High contrast multimode fiber imaging based on wavelength modulation

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.