Universidade de Aveiro Departamento de F´ısica 2018
Telmo David
Pelicano de Almeida
T´
ecnicas e Dispositivos para a pr´
oxima gera¸
c˜
ao de
Redes de Comunica¸
c˜
oes ´
Oticas
Devices and Techniques for the Next Generation of
Optical Networks
Universidade de Aveiro Departamento de F´ısica 2018
Telmo David
Pelicano de Almeida
T´
ecnicas e Dispositivos para a pr´
oxima gera¸
c˜
ao de
Redes de Comunica¸
c˜
oes ´
Oticas
Devices and Techniques for the Next Generation of
Optical Networks
Disserta¸c˜ao apresentada `a Universidade de Aveiro para cumprimento dos requesitos necess´arios `a obten¸c˜ao do grau de Doutor em Engenharia F´ısica, realizada sob a orienta¸c˜ao cient´ıfica de Rog´erio Nunes Nogueira, Investi-gador Principal do Instituto de Telecomunica¸c˜oes da Universidade de Aveiro, Paulo S´ergio de Brito Andr´e, Professor Associado com Agrega¸c˜ao do IST e Ana Maria Sousa da Rocha, Investigadora - Instituto de Telecomunica¸c˜oes da Universidade de Aveiro.
Apoio financeiro da Fundac˜ao para a Ciˆencia e Tecnolo-gia - FCT atrav´es da bolsa SFRH/BD/88872/2012 e do FSE no ˆambito do Programa Operacional Humano (POPH) do
o j´uri / the jury
presidente / president Doutor Artur da Rosa Pires
Professor Catedr´atico, Universidade de Aveiro.
vogais / examiners committee Doutor Jo˜ao de Lemos Pinto
Professor Catedr´atico, Universidade de Aveiro.
Doutor Henrique Manuel de Castro Faria Salgado
Professor Associado, Universidade do Porto.
Doutor Jos´e Maria Longras Figueiredo
Professor Auxiliar, Universidade de Lisboa.
Doutor Manuel Filipe Pereira da Cunha Martins Costa
Professor Auxiliar, Universidade do Minho.
Doutor Rog´erio Nunes Nogueira
agradecimentos / acknowledgements
Primeiramente, gostaria de agradecer aos meus orientadores: Ao professor Rog´erio Nogueira agrade¸co toda a disponibilidade que sempre teve para me ouvir e aconselhar. Teve sempre ideias ´uteis para oferecer e palavras de encorajamento certas no momento certo. Ao professor Paulo Andr´e quero agradecer todas as oportunidades que me deu para crescer dentro do IT desde que cheguei em 2008, pela calma que sempre me conseguiu transmi-tir e pela disponibilidade constante, mesmo estando agora a lecionar fora de Aveiro. `A Ana Rocha quero agradecer todo o entusiasmo e rigor que sempre demonstrou em orientar-me, mesmo quando ainda n˜ao era de forma oficial minha orientadora.
Quero tamb´em agradecer aos meus colegas, co-autores dos meus artigos: Ao Ricardo Oliveira, por toda a ajuda que me deu na montagem de setups para grande parte das experiˆencias, pelas ideias que me deu que permiti-ram explorar novas facetas do meu trabalho. Ao Miguel Drummond, por me ter ajudado a implementar o GN model, `a Natasa Pavlovic por me ter fornecido parˆametros importantes para as minhas simula¸c˜oes. Um agradec-imento muito especial ao Ali Shappari por ter disponibilizado o seu tempo, ajuda e conhecimentos com o seu sistema de transmiss˜ao para testar o meu acoplador multicore na altura do nascimento do seu primeiro filho. Foi um gesto especial, que me permitiu publicar um paper importante na OFC, do qual nunca me esquecerei.
Agrade¸co ao Bruno Faria todo o conhecimento e suporte que facilitaram a escrita deste documento em LATEX.
Agrade¸co tamb´em ao Carlos Marques, pela ajuda, conselhos e recursos que me deu antes de eu ter iniciado o meu trabalho experimental. Sem o soft-ware em Labview que ele desenvolveu durante o seu doutoramento, n˜ao seria possvel gravar LPGs com o Laser UV.
Durante o meu doutoramento tive tamb´em a oportunidade de fundar um Student Chapter da OSA, e isso abriu-me portas para experiˆencias ex-traordin´arias. Organizei diversos eventos e conferˆencias, viajei pelo mundo inteiro e conheci pessoas incr´ıveis. Tudo isto porque os meus colegas ´Alvaro Almeida, Gil Fernandes, Andr´e Albuquerque e Vanessa Duarte decidiram fundar o chapter comigo. Estou eternamente agradecido por terem aceitado o desafio. Tamb´em quero agradecer a todos os membros que posteriormente se juntaram ao chapter e que v˜ao assegurar no futuro que mais estudantes da nossa universidade tenham acesso aos mesmos recursos e experiˆencias. Quero deixar aqui tamb´em um grande abra¸co aos meus amigos de sempre: Sabina, Daniel e Lu´ıs. Porque apesar de agora nos vermos cada vez menos, os momentos em que conseguirmos estar juntos durante o meu doutora-mento foram muito especiais para mim.
Palavras-Chave Comunica¸c˜oes ´Oticas, Redes de Longo Per´ıodo, Multiplexagem Espacial, Fi-bra de M´ultiplos M´ucleos, Otimiza¸c˜ao de Parˆametros de Entrada em Redes com Compensa¸c˜ao da Dispers˜ao.
Resumo O trabalho descrito nesta tese lida com o estudo de novos conceitos e fer-ramentas para a otimiza¸c˜ao da capacidade e eficˆencia energ´etica das redes em fibra ´otica, quer sejam elas as redes que j´a est˜ao instaladas por todo o mundo ou as redes da pr´oxima gera¸c˜ao. Um novo m´etodo para otimizar dois parˆametros de transmiss˜ao em redes com mapeamento de dispers˜ao ´e apre-sentado. Um algoritmo foi desenvolvido para para calcular os valores ideais de pr´e dispers˜ao e potˆencia. Os resultados desta nova metodologia foram comparados com os de metodologias aplicadas por um software comercial. Neste caso de estudo,efectuado para uma rede de fibra ´otica submarina, os resultados da nova metodologia foram obtidos mais rapidamente, preser-vando a exactid˜ao dos mesmos. As redes de longo per´ıodo (LPGs) s˜ao propostas nesta tese como meio para produzir componentes passivos para as redes da pr´oxima gera¸c˜ao, que usam o conceito de multiplexagem espa-cial e fibras com m´ultiplos n´ucleos, redes essas que apresentam uma maior capacidade de transmiss˜ao por fibra e usam menos componentes. Um es-tudo comparado trˆes t´ecnicas de inscri¸c˜ao em fibras standard monomodo ´e apresentado. Uma t´ecnica melhorada para gravar LPGs foi proposta. Com esta t´ecnica ´e poss´ıvel gravrar LPGs com picos de atenua¸c˜ao de -25 dB e perdas induzidas pela polariza¸c˜ao inferiores a 2 dB, com uma alta taxa de reprodutibilidade, com os valores entre os picos de ressonˆancia de v´arias amostras a apresentarem uma diferen¸ca m´axima de 1 nm . O uso de uma LPG para promover transferˆencia de potˆencia ´otica entre dois n´ucleos de uma fibra com quatro n´ucleos foi demonstrado experimentalmente com um sinal modulado a 200 Gb/s DP-16QAM.
Keywords Optical Communications, Long Period Gratings, Space Divisision Multiplex-ing, Multicore Fiber, Launch Parameter Optimization in Dispersion Man-aged Networks.
