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Focus field



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The high-energy focused field-effect ablation system brings new hope for prostate patients to get rid of chronic diseases. The system has been recognized by many domestic medical units such as Peking University Hospital and Peking Union Medical College Hospital, and has become the preferred technology for the treatment of prostate diseases. It uses the detoxification, biological and thermal effects produced by the high-energy focused current field to dredge the small glands of the prostate, prompting the removal of bacterial toxins and purulent obstructions with the catheter, thereby improving the internal environment of the glands, eliminating the symptoms of prostate disease, and achieving treatment Purpose. The treatment requires no surgery, no pain, no trauma, and no side effects. Clinical experiments have proved that the system has an effective rate of 96% for prostatitis and benign prostatic hyperplasia.

◆Detox effect: The high-frequency focused current field directly acts on the lesion, killing or inhibiting the growth of bacteria, and expanding various glands in the prostate, increasing its metabolic function. During the treatment, pus can be seen with urine. The fluid was excreted from the body, and the patient's symptoms such as frequent urination, urgency, and dysuria disappeared quickly.

◆Biological effects: Induces certain physical and chemical processes and physiological effects in the body, restores the normal physiological functions of diseased cells and tissues, and promotes drug absorption. After treatment, the combination of drugs for about 3 days can make the treatment of prostatitis more rapid and more effective, especially for the refractory prostatitis that does not heal for a long time. The effect is very obvious.

◆Thermal effect: It can make the protein irreversible at high temperature of 70℃, and it can kill and inhibit various pathogenic bacteria at 45℃, 46℃ and 47℃.

The focus field characteristics of Hermite-Gaussian beams

The focus characteristics of the light field have always been the subject of optical workers. The classic theory of the focus field is given in Bom and Wlof's "Principles of Optics". In many practical applications of lasers, for example, laser processing (punching, cutting), laser fusion, etc., it is necessary to focus the laser for use.

The characteristics of the focus field when the diaphragm aperture is infinite

When the diaphragm aperture is infinite (no diaphragm), the beam waist of the focused H-G beam ( Waist spot) is located at △z/ f =一1/( 1 十π2N2G) At (or written as (15)), the minus sign indicates that the beam waist is located between the geometric focus and the aperture, and its position has nothing to do with the H-G beam mode m and n. The beam waist width of the focused H-G beam is related to the m and n of the H-G beam, and the relative focal shift and relative light intensity increment of the focused H-G beam are both the same as the H-G beam's modulus m or n ( m and n are even numbers) irrelevant, that is to say, in the case of no aperture limit, for different modes of H-G beam TEMmn (m and n are both even numbers) after focusing on its axis The point of maximum light intensity is located on the same axis, and its relative light intensity increment is the same.

The focus field characteristics of the finite size of the diaphragm aperture

Calculated by Gauss quadrature method on the AST386 computer a = 2.0, 3.0, NG=0.5, 1.0, 1.5 of TEM00, TEM02, TEM22 The on-axis light intensity distribution of NG=5.0, TEM00, TEM22, TEM24,TEM44 Relative focal shift even Δf/f with truncation parameter a NG= TEM22, TEM24 at 5.0, The maximum light intensity of TEM44ImaX (normalized) and the light intensity at the geometric focusI0With the change of the truncation parameter a, when the truncation parameter a≥3.0, for the same NG ,TEM00, TEM02, TEM22 The light intensity distribution on the axis is exactly the same, this is because a≥3.0 It is equivalent to the situation that the aperture of the diaphragm is infinite; as far as the calculation is concerned, the relative focal shift Δf/f varies with N< i>G decreases and increases, and the sub-maximum value of the light intensity is also large when the relative focus shift is large. The relative focal shift is related to the Fresnel number of the beam, the truncation parameter and the beam mode.

Regulating the reverse construction of the focus field of the vector beam

According to the electromagnetic radiation theory and the vector light field integration theory, the relationship between the focus field characteristics of the vector beam and the numerical aperture of the focusing lens is studied . Set the electric dipole array and the magnetic dipole array in the lens focal field area, collect their radiation field and focus in reverse, by adjusting and optimizing the parameters of the dipole array, inverting the focused light field under different numerical aperture lenses, The changes of the focused light needle field and the three-dimensional diffraction limited light tube field with the numerical aperture of the lens are obtained.

