Abstract:
A vector velocimeter includes a laser emitting a measurement beam with a wavelength λ, for illumination of an object in a measurement volume to create a signal beam, a reference beam generator generating a reference beam, and a first detector arranged such that the signal beam and the reference beam, propagating at a first angle θ relative to the signal beam, are incident thereon. The first detector includes an array of first detector elements to convert the intensity of the interfering signal beam and reference beam incident thereon into an oscillating electronic detector element signal when the fringe pattern formed thereby moves across the first detector array. A signal processor generates a velocity signal corresponding to a first velocity component of movement of the object in the measurement volume in the longitudinal direction thereof based on the electronic detector element signals from each of the first detector elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is the U.S. national phase application based on PCT Application No. PCT/EP2011/062239 filed Jul. 18, 2011, which is based on European Application No. 10172261.9 filed Aug. 9, 2010 and U.S. Provisional Application No. 61/371,830 filed Aug. 9, 2010, the entire contents of all of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to a compact, reliable and low-cost vector velocimeter for example for determining velocities of particles suspended in a gas or fluid flow, or for determining velocity, displacement, rotation, or vibration of a solid surface. 
     2. Description of the Related Art 
     LIDAR (Light Detection And Ranging) systems are well-known in the art. LIDAR determines velocity in the direction of line-of-sight based on detection of backscattered coherent light from airborne aerosols or particles in a measurement volume formed by a laser beam emitted by the LIDAR. 
     WO 2009/046717, which is hereby incorporated in its entirety by reference, discloses a LIDAR system with an all-semiconductor light source for emission of a laser beam for illumination of aerosols or particles in the measurement volume. The disclosed LIDAR system determines velocity magnitudes along the direction of propagation of the emitted laser beam. Possible velocity components in directions perpendicular to the direction of propagation of the emitted laser beam are not determined. 
     SUMMARY 
     Embodiments provide a vector velocimeter that is capable of determining a velocity vector, i.e. the magnitude of the velocity and the direction in one, two, or three dimensions, and which is compact, reliable and can be manufactured at low cost. 
     According to embodiments the above-mentioned and other features are obtained by provision of a vector velocimeter comprising a source of electromagnetic radiation that is arranged for emission of a measurement beam of electromagnetic radiation, e.g. spatially coherent light, directed towards a measurement volume for illumination of an object in the measurement volume. 
     According to embodiments a vector velocimeter is provided, wherein the vector velocimeter comprises a laser assembly for emission of a measurement beam for illumination of an object in a measurement volume with coherent light whereby a signal beam emanating from the object in the measurement volume is formed in response to illumination of the object by the measurement beam. 
     The vector velocimeter may further comprise a reference beam generator for generation of a reference beam. The vector velocimeter may be configured such that the reference beam and the measurement beam are emitted from the same laser source. The reference beam and the measurement beam may thereby be mutually coherent. 
     The vector velocimeter may further comprise a detector system comprising a first detector arrangement arranged in such a way that the signal beam and the reference beam are incident upon the first detector arrangement with the reference beam propagating at a first angle relative to a signal beam. 
     The first detector arrangement may comprise a first detector array of first detector elements, each of the first detector elements converting the intensity of the interfering signal beam and reference beam incident thereupon into a corresponding electronic detector element signal thereby generating an oscillating electronic detector element signal when the fringe pattern formed by the interfering signal beam and reference beam moves across the first detector array. 
     The vector velocimeter may further comprise a signal processor that is adapted for generation of a velocity signal corresponding to a first velocity component of movement of the object in the measurement volume in the longitudinal direction of the measurement volume based on the electronic detector element signals from each of the first detector elements. 
     The source of electromagnetic radiation may be a laser, such as a He—Ne laser or a semiconductor laser, e.g. included in a laser assembly, arranged for emission of the measurement beam. 
     The semiconductor laser may be a vertical external cavity surface-emitting laser (VECSEL) for emission of a high power beam. In a VECSEL, electromagnetic radiation is emitted perpendicular to the junction and the surface of the diode chip. The semiconductor chip or device, also denoted the gain chip, may contain a single semiconductor Bragg mirror and the active region (gain region) with typically several quantum wells (QWs). The device may have a total thickness of only a few micrometers. The laser resonator is completed with an external mirror. 
     The large transverse area of a VECSEL facilitates fundamental mode operation and leads to a high beam quality. Furthermore, the output beam of the VECSEL may be circular symmetrical with an insignificant amount of astigmatism leading to simple imaging properties. 
     The laser material in the electromagnetic cavity may be pumped optically. Optical pumping facilitates uniform pumping of large active areas. The optical pump source may for example be a high-brightness edge emitting broad-area diode or a diode laser bar. It is possible to achieve tens of watts of output power when pumping with a diode bar. Utilisation of an external resonator may facilitate provision of a diffraction-limited output. 
     The semiconductor laser may be a tapered semiconductor laser. Due to its tapered structure, the tapered semiconductor laser provides a high output power at its large area output facet, e.g. having a width of app. 250 μm, with a high beam quality since the ridge-waveguide at the narrow end, e.g. having a width of app. 3 μm, of the tapered laser forms a single mode spatial filter. 
     The vector velocimeter may furthermore comprise a semiconductor tapered power amplifier for amplification of the beam emitted by the semiconductor laser, for example the semiconductor laser and amplifier are of the semiconductor master-oscillator-power-amplifier (MOPA) type. 
     For example, a MOPA assembly may have a semiconductor master oscillator followed by a semiconductor tapered power amplifier; both realized on the same substrate constituting a cheap, rugged solution, ideal for industrial applications. 
     For example, an output power of approximately 1 Watt has been provided by a semiconductor MOPA assembly, even at a wavelength of 1.5 μm where the electron to photon conversion is less efficient as compared to the case for the 800 nm range. A wavelength of 1.5 μm or more is important for practical vector velocimeter use, since 1.5 μm is within the eye-safe region of the optical spectrum. The laser safety requirements during operation are more easily met when operated at eye-safe wavelengths. 
     Furthermore, the temporal coherency of tapered semiconductor laser assemblies, i.e. the coherence length, is sufficient for coherent vector velocimeter applications. 
     Still further, even though the spatial coherence of the output beam of a tapered semiconductor device is not perfect when compared to that of a diffraction-limited Gaussian beam, the laser assembly output beam is of sufficient quality to be used in a vector velocimeter. Spatial low-pass filtering of the output beam can remove or reduce the non-Gaussian spatial components that otherwise may lead to a reduced signal-to-noise ratio of the detector signal. 
     A vector velocimeter with a continuous wave (CW) coherent laser source of electromagnetic radiation relies on the focusing properties (the M 2  factor) of the laser to confine the actual measurement volume. The width of the measurement volume is confined by the diameter of the laser beam in the focused region (i.e. the confocal region). The length of the measurement volume along the beam axis is confined approximately by the Rayleigh length of the focused laser beam. For a CW vector velocimeter focused at a distance of one hundred meters from the system, the width of the measurement volume is typically in the order of one millimeter and the length of the measurement volume is in the order of ten meters depending on wavelength and focusing optics (the telescope). For pulsed systems the width of the measurement volume is the same as for the CW laser case, but the length of the measurement volume is given by the confocal parameter or the spatial length of the emitted pulse, whichever is the smallest. 
     The object in the measurement volume illuminated by the measurement beam may comprise one or more particles, molecules, atoms, or aerosols, such as water droplets, dust, etc., in the measurement volume, each of which scatters, diffracts, reflects, or refracts electromagnetic radiation in response to being illuminated by the measurement beam, thereby forming signal radiation emitted from the object in the measurement volume in response to the illumination by the measurement beam. 
     Throughout the present disclosure, the teen “particles” includes aerosols, molecules, atoms, dust, etc. 
     In the following, the part of the signal radiation that is received by a detector of the vector velocimeter is termed a signal beam. 
