A cold atom gravity gradiometer based on hollow-core optical fiber guidance

Abstract

The present invention relates to a cold atom gravity gradiometer based on hollow-core fiber guidance, comprising a beam coupling module: a first 780nm laser is coupled into the optical fiber after transmission and polarization state adjustment, and a second 780nm laser is combined with an 852nm guide light after transmission and polarization state adjustment, and then enters the optical fiber; an atom guidance module: a first gradient magnetic field coil and a first cooling light form a two-dimensional magneto-optical trap, and the resulting low-speed atomic beam flows through the 852nm guide light and enters the optical fiber; an atom cooling module: two magneto-optical traps are provided in the hollow-core optical fiber, forming a first cold atom cluster and a second cold atom cluster at the center of each magnetic field, and the line connecting the centers of the two cold atom clusters is collinear with the optical fiber axis, which is the measurement reference line; and a gradient measurement module: the fluorescence signal generated by the spontaneous radiation of the two cold atom clusters in the hollow-core optical fiber is collected and the gravity gradient between the two measurement points is derived. The present invention can significantly reduce the system size, complexity and power consumption.

Description

Technical Field

[0001] The present invention relates to the field of cold atom interferometric inertial measurement technology, and in particular to a cold atom gravity gradiometer guided by a hollow-core optical fiber. Background Art

[0002] Since the new century, with the maturity of quantum precision measurement technology, the interferometric cold atom gravity gradiometer has developed rapidly. The cold atom gravity gradiometer consists of two cold atom gravimeters of the same specifications on the same measuring platform with a fixed spacing in the direction of the measurement reference line. Compared with the relative gravity measurement instrument based on the force balance structure, the advantages of the cold atom gravity gradiometer include: ① high data refresh rate; ② no mechanical moving parts; ③ the measurement results reflect the absolute gravity information, which is not affected by drift and theoretically has super high measurement sensitivity. The current cold atom gravity gradiometer is mainly based on the free-space cold atom gravimeter: In 2009, Stanford University used a spatially separated Raman laser scheme to build a gravity gradient measurement system, which measured the 250×10 -9 s -2 Gravity gradient characteristics; in the same year, Wu of Stanford University achieved 7×10 -9 s -2 In 2022, the University of Birmingham used a mobile cold-atom gravity gradiometer to achieve high-precision mapping of underground tunnels, achieving a measurement sensitivity of 20E in a high-noise outdoor environment (Nature 602, 590–594(2022)). In this cold-atom gravity gradiometer, common-mode noise such as platform vibration noise, Raman optical phase noise, and laser intensity noise in the measurement system are all suppressed.

[0003] However, when the above system is measuring a non-vertical gravity gradient, the free fall of atoms will cause the atomic position to move vertically relative to the Raman laser. At this time, a large-scale or spatially separated Raman laser solution is required, which greatly increases the system power consumption, size and complexity. Summary of the Invention

[0004] The technical problems to be solved by the present invention are:

[0005] To address these issues, the present invention proposes a cold-atom gravity gradiometer based on hollow-core fiber guidance. This approach utilizes a far-detuned guiding light field to load atoms into the hollow-core fiber. The atoms are trapped in a dipole potential well radially along the hollow-core fiber, allowing them to move freely along the fiber's axis. A Raman laser with a diameter of only tens of microns is required to measure the gravity gradient along the fiber's axis, significantly reducing system size, complexity, and power consumption.

[0006] The technical solutions adopted are as follows:

[0007] A cold atom gravity gradiometer based on hollow-core optical fiber guidance, comprising:

[0008] A beam coupling module includes a first 780nm laser, a second 780nm laser, and an 852nm guide light; the first 780nm laser is coupled into the hollow-core optical fiber after transmission and polarization adjustment, and the second 780nm laser is combined with the 852nm guide light after transmission and polarization adjustment, and the combined lasers are coupled into the hollow-core optical fiber;

[0009] The atom guidance module includes a two-dimensional magneto-optical trap; the low-speed atomic beam formed by the two-dimensional magneto-optical trap enters the hollow-core optical fiber under the action of 852nm guiding light;

[0010] The atomic cooling module includes a first magneto-optical trap and a second magneto-optical trap within a hollow-core optical fiber. The two magneto-optical traps form a first cold atom cluster and a second cold atom cluster at the centers of their respective magnetic fields. The line connecting the centers of the two cold atom clusters is collinear with the axis of the hollow-core optical fiber, which serves as the measurement reference line.

