EFFPI Based on Bubble
Microbubbles or microspheres have been used for developing high Q factor optical resonators, which have supported whispering gallery modes for decades. Thanks to the development of high-end fusion splicers, microbubbles can be fabricated with good controllability , as shown in Figure 2.11. A unique rectangle air-bubble-based EFFPI was reported recently by fusion
Figure 2.11 (a, b) EFFPI structures based on microbubble formed by fusion splicing and (c, d)
corresponding interference spectra. The parameters for fusion splicing were different between (a) and (b).
splicing two cleaved SMFs and tapering the splicing point using the same fusion splicer. The surface was very smooth as the fabrication process occurs after melting of the material and the wall thickness can be controlled as thin as 1 p,m. Due to this unique microstructure, the strain sensitivity was as high as 43 pm/|i?, and the temperature cross sensitivity was low, 0.046 p?/°C.
EFFPI Based on Reflective Film
Most of the previous EFFPI structures were formed by two surfaces with low reflectance, often determined by the Fresnel reflection formula. In these low-finesse cavities, a two-beam interference model can be used to describe it, as the intensities of higher order reflections are much lower than the first reflection from each reflective surface. Usually, the reflection spectra were detected in the case of low-finesse FP cavities. If we look at the transmission of the low-finesse cavities, the intensity of the first transmission is much stronger than the second or even higher order transmission, so that the fringe visibility is very low due to the strong dc signal.
High-finesse cavities were not preferred in the field of FFP sensors owing to their relatively complicated fabrication process. However,
Figure 2.12 High-finesse EFFPI based on coated SMFs and microlens pairs.
high-finesse cavities correspond to a much narrower linewidth of resonant peaks, compared to the interference fringes in low-finesse cavities.
High spectral resolution leads to high resolution for sensing applications. In 2008, Jiang et al. developed a high-finesse microlens-based EFFPI by curing epoxy droplets on the fiber ends, as shown in Figure 2.12. This method is simple and cost effective .
The manufacturing procedure is described as follows. First, two SMFs were cleaved, polished, and coated with highly reflective films. The reflectance was found to be about 96%. Second, another bare optical fiber was cleaved and dipped into the epoxy adhesive (NOA61, Norland) with a depth of 1 mm. When the fiber was withdrawn from the liquid epoxy, a small droplet of epoxy was formed at the end of the fiber. This epoxy added fiber and the coated fiber were fixed at each side of the fusion splicer. The two fibers were pushed together and then separated, resulting in a small epoxy droplet being attached on the fiber end with a mirror. Then the mirror-coated fiber with a small epoxy droplet was put downward letting gravity make the curve of the lens symmetrical, and was finally cured using a UV light for 10 min. To form the highfinesse EFFPI, two mirror-coated SMFs with epoxy microlens were inserted into a capillary for alignment.
This high-finesse EFFPI structure has two differences from other EFFPIs. One is that highly reflective films were coated on the fiber ends, reducing the transmission loss of the cavity. The other is using the epoxy-based microlens, which reduces the propagation loss of multiple reflected beams in the cavity. Due to the high reflectance of the coated mirror, the intensities of the transmitted beams are comparable and multi-beam interference, rather than two-beam interference, occurs in the cavity. Indicated by the transmission spectrum of the high-finesse EFFPI, the FSR is 14.5 nm and the finesse of this kind of EFFPI is around 37 and 78, which is not as high as expected from the high reflectance of 96% because of the divergent angle of light emitted from each fiber end. But the finesse is much higher than the EFFPIs with Fresnel reflections. The high-finesse EFFPI has high sensitivities for strain and temperature sensing, with resolution of 0.0625 p? and 0.025°C, respectively.
Jiang and his coworkers  further fabricated a high-finesse EFFPI for magnetic field sensing. The fabrication process was the same as described above, except one TbDyFe rod was used at one end instead of SMF. The light was incident from the lead-in SMF and reflected from the coated film on the TbDyFe rod. The reflected light was detected as the TbDyFe rod was not transparent. The length of the TbDyFe rod was changed with the ambient dc magnetic field, introducing a cavity length change of the EFFPI. The wavelength shift was measured with the magnetic field. The sensor had a high sensitivity of 1510 nm/mT, with a magnetic resolution of 25 nT. Again it is proved that the high spectral resolution of the resonant wavelength peaks is beneficial for high-resolution sensing.