Whether the excimer mode or femtosecond laser mode is adopted, itstechnical characteristics make it difficult to carry out mass production withhigh efficiency. This limits the cost and application scope of grating, thatis, the cost is too high and it is difficult to use in large-scale projects.

Therefore, people think of the way of grating writing in theprocess of fiber drawing. Its principle is shown in Figure 1:

Fig. 1 Schematic diagram of online writing grating

On line grating writing is a comprehensive technology combiningfiber drawing technology with grating writing technology. The optical fiberpreform is placed on the fixture of the rod feeding system, and the rod feedercan control the rod feeding speed through the industrial computer; after thepreform is sent to the graphite heating furnace, after nearly 2000 degrees ofheating, the preform begins to melt and sag and form a filiform shape; theoptical fiber enters the writing system after passing the caliper, and thelaser pulse trigger of the laser of the writing system can be controlledthrough the industrial computer; it has been written The optical fiber ofgrating can be coated by heat curing or light curing device after coating thecoating Cup; then it can be wound on the optical disk after passing the speedmeasuring wheel and automatic marking device. The feeding speed of the rodfeeder, the traction speed of the traction wheel, the control of the gratingwriting pulse, and the control of the fiber drawing diameter are all carriedout in real time and dynamically by the industrial computer.

There are several important characteristics of grating productionin this way：

l   High efficiency, adjustable fiber drawing speed. In the drawingprocess, the high-energy excimer laser is directly integrated, which canachieve high-speed grating writing, and the speed can reach hundreds ofgratings per minute;

l   Low cost, all processes are fully automatic production process, sowhen high-density fiber grating array is needed, the process has highcost-effectiveness.

l   High strength, compared with the traditional stripping andrecoating lettering process, the online writing grating maintains the originalhigh mechanical strength of the fiber. The tensile strength is usually morethan 5gpa, which is equivalent to more than 4% of the fracture strain.

l   High repeatability. Due to the use of automatic production processmanufacturing, production parameters can be more accurately controlled toensure high repeatability and high quality.

l   The coating is uniform, and this process can be produced withoutstripping and recoating. It has a unique characteristic that the coating isuniform over the entire length of the fiber, including the position of thefiber grating.

l   Long distance, continuous drawing and engraving, up to tens orhundreds of kilometers;

l   It is easy to string and there is no fusion point betweengratings;

l   Easy to deploy, high adhesion coating can be applied to opticalfiber, such as polyimide. These coatings show excellent adhesion to the glassfiber surface, which makes the grating produced by this process suitable fordirect fixation on the structure without removing the coating. This not onlyfacilitates the installation process, but also improves the reliability.

l   Low bending loss, writing gratings can be produced in the type offiber with very low bending loss, which enables them to operate in theconfiguration with low bending radius without significant attenuation effect.