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Fiber Tutorial


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By Ronny Sletteng, Product Manager, M.Sc.

Why fiber? Optical fiber has gained incredible popularity as cabling medium within the broadcast business the last few years. Below are some of the reasons why more and more people are using optical to future proof their cabling infrastructure. • Fiber has no high frequency roll-offs • Fiber supports bit rates of 10Gbps and beyond • Fiber attenuation is very low • Fiber core is much smaller than copper • Fiber is extremely lightweight • Fiber is not sensitive to cross-talk • Fiber is glass – i.e. it is a dielectric, not a conductor • Fiber has a transmission capacity of up to 100Tbps

Optical fiber deployment means future proofing of the cabling infrastructure

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Jacket/ Sheath material

Belden 1694A


Single cable


4xSDI with



Chloride (PVC)

40xSDI with


Coaxial cable Belden 7713A


Multi cable


Coaxial cable

(10 off 1694A)

Nexans G24-9 UT 6.5mm

24 fibers

TDM 4.5kg/100m

Fiber cable


Low Smoke Zero

with TDM

Halogen (LSZH)

and DWDM

Belden 1694A

Nexans G24-9 UT

Belden 7713A

Attenuation [dB/100m]

45 40


35 30 25 20 15 10


5 0 0




2000 2500 3000 Frequency [MHz]




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Fiber Tutorial A short, non-mathematical introduction to the interesting world of fiber optics. CROSS SECTION OF OPTICAL Fiber The figure below shows a typical cross section of an optical fiber.

This is necessary in order for the light to be guided within the core.

The actual size of the fiber is very small; the core-diameter of a single mode fiber is typically 9Âľm.

Outside of the cladding is often a protective polymer coating of the fiber.

The core of the fiber (drawn yellow to symbolise that light passes through) has a slightly higher refractive index than the cladding surrounding it.

The refractive index of the coating is higher than the index of the cladding, to ensure that cladding modes (light travelling in the cladding instead of the core) are guided away from the core, and not reflected back towards the core causing signal disturbances.




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REFRACTION OF LIGHT The speed of light depends on the medium light travels through. In vacuum (air) 300 000 km/sec. In water 225 000 km/sec. In glass (optical fiber) 200 000 km/sec.

We then have the following indices of refraction (all values are approximate values): Air (vacuum): 1.0 Water: 1.3 Optical fiber: 1.45–1.5

An optical property for a given medium, called refractive index or index of refraction is defined as the ratio of speed of light in vacuum to the speed of light in the medium.

What does this mean? When light is incident on a boundary between two materials with different index of refraction, the light is refracted. An example is shown below.

large angle

The man sees the fish at an apparent position, different from the real position. We note that the angle in water, with the largest index of refraction, is smaller than the angle in air. This is a physical law, and is always true.

apparent position

small angle real position

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This optical property is used to guide light in graded index multi mode optical fibers: cladding core 125 cladding 50/62.5

Multi Mode (MM) and Single mode (SM) Fiber Today all telecommunication networks use SM fiber for long distance transmission (more than a few kilometers). Let’s compare some of the properties of SM and MM fibers: Property

Multi mode

Single mode

Core diameter (approx.)

50 um graded index fiber

9 um step index fiber

62.5 um graded index fiber

Cladding diameter

125 um graded index fiber

125 um

Modal dispersion



Modal noise



Connector termination

Special crimp tool kits available

Requires fusion splicing

The difference in properties occur mainly because of the smaller core diameter of SM fiber.

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Multi mode fiber

Single mode fiber









Index profile

A comparison of the two cross sections is shown above. The index profile shown in the bottom of the picture shows that the refractive index of the core is higher than the refractive index of the cladding.

MM fibers guides light inside the core through multiple reflections from the core-cladding boundary. SM fibers has so small corediameter, that light entering the core at the appropriate angles will be confined in the core without any reflections.

Another difference between the two types of fiber, is how they guide light.

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Attenuation (db/km) SMF – Single mode fiber, ITU-T G.652 A and B LWPF – Low water peak single mode fiber, ITU-T G.652 C and D 1st window

2nd window

3rd window




IR-absorption OHabs.