Abstract The work described in this thesis deals with studying new concepts and developing tools for improving the capacity and efficiency of optical net-works, whether they are the existing networks that already span the globe or those that will be deployed in the future. A new method for optimizing two key launch parameters in legacy dispersion managed (DM) networks is presented. An algorithm was developed to calculate optimum pre-dispersion and launch power values. The results of this methodology were compared to those of a commercial software. For this case-study, in a submarine link scenario, the new method is found to be faster while preserving accuracy. Long period gratings (LPGs) are proposed in this thesis as the foundation for producing passive devices that will operate in future networks that use the concept of space division multiplexing (SDM), which uses new types of fibers such as multicore fibers (MCFs), reducing components and upgrading the capacity per fiber. A study comparing three LPG inscription techniques in both standard single mode fibers (SSMFs) and (MCFs) was performed. An improved technique to inscribe LPGs in SSMFs was proposed. With this technique, LPGs can be produced with a maximum difference between resonant wavelength values of less than 1 nm. Furthermore, it is possible to inscribe LPGs, with attenuation dips of -25 dB while at the same time obtaining polarization-dependent losses as low as 2 dB. The use of a LPG to promote selective coupling between two cores of a 4-core fiber was ex-perimentally demonstrated. The device was successfully tested for a 200 Gb/s DP-16QAM signal transmission.
Contents
Contents i
List of Figures iii
List of Tables vii
List of Acronyms ix
1 Introduction 1
1.1 Framework and Motivation . . . 1
1.2 Thesis Outline . . . 5
1.3 Objectives . . . 5
1.4 Main achievements . . . 5
2 Grating Inscription in Standard Single Mode Fibers 7 2.1 Fiber Gratings . . . 7
2.2 Long Period Grating Inscription Techniques . . . 9
2.2.1 The Ultraviolet Laser Point-by-Point Method . . . 10
2.2.2 The Electric Arc Discharge Method . . . 12
2.2.3 The CO2 Laser Method . . . 17
2.3 Summary . . . 23
3 Space Division Multiplexing and Long Period Grating Inscription in Multi-core Fibers 25 3.1 Space Division Multiplexing . . . 25
3.1.1 Main Challenges and Opportunities in Multicore Fiber Technology . . 27
3.2 LPGs in Multicore Fibers . . . 30
3.3 Inscription of Long Period Gratings in Multicore Fibers . . . 30
3.3.1 UV Laser Method . . . 32
3.3.2 The Electric Arc Discharge Method . . . 35
3.3.3 The CO2 Laser Method . . . 40
3.4 Summary and final chapter conclusions . . . 42
4 Launch Parameter Optimization for Dispersion Managed Long-Haul Opti-cal Links 43 4.1 Signal Propagation in Standard Single mode Fiber . . . 43
4.3 Historical Perspective of optical Link Deployment and Modeling . . . 46
4.4 Optical Link Modelling Methodologies . . . 48
4.4.1 Commercial Software methodology . . . 49
4.4.2 Perturbative Models and the GN Model . . . 50
4.5 Pre-dispersion Optimization Tool . . . 54
4.6 New Methodology: Validation Case Study . . . 55
4.6.1 Scenario and Simulation Description . . . 55
4.7 Results and Discussion . . . 58
4.8 Summary . . . 62
5 General Conclusions and Future Work 65 5.1 Conclusions . . . 65
5.2 Future Work . . . 66
List of Figures
1.1 Projection of the growth of worldwide IP Traffic [6] . . . 2 1.2 Transmission capacity per optical fiber in research and commercial systems
from 1980 to 2030 [8]. . . 3 1.3 Capacity and distance over several transmission experiments using SDM fiber.[16] 4 2.1 Illustration of a cross section of a fiber inscribed with a FBG and the
corre-sponding diffraction effect on an input signal. . . 8 2.2 Transmission spectra of an LPG inscribed with a CO2 laser. Grating period
Λ = 800µm, grating length L= 14.4 mm. . . 9 2.3 Photograph of the hydrogenation system. . . 11 2.4 Schematic of the hydrogenation system. A, B, C and D are hand valves. C is
the purge valve. . . 11 2.5 Photograph of the UV Laser Setup. . . 13 2.6 Schematic diagram illustrating the fiber point-by-point method. . . 14 2.7 LPG inscription with the UV point-by-point technique and a table listing the
corresponding parameters. . . 14 2.8 Illustrative Diagram of the electric arc system setup used to inscribe LPGs. . 15 2.9 Photograph of the components of the electric arc system setup used to inscribe
LPGs. . . 15 2.10 Microscope image of an irradiated fiber used to determine the exposure length
LPGs. . . 15 2.11 LPG peak evolution with the number of electric arc discharges applied (”shots”)
and a table listing the fabrication parameters. . . 16 2.12 Optical path of the laser beam of the LZM-100 (top) and top view of the main
chamber of the LZM-100 (bottom). . . 18 2.13 Diagram of the programming steps used for LPG inscription. . . 18 2.14 Images of a fiber after irradiation with tension of: a) 555 µm ; b) 1800 µm . . 19 2.15 Transmission spectra of three different LPG samples with different periods:
700 µm, 750 µm, 800 µm and the corresponding list of fabrication parameters. 19 2.16 LPG transmission spectra of three different samples inscribed with the a period
of 700 µm, but different lengths (number of points). . . 20 2.17 Beam heating profile in the fiber and correspondent supergaussian fit,
esti-mated thought a fiber heating image (inset) obtained with the LZM cameras. The black dots represent the collected data points and the red line a super-gaussian fit. . . 21
2.18 LPG transmission spectra of three different samples inscribed with a period of 950 µm. . . 21 2.19 Transmission and PDL spectra of a LPG with a period of 725 µm. . . 22 2.20 LPG PDL spectra off the samples of figure 2.18. . . 22 2.21 Near-field profiles of the output of a 950 µm LPG at 1580 nm (a) and at the
resonant wavelength of 1542 nm (b). . . 23 3.1 The pictures of the end-faces of several different types of MCF geometries. 1)
19-core trench assisted MCF [48]; 2) 8-core ring-geometry trench assisted MCF [49]; 3) 7-core trench assisted MCF [50] ; 4) 7-core MCF from Fibercore. . . 26 3.2 Picture of the end-face of the two types of fibers used in the work described
in this thesis. Image and parameter tables provided by Fibercore in the fiber’s data-sheets. . . 27 3.3 Direct coupling methods using a tapered cladding (a) and a waveguide
mod-ule(b). (c) Free space optics, an indirect coupling method. (adapted from [56]). (d) butt coupling method . . . 29 3.4 Transmission spectra measured in all cores of a 4-core MCF connected to a
fan-in-fan-out device. This characterization was performed on the original fiber spool (50 m) (a) provided by the manufacturer and after the fiber was cut in the middle of the spool and spliced again with the LZM-100 (b). . . 29 3.5 Illustration of the phenomenon of optical power coupling between two cores of
an MCF with identical LPGs. . . 31 3.6 Simulated transmission spectra measured in all cores of a 4-core MCF with an
identical LPG inscribed in all cores. The optical power inserted in the input core is equally distributed in all cores of the fiber [18]. . . 31 3.7 The spectra of a long period grating inscribed in a 4 core MCF, in this case
MCF1. The UV point-by-point method was used. The exposure length and time were 0.16 mm and 10 s respectively. . . 32 3.8 The lens effect simulated in COMSOL Multiphysics with the ray optics module.
