An absolute gravimeter based on a michelson laser

Abstract

This invention discloses an absolute gravimeter based on a Michelson laser, characterized by comprising a Michelson laser, a spectrum analyzer, a vacuum cavity, a vibration isolation system, and a data processing unit; wherein, the front cavity mirror, laser gain medium, beam splitter, and feedback output mirror of the Michelson laser are placed sequentially along the horizontal optical axis; a falling prism and a reference prism are placed along the vertical optical axis; the beam splitter is located at the intersection of the horizontal and vertical optical axes; the horizontally oscillating optical path is used as the reference optical path, and the corresponding laser is used as the reference laser; an oscillating optical path outside the reference optical path is used as the measurement optical path, and the corresponding laser is used as the measurement laser; the spectrum analyzer is placed after the feedback output mirror to measure the beat frequency signals of the reference laser and the measurement laser; the data processing unit is used to perform a secondary fitting based on the clock signal corresponding to each beat frequency signal measurement point and the calculated falling trajectory data of the falling prism to obtain the gravitational acceleration value.

Description

Technical Field

[0001] This invention belongs to the field of gravity measurement, specifically relating to an absolute gravimeter based on a Michelson laser. Background Technology

[0002] Absolute gravity measurement is the direct measurement of the acceleration g at the Earth's surface, and it has wide applications in earth science, resource exploration, navigation aids, and metrology. Since 1960, with the development of laser technology, people have begun to develop and use laser interferometric absolute gravimeters for absolute gravity measurement. Traditional laser interferometric absolute gravimeters employ a Michelson interferometer structure. When a freely falling prism moves a distance of half a wavelength, the interference fringe signal changes for one cycle. The interference fringe signal is collected using a photodetector and processed for analysis. By collecting the time series where the amplitude of all interference signals is zero, the trajectory signal of the falling body is obtained. A second fitting of this sequence yields the acceleration of the falling body relative to the reference prism, which is the gravitational acceleration.

[0003] Traditional laser interferometric absolute gravimeters determine the distance a falling object travels by observing interference fringes. A clock signal is recorded when the brightness of the interference fringes changes for one cycle, thus obtaining the object's trajectory. In this case, a record is made every half-wavelength of the laser light cascaded, meaning the resolution of the falling distance is on the order of nanometers. Improving this distance resolution could further enhance the accuracy of gravitational acceleration measurements, making it of significant application value.

[0004] The Michelson laser was first proposed in the patent "A Michelson Laser and Its Implementation Method, Displacement Measurement Method" (application number: 202210393703.X). This is a novel laser technology and principle implemented using a Michelson interferometer optical path. Compared to traditional laser interferometers with only nanometer-level displacement measurement resolution, the displacement measurement system implemented by this invention using a Michelson laser combined with a spectrum analyzer can achieve picometer-level or even higher displacement measurement resolution, which has extremely high research value in the field of precision metrology. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides an absolute gravity interferometer based on a Michelson laser to improve the measurement accuracy of gravitational acceleration.

[0006] First, let's introduce the structure and displacement measurement method of the Michelson laser: The Michelson laser borrows from the Michelson interferometer in its optical path structure. It uses four high-reflectivity laser cavity mirrors to form a closed cavity. Multiple loops exist within the cavity, and after satisfying the laser resonance condition, multiple lasers of different frequencies can be generated. A spectrum analyzer is placed behind the output mirror to observe the interference signal of the output laser. By comparing the changes in the laser interference beat frequency signals of the measuring arm and the reference arm, the displacement change of the measuring arm can be derived. This approach overcomes the limitations of traditional laser interferometers, namely, that the displacement measurement resolution due to calculating the displacement change by observing interference fringes is no more than the nanometer scale. By converting displacement measurement into frequency measurement, the displacement measurement resolution can be improved to the picometer scale or even beyond.

[0007] By using this displacement measurement scheme based on a Michelson laser to replace the Michelson interferometer and its signal processing part in a traditional laser interferometric absolute gravimeter, a larger amount of data can be measured when a falling object is at the same displacement, thereby improving the measurement accuracy of gravitational acceleration.

[0008] Based on the above ideas, this invention proposes an absolute gravimeter based on a Michelson laser, employing the following technical solution:

[0009] An absolute gravimeter based on a Michelson laser is characterized by its structure comprising a front cavity mirror 1, a laser gain medium 2, a beam splitter 3, a falling prism 4, a reference prism 5, a feedback output mirror 6, a spectrum analyzer 7, a vacuum cavity 8, a vibration isolation system 9, and a data processing unit; wherein,

[0010] The falling prism 4 is a corner prism fixed inside the free-falling object. Using a corner prism can ensure that the incident light and the reflected light are absolutely parallel, eliminating the error caused by horizontal disturbances in the measurement of gravitational acceleration.

[0011] The front cavity mirror 1, laser gain medium 2, beam splitter 3, falling prism 4, reference prism 5, and feedback output mirror 6 together constitute a Michelson laser.

