Understanding Optical Communications:Optical Fibre - IMEDEA

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2.2.1.1 Attenuation (Absorption) Characteristics of Glasses

Figure 13 on page 31 shows the attenuation characteristics of typical modern fibres in the infrared range. Light becomes invisible (infrared) at wavelengths longer than about 730 nanometers (nm).

Note:  1 nm = 10 Å (Angstrom)

There are a wide range of glasses available and characteristics vary depending on their chemical composition. Over the past few years the transmission properties of glass have been improved considerably. In the “ballpark” attenuation of a silicon fibre was 20 dB/km. By research had improved this to 1 dB/km. In the figure was 0.2 dB/km. As the figures show, absorption varies considerably with wavelength and the two curves show just how different the characteristics of different glasses can be.

Figure 13.  Typical Fibre Infrared Absorption Spectrum. The lower curve shows the characteristics of a single-mode fibre made from a glass containing about 4% of germanium dioxide (GeO2) dopant in the core. The upper curve is for modern graded index multimode fibre. Attenuation in multimode fibre is higher than in single-mode because higher levels of dopant are used. The peak at around nm is due to the effects of traces of water in the glass.

Most of the attenuation in fibre is caused by light being scattered by minute variations (less than 1/10th of the wavelength) in the density or composition of the glass. This is called “Rayleigh Scattering”. Rayleigh scattering is also the reason that the sky is blue and that sunsets are red.

In fibre, Rayleigh scattering is inversely proportional to the fourth power of the wavelength! This accounts for perhaps 90% of the enormous difference in attenuation of light at 850 nm wavelength from that at nm. Unfortunately, we can’t do a lot about Rayleigh scattering by improving fibre manufacturing techniques.

There is another form of scattering called “Mie Scattering”. Mie scattering is caused by imperfections in the fibre of a size roughly comparable with the wavelength. This is not a significant concern with modern fibres as recent improvements in manufacturing techniques have all but eliminated the problem.

The absorption peak shown in Figure 13 is centered at nm but it is “broadened” by several factors including the action of ambient heat. This absorption is caused by the presence of the -OH atomic bond, that is, the presence of water. The bond is resonant at the wavelength of nm. Water is extremely hard to eliminate from the fibre during manufacturing and the small residual peak shown in the diagram is typical of current, good quality fibres. In the past this peak was significantly greater in height than shown in the figure (up to 4 dB/km).

In the early days of optical fibre communications impurities in the glass were the chief source of attenuation. Iron (Fe), chromium (Cr) and nickel (Ni) can cause significant absorption even in quantities as low as one part per billion. Today, techniques of purifying silica have improved to the point where impurities are no longer a significant concern.

Some of the dopants added to the glass to modify the refractive index of the fibre have the unwanted side effect of significantly increasing the absorption. This is why single-mode fibre has typically lower absorption than multimode - it has less dopant. The conclusion that can be drawn from the absorption spectrum is that some wavelengths will be significantly better for transmission purposes than others.

2.2.1.2 Fibre Transmission Windows (Bands)

Figure 14.  Transmission Windows. The upper curve shows the absorption characteristics of fibre in the s. The lower one is for modern fibre.

In the early days of optical fibre communication, fibre attenuation was best represented by the upper curve in Figure 14 (a large difference from today). Partly for historic reasons, there are considered to be three “windows” or bands in the transmission spectrum of optical fibre. The wavelength band used by a system is an extremely important defining characteristic of that optical system.

Short Wavelength Band (First Window)

This is the band around 800-900 nm. This was the first band used for optical fibre communication in the s and early s. It was attractive because of a local dip in the attenuation profile (of fibre at the time) but also (mainly) because you can use low cost optical sources and detectors in this band.

Medium Wavelength Band (Second Window)

This is the band around nm which came into use in the mid s. This band is attractive today because there is zero fibre dispersion here (on single-mode fibre). While sources and detectors for this band are more costly than for the short wave band the fibre attenuation is only about 0.4 dB/km. This is the band in which the majority of long distance communications systems operate today.

Long Wavelength Band (Third Window)

The band between about nm and nm has the lowest attenuation available on current optical fibre (about 0.26 dB/km). In addition optical amplifiers are available which operate in this band. However, it is difficult (expensive) to make optical sources and detectors that operate here. Also, standard fibre disperses signal in this band.

In the late s this band is where almost all new communications systems operate.

