RU2157034C2 - Soft diaphragm for lasers - Google Patents

Abstract

FIELD: laser optics; solid-state and gas lasers for laser fusion technology. SUBSTANCE: soft diaphragm built around dish with two windows interconnected through sealed joint by means of spacer ring with holes which forms casing has optically transparent insert (refraction index n_i) in the form of convex-concave meniscus with spherical or aspherical surfaces placed between windows with two liquid- filled clearances between them. Only one clearance between window and convex surface of meniscus is filled with liquid (refraction index n_l1) absorbing radiation from laser beam of wavelength λ incident on dish; second clearance is filled with liquid having refraction index n_l2 that does not absorb laser radiation. Thickness of absorbing liquid layer increasing as function of dish radius r from its axis to periphery smooths down (anodizes) flow section of dish obeying desired law with contrast K in the range of 10²-10⁶. Differences between refraction indices Δn₁ and Δn₂ of insert and liquid Δn₁= n_i-n_l1 and Δn₂= n_i-n_l2, respectively, are chosen are chosen so as to ensure that maximal deflection angle φ(a) of beam incident on dish periphery at r = a is within diffraction angle φ(a)<1,22λ/2a for dish aperture diameter 2a. Dish components may be made of various optical materials and may use various working liquids with permissible deviation of their refraction indices Δn < 0,1. EFFECT: improved stability at temperature fluctuations within several degrees. 2 cl, 3 dwg

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, in laser installations developed according to the program of laser fusion.

 Soft diaphragms-apodizers of light beams are currently widely used devices in the optical path of modern high-power laser systems. They are used to smooth the spatial distribution of intensity in laser beams; their application allows one to suppress sharp bursts of intensity arising in the aperture of beams when they are diffracted by conventional ("hard") diaphragms. Thus, the use of soft diaphragms increases the stability of high-power laser beams with respect to self-focusing. It also makes it possible to optimize energy removal in the active medium by increasing the factor of filling the working aperture of the amplifier with radiation, and increasing the efficiency of converting laser radiation to higher harmonics [1-8].

Despite the large number of proposed methods and technologies for forming soft diaphragms, only a few types of apodizers have found the real use of high-power lasers in the optical path, which are distinguished by a sufficiently high resistance to laser radiation (1-5 J / cm ² ) and high contrast - the ratio of the transmittance of radiation to axis and on the periphery of the beam, K = 10 ² -10 ³ . Among them are the so-called serrated metal diaphragms [8], diaphragms based on partially frosted glass plates [5] diaphragms based on multilayer dielectric coatings, and some others [6, 7].

а профиль оптических элементов, ограничивающих слой поглощающей жидкости, должен быть, вообще говоря, асферическим. При этом зависимость толщины слоя поглотителя от радиуса r описывается функцией вида

Здесь: Т(о) - пропускание на оси диафрагмы,
а - радиус диафрагмы, при котором Т(r) уменьшается в К раз,
h₀ ≥ 0 - толщина слоя поглогителя на оси диафрагмы.It is possible to obtain the highest contrast values up to K ≥ 10 ⁶ in soft diaphragms based on layers of variable thickness from substances absorbing laser radiation [2-6]. The radiation strength of such diaphragms can exceed 1 J / cm ² [2-6]. A known device that implements this opportunity is a cell with an internal cavity of variable thickness filled with a liquid with an absorption coefficient k [2, 3, 5, 6]. In order for a laser beam with a uniform intensity distribution incident on such a cuvette to acquire a soft intensity distribution at the exit from it, described by a super-Gaussian function with contrast K, the dependence of the transmittance of the diaphragm on the radius r Tr should be described by a function of the form

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

Here: T (o) - transmission on the axis of the diaphragm,
a is the radius of the diaphragm at which T (r) decreases by a factor of K,
h ₀ ≥ 0 is the thickness of the absorber layer on the axis of the diaphragm.

A soft diaphragm based on a glass cuvette is known, one of the windows of which is a plane-parallel plate, and the other is a plano-convex spherical lens [3, 5, 6]. A solution of copper sulfate was used as an absorbing liquid at a wavelength of a neodymium laser λ = 1.06 μm in a cuvette [3, 5, 6]. The transmission function of such an aperture is close to Gaussian, and the contrast is low K≤10 ² [3, 5, 6].

