CN109814165B - Light power cooling miniaturized high-precision optical gravimeter - Google Patents
Light power cooling miniaturized high-precision optical gravimeter Download PDFInfo
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Abstract
The invention discloses a light force cooling miniaturized high-precision optical gravimeter. The device comprises a vacuum cavity, a release device, a laser cooling cavity, an optical detection cavity and a micro-nano small ball; the laser cooling cavity and the optical detection cavity are respectively positioned above and below the middle part of the vacuum cavity; the laser cooling cavity comprises two lenses, and two beams of horizontal parallel light adopt two vertical polarization states output by the same laser; the focal points of the two lenses are superposed and form a light well region at the focal points; the optical detection cavity comprises two lenses, and the focal points of the two lenses are superposed to form a detection area at the focal point; the releasing device is provided with a releasing outlet, a cooling light source is arranged outside the right lower part of the vacuum cavity, and the cooling light source emits cooling light upwards to irradiate the light well region. According to the invention, the micro-nano small balls are used as a measurement carrier, and the laser cooling technology is combined, so that the release position and the capture position of the small balls are accurately measured to obtain the accurate value of the gravitational acceleration, the environmental interference is eliminated, the gravitational acceleration measurement accuracy is improved, and the measurement speed and efficiency are improved.
Description
Technical Field
The invention relates to a light force cooling miniaturized high-precision optical gravimeter, belongs to the technical field of precision measurement, and belongs to a sensor and an instrument for measuring a gravity acceleration value.
Background
The gravimeter is a precise instrument for measuring gravity acceleration (gravity), can be widely applied to the application fields of earth exploration, mine exploration, exploration and the like, and has important significance for the development of national economy. The basic measurement principle of a gravimeter is that the free fall time dt and the distance s are passed through the proof mass according to newton's second law, then according to g-2 s/t2A measured value g of the gravitational acceleration is obtained.
The initial speed of the mass block caused by the releasing process of loosening the mechanical fixing structure in the free falling process of the mass block and the disturbance of gas disturbance in the free falling process of the mass block can influence the measuring precision due to the surface shape of the adopted mass block object, and further influence the actual gravity measuring precision.
The existing gravimeter generally adopts a measuring distance from decimeter to meter to obtain high measuring precision, and adopts a data fitting mode to reduce the influence of the initial speed of the mass block in free falling, and simultaneously reduces the disturbance of air to the movement of an object through a vacuum system, thereby comprehensively improving the measuring precision of the gravimeter, but the traditional gravimeter needs to mechanically fix the mass block before the measurement is started, and the uncontrollable and unpredictable initial speed is easily introduced in the releasing process; meanwhile, the mass block needs to be reused, and after a measurement process, the mass block needs to be reset by a reset mechanism, so that the problems of low measurement efficiency, large and complex system, limited measurement speed and the like are caused.
The optical suspension system is a new subject established on the basis of laser control, laser cooling, laser capture and other technologies, and suspends micro-nano spheres (typically micro-nano spheres) under the action of the optical force of a laser beam, supports particles by utilizing the optical force and replaces a mechanical support structure; therefore, the interaction between the particles and the environment is isolated, and the interference of various environmental factors such as vibration, heat conduction and the like on the motion of the particles is avoided, so that better anti-interference performance can be obtained if the micro-nano small balls are used as mass blocks in measurement sensing, and further, the performance precision is better.
The suspended particles in the optical force suspension system adopt micro-nano spheres, an optical trap is formed by two beams of light which are transmitted in opposite directions, the particles are captured by utilizing the gradient force and the scattering force of the optical trap, and the particles can be suspended by resisting the earth attraction through the gradient force of the optical trap, so that no mechanical contact exists between the suspended particles and the external environment. The suspended micro-nano small ball is used as a mass block in the gravity meter measurement, so that in the releasing process, only the laser power of the suspended optical trap needs to be controlled to be reduced to 0, the optical trap force disappears at once, and compared with mechanical contact, unpredictable random initial speed cannot be introduced into the mass block by the method, so that the measurement precision can be improved.
The micro-nano small balls captured by laser can be processed according to the harmonic oscillator, and for the harmonic oscillator in a free state, the harmonic oscillator is in a random motion state even when the harmonic oscillator is not in external contact due to internal thermal motion of the harmonic oscillator, and the harmonic oscillator generally adopts the principle that the equivalent temperature is high or lowThe strength of this motion state is described. In an optical suspension system, a set of laser cooling system is usually adopted to perform feedback control on the motion of the micro-nano balls captured by laser, so as to reduce the random motion amplitude, and the process is called laser cooling. The typical laser cooling system comprises a small ball position information sensing unit, an electrical information processing system and a feedback system, wherein the cooling light can adopt a wavelength different from that of the capture light, and the capture and particle cooling can be simultaneously realized by adopting the same optical path. According to the current technology, the displacement of the random movement of the particles can be controlled to picometers (pm: 10) by the laser cooling technology-12Meters), equivalent temperature is close to milli-kelvin (10)-3On). The cooling technology is applied to the gravimeter, so that the displacement initial value of the mass block of the free falling body, namely the micro-nano small ball when released, can be greatly improved, the error items in the processes of measuring, calculating and calibrating the displacement of the free falling body are reduced, and the measuring precision of the gravimeter can also be improved; the same position information measuring method in the laser cooling technology is also used for realizing the position information measuring method better than picometer (pm: 10)-12Meters) so that accurate measurement of the start and end positions can be achieved.
