What is an optical network?
An optical network is a collection of interconnected terminals who share data in the form of light. The data travels through special kind of cables called optical fibers. Today’s internet, television and telephones are highly dependent on optical networks. Although we don’t see optical fiber cables connecting to our Wi-Fi routers or computers directly but the global optical network is the backbone of the world.
But light travels in a straight line. Does that mean optical fibers have to be laid straight to transmit data?
Although, light travels in a straight line, optical fibers use a very important property of light called refraction. When light travels from one medium to another it tends to bend towards or away from the normal (line perpendicular to the cross section of the mediums). When light travels from an optically denser medium to a rarer medium it tends to deviate away from the normal. At a certain incidence angle (>critical angle) light starts to get reflected from the boundary region. This is called Total Internal Reflection.
This property of light is used by optical fibers to make light through them along its path without ensuring that the fiber is straight.
Optical fiber cables are made of silica and the ones used in the communication industry are generally thinner than human hair. The glass body called the core, is covered with a cladding which generally has a lower refractive index to allow Total Internal Reflection. There are two types of optical fiber cables used in the communication industry. Single Mode Fibers(SMF), which allow only one propagation path and Multi-Mode Fibers(MMF), that allow multiple propagation paths. Normally, SMFs have a smaller core diameter and are used where long-distance data transmission is involved. MMFs on the other hand have a thicker core and are used at places where high power data is transmitted over a short distance.
Light rays in a multi-mode fiber
But light is a form of energy. Isn’t there any loss involved in this kind of transmission?
As a matter of fact, no system can perform at 100% efficiency. The same is true for optical networks. It is a lossy system; however, the losses are very low when compared to electrical networks. The reason being low resistance. The fibers also do not suffer from electromagnetic interference.
But, the reduction in intensity of the source light beam is still a major concern in fiber optics. Attenuation basically happens in two ways in fiber optics.
- Scattering – Light travels through the core of an optical fiber by reflection and roughness even at a molecular level can make light rays reflect in random directions. This random reflection is called scattering.
- Absorption – Light traveling through the optical fiber might also get absorbed due to impurities in the lattice of the silica the fiber is made of. It can be correlated to resistance in electrical transmission.
For better performance it is desired to have lesser number of interconnects and splices.
So, we achieved better transmission efficiency using optical networks but the devices we use are electrical. How do we connect the electrical domain to the photonic domain?
The concept is very similar to modems we use everyday. Modems convert analog signals travelling through coaxial cables to digital signals for our devices and vice versa. Similarly, optical trans-receivers convert optical to electrical signals and vice versa. They consist of a laser diode for converting electrical to optical signals and a photo diode to do the reverse. A basic optical trans-receiver module has two ports – One for sending the encoded messages and one for receiving. The transmitter and the receiver operate independently.
While choosing an optical trans-receiver module some key factors have to be kept in mind.
Sensitivity – This characterizes the weakest light impulse the module will detect correctly.
Responsivity– This is the ratio of the radiant energy, which is expressed in watts on the device, with respect to the resulting photocurrent, which is expressed in amperes.
Receiver rise time– Speed of the receiver.
Light source – The light source can either be a light emitting diode (LED) or a Laser Diode. LEDs are not preferred much because of their larger emitting areas as compared to the laser diodes. In short to medium distances, they are used because they are quite economical. The laser diodes are very powerful source of light emission and are used in areas that require signal transmission over long distances.
An optical Trans-receiver.
How do multiple signals get transmitted over a single cable?
Multiple signals are transmitted over a single cable by using multiplexing. There are mainly four types of multiplexing.
- Frequency Division Multiplexing (modulating data into different carrier frequencies)
- Wavelength Division Multiplexing
- Time division Multiplexing (dividing available time among various signals)
- Statistical Multiplexing (dynamic allocation of time spaces depending on the traffic pattern)
In modern day optical networks the most frequently used multiplexing is WDM(Wavelength division multiplexing). A WDM system has a multiplexer in the transmitter end and a demultiplexer in the receiving end. Some devices can do both simultaneously and are called add-drop multiplexers. WDM enables to expand the load on a network without putting more infrastructure. More channels of communication can be installed on a same fiber by changing the multiplexers and demultiplexers on both ends.
WDM systems are also sub-divided into three types:-
- Regular WDM – It uses two normal wavelengths 1310 and 1550 on one fiber
- Coarse WDM – It provides up to 16 channels across multiple transmission windows
- Dense WDM – It uses the C-Band (1530 nm-1565 nm) transmission window but with denser channel spacing. The channels vary from 40 – 80 in typical networks.
Do we need an electrical interface for switching and routing optical signals always?
No we dont need an electrical interface always. Optical switches help to switch and route optical signals in an optical network. Basically, whenever light falls on this switch another light is switched on in a different channel corresponding to it. This enabled optical cross connects and switching in optical domain. With all these developments the optical network can take a shape which is similar to electronic networks and adding or removing information at various nodes would be easier.
In the next article we will go deeper into how modern optical network works, we will study SONET, SDH in detail.