Reflecting surface

FIELD: optical instrument engineering.

SUBSTANCE: invention can be used for wide-band light reflecting. Reflecting surface has dielectric layers A, B and C. A layer is made of material having low refractivity, B layer is made of material with average refractivity and C layer is made of material having high reflectivity. Optical thickness of layers equals to λr/4, where λr is wavelength of middle part of interval having high refractivity. Sequence of layer alternation looks like (CDCABA)KCBC, where K>=and has to be integer. Spectrum range with high reflectivity is widened due to shift in adjacent bandwidths at opposite sides along wavelength scale.

EFFECT: widened spectrum range with higher refractivity.

5 dwg

 

The invention relates to an interference coatings and, in particular, can be used in optical instrumentation for broadband light reflection.

Known broadband multilayer mirror [Asssss No. 1062636, CL G 02 B 5/28, bull. # 47 23.12.83]containing substrate with on her multilayer coating, which is a system of elementary mirrors consisting of alternating layers with high and low refractive indices, while the optical thickness of the layers in the elementary mirror connected between a defined ratio, the average optical thickness of the layer in the elementary mirror increases with their number, the range of variation of the average layer thickness is determined by the interval of wavelength high reflection, and the number of elementary mirrors is determined by the formula, linking it with the boundary wavelengths and refractive indices of the layers. To obtain a high reflectance coating such mirrors must contain a large (more than 20) number of layers.

The closest to the essential features analog is a multilayer interference coating [N.A. Macleod, Thin-film optical filters, Adam Hilger Ltd.: London, 1969, p. 149-151 (prototype)], having the structure A(ABCBA)Kin which a, b and C is respectively low, medium and high display is the prevalence of refraction, K=10, and the optical thickness of each layer is equal to λ0/10, where λ0- wavelength mid region with high reflection. This procedure of alternating layers of spectral high reflectance get narrow.

The technical result of the invention is the extension region of the spectrum with a high coefficient of reflection.

This technical result in the implementation of the invention is achieved in that in the reflective coating, consisting of a system of alternating layers a, b and C, the optical thickness of each of which is equal to 0.25λ0where λ0- wavelength of the center strip reflection, and the values of the refractive indices of the layers are related as nA<nB<nCwhat's new is that the sequence of layers is selected as (SSAVA)KSVS, where It≥1 is an integer.

The differences between the claimed device from the closest analogue is that the optical thickness of the layers is 0.25λ0and not 0.1λ0and the sequence of layers (SSAVA)KSHS, and not A(ABCBA)K.

The invention is illustrated by drawings, which depicts the design of the inventive reflective coatings (Figure 1), microstrip model interference coatings (2, 4) and spectral (frequency) characteristics (Fig 3, 5), explain what their principle of operation of the inventive coating.

The claimed coating contains layers, each of which is made of transparent in the range of operating frequencies (wavelengths) of material (a, b and C in figure 1), and the optical thickness of the layers is equal to 0.25λ0where λ0the wavelength in the center of the spectral interval with a high reflection coefficient and the refractive indices such that nAnd<nB<nC. The sequence of layers (CBCABA)KCBC, where K≥1. Depicted in figure 1, the design corresponds To=1.

Reflective coating works as follows.

As in the case of coatings of the type AB, consisting of alternating layers with different refractive indices nAndand nBthanks to the phenomenon of interference of electromagnetic radiation incident on the coating at an angle close to normal is reflected in the range of wavelengths, the center of which is in the wavelength λ0one quarter of which is equal to the optical thickness of the coating layers, and the width of the interval is greater, the greater the ratio of the refractive indices. The proposed sequence of alternating layers covering the spectral band of high reflectivity in it is significantly wider than in conventional coating consisting of layers of two types, and even when K=1, i.e. the number of layers 9, is high (>99%) reflectance.

