Optical Communication Systems: Evolution and Fiber Types
Optical Communication: Core Requirements
Optical communication requires a coherent source and a suitable transmission medium.
Evolution of Lightwave Transmission Systems
- 1st Generation (0.85µm): Operated at a bit rate of 45 Mb/s with a repeater spacing of 10 km. The main advantage was larger repeater spacing compared to coaxial systems, leading to reduced installation and maintenance costs.
- 2nd Generation (1.3µm): Utilized single-mode fibers at 1.7 Gb/s with a repeater spacing of 50 km. Repeater spacing was primarily limited by signal loss.
- 3rd Generation (1.55µm): This spectral region offers the lowest signal loss. However, high dispersion necessitated the use of dispersion-shifted fibers and single-longitudinal-mode lasers, achieving speeds up to 10 Gb/s.
- 4th Generation: Introduced optical amplification to compensate for signal loss, thereby increasing repeater spacing. Wavelength division multiplexing (WDM) further enhanced the bit rate to 5 Gb/s.
- 5th Generation: Addressed fiber dispersion using optical solitons, which are pulses that maintain their shape due to a balance between dispersion and fiber nonlinearity.
Repeaters in Optical Communication
Repeaters play a crucial role by converting the optical signal to an electrical signal and using it to modulate a laser.
Multiplexing Techniques in Telecommunications
Time Division Multiplexing (TDM)
In Time Division Multiplexing (TDM), bits from different channels are interleaved in the time domain, creating a composite bit stream. TDM is widely used in telecommunications networks.
Frequency Division Multiplexing (FDM)
Frequency Division Multiplexing (FDM) assigns each channel its own carrier frequency. The spacing between carrier frequencies is greater than the channel bandwidth to prevent overlapping of channel spectra. FDM is employed for both analog and digital signals in radio and TV broadcasting. In optical communications, FDM is commonly referred to as Wavelength Division Multiplexing (WDM).
Fiber Types and Modal Dispersion
Single-Mode Fiber
Single-mode fiber is preferred for long-distance communication due to its low signal loss and dispersion characteristics.
Multimode Fiber
In multimode fiber, different modes have varying group velocities, leading to modal dispersion and pulse broadening.
Solution to Modal Dispersion
Modal dispersion can be mitigated by grading the refractive index. The refractive index is highest at the center and gradually decreases towards the core-cladding boundary. This causes the velocity to increase with distance from the core. Consequently, rays with greater inclination travel further but faster, equalizing the travel times of different rays.
Numerical Aperture
Numerical Aperture is a measure of the light-gathering capacity of the fiber.