     The object in the measurement volume may also be an object of a size similar to the width of the measurement volume; or larger, and the surface of the object may scatter, diffract, reflect, or refract electromagnetic radiation in response to being illuminated by the measurement beam, thereby forming signal radiation emitted from the object in the measurement volume in response to the illumination by the measurement beam. Hereby, the vector velocimeter may determine the velocity of the surface of the object, or vibration, or rotation, of the object. 
     The vector velocimeter also has a reference beam generator that is arranged for emission of a reference beam of electromagnetic radiation at the wavelength of the measurement beam and propagating at an angle relative to the signal beam. The reference beam generator may for example be a beam splitter in which case the source of electromagnetic radiation also generates the reference beam. 
     In the vector velocimeter, the reference beam and the signal beam are arranged for interfering with each other at one or more detectors of the velocimeter. 
     Although the use of a collimated reference beam is envisaged, an arbitrary curvature of the reference beam might be useful. In this case, a given curvature will change the axial distance at which a given object will give the best signal. Thus, especially in case of measurement in the atmosphere, a change in reference beam curvature will facilitate the probing of various axial distances. 
     The vector velocimeter may in one or more embodiments comprise a detector system comprising a first detector arrangement with one or a plurality of detector arrays including a first detector array arranged in such a way that a signal beam emanating from the object together with the reference beam propagating at a first angle relative to the signal beam are incident directly upon a first detector array of first detector elements, wherein each of the first detector elements converts the intensity of the electromagnetic radiation incident thereupon into a corresponding electronic detector element signal. The operation of the detector array is explained in more detail below. 
     The vector velocimeter may in one or more embodiments comprise a detector system comprising a first detector arrangement with a first optical array of first optical elements arranged in such a way that a signal beam emanating from the object is incident upon the first optical array together with the reference beam propagating at a first angle relative to the signal beam, and wherein each of the first optical elements redirect the incident signal beam and reference beam towards a first detector array of first detector elements. Each of the first detector elements converts the intensity of the electromagnetic radiation incident thereupon into a corresponding electronic detector element signal. The operation of the optical array in cooperation with the detector array is explained in more detail below. 
     Utilizing a first optical array for directing the interfering signal beam and reference beam onto the first detector array allows one to focus the beams to a narrower area on the first detector elements thereby obtaining higher beam intensity at the first detector elements. 
     Due to the non-zero angle between the reference beam and the signal beam at the first detector arrangement in the detector system, and due to the fact that the reference beam and the signal beam are mutually coherent beams, an intensity fringe pattern is formed when the signal beam and the reference beam interfere and the intensity distribution of the interference pattern is detected. 
     More generally, the angle (θ) may be related to the period (Λ) of the detector arrangement and the wavelength of the measurement beam (λ) as λ/(2Λ)&lt;θ&lt;2λ/Λ. At angles below λ/(2Λ), the fringe signal disappears, whereas for angles above 2λ/Λ, the direction information is lost. 
     The period (Λ) of the detector arrangement in embodiments of the vector velocimeter where the signal beam and the reference beam are incident directly on the first detector array is the width of a detector array unit comprising 1, 2, 3, 4, 5 or more individual detector elements. Normally, the detector array unit comprises 2-4 detector elements. 
     The period (Λ) of the detector arrangement in embodiments of the vector velocimeter where the signal beam and the reference beam are incident on the first optical array is normally the width of an individual optical element. 
     The angle between signal beam and the reference beam may therefore be at least 1°, at least 2°, at least 3°, at least 4° or at least 5°. Suitable angle degrees may be between 1-10°. 
     The detected intensity fringe pattern is formed by alternating dark and bright lines as for example known from a Michelson type interferometer. The fringe distance is determined by the angle between the signal beam and the reference beam on incidence on the first detector arrangement. 
     Although the fringe pattern is a detected intensity pattern, for example by the eye for visible wavelengths of radiation, the term “fringe pattern” as used throughout the present disclosure, includes the electromagnetic field in a certain area or volume that would cause a fringe pattern of intensity variations to be detected in the event that a detector was positioned in the area or volume of the electromagnetic field in question. 
     When an object, such as particles, aerosols, a solid surface, etc., moves in the measurement volume in the direction of propagation of the measurement beam, the Doppler effect causes a corresponding movement of the fringe pattern formed by the signal beam and the reference beam in a direction perpendicular to the alternating dark and bright lines of the fringe pattern. Movement in the opposite direction in the measurement volume also leads to movement of the fringe pattern in the opposite direction. 
     In embodiments of the vector velocimeter where the signal beam and the reference beam are incident directly on the first detector array of the first detector arrangement with a first angle between the signal beam and reference beam, the first detector array are utilized directly for detection of movement of the fringe pattern formed at the first detection array by interference between the signal beam and the reference beam at the first detector array. Each of the first detector elements generates an oscillating electronic detector element signal in response to a fringe pattern moving across the first detector array. Thus, the first detector array is arranged so that the signal beam and reference beam are incident on the first detector array of first detector elements, each of the first detector elements converting intensity of radiation incident thereupon into a corresponding electronic detector element signal. When the fringe pattern of the interfering signal beam and reference beam move across the first detector array, each of the electronic detector element signals will oscillate due to the movement of the fringe pattern on the first detector elements. 
     In embodiments of the vector velocimeter comprising a first detector arrangement with a first optical array and a first detector array, the first optical array of first optical elements in cooperation with the first detector array are utilized for detection of movement of the fringe pattern formed at the input plane directly in front of the first optical array by interference between the signal beam and the reference beam. A moving fringe pattern is redirected repetitively by the first optical elements towards the first detector array of first detector elements so that each of the first detector elements generates an oscillating electronic detector element signal in response to a fringe pattern moving across the first optical array. Thus, the first optical array and the first detector array are arranged so that the signal beam and reference beam are incident on the first optical elements at a first non-zero angle and redirected by the first optical elements towards the first detector array of first detector elements, each of the first detector elements converting intensity of radiation incident thereupon into a corresponding electronic detector element signal. When the fringe pattern of the interfering signal beam and reference beam move across the first optical array, each of the electronic detector element signals will oscillate due to the repeated redirection of the fringe pattern towards each of the first detector elements. 
     Further, the vector velocimeter has a signal processor that is configured for generation of a velocity signal corresponding to the velocity of movement of the fringe pattern across the first optical array based on the electronic detector element signals, for example the frequency or other signal properties of the detector element signals. The fringe pattern velocity corresponds to a first velocity component of movement of the object in the measurement volume in the longitudinal direction of the measurement volume. 
     For example in a vector velocimeter wherein the measurement beam and the signal beam propagate along the same path, but in opposite directions, the longitudinal direction of the measurement volume coincides with the direction of propagation of the measurement beam (and the signal beam), and thus the velocity component determined by determination of fringe pattern movement as described above, is the velocity component of object movement in the direction of propagation of the measurement beam. When direction of fringe pattern movement is determined, the direction of object movement along the direction of propagation of the measurement beam is also determined. 
     In a vector velocimeter wherein the signal beam propagates in a direction that forms an angle with the direction of propagation of the measurement beam, the fringe distance is still determined by the angle between the reference beam and the signal beam at the detector array in question, but the measurement volume is formed in cooperation by the transmitter optics transmitting the measurement beam towards the measurement volume and the receiver optics receiving the signal beam emitted from the measurement volume so that the longitudinal direction of the measurement volume in this case does not coincide with the direction of propagation of the measurement beam. Instead, the longitudinal direction of the measurement volume forms an angle with the measurement beam and also with the signal beam. This angle is half the angle formed between the measurement beam and the signal beam, and extends in a plane defined by the measurement beam and the signal beam. 
     Thus, in this case, the direction of maximum Doppler shift does not coincide with the direction of propagation of the measurement beam. Instead, the direction of maximum Doppler shift forms an angle with the measurement beam, and also with the signal beam, which is half the angle between the measurement beam and the signal beam and extends in a plane defined by the measurement beam and the signal beam. 
     When, the measurement beam illuminates more than one particle or a large object with a rough surface in the measurement volume speckles can be formed in addition to the fringe pattern. 
     If the receiver optics can resolve objects with a size less than the cross-section of the measurement volume in a plane perpendicular to the signal beam then speckles are formed. 
