Cascadability of Uniform Fibre Bragg Grating for 40 Gbit/s RZ-OOK to NRZ-OOK Conversion

doi:10.4236/opj.2013.32B077

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Optics and Photonics Journal, 2013, 3, 337-341

doi:10.4236/opj.2013.32B077 Published Online June 2013 (http://www.scirp.org/journal/opj)

Cascadability of Uniform Fibre Bragg Grating for 40

Gbit/s RZ-OOK to NRZ-OOK Conversion

O. Ozolins, V. Bobrovs, G. Ivanovs

Institute of Telecommunications, Riga Technical University, Riga, Latvia

Email: [email protected]

Received 2013

ABSTRACT

The cascadability of uniform fibre Bragg grating for 40 Gbit/s return to zero on-off keying to non-return to zero on-off

keying format conversion has been shown using OptSim simulation program. The main idea of this approach is use of

specially designed uniform fibre Bragg grating with appropriate transfer function for shaping of 40 Gbit/s return to zero

on-off keying optical spectrum. Error free performance is achieved after four cascades of uniform fibre Bragg grating

with different reflectivity values.

Keywords: All-optical Devices; Fibre Bragg Grating; Fibre Optic Transmission Systems; Format Conversion; Intensity

Modulation; Optical Signal Processing

1. Introduction

In last year’s photonic technologies for optical transmission

have gained great breakthrough because of high demand

for bandwidth and need to reduce costs per every trans-

mitted bit [1,2]. To overcome the emerging challenges

various optical modulation formats have been utilized in

optical transmission depending on the bit rate, transmis-

sion distance and media [3].

Historical circumstances have made on-off keying

(OOK) in both non-return to zero (NRZ) and return to

zero (RZ) in the modulation format of choice for the

most of optical transmission systems [3]. Optical modu-

lation format NRZ-OOK is simplest in generation. The

Mach Zender modulator (MZM) is biased at 50% of

transmission and driven from minimum to maximum.

NRZ-OOK modulation format is more spectrally effi-

cient comparing to RZ-OOK and thus is appropriate for

metro and access transmission systems because of rather

short span lengths and lower bit-rates [1,3]. In turn the

RZ pulse fills a part of the bit period which can have

three different types of duty cycle: 33%, 50% and 67%.

Shorter pulses results in broad spectrum which is clear

from properties of the Fourier transform. This is the main

drawback of RZ-modulated signals resulting in a reduced

tolerance to chromatic dispersion and a smaller spectral

efficiency. Still, RZ-OOK format is preferred in transport

optical transmission systems thanks to superior tolerance

to nonlinear optical effects (NOE), inter-symbol inter-

ference (ISI) and polarization mode dispersion (PMD) [3,

4]. Interconnections among transport, metro and access

networks are necessary because of different modulation

formats in use. Optical-electrical and electrical-optical

conversion degrades system efficiency. This is mainly

related to increase of time and cost. Therefore it is im-

portant to perform all-optical signal processing [1]. This

will lead to increase of systems overall efficiency. One of

the functionalities of all-optical signal processing is

modulation format conversion in optical domain. It will

be demanded at the gateways to ensure transparent and

efficient interconnection especially for future ubiquitous

transparent optical networks.

Different approaches have been worked out for all-

optical format conversion. One part of this conversion is

related to nonlinear signal processing: semiconductor

optical amplifier (SOA)/distributed feedback laser (DFB-

LD) [5], SOA-loop-mirror [6], SOA/fibre Bragg grating

(FBG) [7], nonlinear optical loop mirror (NOLM) and

four-wave mixing (FWM) [8]. Other part is related to

linear optical signal processing: specially designed silicon

mirroring resonators (MRR) have been used for modula-

tion format conversion [9,10]. The main idea of this ap-

proach is use of optimized MRR with appropriate transfer

function for shaping of input optical spectrum. Different

approach could be based on uniform FBG [11]. In this

paper cascadability of uniform fibre Bragg grating for 40

Gbit/s RZ-OOK to NRZ-OOK conversion has been shown

using OptSim simulation program.

