RU2034318C1 - Polarization-selective laser mirror - Google Patents

Abstract

FIELD: laser equipment. SUBSTANCE: polarization-selective laser mirror is manufactured in the form of multilayer dielectric coat applied on to optical backing 1. Coat includes alternating layers 2 and 3 of two dielectrics with relatively high and low index of refraction. One of dielectric layers - layer 4 has single-side or double-side corrugation of surface with period of corrugation 5 determined depending on length of wave laser radiation and indices of refraction of layers. Indices of refraction, optical thicknesses and number of layers 2 and 3 of coat are defined by condition of obtainment of specified reflection factor. In consequence of such arrangement only one component will interfere due to presence of difference of phase hop for P and S components of reflected radiation. EFFECT: amplified polarization-selective properties of mirror. 3 dwg

Description

 The invention relates to laser technology, namely to polarizing laser mirrors.

 The purpose of the invention is the strengthening of the polarization-selective properties of the mirror while maintaining a high reflection coefficient for the allocated polarization.

 In FIG. 1 shows the construction of a mirror with a corrugated surface of the outer layer; figure 2 conventional scheme of diffraction in this layer; figure 3 the design of the mirror with the corrugated surface of the layer adjacent to the substrate.

< d <

где λ длина волны лазерного излучения;
n₁ показатель преломления слоя 4 диэлектрика с гофрированной поверхностью;
n₂,n₃ показатели преломления сред, обрамляющих слой 4.The polarization-selective laser mirror is made in the form of a multilayer dielectric coating deposited on an optical substrate 1, and the coating consists of alternating layers 2 and 3 of two dielectrics with relatively high and low refractive indices. One of several layers, the dielectric layer 4 is made with one-sided or two-sided corrugation of the surface of the layer with a period d of the corrugation 5, determined from the condition

<d <

where λ is the wavelength of the laser radiation;
n ₁ the refractive index of the layer 4 of the dielectric with a corrugated surface;
n ₂ , n ₃ refractive indices of the media framing layer 4.

 The refractive indices, optical thicknesses and the number of layers 2 and 3 of the coating are determined by the condition for obtaining a given reflection coefficient.

. Дифракционные волны 8 и 9 испытывают полное внутреннее отражение на границах 5 и 12 слоя и ограничены слоем 4. В результате повторной дифракции на гофре 5 часть энергии дифракционных порядков волн 8 и 9 перейдет в энергию волн 13 и 14 нулевого порядка в отраженном от зеркала пучке. Волна 7 нулевого порядка дифракции последовательно отражается от границ слоев 4,3 и 2 и после прохождения поверхности раздела (гофра 5) дает вклад в волну 10 нулевого порядка дифракции. Для световых пучков с поперечными размерами много большими толщин слоев многослойника можно пренебречь влиянием поперечного смещения волн 8 и 9 при отражении от слоев многослойника. В результате интерференции полей волн 13 и 14 и отраженной волны 10 нулевого порядка дифракции изменяется амплитуда волны, отраженной от зеркала. Результат интерференции зависит от величины набега фаз интерферирующих волн. Фазовый набег волны 7 при двойном проходе слоя 4 равен

n₁H′+φ где Н' толщина слоя 4, φ скачок фазы коэффициента отражения от многослойного покрытия в плоскости границы 12 раздела сред. Аналогично для дифрагированного поля фазовый набег равен

n₁H′cosα+φ₁, где φ₁ скачок фазы волн 8 и 9 при отражении от границы 12 раздела сред. Величина скачка фазы φ₁ дифрагированного поля волн 8 и 9 существенно различна для двух положений плоскости поляризации падающего света: φ₁ ^Е, когда вектор электрического поля падающей на зеркало волны 6 направлен вдоль полос гофра и φ₁ ^Н, когда вектор перпендикулярен направлению полос гофра. Разность φ₁ ^E и φ₁ ^Н отлична от нуля и определяется выражением
φ $\genfrac{}{}{0ex}{}{\text{E}}{\text{1}}$ -φ $\genfrac{}{}{0ex}{}{\text{H}}{\text{1}}$ 2arctg

Следовательно, различен и коэффициент отражения зеркала для двух состояний поляризации светового пучка.The principle of operation of the mirror is illustrated in figure 2. The light wave 6 incident behind the mirror as a result of diffraction on the corrugation 5 gives waves of zero 7 and first 8 and 9 diffraction orders propagating in layer 4. Waves 8 and 9 in accordance with the diffraction equation propagate at an angle α to the direction of wave 7 of the zero diffraction order, sin α = λ / dn ₁ . Outside layer 4, only waves 10 and 11 of the zeroth diffraction order propagate, since the condition d <

