The use of a frequency modulated continuous wave (FMCW) coherent source for both the illumination and reference beam enables range-selective digital holographic imaging. When the frequency of the reference beam is shifted by the FMCW beat frequency associated with an object at a particular range, only the light scattered from objects at that selected range forms a temporally stable interference pattern on the sensor. Thus, only objects at the selected range are present in the reconstructed image. Interference patterns from light scattered from objects outside the selected range integrate towards zero. This enables the removal of foreground scattered light from particles (for example, dense fog) that obstructs the object of interest and reduces contrast. To demonstrate this, a four-meter-tall range-extending tower with mirrors at the top and base (which create multiple reflections, and multiple passes through the tower), is used to achieve beam path lengths up to 50 meters on the optical bench. Scattering plates in the tower are used to emulate volumetric Mie scattering, analogous to dense fog. The holographic images are recorded using an off-axis digital holography setup that operates at 1550 nanometers. We demonstrate range-selective digital holographic imaging of objects up to 24 meters with range selectivity much shorter than the optical system’s depth of field.
This study demonstrates a coherent frequency modulated continuous wave (FMCW) scanning lidar system coupled with range-selective coherent FMCW digital holography (DH), yielding broad field-of-view point cloud images and high-resolution holographic images of objects at selected ranges. The integration of DH into a Cassegrain telescope enables long-range phase-sensitive imaging, improving upon traditional DH systems. A chirped frequency modulation technique is used to capture holograms at specified ranges, with a current range resolution of 7 cm. The research details the technicalities, challenges, and future directions of this approach, which marks a substantial advancement in coherent 3D imaging and sensing technologies.
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