Remote measurement of shallow depths in semi-transparent media
Through discrimination of the scattered signal polarization state, a lidar system measures a distance through semi-transparent media by the reception of single or multiple scattered signals from a scattering medium. Combined and overlapped single or multiple scattered light signals from the medium can be separated by exploiting varying polarization characteristics. This removes the traditional laser and detector pulse width limitations that determine the system's operational bandwidth, translating relative depth measurements into the conditions of two surface timing measurements and achieving sub-pulse width resolution.
1. A method of mapping a surface topography, comprising:
generating a pulse of polarized light;
scattering at least some portion of the pulse of polarized light onto two or more surfaces comprising a first surface and a second surface;
receiving the scattered light from the first surface and the second surface as a first received pulse and a second received pulse, wherein the first received pulse and the second received pulse have a one or more portions that overlap in an ambiguous intrapulse overlap portion;
separating the first received pulse and the second received pulse into a first component and a second component, the first component and the second component having a relative difference in polarization between each other, wherein the separation removes the ambiguous intrapulse overlap portion;
determining an amount of time elapsed between the first component and the second component; and
calculating a relative distance between the first surface and the second surface based on the amount of time elapsed, thereby achieving a sub-pulse width resolution.
2. The method of claim 1 , wherein one of the two or more surfaces comprises a relatively polarization preserving surface.
3. The method of claim 1 , wherein the generating a pulse of polarized light step comprises the steps of generating a light pulse with a laser; and passing the light pulse through a polarizer.
4. The method of claim 1 , wherein the first component is a co-planar polarization component of the scattered light and the second component is a cross-planar component of the scattered light.
5. The method of claim 1 , wherein the receiving the scattered light step comprises the step of collecting the scattered light through a telescope.
6. The method of claim 1 , further comprising dynamically adjusting scanner parameters to keep a spot density relatively constant as at least one of an aircraft flying height and ground terrain elevation changes during a survey mission.
7. The method of claim 6 , wherein the dynamically adjusting scanner parameters comprises a galvanometer-based scanner configured to execute a swath-tracking algorithm for maintaining a predetermined laser spot density on the one or more surfaces.
8. The method of claim 7 , wherein laser spot density is constant as an elevation changes between one or more surfaces.