Weak electric field measurement method based on orbital angular momentum light

Abstract

The invention discloses a weak electric field measurement method based on orbital angular momentum light, and relates to the field of electric field measurement. A 780nm laser beam is emitted by an external cavity semiconductor laser, the laser beam passes through a first half-wave plate and a quarter-wave plate and then enters a Sagnac loop interferometer with a first polarization beam splitter as an incidence end, and a spiral phase plate is inserted into a Sagnac loop to introduce orbital angular momentum for the laser beam; laser emitted from the first polarization beam splitter enters an electro-optical modulator in parallel, and light beams output by the electro-optical modulator respectively enter a second half-wave plate and a second polarization beam splitter for rotary polarization and horizontal polarization; a CCD is arranged on an output light path of the second polarization beam splitter to collect light spot signals, the light spot signals are connected into a computer to be subjected to image processing, and the electric field intensity is calculated. According to the scheme, the electric field intensity can be accurately measured without any interference on the measured electric field, and the method is suitable for measuring a sensitive sample and can be used for precisely measuring a weak electric field.

Description

Technical Field

[0001] The embodiments of the present application relate to the field of electric field measurement, and in particular to a weak electric field measurement method based on orbital angular momentum light. Background Art

[0002] Orbital angular momentum (OAM) light is a special light beam with a rotating phase structure that can carry additional angular momentum. This light beam has potential applications in many fields, including quantum information processing, optical trapping, and micromanipulation. In related technologies, some use the characteristics of OAM light to design a new type of electric field sensing technology. By superimposing light beams with different OAM states, a light field with a complex phase structure can be generated. When this light field interacts with an external electric field, its phase and intensity distribution will change. These changes can be detected by specific optoelectronic devices (such as photodetectors PD), thereby realizing the sensing of the electric field.

[0003] However, this technology is only suitable for measuring low-frequency electric fields, and the electric field needs to change periodically in order to infer the electric field strength from the center frequency signal strength. It is impossible to accurately measure the electric field strength when it does not change periodically, and the relationship between the electric field strength and the center frequency strength is nonlinear and logarithmic, which means that the relationship between the electric field strength and the center frequency strength is more complicated, and nonlinear calibration is required to accurately measure the electric field strength. Summary of the invention

[0004] The embodiment of the present application provides a weak electric field measurement method based on orbital angular momentum light, which solves the problems of limitations and complex calculations in the related art of using photodetectors to measure periodic low-frequency electric fields.

[0005] On the one hand, the present application provides a method for measuring weak electric fields based on orbital angular momentum light, the system comprising:

[0006] An external cavity semiconductor laser emits a 780nm laser beam, and after passing through a first half-wave plate and a quarter glass plate, the laser beam is incident on a Sagnac loop interferometer with a first polarization beam splitter as an incident end, and a spiral phase plate is inserted into the Sagnac loop to introduce orbital angular momentum into the laser beam;

[0007] The laser beam emitted from the first polarization beam splitter is incident on the electro-optic modulator in parallel, and the light beam outputted by the photoelectric regulator is incident on the second half-wave plate and the second polarization beam splitter respectively for rotational polarization and horizontal polarization; wherein the light beam incident on the electro-optic modulator is perpendicular to the direction of the electric field;

[0008] A beam quality analyzer CCD is set on the output optical path of the second polarization beam splitter to collect the spot signal, and the spot signal is connected to a computer for image processing to calculate the electric field intensity.

[0009] Specifically, the Sagnac loop includes three reflectors, and the spiral phase plate is located between the second reflector and the third reflector in the Sagnac loop; the horizontal polarization state H and the vertical polarization state V in the loop pass through the spiral phase plate in opposite directions to obtain orbital angular momentums of l and -l, respectively.

[0010] Specifically, the initial polarization state |Φ0> of the laser beam emitted by the external cavity semiconductor laser is expressed as follows:

[0011]

[0012] Wherein, H and V represent the horizontal polarization state H and vertical polarization state V components respectively.

[0013] Specifically, light beams carrying different polarization directions and opposite OAM modes are output from the first polarization beam splitter, and a polarization superposition state |Φ1> is generated along the same path, which is expressed as follows:

[0014]

[0015] Here, l represents orbital angular momentum, |l> and |-l> represent opposite OAM modes, respectively.

[0016] Specifically, in the electro-optic modulator, the temperature of the crystal is stably set at 25°C, and the superposition state |Φ2> of the output beam is expressed as follows:

[0017]

[0018] in, is the phase difference produced by the Pockels effect, λ is the free space wavelength, n e is the undisturbed refractive index, γ 33 It is the electro-optic tensor, the electric field intensity E = U / d.

