Interference mirror

 

(57) Abstract:

Use: for the manufacture of dielectric and metal-dielectric mirrors mainly for the infrared region, including technological CO laser systems. The essence of the invention: interference in the mirror comprising a substrate and a deposited coating of alternating layers with a low value of the refractive index of the metal fluoride and high value - from Germany introduced an additional layer of optical thickness (0.1 to 0.5)Lambda/4 from materials having an intermediate value of the refractive index and compressive mechanical stress, and the coating has the form P(PR-fluoride-PR-Ge) or the PDE (PR-fluoride-PR-Ge), where P - substrate, a CR-layer, n > 1 - the number of periods in the thickness of Lambda/2, and the thickness of Germany, adjacent to the substrate, is Lambda/4. As a material with an intermediate value of the refractive index and compressive mechanical stresses can be selected or zinc sulfide or zinc selenide or sulphide of arsenic, or arsenic selenide or sulphide of antimony. 1 C.p. f-crystals, 4 Il.

The invention relates to an optical instrument, in particular to the technology of optical coatings, and mo is blast (8-12 microns), including technological CO2-laser systems.

Interference coatings have found wide application in optical instrumentation, including laser technology. In particular, in high power industrial CO2lasers are used resonator mirrors with an interference coating. Under the influence of powerful laser beams due to the presence of losses by absorption in the interference layers part of the optical energy is converted into heat, stimulating the layers of diffusion, recrystallization, solid-phase reactions and mechanical stress, which leads to instability of the optical characteristics and fracture coatings.

Spread the design of the interference mirror type P (HB)nor PV (NV)nwhere P substrate N and a quarter-wavelength (at the operating wavelength) of alternating layers of low and high values of refractive index, respectively, n is an integer multiplier of not less than one, indicating the number of pairs (periods) layers, where the period is formed by layers with low and high values of refractive index and has an optical thickness equal to /2 (So-called Krylov. Interference coatings. L. Engineering, 1973).

For IR onium refractive index: fluoride thorium (ThF4), barium (BaF2), strontium (SrF2), lead (PbF2) and bismuth (BiF3), having a refractive index in the region (1,4-1,65), among which only ThF4and BiF3insoluble in water. For all of fluoride characteristic tensile mechanical stress in the layers. Known IR POM with a higher value of refractive index of ZnS, ZnSe, As2S3, As2Se3, Sb2S3, Ge, PbTe, etc. among which Ge and PbTe have a maximum refractive index in region (8-12) of 4.0 μm and 5.2, respectively, characterized by mechanical tension strains, for the rest of compressive stress (Physics of thin films./ Edited by Huss, and others M the World, so 8, 1978, S. 36-46).

Calculations of the optical characteristics show that it is possible to create was highly reflective interference mirror based on a set of pairs of layers of arbitrarily different refractive index POM, and the number of layers in them will be greater, the smaller the difference between the values of the refractive index.

Practically it is known that the greater the number of layers in the interference coating, the more optical loss and less mechanical stability of the coating due to nscompositesourceover mirrors, especially working under the influence of a powerful laser beam or thermal effects.

For the infrared region considered more significant as the geometric thickness of the interference layer, for example, at a wavelength of 10.6 µm reach quantities (1-2 microns), which significantly increases the internal stresses in the coating. Therefore, with that said, in practice it is advisable to seek to use AID with a maximum difference in the values of refractive index, which will reduce the number of layers of the interference coating and, consequently, increase its mechanical stability (provided razroznennyh and equal mechanical stresses). Other limitations in the choice of SIP in some cases are diffusion and solid state reactions between the layers, the result of which are the products of their interaction with unsatisfactory mechanical properties, or increased absorption, which ultimately leads to the destruction of the coating. Taking into account the above limitations in the choice of SIP, the number of possible design solutions IR interference mirror coatings with low losses is significantly reduced.

It is advisable to pay unline the distribution of losses in the coating thickness and decreases in the direction to the substrate. In a quarter-wave interference mirror coating antinodes (maxima) of the intensity fields correspond to the interfaces between layers 1-2, 3-4, and so on, counting from the side of incidence of light (from the air), and nodes (minima) of the intensity fields correspond to the boundary air-1, 2-3, 4-5, etc.

