Fiber Optic Products

Single-mode fiber (OS2, per ITU-T G.652.D) has a 9 micrometre core, uses laser light at 1310 or 1550 nm and covers links of 10 km and more. Multi-mode fiber (OM1 to OM5) has a 50 or 62.5 micrometre core and normally works at 850 nm over only a few hundred metres (for example up to 550 m at 1 Gigabit and 300 to 400 m at 10 Gigabit on OM3 and OM4). For a new project, a link between buildings or any run longer than 300 m, single-mode is the right choice; multi-mode only makes sense for short runs inside one building or server room where multi-mode cabling already exists.

First read the print on the cable jacket: markings such as 9/125 or OS2 mean single-mode, while 50/125, 62.5/125 or OM3 mean multi-mode. In the usual colour convention a single-mode patch cord is yellow, OM1 and OM2 are orange, OM3 and OM4 are aqua (OM4 is sometimes violet) and OM5 is lime green. Colour is only a convention and not a guarantee, so if the printing or the purchase record is not available, verify the type with a test instrument or the supplier instead of guessing.

OM1 has a 62.5 micrometre core and the others have a 50 micrometre core; the main difference is modal bandwidth, expressed in MHz·km. The minimum standard values at 850 nm are 200 for OM1, 500 for OM2, 2000 (effective) for OM3 and 4700 for OM4. OM5 has the same 4700 at 850 nm and is also specified for several nearby wavelengths (up to about 953 nm) so that SWDM modules can carry more capacity on one fiber. For 10 Gigabit and above, OM3 is the minimum and OM4 is recommended; OM1 and OM2 are only suitable for maintaining older networks.

850 nm and 1300 nm are multi-mode wavelengths: 850 nm is used by the low-cost SX and SR modules, and 1300 nm (about 1310) by the older 100BASE-FX standard. 1310 nm and 1550 nm are single-mode wavelengths: 1310 nm for links up to 10 km (LX, LR) and 1550 nm for longer links (typically 80 km modules and many 40 km modules) because it has lower attenuation. Both ends of a link must use the same wavelength and standard, for example SX with SX and LX with LX.

Typical values for OS2 single-mode fiber are about 0.35 dB/km at 1310 nm and about 0.22 dB/km at 1550 nm; the standard ceiling for OS2 cable is 0.4 dB/km. For multi-mode the typical figures are about 3.0 dB/km at 850 nm and 1.0 dB/km at 1300 nm (standard ceilings 3.5 and 1.5). So 10 km of single-mode fiber loses about 3.5 dB at 1310 nm and about 2.2 dB at 1550 nm from the fiber alone.

Total link loss = length × attenuation per km + number of connector pairs × loss of each + number of splices × loss of each. Example: an 8 km single-mode link at 1310 nm gives 8 × 0.35 = 2.8 dB, four connector pairs at a conservative 0.75 dB each (the maximum allowed by cabling standards) give 3.0 dB and two fusion splices at a typical 0.1 dB give 0.2 dB, for a total of 6.0 dB. The module budget is the minimum transmit power minus the receiver sensitivity; for a module with a minimum transmit power of -9 dBm and a sensitivity of -20 dBm that is 11 dB, leaving a 5 dB margin, which is above the commonly used minimum of 3 dB.

G.657 is a type of single-mode fiber that loses less light in tight bends and is compatible with G.652.D. The minimum bend radius of ordinary G.652.D fiber is about 30 mm, while G.657.A1 allows 10 mm and G.657.A2 allows 7.5 mm. It is a good choice for indoor cabling, patch cords, crowded ODFs and installations with sharp corners; sharp bends of the cable under tension or pressure still cause damage and must be avoided.

1000BASE-SX works at 850 nm over multi-mode and its standard reach is about 275 m on OM1 and 550 m on OM2 or better. 1000BASE-LX works at 1310 nm over single-mode; the original IEEE standard gave 5 km and 1000BASE-LX10 was defined for 10 km, but most LX modules on the market are rated 10 km. Modules for 40 and 80 km (for example ZX at 1550 nm) are not IEEE standards but manufacturer specifications. These figures are nominal, and the real reach depends on the loss of the cable and connectors.

10GBASE-SR works at 850 nm over multi-mode and its nominal reach is about 33 m on OM1, 82 m on OM2, 300 m on OM3 and 400 m on OM4. 10GBASE-LR covers 10 km over single-mode at 1310 nm, and 10GBASE-ER reaches 40 km at 1550 nm. These are nominal values; that is why an LR module over single-mode is recommended for runs above 300 m or for runs that may become longer in the future.

