Lecture 7b
DWDM
1. Introduction
2. Principles of Wavelength Division Multiplexing
3. WDM System Components
4. Wavelength-Independent Coupler
5. Construction of Wavelength Independent Couplers
6. Wavelength-Dependent Coupler
(Multiplexer/Demultiplexer)
7. WDM Communication System
8. Dense Wavelength Division Multiplexing (DWDM)
9. Add-drop Multiplexer
10. Conclusion
11. Appendix
1
• Time Division Multiplexing (TDM)
• Wavelength Division Multiplexing (WDM)
• TDM divides a high-bandwidth transmitted signal
into time slots. Each time slot carries a different
low-bandwidth signal.
• In WDM, several high-bandwidth signals travel
on the same fiber, in the same time, each using a
different light wavelength.
• DWDM uses the same principles as a WDM, but
with high density of light wavelength alocation.
• A common application of multiplexing is in longdistance data and voice communications.
2
Access techniques for mobile
communications
FDMA (TACS)
P
F
TDMA (GSM, DECT)
ATDMA (UMTS)
T
P
F
CDMA (UMTS)
T
P
P - Power
T - Time
F - Frequency
F
T
3
Principles of Wavelength Division Multiplexing (WDM)
m1 (t)
m2 (t)
m3 (t)
Transmitter
λ1
Transmitter
λ2
Transmitter
λ3
Multiplexer
optic fiber
λn
λ1
Receiver
λ2
Receiver
λ3
Receiver
m1 (t)
m2 (t)
m3 (t)
λn
Demultiplexer
mn (t)
Transmitter
Receiver
mn (t)
4
WDM System Components
• A coupler combiner, and a splitter,
• Couplers are bi-directional devices;
Wavelength Independent
Wavelength Dependent
5
Wavelength-Independent Coupler
1
2
3
 P3 (unwanted output) 
 P1


loss D  10  log10
 10  log10 


 P3
P1 (input)



lossTHROUGHPUT
 P1 

 10  log10 
 P2 


loss EXCESS
lossTAP
 P1  P3
 10  log10 
 P2

Splitting ratio 




 P3 

 10  log10 
 P2 






P1
P3
6
Input power = 1 mW
Splitting ratio of each coupler = 9:1
Excess loss of each coupler = 0.3 dB
Insertion loss of each connector = 0.2 dB
1
2
3
1
2
Terminal 1
3
1
2
Terminal 2
3
Terminal 3
The input power is 1 mW, so we can express the power as zero decibel milliwatts
(0 dBm). The incoming power level to first splitter is reduced by 0.2 dB by the first
connector, and 0.3 dB by the excess loss. At the first splitter, we have total power of 0.5 dBm, or:
Total power  1 mW  100.1( 0.5dBm)  891.3  W
7
Construction of a Wavelength Independent Coupler
(input)
Port 2
(throughput)
Port 1
Fused area
fibers
Port 3
(tap)
In the fused area, some light is able
to enter the other fiber.
Light enters
both fibers
Port 2
output
Port 1
light input
Port 3
output
The screw adjuster moves
the input fiber to vary the
splitting ratio
8
Wavelength-Dependent Couplers (Multiplexer /
Demultiplexer)
P (Bright area)
max
min
(a) Constructive Interference
(waves combine at point P)
max
min
R (Dark area)
max
(b) Destructive Interference
(waves are opposite in magnitude at point R)
9
B
A
Output
P fiber
λ
Input
Q
fiber
θ
d
d
θ
λ = d sin( θ )
d  sin(  )  m  
(m  0,1, 2, 3, ...)
(8.5)
10
B
A
λ1
P1 Fiber 1
λ1 , λ2
P2 Fiber 2
Input
fiber Q
λ2
d
θ2
θ1
d
d  sin(  )  m  
λ1 = d sin( θ1 )
λ2 = d sin( θ2 )
(m  0,1, 2, 3, ...)
11
WDM Communication System
Transmitter 1
λ1 = 850 nm
Receiver 1
λ1 = 850 nm
λ1
λ1
λ 1 , λ2
Multiplexer
λ2
Transmitter 2
λ2 = 1300 nm
λ1 , λ2
Optical fiber
Demultiplexer
(8.6)
λ2
Ceff  C850 nm  C1300 nm  2  C
Receiver 2
λ2 = 1300 nm
CrT850 nm
 P 850 nm out , 1300 nm port
 10  log 10 
 P 850 nm in , combined port





CrT1300 nm
 P 1300 nm out , 850 nm port 

 10  log 10 
 P 1300 nm in , combined port 


12
λpump = 980 nm
doped fiber
EDFA
optic fiber
optic fiber
Multiplexer
 1 = 1530.3
Demultiplexer
λ1 = 1530.3 nm
nm
= 1531.9 nm
λ2 = 1531.9 nm
2
λ3 = 1533.5 nm
λ4 = 1535.0 nm
λ3 = 1533.5 nm
λ4 = 1535.0 nm
13
λ20 = 1560.6 nm
λ20 = 1560.6 nm
• Several modulated laser sources, one for each optical
channel.
• A Distributed Feedback (DFB) laser is the best source for
a DWDM system, narrow bandwidth, (less than 0.4 nm).
Modulation
• Erbium Doped Fiber Amplifier (EDFA), An optical
demultiplexer to separate each signal at its destination.
• Suitable detectors for each signal to extract the
information in that.
• A DWDM system can be designed with 41 channels in
the range of 1528 nm to 1561 nm. DWDM can increase
the capacity of a single fiber to as much as several
hundred gigabits per second. This is the same capacity as
several thousand 100Base-T Ethernet cables.
14
Longerwavelength
Source (1480 nm)
Shortwavelength
Source (980 nm)
Excited erbium atoms at high energy level
-1 mks
Atoms at
metastable energy
(-10 ms)
Stimulated
Emission
1520-1620 nm
Erbium atoms at low energy level
15