![]() ![]() The problem arises when two factors are present: 1. It is different from most noise in the electronic world in that it does not come from external sources, but originates from the signal itself interacting with the optical channel. Modal noise appears as unwanted amplitude modulation of the received signal. The resulting interference-induced energy pattern, which appears at a multimode fiber interface, is referred to as speckle pattern. While the LED produces homogeneous illumination, the laser spot does not. LEDs on the other hand, are highly incoherent light sources with broad spectra. The problem is almost exclusively confined to the use of laser sources such as VCSELs or DFB lasers with a narrow linewidth and a long coherence length. ![]() In many cases, this will have serious effects on the BER of the communication channel and, therefore, on the achievable throughput. Modal noise Modal noise is a distortion of the signal caused by the fiber and joints (connectors or splices) themselves interacting with the signal in a way which produces ‘random’ fluctuations in signal power at the receiver. This combination of VCSEL-coupling and laser optimized fiber design is a basic requirement for digit rates of 10 GBit/s and more with Multi-mode fibers.įaster, but… We’ve now established why applications above 1GBit/s work with VCSEL - but we also need to take a new parameter into account. Laser optimized Multi-Mode fibers - known as OM3 and OM4 – are created with optimized Gradient index profiles, which don’t have dips in the center axis, allowing them to fulfill the requirements of laser coupling. This profile imperfection increases dispersion and BER (bit error rate). With conventional, older Multi-Mode fiber types (OM1/OM2) the gradient index profile in the center core area is has dips, peaks and flat tops. This coupling method is called Underfilled Launch (UFL) or Restricted Mode Launch (RML). VCSELs (Vertical Cavity Surface Emitting Lasers) offer far better focusing of light than LEDs, and only activate modes which lie close to the fiber core. With speeds of 1 GBit/s or higher over OM3 and OM4, laser diodes are used as the coupling method. However, because the operational frequency of LEDs is limited, they can’t submit digit rates higher than 622 Mbit/s. When using this method, all modes in the Multi-Mode fiber are activated. Evolution from LED to VCSEL sources Testing of fibers in categories OM1 and OM2 was originally carried out with light coupling by LED.
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