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.

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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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Parameter Standard grade Precision grade Ultra-precision grade Selection code WIN00x WIN10x WIN22x Material BK7 BK7 BK7 Dimensional tolerance +/-0.15 +/-0.15 +/-0.15 Clear aperture >80% >80% >80% Surface quality 60-40 40-20 20-10 Parallelism 1 arc min 30 arc seconds 1 arc min Wavefront distortion 1l per 25mm  l/4  l/10 Protective bevel <0.25x45 deg <0.25x45 deg <0.25x45 deg Coating Optional Optional Optional

PN: WIN006 Description: Optical window, BK7, D=15, t=3 Diameter: 15 Thickness: 3 Volume Price: Request Volume Quote

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