SFP and SFP+ look and measure the same, but SFP is for 1 Gigabit (and in some types up to 4.25 Gigabit) while SFP+ is for 10 Gigabit. An SFP+ module does not work in an SFP-only port because that port does not support 10 Gigabit. A standard SFP module in an SFP+ port works at 1 Gigabit on most switches, provided the switch supports 1 Gigabit on that port; check the switch specifications before buying.

DDM (also called DOM) lets the switch or media converter read the module temperature, supply voltage, laser bias current, transmit power and receive power. The most important number is the receive power (Rx) in dBm: if it is close to the receiver sensitivity (for example -18 to -20 dBm), the link has little margin and the connectors should be cleaned and inspected. With DDM you can spot a problem before the link drops, without a measuring instrument. According to their product specifications, several KDT modules such as KG-GLC20SC, KG-GLC80SC, KG-10GLC10SC and KG-GLC20BiDi include DDM.

The brand or model does not have to be identical, but the speed, wavelength and fiber type must match: SX with SX, LX with LX and a 10 Gigabit module with a 10 Gigabit module. Combining a 10 km module with a 20 km module on a short link usually works, provided the wavelength is the same and the received power does not exceed the receiver overload limit. An SX module never links with an LX module because the wavelength and fiber type are different. If a switch accepts only its own vendor-approved modules, ask about compatibility before buying.

A long-reach module has high transmit power (depending on the model, roughly -5 to +5 dBm) and on a short fiber the received power exceeds the receiver overload limit (typically about -3 dBm); the result is an unstable link or long-term damage to the receiver. The fix is an optical attenuator (typically 5 to 10 dB, according to the calculation) at one end. Choose the attenuator from the real received power read through DDM, and do not rely on coiling fiber as an attenuator. If the run is short, simply choose a 10 or 20 km module in the first place.

A BiDi module uses WDM technology to transmit and receive over a single strand of fiber, halving the number of fibers needed. One end is fitted with a module that transmits at 1310 nm and receives at 1550 nm, and the other end with one that transmits at 1550 nm and receives at 1310 nm; the modules are usually labelled A/B or by wavelength. If both ends are the same type the link does not come up, and a BiDi module is also not compatible with an ordinary two-fiber module. The IEEE 1000BASE-BX10 standard defines the 1310/1490 nm pair and the 1310/1550 nm pair is also common on the market; the pairs are not interchangeable, so read the label. KDT offers the KG-GLC20BiDi module (20 km, LC connector) with WDM and DDM; its product page lists 1550 nm (B side), so the other end must be the matching A-side module.

A duplex link uses two fibers, one to transmit and one to receive, with the two connectors side by side. A simplex link has only one fiber and is used for BiDi. On a duplex link the transmit (Tx) of one end must reach the receive (Rx) of the other end, so the whole path needs an odd number of crossovers, and if the link does not come up you should first swap the two connectors at one end (a polarity problem). This is one of the most common reasons for a link not coming up in a fresh installation.

A media converter converts copper Ethernet (RJ45) to light and back, so a device with no fiber port, such as a camera or a simple switch, can be brought onto the network over fiber. The most common use is a pair of converters, one at each end of the run, for example for a camera 400 m from the server room where copper (100 m at most) cannot reach. The speed at both ends must be the same (100 Megabit or 1 Gigabit), and the wavelength and fiber type of the two converters must match. If the switch itself has an SFP port, an SFP module plugged directly into it is usually simpler than a converter.

The MC-0202H4ST belongs to the Platinum series and, according to the product data, has two Gigabit SFP ports, one Gigabit RJ45 port and one Fast Ethernet RJ45 port, with 8 Gbps of internal bandwidth. It has a fanless all-metal chassis and can be installed in a rack, on a desktop or on a DIN-Rail. The SFP module is chosen separately (for example a single-mode or BiDi module from KDT), so the final reach is set by the module and not by the unit itself. See the datasheet on the product page for full details.

The fiber cable itself is the same, but the module and port differ. 100BASE-FX works at 1310 nm over multi-mode up to 2 km (full duplex) and 100BASE-LX10 is for single-mode up to 10 km. A 100 Megabit fiber port does not link with a 1 Gigabit module and vice versa, because speed is not negotiated over fiber and both ends must match exactly. So check the speed of the SFP port or converter before buying.

