RU2504805C2 - Multispectral interference light filter for protection from laser radiation - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: light filter includes a transparent substrate and three elements deposited thereon, said elements having interference coatings made of alternating layers with high and low refraction indices (BH)^k. In the first element, which is a multilayer interference filter in form of a second-order mirror (λ₀=1565 nm) with high reflection in the 530-540 nm range and maximum transmission in the 470-505 nm and 545-620 nm range, at the filter-substrate and filter-air boundary, there are additional layers of (CH)³ and (CH)² C1.24H. In the second element, which is a long-wave cut-off filter (λ₀=680 nm) with high reflection in the 635-740 nm range and maximum transmission in the 470-620 nm range, situated on the side of the substrate opposite the first element and directly adjacent to it, there are additional layers of 0.5C(CH)⁴ and (CH)³C 0.54H. In the third element, which is a short-wave cut-off filter (λ₀=425 nm) with high reflection in the 380-460 nm range and maximum transmission in the 470-620 nm range and situated on top of the second element, there are additional layers of 0.5 BH(CH)³ and 0.5B.

EFFECT: high filter transparency in the short-wave and long-wave region of the spectrum from the high reflection band while blocking laser radiation with a given wavelength.

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Description

The invention relates to optical instrumentation, and in particular to devices for protecting organs of vision and equipment from the blinding effects of laser radiation. Known protective devices using various types of filters that block the laser radiation of certain frequencies and a certain power. Blocking of radiation occurs either due to absorption of radiation of a given wavelength (λ ₀ ), or due to its reflection. Absorption in color filters is created by introducing impurities, dyes, pigments, semiconductor elements into materials. The absorbed light is converted to heat, as a result of which the filters may become discolored and lose their ability to protect against powerful laser radiation. As reflecting systems, various types of interference coatings from alternating layers with high (n _in ) and low (n _n ) refractive indices can be used: dielectric mirrors, polarizers, beam splitters, band-pass, cut-off filters [1]. The main requirements for such coatings are high reflection in the spectral region of laser radiation, high transmittance in the remaining parts of the visible part of the spectrum and the maximum possible color reproduction. To fulfill these requirements, coatings are usually created that block the laser radiation of one specific frequency.

Currently, the creation of coatings is required to protect against laser radiation of several wavelengths simultaneously. For this, the blocking band in the visible part of the spectrum should be large enough to provide deep suppression of the laser wavelength, including when the light is obliquely incident, and small enough not to block the rest of the spectrum. The optimal reflection bandwidth is Δλ _0.01 / λ ₀ = (0.04-008) (Δλ _0.01 is the reflection bandwidth at 1% transmittance, λ ₀ is the blocking wavelength). An obstacle to obtaining maximum transmission in the transparency region is the presence in the transmission spectra of side reflection maxima (oscillations), the height of which depends both on the difference in refractive indices and on the absolute values of the refractive indices of alternating layers. The deepest dip is near the edge of the reflection band. The number and height of side maxima increases as the number of layers increases. An increase in the number of layers is necessary to obtain maximum reflection (high optical density).

Fairly high requirements are also imposed on the performance characteristics of coatings. Coatings must be moisture resistant, withstand cleaning with organic solvents (alcohol, acetone).

A known design of a filter from alternating layers with a high and low refractive index, with a ratio of optical thicknesses equal to five (copyright certificate No. 381055 of 05.15.1973) [2]. This embodiment of the filter provides selective reflection in several parts of the spectrum, for example, in the region of 0.53; 0.69 and 1.06 microns, while maintaining transparency outside these areas. The design of the filter P (5VN) ^to 5V, where the symbol P indicates the substrate; B and H are quarter-wave films with high and low refractive indices; k is the multiplicity parameter that determines the total number of layers in the system.

