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The Fibre Optic Path
November 2022
Geoff Huston
In August 1858, Queen Victoria sent the first transatlantic telegram
to U.S. President James Buchanan.
The cable system had taken a total of four years to build, and used 7
copper wires, wrapped in a sheath of gutta percha, then covered with
a tarred hemp wrap and then sheathed in an 18-strand wrap, each
strand made of 7 iron wires. It weighed 550kg per km, with a total
weight of over 1.3Mkg. This was so heavy that it took two vessels to
undertake the cable lay, and meeting in middle of the Atlantic,
splicing the two cables together, then setting out east and west
respectively. The result could hardly be described as a high-speed
system, even by the telegraphic standards of the day, as Queen
Victoria's 98-word message took a total of 16 hours of attempts and
retries to send.
But poor signal reception was not the only problem with the cable.
The copper conductor was powered with a massive high-voltage
induction coil producing several thousand volts, so enough current
would be available to drive the standard electromechanical printing
telegraph station at the receiving end. This, coupled with the weight
and cost saving measure of using thinner copper wires, were important
reasons for the cable's receptions problems, and for its subsequent
demise. To compensate for the deteriorating quality of the signal,
the cable company's chief engineer, Wildman Whitehouse, responded by
progressively increasing the voltage applied to the cable system. In
a DC system increasing the voltage increases the current, which
raises the temperature of the copper conductor and in this case
caused failure in the insulation, which obviously exacerbated the
deterioration of the signal to a catastrophic extent. After three
weeks, and just some 732 messages, the cable failed completely.
The cable company's inquiry into the failure found that Whitehouse
was responsible for the cable's failure and was dismissed. But the
problems were not solely due to Whitehouse's efforts in increasing
the voltage, but in poor cable construction in the first place. Not
only did they use a thin copper conductor, but in places the copper
was off centre and could easily break through the insulation when the
cable was laid. A test sample was compromised with a pinprick hole
that "lit up like a lantern" when tested, and a large hole was burned
in the insulation.
[optics-fig1]
Figure 1 - Isambard Kingdom Brunel's SS Great Eastern, the ship that
laid the first lasting transatlantic cable in 1866
But this was not seen as a failure in the entire concept of
trans-oceanic cables. It was an early proof that the concept was
workable, but it needed a higher quality implementation. Perhaps more
sensitive reception equipment, such as Thompson's mirror galvanometer
and siphon recorder. Perhaps thicker conductors, and larger cable lay
ships to avoid at-sea splicing with its attendant cable handling
issues. Similar to the railway boom some 10 years earlier, a flurry
of companies formed to lay more undersea cables that very quickly
wrapped the globe.
Throughout the 1860s and 1870s, British-funded cables expanded
eastward, into the Mediterranean Sea and the Indian Ocean. An 1863
cable to Bombay (now Mumbai), India, provided a crucial link to Saudi
Arabia. In 1870, Bombay was linked to London via submarine cable in a
combined operation by four cable companies, at the behest of the
British Government. In 1872, these four companies were combined to
form the mammoth Eastern Telegraph Company, owned by John Pender. A
spin-off from Eastern Telegraph Company was a second sister company,
the Eastern Extension, China and Australasia Telegraph Company,
commonly known simply as "the Extension". In 1872, Australia was
linked by cable to Bombay via Singapore and in 1876, the cable linked
the British Empire from London to New Zealand.
[optics-fig2]
Figure 2 - Map of the All Red Line, c 1902
Immediately thereafter there were a number of pressures on this
burgeoning new industry: make the cables more reliable, increase the
reach of the cable system and reduce the cost of messages and at the
same time increase the speed. We settled into a few decades of
progressive refinement of the basic model, where improvements were
incremental rather than meteoric. It was not until the early 20th
century that message speeds on trans-Atlantic cables would exceed 120
words per minute.
