Optical multilayer filter

FIELD: fibre-optic communication, optical multilayer filters.

SUBSTANCE: optical multilayer filter (OMLF) consists of an input optical transformer (In. OT 1), a selective part (SP 2), and output optical transformer (Out. OT 3) and substrates 5, 6. The In. OT 1, SP 2, and Out. OT 3 consist of NIn=2s, Nsp=4k and Nout=2r alternating layers 7 and 8, respectively, with high nh and low nl values of refractive indices of materials they are made of. The thickness of every layer d=0.25λ, where λ is the mean OMFL bandwidth wave length. Refractive indices of adjoining layers of the In OT and SP, and those of SP and Out.OT are equal. Note that the SP alternating layers are made from materials with refractive indices mirror-symmetric relative to the SP centre. The first layer of In. OT and the last layer of Out. OT are connected to substrates. Proposed are the relations to calculate the parameters of claimed arbitrary type OMLF.

EFFECT: reduction of signal distortion to preset magnitude in a wide frequency range of the filter attenuation in the preset bandwidth and increase in attenuation to the preset magnitude that allows wider application of the aforesaid filters.

3 cl, 7 dwg

 

The invention relates to radio engineering, in particular to the technical field of fiber-optic transmission systems, in particular to multilayer optical filters included in the device multiplexing wavelength for the formation of many spectral channels when working with fiber-optic communication cables.

Known optical multilayer filters (OMSF) interference type which operates as a bandpass or bandreject filters.

Known bandpass filter (see RF patent № 2079861, IPC 6 G02B 5/28, publ. 20.05.1997 year). The bandpass filter includes a substrate and a multilayer interference coating of alternating layers of optical thickness λ/4 with high and low refractive indices. As the substrate material used film optical filter having the characteristics of the filter, low pass wavelength region transmission.

The disadvantage of this bandpass filter is a relatively high level of distortion of the signal due to non-optimal choice of the sequence and thickness of layers in the structure of the multilayer coating.

Known reflective coating is used as a band-stop filter (see U.S. Pat. Of the Russian Federation No. 2256942, IPC 7 G02B 5/08, G02B 5/28, G02B 5/26, publ. 20.07.2005,). The band-stop filter comprises dielectric layers a, b and C, where a layer And made of m is material with a low refractive index, the layer of material with an average refractive index and the layer of material with a high refractive index and thickness of the layers is λ0/4, where λ0- wavelength mid-interval with high reflectance, and the sequence of layers has the form (CBCABA)kCDC, where k≥1 an integer.

The disadvantage of this band-stop filter is a relatively high level of distortion of the signal due to non-optimal choice of the sequence and thickness of layers in the structure of the multilayer coating.

The closest to the technical nature of the claimed is an optical multilayer filter (multilayer interference system for the far infrared region of the spectrum) (see ed. St. USSR № 1626916, IPC 5 G02B 5/28, publ. 15.02.1988 G., Appl. 08.12.1988 year). The optical multilayer filter prototype consists of alternating layers with high and low refractive indices are made of polymer material, and in order to reduce bandwidth in the shortwave region of the spectrum at least one layer with high refractive index is made is not continuous, but discrete, consisting of the set of elementary layers with high and low refractive indices.

A disadvantage of the known optical multilayer filter is a low technology, obuslovlen what I need to use layers of small thickness, requiring precision technologies of their manufacture, which leads to significant errors of parameters OMSF in their production.

The purpose of the invention to provide an optical multilayer filter with a regular structure layers having a thickness that does not require precision technology, and consequently with less stringent requirements for tolerances on the accuracy of their production, which ensures minimum error frequency selectivity parameters of filters in their production.

This objective is achieved in that in the known OMSF, including N dielectric layers made of materials with differing refractive indices and the optical thickness of each layer, d, is made consisting of the input optical transformer (What), selective part (ICH) and optical output transformer (What). What, ICH and What consist respectively of NI=2s, NICH=4k and No=2r alternating layers made of materials with low nnand high ninrefractive indices, where k, s, and g is the corresponding integer number of pairs of layers, which is calculated by the formula

where ν=nn/nin,Δf and f0- accordingly, the working the bandwidth and Central frequency of the filter, and m is an integer odd number indicating the bandwidth of the filter. The refractive indices are adjacent to each other layers What and ICH, as well as adjacent to each other layers ICH and What the same. Alternating layers ICH made of materials with refractive indices are mirror-symmetrical relative to the center of the electoral part (ICH). To the first layer What and the last layer Wiht connected to the substrate. The thickness of each optical layer is selected from the condition d=mλ0/4, where λ0- the average wavelength bandwidth of the filter. The number m is chosen in the range m=3...201.

