ijet simulation of ultra high bit rate dwdm/pdm/sdm...

International Journal of Engineering and Technology Volume 6 No.12, December, 2016

ISSN: 2049-3444 © 2016 – IJET Publications UK. All rights reserved. 443

Simulation of Ultra High Bit Rate DWDM/PDM/SDM System Using FMF

Supporting 20 Spatial and Polarization Modes Enabling Multi-Mode Splicers

with QPSK Modulation Format

Ibraheem Abdullah Murdas and Musaddak Maher Abdul Zahra

Department of Electrical Engineering, University of Babylon – Hillah – Iraq.

ABSTRACT

Optical fiber communication is the backbone for the telecommunications infrastructure that supports the internet. Single-mode fiber

transmission can no longer satisfy exponentially growing capacity demand. It is well known that the capacity of a communication

channel cannot exceed the Shannon limit. This paper demonstrates simulation framework for few-mode fiber based space division

multiplexing (SDM) transmission system. The technique has been proposed as an option for further capacity increase of transmission

fibers. Polarization dual multiplexing (PDM) and dense wavelength multiplexing (DWDM) techniques are also used in this system to

increase total system data rate. An extra dimension that a fiber can offer for achieving more information is space. For the ultra-high

capacity need of SDM, we have proposed the FMF as SDM highest technology for obtaining ultra-high bit rate systems. The

challenge is the inter-core crosstalk of the high-order modes. The properties of modes were investigated using the uncoupled-mode

theory. In this paper, we explore the design and modeling of DWDM technique with sixteen channels over ten cores SDM/PDM

system proposed as future of ultra-high capacity optical system. The approach of adaptive multi-input multi-output (MIMO) digital

signal processing (DSP) has been proposed and demonstrated to untangle the crosstalk between the spatial modes and compensate

the DMGD. Different sub-channels are synchronously sampled, and the sampled signals from adjacent sub-channels are processed

jointly using DSP in a form of float matrix to remove different fiber impairments.

Keywords: SDM, FMF, FM-EDFA, QPSK, MIMO

1. INTRODUCTION

Since its conception and implementation; optical fiber

communication has become the backbone for the

telecommunications infrastructure that supports the internet

and various other high-volume data applications; nearly every

call we make, every text message we send, every movie we

download and every internet based application is then

converted into a ray of photons that travels through the

billions of kilometers of optical fiber that’s been deployed

around the globe [1]. In order to respond to these growing and

urgent demands for bit rates, research in optical-fiber

communications introduced a number of innovative solutions

that led to the development of high bitrate optical

transmission systems [2]. To satisfy the ever-increasing

capacity demand in optical fiber communications, both the

spectral efficiency (SE) and the data rate carried by a

wavelength channel have been increasing dramatically [3].

Space-division multiplexing (SDM) is being considered as a

promising candidate technology to dramatically increase per-

fiber capacity [4]. Per-fiber capacities of over 100 Tb/s have

been recently demonstrated by using SDM with multi-core

fibers (MCFs) [5], surpassing with ease the highest capacity

reported over single-mode fiber [6].

Space-division multiplexing (SDM) has emerged as a next-

generation technology to sustain the continuous traffic growth,

in order to keep up with the future of Internet bandwidth

requirement [7].Among SDM technologies, SDM using few-

mode fiber (FMF) transmission has been extensively explored

[8-9]. Since the conventional multi-mode fiber (MMF) is not

suitable for long distance SDM transmission because of its

very large differential mode group delay (DMGD) and more

than hundreds of spatial modes, the few-mode fiber is

developed with only the support of small number of spatial

modes at relatively small DMGD [10]. Figure (1) shows the

evolution of transmission capacity in optical fibers as

evidenced by state of the art laboratory transmission

demonstrations over the years.

International Journal of Engineering and Technology (IJET) – Volume 6 No. 12, December, 2016


Fig. 1: The evolution of transmission capacity in optical fibers

i. Architecture of SDM system

The concept of SDM was demonstrated to transmit signals

over multiple fiber cores as early as 1970, but the crosstalk

between any two cores cannot be finely controlled. Until

recently, as the predicted capacity crunch of single-mode

fiber for rapidly growing traffic, SDM technology is re-

considered as the promising way to sustain the bandwidth

requirement in future decades. On one side, the manufactural

progress of optical components, like specialized design of

large DMGD or low crosstalk MMF, fine fabrication control

of multicore or hollow-core fiber, and specialized mode

generator and so on [11].

