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Draw Tower Grating Draw Tower Gratings (DTG's®) are produced using a process that combines the drawing of the optical fibre with the writing of the grating. The input of the process is a glass preform. After heating the preform, the pulling and formation of the fibre is initiated. Further in the production process, the fibre crosses the optical axis of a laser and the interferometer that create a periodical UV-light interference pattern in order to write the grating.

Using a pulse selector and whilst monitoring the draw speed, Fibre Bragg Gratings (FBG's) can be accurately positioned in the fibre. When the grating has been written the fibre is coated by entering a coating reservoir, followed by a curing step of the coating. Finally the location of the FBG is marked automatically and the fibre is reeled onto a spool.



This process of simultaneously drawing the fibre, writing the grating and coating the fibre directly after the grating inscription, results in high strength grating chains. As such the commonly used stripping and recoating process of standard FBGs is not necessary and the pristine fibre strength is maintained during the DTG® manufacturing process.

Within the field of Fibre Bragg Grating sensing, Draw Tower Gratings have some unique properties that makes them the preferred product for many FBG applications including:


 * Extremely high mechanical strength - Draw Tower Gratings show extremely high mechanical strength compared to conventional gratings. The mechanical strength of DTGs® and standard recoated FBGs is tested by applying increasing tensile stress to the samples until the fiber breaks. The test has been performed using a commercial material testing machine according to IEC 793-1 and consists of a fatigue test with three different elongation speeds of 3, 30, 300 mm/min. 15 samples per speed were tested. The breaking force is recorded and plotted in a diagram of relative breaking probability (Weibull plots).




 * Spliceless Arrays - The Draw Tower production process results in spliceless grating chains with a high number of sensor elements. Figure 2 shows the spectrum of an array of 80 DTGs® on a single fiber, all with a varying wavelength.
 * Uniform Coating Coverage - The coating is applied after the writing of the DTG® and no stripping and recoating process is needed. Therefore the coating is uniform over the full length of the fiber, even at the position of the DTG itself.
 * High Repeatability - DTGs® are fabricated using an automated production process. This means production parameters can be carefully controlled and quality maintained.


 * Relatively Low Cost - Unlike traditional FBG's, DTGs® are produced using a fully automated process. Therefore production can be highly cost-effective for medium and high volumes especially.