Hybrid FFPI Sensors

Up to now, we introduced extrinsic and intrinsic FFPIs based on either SMFs or MMFs. IFFPIs have the advantages of high- temperature sensitivity and high mechanical strength, while EFFPIs have low-temperature cross sensitivity and are especially useful for temperature-insensitive sensing of other kinds of parameters such as pressure, strain, etc. SMF-based FFPIs have the advantages of good transverse mode distribution and no cross talk from multimode excitation, while MMFs are much easier for alignment and easy-to- use for high-efficiency light coupling during the fabrication of FFPIs. Therefore, researchers in this field have made further efforts on making full use of all these advantages by fabricating hybrid FFPIs.

Intrinsic-Extrinsic Hybrid FFPIs

As described in Section 2.1.2, the in-line EFFPI based on an air cavity was first used for strain sensing. By simply cleaving one end of the air-cavity-based EFFPI, an intrinsic-extrinsic hybrid FFPI was fabricated, as shown in Figure 2.14. The end face can be exposed to aqueous solutions to measure the refractive index [71], without the complicated process of fabricating the microchannels for sampling. It has the advantages of a large measurement dynamic range for refractive index sensing (Figure 2.14).

Another similar structure, by fusion splicing a short section of hollow-core fiber and SMF to the lead-in PCF [72], was developed for high-temperature sensing. By fusion splicing a SMF with a large- diameter capillary and a small-diameter capillary in sequence, an intrinsic-extrinsic hybrid FFPI was developed for gas sensing [73]. The air gap based on the large-diameter capillary formed an EFFPI, while the small-diameter capillary formed an IFFPI and also served as the sampling channel for gas.

Another advantage of using the intrinsic-extrinsic hybrid FFPI is enhancing the fringe visibility of the reflected spectra by three-beam interference, instead of two-beam interference. The schematic diagram of the three-beam interference model is shown in Figure 2.15.

Intrinsic-extrinsic hybrid FFPI for refractive index sensing

Figure 2.14 Intrinsic-extrinsic hybrid FFPI for refractive index sensing.

Schematic diagram of the three-beam interference model for the intrinsic-extrinsic hybrid FFPI

Figure 2.15 Schematic diagram of the three-beam interference model for the intrinsic-extrinsic hybrid FFPI.

The theoretical expressions were described in detail in Reference 74 and also in Chapter 1.

As is well-known, the fringe visibility of two-beam interference reaches its maximum value when the reflectance of the two FFPI surfaces equals, that is, Th = Rn. Th, Rn, and Rm are the reflectance of the reflective surfaces of the FFPI. If R = Rn is not strictly satisfied, the fringe visibility will not be very high.

Considering the three-beam interference, the fringe visibility becomes maximum when the reflectance of three reflective surfaces meets the following inequality:

This makes it much easier for the three-surface-based hybrid FFPI sensor to obtain fringe visibility higher than the conventional two- surface FFP sensors, as shown by the gray curve in Figure 1.6. The fringe visibility can be as high as 35 dB [75]. The high fringe visibility is helpful to enhance the sensing resolution due to the higher spectral resolution, if the wavelength shift is detected, and also to enhance the sensitivity if the fringe visibility is detected as a function of the measurand.

By combining the singlemode-multimode hybrid FFPI structure, not only can the fabrication process be simplified by reducing the precision of alignment, but this is also useful for multi-parameter sensing. The use of graded-index MMF is also helpful for enhancing the fringe visibility of the reflected spectra of the FFPIs, which can be as high as 35 dB [76]. For refractive index sensing based on fringe visibility detection, the sensitivity of 160 dB/RIU can be achieved [74].

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