In so many ways the commercial pressures for increased speed,
reliability and reach have continued to drive the cable industry for
the ensuring 150 years. The industry initially concentrated on
metallic conductors for the signal, namely copper, matching the
capabilities of the electric technology of the day. But metallic
conductors have some severe limitations. There are noise components,
frequency limitations, and current limitations. So, in the effort to
increase the capacity of the cable system in response to the growing
demands of telephony, we turned to fibre optic systems. The first
transatlantic telephone cable to use optical fibre was TAT-8, which
went into operation in 1988.
Fibre Optic Cables
There is a curious physical property of light that when it passes
from one medium to another with a different index of refraction, some
of the light will be reflected instead of being transmitted. When you
look through a clear glass window you will see a faint reflection of
yourself.
Depending on the angle of incidence of light meeting a refractive
boundary, the proportion of reflected light can be varied. For low
incidence light, the reflection rate can approach 100%, or total
internal reflection.
References to the original work that describe this total internal
reflection property of light appear to date back to the work of
Theodoric of Freiberg in the early 1300's, looking at the internal
reflection of sunlight in a spherical raindrop. The behaviour of
light was re-discovered by Johannes Kepler in 1611 and again by Rene
Descartes in 1637 who described it as a law of refraction. Christiaan
Huygens, in his 1690 Treatise on Light, examined the property of a
threshold angle of incidence at which the incident ray cannot
penetrate the other transparent substance. Although he didn't provide
a way to calculate this critical angle, he described examples of
glass-to-air and water-to-air incidence. Isaac Newton with his 1704
corpuscular theory of light explained light propagation more simply,
and it accounted for the ordinary laws of refraction and reflection,
including total internal reflection on the hypothesis that the
corpuscles of light were subject to a force acting perpendicular to
the interface. William Wollaston in 1802 invented a refractometer to
measure the so-called refractive powers of numerous materials.
Pierre-Simon Laplace took up this work and proposed a single formula
for the relative refractive index in terms of the minimum angle of
incidence for total internal refraction. Augustin-Jean Fresnel came
to the study of total internal reflection through his research on
polarization. In 1816, Fresnel offered his first attempt at a
wave-based theory of chromatic polarization. Fresnel's theory treated
the light as consisting of two perpendicularly polarized components.
In 1817 he noticed that plane-polarized light seemed to be partly
depolarized by total internal reflection, if initially polarized at
an acute angle to the plane of incidence.
While the behaviour of total internal reflection had been useful in
gaining a better understanding of the nature of light, it was still a
solution in term of a practical problem. In 1842 a Swiss physicist,
Jean-Daniel Colladon, demonstrated the use of a tube of water as a
wave guide for light. This was also demonstrated some 17 years later
by John Tyndall in 1859. The subsequent years saw these "light
bending" experiments shift from columns of water to fine strands of
glass (fibres).
[optics-fig3]
Figure 3 - Daniel Colladon's demonstration of total internal
reflection of light in a water tube
It was not until 1930 when Heinrich Lamm invented the "medical
endoscope" which was a bundle of fibres carrying light into the body,
and fibre endoscopes became a common tool in the medical space. Some
30 years later, in 1965 we turned our attention to using guided light
in the context of communications, when Manfred Borner of Telefunken
in Germany patented the first fibre optic communication system. At
much the same time STC's Sir Charles Kao offered the perspective that
if we could device of a combination of glass fibre and a light source
that had a loss rate of less than 20db per km, then the result would
be a communications system that could compete cost effectively with
existing metallic conductor cable systems.
20db per kilometer?
Loss is measured in decibels, which is a logarithmic unit. A loss
rate of 10db per kilometre is equivalent to a degradation in the
signal of 90% (1 in 10) while a loss rate or 20db is a degradation of
99% (1 in 100).
At 20db/km a 10km fibre cable has a transmission rate of 1 in 10^20.
A 40km cable has a transmission rate of 1 in 10^80. A photon may be
small, but that's still a lot of photons! By the time you try and
push this 20db/km cable beyond 42km you need to inject more photons
than exist in the visible universe to get even 1 photon out!
In Kao's vision, he was probably thinking about very short cable runs
indeed!