A soft diaphragm cuvette based on a dye solution is also known, in which one of the windows is plane-parallel and the other is a plano-convex aspherical lens [2]. In such a cuvette — a soft diaphragm, it is possible to obtain a super-Gaussian transmission function and, due to the high values of the dye absorption coefficient, contrast values up to K ≥ 10 ⁶ [2].

The main drawback of the designs of cuvettes – soft diaphragms proposed in [2, 3, 5, 6] is the need to select with very high accuracy the equality of the refractive indices of the liquid filling the cuvette, n _w , and the material of the window with a curved surface n ₁ , as well as the need to strictly control temperature conditions of such ditches - soft diaphragms. Indeed, a cuvette with a lens-like window has significant optical power, unless the working fluid filling the cuvette is a strictly immersion liquid for the material of the window with a curved surface. A deviation from immersion will naturally cause strong phase distortions in the beam passing through such a cuvette. According to estimates, for diaphragms with apertures ≅ 70 mm, the permissible deviation of the refractive indices Δn = n ₁ -n _w should not exceed 10 ^-5 . In this case, phase distortions in the beam passing through the cell — the soft diaphragm — will fit within the diffraction angle of ≅ 10 ^-5 rad. This condition, as is easy to see, imposes stringent requirements on the temperature regime of such a device. Indeed, the temperature coefficient of refractive indices β of most organic solvents for dyes used in lasers in the visible and near IR wavelength ranges λ is β _l = (- 2) - (- 5) • 10 ⁴ / K [9]. For most glasses and crystals that can be used in the design of the cell, in the same wavelength range β ₁ = 10 ^-6 - 10 ^-7 / K [10]. Thus, the main reason for the violation of the immersion condition in the cuvette will be variations in the value of n _W associated with small deviations in the temperature of the cuvette. To maintain the immersion condition in the cuvette, it will therefore be necessary to stabilize it with an accuracy of Δt ≈ 0.1 K. These features of the operation of soft diaphragms — the cuvette [2, 3, 5, 6] with a profiled layer of liquid, make their practical use difficult, of course.

 A known design of the cell with two plane-parallel windows, separated by an insert (insert) of plane-parallel quartz plate [11]. Such an insert formed gaps with a constant thickness with the entrance and exit windows of the cell, filled with the same solution of a bleachable dye [11]. The cell in [11] was used as an interstage decoupling of an amplifier-sharpener of laser pulses. A cuvette with a plane-parallel insert could not provide the functions of a soft diaphragm.

Closest to the invention (prototype) is a device of a soft diaphragm-cell, considered in [12]. In this work, the design of the cuvette of Fig. 1 with two windows hermetically connected to each other with a gap filled with a working fluid is given, and the thickness of the gap increases from the axis of the cuvette to its periphery due to the lens-like surface profile of one of the windows. In figure 1 and in the text, the following notation:
1 - cell windows (glass, quartz);
2 - profiled area of the window;
3 - layer of absorbing liquid,
4 - plane of the optical contact.

 It is noted that in order to create a profiled layer of the working fluid, an insert can be placed in the cuvette, however, the design of the cuvette with the insert is not given. An advantage of the design of a soft diaphragm-cell from [12] is its tightness, which is essential when working with aggressive and toxic working fluids. When using dye solutions of a cuvette dye that is illuminated by laser radiation in a cuvette, a soft diaphragm can also be used as a passive Q-switch in a generator or interstage decoupling in a laser amplifier. It should be noted that when using a bleachable dye, the transmittance profile of the cell begins to depend on the intensity of the laser radiation incident on it. However, the design of the cuvette from [12] is also characterized by the drawbacks noted above that make it difficult to use as a soft diaphragm and a passive shutter: the need to select with high accuracy the equality of the refractive indices of the working fluid and the material of the window with a curved surface and, as a consequence, the need for rigorous thermal stabilization ditches.

 The objective of this invention was to create a design of a soft diaphragm for lasers based on a cuvette with a laser absorbing liquid with a smooth transmission function with high contrast and providing apodization of laser beams in the near UV, visible and near infrared wavelengths, and without requiring a rigid thermal stabilization and accurate selection of the equality of the refractive indices of the materials of the optical parts of the cell and the working fluid. When using a translucent dye in a cuvette, the cuvette should also serve as a passive optical shutter.

