Optical remote sensing of vibrations

Systems and methods are provided, which use at least two coherent light sources with known phase relations between them, which are configured to illuminate a target with at least two corresponding spots, an optical unit comprising a mask and configured to focus, onto a sensor, interfered scattered illumination from the spots of the target, passing through the mask, to yield a signal, at least one shifter configured to shift a frequency of at least one of the coherent light sources to provide a carrier frequency in the signal, and a processing unit configured to derive a vibration frequency of the target from the sensor signal with respect to the carrier frequency. The vibration frequency of the target is separated from the carrier frequency and speckle disturbances may be attenuated or avoided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Application of PCT International Application No. PCT/IL2017/051116, International Filing Date Oct. 2, 2017, entitled “OPTICAL REMOTE SENSING OF VIBRATIONS”, published on Apr. 12, 2018, under publication No. WO 2018/065982, which claims priority of Israel Patent Application No. 248274, filed on Oct. 9, 2016, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to the field of remotely sensing vibrations, and more particularly, to sensing vibrations remotely using optical means.

2. Discussion of Related Art

Various ways were suggested for sensing vibrations, such various mechanical, acoustic and optical methods; the latter including speckle interferometry and laser Doppler vibrometers.

SUMMARY OF THE INVENTION

The following is a simplified summary providing an initial understanding of the invention. The summary does not necessarily identify key elements nor limit the scope of the invention, but merely serves as an introduction to the following description.

One aspect of the present invention provides a system comprising: at least two coherent light sources with known phase relations between them, configured to illuminate a target with at least two corresponding spots, an optical unit comprising a mask and configured to focus, onto a sensor, interfered scattered illumination from the spots of the target, passing through the mask, to yield a signal, at least one shifter configured to shift a frequency of at least one of the coherent light sources to provide a carrier frequency in the signal, and a processing unit configured to derive a vibration frequency of the target from the sensor signal with respect to the carrier frequency.

One aspect of the present invention provides a system comprising: at least one coherent light source configured to illuminate a target, an optical unit comprising a mask and configured to focus, onto a sensor, scattered illumination from the target passing through the mask, to yield a signal, at least one mechanical unit configured to move the mask over an aperture of the optical unit at a specified speed to yield a carrier frequency, and a processing unit configured to derive a vibration frequency of the target from an analysis of the sensor signal with respect to the carrier frequency.

DETAILED DESCRIPTION OF THE INVENTION

Reflected signals from targets which are illuminated by a laser are characterized by speckle patterns, which are non-uniform intensity patterns resulting from interference of wavefronts of the reflected laser illumination. When targets are vibrating, the reflected speckle patterns move and, when integrated on a detector, result in a signal with fluctuating intensity due to the small changes of the part of the speckle pattern which is integrated in an aperture associated with the detector. Signal fluctuations may be intensified by a mask set at a pupil plane in front of the detector, which provides more entry and exit lines for speckle with respect to the integration area of the signal.

However, disadvantageously, the amplitude of the detected signal fluctuations depends on the characteristics of the speckle pattern (e.g., dimension and density of speckles) and not on the characteristics of the target vibrations. The inventors have found out that the target vibration amplitude that produces a translation larger than 5-10% of mean speckle size, influences the detected signal frequency rather than its amplitude.

FIG. 1is a high level schematic block diagram of a system100with a moving mask122, according to some embodiments of the invention. System100may be configured to detect vibrating elements91in a scene and derive parameters relating to their vibrations. System100comprises a coherent light source95configured to illuminate a target91, typically by a narrow laser beam, and an optical unit120comprising mask122and configured to focus, via optical element(s)125and onto a sensor99, scattered illumination90from target91passing through mask122, to yield a signal140.

Starting from a static target91, illumination95causes a speckle pattern90A (illustrated schematically) in scattered radiation90, which is collected by optical unit120and focused onto detector99. Detector or sensor99may comprise one or more pixels and may be implemented as a single pixel detector. When target91vibrates, speckle pattern90A vibrates as well on the aperture of optical unit120, resulting in intensity fluctuations on detector99, which are termed herein “modulation” of the signal. The modulation is at vibration frequency(ies) of target91. A mask122at the aperture may be designed or selected to enhance the amplitude of the modulation, depending on the relation between the geometrical parameters of mask122, speckle dimensions and vibration amplitude. Therefore, in the prior art, a vibration frequency of target91may be derived from the frequencies in the modulation.

