Functionally integrated waveguide illuminator for video-rate underwater digital holographic microscopy with extended range of observations.

We present an improved functionally integrated waveguide illuminator designed for compact and robust digital holographic microscopy for use both in air and underwater environments with an extended range of observations. The device integrates a thermo-optic phase shifter directly onto a silica-based planar waveguide, enabling calibrated four-step thermo-optic phase shifting without any external phase-shifting components. Two adjacent on-chip point sources generate a pair of spherical waves for highly stable common-path illumination, providing digital holographic microscopy with strong immunity to environmental disturbances and refractive-index fluctuations in water. Quantitative experiments demonstrate near-diffraction-limited lateral resolution of 1.7 µm (theoretical limit being 1.62 µm) and accurate determination of three-dimensional object location with < 1% error in air and ≤ 0.35% error in air-glass-water mixed medium. The system maintains high-fidelity reconstruction over an extended 25 mm axial range and demonstrates underwater imaging of freely moving microorganisms from holograms acquired at 50 frames per second using four-frame sliding-window reconstruction. These results demonstrate that digital holographic microscopy based on the functionally integrated waveguide illuminator provides a compact, alignment-free, and environmentally robust platform for real-time three-dimensional holographic imaging, with promising applications in biological microscopy, environmental monitoring, and field-deployable underwater sensing.