H2 abs. LWPF

0 600








Wavelength (nm)

Attenuation in optical fibers The attenuation of signal power in optical fibers is highly dependent of the transmission wavelength. In the figure above we see a typical sketch of attenuation in optical silica-fiber versus wavelength. In the figure we see the three telecom windows: The first window (around 850 nm) was used in the beginning, because of the low cost GaAs laser diodes available then emitting light at these wavelengths. This wavelength window is only used for shortrange datacommunications (Ethernet) today, and not in telecommunications. The second window (around 1300 nm) is where the dispersion of silica fiber is close to zero, with an attenuation of less than 0.5 dB/km.

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The Flashlink range offers transmission in this window, as well as in the third telecom window (around 1550 nm), where the loss in dB/km for silica fiber has its lowest value, less than 0.3 dB/km. New fiber cables use fibers with attenuation in the range of 0.4dB/km at 1310nm and 0.25dB/ km at 1550nm. The figure also shows how the attenuation increases for short wavelengths due to scattering of the signal in the fiber (Rayleighscattering). For longer wavelengths (> 1700 nm) the IR-absorption increases rapidly. At 1250 nm an absorption peak is caused by H2-molecules diffusing into the fiber. At 1383 +/-6nm we have a peak of absorption, sometimes called the water peak limiting the number of long-haul CWDM channels on G.652 A and B fibers.

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DFb 1300nm laser

F-P 1300nm laser

FWHM = 0.2nm



FWHM = 4nm





Wavelength (nm)





Wavelength (nm)

Comparison of DFB and F-P Lasers DFB laser is an abbreviation for Distributed Feedback laser. F-P laser is an abbreviation for Fabry-Perot laser. The main difference between the two types of lasers, is the way they amplify the light signal. A semiconductor F-P laser has a constant index of refraction. At one end of the semiconductor material, the light is partially reflected for amplification, and partially coupled out. An F-P laser, emits light at different wavelengths (modes) simultaneously. A DFB laser has a periodically modulated index of refraction. This modulation enables laser operation at a “single� wavelength. The sketch to the left compares the output spectra of a DFB laser and an F-P laser at 1300 nm.

The Full Width of a DFB laser at Half Maximum (FWHM) bandwidth is about 0.2 nm compared to the F-P laser which has an FWHM of approximately 4 nm. The FWHM numbers are meant to illustrate the difference in bandwidth between the two types of lasers, and must not be taken as specifications of the lasers in the Flashlink range. The fabrication of a DFB laser is more difficult than the fabrication of an F-P laser, resulting in a higher price for the DFB laser. Used in systems employing CWDM or DWDM, the F-P laser is no choice at all. The large bandwidth of F-P lasers makes crosstalk a serious problem if the lasers are spaced too closely. DFB lasers with their better performance are preferred to F-P lasers.

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TDM (Time Division Multiplexing) With today’s increasing demands of transmission capacity (bandwidth), several techniques have been developed to utilize available bandwidth. One of these is TDM – Time Division Multiplexing in the electrical domain. To explain the principle we use a figure:

Ch. 1

Ch. 1 1 2 3 4 1 2 3 4 Ch. 2


Ch. 3


Ch. 4

4 ch. per wavelength


Ch. 2 Ch. 3

Single mode fiber

Ch. 4

From the figure we see that the system is a 4-channel TDM system. Each channel has its own transmitter sending at a given bit-rate. By sending 4 channels simultaneously with a small time delay between the channels, we increase the system capacity. The process of sending several channels at the same time is called multiplexing. The time is divided among the other channels, therefore the name time division multiplexing. As we see in the figure, this multiplexing from four parallel signal stream into a serial signal bit stream gives in this case a transmission bit-rate of four times the original bit-rate. At the receiver end is a demultiplexer, separating the different channels again.

