RU2061249C1 - Device for phase shift of laser beam structure - Google Patents

Device for phase shift of laser beam structure Download PDF

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RU2061249C1
RU2061249C1 RU93006533A RU93006533A RU2061249C1 RU 2061249 C1 RU2061249 C1 RU 2061249C1 RU 93006533 A RU93006533 A RU 93006533A RU 93006533 A RU93006533 A RU 93006533A RU 2061249 C1 RU2061249 C1 RU 2061249C1
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lens
crystal
laser
maxima
waves
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RU93006533A
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RU93006533A (en
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В.Г. Бородин
С.В. Красов
А.В. Чарухчев
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Научно-исследовательский институт комплексных испытаний оптико-электронных приборов и систем Всесоюзного научного центра "Государственный оптический институт им.С.И.Вавилова"
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Abstract

FIELD: methods for exposing physical and medical objects to laser light. SUBSTANCE: laser beam runs through phase plate and lens which is made from crystal. Two waves are generated at output of this unit. Usual wave and unusual waves have own distribution of intensity shaped as periodical maximums inside dispersal circle. Common pattern from two waves has greater degree of density of filling dispersal circle with maximums with respect to density of one wave. EFFECT: increased quality of phase shift. 2 dwg

Description

 The invention relates to laser technology and can be used, for example, in multichannel installations for laser thermonuclear fusion (LTS).

 One of the urgent tasks of LTS is the problem of uniform irradiation of a spherical target. This is prevented by amplitude-phase distortions of the laser beam and aberrations of the focusing lenses, which cause a complex inhomogeneous intensity distribution on the target surface. To eliminate the influence of beam distortions and optical aberrations, devices are used to form the intensity distribution of laser beams, in particular, due to phase transformation of the laser beam structure. The intensity distribution during focusing of the transformed beam is modulated: the scattering circle is an array of narrow intensity maxima separated by minima. When a target is irradiated, the inhomogeneity caused by high-frequency modulation is eroded due to the electronic thermal conductivity of the target material, and the irregularity of the irradiation is determined only by the envelope of the intensity distribution. Moreover, the higher the density distribution of the maxima in the scattering circle, the higher the quality of the beam transformation, since blurring is faster and more complete and, therefore, the beam properties are determined to a greater extent by the envelope.

 A device for phase transformation of the structure of a laser beam, including a lens microraster located at the exit of the laser. When focusing such a beam, the intensity distribution is an array of intensity maxima distributed over the area of the scattering circle. However, the density of the occupation by the maxima of the scattering circle is low. This reduces the conversion quality of the lazar beam.

 The closest in technical essence to this invention is a device for phase transformation of the structure of a laser beam, including an optically transparent substrate, one of the surfaces of which is divided into zones with different phase delays having a random value. Such a device is called a phase plate.

 When focusing the transformed beam, the scattering circle is an array of intensity maxima distributed over its site. However, the filling density of the scattering circle with intensity maxima is low. This reduces the quality of the beam conversion.

 An object of the invention is to improve the quality of the phase transformation of the structure of the laser beam by increasing the density of filling with maxima of the intensity of the scattering circle.

 This problem is achieved by the fact that in the known device for phase transformation of the laser beam structure, including an optically transparent substrate, one of the surfaces of which is divided into zones introducing a phase delay into the beam, a crystal lens is installed in series with the substrate, the optical axis of which is located at an angle to axis of the lens. Moreover, the lens is made with the possibility of rotation around its optical axis until the plane of the main section of the crystal and the normal to it coincide with the polarization vector of the converted beam.

Figure 1 shows the optical scheme of the proposed device, where the optically transparent substrate 1, zone 2, introducing a phase delay into the beam, a crystal lens 3, an optical axis 4 of the crystal, an angle 5 between the axis of the crystal and the axis of the lens, axis 6 of the lens, plane 7 the main section of the crystal, normal 8 to the plane of the main section, the polarization vector 9 of the converted beam, the converted beam 10; on figa-e presents the intensity distribution of the converted beam near the focal plane, where a polarization vector coincides with the plane of the main section of the crystal, also on an enlarged scale, the angle between the polarization vector and the plane of the main section of the crystal is 15 ° , the angle is 45 ° , q angle is 70 ° , e angle is 90 ° , i.e. the polarization vector coincides with the normal to the plane of the main section of the crystal.