As shown in Figure 2, N optical channels are spatially

multiplexed with mode multiplexer (MUX) and transmitted

through SDM fibers. After fiber transmission, the mode de-

multiplexed (DEMUX) is used to separate the N spatial

channels. Since most of MUX and DEMUX are still free

space devices, they can induce significant power loss and

mode coupling. There have been proposed different types of

SDM fibers including bundled SMF, FMF, and MCF. The

inter-modal crosstalk and link modal dispersion varies by the

fiber type.

Fig.2: Architecture of SDM systems

Table 1 comprehensively compared three fiber or cable types

for SDM transmission: bundled SMF, MCF, and FMF.

Because the MMF needs to support numerous spatial modes,

which requires larger fiber numerical aperture, it may have

very low loss in fusion splicing, which can save more link

budget on SDM transmission. Because all spatial modes share

the same transmission media, there is inevitable inter-modal

crosstalk between any two spatial or polarization modes.



Table 1: Comparison of different SDM technologies

Parameter Bundled SMF MCF FMF

Fiber loss Standard As low as SMF As low as

SMF

Intra-mode

nonlinearity

Standard Standard or

high

Low

Inter-mode

nonlinearity

No Low Low to

medium

Mode

coupling

crosstalk

No Medium Low to high,

can be

optimized

No. of

amplifiers

N N 1

Fusion

splicing

Easy Special splicer,

Probably high

loss

Easy, low

loss

No. of

ROADM

N N 1

DSP

complexity

Low Low to

medium

Medium to

high

Application Interconnect &

long

reach

Interconnect &

long

reach

Long reach

ii. FMF system modal

Fig. 3: Supported LP-Modes for given values of the

normalized frequency and normalized propagation constant

A few-mode fiber is similar to a multi-mode fiber but with

reduced number of modes so that each mode can be handled

with care. The fiber mode concept is presented in the

following as preliminary knowledge for the discussion

afterwards. In MDM, it has been explored other modes other

than the fundamental mode that can be supported in optical

fibers. Figure (3) shows the supported modes given the

normalized frequency and the normalized propagation

constant.

The propagation constant β of each mode can be found by

numerically solving the eigenvalue equation derived from

Maxwell’s equations for a cylindrical dielectric waveguide

[14]. The definition of β is:

…………………………

……………………………

………………………………………………………1

where 𝜆 is the wavelength in vacuum and is the

effective index of each mode. An important parameter for

optical fiber is the V number, also called the normalized

frequency, represented as [14]:



……………

………………..2

where d is the diameter of fiber core, a is the radius of the

fiber, and and are the refractive indices of the core

and cladding, respectively.

Figure 4 shows the effective index of lower order

vector modes as a function of d (bottom axis) and V number

(top horizontal axis) in a step-index (SI) fiber with

and . As shown in Figure 4,

some vector modes have similar , due to the small

index difference defined by:

Fig. 4: The effective index of lower order vector modes as a

function of d (bottom axis) and V number (top axis) in an SI

fiber

in the so-called weakly guiding case. Therefore, linearly

polarized (LP) modes, which are called scalar modes, are

usually used to represent fiber modes as an approximation of

the vector modes.

This fiber will always support the fundamental mode, the

mode, characterized by its propagation constant and the

normalized mode profile such that the power

contained in the mode

is

……………………………………………..3.

When the fiber diameter is increase to a point where the V

number, of the fiber is greater than 2.405, the fiber can guide

light in the next higher order mode, the mode,

characterized by its propagation constant and the

normalized mode profile .

The mode has a two-fold degeneracy, rotated by

illustrated in Figure 5. Fibers guide light using a high-

index core and low-index cladding, which can be intuitively

understood as by means of total internal reflection at the core-

cladding boundary.

In step-index fiber, the refractive index is uniformly

distributed across the core surrounded by a cladding with

refractive index . The propagation constant β of any guide

mode is thus bounded by where is the

propagation constant of light in vacuum.

Under the weakly-guided approximation, the vectorial modes

of the fiber can be simplified using linearly polarization (LP)

modes whose transverse field in the core is of the form:

…………

……………………………………………………….4



Where is the radius of the core a,

and p is a non-negative integer

referred to as the azimuthal mode number. For the same p

value, can take on discrete values, labeled by a non-

negative integer q corresponding to the number of zero

crossings of the field along radial direction.

