Photonics Crystal Fiber Loop Mirrors and Their Applications

Fiber loop mirrors (FLMs), also called Sagnac interferometers, are interesting and very useful components for use in optical devices and systems [1, 2]. Many components based on FLMs have been demonstrated for applications in wavelength-division-multiplexing filters and in sensors, among others [3-7]. In FLM, the two interfering waves counter-propagate through the same fiber and are exposed to the same environment. This makes it less sensitive to noise from the environment. In general, a conventional fiber loop mirror made of high-birefringent fibers (HiBi fibers) or polarization-maintaining fibers (PMFs) has several advantages compared with a Mach–Zehnder interferometer, such as insensitivity, high extinction ratio, in-dependence of input polarization, easy to manufacture and low cost [1, 2]. However, conventional PMFs (e.g., Panda and bow-tie PMFs) have a high thermal sensitivity due to the large thermal expansion coefficient difference between boron-doped stress-applying parts and the cladding (normally pure silica). Consequently, conventional PMFs exhibit temperature-sensitive birefringence [8]. Therefore, conventional PMF based Sagnac interferometers exhibit relatively high temperature sensitivity, which is about 1 and 2 orders of magnitude higher than that of long-period fiber grating (LPG) and fiber Bragg grating (FBG) sensors [9, 10]. This can limit the practical use of the devices in some applications. Various kinds of sensors based on HiBi-FLMs have been proposed and realized since HiBiFLMs are sensitive to many parameters and have a high sensitivity, such as temperature sensors, level liquid sensors, refractive index sensors, strain sensors and biochemical sensors [7, 9-12]. However, when a HiBi-FLM is used to measure strain or other parameters, its cross-sensitivity to temperature may degrade sensor performance since the optical path length of the HiBi-FLM shows temperature dependence caused by thermal refractive-index change and thermal expansion effect. Thus, the temperature effect must be discriminated or eliminated when they are used for sensing [13-15]. The photonic crystal fiber (PCF) is a new class of optical fiber that emerged in recent years. Typically, these fibers incorporate a number of air holes that run along the length of the fiber and have a variety of different shapes, sizes, and distributions [16-17]. Of the many unusual properties exhibited by a PCF, a particularly exciting feature is that the PCF can be made HiBi by arranging the core and the air-hole cladding geometry, thereby introducing