Fiber Optics

Optical Fibre
Optical fibres are waveguides (they guides lightwaves along a path). Most fibres are made of silica (SiO₂), although fibres of different materials exists (e.g. fluoride fibres or hollow core fibres). A fibre consist of a core and surrounding cladding, both protected by insulation around it. The core has a slightly higher diffractive index than the cladding, so that light in the core reflects against the cladding and keeps propagating.

The two most common types of fibre are multimode fibre and single mode fibre. Single mode fibre has a thin core (9 micrometer), while multi mode fibre a a thicker core (62.5 micrometer). For single mode, the light will travel more or less parallel with the core, while for multimode the light will bounce back and forth at the edge between the core and the cladding. Single mode fibre is slightly more expensive then multi mode fibre. For single mode fibre it is possible to use WDM (wavelength division multiplexing), while this is not possible for multi mode fibre.

A waveguide has a limited number of guided propagation modes, sort of the number of standing waves in a fiber. As the name suggests, single mode fiber has only one mode, while multimode has multiple modes.

As a comparison, a human hair is 17 to 181 µm thick.

It is interesting to note that most receivers have a very broad range, so it may be possible to have a transmitter sending at 1310 nm, and a receiver which is designed 1550 nm, and vice-versa.

The different types of fibres can be distinguished by their colour:

Refractive Index
core has a higher refractive index than the surrounding

Holow core

speed of light

Wavelengths
Wavelengths are often grouped in windows:

Over time, the wavelength of lasers has increased.

Optical fiber communications typically operate in a wavelength region corresponding to one of the following “telecom windows”:

The first window at 800–900 nm was originally used. GaAs/AlGaAs-based laser diodes and light-emitting diodes (LEDs) served as transmitters, and silicon photodiodes were suitable for the receivers. However, the fiber losses are relatively high in this region, and fiber amplifiers are not well developed for this spectral region. Therefore, the first telecom window is suitable only for short-distance transmission.

The second telecom window utilizes wavelengths around 1.3 μm, where the loss of silica fibers is much lower and the fibers' chromatic dispersion is very weak, so that dispersive broadening is minimized. This window was originally used for long-haul transmission. However, fiber amplifiers for 1.3 μm (based on, e.g. on praseodymium-doped glass) are not as good as their 1.5-μm counterparts based on erbium. Also, low dispersion is not necessarily ideal for long-haul transmission, as it can increase the effect of optical nonlinearities.

The third telecom window, which is now very widely used, utilizes wavelengths around 1.5 μm. The losses of silica fibers are lowest in this region, and erbium-doped fiber amplifiers are available which offer very high performance. Fiber dispersion is usually anomalous but can be tailored with great flexibility (→ dispersion-shifted fibers).

Fiber Types
Multi mode fibres

Signal Distortion
The maximum distance of a signal in a fiber is caused by distortion of the signal over time: the maximum distance is reached when it is just barely possible to still distinguish the zeroes and ones in the signal.

In multi mode fiber, the primary cause of distortion is due to modal dispersion. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle. Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than rays that enter at a steeper angle. This effect is a direct cause of the relative thickness of the core of multi mode fibers. The different angles are often referred to as different "modes" of light, hence the name multi mode fiber.

Since single mode fibers have a much smaller core, the model dispersion is not a significant factor in the signal degradation. The maximum distance of a signal in a single mode fiber is typically reached due to attenuation of the signal. So a stronger source of light will yield a better reach. For example ER (Extended Range) optics have brighter lasers and have longer reach (about 40km) than LR (long range) optics (about 10km). It is possible to extend the reach of a signal even further using amplifiers, like EDFA (Erbium Doped Fibre Amplifier). However, at some point other signal distortions will take its toll, meaning that the signal needs full (3R) regeneration: Reamplification (of signal strength), Reshaping (of the signal waveform), and Retiming (of signal synchronisation).

Beside attenuation, chromatic dispersion and to a lesser extend polarisation dispersion play a role in the signal degradation. These two are different from modal dispersion. Chromatic dispersion is the signal degradation because a signal is not perfectly monochromatic; a laser with 1550nm wavelength actually will have light between 1549.9 and 1550.1 nm. Because the diffractive index is different for different wavelengths (which you can beautifully see with a rainbow or a prism), and the speed of light in a fiber depends on the refractive index, light of larger wavelength travels faster through fibre than light of a smaller wavelength (This can be calculated with the Sellmeier dispersion formula). This causes the signal to deform. Fortunately, dispersion compensating fibre exist with a crafted waveguide dispersion that cancels the chromatic dispersion.

The cancellation of chromatic dispersion with waveguide dispersion is a field of study on its own, called dispersion management.

To give an indication of the importance: at 160Gbit/s, with 'only' first and second order dispersion compensation, the maximum distance of a signal is about 50km before consecutive pulses start to overlap. Third order compensation is required to reach longer distances.

In silica-based glasses (such as most fibres), the dispersion is minimal at about 1300nm (the zero-dispersion wavelength). By introducing dopants (minor pollution) into a fiber, such as erbium doped fibre, the zero-dispersion wavelength shifts to about 1550nm (this type of fiber is called dispersion-shifted fibre). This explains why Erbium-doped amplifier are quite popular, since they can easily be used as dispersion-compensating fiber as well. A disadvantage of dispersion-shifted fibers is that they are more prone to interference (intermodulation) of signals of different wavelengths, such as in DWDM systems.