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When it comes to the format of the TDM signal, there are different approaches. Most vendors use a proprietary format, which is useful for point to point connections. The downside by using a proprietary TDM signal format is that the TDM signal must be decoded back to baseband at the entrance point to the facility for inter-facility distribution, meaning extra cabling cost. When the TDM signal is non proprietary like the widespread HD-SDI or 3G-SDI video formats this is useful for point to point connections, but it also enables you as the user to take your valued signal formats deep within a broadcast centre through for instance a Network Electronics / VPG routing system to deliver the TDM signal to the location you actually need the signal. No additional cabling needed, the existing broadcast infrastructure is used to transport multiple signals. SMPTE 346M:2000 has standardized TDM of four 270Mbps SDI, SDTI or DVB-ASI

signals into a 1.485Gbps HDSDI transport stream. This is implemented in Flashlink, and allows for straight forward transport of the multiplexed stream through standard HD-SDI infrastructure. SMPTE 424M and SMPTE 425M describes the 3G-SDI signal format. Using this signal as the TDM signal format, as done in the Flashlink module SDI-TD-3GMX-5 allows for transport of multiple asynchronous signal streams up to • 8x SD-SDI, SDTI or DVB-ASI or • 2x HD-SDI or • 1x HD-SDI and 4x SD-SDI, SDTI or DVB-ASI or • 1x 3G-SDI. As we see TDM is a technology that enables several signals to be transported as a single signal at a higher bit rate, using a standardized TDM signal format gives more flexibility in network designs, as the decoding to baseband a the entrance point of the facility is not needed.

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WDM (Wavelength Division Multiplexing) WDM is a method to send multiple signals on the same fiber instead of using multiple fibers. WDM technology is easier to understand if we think of the school experiment with white light being split into the different colors by a prism.

Each transmitter is having a unique wavelength (color), and the filtering needs to match the channel scheme. A simple 2-channel WDM system sends one channel at 1310 nm, and one channel at 1550 nm. This technique is both bit rate and signal independent, meaning that the signals running at the different channels can be different without extra signal processing.

nm to transmit at different wavelengths in this range. In contrast to 2-channel 1310/1550nm WDM systems, DWDM systems require more electronics and sharper optical filters due to the need for temperature stabilization of the lasers. The stability of the laser is a critical issue. At the receiver end the filtering of the channels is a separate task.

The increasing demands of bandwidth, introduced dense WDM (DWDM) systems back in 1996.

The Flashlink DWDM system was introduced in 2000 and have been deployed world-wide since then.

These systems use the wide low attenuation band from 1525–1575

The ITU T-G.694.1 has specified a channel-spacing of 100 GHz

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Digital Signal In


Ch.1 Ch.2 Ch.3 Ch.4

Ch.1 Ch.2 Ch.3 Ch.4

Ch.5 Ch.6 Ch.7 Ch.8

Ch.5 Ch.6 Ch.7 Ch.8

Ch.9 Ch.10 Ch.11 Ch.12

Ch.9 Ch.10 Ch.11 Ch.12

Ch.13 Ch.14 Ch.15 Ch.16

Ch.13 Ch.14 Ch.15 Ch.16

Ch.17 Ch.18 Ch.19 Ch.20 Ch.21 Ch.22 Ch.23 Ch.24

Flashlink DWDM 40 ch. Mux frame

40 independent channels optical fiber

Flashlink DWDM 40 ch. DMux frame

Ch.17 Ch.18 Ch.19 Ch.20 Ch.21 Ch.22 Ch.23 Ch.24

Ch.25 Ch.26 Ch.27 Ch.28

Ch.25 Ch.26 Ch.27 Ch.28

Ch.29 Ch.30 Ch.31 Ch.32

Ch.29 Ch.30 Ch.31 Ch.32

Ch.33 Ch.34 Ch.35 Ch.36

Ch.33 Ch.34 Ch.35 Ch.36

Ch.37 Ch.38 Ch.39 Ch.40

Ch.37 Ch.38 Ch.39 Ch.40

(approximately 0.8nm in the 1550nm range). Each channel has its own transmitter sending at its own bit rate and wavelength. By sending 40 channels simultaneously, each at a separate wavelength, we enhance the fiber capacity by using the same fiber 40 times instead of using 40 separate fibers. The Flashlink range offers currently a 40-channel DWDM system, with bit rates in the range from 143 Mbps to 3Gbps.


