This document discusses materials used for optical fibers and losses in optical fibers. It describes how silica is commonly used due to its good optical transmission, mechanical strength, and chemical inertness. Fluoride glasses are also mentioned but are difficult to manufacture without crystallization. The main losses discussed are absorption, scattering, and bending losses. Absorption can be intrinsic to the material or due to impurities. Scattering transfers power between modes. Bending losses include macro bending from sharp curves and micro bending from microscopic bends in cables. The wavelength of minimum attenuation is around 1550 nm.

1. Presented To- Presented By-
Prof. M.S. Narkhede Er. Sanyam S. Saini
NITTTR, Chandigarh ME (I&CE) (Regular)
2012-14

2. Question Based on University Exam
1. Enlist the material used for Fluoride Glass fabric. Discuss the
difficulties involve in manufacturing of such fiber using conventional
techniques.
2. State & Explain the various Losses in Optical Fiber. Mention the
wavelength at which attenuation is minimum?

3. Answer #01
Basic Requirements
 It must be possible to make a long ,thin and flexible fibers from the
materials.
 For the fiber to guide light efficiently, the material must be transparent
at a particular wavelength.
 Materials having slightly different refractive indices for the core and
cladding must be available.

4. Silica
 Silica exhibits fairly good optical transmission over a wide range of
wavelengths.
 In the near-infrared (near IR) portion of the spectrum, particularly
around 1.5 μm, silica can have extremely low absorption and scattering
losses of the order of 0.2 dB/km.
 Silica fiber has high mechanical strength .
 Silica is also relatively chemically inert.
 Silica is not hygroscopic.

5. Silica
 Silica glass can be doped with various materials. The purpose of
doping is to raise the refractive index or to lower it.
 To raise the RI, doping materials are: (GeO2) or (Al2O3))
 To lower the RI, doping materials are: fluorine or B2O3
 Silica fiber also exhibits a high threshold for optical damage.
 Because of these properties silica fibers are the material of choice in
many optical applications, such as communications , fiber lasers, fiber
amplifiers, and fiber-optic sensors.

6. Fluorides
 Fluoride glass is a class of non-oxide optical quality glasses composed of
fluorides of various metals.
 Because of their low viscosity, it is very difficult to completely avoid
crystallization while processing it through the glass transition.
 As heavy metal fluoride glasses exhibit very low optical attenuation, they are
not only difficult to manufacture but they have poor resistance to moisture and
other environmental attacks.
 An example of a heavy metal fluoride glass is the ZBLAN glass
group, composed of zirconium, barium, lanthanum, aluminum , and sodium
fluorides.
 They are advantageous especially in the mid-infrared (2000–5000 nm) range.
 The optical applications of fluoride fibers include mid-IR spectroscopy, fiber
optic sensors, thermometry, imaging etc.

7. Answer #02
Attenuation
Scattering Bending
Absorption Losses losses
Intrinsic Atomic
Extrinsic
Absorption Defects
Microscopic Macroscopic
bends bends
Absorption Absorption
in in
Infrared Ultraviolet
Region Region
Homogeneities Compositional
in fiber fluctuations

8. Attenuation
 Signal attenuation (fiber loss) largely determines the maximum
Repeater less separation between optical transmitter & receiver.
 Signal distortion cause that optical pulses to broaden as they travel
along a fiber, the overlap between neighboring pulses, creating errors
in the receiver output, resulting in the limitation of information-
carrying capacity of a fiber.

9. Attenuation
Power loss along a fiber:
Z=0 Z= l
P(0) mW  p l
P (l )  P (0)e
 p z
P ( z )  P ( 0) e
The parameter  p is called fiber attenuation coefficient in a units of for
example [1/km] or [nepers/km]. A more common unit is [dB/km] that is defined
by:
10  P(0) 
 [dB/km]  log    4.343 p [1 / km]
l  P(l ) 
Fiber loss in dB/km P(l )[dBm ]  P(0)[dBm ]  [dB/km]  l[km]

10. Attenuation v/s Wavelength

11. Absorption
Absorption is caused by three different mechanisms:
1- Impurities in fiber material: from transition metal ions & particularly
from OH ions with absorption peaks at wavelengths 2700 nm, 400
nm, 950 nm & 725nm
2- Intrinsic absorption (fundamental lower limit): electronic absorption
band (UV region) & atomic bond vibration band (IR region) in basic SiO2.
3- Radiation defects

12. Intrinsic Absorption
• Less significant than extrinsic absorption. For a pure (no impurities) silica
fiber a low loss window exists between 800 nm and 1600 nm.
• Graph shows attenuation spectrum for pure silica glass.
• Intrinsic absorption is very loother forms of loss.
• It is for this reason that fibers are made up of silica and optical
communications systems work between about 800 to 1600 nm.

13. Extrinsic Absorption (metallic ions)
• Extrinsic absorption is much more significant than intrinsic
• Caused by impurities introduced into the fiber material during
manufacture
• Iron, nickel, and chromium
• Caused by transition of metal ions to a higher energy level
• Modern fabrication techniques can reduce impurity levels below 1 part in
1010.
• For some of the more
common metallic impurities
in silica fibre the table
shows the peak attenuation
wavelength and the
attenuation caused by an
impurity concentration of 1
in 109

14. Scattering Losses in Fibre
• Scattering is a process whereby all or some of the optical power in a mode
is transferred into another mode.
• Frequently causes attenuation, since the transfer is often to a mode which
does not propagate well. (also called a leaky or radiation mode).

15. Bending Loss
(Macro bending & Micro bending)
Macro bending Loss:
• The curvature of the bend is much
larger than fiber diameter. Light
wave suffers sever loss due to
radiation of the evanescent field in
the cladding region.
• As the radius of the curvature
decreases, the loss increases
exponentially until it reaches at a
certain critical radius.
• For any radius a bit smaller than
this point, the losses suddenly
becomes extremely large.
• Higher order modes radiate away
faster than lower order modes.

16. Micro bending Loss
Micro bending Loss:
• Microscopic bends of the fiber
axis that can arise when the
fibers are incorporated into
cables.
• The power is dissipated
through the micro bended
fiber, because of the repetitive
coupling of energy between
guided modes & the leaky or
radiation modes in the fiber.

17. Thank You