Data link performance and link optical power budget

Measurement of data transmission quality

As with copper or radio cable transmission, the performance of an optical data link can be determined by how it transmits the data; how the reconverted electrical signal that leaves the receiver adapts to the transmitter input.

The ability of any fiber optic system to transmit data basically depends on the optical power in the receiver, as illustrated in the image above, in which the erroneous bit rate (BER) of the data link is shown as a function of the optical power in the receiver. (The wrong bit rate is inverse to the signal to noise ratio, for example, a high bit rate implies a poor signal-to-noise ratio). In the case of insufficient power or excess power, a high bit rate will be generated. If there is excess power, the receiver amplifier becomes saturated; and if there is insufficient power, noise becomes a problem since it interferes with the signal. The power of the receiver depends on two basic factors: how much power the transmitter throws on the fiber and how much power is lost by attenuation in the fiber optic cable network that connects the transmitter with the receiver.

Link optical power budget

The budget of the optical power of the link is determined taking into account two factors: the sensitivity of the receiver (which in turn is determined in the wrong bit rate curve as illustrated above) and the output power of the transmitter in the fiber. The minimum power level that generates an acceptable erroneous bit rate determines the sensitivity of the receiver. This transmitter power coupled to the fiber determines the transmitted power. The difference between these two power levels determines the loss margin (optical power budget) of the link.

High-speed data links such as gigabit local area networks or 10gigabit Ethernet over multimode fiber have decrease factors in fiber bandwidth power caused by the dispersion of digital data pulses. Old OM1 62.5 / 125 fibers generally operate on short links while transmissions through OM3 fiber optimized for 50/125 lasers are for greater distances. Even long-distance single-mode fiber links may have limitations caused by chromatic dispersion or polarization mode dispersion.

If the link is designed to operate at different bit rates, it is necessary to generate a performance curve for each bit rate. Since the total power in the signal is a function of the pulse width and this varies depending on the bit rate (at a higher bit rate, shorter pulses), the sensitivity of the receiver will cause degradation at high bit rates.

 Fiber optic certification

The manufacturers of systems and components for data links specify for each type of link, the sensitivity of the receiver (maybe a minimum required power) and the minimum power coupled to the fiber from the source. The standard values ​​for these parameters are shown in the following table. In order for the manufacturer or who designs the system to test them properly, it is necessary to know the test conditions. For components for data links, these conditions include data input frequency or bit rate and duty cycle, power source voltage and the type of fiber coupled to the source. For the systems,