Multimode fiber loss is typically calculated based on the fiber's attenuation rate per kilometer (km) at specific wavelengths. This loss, primarily due to absorption and scattering, reduces the signal strength over distance.
Here's a breakdown of how it's calculated and what factors influence it:
Understanding Multimode Fiber Loss
Multimode fiber, characterized by its larger core diameter (compared to single-mode fiber), allows multiple light modes to propagate simultaneously. While this simplifies connections and lowers component costs, it also introduces modal dispersion and higher attenuation.
Key Factors Influencing Loss
- Wavelength: The wavelength of the light source significantly affects loss.
- Fiber Material: The composition and quality of the glass or plastic used in the fiber core and cladding play a role.
- Manufacturing Imperfections: Microscopic variations in the fiber's structure can scatter light.
- Bending: Excessive bending of the fiber can cause light to leak out, increasing loss (bending loss).
- Splices and Connectors: Each splice and connector introduces loss due to imperfections in alignment or contamination.
Typical Attenuation Rates
Multimode fiber loss is generally specified as decibels per kilometer (dB/km) at particular wavelengths. Common values are:
- 850 nm: Approximately 3 dB/km. This wavelength is typically used for shorter distances.
- 1300 nm: Approximately 1 dB/km. This wavelength is generally used for longer distances than 850 nm within multimode applications.
EIA/TIA 568 standards specify maximum attenuation as 3.5 dB/km at 850 nm and 1.5 dB/km at 1300 nm.
Calculation Example
To calculate the loss over a specific length of fiber, you can use the following formula:
Total Loss (dB) = Attenuation Rate (dB/km) x Fiber Length (km)
For example:
If you have 500 meters (0.5 km) of multimode fiber operating at 850 nm with an attenuation rate of 3 dB/km, the estimated loss would be:
Total Loss = 3 dB/km * 0.5 km = 1.5 dB
Practical Considerations
- Measurement Tools: Optical Loss Test Sets (OLTS) and Optical Time Domain Reflectometers (OTDRs) are used to accurately measure fiber loss in the field.
- Margin: When designing a fiber optic link, it's important to include a margin for unexpected losses, such as those from splices, connectors, and aging.
- Standards: Always refer to relevant industry standards (e.g., EIA/TIA 568) for acceptable loss budgets.
Summary
Calculating multimode fiber loss involves considering the fiber's attenuation rate at specific wavelengths, the length of the fiber, and any additional losses from connectors and splices. Understanding these factors is crucial for designing reliable fiber optic communication systems.
