Beam-splitting interference coating

 

(57) Abstract:

Usage: in the optical instrument when receiving interference coatings, including when creating the output mirror of the resonator powerful technology WITH lasers. The inventive beam splitter interference coating having the form of Pochv, where P is the substrate of zinc selenide, O - ar coating, KrB - outer layer with a high value of refractive index with variable thickness, determined by the law of Kr changes thickness depending on the radial coordinate, and the thickness of the outer layer KrB does not exceed /4, where is the working wavelength ar coating is a construct of the form BH, where B is a quarter-wave layer of telluride Germany, H - quarter-wave layer of film-forming material with a refractive index n = 2.2 to 2.7, and and the outer layer with variable thickness made of telluride Germany. 1 C.p. f-crystals, 4 Il.

The invention relates to an optical instrument, in particular an interference coatings, and can be used to create the output mirror of the resonator powerful technological CO2-lasers.

One of the important parameters that determine the quality of obrazuemogo beam. As shown in several works [1-4] one possible means of reducing the divergence of the radiation is used in the resonator technology laser output mirror with a beam splitter (mirror) coating having a variable radius reflectance. This coating has a maximum value of the reflection coefficient in the Central zone, gradually decreasing in the peripheral zone to the minimum value, close to zero, and the phase change of the past (or reflected) radiation within the beam splitting coating should not exceed /2.

For technological CO2lasers with output power of radiation of more than 1 kW important parameter of the cavity mirrors is the optical absorption, the value of which should be negligible. Due to absorption of the optical energy in the output mirror (including enlightening and beam-splitting coating), is heated mirror, which leads to its deformation and, consequently, distortion of the front radiation. In addition, increases the heat load on the mirror, which reduces its service life.

In the practice of manufacturing CO2lasers as a base (substrate) is (3-10)10-4cm-1depending on the quality of the material. The optical absorption of the output mirror includes, in addition to the volumetric absorption of the substratevsurface absorption of two optically polished surfaces 2s. Moreover, the respective contribution of absorption interference coatings applied to both surfaces 2cIn turn, the absorption interference coatings depends on the absorption coefficient (k) of film-forming materials (POM) and cover design. In the latter case, a significant factor in the achievement of the minimum absorption is the optimal distribution of intensity of the electric field in the cross section of the coating [5]

In effect a large enough gain in the resonator technological CO2lasers with output exceeding 1 kW, maximum reflectivity beamsplitter coating typically does not exceed (60 to 80)%

Known for the design of beam-splitting coatings [6] provides a variable along the radius of the reflection coefficient for the output mirror CO2lasers of the form P H B Kr H B where: P substrate made of ZnSe, H and B - a quarter-wavelength at the operating wavelength layers fluoride thorium (ThF4and ZnSe ratio is r. Moreover, the maximum value of Kr is in the Central region (r 0) and minimum (close to 0) in the peripheral area of the mirror. In this case, the range of variation of the thickness of the outer layer ThF4does not exceed /4, which ensures the fulfillment of the condition change of the phase is not more than p/2. This design of the beam-splitting coating ensured the maximum range of variation of the reflection coefficient from 1.9 to 82% In the peripheral zone of the cover (due to Kr 0) design takes the form P H B B, for which the presence of two quarter-wave layers of ZnSe is equivalent to the case (from the point of view of the magnitude of the reflection coefficient), therefore the design is equivalent to the antireflection coating of the form P H with the reflection coefficient r of 1.9%

The main disadvantages are considered technical solutions can be considered: a) the increased absorption due to the use of ThF4with a sufficiently large absorption coefficient (k (3-4)10-4), and also due to the "extra" two quarter-wave layers of ZnSe in the peripheral zone; b) imposed more stringent requirements for precision control of all layer thicknesses in the process of their application to create the necessary law changes of the reflection coefficient; C) the complexity UB>4with variable thickness, as required installation and then removal of special tooling, providing application layer ThF4with variable thickness.