Digital Signal Out



Coarse WDM (CWDM) systems have gained high popularity for channel counts up to 16 and for medium distances. The wavelengths are standardized in ITU-T G.694.2, with channel spacing of 20nm. The Flashlink range uses 8 wavelengths in the blue range from 1270nm up to 1410nm and 8 wavelengths in the red range from 1470nm up to 1610nm. The following table shows a comparison of DWDM and CWDM technology at a glance.

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Comparison of WDM technologies Flashlink DWDM/CWDM



ITU-T G.694.1

ITU-T G.694.2

Power consumption per SDI-Tx

3W typically


Laser wavelength variation (0-40°C)



Channel spacing

0.8nm (100GHz)


Max number of channels today



Channels per frame

8+1 upgrade port 8 or 16

Optical amplification well proven for long haul applications



Fiber Attenuation

Fiber type 9/125um single mode

9/125um single mode or 50/125um multi mode

SMF – Single mode fiber, ITU-T G.652 A and B LWPF – Low water peak single mode fiber, ITU-T G.652 C and D

SMF LWPF O-band 1250


E-band 1350



1450 1500 Wavelength (nm)









2 ch WDM




16 ch CWDM





40 ch DWDM

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Long haul optical transport solutions Even though the attenuation in optical cables is insignificant compared to coaxial cables, there are still some limiting factors in the optical domain. Without going into details, the following list gives an overview: Linear effects • Signal attenuation • Dispersion – Modal dispersion in multi mode fibres – Chromatic Dispersion (CD) – Polarisation Mode Dispersion (PMD) Non-linear effects • Four-Wave Mixing in DWDM systems (FWM) • Self and Cross Phase Modulation (SPM/XPM) • Stimulated Raman and Brillouin Scattering (SRS/SBS) When it comes to long distance networks, planning is important. We have the following general guidelines: • Longer fibre distances means more cost, each dB of optical budget has a price • If possible, work with real, measured fibre attenuation values • Add to this number the fibre margin (ideally >3dB)

• Let the manufacturer or its local representatives propose the system based on the optical budget needed • Note: X dB can cost Y $, (X+1) dB can sometimes double the price, or in worst case not be possible to achieve

Signal regeneration In case of a long distance there are basically two ways to regenerate the signals. • 3R regeneration – The original way of doing long-haul • All-optical amplification – Replacing 3R in many circumstances In the term 3R regeneration, each R has a meaning, 3R stands for Re-amplification, Re-shaping and Re-clocking. This is normally done with an optical to electrical and electrical to optical conversion process, sometimes also called OEO (Optical-Electrical-Optical). This must be done on a signal by signal level, one process per signal. The positive aspect is that this can easily be deployed for CWDM and DWDM.

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Ch.1 Ch.2 Ch.3 DWDM DMux

Ch.4 Ch.4 Ch.4 Ch.4 Ch.4

All-optical amplification can be done e.g. by deploying a so-called Erbium-Doped Fiber Amplifier (EDFA). An EDFA can be used to amplify DWDM signals in the range from 1528 to 1565nm. The DWDM signals will pass through a section of erbium doped fiber where the amplification will take place. An EDFA is a uni-directional device passing light only in one direction, so in cases where a bi-directional signal is transported over a long distance, one fiber and one EDFA is needed per direction.

3R 3R 3R 3R 3R 3R 3R 3R

Ch.1 Ch.2 Ch.3 Ch.4 Ch.4


Ch.4 Ch.4 Ch.4

In contrast to 3R regeneration, an EDFA will not do re-clocking or reshaping, but only re-amplification, on the other hand, the EDFA is bit rate and protocol transparent and can amplify up to 40 wavelengths at the same time. What is important when purchasing an EDFA is that all 40 DWDM wavelengths are amplified equally much, this is done by using a gain-flattening filter, and the parameter is called gainflatness. A full-band EDFA will have this parameter specified, whereas a narrowband EDFA will not specify this.



Typical EDFA setup

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Transmission Budget Description of parameters in transmission budget Transmitter output power

Attenuation per fiber length

The output power of the laser.

As the signal propagates through optical fiber, the power of the signal is attenuated. The attenuation is proportional to fiber length.

Receiver sensitivity The sensitivity of the receiver is the smallest amount of input optical power, which the receiver (OEunit) needs to detect the incoming signal properly.

