one of the most remarkable and important pieces of technology to emerge
in the last quarter of the 20th century, the optical fibre is literally
the backbone of the "digital revolution". In the two decades since its
entry into the market, the optical fibre has almost completely displaced
the copper cable in long haul applications, and continues to displace
copper in medium and short haul applications.
The most recent development in optical communications is
the idea of optical amplification, which yields significant gains over
the established technology of semiconductor based amplification. In
this month's article, I will discuss the idiosyncrasies and limitations
of established technology, and explore the basic ideas behind optical
Conventional Optical Links
The decisive measure of the worth of any communications
medium is its transmission performance, which at the most fundamental
level is measured by the signal loss through a finite length of a
channel, and the bandwidth limiting effects which constrain the speed
of the channel.
Signal loss is a very simple metric, which compares the
power fed in at one end of a channel with the power extracted at the
other end. The greater the loss, the less power is available to a
receiver, and the more difficult it becomes to extract a signal from
the ever present noise with a reasonable error rate. Indeed this is a
central aspect of Shannon's communications theory. Therefore the lower
the loss rate of the medium, the more useful it becomes for carrying
"Bandwidth limiting effects" are a very rough metric,
which really covers a whole range of possible phenomena, all of which
result in limitations in the data rate we can push through a medium
regardless of the throughput limiting effects of signal loss itself.
Therefore the ideal communications channel is one with
infinite bandwidth and zero loss. The closer we can get to this ideal
with a practical channel, the greater the possible throughput through
Prior to the advent of the optical fibre, the best cable
technology available was the coaxial cable, colloquially known as coax.
Coax is still widely used, and is the primary distribution medium for
the cable TV network (a somewhat short-sighted decision in this
A coaxial cable is essentially a annular waveguide, with
a tubular metal shield, and a solid metal core. The annular waveguide
cavity between the core and the shield contains a dielectric, either
gas, or a solid or foamed plastic like polyethylene or Teflon. The
performance of a coax cable is mostly limited by the frequency
dependency of the dielectric losses, and the ever present skin effect.
Even with an excellent dielectric like air or gas, the skin effect,
which causes electrical currents to be confined to an increasingly
shallower depth in the shield and core, will ultimately limit the
usefulness of coax for high speed data. A coaxial cable which has a loss
rate of about 30 deciBels/km at 100 MHz will typically lose thousands
or more deciBels/km at 10 GHz, and is thus unusable.
A typical communications link of this period was built
using transistor analogue, and later digital repeaters, separated by
lengths of coax of the order of a kilometre in length. To span hundreds
of kilometres, you needed hundreds of repeaters. Therefore the
economics of coax for long haul transmission are not spectacular,
exacerbated by the severe degradation of cable performance with aging
and corrosion. A decade is considered to be a reasonable life for a
The basic idea behind optical fibres was to create an
optical rather than electrical waveguide, to exploit the potentially
very low loss rates of very pure glass materials. While very simple in
concept, the implementation of an optical cable is anything but
trivial, indeed major obstacles in pure glass manufacture and fibre
fabrication techniques, as well as materials and design techniques in
semiconductor lasers, had to be overcome before the potential of the
"light pipe" could be realised.
Since the basic material is glass, the durability
problems of metal/dielectric coaxial cables disappear. The primary
limitation to the life of contemporary fibre cables is the degradation
of the organic materials used in the mechanical construction of the
cable itself !
The optical fibre traps light internally by using a
cladding and core with different optical refractive indices. Light fed
into the core cannot escape since it is internally reflected at the
boundary between the core and cladding.
An optical communications link therefore comprises a
high speed laser at one end, a length of fibre, and a high speed
receiver at the other end. An electrical signal is used to modulate the
laser, and an optical detector in the receiver produces a faint
electrical output which is electrically amplified and then demodulated
to extract the signal. A simple model, not unlike the coaxial cable
link, but using an electrical/optical and optical/electrical conversion
at either end of the cable.
