Bit Error Rate Performance in Coherent Optical Systems

The probability of a transmitted bit error is generally termed as BER. It is calculated by the ratio of incorrectly transmitted bits to total transmitted bits. Regardless of the data destination, an optical transport system (OTS) must provide the predefined bit-error rate (BER) performance. Optical data links also face a tradeoff between optical power and unrepeated distance. Although bit error rates are typically on the order of 10-6 to 10-9at 155 Mb/s, this is adequate for some applications such as voice and video transmission; recent experiments have shown that error rates as low as 10-l2 can be obtained at 200 Mb/s using different protocols on free-space optical links. This paper provides enough background material for calculating the bit-error rate (BER) of coherent light wave systems. However, the BER, and hence the receiver sensitivity, depend on the modulation format as well as on the demodulation scheme used by the coherent receiver. This paper explains each system separately.