Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
  • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
  • G01S7/481—Constructional features, e.g. arrangements of optical elements
  • G01S7/4817—Constructional features, e.g. arrangements of optical elements relating to scanning
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
  • G01S17/88—Lidar systems specially adapted for specific applications
  • G01S17/95—Lidar systems specially adapted for specific applications for meteorological use
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
  • F05B2270/00—Control
  • F05B2270/30—Control parameters, e.g. input parameters
  • F05B2270/32—Wind speeds
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
  • F05B2270/00—Control
  • F05B2270/80—Devices generating input signals, e.g. transducers, sensors, cameras or strain gauges
  • F05B2270/804—Optical devices
  • F05B2270/8042—Lidar systems
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
  • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
  • G02B26/0816—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements
  • G02B26/0833—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
  • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
  • G02B26/10—Scanning systems
  • G02B26/101—Scanning systems with both horizontal and vertical deflecting means, e.g. raster or XY scanners
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
  • G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
  • G02B26/10—Scanning systems
  • G02B26/105—Scanning systems with one or more pivoting mirrors or galvano-mirrors
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
  • Y02A90/00—Technologies having an indirect contribution to adaptation to climate change
  • Y02A90/10—Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation

Definitions

• the present invention relates to an eye-safe LIDAR system, more specifically a LIDAR system with a micro-electro-mechanical system (MEMS) used for spatially dithering an output beam.
• MEMS micro-electro-mechanical system
• a LIDAR system transmits at least one laser beam to a probed target.
• the probed target can either be a single solid object or an ensemble of minute objects or particulates in fluid suspension.
• the LIDAR also receives light emanating from the probed target (usually back-reflected light) which, in combination with particular optoelectronic detection and signal processing means, enables it to deduce some properties of the probed target such as position, speed, reflectance, absorption, concentration, temperature, etc.
• the laser beam transmitted from the LIDAR can either be divergent, collimated, or convergent (that is, focused at a certain distance).
• MPE is "the highest power (or energy) density in W/m 2 (or J/m 2 ) of a light source that is considered safe", i.e. that has a negligible probability for creating damage. It is usually about 10% of the dose that has a 50% chance of creating damage under worst-case conditions.
• the transmitted beam is focused at a remote distance R such that the 1/e 2 beam radius ⁇ ⁇ at the focal plane is significantly smaller than the beam radius ⁇ ⁇ at the plane of the exit-pupil.
• the back-reflection from the airborne particles that serve as tracers of the wind field being probed is very small. This means that a relatively high power laser beam in the order of few hundreds of milliwatts (the precise amount depends on the concentration of airborne particles) is typically required to detect the Doppler shift from a sufficient amount of back-reflected signal that overcomes the noise level and thereby deduce radial wind speed.
• One way to reduce the mean power density for a given exposure time for the purpose of satisfying the eye safety requirement is to dither or scan the laser light that is emitted from a LIDAR system, for example by rotating a wedge or a mirror.
• the wedge or the mirror is set up to direct a beam into a telescope in different degrees of inclination to produce the spatial dither of the beam at the focal plane.
• rotating a wedge or a mirror requires a rather great power in particular because the wedge or mirror has a great mass that needs to be moved and/or rotated.
• directing a beam into a telescope using a wedge or a mirror is rather inefficient in terms of how the
• An advantage of the present invention over known LIDAR systems having reduced mean power is that it provides a LIDAR system where the at least one reflecting element is only required to move very little in comparison to systems where the scanning member, i.e. wedge mirror or, is placed outside the telescope.
• the present invention provides an efficient dithering of an output beam in a LIDAR setup.
• efficient dithering is also achieved by providing that that the inner point of focus is located before or after the at least one reflecting element.
• the inner point of focus is located before or after the at least one reflecting element.
• the output beam is easily moved.
• the present invention provides thus several advantages as can be seen directly from the formula, and as will be described in the following.
• the lateral displacement, or dithering distance is proportional to the distance by which the inner point of focus lies from the plane of the at least one reflecting element, more specifically to a point in the plane of the at least one reflecting element, wherein that point would have been the inner focus point on the at least one reflecting element if the inner point of focus would lie on the plane of the at least one reflecting element.
