CN115826122A

CN115826122A - Depolarization flat-plate thin film cut-off filter

Info

Publication number: CN115826122A
Application number: CN202211576660.5A
Authority: CN
Inventors: 鲍刚华
Original assignee: Changzhou Wanhua Laser Technology Co ltd
Current assignee: Changzhou Wanhua Laser Technology Co ltd
Priority date: 2022-12-09
Filing date: 2022-12-09
Publication date: 2023-03-21

Abstract

The invention relates to the technical field of optical thin films, in particular to a depolarized flat-plate thin-film cut-off filter, which comprises a thin film deposited on a flat-plate substrate, wherein the basic periodic structure of the thin film is as follows: (aHbMcLbMaH) ^ P; h represents a high-refractive-index film layer with quarter optical thickness, M represents a medium-refractive-index film layer with middle quarter optical thickness, L represents a low-refractive-index film layer with quarter optical thickness, and P is the period number; a is a multiple of film structure H, b is a multiple of film structure M, c is a multiple of film structure L, and P is the number of cycles. The method is simple and easy to implement, and can be completed by using a computer and any conventional film design software; the method has strong pertinence, wide application range and high quality, and is suitable for all medium film materials in the current optical film industry; the designed film system does not contain an extremely thick layer, and the total thickness of the film layer is relatively thin, so that the precise preparation is facilitated, and the yield of products is greatly improved.

Description

Depolarization flat-plate thin film cut-off filter

Technical Field

The invention relates to the technical field of optical films, in particular to a depolarized flat-plate film cut-off filter.

Background

With the development of optical communication technology and capacity, the application of the broadband tunable filter in dense wavelength division multiplexing systems and optical switching networks is more and more demanding. The space quantum communication technology developed on the basis of space laser communication is the hot research currently. In the quantum communication process, information carriers distributed by quantum keys are different polarization states of linearly polarized light, so that the polarization effect of the film due to oblique incidence in an optical path must be eliminated as much as possible.

The initial structures of conventional short-pass or long-pass cut-off filters are G (0.5lh0.5l) pA and G (0.5hl0.5h) pA, respectively. Where G and a denote the plate substrate and air, respectively, H and L denote the quarter optical thickness of the high and low index material, respectively, and p is the periodicity. Cut-off filters based on this initial structure have been widely used to date. However, at a large-angle oblique incidence, the P-and S-polarization separation at the transition zone is very severe, and thus cannot meet the increasing angle requirement, and thus a new film design method is urgently needed.

The design method of the multi-cavity narrowband thin film optical filter film system has the advantages of low insertion loss, insensitivity to temperature, high rectangularity and the like, so that the design method is widely applied to the field of optical communication. However, when the thin film filter is at an oblique incidence, the decrease in the equivalent optical thickness of the spacer layer causes the center wavelength of the light transmitted by the thin film filter to shift toward a short wavelength. Meanwhile, the center wavelength of the P polarized light is obviously separated from the center wavelength of the S polarized light, so that the average light generates serious polarization-dependent loss and the inclined incidence angle of the filter is limited. The polarization effect referred to above means that when light is obliquely incident on the film, separation between the P-and S-polarized components typically occurs. It is therefore desirable to design a short-pass or long-pass cut-off filter for 45 ° incidence, which requires the film to be able to separate two close wavelengths, so that one of the wavelengths is highly reflective and the other highly transmissive, and requires the transition band P-and S-polarized light to have as little polarization spectral separation as possible at 50% transmission.

The multi-cavity narrowband thin film filter film system design approach addresses some of the polarization effects, but the drawbacks of this approach are also significant. First, the spacer layer has a high order, which makes the overall thickness of the thin film thick, which is not favorable for practical fabrication. Second, the width of the pass-through band and the cut-off band is still not wide enough and may no longer be suitable in some cases. Finally, when the H and L indices of refraction are not matched, problems may arise in that the amount of P-and S-polarization separation at the transition zone cannot be eliminated and mass production is not possible.