The claimed technical solution is a soft diaphragm for lasers based on a cuvette, figure 2, with two windows hermetically connected by a spacer ring with holes and forming a housing containing a liner of optically transparent material with a refractive index n _in placed between the windows with two gaps filled with liquid through the holes with a refractive index n _x, absorbing radiation with a wavelength λ of the laser beam incident on the cell, wherein a thickness of at least one of the gaps, s complements liquid increases from the cell axis to its periphery. When this insert is formed as a convex-concave meniscus with spherical or aspherical surfaces, and the gaps are filled independently with various liquids, the absorbing laser light liquid with a refractive index n _w filled only the clearance between one of the cell windows and the convex surface of the meniscus, and a second gap is filled a liquid transparent to laser radiation with a refractive index of n _Ж1 , while the discrepancies Δn _{1,2 of} the refractive indices of the liner and liquids Δn _1,2 = n _in -n _Ж2 are chosen so that the max the maximum deflection angle of the radiation beam emerging at the periphery of the cell at ψ (a) was within the diffraction angle for the aperture of the cell with a diameter of 2a, ψ (a) <1.22 λ / 2a.

 In addition, when used in a cuvette as an absorbing liquid, a bleachable colorant can be used as a passive shutter for lasers.

The design of the cell is shown in figure 2. In figure 2 and in the text, the following notation:
1 - cell windows (glass, quartz),
2 - a spacer ring (glass, quartz) at the optical contact with the windows,
3 - layer of absorbing liquid,
4 - insert in the form of a convex-concave meniscus (glass, quartz, plastic),
5 - seals,
6 - holes for pouring liquid,
7 - a layer of non-absorbing liquid.

 The cuvette body is formed by two plane-parallel windows 1 of an optically transparent material, hermetically connected to each other by a spacer ring 2. In the case between the cell windows there is an insert 4 of optically transparent material in the form of a convex-concave meniscus. The gaps formed between the windows and the liner are filled with a liquid, and only one of the gaps between the cell window and the convex meniscus surface is filled with a liquid 3 absorbing laser radiation (for example, a dye solution). This gap forms a soft transmission profile of the cell. The second gap - between the concave surface of the meniscus and another window is filled with a liquid 7 transparent to laser radiation (for example, with a clean solvent without dye). The design of the cuvette includes seals 5 and holes 6, made in the spacer ring for separate filling of the gaps with working substances.

Such a design ensures the operation of the cuvette as a soft diaphragm under practically acceptable, non-critical in terms of deviations of refractive indices Δn = n _in -n _w and thermal stabilization conditions. Indeed, the optical power of a meniscus located in a medium with a close refractive index is very small. In this regard, for the design of the claimed cell, one should expect a significant softening of the requirements for acceptable deviations of the refractive indices of the liner and the liquid Δn and, accordingly, for temperature deviations. To clarify the essence of the claimed technical solution, FIGS. 3a and b compare the path of the rays of the radiation beam passing through the cell with an equal-thickness meniscus insert, FIG. 3a, and through the cell in which the insert is a plano-convex lens, FIG. 3b. On figa and b and in the text the following notation:
H is the thickness of the meniscus,
h (r) is the profile of the absorbing layer,
r is the radius of the cell
R are the radii of curvature of the spherical meniscus surfaces,
φ is the angle of deviation of the beam passing through the cell with an equal thickness meniscus,
φ ₁ - the angle of deviation of the beam at the exit of the cell with a plano-convex lens.

Здесь n^* = n_в/n_ж - относительный показатель преломления для границы раздела мениск - жидкость, n_в - показатель преломления для границы раздела мениск - воздух, т. е. Δn-n_ж(n^*-1). Для сравнения угол отклонения φ₁ для кюветы с линзой, фиг.3б, описывается формулой

Применение кюветы с мениском может обеспечить, таким образом, выигрыш в угле отклонения порядка

Полученные соотношения справедливы для малых углов падения, dh(r)/d_r << 1. (4)
Нижеследующие примеры сопоставляют величины характерных угловых отклонений пучка на периферии кюветы с менисковым вкладышем со сферическими и асферическими поверхностями с отклонениями пучка в случае применения линзоподобного вкладыша. Для простоты расчетов менисковые вкладыши считались равнотолщинными, а линзовый вкладыш - плоско-выпуклым. Условие (4) во всех примерах выполнено n_ж1 = n_ж2 = n_ж
Пример 1. Сферические поверхности стеклянного мениска радиуса R. Выражение (1) для φ при этом

