Date of Award
Doctor of Philosophy (PhD)
Aerospace Engineering Sciences
Jeffrey P. Thayer
Active noncontact range measurement sensors transmit electromagnetic radiation onto a remote target and process the received scattered signals to resolve the separation distance, or range, between the sensor and target. For lidar sensors, range is resolved by halving the roundtrip transit time multiplied by the speed of light, accounting for the refractive indices of the transit media. The ranging technique enables remote measurement of depth by resolving the range to sequential surfaces. Depth measurement in the shallow regime has conventionally been limited by the presence of ambiguous, overlapping optical pulses scattered from sequential surfaces. Enhanced performance in the shallow regime has conventionally come at the expense of the increased cost and complexity associated with high performance componentry. The issue of remote shallow depth measurement presents an opportunity for a novel approach to lidar sensor development. In this work, I discuss how the issue of ambiguity in the shallow depth measurement may be addressed by exploiting the polarization orientation of the transmitted and received optical signals, the components of which are modified during the range observation by naturally-occurring phenomena. Conventional pulsed time of flight laser ranging sensors are unable to resolve the shallow depth between overlapping pulses received from sequential surfaces due to operation in the scalar lidar regime, where the intensity of the received scattered signal is measured with no regard for polarization information. Enhanced performance by scalar lidar sensors in the shallow media regime has been conventionally enabled through incorporation of picosecond pulse width lasers and fast photodetectors, along with their associated increase in cost and complexity. The polarization lidar approach to shallow depth measurement developed in the dissertation facilitates the use of common lasers, optics, and detection componentry, making it comparatively less complex and costly while achieving two orders of magnitude improvement in the depth resolution of distant targets. Evolution of the measurement is presented, from concept and laboratory demonstration to development of prototype instrumentation. The approach is presented within the context of lidar bathymetry, with demonstrated measurement of 1 cm water depths with an uncertainty of +/- 3 mm. Furthermore, the approach provides an estimate of the first surface linear depolarization ratio, enabling differentiation between surfaces defined by variable scattering matrices. The theory is sufficiently generalized for future application to depth measurement of additional media with bounding surfaces defined by unique scattering matrices.
Mitchell, Steven Edward, "Remote Measurement of Shallow Media Depth Using Polarization Lidar" (2013). Aerospace Engineering Sciences Graduate Theses & Dissertations. 71.