In 1970 a team at Corning made a fibre that had a loss rate of 17db/
km with a light source of 630nm (this 3db gain is the same as halving
the attenuation of a 20db/km system). This introduction of low loss
fibre was a critical milestone in fibre optic cable development. In
work on how to further reduce this loss rate, it was noted that
longer wavelengths had lower rates. This led to work in the early
1970's to increase the wavelength of the light source, using near
infrared light. There were also experiments with changing the doping
agent in the fibre from titanium oxide to germanium oxide. With these
two changes, using a 850nm light source (instead of 630nm) and fibre
cable with germanium oxide doping, the achievable fibre loss rate
dropped to less than 5db/km, which is a truly awesome change in the
loss profile! This 12db drop is a 16-fold improvement in the
'transparency' of the fibre. This result is close to the theoretical
minimum attenuation at 850nm, and the effort then concentrated on
using light sources with even longer wavelengths. By 1976 we had
developed laser light sources that operated at 1200nm, which could
drive fibre with an attention of less than 0.46db/km, and by the end
of the 1970s they were using 1550nm lasers and achieving less than
0.2db/km.
Why is 1550nm important?
The fundamental loss limits for a silica-based glass fibre are
Rayleigh scattering at short wavelengths and material absorption in
the infrared part of the spectrum. A theoretical attenuation minimum
for silica fibres can be predicted at a wavelength of 1550nm where
the two curves cross (Figure 4). This has been one reason for laser
sources and receivers that work in this portion of the spectrum.
[optics-fig4]
Figure 4 - Loss Profile for Silica Fibre
One area of refinement in recent work in germanium-doped silica-based
fibre has been in reducing the water loss peak at 1383nm.
Conventional single-mode fibres have very high loss rate at this
wavelength because the fibre absorbs OH ions during manufacturing.
This water loss point can continue to increase even after cable
installation. The high attenuation makes transmission in this
particular spectral region impractical for traditional single-mode
fibres. There are two types of fibres that address this limitation:
Low Water Peak (LWP) fibres, which lower the loss in the water peak
band of the spectrum, and Zero Water Peak (ZWP) fibres, which
eliminates the heightened loss at the water peak line.
Semi-Conductor Lasers
A parallel thread of development has taken place in the area of
semi-conductor lasers.
This work can be traced back to a period of a renaissance in physics
at the start of the twentieth century. In 1917 Albert Einstein
described the concept of stimulated emission, where if you inject a
photon of precisely the 'right' wavelength into a medium containing a
collection of electrons which have previously been pushed into an
excited state, they will be stimulated to decay back to their
original stable state by emitting photons of the same wavelength of
the trigger photon, and this stimulated emission will occur at the
same time for all of these stimulated electrons. It took another
twenty years for this theoretical conjecture to be confirmed in an
experiment conducted by Rudolph Ladenburg in 1937.
In 1947 Willis Lamb demonstrated stimulated emission in hydrogen
spectra in a gaseous medium. Alfred Kastler then proposed the idea of
optical pumping where shining light of a certain wavelength will push
the electrons in the medium into an excited state which can then be
triggered to emit stimulated emissions in a simultaneous manner. In
1951 Joseph Weber proposed the MASER, or Microwave Amplifier for
Stimulated Emission of Radiation, which operated at the wavelength of
microwaves, with direct application for radar applications,
subsequently developed in 1954 by Charles Townes (Figure 5).
[optics-fig5]
Figure 5 - Charles Townes (left) with the Maser he developed in 1954
In 1957 a group from Bell Labs theorised a MASER that operated at the
frequencies of visible light, and they changed the name to Light
Amplifier for Stimulated Emission of Radiation, or LASER, which was
then demonstrated by Theodore Mainam in 1960 with a pulsed ruby
laser. In this experiment the medium was a ruby crystal, and the
stimulation was provided by a flashbulb. In 1962 Robert Hall proposed
the use of semiconductor material as the gain medium and demonstrated
the pulsed semiconductor laser.