The fiber acts as a cylindrical lens and focuses the incoming UV light from the laser. . . 33 3.9 The spectra of a long period grating inscribed in a 4 core MCF using the UV
point-by-point technique and the defocused beam method, in this case MCF2. 34 3.10 Images taken with a CCD camera of the end-face of the fiber with the grating
from fig.3.7 when light with 1580 nm wavelength is inserted in each core (1, 2, 3 or 4) of the fiber individually. The exposure in the CCD is saturated to show residual optical power coupling. . . 34 3.11 Images taken with a CCD camera of the end-face of the fiber with the grating
from fig.3.7 when light with 1480 nm wavelength is inserted in each core of the fiber individually. . . 35 3.12 The spectra of a long period grating inscribed in a 4 core MCF using the
electric arc method. MCF 1 was used and all the used fabrication parameters. 36 3.13 Transmission spectra measured in all cores when light is being injected into a
specific core. MCF1 was used. All the parameters are listed on table 3.1. . . 38 3.14 Schematic representation of the LPG and the end face of the used multicore
3.15 CCD images of the output end-face of the fiber when light is being injected into a specific core. MCF1 was used. All the parameters are listed on table 3.1. In the top row a signal with 1555 nm wavelength is used. In the bottom row a signal with 1580 nm wavelength is used . . . 39 3.16 Schematic of the experimental transmission setup used to test the selective core
coupler. . . 39 3.17 OSNR vs BER curves for back to back, all the direct detection configurations
and also for coupling (3-1) . . . 40 3.18 Transmission spectra of an LPG inscribed with the LZM-100 in a MCF
rotat-ing. The parameters are listed on the table displayed on fig. 2.15. The period is 950 µm. . . 41 3.19 Transmission spectra of an LPG inscribed with the LZM-100 in a MCF (no
rotation). The parameters are listed on the table displayed on fig. 2.15. The period is 950 µm. . . 41 4.1 Comparison of total dispersion in several fiber types (adapted from www.thorlabs.com). 45 4.2 Map of all active undersea optical cables as of February 2017. Taken from
www.cablemap.info under the GPLv3 license. . . 47 4.3 Schematic of the link topology of DM (top) and UT (bottom) links. DM links
use either hybrid links, DC modules at amplifier locations, or a combination of both. . . 47 4.4 Example of a Q factor vs Launch power per channel plot obtained by the
commercial software. . . 49 4.5 Flowchart detailing the key steps of a launch parameter optimization task using
the commercial software. . . 50 4.6 Flowchart detailing the key steps of an optimal launch power optimization task
using the GN Model. . . 52 4.7 Example of a spectra of a typical WDM signal used in simulations. It has 9
WDM channels, ∆f =100 Hz and Rs=31.25 Gsym/s. . . 52 4.8 Flowchart describing the newly proposed dispersion optimization algorithm. . 54 4.9 Depiction of the simulated loop. The base link consists of 10 loops of a hybrid
span (LMF + HDF ) and a single span of NZDSF. For simulation of greater distances, the base link was looped N times, up to 4 times. . . 56 4.10 Dispersion map of the simulated scenario for 1540 nm , 1550 nm and 1560 nm,
which were tested wavelengths in the simulations. . . 57 4.11 The ideal dispersion map for 2423.2 km calculated by the proposed algorithm. 58 4.12 Comparison of predicted optimum launch power per channel vs tested distance
between the GN model and the commercial software for three distinct wave-lengths: 1540 nm (top), 1550 nm (middle) and 1560 nm (bottom). . . 59 4.13 Pre-Dispersion sweeps performed in the commercial software for 1540 nm, 1550
nm and 1560 nm for 605.8 km, 1211.6 km, 1817.15 km and 2423.2 km. The dash-dotted line marks the optimum pre-dispersion value predicted by the de-veloped algorithm. . . 61
4.14 Pre-Dispersion sweeps performed in the commercial software for 1560 nm and a distance of 2523.2 km, for OLP (left) and OLP + 2 dB. The dash-dotted line marks the optimum pre-dispersion value predicted by the developed algorithm. The highlighted area marks a ± 0.1 Q factor band around the point where the prediction line intercepts the plot of the sweep performed by the commercial software. . . 63
List of Tables
3.1 List of parameters of an LPG inscribed in MCF1 with the electric arc method. The LPG Transmission spectra of all cores is displayed in figure 3.13. . . 37 4.1 Parameters Required to Characterize the Spans in the GN Model. . . 51 4.2 Parameters Required to generate the signal spectrum in the GN Model. . . . 51 4.3 Physical parameters of the three fiber types used in the simulations. . . 56
List of Acronyms
AWG Arrayed Waveguide Grating
BER Bit Error Ratio
DCF Dispersion Compensated Fiber
DM Dispersion Managed
DMGD Differential Modal Group Delay Dispersion
DSF Dispersion Shifted Fiber
ECL External Cavity Laser
EDFA Erbium Doped Fiber Amplifier
FBG Fiber Bragg Grating
FMF Few Mode Fiber
FWM Four Wave Mixing
GN Gaussian Noise
HDF High Dispersion Fiber
IP Internet Protocol
LMF Large Mode Field Diameter Fiber
LPG Long Period Grating
MCF Multicore Fiber
MMF Multimode Fiber
NLI Nonlinear Interference
NZDSF Non Zero Dispersion Shifted Fiber
ODL Optical Delay Line
OLP Optimal Launch Power
ONA Optical Network Analyzer
OSNR Optical Signal to Noise Ratio
PBC Polarization Beam Combiner
PDL Polarization Dependent Loss
PSD Power Spectral Density
RDF Reverse Dispersion Fiber
SDM Space Division Multiplexing
SOP State of Polarization
SSMF Standard Single Mode Fiber
UT Uncompensated Transmission
UV Ultraviolet
WDM Wavelength Division Multiplexing
Chapter 1
Introduction
This chapter discusses the motivation and objectives of the work developed during this thesis. The idea was to explore two different concepts of advancing optical network technolo-gies. On one side, there is an impending need for a new type of fiber technology that will enable the optical networks of the future. These new fiber links will boast more traffic capac-ity and unprecedented energy efficiency. On the other side, there are thousands of kilometers of already installed fiber networks, which are costly to replace and will have to be a part of these new optical networks, albeit with critical optimization, supported by fast but yet accurate modeling tools that will constantly optimize the parameters of the optical signals launched into them.
1.1
Framework and Motivation
Since Nobel laureate Charles Kao demonstrated that optical fibers could be used to carry vast quantities of information over large distances [1], optical fiber technology never stopped advancing. Today, in the telecom industry, optical fiber is the unmatched physical media of choice, the key element that defined our era as the information age. No other medium matches its capacity of carrying such high data rates, cost effectiveness, reliability and energy efficiency [2].
The success of optical fibers was sustained by a series of key innovations, such has the erbium doped fiber amplifier (EDFA), new modulation formats and multiplexing schemes, which themselves propped up new technologies that continued the evolution of the networks and the generation of more demand. Figure 1.1 shows the rising trend on consumer internet protocol internet protocol (IP) traffic. Today, the combination of an expanding broadband internet user base and new technologies such as ultra-high definition video streaming and cloud computing keeps pushing on the limits of data transfer in the telecommunications in-frastructure. At the current rate of growth in traffic demand, it is expected that by the year 2029 optical networks will have to boast a capacity increase of five orders of magnitude [3]. As this demand grew over the years, technological advances around standard single mode fibers (SSMF) pushed the capacity over 100 Tb/s [4]. These efforts swayed towards exploring every available degree of freedom available in SSMF systems such as time, wavelength and polar-ization multiplexing, while taking advantage of the most advanced digital signal processing (DSP) techniques. Over the past forty years technological advances kept up with demand, not only by allowing capacity-per-fiber to increase around ten times every four years (as shown
in figure fig.1.2), but also by doing it while mitigating costs via component reduction. Also, operational costs were kept down thanks to improvements that mostly consisted of equipment upgrades performed solely at the network end nodes (transmitters and receivers) [5], avoiding the cost-heavy operation of fiber replacement or installation.
2015 2016 2017 2018 2019 2020 60 80 100 120 140 160 180 200 P e t a b y t e p e r m o n t h Year
Figure 1.1: Projection of the growth of worldwide IP Traffic [6]
The work discussed in this thesis deals with studying new concepts and developing tools for improving the capacity and efficiency of optical networks, whether they are the existing networks that already span the globe or those that are yet to be deployed. The aim is to reduce the cost per bit, the energy per bit and the number of required components [7].
