Multiplexed and distributed optical fibre sensors

Abstract

The viability and economy of optical fibre sensors is greatly improved by multiplexing, the method by which many individual passive heads can be interrogated by a common opto-electronics terminal. For space applications, multiplexing offers the added attraction of greatly reduced weight, as the fibre cable network and passive sensor units will generally be much lighter than their electrical counterparts and there is no need for heavy electromagnetic screening materials. The ultimate form of multiplexing, and the maximum weight saving is achieved when the sensor consists of a single length of fibre cable and where the terminal unit can monitor and resolve the variation of a physical parameter, as a continuous function along the length of the fibre. Such a sensor is called a distributed sensing system.

A fuller description of multiplexing and distributed fibre sensing methods has been given in a general textbook [1] on optical fibre sensors. In addition, a collection of more specific published papers is available in a recent text [2]. This paper will therefore simply outline the basic concepts, then concentrate on a number of methods which show promise for space applications. Finally, it will point the way in which new systems may provide attractive possibilities for future space systems. The lecture presentation will include many other case studies not covered in this short written text. The operating principles of many of these are, however, described in more detail in the references.

In the following discussions, we shall initially assume that the optical sensor is a simple amplitude modulation device, incorporated in a fibre path, although most sensors tend to be a little more complex in practice.

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