Comparison of different polymers and printing technologies for realizing flexible optical waveguide Bragg grating strain sensor foils

Abstract

Waveguides with Bragg gratings realized on a flat polymer foil are promising candidates for advanced strain sensors since such a planar approach allows precise positioning of multiple sensors in various well-defined directions, in the same foil. As such, an optical version of an electrical strain gage can be realized. Herein, several parameters are discussed which define the behaviour of such sensor foils, in particular the grating design, including the wavelength of operation and mechanical and optical properties of the used polymers. Epoxy and Ormocer®-based Bragg grating sensors operating at 850 nm and 1550 nm wavelength were realized using nano-imprint lithography and laser directwrite lithography and their strain and temperature sensitivities were compared. Finally, it is demonstrated that optical strain gage rosettes can be realized by multiplexing 3 angularly displaced sensors in the same waveguide on a single foil.

Manufacturing and Characterization of Femtosecond Laser-Inscribed Bragg Grating in Polymer Waveguide Operation in an IR-A Wavelength Range

“…To exclude this source of error and to obtain precise measurements, appropriate measures should be taken, for instance by rapidly varying the SOP and recording of averaged values. We focused on the quasi-static sensitivity with respect to temperature, strain, and humidity, and found our femtosecond laser-inscribed Bragg grating to perform similarly to those reported for other polymers and grating inscription techniques [6,10,32]. The sensitivity to strain is 0.26 pm/µε, sensitivity to temperature is −44 pm/K (at 43% relative humidity), and sensitivity to humidity is 19 pm/%, although these values are still influenced by the utilized substrate and cannot satisfy the requirement of general validity.…”

“…Missinne et al recently reported that their polymeric Bragg sensors have strain sensitivities in the range of 0.85-1.41 pm/µε. They further postulate that the photoelastic constant of optical polymers might be dependent on the Bragg wavelength, since the sensitivity did not scale solely with wavelength for their sensors with different grating pitches [10]. Nevertheless, simple strain variation of the Bragg sensor does not allow precise determination of the photoelastic constant, and further work has to be done.…”

“…Among others, polymethyl-methacrylate (PMMA) is one example for a widely used polymer material, which offers sufficiently low optical loss for light transmission in optical telecommunication or sensor networks that are typically fiber-based. In recent years, however, many other polymeric materials have surfaced that offer beneficial optical properties and enable easy manufacturing with techniques such as laser direct lithography to produce microphotonic systems [10][11][12]. EpoCore, for instance, is a negative epoxy-based photoresist, designed to fabricate waveguides that have long-term stable material properties after processing [13].…”

Imprinted Polymer-Based Guided Mode Resonance Grating Strain Sensors

“…The change in RI of a material as a function of strain is given by the strain-optic coefficient, denoted as ρ [36]. Based on previously reported strain measurements with waveguide Bragg grating sensors made with the same materials [6,7,37], an estimation is made of the strain-optic coefficient for the used grating and waveguide materials, i.e., EpoCore: ρ = 0.31, and OrmoCore: ρ = 0.08. This extracted material data is used to simulate the effect of strain on the GMR grating signal.…”