Chapter 5
Passive Components
San-Liang Lee
National Taiwan University of Science and Technology
sanlee@et.ntust.edu.tw
Outline
• Optical couplers
– Optical splitter
– Optical combiner
– Star coupler
• Wavelength Selective and Routing Devices
–
–
–
–
Optical filters
Wavelength Mux/Demux
Optical add/drop (OADM)
Wavelength router
• Other Passive Devices
– Optical isolators
– Optical circulators
• Optical Switches
2
Key Parameters
•
•
•
•
•
•
•
•
•
Insertion loss
Excess loss
Back Reflection
Polarization dependent loss (PDL)
Splitting Ratio
Splitting uniformity
Channel Isolation or Crosstalk
Optical bandwidth (-1, -3, -20dB)
Isolation ratio (isolators or circulators)
3
Optical Couplers
(1) 2x2 Directional coupler
1
3
2
4
• Fabrication: can be made out of fiber fusing, bulk optics, and
integrated optics
• Excess loss = the ratio of power input to power output
P1
(dB )
= 10 log
P3 + P4
• Insertion loss = the loss from a given input (1) to an output (4)
P1
= 10 log
(dB )
P4
–
–
Input (1) to output (4)
Through loss : insertion loss for through path
Tap loss: insertion loss for cross path
4
Optical Couplers (Cont.)
1
3
2
4
• Directivity: the ratio of the backscattered power at the other
port to the input power
P1
= 10 log
(dB)
P2
• Splitting ratio: the percentage division of the input optical
power between the output ports
 P3 

 x 100%
=
 P3 + P4 
5
Optical splitter and Combiner
• For an ideal 1xN splitter (N > 1),
Pout = Pin N
at each output port
• For an ideal Nx1 beam combiner, the output is the
sum of all inputs, but in fact
Pout
1
= ∑ Pin,i
N i
6
Star Coupler
• The power of each input is
distributed evenly to all
outputs
• Device types: fused fibers,
cascade of directional
couplers, planar waveguides
Splitting loss = 10 log N
Slab Waveguide Region
Input Waveguides
( dB )
Output Waveguides
Fused Portion
N