The year 1970 brought all these ideas together into a practical
device. It was a continuous operation device (rather than "pulsed"),
that operated at room temperature (and not cooled by liquid nitrogen)
and used a Gallium Arsenide semiconductor medium, demonstrated by
Zhores Alferov, Izuo Hayashi and Morton Parish. These days the
industry uses a number of alloys for semi-conductor fibre optic
lasers, such as the InGaAsP semiconductor, which uses an alloy of
gallium arsenide, gallium phosphide, and indium phosphide.
Further Refinements
The question was then how to make this approach scale to provide a
high-capacity communications system. We had been through a similar
path in telephony some decades earlier when trying to place multiple
conversations on a single conductor. One approach was to multiplex
the conversations and use a higher common base data rate and
intertwine the signals from each conversation (time division
multiplexing). Another approach is to use frequency division
multiplexing and encode each conversation over a different base
frequency. The same applies to fibre communications. The scaling
options are to increase the data rate of the signal being encoded
into light pulses, or to use a number of parallel data streams and
encode them into carrier beams of different wavelengths, or even to
use both approaches at the same time. Which approach is more cost
effective at any time depends on the current state of lasers and
fibres, and the current state of the encoding and decoding
electronics (codecs) and digital signal processors (DSPs). More
capable signal processors allow for denser encoding of signals into a
wavelength, while more capable fibre allows for more wavelengths to
be used simultaneously.
The period from 1975 to the 1990s saw a set of changes to lasers that
allowed them to operate at longer wavelengths, with sufficient power
to drive 10um core fibre (single mode fibre, or SMF). These are
single mode lasers with external modulators.
The next breakthrough occurred in 1986 where work in the UK by David
Payne at Southampton University and the US by Emmanuel Desurvivre at
Bell Labs both discovered the Erbium Doped Fibre Amplifier (EFDA).
Long fibre runs require regular amplification (or repeaters). The
optical repeater model used prior to EDFA was to pass the signal
through a diffraction grating to separate out the component
wavelengths, send each signal to an optical to digital signal
processor to recover the original binary data, then pass this data to
a laser modulator to convert the signal to an optical signal for the
next fibre leg. Each wavelength required its own
optical-electrical-optical amplifier. When a length of fibre is doped
with erbium, and excited with its own pumped light, then incoming
signals are amplified across the spectrum of wavelengths of the
original signal. Rather than demuxing the composite signal into
individual signals and amplifying them separately, an EDFA system can
use a single piece of doped cable in a repeater to amplify the entire
composite signal.
The implication of EDFA was that the amplification function was
simplified, allowing high wavelength counts to be applied to a
EDFA-based cable system without changing the repeaters.
Fibre Impairments
Modal dispersion is seen in multi-mode fibre. There are a number of
paths through a large core fibre. Some paths use internal reflection
off the edges of the core, while other paths are more direct without
any internal reflection. The reflected paths are longer so a single
square wave input pulse will be dispersed into a waveform that has a
strong resemblance to a Gaussian distribution. Single mode fibre that
uses a 8-9m core diameter does not have this form of signal
impairment, because as long as the core diameter is sufficiently
small, at no more than 10x the signal wavelength, the internal
reflection paths are minimal compared to the path directly along the
core.
Chromatic dispersion is due to the property that lower frequencies
travel faster, and if a signal uses a broad spectrum of frequencies,
then the signal will become dispersed on the transit through the
fibre and the lower frequency component will arrive earlier than the
higher frequency component. To reduce the level of signal degradation
and increase the signal bandwidth G.655 fibre (the most common fibre
used) was engineered to have zero chromatic dispersion at 1310nm as
this was the most common laser source at the time. Signal dispersion
can be compensated by placing dispersion compensating fibre lengths
in every repeater, assuming that you wanted to compensate for
chromatic dispersion (which in Dense Wave Division Multiplexing
(DWDM) is not necessarily the case).