The value of m is chosen within the specified limits, as when m<3 it is almost impossible to realize the stated aim, and if m>201 is limited reliable protection from interference from adjacent frequency bands.

Thanks to this new essential features is an opportunity to build a filter with a regular structure of layers of equal thickness with the simultaneous possibility of implementing the required parameters using filter layers of relatively large thickness, which ensures the achievement of the formulated technical result - the creation of a filter more adaptable to mass production.

The analysis of patent and special sources of information on vail to install that there were no analogues characterized by a set of characteristics is identical for all essential features of the claimed solution, which indicates that conform optical multilayer filter condition of patentability "novelty".

Search results known solutions in this and related fields of knowledge in order to identify the distinctive features of the declared object has shown that they do not follow explicitly from known sources. Also not revealed the popularity of the distinctive influence of material characteristics on the achievement of the stated result that indicates compliance of the declared object condition of patentability "inventive step".

Declared OMSF illustrated by drawings on which is shown:

figure 1 - General structure (topology) of the filter.

figure 2 - profile of the refractive indices of the materials of the layers OMSF and substrates;

figure 3 - frequency dependence of working attenuation OMSF with Chebyshev characteristic in the vicinity of the average frequency f0given bandwidth;

figure 4 - the topological structure of the experimental sample OMSF when m=1 and the thickness of each layer d=d10/4;

figure 5 - frequency response of the attenuation according to the results of an experimental design OMSF when m=1, with the characteristic of ebisawa in the vicinity of the average frequency f 0=450 THz given bandwidth, with the boundary frequency f01=440 THz and f02=460 THz;

figure 6 - the topological structure of the experimental sample stated OMSF when m=9 and the thickness of each layer d=dm=mλ0/4.

figure 7 - frequency response of the damping characteristic of the multi-band Chebyshev OMSF the experimental calculation of the claimed filter that includes the characteristic of attenuation in a given bandwidth with the boundary frequency f01=445 THz and f02=455 THz when m=9 in the vicinity of the average frequency f0=THC and characterization of damping in additional bandwidth with the boundary frequency fm1=45 THz and fm2=55 THz when m=9 in the vicinity of the average frequency fm=50 THz.

From a consideration of figure 4, 5, 6 and 7 show that the implementation of the topology of the multilayer coating OMSF with an average frequency of a given bandwidth equal to f0can be done in two main ways. In the first embodiment of the multilayer coating are made of alternating layers of thickness d=d10/4, while the relative error in the thickness of the coating layer is δ1=Δd1/d1where Δd1- absolute error of the thickness of the coating layer for m=1. According to the second variant of the multilayer coating is performed from eredvisie layers of thickness d=d m=mλ0/4, while the relative error in the thickness of the coating layer is δm=Δdm/dmfor values of m≥3. The condition of the equality of relative errors of the thickness of the deposition layer on the first and second choices of the layer thickness of the absolute error will be according to Δdm=mΔd1from which it follows that the thickness tolerance of the dmlayer will be m times higher than the permissible deviation of thickness d1layer, which defines a higher adaptability of the second option.

Declared OMSF shown in figure 1, consists of What 1, ICH 2, What 3 and substrate 5, 6. What 1, ICH 2 and What 3 are composed of NI=2s, NICH=4k and No=2r alternating layers 7 and 8, respectively, with high ninand low nnvalues of the refractive indices of the materials from which they are made.

Figure 1 layers with a high refractive index of ninindicated by the symbol "b" and cross-hatched, and low-nnsymbol "n".

Each of the layers "b" and "h" has a thickness of dmequal to m-quarters of the average wavelength λ0bandwidth OMSF, i.e. dm.=m·λ0/4.

The value of s, k and r, are included in the expression for NINICHand No, calculated by formulas (1) and (2). Indicators rotten the value of the materials adjacent to each other layers What and ICH 2 (section a-a' in figure 1), as well as the refractive indices of the materials adjacent to each other layers ICH 2 and What 3 (cross-section b-b'), are the same. Moreover, the layers ICH 2 is made of materials with indices of refraction, mirror-symmetrical relative to the center (cross-section C-C') ICH 2.

The profile of the refractive indices of the materials with high b - 7 and a low "n" - 8 values, made from layers OMSF thickness of dmshown in figure 2.