2. Simulation Method and Model

The OptSim 5.2 simulation program is software tool for

O. OZOLINS ET AL.

338

the design and simulation of optical communication sys-

tems at the signal propagation level. It was applied for

demonstration of 40 Gbit/s RZ-OOK to NRZ-OOK format

conversion and assessment of cascadability of uniform

FBG impact on conversions efficiency. For uniform FBG

transfer function calculation Bragg Grating Filters Syn-

thesis (BGFS) 2.6 simulation program was used. The

main results of this paper are obtained with two different

simulation tools: for physical layer simulation of optical

transmission systems and other for synthesis of physical

components transfer function depending on different

parameters. Calculation method of simulation program

OptSim 5.2 solves set of complex differential equations

and takes into consideration linear and nonlinear im-

pairments on electromagnetic signal propagation in opti-

cal and electrical components of transmission system.

In this research the time domain split step (TDSS)

method was used which despite its complexity grants a

precise outcome of simulation. This calculation method

is used in all commercial simulation tools to perform the

calculation of the electromagnetic signal propagation

equation in optical fibre:





Atz LNAtz







(1)

A (t, z) represents the optical field, L is the linear op-

erator which stands for linear impairments and N is the

nonlinear operator which is responsible for nonlinear

impairments. The equation (1) is calculated over small

spans of fibre by including either linear or nonlinear op-

erator. For instance, on the first span only L is considered,

on the second span only N operator and so on [12].

The BGFS 2.6 simulation program was used to obtain

FBG transfer function. Transfer matrix method is employed

in BGFS 2.6 simulation program to simulate different

configurations of FBG filters. It is applied to solve the

coupled mode equations and to obtain the spectral response

of the FBG filter. The transfer function was for reflection

spectra of FBG filter. The FBG parameters were opti-

mized by altering the length of grating and reflectivity

parameter. Two different transfer functions of FBG filter

reflection spectra were synthesized with uniform apodi-

zation profile and 60% and 97.5% reflectivity values. In

Figure 1 and Figure 2 uniform FBG filter reflection

spectrums with 97.5% and 60% reflectivity are shown

accordingly.

Us it is noticed two simulation programs were used in

simulations. In BGFS 2.6 simulation program calculated

reflection spectra of FBG filter was recorded in the data

file. After simple mathematical calculations compatible

data file format were created for OptSim 5.2 simulation

program. User defined optical filter block was used for

building of 40 Gbit/s RZ-OOK to NRZ-OOK format

converter based on uniform FBG. The synthesis of the

user defined optical filter is based on the Overlap and

Add algorithm [13]. At first the data points are interpo-

lated so to obtain a continuous transfer function. The

algorithms developed to implement user defined filter

component are very accurate and the transfer function is

synthesized. Then it is applied for investigation of uni-

form FBG filter cascadability impact on format conver-

sion.

3. Results and Discussion

The simulation setup for investigating of the uniform

FBG filter cascadability impact on 40 Gbit/s RZ-OOK to

NRZ-OOK format conversion is illustrated in Figure 3.

The setup consists of three parts: optical transmitter,

1549,0 1549,5 1550,0 1550,5 1551,0

-100

-50

Reflection (dB)

Wavelength (nm)

First

Second

Third

Fourth

Figure 1. Amplitude transfer function of uniform FBG op-

tical filter reflection spectra with 97.5% reflectivity after

different number of cascades shown in inset.

1549,0 1549,5 1550,0 1550,5 1551,0

-100

-50

Reflection (dB)

Wavelength (nm)

First

Second

Third

Fourth

Figure 2. Amplitude transfer function of uniform FBG op-

tical filter reflection spectra with 60% reflectivity after dif-

ferent number of cascades shown in inset.