Standard telecom fibers exhibit zero chromatic dispersion in the 1.3-μm wavelength region. This was convenient for early optical fiber communications systems, which often operated around 1310 nm. However, the 1.5-μm region later became more important, because the fiber losses are lower there, and erbium-doped fiber amplifiers (EDFAs) are available for this region (whereas 1.3-μm amplifiers do not reach comparable performance). In this wavelength region, however, standard single-mode fibers (now sometimes called dispersion-unshifted fibers) exhibit significant anomalous dispersion. For linear transmission, this can be a problem, because it leads to significant dispersive pulse broadening, limiting the achievable transmission rates or distances. Therefore, so-called dispersion-shifted fibers [6] have been developed, which have modified waveguide dispersion so as to shift the zero dispersion wavelength into the 1.5-μm region. Zero chromatic dispersion is not necessarily ideal for data transmission. Particularly for the transmission of multiple channels (→ wavelength division multiplexing), four-wave mixing effects can be phase-matched and thus introduce significant distortions, if the dispersion is too weak. Therefore, it can be advantageous to use non-zero dispersion-shifted fibers [7], which are designed to have a small dispersion in the wavelength range of the data transmission, with the zero dispersion wavelength lying just outside this region. An alternative is to use dispersion-unshifted (i.e., standard) fiber with larger dispersion at 1.5 μm, combined with some kind of dispersion compensation.

Another cause for signal degradation is polarisation mode dispersion (PMD), which is

A dis (a few hundred to 2000km), other That said, it is not possible to infinitely

A special case of modal dispersion is polarization mode dispersion (PMD)

usually core has higher refractive index than cladding. holow core:

https://www.rp-photonics.com/encyclopedia.html

Standards
See http://www.bicsi.org/uploadedfiles/pdfs/presentations/region_events/SturbridgeMA_Nov2014/OverviewTodaysOpticalFibers.pdf

Single mode fibres


 * IEC 60793-2-50:2008 Type B6_a komt overeen met G.657 A1/2.
 * IEC 60793-2-50:2008 Type B6_b komt overeen met G.657 B2/3.
 * EN 50173-1:2007 OS1 en OS2 kabels kunnen alleen van G.652.D of G.657 vezels gemaakt worden. (OS1: indoor, tight buffered en OS2: outdoor, loose tube).

The fiber colours are not part of the formal specification, but (apparently) a de-facto standard.

(The links lead to the table of contents; the actual standard is available at the IEC webstore at a a cost.)

ITU G.657 (bend-insensitive single mode fibers, improved over G.652) lists two categories:
 * Categorie A: geschikt voor de transmissie in de O, E, S, C en L-Band (van 1260 tot 1625 nm). Vezels hebben dezelfde transmissie en inter- connectie eigenschappen als G.652.D. Bedoeld voor elke plek in het netwerk. Max. buigradius 7.5 - 10 mm.
 * Categorie B: geschikt voor de transmissie op 1310, 1550 en 1625 nm voor beperkte afstanden bedoeld voor inpandige netwerken. Vezels hebben andere las en connectie eigenschappen dan G.652.D vezels maar zeer kleine buigstraal. Bedoeld voor korte lengtes waar CD en PMD geen rol spelen. Max. buigradius 5 - 7.5 mm.

More information can be found at the OpenLearn website of the British Open University. Course T305 (given until 2008) covers optical communication as part of it curriculum. Unfortunately, the website seems to redirect back the home page. You can find a wealth of information by getting the Google cache.

http://ict.open.ac.uk/courses/t305/ http://openlearn.open.ac.uk/course/category.php?id=13

Power
The terms bellow do only apply to 1 Gbit ethernet framing, not to SONET/SDH. For 10 GbE this might be slightly different. See the technical white papers for details.

For the Cisco, the SX and LX power levels are in the following range. I haven't looked up the specification, but I've understand that this is a wide range for receivers (about 15 dB). For most equipment the receiver range is about 8 dB.

Please note that the naming is sometimes not so obvious. For example, the AMS-IX technical specs notes: What Foundry calls "LX", Cisco calls "LH". The Foundry LH optics are compatible with the Cisco ZX optics. What is listed above are the Cisco LH specs. For the record, the only difference of the Foundry is the mimumum Rx power: -20.0 dBm instead of -19.0 dBm.

Power losses
With the distance, you will lose power. However, for long distances, typically regenerators are used by the telecom operators who offer the dark fibers. We only deal with short distances (within a building). For that, the loss due to distance is negligible. The loss due to cuts (one for each patch cable), and the loss within an optical cross connect, is significant.

For used fibers, or single mode fibers, it is highly recommended to clean the fibre ends before reusing the fiber. Especially with a core as small as with single mode fibers, a single spec of dust could easily yield to 10 dB power loss.

Connectors

 * LC (small connector type)
 * SC (big connector type)

LC and SC are both used for either single mode or multimode fibre, so having a connector that fits doesn't guarantee a working connection.

Other connectors in use are ST and FC. ST looks a lot like the BNC connectors on old COAX cables (10 Base-T, early 90's), and has been used in our group. FC has a screw threads (schroefdraad), and was the expensive counterpart of ST. It was never used in our group. Fiber polishes (cuts)

Fibres can have different type of cut offs ("polishes"). The most common is "PC", which is a straight cut. There are other types, but we don't use those. Though both LC and SC connectors can have different type of polishes, somehow, people only list the polish with SC connectors (e.g. SC/PC), but hardly with LC connectors.

Transceivers (Optics)
Transceivers are replaceable optics, and allow you to replace a short distance (SR) optic with a long distance (LR) optic or vice versa.

SPF and SFP have the same form factor. QSFP+ and QSFP28 also have the same form factor, and many devices will accept either optic.

The following optics are defined:

Other (obsolete or uncommon) standards: