Principle of using ultraviolet laser to write fiber Bragg grating

   There are many ways to make fiber Bragggrating (FBG), such as standing wave method, photoetching method, physicalpressing method and so on. But the method of using UV laser with phase mask isthe most efficient, stable and economical one. Its basic principle is asfollows.

Figure 1 Schematic diagramof fiber grating writing

The ultraviolet light from the laser passes through the reflectorof the optical path and enters the cylindrical lens. The role of thecylindrical lens is to compress the laser into a line in the verticaldirection, so as to improve the energy density of the light. Before the focusedlaser line enters the core, it passes through the phase mask. The mask isgenerally designed as positive and negative first-order diffraction. Afterpassing through the phase mask, the laser is divided into two beams, andinterference fringes are formed near the phase mask. The fiber is placed in thearea of interference fringes. These interference fringes directly act on thefiber core, and periodically modulate the refractive index of the fiber core toform a grating.

The principle of laser writing after passing through the phasemask is as follows.

Fig. 2 Schematic diagram of laser passing through phase mask

Fig. 3 spectrum of uniform FBG

   As can be seen from the spectral line inFig. 3, there are a large number of stray peaks on both sides of the reflectionpeak of the uniform grating, which is called side lobe here. The existence ofthese sidelobes sometimes causes trouble to the demodulator, that is, it isdifficult to determine the precise drift position of the reflection peak. Forthis reason, a grating apodization technique is developed, that is, in theprocess of writing, the exposure intensity is reduced at both ends of thegrating area, so that the exposure energy is strong in the middle and weak onboth sides. This is equivalent to adding a modulation function on the originalspectral line to remove the sidelobes on both sides of the main peak. As shownin Figure 4.

Fig. 4 Schematic diagram ofapodization FBG

To write FBG Using excimerlaser and phase mask has the following advantages and disadvantages.

Advantages:

l   The system is simple to build;

l   High efficiency, supporting mass production;

l   Stable system and low maintenance cost;

l   It is easy to adjust and control the central wavelength;

l   It is suitable for industrial production;

l   It is easy to write grating with arbitrary reflectivity;

Disadvantages:

l   It is not suitable for strain sensing because the fiber grating iseasy to break when the coating is peeled off;

l   The stripping area needs secondary coating;

l   Full manual operation;