The principle of writing FBG with femtosecond laser

A remarkable feature of femtosecond laser is its ultra short pulse(femtosecond level). The energy of micro focus level is released in such ashort time. Its peak power is very high (up to 2gw). It can be used to cut andprocess various materials including quartz. Grating writing is realized byusing the feature of femtosecond laser.

Through precise optical path design, the laser is focused on thecore directly through the coating layer and cladding. By controlling the laserenergy threshold, the laser can not affect the coating layer and cladding, butonly etch the core. As shown in the figure below:

Figure 1 Schematic diagramof ultrafast laser focusing on core

   With the laser point focusing on the core,the next important problem is how to form Periodic etching points along theoptical axis to form a grating. Generally speaking, there are three ways toachieve this.

    A. Moving optical fiber method: the laserbeam and optical path are invariable, and a very high precision displacementtable is used to move the optical fiber at a certain speed along the axisdirection, while adjusting the laser to release the optical pulse at a fixedfrequency. In this way, etching points with specific period can be formed onthe fiber core to form a grating. As shown in Figure 2.

Fig. 2 grating writing bymoving fiber method

B. This method is similar to the etching method of excimer laser.Femtosecond laser is used as the light source similar to excimer laser, and theinterference fringes with fixed period are formed by the phase mask to etch thefiber. We can refer to the grating technology using excimer laser, which willnot be discussed here.

C. Projection method: the principle of this method is to make auniform periodic diaphragm, and the laser spot is hit on the diaphragm, whichis divided by the diaphragm, equivalent to a number of light-emitting points;then through imaging technology, the diaphragm is imaged as a reduced realphase, projected to the fiber core of the fiber, so as to complete the periodicetching of the fiber.

Using femtosecond laser for grating writing adds an etchingmethod, which also brings its very special characteristics:

l   High intensity: because femtosecond laser directly etches thefiber core through the coating layer and cladding, the coating layer does notneed to be peeled off, which brings a very important feature: the intensity ofgrating is very high, which can reach the intrinsic mechanical performanceindex of the fiber.

l   High temperature resistance: compared with excimer laser,femtosecond has different effects on the fiber core. Excimer laser focuses onchanging the refractive index of the fiber core and can be recovered undercertain conditions (such as high temperature), while femtosecond laser cancause certain structural changes to the fiber core and cannot be recovered. Sofemtosecond gratings can be used at very high temperature, as long as the fiberitself can tolerate it.

l   It is easy to form a short grating array; femtosecond gratingwriting can be used for precise grating length control and spacing controlbetween gratings, and the control accuracy of grating length and spacing canreach the order of micrometer. Therefore, it is very convenient to write OFDRdemodulated high-density grating.

l   Flexible bandwidth and reflectivity control; through processadjustment, grating with any width from 200 PI meters to 5nm can be engraved,and reflectivity can be adjusted as required.

However, the femtosecond direct writing grating has a disadvantage.The insertion loss of the FBG is relatively high, which may be a difficultproblem for the application of large volume (a large number of gratings arewritten by a single fiber). In addition, it is difficult to cope with verylarge volume application by using femtosecond writing method, and its technicalcharacteristics make it difficult to carry out efficient writing.