However, this filter does not have sufficient optical density (D). At a wavelength of 0.53 μm, D at almost one point is equal to unity (Δλ _0.01 / λ ₀ <0.01, i.e., Δλ≈2-3 nm). In addition, the filter has low mechanical strength, since with a large difference in the thickness of adjacent layers, too high internal stresses are created. Such filters are subject to mandatory gluing immediately after manufacture.

Known systems of dielectric mirrors with increased selectivity (Δλ _0.01 / λ ₀ = 0.1-0.2) based on layers of unequal optical thickness, and this inequality varies from layer to layer according to a given program. For example, P (0.07B- 1.93H-0.09B-1.91H-0.12B- 1.88H- ... 0.07B). However, such systems are difficult to manufacture, since they contain very thin layers, the thickness of which cannot be controlled with the necessary accuracy in the visible part of the spectrum. Such coatings can be used for the far infrared region of the spectrum.

Also known are systems of alternating layers of equal optical thickness from three different materials. The selectivity of systems can be Δλ _0.01 / λ ₀ = 0.08-0.16. The symbol for such systems is П (ВСННС) ^к , П (НСНВС) ^к ипп, where N is an integer denoting the thickness of the layer following it in quarters of the wavelength, С is a quarter-wave film with an average value of the refractive index. However, such systems have a low optical density and additional reflection bands in the transparency zone.

Various coating designs are known that are used to reduce oscillations and increase transmittance in the transparency working region, with the condition that the high reflection region is preserved [3-6]. To obtain the maximum possible transparency on the long-wave side from the reflection band, the designs P (0.5VN0.5V) ^k or P (0.5NV0.5N) ^k are usually used. To obtain maximum transparency on the short-wave side from the reflection band, the structures P (BH) ^to B0.5H are used.

Depending on the specific requirements for the spectral characteristics of the coatings, a large number of filter designs have been developed to smooth out oscillations and increase transmission in given spectral regions. For example, it is proposed, except for the upper layer of low refractive index (n _m) of thickness λ _0/8 (0.5H), between the substrate and the interference system to place an additional layer of high refractive index (n _c) of thickness 3/4 λ ₀ (3B ) (copyright certificate No. 386363 dated 06/14/1973) [7]. The inventor's certificate №471568 from 05.25.75 proposed interference between the framing system and the media placed further layers with an average refractive index (n _sr), calculated according to a formula, the optical thickness of λ _0/8. The inventor's certificate №448418 from 10.30.74 [9] The same problem can be solved by introducing two additional words thickness λ _0/8 at the boundaries of the interference coating. Moreover, the layers adjacent to the coating have a refractive index equal to n = n _in ^3/2 · nn ^-1/2 , and the refractive indices of the outer layers are (nn ₀ ) ^1/2 and (nn _vap ) ^1/2 . In the certificate of authorship No. 553564 dated 04/05/1977 [10] it is proposed to introduce additional layers with n _in and an optical thickness of 0.3λ ₀ and an upper layer with n _n and an optical thickness of 0.15λ ₀ on both sides of the interference system.

Such a wide variety of ways to smooth and increase transmission in the field of filter transparency is due to the fact that interference systems are very sensitive to changes in optical parameters: the refractive indices of the substrate and layers, the difference between them, the number of layers, and the spectral range of application. Therefore, for each optical device, additional calculations of the layer parameters are necessary to ensure the required spectral characteristics.

The most complete variety of methods for smoothing oscillations in interference systems are presented in Sh.A. Furman (prototype) [4]. Using the equivalent layer method and other analytical methods, the author presented some special cases of smoothing oscillations for specific optical systems. For a short-wave cut-off filter, an increase in transmission in the short-wave region is proposed due to the introduction of two additional layers with an optical thickness of 0.125λ ₀ and refractive indices calculated by approximate formulas at the substrate-filter and filter-air boundaries. For a long-wavelength cut-off filter, to obtain high transmission in the long-wavelength region, the author proposes the use of the classical design P (0.5VN0.5V) ^k , with a refractive index of the first and last framing layer in accordance with the calculation. However, solutions found by calculation can not always be implemented in practice, since it is not possible to find film-forming materials with the necessary parameters.

