CN104808254B - High-precision absolute gravimeter optics frequency multiplier type laser interference system and application - Google Patents

Abstract

The invention discloses an optical frequency-doubling laser interference system for a high-precision absolute gravimeter, which includes a frequency-stabilized laser, a collimating beam expander, an adjustable reflector, a beam splitter, a reference prism, a falling prism, and a horizontal liquid surface , reflective prism, aperture, focusing lens, photodetector; the laser light is emitted by a frequency-stabilized laser, is reflected by an adjustable mirror after being collimated by a beam expander, and then transmitted vertically downwards, and then divided into vertical beams by a beam splitter. The test beam transmitted downward and the reference beam transmitted horizontally to the right, in which the test beam is reflected by the reference prism and the falling prism for multiple times, and then shoots to the horizontal liquid surface, and then returns to the reference beam according to the original optical path after reflection, resulting in interference The fringe, the interference beam passes through the open aperture and is focused on the photodetector by the focusing lens. An absolute gravimeter employing the system. The invention can improve the test accuracy by multiple times, and can simply and effectively adjust the direction of the test beam to ensure that it is parallel to the direction of the acceleration of gravity.

Description

Optical Frequency Doubling Laser Interferometry System and Application for High Precision Absolute Gravity Meter

technical field

The invention relates to absolute gravity testing technology, in particular to an optical frequency-doubling laser interference system for a high-precision absolute gravimeter, which can accurately detect the falling position of a falling prism during the absolute gravity testing process.

Background technique

The gravity field is the basic physical field of the earth. High-precision absolute gravity observation data are widely used in the fields of geodesy, geophysics, geodynamics and seismology, as well as in the fields of military, aerospace, navigation, resource exploration and development engineering. There are a wide range of applications. At present, an absolute gravimeter is generally used to measure the acceleration of gravity.

Fig. 1 is a schematic structural diagram of an absolute gravimeter in the prior art. As shown in Figure 1, the absolute gravimeter in the prior art uses laser interferometry to accurately measure the position and time data (h _i , t _i ) of the falling prism when it falls freely in a vacuum environment, and then brings the data into the free fall motion equation And use the polynomial fitting method to get the best estimate of the acceleration of gravity. Absolute gravimeter generally consists of five parts: laser interference system, vacuum free fall control system, ultra-low frequency vertical vibration isolation system, high-speed signal acquisition system, data processing and instrument control system.

The laser interferometry system provides a length reference for the absolute gravity test and ensures that the test beam is in the same direction as the gravitational acceleration, which is an important part of the absolute gravimeter.

Fig. 2 is a structural schematic diagram of a laser interference system for an absolute gravimeter in the prior art. As shown in Figure 2, the laser is emitted by a frequency-stabilized laser and is divided into a horizontally transmitted reference beam and an upwardly transmitted test beam by the first beam splitter, and the test beam is reflected upward by a falling prism that is free-falling in a vacuum cavity It is transmitted downward, and then reflected by the reference prism installed on the ultra-long spring vibration isolation system. After passing through two plane mirrors that adjust the optical path, it merges with the reference beam in the second beam splitter to generate interference. The photoelectric detector interferes with the light The signal is converted into an electrical signal for data processing in subsequent systems.

To detect the absolute gravitational acceleration at a certain point with an absolute gravimeter, it is necessary to ensure that the test beam in the laser interferometer system is parallel to the direction of the absolute gravitational acceleration. Therefore, in the existing absolute gravimeter laser interferometry system, there is a corresponding laser vertical direction adjustment unit.

Fig. 3 is a laser vertical direction adjustment unit in the prior art absolute gravimeter laser interference system. As shown in Figure 3, before the start of the absolute gravity test, the direction of the test beam in the laser interference system needs to be adjusted to make it parallel to the direction of the acceleration of gravity. Generally, the horizontal liquid surface is used to reflect the test beam in the original optical path to return along the original path. And interfere with the reference beam, and then use the telescopic system to observe whether the interference image is a uniform circular spot, so as to judge whether the test beam has been adjusted to the vertical direction. After confirming that the test beam has been adjusted to the vertical direction, remove the horizontal liquid surface , to measure the absolute acceleration of gravity.

