GB2206258A

GB2206258A - Water-surface profilometer

Info

Publication number: GB2206258A
Application number: GB08715162A
Authority: GB
Inventors: John Mansbridge; George Malcolm Swift Joynes
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-06-27
Filing date: 1987-06-27
Publication date: 1988-12-29
Anticipated expiration: 2007-06-27
Also published as: GB2206258B; GB8715162D0

Abstract

A water-surface profilometer comprises a light source 1, a modulator 3 controlled by an oscillator 4 which is capable of amplitude modulating a light beam from the source at a suitable frequency, the beam being directed through a lens 6 onto the water surface 8 to be measured, a proportion of the said transmitted beam being scattered from the body of the water adjacent to the surface and forming a return beam which is directed onto a photodetector 13 to produce an electrical signal. This signal is compared in a phase detector 14 with the oscillator 4 output to produce a phase difference representative of the water-surface distance from the apparatus. <IMAGE>

Description

WATER-SURFACE PROPILOMETER APPARATUS This invention relates to a water surface profilometer apparatus which can be particularly suitable for measuring the profile of a water surface, such as the surface of an ocean, sea or river. Such profiles can be used for the investigation of oceanographic or hydrodynamic phenomena and the measurement of wave heights. The system is intended for use primarily in situations where the optical apparatus can be positioned within about ten metres of the water surface.

The profilometer apparatus relies on use of an optical beam to probe the surface under investigation and this can be done without significant perturbation of the water state. The design of the profilometer is particularly suitable for being mounted on a ship or other floating platform because there are no parts of the system required to be located below the water surface and which would thus require a modification of the vessel construction.

According to the invention there is provided a water-surface profilometer apparatus comprising a light source, a modulator controlled by an oscillator which is capable of modulating a light beam from the source at a suitable frequency, the beam being directed through a lens onto the water surface to be measured, a proportion of the said transmitted beam being scattered from a body of water at or adjacent said surface, said proportion forming a return beam which is directed onto a photodetector of the apparatus which produces a corresponding electrical signal, a phase detector arranged for comparing phase differences between said return beam signal and a signal received directly from said oscillator, and said phase detector producing an output signal representative of the distance between said water surface and the profilometer.

The return beam may be arranged to enter the apparatus along the same path as that of the transmitted beam, a beam splitter being positioned to divert said return beam to the photodetector. The lens may include a focussing mechanism operated by a servo system which is effective to maintain a focussed light spot on the water surface.

The light source may be a laser. The modulator may act to modulate the intensity or the amplitude of light from the source.

Preferably, the transmitted beam is directed onto scanning mirror arranged such that the beam will be scanned repeatedly over a predetermined path on the water surface. The apparatus may include an optical band-pass filter through which the said return beam is passed before the beam enters the photodetector.

By way of example, a particular embodiment of the invention will now be described with reference to the accompanying drawing, the single Figure of which shows a block diagram of the profilometer apparatus.

The system is based on the measurement of changes in the propagation time for the optical beam to travel to the water surface and return, owing to changes in the surface height. This measurement is carried out by the use of a sub-carrier modulation technique. Under this system, the optical beam has its intensity (or amplitude) modulated with a sine wave known as a sub-carrier. The time differences can then be measured by determining the phase of the received signal relative to the transmitted sine wave. This is in effect, an interferometer system based on the wavelength of the sub-carrier modulation (as opposed to that of the light itself). The wavelength of the sub-carrier is arranged to be as short as possible to give the maxImum resolution, but sufficiently long so that the maxImum change in distance does not exceed half a wavelength.The changes in distance are due to both thewave height changes and also to geometrical effects resulting from the scanning process. The present invention is able to make use of a shorter wavelength of light than might be expected because the effect of focussing is utilised to resolve the ambiguity.

As depicted in the Figure, the apparatus comprises a light source 1 whIch in this embodiment is a laser light source. An output light beam 2 from the source 1 is directed through a modulator 3 which is driven by a sine wave oscillator 4 at a suitable modulation frequency.

The modulated light beam is next passed through a lens 6, and is reflected from a mirror 7 to form a light spot on a surface 8 of a body of water. The profilometer apparatus is required to measure the distance of the body of water from the apparatus.

The lens 6 is mounted with a focussing mechanism (not shown) which is operated by a mechanical servo system and which is effective to maintain a focussed light spot on the water surface 8. The servo system derives its error signal from a measurement of the power received and, if necessary, information derived from the system output.

The mirror 7 is also movable mechanically by a scanning mechanism (not shown) so that the beam will be scanned repeatedly over a predetermined path on the water surface 8.

Part of a light beam scattered from the distant water surface 8 forms a return beam which travels back to the apparatus along the path of the transmitted beam. The return beam is reflected by the mirror 7, is focussed by the lens 6 and then is reflected away from the path of the transmitted beam by a beam splitter 9.

The return beam 11 next passes through an optical band-pass filter 12 and strikes a photodetector 13. The photodetector 13 serves to convert the light energy into an electrical signal and this is amplified by an amplifier mounted with the photodetector 13. The amplified electrical signal is applied to one input of a phase detector 14 and a second input signal is obtained directly from the oscillator 4. The phase detector 14 serves to compare phase differences between the electrical signal receIved from the oscillator 4 and that which has had to pass along the optical path to the water surface 8 and back.