To solve the problem posed before us, it is necessary to increase the transparency of optical systems both from the long-wavelength and short-wavelength sides of the high reflection band for all transition boundaries from high to low transparency.

The proposed coating differs from the known ones in that it consists of a composition of three different elements providing a given spectral characteristic of the coating as a whole. The goal is achieved in that each of the three elements is optimized in its structure so as to obtain an optical density of at least three in the spectral regions of 380-460 nm, 510-540 nm and 635-740 nm, respectively, with a maximum transmission in the spectral regions of 470 -505 nm and 545-620 nm to preserve color reproduction.

Figure 1 shows the design diagram of the filter. The light filter is a substrate 4, on both sides of which interference optical coatings are located. On one side of the substrate there is a multilayer interference filter in the form of a second-order mirror (λ ₀ = 1565 nm) with high reflection in the region of 510-540 nm (element 1) and maximum transmission in the range of 470-505 nm and 545-620 nm. On the second side of the substrate is a composition of two interference coatings. Directly adjacent to the substrate is a long-wave cut-off filter (λ ₀ = 680 nm) with high reflection in the region of 635–740 nm and maximum transmission in the region of 470–620 nm (element 2). This filter has a short-wave cut-off filter (λ ₀ = 425 nm) with high reflection in the region of 380-460 nm and maximum transmission in the region of 470-620 nm (element 3).

To increase the transparency of the filter in the short and long wavelength regions of the spectrum from the high reflection band of 510–540 nm when blocking laser radiation with a wavelength of 532 nm, additional layers are introduced into the main structure of element 1 at the filter – substrate – filter – air interface:

P (CH) ³ (BH) ¹⁴ (CH) ² C, 1.24H.

When blocking laser radiation with a wavelength of 650 nm, in order to increase transparency on the short-wave side from the maximum reflection band in the spectral regions of 545-620 nm and 470-505 nm, additional layers at the filter-substrate and filter-element 3 boundary were introduced into the design of element 2:

P0.5C (CH) ⁴ (BH) ¹¹ (CH) ³ C 0.54H.

When blocking laser radiation with a wavelength of 405 nm and 445 nm, in order to increase transparency on the long-wave side from the maximum reflection band in the spectral regions 470-505 nm and 545-620 nm, additional layers are introduced into the design of element 3 at the interface between the 2-filter and filter-air elements :

Ф0.5 VN (CH) ³ (VN) ¹⁵ 0.5V.

The composition applied to the second side of the substrate is written as follows:

П0.5С (СН) ⁴ (ВН) ¹¹ (СН) ³ С0.54Н0.5В Н (СН) ³ (ВН) ¹⁵ 0.5В.

Figure 2 shows the calculated spectral transmittance of the filter. The calculated average transmittance in the 470-505 nm region is 83.4%, in the 545-620 nm region - 88%. The visual transmittance for standard source A is 62.6%. The calculated values of the optical density at the wavelengths of the radiation of the laser pointers of purple, blue, green and red are shown in table 1.

Table 1 Wavelength nm 405 445 532 650 Optical density four 3,5 3.4 3.2

In accordance with the calculated data, pilot batches of filters were manufactured under production conditions. As layers with a high refractive index B, zirconium oxide was used. Yttrium oxide was used as layers with an average refractive index C. As layers with a low refractive index H, quartz was used. As substrates, optical optically transparent glass was used. For the manufacture of prototypes of coatings, the A-700QE vacuum system from Leubold-Heraus was used. The technological process for manufacturing mirrors is standard and consists of cleaning the substrates before coating, heating the substrates and sequential deposition of layers in accordance with the calculation. For applying the layers used electron beam evaporators. The thickness control of the layers was carried out by transmission spectrophotometric method. To obtain maximum accuracy, control schemes were calculated in advance. The residual gas pressure in the coating chamber was 2 ÷ 5 · 10 ^-5 mm Hg. Substrate heating temperature 180 ° C-220 ° C. The deposition rate of ZrO ₂ layers was 17 Å / min, the deposition rate of Y ₂ O ₃ layers was 20 Å / s, and the condensation rate of SiO ₂ layers was 25 Å / min. The selected evaporation modes are optimal from the point of view of obtaining the most stable and reproducible optical characteristics of the formed optical systems.

