Fiber Optics Networking and technologies related explained

Fiber Optics was invented by Charles Kao and George Hockham in England in 1966. Fiber Optics is a thin hair like structure made of either glass or plastic.

Entire Fiber networking is based only on the concept of Total Internal Reflection.

What type of material used in Fiber Optics?

As mentioned earlier, Fiber can be made of Silica Glass or Plastic.

Silica Glass Fibers uses Infrared Light for transmission. It results in less attenuation and high distance transmission which can be around 100 kms.

Plastic Fibers are made of Polycarbonate, Polystyrene, Polymethyl, Methacrylite). These are cheaper and more flexible than glass fibers. But it results in more attenuation and covers lesser distances.

Why Infrared Lights are used in Fiber Networking?

LED and Lasers generate Infrared Light of wavelength : 0.8-0.9 um or 1.3-1.6um.

They suffer least attenuation in glass fibers.

The figure below describes the Wavelength and Attenuation relation with respect to fiber networking :-

As can be seen with wavelength w = 0.8 – 0.9 nm the Attenuation (A) ~ 0.8. For w ~ 1.3nm, A~ 0.3 and w ~ 1.5-1.6, A ~ 0.2-0.5. Beyond these Attenuation increases. Fiber suffers nothing from Electromagnetic Interference. This is added advantage with respect to Copper based Ethernet.

In Medical industry, professionals use visible light.

What is Total Internal Reflection?

Total Internal Reflection is the main concept of the Fiber Optics Networking. This is a refraction concept.

It happens when light enters from a higher refractive index medium to lower refractive index medium. If the angle of incidence is more than the critical angle, the light is reflected back in the same medium. This goes on in the case of fiber networking till the light reaches the other end.

In fiber optics, the Core is made of higher refractive index material and Cladding has lower refractive index as well as it does not absorb any light. Hence, the light suffers minimum loss and travels large distances.

What is the structure of optical fiber cable?

The following diagram shows the structure of optical fiber cable :-

As can be seen, the fiber cable has four major parts – Core, Cladding, Buffer, and Jacket. Below diagram gives the typical size of single mode fiber.

A typical Single Mode fiber is represented above with thickness

What are the various Modes of Fiber Transmission?

There are three modes for fiber transmission : Multi Mode Step Index Fiber, Multi Mode Graded Index Fiber and Single Mode Fiber. A mode means the number of light rays allowed in the fiber cable. Single Mode refers to only one light ray allowed in the fiber. Multi Mode means multiple light rays allowed inside the fiber cable.

What is Multi-Mode Step Index Fiber?

Below diagram describes the Multi-Mode Step Index Fiber as follows :-

In this setup, the core diameter is high but the cladding diameter is less. The refractive index is always larger than the cladding refractive index. In the above example, Core diameter is 200 um and entire fiber has diameter of 380 um.

Lets see the modal dispersion as below :-

The ray enters at different angles of incidence and therefore paths of rays are different. As can be seen, the incident pules shape is distorted at output pulse if the data rate is high. So, 101 is distorted to 111.

In the multi mode step index, only limited data rate could be sent. If higher data rate is sent bit error increases. Therefore, multi mode works with lower data rate only. This will limit the speed of the network. Hence, this cable is not used in the market and industry.

What is Multi Mode Graded Index Fiber?

Below diagram describes the Multi-Mode Graded Index Fiber as follows :-

In Multi Mode Graded Index Fiber, the Core diameter is between 50-100 um. So, the diameter of the cladding is more than the Core. While, the refractive index of core is much more than that of the cladding.

This mode is better than the Step Index as modal dispersion is far less than the Step Index setup. This is because the path deviation of the rays are much lower than the step index setup. The paths are close to each other.

In the above example, the bits are 101 and the output also results in 101. So, the error rate is lower than step index setup.

What is Single Mode Fiber?

Below diagram describes the Single Mode Fiber as follows :-

In this setup, the Core diameter is around ~10 um. This is much lower than 125 um cladding. Here, only 1 mode is allowed. So, only 1 ray enters. This eliminates modal distortion and results in best performance.

As can be observed, the bits sent are 101 and received with full clarity as 101. So, Single Mode Fiber gives the best performance in fiber optic connectivity.