WDM ~ Wavelength Division Multiplexer A WDM sends different wavelengths into different fibers, or combines different wavelengths onto the same fiber. Different versions like 1310/1550nm WDM, CWDM or DWDM exist. The Flashlink products have insertion loss stated including connectors.

Transmission wavelength The attenuation in optical fiber depends on the transmission wavelength. At 1310 nm we estimate 0.4 dB/km of fiber, at 1550 nm we use 0.25 dB/km of fiber.

Fiber length The distance between the transmitter and the receiver in km. Different versions like 1310/1550nm WDM, CWDM or DWDM exist. The Flashlink products have insertion loss stated including connectors.

Fiber attenuation Fiber attenuation is the product of attenuation per length and length of fiber.

# Connectors Each connector pair contributes to the attenuation with ca. 0.5dB. The attenuation in the couplers and the WDMs includes attenuation in the connectors.

Couplers The Flashlink product range, includes couplers where you can drop a signal or split the signal. This process contributes to loss in

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Attenuation margin

the optical link. All the different Flashlink couplers are available in the drop-down menu. The coupling ratio is given as x:100-x in %. E.g. 90:10 means that 10% of the signal power is tapped (coupled) from the main signal and 90 % travels towards the receiver. The attenuation in dB, includes the attenuation in the connectors connected to the couplers.

A safety margin, in case the attenuation exceeds the estimates. This margin allows for repair, splices etc. A margin of more than 3 dB is preferred.

Total link attenuation The total link attenuation is the sum of all the attenuations in signal between the transmitter (EO-unit) and the receiver (OEunit). We denote the components between the two units an optical link.

Example of Calculation We have studios A and C, located 24 km apart. In addition a post production house B shall have access to the content transported on the optical fiber. We calculate budgets for

The figure to the right is a sketch of an optical link. We will show you how to calculate the transmission budget for this example.


SDi-EO-13T, –7.5dBm A

90% 4km





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Link name: A to C Fiber type: Corning SMF-28 Transmission wavelength (λ):

1310 nm

Attenuation per fi ber length at λ: 0.4 dB/km Fiber link length:

24 km

Number of connectors: 2 x 0.5

Fiber attenuation: Connector att.:

Coupler #1 attenuation (90-10, 90% port):

≈9.6 dB 1 dB 1.4 dB

Attenuation margin:

3 dB

+ Excess attenuation:

2 dB

Optical transmission budget needed from A to C:

17 dB

Link name: A to B Fiber type: Corning SMF-28 Transmission wavelength (λ):

1310 nm

Attenuation per fi ber length at λ: 0.4 dB/km Fiber link length:

4.2 km

Number of connectors: 2 x 0.5

Fiber attenuation: Connector att.:

Coupler #1 attenuation (90-10, 90% port): Attenuation margin:


SDI-EO-13T, -7.5dBm


SDI-OE-L, sensitivity -30dBm

Optical budget: 22.5dB

1 dB 11.7 dB 3 dB

Optical transmission budget needed from A to B:

Confi guration I:

≈1.7 dB

17.4 dB

As we see, both links should work well with either of the two suggested confi gurations. The overall cost of confi guration B is less than confi guration A.

Confi guration II: Transmitter:

SDI-EO-13T, 0dBm


SDI-OE-S, sensitivity -20dBm

Optical budget: 20dB

To help you calculate your own transmission budgets we provide a blank form on the next page.

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CALCuLATE yOuR OWN TRANSMISSION buDGET Link name: Fiber type: Transmission wavelength (λ):


Attenuation per fi ber length at λ:


Fiber link length:

Fiber attenuation:


Number of connectors:

Connector att.:


Coupler #1 attenuation:


Coupler #2 attenuation:


Coupler #3 attenuation:


Coupler #4 attenuation:


Optical fi lter #1 attenuation:


Optical fi lter #2 attenuation:


Optical fi lter #3 attenuation:


Optical fi lter #4 attenuation:


Optical switch #1 attenuation:


Optical switch #2 attenuation:


Excess attenuation:


+ Attenuation margin:

= Optical transmission budget needed

Transmitter output power (laser): – Optical transmission budget needed = Estimated input power on receiver incl. margin =


dB dB

dBm dB dBm

Remarks: Attenuation (insertion loss), transmitter output power and receiver sensitivity for the different devices can be found in technical data sheets. This calculation sheet may include devices not found in the actual optical link. These lines can be omitted in the calculation. Devices not specifi ed here can be included in “Excess attenuation”.

