Mode coupling occurs in between forward and backward modes of fiber Bragg grating（ FBG）

to understand physical properties of fiber grating

improve precision of reflectivity

coupled mode theory of FBG must be researched deeply. Through coupled equations of FBG

Bragg grating coupled model will be researched on different refractive index modulation depth and simulation experiment is carried out by Matlab

it is found the grating length take 2

5

10 mm

the maximum values of the reflection spectrum bandwidth of week grating moderate grating and strong grating in the refractive index change 0. 000 1

are 2. 015 0 × 10

-11

5. 2767 × 10

-11

9. 2143 ×10

-11

. Calculation suggests that under the fixed length of grating

the valuation of reflection spectrum bandwidth resulting from every 0. 000 1 reflective index is relatively stable in any reflection band in which the reflection spectrum of weak grating is the minimum

followed by that of moderate grating

while the greatest one is strong grating.