fiber pres 3
DESCRIPTION
Fiber Pres 3TRANSCRIPT
1
LIGHTWAVE
ENGINEERING
Fiber Attenuation
Dr. Abid [email protected]
• Loss or Attenuation – Reduction in amplitude of pulse
• Dispersion – Pulse broadening or spreading
Loss and Dispersion
2
The fraction of power that is transmitted from input stage to output stage is
Input
Pi
Output
Po
Po
Pi
r =
Loss
rP
PgR
i
o log 10 log 10 (dB) or 1010
Example: For the example of 1mW out for 3mW in, we have:
R dB( ) log
10
1
310
10 0 333
10 0 44712
4 7712
10log ( . ...)
[ . ...]
. ...
Example: An amplifier produces 30 mW of output power with 2mW of input
R dBP
P( ) log
10 10
2
1
1030
2
10 1176
1176
10log
[ . ...]
.
Loss
3
Attenuation Characteristics (Theoretical)
Fiber Losses
window #1: 0.8-0.9µm
window #2: 1.25-1.35µm
windows #3: 1.5-1.6 µm
why not #4: 1.0-1.2 µm?
Rayleigh Scattering
Total Loss
Second Window
First Window
Third Window
800 900 1000 1100 1200 1300 1400 1500 1600 1700
3.0
2.5 2.0
1.5
1.0
0.5
OH Absorption Peak
Wavelength (nm)
Att
enu
ation
(d
B/k
m)
Fiber Losses
• Attenuation Characteristics (Experimental)
• Total Attenuation
Waveguide
losses
4
Losses in Fiber
• Attenuation happens because:
– Material Absorption (extrinsic and
intrinsic)
• Due to Atomic Defects
– Scattering losses (Rayleigh)
– Waveguide losses
– Bending losses (macro and micro
bending)
Material Absorption
• Impurities (SiO2 fibers)
- Metals (Fe, Cu,N, Mn, Cr...)
Now controlled to < 1 ppb
- OH (hydroxyions) - loss is due to
resonances with the O-H bond
• - r ~ 0.9,~ 1.2,~ 1.4m
5
Phenomena of Scattering
• If no clad -- Scattering on surface defects (Sources of light
losses )
• If clad is used -- light cannot reach them
• Scattering only on defects
Scattering Losses
• Rayleigh Scattering -
- Randomly connected
molecular network -- results in
localized variations of index of
refraction.
- small scattering sources
• Mie Scattering at longer
wavelengths – Open air
communication
6
Waveguide Losses
- imperfect processing
•rough interfaces
•local index variations
m
nc
n + c
Waveguide Losses
Micro-bending losses
7
Power loss in a curved fiber
Power loss in curved fiber
Attenuation Measurements
• Cut off a length L
• Re-measure Pout
L
FiberP
in outP
8
Example
• Suppose 100 µW of power coupled in to a
fiber. In calculating attenuation losses of
this fiber it was observed that 8% of input
power to the fiber was absorbed/lost within
the fiber material. Calculate the output
power at the output of the fiber and losses
in dB. Take ncore = 1.45 and ncl = 1.42.
• Pin = 100 x 0.92 = 92 µW
• α = 0.362 dB
• Calculate the length of the fiber if losses
are 1 dB/km.
L = 362 m
10
Dispersion (Track Team Example)Suppose three members of the track team are running simultaneously. Let us further suppose that the three can run
mile i ,for 4 minutes for persons A, B, and C respectively.
B
1mileX=0
1mileX=0
t=0
t=4min
A
C
ABC
4/5miles
4/6miles
Example Cont’d
t = 0 X
13
Intramodal or Chromatic
Dispersion
• In a single-mode fiber, the Intermodal (multi-path) dispersion is eliminated
• However, there is Chromatic dispersion
• It contains Material dispersion + Waveguide dispersion
• In a fiber, typically the material and waveguide dispersions counteract
• At about 1.3 -1.5 µm, the resulting dispersion may be close to zero.
