Questions
1 (a)
Explain clearly how each of the following physical mechanisms causes attenuation in a
silica optical fibre, stating in each case how the attenuation depends on wavelength:
(i) Intrinsic absorption;
(ii) Extrinsic absorption;
(iii) Rayleigh scattering.
[6 Marks]
1 (b) List three common types of dispersion that affect the bandwidth of an optical fibre. For
each type of dispersion explain briefly how dispersion takes place and list two factors,
apart from lowering the fibre span, which can reduce the level of dispersion.
[6 Marks]
1 (c) In an optical fibre the time taken m for an impulse to propagate a distance L is given
by: Where n1 is the core refractive index,  is the operating wavelength and c is the
velocity of light in a vacuum.
L
dn 
m  n1   1 
c
d 
How does this equation underpin the concept of material dispersion in a fibre?
Derive an expression for the total impulse spread and show that the condition for zero
material dispersion is given by:
d2 n1
 0
d 2
State clearly any assumptions made. In practice how is this condition commonly
approximated?
[13 Marks]
2 (a) What is meant by the term parametric mismatch used in reference to fibre-to-fibre
joints? Illustrate your answer by concisely defining the three most common forms of
parametric mismatch for a multimode fibre.
[5 Marks]
2 (b) Discuss how modern connectors used in ‘Fibre to the Desk’ solutions achieve cost
savings in comparison to traditional connectors such as ST or FC.
[10 Marks]
Questions
2 (c) Describe an experiment to measure the light current characteristics of a Vertical Cavity
Surface Emitting Laser diode (VCSEL) stating any precautions that may be taken.
Show how the following characteristics can be determined.
1. The lasing threshold
2. The differential quantum efficiency (above and below threshold)
[10 Marks]
Solutions
1 (a) Intrinsic absorption loss:
Intrinsic absorption is caused by the interaction of the light with one or more of the
components of the glass itself. For silica glass there is a low loss window between 800
and 1600 nm where intrinsic absorption is negligible, by comparison with other loss
mechanisms, such as scattering loss (see below). Intrinsic absorption in this window
falls between 700 nm and 1500 nm, then rises again toward 1700 nm
Extrinsic absorption loss:
Absorption of light caused by impurities in the fibre, such as water and metals ions.
One of the most common impurities is dissolved water in the glass, present as the
hydroxyl or OH ion. In this case the fundamental processes takes place between 2700
nm and 4200 nm, but gives rise to so called absorption overtones at 1380, 950 and 720
nm. Extrinsic absorption depends only on the absorption wavelength of a particular
impurity and on the level of the impurity. Very recently newly developed fibre
manufacturing techniques have virtually eliminated absorption loss peaks giving rise to
silica fibres which show no absorption peaks, This in turn opens up transmission at
wavelengths circa 1400 nm and 1000 nm, which have not been utilised to date.
Scattering Loss:
Scattering is a process whereby all or some of the optical power in a mode is
transferred into another mode. This frequently causes attenuation, since the transfer is
often to a mode which does not propagate well. (also called a leaky or radiation mode).
One of the most common forms of scattering is Rayleigh, a the dominant loss
mechanism in the low loss silica window between 800 nm and 1600 nm. The
attenuation caused by Rayleigh scattering falls off with wavelength as a function of the
4th power of wavelength.
[6 Marks]
1 (b) Modal dispersion
In a multimode fibre different modes travel at different velocities. As a result if an
optical pulse is constituted from different modes then as different modes reach the end
of the fibre at different times then intermodal dispersion occurs. The ray diagram
model below where modes are approximated by rays can give an approximate
description of modal dispersion
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Modal dispersion can be reduced by:
(1) Using a Graded index fibre design in which the propagation velocities of the
various modes are partially equalised
Solutions
(2) By reducing the core diameter of the fibre, transmission is restricted to a
singlemode which by definition cannot undergo modal dispersion.
Material Dispersion
Material dispersion is pulse broadening in an optical fibre resulting from the different
group velocities of the various spectral components at different wavelengths launched
into the fibre by a source. It occurs when the velocity of a plane wave propagating in
the dielectric medium varies non-linearly with wavelength. This is a result of a
refractive index which varies non-linearly with wavelength, and since the velocity of
propagation is a function of refractive index, the result is a non-linear variation of
velocity with wavelength. Material dispersion can be reduced by using a singlemode
laser with a narrow spectral width, e.g. a Distributed Feedback laser, (DFB) and/or by
operating at a wavelength at which material dispersion reaches a minimum, around
1320 nm for normal silica fibres
Polarization Mode dispersion
In a singlemode optical fibre two orthogonal polarization states exists. If the
polarization components propagate at different velocities then pulse broadening
(dispersion) takes place. PMD is a key factor at bit rates above STM-16 (2.5
Gbits/sec), eg. at STM-64. PMD is caused by cylindrical asymmetry due to
manufacturing, temperature, bends, and so forth that lead to birefringence. Improving
core circularity at manufacture and reducing cable induce stress on fibre are two
approaches to reducing PMD
[6 Marks]
1 (c)
As given for an optical fibre the time taken m for an impulse to propagate a distance
L is given by:
In an optical fibre the velocity of propagation is a function of the refractive index of
the core material as expected. However the differential term in the above equation
reminds us that the refractive index itself is a complex function of wavelength and
thus the velocity of propagation becomes a function of wavelength. Thus propagation
time for a pulse becomes wavelength dependent. This is not a problem for truly
monochromatic sources but for conventional sources with a non-zero spectral width
pulse broadening, so called material or chromatic dispersion takes place.
[3 Marks]
Derivation:
L
dn 
m  n1   1 
c
d 
Assume an optical source with an RMS optical spectral width of  and a mean
wavelength of . The RMS pulse broadening in time due to material dispersion m
may be found by expanding the equation above using a Taylor series:
Solutions













