2.4        Experimental results

2.4.1        FBG fabrication

a)   Standard phase mask technique

We present in Fig. 2-11 the spectral response of a FBG fabricated in our institute with the standard phase mask technique. This spectral response is compared to the simulated response for an homogeneous grating of 2.7 mm length and a refractive index modulation is 2.5×10^-4. The agreement between both spectral response is good indicating a nearly homogeneous UV light beam.

Fig. 2-11 FBG written in standard fiber and theoretical calculation

b)   Modified phase technique for Bragg wavelength tuning

Fig. 2-12 FBG writing set-up with the modified phase mask technique

For given phase mask and fiber, the Bragg wavelength is determined by the effective index of the fiber, n_eff, and the phase mask period L_M : l_B=n_effL_M, where the grating period is L=L_M/2. It is possible to tune l_B to higher wavelengths using post-exposure or to tune l_B to lower wavelengths by stretching the fiber during the writing process. To have more flexibility with the same phase mask, an optical system including several lenses has been studied. The basic idea was to magnify the image of the grating onto the fiber as shown by Fig. 2-12. Compared to the standard writing set-up (top view of Fig. 2-4), two cylindrical lenses have been added, a convex one with focal length f_x and a concave one with focal length f_v.

Using ray optics, we find that the Bragg wavelength change Dl_b, with respect to the Bragg wavelength with a parallel beam l_b,0, is given by (Appendix B)

where d is the distance between the phase mask and the fiber core, d₁ the distance between the lenses and d₂ the distance between the concave lens and the phase mask. We observe that a Bragg wavelength change is obtained even with the fiber touching the phase mask due to the cladding thickness. The parallel alignment between the phase mask and the fiber is very important, since the Bragg condition strongly changes with d. Misalignment will lead to an important chirp.

We fabricated several FBGs with this set-up formed by two lenses and the same phase mask. The reflection spectrum of six different of such FBGs are presented in Fig. 2-13. We observe a tuning range of 10 nm and a small bandwidth change, indicating a good alignment of the fiber in front of the grating. The smaller reflectivity of FBG 6 is probably due to a misalignment of the fiber in the laser beam (then reducing the total dose).

Fig. 2-13 Reflection spectrum of six FBG's written with the modified phase mask technique