Optical attenuation coefficient meter

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.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention is a meter and method of use for measuring an optical beam attenuation coefficient and an optical diffuse attenuation coefficient in a liquid medium. The beam attenuation co-efficient accounts for the loss of a narrow, collimated beam of monochromatic light by absorption and scattering per unit distance while the diffuse attenuation co-efficient accounts for absorption loss from both direct and scattered paths of a directional light field.

(2) Description of the Prior Art

Numerous commercial meters are available to measure an optical beam attenuation coefficient “c” and a diffuse attenuation coefficient “K” in water. To limit size, the meters use optical propagation paths that are generally less than 1 meter in length. In clear water, the attenuation lengths (1/attenuation coefficient) are often greater than eight meters. This circumstance imposes demands on the cleanliness of the optical surfaces, the accuracy of the measuring electronics, and the accuracy of the calibration procedures. The demands include the avoidance of absorption and scattering in the meter. Because of this circumstance, the measurements provided by the meters in clear water are generally non-repeatable and inaccurate to the extent that the measurements are generally unusable.

As such, there is a need for a meter, recognizing back scattering by a pulsed laser source, that would allow a propagation path, which is not confined by the size of the meter.

SUMMARY OF THE INVENTION

Accordingly, it is a general purpose and primary object of the present invention to provide an attenuation meter for measurements of an optical beam attenuation coefficient in a water environment.

It is a further object of the present invention to provide an attenuation meter for measurement of an optical diffuse attenuation coefficient in a water environment.

It is a still further object of the present invention to provide an attenuation meter for measurements of an optical beam attenuation coefficient in a liquid medium.

It is a still further object of the present invention to provide an attenuation meter for measurement of an optical diffuse attenuation coefficient in a liquid medium.

In order to attain the objects described, an attenuation meter with a transmitter and receiver is provided in which the transmitter produces a laser pulse of a duration and in-water wavelength that is focused to a sized location at a range from the attenuation meter. As the laser pulse propagates thru water, some of the light becomes back scattered. A partial rejection of the back scattered light is achieved by filtering an angular region to only admit light back scattered within a calculated solid angle. The time bandwidth of receiver detection is set so that the receiver response time matches the pulse width.

An output sample from the receiver is averaged over numerous pulses; thereby, allowing for multiple and independent scattering realizations to produce an average output result. The laser output can then be focused to a sized location at a larger and different range to produce an average output result. The beam attenuation coefficient of the water is then calculated by using this time average.

The laser of the transmitter produces nanosecond pulses of linearly polarized light at a predetermined repetition rate. A lens of the transmitter collimates the light and a half wavelength plate rotates the polarization of the light until the light polarization is horizontal. Mirrors direct the light onto a lens that focuses the light to a 50 micron diameter in the plane of a pinhole. Lenses project a virtual image of a plane of the pinhole in a region between a negative lens and a positive lens.

The light output passes through a quarter waveplate that converts the light to a circular polarization. The light then forms an image in the water. Light that is back scattered in a region about the sized location is reflected back to the receiver. To the extent that the circular polarization is preserved; the back scattered light is converted to linear polarization by the quarter waveplate in that the returning light is directed to the receiver.

When the back scattered light reaches a polarized beam splitter, the light is reflected toward a mirror. A small portion of the output light is reflected toward a high speed detector that can calculate the duration of the laser pulses. The output of the high speed detector is sent to a channel of a Pico Scope (a portable oscilloscope). The output of the high speed detector is measured and recorded at the Pico Scope to validate the laser pulse strength and time wave shape.

A unit magnification image relay telescope images the water focal region onto a pinhole. The comparatively small size of the pinhole is matched to an ideal pinhole image formed in the water. This matched filtering rejects light that was forward scattered into regions outside the ideal water image. Another pinhole is positioned in the far field of the first pinhole. The size of the pinhole is matched to the angular region of light that is used to focus onto the first pinhole. Thus, the pinhole also rejects multiple scattered light.

To further reject background light, an interference filter (tuned to the laser light wavelength) is positioned at the detector. The output of the detector is measured, recorded, and processed by the Pico Scope to form multiple pulse averages that are then accessed by a PC104 controller for processing and storage.

After a preset time interval (determined by the desired number of pulses to be averaged); the PC104 controller commands a translator controller to move the telescope in order to focus the light at a different range and position. When the results for the times for different ranges are processed; the PC104 controller divides the results to generate a result that can be used to calculate the beam attenuation coefficient.

