Bandwidth requirements for bit-sequence codes used in fibre optic multiplexing systems

Maximal length sequence spread spectrum techniques have been identified as an attractive means of networking a number of fibre optic sensors, since they provide a means of increasing the mean launch power of the interrogating signal while retaining sufficient measurand bandwidth. In such applications it is important to minimise the crosstalk between sensors, while maximising the number of sensors supported. The paper considers and presents formally the coupling requirements of such a system and the impact upon system performance. It then goes on to derive an expression for the effect of high frequency bandwidth constraints upon the autocorrelation function as a function of the channel response and the code length. Experimental verification of this theory is also reported. The results indicate that, for a network that operates with an upper 3 dB cut-off frequency equal to the code rate, the suppression ratio penalty is less than 1 dB. As a consequence of the bandwidth constraint, the correlation peak is broadened, so that operation under the condition described above leads to a minimum sensor spacing of approximately two chip periods