CN212134987U

CN212134987U - Ultra-narrow band optical filter with high optical stability

Info

Publication number: CN212134987U
Application number: CN202020617237.5U
Authority: CN
Inventors: 吴江波; 金波; 艾曼灵; 李冰霞; 顾培夫; 刘璐
Original assignee: Hangzhou Koti Optical Technology Co ltd
Current assignee: Hangzhou Koti Optical Technology Co ltd
Priority date: 2020-04-22
Filing date: 2020-04-22
Publication date: 2020-12-11
Anticipated expiration: 2030-04-22

Abstract

The utility model discloses a high optical stability's super narrow band light filter, including basement and the super narrow band filter membrane of setting on the basement. The substrate is made of FEP optical plastic, and the ultra-narrow band filter film is of a modified three-cavity structure. Central wavelength lambda of optical filter₀530nm, the number of film layers was 60, and the total geometric thickness was 4771 nm. The film system is made of high-refractivity TiO₂Film and low refractive index SiO₂All film layers are subjected to ion-assisted deposition, and the ion energy is 350 eV. The coefficient of thermal linear expansion of FEP plastic reaches 140 multiplied by 10^‑6And/degree, and selecting a film material with small coefficient of thermal linear expansion and positive and negative matching of temperature coefficient of refractive index to eliminate wavelength drift caused by environmental temperature. The ultra-narrow band filter can be widely appliedThe filter is used for optical, photoelectric and laser systems and instruments as a stable signal filter.

Description

Ultra-narrow band optical filter with high optical stability

Technical Field

The utility model relates to a high optical stability's super narrow band filter belongs to the optical film field, but wide application in optics, photoelectricity and laser system and instrument regard as signal filter.

Background

Thin film bandpass filters have irreplaceably important applications as optical filters in the fields of optics, spectroscopy, lasers, astronomical physics, and the like. The main parameters characterizing the bandpass filter include the center or peak wavelength λ₀Peak transmittance T_mAnd half width Δ λ. As shown in FIG. 1, the half-width is the wavelength width at half-peak transmission, and is often Δ λ/λ₀Indicating the relative width. Delta lambda/lambda₀The filter with the weight percentage more than 5 percent is called a broadband filter, the filter with the weight percentage between 5 percent and 1 percent is called a narrow-band filter, and the filter with the weight percentage less than 1 percent is called an ultra-narrow-band filter. The half width of the optical filter related to in the utility model is 1nm, and the relative width is 1/530 ═ 0.19%, belongs to typical ultra-narrow band optical filter.

The ultra-narrow band filter has a very difficult problem of optical stability during use, where the optical stability refers to instability of optical characteristics of the filter caused by changes in relative humidity and ambient temperature of the use environment during use: i.e. the central or peak wavelength lambda of the filter₀Peak transmittance T_mAnd the half width DeltaLambda can change along with the change of the relative humidity of the use environment and the environmental temperature, wherein the maximum change is the central wavelength or the peak wavelength Lambda₀Is customarily called λ₀The shift of (2) is wavelength drift, which causes the filter filtering function to be reduced if the wavelength drift is light, and completely cuts off the optical path if the wavelength drift is heavy, and loses signals.

The mechanisms responsible for optical instability of the filter are now substantially well understood. In the last 70 th century, film workers found that any film produced by thermal evaporation was, without exception, porous columnar microstructure (as shown in FIG. 2), meaning that the surface of the pillars inside the film was exposed to the ambient atmosphere virtually the same as the outer surface of the film, and the area of the inner surface of the pillars was more than ten times larger than the area of the outer surface of the film. The gaps among the thin film columns are like capillary holes penetrating through the thin films, and the water adsorption effect is extremely strong in the ambient atmosphere. To study this porous columnar structure, a well-known physical concept, the packing density P, was proposed, which is defined as:

tests on various films prepared by the conventional process show that P is generally between 0.75 and 0.95, which forces people to know again that: the original film is not dense but loose inside. Even if the film prepared by the optimized process is adopted, P can only reach 0.9-0.95 generally. If P is 0.92, the change value of the high and low refractive indexes of the filter when the relative humidity of the environment changes from 10% to 95% is substituted into the commercial TFC program, and then the center wavelength or peak wavelength λ of the filter can be calculated₀The drift is around 10nm, which is why filters have to be used in a constant humidity sealed cell before. Lambda [ alpha ]₀The reason for the drift is straightforward: when the relative humidity is 10%, the film voids are filled with substantially air having a refractive index of 1.0, and when the relative humidity is raised to 95%, the film voids are filled with substantially water having a refractive index of 1.33. That is, in a high-humidity environment, the refractive index of the thin film increases, resulting in an increase in optical thickness, so that the filter characteristics drift toward a long wavelength; on the other hand, if the relative humidity changes from high to low, the filter characteristics shift to short wavelengths. This is a fluctuating optical instability caused by the relative humidity of the environment. This optical instability has not been substantially resolved until the break through of optical communication wavelength division multiplexing devices has emerged at the century, which is ion assisted deposition: the kinetic energy and mobility of the deposited film molecules are improved by means of the momentum transfer of the auxiliary ions, and a compact film with the aggregation density of 1 is obtained. However, ion-assisted deposition must be based on reasonably accurate use of the ion source, otherwise the nm drift occurs even with ion assistanceAre still frequently present.

