RU2345388C1 - Optical device of laser resonator (versions) - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: optical device of the laser resonator consisting of a plate of an optical material with a blanket, executed in the form of a diffraction grating with accents in the form of sequence of dimples or the blanket salients, differing that area S_xy of dimple or salient in the field of a blanket with in co-ordinates X and Y is spotted from expression

where T - a continuance of following of dimples or salients, G₁ - reflectivity or blanket gear transmissions, G₂ - a reflectivity or gear transmissions of blanket in the field of dimple or salient, G (x, y) - demanded function of change of reflectivity or a gear transmission of an optical device of the laser resonator along co-ordinates X and Y.

EFFECT: obtaining of device with high coefficient of reflexion/transmission with any way given law of the change providing an apodisation of laser bundles in a wide spectroscopic range with high beam firmness.

6 cl, 5 dwg

Description

The invention relates to laser optics and can be used as an optical element of a laser resonator (gradient mirror or “soft” aperture) when working with solid-state and gas lasers to form a given law for the distribution of optical radiation, as well as in astronomy and spectroscopy to correct the shape of the optical transfer function .

Known optical element of the laser resonator, made in the form of a mirror with a Gaussian distribution of the reflection coefficient (N.R. Belashenkov, V. B. Karasev, E. S. Putilin, P. N. Fimin, V. Yu. Khramov. Gradient laser mirrors. Optical Journal, 2001, vol. 68, No. 5, pp. 9-19). The multilayer dielectric coating is made with a variable thickness distribution over the surface of the mirror. In the far zone, the distribution of the laser radiation intensity with such a mirror has no additional maxima.

However, the manufacture of such mirrors is associated with the great difficulty of creating multilayer mirrors with a variable surface thickness and the need to produce special diaphragms for this. In addition, a significant drawback of this optical element of the laser cavity is the impossibility of providing an arbitrarily specified transmission law. In addition, due to the fact that the dielectric structures are performed with a thickness that is variable over the surface, spurious phase shifts occur that disrupt the operation of the laser cavity.

Also known is the optical element of the laser cavity, made in the form of a cuvette with two windows installed with a gap growing from the axis of the cuvette to the periphery and filled with a working substance, attenuating optical radiation and containing optical inhomogeneities with a refractive index different from the refractive index of the main working substance (Patent RF №2163386, G02B 27/58 Soft diaphragm for lasers. Senate Yu.V. Priority March 19, 1999 Published on February 20, 2001).

The disadvantages of this invention are the design complexity and the impossibility of obtaining an arbitrary intensity distribution profile due to technological difficulties in manufacturing a gap of arbitrary thickness, the complexity of manufacturing a cuvette and a working substance, low radiation resistance of a cell and a working substance, low radiation resistance of an optical element and high light losses.

Also known is an optical element of a laser resonator made in the form of a substrate with a relief metal diffraction grating (DR) deposited on it (RF Patent No. 2166819, G02B 5/18, Optical element of a laser resonator. Nizyev VG Priority June 15, 1999 Published May 10, 2001).

The disadvantages of this invention are the low transmittance (or reflection), low radiation resistance due to the use of absorbing metal DR, and large light losses.

The closest in design to the proposed invention is a device consisting of a plate of optical material with a surface layer made in the form of DR with strokes in the form of a sequence of recesses of the surface layer (RF Patent No. 2137163, G02B 5/20. Optical filter of optical radiation of variable density. A. G. Poleshchuk. Priority 12/10/1996, published 10/09/1999). The strokes of the DR are made in the form of concentric rings with a variable width along the circumference. The specified device is designed to create a variable transmission filter, and it cannot be used as the optical element of the laser resonator proposed in the application for the following reasons:

1. It has a low transmittance (or reflection) due to the presence of continuous concentric rings with a variable width. The transmittance of a known element is determined by the expression:

where d is the stroke width, T is the period of the diffraction grating.