The test beam of the laser interference system in the prior art absolute gravimeter is only reflected once between the falling prism and the reference prism, and the sensitivity to the falling distance of the test prism is low; at the same time, the vertical direction adjustment of the test beam needs to be assisted by the telescopic system, and the system is relatively complicated. , and after the vertical direction adjustment of the test beam is completed, the horizontal liquid surface needs to be removed to start the absolute gravity test, which changes the state of the instrument and cannot monitor the vertical direction of the test beam in real time.

Contents of the invention

In order to overcome the above disadvantages, the present invention provides an optical frequency-doubling laser interference system and its application for a high-precision absolute gravimeter.

An optical frequency-doubling laser interference system for a high-precision absolute gravimeter, which includes a frequency-stabilized laser, a collimating beam expander, an adjustable mirror, a beam splitter, a reference prism, a falling prism, a horizontal liquid surface, a reflecting prism, Aperture, focusing lens, photoelectric detector; laser light is emitted by a frequency-stabilized laser, reflected by an adjustable reflector after being collimated by a beam expander, and then transmitted vertically downwards, and then divided into vertically transmitted downwards by a beam splitter The test beam and the reference beam transmitted horizontally to the right, in which the test beam is reflected by the reference prism and the falling prism for multiple times, and then shoots to the horizontal liquid surface, and then returns to the beam splitter according to the original optical path after being reflected by the horizontal liquid surface. The reference beams reflected by the reflective prism merge to generate interference fringes, and the interference beams pass through the opened diaphragm and are focused on the photodetector by the focusing lens.

The horizontal liquid level is alcohol or mercury.

The transmittance of the beam splitter can make the light intensity of the test beam and the reference beam equal when interference occurs.

An absolute gravimeter employing the system.

According to the application method of the system, before the absolute gravity test starts, the aperture is closed, and when the incident light beam has an included angle θ with the vertical direction, the test error will be The oblique incident beam is reflected by the reference prism and the falling prism for many times and becomes a beam with the same inclination angle. The multiple reflections with the reference prism become beams, and the beams are reflected by the beam splitter and become test beams. The test beams interfere with the reference beams reflected by the reflective prism, and the angle is α=2θ, forming equidistant distances on the diaphragm surface. Adjust the adjustable mirror to make the interference image into a circular spot with first-class light intensity. At this time, the test beam is parallel to the direction of gravitational acceleration, that is, θ=0. Then open the diaphragm and start the absolute gravity test. During the test In the middle, the aperture can be closed at any time, and the interference image on the aperture surface can be observed to maintain a first-class light intensity circular spot to monitor whether the test beam is parallel to the vertical direction in real time. If the interference image has changed, adjust the adjustable mirror to the interference The image changes back to a circular spot of equal light intensity, and then the aperture is closed to continue the absolute gravity test.

Beneficial effects of the present invention:

1. Make full use of the caliber of the reference prism and the falling prism, and adopt the optical frequency doubling method to make the test laser reflect N times between the reference prism and the falling prism. The test accuracy can be improved without changing other components of the absolute gravimeter. Improve N times;

2. A laser interference system test beam vertical direction adjustment unit is designed, which can simply and effectively adjust the direction of the test beam to ensure that it is parallel to the direction of the acceleration of gravity, and at the same time, the direction of the test beam can be real-time during the absolute gravity test. Monitor and adjust.