As always, the devil is in the details. To best
understand the limitations of this scheme, it is helpful to digress
into the materials issues, and the evolutionary history of optical
The loss performance of a fibre is determined by the
scattering and absorption the optical signal experiences in the glass.
This depends upon the purity of the glass, its homogeneity, and the
material properties of the glass itself.
Absorption losses arise from impurities, such as OH (ie
water) ions and transition metals in silica glasses, but also from
resonances in the glass itself, and harmonics of these. These losses
vary with an inverse sixth power law of the wavelength. For typical
silica glasses, "windows" of very low loss arise at 0.81 microns, 1.3
microns and 1.55 microns of wavelength. These intrinsic losses are
typically about 0.3 and 0.15 dB/km respectively, for the latter
wavelengths. This is between 1/100 and 1/200 the dB/km loss of a
coaxial cable ! An interesting statistic is that the first
transatlantic fibre cable used 95 repeaters, spaced about 70 km apart.
In practice many silica fibres can get very close to
this loss performance, although fabrication becomes quite finicky.
Inhomogeneous glass structure, at a microscopic level, introduces an
inverse fourth power Raleigh scattering loss.
Fibre fabrication technology converged quite rapidly to
very low loss fibre designs, and the required solid state laser
technology for 1.31 and 1.55 micron operation appeared by the mid
The bigger issue in fibre design was achieving Gigabit/s
and faster transmission speeds over significant distances. To best
understand why we have to explore the structure of the fibre, and the
limitations of the receivers and laser transmitters.
At this point in time we can classify optical fibre
designs into four discrete generations. These are step index multimode
fibres, graded index multimode fibres, single mode fibres and advanced
single mode fibres.
The simplest fibre design is the step index (SI) fibre,
in which the refractive indices of the core and cladding are constant
through the cross section of the fibre. A typical fibre of this ilk has
a 100 micron core and 140 micron cladding. It has one very ugly
limitation, which is termed modal dispersion. The light which is coupled
into the fibre travels different distances, depending upon the angle at
which it entered the fibre. As a result, part of your signal arrives
earlier, and part later. This effect gets worse, the longer the fibre
cable is. For all practical purposes, the step index fibre is limited
to short haul low speed links.
By the early eighties, the step index fibre was
supplanted by the more sophisticated graded index (GI) fibre, in which
the refractive index between the core and cladding changed smoothly,
rather than abruptly, following a power law or Gaussian profile. The
idea was to cleverly bend the light rays in such a manner, to minimise
the modal dispersion. A typical GI fibre has a 62.5 micron core and 125
micron cladding diameter. While the GI fibre was an enormous
improvement over the SI fibre, it still fell short.
The mainstay of today's long haul communications is the
single mode (SM) fibre. An SM fibre has a core so small in diameter,
that it supports only a single mode of transmission, and thus modal
dispersion is eliminated. Such fibres typically have core diameters of
single microns. The price to be paid for almost unlimited speed is
however the small diameter, and thus much more difficult coupling of
light into the fibre. Indeed a good part of the higher cost of SM
lasers is need to add a precise optical coupling arrangement to get the
light from the laser into the fibre with minimal losses.
Once the modal dispersion speed limit was broken,
another one was found. This was chromatic or colour dispersion.
Chromatic dispersion arises as a result of the dependency of
propagation velocity of light upon its wavelength. This in turn is a
result of the refractive index of the fibre varying with wavelength. In
practical terms this means that a pulse of light transmitted at
slightly differing wavelengths arrives slightly earlier or later at the
far end of the fibre, depending on the wavelength of the light.
The conventional semiconductor laser is an "impure"
light source. Due the physics of the laser cavity design, it typically
puts out light with a Gaussian colour spectrum, centred on its nominal
If we are trying to pump pulses through a fibre at
Gigabit/s rates, the pulses will be spread in time, in a manner
reflecting the colour spectrum of the laser we are using and the length
of the fibre.