d m
2 d2 m ...
 m  

d   d2
In practice it is found that the first term normally dominates, thus:
d
m

m
d
But from the original equation given we know that:
2
d
d
n
d
n
d
n
L


m
1
1
1



2


d
d
d
d
c








2
d
n

L


1

2


d
c


2
d
n
L
1

2
m
d
c
Thus we can write an expression for the RMS pulse broadening:
From this equation the condition for zero pulse broadening or dispersion is that:
d2 n1
 0
d 2
[8 Marks]
In practice this condition can be approximated in silica fibre by operating close to
1320 nm where the second differential goes through zero (1550 nm for Dispersion
Shifted Fibre) or alternatively over a span of fibre by using a commercial dispersion
compensation module.
[2 Marks]
Solutions
2 (a) Parametric mismatch: In a fibre-to-fibre joint even where there is perfect
“mechanical” alignment of the fibre cores (e.g. no lateral misalignment etc.) there still
exists the possibility of loss because of slight parameter differences between the two
fibres, this is referred to as parametric loss resulting from parametric mismatch.
The three most common forms of parametric mismatch are:

Core diameter mismatch: If the exit fibre core in a joint is smaller any mismatch
will cause loss

Numerical aperture mismatch: If the exit fibre has a lower NA then loss will occur

Core concentricity. If the core of either is not centred within the cladding loss will
occur in joint types where the claddings are aligned (eg. connectors, mechanical
splices) rather than the actual core (eg. a fusion splice from a full three axis fusion
splice machine).
[5 Marks]
2 (b)





Higher densities – Connectors are much smaller (lucent) or a traditional sized
connector now has two fibres in it (AMP mtrj, 3M volition)
Improved plastics technologies (injection moulding) sufficiently accurate.
Labour Costs
Steps removed in the preparation of the connector e.g. no adhesive and hence
no curing. Minimal polishing . Multiple fibres prepared at the same time (3M
volition)
Use of VCSELs means that relatively high bit rates can be achieved over short
runs of multimode fibre. Hence single mode fibre can be avoided and the
resultant connector issues. E.g. increased tolerances.
[10 Marks]
2 (c) Equipment:




Honeywell HFE4080-32X VCSEL Laser mounted in an ST package on a laser heatsink
ILX 3412 precision Laser diode Driver and associated mains supply and laser interconnect
lead
Interconnecting ST connectorised 62.5/125 µm fibre patchcord
Fiber Optic power level meter, Megger OTP 620.
Connect the laser to the ILX laser driver (the front cover is shown below) and the laser
optical output to the optical power meter. The laser driver output can be controlled to
within 0.1 mA. Note that to protect the laser the current output is initially off when the
mains power switch is turned on.
Turn on the laser driver using the power button. Turn the laser driver output current
control to zero (fully anti-clockwise) prior to turning on the laser current. To turn on
the laser current press the small output button once (below the current set knob).
Rotating the laser output knob clockwise will increase the current, the value of
which is shown on the display. If the display does not show an increase in current
recheck your connections.
To switch off the laser current depress output button once more. To protect the
Solutions
laser from transient damage do not disconnect the laser from the laser driver at any
time when the laser current is enabled. The correct sequence to disassemble the
experiment is to turn off the laser output, then disconnect the laser from the ILX laser
driver, then turn off the ILX driver using the power button.
Measure the laser diode light-current characteristic, by varying the laser diode current
from about 1 mA up to a maximum of 12 mA, in small increments (typically 0.2 mA,
but close to threshold smaller increments of 0.1 mA will be needed for accuracy.
Monitor the output of the laser via the supplied ST connectorised 62.5/125 µm optical
fibre patchcord, connected to the optical power meter. Set the optical power meter
to measure µW at 850 nm. WARNING: The maximum laser diode current must never
exceed 12 mA otherwise permanent damage to the laser will result. Plot the laser diode
light-current curve.
The laser threshold is the point where the laser changes its operating mode from a
spontaneous emission (like a light emitting diode (LED)) to stimulated emission
(Lasing). By convention the threshold can be found by drawing a line parallel to
the characteristic above threshold. The point where the line intercepts the X or Current
axis is the threshold.
The differential slope efficiency (dL/dI) is the slope of the characteristic at a particular
current. The SE has units of µW per mA (or mW per mA depending on the laser
optical power). The differential slope efficiency can be found by calculating the slope
at a number of sections along the characteristic. E.g. above threshold identify a linear
section and picking two points at the extremes of the sections calculate the slope using.
y 2  y1
x2  x1
Solutions
[10 Marks]