Asymmetric IFFPI Structures
We have already introduced several types of reflective mirrors for constructing the IFFPI structures by using a pair of such mirrors, including FBGs, reflective films, and Fresnel reflections. The Fresnel reflection mirrors include three different structures: (1) the air gaps by fusion splicing a chemically etched fiber and a cleaved fiber, (2) the reflection from the splicing joint of two different kinds of optical fibers, and (3) a cleaved fiber end face. Theoretically speaking, IFFPI structures can be formed by choosing either two of these reflective mirrors. Here, the asymmetric IFFPI structures can be classified into two types. One is asymmetric in the axial direction, mainly the structure by using two different kinds of reflective elements for each mirror, or using the same kind of reflective element but with different reflectance. It is simple to combine one reflective element with a cleaved fiber as the other mirror. The other is asymmetric in the radial direction. An example is the asymmetric IFFPI structure based on the off-axis fusion splicing. We call them axially asymmetric IFFPI sensor and radially asymmetric IFFPI sensor, respectively.
In the last section, two examples of the axially asymmetric IFFPI structures were introduced. One is the IFFPI structure with an air gap and a cleaved fiber end face as the mirrors . The other is by using the splicing point between two different optical fibers , and a cleaved fiber end face as the mirrors. Such asymmetric IFFPI structures are simple to fabricate. As one of the mirrors is fixed by using the Fresnel reflection, it is easy to optimize the interference fringe contrast by designing the structure and making the reflection of the other mirror similar to that of the Fresnel reflection.
Axially asymmetric IFFPI structure based on a pair of FBGs with different reflectance was developed by Yongming Hu and coworkers . Interference between multiple reflections from two strong FBGs was used to enhance the sensitivity. By using a path-matched Michelson interferometer, the sensitivity of the system was enhanced by more than 19 dB when the light was reflected nine times between the FBG mirrors.
Radially asymmetric IFFPI structure often has a lateral offset fabricated by using the fusion splicing method . The fabrication process is rather simple, only by cleaving and fusion splicing. A large lateral offset of 62.5 |lm was introduced during splicing two sections of SMFs. The reflections from the splicing point and the Fresnel reflection of the cleaved end show interference with each other. The interference visibility can be as high as 15 dB. This kind of radially asymmetric IFFPI sensor was used to measure high temperatures up to 1200°C with a sensitivity of 41 nm/°C. Experimental results also show that there was no degradation of the sensor performance during putting the sensor in a 1000°C environment for 160 min.