We demonstrate full-field optical coherence microscopy (OCM) using an ultrathin forward-imaging short multimode fiber (SMMF) probe with a core diameter of 50 μm, outer diameter of 125 μm, and length of 7.4 mm, which is a typical graded-index multimode fiber used for optical communications. The axial and lateral resolutions were measured to be 2.14 μm and 2.3 μm, respectively. By inserting the SMMF 4 mm into the cortex of an in vivo rat brain, scanning was performed to a depth of 147 μm from the SMMF facet with a field of view of 47 μm. Three-dimensional (3D) OCM images were obtained at depths ranging from approximately 20 μm to 90 μm. Based on the morphological information of the resliced 3D images and the dependence of the integration of the OCM image signal on the insertion length, the obtained 3D information of nerve fibers has been presented.

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To improve frequency resolution, assignments of sub-bins within the bin interval of the conventional discrete Fourier transform are proposed by decreasing the number of sampled data. The simulated and experimental results of the basic sub-bin spectrum characteristics are presented. Using a signal oscillator, digital oscilloscope, and PC, the sub-bin line spectrum between two conventional bins is measured, and nine line spectra of sub-bins corresponding to the simulated results are measured. The linearity of the sub-bin spectrum intensity for amplitudes of 0.001–10 V of the input signal is also measured. The Doppler signals from the optical interferometer using a He–Ne laser are measured using a mixer to tune the frequency. A sub-bin spectrum of 104.89 kHz between two bin frequencies of 104.49 and 105.47 kHz is measured using 256 data points at a sampling frequency of 250 kHz and an acquisition time of 1.024 ms. The precision of displacement for the acquisition time is improved by one order to 32 nm for a full displacement of 2.342 µm. The sub-bins, as a composition of bin structure, are applicable to various fields.

We demonstrate phase imaging that reduces the common phase noise in full-field optical coherence microscopy using a short multimode fiber (SMMF) probe. Using a cover glass, phase images of the SMMF and sample surfaces were measured simultaneously. Subtracting the phase of the SMMF surface as a reference, the phase drifts in the sample region are reduced. The axial and lateral resolutions were 2.3 µm and <2020