A photodetector measures the diffuse attenuation coefficient. In operation, the output voltage of the photodetector is measured and processed by the Pico Scope that produces an average voltage over a preset number of pulses. Next, the voltage is sent to the PC104 controller where the output voltage is recorded and processed. The controller makes a best fit of voltage to calculated time dependence in order to produce a measurement of the diffuse attenuation coefficient.

DETAILED DESCRIPTION OF THE INVENTION

An attenuation meter10and a water environment300to be measured are depicted inFIG. 1. In the figure, the attenuation meter10comprises an afocal LIDAR transmitter/receiver20(with a lateral magnification M and a longitudinal magnification M2) which transmits a laser pulse200of duration, τ, and in-water wavelength, λW, that is focused to a location with a size, S (the diameter of the laser pulse at the image), at a range, R.

As the laser pulse200propagates thru the water300, some of the light becomes back scattered light202. The back scattered light202travels to the attenuation meter10after scattering by thermodynamic density fluctuations and particles within the water300. The back scattered light202is from a focused spot or location in the water300or liquid medium. At any time, t>τ, light that is scattered only once in the backward direction is scattered within a range segment that is

where cWis the speed of light at the wavelength of the laser in the liquid medium.

A receiver component of the transmitter/receiver20images the focused spot or location onto a hole (aperture) of size
S/M.  (2)

This imaging minimizes light that undergoes multiple scattering. To avoid the effects of diffractive spreading within a back scattering region of interest; the spot size is chosen so that

If the transmitted pulse begins at the time, t=0; the received signal at the time t is due to light scattered within the ranges

which provides a range resolution of

The time bandwidth of the transmitter/receiver20is set so that the response time of the back scattered light matches the pulse width or pulse duration, τ. An output telescope is mounted on a motorized translation stage (not shown) so that the laser pulse200can be focused at different ranges. For the fixed focal range, R1; the output of a photodetector16of the attenuation meter10is sampled at

A sampled output pulse power, P1, is averaged over numerous pulses; thereby, allowing numerous independent scattering realizations to produce an average result,P1. The laser output is then focused to the same size, S, at a different range, R2>R1, to produce an average resultP2. The beam attenuation coefficient, c, of the water can then be determined by

FIG. 2depicts a detailed design of the afocal LIDAR transmitter/receiver20. In the figure, a microchip laser22produces one nanosecond (ns) duration pulses of linearly polarized light at a 532 nanometer (nm) vacuum wavelength and a 6 kHz repetition rate.

A first lens24collimates the light and a half wavelength plate26rotates the polarization of the light to horizontal. A first mirror28and a second mirror30direct the light onto a second lens32that focuses the light to a 50 micron diameter in the plane of a 50 micron diameter pinhole (aperture) of a first section34. A third lens36and a fourth lens38form a first unit magnification image relay telescope that projects a virtual image of the pinhole of the first section34to a power telescope60of a region between a negative lens62(first power telescope lens) and a positive lens64(second power telescope lens). In the example shown inFIG. 2; a transverse magnification of the telescope60is M=117 and the longitudinal magnification is M2=188.9.

The light output of the telescope60passes through a quarter waveplate70that converts the light pulse to a circular polarization. The light pulse then forms an image of size S=13.7*50=685 microns in the water300. Light that is back scattered in the region

cW⁢τ2=3×1081.34×10-9/2=0.11⁢⁢meters⁡(<S2λW=1.18⁢⁢meters)
about the focus of the light pulse is reflected back to the transmitter/receiver20. To the extent that the circular polarization is preserved; the back scattered light is converted to linear polarization by the quarter waveplate70, which is rotated ninety degrees from the outgoing light that enters the waveplate.

When the back scattered light202reaches a first polarized beam splitter40, the back scattered light is reflected toward a third mirror42. For the outgoing light, the light polarization is such that the light is transmitted by the first beam splitter40. Also, when the outgoing light strikes the first beam splitter40; a portion of the light is reflected (due to surface reflections and polarization errors) toward a high speed detector43. The output of the high speed detector43is sent to a channel of a Pico Scope80(portable oscilloscope).

The output of the high speed detector43is measured and recorded at the Pico Scope80to validate the laser pulse strength (which is proportional to the output and time wave shape). The telescope60and a second unit magnification image relay telescope formed by a fifth lens37and the fourth lens38; image the water focal region onto a pinhole (aperture) of a second section46via the fifth lens37. The 50 micron size of the pinhole of the second section46is matched to an ideal image formed in the water300. This matched filtering rejects light that was forward scattered into regions outside the ideal water image.