Description of drawings

Fig. 1 is the structural representation of prior art absolute gravimeter;

Fig. 2 is a schematic structural diagram of a laser interference system for an absolute gravimeter in the prior art;

Fig. 3 is the laser vertical direction adjustment unit in the prior art absolute gravimeter laser interference system;

Figure 4 is a schematic diagram of the measurement error caused by the non-parallel test beam and the gravitational acceleration direction;

Figure 5 is a schematic diagram of laser interference;

Fig. 6 shows a schematic structural diagram of a laser interference system for an absolute gravimeter according to an embodiment of the present invention; wherein, 1—frequency-stabilized laser, 2—collimating beam expander, 3—adjustable mirror, 4—beam splitter, 5—reference prism, 6—falling prism, 7—horizontal liquid surface, 8—reflecting prism, 9—diaphragm, 10—focusing lens, 11—photoelectric detector.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

The absolute gravimeter uses laser interferometry to accurately measure the position and time data (h _i , t _i ) of the falling prism in a vacuum environment, and then bring it into the free fall formula The method of polynomial fitting is used to obtain the best estimate of the acceleration of gravity. Its interference principle is similar to Michelson interferometer. Every time the falling prism falls λ/2 (where λ laser wavelength), an interference fringe will be generated.

When the absolute gravimeter tests the gravitational acceleration at a certain point, it is necessary to ensure that the test beam is parallel to the direction of the gravitational acceleration. If the two are not parallel, errors will be introduced during the measurement process. As shown in Figure 4, when the test beam has an angle θ with the vertical direction, the test error is Therefore, the absolute gravimeter laser interferometry system must have a test beam vertical direction adjustment unit to ensure that the test beam direction is vertical and reduce test errors.

The laser interference image is determined by the optical path difference of the beams participating in the interference. The interference fringe is the path of equal optical path difference. As shown in Figure 5, λ is the wavelength of the interference laser, and ω is the angle between the two interference beams. When ω=0, that is, the interference beam is parallel, the fringe spacing e approaches infinity, and the observation screen displays a circular spot with first-class light intensity. Using this property, the direction of the beam can be tested for the laser interference system in the absolute gravimeter Detect and adjust to make it parallel to the direction of gravitational acceleration.

Fig. 6 is a schematic structural diagram of an optical frequency-doubling laser interference system for a high-precision absolute gravimeter of the present invention. The basic principle is to use the method of optical frequency doubling to achieve high-precision measurement of the position of the falling prism in the absolute gravity test, and to monitor and adjust the vertical direction of the test laser beam in real time during the absolute gravity test. Frequency laser 1, collimating beam expander 2, adjustable mirror 3, beam splitter 4, reference prism 5, falling prism 6, horizontal liquid surface 7, reflecting prism 8, diaphragm 9, focusing lens 10, photodetector 11.

The laser light is emitted by a frequency-stabilized laser 1, and after being reflected by an adjustable reflector 3 after passing through a collimating beam expander 2, it is transmitted vertically downward, and then divided into a test beam transmitted vertically downward and a horizontal horizontal beam by a beam splitter 4. The reference beam transmitted on the right, in which the test beam is reflected by the reference prism 5 and the falling prism 6 for multiple times (the figure is 4 times) and shoots to the horizontal liquid surface 7, and then returns to the branch according to the original optical path after being reflected by the horizontal liquid surface. The beamer 4 merges with the reference beam reflected by the reflective prism 8 to generate interference fringes, and the interference beam passes through the opened diaphragm 9 and is converged on the photodetector 11 by the focusing lens 10, and the photodetector 11 converts the optical interference signal into The electrical signal is used by the absolute gravimeter data processing system to fit and calculate the absolute gravitational acceleration estimate.

The horizontal liquid surface 7 is a liquid surface that can reflect light such as alcohol or mercury. When the light beam is incident on the horizontal liquid surface 7 in the vertical direction, it can return according to the original optical path. When the incident light beam is incident at an angle relative to the vertical direction, the reflection The beam is separated from the incident beam.

During the test, the test beam is reflected by the horizontal liquid surface 7 and the reference prism 5 and the falling prism 6 multiple times, and the attenuation is relatively strong. The transmittance of the beam splitter 4 is specially designed so that the light intensity of the test beam and the reference beam when interference occurs Equal, the contrast of the interference fringes approaches 1, which improves the signal-to-noise ratio of subsequent measurements.

