B.3      Modified phase mask technique

From Fig. B-1 we see that r₀ = ¥, s₀ = f_x and L₀ = 2h₀. We then obtain

The distances L and L+DL are given by

The relative Bragg wavelength change l_b/l_b,0 and the absolute Bragg wavelength shift Dl_b correspond to

where he magnification is close to 1 and then the |.| is omitted. We have realized a set-up with the following parameters :

-         f_x  = 100 mm

-         f_v = -20 mm

-         d₁ = 32 mm

-         d₂ = 13 mm

We observe that for these parameters, the beam width is identical at the phase mask location to the beam size at the entrance (L = h₀). For an initial Bragg wavelength of 1550 nm, the parameter a has a value of 24.2 m^-1. For the fiber touching the phase mask, the distance d₃ = 62.5 mm (fiber radius) and then the Bragg wavelength shift is 2.35 nm as can be seen in Fig. B-4.

Fig. B-4 Wavelength shift for the realized set-up and a phase mask for FBG writing at 1550 nm