Multiplexed fibre-optic sensors using ring interferometers

We describe a number of multiplexing techniques which may be used for fibre-optic sensors in which the sensing element is a ring resonator . The basic optical unit comprises a sensing ring resonator connected to a second interrogating ring resonator of nearly equal length, and illuminated by a source of short coherence length. A number of such units may be combined to form an array, using coherence multiplexing techniques . Using such an arrangement, the high finesse of the ring resonator may be exploited to yield a sensor of increased sensitivity over that obtainable with a two-beam interferometer . When a highcoherence source is used, then multiplexing may be accomplished using frequency-division techniques . However, in this case, the resonators must have low finesse, in order to avoid cross-talk . We also show that when a source of moderate coherence length is used, it is possible to use both frequency division and coherence multiplexing simultaneously, with the potential of realizing a large sensor array . 1 . Introduction A large variety of fibre-optic interferometric sensors has been developed using different optical configurations and signal processing techniques [1] . Recently there has been increasing interest in multiplexing such sensors in one system to avoid unnecessary duplication of light sources, photodetectors, and fibre transmission lines. Several techniques have been previously reported including time-division multiplexing [2, 3], coherence multiplexing [4, 5], wavelength-division multiplexing [6], and frequency-division multiplexing [7] . The combination of time addressing with coherence multiplexing [8] and time addressing with frequency division [9] has also been reported with the advantage of increasing the number of sensors in a sensing system . Most multiplexed sensor systems described to date have used the Mach-Zehnder interferometer as the primary sensing element although multiplexed systems based upon miniature low-finesse fibre Fabry-Perot interferometer sensing elements have recently been reported [10, 11] . The ring resonator is a multiple-beam interferometer, and has high finesse provided that a beam-splitting element (directional coupler), which satisfies the resonance conditions, is used [12,13] ; an advantage in sensitivity over that of a two-beam interferometer may therefore be achieved . The ring resonator has no separate fibre reference arm, and is therefore capable of absolute measurement ; the Mach-Zehnder effectively makes differential measurements since only the optical path difference between the signal and reference arms can be determined . j' Permanent address: Department of Electronics, Himeji Institute of Technology, 2167, Shosha Himeji-Shi, Hyogo 671-22, Japan . 0950-0340/89 $3.00 © 1989 Taylor & Francis Ltd . 338 F. Farahi et al . An objective of the present work was to realize a multiplexed sensor array, making use of high-finesse monomode optical-fibre ring resonators . A disadvantage of the ring resonator is that it is an inherently unbalanced interferometer, so that it requires a source with a long coherence length, and the phase of the interference is a strong function of the source wavelength . This effect limits the phase resolution of the ring resonator, especially when a diode laser source is used, since the wavelength of these lasers shows substantial drift and noise. Accordingly, in the present work we have adopted a tandem arrangement of ring resonators, in which each ring is of nearly equal length . A source is used whose coherence length is short in comparison with the length of either ring, but is longer than the difference between the optical paths of the two rings . Hence interference is only observed between nearly equal optical paths, so that the arrangement is balanced, and is hence relatively insensitive to wavelength variations . It is convenient to use one of the rings in the tandem arrangement as the sensor, and the second ring as the receiver . The sensor ring may be deployed remotely from the receiver, whereas the receiver may incorporate any active components required for signal processing . The tandem arrangement is also well suited to coherence multiplexing . An array of sensing rings, all of different lengths, may be deployed . These sensors may then be connected by a single fibre cable to a corresponding set of receiving rings . Each receiving ring will show visible interference only in conjunction with the sensor ring of matching path length . We describe below an experimental demonstration of a multiplexed array based on these principles . In situations where a raised noise floor due to source frequency variation may be tolerated, then unbalanced optical arrangements may be used. Under these circumstances, frequency-division multiplexing is feasible . In this method, a frequency-chirped laser source is used to generate a pseudo-heterodyne carrier at the output of the unbalanced ring pair . The frequency of the carrier is dependent on the path imbalance, so that demultiplexing may be achieved by band-pass filtering . However, it is shown below that when high-finesse ring resonators are used in this way, then considerable cross-talk is introduced between the outputs . Nevertheless, it is demonstrated that frequency-division multiplexing may be satisfactorily employed when low-finesse rings are used . It is further shown that, with such lowfinesse rings, it is possible to combine the techniques of coherence multiplexing and frequency-division multiplexing, and so to realize large arrays . 2 . Tandem ring interferometers system 2.1 . Theory A tandem fibre ring system is shown in figure 1 . The system is illuminated by a relatively short coherence-length source such that no interference occurs at each individual ring interferometer but the interference can be observed when the interferometers' path imbalances are matched within the coherence length of the source. Following the notation used in references [12, 13], the general form of a ringinterferometer transfer function-assuming polarization insensitive componentsis given by : E4 E1 2 =(1 -Yo) 1(1-k)(1-k,) (1) [1 + (kk,) 1 J 2 ] 2 -4(kk,) 1 / 2 sine CAL ~~ 2 4 Multiplexed fibre-optic sensors using ring interferometers