2.2.2 Transmission Capacity

The potential transmission capacity of optical fibre is enormous. Looking again at Figure 14 on page 32 both the medium and long wavelength bands are very low in loss. The medium wavelength band (second window) is about 100 nm wide and ranges from nm to nm (loss of about .4 dB per km). The long wavelength band (third window) is around 150 nm wide and ranges from nm to nm (loss of about .2 dB per km). The loss peaks at and nm are due to traces of water in the glass. The useful (low loss) range is therefore around 250 nm.

Expressed in terms of analogue bandwidth, a 1 nm wide waveband at nm has a bandwidth of about 133 GHz. A 1 nm wide waveband at nm has a bandwidth of 177 GHz. In total, this gives a usable range of about 30 Tera Hertz (3 × Hz).

Capacity depends on the modulation technique used. In the electronic world we are used to getting a digital bandwidth of up to 8 bits per Hz of analog bandwidth. In the optical world, that objective is a long way off (and a trifle unnecessary). But assuming that a modulation technique resulting in one bit per Hz of analog bandwidth is available, then we can expect a digital bandwidth of 3 × bits per second.

Current technology limits electronic systems to a rate of about 10 Gbps, although higher speeds are being experimented with in research. Current practical fibre systems are also limited to this speed because of the speed of the electronics needed for transmission and reception.

The above suggests that, even if fibre quality is not improved, we could get 10,000 times greater throughput from a single fibre than the current practical limit.

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All electromagnetic signals propagate at different frequencies.  The electromagnetic (EM) spectrum is the range of all types of EM radiation.  Radiation is energy that travels and spreads out as it goes &#; the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma-rays.

You know more about the electromagnetic spectrum than you may think.

The image to the left shows where you might encounter each portion of the EM spectrum in your day-to-day life.

Radio: Your radio captures radio waves emitted by radio stations, bringing your favorite tunes. Radio waves are also emitted by stars and gases in space.

Microwave: Microwave radiation will cook your popcorn in just a few minutes, but is also used by astronomers to learn about the structure of nearby galaxies.

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Infrared: Night vision goggles pick up the infrared light emitted by our skin and objects with heat. In space, infrared light helps us map the dust between stars.

Visible: Our eyes detect visible light. Fireflies, light bulbs, and stars all emit visible light.

Ultraviolet: Ultraviolet radiation is emitted by the Sun and are the reason skin tans and burns. &#;Hot&#; objects in space emit UV radiation as well.

X-ray: A dentist uses X-rays to image your teeth, and airport security uses them to see through your bag. Hot gases in the Universe also emit X-rays.

Gamma ray: Doctors use gamma-ray imaging to see inside your body. The biggest gamma-ray generator of all is the Universe.

Most people commonly know how we refer to frequency.  For example, in Wi-Fi you hear about 2.4 GHz and 5.0 GHz.  In other applications, we here about 900 MHz.  These are simple ways to describe the frequency of the signals.  Science loves simplicity, so we often use terms that are the simplest to describe the signal.  Electromagnetic radiation can be expressed in terms of energy, wavelength, or frequency.  Frequency is measured in cycles per second, or Hertz.  Wavelength is measured in meters.  Energy is measured in electron volts.  Each of these three quantities for describing EM radiation are related to each other in a precise mathematical way.  But why have three ways of describing things, each with a different set of physical units?  They are all describing the same thing, the signal.

The short answer is that scientists don&#;t like to use numbers any bigger or smaller than they have to. It is much easier to say or write &#;two kilometers&#; than &#;two thousand meters.&#; Generally, scientists use whatever units are easiest for the type of EM radiation they work with.

Astronomers who study radio waves tend to use wavelengths or frequencies. Most of the radio part of the EM spectrum falls in the range from about 1 cm to 1 km, which is 30 gigahertz (GHz) to 300 kilohertz (kHz) in frequencies. The radio is a very broad part of the EM spectrum.

Infrared and optical astronomers generally use wavelength. Infrared astronomers use microns (millionths of a meter) for wavelengths, so their part of the EM spectrum falls in the range of 1 to 100 microns. Optical astronomers use both angstroms (0. cm, or 10-8 cm) and nanometers (0. cm, or 10-7 cm). Using nanometers, violet, blue, green, yellow, orange, and red light have wavelengths between 400 and 700 nanometers. (This range is just a tiny part of the entire EM spectrum, so the light our eyes can see is just a little fraction of all the EM radiation around us.)