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Laser Safety Precautions uv


1st window

2nd window

3rd window

visible light










Wavelength (nm)

Guidelines to limit hazards from laser exposure. All the available EO units in the Flashlink range include a laser. The lasers emit light in the range from 1270 up to 1610nm.

This means that the human eye cannot see the radiation and the blink reflex will not be activated. The human eye can see radiation from 400 nm to 700 nm, thus called visible light.

Always read the data sheet of the Flashlink product, as well as the laser safety label before the laser power is switched on. Always switch off the laser power before connecting/disconnecting fiber patchcords. Laser radiation can be harmful to the human eye (depending on laser power and exposure time). Therefore … • Never look directly into a fiber pigtail. • Never use microscopes, magnifying glasses or eye loupes to look into a fiber end.

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Fiber Handling Even though a fiber optical cable can look almost the same as an electrical wire, special care must be taken. Inside the cable is a fiber made of glass. Glass has very different physical properties than copper used in electrical wires. In practical terms this means that the following precautions must be taken: SC/PC connector protective cap


• Do not bend the fiber too much • Do not put heavy or sharp items on top of the optical fiber • Keep the connectors clean from dust If a fiber is bent to much, parts of the transmitted light is lost. All datasheets of optical fibers have a point called “minimum bend radius”. This means that any bending of the fiber corresponding to a bend radius less than the given value, will make the light leak out of the fiber. A typical value is 20 mm to 40 mm (Bellcore/Telcordia standard) for single mode fibers.

plastic housing (blue)

optical fiber

plastic coating

Good connection

bad connection


You should also avoid putting any heavy items on top of the optical fibers, because this will change the optical properties of the fiber, and contribute to errors in the transmitted signal. Unless the fiber is damaged, it will regain its optical properties after a bend is straightened out or the items are removed or the squeeze is released.

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Fiber Optical Connectors The Flashlink product range utilises ultra polished SC/PC connectors (SC/UPC).

is large compared to the crosssectional area of a single mode fiber some light will pass the lens.

These connectors have a return loss of better than 40 dB typically with single mode fiber.

To protect the connectors from dust, one shall always put on the protective dust cap which is on the connector end at delivery (or enclosed in a plastic bag) whenever a fiber end is disconnected.

As compared to an electrical connection between two points, an optical connection is much more dependent on clean connectors. A dirty connector can add up to 10dB of attenuation to your link. If there is no light at the receiver, then no signal will be detected. So the difference between an errorless connection over fiber, and no connection at all can be a dirty connector. Therefore: Clean connectors are of crucial importance. This also implies that people working with patching of fiber connectors must be aware of this. An optical fiber is made of glass and must be treated as such, not as an electrical wire.

If there is a chance that a fiber connector is dirty, one should clean the connector before connecting it to a clean fiber ferrule. If a clean connector is pressed against a dirty connector, both connectors will become dirty resulting in degradation of signal quality. Or even worse, you may damage the surface of the connector(s). Using a special cleaning-tape (called CleTop) suitable for this purpose does cleaning of the connectors. This is described in four easy steps – see next spread.

Compare it to the lens of camera. You don’t want fingerprints on the lens, but since the lens area

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4 easy steps

cleaning of the connectors

Step 1: Hold the CleTop in your hand with the metal shield upwards.

Step 2: Press the metal handle on the side of the box to reveal the cleaning tape by removing the metal shield.

Step 3: Place the connector ferrule in one of the two tracks and make a circular movement with the ferrule pressed against the tape.

Step 4: Pull the ferrule on the cleaning tape to the other end of the guiding track. The connector should now be clean.

Note: For economical reasons we recommend that both tracks of the tape is used whenever practical possible.