A pinhole (aperture) of a third section44is positioned approximately in the far field of the pinhole of the second section46. The size of the pinhole of the third section44is matched to the angular spectrum of light that would be reflected by a perpendicular mirror placed at the water focal plane when no scattering takes place. Thus, the pinhole of the third section44further rejects multiple scattered light.

To additionally reject background light, an interference filter48(tuned to the laser light wavelength) is positioned at a detector50. The output of the detector50is measured, recorded, and processed by the Pico Scope80to form multiple pulse averages which are then accessed by a PC104 Controller90for final processing and storage by implementation of Equation (7).

Because of the (matched filters) of the third section44and the second section46; the average recorded output is approximately proportional to exp(−2cR). After a preset time interval (determined by the desired number of pulses to be averaged); the PC104 controller90commands a translator controller100to move the telescope60in order to focus the light at a different range or position.

In the example and using the components ofFIG. 2, the range difference is chosen to be five meters which causes a ten meter difference in light propagation distance. In a vacuum, the five meter difference is reduced by the water index of refraction, n=1.34; thereby, producing a vacuum focal difference of 5/1.34=3.73 meters. For the longitudinal magnification of M2=188.9, the telescope60must be translated 3.73/188.9=0.0197 meters. When the results for the times

t=2⁢RcW+τ2
for the different ranges are processed; the PC104 controller90divides the two results to generateP1/P2which can be used along with R2−R1=5 meters in Equation (7) to determine the beam attenuation coefficient, c, at the 532 nm wavelength.

Returning toFIG. 1, the photodetector16is used to measure the diffuse attenuation coefficient, K. The interference filter14is tuned to the laser wavelength in order to discriminate against background light. For times t>τ and pulse durations that satisfy

the back scattered optical power, PD(t), that reaches the photodetector16is approximately given by

PD⁡(t)=TAEp⁢b180⁢°⁢cW⁢exp⁡(-KcW⁢t)(cW⁢t2)2.(9)
where T is the combined transmission of the filter14and a window18, A is the area of photosensitive portion of the photodetector16, EPis the pulse energy, and b180°is the volume scattering coefficient in the backward direction at the laser light wavelength.

The output voltage, VD(t), of the photodetector16is given by VD(t)=gPD(t) where g is the overall gain of the photodetector. VD(t) is then measured and processed by the Pico Scope80that produces an average,VD(t), over the preset number of pulses. Next,VD(t) is sent to the PC 104 controller90where the output voltage is recorded and processed. The PC 104 controller90makes a best fit ofVD(t) to the time dependence in Equation (9) to produce the measurement of K.

InFIG. 1, a baffle12is used to avoid light scattered within the attenuation meter10and at the window18. The relative position of the photodetector16and the baffle12can be used to reduce the signal at the photodetector due to intense light that is back scattered at short ranges. If the short ranges are obstructed,VD(t), must be compared with Equation (9) in the unobstructed region.

Returning toFIG. 2, a depth sensor120is used to activate the attenuation meter10after the meter has reached a desired depth to avoid surface effects. A power supply130is used to supply power to the components of the attenuation meter10that require power including a laser controller140that activates the laser22.

A major advantage of the attenuation meter10is long measurement paths that allow for more accurate measurements than those provided by currently-available meters with short optical paths. Another advantage is that the beam attenuation measurement is derived from the sensor response by evaluating the ratio of the responses at two or possibly more ranges. This evaluation eliminates the calibration needed for conventional meters.

In the case of the diffuse attenuation coefficient; only the shape of the receiver time dependence and not the absolute level is required to provide the diffuse attenuation coefficient measurement by fitting the results of Equation (9). This comparison is accomplished by comparing the logarithms of Equation (9) and the logarithm detected signal. Yet another advantage is that the optical path length is easily adjusted to accommodate media with different clarity by adjusting the focal ranges with the controller100.

The attenuation meter10can be deployed as a self-contained module and powered by appurtenant batteries and deployed on vehicles such as unmanned underwater vehicles (UUVs) or deployed from a separate platform with a cord connection that supplies electrical power and provides access to stored data. The attenuation meter10can contain more than a single color light source (preferably blue) to provide measurements at more than one wavelength.