Not only the above relative humidity but also the ambient temperature can cause the wavelength of the filter to drift. For the change of the environmental temperature, on one hand, the film and the substrate can expand with heat and contract with cold, so that the geometric thicknesses of the film and the substrate are changed; on the other hand, it also causes a change in the refractive index of the film and the substrate. Since the influence of the geometric thickness change and the refractive index change of the substrate on the characteristics of the optical filter can be ignored, only the influence of the geometric thickness change and the refractive index change of the thin film on the characteristics of the optical filter needs to be considered. Assuming that the maximum temperature difference of the environmental temperature in winter and summer is 80 ℃, the variation of the geometric thickness can be conveniently calculated according to the thermal linear expansion coefficients of the high-refractive-index film and the low-refractive-index film of the optical filter; and calculating the variation of the refractive index according to the temperature coefficients of the refractive indexes of the high-refractive-index film and the low-refractive-index film of the optical filter. By substituting these variations into a commercial TFC program, the center wavelength or peak wavelength λ of the filter can be calculated₀The amount of drift is also in the order of nm. Obviously, the amount of wavelength drift caused by ambient temperature is much smaller than that caused by the absorption of water in the film voids, but even so, it cannot be tolerated in the ultra-narrow band filters, so the ultra-narrow band filters used in harsh environments in the field must be contained in a constant temperature sealed box.

To ensure high signal-to-noise ratio of optical, optoelectronic and laser systems and instruments, the allowable wavelength drift amount during practical use of the optical filter requires 1/3 smaller than half width, it is right that the actual total drift caused by the variation of humidity and temperature must be smaller than 0.33nm for the ultra-narrow band optical filter with 1nm half width of the present invention, which is not easy to do.

The utility model discloses starting with super narrowband optical filter membrane system design and preparation technology, developing some exploration and research to film gathering density, thermal expansion coefficient and refracting index temperature coefficient. Particularly, the ion source is controlled, the film aggregation density is ensured to reach 1 under the simple and convenient condition, and the optical instability caused by water absorption of the film is eliminated; meanwhile, optical instability of the film caused by environmental temperature change is eliminated by matching the thermal linear expansion coefficient and the temperature coefficient of the refractive index of the film and selecting a substrate with a high expansion coefficient. Finally, the total drift amount caused by the change of the relative humidity and the environmental temperature is less than 0.33 nm.

SUMMERY OF THE UTILITY MODEL

The utility model aims at providing a high optical stability's super narrowband filter eliminates the instability of the produced optical characteristic of light filter use in-process because of relative humidity and ambient temperature change, ensures optics, photoelectricity and laser system and instrument and remains high SNR throughout.

To achieve the above object, the present invention provides the following three aspects of concepts and experiments.

Ultra-narrow band filter film system design

The first step of designing the ultra-narrow band filter is to determine the thin film material according to the use requirement. Many high refractive index materials for the visible region, including the oxide TiO₂，Nb₂O₅，Ta₂O₅，CeO₂And ZnS sulfide, etc., but among them, TiO is the highest refractive index₂The refractive index of the material at the wavelength of 530nm can reach 2.44; TiO 2₂The refractive index is high, and the temperature coefficient of the refractive index is negative, so that the temperature drift of the optical filter can be reduced; TiO 2₂Also has the advantages of high mechanical strength, small film stress, cheap material and the like, which is TiO₂Which is the reason why film designers most often choose as a high index material. The low refractive index material used in the visible region is less, mainly oxide SiO₂And the fluoride MgF₂Of these two materials, MgF₂Although the refractive index is SiO₂It is low, but it is a soft film material, unless the number of layers is small, and can be mixed with TiO₂And the like are matched for use. As hard coat material, SiO₂Is a unique material with extremely excellent performance and low refractive index, has the attractive advantages of low optical loss, high laser damage threshold, high mechanical strength, small thermal expansion coefficient, low film stress and the like, and is most important with TiO₂The matching can meet the design requirements with a minimum number of layers.