     Like “fringe pattern”, speckle pattern is a pattern of intensity variations as detected by a detector. For example, when a surface is illuminated with visible laser light, speckles may be observed by the human eye. The speckle pattern appears as a grainy intensity pattern when the intensity is detected, e.g. by the human eye. Surface roughness of the object causes formation of the speckle pattern since surface deviations modify the phase of various parts of the incident electromagnetic field differently, and produces the speckle pattern by mutual interference of various parts of the electromagnetic field as received by the detector. 
     Like the term “fringe pattern”, throughout the present disclosure, the term “speckle pattern” also includes the electromagnetic field that would cause detection of a speckle pattern (of intensity) with an intensity detector. 
     When the object moves in the measurement volume in a direction in a plane substantially perpendicular to the signal beam, the speckle pattern in the measurement volume as detected with an intensity detector in a position where a signal beam can be received, moves across the surface of the detector with a velocity and direction corresponding to the velocity and direction of the object in the measurement volume in a direction in a plane substantially perpendicular to the signal beam. 
     In embodiments of the vector velocimeter with a detector system where the signal beam and the reference beam are incident directly on the first detector array of the first detector arrangement, a second detector arrangement comprising a second detector array of second detector elements may be included into the vector velocimeter, wherein each of the second detector elements converts the intensity of the signal beam (and possibly the reference beam) incident thereupon into a corresponding electronic detector element signal thereby generating an oscillating electronic detector element signal when the speckle pattern formed by the signal beam moves across the second optical array. The reference beam may be overlapped with the signal beam on incidence on the second detector array in order to amplify the intensity of the speckles pattern, but it is not a requirement in order to obtain a speckles pattern. The signal beam and the reference beam may further be incident on the second detector array at an angle. 
     In embodiments of the vector velocimeter comprising a first detector arrangement with a first optical array and a first detector array, a second detector arrangement comprising a second detector array of second detector elements and a second optical array of second optical elements may be included into the detector system. The second optical array is arranged in such a way that the signal beam is incident upon the second optical array, wherein each of the second optical elements redirects the incident signal beam (and possibly the reference beam) towards the second detector array arranged so that the redirected signal beam from the second optical elements are incident upon the second detector array. Each of the second detector elements converts the intensity of the beam(s) incident thereupon into a corresponding electronic detector element signal thereby generating an oscillating electronic detector element signal when the speckle pattern formed by the signal beam at incidence upon the second optical elements of the second optical array moves across the second optical array. The reference beam may be arranged such that the signal beam and the reference beam overlap and interact when the beams are incident on the second optical array in order to amplify the intensity of the speckles pattern, but it is not a requirement in order to obtain a speckles pattern. The signal beam and the reference beam may be incident on the second optical array at an angle. 
     Other embodiments of the vector velocimeter incorporate a mixture of the two detector arrangements described above. Thus, a first detector arrangement comprising a first optical array in combination with a first detector array may be used for the detection of the first velocity component and a second detector arrangement wherein the signal beam and the reference beam are incident directly on the second detector array of the second detector arrangement may be used for the detection of the second velocity component. The first and the second detector arrangements may also be interchanged for the detection of the first and the second velocity component, respectively. 
     The signal processor of the velocimeter is adapted for generation of a velocity signal corresponding to a second velocity component of movement of the object in the measurement volume based on the electronic detector element signals from each of the second detector elements. The second velocity component is substantially perpendicular to the first velocity component. 
     In order to determine a third velocity component in a direction in a plane substantially perpendicular to the measurement beam, for example perpendicular to the second velocity component, the detector system of the vector velocimeter may further comprise a third detector arrangement comprising a third detector array of third detector elements, wherein each of the third detector elements converts the intensity of the signal beam (and possibly the reference beam) incident thereupon into a corresponding electronic detector element signal thereby generating an oscillating electronic detector element signal when the speckle pattern formed by the signal beam moves across the third detector array. The reference beam may be arranged such that the signal beam and the reference beam overlap and interact when the beams are incident on the third detector array in order to amplify the intensity of the speckles pattern. The signal beam and the reference beam may be incident on the third detector array at an angle, but it is not a requirement in order to obtain a speckles pattern. 
     The third detector arrangement may further comprise a third optical array of third optical elements arranged in such a way that the signal beam is incident upon the third optical array, wherein each of the third optical elements redirects the incident signal beam (and possibly the reference beam) towards the third detector array of third detector elements arranged so that the redirected signal beam from the third optical elements are incident upon the third detector array. Each of the third detector elements converts the intensity of the beam(s) incident thereupon into a corresponding electronic detector element signal thereby generating an oscillating electronic detector element signal when the speckle pattern formed by the signal beam at incidence upon the third optical elements of the third optical array moves across the third optical array. The reference beam may be arranged such that the signal beam and the reference beam overlap and interact when the beams are incident on the third optical array in order to amplify the intensity of the speckles pattern, but it is not a requirement in order to obtain a speckles pattern. The signal beam and the reference beam may be incident on the third optical array at an angle. 
     The signal processor is adapted for generation of a velocity signal corresponding to a third velocity component of movement of the object in the measurement volume based on the electronic detector element signals from each of the third detector elements. The third velocity component is substantially perpendicular to the first velocity component. 
     The first and second optical arrays may be integrated into a single optical array. 
     The first and third optical arrays may be integrated into a single optical array. 
     The second and third optical arrays may be integrated into a single optical array. 
     The first and second and third optical arrays may be integrated into a single optical array. 
     The first and second detector arrays may be integrated into a single detector array. 
     The first and third detector arrays may be integrated into a single detector array. 
     The second and third detector arrays may be integrated into a single detector array. 
     The first and second and third detector arrays may be integrated into a single detector array. 
     Examples of optical arrays and detector arrays are disclosed in WO 03/069278, which is hereby incorporated in its entirety by reference. 
     An optical array may comprise at least three optical elements for mapping of different specific areas of the measurement volume onto substantially the same area of the corresponding detector in space thereby generating an oscillating electronic detector signal caused by phase variations of light emanating from the object moving in the measurement volume. For example, the first optical array may comprise at least three first optical elements. The second optical array may comprise at least three second optical elements and/or the third optical array may comprise at least three third optical elements. Accordingly, each of the first, second and third optical arrays may have at least three first, second, and third optical elements, respectively, for mapping of different specific areas of the measurement volume onto substantially the same area of the first, second, and third optical detectors, respectively, in space thereby generating an oscillating electronic detector signal caused by phase variations of light emanating from the object moving in the measurement volume. 
     In embodiments of the velocimeter where detector system is such that the signal beam and the reference beam are incident directly on the detector array(s), the formation of an oscillating optical signal emitted by an illuminated moving object can be obtained by 
     1) Illuminating the object moving in the measurement volume with the measurement beam and allowing electromagnetic radiation emitted by the object in response to the illumination to interfere with the reference beam whereby a moving fringe pattern is formed when movement of the object has a velocity component in the direction of propagation of the measurement beam. The first detector array is positioned so that the moving fringe pattern moves across the first detector array thereby generating an oscillating electronic signal in response to the incident electromagnetic radiation, and in vector velocimeters with second and/or third optical arrays by: 
     2) Illuminating the object moving in the measurement volume with the measurement beam whereby speckles are formed that move when the object has a velocity component perpendicular to the direction of propagation of the signal beam. The second and third optical arrays are positioned so that the moving speckles move across the second and third detector arrays, respectively, thereby generating respective oscillating electronic signals in response to the incident electromagnetic radiation, respectively. 
     The direction of the velocity component determined by each of the first, second, and third detector arrays in the embodiment of the vector velocimeter where the signal beam and the reference beam are incident directly on the detector arrays is determined by the orientation of the detector array in question. 
