Single-Mode Fibers (SMFs) have been successfully exploited for long-distance optical transmission for over the decades. At the same time their capacity continuously grew by three orders of magnitudes. The growth was improved by the successive introduction of Wavelength- Division Multiplexing (WDM) technique, Polarization-Division Multiplexing (PDM) technique, and higher-order modulation formats [1] known. In fiber-optic communication, a Single-Mode optical Fiber (SMF) is an optical fiber designed to carry only light directly down the fiber - the transverse mode. These modes define the way the wave travels through space, i.e. how the wave is distributed in space. Waves can have the same mode but have different frequencies.

Eventhough the capacity of SMFs is now approaching, the limits are imposed and calculated by the combination of Shannons information theory and nonlinear fiber effects [2]. In order to continue to grow the capacity and fulfill the demands, a new dimension is now demanded and it has been suggested [3] that Space Division Multiplexing (SDM) be utilised as a technique for enhancement in capacity of the optical transmission systems. SDM has an unique advantage of not requiring any multiplexing equipment. It is usually combined with other multiplexing techniques to better utilize the individual physical channels. Space- Division Multiple Access (SDMA) is a channel access method based on creating parallel spatial pipes next to higher capacity pipes through spatial multiplexing and/or diversity, by which it is able to offer superior performance in radio multiple access communication systems.

In SDM, spatially distinct paths are used to transmit multiple channels, and if realized over a single fiber, SDM offers a significant potential for cost-, space- and energy savings [4].

SDM over a single fiber can be achieved in two different ways. The very first technique consists of using waveguides that support multiple distinct waveguide modes, such as, in a Multi Mode Optical Fiber (MMF). Earlier attempts of SDM over MMF [5]-[8] were limited in lower transmission distance and bandwidth, because the waveguide modes supported could not be selectively excited and detected and also because of the increased modal Differential Group Delay (DGD) present in the standard multimode fiber.

Most recently, the transmission distance and bandwidth has been increased and enhanced by using Few Mode Fibers (FMFs) [9]-[14] which are MMF, that support only a small and fixed number of waveguide modes. For such a larger transmission distance, significant crosstalk between all the modes offered by the FMF can be observed, and MIMO DSP for crosstalk reduction is required. Few Mode Fiber is the alternative applied in place of Single Mode Fiber as it offers a number of advantages comparitively.

The second technique to implement SDM consists of multiple spatially distinct parallel waveguides formed and are consisted inside the fiber. The simplest implementation is given by the Multi-Core Fiber (MCF), and consists of multiple cores distributed and placed across the fiber section.

In this technique, it is desirable to reduce the crosstalk between the cores so that, the individual cores can be considered as individual channels, thus greatly simplifying the communication system design and modelling. Eventhough the cores are spatially separated, achieving low crosstalk between cores for long haul transmission, can be a challenging task because the light is not completely confined and constricted in the cores of fiber.

The general method to perform SDM transmission over FMF or CCF in the presence of crosstalk or coupling between the SDM channel is shown in Figure 1.

Figure 1. NxN Coherent SDM Transmission Based on MIMO DSP

The signals from N transmitters are coupled into the SDM fiber by an SDM Multiplexer (SDM-MUX). After transmission through the SDM fiber, the received signals are demultiplexed by a SDM Demultiplexer (SDM-DEMUX) and fed into N coherent receivers. The received signals are subsequently processed using MIMO DSP.

In order to achieve the full SDM capacity gain of a factor N at high reliability (i.e. low outage), it is required that the NxN transmission channel consisting of SDM-MUX, SDM fiber, and SDM-DEMUX is to be described by a unitary linear transfer function [15]. In particular, this requires that the SDM-MUX and DEMUX is capable of exciting all the modes supported by the SDM-fiber in a selective way.

For the SDM fiber, the requirement implies negligible Polarization Dependent Loss (PDL) and negligible Mode- Dependent Loss (MDL). These conditions are fulfilled for both the FMF and CCF.