. Diffraction waves 8 and 9 experience total internal reflection at the boundaries of layer 5 and 12 and are limited to layer 4. As a result of repeated diffraction on the corrugation 5, part of the energy of diffraction orders of waves 8 and 9 will go into the energy of waves 13 and 14 of the zero order in the beam reflected from the mirror. Wave 7 of the zeroth diffraction order is successively reflected from the boundaries of layers 4.3 and 2 and, after passing through the interface (corrugation 5), contributes to wave 10 of the zeroth diffraction order. For light beams with transverse dimensions much larger than the thicknesses of the layers of the multilayer, we can neglect the influence of the transverse displacement of waves 8 and 9 when reflected from the layers of the multilayer. As a result of the interference of the fields of waves 13 and 14 and the reflected wave 10 of the zero diffraction order, the amplitude of the wave reflected from the mirror changes. The result of interference depends on the phase shift of the interfering waves. The phase incursion of wave 7 with a double pass of layer 4 is

n ₁ H ′ + φ where H 'is the thickness of the layer 4, φ is the phase jump of the reflection coefficient from the multilayer coating in the plane of the interface 12. Similarly, for a diffracted field, the phase shift is

n ₁ H′cosα + φ ₁ , where φ _{1 is the} phase jump of waves 8 and 9 upon reflection from the medium boundary 12. The phase jump φ _{1 of the} diffracted field of waves 8 and 9 is significantly different for two positions of the plane of polarization of the incident light: φ ₁ ^Е , when the electric field vector of the wave 6 incident on the mirror is directed along the corrugation bands and φ ₁ ^Н , when the vector is perpendicular to the direction of the corrugation bands. The difference φ ₁ ^E and φ ₁ ^{N is} non-zero and is determined by the expression
φ $\genfrac{}{}{0ex}{}{\text{E}}{\text{1}}$ -φ $\genfrac{}{}{0ex}{}{\text{H}}{}$$\genfrac{}{}{0ex}{}{}{\text{1}}$ 2arctg

Consequently, the reflection coefficient of the mirror is also different for the two states of polarization of the light beam.

EXAMPLE On a substrate 1 of fused silica there is a multilayer coating of two dielectrics: zirconium dioxide (layer 2) and silicon dioxide (layer 3). The optical thickness of each layer is a quarter of a wavelength of 0.6328 microns. Layer 4 is made of zirconia and has a thickness H 'of 0.11 microns. The surface of layer 4 is made with a corrugation close to rectangular and has a corrugation amplitude of 0.025 μm, a corrugation period of 0.4 μm, and a strip width of 0.2 μm. The total number of layers in the multilayer, including layer 4, is 11. The reflection coefficients of the mirror for two positions of the plane of polarization of the incident light are 99.9% and 91%
PRI me R 2. Polarization-selective mirror made according to Fig. 3, is a fused silica substrate 1. The surface is made with a corrugation 5 having a pro-angular shape of the strip profile. The corrugation period is 0.4 microns, the width of the protrusions is 0.2 microns, the height of the protrusions is 0.025 microns. On top of the substrate is a multilayer dielectric coating of alternating layers of two dielectrics: zirconia (layer 2) and silicon dioxide (layer 3). Layers 2 and 3 have an optical thickness equal to a quarter of a wavelength of 0.6328 microns. Layer 4 adjacent to the substrate 1 is made of zirconia and has a thickness H 'equal to 0.015 μm. The total number of layers in the multilayer is 9. The reflection coefficients for the two positions of the plane of polarization of light are 99.7% and 81% at a wavelength of 0.6328 μm.

Claims (1)

A POLARIZATION-SELECTIVE LASER MIRROR made in the form of a multilayer dielectric coating deposited on an optical substrate, the coating consisting of alternating layers of two dielectrics with relatively high and low refractive indices, characterized in that, in order to enhance the polarization-selective properties, one of several layers of the dielectric is made with one or two-sided corrugation of the surface of the layer with a period d of the corrugation, determined from the condition

where λ is the wavelength of the laser radiation;
n _{1 is} the refractive index of a dielectric layer with a corrugated surface;
n ₂ , n ₃ refractive indices of media framing a dielectric layer with a corrugated surface.

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