Draw Tower Gratings (DTG^®)

Draw Tower Gratings (DTG^®s) are produced using a process that combines the drawing of the optical fiber with the writing of the grating.

The figure below shows the set-up of this production process. The input of the process is a glass pre-form. After heating the pre-form, the pulling and formation of the fiber will be initiated.

Further in the production process, the fiber 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 taking into account the draw speed, FBGs can be accurately positioned in the fiber. When the grating has been written the fiber 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 fiber is reeled onto a drum. This process of simultaneously drawing the fiber, writing the grating and coating the fiber 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 fiber strength is maintained during the DTG^® manufacturing process.

FemtoSecond Gratings (FSG^®)

FBGS’s FemtoSecond Grating (FSG^®) technology complements the existing DTG^® technology by adding new manufacturing capabilities due to special demands for Fiber Bragg Gratings (FBGs).

The fabrication processes of both techniques maintain the pristine high mechanical- strength of the optical fiber. In addition, FSG^®s can provide sensor configurations and features needed for special applications.

FSG^® exploits the through-coating FBG inscription process, which utilizes ultrafast laser pulses launched via special optics to the fiber core without damaging the fiber coating. See below the schematic for FSG^® inscription.

The focused high-power laser pulses change the index of refraction of the glass material in the core of the optical fiber due to nonlinear absorption processes. These effects are nearly independent of the doping level of the optical fiber or the type of glass, which uniquely sets this process to offer an advantage in its ability to modify any type of optical fiber.
The laser pulses are guided via special optics to form a unique interference pattern, which is geometrically co-aligned with the longitudinal axis of a stationary standard optical fiber. The impinging pattern creates the desired modulated index of refraction onto a selected local length along the core of the optical fiber.

This inscription defines the required reflected Bragg wavelength. In addition, an array of desired FBGs can be inscribed by translating the optical fiber along its longitudinal axis.

Advantages of FSG^®

FSG^® inscription offers additional advantages to the arsenal of DTG^® sensors.

• For one, a standard commercially-available telecom optical fiber, such as an SMF-28 (or similar type) or a pure silica core fiber, with different coatings (polyimide, acrylate, etc.) can be used to inscribe the gratings.
• System designers can take advantage of new features, such as; enhanced reflectivity, which can exceed 90%, have a narrow spectral bandwidth (FWHM) down to 200pm and more flexibility in configuring FSG^® chains, where spectral spacing between consecutive FBGs is less restricted.
• Additionally, the process offers a capability to fabricate FSG^®s with broad FWHM of higher than 5nm with an option to offer low or high reflection values.
• Furthermore, FSG^®s offer a higher thermal stability across a wide band of temperatures and can be designed to operate at the temperature limit of the optical fibers.

Advantages of DTG^® & FSG^®

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:

1 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.

2 Spliceless arrays

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.

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3 Direct Fixation

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.

4 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.

5 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.

6 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.

7 High Repeatability

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.

8 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.

Comparison of DTG^® & FSG^® Technology