The modal noise is associate with the multimode fiber. Various propagating modes inside a multimode fiber interfere to produce a speckle pattern on the photo detector.

Since the speckle is a spatial redistribution of the energy, and the photo-detector detects the total energy falling on the detector, as such the modal noise degrade the system performance.

However, in practice, the photo-detector does not have a uniform response over its surface. Also the speckle pattern varies as function of time due to temperature and pressure variations giving a fluctuating photo current. This is the modal nose.

The formation of speckle due to coherent interference of the various modes. Now, since various modes travel with different speeds, the coherent addition of different modes is possible only if the differential delay between different modes is much smaller compared to the coherence time.

The coherence time of light is of the order of , where is the bandwidth of the source. The speckles would vanish if the modal delay is much larger than coherence time reducing the modal noise.

For a source like LED, the spectral width is about 30-50 nm, giving spectral width of 3-5 THz. The coherence time of LED would be 0.2-0.3 psec.

Since the typical inter-modal dispersion inside a multimode fiber is few nsec/Km, even for small networks, where usually the multi-mode fiber is used, the modal noise is very small.

Modal noise becomes a problem when laser diodes are used as optical sources.