This document discusses transmission characteristics of optical fibers, including:
- Attenuation losses such as absorption and scattering losses that reduce signal strength.
- Dispersion which causes pulses to spread, interfering with each other (intersymbol interference) and limiting bandwidth. Types of dispersion include material dispersion and waveguide dispersion.
- Single mode fibers which only propagate a single mode and have low dispersion. Dispersion can be optimized by designing the refractive index profile, such as in dispersion shifted or flattened fibers.
Oc unit 2 - Part II - Transmission Characteristics
1. R.M.K. COLLEGE OF ENGINEERING AND TECHNOLOGY
Puduvoyal,Chennai.
DEPARTMENT OF ECE
OPTICAL COMMUNICATION
UNIT II
TRANSMISSION CHARACTERISTIC OF OPTICAL FIBER
Dr.K.Kannan
ASP/ECE
2. Attenuation Losses
Absorption Losses
Scattering Losses
Linear and Non Linear Scattering losses
Bending Losses
Micro and Macro bending losses
Core and Cladding Losses
Signal Dispersion
Inter Symbol Interference and Bandwidth
Intra Model Dispersion
Material Dispersion and Waveguide Dispersion
Polarization Mode Dispersion
Intermodal Dispersion
Dispersion Optimization of Single Mode Fiber
Characteristics of Single Mode Fiber
RI Profile
Cutoff wave length
3. Signal Degradation in the Optical Fiber
SignalAttenuation
Signal Distortion/Dispersion
signal strength
10. INTER SYMBOL INTERFERENCE AND BANDWIDTH
Dispersion (intramodal and intermodal) affects the transmission of optical signal in
case of both analog and digital transmission of optical signal.
Dispersion causes overlapping of spread pulses making them unrecognizable and
resulting into Inter Symbol Interference (ISI). This overlapping finally leads to errors
in making decisions regarding 1’s and 0’s
A large ISI may lead to increased number of errors. The error in digital optical
communication is measured in terms of Bit-Error-Rate (BER), which is measured in
terms of the number of errors incurred in a given bit stream
ISI restricts bit rate and Bit rate may be viewed as the analogue of bandwidth in the
case of analog optical communication. Similarly, the bit-error-rate and signal-to-
noise ratio are also related to each other. The exact relationship depends on the
characteristics of the channel (fiber).
11. In order to avoid the effect of Intersymbol interference one must control the rate
at which the bits are being transmitted.
If Tb is the duration of a single pulse, a conservative estimate of the maximum bit
rate that can be obtained on an optical channel without overlapping of bits as
The above estimation is based on the assumption that spreading of the pulses
due to dispersion in the channel is also Tb.
Thus the optical bandwidth can be written as
In terms of bit rate,
32. Polarization Mode dispersion
Polarization mode dispersion (PMD) is a source of pulse broadening which results from fiber
birefringence
It is a random effect due to both intrinsic (caused by noncircular fiber core geometry and residual
stresses in the glass material near the core region) and extrinsic (caused by stress from
mechanical loading, bending or twisting of the fiber) factors which in actual manufactured fibers
result in group velocity variation with polarization state.
33. Two modes have different phase propagation constants βx and βy they exhibit different
specific group delays In the time domain for a short section of fiber, the differential group delay
(DGD), Δτ = δτgL, is defined as the group delay difference between the slow and the fast modes
over the fiber lengths as indicated in Figure
The differential group delay per unit length is referred to as the polarization mode dispersion
(PMD) of the fiber and is usually expressed in units of picoseconds per kilometer of fiber
34. DISPERSION OPTIMIZATION OF SINGLE MODE FIBER
The pulse broadening in single-mode fibers results almost entirely from chromatic or intramodal dispersion as only a
single-mode is allowed to propagate
Hence the bandwidth is limited by the finite spectral width of the source.
The transit time or specific group delay τg for a light pulse propagating along a unit length of single-mode fiber may be
given, following
where c is the velocity of light in a vacuum,
β is the propagation constant for a mode within the fiber core of refractive index n1 and
k is the propagation constant for the mode in a vacuum.
The total first-order dispersion parameter or the chromatic dispersion of a single-mode fiber, DT, is given by the derivative
of the specific group delay with respect to the vacuum wavelength λ as
The material dispersion parameter it is usually expressed in units of ps nm−1 km−1. When the variable λ is replaced by ω,
then the total dispersion parameter becomes:
The fiber exhibits intramodal dispersion when β varies nonlinearly with wavelength.
The total first-order dispersion DT in a practical single-mode fiber as comprising:
which is simply the addition of the material dispersion DM, the waveguide dispersion DW and the profile dispersion DP
components.
35. However, the overall dispersion of single mode fibers is much less than that of multimode fibers. Therefore,
single mode fibers are widely used for high-speed long-haul optical communication systems
The single mode fibers do not suffer from intermodal dispersion. The overall dispersion of the fiber is thus
determined by the intramodal dispersion which has two components, e.g., material dispersion and waveguide
dispersion.
Out of these two components, the material dispersion of the fiber cannot be changed much.
On the other hand, the waveguide dispersion component can be varied significantly by changing the refractive
index profile from the conventional step-index profile to a more complex index profiles
36. Different Refractive index structures that give different waveguide dispersion and they can be added with
material dispersion which is constant for selected material.