Despite the successive improvements made over the years, the limitations for systems based on the widely used SSMF started to surface when higher power budgets and bitrates were required. Non-linear phenomena become more prominent and optical signal to noise ratio (OSNR) approaches the Shannon limit, where the bit-error ratio (BER) reaches intolerable levels [3]. At this limit, state-of-the-art DSP cannot recover the signal. There are also the issues of the fiber fuse effect and the bottleneck of amplifier bandwidths.
In the urgency of coming up with a solution for drastically increasing the capacity of optical networks, researchers started to explore the concept of space division multiplexing (SDM). SDM is the concept of exploring more than one single spatial channel in a single fiber [3, 5, 9]. Currently there are two different fiber technologies that seem to be promis-ing candidates: Multicore fibers (MCFs), consistpromis-ing of fibers with multiple spromis-ingle-mode cores (up to 30 [10]) arranged in a wide variety geometries, and few-mode fibers (FMFs), which are multimode fibers that only propagate a few number of modes. Currently, The National Institute of Information and Communications Technology (NICT), Sumitomo Electric Indus-tries and RAM Photonics set the new world record fiber-capacity of 2.15 Pb/s [11], using a 22-core MCF. The distance record was recently set by Turukhin et.al [12] by demonstrating that a 9-core fiber could be used for transoceanic grade transmission of over 14000 km. More recently there are demonstrations of a combination of these two technologies in the same fiber: few-mode-multicore fiber transmission experiments [13]. Figure 1.3 shows an overview
Figure 1.2: Transmission capacity per optical fiber in research and commercial systems from 1980 to 2030 [8].
of several experiments made over the years, their achived transmission distances, capacities and type of fiber used.
Since one SDM fiber can carry much more information than one SSMF fiber, SDM net-works have the potential to be more efficient, meaning that they will operate with much less components than SSMF fiber networks. The development of passive components such as filters, routable add drop multiplexers ( ROADMs) and others could further lower these networks energy consumption. One very promising example of this component reduction phi-losophy that occurs in SDM networks is that of the number of pump lasers used in MCF amplification schemes. In traditional SSMF systems a single pump is used for each fiber. A direct conversion to an MCF system would mean that the number of required pumps would be equal to the number of cores or modes in the MCF, but this would not result in any en-ergy saving. Instead, there is a concerted effort in research dedicated to amplification schemes that reduce the number of pumps in both EDFA and Raman amplification schemes for MCFs. This is still work in progress, and so far the proposed solutions only managed to reduce the number of pumps in half using complex free space optics setups or employ cladding pumping, which is still a form of very inefficient and cumbersome form of amplification, as the energy of a highly powerful multimode pump is attenuated.
With the intent of contributing to the important topic of passive component development for SDM and further network optimization through component reduction, this thesis explored the concept of Long Period Gratings (LPGs). In SSMFs, depending on the characteristics (length of the grating, induced refractive index modulation) of the grating these devices operate by coupling light with a specific wavelength from the forward propagating core mode to a cladding mode of the fiber with very high attenuation, essentially producing a wavelength filter. These LPGs in SSMF fibers are already present in several applications for sensing (in diverse types of fibers) and passive operation in SSMF networks, with devices such as low back-reflection filters and gain equalizers [14, 15]. However, in LPGs inscribed in MCFs with
multiple cores, the light that is coupled to cladding modes can be coupled to the forward propagating modes of other cores, provided that these cores have identical LPGs inscribed in them. This new paradigm can potentially be used for producing a new host of relevant in-fiber passive devices specifically tailored for SDM operation. Also, one new and very promising application of LPGs in MCFs consists in developing an amplification scheme where the light of a pump laser is injected in one of the cores of an MCF with identical LPGs inscribed in all the cores. The grating could be tailored to couple light with the pump wavelength to the remaining cores of the fiber, representing a highly efficient form of amplification.
Figure 1.3: Capacity and distance over several transmission experiments using SDM fiber.[16] Regardless of what new types of fibers will be installed in the future, there will be always a vast network with legacy optical links which are too expensive to replace and need constant revisions and updates to keep up with demand. These links are mainly submarine links or metropolitan links located in heavily populated areas that were installed between 1993 and 2009 [2]. They need to be improved exclusively at the transmission and reception ends. These types of links are denominated as dispersion managed (DM) links, as they consist in hybrid fiber spans with balanced negative and positive dispersion values. DM links are no longer considered for future higher capacity optical networks, as new, uncompensated transmission (UT) links that use DSP to compensate for dispersion in the electronic domain are becoming the norm. Dispersion pre-compensation and optimal launch power are two key parameters that need to be optimized when testing new modulation formats, bitrates and channel capacity upgrades in existing DM links. This type of optimization and design planning tasks emphasize the need to have accurate and reliable simulation tools, that can quickly predict what values of power and pre-dispersion need to be launched in each individual scenario [17]. There have been significant advancements in modelling UT links, with a new model, the Gaussian Noise Model, but DM link optimization still relies mainly in computationally intensive and time consuming numerical methods. The work discussed in chapter 2 of this thesis aims to use the recently successfully developed UT models for this types of DM links, in order to decrease computational effort while preserving model accuracy.
1.2
Thesis Outline
This thesis is divided in 5 chapters. In this first chapter the work is contextualized, the main objectives are presented, as well as the main contributions.
In Chapter 2, the inscription of Long Period Gratings in Standard Single Mode fibers is discussed. After a brief theoretical introduction on both fiber Bragg Gratings (FBGs) and LPGs, several recording methods for LPGs are discussed, with a particular emphasis on the three methods that were employed in the course of this thesis: ultraviolet (UV) Laser, electric arc discharge, and the carbon-dioxide (CO2) laser methods. Finally, a new, completely automated technique to inscribe reproducible Long Period Gratings using a CO2Laser Splicer is presented at the end of this chapter.
In chapter 3 the concept of SDM is firstly introduced, with a particular emphasis on MCFs. The potential applications of using MCFs with LPGs inscribed are explained. An experimental report on inscribing LPGs in multi-core fibers trough three different methods (UV, CO2laser radiation, as well as electric arc discharge) is then presented. The chapter also includes an application showcase of an experimental demonstration of selective core coupling in MCFs of a 200 Gb/s DP-16QAM signal.
Chapter 4 deals with methods for optimization of legacy Long-Haul Optical Links. Basic concepts on both signal propagation in SSMFs fibers and in dispersion management fibers are firstly introduced. A brief historical perspective on optical link deployment and modelling precedes a sub chapter explaining the current state of affairs in non-linear fiber propagation models, both from a commercial software (Optiworks Optysystem) and research standpoints. The development of a new pre-dispersion optimization tool is then explained. A case-study combining the use of state-of-the art propagation model for UT links, the Gaussian noise model, and the new pre dispersion optimization tool is presented. The aim of the case-study was to validate a new methodology that combines booth the Gaussian Noise Model and the pre-dispersion optimization tool, comparing it with a conventional methodology the commercial software.
Conclusions and a discussion about future work take place in chapter 5.
1.3
Objectives
The main objectives this thesis aimed to accomplish were:
• To demonstrate and further validate experimentally the concept of optical power cou-pling between the cores of a MCF using Long Period Gratings.
• To improve Long Period Grating inscription techniques in both SSMF and MCF. • To study and develop new methods of decreasing computational effort for modeling
long-haul optical networks, without compromising accuracy.
1.4
Main achievements
• Demonstration of distributed optical power coupling between all the cores of a 4-core multi-core fiber, by using identical and uniform inscribed Long-Period Gratings in all the cores of the fiber [18].