The substrate 5 and 6 are made of a solid dielectric material with a refractive index of n0whose value is selected between the values of ninand nni.e. nin>n0>nn. The substrate 5 and 6 are designed for consistent input in OMSF and output from OMSF signal (optical beam). The substrate 5 and 6 perform the role of support elements between the actual OMSF and transmitting optical media. As substrates 5 and 6 can be used in optical fibers, rods, plates, lenses, etc.

The first layer What 1 "to" 7 adjacent to the substrate 5 made of a material with nin, i.e. the alternation of layers

"in"→"h"→""→...→"h"→""→"n / a".

This topology What 1 can be designated as

where s is the number of pairs of layers of thickness dmwith ninand nnWhat 1 OMSF.

Similarly, in ICH 2 them is no place alternating layers

"h"→""→"h"→...→""→"h"→""→""→"h"→""→...→h"→""→"n / a", topology which can be designated as

where k is the number of pairs of layers of thickness dmwith ninand nnhalf of ICH 2 OMSF.

Similarly, in What 3 takes place alternating layers

"h"→""→"h"→...→""→"h"→""

the topology of which can be designated as

where r is the number of pairs of layers of thickness dmwith ninand nnWhat 3 OMSF.

Thus, the overall topology (NR)Σdeclared OMSF can be represented in the form

The profile values of the refractive indices OMSF depicted in figure 1, includes the refractive index of the substrate-side input filter n0refractive indices ninand nnalternating layers OMSF and the refractive index of the substrate from the outlet side of the filter (figure 2).

The claimed device operates as follows. Optical multi-layer coating as OMSF can be illustrated microwave models representing cascade (chain) connection of a quarter-wavelength line segments with alternating sequence of segments with high and low values of the impedances, the combination of which about is ashet chained microwave filter with predetermined frequency characteristics of attenuation (Chebyshev, Butterworth, Bessel, and so on).

The minimum optical filter unit is original polozeno filter in the form of a two-layer coating layer with high ninand low nnrefractive indices and the thickness of each of these layers dm.=m·λ0/4. Specified source polozeno has properties as filter periodically repeated with a period of 2f0/m bandwidth and retaining strips and transformer refractive indices are periodically repeated with a period of 2f0/m bandwidth, each bandwidth two-layer filter-transformer depends on the attitude of ν refraction ν=nn/nin. It is known (see, for example, Basatin. Theory and design of filters and transformers on sections of transmission lines. SPb.: Science, 1998, 181 C.), that νseeking code (ν→1), the bandwidth increases and the band retaining decreases. When ν<<1, the bandwidth decreases, and the band retaining increases.

Practical OMSF are super narrow, because of their relative bandwidth Δf/f0in the optical range is about (0,01 0,05...)%. For the manufacture of such original dual-layer super narrow polozenie filter is s (NRF) is necessary to highest indicatorwas of the order of 104...108that is practically unrealizable. Moreover, the coefficient of transformation of such NRF determined by reverse size νNRFi.e. the value ofwill be of the same order i.e. 1/νNRF=104...108. At the same time practical maximum values of the refractive indices of the various materials used in OMSF, does not exceed, as a rule, size 4...5. This condition imposes constraints on the feasibility of the filter.

With the aim of building a super narrow (highly selective) NRF implemented with predetermined refractive indices of the layers of the ninand nnin the claimed OMF provides conversion topology NRF with a very large transformation ratio 1/νNRFin the topology of the cascade connection k transformers in ICH 2 (EXT)kwith the same ongoing transformation ratios, which is defined aswhere νNRFratio of the low refractive index of nnand unrealizable highk is determined using formulas (1...2). In this case, each new NRF with a reduced ratio k becomes more broadband, h is due to the addition of the selectivity of k broadband NRF their total selectivity will be restored to the original. Figure 1 and figure 2 topology converted NRF presents the topology half of the polling part ICH 2 OMSF, i.e. (HB)kwith the numbers of layers from 1 to 2k and the topology of the half mirror ICH 2 OSF (NR)kwith the numbers of layers from 2k+1 to NICH=4k.

In the claimed OMSF in figure 1 and figure 2 ICH 2 made in the form of two mirror-symmetrical converted NRF topology (HB)k(NR)k. This decision is due to two reasons: first, a consistent connection between two NRF allows to obtain a symmetric link filter (ICH 2) with doubled selectivity; secondly, it simplifies the task of matching input and output ICH 2 substrates using respectively What 1 and What 3. What 1 made of s pairs of alternating layers of materials with a low nnand high ninrefractive indices in the topology (NR)s. What 3 made of r pairs of alternating layers of materials with a low nnand high ninrefractive indices in the topology (HB)r.