• the lateral displacement, or dithering distance is inverse proportional to the focal length of the one or more outer optical elements.
• the lateral displacement, or dithering distance is the lateral distance from the outer point of focus.
• Fig. 1 shows one embodiment of an eye-safe LIDAR according to the present invention.
• Fig. 2 shows a second embodiment of an eye-safe LIDAR according to the present invention.
• Fig. 3 shows a third embodiment of an eye-safe LIDAR according to the present invention. Detailed description of the invention
• the present disclosure relates to a LIDAR system to reduce the mean power density for a given exposure time and for the purpose of satisfying the eye safety requirement.
• the LIDAR system as disclosed herein may be a coherent Doppler LIDAR system.
• the system transmits a light beam and receives a part of backscattered light from a target, such that the backscattered light is coherently superpositioned with a reference beam generated by a local oscillator.
• the LIDAR system may comprise a local oscillator.
• the local oscillator may be comprised of optical elements, such as a reference wedge, for example responsible for generating a reflected signal.
• the backscattered light may be received with the reference beam on a detector, from where the line-of-sight or radial speed of the target may be deduced.
• the backscattered light may be a Doppler-shifted target signal and an unshifted reference signal, i.e. the detector may receive a Doppler spectrum from which an analysis, such as a frequency analysis, can be performed, for example in a signal processor.
• an analysis such as a frequency analysis
• the LIDAR system further comprising an optical circulator comprising at least two ports configured to be in optical connection with at least the beam-generating section and the beam-steering element.
• a first port is connected to the beam generating section, and a second port is where the output beam is transmitted from and further into the MEMS via the inner optical element.
• the beam Prior the MEMS, the beam may have passed through a local oscillator generating optics, such as a reference wedge, when setup as a coherent LIDAR Doppler LIDAR.
• the optical circulator comprises three ports, such that a third port is in optical connection with a detector.
• the third port may be where the Doppler- shifted target signal and the reference signal is transmitted to the detector.
• the inner point of focus is located before or after the at least one reflecting element.
• the inner point of focus does not lie on a plane of the at least one reflecting element.
• lateral dithering converges to zero as the defocus approaches zero.
• a very inefficient dithering system would be provided if the defocus was zero. Due to a point source having a diffraction-limited size, it would be possible to move the output beam when the inner point of focus does lie on a plane of the at least one reflecting element, i.e. when the focus spot is diffraction limited.
• the inner point of focus is located at a focal plane such that the output beam from the inner optical element is at least partly truncated by an exit-pupil of the beam-focusing optical unit.
• an exit-pupil of the beam focusing optical unit may be the one or more outer optical elements, for example the clear aperture of the one or more outer optical elements, or a circular exit-pupil defined elsewhere in the beam-focusing optical unit.
• the truncation by the exit-pupil of the beam-focusing optical unit may be the dominant truncation.
• the inner point of focus is located at a focal plane such that the output beam from the inner optical element is at least partly truncated by the at least one reflecting element.
• the tails of the beam may be truncated by the at least one reflecting element.
• the beam generating section is adapted for generating a collimated output beam toward the inner optical element.
• this may facilitate that the inner optical element is focusing the collimated beam towards the beam-steering element, however without focusing the output beam on the beam-steering element, or on the at least one reflecting element.
• the inner optical element is configured with a positive focal length.
• the output beam is redirected to the one or more outer optical elements such that output beam, as defined by a measure of radius, is at least equal to 80% of a radius of the one or more outer optical elements or a radius of the exit-pupil of the beam-focusing optical unit.
• the measure of radius may be defined in many ways, but in common practice, a typical measure of radius is the 1/e 2 beam radius.
• the output beam significantly fills the exit lens, i.e. the one or more outer optical elements, to achieve tight focusing at remote distance R. Under this condition, there is provided a specific optimal LIDAR condition, called optimal heterodyne efficiency.
• the first movement pattern corresponds to the at least one reflecting element being switched with at least angular positions, whereby said outer point of focus is at least laterally shifted by a lateral displacement.