Known technical solution was highly reflective interference mirrors for 10.6 µm of the form P (HB)nfrom POM ZnSe and Ge, respectively (Kulikov E. N. Investigation of the mechanism of optical losses in the films Germany. Optics and spectroscopy, I. 69, V. 4, 1990, S. 846), in which to achieve a reflectivity of about 99% was used (5-6) pairs of layers. The main disadvantages of this solution are quite large total losses ( 1%) and a low resistance coating during thermal Cycling effects. In our opinion, this was determined by a large number of layers, leading to increased losses and unbalanced mechanical stresses in multilayer coating. These tensions led to the fact that the floor was cracked and crumbled under cyclic temperature effects.

Known technical solution (prototype) IR interference mirrors for CO2laser is tive of the metal fluoride, for example, ThF4, BaF2, NaAlF6etc. and layers with a high value of ZnS, ZnSe, Ge, and others (ed. St. Bulgaria N 30286 from 26.05.81).

In case of creation of such SIP was highly reflective mirrors with a reflectivity of 99% for example, from BaF2and ZnSe necessary (14-16) quarter-wave layers, which significantly deteriorates the mechanical stability of the coating and increases optical loss. If you create a mirrored coating on the basis of BaF2and Ge can theoretically do 6 layers, however, the direct combination of fluorides with GE, it is undesirable for the following reasons. First, as the layers of fluorides, and the Ge layers are tensile mechanical stress, which does not ensure the mechanical stability of the surface due to their cracking (what we have seen in practice). Secondly, between fluoride and possible Ge solid-phase reaction, which affects the optical characteristics of the mirror coatings due to the increase of loss absorption.

The aim of the invention is to improve performance, reduce total losses and the cost of the mirrors.

The aim is achieved in that in an interference mirror comprising a substrate and a deposited coating and the Germany, in the coating composition introduced an additional layer of optical thickness (0.1 to 0.5)/4 made of materials having an intermediate value of the refractive index and compressive mechanical stress, and the coating has the form P (PR fluoride PR Ge)nor P Ge (PR fluoride PR Ge)nwhere P substrate, a CR layer, n1 is the number of periods thickness /2, and the thickness of Germany, adjacent to the substrate, is /4.

As a material with an intermediate value of the refractive index and compressive stresses selected or zinc sulfide or zinc selenide or sulphide of arsenic, or arsenic selenide or sulphide of antimony.

The proposed interference mirror is implemented in the following way.

Consider the case was highly reflective interference mirrors on a substrate of Ge, then the design of the cover is written in the following form: P(0,1 ZnSe 0,8 BaF20,3 ZnSe 0,8 Ge)3. Such a coating can be applied, for example, by thermal evaporation in vacuum. As you can see, the thickness of the layer of ZnSe chosen minimal in places with a minimum intensity of the field, where the first does mostly functions pestiviruses and compensating mechanical stress layer. The layer thickness of 0.3 s, to reduce losses by scattering and absorption in the reflective coating.

Another case of an opaque metal-dielectric mirror with a reflectivity of not less than 99,5% on the metallic substrate, which is applied to the interference coating type Psi (0,1 ZnSe 0,8 BaF20,3 ZnSe 0,8 Ge)2in which layer is selected from the same reasons. Design characteristics: reflectance of 99.7% loss absorption A=a 0.3% loss on the scattering are not taken into account.

The third output of the interference mirror on the ZnSe substrate. In this case, the coating can be written in the following form: P Ge (0,1 ZnSe 0,8 BaF20,3 ZnSe 0,8 Ge), in which a layer of ZnSe selected on the proposed principle. The calculated optical characteristics: r 91,5% transmittance t of 8.4% AND 0,1%

In Fig. 1 shows the relative design of a quarter-wave interference mirrors; Fig.2 diagram of the intensity distribution of the optical field in the cross section of the mirror coating; Fig.3 design of the interference mirror; Fig.4 diagram of the intensity distribution of the optical field in the interference coating.

In Fig.1 shows a hypothetical construct was highly reflective interference mirrors (the specifications specifications such mirrors (when the radiation direction side of the coating), excluding losses on the light diffusing the following: r 99,6% t 0,25% 0,15% From Fig.2 shows, the intensity distribution of the optical field (J2in the cross-section of the coating is such that the antinodes (maxima) are in the boundary layers 1-2, 3-4, and so on, and the nodes (minima) at the boundaries of air-1, 2-3, and so on, the total geometrical thickness of such reflective coatings 7.5 μm.

