Dr. Mohammad Faisal

Dept. of EEE, BUET

Transmission Characteristics of Optical Fiber

Transmission characteristics

Attenuation or Fiber Loss

Material Absorption

Material Scattering

Waveguide Imperfections

Delay Distortion

Signal attenuation (also known as ‘fiber loss’ or ‘signal loss’) is one of the most important properties of an optical fiber, because it determines the maximum repeaterless separation between transmitter and receiver. Fiber attenuation is important because a lightwave receiver requires at least a minimum amount of signal power to detect a transmitted bit with an acceptable error rate.

Of equal importance is the signal distortion in fiber,

which causes optical signal pulses to broaden as

they travel along the fiber. The signal distortion limits

the information-carrying capacity of a fiber.

These are the two principal factors to determine the

optical transmission characteristics of fiber.

Mathematical analysis of fiber loss: Changes in power P of a bit stream propagating inside an optical fiber

Fiber Loss

dPP

dz

α is attenuation constant which includes all sources of power loss

Pin Pout

10

1numerical ln

10dB/km log

10log log

L

out in

in

out

out

in

oute

in

P P e

P

L P

P

L P

Pe

L P

1

dB/km 10 0.434294 ln

4.434

out

in

P

L P

Material Absorption

Intrinsic Absorption: absorption by fused silica (SiO2)

electronic and vibrational rasononaces associated with specific molecules due to absorption of power at certain wavelength

for silica molecules, electronic resonances occur in the UV region (λ < 0.4 μm) whereas vibrational resonances occur in the infrared region (λ > 7 μm)

intrinsic absorption for silica; λ-range: 0.8‒1.6 μm, below 0.1 dB/km

in fact below 0.03 dB/km in the 1.3 ~1.6 μm range

Extrinsic Absorption: absorption by impurities within silica (SiO2)

transition metal impurities such as Fe, Cu, Co, Ni, Mn, and Cr absorb in the wavelength range 0.6‒1.6 μm

the main source of extrinsic absorption is the presence of water vapors. OH ion dissolves in glass. Three absorption peaks occur near 1.39-, 1.24-, 0.95- μm wavelengths due to presence of residual water vapor in silica.

Factors affecting the fiber loss:

Attenuation Spectrum for SMF

Extrinsic Absorption

Material Scattering Rayleigh Scattering: This is the dominant loss mechanism arising from

local microscopic fluctuations in density

the density and compositional variations are frozen into the glass on cooling

density fluctuations lead to random fluctuations of refractive index which cause light scattering- Rayleigh scattering

the loss due to Rayleigh scattering:

Where C is constant in the range of 0.7-09 (dB/km)- μm4

depending on the constituents of the core

αR = 0.12‒0.16 dB/km at λ=1.55 μm

• Waveguide Imperfections:

Mie Scattering: Due to imperfections at the core-cladding interface

(say core radius variation), scattering of light occurs because of index inhomogeneities

this loss is typically below 0.03 dB/km

4

R C

Macro-bending Loss According to ray optics theory: a guided ray hits the core-cladding

interface at an angle greater than critical angle to experience total internal reflection. The angle decreases near a bend and may be smaller than critical angle for tight bends. Hence, ray would escape out of fiber.

In terms of mode theory: the part of mode outside the bend is required to travel faster than that on the inside so that a wavefront perpendicular to the direction of propagation is maintained. Hence, part of the mode in the cladding region needs to travel faster than the velocity of light in that medium. Since it is not possible, energy associated with this part of the mode is lost through radiation.

Bending loss is negligible (<0.01 dB/km) for bend radius R>5mm, practically most bends exceed R=5mm.

Micro-bending loss: Microscopic meandering of core axis is known as micro-bending Slight surface imperfections during manufacturing, cabling process or

cable installation, during service, due to stress for temperature variation etc.

It can cause mode coupling between adjacent modes which in turn cause radiation loss.

Dry Fiber

Dry fiber is developed which has very low loss over the entire wavelength range of 1.3 to 1.65 μm.

Lightwave systems with thousands of channels are possible

Dispersion Intermodal Dispersion: only in MMF In multimode fiber,

intermodal dispersion is due to the difference in propagation of various modes of the same signal

Intramodal Dispersion (Chromatic Dispersion): Both SMF and MMF: intramodal dispersion occurs within a single mode, because of group velocity being a function of wavelength

Signal distortion occurs from the effect that the velocity of propagation of a light becomes frequency dependent in the fiber. This dependence is expressed by the following equation

g

cv

dnn

d

Where vg is the group velocity, n is refractive index of fiber medium, is wavelength of light and c is the light velocity.

Thus different frequency components of the optical signal propagate at different velocities. The time delay between different spectral components causes spectral broadening of the optical pulses.

