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In this Letter, we propose a compact multimode fiber endoscope which employs wavefront shaping with a digital micromirror device (DMD). An automated single calibration step allows us to correct for optical misalignment, and the method achieves accurate focusing at various depths in the sample through rapid switching of holographic patterns by the DMD. The speed of calibration is one or two orders of magnitude faster than existing methods. The method, single calibration multimode fiber imaging (SCMFI), is compared with existing methods, and its performance is validated. We show a near diffraction limited focusing capability at imaging depths up to 110 µm with near constant lateral resolutions of 1.4 µm. Finally, we demonstrate the method for the imaging of small fluorescent beads embedded in a 3D matrix. The results indicate excellent power penetration and focusing performance. Combined with the high speed of SCMFI, this paves the way for volumetric tissue endoscopy at depth.

The property of the multimode fiber (MMF) to remain minimally invasive when performing high-resolution observations, makes MMF imaging of particular interest in many related fields recently, especially in bioendoscopic imaging. Imaging through point scanning is the most common method of MMF imaging now, which means modulating a scanning focal spot on the end face of fiber by controlling modes in the fiber. However, due to mode interference, there is always a background speckle around the focal spot formed, which affects imaging quality seriously. Increasing controllable modes number can effectively suppress the effects of the background speckle, but it is limited by the number of controllable elements (the elements number of wavefront shaping devices). Here, we propose a new, to the best of our knowledge, method to increase the contrast-to-noise ratio (CNR) of MMF imaging without increasing the number of controllable modes. Wavelength modulation is introduced to suppress the background. The background speckles turn to be uncorrelated, whereas the signal patterns turn to be strongly correlated and can be added when 20 different wavelengths of light form a focal spot at the same position at the distal end of MMF, respectively. Thus, a four-fold enhancement can be gained in CNR at a 200 µm field-of-view (FOV) by suppressing background speckles.