LC is small (1.25 mm ferrule), latches with a click clip and is the standard connector of SFP and SFP+ modules and dense patch panels. SC is square and push-pull with a 2.5 mm ferrule and is common on ODFs and network equipment. ST is bayonet-style (twist) and is seen on older multi-mode networks, while FC is threaded and is used mostly in telecom and laboratory equipment. To convert between them use a hybrid patch cord such as SC to LC rather than changing the connector.

In UPC the end face of the ferrule is polished flat with a slight curve and the return loss is typically about 50 dB or better; in APC the end face is polished at an 8 degree angle and the return loss is typically 60 dB or better. A UPC connector is usually blue and an APC connector green. APC is used in PON networks, cable TV and reflection-sensitive signals, while ordinary Ethernet is usually fine with UPC. Mating UPC to APC causes high loss and can scratch the end faces of both connectors, so always check the colour and marking before plugging in. For example, the KDT PG-150CMSC pigtail is an SC/UPC type.

A patch cord has factory connectors at both ends and is used to connect a patch panel to equipment (for example an ODF to a switch). A pigtail has a connector at one end only, and the other end is bare fiber that is fusion-spliced to the fiber of the main cable, forming the connector inside the ODF. Among KDT products there are the single-mode patch cords PF-SCLC, PF-SCSC and PF-LCLC (yellow PVC jacket) and the PG-150CMSC pigtail with an SC/UPC connector. Splicing a pigtail instead of fitting a connector on the cable in the field gives lower loss and more consistent quality.

Fusion splicing melts and joins the fibers with an electric arc; its typical loss is 0.02 to 0.1 dB with almost no reflection, and it is the best choice for permanent work and long routes. A mechanical splice uses gel and a V-groove alignment, needs no expensive machine and suits quick repairs or a few joints, but its typical loss is 0.2 to 0.5 dB and it is less stable. The standards allow about 0.3 dB per splice, so on long links where every decibel counts choose fusion. A clean cleave (angle under 1 degree) is the key requirement in both methods.

An ODF is an optical patch panel: it fusion-splices the fibers of the incoming cable to pigtails inside a splice tray and presents them as adapters (SC or LC) on the panel so they can be connected to equipment with patch cords; it protects the bare fiber and makes management and tracing easier. A splice closure is for the middle of a route: where two cables (for example from two drums) are spliced together outdoors and sealed in a watertight enclosure (typically IP67 or IP68) on a pole, in a manhole or underground. KDT offers the FA-2SC-SC duplex adapter for SC patch panels.

A dirty connector end face is the most common cause of high loss, an unstable link and CRC errors, because a speck of dust on the 9 micrometre core blocks or scatters the light. Clean first dry (with a click cleaner or a lint-free fiber wipe); if that is not enough, use a wipe moistened with pure isopropyl alcohol (99 percent) and then dry it. Never wipe with a finger or clothing, inspect with a fiber microscope after cleaning, and keep the dust cap on until the moment of connecting. This is the first thing to do before any test or troubleshooting.

Proceed in this order: 1) clean both modules and the patch cords; 2) make sure the module type, wavelength and speed are the same and that the single-mode or multi-mode fiber matches the module; 3) swap Tx and Rx at one end; 4) read the received power with DDM (a value near -40 dBm or 'no signal' means no light is arriving; a very high value means overload); 5) if it is a BiDi module, the two ends must be a matched pair; 6) find the loss and the location of a break with a power meter and light source or an OTDR. A red laser (VFL) also helps to find a bend or break within the first few kilometres.

dB expresses the ratio of two powers (for example the loss of a link) and dBm is absolute power relative to 1 milliwatt. A value of 0 dBm equals 1 mW, -10 dBm equals 0.1 mW and -20 dBm equals 0.01 mW. Every 3 dB of loss roughly halves the power and every 10 dB of loss reduces it to one tenth. So if the transmit power is -5 dBm and the received power is -12 dBm, the link loss is 7 dB (no conversion is needed).

The standard method (OLTS) is to connect the light source directly to the power meter with a reference patch cord and store the reading as the reference (0 dB); then connect the source to one end of the link and the power meter to the other and read the loss. For example, if the reference is -5.0 dBm and you read -9.2 dBm through the link, the loss is 4.2 dB and it should be compared with the budget calculation (length × attenuation + connectors + splices). The test wavelength must be the operating wavelength (1310 or 1550 nm for single-mode, 850 nm for multi-mode) and the connectors must be cleaned before the test.

