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Coherence Multiplexing Based on Interferometric Scanning WLI

The WLI monitoring scheme can be extended to the interrogation of FFP sensors with different lengths in a sensor network, as illustrated in Figure 5.23. This scheme is known as coherence multiplexing (CM) [39]. CM also requires the use of a reference interferometer matched in length to within the coherence length of the light source (about 10 |lm for a typical LED or SLD) to the sensor FFP being interrogated. Light from the broadband source must be transmitted or reflected by both the sensor and reference interferometers before reaching the photodetector. If the reference interferometer is scanned, one fringe peak is observed. In one experiment using a quartz halogen lamp as the light source and a scanned Michelson reference interferometer, six FFP strain sensors with different optical cavity lengths are multiplexed in series along the length of an SMF [40]. In another

Arrangement for CM

Figure 5.23 Arrangement for CM: (a) optical system for sensor interrogation and (b) calculated fringe patterns from three FFP interferometers of lengths L1, L2, and L3.

case, two FFP strain sensors are multiplexed in serial and parallel arrangements using a multimode LD as the light source [41].

Comparison of Multiplexing Methods

Comparison of multiplexing methods is given in Table 5.3. It can be seen that different methods have different advantages and disadvantages.

Table 5.3 Comparison of Different Multiplexing Methods

METHODS

CAPACITY

REFERENCES

SFDM

Medium

33

CWDM

Low

34

SFDM/CWDM

High

35

TDM

Medium, depends on sensor insertion loss

36,37

Microwave-assisted reconstruction

Medium, depends on sensor insertion loss

38

Interferometric scanning WLI

Low

39-41

 
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