In certain embodiments, mask122is moved at a specified speed over the aperture. Assuming a static target, speckle pattern90A is also static and the motion of mask122results in a specified modulation having a specified frequency termed herein the carrier frequency fc, denoted131inFIG. 1. Once target91vibrates as well, and assuming the carrier frequency is much larger than the vibration frequency(ies) of target91, fcis frequency-modulated (FM) by the target vibration frequency (see frequency shift in the frequency domain, denoted by131A and illustrated schematically as signal140inFIG. 1). (It is noted that the spectrum is shown schematically in a non-limiting manner and may include larger side lobes, depending on the modulation index.) The vibration frequency(ies) of target91may therefore be extracted from the measured modulation by known methods. It is noted that in these embodiments, mask122plays a dual role of amplifying the modulation (resulting from the vibrating speckle pattern) and of providing the carrier frequency; and speckle pattern90A provides the information about the vibration frequency(ies) of target91. System100further comprises a mechanical unit124, e.g., a chopper, configured to move mask122across the aperture of optical unit120at a specified speed to yield and provide the carrier frequency fc. System100further comprises a processing unit150configured to derive vibration frequency131A of target91from an analysis of sensor signal140with respect to carrier frequency131. Advantageously, the vibration intensity may also be estimated by processing unit150from signal140. Mechanical unit124and the carrier frequency fcmay be optimized with respect to required signal amplification and with respect to speckle characteristics

FIGS. 2A and 2Bare high level schematic block diagrams of system100with stationary mask122, according to some embodiments of the invention. Referring toFIG. 2A, system100may be configured to detect vibrating elements91in a scene and derive parameters relating to their vibrations. System100may comprise at least two coherent light sources95with known phase relations between them, which are configured to illuminate target91by at least two corresponding spots96(indicated schematically). Coherent light sources95are further configured to have correlated phases, e.g., be derived from a single coherent light source. In certain embodiments, coherent light sources95are derived from a single laser source97split by a splitter97A (see examples inFIGS. 4A-4Dbelow). System100may further comprise optical unit120comprising static mask122and configured to focus, onto sensor99, interfered scattered illumination from target91, passing through mask122, to yield signal140. Sensor/detector99may comprise one or more pixels and may be implemented as a single pixel detector. It is noted that the term vibrations is used in a broad sense to refer to various types of kinetic changes of elements91, such as any change (e.g., movements, rotations etc.) that causes a change of the relative phase between radiation reflected from spots96. The relative phase is detectable as described herein and used to detect the respective kinetic change.

Two (or more) illumination sources95, having correlated phases, are used to illuminate two close areas on target91(denoted as spots96), so that the scattered radiation therefrom interferes on the aperture of optical unit120to provide an interference pattern92(shown schematically) which is collected to yield detected signal140. Assuming a static target, interference pattern92is static, and speckle pattern90A is expressed as deformations with respect to a clean interference pattern (see an example inFIG. 3below). Once target91vibrates, interference pattern92vibrates as well, resulting in a signal modulation.

In order to provide frequency modulation, carrier frequency fc131may be generated by controlling the relative phase between illumination sources95and/or shifting the frequency of one source95with respect to another source95, to yield carrier frequency fc>>target vibration frequency(ies)131A. (It is noted that the spectrum is shown schematically in a non-limiting manner and may include larger side lobes, depending on the modulation index.) The modulation, resulting from target vibrations, is turned into a FM modulation of fcand may be derived by known methods. It is noted that in these embodiments, mask122may be selected only to amplify the modulation (resulting from interference pattern92), and the speckle pattern is merely a disturbance that deforms interference pattern92, as illustrated inFIG. 3below.

FIG. 2Billustrates schematically a somewhat more complex signal spectrum140having additional vibration frequency harmonics that may be used for FM demodulation152followed by further processing154by processing unit, leading to identification of vibration frequency131A in analysis results155of the vibration frequencies. In some embodiments, additional detector(s)99A and/or optical unit(s)120A (shown schematically) may be used to derive additional spectra140A which may be FM-demodulated152A and incorporated in processing154and analysis155.

The inventors have shown in simulation and experimentally, that changing the distance between spots96controls the density of interference pattern92which in turn changes the intensity of vibration signal133but not the modulation. Therefore, increasing the distance between spots96improves the SNR (signal to noise ratio). Moreover, diminishing spot size increases the SNR, but also the sensitivity to atmospheric conditions. The inventors have further shown, both analytically and experimentally that, unlike speckle interferometry, disclosed methods measure translational difference between the spots with the same sensitivity as tilt angles of the surface on which the spots fall. The disclosed methods and systems thus combine the advantages of speckle interferometry and laser Doppler vibrometers to provide improved systems. Any of the distance between spots96, spot sizes, parameters of mask122, parameters of optical unit120and the modulation of illumination sources (i.e., fc) may be optimized with respect to each other to optimize system performance. In certain embodiments, optical unit120may be configured to comprise at least one polarizer configured to further improve the SNR, as explained below.

System100may comprise at least one shifter130such as at least one frequency shifter132and/or at least one phase shifter134, configured to shift the frequency of at least one of coherent light sources95according to predetermined shifting characteristics, and processing unit150may be configured to derive vibration frequency131A of target91from a relation between sensor signal140and the predetermined shifting characteristics. The configuration of shifter(s)130to determine carrier frequency fcis shown schematically inFIG. 2Aby an arrow, near interference pattern92. In embodiments, shifter(s)130may comprise at least one RF (radiofrequency) shifter132configured to shift an illumination frequency of one or more phase-correlated or possibly phase-locked coherent light source(s)95by a predetermined RF frequency131. The vibration frequency may then be derived from a component133in sensor signal140at a frequency that is shifted from predetermined RF frequency131. In embodiments, shifter(s)130may comprise at least one phase shifter134configured to shift a phase of one or more coherent light source(s)95by a predetermined phase shift pattern. The vibration frequency may then be derived from a relation between sensor signal140and the predetermined phase shift pattern.