• This is why the wavelength range of 1.3 -1.5 µm is the most promising for optical fiber communications
• Chromatic dispersion can be calculated as:
Chromatic Dispersion
LDtch
– D(λ) is the dispersion
coefficient or parameter
[ps/(nm.km)]
– Δλ is the linewidth
– L is the length
14
Different Dispersion Profiles
Material Dispersion • Any light source has a linewidth ∆λ , i.e., it emits
in the range from λ -∆λ to λ +∆λ
• Refractive index n is a function of wavelength. Because of that, travel times for light at λ -∆λ and λ +∆λ are different
• Material dispersion depends on the material properties and difficult to alter
15
Material Dispersion
Zero
Dispersion
Wavelength
Waveguide Dispersion
• In a optical waveguide, the travel path
depends on λ. Again, this results in
different travel times for light at λ -∆λ and λ
+∆λ.
• Waveguide dispersion can be altered by
changing the fiber refractive index profile
16
Total Dispersion
22
wmch ttt
For Single Mode Fibers:
For Multi Mode Fibers:
22
int cherTotal ttt
A fiber has the following specifications:
– Intermodal Modulation = 5 ns/km
– Chromatic Dispersion Parameter = 100 ps/(nm.km)
– Fiber Length = 5 km
– Linewidth = 40 nm
Calculate:
1. Total Intermodal Dispersion
2. Total Chromatic Dispersion
3. Total Dispersion
Dispersion -- Example
25 ns
20 ns
32 ns
17
Dispersion -- Example
Example: Find the amount of pulse spreading for a single mode fiber system with an LED at 0.82 µm with a 20 nm spectral width . Length of the fiber is 10 km. Take D(λ) = 10 ps/(nm.Km)
Solution:
A single mode fiber has the following specifications:
– Intermodal Modulation = 5 ns/km
– Chromatic Dispersion Parameter = 100 ps/(nm.km)
– Fiber Length = 10 km
– Linewidth = 10 nm
Calculate:
1. Total Intermodal Dispersion
2. Total Chromatic Dispersion
3. Total Dispersion
Question
N/A
10 ns
10 ns
18
Chromatic Dispersion• Different spectral components of a pulse travel at
different velocities
• Also called group-velocity-dispersion (GVD),
Bandwidth & Bit Rate
• Bit Rate (BR) = Data Rate
• BW = 2 BR
• What would be the BW in last two examples
t
2
1BW
cher tt 2int
2chinter
4
1BRBRBR
15.625 MHz 50 MHz
20
window #1: 0.8-0.9µm
window #2: 1.25-1.35µm
windows #3: 1.5-1.6 µm
Rayleigh Scattering
Total Loss
Second Window
First Window
Third Window
800 900 1000 1100 1200 1300 1400 1500 1600 1700
3.0
2.5 2.0
1.5
1.0
0.5
OH Absorption Peak
Wavelength (nm)
Att
enu
ation
(d
B/k
m)
• Attenuation Characteristics of Fiber
Waveguide
losses
Choice of Wavelength
Choice of Wavelength
21
Transmission Bands
Bandwidth: over 35000 Ghz, but limited by bandwidth of EDFAs
(optical amplifiers): studied later…
Choice of Wavelength
Window Wavelength
(µm)
Source
Material
Applications
1 0.8-0.9 GaAs/
AlGaAs
Old
Communication
Systems
2 1.25-1.35 InP/
InGaAsP
LAN
3 1.5-1.6 Long-haul
Communication
Systems
Now LAN as well?
22
Modifying Chromatic Dispersion
• Material dispersion depends on the material properties and difficult to alter
• Waveguide dispersion can be altered by changing the fiber refractive index profile
– 1300 nm optimized
– Dispersion Shifting (to 1550 nm)
– Dispersion Flattening (from 1300 to 1550 nm)
Dispersion Shifting/Flattening