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Some fiber standards ITU-T G.650

Definition and test methods for the relevant parameters of singlemode fibres

ITU-T G.651

Characteristics of a 50/125 um multi mode graded index optical fibre cable

ITU-T G.652-2003

Characteristics of a single-mode optical fibre and cable

ITU-T G.653

Characteristics of a dispersionshifted single-mode optical fibre 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 cable

ITU-T G.656

Characteristics of a fibre and cable with non-zero dispersion for wideband optical transport

ITU-T G.694.1

DWDM frequency grid

ITU-T G.694.2

CWDM wavelength grid

Source: International Telecommunication Union,

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Glossary ADM – Add Drop Multiplex

DMUX/DEMUX – Demultiplexer

An ADM-node is a place where channels can be extracted from the main stream (dropped) and new channels can be multiplexed (added) into the stream. In video networking, it is most cost-efficient to do this by putting a digital video router at the node.

Used for extracting the different channels in a multiplexed stream into single channel streams. Used at the receiver end or in an ADM-node. See also MUX.

APC – Angle Polished Connector The tip of the connector ferrule is polished at an angle of normally 8° in order to minimize back-reflection of the incoming signal. Typically back reflection (return loss) of a single mode APC connector is better than –60dB.

EDFA – Erbium Doped Fiber Amplifier An all-optical amplifier operating in the 1550nm range, widespread in long-haul optical networks. Enables in-line signal amplification without time- and powerconsuming optical-to-electrical-to-optical conversion. Amplifies the incoming signal (including noise). Some EDFAs have problems handling the SDI-pathological signal. Versions operating in the C-band as well as in the L-band are available.

APS – Automatic Protection Switching A term originated from SDH networks. If a signal path is broken due to e.g. destruction of a fiber the optical signal is automatically re-routed to a redundant fiber path, thus minimizing down time of the fiber link. In the Flashlink range, the APS functionality is incorporated in the Wideband Optical Switches when used together with the GYDA system controller. See also WOS.

F-P – Fabry-Perot

C-band – Conventional band

L-band – Long wavelength band

The spectral band from 1530-1565nm as proposed by the International Telecommunication Union.

The spectral band from 1565-1625nm as proposed by the International Telecommunication Union.

CWDM – Coarse Wavelength Division Multiplexing

Laser – Light Amplification by Stimulated Emission of Radiation

A standardized multiplexing method using unstabilised lasers and broadband filters as specified in ITU-T recommendation G.694.2. See also DWDM.

Stimulated emission is an atomic process giving laser light its special properties like coherence, directivity and spectral purity.


MM – Multi Mode

Optical power is expressed in dBm. This is an absolute measure of signal strength related to 1mW (this is where the m comes in). 0dBm corresponds to 1mW, -3dBm corresponds to 0.5mW, and +3dBm corresponds to 2mW.

In a multi mode fiber a signal can propagate through the fiber in different paths (modes). This means that a bit travelling along a shorter path than the preceding bi t may arrive earlier at the receiver, thus generating errors in the signal. This effect is called modal dispersion and limits the maximum length a signal can travel at a given bit rate. Higher bit rate means shorter maximum length between transmitter and receiver. An MM fiber comes in different versions dependent on the diameter of the core and the cladding. Different types are called OM1, OM2 and OM3.

DFB – Distributed FeedBack A distributed feedback laser is a device with a periodic variation of the refractive index in the amplifying region. This ensures a narrow bandwidth of the laser light and is the preferred choice of laser type for dense wavelength division multiplexing applications. See also F-P.

DWDM – Dense Wavelength Division Multiplexing A standardized multiplexing technique (ITU-T recommendation G.694.1) for simultaneous transport of multiple signals at different wavelengths in a single mode optical fiber. Temperature stabilised lasers and very narrowband optical filters ensure errorless transmission. See also CWDM.

A Fabry-Perot laser consists of a semiconductor material with the same refractive index in the amplifying region. This makes a standing wave pattern and lasing occurs at several equidistant wavelengths simultaneously, making the laser unattractive for dense channel spacing but a very cost-effective laser for 1310nm. See also DFB.