     In embodiments of the detector system, wherein the detector arrangements comprise optical arrays, the repetitive optical structure of the optical arrays is utilized for formation of an oscillating optical signal emitted by an illuminated moving object by 
     1) Illuminating the object moving in the measurement volume with the measurement beam and allowing electromagnetic radiation emitted by the object in response to the illumination to interfere with the reference beam whereby a moving fringe pattern is formed when movement of the object has a velocity component in the direction of propagation of the measurement beam. The first optical array is positioned so that the moving fringe pattern moves across the repetitive optical structure of the first optical array and is redirected repetitively onto the first detector array that generates an oscillating electronic signal in response to the incident electromagnetic radiation from the first optical array, and in vector velocimeters with second and/or third optical arrays by: 
     2) Illuminating the object moving in the measurement volume with the measurement beam whereby speckles are formed that move when the object has a velocity component perpendicular to the direction of propagation of the signal beam. The second and third optical arrays are positioned so that the moving speckles move across the repetitive optical structure of the second and third optical arrays, respectively, and are redirected repetitively onto the second and third detector arrays, respectively, that generate respective oscillating electronic signals in response to the incident electromagnetic radiation from the second and third optical arrays, respectively. 
     The direction of the velocity components determined by each of the first, second, and third optical arrays is determined by the orientation of the optical array in question in cooperation with the orientation with the corresponding detector array, i.e. the direction between neighbouring optical elements and neighbouring detector elements, respectively. 
     The optical array may for example comprise a linear array of cylindrical lenses. The focal length of the lenses may be positive or negative. For the sake of explanation, the input plane may be defined in front of the array of lenses, e.g., at a distance equal to the focal length of the lenses and perpendicular to the direction of propagation of the incoming electromagnetic radiation. 
     The fringe pattern is formed at the input plane when the overlapping reference beam and signal beam interact. The speckle pattern and/or the fringe pattern can be detected at the input plane, e.g. by intensity measurements, caused by variations of the electromagnetic field along the input plane. 
     When the object moves in the measurement volume, the speckle pattern and/or the fringe pattern move across the input plane with a velocity proportional to the velocity of the object in the measurement volume in the direction of movement corresponding to the orientation of the optical array, i.e. the direction perpendicular to the direction of length of the cylindrical lenses. Each of the individual optical elements, in this example constituted by cylindrical lenses, directs the incoming electromagnetic radiation towards a detector array of detector elements. The electromagnetic radiation that is redirected by the individual optical elements sweeps across the detector array, when the speckle pattern or fringe pattern moves a distance that is equal to the width of an individual optical element across the input plane. This is repeated for each optical element when the speckle pattern or fringe pattern travels across the input plane, and when the pattern has traversed a distance equal to the length of the optical array, i.e. across all of the cylindrical lenses in this example, the redirected electronic radiation has been swept repetitively across the detector array. The average number of sweeps is equal to the number of individual optical elements of the linear array passed by the moving fringe pattern, or, moving speckle pattern during the time it takes the speckle pattern to either decorrelate or to move across the entire optical array. The repeated sweeps cause generation of an oscillating electronic signal by each of the detector elements. 
     Preferably, the width of the individual optical elements is matched to the fringe distance in the fringe pattern or to approximately 2-3 times the width of individual speckles in the speckle pattern at the input plane in order to generate an electronic detector signal with a large signal to noise ratio. The width of the individual optical elements is determined so that the intensity of the electromagnetic field at a detector element varies between a high intensity when high amplitude parts of the fringe pattern or speckle pattern are aligned with the optical elements and a low intensity when low amplitude parts of the fringe pattern or speckle pattern are aligned with the optical elements. 
     When a fringe pattern and a speckle pattern both move across the input plane of an optical array, the fringe pattern is preferably arranged with a fringe distance that is significantly different from a characteristic size of the speckles so that the velocity components can be separated by spatial filtering velocimetry, i.e. the velocity component of the fringe pattern can be determined with an optical array with a certain pitch, and the velocity component of the fringe pattern can be determined with an optical array with a different pitch. The pitch or period of an optical array is the distance between individual neighbouring optical elements, e.g. for an array of identical cylindrical lenses, the pitch equals the width of the individual cylindrical lenses. 
     The frequency of the oscillations of the electronic detector element signal corresponds to the velocity of displacement of the fringe pattern or speckle pattern across the input plane in the direction defined by neighbouring optical elements in cooperation with the direction defined by neighbouring detector elements, divided by the array pitch, i.e. the distance between individual neighbouring optical elements in the direction in question. For an array of cylindrical lenses, the direction is perpendicular to the direction of length of the individual cylindrical lenses, and the orientation of the detector array defined by the direction between neighbouring individual detector elements is aligned with the direction perpendicular to the direction of length of the individual cylindrical lenses. 
     This principle of operation applies in general to any type of optical array utilized and regardless of whether or not an image of the object in the measurement volume is formed at the input plane or directly at the optical array. 
     Two-dimensional speckle pattern displacement may be determined with a two-dimensional array of optical elements, e.g. circular lenticular lenses, arranged along perpendicular directions of the array and cooperating with a two-dimensional detector array aligned with the two-dimensional optical array. 
     The vector velocity meter may further comprise an imaging system, e.g. a lens, for imaging part of the input plane onto the detector elements whereby each of the individual optical elements in combination with the imaging system images specific parts of the input plane onto the same specific area of the detector array. Hereby, points at the input plane that are positioned at the same relative positions in relation to adjacent respective optical elements will be imaged onto the same point of the detector array, whereby the signal-to-noise ratio may be improved. Without the imaging system, there will be a small distance between mapped points at the detector array for corresponding points at the input plane having the same relative position in relation to respective neighbouring optical elements. However, the accuracy of the system may still be sufficient and will depend on the actual size of the system. 
     The optical array and the imaging system may be merged into a single physical component, such as a moulded plastic component, in order to obtain a further compact system suited for mass production. 
     The individual optical elements of the optical arrays may interact with light by reflection, refraction, scattering, diffraction, etc, either alone or in any combination, of light incident upon them. Thus, the individual optical elements may be lenses, such as cylindrical lenses, spherical lenses, Fresnel lenses, ball lenses, or phase gratings, amplitude gratings, diffractive gratings, Ronchi rulings, prisms, prism stubs, mirrors, liquid crystals, etc. 
     The optical array may further be formed by a diffractive optical element, such as holographically produced lenses, etc. 
     Still further, the optical array may comprise a linear phase grating with a sinusoidal modulation of the film thickness, e.g. in a photo resist film. 
     In the vector velocimeter, the electronic signals output from the individual detector elements from each of the detector arrays may be combined in order to suppress undesired signal components in the electronic signal output from the detector array in question as also disclosed in WO 03/069278, whereby signal detection is simplified. For example, subtraction may be used to suppress the pedestal of the signal, i.e. a low frequency part of the electronic signal, and also harmonics in the electronic output signal may be suppressed. 
     Also, the direction of movement of the fringe pattern or speckle pattern may be determined by suitable arrangement of the detector array elements in combination with suitable signal processing, e.g. whereby a quadrature, or substantially quadrature, signal may be obtained, thereby simplifying detection of direction as for example compared to conventional LDA (Laser Doppler Anemometer) or LIDAR systems. 
     Occurrence of velocity signal drop out may be reduced by provision of a second set of optical detector elements that is displaced in relation to the existing set of detector elements so that a signal that is statistically independent of the other signal may be available from one set of detector elements during absence of a signal from the other set of detector elements. Thus by proper processing of the two signals, e.g. switching to a set of detector elements generating a velocity signal, occurrence of signal drop out is minimized. 
     It is an important advantage of the vector velocimeter that formation of a fringe pattern by the non-zero angle between the reference beam and the signal beam makes it possible to utilize a compact electro-optical device with the first optical array and the first detector array for determination of movement of the fringe pattern and thereby the corresponding velocity of the object in the measurement volume including the direction of the velocity. Furthermore, optical arrays cooperating with respective further detector arrays can be added for determination of speckle movement in one or two dimensions whereby two-dimensional and three-dimensional velocity of the object may be determined. 
     For embodiments of the vector velocimeter where the signal beam and the reference beam are incident directly on the detector array, the non-zero angle between the reference beam and the signal beam makes it possible to utilize the even more compact detector scheme for determination of movement of the fringe pattern and thereby the corresponding velocity of the object in the measurement volume including the direction of the velocity directly on the detector array. 