It is a fortuitous accident that the chromatic
dispersion performance of a silica glass is minimised at 1.3 microns,
which also happens to be region of decent loss performance. As a result
most early SM fibre systems operated at 1.3 microns.
Advances were subsequently made in laser technology, to
produce lasers which had a single dominant spectral line, ie truly
spectrally pure lasers. The distributed feedback laser (DFB)
incorporates internal corrugations which form a diffraction grating.
This in effect tunes the laser very sharply to a single wavelength,
thereby defeating the chromatic dispersion effect in the glass.
The DFB laser allowed the much lower loss 1.55 micron
transmission region to be exploited, virtually halving the dB/km loss
seen in a basic silica glass fibre.
This generation of fibre transmission equipment first
appeared in the mid eighties. I had the opportunity to participate in
the design of a 140 Megabit/s commercial fibre system, using 1.31
micron technology, in 1984, while also performing performance test work
on 400 Megabit/s 1.55 micron hardware. At that stage we began to run
into the next performance barrier to be beaten, which was the speed
(and cost) of the electronics at either end of the fibre, and by
default in every repeater.
While the digital portions of the designs could be
readily built using Emitter Coupled Logic (ECL) gate arrays and glue
chips, the analogue portions of the designs required to interface to
optical detectors and lasers were a genuine headache. The speed at
which the ones and zeroes are being clocked is significantly faster
than the carrier frequencies of most TV stations !
Laser drivers and wideband front end receivers for such
speeds are significantly more difficult to design well than narrowband
RF hardware. This is because the circuits must have very low phase
distortion, to avoid unwanted shape distortion of the transmitted and
received optical signal. I still take much pleasure in the knowledge
that I was one of a select few who had mastered this artform.
The result is increasingly more expensive hardware, with
increasing speeds. With the need to insert repeaters in every 50-200 km
apart, this severely impacted the economics of link design.
Laser performance also proved to be a big issue, since
the startup transients of the lasers were often quite nasty.
The basic modulation speed of the lasers became the next
target for improvement. In the decade between 1984 and 1994 we have seen
laser modulation speeds go up from 4,5 GHz up to a staggering 20 GHz.
While the availability of such lasers raised the
achievable link speeds, the economics of the electrical repeaters
inserted between fibre spans did not dramatically improve. Using GaAs
MMIC receivers, GaAs logic and very fast ECL, on printed circuit board
layouts which must be hand crafted by high speed analogue design
engineers, is not a recipe for cheap, economical mass production
designs ! The problem is further complicated by the basic inflexibility
of such hardware, which must be crafted around a specific bit rate and
modulation technique. If you want to change either, you have to ditch
every single repeater in the link. In an extremely competitive,
deregulated telecommunications market, this imposes strong pressures
for the rapid amortisation of the investment, yet the competitive
pressures force a minimal profit margin per bandwidth.
Detector sensitivity also became an issue for long haul
links, since with increasing distance between repeaters the number of
photons in a single bit dropped to dozens. Extracting them from the
noise in the detector became increasingly difficult, imposing limits on
The solution to these problems is direct optical
The basic idea behind optical amplification dates back
to the early days of laser research. The basic physics of a laser are
based on the idea of exciting ("pumping") a volume of a suitable
material (solid or gas) in such a manner, that impinging photons of a
specific wavelength can stimulate the emission of further photons of
the same wavelength. Therefore if you shine light at that wavelength
into one end of a volume of so excited material, you get much brighter
light coming out of the opposite end. If you place suitable mirrors at
either end, the photons will bounce back and forth and you get an
optical oscillator, termed a laser. If one of the mirrors is slightly
leaky, you can extract optical power from the laser.
The first serious application of optical amplification
was in high power military laser experiments performed in the US during
the late sixties and seventies. A low power laser was used as a "master
oscillator", and the light from it was fed into a cascade of "power
amplifier" stages, which boosted it to a much higher power level. This
is termed a MOPA (Master Oscillator Power Amplifier) arrangement.
While the MOPA idea was well understood very early,
adapting it to the world of optical fibres was not that simple.
