Optical Fiber

Optical fiber is a type of cable used in telecommunications using light as its medium.

You can read a post about telecommunication cabling.

Optical fiber reaches a speed of 10 gigabit per second.

Unlike metallic conductor, there is no thermal dissipation.

Optic fiber is limited by the difussion effect.

They use a carrier (portadora, in Spanish) that is similar to visible light.

Wavelength Division Multiplexing (WDM) is used in optic fiber.

Optical Fiber Composition

Component of an optic fiber cable:

Core (núcleo in Spanish)
Cladding (revestimiento in Spanish)
Buffer (cobertura in Spansih)

The refractive index is different in the core and cladding are slightly different.

The buffer avoids external interference.

The Snell law states that it is necessary that n1 > n2, where n1 is the core’s refractive index.

A laser narrows the direction and bandwitdth of a LED. This si why lasers are used in optic fiber.

Vertical-cavity surface-emitting laser (VCSEL) is integrated in a circuit.

A plastic optic fiber (POF) is made essentially of platic, built of polymetilmethacrilate wrapped in fluorure polymeres. The cable is very flexible, what makes it suitable to use in facilities where the tubes are old. It allows to use all the bandwidth without loss.

Transmission in Optical Fiber

Optical Fiber Modes

Fiber optic modes:

Step-index Multimode
Graded-index Multimode
Monomode

Step-index

The step-index is the biggest core. It was the first to be obtained.

it has two effects:

Loss of energy
Modal dispersion

Modal dispersion occurs because the signal in different frequencies does not arrive at the same time, and the signal receives has a widen bandwidth in relation to the original.

It uses a laser.

The core has 62,5 micrometres.

300 micrometres

Graded-index

A graded-index optical fiber receive all frequencies at the same time.

Monomode

Monomode avoids the phenomenon of dispersion using a single mode. It has a very small core.

Graded-index and monomode has a ~50 micrometre core?

The whole cable for 125 micrometres.

Monomode avoid any reflection.

It is used on long distance.

Monomode has a 9 micrometre core.

Multiplexion in Optical Fiber

Wavelength Division Multiplexing (WDM) is a variant technology of FDM used in optical fibers.

WDM types:

Normal
Coarse
Dense

Coarse WDM

Coarse WDM (CWDM) provides a greater number of optical channels than CWDM with a greater chromatic dispersion of the same.

CWDM allows shorter transmission distances (50-70 km), with the trade-off of a greater optical spectrum and lower equipment costs.

Dense WDM

Dense WDM (DWDM) typically operates in the C and L bands, with channel spacings from 50 GHz.

The use of erbium-doped fiber amplifiers (EDFA) allows the transmission distance of DWDM systems to be increased.

It is more expensive but allows longer distances (500-700 km).

DWDM is used to connect data centers.

Optical Fiber Networks

The optical line terminal (OLT) is a device located at the central hub of the ISP. It acts as the central command for the PON network, managing network-wide traffic.

An optical network terminal (ONT) is a device that serves as the telecommunication chain’s endpoint of the PON on the end user. Simply put, an ONT is a user-side equipment.

A Passive Optical Network (PON) utilizes fiber-optic technology to distribute data from one source to multiple endpoints. The term “passive” denotes the use of optical fiber cables linked to an unpowered splitter, which transmits data from the service provider’s network to numerous customers.

A Passive Optical LAN (POL or POLAN), short for Passive Optical Local Area Network, is based on the Passive Optical Network (PON) architecture. Like any PON system, POL is a point-to-multipoint indoor network infrastructure that uses optical splitters to distribute data from a single source to multiple user endpoints.