     Furthermore, additional detector arrays can be added for determination of speckle movement in one or two dimensions whereby two-dimensional and three-dimensional velocity of the object may be determined. 
     Even further, the utilization of a combined detector array allowing for detection of two or three velocity components by the same detector array makes the velocimeter even more compact. 
     For all embodiments of the vector velocimeter, this makes the one-dimensional, two-dimensional, or three-dimensional vector velocimeter simple, robust, compact and easy to manufacture. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the vector velocimeter will become readily apparent to those skilled in the art by the following detailed description of exemplary vector velocimeters with reference to the attached drawings, in which: 
         FIG. 1  schematically illustrates a conventional LIDAR system, 
         FIG. 2  schematically illustrates a vector velocimeter according to the invention, 
         FIG. 3  schematically illustrates the operation of an optical array in cooperation with a detector array, 
         FIG. 4  is a plot of a detector element signal from the detector array shown in  FIG. 3 , 
         FIG. 5  is a plot of a signal from a displaced optical detector element having a phase lag in relation to the signal shown in  FIG. 4 , 
         FIG. 6  is a plot of the difference between the signals shown in  FIGS. 4 and 5 , 
         FIG. 7  schematically illustrates a simple detector circuit with subtraction of detector element signals, 
         FIG. 8  is a plot of the output signal provided by the detector circuit shown in  FIG. 7 , 
         FIG. 9  is a plot of the power spectrum of the signal shown in  FIG. 8 , 
         FIG. 10  schematically illustrates another detector circuit with subtraction of detector element signals, 
         FIG. 11  is a plot of the output signal provided by the detector circuit shown in 
         FIG. 10 , 
         FIG. 12  is a plot of the power spectrum of the signal shown in  FIG. 11 , 
         FIG. 13  schematically illustrates yet another detector circuit with subtraction of detector element signals, 
         FIG. 14  is a plot of the almost phase quadrature signal provided by the detector circuit shown in  FIG. 13 , 
         FIG. 15  is a phase plot of the signal shown in  FIG. 14 , 
         FIG. 16  schematically illustrates still another detector circuit with subtraction of detector element signals, 
         FIG. 17  is a plot of the phase quadrature signal provided by the detector circuit shown in  FIG. 16 , 
         FIG. 18  is a phase plot of the signal shown in  FIG. 17 , 
         FIG. 19  schematically illustrates an exemplary vector velocimeter according to the invention, 
         FIG. 20  schematically illustrates another exemplary vector velocimeter according to the invention, 
         FIG. 21  schematically illustrates yet another exemplary vector velocimeter according to the invention, 
         FIG. 22  schematically illustrates still another example of a vector velocimeter according to the invention, 
         FIG. 23  schematically illustrates yet, still another example of a vector velocimeter, according to the invention, 
         FIG. 24  schematically illustrates an exemplary vector velocimeter according to the invention, 
         FIG. 25  schematically illustrates an exemplary vector velocimeter according to the invention, 
         FIG. 26  schematically illustrates an exemplary a detector array and detector circuit, 
         FIG. 27  schematically illustrates another example of a vector velocimeter according to the invention, and 
         FIG. 28  schematically illustrates the detector array of  FIG. 27  in detail. 
     
    
    
     DETAILED DESCRIPTION 
     The figures are schematic and simplified for clarity, and they merely show details which are important to the understanding of the operation of the vector velocimeter including non-essential features that may have many alternatives. For simplicity, details that are well-known to the person skilled in the art may have been left out. Throughout, the same reference numerals are used for identical or corresponding parts. 
     In addition to the exemplary vector velocimeters described more fully hereinafter with reference to the accompanying drawings, the principles of the vector velocimeter may also be applied in further different ways and should not be construed as limited to the examples set forth herein. Rather, these exemplary vector velocimeters are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the vector velocimeter to those skilled in the art. 
       FIG. 1  schematically illustrates a conventional LIDAR system  1 . A laser  2  emits a first coherent light beam  3  of high spatial and spectral beam quality. A beam splitter  4  divides the emitted light beam  3  into a reference beam  5  and a measurement beam  6 , and imaging optics  7  focuses the measurement beam  6  at the measurement volume  8 . The imaging optics  7  may be a Galilean or Keplerian telescope. When the object  9  constituted by aerosols in the measurement volume  8  are illuminated by the measurement beam  6 , the aerosols back scatter a small amount of light forming a signal beam  10  towards the LIDAR detector  11 . For aerosols, the intensity of the back scattered signal beam  10  is in the order of 1 pW when illuminated by a measurement beam  6  of 1 W. 
     The signal beam  10  propagates through the imaging optics  7  and is redirected by the beam splitter  4  towards the opto-electronic detector  11  on the surface of which, the signal beam  10  interferes with the reference beam  5  and is mixed with the reference beam  5  in the opto-electronic detector  11  so that the opto-electronic detector  11  outputs an a measurement signal containing spectral components corresponding to the difference frequency between the reference beam  5  and the signal beam  10 . The difference frequency corresponds to Doppler frequency of the moving aerosols  9  in measurement volume  8  relatively to the LIDAR system. By processing the measurement signal using a signal processor, the corresponding speed of the aerosols in the direction of propagation of the measurement beam  6  can be calculated. 
     In the conventional LIDAR shown in  FIG. 1 , the direction of movement of the aerosols  9  along the direction of propagation of the measurement beam  6  is not determined, i.e. the same speed will be measured for aerosols moving with the same speed, but in opposite directions along the direction of propagation of the measurement volume. 
     Complex and expensive components have to be added to the illustrated LIDAR for provision of determination of the direction of movement of the aerosols, for example frequency shifting components, such as a Bragg-cell, shifting the frequency of the measurement beam  6  or the reference beam  5 . 
     In the vector velocimeter, this problem is solved in a simple and cost effective way. 
     The vector velocimeter also provides determination of two-dimensional or three-dimensional velocity vectors. 
       FIG. 2  schematically illustrates a vector velocimeter  100  in which the reference beam  18  and the signal beam  28  are incident on the detector system  32  forming a non-zero angle  34 , contrary to the conventional LIDAR  1  shown in  FIG. 1  wherein the reference beam  5  and the signal beam  10  propagate along the same path and are incident on the detector  11  at an angle of 0 degrees. 
     The angle (θ)  34  may be related to the wavelength of the measurement beam (κ) and the period (Λ) of one or more detector arrangements  33  included in the detector system  32  as λ/(2Λ)&lt;θ&lt;2λ/Λ. At angles below 2/(2Λ), the fringe signal disappears, whereas for angles above 2λ/Λ, the direction information is lost. 
     The angle  34  may in one or embodiments be at least 1°, at least 2°, at least 3°, at least 4° or at least 5°. An angle  34  of e.g. 3.3° corresponds to a fringe distance of 15 μm at a wavelength of 850 nm. Suitable angle  34  degrees may be between 1-10°. However, as apparent from the above relation, suitable angle ranges are dependent on the wavelength of the measurement beam  20 . 
     A laser in a laser assembly  12 , for example as disclosed in WO 2009/046717 A2, emits a first coherent light beam  14  of high spatial and spectral beam quality. A beam splitter  16  divides the emitted light beam  14  into a reference beam  18  and a measurement beam  20 , and an optical transmitter  22  focuses the measurement beam  20  at the measurement volume  24 . The optical transmitter  22  may be a Galilean or Keplerian telescope. When the object  26 , in the illustrated example constituted by aerosols  26 , in the measurement volume  24  are illuminated by the measurement beam  20 , the aerosols back scatter a small amount of light forming a signal beam  28  towards an optical receiver  30  that images the measurement volume  24  onto one or more detector arrangements  33  in the detector system  32 , the operation of which is further explained below. 
