The fiber etalons in Fig. 1 can also be used as sensors for measuring strain, as the distance between mirrors in the fiber determines their transmission characteristics. The mirrors can be fabricated directly into the fiber by cleaving the fiber, coating the end with titanium dioxide, and then resplicing. An alternative approach is to cleave the fiber ends and insert them into a capillary tube with an air gap. Both of these approaches are being investigated for applications where multiple in-line fiber sensors are required.

For many applications a single point sensor is adequate. In these situations an etalon can be fabricated independently and attached to the end of the fiber. Fig 2 shows a series of etalons that have been configured to measure pressure, temperature, and refractive index, respectively.

In the case of pressure, the diaphragm has been designed to deflect. Pressure ranges of 15 to 2000 psi can be accommodated by changing the diaphragm thickness with an accuracy of about 0.1% full scale. For temperature the etalon has been formed by silicon–silicon dioxide interfaces. Temperature ranges of 70 Degree to 500 Degree Kcan be selected, and for a range of about 100 Degree K a resolution of about 0.1 Degree K is achievable. For refractive index of liquids, a hole has been formed to allow the flow of the liquid to be measured without the diaphragm deflecting. These devices have been commercialized and are sold with instrument packages.

Fig 2. Hybrid etalon-based fiber optic sensors often consist of micromachined cavities that are placed on the end of optical fibers and can be configured so that sensitivity to one environmental effect is optimized.