Apodization for a beam of laser radiation

 

Use: laser optics, when working with solid-state and gas lasers used in laser technology, laser medicine, research. The essence of the invention: Apodization for a beam of laser radiation with a wavelengthand cross-sectional dimension 2R, R is the radius of the beam, on the basis of transparent to the radiation environment with a refractive index ofcontains formed in the aperture of the beam in the transverse coordinates 0rR, 02and in the direction of the axis of propagation of the beam z layer consisting of diffuse radiation particles with a concentration of m, with a refractive index nand with radii,<<R, and the profile of the transmission radiation layer T (r) falls from the beam axis to its periphery. Layer formed from moving in an orderly fashion in the aperture of the beam is symmetrical about its axis in the coordinates r,z particle j (j=1, 2... ) with radiij<<R and nj. Wednesday particles may be in the form symmetrical with respect to the axis of the laser beam flow through the sample cell. The technical result of the invention is the creation of Apodization for a beam of laser radiation with smooth transmission function, high values of contrast and "fill factor", which is the Apodization in a wide spectral range with low intrinsic absorption of transmitted radiation and allowing the possibility of rebuilding the transmission functions. 3 C.p. f-crystals, 1 Il., 1 PL.

The invention relates to laser optics and can be used when working with solid-state and gas lasers used in laser technology, laser medicine, research. Apodization (soft aperture) are widely used devices in the optical path of the laser devices [1-14]. They are used to smooth the spatial intensity distribution of the laser beams, their use allows to suppress the re. who eat the most use of Apodization increases the resistance of powerful laser beams with respect to the self-focusing equalize the intensity distribution over the cross section. It allows you to optimize the power output of the active medium by increasing the fill factor of the radiation of the working aperture of the amplifier. Resonator lasers Apodization used for selection of transverse modes and the formation of a profile of intensity distribution of radiation [6, 8]. The most widespread are the so-called “amplitude” Apodization representing, essentially, optical filters with variable beam transmission. Also known “phase” Apodization, the aperture of which entered phase heterogeneity of type dispersing lenses /1, 10/, as well as Apodization mixed (amplitude-phase) type /10/.

Despite the large number of proposed methods and technologies of manufacturing of Apodization [1-14], the actual application have found only a few types of the amplitude of Apodization highly resistant to laser radiation (radiation exposure 1-3 j/cm2) and high contrast (ratio of the transmittance of radiation on the axis and at the periphery ,232D/chr/8805.gif">70%). Among them the so-called toothed aperture of the metal /11/, Apodization based on partially frosted glass plates /12, 13/, the aperture on the basis of multilayer dielectric coatings on the plates and some other /7, 8/.

A common characteristic of the amplitude of Apodization for light beams is the presence in their aperture with a characteristic size of 2R, Rr0 (r is the transverse coordinate, R is the radius of the beam) scattering, reflecting, or absorbing radiation with a wavelengthzone widthrR with smooth spatial profile of the transmittance T(r), growing from the edge of Apodization to its axis. For the radiation beam with a uniform spatial distribution of the phase and the intensity I(r)=I0=const, falling on Apodization, at a distance of LFfrom him, 0<L<L max2rR/in the field of Fresnel diffraction is formed spatial distribution I(r)I0T(r) with smooth (soft) profile.

Known devices proposed as the amplitude apt what PR) of variable thickness, filled with the working substance (immersion liquid), absorbing the laser light (a solution of salt or dye) [2-6]. Known cell-Apodization with inserts in the inner cavity forming a gap of variable thickness, fill the absorbing liquid [4-6]. For a beam with uniform intensity distribution incident on the cuvette on the way out of it acquired a smooth intensity distribution described supergaussian function with an indicator of “rigidity” N and contrast To, the dependence of the transmittance of the aperture radius r, T(r) should be described by the function type /4/

and the profile of optical elements, limiting layer of the absorbing liquid, must be, generally speaking, aspherical. Thus the dependence of the layer thickness of the absorber radius r is described by a function of the form /4/

Here T0=T(o) - transmission axis of the aperture, R is the radius of the aperture, in which T(r) is reduced To h0>0 is the layer thickness of the absorber on the axis of the diaphragm, k1is the absorption coefficient of the solution.