[0011] The gradient measurement module includes a bias magnetic field coil, a first atomic interference system, a second atomic interference system, a first detection system, and a second detection system; the bias magnetic field coil defines the quantization axis by providing a bias magnetic field, causing the Zeeman sub-energy levels of the atoms to split, thereby facilitating subsequent initial state preparation and velocity selection of the atoms; the first and second atomic interference systems respectively perform interference measurement at two measurement points on the measurement baseline, and the generated output signals are collected by the first and second detection systems respectively.

[0012] Furthermore, the beam coupling module also includes a first single-mode polarization-maintaining fiber, a first fiber collimator, a first liquid crystal variable retarder, a first long-wavelength pass dichroic mirror, a first focusing lens, a second single-mode polarization-maintaining fiber, a second fiber collimator, a second liquid crystal variable retarder, a second long-wavelength pass dichroic mirror, a third single-mode polarization-maintaining fiber and a third fiber collimator; the first 780nm laser is transmitted through the first single-mode polarization-maintaining fiber and the first fiber collimator, and after transmission, enters the first liquid crystal variable retarder to adjust its polarization state, and then the output laser is reflected by the first long-wavelength pass dichroic mirror and enters the first focusing lens, and then coupled into the hollow-core fiber; the second 780nm laser is transmitted through the second single-mode polarization-maintaining fiber and the second fiber collimator, and then enters the second liquid crystal variable retarder to adjust its polarization state, and then the output laser passes through the second long-wavelength pass dichroic mirror and is combined with the 852nm guide light; the 852nm guide light is transmitted by the third single-mode polarization-maintaining fiber and the third fiber collimator; the combined laser is coupled into the hollow-core fiber through the second focusing lens.

[0013] Furthermore, the first 780nm laser and the second 780nm laser in the beam coupling module correspond to lasers for different purposes at different stages of gravity gradient measurement, including cooling light, detection light, resonance blow-off light and Raman light.

[0014] Furthermore, the two-dimensional magneto-optical trap in the atom guiding module is composed of a first gradient magnetic field coil and a first cooling light, and forms a low-speed atomic beam under the joint action of the first 780nm laser and the cooling light of the second 780nm laser.

[0015] Furthermore, in the atomic cooling module, the first magneto-optical trap is composed of a second gradient magnetic field coil, a second cooling light, the cooling light in the first 780nm laser, and the cooling light in the second 780nm laser; the second magneto-optical trap is composed of a third gradient magnetic field coil, a third cooling light, the cooling light in the first 780nm laser, and the cooling light in the second 780nm laser.

[0016] Furthermore, the cooling light is composed of two groups of mutually orthogonal opposing lasers, wherein the second cooling light and the third cooling light are vertically incident into the hollow-core optical fiber from the side.

[0017] Furthermore, in the gradient measurement module, the first detection system consists of a first fluorescence detection lens, a first photodetector, and detection light from a first 780nm laser; the second detection system consists of a second fluorescence detection lens, a second photodetector, and detection light from a second 780nm laser.

[0018] Furthermore, in the gradient measurement module, the first atomic interference system is composed of the first cold atomic group, the Raman light of the first 780nm laser and the second 780nm laser; the second atomic interference system is composed of the second cold atomic group, the Raman light of the first 780nm laser and the second 780nm laser; wherein the Raman light of the first 780nm laser and the second 780nm laser is composed of a pair of pulsed lasers with a frequency difference of 6.834Ghz and a constant phase difference, which are divided into "π / 2-π-π / 2" Raman light pulses according to the action time; after the cold atomic group interacts with the "π / 2-π-π / 2" Raman light pulse, it will split, reflect, and combine the beams to form a closed interference loop and output a signal related to the gravity of the measurement point; finally, the gravity gradient between the two measurement points can be derived based on the output signals of the two atomic interference systems.

