Photonics Crystal Fiber (PCF) is a new type of optical fiber that has emerged recently, which show very interesting guiding properties [17
]. These fibers have an arrangement of microscopic air holes running along their length, and have the great advantage that by varying the size and location of the holes, the fiber mode shape, non-linearity, dispersion and birefringence can reach values that are not achievable in conventional fibers. Several authors have presented the PCF as a new and promising alternative solution for application in optical communication systems [18
] and in optical fiber sensing [19
]. In this type of fiber the internal birefringence is created by the GE. Applications combining the Hi-Bi PCF with the FLMs have been also presented, for instance, a Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with temperature insensitivity [20
]. Yang et al
have proposed a fiber Bragg grating sensor interrogation technique with temperature insensitivity by using a highly birefringent photonic crystal fiber Sagnac loop filter [22
]. This device was also demonstrated as temperature-insensitive strain sensor or load sensor [23
]. Due to the properties of the uncoated Hi-Bi PCF fiber, its incorporation in fiber loop mirrors turns this structure insensitive to temperature variations when compared with the case of using standard Hi-Bi fibers. Actually, the Hi-Bi PCF loop mirror was characterized for two different cases, uncoated and coated. The results are different for each situation [25
]. The acrylate material of the coating changes the strain and temperature sensitivities. This aspect can be very important in future applications, namely when special re-coatings are applied to create high sensitivity to chemical or biological parameters.
The experimental setup of a Hi-Bi PCF FLM sensing system is based on the one described in Figure 1
and consists in an optical broadband source, a fiber loop mirror containing a section of Hi-Bi PCF and an optical spectrum analyzer (OSA) with a maximum resolution of 0.05 nm. The optical source is an Erbium-doped broadband source, with a central wavelength of 1550 nm and a spectral bandwidth of 100 nm. The Hi-Bi PCF FLM is formed by a 3dB (2×2) optical coupler with low insertion loss, an optical polarization controller (PC) and a Hi-Bi PCF section. This Hi-Bi PCF (Thorlabs
PM-1550-01) is a polarization maintaining PCF fiber containing two large holes, with a beat length smaller than 4 mm and an attenuation inferior to 1.0 dB/Km for a wavelength of 1550 nm (Figure 6
The diameters of the two holes are 4.5 μm and 2.2 μm, respectively, the pitch (spacing between holes) is 4.4 μm, and the diameter of the perforated region is 40 μm, while the fiber outer diameter is 125 μm. A low loss splice (∼2 dB) between SMF 28 from the optical coupler and the PCF was achieved.
represents the evolution of the wavelength shift when temperature is applied in the situations of uncoated and coated Hi-Bi PCF. As can be observed, for the case of utilization of uncoated PCF the FLM structure was essentially insensitive to temperature variations (-0.29 pm/ °C). To explain this behavior, one has to bear in mind that the Hi-Bi PCF has an internal structure of pure silica with two air holes. Therefore, this single material composition is in the origin of the low temperature sensitivity. On the other hand, when the Hi-Bi PCF has an acrylate coating, a non-linear response to temperature was observed, surely a consequence of the non-linear temperature behavior of the acrylate polymer.
For the case of applied strain, as expected the spectral response of the fiber configuration is similar for the cases on uncoated and coated Hi-Bi PCF. Table 3
summarizes the strain and temperature coefficients of the Hi-Bi PCF based fiber loop mirror.