=0.35-0.65. Таким образом, более половины световой энергии теряется. В известном устройстве закон изменения пропускания связан с изменением ширины штрихов вдоль окружности.From the expression (1) it follows that at values of T = 5-10 μm and d _min = 1-2 μm, the maximum light transmission of the known device is

= 0.35-0.65. Thus, more than half of the light energy is lost. In the known device, the law of variation in transmission is associated with a change in the width of the strokes along the circle.

2. When using the specified device, only the angular law of the transmission variation is possible, which does not allow the well-known invention to form the super Gaussian radial dependence of the transmittance (reflection) of the element most often encountered in practice:

where r is the distance to the center, G ₀ is the transmittance (reflection) in the center, w is the distance to the center at which the reflection coefficient drops e times, N is the order of the super-Gaussian law.

However, in design this device is the closest to the proposed invention.

The technical task of the invention is the creation of an optical element of a laser resonator having a high transmittance (reflection), having a reflection or transmitting function with an arbitrarily specified law of change, and providing apodization of laser beams in a wide spectral range (for example, with a two-dimensional super-Gaussian dependence of the transmittance / reflection ) and at the same time having high radiation resistance.

The task of fulfilling an arbitrary given law of the radial dependence of the transmittance (reflection) is achieved in the first embodiment due to the fact that in the optical element of the laser resonator, consisting of a plate of optical material (for example, quartz) with a surface layer made in the form of a diffraction grating with strokes in in the form of a sequence of recesses or protrusions of the surface layer, the area S _{xy of the} recess in the region of the surface layer with x and y coordinates is determined by the form proposed by the authors le

- коэффициент отражения или пропускания поверхностного слоя в области углубления, G(x,y) - закон изменения коэффициента отражения или пропускания оптического элемента лазерного резонатора вдоль координат x и y, при этом огибающей пластины является окружность, а начало координат совпадает с центром окружности. Задаваясь значениями G(x,y), мы можем рассчитать S_xy в каждой точке пластины и тем самым обеспечить требуемый закон изменения коэффициента пропускания или отражения. При этом глубина h углублений штрихов решетки выбирается в пределах от h=0 до h=

/2(n-1), где n - коэффициент преломления материала поверхностного слоя. Величина площади углублений S_min выбирается по формуле S

>(25-100)·h².where T is the period of the recesses or protrusions, G ₁ is the reflection or transmission coefficient of the surface layer,

is the reflection or transmission coefficient of the surface layer in the region of the recess, G (x, y) is the law of variation of the reflection or transmission coefficient of the optical element of the laser resonator along the x and y coordinates, while the envelope of the plate is a circle, and the origin coincides with the center of the circle. Given the values of G (x, y), we can calculate S _xy at each point of the plate and thereby ensure the required law of change in the transmittance or reflection. The depth h of the grooves of the grooves of the lattice is selected in the range from h = 0 to h =

/ 2 (n-1), where n is the refractive index of the material of the surface layer. The size of the area of the recesses S _min is selected by the formula S

> (25-100) h ² .

In order to increase the reflection coefficient (transmittance), a multilayer dielectric structure is applied to the surface of the plate. Such a highly reflective structure is used in laser mirrors, as well as in antireflection coatings.

In this case, in the second embodiment of the optical element of the laser resonator, the multilayer dielectric structure is additionally deposited on the diffraction grating made in the surface layer of the plate in the form of a sequence of recesses or protrusions differing in area, and in the third embodiment of the optical element of the laser resonator, the surface layer is formed by applying a multilayer dielectric to the plate structures, and a diffraction grating consisting of a sequence of recesses differing in area or protrusions made in the outer layer of a multilayer dielectric structure. The repetition period T of the recesses or protrusions of the grating strokes is selected for all variants according to the formula

- максимальный коэффициент отражения или пропускания оптического элемента лазерного резонатора.Where

- the maximum reflection or transmission coefficient of the optical element of the laser resonator.