• The first experimental assessment of the inscription of Long Period Gratings in multi-core fibers was performed and reported [19].
• A new automated technique to inscribe reproducible Long Period Gratings using a CO2 Laser Splicer was developed [20].
• Selective Core Coupling was experimentally demonstrated for the first time in Multi-core fibers [21].
• A new method for optimizing two key launch parameters, input power per channel and pre-dispersion, was presented and its predictions were validated by comparing them to those made by a commercial simulation tool [17, 22].
Chapter 2
Grating Inscription in Standard
Single Mode Fibers
In this chapter, theoretical aspects about FBGs and LPGs will be covered, as well a brief description and comparison of three different LPG recording techniques used on the course of this thesis: UV laser irradiation, electric arc discharge and CO2 laser irradiation. One of the objectives this thesis was to study and record LPGs in MCFs for SDM applications. In order to do so three techniques were first studied in SSMF. The objective was to access and optimize each of the techniques capabilities.
2.1
Fiber Gratings
FBGs were first proposed by Hill in 1978 [23], after discovering the phenomenon of fiber photosensitivity. Hill discovered that by exposing the fiber with Argon laser radiation, its refractive index could be changed. FBGs consist of an induced, periodic variation of the refractive index of the core of an optical fiber along a small portion of its longitudinal axis. This periodic structure, as well as its effect is represented in fig.2.1. The resonant wavelength is defined by the Bragg condition of the grating, condition which depends on two key grating characteristics: grating period, Λ, and refractive index modulation ∆n. A single peak in the reflection spectrum represents coupling of light from the forward propagating mode to the backward propagating mode in the fiber. Typically, in order to produce an FBG, UV light is used to expose the fiber, and since SSMF is not originally photosensitive to UV light, the fiber is either doped with more germanium/boron or loaded with hydrogen in a pressurized chamber in order to increase its photosensitivity. In telecommunications FBGs are important not only as wavelength filters, which are used in optical communications for add/drop multiplexing, but also for other important tasks such as amplifier gain flattening [24] and dispersion compensation [23]. Even more substantially, FBGs are responsible for an entire new field of optical sensors, as the resonant wavelength is very sensitive to several of external stimuli such as temperature, humidity, pressure, stress, etc. [25].
At the time LPGs were first proposed by Vengarskar [24], they were introduced as a cheaper, more sensitive and resilient to higher temperature form of an optical sensor. As the name indicates, LPGs have periods in the range of hundreds of micrometers [26], which are longer than those of FBGs (which are typically hundreds of nanometers [27] in length). LPGs can be obtained by modulating the refractive index of the core, cladding or the whole
Figure 2.1: Illustration of a cross section of a fiber inscribed with a FBG and the corresponding diffraction effect on an input signal.
fiber. This means that LPGs can be fabricated with other types of radiation other than UV, such as CO2 laser radiation (10.6 µm) [28], electric arc discharge [29] and even mechanical pressing [30]. All of these methods, even UV-laser based methods, can be potentially cheaper and simpler, as they do not need the interferometer or phase mask based techniques required to produce the smaller grating pitches required for FBGs.
The phase matching condition (Bragg condition) can be approximated by the following relation:
β1− β2= 2π
Λ (2.1)
where β1 and β2 are the propagation constants of modes 1 and 2 respectively. Considering a fiber with the propagation constants of the guided modes, LPlm, denoted by βlm. For the case of an FBG, coupling occurs from the cores forward propagating mode to a backward propagating mode:
2π
Λ = β1− β2= β01− (−β01) = 2β01 (2.2) This addition of propagation constants requires a short period grating. For the case of an LPG, coupling occurs between the forward propagating core mode and an equally forward propagating cladding mode, turning the operation into a subtraction This means that 2π/Λ is very small, and hence high values for Λ, typically between 100 µm and 1000 µm [31]. LPGs couple light to several co-propagating cladding modes. The high attenuation of cladding modes results in a transmission spectrum with several attenuation bands. As coupling can occur to more than on mode, the transmission spectrum has multiple dips. An example of a transmission spectra of a LPG produced with a CO2 laser in a SSMF in fig. 2.2 illustrates this phenomenon, presenting a spectrum with several dips, each representing coupling to a different cladding mode. For a given value of induced refractive index modulation, an increase in length of the grating produces different shapes of the transmission spectra, both in dip strength (maximum attenuation value) and spectral bandwidth. Usually, in order to obtain the desired spectral characteristic of the grating, the spectra is monitored on a period-by-period basis so that the length of the grating can be controlled in order to produce the desired spectral response [31].
1200 1300 1400 1500 1600 1700 1800 -25 -20 -15 -10 -5 0 T r a n s m i s s i o n ( d B ) wavelength (nm)
Figure 2.2: Transmission spectra of an LPG inscribed with a CO2 laser. Grating period Λ = 800µm, grating length L= 14.4 mm.
2.2
Long Period Grating Inscription Techniques
This subchapter is an essay on three methods of LPG inscription on SSMF along this thesis: the UV method, electric-arc method and the CO2 laser method. The objective was to study, test, optimize if there was room for improvement and ultimately compare these three techniques in order to access their viability for the production of LPGs in MCFs. The LPGs inscribed in SSMF by the three methods were judged on three key components: overall serviceability (associated costs, production time, practicality, etc.), reproducibility of results and polarization dependent loss. Reproducibility is a common problem associated with all LPG inscription techniques.
In order to achieve good reproducibility, all the parameters associated with the production of LPGs, such as laser power, exposure time, fiber tension and fiber movement, need to be precisely controlled, which can be hard to achieve. Good reproducibility is vital in an experimental study where different parameters are constantly being tested and tuned, and also relevant for an eventual development of a commercial application based on LPG production. Unless there are polarization-specific applications where polarization dependent loss (PDL) needs to be tailored in order to suppress certain polarization states, PDL needs to be mini-mized in any device used in optical communications and in some cases, sensing applications. Multiplexers, couplers and dispersion compensators need to have minimal PDL across all polarization states so they do not introduce additional losses to optical networks. In sensing applications, PDL can also affect the accuracy and stability of optical fiber based gyroscopes. Birefringence in LPGs arises from a variation in the azimuthal refractive index profile in an optical fiber. Because of the birefringence, the LPG properties such as resonant wavelength and coupling strength will vary for different states of polarization (SOP) of the light that travels trough the fiber. The absolute difference between the maximum an minimum trans-mitted power over all SOPs defines the PDL. The sources of PDL can be intrinsic to the fiber where the LPG was inscripted but they can also be induced by the method of inscription. [32]. A method that leads to an azimuthally asymmetric distribution of the change in the
index of refraction of the core and cladding of the fiber will cause birefringence and hence PDL. This is translated in the coupling between the fundamental core mode and azimuthally asymmetric cladding modes. If the method leads to an azimuthally symmetric change in the refractive index of the core and cladding of the fiber, the fundamental core mode will couple to azimuthally symmetric cladding modes [33]. This will result in LPGs with reduced PDL [32].
2.2.1 The Ultraviolet Laser Point-by-Point Method
Photo-sensibility is the key phenomenon behind the production with LPGs with an UV laser. There is no definitive model on how it affects the inscription process. Besides inducing the key refractive index change in order to produce a FBG or a LPG, other physical param-eters are altered when fibers are exposed to UV light. These include absorption, tension, birefringence, thermal expansion coefficient and density [34]. Since the UV method is based on the photo-sensibility of the core of an optical fiber, the LPG is created by only altering the refractive index of the core, which is an advantage when it comes to produce LPGs in SSMFs, as these fibers have intrinsic low birefringence and the remaining birefringence introduced by the inscription method will be confined to the core. This results in LPGs with PDL values very close to zero.