O. OZOLINS ET AL.

339

format converter in cascade configuration and preampli-

fied optical receiver. The optical transmitter for 40 Gbit/s

RZ- OOK optical signal generations consists of two

LiNbO3 MZMs and continuous wave (CW) light laser.

The first MZM was used as a pulse carver driven by a

half clock, while the second one was driven by a 40

Gbit/s pseudo random binary sequence (PRBS) with a

pattern length of 231-1. The optical signal was then cou-

pled into the optical format converter which consists of

uniform FBG filter and the three port optical circulator.

In this investigation four optical format converters were

used to see the impact of cascadability on format conver-

sion efficiency. These converters were connected with

optical circulators in cascade by connecting third fort of

first circulator with first port of next. After each con-

verter 40 Gbit/s NRZ-OOK optical signal was detected in

a preamplified receiver. This receiver consists of erbium

doped fibre amplifier (EDFA), 100 GHz Gaussian optical

band pass filter (OBPF), 45-GHz photodiode, electrical

low pass filter (ELPF), optical spectrum analyser (OSA),

electrical scope and bit error rate (BER) tester.

Figure 4 shows the power spectrum density (PSD) of

the input 33% RZ-OOK, converted NRZ-OOK after dif-

ferent number of uniform FBG with 97.5% reflectivity

cascades and generated NRZ-OOK. Figure 5 shows bit

error rate (BER) as a function of received power for the

input 33% RZ-OOK and converted NRZ-OOK after dif-

ferent number of uniform FBG with 97.5% reflectivity

cascades. Insets: eye diagrams of back to back 33% RZ-

OOK signal and converted NRZ-OOK signals. In this

case uniform FBG with 97.5% reflectivity was applied

for the conversion of signal formats. Increased reflective-

ity for uniform FBG leads to lower insertion loss of the

device and flat top of the pass band (see Figure 1). Ac-

cumulation of insertion loss is lower and full width half

maximum (FWHM) bandwidth reduction effect is also

reduced while filters are cascaded. Due to imperfections

of the employed uniform FBG: (the side lobe minimum

values are not matched to tones of RZ-OOK signal zeros

and pass band shape is not optimized) the RZ-OOK to

Figure 3. Setup for investigating of the uniform FBG filter cascadability impact on 40 Gbit/s RZ-OOK to NRZ-OOK format

conversion.

-36 -33 -30 -27 -24-21 -18 -15-12

-log(BER)

Received power (dBm)

RZ-OOK b2b

First FBG

Second FBG

Third FBG

Fourth FBG

193,2 193,3 193,4 193,5 193,6

-30

-20

-10

PSD(dBm/nm)

Frequency (THz)

RZ-OOK

First FBG

Second FBG

Third FBG

Fourth FBG

NRZ-OOK

Figure 5. BER as a function of rec eived power for the input

33% RZ-OOK and converted NRZ-OOK after different

number of uniform FBG with 97.5% reflectivity cascades.

Insets: eye diagrams of back to back 33% RZ-OOK signal

and converted NRZ-OOK signals.

Figure 4. Power spectrum density of the input 33% RZ-OOK

converted NRZ-OOK after different number of uniform

FBG with 97.5% reflectivity cascades and generated NRZ-

OOK. Details are shown in inset.

O. OZOLINS ET AL.

340

NRZ-OOK conversion is not successful. Adding more

filters in cascade leads to better BER performance. After

third and fourth cascade the eye diagrams shows partial

format conversion, but with large amount of amplitude

ripples. These ripples could be reduced by adding addi-

tional Gaussian band pass filter. Therefore this leads to

finding that reflectivity of uniform FBG must not be high.

This also leads to drawback which is related to increased

insertion loss value.

To obtain more convincing results the additional simu-

lations was carried out with device which has lower re-

flectivity and less imperfections for transfer function.