The patent with application publication number CN109683225A is named as a patent of a flat-plate thin-film cut-off filter for depolarization, and for the design of a film system structure adopting G (a (HL) b (HL)). Times.PA, a spectrum for cutting off short wave and long wave is easier to design, the offset of PS light in the spectrum at the short wave is less than 1nm, but the spectrum required by long wave passing is not suitable, because the offset of the film system at the long wave is more than 15nm, the structure of the film system can be changed by forced optimization, and the difficulty in film system preparation is increased. New membrane systems are proposed which have a small offset to long waves, typically less than 1nm. After the initial structure is given, a better design can be obtained under the condition of slightly not damaging the basic structure, and the difficulty of film coating is reduced.

Disclosure of Invention

In order to overcome the defects in the prior art, the invention provides a depolarized flat-plate thin-film cut-off filter.

The technical scheme adopted by the invention for solving the technical problems is as follows: a depolarized, flat-panel thin film cut-off filter comprising a thin film deposited on a flat-panel substrate, said thin film having a fundamental periodic structure: (aHbMcLbMaH) ^ P; h represents a high-refractive-index film layer with quarter optical thickness, M represents a medium-refractive-index film layer with middle quarter optical thickness, L represents a low-refractive-index film layer with quarter optical thickness, and P is the period number; a is a multiple of film structure H, b is a multiple of film structure M, c is a multiple of film structure L, and P is the number of cycles.

According to another embodiment of the present invention, the film further comprises adjusting the relative proportions of a, b, and c to adjust the amount of spectral separation of the polarization at 50% transmission of the transition bands P-and S-at oblique incidence and to adjust the widths of the transmission and cut-off bands until complete depolarization.

According to another embodiment of the invention, the thin film adjusts the amount of the P light and the S light shift by adjusting the number of layers of the H, M and L films.

According to another embodiment of the present invention, further comprising that the substrate on which the thin film is deposited is in a flat plate shape.

According to another embodiment of the present invention, further comprising the initial structure of the film is: g (aHbMcLbMaH) ^ PLA; wherein G represents a flat substrate, and A represents air.

The invention has the advantages that for the flat substrate, the polarization spectrum separation quantity of the transition zones P-and S-at the position of 50% transmittance can be effectively eliminated at any incident angle; the offset of the P light and the S light can be conveniently adjusted through the coefficient in the basic periodic structure; the widths of the transmission belt and the stop belt can be conveniently adjusted through the number of film layers in the basic periodic structure; the method is simple and easy to implement, and can be completed by using a computer and any conventional film design software; the method has strong pertinence, wide application range and high quality, and is suitable for all medium film materials in the current optical film industry; the designed film system does not contain an extremely thick layer, and the total thickness of the film layer is relatively thin, so that the precise preparation is facilitated, and the yield of products is greatly improved.

Drawings

The invention is further illustrated with reference to the following figures and examples.

FIG. 1 is a schematic diagram showing the basic periodic structure of a thin film cut-off filter for depolarization;

FIG. 2 is a plot of the spectrum of initial structure G (0.70H 0.8M 1.2L 0.8M 0.70H) ^20LA at 45 ° incidence;

FIG. 3 is a plot of the spectrum of initial structure G (0.75H 0.8M 1.2L 0.8M 0.75H) ^30LA at 45 ° incidence;

FIG. 4 is a design curve for the initial structure G (0.75H 0.8M 1.2L 0.8M 0.75H) ^20LA at 45 ° incidence;

FIG. 5 is a plot of the initial structure G (0.75H 0.8M 1.2L 0.8M 0.75H) ^20LA at 45 incident.

Detailed Description

A depolarized, flat-panel thin film cut-off filter comprising a thin film deposited on a flat-panel substrate, said thin film having a fundamental periodic structure: (aHbMcLbMaH) ^ P; h represents a high-refractive-index film layer with quarter optical thickness, M represents a medium-refractive-index film layer with middle quarter optical thickness, L represents a low-refractive-index film layer with quarter optical thickness, and P is the period number; a is a multiple of film structure H, b is a multiple of film structure M, c is a multiple of film structure L, and P is the number of cycles.

