GB2070275A

GB2070275A - Interference mirrors

Info

Publication number: GB2070275A
Application number: GB8102810A
Authority: GB
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1980-02-01
Filing date: 1981-01-30
Publication date: 1981-09-03
Also published as: HU188519B; RO84537A; GB2070275B; BG34870A1; FR2475237A1; PL229483A1; RO84537B; DE3102301A1; DD153511A3; PL128372B1

Abstract

An interference mirror having a high reflection for several spectral bands, consisting of a substrate on which alternating layers of non-metallic individual layers, which have low optical losses, are arranged one above the other in such a manner that the individual layers have alternately a high refractive index and a low refractive index, one or more non-metallic coupling layers being located between adjacent groups of alternating layers, these groups consisting of 2 to 9 individual layers and being built up identically or differently, the individual layers of all groups of alternating layers having an optical thickness of 1/4 of a standard wavelength ???0 and the optical thickness of the non-metallic coupling layers amounting to an even-number multiple of ???0/4.

Description

SPECIFICATION Interference mirror which is highly reflective for several spectral bands The invention relates to interference mirrors which have a high reflection for several separate spectral bands and reflect light in these spectral bands with extremely low absorption losses and scattering losses. In the construction of optical equipment, mirrors of this type are used, above all, when it is important to minimize losses of light containing several distinctly separate spectral lines or spectral bands. One of the most significant applications is their use as resonator mirrors in gas lasers having several laser wavelengths, in which small absorption and scattering losses already reduce the output power of the different wavelengths to a considerable extent.

The construction of highly reflecting mirrors which operate according to the interference principle, is known, these mirrors consisting of a system of alternating layers of many alternating strongly refracting and weakly refracting non-metallic layers having low optical losses, these layers being superimposed on each other on the surface of mirror bodies. The layers have an optical thickness of 1/4 of the wavelength Am, which is intended to be reflected to the greatest extent, the number of layers having to be large in order to achieve a high reflection. (Z.f. Physik 142, 21 to 41, 1955)) Highly reflecting interference mirrors of this type possess the defect that, except in the case of relatively high orders of interference, the high reflection which is aimed at is achievable only in a narrow spectral range in the region of the wavelength Am.The simultaneous reflection of light having wavelengths outside the immediate region of Am is impossible by means of these interference mirrors.

Interference mirrors having a small number of layers are known from the same source, these mirrors certainly having a wider refraction range, but possessing the disadvantage of a lower degree of reflection. In order, at the same time, to obtain mirrors having high reflection for expanded spectral bands, or for several spectral bands, it is further known to superimpose different groups of alternating layers on each other, either on the front face and rear face of a glass plate, or on the surface of a layer-substrate, these groups having a large number of layers and accordingly a high reflection. Each individual group of alternating layers is matched to a different spectral band having wavelengths that are to be reflected most strongly Ambi, A,,,2 .A,,, .Amn, by means of separate measurements of the optical thicknesses of their A/4 component layers. (Optics of Thin Films, John Wiley + Sons, London 1976, Pages 148 to 157; Soviet Union Originator's Certificate 141,659; Swiss Patent 417,997).

This arrangement, however, entails the disadvantageous fact that the light of the spectral bands, which is reflected from groups of alternating layers located at increasing depths, must still pass through the many component layers of the overlying groups of alternating layers of other spectral bands, before and after reflection, and, in doing so, suffers appreciable absorption and scattering losses, in comparison to the light which is already reflected from the foremost or uppermost group, referred to the light-incidence direction, of alternating layers. The simultaneous achievement of several reflection bands having a high reflection and low absorption and scattering losses is thus possible by this means.

In order to obtain a reflected band of sufficient width and a steep edge between the reflection region and the transmission region, interference filters are known in which so-called transition layers are located between two different groups of alternating layers having a high reflectivity.

(Swiss Patent 458,780) However, the most significant defect resides in the fact that although wide spectral ranges having a low transmission are displayed, these ranges nevertheless largely derive, in practice, from absorption effects and not from a high reflection.

In addition, simple and coupled bandpass filters are known, which consist of purely dielectric layers or of a combination of metal layers with dielectric layers. [Optics of Thin Films; John Wiley + Sons. London 1976, Pages 1 62 to 177). These bandpass filters are intended to give rise to a bandpass region having a high transmission, bounded on both sides by blocking regions of low transmission. In this case also, it is particularly disadvantageous that the blocking action in the blocking regions likewise derives from the absorption effects of the layers, besides the reflection. In connection with the manufacture and function of bandpass filters, no additional teaching emerges to the effect that layer systems of this type can advantageously be employed as mirror for two spectral bands, compared to the known interference mirrors.