[0012] The light output from the laser gain medium 2 oscillates back and forth within the cavity, achieving laser output once the gain exceeds the loss. The horizontally propagating optical path serves as the reference optical path, maintaining a constant optical path length. The laser sub-cavity forming this mode laser consists of a front cavity mirror 1 and a feedback output mirror 6. The resonant cavity corresponding to the measurement laser path consists of the front cavity mirror 1, a falling prism 4, a reference prism 5, and the feedback output mirror 6. This measurement laser reaches the beam splitter 3, is transmitted vertically upwards to the falling prism 4, is reflected by the falling prism 4, and then vertically downwards into the reference prism 5. It is then reflected back to the beam splitter 3 and output from the feedback output mirror 6. A spectrum analyzer 7 is placed after the feedback output mirror 6, which can be used to measure the beat frequency signal between the reference laser and the measurement laser.

[0013] The data analysis process of the data processing unit is as follows: The change in cavity length dl of the laser resonator cavity and the change in laser frequency Δν have the following corresponding relationship: Where L is the cavity length of the measuring arm resonant cavity, and ν is the measuring laser frequency, the change in cavity length dl corresponding to the measuring optical path can be inferred from the change in beat frequency signal Δν. Therefore, in practical applications, the falling height of the falling prism 4 can be obtained by reading the beat frequency signal on the spectrum analyzer and further processing the frequency data. By using a high-precision atomic clock to record the clock signal at each beat frequency test point, the falling trajectory data is obtained. Based on the kinematic relationship of Newton's second law, the data is fitted twice to calculate the value of gravitational acceleration.

[0014] Furthermore, the laser gain medium 2 has an antireflection coating on its exit end face; the front cavity mirror 1 is a total reflection mirror, and the falling prism 4, reference prism 5, and feedback output mirror 6 are high reflection mirrors. Together, they form a closed laser resonant cavity, wherein the feedback output mirror 6 is used to output laser light.

[0015] Furthermore, to satisfy Abbe's principle and reduce the influence of horizontal micro-vibrations, the falling prism 4 and the reference prism 5 should be placed on a vertical line. Simultaneously, the front cavity mirror 1 and the feedback output mirror 6 should be placed on a horizontal line.

[0016] Furthermore, the beam splitter 3 has a beam splitting ratio of 1:1.

[0017] Furthermore, the falling prism 4 can be reset by a transmission device after free fall, enabling repeated measurements.

[0018] Furthermore, the vacuum chamber 8 maintains a vacuum environment within the cavity containing the falling body and the falling prism 4 via an ion pump, eliminating air interference and ensuring that the measured acceleration of the falling body is the acceleration due to gravity.

[0019] Furthermore, the vibration isolation system 9 is used to reduce the interference of external vibrations on the reference prism 5, so that the reference prism 5 remains stationary relative to the inertial frame.

[0020] Compared with existing laser interferometric absolute gravimeters, the positive effects of the Michelson laser-based absolute gravimeter proposed in this invention are as follows:

[0021] Traditional laser interferometric absolute gravimeters can only acquire data in units of half the laser wavelength when obtaining the trajectory of a falling object. This invention uses a combination of a Michelson laser and a spectrum analyzer to acquire the displacement changes during the free fall of the object. This breaks the limitation of traditional absolute gravimeters that only measure the trajectory of the falling object in units of half the laser wavelength, achieving more precise displacement measurement. Its advantage lies in that when the falling distance is the same, more sets of data can be measured, thereby improving the accuracy of the secondary fitting of the gravitational acceleration value. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of an absolute gravimeter based on a Michelson laser.

[0023] Among them, 1-front cavity mirror; 2-laser gain medium; 3-beam splitter; 4-falling prism; 5-reference prism; 6-feedback output mirror; 7-spectrum analyzer; 8-vacuum cavity; 9-vibration isolation system. Detailed Implementation

[0024] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0025] The present invention proposes an absolute gravimeter structure based on a Michelson laser, as shown in the figure, comprising a front cavity mirror 1, a laser gain medium 2, a beam splitter 3, a falling prism 4, a reference prism 5, a feedback output mirror 6, a spectrum analyzer 7, a vacuum cavity 8, and a vibration isolation system 9. The front cavity mirror 1 is a total reflection mirror; the laser gain medium 2 can be a helium-neon laser tube, a semiconductor laser diode, a YAG crystal, etc., with an anti-reflection coating on the light-emitting end; the beam splitter 3 has a semi-reflective coating on its front end and an anti-reflection coating on its rear surface; the falling prism 4 is a pyramidal prism fixed inside a freely falling object; the falling prism 4, the reference prism 5, and the feedback output mirror 6 are all high-reflection mirrors; the front cavity mirror 1, the laser gain medium 2, the beam splitter 3, the falling prism 4, the reference prism 5, and the feedback output mirror 6 together constitute a Michelson laser.

