A thermomechanical sensor using photo-inscribed volume Bragg gratings

To offset the lack of tools capable of efficiently monitoring local and dynamic contact conditions relating to stress and heat simultaneously, a sensor using a waveguide Bragg grating has been developed and tested thermomechanically. We demonstrate that temperature and strain measurements inside transparent materials can be performed by fabricating photonic sensors directly in the bulk of a material. To do this, temperature and strain sensing is performed using an optical system comprising an embedded ultrafast laser fabricated waveguide and a Bragg grating inscribed inside the bulk of a pure silica glass. An isotropic modification of the refractive index is first generated inside the material to fabricate a 10 μm-core cylindrical waveguide then an anisotropic modification generated by a scanning technique is used to inscribe the Bragg grating superposed on the waveguide. Three sensors were fabricated and characterized with periods of 1.07 μm, 1.60 μm and 2.14 μm. The results show that the Bragg grating waveguide sensor fabricated inside the bulk of a transparent material can perform both temperature and strain measurements with only one grating. This proof of concept sensor is so a promising tool that can help tribologists to access quantitatively to local contact data (around a few μm2 area and few μm below the contact surface, normal stress only in this example) and finally validate their models more quantitatively. To convert it into a fully efficient tool for tribologists, an extension of the sensor capability is under development, based on the same principles, to help measuring simultaneously tangential stress and temperature evolution in a contact under dynamic boundary conditions.