[0019] The method for measuring using the cold atom gravity gradiometer based on hollow-core optical fiber guidance specifically includes the following steps:

[0020] S1: In the cooling stage, the following steps are included:

[0021] S11: Turn on the two-dimensional magneto-optical trap to form a long strip-shaped atomic ensemble in the hollow fiber cavity, then turn on the cooling light in the first and second 780nm lasers and couple them into the fiber from both sides of the end face. When the intensities of the two 780nm lasers do not match, the two 780nm lasers and It is equivalent to a beam of push light

[0022] S12: Turn on the 852nm guiding light. The far-infrared detuned guiding light causes the atoms to produce an energy level frequency shift proportional to the light intensity. Under the action of the guiding light dipole force, the low-speed atomic beam will be coupled into the hollow-core fiber, forming a continuous atomic beam in the fiber core;

[0023] S13: The two magneto-optical traps (MOTs) in the hollow-core fiber are opened sequentially to cool and capture atoms from the low-speed atomic beam. The gradient magnetic fields of the two MOTs are then turned off, gradually reducing the power of the cooling light in the MOTs and increasing the detuning over a few milliseconds. This allows for polarization gradient cooling of the trapped cold atomic clusters, further reducing the average atomic temperature and breaking the Doppler cooling limit.

[0024] S2: In the state selection stage, the cooled atoms are at various Zeeman sub-levels of the |F=2> state. The bias magnetic field in the axial direction of the hollow-core fiber is turned on, and the quantization axis is defined by the bias magnetic field, so that the Zeeman sub-levels of the atoms are split. Then, the Raman light π pulses in two 780nm lasers are used to split the atoms at |F=2, m F =0> state atoms are pumped to |F=1,m F =0> state; finally, the resonance blow-out light in the two 780nm laser beams is turned on to blow away the atoms in other Zeeman sub-levels and away from the interference path; through the initial state preparation, the atoms are now in the magnetically insensitive state|m F =0>, which can eliminate the atomic interference phase shift caused by the ambient magnetic field in the subsequent interference process;

[0025] S3: During the interferometry phase, two cold atomic clusters interact with two Raman pulses from 780nm lasers, forming two atomic interferometry systems. These systems can determine the gravitational force components along the fiber axis at two measurement points. A typical atomic interferometry sequence consists of three sets of Raman pulses, "π / 2-π-π / 2."

[0026] Before the interference sequence, the atoms are in the same initial state |F=1, p>, where |F=1> is the energy level of the atom, i.e., the initial internal state; p represents the initial momentum of the atom, i.e., the initial external state; Raman light can make the atoms transition between different ground state energy levels (|F=1>, |F=2>), while changing the momentum of the atomic matter wave packet after the transition; after the first group of π / 2 Raman light pulses act on the atoms, the atomic matter wave packet is split, and the atoms are evenly distributed between |F=1, p> and In terms of state, is the reduced Planck constant, k eff is the equivalent wave vector of the Raman light pulse; after a time T of evolution, atoms in different states separate due to momentum difference, forming the two arms of the interference system; after the second group of π Raman light pulses acts on the atoms, the atomic states flip and the atomic matter wave packets are reflected; after another time T of evolution, the third group of π / 2 Raman light pulses acts on the atoms, causing the atomic matter wave packets of different paths to merge into a beam, forming a closed interference loop;

[0027] S4: In the detection phase, the internal states of the two cold atomic clusters after the combined beams are detected using the probe light from two 780nm laser beams. The population distribution of atoms on the two ground state energy levels |F=1> and |F=2> depends on the phase difference experienced by the atoms on different interference paths. After a typical atomic interference sequence is completed, the ratio P of atoms in the |F=2> energy level to the total number of atoms in |F=1> and |F=2> can be expressed as:

[0028]

[0029] Among them, the phase difference It can be expressed as:

[0030]

[0031] Among them, k eff is the equivalent wave vector of the Raman light pulse, g is the gravitational acceleration component along the optical fiber axis at the measurement point, T is the interference evolution time, and are the initial phases of three groups of Raman light pulses;

[0032] In the atomic interference system, the ratio P can be obtained through the detection system, and the phase difference can be calculated The gravitational acceleration component g along the optical fiber axis at the measurement point can be further calculated;

[0033] In summary, the two atomic interferometer systems can respectively obtain the gravity components along the optical fiber axis at the two measurement points. Further deduction can be used to obtain the gravity gradient between the two measurement points:

[0034]

[0035] Among them, g 1a and g 2a are the components of the gravitational acceleration along the optical fiber axis at the two measurement points, and L is the distance between the two measurement points.

[0036] The beneficial effects of the present invention are:

[0037] 1. During the entire measurement process of the gradiometer of the present invention, the atomic clusters and Raman lasers are always located within a quasi-one-dimensional hollow-core optical fiber. Therefore, only a Raman laser with a diameter of tens of microns is needed to measure the gravity gradient along the axis of the optical fiber, which can significantly reduce the system size and power consumption.

