Optical fiber-based homogeneous illumination system for microscopy applications

We have engineered an optical fiber-based system for homogeneous laser illumination and tested it on a home-built 3D-printed microscope in reflection mode. The sample is illuminated using an array of multimode fibers (MMFs) strategically positioned around a custom-made objective lens. Homogeneous illumination is achieved through the superposition of dynamic speckle patterns emerging from the illuminating MMFs that accumulate over the camera’s exposure time. Time-varying speckle patterns are generated by imparting random perturbations at a rate of tens of kilohertz with a radial piezoelectric actuator, around which the MMFs are tightly coiled. At the output facet of each MMF, the speckle contrast is reduced in the first decimal digit, which then results in a uniform illumination over the entire field-of-view regardless of wavelength and the magnification of the imaging system. The spatial extent of the region of homogeneous illumination is determined by the characteristics of the MMFs and their geometrical arrangement, not by the imaging lens. We present detailed descriptions of both the optimization of the speckle reduction strategy and the characterization of the intensity distribution obtained with different numbers of illuminating fibers. Ultimately, we demonstrate that the proposed fiber-based system provides homogeneous laser illumination at three different wavelengths across the visible spectrum and confirm its suitability for low-cost fluorescence imaging.