What are the types of Dispersion?

There are three types of Dispersions :-

  1. Polarization
  2. Chromatic Dispersion
  3. Modal Dispersion


Below diagram shows the polarization effect in light. This effect takes place due to bends and curves in the fiber wiring.

Light has vertical and horizontal polarization travelling at same velocity. If the wiring is done as perfectly round then the entry and exit signals are received at same time.

If the wiring is bent then entry and exit are made at different times.

Solution for this is single polarization fiber. This allows either the Horizontal or the vertical pole to enter. This is known as Polarization compensation system.

Chromatic Dispersion

Chromatic Dispersion takes place in Multi Mode only. The rays travels with different wavelength travels at different velocity. They arrive at receiver end at different times. This results in distortion.

It is compensated at either Transmission end or Receiver end by delaying the faster one.

What are the types of Connectors used in Fiber Optics?

Below diagram explains the types of connectors used in Fiber Optics :-

ConnectorDescriptionSupported ModesSetup
STStraight TipSingle/MultiOld
FCFiber Channel/FerruleSingleOld
SCStandard ConnectorSingle/MultiCurrent
LCLocal/Little ConnectorSingle/MultiCurrent

The Straight Tip was used in 1980s and 1990s. It uses 2.5mm ferrule. But the setup is Old.

FC Connectors are used in high vibration environments. These are replaced now by SC/LC Connectors.

SC Connectors are latest that uses Snap/Push Pull method. It uses 2.5mm ferrule and can be used for Single and Multi Mode fiber.

The Little Connectors are very popular at present. It is already replacing SC/FC. It has 1.25mm ferrule only.

Below table shows the characteristics of Fiber Connectors :-

As we know, an Ethernet supports maximum 100 meters without distortion. Similarly, Single Mode Fiber supports 5,000 to 10,000 meter. And Multi mode fiber supports 300 to 2,000 meter only.

What are the types of Ferrules and Losses – Insertion and Return?

We have already discussed about the connectors. These connectors are used to connect end points. These end points are ferrules.

There are two types of loss that occurs in Ferrules. One is Insertion Loss and the other is Return Loss. Insertion Loss occurs due absorption, misalignment, mismatch and air gaps.

Return loss occurs when two ferrules meet. Some signal are reflected back. This is known as Back Reflection or Return Loss.

Ferrules are of four types as mentioned below :-

Types of FerrulesDescriptionLossColor Coding
FLATFlat Fiber-30 dBNO COLOR
PCPhysical Contact-35 dBNO COLOR
UPCUltra Polished -55 dBBLUE
APCAngles Polised-65 dBGREEN

Lower the number in Loss column, better is the signal. So. APC ferrules are much better than PC/FLAT. APC comes in GREEN Color Coding.

Below figure shows the design structure of these ferrules :-

What is a GBIC?

GBIC is a Gigabit Interface Convertor. It converts Gigabit ethernet to Fiber. It is represented by SFP/SFP+ and supported by SC/LC connector. SFP/SFP+ are small factor pluggable transistor.

But this GBIC is OLD.

What are Simplex and Duplex Connectors?

Simplex Connectors has single connectors for Half-Duplex use. While, Duplex connectors has double connectors for Full Duplex. Half-Duplex is single channel for either transmit or receive. Full-Duplex is dual channel for both transmit and receive at same time.

What are various Multi-Mode Graded Index Fiber?

Various Multi-Mode Graded Index Fibers are mentioned below :-

As can be seen the diameter ranges from 50 um to 62.5 um in multi mode fiber as compared to Single mode 9 um.

What are the different multi mode fiber cores OM1, OM2, OM3, OM4, OM5?

Below diagram shows the entire range of multi mode fiber types available in the market :-

The Multi-Mode fiber finds its application in as a replacement of Ethernet LAN Cables in short distances such as data centers and office premises. It allows high speed networks to reach speeds upto 10Gbps at ease.

Below chart explains the difference among the OM1, OM2, OM3, OM4, OM5. The Green highlight shows its main applications. Also, the 40Gbps and 100Gbps range is shown with SWDM technology applied (SWDM is discussed later) :-

OM1 (1989) is a Legacy Orange Jacket fiber that supports LED light source. It is mainly used for 100 Mbps data bandwidth. OM2 (1998) was established to support 1 Gbps fiber network over 550 meters. OM2 also comes with Orange jacket.

