DD263836A1 - MILL LINE - NARROW BAND FILTER - Google Patents

Abstract

The multi-line narrowband pass filter is primarily used to select multiple limited wavelength ranges. It has a plurality of fitting ribs which are symmetrical about the filter center-line wavelength l0 and have a predetermined transmission. The filter according to the invention consists of a symmetrical interference layer system that is composed of several asymmetric cavity filter layer groups. At the same distance from the geometric center of the interference layer system, there are profile layers which approximate differently monotonous mathematical functions which either increase or decrease with respect to the geometric center.

Description

Field of application of the invention

The multi-line narrowband pass filter is applicable in optical arrangements and devices in which limited wavelength ranges or monochromatizing elements are required in the ultraviolet, visual or infrared spectral range. Multi-line filters are of particular interest for the simultaneous detection of several spectral intervals and are used, for example, in phytometry, plasma diagnostics, astrophysics and in signal and image processing.

Characteristic of the known state of the art

Interference layer filters have gained outstanding importance in the spectral selection of light because they allow high flexibility and quality in the generation of desired optical properties and have a high resistance to external influences.

The simplest filters of these groups are metal interference filters based on the principle of the FABRY-PEROT interferometer.

A major shortcoming of these filters is the use of metal layers that greatly reduce transmission. Is where a dielectric λ / 2 spacer layer (cavity), symmetrically enclosed with an optical layer thickness which is a ganzzahiiges multiple of half the filter center wavelength A ₀ of reflectors made of dielectric Wechselschichtsyotemen avoid these disadvantages cavity filter. To design certain filter designs, several symmetric cavity filter layer groups have been coupled through intermediary layers (eg, in "Optics of Thin Films" by Z. Knittl, John Wiley and Sons, London-New York, 1976) .The use of various filter group designs as well as styles and widths The non-symmetrical systems of this type are listed in the specification of patent specification GB 1270042. In order to increase the transmission of the passband ranges, it is proposed in WP G 02 B / 302 2281, within the cavity Filter layer groups, preferably instead of a cavity layer to use a Brechzahlinhomogene layer.

The abovementioned solutions, however, merely show the possibility of improving the transmission of bandpass filters and eliminating disruptive band structures. However, they are not sufficient for multi-line narrow-band pass filters since the most varied spectral positions and transmittances of pass bands are required. Another limit of the above-mentioned solutions is the transmission condition of the FABRY-PEROT filter, which is equally applicable to the cavity filters.

2n-n-dcos9 0, + 0 _b

mn -

λ 2

with the angle of incidence Θ, the light wavelength λ, the phase changes at the two boundary surfaces of the spacer layer (or cavity) 0 _a and 0 _{b of} the atomic number m = 0, ± 1, ± 2, ± 3, ... and the thickness d and the refractive index η of the spacer layer.

Due to this condition, the filters have, in addition to the filter centroid wavelength A _0, even more imperfect maxima, which are bound in their spectral position to gonzzahlige parts of A ₀ .

The realization of closely adjacent transmission bands is not possible due to the high blocking in the side areas.

For the production of two-line filters, IFHodgkinson, F.Hovowitz, HA Macleod, M.Sikkens and JJWhartonin "Birefringence in optical coatings" J.Opt.Soc.Am.Vol.73 No. 12 (1983) 1871 states that in a üirkondioxid and silicon dioxide for X ₀ = 627.8 nm constructed cavity filter with the structure:

Substrate-fHL ^ HHHfLH) ³ - ^

(where H is a high refractive index, L is a low refractive index layer having an optical layer thickness of A ₀ M), birefringent zirconia produced by a specific method is used as a cavity material.

However, the resulting double peak shows only a small blocking between the adjacent transmission maxima. In principle, this method only allows the generation of two. lienfiltern with little opportunity to change the tapes and this transmission. They are also technologically complex and not geeingnet for any Materialiet.

In another solution, immediately adjacent passbands in the far infrared spectral range are produced by photolithographically produced metal mesh structures on either side of the spacer layer of a FABRY-PEROT filter. The optical properties are adjustable by the metal structure (DD Nolte, AE Lange, PL Richards in "For infrared dichroic bandpassfilters" Appl., Opt., Vol., 24, No. 10 (1985) 1541.) Such filters, however, are limited to applications in the far infrared spectral range ,

Object of the invention

Ziei of the invention are in a simple manner, preferably by means of known coating method producible multiline narrow bandpass filter with improved optical properties.

Explanation of the essence of the invention

The object of the invention is a multi-line narrow band pass filter whose. With respect to the centroid wavelength A ₀ symmetrically arranged pass bands have a predetermined transmission.

Dio object is achieved with a multi-line narrow bandpass filter, consisting of an interference layer system, which is constructed of interlayer coupled cavity filter layer groups, wherein at least one layer of the filter layer groups is formed as a profile layer with a variable refractive index perpendicular to the substrate surface, achieved in that the interference layer system symmetrical is constructed of asymmetric cavity filter layer groups and contains at least one pair of profile layers, wherein the layers of a pair have the same distance to the geometric center of the interference layer system and their Brachzahlverläufe either mutually different continuous monotonically increasing or mutually different continuous monotonically falling mathematical functions with respect to the geometric center approximate.

The number of cavity filter layer groups determines the number of passbands of a filter. A cavity filter layer group has the following general structure:

H (LH) ^k 2 mL (HL) ¹ H or (HL) ^k 2 mH (LH) ¹

with freely selectable k, I = 0,1,2, ... andm = 1,2,3 ,. .., where H is a high refractive index and L is a low refractive index layer with the optical layer thickness of ¹ A of the filter centroid wavelength A ₀ . The choice of k, I and m determines the spectral position of the passbands. The transmission of the individual pass bands is determined by the mathematical function of the refractive index profile and the position of the profile layers in the layer system. With the aid of computational means, a layer system modeling to adapt to specific objectives is easily possible on the basis of the solution according to the invention.