The technical result consists in the fact that the inventive optical element of the laser resonator has a high transmittance / reflection coefficient and a reflection / transmittance function with an arbitrarily specified law of change, provides apodization of laser beams in a wide spectral range and at the same time has high radiation resistance due to lack of absorbing substances in multilayer dielectric coatings. The increase in transmittance achieved in the device simultaneously increases contrast.

The invention is illustrated graphic materials.

Figure 1 presents a diagram of an optical element of a laser resonator in two projections (a and b).

In figure 2. examples of the cross section of an optical element with a surface layer made in the form of a multilayer diffraction structure (a and b) are presented.

Figure 3 presents an example implementation of the claimed invention as a gradient mirror of the laser.

In figure 4. The dependences of the transmittance (reflection) on the area of the depression are presented.

In Fig.5. two graphs of the dependence of the reflection coefficient of a ten-layer dielectric mirror on the material of the last layer are presented for comparison. In graph 1, the last layer is made of ZnO ₂ , in graph 2 - of SiO ₂ .

The optical element of the laser resonator (figa) consists of a plate of optical material (substrate) 1 with a surface layer 2 made in the form of a diffraction grating with strokes 3 in the form of a sequence of recesses of the surface layer. The diffraction grating has a period T and a profile depth h (Fig. 1b).

Figure 2 shows the substrate 1, which is additionally coated with a multilayer dielectric structure 4. Figure 2a illustrates the implementation of the plate of the optical element of the laser resonator according to the first embodiment of the invention, and Fig. 2b - according to the second.

/2(n-1) для оптического элемента, работающего на пропускание, и не более h=λ/4 для оптического элемента, работающего на отражение, где λ - длина волны оптического излучения, n - коэффициент преломления материала поверхностного слоя пластины. Вид штрихов ДР в поперечном сечении вдоль оси x показан на фиг.1б. Период Т и глубина штрихов h выбираются везде одинаковы, а площадь углублений изменяется от S_min до S_max.Let us explain in more detail the design of the proposed device. The surface layer 2 (Fig. 1) is made in the form of a diffraction grating with strokes in the form of a sequence of recesses (or protrusions) 3. The area S of each recess varies depending on the location on the plate. The recesses in the surface layer of the plate have a depth h of not more than h =

/ 2 (n-1) for an optical element operating on transmission, and not more than h = λ / 4 for an optical element operating on reflection, where λ is the wavelength of optical radiation, n is the refractive index of the material of the surface layer of the plate. A cross-sectional view of the DR strokes along the x axis is shown in FIG. The period T and the stroke depth h are chosen everywhere the same, and the area of the recesses varies from S _min to S _max .

The surface layer of the substrate can serve as an additionally applied multilayer dielectric structure (MDS) 4, shown in figure 2. MDS is applied to increase transmittance or reflection. MDS 4 (Fig. 2a) can be deposited on the surface of a diffraction grating made in a substrate material (manufacturing an optical element according to the first embodiment of the present invention). And according to the second variant of the proposed device, DR is performed not in the substrate material, but in the outer layer of the MDS (fig.2b). In the first case, a DR is first manufactured, and then N layers of MDS are sprayed onto it. In the second case, N layers of MDS are sprayed onto the surface of the plate of the optical material 1. In the outer layer, a relief relief is formed by photolithography and reactive-ion etching. Since the MDS layers are usually made of various materials, the etching of the DR pattern in depth is carried out to completely etch the outer layer, preferably made of SiO ₂ . In this case, the lower layer plays the role of a stop layer, which allows etching in depth with high accuracy.