Advantageously, with respect to certain embodiments of mechanical unit(s)124, system100with two or more illumination sources95and shifter(s)130may comprise fewer or no moving mechanical parts, overcome challenges of designing mask122as well as be less sensitive to small speckles. Embodiments illustrated inFIGS. 2A and 2Bprovide an interferometric approach to vibrations sensing, with all interferometer paths being outside optical unit120, namely being between illumination sources95, target91and optical unit120.

FIG. 3is an exemplary illustration of interference patterns92in simulation92A and in experiment92B, according to some embodiments of the invention. Mask122is illustrated schematically as having parallel straight lines, and interference patterns92are shown as derived from simulation92A and from experiments92B in an exemplary manner. Speckle pattern90A is transformed through the use of two illumination sources95and interference pattern of the respective reflected radiation, into modulated line patterns which are then focused on detector99through mask122. In the embodiments illustrated inFIGS. 2A and 2B, speckle pattern90A is expressed in the disturbances in interference pattern and do not convey the information concerning the vibrations of the target. Instead of mechanical modulation with the predetermined modulation characteristics, embodiments with interference patterns may use modulation of illumination sources95to enable isolation of a signal proportional to the vibration intensity of target91, as explained below.

FIGS. 4A-4Dare high level block diagrams of illumination sources95, according to some embodiments of the invention. In certain embodiments, multiple illumination sources95may be implemented using a single illumination source97which is split (by a splitter97A) into two or more illumination sources95A,95B by a beam splitter97A, with shifter(s)130implemented as one or more phase shifter(s)134, acousto-optic deflector(s) (AOD)136A,136B and/or other optical components. illumination sources95A,95B may be configured to provide spots96A,96B with specified characteristics of target91. For example, as illustrated inFIG. 4C, illumination sources95may comprise two lasers amplifiers137A,137B configured to receive the outputs of AODs136A,136B and be phase-locked with laser seeder97. Illumination from lasers amplifiers137A,137B may then be combined by a combiner138, collimated by a collimator139and used as illumination sources95A,95B to provide spots96A,96B. In an example illustrated in FIG.4D, both AODs136A,136B may be located on one split channel. Elements fromFIGS. 4A-4Dmay be combined to form additional embodiments.

FIG. 5is a high level block diagram of polarized detection, according to some embodiments of the invention. In certain embodiments, polarizer(s)98A,98B may be implemented to reduce speckle noise. Spots96A,96B from target91create interference pattern92which is then split, e.g., by a polarizing beam splitter (PBS)93in optical unit120into different polarizations by polarizer(s)98A,98B which are detected separately on detector99and/or on separate detectors99A,99B.

FIG. 6is a high level flowchart illustrating a method200, according to some embodiments of the invention. The method stages may be carried out with respect to system100described above, which may optionally be configured to implement method200. Method200may be at least partially implemented by at least one computer processor. Certain embodiments comprise computer program products comprising a computer readable storage medium having computer readable program embodied therewith and configured to carry out of the relevant stages of method200. Method200may comprise stages for producing, preparing and/or using system100, such as any of the following stages, irrespective of their order.

Method200may comprise illuminating a target by at least two phase-correlated coherent light sources to form at least two corresponding spots (stage210), possibly splitting the at least two coherent light sources from a single illumination source (stage212) (see e.g.,FIGS. 4A-4D).

Method200may comprise focusing, onto a sensor and through a mask, interfered scattered illumination from the spots on the target, to yield a signal (stage225), shifting a frequency and/or a phase of at least one of the coherent light sources to provide a carrier frequency in the signal (stage230) and deriving a vibration frequency of the target from the sensor signal with respect to the carrier frequency (stage250), e.g., from a component in the sensor signal at a frequency that is shifted from the carrier frequency (stage252), as illustrated e.g., inFIGS. 2A and 2Band in the accompanying description.

Method200may further comprise optimizing sizes and spacing of the spots, with respect to parameters of the optical unit and the mask (stage260). Method200may further comprise generating the signal with respect to different polarizations of the interfered scattered illumination, thereby detecting, separately, polarization components of the interference pattern to improve the SNR (stage262). Method200may further comprise configuring the mask with respect to an expected interference pattern of the scattered illumination.

In certain embodiments, method200may comprise illuminating the target with one (or more) illumination source(s) (stage210), focusing, onto the sensor, scattered illumination from the target through the mask, to yield a signal (stage220), moving, mechanically, the mask across the aperture, according to predetermined characteristics, to yield a carrier frequency (stage240) and deriving a vibration frequency of the target from an analysis of the sensor signal with respect to the carrier frequency (stage250) as illustrated e.g., inFIG. 1and the accompanying description. The mask parameters and movement parameters, as well as illumination and focusing parameters may be optimized with respect to the expected speckle patterns as described above.