Polarisation mode dispersion. Silica fibre has small scale radial
variations, which means that different polarisation components will
propagate through the fibre at various speeds. This can be corrected
by placing a Polarisation Mode Dispersion (PMD) compensator just in
front of the receiver. This is performed after the wavelength
splitting so one unit is required for every wavelength. This
requirement of PMD compensation per wavelength was an inhibitory
factor in the adoption of 40Gbps per wavelength in long cable systems
in DWDM systems.
These three forms of optical impairments are shown in Figure 6.
[optics-fig6]
Figure 6 - Optical impairments
Non-linear effects. A linear effect is where the effect is
proportional to power and distance, whereas non-linear effects are
threshold based and only appear when a particular threshold is
crossed. For linear effects a common response to compensate is to
increase the input power, but for non-linear effects this will not
necessarily provide compensation. This was first discovered in 1875
by Jon Kerr and was named the Kerr Effect. In a light pulse the
leading part of the pulse changes in power very quickly and this
rapid change causes a change in the refractive index of the medium if
the optical power level is high enough. The result is that there is
self-phase modulation where the pulse interferes with itself. In a
Wave Division Multiplexed (WDM) system there is also cross-talk
modulation, where the signals (pulses) in one wavelength interferes
with the signals in other wavelengths, There is also four wave mixing
where two wavelengths of sufficiently high power levels create two
phantom side signals spaced equally apart, which will interfere with
adjacent signals in a DWDM configuration.
The conventional approach to these non-linear effects was to limit
the optical launch power (which limits the reach, or inter-amplifier
distance in a cable run), and also to use cable with high levels of
chromatic dispersion (so that the pulse energy is dispersed by
chromatic dispersion effects) and to use a large effective core area
in the fibre, both of which were properties of G.652 fibre.
In the search for longer reach and higher capacity of optical systems
we have devised a new set of non-linear mitigations, which include
Nyquist subcarriers, Soft Decision Forward Error Correcting codes.
Super-Gaussian PCS and Non-linear compensation. All these techniques
rely in improvements in the digital signal processing (DSP) in the
transceivers.
In the 1980's the G.652 fibre systems were suited to the 1310nm
lasers that were used at the time. In the search for lower
attenuation, we shifted to 1550nm lasers, which were placed at the
minimum attenuation point for the media, but the large core area
(80um^2) meant high chromatic dispersion which had to be compensated
for with lengths of Dispersion Compensating Fibre (DCF), which
effectively cancelled out the advantages of 1550nm lasers.
There was a brief time when we used G.653 DSF (dispersion shift
compensating fibre), which used a narrower core to shift the zero
chromatic dispersion point up to 1550nm. However, in the mid 1990's
we started to use DWDM systems, and here the aim was to have some
chromatic dispersion to reduce the cross-talk factors in the fibre.
Today we tend to use G.655 Non Zero Dispersion Shift Fibre (NZDSF)
that provides some level of chromatic dispersion at 1550 nm in order
to reduce the cross-talk effects in DWDM systems centred around
1550nm, with a core area of 55um^2.
The next step was G.655 Large Effect Area Fibre (LEAF) with a larger
core area of 72um^2. This is a variant of G.655 with a low band
dispersion area, and a large effective area. This is a better
configuration for single wavelength 10Gb transmission.
Coherent Technology
In 2010 coherent technology was introduced. This form of denser
packing of the signal encoding into the signal through phase and
amplitude modulation, borrowed from the earlier work in various forms
of radio and voice digital models was now being applied to photons
with the advent of more capable DSPs that exploited ever narrower
ASIC track width and higher capabilities at constant power).
The first generation of all-optical systems used simple on/off keying
(OOK) of the digital signal into light on the wire. This OOK signal
encoding technique has been used for signal speeds of up to 10Gbps
per lambda in a WDM system, achieved in 2000 in deployed systems, but
cables with yet higher capacity per lambda are infeasible for long
cable runs due to the combination of chromatic dispersion and
polarisation mode dispersion.
At this point coherent radio frequency modulation techniques were
introduced into the digital signal processors used for optical
signals, combined with wave division multiplexing. This was enabled
with the development of improved digital signal processing (DSP)
techniques borrowed from the radio domain, where receiving equipment
was able to detect rapid changes in the phase of in incoming carrier
signal as well as changes in amplitude and polarization.
