RU2229762C2 - Laser beam apodizer - Google Patents

Abstract

FIELD: lasing optics; solid-state gas lasers; laser medicine and scientific research. SUBSTANCE: laser beam apodizer with wavelength λ and transversal dimension 2R, where R is beam radius, based on medium transparent for emission at refraction index μ has layer z formed in beam aperture on cross coordinates 0≤r≤R,0≤φ≤2π and in direction of beam propagation that incorporates emission dissipating particles of concentration m, refraction index n≠μ and radii ρ,λ≤ρ≪R layer T (r) beam transmission profile from beam axis to its periphery being reduced. Layer is formed of particles of j kinds (j = 1, 2...) moving in ordered manner in beam aperture symmetrical to its axis in coordinates r, φ, z, having radius ρ_j, concentrations m_j, (r, φ, z) and refraction indices n_j; λ≤ρ_j≪R and n_j≠μ Particles medium may pass in the form of laser beam flow through pan. Apodizer provides for apodization in broad spectral range, it is distinguished by negligible inherent absorption of passing beam and at the same time permits transmission function control. EFFECT: smoothed transmission function, enhanced contrast and duty factor. 4 cl, 1 dwg, 1 tbl

Description

The invention relates to laser optics and can be used when working with solid-state and gas lasers used in laser technology, laser medicine, in scientific research. Apodizers (“soft” diaphragms) are quite widely used devices in the optical path of laser systems [1-14]. They are used to smooth the spatial distribution of intensity in laser beams; their application allows one to suppress sharp bursts of intensity that arise in the aperture of beams when they are diffracted by conventional ("hard") diaphragms. Thus, the use of apodizers enhances the stability of high-power laser beams with respect to self-focusing and evens out the intensity distribution over the cross section. It allows you to optimize the energy removal in the active medium by increasing the factor of radiation filling the working aperture of the amplifier. In laser cavities, apodizers are used to select the transverse modes and form the radiation intensity distribution profile [6, 8]. The most widely used are the so-called “amplitude” apodizers, which are essentially optical filters with a variable transmission cross section. Also known are “phase” apodizers, into the aperture of which phase inhomogeneity of the type of scattering lens / 1, 10 /, as well as mixed (amplitude-phase) type apodizers / 10 / are introduced.

Despite the large number of proposed methods and manufacturing techniques of apodizers [1-14], only a few types of amplitude apodizers have real application, which are distinguished by a fairly high resistance to laser radiation (radiation load of 1-3 J / cm ² ) and high contrast values (ratio radiation transmission coefficients on the axis and on the periphery of the beam, K = 10 ² -10 ³ ) and the “filling factor” (integral over the aperture of the transmission apodizer, F≥ 70%). Among them are the so-called serrated metal diaphragms / 11 /, apodizers based on partially frosted glass plates / 12, 13 /, diaphragms based on multilayer dielectric coatings on plates, and some others / 7, 8 /.

A common characteristic of amplitude apodizers for light beams is the presence in their aperture with a characteristic size of 2R, R≥ r≥ 0 (r is the transverse coordinate, R is the beam radius) scattering, reflecting or absorbing radiation with a wavelength λ of a zone of width Δ r≤ R s smooth spatial transmission profile T (r), growing from the edge of the apodizer to its axis. Moreover, for a radiation beam with a uniform spatial distribution of phase and intensity I (r) = I ₀ = const incident on the apodizer, at a distance L _F from it, 0 <L _F <L _max ≈ 2Δ rR / λ, in the Fresnel diffraction region the spatial distribution I (r) ≈ I ₀ T (r) with a smooth (soft) profile is formed.

Known devices proposed as amplitude apodizers for lasers are cuvettes with radiation-transparent lens windows and an internal cavity (gap) of variable thickness filled with a working substance (immersion liquid) that absorbs laser radiation (salt or dye solution) [2- 6]. Known apodizer cuvettes with inserts in the internal cavity, forming gaps of variable thickness, filled with absorbing liquid [4-6]. In order for a beam with a uniform intensity distribution incident on the cuvette to obtain a smooth intensity distribution at the exit from it, described by a super-Gaussian function with the “hardness” index N and contrast K, the dependence of the transmittance of the diaphragm on the radius r, T (r) should be described by the function type / 4 /

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

Here T ₀ = T (o) is the transmission on the axis of the diaphragm, R is the radius of the diaphragm at which T (r) decreases by K times, h ₀ > 0 is the thickness of the absorber layer on the axis of the diaphragm, and k _{1 is the} absorption coefficient of the solution.