After the film material is determined, the second step is to determine the film system structure according to the technical indexes. To increase the steepness of an ultra-narrow band filter, a membraneA modified three-cavity structure is adopted; meanwhile, SiO is selected to reduce absorption and increase the peak transmittance of the optical filter₂As the spacer layer, specific film structures are listed below: s < lambda > electrically ⁴ ⁴(HL)H2LH(LH)L ⁴ ⁴(HL)H4LH(LH)L ⁴(HL)H2LH ⁴(LH)L' | A, the center wavelength λ of the film system₀530nm, which is the frequency doubling wavelength of a 1060nm laser, 60 total film layers, and a total geometric thickness of 4771nm, where S denotes the filter substrate, A denotes the incident medium air, and H denotes a quarter-center wavelength thick high refractive index TiO₂Film, L represents a low refractive index SiO of quarter center wavelength thickness₂Film, L' is MgF of any thickness₂An antireflection film. It can be seen that the spacing layer of the three cavities is 2L-4L-2L, and the cavities are coupled by L connection.

The above membrane system has two innovations: 1. the substrate S is selected from an optical plastic called Fluorinated Ethylene Propylene copolymer (FEP). FEP has two specific properties, namely, extremely low water absorption and extremely high coefficient of thermal expansion (see n. kaiser, h.k. pulker, Optical Interference Coatings, p362), which is very beneficial for improving the stability of the filter. FEP has a water absorption of about 0.01%, being the lowest among plastic substrates; and its thermal expansion coefficient is 140X 10^-6The/degree, which is almost 20 times that of ordinary optical glass, is the highest of all substrates including inorganic glass, and as will be seen below, is very useful for reducing temperature drift. 2. In the design of three chamber light filters, the distance layer in three chamber always equals, but the utility model discloses adopt 2L-4L-2L to revise the structure, this be favorable to improving peak value transmissivity with FEP basement refractive index matching. Through the calculation of the equivalent refractive index of the symmetry period, the equivalent refractive index E of the 2L-4L-2L structure is found to be closer to the refractive index 1.34 of the substrate FEP, as shown in FIG. 3; on the air side, an optimized L' film with any thickness of MgF needs to be added₂Antireflection film of MgF₂It is useful to increase the peak transmittance (see FIG. 4), so that λ shown in FIG. 5 is obtained₀530nm ultra narrow band filter.

Secondly, eliminating the optical instability caused by the relative humidity change of the film

It has been pointed out that the influence of the ambient relative humidity on the optical instability of the film is reflected by the concentration density of the film, if the concentration density of the film is equal to 1, water vapor cannot be filled into the film, the refractive index of the film can be kept stable regardless of the change of the ambient relative humidity, the characteristic of the film does not have any drift, and the optical stability of the film is good. To achieve the above object, it is necessary to establish a convenient and feasible method for detecting the film aggregation density in the first step, and although the film aggregation density can be calculated by measuring the refractive index change of the film before and after water absorption or by measuring the wavelength drift of the film before and after water absorption, the two methods are really and directly applicable, but the problem is that the dynamic real-time measuring instrument is extremely complex and has low precision, so the utility model provides a test method using a quartz crystal oscillator. Because most of the existing film plating machines are provided with quartz crystal oscillators to control the thickness of the film and the deposition rate, the working principle is that the piezoelectric effect and the mass load effect of the quartz crystal are simply utilized, the piezoelectric effect means that the quartz crystal can generate mechanical oscillation with determined frequency by adding voltage, and the mass load effect means that the oscillation frequency can be changed by the tiny change of the mass on the quartz crystal, so the change of the mass before and after the film absorbs water can be called by the change of the frequency of the quartz crystal, and obviously, the quartz crystal acts as a microbalance. If the frequency of the quartz crystal before film coating is recorded as f₀After the film is coated, the frequency is changed to f₁Then, the frequency variation quantity generated by the film thickness d is obtained as f₁-f₀＝c·△m_s＝c·P·ρ_sA.d; further, a high-humidity gas is introduced into the vacuum chamber, and the frequency of the film on the quartz crystal is changed to f by sufficient absorption of water₁It is clear that the frequency change produced by the film absorbing water is Δ f ═ f₁*-f₁＝c·△m_w＝c·(1-P)·ρ_wA · d. In the above formula, c is a constant,. DELTA.m_sAnd Δ m_wFilm solid part and water absorption generation, respectively, of thickness dP is the film packing density, P_sAnd ρ_wThe density of the solid part of the film and the density of water are respectively shown, and A is the area of the quartz crystal plating film. Accordingly, c, A, and d are eliminated and substituted into ρ_wThe film has an aggregate density of P ═ Δ f/(. DELTA.f + ρ) of 1, and the film is obtained_sΔ f). By utilizing the formula, under the condition that equipment is not increased or improved, the utility model expands a new application of the quartz crystal oscillator of the existing film plating machine, namely the detection of the gathering density of the film.