In addition, the micro-nano small ball has a very small volume which is far smaller than the pyramid edge angle adopted in most of the prior gravimeters, can be arranged in a container in advance and matched with a corresponding release device, such as high-energy laser emission, ultrasonic emission and other modes, and is released when in application and captured by laser, and the mode eliminates the process that the pyramid edge angle needs to be reloaded and reset in one test in the traditional gravimeter, so that the feasibility is high. In addition, the micro-nano small balls typically adopted are silicon materials, the diameter is between 100nm and 10um, and if 1 small ball is adopted in one test, the theoretical diameter is 1mm3The volume of the micro-nano globule can be realized by 106～1012And (6) testing. The measuring mass block does not need to be repeatedly reset and utilized, the testing speed is improved, and a time blind area in the testing process is avoided.
The existing gravity measurement mode mainly utilizes a mode of mechanically releasing a mass block, and realizes repeated measurement of gravity by utilizing the reset of the mass block, the measurement mode has the defects of complex reset, large volume, multiple error factors and the like, and a new gravimeter technology is developed on the basis of the new technologies according to the current optical technology development, particularly the technical development in the field of optical force and optical trap, so that the measurement precision of the gravity is improved, the complexity of a system is reduced, and meanwhile, the high-precision rapid measurement mode of the gravity is realized, thereby meeting the urgent requirements of the fields of inertial navigation, geodetic measurement, mineral product general survey and the like on the measurement of the gravity angular velocity.
Disclosure of Invention
Aiming at the technical current situation of gravity measurement, the current gravimeter is large in size, a measuring mass block needs to be mechanically supported and released, is easy to be interfered, needs to be reset and is difficult to continuously measure, and the invention provides the light force cooling miniaturized high-precision optical gravimeter by utilizing the technology in the new fields of quantum theory, micro-nano technology, light force technology and the like.
The basic principle of the gravimeter is relatively simple, and the current core difficulty and key technology are that in order to control several major error sources, the influence of the environment on the measurement is not negligible in practical application, and the major resulting core errors include:
the first technical problem is the influence of air resistance. Due to the existence of air molecules in the system, the mass block is influenced by air flow and Brownian motion of the air molecules in the descending process, and the actual mass block is not only subjected to gravity acceleration but also subjected to the reaction force of the air. This problem can be solved by a vacuum system, by which the gas in the environment during the free fall of the mass is removed, so that this error source can be reduced considerably.
A second technical problem is the environmental disturbance of the mass. In a conventional gravimeter, the measuring mass (usually a pyramid corner angle) is fixed by a mechanical structure and released by the mechanical structure at the beginning of the measurement. In this way, due to the existence of mechanical contact environment, the initial speed of the mass block is nonzero in the middle century of the release process, the speed direction is random and uncontrollable, and modeling elimination cannot be realized, which is a main error source.
In the invention, the mass block is supported in an optical force mode (micro-nano balls are used as the mass block in the invention). According to the theory related to quantum optics, how photons are incident on an object with momentum generates a pressure on the object, called optical pressure, and the force is related to the intensity of light, the power density and the irradiation area, but the force is also quite small. With the development of laser technology, a high-power and high-power-density optical field can be realized, so that a larger optical pressure can be generated, calculation can be performed, for small balls with the diameter of 100 nm-10 um, the pressure generated by mW laser can be used for realizing the force equivalent to gravity, so that an optical trap can be formed by using a capture light beam in vacuum, the small balls are suspended (i.e. the small balls are captured) by using the gradient force or the scattering force of the optical trap, the small balls do not have any other mechanical contact in the capturing process, and the release of the small balls can be realized by laser turn-off, so that the initial speed of the small balls caused by the mechanical contact can be eliminated.
The micro-nano small spheres collide with air molecules in the air to generate momentum exchange, so that the micro-nano small spheres can make Brownian motion, the motion is sometimes called as thermal motion, and the motion can also influence the position and the initial speed value of the micro-nano small spheres to a certain extent. In the prior art, the thermal motion can be reduced by reducing the molecular density through a vacuum system. The thermal movement of the pellets caused by the residual molecules in the vacuum system can be cooled by a vacuum pellet cooling system. By adding a cooling light and a motion measurement system outside the captured light, the position and the motion measurement of the micro-nano small ball are monitored constantly, the cooling light beam is used for controlling, the impulse of air molecules is offset, and the position fluctuation and the speed of the small ball are reduced, so that the laser cooling of the micro-nano small ball is realized, and the initial position precision of the micro-nano small ball is improved.
At present, the cooling technology of the micro-nano ball is relatively mature, and the relevant documents in the field of optical force can be referred. The micro-nano small ball thermal motion cooling can reach the mK temperature magnitude, and the position precision of the mass center of the micro-nano small ball can be controlled to be superior to the pm magnitude, so that the small ball can realize high-precision initial value positioning, the position error is reduced, and the gravity measurement precision is improved.
The method adopting the technical scheme comprises the following steps:
the gravity meter comprises a vacuum cavity, a release device, a laser cooling cavity, an optical detection cavity and a micro-nano small ball, wherein the release device, the laser cooling cavity, the optical detection cavity and the micro-nano small ball are positioned in the vacuum cavity; the release device is positioned at the top of the vacuum cavity, the micro-nano balls are loaded in the release device, and the laser cooling cavity and the optical detection cavity are respectively positioned above and below the middle part of the vacuum cavity;
the laser cooling cavity is internally provided with a laser capturing device of a micro-nano small ball, the laser capturing device comprises two lenses L1 and L2 which are arranged in a way of being opposite to each other horizontally and in the same optical axis, and two beams of horizontal parallel light which are respectively incident from two sides of the two lenses L1 and L2 are used as capturing light, and the two beams of horizontal parallel light adopt two vertical polarization states output by the same laser; focal points of the two lenses L1 and L2 are superposed, an optical trap area for capturing the micro-nano small ball is formed near the focal point, and the optical trap area is used as an initial position area of the micro-nano small ball which freely falls to the ground;
the optical detection cavity is internally provided with a small ball detection device of a micro-nano small ball, the small ball detection device comprises two lenses L3 and L4 which are horizontally arranged opposite to each other along the same optical axis, and a beam of horizontal light which is respectively incident from the side of one of the two lenses L3 and L4 and is used as detection light, the focal points of the two lenses L3 and L4 are overlapped and form a detection area of the position of the micro-nano small ball near the focal point, and the detection area is used as an end position area of the free landing of the micro-nano small ball;
the release device is provided with a micro-nano small ball release outlet, the light trap area in the laser cooling cavity is positioned right below the micro-nano small ball release outlet of the release device, and the detection area in the optical detection cavity is positioned right below the light trap area in the laser cooling cavity; and a cooling light source is arranged outside the vacuum cavity right below the detection region, and the cooling light source upwards emits cooling light to irradiate the light well region.
