Unambiguous refractive-index measurement in a wide dynamic-range using a hybrid fiber Fabry-Perot interferometer assisted by a fiber Bragg grating

Abstract

We demonstrate the unambiguous measurement of the refractive index (RI) in a wide measurement range (from 1.0003 to 1.6068 RIU at 1550 nm), using a fiber Bragg grating (FBG) and a hybrid fiber Fabry-Perot interferometer (HFFPI) arranged in series. The spectrum of the reflectivity (RHFFPI) is composed by the interference pattern and the narrowband grating peak. In optical domain the wavelength of the grating peak is used to monitor the temperature, while the RI changes are monitored by the fringe contrast of the interference pattern. Nevertheless, the fringe contrast exhibited an ambiguous behavior that limits the RI measurement range to substances with a RI smaller than that of the core of the optical fiber used. In Fourier domain the spectrum of the RHFFPI showed the three characteristic interference peaks of an HFFPI but superimposed on a broad pedestal, due to the FBG. The amplitude of the three peaks was maximum when the fiber tip is surrounded by air, but two of them diminished when the RI of the external medium (next) increased. The amplitude of the second and third peaks decreased below the level of the pedestal, that is, peaks turned into dips, when next was larger than the single-mode fiber refractive index (nSMF). Importantly, to the best of our knowledge, a one to one relationship between the amplitude of the Fourier peaks and next was established even for values larger than nSMF, for the first time. Additionally, this device allows the simultaneous measurement of refractive index and temperature with good sensitivity and stability since both devices (HFFPI and FBG) are intrinsically immune to the power fluctuations of the optical signal, which is crucial for strict real-world applications such as biosensing.