At the same time, all considered cuvette apodizer [2–7] using a profiled absorbing layer have disadvantages. These include, firstly, the inevitable spectral selectivity of such devices, associated with certain limited spectral ranges, which usually occupy the absorption bands of salt and dye solutions. Another significant drawback is the inevitable significant heat release in the cell for these devices when radiation passes through the absorber layer, which leads to phase distortions of the beam passing through the cell. In lasers operating in pulsed-periodic or continuous modes, thermal distortions in the optical components of the cell will accumulate, which will lead to degradation of the spatial-angular parameters of the laser beam and can lead to destruction of the cell components due to thermal stresses. The disadvantages of the considered apodizers include the difficulty in manufacturing aspherical optics for the formation of a profiled absorption layer in cuvettes.

Known apodizers based on laser-scattering centers formed on the surface [12, 13] or in the volume [6, 14] of a dielectric (glass) plate transparent to radiation by matting the surface [12, 13] or processing the plate volume with laser radiation [6 , 14]. These apodizers have radiation resistance comparable to the optical strength of glass, insignificant absorption losses and can be used with lasers in a wide wavelength range from near UV to IR spectral regions, including pulsed-periodic and continuous-wave lasers.

However, a common drawback for all types of apodizers considered is the lack of the ability to rearrange their transmission function. At the same time, there is a need for optical apodizer elements with controlled (adaptive) characteristics. Apodized adaptive mirrors are known for tuning the mode composition and radiation intensity distribution in laser cavities and in the field of interaction of laser radiation with the processed material [15]. Adaptive optics is also necessary to compensate for well-known distortions arising in the active medium of a laser due to heat generation during pumping [16]. Adaptive transmit profile apodizers can be used to solve these problems. In [17], a restructuring of the radiation transmission profile in a cell with a flow of a colloidal solution of radiation-absorbing particles was observed. Loss of light scattering in a colloidal solution did not exceed a tenth of a percent / 17, 18 /. Essentially, the well-known principle of hydrocyclone / 19 /, which, along with a centrifuge / 20 /, is widely used in the technique for separating various mixtures based on liquid and gaseous media, was used in / 17 / for separating particles by size and tuning the transmission profile. Note, however, that when using a cell type / 17 / operating on the principle of a hydrocyclone for particle separation, distortions in the transmission profile are inevitable due to the asymmetry of the input and output of the flow in the cell and the appearance of an axial countercurrent / 19 /. We also note that when using moving particles absorbing laser radiation in a cuvette, it is only possible to create an apodizer that is not free from the common drawbacks of cuvettes with absorbing media: spectral selectivity and significant heat release in the absorber.

Closest to the claimed technical solution is the device of the apodizer based on a cell with a turbid medium in accordance with RF patent No. 2163386 [21]. Such a cuvette is filled with a medium transparent to laser radiation with a refractive index μ containing optical microinhomogeneities (small particles) with a refractive index n ≠ μ, with radii ρ ≥ λ, which scatter laser radiation. Formed due to the design of the cuvette, a thickness-profiled layer of such scatterers with a maximum number of particles on the periphery of the cuvette provides a smoothed transmission profile of the apodizer T (r), decreasing from the axis of the cuvette to its periphery. To calculate T (r) in expressions (1, 2) for the amplitude apodizer, instead of the absorption coefficient k _1, we need to use the attenuation coefficient γ = σ m ₀ , where σ = 2π ρ ² is the scattering cross section, and m ₀ is the concentration constant of the scattering particles. For a cuvette filled with a gas or liquid containing thin-walled glass shells with a characteristic radius ρ = 5 μm at m ₀ = 8 · 10 ⁶ m ^-3 , the contrast of the apodizer is calculated (the transmission functions for the layers of a turbid medium on the axis and on the periphery are respectively 0 , 1 and 5 mm) gives a value of K≅ 400 [21]. To prevent particles from settling due to gravity, a medium flow with microparticles through a cuvette is provided [21]. The advantages of the apodizer in accordance with the patent [21] are its low intrinsic absorption, high optical strength and the ability to be used with lasers in a wide spectral range of radiation wavelengths. The disadvantages include the difficulty in manufacturing the aspherical optics necessary for the formation of a profiled scattering layer, as well as the immutability of the transmission profile of the apodizer. Indeed, when the concentration of scatterers is constant over the volume and the medium flows through the cell and maintains this concentration, the distribution of particles over the volume of the cell and, consequently, the transmission profile of the apodizer do not change.