Dispersion

After certain overlap, the adjacent pulses can no longer be individually distinguishable. This is known as intersymbol interference (ISI) as illustrated in Figure.

Input pulses Output pulses Optical fiber

Dispersion

1

11

11

1 1 112

1

11

Phase velocity:

2 2group velocity: ; ;

1 2 2;

2

p

g

g

g

cv

n

nd c c cv n

dnd n cn

d

d d dv

d d d

dn n dndn

d d d dn

d

c

dnn

d

ng is group index

Group Velocity Dispersion (GVD)

Consider a fiber with length L, if is the spectral width of the pulse, the extent of pulse broadening for L:

2

2

2

2 2 2

;

;

g

g

dT d L dT v

d d v d

d dL

d d

dL

d

dL where

d

The parameter β2 is known as GVD parameter which determines the how much an pulse would broaden on propagation inside the fiber.

Unit: ps2/km

Dispersion Parameter

D is the dispersion parameter

Unit: ps/(nm-km)

Fiber dispersion 16 ps/(nm-km) means pulse will broaden 16 ps per nm wavelength after propagating 1 km

2

2

2 2

2

2 22 2

2 2; ,

1

1

2

2;

g

g

g

d L c cT

d v

DL

dD

d v

d d

d d v

d d d

d d d

c d

d

c dwhere

d

Types of Dispersion

Material Dispersion (DM): It occurs due to refractive index of silica, which changes with optical frequency.

n=f(λ)

Chromatic Dispersion

Material Dispersion

Waveguide Dispersion

Profile Dispersion

Waveguide Dispersion (DW): Due to waveguide design or structure

Core radius, index DW is negative in the range 0-1.7μm

It shifts the λZD so that the total

is zero near 1.3μm or 1.55μm

2

1 22

1; put

; or

122 1 ;Empirical relation

is zero-dispersion wavelength

since at = , 0.

M g

g

ZDM

ZD

ZD M

d cD v

dnd vn

d

d nn n n n

c d

D

D

2

1 2

2

2where,

W

n n d VbD V

c dV

V a NA

Profile Dispersion (DP): Due to variation of index difference with frequency

Negligible

Total Dispersion:

P

dD

d

0

T M W P

P

D D D D

D

ZMD: Zero material dispersion point

The fact that waveguide dispersion has opposite sign compared to the material dispersion is of considerable practical interest, which can be utilized to develop special fibers, such as dispersion flattened fiber (DF), dispersion shifted fiber (DSF) and nonzero dispersion shifted fiber (NZDSF) etc.

Dc (ns/nm-km)

Wavelength (nm)

DSF

NZDSF

SMF

- 3 0

- 2 0

- 1 0

0

1 0

2 0

3 0

1 2 5 0 1 3 5 0 1 4 5 0 1 5 5 0 1 6 5 0

Different Types of Fibers

Fiber Parameters

Dispersion (DSF) 0 ps/nm-km @1550nm

Dispersion (NZDSF) ±2 to ±5 ps/nm-km

Dispersion (SMF) 17 ps/nm-km @1550nm

But 0@1300nm

Dispersion slope (DSF) 0.055 ps/nm2-km

Dispersion slope (NZDSF) 0.07 ps/nm2-km

Dispersion slope (SMF) 0.09 ps/nm2-km

DFF: low loss, low dispersion: 1.3 to 1.6μm

Dispersion Compensating Fiber (DCF)

DCF has negative dispersion which is used to compensate for the accumulated

dispersion of SSMF and NZDSF. An appropriate length of DCF (with dispersion like

-70 to -300 ps/nm-km) is inserted into SSMF/NZDSF. Overall dispersion can be

Kept zero using DCF properly at a particular wavelength.

Dispersion Slope The variation of chromatic dispersion with wavelength is usually

characterized by second-order dispersion parameter or dispersion slope S.

𝑆 = (𝑑𝐷𝑇)/𝑑𝜆 =𝑑2𝜏𝑔

𝑑𝜆2

𝜏𝑔 =1

𝑣𝑔=𝑑𝛽

𝑑𝜔=1

𝑐𝑛1 − 𝜆

𝑑𝑛1𝑑𝜆

τg is group delay which is the reciprocal of group velocity vg. S is related to both second and third derivative of β. Total chromatic dispersion at an arbitrary wavelength can be estimated as

𝑆 =2𝜋𝑐 3

𝜆4𝑑3𝛽

𝑑𝜔3 +4𝜋𝑐

𝜆3𝑑2𝛽

𝑑𝜔2

𝐷𝑇 𝜆 =𝜆𝑆04

1 −𝜆0𝜆

4

𝑆0 = 𝑆 𝜆0