A Polarization Beam Splitter (PBS) is arranged outside the vacuum cavity beside one lens L1 of the two lenses L1 and L2, and a first position detection sensor (QPDA) is arranged below the Polarization Beam Splitter (PBS); a horizontal light beam horizontally enters the polarization beam splitter Prism (PBS) from the side of the polarization beam splitter Prism (PBS), is transmitted by the polarization beam splitter Prism (PBS), enters the lens L1 and is converged to a focus through the lens L1; another beam of horizontal light horizontally enters the lens L2 from the side of the lens L2, is converged to a focal point through the lens L2, then enters the polarization beam splitter Prism (PBS) after being recovered and expanded through the lens L1, and enters the first position detection sensor (QPDA) after being reflected by the polarization beam splitter Prism (PBS) to be detected and received; and detecting by a first position detection sensor (QPDA) to obtain the position information of the micro-nano small ball deviating from the center of the optical trap along the radial direction of the horizontal optical axis.
A reflector is arranged outside the vacuum cavity at the side of one lens L4 of the two lenses L3 and L4, and a second position detection sensor (QPDB) is arranged below the reflector; a beam of horizontal detection light horizontally enters a lens L4 from the side of a lens L4 which is not provided with a reflector on the side, is converged to a focal point through a lens L4, is used as a detection center at the focal point, enters the reflector after being recovered and expanded through a lens L3, and is reflected to a second position detection sensor (QPDB) through the reflector to be detected and received; and detecting by a second position detection sensor (QPDB) to obtain the position information of the micro-nano small ball falling to the position deviated from the detection center along the radial direction of the horizontal optical axis.
The two lasers for capturing the light source in the laser capturing device are 1064nm lasers, and the cooling light source specifically adopts 532nm laser.
The release device adopts piezoelectric ceramics (PZT), and micro-nano small spherical particles which are placed on the PZT in advance are sequentially released in a single particle mode through vibration of the PZT.
The device is characterized by further comprising an information processing circuit, wherein the information processing circuit is respectively connected with the first position detection sensor (QPDA), the second position detection sensor (QPDB) and the piezoelectric ceramics of the release device.
The micro-nano small spheres are micro-nano particles made of quartz materials.
The vacuum cavity is provided with a glass transparent hole, and the glass transparent hole allows the injection and the output of laser beams of captured light, detection light and cooling light.
The optical gravity test process of the gravimeter is as follows:
firstly, sequentially releasing micro-nano pellets in a single particle mode through a releasing device, starting two beams of trapping light and cooling light, enabling each micro-nano pellet to fall to a light well region and be stabilized at the center of the light well, and detecting and recording the position height of the micro-nano pellet as a first height value through one beam of trapping light;
then, simultaneously cutting off the two beams of captured light and cooling light and starting timing, simultaneously eliminating the light trap scattering force and gradient force borne by the micro-nano spheres and the feedback force of the cooling light at the cutting-off moment, and enabling the micro-nano spheres to freely fall to the optical detection cavity along the gravity direction under the action of the gravity field only under the action of gravity;
and then, when the micro-nano ball approaches or reaches the optical detection cavity, starting detection light to detect and record the position height of the micro-nano ball as a second height value, finishing timing, and calculating the gravity acceleration by using the first height value, the second height value and timing time difference.
The optical gravity test process of the gravimeter is as follows:
1) for one micro-nano small ball released by the releasing device each time, obtaining a gravity acceleration value g (n) by adopting the following mode, wherein n represents the ordinal number of the test times;
1.1) after the micro-nano small balls are released by a releasing device, the micro-nano small balls fall to an optical trap area of a laser cooling cavity along gravity under the action of gravity;
1.2) starting a laser cooling cavity to capture light, capturing the micro-nano spheres near the center of an optical trap by using an optical trap region formed by the captured light under the action of gradient force and scattering force in the optical trap region, and detecting by using a beam of captured light to obtain the height position of the micro-nano spheres in the optical trap region and recording as 0;
1.3) starting cooling light, and performing laser closed-loop feedback cooling on the micro-nano small balls by using the cooling light to reduce the thermal motion of the small balls;
1.4) after the thermal motion of the micro-nano small balls is reduced and reaches a preset target, simultaneously cutting off the capture light and the cooling light, starting timing from the cut-off time and setting a target timing duration dTg (n),
1.5) after the turn-off time, the micro-nano small ball freely falls to the ground under the action of a gravity field, vertically accelerates along a gravity vertical line until reaching an optical detection cavity, starts detection light after a target timing duration dTg (n), and obtains the height position Z (n) of the micro-nano small ball in a detection area by utilizing the detection light;
1.6) obtaining a gravity acceleration value g (n) as a single measurement output of the optical gravimeter by adopting the following formula:
g(n)＝2dZ/[dTg(n)]2
wherein dZ represents the height position of the detection center relative to the center of the optical trap;
2) continuously repeating the step 1) to realize the continuous measurement value g (n) of the gravity acceleration; and after obtaining the gravity acceleration value g (n) each time, according to the relationship between the height position Z (n) of the micro-nano small ball and the height position dZ of the detection center, simultaneously updating the target timing duration dTg (n +1) of the next measurement according to the following formula:
dTg(n+1)＝dTg0when n is 0
dTg(n+1)＝dTg(n)-dt0When n ≠ 0, and Z (n)<When dZ is
dTg(n+1)＝dTg(n)+dt0When n ≠ 0, and Z (n)>When dZ is
dTg (n +1) ═ dTg (n), when n ≠ 0, and Z (n) ═ dZ
Therein, dt0Indicating the adjustment amount of the target time period, dTg0An initial value of a target timing period dTg (n) is obtained based on a local gravitational acceleration estimated value g at a measurement position0Calculated according to the following formula:
dTg0＝(2dZ/g0)1/2
wherein, g0Representing a local gravitational acceleration estimate.