     The angle  34  between the reference beam  18  and signal beam  28  incident at the detector system  32  comprising one or more detector arrangements  33  leads to formation of a fringe pattern  36  of intensity variations overlaying a speckle pattern  38  that is formed by illumination of the object  26  in the measurement volume  24  by the measurement beam  20 . The combined fringe pattern  36  and speckle pattern  38  is illustrated to the right in  FIG. 2  showing the intensity pattern as it could be detected at the surface of a detector arrangement  33  in the detector system  32 . The fringe distance is determined by the angle  34 . 
     The longitudinal direction of the measurement beam is equal to the common direction of propagation of the measurement beam and the signal beam. When the object moves in the direction of propagation of the measurement beam  20 , the fringe pattern  36  shown in  FIG. 2  moves in the speckle pattern  38  to the left or right, i.e. perpendicular to the direction of the individual fringes, as determined by the direction of movement of the object in the direction of propagation of the measurement beam  20 . Thus, both speed and direction can be determined. 
     In case the object  26  moves in the transverse direction, i.e. in a direction perpendicular to the direction of propagation of the measurement beam  20 , the fringe pattern  36  does not move while the speckle pattern  38  will move accordingly following the movement of the object. 
     In case, the velocity of the object does not have components perpendicular to the direction of propagation of the measurement beam  20 , the speckle pattern  38  will remain in its current position; however, the statistics of phase changes of the signal beam may lead to changed occurrence of speckles also known as “speckle boiling”. 
     The possible movement of the fringe pattern  36  and of the speckle pattern  38  is determined by the detector system  32  comprising one or more detector arrangements  33 , the operation of which is further explained below, whereby the velocity of the object including the direction of the object may be determined in one, two or three dimensions. 
       FIG. 3  schematically illustrates the operation of a detector arrangement  33  comprising an optical array  112  and a detector array  125  with detector elements  126 ,  128 ,  130 . In the illustrated example, the optical array  112  is a linear array  112  of substantially identical cylindrical lenses  118 . f 1  is the focal length of the cylindrical lenses  118 . For the sake of explanation, an input plane  114  is defined at a distance equal to the focal length f 1  of the lenses  118  and perpendicular to the direction  116  of propagation of the incoming light  18 ,  28 . 
     When the object is displaced in the measurement volume (not shown), the intensity pattern in question, i.e. the speckle pattern  38  and/or the fringe pattern  36 , moves correspondingly along the input plane  114 . The individual cylindrical lenses  118  redirect the light  18 ,  28  towards a refractive lens  122  having a focal length f 2  and being positioned a distance equal to f 1 +f 2  from the linear array  112 . The lens  122  further refracts the redirected light  120  into light  124  propagating towards detector array  125  having detector elements  126 ,  128 , and  130  positioned at the focal plane of lens  122 . In this way, each of the individual lenses  118  of the optical array  112  in combination with lens  122  images the input plane  114  onto the same area of an output plane  115 . The detector elements  126 ,  128 ,  130  of the detector array  125  are positioned so that their individual surfaces for reception of light coincide with the output plane  115 . 
     Thus, an area  132  of the input plane is imaged by a respective adjacent lens  118  onto an area  134  of a detector element  128  and corresponding areas  136  that are located at the same relative positions in relation to other respective adjacent cylindrical lenses  118  are imaged onto the same area  134  of the detector element  128 . 
     It should be noted that the distance between the linear array  112  and the lens  122  is chosen to be equal to f 1 +f 2  in the present example for ease of explanation of the operation of the detector arrangement  33 . However, the detector arrangement  33  operates with any distance between the linear array  112  and the lens  122 . For compactness it may be preferred to set the distance to zero. 
     Thus, when an intensity feature at the input plane  114  has moved a distance  138  that is equal to the width Λ 0 , i.e. the pitch, of an individual optical element  118 , the corresponding image formed by the combination of lens  122  and the respective cylindrical lens  118  sweeps across the area of the detector array  125  with detector elements  126 ,  128 , and  130 . This is repeated for the other cylindrical lenses  118 , and it is seen that when an intensity feature has traversed a number of individual cylindrical lenses  118  of the linear array  112 , the detector elements  126 ,  128 ,  130  are swept repetitively a number of times equal to the number of individual cylindrical lenses  118  the intensity feature has passed. It is seen that for a regular intensity pattern at the input plane, the width of the individual optical elements, in the illustrated example cylindrical lenses  118 , can be matched to the size of features of the intensity pattern, such as fringe distance or speckle size, for optimization of the signal to noise ratio of the output signal. 
     Furthermore, the optical array  112  is preferably aligned with the desired direction of movement to be determined. Thus, if the detector arrangement  33  shown in  FIG. 3  is used for determination of the velocity of the fringe pattern  36 , the optical array  112  is preferably positioned so that its longitudinal direction is perpendicular to the fringe pattern movement and the size of the individual optical elements  118  is matched to the fringe distance. Likewise, if the detector arrangement  33  is used for determination of the velocity of the speckle pattern  38  in a certain direction, the optical array  112  is preferably positioned so that its longitudinal direction is aligned perpendicular to the desired direction of speckle pattern movement and the size of the individual optical elements  118  is matched to approximately 2-5 times the speckle size. If the detector arrangement  33  is used for determination of the velocity of both the fringe pattern  36  and the speckle pattern  38  in e.g. the same direction the system is designed so that the speckle size is an order of magnitude larger than the fringe distance. In this way, fringe pattern movement and speckle pattern movement in various directions can be separated by spatial filtering velocimetry provided by the optical array  112 . 
     The frequency of the signal generated by each of the detector elements  126 ,  128 ,  130  corresponds to the velocity of the intensity pattern in question in the direction Δx along the length of the linear optical array  112  divided by the array pitch, i.e. the distance between individual neighbouring optical elements. 
     The lens  122  is not required in the detector arrangement  33 . In a detector arrangement  33  without the lens  122 , the individual mappings of the input plane  114  onto the output plane  115  by the individual optical elements  118  of the optical array  112  will be displaced slightly with respect to each other. The amount of displacement depends on the size of the detector arrangement  33 ; however, the assembly still operates substantially according to the principles explained above. 
     The same principle of operation applies in general to other detector arrangements  33  regardless of the type of optical element utilized and regardless of whether or not an image of the object is formed at the input plane  114 . 
       FIG. 4  is a plot of the output signal  150  from one of the detector elements  126 ,  128 ,  130  shown in  FIG. 3 . A corresponding signal  152  from an adjacent detector element is shown in  FIG. 5 . This signal  152  is phase shifted in relation to the signal  150  shown in  FIG. 4  because of the physical displacement of the detector elements  126 ,  128 ,  130 . Since the low frequency pedestals of the two signals  150 ,  152  are substantially identical, the difference between the two signals  150 ,  152  is an AC-signal  154  as shown in  FIG. 6 . 
       FIG. 7  shows a detector array  125  with six detector elements that are combined two by two for generation of an output signal  150 . As explained above, the detector array  125  is swept once for each passage of an intensity pattern across an individual optical element  118  of the optical array  112 . Thus, neighbouring detector elements of the detector array  125  output signals with a 60° phase shift with relation to each other, and the first element and the fourth element of the detector array outputs signals with a 180° phase shift with relation to each other. In  FIG. 7 , a detector circuit configuration is used, wherein the output signals from the first two elements are combined, and  FIG. 8  shows a plot of the resulting signal  150  and  FIG. 9  shows the power spectrum  156  of the signal  150 . It should be noted that the low frequency part  158  and the second harmonic  160  of the spectrum  156  are quite significant. The low frequency noise leads to a variation of the running mean value which will introduce significant errors in velocity determinations based on zero-crossing detection. The width of the detector has been selected for optimum suppression of the third harmonic of the fundamental frequency. The detector element is assumed to have a rectangular shape and thus, the power spectrum of the detector function is a sinc-squared function. In order to eliminate every third harmonic of the detector output signal, the width of each detector element is selected to be substantially equal to one third of the full width of the detector array that is selected to be equal to the width repetitively swept by an intensity pattern traversing the input plane. 