[0019] Specifically, in the electro-optic modulator, the crystal length is L, the thickness is d, the electric field voltage U is generated by a signal generator and applied along the crystal axis transverse to the light propagation direction, and the value of the electric field voltage U is half of the peak-to-peak voltage Vpp.

[0020] Specifically, the optical axis direction of the second half-wave plate differs from the horizontal direction by 22.5 degrees.

[0021] Specifically, the two vortex lights passing through the second polarization beam splitter interfere with each other, and the beam superposition state |Φ3> is expressed as follows:

[0022]

[0023] The light signal collected by the CCD generates a petal-shaped light spot, which includes 21 petal-shaped interference light spots, which are centrally symmetrical and alternate between light and dark.

[0024] Specifically, the process of calculating the electric field strength includes:

[0025] The rotation center of the petals is extracted by small-step scanning, the image is rotated, and the maximum correlation coefficient method is used to obtain the relative rotation angle α between the two reference images and the rotated target image; it is expressed as follows:

[0026]

[0027] The actual value of the electric field strength E is obtained according to the rotation angle calculation formula.

[0028] Specifically, when the photoelectric regulator outputs a non-periodic electric field, the rotation center of the petals is extracted by small-step scanning under different electric field states, and the image is rotated to obtain the correlation coefficient between the reference image and the target image, and a relationship curve between the electric field strength and the rotation angle is plotted.

[0029] The beneficial effects brought by the technical solution provided by the embodiment of the present application include at least:

[0030] (1) High precision: This method can achieve 4.57×10 -2 The resolution of V / mm can accurately measure the electric field strength.

[0031] (2) Non-invasive: It will not cause any interference to the measured electric field and is suitable for measuring sensitive samples.

[0032] (3) Real-time measurement: The optical method can achieve real-time measurement and can be used to measure dynamic DC electric fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a system schematic diagram of a weak electric field measurement method based on orbital angular momentum light provided in an embodiment of the present application;

[0034] Figure 2 is a reference schematic diagram of a rotated image provided in an embodiment of the present application;

[0035] Figure 3 This is a curve diagram showing the change of rotation angle with DC electric field when l=2. DETAILED DESCRIPTION

[0036] In order to make the objectives, technical solutions and advantages of the present application clearer, the implementation methods of the present application will be further described in detail below with reference to the accompanying drawings.

[0037] The term "multiple" as used herein refers to two or more than two. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. The character " / " generally indicates that the related objects are in an "or" relationship.

[0038] Figure 1 It is a system schematic diagram of a weak electric field measurement method based on orbital angular momentum light provided by an embodiment of the present application. The system needs to emit a 780nm laser beam in an external cavity semiconductor laser as an emission source, and sequentially set a first half-wave plate (HWP) and a quarter glass plate (QWP) on the optical path of the laser beam to adjust the polarization state of the light beam. The laser beam after passing through the quarter glass plate (QWP) is incident on a Sagnac loop interferometer with a first polarization beam splitter (PBS1) as the incident end, and a spiral phase plate (VPP) is inserted from the Sagnac loop to introduce orbital angular momentum (OAM) into the laser beam.

[0039] See also Figure 1 As shown, the Sagnac loop interferometer in the present application uses the Sagnac effect to design a loop structure, which includes a PBS1, three reflectors M1-M3 placed at different angles, and the MPP is located between the M2 and M3 reflectors in the Sagnac loop. The laser emitted from PBS1 continues to be incident on the electro-optic modulator (EOM) in parallel. The EOM is connected to a high-voltage amplifier (HVA), and the HVA is controlled by a function generator (FG) input. The EOM to be measured in the present application is not limited to a low-frequency and periodically changing electric field environment, and even non-periodic measurements can be achieved. The light beams output by the EOM are respectively incident on the second half-wave plate (HWP2) and the second polarization beam splitter (PBS2) for rotational polarization and horizontal polarization. The light beam incident on the electro-optic modulator is perpendicular to the direction of the electric field.

[0040] A beam quality analyzer CCD is set on the output optical path of PBS2 to collect the spot signal. The CCD is connected to the computer PC through a harness, and the spot signal is connected to the computer for image processing. The electric field strength is indirectly calculated by calculating the spot rotation angle through the PC. Furthermore, for EOM with variable electric field, multiple spot signal images can be obtained, and then the transformation relationship diagram between electric field strength and rotation angle can be established, providing a more accurate measurement formula.

[0041] exist Figure 1In the design of the Sagnac loop, the horizontal polarization state H and the vertical polarization state V in the original loop can be passed through the spiral phase plate in the opposite direction to obtain the orbital angular momentum of l and -l respectively, and then the electric field can be measured based on the OAM principle in the subsequent process. The initial polarization state |Φ0> before the laser beam enters the Sagnac loop is expressed as follows:

[0042]

[0043] H and V represent the horizontal polarization state H and vertical polarization state V components respectively. After passing through the Sagnac loop, the light is focused, and beams with different polarization directions and opposite OAM modes are output from PBS1, and finally a polarization superposition state |Φ1> is generated along the same path, which can be expressed in the language of quantum mechanics as follows:

[0044]

[0045] Here, l represents orbital angular momentum, and |l> and |-l> represent opposite OAM modes, respectively. The VPP here converts Gaussian light into orbital angular momentum light.

