Noise sources can be categorised as an active and passive.
Active sources such as optical plugs,lasers, receivers, and amplifiers generate noise in the fiber link.
Passive sources such as connectors, fiber, splices, and WDMs cause interference by distorting or reflecting the propagating signal power.
Below are ten major noise sources:
1. Signal-spontaneous noise (si-sp): This type of noise is generated by the signal mixing with amplified spontaneous emission noise generated in an optical amplifier. It is typically the dominant noise source in an amplified optical link. It is also known as amplified spontaneous emissions (ASE) noise.
2. Spontaneous-spontaneous noise (sp-sp): This type of noise is generated by ASE mixing with itself.
3. Shot noise (sh): This kind of electrical noise is generated by the receiver photodiode in both PIN and APD type receivers. APD type receivers have a better signal to noise ratio (SNR) due to their internal multiplication gain mechanism. Receiver noise level and minimum OSNR are established in transceiver design and cannot be controlled by system planning other than by selecting better transceivers.
4. Shot-spontaneous noise (sh-sp): This kind of noise is generated by shot noise added along-with ASE noise in the receiver. It is accounted for the transceiver’s OSNR specifications.
5. Thermal noise (th): This type of noise is generated by the front end of the receiver diode due to thermal activity and is accounted for in the transceiver’s OSNR specifications. The RMS photodiode
6. Multiple path interference noise (MPI): This noise is generated by the signal reflecting multiple times in the fiber and interfering with itself. Reflections are due to Rayleigh scattering and other reflective events such as connectors and splices. It is typically a concern for high signal powers and in distributed Raman amplifiers with high gains. This noise source is also known as double Rayleigh scattering (DRS). To keep MPI noise to a minimum, use good-quality, and clean ultra-polish (UPC) or angle polish (AP) type fiber connectors. Fiber splices that show OTDR reflections should be re-spliced.
7. Source spontaneous emissions (SSE): This noise is due to spontaneous photon emissions during the lasing process. These random photons add to the laser’s amplitude and phase, causing noise.
8. Mode partition noise (MPN): This noise occurs because of random variations of individual laser modes even though the total laser output power remains constant. The noise is generated in fibers where the signal dispersion wavelength is not zero. The fluctuating modes travel at different group velocities due to chromatic dispersion, which results in mode desynchronization and adds receiver noise. MPN occurs for MLM lasers but can also occur for SLM lasers that have significantly large side nodes and where the side mode suppression ratio (SMSR) is less than 20 dB.
9. Cross talk noise: DWDM (dense wavelength-division multiplexing) can cause interference noise as channel cross talk. The signal from one WDM channel appears on another channel resulting in interference. This is because a WDM cannot provide total 100% channel isolation. Typical adjacent channel isolation is approximately 30 dB down from adjacent channel signal power. At this level, the interference is not significant in most systems. However, it can be a concern for high channel launch powers.
10. Nonlinear distortions: Fiber can also cause noise interference when high power signals interact with the fiber, resulting in nonlinear distortions such as four-wave mixing. Maintaining low signal power below recommended limits can help keep this interference to a minimal level.
