Using optical noise loading to estimate margin in optical amplifier systems

Digital transmission systems are designed with margin to compensate for impairments and it is important to determine that a system does in fact possess adequate margin. Margin is difficult to measure because it is based on beginning and end of life bit error ratios (BERs), which are usually too low to measure. The solution is to measure margin indirectly by increasing the BER in a controlled way to measurable levels. The current method uses the dependence of BER on the decision circuit threshold voltage, V_th. This method requires a receiver with a linear amplifier, which is not universally available. It also assumes that V_th will be at optimum both at the beginning and end of system life. The method proposed here has the advantages of not requiring a linear amplifier and gives a more accurate value for the margin by measuring the latter for the value of V_th as it exists in the receiver. In the new method, measurable BERs are produced by adding amplified spontaneous emission (ASE) power to the system. The ASE can be added by attenuating the launch power of the transmitter or from an ASE generator at the receiver. The new method is only applicable, at present, to systems where there is no line broadening due to nonlinear effects