Thu 10 Nov: Optical Fibers Info (1)

Optical Fibers - by James McLeman

Most of this information comes from:

http://hw-server.com/fiber-optic-cable-utp-categories

I strongly suggest everyone looks at this because it contains a lot of really good information on optical fibres/splicing and LED requirements for a multimode fibre.

An optical fibre consists of the following configuration:

Terminology

Cladding: The outside optical layer of the fibre that traps the light in the core and guides it along - even through curves.

Buffer coating or primary coating: A hard plastic coating on the outside of the fibre that protects the glass from moisture or physical damage.

Mode: A single electromagnetic field pattern (think of a ray of light) that travels in fibre.

Multimode fibre: has a bigger core (almost always 62.5 microns - a micron is one one millionth of a meter - but sometimes 50 microns) and is used with LED sources at wavelengths of 850 and 1300 nm for short distance, lower speed networks like LANs.

Singlemode fibre: has a much smaller core, only about 9 microns, and is used for telephony and CATV with laser sources at 1300 and 1550 nm. It can go very long distances at very high speeds.

Plastic optical fiber (POF): is a large core (about 1mm) multimode fiber that can be used for short, low speed networks. POF is used in consumer HiFi and starting to be used as part of a new standard for car communication systems called MOST.

Step Index (SI) fibre:

Step index fibre has a core of ultra-pure glass surrounded by a cladding layer of standard glass with a lower refractive index.  This causes light travelling within the fibre to continually “bounce” between the walls of the core much like a ball bouncing through a pipe.

Graded index fibre on the other hand operates by refracting (or bending) light continually toward the centre of the fibre like a long lens.  In a graded index fibre the entire fibre is made of ultra-pure glass.  In both types of fibre however, the light is effectively trapped and does not normally exit except at the far end.

In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.

Losses in an optical fibre are the result of absorption and impurities within the glass as well as mechanical strains that bend the fibre at an angle that is so sharp that light is actually able to “leak out” through the cladding region.  Losses are also dependent on the wavelength of the light employed in a system since the degree of light absorption by glass varies for different wavelengths.  At 850 nanometres, the wavelength most commonly used in short-range transmission systems, typical fibre has a loss of 4 to 5 dB per kilometre of length.  At 1300 nanometres this loss drops to under 3 dB per kilometre and at 1550 nanometres, the loss is a dB or so.  The last two wavelengths are therefore obviously used for longer transmission distances.

The losses described above are independent of the frequency or data rate of the signals being transmitted.  There is another loss factor however that is frequency (and wavelength) related and is due to the fact that light can have many paths through the fiber.  Figure 4 shows the mechanism of this loss through step-index fiber.

A light path straighter through a fiber is shorter than a light path with maximum “bouncing”.  This means that for a fast rise-time pulse of light, some paths will result in light reaching the end of the fiber sooner than through other paths.  This causes a smearing or spreading effect on the output rise-time of the light pulse which limits the maximum speed of light changes that the fiber will allow.  Since data is usually transmitted by pulses of light, this in essence limits the maximum data rate of the fiber.  The spreading effect for a fiber is expressed in terms of MHz per kilometer.  Standard 62.5 micron core multimode fiber usually has a bandwidth limitation of 160 MHz per kilometer at 850 nanometers and 500 MHz per kilometer at 1300 nanometers due to its large core size compared to the wavelength of the propagated light.  Single mode fiber, because of its very small 8 micron core diameter has a bandwidth of thousands of MHz per kilometer at 1300 nanometers.  For most low frequency applications however, the loss of light due to absorption will limit the transmission distance rather than the pulse spreading effect.

The smearing can distort the original signal:

A special case of modal dispersion is polarization mode dispersion (PMD), a fibre dispersion phenomena usually associated with single-mode fibres. PMD results when two modes that normally travel at the same speed due to fibre core geometric and stress symmetry (for example, two orthogonal polarizations in a waveguide of circular or square cross-section), travel at different speeds due to random imperfections that break the symmetry.

Note: Can we use group theory to identify where the symmetry has been broken?

Pulse dispersion in a graded index optical fibre is significantly less than a step index fibre and is given by

where
∂n is the difference in refractive indices of core and cladding,
n1 is the refractive index of the cladding,
l is the length of the fiber taken for observing the pulse dispersion,
c = 3 x 10^8 is the speed of light, and
k is the constant of graded index profile.

UTP cable: Unshielded Twisted Pair cable.

Acceptance Cone: The range of angles that cause total internal reflection.

Attenuation (transmission loss): Attenuation in fibre optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance travelled through a transmission medium. Attenuation coefficients in fibre optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media. Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. Empirical research has shown that attenuation in optical fibre is caused primarily by both scattering and absorption.

