Optical monitoring system

An optical monitoring system is configured such that no wavelength is carried bi-directionally in the first and second optical connections. The system comprises first and second optical signal sources 105, 106 for emitting first and second interrogation signals having distinct wavelengths. First and second optical sensor arrays 101, 102 are interrogated by the first and second interrogation signals respectively and then first and second optical receivers 107, 108 respectively receive the first and second interrogation signals. A first optical connection couples the first interrogation signal from the first source to the first sensor array and the signal from the second sensor array to the second receiver, while a second optical connection couples the second interrogation signal from the second source to the second sensor array and the signal from the first sensor array to the first receiver. Further embodiments introduce additional sensor arrays still without using bi-directionality in the fibres (Fig 4), and the ability to interrogate either array with different source and receiver combinations to provide redundancy against failure of individual components or connections (Fig 5).