лежат на оптической оси лазерного резонатора, но по разные стороны пластины. Оптические оси светового потока дифракционных порядков с интенсивностью I_-1 и I₊₁ (другие более высокие дифракционные порядки на фиг.3 не показаны) лежат в плоскости светового потока и наклонены под углами α_k=λ/T к оптической оси светового потока, где k=1, 2, 3… - номер дифракционного порядка.Figure 3 presents an example of the use of the claimed invention as a gradient mirror. The optical element 1 of the laser resonator is mounted in series with the active medium 5 and the mirror 6. The optical axis of the reflected light flux I _ref and the transmitted light flux I _out

lie on the optical axis of the laser cavity, but on opposite sides of the plate. The optical axes of the light flux of diffraction orders of intensity I _-1 and I ₊₁ (other higher diffraction orders are not shown in FIG. 3) lie in the plane of the light flux and are inclined at angles α _k = λ / T to the optical axis of the light flux, where k = 1, 2, 3 ... is the number of the diffraction order.

The light flux, reflected from the DR, is decomposed into the angular spectrum into a number of diffraction orders. The zero diffraction order with intensity I ₀ does not change the direction of propagation, and the lateral diffraction orders propagate at angles α _k to the optical axis (Fig. 3) and are delayed by the input aperture of the active medium 5. The radiation intensity in the zero diffraction order, which is working, at the output element in the scalar approximation is described by the expression

where φ = 2π · h (n-1) is the phase shift introduced by the strokes of the DR (V.N. Kotletsov. Microimages. Optical methods of obtaining and control. L., Mashinostroenie, 1985, p. 210).

For a DR with φ = π, it follows from expression (3) that the transmittance or reflection of an optical element is described by the expression

where G ₁ is the reflection or transmission coefficient of the surface layer of the substrate, G ₂ is the reflection or transmission coefficient of the surface layer of the substrate in the region of the recess.

If it is necessary to have a given function G (x, y) of changing the reflection coefficient of the element or transmittance depending on the coordinates x and y, then, as follows from formula (4), the area of the recesses S of the lattice should change according to the following law:

where G (x, y) is the given function of the reflection or transmission coefficient of the optical element of the laser resonator along the x and y coordinates.

=0.95 и

=0.9. Из графика следует, что при минимальной площади области углубления S=1 мкм² (линейные размеры 1×1 мкм) коэффициент пропускания (отражения) равен R=0.96, а при S=50 мкм² (линейные размеры ~7×7 мкм) коэффициент пропускания (отражения) стремится к нулю. Таким образом, оптический элемент обеспечивает большой диапазон регулировки интенсивности оптического излучения, в то время как у устройства, являющегося ближайшим аналогом, максимальное пропускание не превышало 0.65 (см. формулу (1)). Т.е. световая энергия в предлагаемом устройстве почти не теряется. Это делает возможным использование предлагаемого оптического элемента в качестве внутрирезонаторного зеркала мощных лазеров.Figure 4 presents the dependence of the transmittance (reflection) on the area of the recess at T = 10 μm,

= 0.95 and

= 0.9. From the graph it follows that with a minimum area of the recess region S = 1 μm ² (linear dimensions 1 × 1 μm), the transmittance (reflection) is R = 0.96, and at S = 50 μm ² (linear dimensions ~ 7 × 7 μm), the coefficient transmittance (reflection) tends to zero. Thus, the optical element provides a large range of adjustment of the intensity of optical radiation, while the device, which is the closest analogue, the maximum transmittance did not exceed 0.65 (see formula (1)). Those. light energy in the proposed device is almost not lost. This makes it possible to use the proposed optical element as an intracavity mirror of high-power lasers.

The reflection coefficient (transmittance) of the MDS varies slightly in the region where the outer layer is removed to form a recess. In figure 5, this change is equal to the difference between the peak values of graphs 1 and 2, which decreases with increasing number of layers in the MDS.

The area of the recesses varies from the minimum value S _min determined by the technology of manufacturing the surface topography and the wavelength of light d _min > λ to a certain maximum value S _max at which the reflection (transmittance) of the element tends to zero G = 0. From expression (4) it follows that for G = 0 this area is determined by the relation

Those. when G ₁ = G ₂ -1 area S _xy = T ^2/2. A significant advantage of the proposed technical solution is the choice of an arbitrary shape of the recess. In accordance with expressions (4) and (5), to ensure the function of a given transmittance or reflection, it is necessary to provide a given area value, and the shape of the recess is not significant (a possible limitation is sharp bends with radii less than the wavelength of light). This makes it possible to significantly simplify the fabrication of the DR of an optical element by applying the direct recording method by a pulsed laser. In this case, the area of the removed material is rather well controlled, while the shape of the treated area changes.