In the laser interference system of the present invention, the test beam needs to pass through N (4 times in the figure, but not limited to this) reflections of the reference prism 5 and the falling prism 6. When the falling prism 6 falls λ/2N, an interference line can be produced. The fringes, compared to the interference fringes generated by the falling prism falling λ/2 in the prior art absolute gravimeter, the invention can improve the measurement accuracy of the absolute gravitational acceleration by N times under the condition that other components of the absolute gravimeter remain unchanged.

Before the absolute gravity test, it is necessary to close the aperture 9 and adjust the direction of the test beam. Due to the reflection of the horizontal liquid surface 7, an interference image will be produced on the aperture 9, as shown by the dotted line in Figure 6. If the incident beam 12 and There is an included angle θ in the vertical direction, after multiple reflections by the reference prism 5 and the falling prism 6, it becomes a beam 13, which is incident on the horizontal liquid surface 7 at the same angle, and after reflection, the outgoing beam 14 and the beam 13 will be separated , the angle is 2θ, the outgoing beam 14 is reflected multiple times by the test prism 6 and the reference prism 5 to become the beam 15, the beam 15 is reflected by the beam splitter 4 to become the test beam 16, and the test beam 16 and the reflected prism 8 The reference beam 17 produces interference, and its included angle is α=2θ, which can form equally spaced interference fringes 18 on the diaphragm surface, adjust the adjustable mirror 3, and make the interference image gradually become a first-class light intensity circular spot 19, then Indicates that the angle between the two interference beams is extremely small, that is, θ=0, and the test beam is parallel to the direction of gravitational acceleration.

Absolute gravimeters generally use λ=632.8nm He-Ne frequency-stabilized laser as the light source, and the beam is expanded to 3mm by the collimator beam expander 2, which can be recognized by human eyes to ensure that the interference image on the surface of the diaphragm 9 is within half Within the fringe range, the maximum angle between the test beam and the vertical direction can be deduced to be The resulting measurement error is:

After the adjustable reflector 3 is adjusted so that the interference image on the surface of the diaphragm 9 becomes a circular spot 19 of first-class light intensity, the diaphragm 9 is opened to start absolute gravity measurement.

During the test, the aperture 9 can be closed at any time to observe whether the interference image on the aperture surface remains a circular spot 19 of equal intensity to monitor in real time whether the test beam is parallel to the vertical direction. If the interference image is not a circle of equal intensity Spot 19, adjust the adjustable reflector 3 until the interference image becomes a circular spot 19 with equal light intensity, close the diaphragm 9, and continue the absolute gravity test.

Claims (3)

1. An optical frequency-doubling laser interference system for a high-precision absolute gravimeter, characterized in that it includes a frequency-stabilized laser (1), a collimating beam expander (2), an adjustable mirror (3), and a beam splitter device (4), reference prism (5), falling prism (6), horizontal liquid surface (7), reflective prism (8), diaphragm (9), focusing lens (10), photodetector (11); laser It is emitted by the frequency-stabilized laser (1), is reflected by the adjustable reflector (3) after being collimated by the beam expander (2), and then transmitted vertically downward, and then divided by the beam splitter (4) for vertical downward transmission The test beam and the reference beam transmitted horizontally to the right, wherein the test beam is respectively reflected by the reference prism (5) and the falling prism (6) multiple times, then shoots to the horizontal liquid surface (7), and then passes through the horizontal liquid surface (7) After reflection, it returns to the beam splitter (4) according to the original optical path and merges with the reference beam reflected by the reflective prism (8) to generate interference fringes. The interference beam passes through the opened diaphragm (9) and is focused on the photoelectric on the detector (11); The transmittance of the beam splitter (4) can make the light intensity of the test beam and the reference beam equal when interference occurs. 2. The system according to claim 1, characterized in that the horizontal liquid level (7) is alcohol or mercury. 3. An absolute gravimeter employing the system according to any one of claims 1-2.