A high sensitivity long period grating Mach-Zehnder refractometer

A refractive index sensing system has been demonstrated, which is based upon an in-line fibre long period grating Mach-Zehnder interferometer with a heterodyne interrogation technique. This sensing system has comparable accuracy to laboratory-based techniques used in industry such as high performance liquid chromatography and UV spectroscopy. The advantage of this system is that measurements can be made in-situ for applications in continuous process control. Compared to other refractive index sensing schemes using LPGs, this approach has two main advantages. Firstly, the system relies on a simple optical interrogation system and therefore has the real potential for being low cost, and secondly, so far as we are aware it provides the highest refractive index resolution reported for any fibre LPG device.

I. Introduction

The long period grating (LPG) is a UV light induced periodic modulation of the refractive index of the core of a single mode optical fibre with a typical modulation depth of ~ 10⁻⁴and a period in the range . The effect of the LPG is to introduce a series of attenuation bands into the transmissivity of the fibre, which occur at wavelengths where the LPG induces phase matched coupling between the guided and cladding modes [1]. Amongst their many potential applications [2], LPGs are attractive for sensing due to the fact that the centre wavelength of the attenuation band is sensitive to strain, temperature and the refractive index of the surrounding medium [3],[4]. The sensitivity arises from the dependence of the effective group refractive index of a cladding mode on the refractive index of the surrounding medium [4].