The value of S _{min the} area of the recesses is determined from the ratio of their area and the depth of the relief DR according to the formula

The repetition period T of the recesses is determined from expression (4) with a maximum reflection / transmission coefficient G (x, y) = G _max and a minimally technologically feasible area S _{min of} recesses

The proposed optical element has a high transmittance, or small residual loss of optical radiation. This makes it possible to use it as an intracavity mirror of high-power lasers. The design of the optical element of the laser resonator provides a reflection or transmission function with high contrast and an arbitrarily specified law of change for apodization of laser beams in a wide spectral range and at the same time has high radiation resistance. Thus, the proposed optical element provides new opportunities that are absent from the known analogues - low light loss, simplicity of design and the ability to effectively generate radiation from high-power lasers with a given distribution of optical radiation.

Claims (6)

1. The optical element of the laser cavity, consisting of a plate of optical material with a surface layer made in the form of a diffraction grating with strokes in the form of a sequence of recesses or protrusions of the surface layer, characterized in that the area S _{xy of the} recess or protrusion in the region of the surface layer with coordinates X and Y is determined from the expression

where T is the period of the recesses or protrusions, G ₁ is the reflection or transmission coefficient of the surface layer, G ₂ is the reflection or transmission coefficient of the surface layer in the region of the recess or protrusion, G (x, y) is the required function of changing the reflection coefficient or transmission of the optical element laser cavity along the X and Y coordinates. 2. The optical element of the laser resonator, consisting of a plate of optical material with a surface layer made in the form of a diffraction grating with strokes in the form of a sequence of recesses or protrusions of the surface layer, characterized in that the diffraction grating made in the form of a sequence of recesses or protrusions varying in area , a multilayer dielectric structure is additionally applied, and the area S _{xy of the} recess or protrusion in the region of the surface layer with coordinates X and Y is determined from rage

where T is the repetition period of the recesses or protrusions, G ₁ is the reflection or transmission coefficient of the surface layer, G ₂ is the reflection or transmission coefficient of the surface layer in the region of the recess or protrusions, G (x, y) is the required function of changing the reflection coefficient or transmission of the optical element laser cavity along the X and Y coordinates. 3. The optical element of the laser cavity, consisting of a plate of optical material with a surface layer made in the form of a diffraction grating with strokes as a sequence of recesses or protrusions of the surface layer, characterized in that the surface layer is formed by applying a multilayer dielectric structure to the plate, and the diffraction grating , consisting of a sequence of recesses or protrusions varying in area, is made in the outer layer of the multilayer dielectric structure, wherein the area of the recess or protrusion in the region of the surface layer with coordinates X and Y is determined from the expression:

where T is the period of the recesses or protrusions, G ₁ is the reflection or transmission coefficient of the surface layer, G ₂ is the reflection or transmission coefficient of the surface layer in the region of the recess or protrusion, G (x, y) is the required function of changing the reflection coefficient or transmission of the optical element laser cavity along the X and Y coordinates. 4. The optical element of the laser resonator according to claims 1, 2 or 3, characterized in that the depth of the grating lines lies in the range from h = 0 to h = λ / 2 (n-1), where n is the refractive index of the material of the surface layer. 5. The optical element of the laser resonator according to claims 1, 2 or 3, characterized in that the value of S _{min the} area of the recesses or protrusions is selected by the formula S _min > (25-100) · h ² . 6. The optical element of the laser resonator according to claims 1, 2 or 3, characterized in that the period T of the recesses or protrusions is selected by the formula

where G _max - the maximum specified reflection or transmittance.

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