[0026] During operation, the light output from the laser gain medium 2 oscillates back and forth within the cavity, achieving laser output once the gain exceeds the loss. The horizontally propagating optical path serves as the reference optical path, maintaining a constant optical path length. The laser sub-cavity forming this mode laser consists of a front cavity mirror 1 and a feedback output mirror 6. The resonant cavity corresponding to the measurement optical path laser is composed of the front cavity mirror 1, a falling prism 4, a reference prism 5, and the output feedback mirror 6. This measurement laser reaches the beam splitter 3, is transmitted vertically upwards to the falling prism 4, is reflected by the falling prism 4, and then vertically downwards into the reference prism 5, before being reflected back to the beam splitter 3 and output from the feedback output mirror 6. A spectrum analyzer 7 is placed after the feedback output mirror 6, which can be used to measure the beat frequency signal between the reference laser and the measurement laser. The change in cavity length dl of the laser resonant cavity and the change in laser frequency Δν have the following corresponding relationship: Where L is the cavity length of the measuring arm resonant cavity and ν is the measuring laser frequency, the change in cavity length corresponding to the measuring optical path can be inferred from the change in the beat frequency signal, thus obtaining the falling height of the falling prism 4. The clock signal at each beat frequency test point is recorded using a highly accurate atomic clock to obtain the falling trajectory data. Based on the kinematic relationship of Newton's second law, a second fitting is performed on this data to calculate the gravitational acceleration value.

[0027] Furthermore, the falling prism 4 can be reset by a transmission device after free fall, enabling repeated measurements.

[0028] Furthermore, to satisfy Abbe's principle and reduce the influence of horizontal micro-vibrations, the falling prism 4 and the reference prism 5 should be placed on a vertical line. Simultaneously, the front cavity mirror 1 and the feedback output mirror 6 should be placed on a horizontal line.

[0029] Furthermore, the vacuum chamber 8 maintains a vacuum environment within the chamber via an ion pump, ensuring that the measured acceleration of the falling motion is the acceleration due to gravity.

[0030] Furthermore, the vibration isolation system 9 is used to reduce the interference of external vibrations on the reference prism 5, so that the reference prism 5 remains stationary relative to the inertial frame.

[0031] The above embodiments are merely illustrative of the technical solutions of the present invention and are not intended to limit it. Those skilled in the art can modify or make equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the specific scope of protection is determined by the scope defined in the claims.

Claims

1. An absolute gravimeter based on a Michelson laser, characterized in that, It includes a Michelson laser, a spectrum analyzer (7), a vacuum cavity (8), a vibration isolation system (9), and a data processing unit; among which, The Michelson laser includes a front cavity mirror (1), a laser gain medium (2), a beam splitter (3), a falling prism (4), a reference prism (5), and a feedback output mirror (6); wherein, the front cavity mirror (1), the laser gain medium (2), the beam splitter (3), and the feedback output mirror (6) are placed sequentially along the horizontal optical axis; the falling prism (4) and the reference prism (5) are placed along the vertical optical axis; the beam splitter (3) is located at the intersection of the horizontal and vertical optical axes, and is used to split the light output from the laser gain medium (2) into beams along the horizontal optical axis. Two paths propagate perpendicular to the optical axis, causing the light output from the laser gain medium (2) to oscillate within the closed cavity of the laser formed by the front cavity mirror (1), the falling prism (4), the reference prism (5), and the feedback output mirror (6), forming multiple lasers of different frequencies; the optical path oscillating in the horizontal direction is used as the reference optical path, and the corresponding laser is used as the reference laser; the oscillating optical path formed by the front cavity mirror (1), the falling prism (4), the reference prism (5), and the feedback output mirror (6) is used as the measurement optical path, and the corresponding laser is used as the measurement laser; The falling prism (4) is placed inside the vacuum chamber (8); the reference prism (5) is placed inside the vibration isolation system (9); The spectrum analyzer (7) is placed behind the feedback output mirror (6) and is used to measure the beat frequency signal of the reference laser and the measurement laser; Data processing unit, used to... The cavity length change dl corresponding to the measurement optical path is obtained, and the falling height of the falling prism (4) is obtained based on the cavity length change dl, where L is the cavity length of the resonant cavity corresponding to the measurement optical path, Δν is the frequency change of the beat frequency signal, and ν is the frequency of the measurement laser; and the clock signal corresponding to each beat frequency signal measurement point and the calculated falling trajectory data of the falling prism (4) are fitted twice to obtain the gravitational acceleration value.

2. The absolute gravimeter according to claim 1, characterized in that, The falling prism (4) is a pyramidal prism, fixed inside the free-falling object used to measure absolute gravity.

3. The absolute gravimeter according to claim 1 or 2, characterized in that, The vacuum chamber (8) is equipped with a transmission device for resetting the falling prism (4) after it has fallen freely.

4. The absolute gravimeter according to claim 1, characterized in that, The laser gain medium (2) is coated with an anti-reflection film on its output end face; the front cavity mirror (1) is a total reflection mirror, and the falling prism (4), reference prism (5), and feedback output mirror (6) are high reflection mirrors.

5. The absolute gravimeter according to claim 1, characterized in that, The beam splitter (3) has a beam splitting ratio of 1:

1.

6. The absolute gravimeter according to claim 1 or 5, characterized in that, The front surface of the beam splitter (3) is coated with a semi-reflective and semi-transparent film, and the rear surface is coated with an anti-reflective film.

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