Imaging characteristics of an 8.8  mm long and 125  μm thick graded-index short multimode fiber probe

We demonstrated the feasibility of an ultrathin imaging probe with a 50-μm core diameter, a 125 μm total diameter, and an 8.8 mm length, which is a typical graded-index multimode fiber for optical communications. We used an ABCD matrix to analyze the imaging conditions and magnification, which corresponded closely to the measured results. The lateral resolution was calculated at 1.2 μm with a wavelength of 730 nm, which reflects the image test pattern where a period of 4.38 μm was measured with a wavelength of 730 nm. In the numerical aperture of the objective lens, we experimentally evaluated the tradeoff between the magnification and the coupling efficiency. At four wavelengths of 540 nm, 632 nm, 730 nm, and 852 nm, the contrast and signal intensity versus the wavelength were investigated to show that the contrast at 632 and 730 nm is relatively high. By using a thin random phase screen model, we explained that as the wavelength decreases the greater the decrease in the optical transfer function at higher spatial frequencies. Using a 635 nm LED light source, we imaged the surfaces of chicken tendons in contact and the surface roughness was visible.