Modulation transfer function of the imaging probe using an 8.8 mm-long and 125 µm-thick graded- index short multimode fiber

We measured the modulation transfer function (MTF) of a short multimode fiber (SMMF) probe and proposed the optical model including the thin random phase screen (TRPS) to explain degradations of imaging characteristics. SMMF is a graded index multimode fiber for optical communication. That length is 8.8 mm and the diameter of core is 50 µm. With the wavelength of 780 nm, for spatial frequencies from128 lp/mm to 228 lp/mm MTFs were measured at 0.179-0.062 in reflection images and these contrasts were 19% to contrasts without TRPS. With parameters of σϕ of 1.5 rad and W of 2-3.5 µm calculated results almost correspond to measured ones. Dependences of contrasts on wavelength were also measured. With the spatial frequency of 200 lp/mm, at the wavelength of 0.8 µm the contrast decreased to 17% due to TRPS and at that of 0.5 µm the contrast decreased to 6%. Measured dependences of contrasts on wavelength with the spatial frequency of 228 lp/mm approximately correspond to calculation results with 200 lp/mm. These basic characteristics are useful to design the imaging optics by means of SMMF.