Figure 3 shows the preset frequency response attenuation is depicted in figure 1 OMSF Chebyshev uneven attenuation Δand the estimated bandwidth limited to frequencies f01and f02and guaranteed attenuation of aein the calculation of the band with retaining boundary frequency fe1and f e2.

The manufacturer stated super narrow optical multilayer filter for the ultraviolet wavelength range on the layer with a larger thickness can be shown on the example OMSF Chebyshev.

Suppose you want to receive bandpass OMSF Chebyshev with the following characteristics (see figure 5):

the uneven attenuation Δa=1 dB in the working bandwidth with the boundary frequency f01=445 THz (674,2 nm) and f02=455 THz (659,3 nm).

guaranteed attenuation of ae=30 dB in the working band of the retaining boundary frequency fe1=435 THz (689,6 nm) and fe2=465 THz (645,2 nm).

Based on the values of f01and f02we have f0=(f01+f02)/2=450 THz.

We choose m=9, then the requirements and characteristics of the claimed filter will be as follows (see Fig.7):

the uneven attenuation Δa=1 dB within the design bandwidth with the boundary frequency fm1=f01/9=45 THz (λm101·9=6666,7 nm) and fm2=55 THz (5454,5 nm);

guaranteed attenuation ande=30 dB in the working band of the retaining boundary frequency fem1=35 THz (8571,4 nm) and fEM2 bid competitively=65 THz (4615,4 nm);

from an admissible set of materials with different indices of refraction selected and specified materials in alternating layers with nn=1,45, nin=2,068 and material substrates with n0=1,52.

Calculated by the formulas (1)...(2) k, s and r gain value

s=k/2=9;

r=k/2=9.

Then the overall optimal topology stated OSF (NR)Σcan be represented in the form (6):

(NR)Σ=(NR)9(HB)18(NR)18(HB)9,

The total number N of layers of the multilayer coating with the thickness of each layer dm=mλ0/4=9·450/4=1500 nm is N=2·9+4·18+2·9=108 layers and the total thickness LOMSFmultilayer coatings LOMSF=N·dm=162000 nm. The substrate 5 and 6 OMSF made of transparent rods with n0=1,52.

For comparison, we give the calculation of the same filter with m=1. Then, using formulas (1)...(4) we obtain k=18, s=9, r=9. Frequency response of bandpass OMSF Chebyshev shown in figure 5.

Topology OMSF can be represented in the form (4)

(NR)Σ=(NR)9(HB)18(NR)18(HB)9,

but the thickness of each layer is equal to d1=dm/m=1500/9=166,7 nm, i.e. in 9 times thinner and, therefore, the manufacture of such layers will be less technology in their production.

The above example shows the possibility of building a super narrow OMSF on thick layers with the desired settings Δand aewhen using materials with realized values of ninand nn. what is also obvious is the possibility of generating optimal topologies OMSF for a wide class of bandpass and bandreject filters. Marked indicates that the use of declared OMSP-m possibly achieving a technical result, greater adaptability of the filter during its production.

1. The optical multilayer filter, consisting of N dielectric layers made of materials with differing refractive indices and the optical thickness of each layer d, characterized in that the optical multilayer filter is made consisting of the input optical transformer, selective parts and optical output transformer, consisting respectively of NI=2·s, NICH=4·k and No=2·r alternating layers made of materials with low nnand high ninrefractive indices, where s, k and r, the corresponding number of pairs of layers, which is calculated by the formula

s=r=k/2,

k=lgq/lgv,

where ν=nn/nin,Δf and f0accordingly, the bandwidth and Central frequency of the filter, a m is an integer odd number indicating the bandwidth of the filter, the refractive index adjacent to each other layers of the input optical transformer and selective part of, and adjacent to each other layers of the electoral part and the output optical transformer Odie the hat, and of alternating layers of selective parts are made of materials with indices of refraction, mirror-symmetrical relative to the center of the election part, and to the first layer of the input optical transformer and the last layer of the optical output of the transformer is connected to the substrate, and the thickness of each optical layer is selected from the condition d=m·λ0/4, where λ0- the average wavelength bandwidth of the filter.

2. The optical multilayer filter according to claim 1, characterized in that the number m is chosen in the range m=3...201.



 

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