The gravity meter provided by the invention utilizes the micro-nano small ball as a measurement carrier, and combines a laser cooling technology to accurately measure the release position and the capture position of the small ball so as to obtain an accurate value of the gravity acceleration. The gravimeter adopts a dual-cavity structure, wherein the first optical cavity realizes laser cooling of the small ball, the initial position precision of the small ball is improved, the second optical cavity realizes accurate extraction of relative position information of the small ball, the falling timing time is updated in real time by combining the position distance of the two optical cavities, and the gravity acceleration is calculated.
The small ball in the gravity meter is cooled by laser in the first cavity, so that the position uncertainty caused by self movement is reduced, the small ball freely falls in the vacuum cavity after being released, the environmental interference is eliminated, no mechanical support change exists in the releasing process, and no initial speed error exists; the second cavity only provides relative position information of the small ball, interference is not introduced in the vertical direction, the gravity acceleration measurement precision is improved, and the measurement speed and efficiency are improved.
The invention has the beneficial effects that:
the invention provides a scheme of a double-cavity optical gravimeter for the first time, a micro-nano ball is used as a mass block for gravity measurement, the laser capture technology is used for eliminating the influence of mechanical release on the initial speed of the measurement mass block, a vacuum system is used for reducing the interference of air on the free falling body movement of the micro-nano ball, the laser cooling technology is used for eliminating the influence of the initial speed displacement caused by the self thermal movement of the micro-nano ball, and the continuous release mode of the micro-nano ball is used for avoiding the process of reloading and resetting the mass block between the front measurement and the back measurement of the mass block, so that the double-cavity.
The invention utilizes the light force support to replace the existing mechanical support, replaces the influence of mechanical release by light beam control, and simultaneously eliminates the interference of the thermal motion of air molecules on the measurement of the micro-nano small ball by a vacuum system, thereby realizing the high-precision gravity rapid measurement.
The scheme provided by the invention fully utilizes the high-precision characteristic of the light-force suspension system, is expected to become a small-sized high-precision wide-bandwidth gravity measurement instrument, provides a brand-new technical approach for gravity measurement, is expected to promote the development of the technical field of gravity measurement, and promotes the popularization and application of the gravity instrument in the fields of prospecting, geological survey and the like.
Drawings
FIG. 1 is a schematic structural diagram of a laser capturing device of a laser cooling cavity for capturing micro-nano pellets;
FIG. 2 is a schematic diagram of the complete structure of the optical gravimeter;
FIG. 3 is a flowchart of a gravitational acceleration test;
FIG. 4 is a graph of the results of the test data of the examples.
Detailed Description
The invention is further illustrated by the following examples in conjunction with the accompanying drawings:
as shown in fig. 2, the gravimeter includes a vacuum cavity, and a release device, a laser cooling cavity, an optical detection cavity and a micro-nano ball which are located in the vacuum cavity; the releasing device is positioned at the top of the vacuum cavity, the initial micro-nano small balls are loaded in the releasing device, the laser cooling cavity and the optical detection cavity are respectively positioned above and below the middle part of the vacuum cavity, and the releasing position of the releasing device, the laser cooling cavity and the optical detection cavity are sequentially arranged from top to bottom along the vertical direction of gravity;
as shown in fig. 2, a laser capturing device with micro-nano beads is arranged in a laser cooling cavity, and the laser capturing device includes two lenses L1 and L2 arranged opposite to each other in the same horizontal optical axis, and two horizontal parallel lights respectively incident from two sides of the two lenses L1 and L2 as captured lights, and the two horizontal parallel lights adopt two vertical polarization states output by the same laser; focal points of the two lenses L1 and L2 are superposed, an optical trap area for capturing the micro-nano small ball is formed near the focal point, and the optical trap area is used as an initial position area of the micro-nano small ball which freely falls to the ground;
as shown in fig. 1, two lenses L1 and L2 focus two flat lights entering from left to right on a focal point position at the center, respectively, and form a light trap region near the focal point, where the focal point is the center of the light trap, and the light trap region at the focal point has a trapping effect on the micro-nano pellet located therein, and traps the pellet at the central position without other external forces, where the central position is the starting position of free landing. As shown in fig. 1, a micro-nano bead is captured at the optical trap region.
When the micro-nano balls leave the center of the optical trap, scattering force and gradient force are applied to the captured micro-nano balls in the laser capture optical trap, and the scattering force and the gradient force point to the center of the optical trap, so that when the micro-nano balls deviate from the center of the optical trap, the micro-nano balls are pulled back to the center of the optical trap by the optical trap area. Wherein the scattering force is a force generated by the micro-nano spheres deviating from the center in the horizontal direction (laser incidence direction), and the gradient force is a force generated by the micro-nano spheres deviating from the center in the vertical direction. The scattering and gradient forces behave like springs in physics.