According to another embodiment of the present invention, it is further included that the substrate on which the thin film is deposited is in a flat plate shape, not a prism.

After the initial structure of the film system is obtained according to the method, the required depolarized flat-plate thin-film cut-off filter can be quickly obtained by using any current film system design software.

Example 1:

in quantum communication, under the condition of 45-degree incidence, two wavelengths of 1040nm and 1080nm need to be separated, so that the reflectivity of 1040nm is more than 99%, the transmittance of 1080nm is more than 98%, and the polarization spectrum separation quantity of P-and S-in a transition band at 50% transmittance is required to be less than 1nm.

The basic periodic structure of the film is: (aHbMcLbMaH) ^ P, with L for SiO2, refractive index set to 1.45, H for Ta2O5, refractive index 2.21, M for AL2O3, refractive index 1.65 this is the most common dielectric film material in the industry, substrate G is the most common H-K9L, and incident medium A is air, where the absorption of the film and substrate is not considered.

Giving the initial structure: g (ahbmlbmah) ^ PLA, temporarily taking a =0.7, b =0.8, c =1.2, p =20 and the reference wavelength 1064nm, gives the spectrum shown in fig. 2. It was found that the amount of polarization spectral separation of the P-and S-of the transition band at 50% transmittance was already small, but the reflectance near 1040nm was not high enough.

Therefore, the further tuning parameters are: a =0.75, b =0.8, c =1.2, p =30, and the reference wavelength is 1064nm, resulting in a spectrum as shown in fig. 3. It was found that the amount of polarization spectral separation of P-and S-in the transition band at 50% transmittance was almost 0, and at the same time, the reflectance around 1040nm was sufficiently high.

To date, it was believed that a suitable initial structure of the film was found: g (0.75H 0.8M 1.2L 0.8M 0.75H) ^30LA.

Based on this, any film design software can be used to obtain ideal depolarization long-wave pass, the design curve is shown in figure 4, and figure 5 is the final result obtained by optimization based on G (0.75H 0.8M 1.2L 0.8M 0.75H) ^30LA. It was found that the reflectance at 1040nm was more than 99.2%, the transmittance at 1080nm was more than 99.1%, and the polarization spectral separation amount at 50% transmittance of P-and S-of the transition band was 0.9nm. Completely meets the use requirement.

The foregoing description is intended to be illustrative rather than limiting, and it will be appreciated by those skilled in the art that many modifications, variations or equivalents may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A depolarized, flat-panel thin film cut-off filter comprising a thin film deposited on a flat substrate, said thin film having a fundamental periodic structure: (aHbMcLbMaH) ^ P; h represents a high-refractive-index film layer with quarter optical thickness, M represents a medium-refractive-index film layer with middle quarter optical thickness, L represents a low-refractive-index film layer with quarter optical thickness, and P is the period number; a is a multiple of film structure H, b is a multiple of film structure M, c is a multiple of film structure L, and P is the number of cycles.

2. The depolarized, flat-panel thin film cutoff filter of claim 1, wherein said thin film is depolarized completely by adjusting the relative proportions of a, b, and c to adjust the amount of spectral separation of the transition bands P-and S-at 50% transmission at oblique incidence and to adjust the widths of the transmission and cut-off bands.

3. The depolarized, flat-panel thin film cutoff filter of claim 1, wherein the amount of P and S light shift is adjusted by adjusting the number of H, M, L film layers.

4. The depolarized, flat-panel thin-film cutoff filter of claim 1, wherein the substrate on which the thin film is deposited is flat-panel shaped.

5. The depolarized, flat-panel thin-film cutoff filter of claims 1-4, characterized in that the initial structure of the thin film is: g (aHbMcLbMaH) ^ PLA; wherein G represents a flat substrate, and A represents air.