[0046] In some possible implementations, in the electro-optic modulator, the propagation direction of light is perpendicular to the direction of the applied electric field. During this process, the temperature of the crystal is controlled by a precision temperature controller (QTECHNICS), and the temperature is stably set at 25C° with an accuracy of 2mK. The superposition state of the output light beam |Φ2< is expressed as follows:

[0047]

[0048] in, is the phase difference produced by the Pockels effect, λ is the free space wavelength, n e is the undisturbed refractive index, γ oo It is the electro-optic tensor, the electric field intensity E = U / d.

[0049] In some embodiments, magnesium oxide-doped lithium niobate (MgO:LiNbO3) is used in the electro-optical modulator, and the phase modulator is composed of an electro-optical crystal with a length of L and a crystal thickness of d. The voltage U is applied along the crystal axis transverse to the light propagation direction, which is generated by a signal generator and has a value of half the peak-to-peak voltage Vpp, and then the electrical signal is directly modulated onto the laser beam through the electro-optic effect.

[0050] After the light beam leaves the crystal, it rotates the light polarization through HWP2 and then projects it to the horizontal polarization through PBS2. The optical axis direction of HWP2 here is 22.5 degrees different from the horizontal direction. After being projected to the horizontal polarization through PBS2, the two vortex lights interfere with each other and a petal-shaped light spot is generated. The beam superposition state |Φ o> means the following:

[0051]

[0052] |Φ o The > state is the result of interference between two OAMs, which contains 21 petal-shaped interference spots, which are centrally symmetrical and alternate between light and dark. The petals will change with the phase difference. The change rotates around the center, and the phase difference Every time the light spot changes by 2π, it moves one cycle, that is, the corresponding rotation angle α is 2π / 2l. At this time, the relationship between the angle α and the electric field E can be obtained, which is expressed as follows:

[0053]

[0054] The specific push process is as follows:

[0055]

[0056] The electric field is generated by a signal generator (RIGOL DG822), and the signal source is amplified by a signal amplifier and finally loaded onto the EOM. The generated light spot is then collected by a CCD and connected to a computer for image processing.

[0057] In some embodiments, the process of calculating the electric field strength includes:

[0058] The rotation center of the petals is extracted by small-step scanning, the image is rotated, and the maximum correlation coefficient method is used to obtain the relative rotation angle α between the two reference images and the rotated target image. Specifically, the image can be analyzed by small-step scanning using matlab programming. When the rotation angle corresponding to the maximum correlation coefficient is obtained according to the image, the electric field intensity and the corresponding angle transformation relationship diagram can be drawn.

[0059] Method for measuring angles: Determining the rotation angle requires two basic operations on the two images: The first operation is image rotation, resulting in two images, one of which is the reference image and the other is the target image.

[0060] Figure 2 : is a reference diagram of a rotated image provided in an embodiment of the present application, in which the image on the left is a reference image and the image on the right is a rotated target image. The actual E value can be calculated according to the rotation angle.

[0061] For a variable electric field environment, the electric field correlation coefficient can be further calculated by scanning the rotation angle of the reference image in small steps and calculating the correlation coefficient between the reference image and the target image, where the value when the correlation coefficient reaches the maximum value is the rotation angle between the reference image and the target image. Then the electric field strength is changed to obtain the relationship between the electric field strength and the angle.

[0062] Figure 3 This is the curve of the rotation angle changing with the DC electric field when l = 2. The slope of the fitting line determines the sensitivity of the angle relative to the crystal temperature. The slope obtained by fitting is 0.219 deg / (v / mm). The numerical calculation angular resolution of our image processing program is 0.01 degrees, corresponding to an electric field intensity resolution of 4.57×10 -2 V / mm.

[0063] In particular, this solution uses CCD to directly collect the spot image and calculate the electric field value according to the rotation angle. It is different from the traditional solution using PD detection and OSC oscilloscope display, because the oscilloscope display solution needs to use ESA spectrometer to measure the spectrum diagram, and reflect the electric field strength according to the center frequency signal strength, and measure the low-frequency electric field of the object with periodic changes. The system and measurement scheme provided by this application no longer rely on the use of PD and spectrometer. The image processing method is used to directly calculate the electric field value according to the rotation angle of the spot interference. The electric field strength and correlation coefficient can be directly calculated according to the rotation angle without the periodic change of the electric field. It has a wider range of applications and can achieve high-precision measurement of the changing electric field.

