Bragg wavelength sensitivity of higher order modes to temperature and strain in highly birefringent microstructured fibers

Summary form only given. Single mode microstructured fibers (MSF) are a subject of extensive research for over a decade now, mainly due to the fact that by changing the topology and distribution of the air holes, the fiber guiding (therefore also sensing) properties can be significantly modified and tailored to desired purposes [1]. Furthermore we have recently reported [2] that greater polarimetric sensitivities can be achieved in higher order modes (as their mode maxima are closer to the cladding hollow regions) of appropriately designed highly birefringent (HB) MSFs.In this paper we report for the llow regions) of appropriately designed highly birefringent (HB) MSFs. first time to our knowledge the results of a success (FBG) inscription in a dual mode HB written with a KrF nanosecond excimer laser in the Talbot interferometer set-up [3 spectrum of a fabricated structure i s presented in Fig. 1a. The three dual peaks visible in the Bragg reflection of the three modes propagated in the fiber. As we used reasonably short pieces (approx. 0.5 m) of the fiber (spliced on both ends with a standard SMF-28) for FBG inscription, we have noticed not only the fundamental and second order mode, but also a third order mode (with confinement loss of approx. 15 dB/m [2]) propagating in the fiber. Furthermore the difference between the effective refractive indices of the polarization modes causes the Bragg peak split into the dual peak structure (with Bragg peak separation strongly correlated with the phase birefringence).In Fig. 1b. and 1c. we present a modes to external strain and temperature changes. The B shift is comparable for all of the propagated modes (with approx. 1.1 pm/με strain sensitivity and 10 pm/K temperature sensitivity) and similar to what was reported for standard FBGs (1.2 pm/με and 13.7 pm/K at 1550 nm [4]). An additional phenomenon in HB fiber based sensors is the birefringence change due to external perturbations (e.g. stra- n, temperature, etc.), which in the Bragg wavelength domain corresponds to the Bragg peak separation (ǻB=Bx-By) change. We have observed the ǻB of the second order mode (E21) of approximately 0.01 nm which is in agreement with the E21 mode polarimetric strain sensitivity of 4.94Â104 rad/εÂm reported in [2]. Therefore we show that HB MSFs with enhanced sensitivity of higher order modes can be successfully used as fiber optic sensors in the convenient FBG configurations.