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Essentials of Coherent Optical

Data Transmission

The Concept of Complex Optical Modulation for

More Efficient Data Transfer

Application Note

Introduction

Data centers are being built across the globe enabling medium and smaller size enterprises to also

store and analyze big collections of structured and unstructured data in the cloud, in order to optimize

the supply chain, marketing activities and more. The storage and analysis capacities are in place but

the more critical question is if the infrastructure outside of the data centers can keep up the pace. The

explosively growing amount of data is becoming an enormous challenge for our backbone networks. If

they don't want to become the bottleneck of the future, the spectral efficiency needs to be increased in

fiber optical networks. Today, fiber optical infrastructure and signal concepts need to support data rates

of 100 Gbit/s, soon 400 Gbit/s and even higher. This is a problem for traditionally applied data coding

schemes.

03 | Keysight | Essentials of Coherent Optical Data Transmission - Application Note

The Beginning

Optical data transport started out like the electronic with the

simplest and therefore cheapest digital coding schemes, which are

`return-to-zero' (RZ) or `non-return-to-zero' (NRZ) on-off-keying

(OOK). The signal here is ideally a rectangular sequence of ones

(power-on) and zeros (power-off). This concept faced a limit when

transfer rates reached for 40 Gb/s.

At 40 Gbit/ s and above, an additional limiting factor comes into

the game. Due to the high clock rate, the bandwidth occupied by

the signal gets larger than the channel bandwidth of a 50 GHz

ITU channel. As can be seen in Figure 1, spectrally broadened

channels start to overlap with the neighboring channel and the

signals are shaped by the wavelength filters, resulting in crosstalk

and degradation of the modulated information. At the latest then,

we have to turn our back on OOK and move to more complex

modulation schemes, like differential quadrature phase shift keying

(DQPSK) for example. Complex modulation reduces the required

bandwidth, depending on the symbol clock rate, and higher data

rates can be transmitted again in the 50 GHz- ITU channel as

illustrated in Figure 1 on the example of DQPSK.

RZ or NRZ modulation

(10 Gb/s)

RZ or NRZ modulation

(100 Gb/s)

Channel interference

Simple Simple

hardware hardware

Wide spectrum high CD/PMD impairment

New modulation scheme

DQPSK modulation

(100 Gb/s)

50 GHz 50 GHz

Complex Complex

hardware hardware

Narrow spectrum lower CD/PMD impairment

Figure 1. With OOK, we face channel interference or degradation at 100 Gb/s and beyond; complex modulation schemes can solve this problem

04 | Keysight | Essentials of Coherent Optical Data Transmission - Application Note

Transmitting symbols instead of bits

The fundamental drawback of OOK methods is that on each

channel, only one bit is transferred at a unit time.

This is where complex transmission comes into the game and

demonstrates its huge potential: instead of transmitting a binary

data stream, several bits are coded to a new `symbol' and a

stream of these symbols is transmitted. Figure 2 illustrates this

for 2 bits being coded to one new symbol.

In this way, twice the amount of data can be accommodated in

the same bandwidth.

Original binary data stream

0 0 1 0 1 1 1 0 0 1 0 1 0 0 1 0 0 0 1 1 0 0 1 0

B D A D C C B D B A B D

Symbol alphabet for coding 2 bits per symbol

Figure 2. Coding concept: Use of symbols to represent a series of bits; here two bits are represented by one alphabetic symbol

Of course you can think of schemes where a much larger number

of bits are defined by a single symbol which allows reaching a

data rate many times higher than in conventional on-off keying

(OOK) where a series of ones and zeroes is transmitted.

05 | Keysight | Essentials of Coherent Optical Data Transmission - Application Note

How does this happen in practice?

In OOK the approach is basically, that when the laser source is

turned on, this is interpreted as a one, and when it is turned off,

this reflects a zero. In other terms, when the light amplitude

exceeds a certain level, this is a one and a zero when the

amplitude falls below this level.

But as a light wave is defined by more parameters than just

amplitude, we also have more possibilities to encode

information by using all degrees of freedom of a light wave. Figure

3 shows the mathematical description of the electric field of

an electromagnetic wave with two polarization components Ex

and Ey. These orthogonal components are used in polarization

division multiplexing (PDM) like two different channels to transfer

independent signals. In wavelength division multiplexing (WDM),

different frequencies are applied as different channels for

independent data transfer at these frequencies or wavelengths.

For complex modulation schemes now, additionally to the

amplitude E, the phase of a light wave is modulated for defining

the above described symbols.

Light is a transversal electromagnetic wave

Ex Ex eix i ( t - k z ) I x + iQx i ( t - k z )

E= = iy

e = e

Ey Eye

I y + iQy

Polarization division Phase

multiplexing modulation

Amplitude Frequency (wavelength)

modulation division multiplexing

Use all degrees of freedom to encode information!

Figure 3. Mathematical description of an electromagnetic wave (electric field)

06 | Keysight | Essentials of Coherent Optical Data Transmission - Application Note

How does this happen in practice? (continued)

The electric field of the modulated light wave can also be

described in the complex plane with an I/Q diagram. Here, I is the

in-phase or real part and Q the quadrature or imaginary part as

shown in Figure 4 (after removal of time and space dependency of

the wave and for one polarization plane only).

A symbol corresponds to a point, also called constellation point,

in this diagram also referred to as constellation diagram and is

defined by a Q and an I value or in polar coordinates by amplitude

E and phase . The constellation points correspond to the symbol

clock times and are also called