Multimode FFPI Sensors
Most of the developed FFPIs were based on SMFs, due to the low cost and good preservation of the single transverse mode. The latter is good for maintaining the high performance of FFPI sensors. However, multimode fibers (MMF) were sometimes employed for forming FFPI structures with special characteristics. The unique characteristic of MMF is that the core diameter, often 62.5, 50, or 105 p,m, is much larger than that of the SMF, 8-10 |lm for SMF at the wavelength of 1550 nm.
One example of multimode FFP is by using the MMF as an efficient light collecting element . The structure of the multimode
Figure 2.13 FFPI using MMF as the input and output fiber. (Reprinted from Jiang, M. and Gerhard, E. 2001. A simple strain sensor using a thin film as a low-finesse fiber-optic Fabry— Perot interferometer. Sensors and Actuators A: Physical, 88(1), 41-46, Copyright 2001, with permission from Elsevier.)
FFPI is shown in Figure 2.13. The FP cavity was formed by the polymer film. The MMF polished at 45° was used as a lead-in and readout channel for the light. The polymer film was attached to a steel plate for constructing an attachable strain sensor. The film was made from polyurethane, which was composed of a mixture of two-phase structure domains called hard and soft segments, which provided the high tensile strength and flexibility to adjust the physical properties of the film. When strain was added on the device, the thickness of the polymer film would change, which can be determined by the shift of interference fringes. Wu and Solgaard  investigated the short- cavity-length EFFPI pressure sensor, which employed MMF as the input and output fiber and the microfabricated diaphragm based on the silicon-on-insulator (SOI) wafer, as shown in Figure 2.13. They also discussed in detail the effects including the mode averaging, phase uncertainty, amplitude reduction, and spectral modal noise.
There were also other kinds of multimode FFPIs where MMF was not only used for the input and output of light, but was engaged directly in forming the structure. Besides its core diameter being larger than SMF, MMF often has higher NA than SMF, 0.12 or 0.14. It is easy to find commercial MMF with NA between 0.2 and 0.4, or even higher, designed for high-efficient collection of light in the field of imaging and spectroscopy. High NA MMF is useful for fabricating FFPIs, thanks to its high doping of Ge, which makes a higher etching rate difference between fiber core and cladding. The dissociation energy of the Si-O bond, 799.6 kJ/mol, is larger than that of the Ge-O bond, 660.3 kJ/mol. The fiber cladding was often composed of SiO2, while the fiber core was doped with high doping concentration of GeO2. The latter can be etched by HF acid with a higher etching rate than that of SiO2. The large core diameter is also helpful for forming a relatively flat reflective surface compared with a small core SMF.
By chemically etching a graded-index MMF with a core diameter of 105 |lm and NA of 0.3, a notch was obtained at the fiber end face with a depth of 25 |lm when etched for 90 s with 40% HF acid . An EFFPI with a cavity length of 17.5 p,m was fabricated after fusion splicing with another cleaved fiber and the fringe visibility was 22 dB. Usually, the cavity length of FFPI was shorter than the depth of the micromachined notch due to the partial overlap of the two fibers and compression of the cavity length during the splicing. This kind of multimode EFFPI sensor preserves the advantages of other EFFPIs like low temperature cross sensitivity.
In the work presented above, the MMF was used as a reflective mirror only, similar to the EFFPI pressure sensors with other kinds of diaphragm  for biomedical applications . Another motivation of using multimode optical fiber in FFPIs is enhancing the sensitivity for pressure sensing by using the large-diameter fibers as the diaphragm.
Ma et al.  proposed an IFFPI based on the SMS structure. Different from the modal interference detection and from the transmission spectra, the authors investigated the multimode excitation- induced phase shifts in the reflected spectra. Further, they fabricated SMS IFFPIs with graded-index MMF . The insertion loss was reduced and the multiplexing capability was greatly enhanced by the refocusing effect of light propagation in the graded-index MMFs.