The LP modes can be labeled as , each having two-

fold degeneracies in polarizations in x and y, and for p≠0 two

fold degeneracies in spatial orientations separated by a

rotation of π/p.

The total number of modes of the step index core fiber is

approximately [1][14][15]:

M≈ ……………………………………………………

……………………………………………………………....5

By definition, modes supported by the fiber are

orthonormal, i.e. [26],

………………

……………………………………………………………….6

which is the basis for mode-division multiplexing:

transmitting and receiving independent information

simultaneously in each fiber mode.The reason that the

orthogonality of modes can only be maintained in practical

application for a very short distance is because of crosstalk

among modes due to fiber imperfections, bending and

twisting as shown in Fig. 6 [16].

Fig. 6: Schematic of (a) an ideal two-mode fiber in which the two

parallel lines represent two orthogonal modes and (b) a real

fiber that has distributed cross talk [16].

iii. System Description:

Ultra-high capacity sixteen DWDM- SDM PDM systems

with ten spatial dimensions (fiber modes) are presented in this

section. By using polarization division multiplexing

technique and 10 spatial modes for 16 channels, the total bit

rate of system become (16Ch.*10 cores*40 GHz = 6.4 THz).

The block diagram of 16 DWDM –10 modes SDM-PDM

System is shown in figure (7).



The description of 16DWDM -10spatial cores SDM PDM

System can by divided into three parts:

Figure 7: Block diagram of 16 DWDM –10 cores SDM-PDM

System

a) TransmitterSection:

The simulated transmitter section of 6cores SDM system is

shown in figure (8).

Fig. 8: Transmitter section of 16DWDM -10core SDM PDM System



Sixteen signals produced with different emission frequencies

to be multiplexed by DWDM multiplexer. The transmission

structure of 16DWDM -10cores SDM system consists of two

parts, 16DWDM transmitter and 10spatial fiber cores. The

characterization of each part is shown below:

I. DWDM Transmitter:

The table of 16DWDM multiplexer parameters shown

below:

Table 2 DWDM multiplexer parameters description

Multiplexer

Parameter Value Units

Channel spacing 50 GHz GHz

Insertion loss 4 dB

Filter type BandPass

Filter order 3

Transfer function Gaussian

Gaussian Order 3

Frequency range 192.725 to 193.475 Thz

No. of channels 16

II. Spatial Dimensions (Fiber modes):

The output signals of power splitter distributed equally into

the ten spatial cores where array waveguides used to

demultiplex 16 combined wavelengths. The description of

AWG parameters is shown in table (3.18).

Table 3 Parameters description of AWG

AWG Mux/Demux

Parameter Value Units

Frequency spacing 50 GHz

Number of input/output ports 1/16 for Mux and 16/1 for Demux

Frequency of the first channel 192.725 THz

Frequency of the last channel 193.475 THz

Reference channel frequency 193.1e12 Hz

Passband type Gaussian

Model type Data sheet

Insertion losses 0 w

The demultiplexed optical signals modulated and dual

polarized by the same way as single channel system but an

array of 16 DP-mQAM Transmitters used in each mode. This

array modulated the incoming optical 16 signals with 40 GHz

bit rate with specified modulation techniques. . Then, the

sixteen polarization-multiplexed optical signals with

quadrature modulation multiplexed again by array

waveguide. Next, the ten spatial modes become ready to be

transmitted over few-mode fiber.

b) Optical fiber channel

In this section the propagation of 10 spatial modes into few-

mode fiber (FMF) illustrated. Multi-mode ideal coupler used

to couple 10 single-mode optical modes into a multimode

optical fiber. The supported LP modes of multi-mode coupler

and MMF are (LP01, LP11a, LP11b, LP21a, LP21b, LP02,

LP31a, LP31b, LP12a, LP12b) modes. The table of coupling

matrix is shown in table (4).



Table 4 Parameters description of Multimode coupler for 16 DWDM 10 modes SDM PDM system

Coupling Matrix

Port Number ModeID Magnitude Phase(deg)

1 0 1 0

2 1 1 0

3 2 1 0

4 3 1 0

5 4 1 0

6 5 1 0

7 6 1 0

8 7 1 0

9 8 1 0

10 9 1 0

The FM-EDFA amplifier array model used in system as

shown in table (5).