Flexible options are the application of hyperbolic, exponential, linear, quintic, sinusoidal and root functions. The technical realization of the profile layers can be effected by controlled simultaneous or quasi-simultaneous vapor deposition processes or by approximation by means of single-value non-uniform layers, for example on the basis of staggered refractive indices or equivalent layers.

The realizable on the basis of the inventive solution multi-line narrow bandpass filter allow close juxtaposed pass bands with a good blocking between the bands. The passbands are symmetrically grouped around the filter centroid wavelength Ao and can be designed with respect to the transmission over large areas with a wide range of relations to one another. This results in a high degree of flexibility with regard to the adaptation of the design to specific tasks.

The interference layer system according to the invention is not very sensitive to production-related tolerances.

embodiment

The invention will be explained in more detail with reference to a Ausführungsjbeispieles for a multi-line narrow bandpass filter with three symmetrically with respect. A filter center wavelength λ "arranged Paßbändern with specifiable transmittance. FIG. 1 and FIG. 2 show:

The transmission of a multi-line narrow bandpass filter according to the invention.

In a particular application, the three passbands should have graduated transmission magnitudes of ~ 20%, ~ 60%, and ~ 90% with increasing wavelength of light, with the two sidebands being at the same distance from the filter centroid wavelength λ o and the barrier between the bands 1CT ³ should be. For the realization of the filter according to the invention, starting from an asymmetric cavity filter layer type H (LH) "2 ml (HL) ¹ H with freely selectable k, I = 0,1,2, ... and m = 1 , 2,3, ... from three filter layer groups H (LH) ² LL (HL) ⁰ H, H (LH) ¹ LL (HL) ¹ H, H (LH) ⁰ LL (HL) ² H constructed by means of L and H are single layers with an optical layer thickness of ¹ A of the filter center wavelength A ₀ and the refractive indices n, = 1.35 (thorium fluoride) and n _H = 4.2 (germanium) .. t arranged on a substrate S (germanium) and adjacent to the external medium air.

Symbolically, this layer system can be represented as follows: S - HLHLHLHLHLHLHLHLHLHLHLHLHH -1

The transmission of this layer arrangement is shown in FIG. 1 as Kvt ve A and amounts to 0.64 at an incidence of 1 without consideration of the substrate surface for all three bands.

In order to influence the transmission of the individual pass bands while maintaining the symmetrical position of the sidebands, according to the invention at least two layers located equidistant from the geometrical center of the interference layer system, which are preferably cavities, are replaced by profile layers having a refractive index progression n (z) between n _L and n have _H and approximate any continuous and monotonous mathematical functions. These refractive index inhomogeneous profile layers must to ensure the symmetrical position of the sidebands and influencing the transmission of the individual passbands necessarily different Blechzahlverläufe n (z) and be arranged so that the Brechzahlverläufe n (z) to the geometric center of the interference layer system either rise or fall. In the given case, the substrate-side LL spacer layer (Cavity-Schlcht) is formed by a profile layer having a hyperbolic refractive index profile n (z) across the thickness of the LL spacer plane according to:

n (z) = π ^ η / ηπη + In ₁ -n _H ) z / d)

is replaced so that n (z) rises toward the geometric center of the interference layer system.

The air-side LL spacer layer is formed by a profile layer with a refractive index n (z), which is an exponential Increase according to:

n (z) = n _L. (n "/ n _L ) ^{I / d}

to the geometric center of the interference layer system has replaced.

The thickness of the profile layers, which is generally an optimization parameter for influencing the optical properties of the Paßbänder is was chosen in the exemplary embodiment so that a mean optical layer thickness for both profile layers

d of λο / 2.

The introduction of the profile layers described above results in the following symbolic filter design: S - HLHLH HYDHLHL L HLHLH exp HLHLH -1

with an increase in both refractive index curves n (z) towards the mean LL spacer layer. As can be seen from the transmission of this layer system into the substrate represented in FIG. 1 by curve B, this rail design fulfills the optical properties required in the specific embodiment.

Changes in the Spoktralabhängigkeit the transmittances, such as a decrease with increasing wavelength at

extensive preservation of the required optical properties can be achieved by simple exchange of the profile layer according to the following:

S-HLHLH exp HLHLHLHLHLHLH hyp HLHLH -1

reach, wherein both refractive index profiles n (z) in turn increase to the LL layer. Figure 2 shows the transmission of such a filter design without regard to the substrate back surface reflection.

By choosing k = 1.1 = Oundm = 1 of the initial cavity filter layer groups, the spectral location of the passbands became

certainly. The number of Paßbünder was determined by the number of coupled filter layer groups. The

Transmission of the individual tapes can be achieved over wide limits by the nature and characteristics of the Brechzahlverläufe as well

their principal course to the geometric center of the interference layer system (rise or fall) are made variable.

Claims (2)

1. multi-line narrow bandpass filter, consisting of an interference layer system, which is composed of interlayer coupled cavity filter layer groups, wherein at least one layer of the filter layer groups is formed as a profile layer with a variable refractive index perpendicular to the substrate surface, characterized in that the interference layer system symmetrically asymmetric cavity Filter layers, is constructed and contains at least a few profile layers, wherein the layers of a pair have the same distance to the geometric center of the interference layer system and their Brechzahlverläufe either mutually different continuous monotonically increasing or mutually different continuous monotonically decreasing mathematical functions with respect to the geometric center approximieren. For this 2 pages drawings