 The device operates as follows. The converted laser beam 10 is incident on a phase plate consisting of a substrate 1 and zones 2 introducing a phase delay into the beam.

The phase plate splits the WF of the beam into small zones with different phase delays. Next, the beam enters the crystalline lens 3. In this case, each beam of the beam is divided into two: ordinary and extraordinary. Since the crystal is given the shape of a lens with optical power, and each of the rays has its own refractive index n o and n e , in accordance with the laws of refraction, two waves will propagate behind the crystal lens: ordinary U o and unusual U e with different radii of curvature since the optical power depends on the refractive index. During the subsequent focusing of the beam transformed by such a device, each of the waves U o and U e will gather in its focus. Moreover, the picture of the intensity distribution in the focal planes and planes parallel to them, is a total picture, respectively, of the distributions of the two waves U o and U e . Since U o and U e converge at different points, the spatial arrangement of the maxima corresponding to each of the waves is different. Therefore, the maxima from one distribution are placed in the regions of the minima from another. This leads to an increase in the density of filling with maxima of the scattering circle.

To implement the regime of splitting the beam in the crystal into ordinary and extraordinary, it is necessary that the rays do not propagate along the optical axis of the crystal; therefore, the axis 4 of the crystal and axis 6 of the lens are located at an angle of 5 to each other. In addition, the polarization vector 8 should not coincide with the principal plane 7 of the crystal, nor with the normal 8 to it, since in the first case one extraordinary wave propagates in the crystal, and in the second one ordinary wave. The possibility of rotation of the lens around its optical axis ensures that the plane of the main section of the crystal does not coincide with the normal to it with the polarization vector of the converted beam, i.e. ensures the formation of two waves U o U e in the transformed beam, which leads to an increase in the filling density of the scattering circle with intensity maxima, and, consequently, an increase in the quality of the beam conversion. In the case of conversion by the device of radiation with an elliptical (circular) polarization, at which the polarization vector rotates, the condition of non-coincidence of the plane of the main section of the crystal and the normal to it with the polarization vector is fulfilled automatically.

 Thus, in cases where the main plane and the normal to it are not parallel to the polarization vector, two patterns of the intensity distribution in the scattering circle, corresponding to ordinary and extraordinary waves, are realized. In this case, the maxima of one are located in the minima of the other. Consequently, the density distribution of the maxima and the conversion quality of the laser beam are increased.

 In addition to the above, the presence of such a feature as a lens made in a certain way from a crystal and in a certain way oriented relative to the polarization vector, designed to solve a similar problem, is not known to the authors. Thus, we can assume that the claimed solution meets the criterion of "inventive step of solving the problem."

 At the enterprise NIIKI OEP made the mock of the claimed device.

The phase conversion device consisted of a phase plate and a crystalline lens. The phase transition was performed by applying non-uniformity zones of SiO 2 with a diameter of 1 mm onto a substrate with a diameter of 50 mm from K-8 glass. The zones were introduced into the WF of the delay beam 0, λ / 2, and λ where λ is 1.06 μm the wavelength of the linearly polarized radiation being converted. The crystalline lens is made of KDP crystal, the optical axis of which is located at right angles to the axis of the lens. The converted beam was focused by a focusing lens with a focal length f 1 50 cm. The registration plane was in the middle between the points F o and F e , at which the ordinary U o and the unusual U e wave were focused.

The sequence of images shown in figure 2, illustrates the process of forming and superimposing intensity distributions corresponding to the waves U about and U e . The first image (see Fig. 2, a) shows the distribution in the scattering circle formed by a single wave U e , which is obtained when the polarization vector coincides with the plane of the main section of the crystal. For convenience of observation, the remaining images on an enlarged scale depict a section near the central region containing four maxima from the wave U e (see Fig. 2, b). The following pictures (see figure 2, 2, f) were obtained during rotation of the lens around its optical axis and correspond to the angles of rotation of the main plane of the crystal relative to the polarization vector at angles of -15 o , r -45 o , d -70 o , e -90 about . In figure 2, in between the maxima of the wave U e appear weak maxima formed by an ordinary wave U o . In figure 2, g, the intensities of both patterns are equal, since they correspond to a rotation angle of 45 ° . In figure 2, e maxima U o exceed in intensity the maxima from U e . Figure 2, e shows nine maxima formed only by the wave U o due to the coincidence of the polarization vector with the normal to the plane of the main section of the crystal.