Table 5 EDFA Parameters Description

Parameter Value Unit

Amplifier type Gain controlled

Modes gain 15 14.5 14.5 14 14.25 14.5

14.5 14.5 14.5 14.5

dBm

Modes Noise Figure 4

Noise bandwidth 2e12 Hz

NP Modes 10



2. RESULTS AND DISCUSSION

The performance of ultra-high capacity 16DWDM 10 modes

SDM PDM system will be illustrated in this section.The

obtained results from this system are:

i. Spectrum Result:

Figure (9 shows the RF scope analyzer of 16DWDM 10

modes SDM PDM QPSK signal before and after 1550km

transmission distance at 40 Gb/s.

a

b

Figure 9: Optical OSA spectrum of 16DWDM 10cores SDM PDM QPSK signal (a) before and (b) after 1000 km transmission distance

i. Received Electrical Constellation Diagram : The received constellation diagram of the received single

channel with QPSK modulation format is presented in this

section. The following cases of constellation results will be

discussed to study the performance of system:

1.Gray mapping:

Gray codes are used in communication to minimize the

number of bit errors in quadrature phase keing

modulation adjacent points in the constellation. In a typical

encoding the horizontal and vertical adjacent constellation

points differ by a single bit and diagonal adjacent points

differ by 2 bits. The constellation diagrams for an example of

Gray code mapping for channel 1X in LP01 mode are shown

in figure (10).



Figure 10: Received constellation diagram of channel 1X LP01 (left) Without using gray mapping ,(right) using gray mapping

The obtained results show that BER of the some received

modes channels shown in table (6).

Table 6 BER using Gray coding for 16DWDM/SDM

system

2. TDE MIMO adapter:

Equalization using multiple-input multiple-output (MIMO)

techniques is required at the receivers to mitigate linear

impairments. MIMO DSP technique is very necessary

algorithm in SDM system to compensate the inter-symbol

interference ISI caused by CD and PMD. Also MIMO used to

compensate fading and DGD losses. For example, the

obtained results using TDE MIMO equalizer for LP01 are

shown in figure (11).

Figure 11: Received constellation diagram of channel1X in LP01

mode (left) without using TDE MIMO equalizer ,(right) using

TDE MIMO equalizer.

Channel per mode

no.

Without using Gray

mapping

Using Gray mapping

1X of LP01 0.0014648 0.0009765

2X of LP11a 0.0014648 0.0012207

3X of LP11b 0.0015869 0.0010986

4X of LP21a 0.0012207 0.0007324

5X of LP21b 0.0014648 0.0010986

6X of LP02 0.0008544 0.00014648

7X of LP31a 0.0014648 0.0009765

8X of LP31b 0.0015869 0.0010986

9X of LP12a 0.0008543 0.0001456

10X of LP12b 0.0014648 0.0009765



4. Input power:

In this section the received constellation diagrams for

1500Km transmission with different input power presented.

According to the self-phase modulation nonlinear effect

(SPM), the higher input power would lead to a larger

nonlinearity distortion. There is limitation in input power

values due to the damage threshold of optical fiber. The

threshold for stopping fiber-fuse signal propagation in

existing optical fiber is being studied in detail, but it is

currently known to be in the range of 1.2–1.4 W. Figure (12)

shows input power versus BER for 16 channels-10modes

SDM/DWDM/PDM-QPSK.

Fig. 12: Input power versus BER for 16 channels-10modes SDM/DWDM/PDM-QPSK

2. CONCLUSION

The problem of obtaining ultra-high capacity optical systems

becomes an important issue to be discussed. In this paper, we

designed powerful simulation system for space division

multiplexing. These system analysed and discussed to explain

its performance in presence of different losses. Our proposed

systems proved the principle of scaling capacity using SDM

in FMF in combination with MIMO signal processing. A total

achieved bit rate that can be obtained from our proposed

system is (6.4)THz with a total distance of 1500 Km. The

FM-EDFA has been demonstrated to be able to amplify

modes and can be used to control gain over large number of

modes and also easy to be fabricated. Although there is still a

long way to go towards real implementation, SDM

transmission has shown its great potential to be a promising

technology for the next generation optical network.

ACKNOWLEDGMENT

The authors sincerely express their gratitude to the great team

of VPI photonics for their great software VPI Transmission

maker v.9.5. Then we would like to thank the great team

working in the Babylon province electricity department for

their valuable support to achieve this work.

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