However, all considered ditch-Apodization [2-7], which uses a shaped absorbent layer is TV, associated with certain limited spectral intervals, which usually take the absorption band of solutions of salts and dyes. Another significant disadvantage is inevitable for these devices a significant dissipation in the cell by passing the radiation absorber layer, which leads to phase distortions passing through the sample cell beam. In lasers operating in the pulse-periodic or continuous mode, thermal distortion in the optical components of the cell will accumulate, which will lead to degradation of the spatial-angular laser beam parameters and can lead to destruction of the component of the cell due to the resulting thermal stresses. Among the shortcomings discussed Apodization should also include the complexity of the manufacture of aspheric optics for forming a profiled layer absorption cuvettes.

Known Apodization based on the scattering of laser radiation centers formed on the surface [12, 13] or [6, 14] is transparent to radiation in the dielectric (glass) plates using matting surface [12, 13] or the processing volume of the plate by laser radiation [6, 14]. These Apodization have radiation stoneslaser in a wide wavelength range from the near UV to the IR portions of the spectrum, including lasers periodic-pulsed and continuous modes.

However, common to all the types of Apodization disadvantage is the inability to rebuild their transmission functions. At the same time, the need for optical elements-Apodization managed (adaptive) features available. Known apovincamine adaptive mirrors for adjustment mode composition and distribution of the radiation intensity in the resonator lasers in the field of interaction of laser radiation with the material to be processed [15]. Adaptive optics necessary to compensate for the well-known distortions that occur in the active medium of the laser due to the heat pumping [16]. Apodization with adaptive bandwidth profile can be used to solve these problems. The /17/ was observed rearrangement profile transmittance of radiation in a ditch with the flow of the colloidal solution absorbing the radiation particles. Loss on light scattering in colloidal solution does not exceed one tenth of one percent /17, 18/. Essentially, the /17/ for the separation of particles by size and adjustment of the profile of the bandwidth was used a well-known principle of cyclone /19/, which, together with C is amended Note, however, that when used for the separation of particles of the cell type /17/, working on the principle of the hydrocyclone, the inevitable distortion of the profile of the bandwidth associated with the asymmetry of the input and output stream in a ditch and the emergence of axial counterflow /19/. Note also that when using cuvette-moving particles that absorb the laser radiation, it is possible to create just such Apodization, which is not free of the common disadvantages of a ditch with absorbing media: spectral selectivity and significant heat generation in the absorber.

Closest to the claimed technical solution is the device of Apodization on the basis of the cell with the turbid medium in accordance with the patent of the Russian Federation No. 2163386 [21]. This cuvette is filled transparent to laser radiation environment with a refractive index ofcontaining optical microheterogeneity (small particles) with a refractive index nwith radiiscattering of laser radiation. Formed due to the structure of the cell profiled on the thickness of the layer of such scatterers with the maximum quantity and the si of the cell to its periphery. To calculate T(r) in expressions (1, 2) for the amplitude of Apodization instead of the absorption coefficient k1you need to use the attenuation=m0where=22- scattering and m0- constant volume concentration of the scattering particles. For the cell filled with gas or liquid containing a thin glass membrane with a characteristic radius=5 µm for m0=8106m-3the calculation of contrast of Apodization (relationship of transmittance functions for the layers of turbid media on the axis and at the periphery of a thickness of respectively 0.1 and 5 mm) gives the value To400 [21]. To prevent settling of particles due to gravity provides duct environment microparticles through the cell [21]. Advantages of Apodization in accordance with the patent [21] are its low internal absorption, high optical strength and the ability to use lasers in a wide spectral range of wavelengths. The disadvantages include the complexity of the manufacture of aspheric optics, neobhodimsoti. Indeed, at constant volume concentration of scatterers and support this concentration, the flow of medium through the sample cell, the distribution of particles by the volume of the cell, and hence the bandwidth profile of Apodization not change.

The objective of the invention is the creation of Apodization for a beam of laser radiation with smooth transmission function, high values of contrast and “fill factor”, which is the Apodization in a wide spectral range from the near UV to the IR region of the spectrum with negligible self-absorption of the transmitted radiation and allowing the possibility of rebuilding the transmission functions. The invention also provides for the simplification of the structure of Apodization due to the exclusion of the construction complex in the manufacture of elements aspherical optics.

The claimed technical solution is Apodization for a beam of laser radiation with a wavelengthand cross-sectional dimension 2R-based transparent to the radiation environment with a refractive index ofcontaining formed in the aperture of the beam in the transverse coordinates 0r, z), with refractive indices njand with radiij,j<<R, and the weakening of the radiation layer increases, and the profile of the transmittance of the layer T(r) falls from the beam axis to its periphery.