Figure 6 shows the power spectrum density (PSD) of the

input 33% RZ-OOK, converted NRZ-OOK after different

number of uniform FBG with 60% reflectivity cascades

and generated NRZ-OOK. Figure 7 shows bit error rate

(BER) as a function of received power for the input 33%

RZ-OOK and converted NRZ-OOK after different num-

ber of uniform FBG with 60% reflectivity cascades. Insets:

eye diagrams of back to back 33% RZ- OOK signal and

converted NRZ-OOK signals. As one can see from Fig-

ure 2 uniform FBG with 60% reflectivity has not so flat

top at the pass band. After number of cascades this pass

band shape tends to be Gaussian like. Also the change in

FWHM bandwidth have been observed due to the fact,

that cascading of band pass filters leads to usable band-

width reduction. In the case of RZ-OOK to NRZ-OOK

format conversion these properties gives some benefits

and limitations. Main contribution from the cascading of

uniform FBG with 60% reflectivity could be seen in

Figure 7 eye diagrams for converted 40 Gbit/s NRZ-

OOK optical signals. After first filter eye diagram (red

colour) has amplitude ripples due to imperfections in

filter pass band shape. Still the eye diagram opening is

rather high to give error free performance. After adding

more uniform FBG filters with 60% reflectivity the eye

diagrams were with less amplitude ripples and with better

BER performance. This trend was observed till third num-

ber of cascade and mainly is caused due to Gaussian like

pass band shape which arises from cascading of the filter.

However at the fourth number of cascade the effect of

FWHM bandwidth reduction leads to over filtering of the

converted NRZ-OOK signal and decreased BER per-

formance.

Figure 8 shows the power penalty (at BER = 10-9) in-

duced by cascading of a uniform FBG versus number of

cascades for uniform FBG with 60% and 97.5% reflec-

tivity. As it can be seen from the results obtained from

BER performance for both 40 Gbit/s RZ-OOK to NRZ-

OOK cascaded converters that power penalty decreases

with the number of cascades. In the case of uniform FBG

with 60% reflectivity minimum point of power penalty is

achieved after third cascade and after more cascades it

increases significantly due to optical signal over filtering.

193,2 193,3 193,4 193,5 193,6

-30

-20

-10

PSD(dBm/nm)

Frequency (THz)

RZ-OOK

First FBG

Second FBG

Third FBG

Fourth FBG

NRZ-OOK

Figure 6. Power spectrum density of the input 33% RZ-

OOK converted NRZ-OOK after different number of uni-

form FBG with 60% reflectivity cascades and generated

NRZ-OOK. Details are shown in inset.

-36 -33 -30-27 -24 -21 -18-15-12

-log(BER)

Received power (dBm)

RZ-OOK b2b

First FBG

Second FBG

Third FBG

Fourth FBG

Figure 7. BER as a function of rec eived power for the input

33% RZ-OOK and converted NRZ-OOK after different

number of uniform FBG with 60% reflectivity cascades.

Insets: eye diagrams of back to back 33% RZ-OOK signal

and converted NRZ-OOK signals.

1234

Power penalty (dB)

Number of FBG

Reflectivity 60 %

Reflectivity 97.5

Figure 8. Power penalty (at BER= 10-9) induced by cascad-

ing of a uniform FBG versus number of cascades for uni-

form FBG with differ ent reflectivity shown in inset.

O. OZOLINS ET AL.

341

4. Conclusions

The cascadability of uniform fibre Bragg grating of 40

Gbit/s RZ-OOK to NRZ-OOK format conversion has

been demonstrated numerically for the first time. The

best BER performance is obtained after three cascades of

uniform FBG with 60% reflectivity. It has been found

that uniform FBG RZ-OOK to NRZ-OOK format con-

verter must have Gaussian like pass band which is con-

nected with rather low reflectivity value to reduce am-

plitude ripples in converted signal waveform and side

lobe minimums must be chosen to match side tones of

RZ-OOK signal for conversion.

5. Acknowledgements

This work has been supported by the European Regional

Development Fund within the project Nr. 2010/0270/

2DP/2.1.1.1.0/10/APIA/VIAA/002.

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