transmission experiments over RCFs include nondegenerate mode multiplexing [
Compared with MIMO-free schemes, MIMO-based MDM schemes can significantly increase the capacity-distance product in RCF-based systems. MDM transmissions at raw data rates of 1.12 (Tbit/s)/ over 1 km and 560 (Gb/s)/ over 24 km RCFs have been demonstrated [
In this work, by employing a specially designed and fabricated RCF with low attenuation and very low inter-MG crosstalk [
2. CHARACTERIZATION OF THE RCF USED IN EXPERIMENT
The novel RCF was specifically designed and fabricated to enable low-loss and low-inter-MG crosstalk transmission of high-order OAM MDM channels [
( | 0.7 | 1.8 | 2.5 | ||||
Attenuation | (dB/km) | 0.209 | 0.211 | 0.208 | 0.210 | ||
DGD | (ns/km) | 0 | 3.2 | 8.5 | 13.1 |
Table 2. Characteristics of the Fabricated RCF
Figure 1.(a) Cross-sectional diagram, (b) RIP, and (c) propagation loss of the RCF used in the experiment.
3. EXPERIMENTAL SETUP
The experimental setup for the OAM-MDM/WDM data transmission is illustrated in Fig.
Figure 2.Experimental setup for OAM-MDM/WDM data transmission. The intensity profiles of MGs (i)
The WDM signals are split into four branches, which are separately delayed for data pattern decorrelation. After optical amplification by erbium-doped fiber amplifiers (EDFAs), collimation, and linearly polarization, they are grouped into two pairs, each pair reflected by a phase-only spatial light modulator (SLM) for the conversion to OAM modes of , or , . Each pair of OAM modes, with one of them passing a half-wave plate (HWP) for a 90-deg rotation of polarization, is combined by a polarization beam combiner (PBC) as orthogonally polarized channels to generate the four OAM modes of , , , and . Here and represent the horizontal and vertical polarization, respectively. Then the four OAM modes of two pairs are combined together by a beam splitter (BS). After that, the combined OAM modes are converted into circular polarizations by a quarter-wave plate (QWP) and polarization multiplexed by using a polarizing beam splitter (PBS) with an optical delay path and a PBC, so the four modes of each MG of or with dual polarizations are generated and multiplexed. The eight multiplexed OAM modes are converted into circular polarizations and coupled into the 100-km RCF.
In Fig.
4. EXPERIMENTAL RESULTS AND DISCUSSION
The crosstalk between the two used MGs in the subsystem consisting of the 100-km RCF and the OAM (de)multiplexer (Mux/Demux) devices (i.e., the blue dashed box in Fig.
Input MG | Input MG | |||
Output MG | Output MG | Output MG | Output MG | |
Crosstalk (dB) | 0 | −12.495 | −11.486 | 0 |
Table 3. Measured Inter-MG Crosstalk among the Two Used High-order OAM MGs over the 100-km RCF Transmission System
After the 100-km RCF transmission, to recover the signals from the four strongly coupled mode channels within each MG coupling, modular MIMO equalization is performed for each transmitted MG. No MIMO equalization is applied between the MGs. Figures
Figure 3.The absolute values of complex tap weights of the 16 FIR filters in
To evaluate the transmission performance of 100-km RCF OAM-MDM system, BER measurements are carried out for both single-wavelength and WDM transmission scenarios. The BERs as a function of optical signal-to-noise ratio (OSNR) for all multiplexed OAM modes of MGs and at 1550.12 nm are plotted in Figs.
Figure 4.Measured BER versus OSNR of MGs (a)
Figures
Figure 5.(a) Measured BERs of all 80 channels after 100-km RCF transmission; (b) optical spectra of the 10 WDM channels at the transmitter side.
5. CONCLUSION
In this paper, using a novel RCF with low attenuation and low inter-MG crosstalk, a two-dimensional wavelength- and mode-division-multiplexed transmission experiment involving ten wavelengths and eight OAM modes belonging to two adjacent MGs and has been implemented over a distance of 100 km, transmitting 16-GBaud QPSK signals over all 80 channels. An aggregate capacity of 2.56 Tbit/s, an overall spectral efficiency of 10.24 bit/(s · Hz), and a capacity-distance product of 256 (Tbit/s) · km have been realized. To the best of our knowledge, this is the first uninterrupted single-span OAM fiber communication experiment at 100-km distance and thus achieves the highest capacity-distance product so far for OAM fiber communications. The system employs a modular MIMO processing scheme that is scalable to higher channel counts without sacrificing signal processing simplicity, which is enabled by the low inter-MG crosstalk of the RCF.
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