The use of alternating layer arrangements in higher orders of interference, in order to obtain several regions of high reflection, also belongs to the state of the art (Glastechn. Ber. 24, pages 147-148, (1951); Soviet Union Originator's Certificate 381,055). In this case, the essential defect resides in the fact that the component layers, corresponding to the order of interference, must be of a large optical thickness. This leads, for the entire layer system, to an overall thickness which is unacceptable for practical manufacture, the thick component layers giving rise to increased absorption and light-scattering losses. Moreover, the mechanical durability of such layer systems is poor.

The object of the invention is to eliminate the defects of the known technical solutions. The invention sets out to indicate interference mirrors, which simultaneously have a high reflection for several separate spectral bands, with extremely low absorption and light-scattering losses.

The invention is based on the problem of producing interference mirrors having layer arrangements, in which the increase in the absorption and light-scattering losses for the spectral bands which are reflected by more deeply located interference layers is prevented, and in which the overall thickness of the layer arrangement is as small as possible.

According to the invention, by means of interference mirrors having a high reflection for several spectral bands, consisting of a substrate on which groups of alternating layers of nonmetallic individual layers, which have low optical losses, are arranged one above the other in such a manner that the individual layers having a high refractive index and a low refractive index alternate, characterised in that one or several non-metallic coupling layers are located between adjacent groups of alternating layers, these groups consisting of 2 to 9 individual layers and being built up identically or differently. All groups of alternating layers are matched to a constant standard wavelength Ao, by arranging for the individual layers of all groups of alternating layers to have an optical thickness of 1/4 of this standard wavelength Ao.The optical thickness of the non-metallic coupling layers amounts to an integral multiple of Ao/4 In this case, the standard wavelength Ao is an average wavelength of the spectral region in which the mirror is to be used, in which region the reflection bands should lie.

A further feature of the invention is that a coupling layer is located between adjacent groups of alternating layers, the optical thickness of this coupling layer amounting to an odd-number multiple of A0/4, or that two coupling layers are located between adjacent groups of alternating layers, the optical thickness of these coupling layers amounting to an even-number multiple of At/4.

It is also possible to solve the problem according to the invention by locating a coupling layer between adjacent groups of alternating layers, the optical thickness of this coupling layer being an even-number multiple of Ao/4 In the case of the groups of alternating layers, employed according to the invention, these groups are expressly groups having a low number of individual layers, so that the individual group of alternating layers does not allow a high reflection for the standard wavelengths A0, but only a partial reflection which is, however, effective over a wide spectral region, on account of the low number of individual layers, in contrast to alternating layer systems according to the state of the art having a high number of individual layers for high reflection, these layers being effective only in a relatively narrow spectral region.

In the groups of alternating layers, employed according to the invention, having a lowreflection, but a reflection which is effective over a relatively wide spectral region, the low number of individual layers can be 2 to 9, depending on the effective refractive index in question.

The high reflection, with very low absorption losses and scattering losses, which can be achieved in spite of using groups of alternating layers of the type described, is explained by the fact that the not highly reflecting effect of the individual groups of alternating layers is strengthened to a high reflection in several spectral bands within their wide reflection region, by means of the coupling, according to the invention, of several such groups of alternating layers via one or more coupling layers, and the absorption losses and scattering losses of such groups of alternating layers, which are in any case low, are further minimised to the same degree.

These effects probably derive from the fact that some of the light, in all spectral bands present, is already largely reflected by the foremost group of alternating layers, referred to the lightincidence direction, or by the foremost groups of alternating layers. Only a little light still reaches the more deeply located groups of alternating layers, which are, however, apparently indispensable for achieving a high reflection, so that these groups cannot contribute decisively to further losses.

The layer arrangements according to the invention can, for example, be advantageously produced according to the high vacuum process, in which, as is known, suitable layer substances are evaporated, in order that they can deposit as layers on the surfaces of customary substrates In the near UV-region, VlS-region and near IR-region the known layer substances of high refractive index can be used, such as, for example, zinc sulphide (ZnS), titanium dioxide (TiO,). tantalum pentoxide (tea.05), and the known layer substances of low refractive index, such as cryolite (Na3AIF6) magnesium fluoride (MgF2), or silicon dioxide (SiO2), and many others, can also be used.

The Invention is explained in greater detail with the aid of the following illustrative embodiments.

In the associated drawing: Figures 1 2 and 3 illustrate the spectral reflection curves R (continuous curves) and the spectral transmission curves T (broken curves) of the interference mirror, according to the invention, described in the illustrative embodiments 1, 2 and 3.

