DD295933A5 - DIELECTRIC LONG PASS FILTER FOR THE CLOSE ULTRAVIOLETTE AND VISIBLE SPECTRAL AREA - Google Patents

Abstract

The invention finds application as a spectral divider or cold mirror in optical systems. In the case of a long-pas filter with a lo / 4 alternating layer system having a high number of layers 25 and optically half as thick first and last, high-index interference layer, the 13 or 15 interference layers of a first layer group close to the carrier have an equal factor of 0.88 to 0, according to the invention. 94 lower optical thickness than the corresponding interference layers of a second, the surrounding medium near the alternating layer group. This achieves an increase in the transmission to T95% in spectral edges near the passband while at the same time ensuring a wide blocking range of high reflection. Fig. 1 {Langpaszfilter; Cold mirror, dielectric; Interference layers; optical thickness; UV-VIS spectral range; spectral edge; Stop band; Reflection; Durchlaszbereich; Transmission}

Description

Field of application of the invention

The invention relates to a dielectric long-pass filter for the near ultraviolet (UV) and the visible (VIS) spectral range.

Such long-pass filters are used as spectral guides or cold-light mirrors in optical systems of scientific devices.

Characteristic of the known state of the art

The use of dielectric alternating layer systems as optical filters has been described in the literature both generally and for specific cases.

For use as a long-pass filter, d. H. the short-wave part of the electromagnetic radiation is to be blocked and the long-wave part transmitted, are known for the visible and the near UV Xo / 4 Wechselschichtsystome the form T / (0.5 HN 0.5 H) VL (Mc leod, Thin Film Optical Filters; Adam Hilger Ltd: London 1969, p. 111 ff.). The symbols used have the following meaning:

T: transparent support, i. general glass;

L: surrounding medium, i. general air;

0.5 H: high refractive layer of optical thickness λ ,, / 8;

N: low refractive layer of optical thickness λ _o / 4

χ: Period number, which indicates how often the period (0.5 H N 0.5 H) is repeated in the shift system;

A ₀ : centroid wavelength of the system indicating the spectral location of the alternating layer system.

The height and width of the stop band of such a shift layer system is determined by the refractive index ratio η H / n N and the number of layers 2 x +1. In order to achieve a wide range of high blocking, ie low transmission, the refractive index ratio and / or the number of layers must be increased. But this has the disadvantage that, especially in the vicinity of the spectral edge of the filter, a large ripple in the transmission range occurs, d. H. the transmission in this range is strongly wavelength dependent and can assume relatively low values. One way to reduce the ripple in the passband, which is particularly applied in the IR spectral range, is to build the alternating-layer systems of layers differing from the optical thickness λ ,, / 4 (JS Seeley, SD Smith, Appl. Opt. 1966) 81-85).

However, such layer systems are difficult to produce due to the different optical thicknesses of the individual layers. A disadvantage makes also noticeable that the alternating layer systems of _λ ο / 4 different optical thicknesses do not have a maximum width of the stop band, as for pure X _o / 4-layer systems is the case (JJ. Vera, Optica Act a 11 (1964) 315 ).

To reduce the ripple of the transmission in the passband, it is also possible to divide the layer system unö ever apply a λο / 4 system low number of layers either on 2 carriers and then cemented together or coat a carrier on both sides (Mc leod, Thin Film Optical Filters, Adam Hilger Ltd: London 1969, p. 111 ff). With sufficiently thick carriers, the two layer systems do not influence each other and, owing to the low number of layers of each layer system, a relatively high transmission with good blocking is obtained, with the relationship for lossless layer systems

1 - R ₁ · R ₂ applies. In this case, T ₁ and T ₂ denote the transmission values and R ₂ and R ₂ the reflection values of the two layer systems.

However, it can also be seen from this relationship that transmission values T> 95% can not be achieved for the double-sided coated glass because then the transmission of each individual system T ₁ Zj must be> 97.6%. However, this value is not realizable in the vicinity of the spectral edge.

Another disadvantage of this solution is the increased Beschchtungsaufwand.

In the case of the coating of 2 carriers Is also the problem of bonding to solve, especially when the filters are to be used at high radiation exposure The proposed to improve the blocking range of dielectric alternating layer systems combination with colored glass (ibid) has the disadvantage that At high radiation exposure as a result of the adsorption in these stained glasses destruction of the filter can occur. The fact that only a limited number of colored glass is available, also the choice of the position of the spectral edge of the filter is limited.

Long-pass filters of this type can not be used as cold-light mirrors because of the absorption losses in the short-wave spectral range.

Object of the invention

The invention aims to provide a dielectric long-pass filter for the near ultraviolet to visible spectral range which is easy and reproducible for as many applications as possible and both a wide stopband and a steep spectral edge and a high transmission a95% in the vicinity of the spectral edge in the passband having.