The objective of this invention is to provide an apodizer for a laser beam with a smoothed transmission function, high contrast and “fill factor”, providing apodization in a wide spectral range from near UV to IR region of the spectrum, which has a slight intrinsic absorption of transmitted radiation and allows for the possibility of tuning transmission functions. The invention also provides for the simplification of the design of the apodizer by eliminating aspheric optics elements that are difficult to manufacture from the design.

The claimed technical solution is an apodizer for a laser beam with a wavelength λ and a transverse dimension of 2R based on a radiation-transparent medium with a refractive index μ containing the beam formed in the aperture of the transverse coordinates 0≤ r≤ R, 0≤ φ ≤ 2π and the direction of the beam propagation axis z, the layer consisting of the beam moving orderly in the aperture is symmetrical about its axis, scattering radiation of particles of j species (j = 1, 2) with concentrations m _j (r, φ, z), with refractive indices n _j ≠ μ and with radius ρ _j , λ ≤ ρ _j << R, and the attenuation of radiation by the layer increases, and the transmission profile of the layer T (r) decreases from the beam axis to its periphery.

The proposed apodizer can be formed in a gas or liquid flow unlimited by the walls, into which microparticles orderly moving in the aperture of the beam are introduced, creating a light-scattering layer with a variable transmission cross section of the laser beam. The claimed technical solution can also be implemented on the basis of cuvettes filled with medium (gas, liquid) containing moving microparticles that form a light-scattering layer. The cuvette may contain a thickness-profiled layer of a medium with moving particles. An apodizer based on a cell with plane-parallel windows with a constant thickness layer of the medium can also be implemented. In this case, the transmission profile can be formed due to the variable concentration of moving scattering particles over the cross section of the cell. When changing the motion parameters of scattering particles or their composition, the transmission profile of the apodizer can be rearranged.

To clarify the essence of the claimed technical solution and its quantitative characteristics, we consider examples of apodizers with a tunable transmission profile based on cuvettes with moving particles scattering laser radiation. 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;

φ is the azimuthal angle;

L is the length of the cell;

j is the particle sort index, j = 1, 2 ...

ρ _j is the radius of particles of the j-th grade;

d _j is the density of particles of the j-th grade;

M _j is the mass of the particle;

d ₀ is the density of the medium;

η is the viscosity coefficient of the medium;

m _{j is the} concentration of particles of the jth grade;

ω = dφ / dt is the angular velocity of rotation of the cell;

T is the temperature of the medium in the cell;

k is the Boltzmann constant.

Various technical solutions are possible to organize the ordered movement of particles so as to form a smoothed transmission function in the cell. It is possible to introduce particles into the medium in several ways, organizing their movement symmetrical with respect to the beam axis in the cell along various trajectories. For example, for a cuvette filled with a gas flow moving symmetrically along the beam axis z with velocity v _z , a smoothing of the transmission profile can be obtained by introducing particles symmetrical in φ along the normal to the z axis and to the wall of the cell with velocities v _r . You can use the well-known dependence of the resistance of the medium to the movement of spherical particles, F _j on the particle radius ρ _j (Stokes formula): F _j = 6π η ρ _j v _r . As the calculation shows, necessary for the formation of a smooth transmission profile of the particle radius distribution can be obtained in a medium flow moving along z with the introduction of particles of different radius with the same initial velocity v _r . It is possible to rebuild such a profile with changes in input rates and particle composition.

Below we consider an example where a centrifugal force acting on particles is used to form a profile. A scattering layer symmetrical with respect to the axis of the beam can be created by rotating the cuvette as a whole together with the working medium around the z axis.