[0038] 2. The gradiometer of the present invention can realize gravity gradient measurement in any direction by changing the direction of the optical fiber, and in principle has the potential for full tensor gravity gradient measurement.

[0039] 3. During the gradiometer measurement process of the present invention, the movement of atoms in the non-measurement direction is restricted by the guiding potential field. Therefore, the relative displacement of the atomic cluster and the Raman laser caused by the carrier movement is greatly suppressed, and the dynamic measurement performance of the system is expected to be greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0041] Figure 1 This is a block diagram of the cold atom gravity gradiometer system based on hollow-core fiber guidance;

[0042] Figure 2 This is the working process of the cold atom gravity gradiometer based on hollow-core fiber guidance.

[0043] The reference numerals are as follows:

[0044] 01-First single-mode polarization-maintaining fiber 02-First fiber collimator 03-First 780nm laser

[0045] 04-First liquid crystal variable retarder 05-First long-wave pass dichroic mirror 06-First focusing lens

[0046] 07-First gradient magnetic field coil 08-First cooling light 09-Low-speed atomic beam

[0047] 10-Hollow core fiber 11-Second gradient magnetic field coil 12-Second cooling light

[0048] 13-first cold atomic group 14-baseline 15-first fluorescence detection lens

[0049] 16-first photodetector 17-third gradient magnetic field coil 18-third cooling light

[0050] 19-second cold atomic group 20-second fluorescence detection lens 21-second photodetector

[0051] 22-second focusing lens 23-second long-wavelength pass dichroic mirror 24-second liquid crystal variable retarder

[0052] 25-second 780nm laser 26-second fiber collimator 27-second single-mode polarization-maintaining fiber

[0053] 28-852nm laser 29-third fiber collimator 30-third single-mode polarization-maintaining fiber

[0054] 31-Vacuum system 32-Bias magnetic field coil DETAILED DESCRIPTION

[0055] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making any creative efforts shall fall within the scope of protection of the present invention.

[0056] The present invention provides a cold atom gravity gradiometer based on hollow-core optical fiber guidance, such as Figure 1 As shown, it includes a first single-mode polarization-maintaining fiber 1, a first fiber collimator 2, a first 780 nm laser 3, a first liquid crystal variable retarder 4, a first long-wavelength pass dichroic mirror 5, a first focusing lens 6, a first gradient magnetic field coil 7, a first cooling light 8, a low-speed atomic beam 9, a hollow-core fiber 10, a second gradient magnetic field coil 11, a second cooling light 12, a first cold atomic group 13, a reference line 14, a first fluorescence detection lens 15, a first photodetector 16, a third gradient magnetic field coil 17, a third cooling light 18, a second cold atomic group 19, a second fluorescence detection lens 20, a second photodetector 21, a second focusing lens 22, a second long-wavelength pass dichroic mirror 23, a second liquid crystal variable retarder 24, a second 780 nm laser 25, a second fiber collimator 26, a second single-mode polarization-maintaining fiber 27, an 852 nm guide light 28, a third fiber collimator 29, a third single-mode polarization-maintaining fiber 30, a vacuum system 31, and a bias magnetic field coil 32.

[0057] A first 780nm laser beam 3 is transmitted through the first single-mode polarization-maintaining fiber 1. This laser beam passes through the first fiber collimator 2 and enters the first liquid crystal variable retarder 4. The first liquid crystal variable retarder 4 can change the polarization state of the first 780nm laser beam 3. The output laser beam passes through the first long-wavelength pass dichroic mirror 5, is reflected by the first focusing lens 6, and then is coupled into the hollow-core fiber 10. A second 780nm laser beam 25 is transmitted through the second single-mode polarization-maintaining fiber 27. This laser beam passes through the second fiber collimator 26 and enters the second liquid crystal variable retarder 24. The second liquid crystal variable retarder 24 can change the polarization state of the second 780nm laser beam 25. The output laser beam passes through the second long-wavelength pass dichroic mirror 23 and is then combined with the 852nm laser beam 28. The 852nm laser beam 28 is transmitted through the third single-mode polarization-maintaining fiber 30 and the third fiber collimator 29. The combined laser beam is coupled into the hollow-core fiber 10 through the second focusing lens 22.