MUX – Multiplexer Used for adding several signals together into a single signal stream. Both electrical and optical multiplexers exist. Electrical multiplexers must have the same input signal format and bit rates, whereas optical multiplexers can have different signal formats and bit rates.

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NMS – Network Management System

allow data streams at different rates to be multiplexed. The bit-rates are exactly the same as used in the international equivalent SDH, but the designations are different. OC-3 (Optical Carrier 3) is the designation of 155.52Mbps, OC-12 is for 622.08Mbps, OC-48 is for 2488.32Mbps and OC-192 is for 9953.28Mbps. See also SDH.

Network Management Systems are used to manage the increasingly complex and heterogeneous computer and telecommunication networks.

PC – Polished Connector (or Physical Contact) A PC-polished connector has a straight ferrule tip. Typically back reflection (return loss) of a single mode PC connector is better than –40dB. See also: APC, SPC, UPC.

SPC – Super Polished Connector An SPC-polished fiber connector has the same physical shape as a PCpolished connector. The difference lies in that the polishing of the ferrule surface. Typically back reflection (return loss) of a single mode SPC connector is better than –45dB. See also APC, PC and UPC.

RI – Refractive Index All optically transparent materials have a refractive index, which is the ratio of speed of light in vacuum to the speed of light in the medium. The RI of an optical fiber is ca. 1.45-1.50, giving a light (signal) speed in optical fibers of circa 200 000 km per second.

TDM – Time Division Multiplexing Adding several slower signals with the same bit rate into a signal stream of a higher bit rate. Increasing the bandwidth per wavelength.

S-band – Short Wavelength band

UPC – Ultra Polished Connector

The spectral band from 1460-1530nm as proposed by the International Telecommunication Union.

An UPC-polished fiber connector has the same physical shape as a PCpolished connector. The difference lies in that the polishing of the ferrule surface. Typically back reflection (return loss) of a single mode UPC connector is better than –50dB. See also APC, PC and SPC.

SDH – Synchronous Digital Hierachy Synchronous Digital Hierarchy is an international standard for synchronous data transmission over fiber optic cables. SDH defines a standard rate of transmission at 155.52 Mbps, which is referred to as STS-3 (Synchronous Transport Signal) at the electrical level and STM-1 (Synchronous Transport Module) for SDH. STM-1 is equivalent to the North American SONET’s OC –3. STM-4 is the designation for 622.08Mbps, STM-16 is for 2488.32Mbps and STM-64 is for 9953.28Mbps. See also SONET.

WDM – Wavelength Division Multiplxing Sending signals at different wavelengths through the same optical fiber, means that a single fiber can replace several fibers. Optical filtering is needed at each end of the link.

WOC – Wideband Optical Coupler

SM – Single Mode In a single mode fiber, the signal can only propagate through the fiber along one path at a given wavelength. The transmission capacity of a single mode fiber is extremely large (50-100 Tbps), and attenuation of down to 0.2dB/km. Standardized in ITU-T.G.652.

SNMP – Simple Network Management Protocol SNMP defines how management information is exchanged between network management applications and management agents. SNMP is a generic protocol used to monitor and configure network hardware and software based on TCP/IP.

SONET – Synchronous Optical Network SONET is a standard for connecting fiber-optic transmission systems found in the USA. SONET is standardised by the American National Standards Institute (ANSI). SONET defines interface standards at the physical layer of the OSI seven-layer model. The standard defines a hierarchy of interface rates that

An optical device that split the optical signal from one fiber onto two or more optical fibers. The split ratio of the device decides how much power shall be transferred to the different fibers. For instance a coupler with one input port and two output ports and a split ratio of 50%-50% (often called a 3dB coupler) will split the incoming signals into two equal parts going to two different receivers. The WOC is used to distribute the same signal to several end users or in redundancy applications.

WOS – Wideband Optical Switch Broadband device, used for automatic protection switching systems. A 2x1 switch can be connected to one input fiber or the other. Controlled through the GYDA system controller or via the GPI interface at accessed through the connector module. Available in both 2x1 (can also be used as 1x2) and 2x2 configurations. See also APS.

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Fiber Tutorial  

A short, non-mathematical introduction to the interesting world of fiber optics. Outside of the cladding is often a protective polymer coat...