[0064] In summary, the beneficial effects of this application agent opening include the following:

[0065] The optical electric field measurement method used for precise measurement of weak electric fields is a non-invasive measurement technology based on optical principles. Compared with traditional electric field measurement methods, this method has the following advantages:

[0066] (1) High precision: This method can achieve 4.57×10 -2 The resolution of V / mm can accurately measure the electric field strength.

[0067] (2) Non-invasive: It will not cause any interference to the measured electric field and is suitable for measuring sensitive samples.

[0068] (3) Real-time measurement: The optical method can achieve real-time measurement and can be used to measure dynamic DC electric fields.

[0069] This specific embodiment is merely an explanation of the present invention and is not a limitation of the present invention. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed. However, as long as they are within the scope of the claims of the present invention, they are protected by the patent law.

Claims

1. A method for measuring weak electric fields based on orbital angular momentum light, characterized in that: The system comprises: An external cavity semiconductor laser emits a 780nm laser beam, and after passing through a first half-wave plate and a quarter glass plate, the laser beam is incident on a Sagnac loop interferometer with a first polarization beam splitter as an incident end, and a spiral phase plate is inserted into the Sagnac loop to introduce orbital angular momentum into the laser beam; The laser beam emitted from the first polarization beam splitter is incident on the electro-optic modulator in parallel, and the light beam outputted by the photoelectric regulator is incident on the second half-wave plate and the second polarization beam splitter respectively for rotational polarization and horizontal polarization; wherein the light beam incident on the electro-optic modulator is perpendicular to the direction of the electric field; A beam quality analyzer CCD is set on the output optical path of the second polarization beam splitter to collect the spot signal, and the spot signal is connected to a computer for image processing to calculate the electric field intensity.

2. The method for measuring weak electric field based on orbital angular momentum light according to claim 1, characterized in that: The Sagnac loop includes three reflectors, and the spiral phase plate is located between the second reflector and the third reflector in the Sagnac loop; the horizontal polarization state H and the vertical polarization state V in the loop pass through the spiral phase plate in opposite directions, and obtain orbital angular momentums of l and -l respectively.

3. The method for measuring weak electric field based on orbital angular momentum light according to claim 2, characterized in that: The initial polarization state |φ0> of the laser beam emitted by an external cavity semiconductor laser is expressed as follows: Wherein, H and V represent the horizontal polarization state H and vertical polarization state V components respectively.

4. The method for measuring weak electric field based on orbital angular momentum light according to claim 3, characterized in that: The first polarization beam splitter outputs light beams with different polarization directions and opposite OAM modes, and generates a polarization superposition state |φ1> along the same path, which is expressed as follows: Here, l represents orbital angular momentum, |l> and |-l> represent opposite OAM modes, respectively.

5. The method for measuring weak electric field based on orbital angular momentum light according to claim 4, characterized in that: In the electro-optic modulator, the temperature of the crystal is stably set at 25°C, and the superposition state |Φ2> of the output beam is expressed as follows: in, is the phase difference produced by the Pockels effect, λ is the free space wavelength, n e is the undisturbed refractive index, γ 33 It is the electro-optic tensor, the electric field intensity E = U / d.

6. The method for measuring weak electric field based on orbital angular momentum light according to claim 5, characterized in that: In the electro-optic modulator, the crystal length is L, the thickness is d, the electric field voltage U is generated by a signal generator and applied along the crystal axis transverse to the light propagation direction, and the value of the electric field voltage U is half of the peak-to-peak voltage Vpp.

7. The method for measuring weak electric field based on orbital angular momentum light according to claim 1, characterized in that: The optical axis direction of the second half-wave plate differs from the horizontal direction by 22.5 degrees.

8. The method for measuring weak electric field based on orbital angular momentum light according to claim 6, characterized in that: The two vortex lights passing through the second polarization beam splitter produce interference, and the beam superposition state |φ o > means the following: The light signal collected by the CCD generates a petal-shaped light spot, which includes 21 petal-shaped interference light spots, which are centrally symmetrical and alternate between light and dark.

9. The method for measuring weak electric field based on orbital angular momentum light according to claim 8, characterized in that: The process of calculating the electric field strength includes: The rotation center of the petals is extracted by small-step scanning, the image is rotated, and the maximum correlation coefficient method is used to obtain the relative rotation angle α between the two reference images and the rotated target image; it is expressed as follows: The actual value of the electric field strength E is obtained according to the rotation angle calculation formula.

10. The method according to any one of claims 1 to 9, characterized in that: When the photoelectric regulator outputs a non-periodic electric field, the rotation center of the petals is extracted by small-step scanning under different electric field states, and the image is rotated to obtain the correlation coefficient between the reference image and the target image, and a relationship curve between the electric field strength and the rotation angle is plotted.

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