The wavelengths of ultraviolet, X-ray, and gamma-ray regions of the EM spectrum are very small. Instead of using wavelengths, astronomers that study these portions of the EM spectrum usually refer to these photons by their energies, measured in electron volts (eV). Ultraviolet radiation falls in the range from a few electron volts to about 100 eV. X-ray photons have energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-rays then are all the photons with energies greater than 100 keV.

Now, with optical fibre links, we use nanometers to describe the wavelength.  The wavelength is so short that others ways to describe it are clumsy and difficult to comprehend.

Since fibre optic signals must propagate through a medium, often glass, this media has an influence on the propagation characteristics.  Not all frequencies propagate equally through all media.  In optical fibre, we have globally settled on three windows when the glass will permit the greatest throughput to flow.  These windows are:

• 850nm &#; normally used for multimode links
• nm &#; normally used for single mode links &#; course wave division multiplexing (CWDM)
• nm &#; normally used for single mode links &#; dense wave division multiplexing (DWDM)

We use nanometers for these three windows since the energy is very low and the distance between the peaks of the oscillations is so tiny that it is hard to describe.

For example:

• 850nm equals 0. inches
• nm equals 0. inches
• nm equals 0. inches

So, you can see why using the term nanometers is far easier to describe the signal.

In the early days of optical &#;bre communication, the LED was employed as a light source. The LED&#;s mostly operated at the 780 nm or the 850 nm wavelength. This region is referred to as the &#;rst transmission window.

The LED&#;s could not be employed for high bandwidth transmissions over a long distance due to their inherent disadvantages and were replaced by lasers.  Laser&#;s operated in two wavelength regions namely nm and nm that are commonly referred to as the second and the third optical transmission windows.

The wavy line in the graphic above shows the propagation characteristics of glass and as can be seen, it rises and falls at different wavelengths.  The three coloured bars are the three most popular windows to permit signal to flow freely.

The effects of dispersion are zero at the nm window, whereas the losses are the least at nm window.  The modern optical fibre networks operate around nm and nm, also nm is gaining steam because of GPON systems.

nm wavelength band is also particularly important to the WDM networks that are increasingly being deployed in networks worldwide.  These networks use ampli&#;ers to counter the effects of attenuation.  The commonly deployed amplifiers are the Erbium-doped Fiber Ampli&#;ers (EDFA) that provides  signal ampli&#;cation across a range of wavelengths around nm and nm.  This window is commonly referred to as the EDFA window.

Does that all make sense?  If you have questions, post them as comments and I will respond as best as I can.

A final note

One major pet peeve of mine is the mass confusion regarding bandwidth and data rate.  Most people use these terms interchangeably.  This is grossly incorrect.  Yes, they are related terms, but they mean very different things.  If you know the bandwidth, you cannot simply transpose that for the data rate.  Data rate is derived from the signal modulation, power / energy in the air / medium, forward error correction, transmission distance, frequency, channel plan, medium used, gross payload, net payload &#; and yes, bandwidth too.  With many different modulation techniques, the data rate cannot be determined simply by knowing the bandwidth, or the frequency of the signal.  While these are all helpful parts of the information, they are not the same and do not indicate that actual data rate, they are elements that contribute to the data rate calculation.

Now you know.

References:

Smale, A, (). The Electromagnetic Spectrum. NASA &#; A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC. Retrieved on December 23, from, https://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html

About the Author:

Michael Martin has more than 35 years of experience in systems design for broadband networks, optical fibre, wireless and digital communications technologies.

He is a Senior Executive with IBM Canada&#;s GTS Network Services Group. Over the past 13 years with IBM, he has worked in the GBS Global Center of Competency for Energy and Utilities and the GTS Global Center of Excellence for Energy and Utilities. He was previously a founding partner and President of MICAN Communications and before that was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation (CYN: TSX).

Martin currently serves on the Board of Directors for TeraGo Inc (TGO: TSX) and previously served on the Board of Directors for Avante Logixx Inc. (XX: TSX.V). 

He serves as a Member, SCC ISO-IEC JTC 1/SC-41 &#; Internet of Things and related technologies, ISO &#; International Organization for Standardization, and as a member of the NIST SP 500-325 Fog Computing Conceptual Model, National Institute of Standards and Technology.

He served on the Board of Governors of the University of Ontario Institute of Technology (UOIT) and on the Board of Advisers of five different Colleges in Ontario.  For 16 years he served on the Board of the Society of Motion Picture and Television Engineers (SMPTE), Toronto Section. 

He holds three master&#;s degrees, in business (MBA), communication (MA), and education (MEd). As well, he has diplomas and certifications in business, computer programming, internetworking, project management, media, photography, and communication technology.

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