Using these DSPs it's possible to modulate the signal in each lambda
by performing phase modulation of the signal. Quadrature Phase Shift
Keying (QPSK) defines four signal points, each separated at 90-degree
phase shifts, allowing 2 bits to be encoded in a single symbol. A
combination of 2-point polarisation mode encoding and QPSK allows for
3 bits per symbol. The practical outcome is that a C-band based 5Thz
optical carriage system using QPSK and DWDM can be configured to
carry a total capacity across all of its channels of some 25Tbps,
assuming a reasonably good signal to noise ratio. The other
beneficial outcome is that these extremely high speeds can be
achieved with far more modest components. A 100G channel is
constructed as 8 x 12.5G individual bearers.
[optics-fig7]
Figure 7 - Phase-Amplitude space mapping of QPSK keying
This encoding can be further augmented with amplitude modulation.
Beyond QPSK there is 8QAM that adds another four points to the QPSK
encoding, adding additional phase offers of 45 degrees and at half
the amplitude. 8QAM permits a group coding of 3 bits per symbol but
requires an improvement in the signal to noise ratio of 4db. 16QAM
defines, as its name suggests 16 discrete points in the phase
amplitude space which allows the encoding of 4 bits per symbol, at a
cost of a further 3db in the minimum acceptable S/N ratio. The
practical limit of increasing the number of encoding points in phase
amplitude space is the signal to noise ratio of the cable, as the
more complex the encoding the greater the demands placed on the
digital signal processor.
[optics-fig8]
Figure 8 - Adaptive Modulation Constellations for QPSK, 8PSK, 16QAM
AND 64QAM
There are two parts to this evolution. The first is increasing the
base signal rate, or baud rate. The second is to increase the signal
to noise ratio of the system, and the third is to increase the
capability of the digital signal processor. Table 1 shows the result
of successive refinements to coherent fibre systems since 2010.
Year Mode Baud Capacity/Lambda Cable Capacity DSP
2010 PM-QPSK 32 GBd 100G 8T, C-Band 40nm
2015 PM-16QAM 32 GBd 200G 19.2T, Ext C 28nm
2017 PM-32QAM 56 GBd 400G 19.2T, Ext C 28nm
2019 PM-64QAM 68 GBd 600G 38T, Ext C 16nm
2020 PS-PM-64QAM 100 GBd 800G 42T, Ext C 7nm
2022 PCS-144QAM 190 GBd 2.2T 105T, Ext C 5nm
Table 1 - Coherent Fibre Evolution
Futures
We are by no means near the end of the path in the evolution of fibre
optic cable systems, and ideas on how to improve the cost and
performance still abound. Optical transmission capability has
increased by a factor of around 100 every decade for the past three
decades and while it would be foolhardy to predict that this pace of
capability refinement will come to an abrupt halt, it also must be
admitted that sustaining this growth will take a considerable degree
of technological innovation in the coming years.
One observation is that the work so far has concentrated on getting
the most we can out of a single fibre pair. The issue is that to
achieve this we are running the system in a very inefficient power
mode where a large proportion of the power gets converted to optical
noise that we are then required to filter out. An alternative
approach is to use a collection of cores within a multi-core fibre
and drive each core at a far lower power level. System capacity and
power efficiency can both be improved with such an approach.
The refinements of DSPs will continue, but we may see changes to the
systems that inject the signal into the cable. In the same way that
vectored DSL systems use pre-compensation of the injected signal in
order to compensate for signal distortion in the copper loop, it may
be possible to use pre-distortion in the laser drivers, or possibly
even in the EDFA segments, in order to achieve even higher
performance from these cable systems.
There is still much we can do in this space!
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Disclaimer
The above views do not necessarily represent the views of the Asia
Pacific Network Information Centre.
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GEOFF HUSTON AM, B.Sc., M.Sc., is the Chief Scientist at APNIC, the
Regional Internet Registry serving the Asia Pacific region.
www.potaroo.net