Optical Distribution Network (ODN)

Optical Fiber Standards

Optical fiber standards:

ISO 11801
ITU-T optical fiber standards
IEEE 802.8

ISO 11801 Optical Fiber Categories

Fiber optic categories according to ISO 11801:

Name	Core/Cover (µm)	Max. attenuation	Max. length	Speed	Mode
OS1	9/125	1 dB/km			Monomode
OS2	9/125	0,4 dB/km			Monomode
OM1	62.5/125		300 m / 1000 m	1 Gbps / 100 Mbps	Multimode
OM2	50/125		500 m	1 Gbps	Multimode
OM3	50/125		1.500 m	10 Gbps	Multimode
OM4	50/125		550 m	10 Gbps	Multimode

ITU-T Optical Fiber Standards

APON
BPON
EPON
GPON

GPON

Gigabit-capable Passive Optical Network (GPON), also known as G.984, is an ITU-T optical fiber standard.

GPON technology in FTTH networks allows distances of up to 60 km to be reached between the OLT and the ONT, thanks to the absence of intermediate active equipment.

The S interface of a GPON network, downstream, is the point on the optical fiber just after the optical connector.

Wavelength is 1,310 nm.

IEEE 802.8

IEEE 802.8 is the IEEE standard for fiber optics

Optical Fiber Windows

There are bandwidth windows that are traditionally used to transfer data though optical fiber.

1st window: 850 nm
2nd window: 1310 nm
3rd window: 1550 nm
- S band: 1510-1535 nm
- C band: 1535-1570 nm
- L band: 1570-1610 nm

There are bandwidth windows that are traditionally used to transfer data though optical fiber.

First (S-band)
Second
Third (C-band)
Fourth (L-band)

First window (s – short). 850 nm.

Second window. 1,310 nm

Third or C-band or E – extra is used for long-distance transmissions (e.g. transatlantic transmissions). 1550 nm.

Fourth or L-band is 1625 nm.

Types of multimode

There are different types of monomode:

Erbium Doped Fiber Amplifier (EDFA) are used to amplify the signal without using electronic components.

Optical fiber distorsion

Optical fiber distorsion factors:

Modal dispersion
Intramodal dispersion
Rayleigh dispersion
Radiation
Difraction
Light absortion

Intermodal or modal dispersion which occurs in multimode fiber the incoming light signal has different modes of propagation. Each wave hence will reach at different time at the exit and hence will cause dispersion.

Intramodal dispersion occurs in monomode optical fiber different colour of light travel at different speed in different material and different waveguide structure. Hence light will exit at different time for each colour hence causing dispersion.

Rayleigh dispersion is because the core material is not uniform and has imperfections.

Optical Fiber Connector

Optical fiber connectors are chosen based on two characteristics:

Structure
Polishing

The optical fiber connector types are defined by the combination of both of this characteristics, mentioning first the structure type and then the polishing type.

For example, there are SC UPC, LC UPC, SC APC or LC APC connector types.

Optical Fiber Connector Structure

Other categories:

LC and SC are the most popular. FC and ST are not much used.

LC is seen in a data center. It has a pair of optical fiber, one on each direction.

SC has a single optical fiber. It is used at home.

Optical Fiber Connector Polishing

Optical fiber connectors:

Flat
PC
UPC
APC

Flat connector has a minimum return loss of -14 dB.

Physical Contact (PC) is polished convexed, but with a small angle so it looks flat. Its minimum return loss is -40 dB. The connector color is black.

Ultra Physical Contact (UPC) is polished with no angle. Its minimum return loss is -55 dB. The connector color is blue.

Angled Physical Contact (APC) is polished with a 8° angle. Its minimum return loss is -65 dB. The connector color is green.

GBIC Transcievers

There are different type of GBIC transcievers.

SFP
SFP+
XFP
QSFP/QSFP+
CSFP
QSFP28

Fiber to the loop

Fiber to the loop (FTTx) types:

Fiber to the Desk (FTTD)
Fiber to the Home (FTTH)
Fiber to the Cabinet (FTTC)
Fiber to the Building (FTTB)
Fiber to the Premises (FTTP)
Fiber to the Neighbourhood (FTTN)

SONET/SDH

Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs).

SDH substituted Plesiochronous Digital Hierarchy (PDH).

SDH is used in ATM.

SDH is standardized by ITU-T.

External References

Optical fiber connectors
- Yana, “APC, UPC, PC Fiber Connector Types Comparison and Selection“, QSFPTEK, 2024-05-23
- “UPC or APC“