[0058] Within vacuum system 31, first gradient magnetic field coil 7 and first cooling light 8 form a two-dimensional magneto-optical trap (2D-MOT). The combined effects of first 780nm laser 3 and cooling light from second 780nm laser 25 form a low-speed atomic beam 9, which is then guided into hollow-core fiber 10 by 852nm laser 28. The first 780nm laser 3 and second 780nm laser 25 serve different purposes at different stages of gravity gradient measurement, including cooling light, probe light, resonance blow-off light, and Raman light.

[0059] Within hollow-core fiber 10, the second gradient magnetic field coil 11, the second cooling light 12, and the cooling light from the first 780nm laser 3 and the second 780nm laser 25 form a first magneto-optical trap (MOTIF). The third gradient magnetic field coil 17, the third cooling light 18, the cooling light from the first 780nm laser 3 and the second 780nm laser 25 form a second magneto-optical trap. These two magneto-optical traps form a first cold atom cluster 13 and a second cold atom cluster 19 at their respective magnetic field centers. The line connecting the centers of the two cold atom clusters is collinear with the axis of hollow-core fiber 10 and serves as the measurement reference line 14.

[0060] The first cooling light 8, the second cooling light 12 and the third cooling light 18 are all composed of two groups of mutually orthogonal opposing lasers, wherein the second cooling light 12 and the third cooling light 18 are vertically incident into the hollow-core optical fiber 10 from the side.

[0061] The bias magnetic field coil 32 defines the quantization axis by providing a bias magnetic field, causing the Zeeman sub-energy levels of the atoms to split, facilitating the subsequent initial state preparation and velocity selection of the atoms; the first fluorescence detection lens 15, the first photodetector 16 and the detection light in the first 780nm laser 3 constitute the first detection system, and the second fluorescence detection lens 20, the second photodetector 21 and the detection light in the first 780nm laser 3 constitute the second detection system. The Raman light in the first cold atomic group 13, the first 780nm laser 3 and the second 780nm laser 25 constitute the first atomic interference system, and the Raman light in the second cold atomic group 19, the first 780nm laser 3 and the second 780nm laser 25 constitute the second atomic interference system. The first atomic interference system and the second atomic interference system perform interference measurements at two measurement points on the measurement reference line 14, and the generated output signals are collected by the first detection system and the second detection system, respectively. Finally, the gravity gradient between the two measurement points can be derived based on the output signals of the two atomic interference systems.

[0062] The basic working process of measuring using the above gradiometer includes cooling, state selection, interference and detection stages, such as Figure 2 As shown, the details are as follows:

[0063] S1: In the cooling stage, the following steps are included:

[0064] S11: Turn on the two-dimensional magneto-optical trap (2D-MOT) to form a long strip-shaped atomic ensemble in the cavity. Then turn on the cooling light from the two 780nm laser beams and couple them into the optical fiber from both ends. When the intensity of the two laser beams does not match, the two 780nm laser beams and It is equivalent to a beam of push light

[0065]

[0066] in, is the reduced Planck constant, is the laser wave vector, Γ is the natural line width, I s is the saturation absorption intensity, I1 and I2 are the intensities of the two cooling beams, and Δ1 and Δ2 are the detuning of the two laser beams relative to the resonant transition frequency. At this time, the atomic ensemble in a non-equilibrium state forms a continuous low-speed atomic beam under the action of the push light.

[0067] S12: Turn on the 852nm guiding light. The far-infrared detuned guiding light causes the atoms to produce an energy level frequency shift (AC-Stark shift) proportional to the light intensity. Because the guiding light intensity in the fiber approximately follows a Gaussian distribution, the dipole potential energy of the atoms at the fiber core is lowest. Therefore, the atoms are always acted upon by the dipole force directed toward the fiber core, and their radial motion is constrained. The magnitude of the potential field U can be calculated using the following formula:

[0068]

[0069] Where c is the speed of light, ω0 is the angular frequency of the guiding light, Δ is the detuning of the guiding light relative to the resonant transition frequency, I(x,y,z) is the Gaussian distributed guiding light intensity, and (x,y,z) is the three-dimensional spatial coordinate.

[0070] Under the action of the guiding light dipole force, the low-speed atomic beam will be coupled into the hollow-core optical fiber, forming a continuous atomic beam in the fiber core.