The proposed Apodization can be formed in unlimited walls of the flow of gas or liquid, into which are introduced in an orderly and moving in the aperture of the beam particles, creating a light-diffusing layer with a variable cross section of the laser beam transmission. The proposed solution can be implemented and on the basis of a ditch, filled with a medium (gas, liquid) containing a moving micro-particles, which form a light-diffusing layer. Cuvette may contain profiled on the thickness of the layer environment with moving particles. Can be also implemented Apodization on the basis of the cell with plane-parallel OK the military cross section of the cell concentration of the moving scattering particles. When changing motion parameters of the scattering particle or composition can be made in the reconstruction of the profile of the transmittance of Apodization.

To clarify the essence of the proposed technical solution and its quantitative characteristics consider the examples of Apodization with tunable bandwidth profile based on a ditch with a moving scattering of laser radiation by the particles. To describe the motion of particles in a cell we introduce the following notation:

R is the radius of the cell (beam);

r is the current radius;

is the azimuthal angle;

L - length of the cuvette;

j - the index of the varieties of particles, j=1, 2...

jis the radius of the particles of the j-th class;

djthe particle density of the j-th class;

Mjis the mass of the particle;

d0the density of the medium;

- coefficient of viscosity;

mjthe concentration of the particles of the j-th class;

=d/dt is the angular velocity of the rotation of the cuvette;

T - temperature environment in a ditch;

k is the Boltzmann constant.

There are various technical solutions to organize an orderly movement of the particles so as to form a smoothed function of the transmission in cuverene in a ditch on different trajectories. For example, for the cell filled with the gas stream moving symmetrically along the beam axis z with velocity vzsmoothing bandwidth profile can be obtained with symmetricalthe introduction of particles normal to the z axis and to the wall of the cell with velocity vr. You can use this well-known dependence of the resistance environment the movement of spherical particles, Fjfrom the radius of particlesj(Stokes formula): Fj=6jvr. The calculation shows that there needed to form a smooth profile of the transmittance distribution of the particle radius can be obtained in moving along the z-flow environment with the introduction of particles of different radius with the same value of the initial velocity vr. To rebuild this profile is possible with changes in the speeds of the input and composition of the particles.

The following is an example, where the formation of the profile uses centrifugal force acting on the particles. Symmetrical with respect to the beam axis of the scattering layer can be created by rotation of the cell as a whole together with the working environment around the z axis.

Example 1. Ditch Korostil. We assume that the particles are microspheres with a radiusjdensity of dj, mass Mj“suspended” in an environment with a density of d0and dj>d0. For particles of small sizejwhere1located in the centrifugal field at thermal equilibrium with the environment, fair Boltzmann distribution of particle concentration along the radius [22], which, taking into account the difference of the densities of the particles and the environmentdj=dj-d0can be written as

where aj=2/Ktdj3jR2- coefficient, and m0j- the initial (uniform) concentration of particles of variety j in a ditch. Taking the scattering 22jfor indicator(r), we obtain

Thus, the expression profile of the transmittance of the cell length L and zapolnenie (3)

In the drawing and in the table presents estimates for rotating with angular velocitythe cell-Apodization. Curves (1-5) transmittance of the cell T(r) obtained with the following fixed parameters: T=300K, L=5 cm, R=5 cm,d=10-4g/cm3and variable values of the radii of the particles and the angular speed of rotation of the cuvette. From the comparison in the drawing, curves 1-3, you can see how the reconstruction of the profile of the transmittance of the cell filled with particles of one species (1=5 µm, m01=104cm-3), depending on the angular speed of rotation. With increasing angular velocity profile, as expected, becomes sharper, closer to the rectangular profile of the transmission. Curves 4-5 illustrate the ability of tuning profile when changing the particle size (1=5,2=10, m0=5103cm-3) at a constant angular velocity. The calculated values of contrast for the considered cases are To105. Supporting after their optical characteristics obtained Apodization.

Estimates show that for the considered examples, we can neglect the effect on the profile shape of Apodization gravitational deposition of particles, and the thickness of the layer of particles trapped on the walls of the cell.

Thus, these examples show the ability to create on the basis of a ditch with a moving scattering of radiation by particles of Apodization with tunable transmission function, high values of contrast and “fill factor” of the aperture of the laser beam. Use as a working environment that is transparent to laser radiation of liquids and gases, as well as transparent rassevayuschim particles creates the possibility to use Apodization in a wide spectral range from UV to IR spectral regions. Low self absorption of radiation increases the resistance of Apodization to radial loads and provides the possibility of using them with lasers pulsed-periodic and continuous modes of operation. One possible interesting applications of Apodization is the formation of directly amplifying the radiation from the active medium of the laser setup. This implementation of tunable Apodization can imagine ykovsky N. E., Zel'dovich B. Ya., Senatsky Yu.V. "Diffraction and selffocusing of the radiation in a high-power light pulse amplifier", Kvantovaya Elektronika (Moscow), v.1, No. 11, p.2435-2458 (1974).

