EE 230: Optical Fiber Communication Lecture 5
Attenuation in Optical Fibers
From the movie
Warriors of the Net
Attenuation/Loss In Optical Fibers
Mechanisms:
Power transmission is governed by the
following differential equation:
Bending loss
Absorption
dP
  P
dz
where  is the attenuation coefficient
Scattering loss
and P is the total power.
Pout (z)=Pin exp  - Z 
dBm refers to a ratio
with respect to a
signal of 1 mW
 is usually expressed in dB/km
 (dB / km )  
P 
10
Log10  out   4.343
L
 Pin 
Note that positive  means loss
Bending Loss
Example bending loss
1 turn at 32 mm diameter
causes 0.5 db loss
Index profile can be adjusted to
reduce loss but this degrades
the fibers other characteristics
Rule of thumb on minimum
bending radius:
Radius>100x Cladding
diameter for short times
13mm for 125mm cladding
Radius>150x Cladding
diameter for long times
19mm
This loss is mode dependent
Can be used in attenuators,
mode filters fiber identifier, fiber
tap, fusion splicing
Microbending loss
Property of fiber, under control
of fabricator, now very small,
usually included in the total
attenuation numbers
Fiber Optics Communication Technology-Mynbaev & Scheiner
Bending Loss in Single Mode Fiber
Bending loss for lowest order modes
Mode Field distributions in straight
and bent fibers
Microbending Loss Sensitivity vs
wavelength
Bending Loss
• Outside portion of evanescent field has
longer path length, must go faster to keep up
• Beyond a critical value of r, this portion of the
field would have to propagate faster than the
speed of light to stay with the rest of the pulse
• Instead, it radiates out into the cladding and
is lost
• Higher-order modes affected more than
lower-order modes; bent fiber guides fewer
modes
Graded-index Fiber

r
nr   n1 1  2 
a
For r between 0 and a. If α=∞, the
formula is that for a step-index fiber
Number of modes is
M

 2
akn1  
2
Mode number reduction caused by
bending
N bent
   2  2a  3  2 / 3  
  
  
 N straight1 
 2  R  2n2 kR   
Absorption
• In the telecom region of the spectrum,
caused primarily by excitation of
chemical bond vibrations
• Overtone and combination bands
predominate near 1550 nm
• Low-energy tail of electronic
absorptions dominate in visible region
• Electronic absorptions by color centers
cause loss for some metal impurities
Electron on a Spring Model
Response as a function of Frequency
Mechanical Oscillator Model
E-Field of a Dipole
Vibrational absorption
• When a chemical bond is dipolar (one atom
more electronegative than the other) its
vibration is an oscillating dipole
• If signal at telecom wavelength is close
enough in frequency to that of the vibration,
the oscillating electric field goes into
resonance with the vibration and loses
energy to it
• Vibrational energies are typically measured in
cm-1 (inverse of wavelength). 1550 nm =
6500 cm-1.
Overtones and combination bands
• Harmonic oscillator selection rule says that
vibrational quantum number can change by
only ±1
• Bonds between light and heavy atoms, or
between atoms with very different
electronegativities, tend to be anharmonic
• To the extent that real vibrations are not
harmonic, overtones and combination bands
are allowed (weakly)
• Each higher overtone is weaker by about an
order of magnitude than the one before it
Overtone absorptions in silica
• Si-O bond fairly polar, but low frequency
• 0→1 at 1100 cm-1; would need six
quanta (five overtones) to interfere with
optical fiber wavelengths
• OH bonds very anharmonic, and strong
• 0→1 at 3600 cm-1; 0→2 at 7100 cm-1;
creates absorption peak between
windows
Attenuation in plastic fibers
• C-H bonds are anharmonic and strong,
about 3000 cm-1
• First overtone (0→2) near 6000 cm-1
• Combination bands right in telecom
region
• Polymer fiber virtually always more
lossy than glass fiber
Absorptive Loss
• Hydrogen impurity leads to OH bonds whose
first overtone absorption causes a loss peak
near 1400 nm
• Transition metal impurities lead to broad
absorptions in various places due to d-d
electronic excitations or color center creation
(ionization)
• For organic materials, C-H overtone and
combination bands cause absorptive loss
Photothermal deflection spectroscopy
Arc
lamp
Lock-in
amplifier
Chopper
Lens
HeNe
Detector
Sample
cuvette
Scattering loss: from index discontinuity
• Scatterers are much smaller than the
wavelength: Rayleigh and Raman
scattering
• Scatterers are much bigger than the
wavelength: geometric ray optics
• Scatterers are about the same size as
the wavelength: Mie scattering
• Scatterers are sound waves: Brillouin
scattering
Raman scattering
• A small fraction of Rayleigh scattered
light comes off at the difference
frequency between the applied light and
the frequency of a molecular vibration (a
Stokes line)
• In addition, some scattered light comes
off at the sum frequency (anti-Stokes)
Mie scattering from dimensional
inhomogeneities
• Similar effect to microbending loss
• Mie scattering depends roughly on λ-2;
scattering angle also depends upon λ
• In planar waveguide devices, roughness
on side walls leads to polarizationdependent loss
Teng immersion technique
Tunable IR laser
Chopper
Lock-in Amplifier
Detector
Motor stage
Intrinsic Material Loss for Silica
Rayleigh Scattering ~ (1/l)4
Due to intrinsic index variations in amorphous silica
Spectral loss profile of a Single Mode
fiber
Spectral loss of single and Multi-mode
silica fiber
Intrinsic and extrinsic loss components for silica fiber
Fundamentals of Photonics - Saleh and Teich