In this paper, present SDM transmission over an FMF supporting six spatial- and polarization modes, referred in the following as six-mode FMF. In order to clarify the nomenclature of the modes, Figure 2 lists the six fiber waveguide modes of the six-mode FMF according to [16] and [17] on the first column, and its relation to the Linearly Polarized (LP) mode LP₀₁ and the two-fold degenerate LP₁₁ mode listed in the forth column.

Figure 2. Relation between the LP modes and the Real Waveguide Modes HE_11x ,HE_11y , TE₀₁ , TM_21a , HE_21a , and HE_21b of the Six- mode FMF

Lp_11a and LP_11b are used to distinguish the degenerate L_p11 mode, and the suffix x and y in the indexes are used to distinguish the two orthogonal linear polarizations [2] .

The six-mode FMF allows for six independent data channels to be simultaneously transmitted at a single wavelength. The six data channels launched are polarization multiplexed into the LP₀₁ , LP_11a , and the LP_11b spatial mode, using a mode multiplexer with high mode selectivity (> 28dB).

The mode multiplexer is based on phase masks [18] , [19] fabricated in glass, which is a simple yet an effective alternative to multiplexers based on programmable spatial light modulators [10], [20].

The design offers low crosstalk and low polarization dependence. After transmission, a second mode multiplexer is used to separate the received optical field into three spatial channels that are detected by three synchronized coherent receivers with polarization diversity.

In order to recover the transmitted data, NxN MIMO DSP [21], [22] is applied to undo coupling effects occurring within the fiber. MIMO processing compensates linear impairments like dispersion, crosstalk, and DGD between modes and polarizations. The term MIMO referred here, mainly use multiple antennas at the transmitter and the receiver. In modern usage, MIMO specifically refers to a practical technique for sending and receiving more than one data signal with the same radio channel simultaneously via multipath propagation. MIMO is fundamentally different from smart antenna techniques developed to enhance the performance of a single data signal.

1.1 Basic Details of Few-Mode Fiber Used

• The FMF used in this work is based on a depressed cladding index profile with normalized frequency V≈ 5.
• The fiber is designed to guide exactly 6 polarization and spatial-modes (the fundamental LP₀₁ mode and the two fold degenerates LP₁₁ mode).
• The DGD has been kept as small as possible because any delay introduced between SDM channels has to be compensated by means of filters with correspondingly large memory as part of MIMO DSP.
• The fiber employed in this project has loss coefficient of 0.205 dB/km at 1550 nm.
• The fiber used has no significant mode-dependent loss.
• The effective areas of the LP₀₁ and LP₁₁ modes are approximately 155 and 159 µm² respectively.
• The chromatic dispersion is 18 ps/(nm km) for both LP₀₁ and LP₁₁ modes.
• The light from port is directly coupled into the LP₀₁ mode of the 6-mode FMF, and port 1 and 2 have orthog-onal phase plates inserted in their optical path and will excite the LP_11a and LP_11b spatial modes, respectively.

1.2 Experimental Setup for SDM Transmission

• The SDM transmission block diagram is shown in Figure 3.
• The source signal for the experiment is generated by modulating an External Cavity Laser (ECL) at 1560 nm wavelength and having a line width of 100 kHz.
• The signal is modulated by a double-nested LiNbO 3 Mach-Zehnder modulator using Quadrature-Phase- Shift-Keying (QPSK), where the in-phase (I) and quadrature (Q) components are driven by two independent De Bruijn Bit Sequences (DBBS) of length 212 [23].
• The use of two independent bit pattern offers the advantage of avoiding correlation effects [24] .
• Subsequently, a polarization-multiplexing stage with a delay of 12 ns generates a PDM-QPSK signal, which is followed by a noise - loading section consisting of a Variable Optical Attenuator (VOA) in front of an Erbium-Doped Fiber Amplifier (EDFA).
• The PDM-QPSK signal is split into three copies with a relative delay of 27 ns and 53 ns, that are connected to different SMF ports of the input MMUX.
• The MMUX is connected to the FMF fiber under test and a second MMUX acting as a mode demultiplexer that is used to terminate the FMF.
• The mode demultiplxed signals are then amplified using low noise EDFAs before being detected by three Polarization-Diversity Coherent Receivers (PD-CRX).
• Each PD-CRX consists of a Polarizing Beam Splitter (PBS) followed by two optical hybrids, whose output ports are terminated by four balanced receivers.
• A second ECL is used as a Local Oscillator (LO) and the resulting 12 electrical high-speed signals from the PDCRXs are captured using 3 high-speed digital oscilloscopes with 4 ports, each operating at a sampling rate of 40 GS/s.
• Each measurement consists of a total of four million samples captured using a common trigger signal.

