Wavelength encoding long stroke fiber optic linear position sensor is a kind of optics displacement sensor

which used a Varied line-space grating (VLS grating) as the hardcore. The resolving power for the displacement depend on the resolving power for the spectrum

so the improvement of the resolving power for the spectrum is very important to the sensor. Based on the Fraunhofer diffraction

the diffraction patterns of VLS grating is obtained when the beam of incidence is parallel and uniform. The intensity of the diffraction facular changing with diffraction angle was figured out and pictured

when the diameter of incident facular was 5mm and 10mm to the same VLS grating

and when the diameter of incident facular was 10mm to different VLS grating.The result was validated by the experimentation for the sensor’s resolving power when the diameter of incident facular was different for the same VLS grating. The result revealed that the diameter of incident facular was very important to the sensor’s resolving power for the diffraction pattern of VLS grating.