При r=a =3,5 см, Δn = 0,015, n^* -1 = 10^-2, H = 0,5 см; R = 17.5 см; n_в = 1,5, n_ж = 1,485. Максимальный угол отклонения пучка на выходе из кюветы φ(a) ≅ 8 • 10^-7, что заведомо меньше, чем дифракционная расходимость пучка на апертуре 2а = 7 см, ψ_д = 1,7 •10^-5.The deflection angle φ for the beam passing the cell with an equal thickness meniscus, figa, is described by the following ratio:

Here n ^* = n _in / n _w is the relative refractive index for the meniscus - liquid interface, n _in is the refractive index for the meniscus - air interface, i.e., Δn-n _w (n ^* -1). For comparison, the deviation angle φ ₁ for the cell with the lens, figb, is described by the formula

The use of a meniscus cell can thus provide a gain in the angle of deviation

The relations obtained are valid for small angles of incidence, dh (r) / d _r << 1. (4)
The following examples compare the characteristic angular deviations of the beam at the periphery of the cell with a meniscus insert with spherical and aspherical surfaces with deviations of the beam in the case of a lens-like insert. For ease of calculation, meniscus inserts were considered equally thick, and the lens insert was considered flat-convex. Condition (4) in all examples is fulfilled n _Ж1 = n _Ж2 = n _Ж
Example 1. Spherical surfaces of a glass meniscus of radius R. Expression (1) for φ in this case

When r = a = 3.5 cm, Δn = 0.015, n ^* -1 = 10 ^-2 , H = 0.5 cm; R = 17.5 cm; n _in = 1.5, n _w = 1.485. The maximum deflection angle of the beam at the exit from the cell is φ (a) • 8 • 10 ^-7 , which is obviously smaller than the diffraction divergence of the beam at the aperture 2a = 7 cm, ψ _d = 1.7 • 10 ^-5 .

где a - радиус, соответствующий спаданию интенсивности в К раз. Для K= 1000 из представленных выше данных можно вычислить требуемый коэффициент поглощения раствора красителя (без просветления) к= 20 см^-1 и толщину его слоя на периферии кюветы h(a)=3,5 мм.The spherical surface of the meniscus creates a transmission profile of a cell of the form of a Gaussian function. For the incident intensity I ₀ (r) = const at h ₀ = 0, the transmitted beam intensity will be

where a is the radius corresponding to a decrease in intensity by a factor of K. For K = 1000, from the above data it is possible to calculate the required absorption coefficient of the dye solution (without bleaching) k = 20 cm ⁻¹ and its layer thickness at the periphery of the cell h (a) = 3.5 mm.

Это отклонение значительно больше дифракционного угла и недопустимо при работе с пучками, имеющими угловую расходимость ≤ 10^-4 рад. Отношение углов отклонения φ/φ₁ = 10^-4, т.е. выигрыш в уменьшении искажений угловой расходимости лазерного пучка в случае использования кюветы с менисковым вкладышем составляет 4 порядка при значении Δn = 0,015.For comparison, we calculate the beam deflection angle φ ₁ for the prototype cell with an insert in the form of a plano-convex lens, Fig.3b:

This deviation is much larger than the diffraction angle and is unacceptable when working with beams with angular divergence ≤ 10 ^-4 rad. The ratio of deviation angles φ / φ ₁ = 10 ^-4 , i.e. The gain in reducing distortions of the angular divergence of the laser beam in the case of using a cell with a meniscus insert is 4 orders of magnitude with a value of Δn = 0.015.

.Example 2. Aspherical surfaces of an equal thickness meniscus insert. The diaphragm transmission profile in this case should ensure the formation of a beam with a distribution of the intensity of the form

.

где h₀ = h(0).Dye layer profile

where h ₀ = h (0).

For K = 1000, the maximum thickness of the dye layer (without bleaching) h (a) = 1.4 mm, for k = 50 cm ^-1 .