• the first movement pattern corresponds to the at least one reflecting element being switched with angular positions differing by less than 1 degree.
• such an embodiment may be achieved by properly choosing the defocus, the focal length of the one or more outer optical element, and the distance to the outer focus point from the one or more outer optical elements.
• the defocus may be between 1 mm and 20 mm, such as between 2 mm and 10 mm, such as between 4 mm and 5 mm.
• the lateral displacement depends on three factors, the defocus, the focal length of the one or more outer optical elements, and the distance to the outer focus point from the one or more outer optical elements, also called the far focus, all of the three factors can contribute to the realization of the embodiment.
• the far focus may be between 30 m and 200 m, such as between 50 m and 100 m, such as 60 m, 70 m, 80 m, or 90 m, preferably around 60 m.
• the one or more outer optical elements are configured with a positive focal length.
• the focal length of the one or more outer optical elements may be between 100 mm and 500 mm, such as between 150 mm, and 250 mm, such as between 200 mm and 220 mm.
• there is provided a lateral displacement of 7 mm using only 0.15 degrees angular deflection of the at least one reflecting element.
• the angular deflection is a difference in angular positions.
• the first movement pattern corresponds to the at least one reflecting element being switched with angular positions differing such that a ratio between a lateral displacement of the outer point of focus and the difference in angular positions is more than 1 mm/mrad, such as more than 2 mm/mrad, such as more than 2.5 mm/mrad, such as 2.7 mm/mrad, or such as more than 3.0 mm/mrad.
• the spatially dithering is applied for a period of time, wherein the period of time corresponds to a spatial dithering pattern.
• the spatial dithering pattern may be a pattern that reduces the overall radiation exposure, such as spiral or a set of concentric rings or a random pattern.
• the eye-safe LIDAR system further comprises two or more outer optical elements and wherein the MEMS and the at least one reflecting element is further capable of switching the output beam from the inner optical element in a second movement pattern interchangeably between said two or more outer optical elements.
• the MEMS and the at least one reflecting element is further capable of switching the output beam from the inner optical element in a second movement pattern interchangeably between said two or more outer optical elements.
• the first movement pattern is different from said second movement pattern.
• the at least one reflecting element when switching the output beam from the inner optical element in a second movement pattern interchangeably between said two or more outer optical elements, the at least one reflecting element may be switched with angular positions differing by more than 1 degree.
• the first movement pattern corresponds to the outer point of focus being spatially dithered in a pattern such as a spiral pattern or a pattern with a set of concentric rings or a random pattern.
• an eye-safe LIDAR system where it is possible to move or scan at least one reflecting element in a fine way, i.e. positioning it with angular positions that differ less than 1 degree, and additionally in a coarse way, i.e. positioning it with angular positions that differ more than 1 degree.
• Such an eye-safe LIDAR system provides a very efficient way of providing eye-safe operation, whilst also providing the ability measure several wind velocity components that fully resolves not only the wind velocity magnitude but also the vector's direction.
• Fig. 1 shows a beam-generating section 1 , (e.g. laser + optical circulator + detector); an output beam 2; a MEMS scanning mirror (SM) 3 (two angular positions are shown); a beam-focusing optical unit 4, comprising an inner optical element (positive lens) 4a; and an outer optical element (positive lens) 4b; an optical axis 5 defined by the two optical elements 4a and 4b or just the line normal to exit lens 4b.
• the inner point of focus formed by the inner optical element 4a occurs before reflection of the output beam by the MEMS-(SM).
• Example 2 An eye-safe LIDAR according to the present invention
• Fig. 2 shows a beam-generating section 1 , (e.g. laser + optical circulator + detector); an output beam 2; a MEMS scanning mirror (SM) 3 (two angular positions are shown); a beam-focusing optical unit 4, comprising an inner optical element (positive lens) 4a; and an outer optical element (positive lens) 4b; an optical axis 5 defined by the two optical elements 4a and 4b or just the line normal to exit lens 4b.
• An additional outer optical element 6 is further shown.
• a new optical axis 7 defined by the line normal to the additional optical element 6.
• the combination of the inner optical element 4a and the additional outer optical element 6 form a beam-focusing optical unit through which the beam is steered by sufficiently large deflection of the MEMS-SM and spatially dithered by appropriately small deflection of the MEMS-SM.
• Example 3 An eye-safe LIDAR according to the present invention

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Citations (4)

• 2016
  • 2016-12-09 WO PCT/EP2016/080416 patent/WO2017097971A1/en active Application Filing

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