1. Standard single mode fiber (1300 nm optimized)
2. Dispersion flattened fiber
3. Dispersion shifted fiber
RI profile design to optimize dispersion
Zero material dispersion(ZMD) point occurs in 1.27 μm. However it can be shifted by designing
different core cladding interface refractive index (RI) profiles.
At wavelengths longer than the ZMD point in most common fiber designs, the DM and DW
components are of opposite sign and can therefore be made to cancel at some longer wavelength.
Hence the wavelength of zero first-order chromatic dispersion can be shifted to the lowest loss
wavelength for silicate glass fibers at 1.55 μm to provide both low dispersion and low-loss fiber.
This may be achieved by such mechanisms as a reduction in the fiber core diameter with an
accompanying increase in the relative or fractional index difference to create so-called dispersion-
shifted single-mode fibers (DSFs)
The step index profile illustrated in Figure gives a shift to longer wavelength by reducing the core
diameter and increasing the fractional index difference.
37. In graph, various design combinations of refractive index profile and dispersion variation with
wavelength are shown.
38. Several of the graded refractive index profile DSF types are illustrated in Figure 2.15. The triangular profile
shown in Figure 2.15 (a) is the simplest and was the first to exhibit the same low loss (i.e. 0.24 dB km−1) at a
wavelength of 1.56 μm (i.e. λ0) as conventional non shifted single-mode fiber.
In the basic triangular profile design the optimum parameters giving low loss together with zero dispersion
at a wavelength of 1.55 μm cause the LP11 mode to cut off in the wavelength region 0.85 to 0.9 μm.
Thus the fiber must be operated far from cutoff, which produces sensitivity to bend-induced losses (in
particular micro bending) at the 1.55 μm wavelength.
One method to overcome this drawback is to employ a triangular index profile combined with a depressed
cladding index, as shown in Figure
39. An alternative modification of the dispersion characteristics of single-mode fibers involves the
achievement of a low-dispersion window over the low-loss wavelength region between 1.3 and 1.6 μm.
Such fibers are known as dispersion-flattened single-mode fibers (DFFs)
A typical W fiber index profile (double clad) is shown in Figure offers flat dispersion profile over a
wavelength region
Triple clad and quad cladding structures also used in DFF design which are shown here.
40. All fiber which are designed by designing different refractive index profile are exhibiting the low
dispersions which are compared in below graph.
41. Cutoff Wavelength
Single-mode operation only occurs above a theoretical cutoff wavelength λc given by:
where Vc is the cutoff normalized frequency. Hence λc is the wavelength above which a particular
fiber becomes single-moded.
Thus for step index fiber where Vc = 2.405, the cutoff wavelength is given by
Mode-field diameter and spot size
The MFD is an important parameter for characterizing single-mode fiber properties which takes into account
the wavelength-dependent field penetration into the fiber cladding.
The MFD is generally taken as the distance between the opposite 1/e = 0.37 field amplitude points and the
power 1/e2 = 0.135 points in relation to the corresponding values on the fiber axis, as shown in Figure 2.18.
42.
43.
44.
45.
46. Consider a 10km optical fiber link using a MMSI fiber with the following parameters: Core refractive index n1= 1.458, Relative
index deviation ∆= 0.002. Estimate the delay time difference between axial ray and most oblique ray. What is the value of rms
pulse broadening due to intermodal dispersion. Estimate the bandwidth and maximum bit rate of transmission assuming RZ
formatting and neglecting intramodal dispersion.
Solution:
The delay time difference between axial ray and most oblique ray can be estimated using
δTmod =
Ln1Δ
c
=
10X 103
X 1.458 X0.002
3X108
= 97.2 nS
(i) The rms pulse broadening due to intermodal dispersion can be calculated using
σmod =
Ln1Δ
2 3c
=
10X 103
X 1.458 X0.002
2 3 X 3X108
= 28 nS
(ii) The bandwidth of transmission can obtained using
B =
0.2
σmod
=
0.2
28 X 10−9
= 7.14MHz
(iii) The maximum bit rate can be obtained using
BT = 2 X B
= 2 X 7.14 MHz
= 14.28 Mbps
47. A multimode step index fiber as numerical aperture of 0.22 and a core refractive index of 1.458.the fiber
exhibits an overall intramodal dispersion of 200ps km-1. calculate overall value of the rms pulse broadening per
kilometer of the fiber when the LED source operating at 850nm has an rms spectral width of 40nm.estimate
the bandwidth of the 10km link based on this fiber.
Solution:
(i) The rms pulse broadening per kilometer due to material dispersion can be obtained as
σintra 1km = σmat = σλ L Dmat
= 40 X 1 X 200
= 80 nS km−1
(ii) The rms pulse broadening due to intermodal dispersion can be obtained as
σmodal 1km = L
NA 2
4 3 n1 c
= 103
X
0.22
4 3 X 1.458 X 3 X 108
= 15.97 nS km−1
(iii) The overall rms pulse broadening can be obtained as
σ = σintra2 + σmodal2
= 82 + 15.972
= 17.86 nS km−1
(iv) For a fiber link of 10km length, the total rms pulse broadening would be
σT = 17.86 X 10−9
X10
=178.6 nS
