Fiber Optics
Optical Fibre
An optical fibre consist of a core and surrounding, both protected by insulation around it. Two types of fibre exists: multimode and single mode. 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 surrounding. Single mode fibre is slightly more expensive then multi mode fibre. For single mode fibre it is possible to use WDM (wavelwength division multiplexing), while this is not possible for multi mode fibre.
| Fibre | Core Diameter | Cladding Diameter | Number of wavelengths | Wavelength | Distance |
|---|---|---|---|---|---|
| Multi mode | 8-10 µm | 125 µm | 1 | 800-850 nm | 100 meter |
| Single mode | 50-62.5 µm | 125 µm | up to 100 | 1310-1550 nm | 1000 km |
The different types of fibres can be distinguished by their colour:
| Fibre | Colour | Colour of transceiver |
|---|---|---|
| Multi mode | Orange or grey | Black or grey |
| Single mode | Yellow | Blue |
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.
Standards
| IEC 60793-2 | Optical fibres - Part 2: Product specifications - General |
| IEC 60793-2-10 | Part 2-10: Category A1 multimode fibres (A1a defines 50/125µm, A1b defines 62.5/125µm) |
| IEC 60793-2-20 | Part 2-20: Category A2 multimode fibres |
| IEC 60793-2-30 | Part 2-30: Category A3 multimode fibres |
| IEC 60793-2-40 | Part 2-40: Category A4 multimode fibres |
| IEC 60793-2-50 | Part 2-50: Category B singlemode fibres |
| IEC 60793-2-60 | Part 2-60: Category C singlemode fibres |
(The links lead to the table of contents; the actual standard is available at the IEC webstore at a a cost.)
| ITU-T G.651.1 | Characteristics of a 50/125 µm multimode graded index optical fibre cable for the optical access network |
| ITU-T G.652 | Characteristics of a single-mode optical fibre and cable |
| ITU-T G.653 | Characteristics of a dispersion-shifted single-mode optical fibre and cable |
| ITU-T G.654 | Characteristics of a cut-off shifted single-mode optical fibre and cable |
| ITU-T G.655 | Characteristics of a non-zero dispersion-shifted single-mode optical fibre and cable |
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
Optical Transceiver
The different standards 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. Type Range Applicability SX Short range virtually always multi mode LX Long range virtually always single mode ZX Very Long range
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. Type Maximum Minimum LX optics Transmitting (Tx) -3.0 dBm -9.5 dBm LX optics Receiving (Rx) -3.0 dBm -19.0 dBm SX optics Transmitting (Tx) -4.0 dBm -9.5 dBm SX optics Receiving (Rx) 0.0 dBm -17.0 dBm
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. However, it was called LX by a SARA engineer. For the record, the only difference of the Foundry is the mimumum Rx power: -20.0 dBm instead of -19.0 dBm.
I currently do now know if all these power ranges are used for both single mode and multi mode fibers. I heard that for multi mode, typically SX is used, and for single mode LX. However, I am not sure about this. If you know, please email me at fdijkstr@science.uva.nl. Thanks! 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. Network element Typical power loss Cut at end of patch cable 0.2 dB Optical Cross Connect 3 to 5 dB (differs by manufacturer)
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. Type Description PC Physical Contact UPC Ultra Physical Contact APC Angled Physical Contact Transeivers (GBICs)
Most devices today allow you to change the connector and power type, by having a small replaceable piece of hardware on the blade, so you can swap a laser with high power for SM fiber with a laser with lower power for MM fiber. These things are called transeivers or GBICs.
For 1 Gb/s these two transeivers exist: GBIC (Gigabit Interface Connector) and SFP (Small Form Factor Pluggable) (also known as mini-GBIC).
For 10 Gb/s these four transeivers exist: XENPAK, XPAK, X2 and XFP, SFP+
For 40 Gb/s there are two types of transeivers: QSFP and CFP.
More information on the bottom of the 10 Gb/s cabeling page of HP.