[0071] S13: The two magneto-optical traps in the optical fiber are opened successively to cool and capture atoms from the low-speed atomic beam; then the gradient magnetic fields of the two magneto-optical traps are turned off, gradually weakening the power of the cooling light in the magneto-optical traps and increasing the detuning within a few milliseconds, performing polarization gradient cooling on the captured cold atomic clusters, further reducing the average temperature of the atoms and breaking the Doppler cooling limit.

[0072] S2: In the state selection stage, the cooled atoms are at various Zeeman sub-levels of the |F=2> state. The bias magnetic field in the hollow-core fiber axis is turned on to define the quantization axis, causing the Zeeman sub-levels of the atoms to split. Then, the Raman light π pulse in the 780nm laser is used to split the atoms at |F=2, m F =0> state atoms are pumped to |F=1,m F =0> state; finally, the resonance blow-out light in the 780nm laser is turned on to blow away the atoms in other Zeeman sub-levels and away from the interference path. Through the initial state preparation, the atoms are now in the magnetically insensitive state|m F =0>, the atomic interference phase shift caused by the ambient magnetic field in the subsequent interference process can be eliminated.

[0073] S3: In the interference stage, two cold atomic groups interact with the Raman light pulses in the 780nm laser to form two atomic interference systems, which can respectively obtain the gravitational components of the two measurement points along the optical fiber axis; a typical atomic interference sequence is composed of three groups of Raman light pulses "π / 2-π-π / 2". Before the interference sequence, the atoms are in the same initial state |F=1, p> (the ground state energy level and momentum are the same). Raman light can make atoms transition between different ground state energy levels (|F=1>, |F=2>), and at the same time change the momentum of the atomic matter wave packet after the transition. After the first group of π / 2 Raman light pulses acts on the atoms, the atomic matter wave packet is split, and the atoms are evenly distributed in |F=1, p> and state; after T time evolution, atoms in different states separate due to momentum difference, forming the two arms of the interference system; after the second group of π Raman light pulses acts on the atoms, the atomic states are flipped and the atomic matter wave packets are reflected; after T time evolution again, the third group of π / 2 Raman light pulses acts on the atoms, causing the atomic matter wave packets of different paths to merge into a beam, forming a closed interference loop.

[0074] S4: In the detection phase, the internal states of the two cold atomic clusters after the combined beam are detected using the probe light from the 780nm laser. The population distribution of atoms on the two ground state energy levels |F=1> and |F=2> depends on the phase difference experienced by the atoms on different interference paths. After a typical atomic interference sequence is completed, the ratio P of atoms in the |F=2> energy level to the total number of atoms (|F=1> and |F=2>) can be expressed as:

[0075]

[0076] Among them, the phase difference It can be expressed as:

[0077]

[0078] Among them, k eff is the equivalent wave vector of the Raman light pulse, g is the gravitational acceleration component along the optical fiber axis at the measurement point, T is the interference evolution time, and are the initial phases of three groups of Raman light pulses respectively.

[0079] In the atomic interference system, the ratio P can be obtained through the detection system, and the phase difference can be calculated by formula (3) The gravitational acceleration component g along the fiber axis at the measurement point can be further calculated from formula (4). In summary, the gravitational components along the fiber axis at the two measurement points can be obtained respectively through the two atomic interference systems. Further deduction can be used to obtain the gravity gradient between the two measurement points:

[0080]

[0081] Among them, g 1a and g 2a are the components of the gravitational acceleration along the optical fiber axis at the two measurement points, and L is the distance between the two measurement points.

[0082] Obviously, the above examples are merely illustrative and are not intended to limit the embodiments. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all embodiments here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A cold atom gravity gradiometer based on hollow-core optical fiber guidance, characterized in that: include: A beam coupling module includes a first 780nm laser, a second 780nm laser, and an 852nm guide light; the first 780nm laser is coupled into the hollow-core optical fiber after transmission and polarization adjustment, and the second 780nm laser is combined with the 852nm guide light after transmission and polarization adjustment, and the combined lasers are coupled into the hollow-core optical fiber; The atom guidance module includes a two-dimensional magneto-optical trap; the low-speed atomic beam formed by the two-dimensional magneto-optical trap enters the hollow-core optical fiber under the action of 852nm guiding light; The atomic cooling module includes a first magneto-optical trap and a second magneto-optical trap within a hollow-core optical fiber. The two magneto-optical traps form a first cold atom cluster and a second cold atom cluster at the centers of their respective magnetic fields. The line connecting the centers of the two cold atom clusters is collinear with the axis of the hollow-core optical fiber, which serves as the measurement reference line. The gradient measurement module includes a bias magnetic field coil, a first atomic interference system, a second atomic interference system, a first detection system, and a second detection system; the bias magnetic field coil defines the quantization axis by providing a bias magnetic field, causing the Zeeman sub-energy levels of the atoms to split, thereby facilitating subsequent initial state preparation and velocity selection of the atoms; the first and second atomic interference systems respectively perform interference measurement at two measurement points on the measurement baseline, and the generated output signals are collected by the first and second detection systems respectively.