Under the gravity field, the micro-nano spheres are also under the action of earth stress, and actually deviate from the center downwards, so that an upward gradient force is generated to counteract the gravity of the micro-nano spheres. Therefore, the micro-nano spheres are captured in the optical trap area and are still at the central position of the optical trap under the action of optical force (scattering force and gradient force) and gravity, and the central position of the optical trap is used as the initial position of free landing.
In specific implementation, due to the scattering force action of parallel light symmetrical on two sides of the center of the optical trap and the unequal action of a gravity field and a gradient force, the micro-nano spheres are substantially at the position right above or below the center of the optical trap and are not at the two sides of the center of the optical trap, so that height deviation can be generated.
As shown in fig. 2, a pellet detection device of the micro-nano pellet is arranged in the optical detection cavity, the pellet detection device includes two lenses L3 and L4 arranged opposite to each other along the horizontal optical axis, and a beam of horizontal light incident from the side of one of the two lenses L3 and L4 as detection light, the focal points of the two lenses L3 and L4 are overlapped and form a detection region of the position of the micro-nano pellet near the focal point, and the detection region is used as an end position region where the micro-nano pellet freely falls to the ground;
the release device is provided with a micro-nano small ball release outlet, the light trap area in the laser cooling cavity is positioned right below the micro-nano small ball release outlet of the release device, and the detection area in the optical detection cavity is positioned right below the light trap area in the laser cooling cavity; and a cooling light source is arranged outside the vacuum cavity right below the detection region, the cooling light source upwards emits cooling light to irradiate the light trap region, and the optical axis of the cooling light passes through the center of the light trap region.
Negative feedback control of thermal motion of the micro-nano small balls in the optical trap area is realized through power control of cooling light perpendicular to the direction of the captured light, and therefore the thermal motion speed of the micro-nano small balls in the optical trap area is reduced.
The vacuum cavity provides a vacuum working environment, and the vacuum degree is controlled to be better than 10-6Is within Pa and is released and cooled as micro-nano spheresThe movement interference of air molecule movement on the micro-nano balls is eliminated through a vacuum working environment provided by the vacuum cavity, so that the precision result is measured.
The vacuum cavity is provided with a glass transparent hole which allows the injection and output of laser beams for capturing light, detecting light and cooling light.
As shown in fig. 2, a Polarization Beam Splitter (PBS) is disposed outside the vacuum chamber at the side of one lens L1 of the two lenses L1 and L2, and a first position detection sensor (QPDA) is disposed below the Polarization Beam Splitter (PBS); a beam of horizontal light horizontally enters the polarization beam splitter Prism (PBS) from the side of the PBS, is transmitted by the PBS, enters the lens L1, is converged to a focus by the lens L1, and is emitted after being expanded by the lens L2; another beam of horizontal light horizontally enters the lens L2 from the side of the lens L2, is converged to a focal point through the lens L2, then enters the polarization beam splitter Prism (PBS) after being recovered and expanded through the lens L1, and enters the first position detection sensor (QPDA) after being reflected by the polarization beam splitter Prism (PBS) to be detected and received; and detecting by a first position detection sensor (QPDA) to obtain the position information of the micro-nano small ball deviating from the center of the optical trap along the radial direction of the horizontal optical axis. The distance difference of the micro-nano small balls deviating from the center of the optical trap along the radial direction of the horizontal optical axis is irradiated by the capture light and reflected to an imaging surface of a first position detection sensor (QPDA) through a Polarization Beam Splitter (PBS) so as to obtain the information of the distance difference and further convert the height position of the micro-nano small balls in the optical trap area. The same applies to the second position detection sensor (QPDB).
As shown in fig. 2, a reflector is arranged outside the vacuum cavity at the side of one lens L4 of the two lenses L3 and L4, and a second position detection sensor (QPDB) is arranged below the reflector; a beam of horizontal detection light horizontally enters a lens L4 from the side of a lens L4 which is not provided with a reflector on the side, is converged to a focal point through a lens L4, is used as a detection center at the focal point, enters the reflector after being recovered and expanded through a lens L3, and is reflected to a second position detection sensor (QPDB) through the reflector to be detected and received; and detecting by a second position detection sensor (QPDB) to obtain the position information of the micro-nano small ball falling to the position deviated from the detection center along the radial direction of the horizontal optical axis. The distance difference of the micro-nano small ball deviated from the detection center along the radial direction of the horizontal optical axis is irradiated by the capture light and reflected to the imaging surface of a second position detection sensor (QPDB) through a reflector, so that the information of the distance difference is obtained, and the height position of the micro-nano small ball in the detection area is converted.
The laser cooling cavity consists of a pair of confocal lenses, an input light path of the laser cooling cavity coincides with a glass light transmitting hole of the vacuum device, captured laser and cooling laser are input from the glass light transmitting hole, a light well region is formed at the focus of the confocal lenses through the confocal lenses, and micro-nano microspheres are captured by utilizing gradient force and scattering force in the light well region. The laser cooling cavity detects the specific position of the micro-nano small ball in the light trap area in a non-confocal detection mode, light is captured to form a focused light beam behind the micro-nano small ball after passing through the micro-nano small ball, the power density of the light beam is provided with the position information of the micro-nano small ball, the photoelectric detector is placed in the rear section of the light beam instead of the focus of the light beam, the detection of the power density is finished, and the position information of the micro-nano small ball is extracted from the power density.
