Transcript Fig. 10-1: Transmission windows

Components for WDM
Networks
Xavier Fernando
ADROIT Group
Ryerson University
Passive Devices
• These operate completely in the optical domain
(no O/E conversion) and does not need
electrical power
• Split/combine light stream Ex: N X N couplers,
power splitters, power taps and star couplers
• Technologies: - Fiber based or
– Optical waveguides based
– Micro (Nano) optics based
• Fabricated using optical fiber or waveguide
(with special material like InP, LiNbO3)
10.2 Passive Components
• Operate completely in optical domain
• N x N couplers, power splitters, power taps,
star couplers etc.
Fig. 10-3: Basic Star Coupler
May have N inputs and M outputs
•
Can be wavelength selective/nonselective
• Up to N =M = 64, typically N, M < 10
Fig. 10-4: Fused-fiber coupler /
Directional coupler
• P3, P4 extremely low ( -70 dB below Po)
• Coupling / Splitting Ratio = P2/(P1+P2)
• If P1=P2  It is called 3-dB coupler
Definitions
Splitting (Coupling) Ratio = P2 ( P1  P2 )
Excess Loss =10 Log[ P0 ( P1  P2 )]
Insertion Loss =10 Log[ Pin Pout ]
Crosstalk = 10 Log( P3 P0 )
Try Ex. 10.2
P1  P0 sin 2 (z)
Coupler
characteristics
P2  P0 cos2 (z)
: Coupling Coefficient
Coupler Characteristics
• By adjusting the draw length of a simple
fused fiber coupler,
– power ratio can be changed
– Can be made wavelength selective
Wavelength Selective Devices
These perform their operation on the incoming
optical signal as a function of the wavelength
Examples:
• Wavelength add/drop multiplexers
• Wavelength selective optical
combiners/splitters
• Wavelength selective switches and routers
Filter, Multiplexer and Router
A Static Wavelength Router
Fig. 10-11: Fused-fiber star coupler
Splitting Loss = -10 Log(1/N) dB
Excess Loss = 10 Log (Total Pin/Total Pout)
Fused couplers have high excess loss
Fig. 10-12: 8x8 bi-directional star coupler by
cascading 3 stages of 3-dB Couplers
1, 2
1, 2
1, 2 5, 6
3, 4 7, 8
N
Number of 3-dB Couplers N c = log 2 N
2
(12 = 4 X 3)
Try Ex. 10.5
Fiber Bragg Grating
• This is invented at Communication
Research Center, Ottawa, Canada
• The FBG has changed the way optical
filtering is done
• The FBG has so many applications
• The FBG changes a single mode fiber (all
pass filter) into a wavelength selective filter
Fiber Brag Grating (FBG)
• Basic FBG is an in-fiber passive optical band
reject filter
• FBG is created by imprinting a periodic
perturbation in the fiber core
• The spacing between two adjacent slits is called
the pitch
• Grating play an important role in:
–
–
–
–
Wavelength filtering
Dispersion compensation
Optical sensing
EDFA Gain flattening and many more areas
Fig. 10-16: Bragg grating formation
2 sin( / 2)  uv
FBG Theory
Exposure to the high intensity UV radiation,
the refractive index of the fiber core (n)
permanently changes to a periodic function
of z
n( z )  ncore  n[1  cos(2z / )]
z: Distance measured along fiber core axis
: Pitch of the grating
ncore: Core refractive index
Reflection at FBG
Fig. 10-17: Simple de-multiplexing function
Reflected Wavelength B  2neff
Peak Reflectivity Rmax = tanh2(kL)
Wavelength Selective DEMUX
Dispersion Compensation using FBG
Longer wavelengths
take more time
Reverse the operation of
dispersive fiber
Shorter wavelengths
take more time
ADD/DROP MUX
FBG Reflects in both directions; it is bidirectional
Fig. 10-27: Extended add/drop Mux
Advanced Grating Profiles
FBG Properties
Advantages
• Easy to manufacture, low cost, ease of coupling
• Minimal insertion losses – approx. 0.1 db or less
• Passive devices
Disadvantages
• Sensitive to temperature and strain.
• Any change in temperature or strain in a FBG causes the
grating period and/or the effective refractive index to change,
which causes the Bragg wavelength to change.
neff
neff
neff 
T 

T

Interferometers
Interferometer
An interferometric device uses 2 interfering paths of
different lengths to resolve wavelengths
Typical configuration: two 3-dB directional couplers
connected with 2 paths having different lengths
Applications:
— wideband filters (coarse WDM)
separate signals at1300 nm from those at 1550 nm
— narrowband filters:
filter bandwidth depends on the number of cascades
(i.e. the number of 3-dB couplers connected)
Fig. 10-13: Basic Mach-Zehnder
interferometer
Phase shift of the propagating wave increases with L,
Constructive or destructive interference depending on L
Mach-Zehnder interferometer
Phase shift at the output due to the propagation path
length difference:
2 neff
 
L

If the power from both inputs (at different
wavelengths) to be added at output port 2, then,
1 1
  2 neff    L
 1 2 
Try Ex. 10-6
Mach-Zehnder interferometer
Fig. 10-14: Four-channel wavelength multiplexer
Mach-Zehnder interferometer
Mach-Zehnder interferometer
MZI- Demux Example
Fiber Grating Filters
• Grating is a periodic structure or
perturbation in a material
• Transmitting or Reflecting gratings
• The spacing between two adjacent slits is
called the pitch
• Grating play an important role in:
– Wavelength filtering
– Dispersion compensation
– EDFA Gain flattening and many more areas
Reflection grating
Different wavelength can be separated/added
Arrayed wave guide grating
Phase Array Based WDM Devices
• The arrayed waveguide is a generalization
of 2x2 MZI multiplexer
• The lengths of adjacent waveguides differ
by a constant L
• Different wavelengths get multiplexed
(multi-inputs one output) or de-multiplexed
(one input multi output)
• For wavelength routing applications multiinput multi-output routers are available
Diffraction gratings
source impinges on a diffraction grating ,each wavelength
is diffracted at a different angle
Using a lens, these wavelengths can be focused onto
individual fibers.
Less channel isolation between closely spaced wavelengths.
Arrayed Waveguide Grating
-- good performance
-- quicker design cycle time
-- more cost effective
--- higher channel count
Multi wavelength sources
• Series of discrete DFB lasers
– Straight forward, but expensive stable sources
• Wavelength tunable lasers
– By changing the temperature (0.1 nm/OC)
– By altering the injection current (0.006 nm/mA)
• Multi-wavelength laser array
– Integrated on the same substrate
– Multiple quantum wells for better optical and
carrier confinement
• Spectral slicing – LED source and comb
filters
Tunable Filters
• At least one branch of the coupler has its
length or ref. index altered by a control
mechanism
• Parameters: tuning range (depends on
amplifier bandwidth), channel spacing (to
minimize crosstalk), maximum number of
channels (N) and tuning speed
Fig. 10-23: Tunable optical filter
Fig. 10-21: Tunable laser characteristics
Typically, tuning range 10-15 nm,
Channel spacing = 10 X Channel width
Summary
• DWDM plays an important role in high
capacity optical networks
• Theoretically enormous capacity is possible
• Practically wavelength selective (optical
signal processing) components decide it
• Passive signal processing elements are
attractive
• Optical amplifications is imperative to
realize DWDM networks