2. The cold atom gravity gradiometer based on hollow-core optical fiber guidance according to claim 1, characterized in that: The beam coupling module also includes a first single-mode polarization-maintaining fiber, a first fiber collimator, a first liquid crystal variable retarder, a first long-wavelength pass dichroic mirror, a first focusing lens, a second single-mode polarization-maintaining fiber, a second fiber collimator, a second liquid crystal variable retarder, a second long-wavelength pass dichroic mirror, a third single-mode polarization-maintaining fiber, and a third fiber collimator. A first 780nm laser is transmitted through the first single-mode polarization-maintaining fiber and the first fiber collimator, and then enters the first liquid crystal variable retarder to adjust its polarization state. The output laser then passes through the first long-wavelength pass dichroic mirror and is reflected into the first focusing lens, and then is coupled into the hollow-core fiber. A second 780nm laser is transmitted through the second single-mode polarization-maintaining fiber and the second fiber collimator, and then enters the second liquid crystal variable retarder to adjust its polarization state. The output laser then passes through the second long-wavelength pass dichroic mirror and is combined with the 852nm guide light. The 852nm guide light is transmitted by the third single-mode polarization-maintaining fiber and the third fiber collimator. The combined laser is coupled into the hollow-core fiber through the second focusing lens.

3. The cold atom gravity gradiometer based on hollow-core optical fiber guidance according to claim 2, characterized in that: The first 780nm laser and the second 780nm laser in the beam coupling module correspond to lasers for different purposes at different stages of gravity gradient measurement, including cooling light, detection light, resonance blow-off light and Raman light.

4. The cold atom gravity gradiometer based on hollow-core optical fiber guidance according to claim 3, characterized in that: The two-dimensional magneto-optical trap in the atom guiding module is composed of a first gradient magnetic field coil and a first cooling light, and forms a low-speed atomic beam under the joint action of the first 780nm laser and the second 780nm laser cooling light.

5. The cold atom gravity gradiometer based on hollow-core optical fiber guidance according to claim 4, characterized in that: The first magneto-optical trap in the atomic cooling module is composed of a second gradient magnetic field coil, a second cooling light, the cooling light in the first 780nm laser, and the cooling light in the second 780nm laser; the second magneto-optical trap is composed of a third gradient magnetic field coil, a third cooling light, the cooling light in the first 780nm laser, and the cooling light in the second 780nm laser.

6. A cold atom gravity gradiometer based on hollow-core optical fiber guidance according to claim 4 or 5, characterized in that: The cooling light is composed of two groups of mutually orthogonal opposite lasers, wherein the second cooling light and the third cooling light are vertically incident into the hollow-core optical fiber from the side.

7. The cold atom gravity gradiometer based on hollow-core optical fiber guidance according to claim 6, characterized in that: The first detection system in the gradient measurement module consists of a first fluorescence detection lens, a first photodetector, and detection light from a first 780nm laser; the second detection system consists of a second fluorescence detection lens, a second photodetector, and detection light from a second 780nm laser.

8. The cold atom gravity gradiometer based on hollow-core optical fiber guidance according to claim 7, characterized in that: In the gradient measurement module, the first atom interference system is composed of a first cold atom group, Raman light from a first 780nm laser and a second 780nm laser; the second atom interference system is composed of a second cold atom group, Raman light from a first 780nm laser and a second 780nm laser; wherein the Raman light from the first 780nm laser and the second 780nm laser is composed of a pair of pulsed lasers with a frequency difference of 6.834Ghz and a constant phase difference, and is divided into "π / 2-π-π / 2" Raman light pulses according to the action time; after the cold atom group interacts with the "π / 2-π-π / 2" Raman light pulses, they will split, reflect, and combine to form a closed interference loop and output a signal related to the gravity of the measurement point; finally, the gravity gradient between the two measurement points is derived based on the output signals of the two atom interference systems.