The laser cooling cavity and the optical detection cavity are similar in optical configuration and are composed of two confocal lenses, a position detection sensor QPD and a polarization splitting prism/light reflecting mirror. The laser cooling cavity plays a capturing role and needs cooling light; the optical detection cavity has a detection function, cooling light is not needed, and a beam of horizontal detection light incident from the right side is replaced.
The laser of two bundles of catch light sources in the laser catch device is 1064nm laser, and 532nm laser is specifically adopted as the cooling light source.
The release device adopts piezoelectric ceramics (PZT), and micro-nano small spherical particles which are placed on the PZT in advance are sequentially released in a single particle mode through vibration of the PZT. The piezoelectric ceramic vibration release mode is selected, and other release modes such as a high-energy pulse laser release mode and the like can be adopted according to application requirements in practical application, for example, other release modes such as a high-energy pulse laser and the like can be adopted.
The releasing device releases micro-nano small ball particles which are placed on the piezoelectric ceramic in advance through vibration of the piezoelectric ceramic, and then the micro-nano small balls move downwards along the gravity direction to reach a laser capturing device of the laser cooling cavity and are captured in the light well region. The movement from the release process to the process in the optical trap area belongs to the micro-nano small ball preparation stage of measurement, and the process does not participate in actual measurement, so that the process does not influence the measurement precision.
The micro-nano small spheres are sequentially released in a single particle mode by the release device, the micro-nano small spheres are not reused, a resetting process is not needed, different particles are used in each measurement course, the measurement efficiency is improved, and the specific weight and size of the particles do not influence the measurement precision and the measurement speed.
The gravity meter is characterized by further comprising an information processing circuit, wherein the information processing circuit is respectively connected with the first position detection sensor (QPDA), the second position detection sensor (QPDB) and the piezoelectric ceramics of the release device, and the work flow of the whole gravity meter is carried out by the information processing circuit according to a preset work flow sequence.
The working flow control of the gravimeter adopts a unified information processing circuit, the timing reference of the information processing circuit adopts an atomic clock as a clock reference, the clock stability of the atomic clock is selected according to the gravity measurement precision requirement, and the clock stability selection index of a typical atomic clock is superior to 10-10。
The micro-nano spheres are micro-nano particles made of quartz materials, the diameter range is between 100 nanometers and 10 micrometers, and the micro-nano spheres with the diameter range of about 1 micrometer are optimal.
As shown in fig. 3, the optical gravity test process of the gravimeter is specifically as follows:
1) for one micro-nano ball released by the releasing device each time, for example, for the nth test (n is a test ordinal number, and is a natural number from 1), obtaining a gravity acceleration value g (n) in the following manner, wherein n represents the ordinal number of the test times;
1.1) after the micro-nano small balls are released by a releasing device, the micro-nano small balls fall to an optical trap area of a laser cooling cavity along gravity under the action of gravity;
1.2) starting a laser cooling cavity to capture light, capturing the micro-nano spheres near the center of an optical trap by using an optical trap region formed by the captured light under the action of gradient force and scattering force in the optical trap region, and detecting by using a beam of captured light to obtain the height position of the micro-nano spheres in the optical trap region and recording as 0;
1.3) starting cooling light, carrying out laser closed-loop feedback cooling on the micro-nano small balls by using the cooling light, and reducing the thermal motion of the small balls until a preset cooling target is met;
1.4) after the thermal motion of the micro-nano small balls is reduced and reaches a preset target, simultaneously cutting off the capture light and the cooling light, starting timing from the cut-off time and setting a target timing duration dTg (n),
1.5) the scattering force and gradient force of the optical trap and the feedback force of the cooling light suffered by the micro-nano ball disappear at the turn-off time, the micro-nano ball is subjected to the action of gravity only, and freely falls to the optical detection cavity along the gravity direction under the action of a gravity field, and is vertically accelerated along a gravity vertical line until reaching the optical detection cavity, after a target timing duration dTg (n), the detection light is started, and the height position Z (n) of the micro-nano ball in a detection area is obtained by utilizing the detection light;
during the on time of the detection light, the second position detection sensor QPDB detects the position information Z (n) of the micro-nano sphere and gives a relative position signal between it and the focal point of the confocal lens group, i.e. above, below or just at the focal point.
The horizontal detection light is in a closed state by default, and is opened only when the timing time is equal to a set value dTg (n), so that a detection signal is provided for the position detection sensor to detect the position of the micro-nano ball in the cavity and update the next timing time set value dTg (n +1), and then the horizontal detection light is closed.
1.6) according to the relation between the height position Z (n) of the micro-nano small ball and the height position dZ of the detection center, calculating by adopting the following formula to obtain a gravity acceleration value g (n) as single measurement output of the optical gravimeter:
g(n)＝2dZ/[dTg(n)]2
wherein dZ represents the height position of the detection center relative to the center of the optical trap; i.e. the laser cooling cavity and the optical detection cavity have a central vertical distance dZ.
In combination with this displacement dZ (displacement of the release position to the focal point of the optical detection chamber, which value can be calibrated beforehand), the gravitational acceleration value g (n) at the measurement can be calculated.
2) Continuously repeating the step 1) to realize the continuous measurement value g (n) of the gravity acceleration; and after obtaining the gravity acceleration value g (n) each time, according to the relationship between the height position Z (n) of the micro-nano small ball and the height position dZ of the detection center, simultaneously updating the target timing duration dTg (n +1) of the next measurement according to the following formula:
dTg(n+1)＝dTg0when n is 0
dTg(n+1)＝dTg(n)-dt0When n ≠ 0, and Z (n)<dZ
dTg(n+1)＝dTg(n)+dt0When n ≠ 0, and Z (n)>dZ
dTg (n +1) ═ dTg (n), when n ≠ 0, and Z (n) ═ dZ
Wherein, dTg0And dt0Are all constant, dt0Indicating the adjustment amount of the target time period, dTg0An initial value of a target timing period dTg (n) is obtained based on a local gravitational acceleration estimated value g at a measurement position0Calculated according to the following formula:
dTg0＝(2dZ/g0)1/2
wherein, g0Representing the local gravitational acceleration estimate, as a known quantity.