Excess loss = 10 log  Pin,i ∑ Pout , j 


1
Splitting uniformity is critical
7
Optical Filters
Fixed-λ filter
• For channel selection, require
– Flat passband
– Low adjacent-channel crosstalk
– Good alignment to wavelength grid
• Wavelength tunable filter is useful for dynamic
channel selection or switching
8
Filter Characteristics
Fixed-λ filter
Filter transmission (dB)
3 dB
1 dB
1-dB
bandwidth
Adjacent
channel
0
3-dB
bandwidth
-10
-20
20-dB
bandwidth
Passband
skirts
-30
Crosstalk
energy
-40
0.996
0.998
1
λ0/λ
1.002
1.004
9
Filter Types
•
•
•
•
•
•
Thin-Film Filters
Mach-Zehnder type
Fabry-Perot Interferometer
Diffraction gratings
Reflective (fiber) gratings
Acousto-optic tunable filter
10
Thin-Film Filters
Input
T(λ )
R(λ )
Common
GRIN
Lenses
Pass
Reflection
WDM filter
11
Fiber Gratings
Cladding
Incident Light
Reflected
Light
Grating planes
Incident light
λ1 λ2 λ3 λ4
Fiber Bragg
grating
Reflected light
λ1
λ3 λ4
Transmitted
light
Grating response
λ2
12
Coupler vs. Multiplexer
Optical coupler is wavelength insensitive, while Mux is
wavelength selective
P1
P2
M
P3
P1
P2
M
P3
λ1
λ2
M
λN
λ1
λ2
M
λN
λ1 λ2 L λN
Pout = ∑ Pi
λ1 λ2 L λN
Pout =
1
∑ Pi
N
λ1
λ2
M
λN
Pout = Pi
λ1 λ2 L λN
λ1 λ2 L λN
Pout =
1
N2
∑
Pi
λ1 λ2 L λN
13
Mux/Demux
Transmission
λ1
λ 1 ,λ 2 ...,λ 8
Fiber
λ2
λ3
λ4
Lens
λ5
λ6
Lens
λ8
Fiber λ
1
λ3
λ5
λ2
λ7
λ4
λ6
λ8
λ7
Fiber
Glass substrate
Narrowband
filter
14
Wavelength Router
Route signals according to
their wavelengths
Input/Output
Waveguide
λ1
λ2
λ 1,λ 2, ..., λ N
λN
Output：v
λ0 λ1 λ2
λN−1 λ 0 λ 1
λN−2 λN−1 λ 0
λN−2 λN−1
λN−3 λN−2
λN−4 λN−3
wavelength：w
Input：u
λ2 λ3 λ4
λ1 λ2 λ3
w=(N-u+v) mod N
λ0 λ1
λN−1 λ 0
SliceWaveplate
Substrate
Array-Waveguide
Slab
Waveguide
15
OADM
λk
λk
OADM
λk
λk
• Selectively extract and Insert optical channels at
intermediate sites
• Manage the WDM traffic in the fiber
• Optical circulator + fiber grating = Fixed OADM
• Flexible and reconfigurable OADMs will be needed in
future DWDM networks
16
OADM Characteristics
0
In -> Drop
&
Add -> Thru
Transmission (dB)
-10
Crosstalk
-20
-30
-40
-50
1530
1532
1534
1536
1538
1540
1542
0
Transmission (dB)
-10
Crosstalk
In -> Thru
-20
-30
-40
-50
1530
1532
1534
1536
1538
1540
1542
17
OADM with Thin-Film Filters
In
Drop
Out
Add
In
Drop-λ 1
Drop-λ 2
Add-λ 1
Add-λ 2
Express
18
OADM with Fiber Gratings
λ1 ,λ2 ....,λn
Add λ1
Fiber Gragg grating
λ1 ,λ2 ....,λn
Drop λ1
Fiber Gragg grating
Circulator
Circulator
Drop
λ1
λ2
λ3
λn
Circ 1
1
2
Add
Fiber Bragg
grating 1
1
2
WDM
M
3
Drop
λ1
λi
Fiber Bragg
grating 2
Circ 2
Fiber Bragg
grating n
Circ n
1
2
3
3
Drop
λ2
Drop
λn
19
Optical Isolators
For reduce or eliminate back reflection
SWP
Faraday
rotator
λ /2 plate SWP
SOP
Fiber in
Forward
(a)
SWP
Backward
Faraday
rotator
λ /2 plate SWP
Fiber in
Fiber out
(b)
20
Optical Circulator
• To de-couple transmitted and received signals
traveling along the same fiber (bi-directional)
• Use Faraday rotators, similar to isolators
2
2
1
1
3
3
4
21
Optical Switches
• To dynamically control the physical connections between
input ports and output ports
• Design criteria:
–
–
–
–
–
Low power loss
Low crosstalk
Polarization independence
Simple interconnection
Low switching energy
• Types of switches
–
–
–
–
Mechanical switch
Waveguide switch
Self-electro-optic effect Devices (SEED)
SOA Gated switch
22
Mechanical Switches
Fiber
Fiber
ON/OFF gate
Fiber
Fiber
1XN switch
Fiber
Fiber
2XN switch
23
Hot Switches
Switch
Mirror
Control signal
• Applications:
Microlens
Silica Substrate
– Circuit switch:
– Packet switch: require fast switching speed (ns)
– Protection switch
• Large-scale switch (>1000 ports)
– MEMS
– Liquid crystal
– Agilent’s ‘champange’ switch
24
SOA Gate Switches
Passive
Splitters
Amplifier
Gates
Passive
Combiners
25