Abstract

The multiplexing capability of optical fiber sensors is one of their main advantages since it allows the possibility of forming sensor networks or arrays. New possibilities for the development of fiber sensors have been open up with the advent of photonic crystal fibers (PCFs). However, most of the PCF sensors devised until now typically operate as point sensors [1]. A few attempts of multiplexing PCF sensors have been reported in the literature, although the proposed techniques allow the multiplexing of a very limited number of sensors or make it necessary complex demodulation schemes, see for example [2,3]. Here we demonstrate an alternative to built PCF-based point and sensor arrays. We exploit the effect of overlapping between modes in the PCF. The proposed sensors consist of a short length (in the 9-12 mm range) of index-guiding PCF fusion spliced between conventional single mode fibers (SMF). The fiber used in our experiments is a cost-effective large-mode-area PCF, commercially known as LMA-10. The cross section of such a fiber is shown in Fig. 1. Two collapsed zones in the PCF-SMF interface with different lengths allow the efficient excitation and overlapping of core and cladding modes in the PCF at specific resonant wavelengths. To understand the operating principle of our devices let us analyzed the evolution of the propagating beam as it travels from the SMF-in to the PCF and finally to the SMF-out. When the fundamental SMF mode enters the collapsed region it diffracts and consequently it broadens. Thus, the beam that reaches the PCF is larger that the PCF core diameter. The mode field mismatch combined with a short section of PCF allows the excitation of a specific cladding mode in the PCF and of course the fundamental HE11 core mode. Owing to the axial symmetry and the longitudinal offset introduced by the collapsed region the excited modes are those that have similar azimuthally symmetry, i.e., the HE11 core mode and most probably the HE22-like cladding mode. The modes experience diffraction again and add up when they reach the SMF-out region. We have found that when the collapsed region in the PCF-SMF-out interface is shorter than that of the PCF-SMF-in interface, then the transmission spectrum of the device exhibits a deep notch at a specific wavelength; see the center plot of Fig. 1. The position of the notch can be controlled with the length of PCF and the collapsed regions. Note from the figure that the overall insertion loss is around ~2 dB. When the device is subjected to axial strain the dip shifts to the blue. The observed shift as a function of the applied strain is linear, as shown in the right-hand side image of Fig. 1. The single dip and the relatively low insertion loss make the multiplexing simple. In the inset of the right-hand side plot shown in Fig.1 we show four devices in series. One of them was subjected to axial strain while the others were not. It can be seen that the position of the dip of the device under strain changes but that of the others do not.

© 2011 Optical Society of America