OM3 (2002) is Aqua Jacket fiber that supports Laser beam. It is LOMMF – Laser Optimized Multi Mode Fiber. It is mainly used for 10 Gbps bandwidth at data centers. The Laser used is called Vertical-cavity surface-emitting laser (VCSEL).

OM4 (2009) is also Aqua Jacket fiber that support Laser beam. This fiber is further enhanced to support upto 40Gbps stable connection over 150 meters. OM4 is also backward compatible with OM3. It further enhances 10 Gbps network to upto 550 meters from 300 meters in OM3.

OM5 (2014) is relatively new technology that supports WBMMF – Wideband Multi Mode Fiber. It uses wavelength in between 846 to 953 nm wavelength range. Earlier, OM3 and OM4 only supported wavelength around 850nm. But, industry needed to improve the utility of fiber that could support multiple channels for transmission. Hence, OM5 was introduced in 2014.

What is WBMMF and SWDM?

Below diagram shows the WBMMF :-

As can be seen the OM3 and OM4 does not support wavelengths beyond 850 nm. Only OM5, supports the wider band from 850 – 940 nm. Hence, OM5 is called the WBMMF – Wide Band Multi Mode Fiber.

We had OFDMA in ethernet 802.11ax, refer the article here. OFDMA was based on Orthogonal Frequency Division. Similarly, SWDM refers to Short Wavelength Division Multiplexing.

In SWDM, four wavelengths are used across the 850 to 940 nm range. Below diagram shows the SWDM application :-

With SWDM, the data rate of the fiber increases by a factor of 4. SWDM transceivers are compatible with OM3/OM4/OM5 optical connectivity solutions.

What are the expected range of SWDM?

Below table mentions the range of OM5/OM4/OM3 :-

What are the range of OS1 and OS2?

Both OS1 and OS2 are the Single Mode Fiber. All single mode fibers are preferably used for long haul distance measured in kilometers. The difference lies in the cable construction. The speed remains the same viz. 1-10 Gbps. Since this is single mode, it will not support SWDM technology.

OS1 is tight buffered construction used only for small distances in indoor medium premises. OS2 is loose buffered that allows outdoor higher transmission of more than 200 kms.

So, the range of OS1 is less than 10km while the range of OS2 is less than 200kms.


FTTX is a term used for terminus. There are four types of terminals used in FTTX :-

FTTXDescriptionEnd Connectivity
FTTH/PFiber to the Home/PremisesCopper 
FTTWFiber to WirelessMobile Tower
FTTBFiber to the BoxCopper 

These are the terms used for commercial application of fibers towards the end users at large.


There are two types of Fibers :-

  1. Active : This setup requires power to run such as Routers, Splitters, etc. Also, these cover large distance and area.
  2. Passive : This setup does not require no power. They are derived from Active Fiber only.

The Passive network is further derived as APON, BPON, EPON and GPON. PON refers to Passive Optic Network.

APON is ATM PON is old legacy system with low bandwidth. BPON is Broadband PON is also slow due to multi load.

EPON and GPON are recent technologies.

EPON uses Ethernet packets instead of ATM cells used in GPON. EPON is used in FTTH/P. But EPON offers only 1Gbps of speed.

While, GPON can provide upto 2.5Gbps speed for FTTH/P applications with QoS (Quality of Service) and advanced security.

What is an OLT and ONU?

A PON network can use TV, Internet and Telephone lines together. OLT refers to Optical Line Termination and ONU/ONT Is Optical Network Unit/Terminus.

Below diagram shows the Fiber Distribution from service providers :-

We all can find ONU/ONT at homes acting as the terminal point for home routers.

Service providers uses OLT to provide services such as – Internet, Leased Line, VoIP, PSTN and Video. A single OT can connect 20-25 end users only. To further distribute the network to users, an ODN is used.

ODN stands for Optical Distribution network. It is a Splitter that can be further connected to other splitters or the end users. An ODN can connect 4, 8, 16.. users.

From the ODN the network is end connected to ONU/ONT. This is how the service for FTTH/P, FTTB, FTTW is achieved.

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