BANDWIDTH REQUIREMENTS FOR M-SEQUENCE CODES USED IN FIBER OPTIC MULTIPLEXING SYSTEMS

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