Your e is said can be illustrated, using microstrip model [Basalaev, Assaassin, Vfrainbow, reports of the Academy of Sciences, 2004, I. 395, No. 6, s-760; .Kitahara, .Kawaguchi, J. Miyashita, M. Wada Takeda, Journal of the Physical Society of Japan, 2003, Vol. 72, No. 4, p. 951-955] one-dimensional superlattices (photonic crystals), a particular case are interference coatings. The applicability of such models for the analysis of propagation of electromagnetic waves in superlattices is based on the fact that in microstrip lines (MICROSTRIP) transfer subject to transverse electromagnetic waves, and the effective dielectric constant of a segment of MICROSTRIP lines depends on the width of its strip conductor. MPL is a substrate (plate) of dielectric material with low loss, one side of which is fully metallized and plays the role of a grounded base (grounded plane), and on the second side of the substrate caused the strip conductor [Reference elements of the strip technique. Edited Allellasala. - M.: Communication, 1979. 336 S.; Vtocco. Microwave circuit. Analysis and computer-aided design. M.: Radio and communication, 1990. 288 C.]. Figure 2 shows the topology of the strip conductors of microstrip model simple interference coatings of the type AB, consisting of nine layers. Wide MICROSTRIP segments, their effective dielectric constant higher model layers with a high score prelola the Oia, and narrow MICROSTRIP segments, their effective dielectric constant lower model layers with a low refractive index. Thus the electrical length of the MICROSTRIP segments that make up the model are analogous to optical thicknesses of the layers in the interference system and equal. Figure 3 shows a typical frequency dependence of the reflection coefficient of the microstrip model simple reflective coatings, calculated in the quasi-static approximation [Helmetta, Lang, EMT Jones. The microwave filters, impedance-matching circuits and communication circuits, So I, M: Communications, 1971, 440 S.]. The bandwidth alternated with strips of reflection. Moreover, the bandwidth of equidistantly on a scale of frequencies. Band reflection will be the greater, the greater the ratio of the widths of the MICROSTRIP segments that make up the model. As mentioned above, the effective permittivity of a segment of MICROSTRIP lines is greater, the wider it is, therefore, this behavior of the bands reflect the microstrip model is similar to the behavior of the bands reflection interference system type AB, the lines of reflection which is the greater, the greater the ratio of the refractive indices of the layers.

Bandwidth in Miropolsky structure formed by the resonances in it. The first bandwidth, starting from the zero frequency, is formed as in the lowpass filter is in its structure such Mick is polaskova model represents a typical low pass filter. The second bandwidth is formed by half-wave resonances in each of the MICROSTRIP segments that make up the model. I.e. the length of each cut Mat fits half the wavelength of the oscillations corresponding to the center frequency of this band. The third bandwidth is formed by two-half-wave resonances, etc. and, thus, each of the MICROSTRIP segments, simulating the dielectric layer is poluvalmovye resonator, and all microstrip structure as a whole system of interacting resonators. Second, the bandwidth corresponds to the first oscillation mode of the resonators, the third bandwidth - second fashion etc.

From the point of view of electrodynamics, the dielectric layer is a one-dimensional dielectric resonator, and a system of several layers with different values of dielectric permittivity is a system of interacting resonators. This is another argument in favor of the applicability of the microstrip models interference coatings for the analysis of the transmission of electromagnetic waves in the past. A similar analysis carried out on models of the interference coating consisting of layers of two types (Figure 2)showed that the expansion of the bands reflect only possible due to the narrowing of the bandwidth that has its limitations.

In ICIE from simple interference system, consisting of layers of two types, the system consisting of layers of three types, i.e. three different refractive indices, it is possible to extend the bandwidth of the reflection due to the shift of bandwidth. Figure 4 shows the topology of the strip conductors of microstrip model of the claimed coating, and figure 5 - frequency response of its reflection coefficient. From Figure 5, in comparison with Figure 3, it is seen that indeed the reflection band in this case has increased due to the displacement of the second and third bandwidths in opposite directions. When this transmission in a third bandwidth has decreased. All this is due to the fact that in such microstrip model, a single half-wave resonators consist of three segments of MICROSTRIP lines. Such crystals are called “resonators with the spike wave resistance” [Basalaev, Tri, Uchiha. The Electronic Equipment. Ser. Microwave Engineering, 1997, issue 2(470), p.20-24]. Their characteristic feature is that they have nequality spectrum of natural oscillations due to the fact that some modes (for example, odd) decrease in frequency, while others (e.g., even) is increased. In figure 4, which shows the topology of the strip conductors of microstrip model of the claimed coating, dashed lines indicate the separation structure is as individual half-wave resonators. By analogy, the inventive coating can be interpreted as consisting of 2K+1 composite layers, each of which consists of three sublayers, characterized by the value of the dielectric constant (refractive indices).

Due to this structure of the coating by increasing the difference in refractive indices of layers width of the reflection grows as by reducing the widths of adjacent frequency bands, and at the expense of their displacement relative to each other in opposite directions. Therefore, to obtain a broadband reflective coatings require fewer layers.

In the near infrared region of the spectrum of the inventive coating can be obtained by thermal evaporation of fluorite (CaF2nA=1.42), sphalerite (ZnS, nB=2.28) and Germany (nC=4.0).

Reflective coating consisting of a dielectric layer, where the layer And made of a material with low refractive index layer is made of material with an average refractive index and the layer of material with a high refractive index, characterized in that the optical thickness of the layers is λ0/4, where λ0- wavelength mid-interval with high reflectance, and the sequence of layers has the form (SSAVA)KSVS, where It ≥ 1 an integer.



 

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