The increase of the film concentration density requires the aid of Ion Assisted Deposition (IAD). However, since IAD has many control parameters and a complex process, the situation that even though ion assisted deposition is adopted, water absorption drift still exists, as shown in fig. 6, the concentration density of the high and low refractive index films reaches 0.99, but the wavelength drift of the optical filter still reaches 1.1 nm. The utility model discloses owing to established the detection method of film gathering density, so can be purposefully improve ion auxiliary process. IAD requires an ion source whose main control parameters include beam voltage, beam current, acceleration voltage, neutralization current, RF source power and argon working gas partial pressure (Ar atomic weight 40, producing momentum-transferred argon ions), oxygen reaction gas partial pressure (oxygen atomic weight 16, producing oxidation-reacted oxygen ions), among others, in addition to which the present invention has found that there is an important parameter, the film deposition rate, that is independent of the ion source parameters themselves but closely related to the concentration density. Generally, one only controls the ion source parameters, neglects the film deposition rate, and adds more ion source parameters and restricts each other, so that the parameters are easily lost and influence the IAD effect. Therefore, the utility model discloses unify earlier the concentration two parameters of ion source parameter: ion energy and ion current density. Ion energy characterizes the strength of the ion momentum transfer, while ion current density characterizes the frequency of the ion momentum transfer. The ion energy is obtained from the beam voltage and the ion current density is obtained from real time measurements, and the desired ion current density is obtained by adjusting ion source parameters other than the beam voltage. When the film is actually evaporated, the ion energy and ion flux density are fixed, and only the film deposition rate needs to be adaptively adjusted, or the film deposition rate and the ion flux density are adjustedDegree is classified as the ratio of the number of auxiliary argon ions arriving at the substrate per unit area per unit time to the number of molecules of the deposited film (J)_I/J_M) The utility model discloses think, this ratio is more important than the ion energy even, is worth appreciating. The ion energy cannot be too low, otherwise argon ions (Ar)⁺) The momentum transfer between the deposited film molecules is too small to generate compact film, but if the ion energy is too high, the film absorption can be increased because the film molecular structure is easy to be damaged, and the concentration density is too high, and the pressure stress is too large, so the utility model discloses a to SiO₂And TiO₂For the measurement of the film aggregation density, an ion energy of 350eV is selected. Ratio of auxiliary argon ion number to deposited film molecule number (J)_I/J_M) It is calculated from the ion current density and the film deposition rate. Under the condition of ion energy of 350eV, the ion source parameters are properly adjusted, and the ion current density is selected to be 50 muA/cm²Since 1 μ a is 6.25 × 10¹²Ion/second, the number of ions J arriving per unit time and unit substrate area_I＝50×6.25×10¹²Ion/s.cm²＝3.1×10¹⁴Ion/s.cm². Now, the number J of molecules of the deposited film arriving per unit time and per unit area of the substrate is calculated_MLet λ be₀/4SiO₂The evaporation time t of the membrane was 120s (. lamda.)₀530nm), then J_M＝dρN_A/Mt＝[(530nm/4/1.46)×10^-7cm]×2.1g/cm³×6×10²³Mu/mol ÷ [60g/mol X120 s]＝1.59×10¹⁵Molecule/s.cm²In the formula, d, rho and N_AAnd M is respectively SiO₂Geometric thickness of the film, density, Afgabor constant, and molecular weight. Thus, SiO was obtained₂J of the film_I/J_M0.195. The same method can calculate lambda₀/4TiO₂J when evaporation time t of film is 150s_I/J_M0.284. The resulting aggregate densities for these two films are shown in fig. 7 and 8. As can be seen from FIGS. 7 and 8, the both films have an aggregation density of substantially 1 at an ion energy of 350eV, and in particular, TiO₂Film, this is mainly attributed to TiO₂Deposition rate ratio of film to SiO₂Edge of membraneTherefore, because of TiO₂The film is prone to oxygen loss and absorption, so it is customary to increase the evaporation time or to reduce the deposition rate to ensure adequate oxidation, which is usually the case with TiO₂Film bulk density ratio SiO₂The reason for the high film. In fact, even with the IAD technique, there is sometimes a shift in the spectral characteristics, often due to SiO₂The deposition rate of the film is too fast, resulting in J_I/J_MToo low for the reason of this. The reason is that, of course, after the low refractive index material absorbs water, the ratio of the increase in refractive index to the original refractive index is high, which corresponds to a large increase in optical thickness. Table 1 below shows TiO₂And SiO₂J of film at different evaporation times or different deposition rates_I/J_MValues, as can be seen from Table 1, evaporation time or deposition rate vs. J_I/J_MThe effect of the value is relatively large, which can significantly affect the packing density of the film. Only the numbers b and b' in table 1 are shown in fig. 7 and 8; the case of the numbers a and a' is lower in the concentration density, in particular SiO₂The membrane has a drift amount exceeding a required value and is not subjected to sodium acquisition; the case of numbers c and c' is high in aggregation density, the amount of drift is certainly negligible, but the film absorption loss and the compressive stress are both significantly increased, which is not a good choice, and the higher the aggregation density is not the better.