The payoff in using this idea for boosting signals in
fibre links is very high. An optical amplifier doesn't care about the
signal modulation, it simply spits out some large number of photons for
every photon it is fed with, faithfully reproducing whatever the
original modulation was. It is thus inherently an extremely fast, low
distortion amplifier. It is also a very simple amplifier, since there
is no need for any conversion between optical and electrical signals.
All that is required is an external source of optical excitation to
"pump" the lasing medium in the amplifier.
The gain of an optical amplifier, ie the ratio of
photons emitted per photons received (or more precisely the ratio of
output optical power to input optical power), is dependent upon the
length of the amplifier and the gain per unit length of the excited
medium. Therefore very high gains can be achieved by cascading multiple
optical amplifier stages. Noise performance can be excellent compared
to conventional electro-optical systems.
By eliminating complex and inflexible electro-optical
repeaters in links, costs can be significantly improved and the
longevity of a cable installation significantly extended. Performance
upgrades in many instances will require only the replacement of the line
terminal equipment at either end of the link.
By the early eighties the race was on in the research
community to find suitable laser technology for adaptation to the unique
fibre environment. Early research explored Raman effect amplification
and adaptations of established semiconductor lasers. Both proved to be
disappointing, with Raman effect designs requiring unreasonably large
pumping power due to their low efficiency, and semiconductor lasers
having high distortion.
The breakthrough came in the mid eighties when a
research group in Southampton in the UK devised a rare earth ion doping
technique for silica fibres, building on the same sixties research which
led to the now standard military Nd:YAG laser, used for bomb guidance.
The first Erbium Doped Fibre Amplifier (EDFA) was published in 1987.
The next important development was the 1.48 micron
InGaAsP laser diode, the pump power source required to optically excite
the Erbium ions embedded in the glass, to amplify at 1.55 microns.
Japan's NTT published their results in 1989.
With a compact and efficient pump source and doped fibre
technology the practical EDFA became a reality.
To build one, the starting point is a spool of Erbium
doped optical fibre of a suitable length. The input, where the EDFA is
coupled to the end of a fibre link, uses a wavelength selective coupler.
The coupler is used to feed the flow of pumping photons from the pump
laser into the EDFA. These photons propagate along the fibre, exciting
it. The "signal" photons pass through the coupler into the EDFA, and
are amplified in number as they pass through the excited fibre. When
they reach the output end of the spool, they are fed into an optical
splitter. Most of the photons go into the next segment of the fibre
link, but some are split off to feed a local optical detector. The
electrical output from the detector is then used in a negative feedback
loop to control the power level produced by the pump laser. In this
manner the EDFA gain can be quite precisely controlled.
While conceptually the EDFA is fairly simple, the laser
physics and system design issues can be quite complex (serious readers
are directed to http://188.8.131.52:591/copl/lco/anglais/index.html,
who have an excellent collection of material online).
Commercially available EDFA technology at this time
covers a wide range of packaging and performance specifications, and a
number of different pumping wavelengths and pumping designs. Current
designs commonly employ a two stage arrangement, using silica glass,
combinations of silica glass and fluoride glass, or fluoride glass
Noise figures typically vary between 4.5 and 9 dB
(competitive with GaAs electrical receivers), gains between 25 and 40
dB, and output power levels between 13 and 20 dBs. We are also seeing
the first commercial designs optimised for 1.3 micron systems, using
praseodymium doped fluoride fibre amplifier (PDFFA) technology, and
achieving similar performance to 1.55 micron EDFAs.
At the time of writing the leading players in the market
were JDS-FITEL, ORTEL, FTI, Bosch Telecom, NTT and Galileo Corp.
Optical amplifiers will see further improvements in
coming years, as the technology matures and is further refined. We are
already seeing significant reductions in the cost of long haul
telecommunications and this trend will continue as the technology
further proliferates in the market. For the forseeable future the
performance bottleneck for long haul fibre links will continue to be in
the line terminal equipment, limited by laser and electronic circuit