Figure 4 illustrates, for comparison, the spectral reflection curve R (continuous curve) and transmission T (broken curve) of an interference mirror according to the state of the art.

In all illustrative embodiments according to the invention, the spectral position and the number of the reflection bands is defined by the standard wavelength Ao and by the sum D of the optical thickness of a group Dw of alternating layers and of the optical thickness of a coupling layer Dk(D = Dw + Dk) There are, for example, three reflection bands having a high reflection and very low absorption losses and light-scattering losses, in the case of the wavelengths

;;43 = 2\0 when the coupling layers have an optical thickness of an odd-number multiple of Ao/4 and an odd number of coupling layers is located between adjacent groups of alternating layers, or when the coupling layers have an optical thickness of an even-number multiple of Ao/4 and the number of coupling layers between adjacent groups of alternating layers is even.

There are, for example, two reflection bands having a high reflection and low absorption losses and light-scattering losses in the case of the wavelengths

when the coupling layers have an optical thickness of an even-number of Ao/4 and an odd number of coupling layers is located between adjacent groups of alternating layers. The spectral optical details of a given system of layers can be determined by measurements on the interference mirrors manufactured.

Illustrative embodiment 1 An interference mirror is manufactured with the aid of high-vacuum vapour-deposition, by depositing an arrangement of groups of alternating layers WO and coupling layers KN and KH on a substrate body G (for example glass). This arrangement can be described by the following formula G/WoKNKHWOKNK,iWoKNKHWOKNKHWO WO represents groups of alternating layers having a small number of individual layers, these groups being matched to the standard wavelength Ao and consisting of an alternating sequence of weakly-refracting (No) and strongly-refracting (Ho) individual layers having an optical thickness of #0/4, the arrangement NoHoNoHo being present, in each case, within WO.

Two coupling layers are arranged, in each case, between the groups of alternating layers WO, the layer KN being equivalent to 2No and KH being equivalent to 2Ho that is to say, in this case, the coupling layers KN and KH each have an optical thickness of At/2. Preferred refractive indices were, for the glass substance, n0 = 1.52, for the low-refracting SiO2 layers, (No) = = 1.455, and for the strongly refracting TiO2 layers (H,), the complex refractive index n"ikH = 2.315 - 0.005 (i = ',/'- 1), with the absorption index KH = 0.005 responsible for the absorption.In accordance with the practical conditions, the absorption index kN of the weakly-refracting SiO2 layers was disregarded. The standard wavelength for the Ao/4 layers is At,= 520nm, so that three reflection bands result, corresponding to the build-up of the groups of alternating layers WO and of the interposed coupling layers KN and KH, these reflection bands iying, in accordance with the relationship given for A1, A2 and A3, in the region of the wavelengths A, = 693 nm, A2 = 416 nm and A3 = Ao = 520 nm.In Fig. 1, the spectral reflection R (continuous upper curve) is represented counter to the transmission T (broken lower curve), so that the shaded region between the curves is a measure of the absorption losses. In the reflection bands, the reflection losses in the working region of the minima amount to only approximately 1 to 3 % and are accordingly only approximately 2 to 5 times higher than those of a single Ao/4 TiO2 layer.

Illustrative embodiment 2 Based on the conditions and definitions described in illustrative embodiment 1, an interference mirror having the layer arrangement G/WoKNWOKNWOKNWOt(NWO is manufactured, the group w0 of alternating layers having the arrangement of individual layers H,N,H,, and the groups WO of alternating layers consisting of the layer sequence H,N,H,N,H,.

In this case, the coupling layers KN consist of 5No layers, that is to say, the coupling layers have an optical thickness of 5 Ao/4 The same materials were used for the substrate body G and for the layers as in Example 1.

The standard wavelength Ao/4 layers is Ao = 520 nm, so that three reflection bands result, corresponding to the build-up of the groups w0 and WO of alternating layers and of the interposed coupling layers KN, these reflection bands lying, according to the relationships for A1, A2 and A3, given above, in the regions of the wavelengths A, = 434 nm; A2 = 650 nm and A3 = A,) = 520 nm.

The spectral reflection characteristics and transmission characteristics are graphically represented in Fig. 2, in the same way as in the case of Example 1 (Fig. 1). In the working region of the minima of the reflection bands, the reflection losses, illustrated by the shaded differenceregion between the R-curve and the T-curve, amount to 1 to 3 %.

