Within the field of Fiber Bragg Grating (FBG) sensing, FBGS Draw Tower Gratings (DTG®) and FemtoSecond Gratings (FSG®) have some unique properties which makes them the preferred product for many FBG based sensing applications.
The most important advantages are summarized below:
Extremely High Mechanical Strength
DTG®s and FSG®s show extreme high mechanical strength. In comparison to the conventional Strip-and-Recoat inscription process, FBGS’s Draw Tower Gratings (DTG®s) and FemtoSecond Gratings (FSG®s) maintain a pristine high mechanical-strength of the optical fiber. Their tensile strength typically exceeds 5GPa, which corresponds to a breakage strain of >7%. In contrast, the alternative femtosecond through-coating process designated as Point-by-Point (PbP) and the classical Strip-and-Recoat manufacturing process suffer from producing sensors with weaker tensile strength of less than 2.9% for the same type optical fiber. The graph below displays the averaged measured breaking force of optical fibers (single FBG in each fiber) with different FBG inscription technologies. It is evident that FBGS´s DTG® and FSG® products offer superior mechanical strength with no significant reduction in the breaking force compared to a pristine optical fiber.
Comparison of tensile strength of different FBG inscription technologies.
The DTG® and FSG® technologies offer unique capabilities to produce spliceless grating chains (arrays) with a high number of sensor elements. The figure below shows the spectrum of an array of 80 DTG®s with different wavelengths.
Spectrum of a DTG array containing 80 different wavelengths.
High adhesion coatings can be applied on the optical fiber such as Organic Modified Ceramic (ORMOCER®1) or Polyimide. These coatings show excellent adhesion properties to the surface of the glass fiber, which makes both of DTG®s and FSG®s applicable for direct fixation on structures without the removal of the coating. This does not only facilitate the installation process, but also increases the reliability.
Uniform Coating Coverage
DTG®s and FSG®s can be produced without the need of a stripping-and-recoating process. DTG®s and FSG®s have therefore the unique property, where the coating is uniform over the full length of the optical fiber, including the location of the FBG.
Extended Operating Temperature Range
The operating temperature is mainly limited by the highest temperature the optical fiber coating can withstand. The ORMOCER®1 coating is rated for operation between -180 and +200°C, whereas the Polyimide coatings can withstand temperatures up to 300°C.
Low Bending Loss
DTG®s and FSG®s can both be produced in optical fiber types with extreme low bending losses, which permits them to operate in configurations with low bending radii without having significant attenuation effects.
DTG®s and FSG®s are fabricated using an automated production process, which offers a more accurate control of the production parameters and assures high repeatability and quality.
Relatively Low Cost
Given that the DTG® process is a fully-automated production process, this process is very cost effective when high-density FBG arrays are needed. For less dense FBG arrays, the FSG® process is more cost-effective as production time is less sensitive to the length of the optical fiber.
1ORMOCER® : Trademark of Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.