TABLE 1

In summary, ion energy of the ion source is 350eV and ion current density is 50 μ A/cm²Under the conditions of (1), SiO is selected₂Evaporation time of 120 seconds and TiO₂The evaporation time of the filter is 150 seconds, and the ultra-narrow band filter with high aggregation density and zero drift of relative humidity change can be obtained.

Thirdly, eliminating the optical instability caused by the change of the environmental temperature of the film

The parameters for constructing the optical characteristics of the ultra-narrow band filter are as follows: the refractive index n of each layer of film and the optical thickness nd or the geometric thickness d. If filtering the lightThe temperature of the using environment of the sheet changes, on one hand, the geometric thickness d of the film and the substrate is changed due to expansion and contraction, and on the other hand, the refractive index n of the film and the substrate is also changed. If the linear expansion coefficients of both the thin film and the substrate are comparable, the geometrical thickness d variation and refractive index n variation of the substrate have negligible effect on the optical properties of the filter, since the substrate is thick and generally does not generate light interference. In this case, only the influence of the geometrical thickness d change and the refractive index n change of the thin film on the characteristics of the filter needs to be considered. For a single layer film, the amount of wavelength drift due to temperature change is λ₀(d/T + n/T) Δ T, where λ₀Since d/T is the linear expansion coefficient, n/T is the temperature coefficient of refractive index, and Δ T is the amount of temperature change, once the linear expansion coefficient and the temperature coefficient of refractive index are known, the calculation of the amount of drift of the single-layer film is not complicated. However, the ultra-narrow band filter of the present invention has different contributions to the drift amount due to each layer of film in the filter, and the difference is very large, because the electric field intensity difference of each layer of film is very large, it is difficult to directly calculate the drift amount by using an analytic formula, and only the geometric thickness increment and the refractive index increment under Δ T can be calculated first according to the linear expansion coefficient and the refractive index temperature coefficient of each thin film, and then the drift amount is actually calculated by inputting the film system structure parameters of the ultra-narrow band filter including these two increments in the commercial TFC program.

As can be seen, TiO is obtained as the first step₂Film and SiO₂The linear expansion coefficient and the temperature coefficient of refractive index of the film are the most important. For simplicity, the linear expansion coefficient is denoted by α ═ d/T, and the temperature coefficient of refractive index is denoted by β ═ n/T. Although TiO may be referred to₂And SiO₂Coefficient of linear expansion of bulk or crystalline materials, but TiO is reported₂Film and SiO₂The linear expansion coefficient of the membrane is few and few, for this reason, the utility model discloses a data that early light and heat deflection method obtained: TiO 2₂The film has a coefficient of thermal linear expansion of 2 to 2.5X 10^-6Degree/degree, 2.5X 10^-6Degree of SiO₂The film had a coefficient of thermal linear expansion of 0.7X 10^-6Degree of reaction, MgF₂Hot wire expansion of membranesCoefficient of about 18 x 10^-6Degree/deg. Obtaining TiO₂Film and SiO₂The temperature coefficient of the refractive index of the film is more difficult, although SiO is found₂Temperature coefficient of refractive index of the film is 1X 10^-5Degree/but no TiO found₂The temperature coefficient of the refractive index of the film, Happy well, is known for TiO₂Film and SiO₂Coefficient of linear expansion of film and SiO₂The temperature coefficient of the refractive index of the film can be fitted according to the drift amount caused by the actually measured temperature change to obtain TiO₂The temperature coefficient of refractive index of the film is about-0.57 x 10^-6Degree/deg. Is this temperature coefficient of refractive index reasonable? Analysis of the expression for temperature coefficient of refractive index: beta-n/T-n [ ("n")²-1)(n²+2)/6n]·[(1/γ)(dγ/dT)-3α]Wherein γ is polarizability and α is linear expansion coefficient. When the temperature rises, thermal expansion leads to a decrease in density on the one hand and an increase in polarizability on the other hand, so that β is positive or negative depending on [ (1/γ) (d γ/dT) -3 α]. Beta is negative for materials with a predominance of thermal expansion. TiO 2₂Although the film has a strong polarizability, α is large, so it is reasonable that the temperature coefficient of refractive index is small negative. From the above, it can be seen that in general, the temperature coefficient of refractive index is larger than the thermal expansion coefficient, but TiO₂The temperature coefficient of the refractive index of the film is very small and exhibits a negative number, which is very beneficial for the present invention to reduce the wavelength drift caused by the ambient temperature variation of the film.