Example 1. The cell is filled with a transparent medium with particles scattering laser radiation and rotates around the z axis with an angular velocity ω. We assume that the particles are microspheres with radius ρ _j , density d _j , and mass M _j “suspended” in a medium with density d ₀ , and d _j > d ₀ . For particles of small sizes ρ _j ≥ λ, where λ ≅ 1μ, located in the field of centrifugal forces at thermal equilibrium with the environment, the Boltzmann distribution of particle concentration over the radius is valid [22], which, taking into account the difference in particle densities and medium Δ d _j = d _j -d ₀ can be written as

where А _j = 2π / 3kТ · Δd _j ρ $\genfrac{}{}{0ex}{}{\text{3}}{\text{j}}$ R ² is the coefficient, and m $\genfrac{}{}{0ex}{}{\text{0}}{\text{j}}$ is the initial (uniform) concentration of particles of type j in the cell. Taking the scattering cross section 2π ρ $\genfrac{}{}{0ex}{}{\text{2}}{\text{j}}$ , for the exponent γ (r) we obtain

The expression for the transmission profile of the cell of length L and filled with particles of sort j takes the form

where m _j (r) is determined by the expression (3)

In the drawing and in the table, the calculated data for an apodizer cell rotating with an angular velocity ω are presented. The transmission curves (1-5) of the cell T (r) were obtained for the following fixed parameter values: T = 300K, L = 5 cm, R = 5 cm, Δ d = 10 ^-4 g / cm ³ and variable values of particle radii and angular cell rotation speed. From a comparison in the drawing of curves 1–3, one can see how the rearrangement of the transmission profile of the cell filled with particles of the same sort occurs (ρ ₁ = 5 μm, m $\genfrac{}{}{0ex}{}{\text{0}}{\text{1}}$ = 10 ⁴ cm ^-3 ), depending on the angular velocity of rotation ω. As the angular velocity increases, the profile, as one would expect, becomes “stiffer", closer to the rectangular transmission profile. Curves 4-5 illustrate the possibility of restructuring the profile when changing particle sizes (ρ ₁ = 5 μ, ρ ₂ = 10 μ, m ₀ = 5 · 10 ³ cm ^-3 ) at a constant angular velocity. The calculated contrast values for the cases considered are K≥10 ⁵ . Maintaining, after adjustment, the particle sizes, their concentration, and the angular velocity of rotation of the cell, the optical characteristics of the resulting apodizer can be fixed.

Estimates show that for the considered examples, one can neglect the influence on the shape of the apodizer profile of gravitational particle deposition, as well as the thickness of the layer of particles that are on the walls of the cell.

Thus, the above examples show the possibility of creating apodizers with a tunable transmission function, high contrast values and the “filling factor” of the laser beam aperture based on cuvettes with moving radiation-scattering microparticles. The use of liquids and gases transparent to laser radiation, as well as transparent scattering particles, as a working medium makes it possible to use apodizers in a wide spectral range from UV to IR spectral regions. Small intrinsic absorption of radiation increases the resistance of the apodizers to radiation loads and makes it possible to use them with lasers of pulse-periodic and continuous operation. One of the possible interesting applications of the apodizer is its formation directly in the laser-amplifying active medium of the laser. Such an implementation of a tunable apodizer may be of interest, for example, for the optical path of high-power technological gas lasers.

Literature

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Claims (4)

1. The apodizer for a laser beam with a wavelength λ and a transverse dimension of 2R, R is the radius of the beam, based on a medium transparent to radiation with a refractive index μ, containing 0≤r≤R, 0≤φ≤ formed in the aperture of the beam along the transverse coordinates 2π and in the direction of the beam propagation axis z, a layer consisting of particles scattering radiation with a concentration m, with a refractive index n ≠ μ and with radii ρ, λ≤ρ << R, and the transmission profile of the radiation by the layer T (r) decreases from the beam axis to its periphery, characterized in that the layer is formed and moving orderly manner in the aperture of the beam symmetrically about its axis in the coordinates r, φ, z j particle types (j = 1, 2 ...) of radius ρ _j, with concentrations of m _j (r, φ, z) and the refractive indices n _j , moreover, λ≤ρ _j << R and n _j ≠ μ. 2. The apodizer according to claim 1, characterized in that the medium with particles flows in the form of a stream symmetrical about the axis of the laser beam through the cuvette. 3. The apodizer according to claim 1, characterized in that the medium and particles with a density of d _j > d ₀ , where d ₀ is the density of the medium, are enclosed in a cuvette with windows transparent to laser radiation, and rotate together with the cuvette around the axis of the laser beam with angular velocity dφ / dt = ω. 4. The apodizer according to claim 1, characterized in that the transparent medium enhances the laser radiation.