An OTDR sends an optical pulse into the fiber and plots the reflected light against distance; the result is a trace showing total length, the attenuation of each section, the position of each splice and connector, and the location of a break or a sharp bend. It is used to find 'the break at which kilometre' and a power meter cannot do that. It has a dead zone at the start and end of the route, so a launch cable of typically 100 to 500 m is placed before the link and one after it. Testing at both 1310 and 1550 nm reveals sharp bends, which lose more at 1550 nm.

Copper network cable (Cat5e, Cat6, Cat6a) covers 100 m at most, while fiber goes from a few hundred metres to tens of kilometres. Fiber is an electrical insulator, so between two buildings whose earths have a potential difference it creates no ground loop, is immune to electromagnetic noise and lightning-induced surges, and carries no copper path that could burn equipment. If an outdoor cable has a metallic element (armour or strength wire) it must be grounded at the entrance; in lightning-prone areas choose an all-dielectric cable. The equipment power (for example a camera) must be supplied locally.

Indoor cable is usually tight-buffer with 900 micrometre fibers and an LSZH (low smoke, zero halogen) jacket; it is flexible and suited to inside buildings and risers, but it cannot tolerate water or UV. Outdoor cable is usually loose-tube with gel or a water-blocking compound and a UV-resistant PE jacket, and is used in underground ducts or aerially. Armoured cable has a metallic layer (steel tape or wire) and resists rodents, impact and direct burial. Do not run an outdoor cable far inside a building; splice it to an indoor cable near the entrance, or use a combined indoor/outdoor cable.

The common rule for cable is a bend radius of at least 10 times the cable outer diameter when not under tension and 20 times while pulling; for the G.652.D fiber itself about 30 mm and for G.657 fiber about 7.5 to 10 mm (in practice the cable or cord jacket needs a larger radius). Always read the maximum pulling tension from the datasheet of that cable (the figure differs greatly from one type to another) and pull gently by hand or with a tension gauge. Do not twist the cable, drag it around corners or fasten it with tight ties, because micro-bends also cause loss.

Light at 850, 1310 and 1550 nm is invisible, so you cannot tell whether a fiber is lit. Ordinary SFP and SFP+ modules are normally Class 1 under IEC 60825-1 and are harmless in normal conditions, but optical amplifiers and some test equipment have more power and may fall into higher classes. Never look into the end of a fiber or the port of a module, and never inspect a live fiber with a magnifier or microscope, because the lens concentrates the light in the eye. Disconnect the link before inspecting, or use microscopes with a safety filter; a red VFL laser is typically 1 to 10 mW, so do not shine it directly into the eye either.

Each duplex link needs two fibers (or one fiber for BiDi), so calculate the number of fibers from the current links and leave about 30 to 50 percent spare, because pulling a new cable costs more than any splice. Outdoor cables usually have 4, 6, 8, 12, 24 or 48 fibers. The 12-colour code is, in order: blue, orange, green, brown, slate, white, red, black, yellow, violet, rose, aqua; in multi-tube cables the tubes themselves are colour-coded in the same order and the fibers inside each tube repeat the same 12 colours. Record the colour map and fiber numbers at the ODF.

Generally no. A single-mode core is 9 micrometres and a multi-mode core is 50 or 62.5 micrometres; when light goes from a large core into a 9 micrometre core, most of the light is lost. The only standard exception is a 1000BASE-LX module over multi-mode fiber, which needs a special mode-conditioning patch cord. Even between two multi-mode types, going from 62.5 to 50 micrometres adds up to about 1.9 dB of loss. Always match the patch cord type to the module and cable: a yellow single-mode patch cord for LX and LR modules and single-mode fiber.

The KDT fiber infrastructure range (Cuprium series) includes the single-mode SFP modules KG-GLC20SC and KG-GLC80SC, the 10 Gigabit SFP+ module KG-10GLC10SC and the BiDi module KG-GLC20BiDi; according to the product specifications, all of them have DDM. The single-mode patch cords PF-LCLC, PF-SCSC and PF-SCLC, the PG-150CMSC pigtail and the FA-2SC-SC adapter are also part of the range. The MC-0202H4ST media converter belongs to the Platinum series. The exact reach, wavelength and speed of each module are given in the datasheet on that product page on the website; for the final selection consult a KDT specialist.