Photosensitive fiber has to be used in order to inscribe LPGs. A special type of SSMF with photosensitive dopants can be bought (it costs roughly 15 times more than SSMF, according to one of the most popular online retailers catalog) or the fiber can be loaded with hydrogen to become photo-sensitive, which is the method of choice in our laboratory. In order to load the fiber with hydrogen the fibers are placed in an isolated chamber with molecular hydrogen at a minimum pressure of 60 bar for at least one week. Figure 2.3 shows a picture of the used hydrogenation system and fig. 2.4 shows a schematic of the system with all its valves.
The method of hydrogenation has the advantage of flexibility and cost. A bottle of H2 can last for several years and a variety of fiber types can be hydrogenated at the same time in large quantities. On the other hand, the fiber loses hydrogen as soon as it is removed from the chamber. This affects the reproducibility of results, as it is nearly impossible to inscribe on two identically hydrogenated fibers.
After the fiber core is made photosensitive and the fiber is spliced in both ends to two pigtails, the LPG inscription process can begin. Two photographs showing the setup are shown in fig.2.5 and an illustrative diagram shown in fig.2.6 explains the inscription process. The coherent UV light source is a KrF BraggStar Laser with 248 nm wavelength and an adjustable energy up to 3 mJ per pulse. The UV beam is steered by two mirrors into a moving optical system stage controlled by a very precise step motor. The moving optical stage has 3 elements: a third mirror, a cylindrical lens and a slit. The mirror deflects the beam towards a fixed stage where the fiber with a portion of the plastic coating removed is placed. One of the fiber holders in the fixed stage is placed atop a moving one axis translation stage controlled by a micrometer. This micrometer can be used to slightly push back one of the holders and apply additional tension to the fiber. If needed, a weight and a pulley attached to the pigtail can be used to ensure that the fiber is under constant strain. Before irradiating the fiber, the beam is concentrated and focused by the lens and then passes through a slit that defines the exposure length of each shot. This slit is one unique feature between all the setups used and adds extra flexibility to the inscription process. The uncoated portion of fiber is placed exactly in the focal point of the lens. For each LPG point (also commonly referred as
Figure 2.3: Photograph of the hydrogenation system.
Figure 2.4: Schematic of the hydrogenation system. A, B, C and D are hand valves. C is the purge valve.
”shot”) the fiber is irradiated for a given time period of exposure. After each irradiation the stage moves a length equal to the period of the LPG. This process is called a point-by-point production method. Both the step motor and the Laser are controlled by a LabView routine with a graphical user interface, making the system fully automated.
The UV method is considered to be flexible as it is able to produce gratings with coupling strengths superior to 20 dB with variable exposure lengths. Fig. 2.7 shows a spectrum of a grating produced in SSMF with this method and setup and the corresponding grating param-eters. The spectrum was measured with an Optical Network Analyser (ONA) from Agilent (model 86038P). However the hydrogenation process is time consuming, and the different con-centrations of hydrogen in the fiber samples make it very difficult to get reproducible results. Also being this a gas laser, the level of KrF in the tank varies, affecting the the energy of the laser pulses.
2.2.2 The Electric Arc Discharge Method
The electric arc method represents a cheaper, simpler, alternative to LPG production as expensive laser equipment is not necessary. Also, the fibers do not need to be photosensitive or doped, meaning that time consuming hydrogenation or the purchase of expensive doped fiber is not required. This also means that LGPs can be inscribed in fibers with air cores such as photonic crystal fibers (PCFs), pure silica fibers and photonic bandgap fibers (PBFs ) [35]. The refractive index modulation is achieved by an electric discharge produced between two electrodes, which triggers a plethora of different thermal based effects. Several mechanisms for LPG formation due to periodical electric discharges have been proposed [36], such as diffusion of dopants (in SSMF, Ge), micro tapering, which is a fairly visible and common phenomenon observed if to much current and/or tension is applied to the fiber during the discharge, microbending, residual stress relaxation and even structural modifications of the silica matrix of the fiber [35]. This method requires less parameters to control in order to produce an LPG, which is both an advantage and a disadvantage, meaning that it is a simpler method but yet the number of options available to tailor different gratings is reduced. When compared to the UV method, there is no way to define exposure length, as the discharge has a fixed exposure length, defined by the size of the two electrodes in the setup being used. Since this is a thermal method, tension and arc current have to be adjusted in order not to cause the fiber to taper, which can lead to increased insertion losses [29].
In this thesis a modified commercial fusion splicer from Fujikura (model FSM-60S) was used as the electric arc discharge source. The modification is easy to perform and it is not permanent. It consists in removing two easily accessible rubber pieces inside two metal enclosures that are used to lock the fibers in place while they are being spliced. The enclosures are screwed up to the fiber clamps and are easily removable. This way, the fiber can be moved inside the splicer when pushed by a step motor. An illustrative diagram of the setup is shown in 2.8. A pulley and a weight are also used to ensure that the fiber is under constant load and does not deviate from its relative position between the two electrodes. A photo of the system components is shown in fig.2.9. The electrodes of this setup produce an electric discharge an approximate exposure length of 95 µm. This value was obtained by analyzing an LPG inscribed in SSMF on a Leica Microscope (model DM750M). A photo taken by the microscope is shown in fig.2.10. The exposure length is estimated by comparing the number of pixels of the fiber diameter with the number of pixels of the region of the discharge.
Figure 2.6: Schematic diagram illustrating the fiber point-by-point method.
Figure 2.7: LPG inscription with the UV point-by-point technique and a table listing the corresponding parameters.
Figure 2.8: Illustrative Diagram of the electric arc system setup used to inscribe LPGs.
Figure 2.9: Photograph of the components of the electric arc system setup used to inscribe LPGs.
Figure 2.10: Microscope image of an irradiated fiber used to determine the exposure length LPGs.
functions, the software has a graphical user interface where the physical buttons of the splicer machine can be triggered. Since the step motor also as a similar interface, an automatic clicker freeware (GS autoclicker) is used to automate the process. The software is programmed to press the arc button, wait 15 seconds so the fiber as enough time to cool before being pushed by another click that pushes the step motor and consequently moves the fiber a distance equal to the desired grating period. The auto clicker software is also instructed to perform this sequence for a pre-defined amount of bursts, so that it stops either when the desired number of discharges is performed or for a periodical spectral measurement. IF so desired, spectral data can be collected after each and every new electric discharge is applied to the fiber.
Figure 2.11 shows a study of spectral evolution with number of periods, as well as a table with the corresponding fabrication parameters. In this case, the setup was programmed to perform 5 irradiations and move the fiber for 920 µm 15 seconds after each irradiation. After 5 irradiations the LPG spectrum is measured by the ONA. With this study, it is was possible to determine that the approximate number of discharges for the highest possible coupling strength to be between 40 and 45 discharges. The splicer uses bits as the power unit, which is a rather unconventional unit. For a weight of 6 grams added on the pulley, the optimal power value for grating inscription without visible fiber deformation was set to be STD-75 bit. STD power is the calibration power obtained calculated by the splicer in order to successfully splice a SSMF.
Figure 2.11: LPG peak evolution with the number of electric arc discharges applied (”shots”) and a table listing the fabrication parameters.
This setup is able to produce LPGs with resonant dips over 20 dB in a simpler and faster manner than the UV setup. However the small electrodes erode quickly, which affects reproducibility of results and increases maintenance costs. High PDL (over 6 dB) is also an issue, since the irradiation method is not azimuthally symmetric. The resonant dips in the spectra of gratings produced with electric arc discharge represent coupling from the fundamental core mode to azimuthally symmetric cladding modes [29], demonstrating that an non-uniform refractive index change occurs in the fiber during inscription.
2.2.3 The CO2 Laser Method
The CO2 laser method of LPG inscription combines the stability and flexibility of a laser based method with the advantage of cost and practicality of the electric arc discharge method. There is no consensus on what the mechanism for refractive index modulation is in the case of CO2 laser irradiation. There is evidence of relaxation of internal stresses that occur in both the core and cladding of the SSMF when the fiber is irradiated with a CO2 laser [37], but there is also a contribution of densification in the process [38].
