IFFPI Structures Based on Reflective Films
Highly reflective mirrors have been widely used for constructing free-space, high-finesse
FP cavities. The same strategy was employed for IFFPI sensors. One of the most promising advantages of coatings on optical fiber is mass production. Thousands of optical fibers can be coated at one time with the same reflective spectral performance. The spectral characteristics of the films can be designed by the mature thin-film optics method. However, it is still challenging to fusion splice the coated fiber with another cleaved fiber to form an in-line reflective mirror. The reflective spectra of the film may distort and the reflectance may decrease greatly during the fusion splicing.
On the other hand, the cross-sectional area of optical fibers is small, with a diameter of ~125 p,m. Dielectric multi-layer stack of optical coatings with a thickness of several micrometers is not as stable as that coated on a free-space substrate, when coated on a cleaved surface of an optical fiber. Therefore, the thinner, the better the reflective films coated on the optical fiber , as long as the reflective characteristics are preserved.
In 1988, H. Taylor and coworkers proposed an IFFPI temperature sensor by introducing a pair of dielectric mirrors into the singlemode optical fiber . Cleaved fibers were coated with TiO2 film with a thickness of 100 nm by the dc magnetron sputtering system. Then the coated fiber was fusion spliced to another cleaved uncoated fiber to form an in-line mirror. The reflectance of such a mirror was 2% and the cavity length is 1.5 mm. The insertion loss of each mirror was reduced to 0.1-0.2 dB by positioning the fiber during the splicing. The sensor was used for temperature sensing from -200°C to 1050°C as the materials, both SiO2 and TiO2, in this structure can withstand high temperature . The reflective film can be fabricated by the vacuum deposition, magnetron sputtering, or e-beam evaporation .
Recently, chromium (Cr), with a melting point of 1907°C was used, instead of TiO2 (1843°C), as the reflective mirror for fabricating the FFPI sensor , as shown in Figure 2.1. This melting temperature is slightly higher so that the long-term stability of the sensor is improved. The interference between the coated Cr film and the Fresnel reflection was used as the read-out signal of the IFFPI sensor. A Cr film with a reflectance of ~10% was coated on the cleaved fiber end. When spliced with another cleaved fiber, the reflectance decreases to ~4%, which is similar to that of the Fresnel reflection so that the fringe contrast is maximized. Temperature up to 1100°C was
Figure 2.1 Schematic of the reflective film-based IFFPI sensor.
measured with a resolution of 4°C. The 300-hour long-term stability was estimated to be about 10°C.