The step-Index Fibre was one of the earliest, if not the first Fibre design available. The propagation of light through a Multi-Mode Step-index Fibre suffers from modal distortion. This type of Fibre design is to slow for Data Comms / Telecoms application, which led to the development of the Graded Index Fibre.
The glass on a Graded-Index is a unique composition, which alters the refractive index across the core profile. The refractive index reduces from its maximum at the Fibre core axis. As the light propagates towards the core-cladding interface, the refractive index will continue to decrease, until it reaches a minimum, the same value as the core-cladding interface. Graded-Index Fibre offers a significant increase in bandwidth.
The refractive index on a Step-Index Fibre is Uniform. However, there is a significant change at the core-cladding interface, a decrease in the refractive index; this is because of the lower refractive index in the cladding. At this point light propagate / reflected along the Fibre core length, "Total Internat Light Reflection". The profile of the light is that of a saw tooth.
Light entering the Fibre from a suitable light source will do so at varying angles of incidence and will therefore propagate different paths at the same speed within the Fibre core. Lengths of these paths will change. The light will arrive at the output at differing times this is Modal dispersion. Modal Dispersion inhibits the performance of the Fibre link.
Suppose we consider that we use light pulses (digital communications) to transmit and receive data. At the source, a pulse of light will be emitted. As a consequence of modal Dispersion, the number of nodes/length of Fibre the pulse of light will distort or widen, this phenomenon modal Dispersion, will increase with the length of the link. Modal Dispersion within a multi-mode step-index Fibre will severely restrict performance, bandwidth and attenuation.
Modal Dispersion can also be responsible for many other performance-related issues
Step-Index Multi-mode Fibre is severely limited as to where it can be deployed within the communication sector. Typically short links no more than 2km at low speeds nothing greater than 10Mb/s. Low-level entry links.
Step-Index Multi-Mode Fibre is used in other industries, Medical/industrial laser delivery, Optical sensing. But increasingly less demand within the Telecoms/Datacomms.
The refractive index on a Graded-Index Fibre is at its highest at the fibre's core axis, which will gradually decrease as you propagate towards the core-cladding interface. The refractive index value at the core-cladding interface will be the minimum value. Because of the lower Refractive index towards the outside of the Fibre core, light travels faster at any point towards the core-cladding interface than that at the core axis of the Fibre. The continually changing refractive index causes refraction and not total internal reflection.
Total Internal Reflection will not occur in Graded Index because the refracted light folds back into the Fibre axis. This happens before the light reaching the core-cladding interface.
Please Note - The comments above are relative to Multi-Mode Fibres only.
Single-mode Fibre is a step-index Fibre. You can get variations of this with Dispersion shifted / nonzero dispersion-shifted. The core in these fibres is very small, typically Ø9um. While they exhibit modal Dispersion, it is very small (Due to the small Fibre core), these fibres are better at retaining the integrity of the light pulse over significantly longer distances. SMF can also handle far bigger bandwidths.
I hope now you have a better understanding of the differences associated with Multi-Mode Graded &Step-Index Fibres. The significant player in the market place is the Graded index because of its speed and bandwith capabilities. These fibres and there capability continue to evolve as the family of 50/125 fibres continues to develop with OM2/3/4 & 5.
Whilst there is small demand for sted-index short links, industrial type applications this area is now also been satisfied by POF -Plastic Optical Fibres.
The glass on a Graded-Index is a unique composition, which alters the refractive index across the core profile. The refractive index reduces from its maximum at the Fibre core axis. As the light propagates towards the core-cladding interface, the refractive index will continue to decrease, until it reaches a minimum, the same value as the core-cladding interface. Graded-Index Fibre offers a significant increase in bandwidth.
Step-Index Multi-Mode Principles and Applications
The refractive index on a Step-Index Fibre is Uniform. However, there is a significant change at the core-cladding interface, a decrease in the refractive index; this is because of the lower refractive index in the cladding. At this point light propagate / reflected along the Fibre core length, "Total Internat Light Reflection". The profile of the light is that of a saw tooth.
Light entering the Fibre from a suitable light source will do so at varying angles of incidence and will therefore propagate different paths at the same speed within the Fibre core. Lengths of these paths will change. The light will arrive at the output at differing times this is Modal dispersion. Modal Dispersion inhibits the performance of the Fibre link.
Suppose we consider that we use light pulses (digital communications) to transmit and receive data. At the source, a pulse of light will be emitted. As a consequence of modal Dispersion, the number of nodes/length of Fibre the pulse of light will distort or widen, this phenomenon modal Dispersion, will increase with the length of the link. Modal Dispersion within a multi-mode step-index Fibre will severely restrict performance, bandwidth and attenuation.
Modal Dispersion can also be responsible for many other performance-related issues
- BER Bit Error Rate
- ISI Inter Symbol Interference
Step-Index Multi-mode Fibre is severely limited as to where it can be deployed within the communication sector. Typically short links no more than 2km at low speeds nothing greater than 10Mb/s. Low-level entry links.
Step-Index Multi-Mode Fibre is used in other industries, Medical/industrial laser delivery, Optical sensing. But increasingly less demand within the Telecoms/Datacomms.
Graded-Index Multimode Principles and Application
The refractive index on a Graded-Index Fibre is at its highest at the fibre's core axis, which will gradually decrease as you propagate towards the core-cladding interface. The refractive index value at the core-cladding interface will be the minimum value. Because of the lower Refractive index towards the outside of the Fibre core, light travels faster at any point towards the core-cladding interface than that at the core axis of the Fibre. The continually changing refractive index causes refraction and not total internal reflection.
Total Internal Reflection will not occur in Graded Index because the refracted light folds back into the Fibre axis. This happens before the light reaching the core-cladding interface.
The light within a Graded-Index Multi-Mode Fibre propagates along the Fibre length in the form of a Sinusoidal Wave. Because of the graded-Index, which reduces from the core axis towards the core-cladding interface, the light will speed up the further it gets from the core axis. So we have established that the speed of a guided light speeds up the further away from the core, but it also travels further, than light travelling in the core. This speed difference compensates for the extra distance travelled. This is the equalisation of the transmission, which significantly reduces modal Dispersion, enabling the Graded-Index Fibre to handle significantly larger bandwidths than step-index Fibre.
It is Graded-Index Fibre OM2-OM5 which are used by the Telecom & Datacomms industry. I suspect demand on Step-index Multi-Mode is decreasing by comparison but more of a niche, specialist type of product in various sectors such as medical/ industrial laser delivery sensing etc.
Please Note - The comments above are relative to Multi-Mode Fibres only.
Step-index SMF (Single-mode Fibre).
Single-mode Fibre is a step-index Fibre. You can get variations of this with Dispersion shifted / nonzero dispersion-shifted. The core in these fibres is very small, typically Ø9um. While they exhibit modal Dispersion, it is very small (Due to the small Fibre core), these fibres are better at retaining the integrity of the light pulse over significantly longer distances. SMF can also handle far bigger bandwidths.
Conclusion
I hope now you have a better understanding of the differences associated with Multi-Mode Graded &Step-Index Fibres. The significant player in the market place is the Graded index because of its speed and bandwith capabilities. These fibres and there capability continue to evolve as the family of 50/125 fibres continues to develop with OM2/3/4 & 5.
Whilst there is small demand for sted-index short links, industrial type applications this area is now also been satisfied by POF -Plastic Optical Fibres.
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