Second step, according to the above TiO₂Film and SiO₂And calculating the change of the geometric thickness d and the refractive index n of the thin film caused by the change of the ambient temperature according to the linear expansion coefficient and the temperature coefficient of the refractive index of the thin film. Assuming that the maximum temperature difference of the outdoor environment temperature in winter and summer is 80 ℃, namely the room temperature is 20 +/-40 ℃, and TiO is used₂The amount of change in the geometric thickness d, TiO, was calculated for the film as an example₂The film had a refractive index of 2.44 at room temperature of 20 ℃ and a wavelength of 530nm, a geometric thickness d (20 ℃) of 530nm/4/2.44 of 54.3033nm, and a coefficient of thermal linear expansion of 2.5X 10^-6Degree/, so that d (60 ℃) is d (20 ℃) plus 2.5 multiplied by 10 at 60 ℃ in summer^-6Degree/40 ℃. 54.3034nm, and d (-20 ℃) d (20 ℃) to-2.5X 10 in winter at-20 ℃^-6Degree/40 ℃ ═ 54.3032nm, in fact, TiO₂The variation of the geometric thickness d of the film is very small, and is only 0.0002 nm; SiO calculated by the same method₂The d change of the film was then substituted into a commercial TFC program to give a change in geometric thickness due to a temperature change of-20 c to 60 c that would result in a shift in the filter wavelength of about 0.1 nm. The change in refractive index can similarly be calculated as SiO₂The film is, for example, SiO, having a refractive index of 1.46 at a wavelength of 530nm at 20 ℃ at room temperature₂Temperature coefficient of refractive index of the film is 1X 10^-5Degree/, so that n (60 ℃) is n (20 ℃) plus 1X 10 at 60 ℃ in summer^-5Degree/40 ℃. 1.4604, and n (-20 ℃) to n (20 ℃) to 1X 10 at-20 ℃ in winter^-5Degree/. times.40 ℃ C. ═ 1.4596, i.e. SiO₂The amount of change in the refractive index n of the film was 0.0008; TiO calculated by the same method₂The n of the film was changed and then substituted into the TFC program to obtain that the refractive index change caused by the temperature change of-20 deg.c to 60 deg.c would cause the wavelength of the filter to drift by about 0.3 nm. As can be seen from the above, the wavelength shift caused by the temperature coefficient of refractive index is larger than the wavelength shift caused by the thermal linear expansion coefficient; due to TiO₂The temperature coefficient of the refractive index of the film is very small and negative, and has important significance for reducing wavelength drift caused by the temperature coefficient of the refractive index; the utility model discloses the ambient temperature change of super narrow band light filter arouses that total wavelength drifts for 0.4nm, though this value is not big, nevertheless right the utility model discloses half-width 1 nm's super narrow band light filter is also unacceptable.

How to solve the drift problem caused by the ambient temperature? The utility model discloses try to select FEP organic plastics that coefficient of thermal expansion is very high as the basement of super narrowband filter to solve. The principle of the idea is as follows: the temperature coefficient of either linear expansion coefficient or refractive index causes the central wavelength of the filter to shift towards a long wave direction with the increase of the environmental temperature, but the thermal expansion coefficient of the FEP substrate is larger than that of TiO₂Film and SiO₂The film is at least 20 times higher, so the expansion of the FEP substrate is much greater than the expansion of the film, plus the substrate thickness is much greater than the film thickness: the thickness of the substrate is several millimeters, and the total thickness of all the films is several micrometers, so that the optical filter film can be subjected to the tensile force of expansion of the substrate in all directions during heating to reduce the thickness of the optical filter film and supplement the optical filter filmThe drift to long wave caused by temperature rise is compensated. On the contrary, when the ambient temperature is reduced, the optical filter film can be subjected to the action of the shrinkage force of the substrate in all directions, so that the thickness of the optical filter film is thickened, and the shortwave drift generated by temperature reduction is compensated.

Unfortunately, the FEP plastic substrate is an organic material, the filter thin film is an inorganic material, the adhesion between the organic material and the inorganic material is generally low, and the film thickness of the filter is reduced by stretching and thinning with thermal expansion and increased by compression with cold shrinkage of the plastic substrate, the substrate and the first layer of TiO₂The adhesion at the film interface must be sufficiently high, and for this purpose, the FEP substrate surface is bombarded with argon ions having an ion energy of 350eV and an ion current density of 50 μ a/cm2 for 5 minutes before starting evaporation of the filter film. Ion bombardment can produce two effects: firstly, the surface of the substrate is cleaned, and secondly, the surface of the substrate can generate a mooring microstructure, so that the mooring energy of the surface of the substrate is increased, and finally, the adhesive force meeting the requirement is achieved.