In Fig.3 shows the proposed design of the interference mirror based on the same SIP, as in the prototype, but with layers of ZnSe, and the thickness of the layers in the nodes of the optical field is chosen equal to 0.1/4, and at the antinodes 0,3/4 (Fig.4). As you can see, the maximum field intensity falls in the middle layer with a thickness of 0,3/4, which allows to reduce the total losses, including those due to light scattering. The calculated optical characteristics of such mirrors as follows: 99,5% t of 0.25% And 0.15% of the total geometrical thickness of the mirror coating is equal to 7 μm.

Thus, in the invention between the main interference layers introduced layer of POM ZnS, ZnSe, As2S3, As2Se3, Sb2S3. In the literature it is known the use of layers in the construction of interference coatings for compensation of internal mechanical stresses and for passivation of the neighboring layers. But the proposed solution is rajini stretching layers of fluorides and Ge; they perform the role of passivator, reduces the processes of diffusion and solid state reactions between the main layers; and, finally, they reduce the total losses. The losses on the absorption decrease due to the fact that the material layers have a substantially lower absorption in comparison with fluorides and Ge, and the scattering losses are reduced by reducing the difference in refractive index between the material layers and the main interference layers, and also due to the fact that the boundaries between the main layers and layer are located in areas with lower intensity of the optical field in comparison with analogues and prototype.

In addition, the optical thickness of the layers selected in the range of values (0.1 to 0.5)l/4. The minimum value is determined from the condition that the geometric thickness of the layer for SIP with a refractive index (2.0 to 3.0) is 0.15-0.1 ám, respectively, and still guarantees the continuity layer that provides passivation and, to some extent, the compensation of mechanical stresses in the mirror coating. The minimum thickness of the layer it is appropriate in areas with a minimum intensity of the optical field in the coating. The maximum thickness of 0.5/4 is the meaning of the names is normal, as further increases in the mirror coating loses its advantage in reflectance compared to the mirror coatings of the SIP BaF2and ZnSe or ZnSe and Ge.

Layer introduced by reducing the thickness of the adjacent layers of fluorides and Germany in the amount of two layers (in the node and the node) within a period of /2. This condition determines the location of the layers in the antinodes of the field so that they perceive a significant proportion of the optical load, therefore, determine the total losses. Thus instead of one border section, located at the antinodes (which is typical for the prototype), the layer forms two boundaries, but at a lower intensity level of the optical field, which leads to reduced losses by absorption and scattering.

Example. Were made was highly reflective resonator "back" mirrors for technological lasers type t is 1.5, released in NCTRL wounds. The design of the interference mirror corresponded to Fig.3. The coating was carried out on domestic technological installation type BU-2M thermal evaporation in vacuum. Used SIP of domestic production, and layers BaF2and G were identified by their optical characteristics, which amounted to (99,0-99,2)% t (0.3 to 0.4)% total losses were determined as A = 1 - - and was not more than 0.5% Mirrors were tested in laboratory conditions in the regime of thermal shock 120+60oC (10 cycles), and was tested in real conditions in the resonator of the laser, t is 1.5 at rated output power for 24 hours Then re-determined optical characteristics and assessed the integrity of the coatings. Within the measurement accuracy (0.2% ) of the optical characteristics have not changed, and the floor had no signs of damage visible in the 100xincrease.

Compared to mirror a similar type P (HB)5from POM ZnSe and Ge total losses were almost two times smaller, and the cost of applying reflective coatings 1.8 times less.

1. Interference mirror comprising a substrate and a deposited coating of alternating layers with a low value of the refractive index of the metal fluoride and a high value of Germany, characterized in that the coating composition introduced an additional layer of optical thickness (0.1 to 0.5)/4 made of materials having an intermediate value of the refractive index and compressive mechanical stress, and the coating has the form P(Germany, adjacent to the substrate, is /4.

2. Mirror under item 1, characterized in that the material with an intermediate value of the refractive index and compressive mechanical stresses selected or zinc sulfide or zinc selenide or sulphide of arsenic, or arsenic selenide or sulphide of antimony.

 

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