The most common setups in literature are similar to the setups described in this thesis for the UV laser method (with a CO2 laser instead), but in the case of this thesis, a CO2 laser splicer from Fujikura (model LZM-100) was used. This represents a novel approach on LPG inscription with a CO2 laser, where rotation during irradiation is available. Figure 2.12 shows both the laser path and the top view of the splicer. The fiber is double sided irradiated. There are two translation motors (Z motors), two rotational motors (θ motors), and two video cameras. Since all the motors and the laser can be programmed to execute instructions at specific time frames, the LZM can be programmed to inscribe gratings. This is an ”all-in-one solution” that boasts some of the strong points of the two previously shown methods. It has the practicality and serviceability of the electric arc discharge and also has the power stability of a laser. When associated with the accurate calibration software of the splicer, it ensures that the fiber is always getting the same power, assuring reproducibility over a very long period of time, as electrode degradation is no longer a problem. On the other hand, there is no slit, but rather a cylindrical lens that defines the exposure length. The LZM has very precise motors, which are the key when it comes to ensure reproducibility of results. No weight is required to ensure constant tension. Instead, after the fiber is clamped, one of the motors can be pushed back a few micrometers in order to ensure constant tension in every inscription. In order to produce gratings with low PDL, the fiber is rotated during lasing. This represents an azimuthally symmetric irradiation method, which has been proved to reduce PDL [32, 39–41] and does not affect reproducibility [33].
To inscribe an LPG with this new method, three distinct programming steps are used (fig.2.13). The first step consists in rotating the fiber 360 degrees while simultaneously irra-diating it at the same time with a defined laser optical power. The lasing time and rotation time are synchronized so that the fiber is irradiated all-around. Next, the fiber is translated by the Z motors accordingly with the desired grating period. In the last step the fiber is unwinded, because continuous rotation in the same direction results in fiber twisting and eventual breakdown. The optical power and the mechanical tension applied to the fiber need to be sufficiently high to modulate the refractive index of the fiber but, at the same time, sufficiently low in order to avoid fiber micro-tapering. Fig. 2.14 shows the fiber micro taper-ing caused by excessive applied tension and/or laser power compared to the result of a good balance between power and tension. The optimal values were chosen after several trial tests. Furthermore, in order to achieve good reproducibility, the optical power and tension must be constant in every LPG inscription. For all LPGs presented, a constant tensile force was applied by moving one of the motors a few micrometers after the fiber is clamped, resulting in a deformation tension of 555 µ. In terms of power, the best value for this particular tension and type of fiber was found to be 4.30 W, which was measured by the two thermopiles that the LZM possesses. All the parameters used for creating the LPGs presented in this subsection are summarized on fig.2.15.
moni-Figure 2.12: Optical path of the laser beam of the LZM-100 (top) and top view of the main chamber of the LZM-100 (bottom).
Figure 2.14: Images of a fiber after irradiation with tension of: a) 555 µm ; b) 1800 µm . toring between each irradiation pulse to estimate the length with the maximum coupling, i.e. the maximum attenuation dip. We have used two methods for measuring the trans-mission and PDL spectra. In one approach the ONA from Agilent, model 86038P, with a measurement range between 1495 nm and 1640 nm, was used to measure the transmission and PDL spectra. In another approach, we have used a supercontinuum broadband optical source from Fianium Whitelase model SC-480-20 in combination with a spectrum analyzer from Yokogawa (AQ6375), which were used to measure transmission spectra in a wider range (1200-2000 nm).
By employing the described method we were able to inscribe LPGs with periods ranging from 700 µm to 980 µm, with attenuation dips exceeding 20 dB. Fig. 2.15 displays the transmission spectra of 3 LPGs with periods of 700 µm, 750 µm and 800 µm. As it can be seen a shift of 50 µm in grating period results in about 200 nm in main dip wavelength shift.
Figure 2.15: Transmission spectra of three different LPG samples with different periods: 700 µm, 750 µm, 800 µm and the corresponding list of fabrication parameters.
spectra have a similar profile, with a main attenuation dip and two secondary dips, around the same resonant wavelengths. Although, there are noteworthy differences in the resonant wavelengths (maximum difference of 6 nm) and dip intensity (maximum difference of 6 dB, for a secondary dip). Moreover, these spectra correspond to LPGs with different lengths (less than 6 periods of difference). These results show some reproducibility, however, careful analysis reveals that improvements could be performed.
1200 1300 1400 1500 1600 1700 1800 -25 -20 -15 -10 -5 0 T r a n s m i s s i o n ( d B ) Wavelenght (nm) 15 points 19 points 13 points
Figure 2.16: LPG transmission spectra of three different samples inscribed with the a period of 700 µm, but different lengths (number of points).
The differences in the LPGs are explained due to the fact that the used period results in the superposition of irradiated regions, affecting the uniformity of the refractive index modulation and hence the reproducibility of results. In this particular setup, with two laser beams directed at the fiber, the profile of the fiber has a supergaussian form shown in fig. 2.17, with a heating region spanning approximately 800 µm. So, in order to avoid the superposition, LPGs with periods longer than 800 µm were produced.
Fig. 2.18 shows the transmission spectra of three gratings produced with a period of 950 µm . The transmission spectra are noticeably identical, being the maximum difference between the resonant wavelengths less than 1 nm and the same dip intensity of 25 dB for all dips. In addition, the difference between the three LPG lengths is smaller than the one presented on samples from fig.2.16. In this case with a maximum difference of 3 points.
We have analyzed the PDL of LPGs inscribed with a short period, 725 µm, and the three samples of 950 µ, using the ONA. The transmission and PDL spectra of the LPG inscribed with a short period is displayed in fig.2.19. It presents an attenuation dip of -29 dB centered at 1590 nm with a PDL maximum of 5 dB. Regarding the gratings PDL value of 3 dB was achieved for the sample with 21 points. The remaining two samples present even lower PDL values, less than 2 dB.
Figure 2.17: Beam heating profile in the fiber and correspondent supergaussian fit, estimated thought a fiber heating image (inset) obtained with the LZM cameras. The black dots repre-sent the collected data points and the red line a supergaussian fit.
1500 1520 1540 1560 1580 1600 -30 -25 -20 -15 -10 -5 0 24 points 23 points 21 points T r a n s m i s s i o n ( d B ) Wavelenght (nm)
Figure 2.18: LPG transmission spectra of three different samples inscribed with a period of 950 µm.
1500 1520 1540 1560 1580 1600 1620 1640 -35 -30 -25 -20 -15 -10 -5 0 0 1 2 3 4 5 P o l a r i z a t i o n D e p e n d e n t L o s s ( d B ) Wavelenght (nm) T r a n s m i s s i o n ( d B )
Figure 2.19: Transmission and PDL spectra of a LPG with a period of 725 µm.
1500 1520 1540 1560 1580 1600 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 24 points 23 points 21 points P D L ( d B ) Wavelenght (nm)
To investigate the mode coupling in the inscribed grating, the mode field profile in the output of a LPG with a period of 950 µm was observed. The fiber was cleaved at one end of the LPG, close to the end of the last irradiated region, no longer than 1 cm apart. An optical sign from a tunable laser (Photonetics OSICS) was launched into the other end of the grating. The mode field profile is observed in the cleaved side of the fiber by a beam profiler (Duma Optronics Beam-On VIS-NIR), while the wavelength was changed in the tunable laser source. As shown in fig. 2.21, for 1580 nm the fundamental mode is observed and at the resonant wavelength (1542 nm), the observed profile is consistent with a circularly symmetrical cladding mode. These results suggest that the grating was originated by a circularly symmetric refractive index change.