Specifically, the technical scheme of the utility model is that:

the ultra-narrow band filter with high optical stability comprises a substrate and an ultra-narrow band filter film arranged on the substrate, wherein the film system structure of the ultra-narrow band filter film is a modified three-cavity structure: s < lambda > electrically ⁴ ⁴(HL)H2LH(LH)L ⁴(HL)H4LH ⁴(LH)L ⁴ ⁴(HL)H2LH(LH)L '| a, where S denotes a filter substrate, a denotes an incident medium air, H denotes a high refractive index film of a quarter center wavelength thickness, L denotes a low refractive index film of a quarter center wavelength thickness, and L' is an antireflection film.

Further, the high refractive index film is titanium dioxide (TiO)₂) A film, said low refractive index film being silicon dioxide (SiO)₂) The antireflection film is magnesium fluoride (MgF)₂)。

The central wavelength lambda of the film system of the ultra-narrow band filter film₀＝530nm。

The titanium dioxide (TiO)₂) The film has a refractive index of 2.44 at a wavelength of 530nm, and the film is made of silicon dioxide (SiO)₂) Membrane waveThe refractive index of the antireflection film is 1.46 at the wavelength of 530nm, and the refractive index of the antireflection film at the wavelength of 530nm is 1.38. High refractive index TiO₂Film, low refractive index SiO₂Film and antireflection MgF₂The films are all subjected to ion-assisted deposition, and the concentration density of the three films can reach 1.

The total number of the membrane layers of the ultra-narrow band filter membrane is 60, the total membrane thickness of the ultra-narrow band filter membrane is about 4771nm, the thickness of each L layer is 90.75nm, the thickness of each H layer is 54.3nm, but the thickness of the last two layers H and L' is optimized, and the thickness is 44.1nm and 66.5nm respectively.

The substrate is optical plastic, further, the substrate is fluorinated ethylene propylene copolymer (FEP for short) optical plastic, and the refractive index of the FEP optical plastic at the wavelength of 530nm is 1.34. Furthermore, FEP plastic with high thermal expansion coefficient is selected as the substrate of the ultra-narrow band filter, and the coefficient of thermal linear expansion of the FEP plastic can reach 140 multiplied by 10^-6And/degree is almost 20 times of that of common optical glass.

In order to eliminate optical instability caused by ambient temperature, FEP plastic with very high thermal expansion coefficient is selected as the optical filter substrate on one hand, and TiO with small thermal linear expansion coefficient, matched refractive index and temperature coefficient and high mechanochemical stability is selected on the other hand₂Film and SiO₂And (4) combining the films.

Further, TiO₂The film had a coefficient of thermal linear expansion of 2.5X 10^-6Degree of SiO₂The film had a coefficient of thermal linear expansion of 0.7X 10^-6Degree of reaction, MgF₂The film had a coefficient of thermal linear expansion of 18X 10^-6Degree/deg.

Further, TiO₂The temperature coefficient of refractive index of the film is about-0.57 x 10^-6Degree of SiO₂Temperature coefficient of refractive index of the film is 1X 10^-5Degree,/degree, said MgF₂The temperature coefficient of refractive index of the film was 2.3X 10^-5Degree/deg.

A preparation method of an ultra-narrow band filter with high optical stability comprises the following steps:

the method comprises the steps of adopting coating equipment with an ion source to pre-treat a substrate, and then adopting an ion-assisted deposition technology to prepare an ultra-narrow band filter film on the substrate.

The coating equipment with the ion source adopts titanium pentoxide as an initial evaporation material of the high-refractive-index film, and the coating equipment with the ion source adopts silicon dioxide (SiO)₂) The crystalline particles act as the initial evaporation material for the low refractive index film.

Further, to add organic FEP substrate and inorganic TiO₂Mechanical adhesion between the film surfaces, first layer of TiO on the starting substrate for vapor deposition₂Before the membrane is coated, for example, the ion energy is 350eV and the ion current density is 50 muA/cm²The argon ions bombard the surface of the FEP substrate for 2-10 minutes. Namely, the pretreatment of the substrate specifically comprises: firstly, using ion energy of 300-400 eV and ion current density of 30-70 muA/cm²Bombarding the surface of the substrate for 5 minutes; more preferably, the ion energy is 330-370 eV, and the ion current density is 40-60 muA/cm²Bombarding the surface of the substrate for 3-8 minutes by the argon ions; most preferably, the ion energy is 350eV, and the ion current density is 50 muA/cm²Bombarding the surface of the substrate for 5 minutes.