Illustrative embodiment 3 An interference mirror having the layer arrangement G/WOKNWOKNWOl',JWOKNWOKNWO is manufactured analogously to Example 1, WO being equivalent to H0N0H0 and KN being equivalent to 2N" Here also, the same material parameters as in Example 1 apply to G, No and H" The standard wavelength for the Ao/4 layers is Ao = 520 nm, so that 2 reflection bands result, corresponding to the build-up of the layer arrangement present in this case, these reflection bands lying, in accordance with the abovementioned relationships for the principal wavelengths of the reflection bands, in the region of A1 = 433 nm and A2 = 650 nm.

The resulting spectral reflection characteristics and transmission characteristics are graphically illustrated in Fig. 3, in the same manner as in the preceding Examples. In the working region of the minima of the reflection bands, the reflection losses, illustrated by the shaded differenceregion between the R-curve and the T-curve, amount to 1 to 2%.

In order to make clear the advance achieved by means of the solution of the problem, according to the invention, the spectral reflection curves and transmission curves of an interference mirror, known from the state of the art, are represented in Fig. 4, for comparison with the interference mirrors according to the invention. This case relates to the arrangement, on a glass substrate body G, of two different alternating layer systems W, and W2 having a large number of layers, so that the overall structure G /W2W, results, W, being equivalent to H,N,...N,H, = 17h,/4 individual layers and W2 being equavalent to H2N2. . H2N2 = 1 6A2/2 individual layers.

The inner system W3 of alternating layers, located directly on the glass surface, is matched to the wavelength A2 = 433 nm, and the outer system W, of alternating layers is matched to the wavelength A, = 650 nm. For manufacturing this mirror, the same substances and the same refractive Indices were used as those on which the calculation was based in the Examples of the invention.

As is evident from the shaded regions of Fig. 4, low absorption losses of approximately 1 to 2%, and consequently high reflection values, are possible only in one spectral band, namely in the vicinity of the wavelength A, = 650 nm used for matching, because only this spectral band is reflected by the foremost system Viol, of alternating layers, referred to the light-incidence direction.The reflection is sharply reduced in the second spectral band, in the vicinity of the wavelength A2 = 433 nm, since the light from this spectral band must first pass, before and after reflection from the system W2 of alternating layers, through many layers of the overlying system W, of alternating layers, and in doing so suffers considerable absorption losses of 10 to 30%, which are thus approximately 10 times higher than in the case of the interference mirrors according to the invention.

CLAIMS. 1. An interference mirror having a high reflection for several spectral bands, consisting of a substrate on which alternating layers of non-metallic individual layers, which have low optical losses, are arranged one above the other in such a manner that the individual layers have alternately a high refractive index and a low refractive index, one or more nonmetallic coupling layers being located between adjacent groups of alternating layers, these groups consisting of 2 to 9 individual layers and being built up identically or differently, the individual layers of all groups of alternating layers having an optical thickness of 1/4 of a standard wavelength Ao and the optical thickness of the non-metallic coupling layers amounting to an even-number multiple of At/4.

2. An interference mirror according to Claim 1, wherein a coupling layer is located between adjacent groups of alternating layers, the optical thickness of this coupling layer amounting to an odd-number of At/4.

3. An interference mirror according to Claim 1, characterised in that two coupling layers are located between adjacent groups of alternating layers, the optical thickness of these coupling layers amounting to an even-number multiple of At/4.

4. An interference mirror substantially as herein described with reference to Figs. 1 to 3 of the accompanying drawings.

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Claims

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30%, which are thus approximately 10 times higher than in the case of the interference mirrors according to the invention.

CLAIMS. 1. An interference mirror having a high reflection for several spectral bands, consisting of a substrate on which alternating layers of non-metallic individual layers, which have low optical losses, are arranged one above the other in such a manner that the individual layers have alternately a high refractive index and a low refractive index, one or more nonmetallic coupling layers being located between adjacent groups of alternating layers, these groups consisting of 2 to 9 individual layers and being built up identically or differently, the individual layers of all groups of alternating layers having an optical thickness of 1/4 of a standard wavelength Ao and the optical thickness of the non-metallic coupling layers amounting to an even-number multiple of At/4.
2. An interference mirror according to Claim 1, wherein a coupling layer is located between adjacent groups of alternating layers, the optical thickness of this coupling layer amounting to an odd-number of At/4.
3. An interference mirror according to Claim 1, characterised in that two coupling layers are located between adjacent groups of alternating layers, the optical thickness of these coupling layers amounting to an even-number multiple of At/4.
4. An interference mirror substantially as herein described with reference to Figs. 1 to 3 of the accompanying drawings.