Known technical solution (prototype) [7] in which the beam splitting coating with a variable reflection coefficient is a construct of the form P H Kr B, where: P substrate made of ZnSe, H quarter-wave antireflection coating, B quarter-wave layer Germany with variable thickness, determined by the law of K depending on the radial coordinate r. Moreover, in the Central region of the mirror coating Kr 1 and the reflectance maximum, and in the peripheral region Kr 0 and the reflection coefficient becomes equal to the value of reflection, antireflection coating. In the Central region of the beam-splitting coating reflectivity can reach 74% and a phase change within the beam splitting coating does not exceed p/2.

The main disadvantage of this technical solution is the increased absorption in the Central region of the beam-splitting coating, which according to our experimental data is 0.3 to 0.5% Is explained by the significant value of the coefficient of the absorption layer Germany, which is k (7-20)10the output mirror of the resonator lasers with output exceeding 1 kW.

The aim of the invention is to reduce absorption while maintaining the range of variation of reflectance of the beam splitter.

This goal is achieved by the fact that the design of the form P KrB, where N - ZnSe substrate, t ar coating, B external quarter-wave layer with a high refractive index with variable thickness, determined by the law of Kr, implements the change of the reflection coefficient from 1.5% to (68 - 74)% ar coating is a construct of the form P B H, where B is a quarter-wave layer of telluride Germany (GeTe), H quarter-wave layer of film-forming materials with a refractive index n (2,2 2,7), for example, zinc sulfide (ZnS), sulphide of arsenic As2S3, zinc selenide (ZnSe) or the sulfide of antimony (Sb2S3).

In Fig. 1 conditional (in cross section) shows the design of the beam-splitting coatings with varying the radius of the reflection coefficient corresponding to the prototype of Fig. 2 graphs of the intensity distribution of the electric field in the cross section of the beam splitting coating of the prototype of Fig. 3 - design of beam-splitting coating in accordance with the invention; Fig. 4 distributions intensionality. 1 shows the design of the beam-splitting coating corresponding to the prototype P H Kr B, where: 1 ZnSe substrate, 2 - ar coating of the quarter-wave layer of fluoride lead-free (PbF2or fluoride, bismuth (BiF3) having the refractive index n of 1,6 - 1,65, In the outer layer of germanium (Ge) having a refractive index n of 4.0, the Kr law of variation of the thickness of the outer layer depending on the radial coordinate r (center r 0).

In Fig. 2 presents graphs of the intensity distribution of the electric field in the cross section of the beam splitting coating, corresponding to the prototype, where: 1 the field intensity in the peripheral zone (Kr 0), 2 - field intensity in the Central zone of the beam-splitting coating (Kr1).

In Fig. 3 given the design of the proposed beam-splitting coatings with variable reflectance type P In N Kr, where: 1 substrate on which is deposited antireflection coating consisting of a layer 2 GeTe (n 3,7) and 3-layer ZnSe (n 2.4), 4 external GeTe layer with variable thickness according to the law of Kr.

In Fig. 4 presents graphs of the intensity distribution of the electric field in the cross section of the beam splitting coating according to the invention, where: 1 intensi is of given distribution of electric field in the peripheral zone of the prototype.

The proposed solution has the following salient features. The first ar coating, which is a part of the beam-splitting coating, made in the form of a two-layer quarter-wave design type P In H, in which the layer has the refractive index n of 3.7 and the layer "N" is the refractive index n (2,2 2,7). Although quarter-wavelength two-layer ar coatings are known, their use in the composition of the beam-splitting coatings with variable reflection coefficient available in the patent and technical literature is not known. The significant advantage of such a two-layer structure in comparison with the prototype is a significant decrease in the intensity of the electric field, which is obvious from a comparison of the graphs in Fig. 2 and Fig. 4. The second selected range of values of the refractive index of the layer "N" (n 2,2 2,7) is justified by the following considerations. On the one hand, this allows to get the minimum value of the reflection coefficient of refractive coating (peripheral zone) not more than 1.5% in many cases satisfactorily. On the other hand, it allows to realize a sufficiently large values of the reflection coefficient in the Central zone with the s2S3, ZnSe and Sb2S3have small values of the absorption coefficient k (0,1 -2,0)10-4[9] and have satisfactory performance characteristics for use in the composition of output mirrors a powerful technology CO2-lasers. Third, as a film-forming AID with a high value of refractive index is used telluride Germany, with the results of our research refractive index n 3.7 and the absorption coefficient k 210-4. GeTe slightly different from the Ge for the refractive index, however, has an absorption coefficient of 5 to 10 times less than satisfactory performance.