Figure 2.21: Near-field profiles of the output of a 950 µm LPG at 1580 nm (a) and at the resonant wavelength of 1542 nm (b).
This new technique is based on a commercial processing and splicing system that uses a CO2 laser and allows the precise control of every parameter associated with LPG production. With point by point monitoring, the results are highly reproducible, especially for higher periods that avoid superposition of irradiated areas, with resonant wavelengths that differ less than 1 nm between resonant wavelength values. The constant rotation of the fiber during irradiation ensures a circularly symmetric refractive index change that results in LPGs with PDL values as low as 2 dB for gratings with coupling strengths of 25 dB. This makes these devices suitable for telecom applications. References [32, 39, 41] demonstrate low PDL values for attenuation dips up to 16 dB. In this work we were able to demonstrate PDL values lower than 2 dB for dips of 25 dB.
2.3
Summary
In this chapter, the most important theoretical concepts regarding LPGs were covered. Also three LPG inscription methods were compared and tested. All three methods are able to produce gratings with attenuation dips over 20 dB.
The UV method is more flexible when it comes to produce several grating types, as it uses a variable slit, allowing gratings with different exposure lengths to be produced. However since it requires fibers to be photosensitive, specialty fibers with air or pure silica cores (no
germanium) cannot be inscribed with this method. Photosensitivity is a key characteristic of the UV method, which has both drawbacks and advantages. The biggest advantage is that only refractive index of the core of the fiber is modulated, resulting in gratings with very little or even zero PDL. The disadvantage is that fibers have to be made photosensitive through the time consuming process of hydrogen loading. A major drawback is the large amount of parameters that have to be precisely controlled in order to not affect the reproducibility of results.
The electric-arc discharge method can be employed by using a cheap commercial fusion splicer, a step motor, a pulley and a set of weights. It is a cheap and simple method that is able to inscribe gratings in almost every type of fiber. In this thesis an automated setup was built by using an auto clicker software that would click in the graphical user interfaces used to control both the motor and the splicer in the desired order so gratings could be produced. But fast electrode degradation affected the reproducibility of results, and the gratings produced have high PDL values.
The CO2 laser method is often regarded in literature as the most flexible, because it is able to produce gratings with variable exposure lengths in all types of fibers. In this thesis, a commercial fusion splicer was used. This splicer uses a cylindrical lens to define exposure length. To this day, there is only one type of lens approved by the manufacturer for use with this splicer. This can be considered as a disadvantage when comparing the method with other CO2 laser based setups and even UV laser based setups. However, the splicer has a very stable laser and very precise motors which can be programmed. The fiber can also be rotated during irradiation. This means that the splicer can be programmed to produce highly reproducible gratings with low PDL in an automated manner.
Chapter 3
Space Division Multiplexing and
Long Period Grating Inscription in
Multi-core Fibers
This chapter introduces the concept of SDM, its challenges and opportunities, particularly in the context of MCFs. Then it focuses on an experimental study of the production of LPGs in MCFs using three different techniques: The UV laser method, the electric arc method, and the CO2 laser method. An application example is also described in this chapter, through an experimental demonstration of a selective core coupler, which was able to multicast a 200 Gb/s DP-16QAM signal between two cores of a 4-core MCF.
3.1
Space Division Multiplexing
The concept of SDM first came to light when multi-core fibers were first reported, in 1979 [42]. Three years later the use of multimode fibers was also proposed [43]. The main aim of SDM is to drastically increase the data capacity of one single optical fiber, but the concept has major implications on network design and operation. The concept of SDM encourages device integration and brings new functionality to network elements. With SDM, the number of components will decrease, resulting in more reliable, energy-efficient networks with less operational expenditure [44].
At the moment, there is no definite imposing fiber technology for SDM, but rather two fiber types acting as the main contending technologies. These are MCFs and also FMFs. FMFs represent a form of SDM that uses multiple modes as distinct channels in the same pathway, which can also be referred to as mode division multiplexing (MDM). Conventional multimode fibers (MMFs) with core/cladding diameters of 50/125 µm and 62.5/125 µm support hundreds of modes and have large differential modal group dispersion (DMGD). These fibers are not suitable for long-haul transmission because the DSP complexity required to demultiplex all the modes at the receiver would be too high [45]. Recent advances have led to the fabrication of fibers supporting a small number of modes, FMFs. These fibers are designed with a core dimension and numerical aperture set to guide a very few number of modes, typically 6 or 12 distinct modes, enabling lower complexity DSP that will demultiplex the modes. Despite the fact that these fibers have reduced DMGD, this is still the main impairment associated with the use of these fibers over long distances. One other unresolved issue is the
mode-dependency of gain on amplification schemes [46, 47], which leads to an unequal amplification of the several modes in the FMF.
Figure 3.1: The pictures of the end-faces of several different types of MCF geometries. 1) 19-core trench assisted MCF [48]; 2) 8-core ring-geometry trench assisted MCF [49]; 3) 7-core trench assisted MCF [50] ; 4) 7-core MCF from Fibercore.
MCFs offer a theoretical transmission capacity increase per fiber proportional to the num-ber of cores in the MCF. This represents a huge boost in transmission capacity per finum-ber as a MCF can have multiple cores. So far, the maximum number of cores reported in literature is 30 [10]. Higher core counts come at the cost of added crosstalk between cores. In order to optimize the performance of MCFs, several core arrangement geometries have been proposed. The idea is to optimize the core count of the fiber in order to minimize crosstalk, increase the per-fiber transmission capacity and the transmission distance. These three performance parameters are hard to balance, as they affect each other, which means that there is very active research in fiber design of MCFs, resulting in a plethora of proposed MCF geometries. The simplest way to manage cross-talk is to increase the distance between fiber cores, but this means that there will be less space to place extra cores. In order to overcome this prob-lem some techniques have been proposed. The use of non-identical cores drastically reduces cross-talk, allowing the fibers to be fabricated with higher core density [51]. The use of trench type refractive index profiles to better confine the mode is also another effective strategy to reduce core coupling to impressively low levels (¡-90 dB/Km) [45]. Figure 3.1 shows pictures of the end-faces of some of the most common MCF geometries in literature. In this thesis, two models of 4-core MCF fabricated by Fibercore were used. Both fibers have a square ar-rangement of the 4 cores, but with different spacing between the cores. The first type of fiber, designated from now on as ”MCF1” for the sake of simplicity, has 51.7 µm core spacing. The
actual model number of the fiber is SM-4C1500 (8.0/125/001). The second fiber, designated as ”MCF2” has a 36.25 µm core spacing (model number SM-4C1500(8.0/1.25)). Figure 3.2 shows a microscope photograph of the end-faces of both fibers, as well as tables with the most important physical parameters of each fiber.
Figure 3.2: Picture of the end-face of the two types of fibers used in the work described in this thesis. Image and parameter tables provided by Fibercore in the fiber’s data-sheets.
When compared to FMFs, MCFs have the advantage of higher capacity per fiber and less crosstalk between propagation channels, which results in lower DSP requirements and longer transmission distances, making these fibers much more suitable for backbone and long haul networks. The current disadvantage lies in the fact that FMFs use less components (ex. less pumps for amplification). This is why research that deals with component reduction in MCFs, particularly pump lasers, is considered to be an important topic.
3.1.1 Main Challenges and Opportunities in Multicore Fiber Technology
There are several issues that need to be considered when adopting SDM technology. The first big issue of this technology has been mentioned already in this thesis and deals with the energy requirements of the networks based on this type of fiber. The jump in capac-ity per fiber as to go hand in hand with a reduction of components used in order to make the technology viable. This is a valid assertion for all transmission distances, meaning that the component reduction is useful from the short distance applications such has huge server rooms who would run more efficiently and would be easier to troubleshoot to the long haul