Explanation of the essence of the invention

The invention is based on the object in long-pass filters by a suitable, applied only on one side of a transparent carrier dielectric alternating layer system, ensuring a maximum width of the stopband and a high slope and free selectability of the position of the spectral edge in a wide range of UV-VIS Spectral region to increase the transmission in the passband, in particular in the vicinity of the spectral edge. This object is achieved by a dielectric long pass filter for the near ultraviolet and visible spectral region, consisting of a transparent support and a system of at least 25 alternating high and low refractive non-metallic interference layers, in which the first, adjacent to the carrier and the last, the Interchangeable layer system to the surrounding medium towards the final interference layer is a high-refractive layer each whose optical thickness is half as large as the optical thicknesses of the intermediate interference layers, the spectral position of the alternating layer system is characterized by a centroid wavelength A ₀ , according to the invention solved in that the alternating layer system of two alternating layer groups is constructed, of which the carrier near the first alternating layer group 13 or 15 comprises interference layers and that their first interference layer with respect to the last interference layer de The second alternating layer group and its further 12 or 14 interference layers each have a smaller optical thickness than the remaining, K ₀ ZA thick interference layers of the second alternating layer group by an equal factor α in the range from 0.88 to 0.94.

It has been found that by the choice of the layer thickness factor α in the range of 0.88 to 0.94 and the layer number 13 or 15 for the adjacent to the carrier first alternating layer group in the vicinity of the spectral edge in the passband, an increase in the transmission occurs Changing the layer thickness factor and the number of layers within the specified range, the spectral position of this range of very high transmission can be moved. Thus, in a part of the transmission range in the vicinity of the spectral edge, values of T a 95% can be achieved. At the same time, in the long-pass filter according to the invention, the stop band for T <1% is wider than in a conventional λ _o / 4 change-layer system with comparably high transmission values. The number of interference layers of the second alternating layer group adjoining the surrounding medium air is freely selectable over a larger range (2:12) and determines the steepness of the spectral edge and, in connection with the total number of layers, the width and the height of the blocking.

Because of their good mechanical properties, such as hardness and layer adhesion, and their good chemical resistance, the high-index substances used for the near UV and VIS spectral range, such as TiO ₂ , Ta ₂ O ₆ , Nb ₂ O ₆ and ZrO ₂ , are used as layer materials. with refractive indices ηη = 2.0 to 2.4 and provided as a low-refractive index substance SiO ₂ with Nn = 1.45. The long-pass filter according to the invention can also be used as a cold-light mirror, since the blocking of the short-wave radiation takes place exclusively by reflection.

embodiment

The invention will be explained in more detail with reference to a AusfUhrungsbeispiels. Show it

Fig. 1: the spectral curve of a prior art dielectric Langpaßfiitors

small number of layers (17) of the form T / (0.5 H ₂ N ₁ 0.5 H ₂ ) ⁸ / L FIG. 2 shows the spectral curve of a corresponding dielectric long-pass filter with a number of layers increased to 25

according to the prior art with the structure T / (0.5 H ₂ N] 0.5 H ₂ ) "/ L FIG. 3 shows the spectral curve of a long-pass filter according to the invention with 35 interference layers

An inventive dielectric long-pass filter for the UV-VIS spectral range is constructed as follows:

₁ Hl ₁₁ N ₁ H ₁ N ₂ H ₂₁₁ -N ₂ H ₂ N ₂ m η

On the glass carrier T with the refractive index n _T = 1.52, an alternating column system consisting of a total of m + η = 35 interference layers is arranged, the high-indexing layers (eg of Ta ₂ O ₆ ) having a refractive index of rin-2.2 and the low-index layers (eg of SiO ₂ ) have a refractive index of n _N = 1.45. The first, adjacent to the carrier m - 13 interference layers according to the invention all by one factor

α = - = - = 0.88 N ₁ H ₂

lower optical thickness (product of refractive index and geometric layer thickness) than the remaining π = 22 interference layers, soft, with respect to the centroid wavelength λ ₀ , except the last high-refractive layer with an optical thickness λο / 8 all an optical thickness of λ _ο / 4 own.

A comparison of the spectral curve of the prior art dielectric longpass filter of the present invention does

Clearly achieved by the invention effect.

Compared to the long-pass filter with only 17 interference layers (see Fig. 1) shows the dielectric according to the invention

Long pass filter a 37% wider stopband and a greater edge steepness with equally good transmission values

> 95% in the passband near the spectral edge.

To a 10% higher transmission in the passband in the vicinity of the spectral edge at the same steepness of the spectral edge and the same maximum width of the stopband, the dielectric long-pass filter according to the invention is distinguished from the constructed from 25 interference layers long-pass filter (see Fig. 2).

Claims (1)

Dielectric long-pass filter for the near ultraviolet and visible spectral region, consisting of a transparent support and a system arranged thereon of at least 25 alternately high and low refractive non-metallic interference layers, in which the first, adjacent to the carrier and the last, the alternating layer system towards the surrounding medium final Interference layer is in each case a high-index layer whose optical thickness is in each case half as large as the optical thicknesses of the intermediate interference layers, wherein the spectral position of the alternating layer system is characterized by a centroid wavelength K _0, characterized in that the alternating layer system is composed of two alternating layer groups of in which the first interchangeable layer group close to the carrier comprises 13 or 15 interference layers and that their first interference layer is opposite to the last interference layer of the second interlayer group and their e further 12 or 14 interference layers with respect to the remaining, λ _ο / 4 thick interference layers of the second alternating layer group each have a smaller by an equal factor α in the range of 0.88 to 0.94 optical thickness. For this 1 page drawing