2. Costich V. L. and Johnson, B. C. "Apertures to shape high-power beams". Laser Focus, September 1974, pp.43-46.

3. Nolen R. L.,Jr., Siebert L. D. "High power laser apodizer", US Patent No. 4017164 (1977).

4. Vinogradsky L. M., Sobolev, S. K., Senatsky Yu.V. et al. "Development of the nonlinear optical element for light beam apodization and large aperture laser amplifier decoupling", Preprint FIAN, Moscow (1998); RF Patent № 2177666 (1998).

5. Senate Y. C., ned L. M. and other "Soft aperture for lasers, RF Patent №2157034 (1998).

6. Vinogradsky L. M., Senatsky Yu.V., Ueda K. et al. "Soft diaphragms for apodization of powerful laser beams", Proc.SPIE, vol.3889, pp.849-860 (2000).

7. S. G. Lukishova, Krasiuk I. K., P. P. Pashinin et.al. "Light beam apodization as the method for increased brightness of a Nd-glass laser system", Trudy IOFAN, No. 7, p.92-147 (1987).

8. Mak A. A. et al. "Nd-glass lasers", Moscow, "Nauka" Publ.House, 1990.

9. Potapova N. And., The flowers of A. D. “Diffraction Fresnel glass apodyterium apertures with supergaussian transmission function” Quant. electronics 15, 10, 2059 (1988).

10. Potapova N. And., The flowers of A. D. “Apodization of laser radiation phase diaphragms” Quant. electronics 19, 5, 460-464 (1992).

11. Van Worterghen B. M. et al. "Performance of a prototype for a large-aperture multipass Nd:glass laser for inertial confinement fusion", Appl.Opt. 36, No. 21, p.4932-4953 (1997).

12. Summers, M. A., Hagen W. F., Boyd, R. D. "Scattering apodizer for laser beams", US Patent No. 4537 475 (1985).

13. Rizvi N., Rodkiss D., Panson C. "Apodizer development", Rutherford Appleton Lab., Ann.rep., -87-041, dryashov A. V. et al. "Formation of a specified intensity distribution of the radiation from an industrial cw CO2laser", Kvant.Electr., v.29, (4), p.339-340 (1999).

16. Vdovin, C., Sharp, S. A. “Active correction of thermal lens in solid-state laser”. “Quantum electronics” 20, 2, 167-171 (1993).

17. Kolerov A. N., Epikhina G. E. "The dispersive two-phase media for laser radiation control parameter", Kvantovaya Elektronika, v.l6, No. 9, p.l841-1843 (1989).

18. Kolerov A. N. Laser with a colloid solution, active medium", Kvantovaya Elektronika, v.l6, No. 5, p.955-957 (1989).

19. Ternovskii, I., Kutepov, A. M. “Gidrotsiklonirovaniya”. - M.: Nauka. (1994).

20. Skoropad D. E. "Centrifuges and separators for chemical production. - M.: Chemistry (1987).

21. Senate Y. C. "Soft aperture for lasers" RF Patent № 2163386.

22. Landau L. D. and Lifshitz E. M. Statistical physics". - M.: Nauka, 1964.

Claims

1. Apodization for a beam of laser radiation with a wavelengthand cross-sectional dimension 2R, R is the radius of the beam, on the basis of transparent to the radiation environment with a refractive index ofcontaining formed in the aperture of the beam in the transverse coordinates 0rR, 02and in the direction of the axis of respose nand with radii,<<R, and the profile of the transmission radiation layer T(r) falls from the beam axis to its periphery, characterized in that the layer formed from moving in an orderly fashion in the aperture of the beam is symmetrical about its axis in the coordinates r,z particle j (j=1, 2...) with radiijconcentrations of mj(r,, z) and a refractive index of njandj<<R and nj.

2. Apodization under item 1, characterized in that environment with particles proceeds in the form symmetrical with respect to the axis of the laser beam flow through the sample cell.

3. Apodization under item 1, characterized in that the medium and particles with a density of dj>d0where d0the density of the medium, enclosed in a cuvette with Windows transparent to laser radiation, and rotate together with the cuvette axis of the laser beam with angular velocity d/dt=.

4. Apodization under item 1, otlichalis

 

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