Adjustment dt of target timing length0The feedback speed and the gravity measurement target accuracy are predetermined according to the design of the system, and 1 nanosecond can be selected typically.
The process forms a closed-loop control process, wherein the detection quantity is the relative position of the micro-nano small ball and the focus, and the control quantity is the next counting time.
Under the condition of closed-loop work, the small ball is finally stabilized near a focus and has weak random fluctuation, and the fluctuation size is determined by the noise condition of the system; and at the moment, the corresponding counting time dTg (n) is the falling time corresponding to the process that the micro-nano small ball falls from the release position of the first optical trap cooling cavity to the focus of the second cavity.
The right side of FIG. 2 is a simplified illustration of the initial release and detection positions of the beadIf the detected position is Z-dZ, the free falling time is dTg (n), and if the ball does not reach the position of dZ, the timing duration of the ball is not enough, so that the timing duration is increased next time, and vice versa. Through the process, time measurement in the free falling process of dZ is finally realized by utilizing dTg (n), so that the measurement value of g (n) is indirectly obtained. During this measurement, the step dt is adjusted by time0One of the most significant tradeoffs between measurement start time and measurement accuracy can be chosen.
Meanwhile, in order to reduce the influence of time technical errors on the g (n) precision, an atomic clock is required to be used as a timing reference, and the stability of the atomic clock is better than 10-10The typical atomic clock with stability is used as a timing reference, and the time error can be suppressed to 10-10Within.
In the optical gravimeter, in order to eliminate the influence of air resistance, a vacuum system is utilized to provide a working vacuum environment of a core measurement component, wherein a release device of a micro-nano small ball, a light path system of a first optical cavity and main components of a second optical detection cavity are placed in the vacuum cavity as shown in a frame in fig. 2, and a light-transmitting glass hole is reserved on the wall of the vacuum cavity, so that light beams placed outside the cavity are allowed to be input and detection light inside the cavity is allowed to be output to a detection light path outside the cavity.
Fig. 4 is an example of test data for an embodiment of the present invention, according to a workflow, by software simulation, considering a closed loop feedback measurement in the presence of noise. The abscissa in the graph is the serial number of the test times, wherein the upper graph is the falling time dTg (n) time, and the lower graph is the proportion of the calculated gravity acceleration measured value g (n) and the theoretical gravity acceleration value g. The gravimeter used in the figure has a descent length of 20 cm, a descent time of about 0.2 ms and time adjustment increments of 1 ns.
Therefore, the implementation shows that the micro-nano small balls are used as measurement carriers, the laser cooling technology is combined, the releasing positions and the capturing positions of the small balls are accurately measured to obtain the accurate value of the gravitational acceleration, the environmental interference is eliminated, the gravitational acceleration measurement accuracy is improved, and the measurement speed and the measurement efficiency are improved.
Claims (8)
1. The utility model provides a miniaturized high accuracy optics gravimeter of light power cooling which characterized in that: the device comprises a vacuum cavity, a release device, a laser cooling cavity, an optical detection cavity and a micro-nano small ball, wherein the release device, the laser cooling cavity, the optical detection cavity and the micro-nano small ball are positioned in the vacuum cavity; the release device is positioned at the top of the vacuum cavity, and the micro-nano balls are loaded in the release device; the laser cooling cavity and the optical detection cavity are respectively positioned above and below the middle part of the vacuum cavity; the laser cooling cavity is internally provided with a laser capturing device of a micro-nano small ball, the laser capturing device comprises two lenses L1 and L2 which are horizontally arranged opposite to each other along the same optical axis, two horizontal parallel lights are incident from two sides of the lenses L1 and L2 and are used as capturing lights, and the two horizontal parallel lights adopt two vertical polarization states output by the same laser; focal points of the lenses L1 and L2 are superposed, an optical trap area for capturing the micro-nano small balls is formed near the focal points, and the optical trap area is used as an initial position area of the micro-nano small balls which freely fall to the ground; a small ball detection device of the micro-nano small ball is arranged in the optical detection cavity, the small ball detection device comprises a lens L3 and a lens L4 which are horizontally arranged opposite to each other along the same optical axis, a beam of horizontal light is incident from the side of one of the two lenses L3 and L4 to serve as detection light, the focal points of the two lenses L3 and L4 are overlapped, a detection area for detecting the position of the micro-nano small ball is formed near the focal point, and the detection area serves as an end position area of the micro-nano small ball falling freely; the release device is provided with a micro-nano small ball release outlet, a light trap area in the laser cooling cavity is positioned right below the micro-nano small ball release outlet, and a detection area in the optical detection cavity is positioned right below the light trap area; and a cooling light source is arranged outside the vacuum cavity right below the detection region, and the cooling light source upwards emits cooling light to irradiate the light well region.