     In  FIG. 10 , a detector circuit for elimination of the low frequency pedestal is shown. The distance between the two pairs of combined elements corresponds to a phase shift of 180°. The output signals from the detector elements are combined for suppression of the low frequency part of the signals and the even harmonic frequencies of the fundamental frequency. The difference signal  154  is plotted in  FIG. 11 , and the power spectrum  156  is plotted in  FIG. 12 . The suppression of the low frequency part  158  and the second harmonic  160  is clearly demonstrated by comparison with  FIG. 9 . 
     An almost-phase-quadrature detector circuit configuration is shown in  FIG. 13 , wherein output signals from six detector elements of equal size are combined to form two signals  154   a ,  154   b  in which the low frequency pedestal has been removed. The two signals  154   a ,  154   b  are 60 degrees out of phase and therefore suitable for determination of the direction of the velocity of the intensity pattern. In this configuration, an exact phase quadrature cannot be achieved without changing the detector width  106  thereby reducing the suppression of the third harmonic. The almost-phase-quadrature signals  154   a ,  154   b  are plotted in  FIG. 14 , and  FIG. 15  is a phase plot  162  of the signals  154   a ,  154   b . The phase plot  162  has an elliptical shape which facilitates determination of the direction of the intensity pattern velocity. 
     The detector circuit configuration shown in  FIG. 16  provides a substantially exact phase-quadrature detector arrangement, wherein output signals from four detector elements of equal size are combined to form two signals  154   a ,  154   b  in which the low frequency pedestal has been removed. The two signals  154   a ,  154   b  are 90° out of phase and therefore suitable for determination of the direction of the velocity of the intensity pattern.  FIG. 17  is a plot of the phase-quadrature signals  154   a ,  154   b , and  FIG. 18  is the corresponding phase plot  162 . The phase plot  162  is circular facilitating determination of the direction of the object velocity and sub-radian measurement accuracy. The circular shape of the traces in the phase plot makes this configuration robust against noise. 
       FIG. 19  schematically illustrates a vector velocimeter  100  wherein a laser in a laser assembly  12 , for example as disclosed in WO 2009/046717 A2, emits a first coherent light beam  14  of high spatial and spectral beam quality. A beam splitter  16  divides the emitted light beam  14  into a reference beam  18  and a measurement beam  20 , and an optical transmitter (not shown) focuses the measurement beam  20  at the measurement volume  24 . The optical transmitter  22  may be a Galilean or Keplerian telescope. When the object (not shown) in the measurement volume  24  are illuminated by the measurement beam  20 , the object, e.g. aerosols, back scatter a small amount of light forming a signal beam  28  towards an optical receiver  30  that images the measurement volume  24  onto an optical array  112  in a detector arrangement  33  also including lenses  122   a ,  122   b , and detector elements  126 ,  128 . The operation of the detector arrangement is further explained below. 
     The beam splitter  16  may be formed according to the principles explained in WO 2009/046717 A2, e.g. in connection with  FIG. 6 , wherein the light assembly  12  comprises a single mode semiconductor laser the optical output of which is collimated into a linearly TM-polarized beam that is fully transmitted through a polarizing beam splitter. A quarter-wave plate changes the transmitted optical output into a circular polarization state. The quarter-wave plate is slightly tilted to avoid back-reflections to reach the laser. Subsequently, the surface of a partly reflecting reference window back-reflects a certain percentage of the laser optical output. The back reflected beam is transmitted back through the quarter-wave plate where it becomes linearly TE-polarized. This TE-polarized beam is fully reflected by the surface of the polarizing beam splitter and forms the reference beam  18 . In the vector velocimeter  100 , the partly reflecting surface is wedged so that the reference beam  18  forms a first angle  35   a  with the signal beam  28  as the signal beam  28  and the reference beam  18  are incident on a first detection arrangement  33   a  in the detection system  32 . A main part of the laser optical output is transmitted as circular polarized light through the reference window. The first angle  35   a  between the reference beam  18  and signal beam  28  leads to formation of a fringe pattern  36  of intensity variations overlaying a speckle pattern  38  that is formed by illumination of the object  26  in the measurement volume  24  by the measurement beam  20  as explained in connection with  FIG. 2 . 
     In the illustrated vector velocimeter  100 , the signal beam  28  propagates in a direction that forms an angle with the direction of propagation of the measurement beam  20 . The fringe distance is determined by first angle  34  between the reference beam  18  and the signal beam  28  at the detector arrangement  33 , but the measurement volume  24  is formed in cooperation by the transmitter optics  22  (not shown) transmitting the measurement beam towards the measurement volume  24  and the receiver optics  30  receiving the signal beam emitted from the measurement volume  24  so that the longitudinal direction  140  of the measurement volume  24  in this case does not coincide with the direction of propagation of the measurement beam  20 . Instead, the longitudinal direction  140  of the measurement volume forms an angle with the measurement beam  20  and also with the signal beam  28 . This angle is half the angle formed between the measurement beam  20  and the signal beam  28 , and extends in a plane defined by the measurement beam  20  and the signal beam  28 . 
     Thus, in this case, the direction  140  of maximum Doppler shift does not coincide with the direction of propagation of the measurement beam  20 . Instead, the direction  140  of maximum Doppler shift forms an angle with the measurement beam  20 , and also with the signal beam  28 , that is half the angle between the measurement beam  20  and the signal beam  28  and extends in a plane defined by the measurement beam  20  and the signal beam  28 . 
     The possible movement of the fringe pattern and/or of the speckle pattern at the input plane (not shown) of the optical array  112  in the detector arrangement  33  is determined based on output signals from the detector elements  126 ,  128  in the detector arrangement  33 . 
     The optical array  112  comprises array elements that in succession redirect features of the intensity pattern towards detector element  126  and detector element  128 , respectively. For example, light and dark areas may in succession be redirected towards the detector elements  126 ,  128  thereby forming an oscillating output signal from the detector elements  126 ,  128 . The optical array  112  may for example be a linear optical array of prisms. The two sides of each prism refract incoming rays of light towards the two respective detector elements  126 ,  128 . The electronic coupling of the detector elements may be performed as explained in connection with  FIGS. 5-19 . 
     Contrary to the detector arrangement  33  shown in  FIG. 3 , light is redirected towards the individual detector elements  126 ,  128  by an individual lens  122   a ,  122   b  so that the detector elements  126 ,  128  need not be positioned in close relationship to each other. 
     The vector velocimeter shown in  FIG. 20  operates in a way similar to the vector velocimeter of  FIG. 19 ; however, in the velocimeter of  FIG. 20 , the angle  34  required for formation of the fringe pattern  36  is formed by the beam splitter  16  and the mirrors  17  in such a way that the measurement beam  20  and the signal beam  28  propagate along the same path whereby the optical transmitter  22  and receiver  30  can be combined, e.g. in a Galilean or Keplerian telescope. Further, a compact detector arrangement  33  is used with a common lens  122  for redirecting light towards both detector elements  126 ,  128  positioned in closely spaced relationship to each other. 
     For determination of velocities in two dimensions, a second detector arrangement  33   b  comprising a second optical array  112   b , a second lens  122   b , and a second detector array  125   b  has been added to the detector system  32  in the velocimeter shown in  FIG. 21  already comprising the first detector arrangement  33   a  with a first optical array  112   a , a first lens  122   a , and a first detector array  125   a  as described in  FIG. 20 . The detector system  32  comprises a semi-transparent beam splitter  164 , which divides the signal beam  28  and the reference beam  18  so that one part of the beams  18 ,  28  propagate towards the first detector arrangement  33   a  and the other part propagate toward second detector arrangement  33   b . The operation of the second detector arrangement  33   b  is explained in connection with  FIG. 3 . A third detector arrangement  33   c  (not shown) may be added to the detector system  32  for determination of velocities in three dimensions, e.g. with an orientation perpendicular to the orientation of detector arrangement  33   b . The signal beam and the reference beam are incident on the first detector arrangement  33   a  at a first angle  35   a  and are incident on the second detector arrangement  33   b  at a second angle  35   b.    