Figure 3 shows a typical spectral transmittance of a prototype filter, manufactured under industrial conditions by the method described above.

The band of the reflection zone in the visible region of the spectrum is Δλ _0.01 / λ ₀ = 0.06-0.08, which is close to the calculated values. The optical density at laser wavelengths was calculated from the transmission spectra measured on an SF8 spectrophotometer with a scale extension and is D> 3 for laser wavelengths of 405, 445, 532 and 650 nm. The average transmittance in the range 470-505 nm is 70.4%, in the range 540-620 nm - 80.5%. The visual transmission of experimental samples is 5-8% less than the calculated values and averages 54% for standard source A. The developed design provided the desired technical characteristics, and mass production of the product can be organized on its basis.

Literature

1. Gainutdinov I.S. and other Properties and methods for producing interference coatings for optical instrumentation. Kazan: Fen, 2003, 424 p.

2. Mironov S.P., Veremey V.V., Soloviev N.G. Interference light filter.// A.s. No. 381055, class G02b 5/28, publ. 05/15/1973. Bull. No. 21.

3. Telen A. Construction of multilayer interference light filters. In the book. Physics of thin films. M .: 1972, v. 5, p. 46-83.

4. Furman Sh.F. Thin layer optical coatings. L .: Engineering, 1977, 264 p.

5. Yakovlev P. P., Meshkov B. B. Design of interference coatings. M .: Engineering, 1987, 192 p.

6. Krylova T.N. Interference coatings. L .: Engineering, 1973. 224 p.

7.L.B. Katznelson and Sh.A. Furman. Interference light filter. // Aut. St. THE USSR. No. 386363, IPC G02b 5/28. Bull. No. 26 dated 06/14/1973.

8. B.B. Meshkov and V.A. Efremenko. Cutting-off optical interference filter with transmission in the short-wave region of the spectrum.// Auth. St. USSR No. 471568. Bull. No. 19 dated 05/25/1975.

9. E.G. Stolov and Sh.A. Furman. Interference optical cut-off filter.// Autosv. USSR No. 448418. Bull. No. 40 dated 10/30/1974.

10. N.F. Markov and E.G. Tables. Optical interference long-wave cut-off filter.// Aut. St. USSR No. 553564. Bull. No. 13 dated 04/05/1977.

Claims (1)

A multispectral interference filter for protection against laser radiation, including a transparent substrate and a composition deposited on it of three different elements containing interference coatings of alternating layers with high and low refractive indices (BH) ^k , characterized in that in the first of the composition elements, representing a multilayer interference filter in the form of a second-order mirror (λ ₀ = 1565 nm) with high reflection in the region of 530-540 nm and maximum transmission in the region of 470-505 nm and 545-620 nm, additional layers of (CH) ³ and (CH) ² C 1.24H are introduced into the filter-substrate and filter-air boundaries, into the second element, which is a long-wave cut-off filter (λ ₀ = 680 nm) with high reflection in the region of 635–740 nm and with a maximum transmission in the region of 470-620 nm, located on the opposite side of the substrate from the first element and directly adjacent to it, additional layers of 0.5C (CH) ⁴ and (CH) ³ C 0.54H are introduced, and in the third element, which represents a short-wave cut-off filter (λ ₀ = 425 nm) with high reflection in the region of 380-460 nm and maximum passing in the region of 470-620 nm and located on top of the second element, layers of 0.5 BH (CH) ³ and 0.5B are additionally introduced.

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