 Thus, as follows from the results presented in Fig. 2, the installation of a crystal lens in series with the phase plate, the optical axis of which is located at an angle to the axis of the lens, and ensuring that the plane of the main section of the crystal and the normal to it do not coincide with the polarization vector of the converted beam to an increase in the density of filling with maxima of the scattering circle, i.e. improve beam conversion quality.

 Here is an example of calculating the parameters of a crystalline lens.

Let the transformed beam be focused by a lens with a focal length f 1 . The wave U e is focused at a point located at a distance F e from the lens, U o at the point F o . Let f o and f e be the focal lengths of the crystalline lens for waves U o and U e, respectively. Then, as follows from the well-known law of geometric optics

Figure 00000001
Figure 00000002
+
Figure 00000003
(one)
Similarly for an extraordinary wave
Figure 00000004
Figure 00000005
+
Figure 00000006
(2)
We will consider the distribution of intensity in a plane located at a distance Z from the point F o . In this plane two intensity distributions are combined: from the wave U o and from the wave U e . In accordance with the similarity relation when focusing beams (see Born M. Wolf, E. Fundamentals of Optics. M. Nauka, 1973, p. 472), there are planes with an intensity distribution identical to each of the two distributions. These planes are at a distance of Z 1 for the wave U o and Z 2 for the wave U e from the focus f 1 of the lens, i.e. are connected by the following relations:
Figure 00000007
Figure 00000008
(3)
Figure 00000009
Figure 00000010
(4)
For a plano-convex form of a crystalline lens, we can write the expression
f o =
Figure 00000011
(5)
f e =
Figure 00000012
(6) where R is the desired radius of curvature of the surface of the crystalline lens.

We substitute (5) and (6) into (1) and (2). The resulting values of F o and F e substitute in (3) and (4). Then from (3) we express Z and substitute in (4). We obtain the cubic equation for the unknown parameter R, which is required to be found
Z 2 R (R + a) 2 Z 1 R (R + b) 2 + (ab) (Ra) (Rb), (7) where af 1 (n o -1) and bf 1 (n e -1)
We substitute the following numerical values in (7):
f 1 50 cm, Z 1 0.4 cm, Z 2 0.5 cm, for KDP crystal: n o 1.4936, n e 1.4598.

 Solving (7), we obtain the radius of curvature of the crystalline lens R 680 cm.Therefore, the crystalline lens has parameters available to conventional manufacturing technology.

 Thus, when a lens made of a crystal is installed in series with the phase plate, the optical axis of which is located at an angle to the axis of the lens and allows the lens to rotate around its optical axis until the plane of the main section and its normal to the polarization vector of the converted beam coincide, it is realized intensity distribution corresponding to two waves: ordinary and extraordinary. In this case, the maxima from one distribution are located at the minima from another, the density of filling with maxima of the scattering circle increases, i.e. Beam conversion quality is improved.

 From the above it follows that the claimed device is effective, reliable, its implementation is simple, it does not require additional development and use of unique or expensive elements. This device can be used in medical devices, the action of which is based on laser radiation: in laser surgery and irradiation of hollow organs, in lighting devices with a laser source.

Claims (1)

  1.  A device for phase transformation of the laser beam structure, including an optically transparent substrate, one of the surfaces of which is divided into zones introducing a phase delay into the beam, characterized in that a crystal lens is installed in series with the substrate, the optical axis of which is located at an angle to the axis of the lens, this lens is made with the possibility of rotation around its optical axis to the mismatch of the plane of the main section of the crystal and normal to it with the polarization vector of the converted beam.
RU93006533A 1992-02-01 1992-02-01 Device for phase shift of laser beam structure RU2061249C1 (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2003072521A2 (en) * 2002-02-21 2003-09-04 Andrey Mikhaylovich Alexeev Method for cutting non-metallic materials and device for carrying out said method

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
Авторское свидетельство СССР N 572868, кл. H 01S 4/00, 1977. Phys. Rev. Lett., 1984, 53, 1057. *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2003072521A2 (en) * 2002-02-21 2003-09-04 Andrey Mikhaylovich Alexeev Method for cutting non-metallic materials and device for carrying out said method
WO2003072521A3 (en) * 2002-02-21 2003-12-18 Andrey Mikhaylovich Alexeev Method for cutting non-metallic materials and device for carrying out said method

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