2. The optical force cooling miniaturized high-precision optical gravimeter according to claim 1, characterized in that: a polarization beam splitter prism is arranged outside the vacuum cavity at the side of the lens L1, and a first position detection sensor is arranged below the polarization beam splitter prism; a horizontal beam of light horizontally enters the polarization beam splitter prism from the side of the polarization beam splitter prism, is transmitted by the polarization beam splitter prism, enters the lens L1 and is converged to a focus through the lens L1; another beam of horizontal light horizontally enters the lens L2 from the side of the lens L2, is converged to a focal point through the lens L2, then enters the polarization beam splitter prism after being recovered and expanded through the lens L1, and enters the first position detection sensor after being reflected by the polarization beam splitter prism to be detected and received; detecting by a first position detection sensor to obtain position information of the micro-nano small balls deviating from the center of the optical trap along the radial direction of the horizontal optical axis;
a reflector is arranged outside the vacuum cavity at the side of the lens L4, and a second position detection sensor is arranged below the reflector; a beam of horizontal detection light horizontally enters a lens L4 from one side of the lens L4 without a reflector, is converged to a focal point through a lens L4, the focal point is used as a detection center, is recovered through a lens L3 and then enters the reflector after being expanded, and is reflected to a second position detection sensor through the reflector to be detected and received; detecting by a second position detection sensor to obtain position information of the micro-nano small balls falling to a position deviating from a detection center along the radial direction of the horizontal optical axis;
the release device adopts piezoelectric ceramics, and micro-nano small spherical particles which are placed on the piezoelectric ceramics in advance are sequentially released in a single particle mode through vibration of the piezoelectric ceramics.
3. The optical force cooling miniaturized high-precision optical gravimeter according to claim 1, characterized in that: the source of the two beams of capture light in the laser capture device is a 1064nm laser, and the cooling light source specifically adopts 532nm laser.
4. The optical force cooling miniaturized high-precision optical gravimeter according to claim 2, characterized in that: the device also comprises an information processing circuit, and the information processing circuit is respectively connected with the piezoelectric ceramics of the first position detection sensor, the second position detection sensor and the release device.
5. The optical force cooling miniaturized high-precision optical gravimeter according to claim 1, characterized in that: the micro-nano small spheres are micro-nano particles made of quartz materials.
6. The optical force cooling miniaturized high-precision optical gravimeter according to claim 1, characterized in that: the vacuum cavity is provided with a glass transparent hole for injecting and outputting laser beams of captured light, detection light and cooling light.
7. The optical force cooling miniaturized high-precision optical gravimeter according to claim 1, characterized in that:
the optical gravity test process of the optical force cooling miniaturized high-precision optical gravimeter is as follows:
firstly, sequentially releasing micro-nano pellets in a single particle mode through a releasing device, starting two beams of trapping light and cooling light, enabling each micro-nano pellet to fall to a light well region and be stabilized at the center of the light well, and detecting and recording the position height of the micro-nano pellet as a first height value through one beam of trapping light;
then, simultaneously cutting off the two beams of capture light and cooling light, and starting timing; at the moment of turning off, the light trap scattering force and gradient force borne by the micro-nano small balls and the feedback force of cooling light disappear at the same time, and the micro-nano small balls are subjected to the action of gravity only and move freely in a falling body towards the optical detection cavity along the gravity direction under the action of a gravity field;
then, when the micro-nano small ball approaches or reaches the optical detection cavity, starting detection light, detecting and recording the position height of the micro-nano small ball as a second height value, and ending timing; and calculating the gravity acceleration by using the first height value, the second height value and the timing time difference.
8. The optical force cooling miniaturized high-precision optical gravimeter according to claim 7, characterized in that: the optical gravity test process of the optical force cooling miniaturized high-precision optical gravimeter specifically comprises the following steps:
1) for one micro-nano small ball released by the releasing device each time, obtaining a gravity acceleration value g (n) by adopting the following mode, wherein n represents the ordinal number of the test times;
1.1) after the micro-nano small balls are released by a releasing device, the micro-nano small balls fall under the action of gravity to reach an optical trap area of a laser cooling cavity;
1.2) starting a laser cooling cavity to capture light, forming a light trap region by using the capture light, capturing the micro-nano spheres near the center of the light trap through the light force action of gradient force and scattering force in the light trap region, and detecting by using a beam of capture light to obtain the height position of the micro-nano spheres in the light trap region and recording as 0;
1.3) starting cooling light, and performing laser closed-loop feedback cooling on the micro-nano small balls by using the cooling light to reduce the thermal motion of the small balls;
1.4) after the thermal motion of the micro-nano spheres is reduced to a preset target, simultaneously cutting off the capture light and the cooling light, starting timing from the cut-off time and setting a target timing duration dTg (n),
1.5) the micro-nano small ball freely falls to the ground under the action of a gravitational field after being turned off, vertically accelerates along a gravity vertical line until reaching an optical detection cavity, starts detection light after a target timing duration dTg (n), and obtains the height position Z (n) of the micro-nano small ball in a detection area by utilizing detection light detection;
1.6) obtaining a gravity acceleration value g (n) as a single measurement output of the optical gravimeter by adopting the following formula:
g(n)＝2dZ/[dTg(n)]2
wherein dZ represents the height position of the detection center relative to the center of the optical trap;
2) continuously repeating the step 1) to realize the continuous measurement of the gravity acceleration value g (n); and after obtaining the gravitational acceleration value g (n) each time, according to the relationship between the height position Z (n) of the micro-nano small ball in the detection region and the height position dZ of the detection center relative to the center of the optical trap, simultaneously updating the target timing duration dTg (n +1) of the next measurement according to the following formula:
dTg(n+1)＝dTg0when n is 0
dTg(n+1)＝dTg(n)-dt0When n ≠ 0, and Z (n)<When dZ is
dTg(n+1)＝dTg(n)+dt0When n ≠ 0, and Z (n)>When dZ is
dTg (n +1) ═ dTg (n), when n ≠ 0, and Z (n) ═ dZ
Therein, dt0Indicating the adjustment amount of the target time period, dTg0An initial value of a target timing period dTg (n) is obtained based on a local gravitational acceleration estimated value g at a measurement position0Calculated according to the following formula:
dTg0＝(2dZ/g0)1/2
wherein, g0Representing a local gravitational acceleration estimate.
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