The propagation of light through the core of an optical fibre is based on total internal reflection of the lightwave. Rough and irregular surfaces, even at the molecular level, can cause light rays to be reflected in random directions. This is called diffuse reflection or scattering, and it is typically characterized by wide variety of reflection angles.

Attenuation results from the incoherent scattering of light at internal surfaces and interfaces. In (poly)crystalline materials such as metals and ceramics, in addition to pores, most of the internal surfaces or interfaces are in the form of grain boundaries that separate tiny regions of crystalline order. It has recently been shown that when the size of the scattering centre (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent. This phenomenon has given rise to the production of transparent ceramic materials.

In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths

Bandwidth: Bandwidth refers to how much data you can send through a network or modem connection. It is usually measured in bits per second, or "bps."

Classification of fibres:

Multi-mode fibers are described using a system of classification determined by the ISO 11801 standard — OM1, OM2, and OM3 — which is based on the modal bandwidth of the multi-mode fiber. OM4 (defined in TIA-492-AAAD) was finalized in August 2009,^[5] and was published by the end of 2009 by the TIA.^[6] OM4 cable will support 125m links at 40 and 100 Gbit/s.

Typical transmission speed and distance limits are 100 Mbit/s for distances up to 2 km (100BASE-FX), 1 Gbit/s to 220–550 m (1000BASE-SX), and 10 Gbit/s to 300 m (10GBASE-SR).

For many years 62.5/125 µm (OM1) and conventional 50/125 µm multi-mode fiber (OM2) were widely deployed in premises applications. These fibers easily support applications ranging from Ethernet (10 Mbit/s) to Gigabit Ethernet (1 Gbit/s) and, because of their relatively large core size, were ideal for use with LED transmitters. Newer deployments often use laser-optimized 50/125 µm multi-mode fiber (OM3). Fibers that meet this designation provide sufficient bandwidth to support 10 Gigabit Ethernet up to 300 meters. Optical fiber manufacturers have greatly refined their manufacturing process since that standard was issued and cables can be made that support 10 GbE up to 550 meters. Laser optimized multi-mode fiber (LOMMF) is designed for use with 850 nm VCSELs.

Numerical Apperature: In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one material to another provided there is no optical power at the interface. The exact definition of the term varies slightly between different areas of optics. In most areas of optics, and especially in microscopy, the numerical aperture of an optical system such as an objective lens is defined by:

NA = nsinθ

where n is the index of refraction of the medium in which the lens is working (1.0 for air, 1.33 for pure water, and up to 1.56 for oils; see also list of refractive indices), and θ is the half-angle of the maximum cone of light that can enter or exit the lens. In general, this is the angle of the real marginal ray in the system. Because the index of refraction is included, the NA of a pencil of rays is an invariant as a pencil of rays passes from one material to another through a flat surface. This is easily shown by rearranging Snell's law to find that nsinθ is constant across an interface.

Full Duplex:

A full-duplex (FDX), or sometimes double-duplex system, allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex, since they allow both callers to speak and be heard at the same time. A good analogy for a full-duplex system would be a two-lane road with one lane for each direction.

So since we are restricted to using LEDs the best we can use so far seems to be a 4-core (full duplex) OM2 multimode graded index loose optical fibre. (Loose Tube C S T Armoured 50/125 LSOH). It's complete specs can be found here:

http://communications.draka.com/sites/eu/Datasheets/MMF%20-%20Graded-Index%20Multimode%20Optical%20Fiber%20%2850_125%20%C2%B5m%29.pdf

http://www.premiumline-cabling.com/download/catalog_PL_FO_final.pdf

The cost of the above is for 45km is:

£38250

http://www.cablemonkey.co.uk/acatalog/Loose_Tube_Armoured_CST_Cable.html

Loss in the system

At the moment I am calculating a loss of 35dB just from the optical fibre alone. So the transmitter needs at least this and the receiver needs to be sensitive enough to detect it.

Other References:

http://www.blackbox.co.uk/documents/datasheets/11330.pdf

http://www.mayflex.com/_assets/downloads/S739DV.pdf

http://www.arcelect.com/Calculating_fiber_loss_and_distance.pdf

http://www.arcelect.com/fibercable.htm

http://www.lgce.net/uploads/lgce_optical_cable.pdf

http://www.leoni.com/uploads/tx_downloadleoni/en_fiber_optics_02.pdf

https://learningnetwork.cisco.com/servlet/JiveServlet/previewBody/3791-102-1-10509/Understanding%20OM1,OM2,OM3,%20OS1,OS2%20Fiber.pdf

http://www.premiumline-cabling.com/download/catalog_PL_FO_final.pdf

http://en.wikipedia.org/wiki/Numerical_aperture

http://en.wikipedia.org/wiki/Optical_fiber#Index_of_refraction

http://www.fiberoptics4sale.com/wordpress/what-is-unshielded-twisted-pair-utp-cable/

http://hw-server.com/fiber-optic-cable-utp-categories