The reverse construction of the electric-magnetic dipole radiation field and the focus field theory

For lenses L with different numerical apertures, near the focus point F, set the electric field with a specific structure , Magnetic dipole array, the reverse radiation field is collected by the lens. The dipole array is mirror-symmetrical with the focal plane as the center, and is arranged along the axial direction. The length of the electric dipole is much smaller than the wavelength of the light wave, and its oscillation direction is along the z-axis; the magnetic dipole is equivalent to a tiny current-carrying ring whose radius is much smaller than the wavelength of the light wave.

Focusing of radially polarized light and optical needle field

Under the condition of the same number of electric dipoles, for systems with different numerical apertures, by adjusting the parameter factor An, dn and βn optimize the focus light field. Set the initial value of the parameter An=1, dn=1. 5λ, β i>n=0. First fix other parameters, adjust dn first, observe the one-dimensional distribution map of axial intensity, select the parameter values ​​that can obtain the long focal depth and the steep edge slope, and then fix the selection. Set the dipole spacing, adjust An in the range of 0 to 1, so that the peak intensity near each electric dipole pair is approximately equal, and finally fine-tune βn.

The light intensity distribution of the focused field obtained at different numerical apertures is called the optical needle field. If the focal depth fDOF is defined as the axial width above 80% of the maximum light intensity along the axial direction, it can be seen that as the numerical aperture changes from 0.9 to 0.7, the focal length The depth drops from 8.12λ to 5.8λ. For this focus field, the full width at half maximum ω1/2 is defined as the lateral full width where the light intensity accounts for more than 50% of the maximum light intensity, and the full width at half maximum increases from 0.49λ to 1.054λ. It can be seen that as NA decreases, the specific gravity of the axial polarization component gradually decreases, and the quality of the optical needle deteriorates. This is because a large numerical aperture objective lens is used to collect the electric dipole radiation field, and the field distribution on the entrance pupil surface is to control the radially polarized beam. After focusing, the vibration direction of each point can be decomposed into longitudinal and radial components due to the deflection of the light. The radial components of the light at the focal point and near the focal point can cancel each other out, leaving only the longitudinal component, and the ones far away from the focal point. Position, the radial component will still be maintained. Therefore, the smaller the numerical aperture, the larger the radial component of the light field after focusing, and the reduction of the longitudinal component, which leads to a shorter focused beam, and the percentage of the longitudinal field component to the total field strength decreases.

Angularly polarized light focusing and diffraction limited light pipe field

After angularly polarized light is focused, the polarization state does not change, and there is a hollow light field near the focal point. Similar to the adjustment process of electric dipoles, the An, dn< between the magnetic dipoles are adjusted and optimized. /sub> and βn make the axial intensity of the hollow light field tube wall uniformly distributed, and produce a hollow light tube field with azimuthal polarization. The reduction of the numerical aperture makes the objective lens's ability to focus the beam weaker. The length of the light pipe (the axial length where the light intensity of the tube wall is greater than 80% of the maximum light intensity) decreases gradually. Due to the sinusoidal relationship between the numerical aperture and θmax, the magnitude of the decrease in the length of the light pipe increases . When the numerical aperture changes from NA=0.9 to 0.85, from 0.85 to 0.8, and from 0.8 to 0.7, the change rates of the length of the light pipe are 1.29%, 5.25%, and 22.16%, respectively.

The incident field on the entrance pupil surface needs to have an azimuthal polarization mode controlled by phase and amplitude. The intensity distribution is an annular band of light and dark, and the center is a dark zone. The strength of the bright ring from the center to the outside gradually increases. The localized polarization directions of adjacent ring zones are opposite. When NA=0.9, 0.85, 0.8 and 0.7, the number of belts is 4, 4, 4, and 2, respectively. As the numerical aperture decreases, the width of the annulus gradually widens and the density decreases.

Compared with the optical needle field, the numerical aperture has less influence on the parameters of the light pipe field. This is because the distribution of the light field before and after converging by the lens is always a pure azimuth polarization mode, and the length of the light pipe varies with The numerical aperture decreases and becomes shorter, and the thickness of the light pipe and the full width at half maximum increase accordingly. However, since the field distribution mode has not changed, the changes in various parameters are relatively small.

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