Figure 3. Experimental set-up. VOA: Variable Optical Attenuator, PBS: Polarization Beam Splitter, QPSK-Mod: QPSK Modulator, BPF: Bandpass Filter, LO: Local Oscillator, PD-CRX: Polarization-Diversity Coherent Receiver

1.3 Block Diagram of Sections used in the Project

The block diagram i.e. Figure 4, shows the elemental design of the work being carried out to accomplish ”Space-Division Multiplexing” between the transmitter and receiver sections keeping Few-Mode Fiber as optical channel and also “Mode-division multiplexing” being carried out between various supported modes of the used optical fiber.

Figure 4. Methodology used in SDM Transmission System

The very first step in any communication system is the generation of signal as shown in Figure 5. Here, the authors using PSK Generator for the generation of signal. They also use different other generators for this purpose. The generators are present in the transmitter library of Optisystem.

Figure 5. Signal Generation

After generation of the signal, polarisation multiplexing is done leading to the generation of PDM-QPSK signal (Polarisation Division Multiplexing-Quadrature Phase Shift Keyed Signals).

Polarisation Division Multiplexing is followed by a Noise- Loading Section consisting of a Variable Optical Attenuator (VOA) in front of an Erbium Doped Fiber Amplifier (EDFA) as shown in Figure 6.

Figure 6. Polarization Mux and Noise Loading Section

Noise-Loading Section is followed by Space Division Multiplexing section, which is utilised as a technique for enhancement in the capacity of the optical transmission systems as shown in Figure 7. In SDM, spatially distinct paths are used to transmit multiple channels, which is realized over a single fiber SDM that offers a significant potential for cost, space and energy savings [25] .

Figure 7. Space Division Multiplexing Section

The PDM-QPSK signals obtained are split into three copies using splitter and followed by optical fibers with different delays. All these three signals are given as an input to the WDM-MUX. The output of the WDM-MUX is transmitted using Few-Mode Fiber (under test) acting as a channel for the overall SDM system. The signal transmitted via FMF is demultiplexed using WDM-DMUX, allowing termination of FMF under test.

Most recently, the transmission distance and bandwidth has been increased and enhanced using Few Mode Fibers (FMFs) which are MMFs that support only a small and fixed number of waveguide modes. For such a larger trans-mission distance, significant crosstalk between all modes offered by the FMF can be observed. The mode demultiplxed signals are amplified using low noise EDFAs before being detected by three Polarization-Diversity Coherent Receivers (PD-CRX), namely PD-CRX0, PD-CRX1 and PD-CRX2 as shown in Figure 8.

Figure 8. Polarization-Diversity Coherent Receivers

• From this paper, the authors conclude that Few-Mode Fiber is much efficient for information transmission compared to a Multi-Mode Fiber.
• Intersymbol interference and Bit Error Rate of the optical link or channel of the system may be easily analyzed using different components of Visualizer library of Optisystem software.
• MIMO Equalization can be used to combat ISI in wide band systems, where the group-delay spread is comparable to a symbol period.
• They also found, that long distance transmission over fibers exist even in the presence of the significantly large crosstalk between the SDM channels.
• MIMO helps in increasing the tolerance (against delay) bearing capability of optical fiber communication system.

The authors would like to thank the anonymous reviewers for their helpful comments and suggestions.