9. A method for measuring using the cold atom gravity gradiometer based on hollow-core fiber guidance according to claim 8, characterized in that: The specific steps include: S1: In the cooling stage, the following steps are included: S11: Turn on the two-dimensional magneto-optical trap to form a long strip-shaped atomic ensemble in the hollow fiber cavity, then turn on the cooling light in the first and second 780nm lasers and couple them into the fiber from both sides of the end face. When the intensities of the two 780nm lasers do not match, the two 780nm lasers and It is equivalent to a beam of push light S12: Turn on the 852nm guiding light. The far-infrared detuned guiding light causes the atoms to produce an energy level frequency shift proportional to the light intensity. Under the action of the guiding light dipole force, the low-speed atomic beam will be coupled into the hollow-core fiber, forming a continuous atomic beam in the fiber core; S13: The two magneto-optical traps (MOTs) in the hollow-core fiber are opened sequentially to cool and capture atoms from the low-speed atomic beam. The gradient magnetic fields of the two MOTs are then turned off, gradually reducing the power of the cooling light in the MOTs and increasing the detuning over a few milliseconds. This allows for polarization gradient cooling of the trapped cold atomic clusters, further reducing the average atomic temperature and breaking the Doppler cooling limit. S2: In the state selection stage, the cooled atoms are at various Zeeman sub-levels of the |F=2> state. The bias magnetic field in the axial direction of the hollow-core fiber is turned on, and the quantization axis is defined by the bias magnetic field, so that the Zeeman sub-levels of the atoms are split. Then, the Raman light π pulses in two 780nm lasers are used to split the atoms at |F=2, m F =0> state atoms are pumped to |F=1,m F =0> state; finally, the resonance blow-out light in the two 780nm laser beams is turned on to blow away the atoms in other Zeeman sub-levels and away from the interference path; through the initial state preparation, the atoms are now in the magnetically insensitive state|m F =0>, which can eliminate the atomic interference phase shift caused by the ambient magnetic field in the subsequent interference process; S3: During the interferometry phase, two cold atomic clusters interact with two Raman pulses from 780nm lasers, forming two atomic interferometry systems. The gravitational components along the fiber axis at two measurement points are obtained. A typical atomic interferometry sequence consists of three groups of Raman pulses, "π / 2-π-π / 2." Before the interference sequence, the atoms are in the same initial state |F=1, p>, where |F=1> is the energy level of the atom, i.e., the initial internal state; p represents the initial momentum of the atom, i.e., the initial external state; Raman light causes the atoms to transition between different ground state energy levels, while changing the momentum of the atomic matter wave packet after the transition; after the first group of π / 2 Raman light pulses acts on the atoms, the atomic matter wave packet is split, and the atoms are evenly distributed between |F=1, p> and In terms of state, is the reduced Planck constant, k eff is the equivalent wave vector of the Raman light pulse; after the evolution of T time, the atoms in different states are separated due to the momentum difference, forming the two arms of the interference system; after the second group of π Raman light pulses acts on the atoms, the atomic state is flipped and the atomic matter wave packet is reflected; after another evolution of T time, the third group of π / 2 Raman light pulses acts on the atoms, causing the atomic matter wave packets of different paths to merge into a beam, forming a closed interference loop; S4: In the detection stage, the detection light in the two 780nm laser beams is used to detect the internal state of the two cold atomic clusters after the beam is merged. The population distribution of atoms on the two ground state energy levels |F=1> and |F=2> depends on the phase difference experienced by the atoms on different interference paths After a typical atomic interference sequence is completed, the ratio P of atoms in the |F=2> energy level to the total number of atoms in |F=1> and |F=2> is expressed as: Among them, the phase difference Expressed as: Among them, k eff is the equivalent wave vector of the Raman light pulse, g is the gravitational acceleration component along the optical fiber axis at the measurement point, T is the interference evolution time, and are the initial phases of three groups of Raman light pulses; In the atomic interference system, the ratio P is obtained by the detection system, and the phase difference is calculated Further calculate the gravitational acceleration component g of the optical fiber axis at the measurement point; In summary, the gravity components along the optical fiber axis at the two measurement points are obtained through two atomic interferometer systems, and the gravity gradient between the two measurement points is further derived: Among them, g 1a and g 2a are the components of the gravitational acceleration along the optical fiber axis at the two measurement points, and L is the distance between the two measurement points.

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