Произведем оценку отклонения пучка в кювете с мениском для следующих значений параметров:
N = 5, а = 3,5 см, H = 0,5 см, n_в = 1,5, n^*-1 = 10^-2, Δn = 0,015, k = 50 см^-1. Получаем φ ≃ 3•10^-6. Отклонение луча в кювете с асферической линзой (фиг.3б) φ₁ ≃ 3 • 10^-3, что неприемлемо для практики.The maximum beam deflection angle at the exit of the cell is described in this case by an expression of the form

Let us estimate the deflection of the beam in the meniscus cell for the following parameter values:
N = 5, a = 3.5 cm, H = 0.5 cm, n _in = 1.5, n ^* -1 = 10 ^-2 , Δn = 0.015, k = 50 cm ^-1 . We obtain φ ≃ 3 • 10 ^-6 . The deviation of the beam in a cuvette with an aspherical lens (Fig.3b) φ ₁ ≃ 3 • 10 ^-3 , which is unacceptable for practice.

The gain from the use of the meniscus in this case is φ / φ ₁ = 1 • 10 ^-3 .

Thus, the use of a meniscus insert in a cuvette with both spherical and aspherical surfaces does not lead to angular deviations of the laser beam beyond the limits of diffraction divergence. The calculations show that the applied cell with a meniscus insert can ensure the operation of such devices as “soft” diaphragms when the refractive indices of the liquid and the insert are mismatched up to 0.1. This means that these ditches are practically insensitive to temperature changes within a few degrees. In addition, it becomes possible to search for various materials for the manufacture of windows and the cell insert among a wide class of media transparent in the near UV, visible and near IR spectral regions, including not only glasses and crystals, but also, for example, optical plastics. The range of possible solvents, dyes, salt solutions for filling the cuvette is also expanding. Of interest may be the use of non-toxic, non-aggressive solvents in cuvettes, such as, for example, water. These advantages should contribute to the wide practical application of the proposed design with lasers operating in the near UV, visible and near infrared spectral regions. The choice of the surface shape of the meniscus liner will allow one to form the intensity distribution in the laser beams of both Gaussian and super-Gaussian shapes, and the choice of the dye concentration in the solution will make it possible to obtain contrast values K = 10 ² - 10 ⁶ for these distributions.

 Example 3

 The use of a bleachable dye in the inventive device gives it the function of a passive shutter: a Q-switch or interstage decoupling in a laser system. The transmittance profile of a cell with a bleachable dye begins to depend on the intensity of the incident radiation. This means that the profile of the laser beam transmitted through the optical element will be different, for example, at the fronts and at the maximum of the laser pulse, although it will be smoothed (soft). The use of a cell with a soft aperture with a bleachable dye as a Q-factor in a laser cavity or as a passive decoupling-sharpener of pulses in amplification stages will be determined by the specific conditions of the laser setup and the type of dye used. It should be noted that the use in a cuvette with a meniscus instead of the usual dye bleach practically does not affect the optical power of the device. Therefore, all conclusions regarding the advantages of a cell with a meniscus insert compared with the lens insert in terms of the angular divergence of the beam passed through the cell remain valid.

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

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Claims (2)

1. A soft diaphragm for lasers based on a cuvette with two windows hermetically connected to each other with a gap filled with a liquid absorbing radiation with a wavelength λ from a laser beam incident on the cuvette, the gap thickness increasing from the axis of the cuvette to its periphery, characterized in that the window of the cell are connected lantern ring containing insert from an optically transparent material having a refractive index n _in a convexo-concave meniscus with spherical or aspherical surfaces, forming with two windows of the cuvette ZAZ pa filled independently via openings with various liquids, the absorbing laser light liquid with a refractive index n _x1 filled with only a gap between one of the cell windows and the convex surface of the meniscus, and a second gap is filled with transparent to laser radiation a liquid having a refractive index n _x2, wherein the mismatch Δn ₁ and Δn ₂ of the insert and the refractive indices Δn = n _{1 in} liquids - n _x1 and Δn ₂ = n _a - n _x2 are selected such that the maximum beam deflection angle φ (a) at the outlet of the cell diameter 2a n hodilsya within the diffraction angle for the aperture of the cell φ (a) <1,22 / 2a.  2. A soft diaphragm for cell-based lasers according to claim 1, characterized in that the absorbing liquid in the gap of the cell is a phototropic medium.

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