Development of a multi-wavelength lensless digital holography system for 3D deformations and shape measurements of tympanic membranes

Lu, Weina

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Development of a multi-wavelength lensless digital holography system for 3D deformations and shape measurements of tympanic membranes

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Current methodologies for characterization of tympanic membranes (TMs) have some limitations. They: are qualitative rather than quantitative, consist of single point mobility measurements, or only include one-dimensional deformation measurements. Furthermore, none of the current clinical tools for diagnosis of hearing losses have the capability to measure the shape of TM, which is very useful for anatomical or pathological investigations. The multi-wavelength lensless digital holography system (MLDHS) reported in this work consists of laser delivery (LD), optical head (OH), and computing platform (CP) subsystems, with capabilities of real-time, non-contact, full-field of view measurements. One version of the LD houses two tunable near-infrared external-cavity diode lasers with central wavelengths of 780.24nm and 779.74nm respectively, an acousto-optic modulator, and a laser-to-fiber mechanism. The output of the LD is delivered to an ultra-fast MEMS-based fiber optic switch and the light beam is directed to the OH, which is arranged to perform imaging and measurements by phase-shifting holography. The second LD version subsystem contains one tunable near-infrared diode laser in the range from 770nm to 789nm, an anamorphic prism pair, an acousto-optic modulator, a half-wave plate, and a fiber coupler assembly. The output of the LD is delivered to the OH directly. The OH is designed by 3D optical ray tracing simulations in which components are rotated at specific angles to overcome reflection issues. A high-resolution digital camera with pixel size of 6.7μm by 6.7μm in the OH is used for image recording at high-rates while the CP acquires and processes images in either time-averaged or double-exposure modes. The choice of working version depends on the requirements of the measurement and the sample under test. MLDHS can obtain shape and one-dimensional deformations along one optical axis (z-axis). In order to recover 3D deformations, assumptions based on elasticity theory are prerequisites for the calculations: (a) the TM is analyzed as a thin shell; (b) shape before and after deformation is considered nearly the same since acoustic pressure typically introduces nanometer scale deformations; and (c) normal vectors remain perpendicular to the deformed mid-plane of the TM. Another part of this Thesis is the design and prototyping of the MLDHS, which translates this holographic platform into a simple and compact holographic instrument for measurements of the visible tympanic-membrane motions in live patients. Therefore, the OH subsystem needs to be light and portable, as it can be mounted on a robotic arm be near the ear canal, while the LD subsystem needs to be stable and safely protected. Preliminary results of acoustically induced 3D deformations and shape measurements by a single instrument that demonstrate the capabilities of the devices developed in this Thesis are presented.

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