Optical attenuation coefficient meter
Inventors
Estes, Lee E • Doyle, Stephen B
Assignees
US Department of Navy • Government of the United States of America
Publication Number
US-9909927-B1
Publication Date
2018-03-06
Expiration Date
2036-06-22
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Abstract
An attenuation meter is provided for use in a water environment. In operation, a transmitter of the meter transmits a laser pulse focused to a size at a predetermined range. A receiver of the meter images a focused spot to minimize unwanted light back scattering and avoid diffractive spreading within the back scattering region. Filtering the angular region can further reject scattered light. The filtered light is received, measured and processed by a oscilloscope as pulse averages. The meter also includes a photodetector to measure a diffuse attenuation coefficient. The output voltage of the photodetector is measured and processed by the oscilloscope that produces an average voltage over a preset number of pulses. A controller best fits voltage to time dependence to produce the diffuse attenuation coefficient. Only the shape of the receiver time dependence is required to provide the diffuse attenuation coefficient measurement.
Core Innovation
The invention provides an attenuation meter designed for use in a water environment to measure both an optical beam attenuation coefficient and an optical diffuse attenuation coefficient in a liquid medium. The meter includes a transmitter generating a laser pulse that is focused at a predetermined range and size, and a receiver imaging the focused spot to minimize unwanted light back scattering and avoid diffractive spreading within the back scattering region. Angular filtering further rejects scattered light, and the received light is measured and processed by an oscilloscope to produce pulse averages. Additionally, a photodetector measures the diffuse attenuation coefficient by processing output voltages and fitting voltage to time dependence to produce the coefficient value.
The problem addressed by the invention arises from limitations of prior art optical meters, which use short optical paths generally less than one meter, leading to demands on optical cleanliness, electronic accuracy, and calibration. These limitations make measurements in clear water non-repeatable, inaccurate, and generally unusable due to scattering and absorption inside the meter. The invention solves this by recognizing back scattering from a pulsed laser source and allowing a propagation path beyond the meter's size, thus enabling longer path lengths and more accurate measurements.
The meter achieves its function by producing nanosecond laser pulses of linearly polarized light that are collimated, polarization rotated, and focused through a series of lenses and apertures to create a sized location in the water. Back scattered light preserving circular polarization is separated and directed to the receiver using polarization optics and matched filtering via pinholes and interference filters to reject forward and multiple scattered light. The receiver output is averaged over numerous pulses, and the telescope is translated to focus at different ranges. These measurements enable calculation of the beam attenuation coefficient by comparing averaged outputs at multiple ranges.
Claims Coverage
The claims cover four independent claims that describe the components and method of the attenuation meter and its operation.
Attenuation meter with transmitter, receiver, photodetector, and baffle
A meter comprising a transmitter with a laser emitting nanosecond pulses focused at a location in a liquid medium with specified magnifications, pulse duration, wavelength, size, and range; a receiver that images the focused spot of back scattered light; a photodetector producing output as a diffuse attenuation coefficient; and a baffle positioned between receiver and photodetector to reduce scattered light within the meter.
Detailed optical system with polarization and matched filtering for back scattered light rejection
An attenuation meter comprising optical components including collimating lens, half waveplate for polarization rotation, mirrors, focusing lens, multiple image relay telescopes, power telescope, quarter waveplate converting light to circular polarization, polarized beam splitter directing back scattered light, high speed detector for pulse validation, pinholes matched to image and angular spectrum for rejecting multiple scattering, interference filter tuned to laser wavelength, detector connected to oscilloscope for multiple pulse averaging, and controller for processing beam attenuation coefficient from range-based measurements.
Depth meter for activation after reaching predetermined depth
Integration of a depth meter that activates the attenuation meter after reaching a set depth in the liquid medium.
Method for measuring attenuation in liquid medium using focused laser pulses and voltage fitting
A method involving setting receiver time bandwidth to match laser pulse duration; transmitting laser pulses focused at specified magnification, wavelength, size, and range; receiving and imaging back scattered light minimizing multiple scattering; outputting photodetector voltage data; processing voltage data by fitting to time dependence of laser pulse to provide diffuse attenuation coefficient; transmitting multiple pulses at different ranges for range-resolved measurement.
The independent claims collectively cover an attenuation meter system with specific optical design features enabling precise measurement of beam and diffuse attenuation coefficients in liquids and a method using focused laser pulses, timed detection, and signal processing to obtain accurate attenuation measurements.
Stated Advantages
Long measurement paths allow for more accurate measurements than conventional meters with short optical paths.
Beam attenuation measurement is derived from ratio of sensor responses at multiple ranges, eliminating the need for calibration required in conventional meters.
Diffuse attenuation coefficient measurement relies only on the shape of the receiver time dependence, not absolute signal level, enabling direct fitting and improved accuracy.
Optical path length is easily adjusted by controller to accommodate media with different clarity by changing the focal ranges.
Documented Applications
Deployment as a self-contained module powered by batteries on unmanned underwater vehicles (UUVs).
Deployment from separate platforms with corded electrical power and data access.
Measurement at multiple wavelengths using multi-color light sources, preferably blue.
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