       FIG. 22  shows a vector velocimeter  100  similar to the vector velocimeters of  FIGS. 21 and 22 , but with another detector system  32  comprising an integrated detector arrangement for determination of velocities in three dimensions. The detector system  32  comprises three optical arrays  112   a ,  112   b ,  112   c  with cylindrical lenses. The optical arrays  112   a ,  112   b  are positioned and sized for detection of speckle movement along orthogonal directions, i.e. the cylindrical axes of lenses of optical array  112   a  are perpendicular to the cylindrical axes of lenses of optical array  112   b . The third optical array  122   c  is positioned and sized for detection of fringe movement in a direction forming an angle of 45° in relation to the cylindrical axes of both optical arrays  112   a ,  112   b  thereby minimizing interference of fringe movement with speckle movement on optical arrays  112   a ,  112   b , since fringes are aligned with the cylindrical axes of the optical array  112   c . The operation of each pair of optical array and detector array  122   a ,  125   a ;  122   b ,  125   b ;  122   c ,  125   c , respectively, is explained in connection with  FIG. 3 . The lenses  122   a ,  122   b ,  122   c  may be combined in a single lens. The electronic coupling of the detector elements may be performed as explained in connection with  FIGS. 5-19 . 
       FIG. 23  shows a vector velocimeter  100  that operates in a way similar to the vector velocimeter of  FIG. 22 ; however, the configuration of the optics is different so that the beams propagate out of plane, i.e. the plane defined by the signal beam  28  as redirected by beam splitter  16  forms an angle with the plane defined by the reference beam  18  as redirected by the beam splitter  16  and mirrors  166 ,  168 . The detector arrangement is identical to the arrangement shown in  FIG. 22 . 
       FIG. 24  shows a velocimeter  100 , wherein the detector system  32  comprises a semi-transparent beam splitter  164  similar to the one in  FIG. 21 , which beam splitter  164  divides the signal beam  28  and the reference beam  18  so that one part of the beams  18 ,  28  propagate towards the first detector arrangement  33   a  comprising the first detector array  125   a  and the other part propagate toward the second detector arrangement  33   b  comprising the second detector array  125   b . The signal beam  28  and the reference beam  18  are incident on the first detector arrangement  33   a  with a first angle  35   a  between the signal beam  28  and the reference beam  18 . Likewise, the signal beam  28  and the reference beam  18  are incident on the second detector arrangement  33   a  with a second angle  35   b  between the signal beam  28  and the reference beam  18 . The velocities in two dimensions may thereby be determined using detector arrangements  33   a ,  33   b  with the two detector arrays  125   a ,  125   b , respectively. A further detector array (not shown) may be added to the detector system  32  for determination of velocities in three dimensions, e.g. with an orientation perpendicular to the orientation of second detector arrangement  33   b.    
       FIG. 25  shows a velocimeter  100 , wherein a semi-transparent beam splitter  164  similar to the one in  FIGS. 21 and 24  divides the signal beam  28  and the reference beam  18  so that one part of the beams  18 ,  28  propagate towards the first detector arrangement  33   a  comprising a first optical array  112   a  directing the beams at the detector elements  126   a ,  128   a  and the other part of the beams  18 ,  28  propagate toward the second detector arrangement  33   b  comprising the detector array  125   b . A further detector arrangement (not shown) may be added to the detector system  32  for determination of velocities in three dimensions, e.g. with an orientation perpendicular to the orientation of second detector arrangement  33   b.    
     A two-dimensional detector arrangement comprising a two-dimensional detector array  225   bc  as shown in  FIG. 26  may also be applied instead of adding an additional detector arrangement to the detector system  32  in the embodiment shown in  FIG. 25  for determination of the velocity in the third dimension. The two-dimensional detector array  225   bc  is constructed such that it enables determination of the second velocity component by using the second detector elements (exemplified by detector element  226   b ) and the third velocity component by using the third detector elements (exemplified by detector element  226   c ) oriented substantially perpendicular in relation to the second detector elements. In this way, the light incident on specific parts  270   a ,  270   b ,  270   c ,  270   d  of the detector elements is used both for the determination of the second and the third velocity component. This provides for a compact solution, wherein the double utilization of the light increases the signal-to-noise ratio. 
     In the shown example of the detector array  225   bc , a detector circuit the output signal  250   a ,  250 ′ a ,  250   b ,  250 ′ b  and the difference spectrum  254 ,  254 ′ are generated as shown and explained in  FIGS. 7-12 . Different signal processing configurations such as those shown and explained in  FIGS. 13-18  could also be used. 
     The detector array  225   bc  may in one example be a complementary metal-oxide-semiconductor (CMOS), possibly coupled to a high-resolution CCD (charge-coupled device) camera. 
       FIG. 27  shows a vector velocimeter  100 , wherein the configuration of the optics before the detector system  32  comprising the detector array  225   abc  is similar to the one shown and explained in  FIG. 23 . In the detector array  225   abc , shown in detail in  FIG. 28 , additional detector elements  226   a  has been added to the detector array  225   bc  shown and explained in  FIG. 26 . The additional detector elements  226   a  are for detection of the fringe movement and therefore oriented such that they form a substantially 45 degree angle with the detector elements for detection of the speckle movement (exemplified by detector elements  226   b ,  226   c ). This enables determination of velocities in three dimensions using only one integrated detector array  225   abc , and provides for an even more compact solution, wherein the multiple utilization of the light increases the signal-to-noise ratio. 
     The detector elements (exemplified by detector element  226   a ) used for detection of the fringe are normally high-resolution detector elements. 
     REFERENCE LIST 
     
         
           1  conventional LIDAR system 
           2  laser 
           3  light beam 
           4  beam splitter 
           5  reference beam 
           6  measurement beam 
           7  imaging optics 
           8  measurement volume 
           9  object 
           10  signal beam 
           11  LIDAR detector 
           12  laser assembly 
           14  first coherent light beam 
           16  beam splitter 
           18  reference beam 
           20  measurement beam 
           22  optical transmitter 
           24  measurement volume 
           26  object 
           28  signal beam 
           30  optical receiver 
           32  detector system 
           33  detector arrangement 
           33   a  first detector arrangement 
           33   b  second detector arrangement 
           33   c  third detector arrangement 
           34  angle between signal beam ( 28 ) and reference beam ( 18 ) 
           35   a  first angle between signal beam ( 28 ) and reference beam ( 18 ) 
           35   b  second angle between signal beam ( 28 ) and reference beam ( 18 ) 
           35   c  third angle between signal beam ( 28 ) and reference beam ( 18 ) 
           36  fringe pattern 
           38  speckle pattern 
           100  vector velocimeter 
           112  optical array 
           112   a  first optical array 
           112   b  second optical array 
           114  input plane 
           115  output plane 
           116  direction of propagation of the incoming light ( 18 ,  28 ) 
           118  cylindrical lenses 
           120  redirected light 
           122  lens 
           122   a  first lens 
           122   b  second lens 
           122   c  third lens 
           124  light propagation towards the detector array ( 125 ) 
           125  detector array 
           125   a  first detector array 
           125   b  second detector array 
           125   c  third detector array 
           126  detector element 
           128  detector element 
           130  detector element 
           132  area of the input plane 
           134  area of a detector element 
           136  area of the input plane 
           138  a distance in the input plane 
           140  longitudinal direction of the measurement volume ( 24 ) 
           150  a first output signal 
           152  a second output signal 
           154  difference spectrum 
           154   a  difference spectrum 
           154   b  difference spectrum 
           156  power spectrum 
           158  low frequency part of the power spectrum ( 156 ) 
           160  second harmonic part of the power spectrum ( 156 ) 
           162  phase plot 
           166  mirror 
           168  mirror 
           225   bc  two-dimensional detector array 
           225   abc  three-dimensional detector array 
           226   a  detector element 
           226   b  detector element 
           226   c  detector element 
           250   a  output signal 
           250 ′ a  output signal 
           250   b  output signal 
           250 ′ b  output signal 
           254  difference spectrum 
           254 ′ difference spectrum 
           270   a  part of a detector element 
           270   b  part of a detector element 
           270   c  part of a detector element 
           270   d  part of a detector element