Fiber optics refers to the technology of transmitting light down thin strands of highly transparent material, usually glass but sometimes plastic. An optical fiber is a long thin strand of glass. A fiber-optic cable is made up of incredibly thin strands of glass known as optical fibers; one cable can have as few as two strands or as many as several hundred. At the heart of this technology lie two phenomena: attenuation and dispersion.
What is Attenuation?
Attenuation refers to the decrease in signal strength as it propagates through a medium, such as an optical fiber. In the context of fiber optics, attenuation specifically refers to the loss of optical power in the form of light intensity as it travels along the length of the fiber. This loss can occur due to factors such as absorption, scattering, and bending of light within the fiber.
Attenuation is measured in decibels per kilometer (dB/km) and is an important parameter in determining the maximum distance over which signals can be reliably transmitted in a fiber optic communication system. Minimizing attenuation is usually essential for maintaining signal integrity and achieving high-quality communication links.
What is Dispersion?
Dispersion in optical fiber refers to the phenomenon where different wavelengths of light experience varying velocities as they travel through the fiber. It causes pulses of light to spread out over time, leading to signal degradation and limiting the transmission capacity of the fiber
A modulated optical signal is not a single wavelength/frequency but occupies a small band (a bit like AM radio). The different frequencies propagate at different speeds and the pulses spread out. This limits the distance over which optical signals can be regenerated. (Especially those with higher bit rates which have shorter periods and are thus more susceptible to dispersion).
Types of dispersion
- Chromatic Dispersion: This type of dispersion occurs because different wavelengths of light travel at slightly different speeds in the fiber. As a result, light pulses composed of multiple wavelengths (as in the case of optical signals) spread out over distance, causing them to overlap and interfere with each other. Chromatic dispersion is expressed in terms of the dispersion coefficient, which indicates the rate of dispersion with respect to wavelength.
- Modal Dispersion: This arises in multimode fibers because of the different propagation paths (modes) that light can take through the fiber core. Because each mode travels a different distance and experiences a different path length, light pulses spread out in time, causing them to arrive at the receiving end with varying delays.
- Polarization Mode Dispersion (PMD): PMD occurs when light polarized in different directions travels at different speeds through the fiber, leading to polarization-dependent delays. As a result, the polarization state of the light pulse changes as it propagates, causing signal distortion and limiting the transmission bandwidth of the fiber.
Attenuation vs. Dispersion: Key Differences
| Basis | Attenuation | Dispersion |
| Definition | Loss of signal strength as light travels through the fiber | Spread of light pulses as they propagate through the fiber |
| Nature | Loss phenomenon | Distortion phenomenon |
| Cause | Absorption, scattering, and bending losses | Material and waveguide imperfections |
| Effect on Signal | Reduces signal intensity over distance | Broadens and distorts signal pulses |
| Types | – Absorption – Scattering – Bending | – Chromatic Dispersion – Modal Dispersion – Polarization Mode Dispersion (PMD) |
| Measurement | Expressed in decibels per kilometer (dB/km) | Expressed in picoseconds per kilometer per nanometer (ps/km/nm) or in terms of dispersion coefficient |
| Impact on System | Limits the transmission distance and system performance | Limits the data rate and bandwidth of the system |