The phase detector 14 also has an output terminal 16 at which a signal representative of the distance between the water surface and the profilometer apparatus appears.

In operation, the light beam 2 output from the laser has a particular modulation, which in the present embodiment is an intensity modulation, impressed upon it in response to the signals from the oscillator 4. The resulting light pulses pass through the automatically focussing lens 6 and are reflected from the scanning mirror 7 to form a focussed line on the water surface 8.

The surface of the water can be considered as a set of facets, each a few millimetres in width and each being essentially flat. Any light incident on a facet will be reflected according to the law of specular reflection, which results in the light being reflected in an unpredictable dIrection, since the local facet wave slope will have a largely random direction. As the spot of light is required (for the purposes of resolution) to be about the same size as a facet, it is possible that, at times, all the light will be reflected from a facet which is at such an angle that the power is reflected away from the system, and no light will be collected. Thus, it is not possible to rely on a system using the property of specular reflection from the water surface.

Therefore the apparatus of the present invention does not rely on light reflected from the surface of the water but upon that scattered from within the water volume after having penetrated the surface. Although the light received from scattering can be expected to be at a much lower level than the light reflected from a facet, the received signal should never In practice fade enough to prevent a measurement being'made at any position.

In order to ensure that the measurement is an accurate representation of the profile of the surface, the system must only receive light scattered from a volume of water near or at the surface. This is done by choosing the light to be of a wavelength which is highly attenuated by absorption of scattering in the water (and its constituents). Thus it is possible to ensure that significant light power only propagates to a limited depth below the surface and thus that none of the scattered light received by the system can have come from below this depth. If a wavelength is chosen which is too highly attenuated, then the scattering volume will be very small and the power received will be too low to be accurately detected. The wavelength is thus a compromise between the measurement uncertainty due to the volume and the amplification power available at the receiver.In addition, a wavelength must be chosen for which there 15 a powerful source of light. Suitable wavelengths have been identlfied in the near-infrared region of the electromagnetic spectrum.

Some of the scattered light travels back along the transmit path and is focussed by the lens 6 to form a beam which travels along the same path as the transmitted beam.

At the beam splitter 9, the return beam is separated off and it is passed through the optical band-pass filter 12 which is intended to block as much of the background light as possible in order to reduce noise in the received signal.

The return beam is then arranged to fall on the photodetector 13, which is followed by a suitable amplifier. The output of the detector is compared with the s-ne-wave from the oscillator 4 in the phase detector 13 which determines the phase difference between the two. . The voltage representing this phase difference appears at the output terminal 16 and this is thus a measure of the distance between the profilometer apparatus and the water surface 8.

Since the light spot has been arranged to forma focussed line on the water surface, it is possible to measure the rate of change of distance along this line so that a value for the profile of the body of water at the surface can be obtained. In a different embodiment, it would be possible for the light spot to scan an area on the water surface such as by usIng a raster or some alternative area-scanning technique.

The foregoing description of an embodiment of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, in order to overcome certain noise sources, it would be possible to use a coherent detection technique, where the received optical frequency was heterodyned or homodyned with the transmitted optical output.

Whilst the embodiment described uses a laser light.

source because of its high radiance and narrow spectral bandwidth, in a different embodiment it should be possible to use a different suitable light source.

Claims

CLAIMS:

1. A water surface profilometer apparatus comprising a light source, a modulator controlled by an oscillator which is capable of modulating a light beam from the source at a suitable frequency, the beam being directed through a lens onto the water surface to be measured, a proportion of the said transmItted beam being scattered from a body of water at or adjacent said surface, said proportion forming a return beam which is directed onto a photodetector of the apparatus which produces a corresponding electrical signal, a phase detector arranged for comparing phase differences between said return beam signal and a signal received directly from said oscillator, and said phase detector producing an output signal representative of the distance between sad water surface and the profilometer.

2. A profilometer apparatus as claimed in Claim 1, in which said return beam is arranged to enter the apparatus along the same path as that of the transmitted beam, a beam splitter being positioned to divert said return beam to the photodetector.

3. A profilometer apparatus as claimed in Claim 1 or 2, in which the said lens includes a focussing mechanism operated by a servo system which is effective to maintain a focussed light spot on the water surface.

4. A profilometer apparatus as claimed in any one of Claims 1 to 3, in which the said light source is a laser.

5. A profilometer apparatus as claimed in any one of Claims 1 to 4, in which the modulator acts to modulate the intensity or amplitude of light from the said source.

6. A profilometer apparatus as claimed in any one of Claims 1 to 5, in which the transmitted beam is directed onto a scanning mirror arranged such that the beam will be scanned repeatedly over a predetermined path on the water surface.

7. A profilometer apparatus as claimed in any one of Claims 1 to 6, including an optical band-pass filter through which the said return beam is passed before the beam enters the photodetector.

8. A water-surface profilometer apparatus substantially as hereinbefore described with reference to the accompanying drawing.