Describes the distinctive characteristics form a new set of features, unknown in the patent and technical literature.

The proposed solution is implemented in the following way. As the source data for the manufacture of beam-splitting coatings for the output mirror of the resonator must be known geometrical dimensions of the substrate of the ZnSe and the law of variation of the thickness Kr outer layer of GeTe, which can be obtained as the calculated and experimentally, on the basis of the required law changes coef thermal evaporation and condensation in vacuum in two stages. The first stage is applied ar coating type P In N, when first applied to the substrate of a quarter-wavelength at the operating wavelength of the GeTe layer, and then a quarter-wave layer of POM ZnS, As2S3, ZnSe or Sb2S3. The second stage is applied outer layer of GeTe with variable thickness, using appropriate tooling, forming the desired distribution of the layer thickness. After coating produce control the reflectivity of the beam-splitting coating on the coordinate r in accordance with a given law.

Practically, we have made the output mirror for technological CO2- "Lantan-3M". The mirror was an optically polished substrate of ZnSe h mm, on the front line which was applied ar coating type P In N of quarter-wave layers GeTe and ZnSe, which reduces the loss of the substrate on the reflection values of 0.2% Then on the other side was inflicted on the beam splitting coating with a variable radius reflection coefficient of the form P N Kr B consisting of ar coatings P N of quarter-wave layers GeTe and ZnSe and external GeTe layer with variable thickness. The law of variation of the layer thickness Kr was experimentally determined during the matching pair is izkuyu to superawesome. All coatings were deposited by thermal evaporation in vacuum on domestic vacuum installing WU-2M.

Achieved the following characteristics of the output mirror. Ar coating on the output face of the form P N had a reflectance of 0.2% absorption of b 0,02% Beamsplitter coating had in the peripheral zone of the reflection coefficient r0,2% in the Central zone of the reflection coefficient was r 72% and the absorption of b 0,03% of the Total absorption of the output mirror, including bulk and surface absorption of the substrate, absorption coating on both faces, was bS0,17% that meets the requirements for work under the influence of powerful laser beams. Thus, the proposed beam-splitting coating had an absorption of at least 10 times less compared to the prototype. Made us the output mirror was successfully tested on "Lantan-5" with output power up to 5 kW in continuous mode, when this was achieved the divergence of the radiation is not more than 0.6 mrad [10]

Sources of information

1. Kuznetsov M. Kulikov, O. L. //Quantum electronics, so 18, No. 6, 1991, c. 697.

2. De Silvestri, S. et. al.//IEEE J. Guantum Electron 26, 1500 (1990).

3. Sherstobitov Century. N. Venoco is n //Appl. Optics, v.16, N 8, 2147, 1977.

6. Emiliani G. at. al. //Appl. Optics, v.28, No. 14, 2832, 1989.

7. Brock D. R. Taylor M. Z. Warren S. W. //Proceedings of the conference LEOS, 90, USA, (4 November 9, 1990.

8. Glebov Century. N. Malyutin, A. M. Yakunin VP //Optical journal, 1992, N 4, S. 32.

9. Takeo Miyata //SPIE, v.660, 131, 1986.

10. Generals N. A. Zimakov B. N., etc. //laser Optics-93. St. Petersburg: VSC GOI them. S. I. Vavilov, I. 1, 1993, S. 277.

1. Beam-splitting interference coating with a variable reflection coefficient of the output mirror of the resonator having the form of Pochv, where P substrate made of zinc selenide, ar coating, GFP outer layer with a high value of refractive index with variable thickness, determined by the law of Kg changes thickness depending on the radial coordinate, and the thickness of the outer layer GFP does not exceed /4, where is the working wavelength, characterized in that the antireflection coating is a construct of the form VN, where the quarter-wave layer of telluride Germany, N quarter-wave layer of film-forming material with a refractive index n of 2.2 to 2.7, and the outer layer with variable thickness made of telluride Germany.

2. The floor under item 1, characterized in that the material with the refractive index n of 2.2 to 2.7

 

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