Fiber Bragg Gratings for DWDM Optical Networks Rogério Nogueira Instituto de Telecomunicações Pólo de Aveiro Departamento de Física Universidade de Aveiro
Overview Introduction. Fabrication. Physical properties. Applications in optical networks. Impairments of optical filtering in DWDM optical networks. Nonlinear optics in FBG. Conclusions.
Introduction Fiber Bragg gratings represent a key element in fiber optical communications systems. Due to their fabrication process, they have very low insertion loss, immunity to electromagnetic interference, electrical isolation and light weight. The wide variety of applications has made it a highly developed technology, becoming one of the most stable and versatile technologies in the optical field.
Fabrication UV beam Phase mask Zero-order block Rotable and laterally translatable mirrors Rotation for blaze Fiber UV fringe pattern λ = Bragg n eff λuv sinα
Physical Properties 2π n( z) = neff + δneff ( z) 1 + s cos z + ϕ( z) Λ Apodization profile (can be controlled for filtering optimization) Fringe visibility (influences the extinction ratio) Period (influences the Bragg wavelength) Chirp (can be controlled for dispersion compensation) 1.0 0.30 1.0 0.30 Normalized Peak Value 0.8 0.6 0.4 0.2 0.25 0.20 0.15 0.10 0.05 Bandwidth [nm] Normalized Peak Value 0.8 0.6 0.4 0.2 0.25 0.20 0.15 0.10 0.05 Bandwidth [nm] 0.0 0.00 5 8 11 14 17 20 dn x10-5 0.0 5000 6000 7000 8000 9000 10000 length (µm) 0.00
Fiber laser Applications Laser wavelength stabilization (980 nm, 1480 nm) Pump reflector in fiber amplifiers (1480 nm) Pump reflector in phase conjugator (1550 nm) and isolation filter in wavelength converter WDM Demultiplexer (1550 nm) WDM add/drop filter (1550 nm) Optical amplifier gain equalizer (1530 1560 nm) Dispersion compensation for long-haul transmission (150 nm) Network physical monitoring Optical code-division multiple-access (OCDMA) Applications High-isolation reflector Blazed Bragg gratings or long period grating Chirped grating Description Narrowband reflector Narrowband reflector Highly reflective mirror Highly reflective mirror Multiple high-isolation reflectors Temperature, strain and humidity sensors Encoding / Decoding Parameters λ=0.1-1 nm R= 1-100 % λ=0.2-3 nm R= 1-10 % λ=2-25 nm R= 100 % λ=1 nm R= 100 % λ=0.1-1 nm Isolation>30 db λ=0.1-1 nm Isolation>50 db λ=30 nm Loss= 0-10 db λ=0.1-15 nm 1600 ps/nm λ=0.1-1 nm R=100% Chirped moiré grating
Applications Examples for Optical Networks Optical Add-Drop Multiplexer Network monitor Central Office Remote Node Subscribers Sensing Channel Power (dbm) 0-20 -40 1 2 3 Power (dbm) 0-10 -20-30 -40 1 2 3-60 -50 1545 1547 1549 1551 1553 1555 1545 1547 1549 1551 1553 1555 Wavelength (nm) Wavelength (nm) A FBG can be used for temperature, stress and humidity (polyimide-coated FBGs) sensors
Applications Examples for Optical Networks Multiple rejection/passband filter EDFA gain equalizer Reflectivity (db) 0-5 -10-15 -20-25 -30 1.546 1.548 1.550 1.552 1.554 Wavelength [µm] A single chirped moiré FBG can be used as multiple pass band/rejection filter (Ex: OCDMA) Transmission [db] 0-2 -4-6 -8-10 -12-14 1.530 1.535 1.540 1.545 1.550 1.555 1.560 Wavelength [µm] 2 long-period FBG can be used as an EDFA gain equalizer
Impairments of Optical Filtering in DWDM Optical Networks Cascaded filtering: Phase distortion Optical Transport Layer Optical add-drop multiplexer Optical cross-connect Bandwidth Narrowing: Leads to performance degradation, especially if the signal has a sharp pulse shape Dispersion: limits the bit rate and leads to pulse distortion, which can result in transmission errors.
Insertion Loss (db) Insertion Loss (db) Bandwidth Narrowing 0-10 -20-30 -40-80 -60-40 -20 0 20 40 60 80 Detuning (GHz) 0-10 -20-30 -40 1 1 2 Fabry-Perot 4 10-3 -30 10 10 AWG -80-60 -40-20 0 20 40 60 80 Detuning (GHz) -3-40 -80-60 -40-20 0 20 40 60 80 Detuning (GHz) The apodized FBG has a flattened filter response, which reduces the effect of bandwidth narrowing in opposition to other types of optical filters. Insertion Loss (db) Insertion Loss (db) 0-10 -20 0-10 -20-30 -40 Multilayer Interference Filter 1 1 Apodized FBG 10-80 -60-40 -20 0 20 40 60 80 Detuning (GHz) -3-3
Dispersion Due to the dispersion in the FBG, there must be a compromise between a good rejection value (higher grating strength) and less dispersion (lower grating strength). A linear-phase (dispersion less) FBG square-filter is still attainable, however it requires complex apodization profiles to achieve such performance. Another method is to use phase equalization techniques. Maximum Bit-Rate (Gbit/s) 20 15 10 5 10 20 30 40 50 60 70 80 90 100 Grating strength = k..l The simulation was made with a super-gaussian pulse with 100 Ghz spacing between channels
Nonlinear optics in FBG Experiments based on Kerr phenomenon, interplay between two or more beams, interplay between dispersion and nonlinearity, pulse shaping and optical switching are possible thanks to the exceptional flexibility in the choice of the grating parameters. Recent results, both theoretical and experimental demonstrated very interesting phenomena which suggest new applications and devices in the optical communications field.
Nonlinear applications The main idea is to use a probe with wavelength near the photonic band gap. A strong pump laser uses XPM to change the refractive index seen by the probe (XPM) thus changing the detuning of the probe from the center of the photonic band gap. Reflected Output Source B Source A PBS FBG Transmitted Output
Optimization of nonlinear effects Use of phase shift FBG (notch bandwidth reduces propagation speed). Use of the latest generation of novel glasses such as the chalocogenides which have a Kerr nonlinearity 100 times that of silica). Use of air-silica microstructure fibers ( holey fibers ).
Conclusions A general overview of the characteristics of FBGs and their application to optical networks have been presented. Also, some of the new developments and applications of FBGs in optical networks have been presented.
Acknowledgments The author gratefully acknowledges the financial support by Instituto de Telecomunicaçõ ções and Portugal Telecom Inovaçã ção. Thank You