Further, in order to eliminate optical instability caused by relative humidity, IAD technology is adopted to prepare high-refractive-index TiO₂Film, low refractive index SiO₂Film and antireflection MgF₂The ion energy of the membrane is 350eV, and the ion current density is 50 muA/cm²By adjusting the deposition rate of each film, the ratio J of the number of auxiliary argon ions reaching the substrate per unit time and per unit area to the number of molecules of the deposited film is maintained_I/J_MHigh enough to achieve an aggregate density equal to 1 for all layers. Namely, in the process of preparing the ultra-narrow band filter film, the ion energy of the ion source is 330-370 eV, and the ion current density is 40-60 muA/cm²(ii) a More preferably, the ion energy of the ion source is 340-360 eV, and the ion current density is 45-55 muA/cm²(ii) a Most preferably, the ion source has an ion energy of 350eV and an ion current density of 50 μ A/cm². By adjusting the deposition rate of each film, the ratio J of the number of argon ions reaching the substrate per unit time and unit area to the number of molecules of the deposited film is maintained_I/J_MAre all greater than 0.19 to achieve an aggregate density of 1 for all film layers.

In the process of preparing the ultra-narrow band light filtering film, SiO is selected₂Evaporation time of 120 seconds and TiO₂Evaporation time of 150 seconds, i.e., the ratio of the number of argon ions to the number of molecules of the deposited film (J)_I/J_M) To SiO₂Film 0.195 to TiO₂When the film is 0.284, the gathering density of the filter can be more than 0.999 by the high and low refractive index films, and the wavelength drift of the ultra-narrow band filter is less than 0.1nm when the relative humidity of the environment is changed from 10% to 95%.

The ion current density is obtained by arranging an ion current detection target at the same position as the substrate for detection.

Furthermore, to eliminate the optical instability caused by the ambient temperature, FEP plastic with very high thermal expansion coefficient is selected as the filter substrate on one hand, and TiO with small thermal linear expansion coefficient, matched refractive index and high mechanochemical stability is selected on the other hand₂Film and SiO₂And (4) combining the films.

Further, the ultra-narrow band filter with high optical stability can be extended to various cut-off filters, band-pass filters, and the like. The ultra-narrow band filter can be widely applied to optical, photoelectric and laser systems and instruments as a stable signal filter.

Compared with the prior art, the beneficial effects of the utility model are that:

1. in the prior art, even if the film prepared by the optimized process is adopted, the aggregation density P can only reach 0.9-0.95, if P is 0.92, when the relative humidity of the environment changes from 10% to 95%, the central wavelength lambda of the optical filter is changed₀The drift is about 10nm, so the ultra-narrow band filter can only be used when being placed in a constant humidity sealing box, the cost is greatly improved, the volume and the weight of the instrument are influenced, the optical loss is increased, and the signal-to-noise ratio is reduced. In recent years, although the application of ion-assisted deposition has achieved significant success in increasing the packing density of thin films, the use of complex ion sources still results in a very common nm drift, which is not expected to achieve high optical stability.

The utility model discloses neither needingThe detection of the film gathering density is realized on the premise of adding equipment and not improving the existing quartz crystal oscillator of the film coating machine. The detection method is simple and feasible, and provides a new technology and a new method for researching the aggregation density and ion assistance. As previously mentioned, the main parameters of the ion source include beam voltage, beam current, acceleration voltage, neutralization current, RF source power, and also the partial pressure of argon working gas and oxygen reactant gas injected into the ion source, in addition to which the present invention finds a film deposition rate that is independent of the parameters of the ion source itself, but is closely related to the concentration density. The prior art not only has difficulty in accurately controlling the parameters of the ion source, but also ignores the film deposition rate, so that the expected effect of IAD can not be achieved. Therefore, the utility model discloses unified two parameters of transferring into to the ion source parameter: ion energy and ion current density. When the film is actually evaporated, the ion energy and ion flux density are fixed, and only by properly adjusting the film deposition rate, the ion flux density and the film deposition rate are classified into the ratio of the number of auxiliary argon ions reaching the substrate per unit area per unit time to the number of molecules of the deposited film (J)_I/J_M) The last monitored parameter is actually only the ion energy and J_I/J_MTwo parameters. Therefore, the film with high aggregation density can be stably obtained, zero drift of the ultra-narrow band filter is realized when the relative humidity is changed from 10-95%, and the mechanical properties of the filter such as optical loss, stress and the like are still excellent.

2. The prior art is still inexistent in the characteristic drift of the optical filter caused by the change of the environmental temperature. The change of the environmental temperature can cause the change of the geometric thickness of the film on one hand and the change of the refractive index of the film on the other hand, if the optical filter is used outdoors, the change of the environmental temperature in winter and summer is inevitable, so the wavelength drift is inevitable, and the ultra-narrow band optical filter used in the severe environment in the field needs to be arranged in a constant temperature sealing box in the prior art.

To solve the problem, the utility model firstly carries out the treatment on TiO₂Film and SiO₂The linear expansion coefficient and the temperature coefficient of refractive index of the film were investigated, and the linear expansion coefficient at the time of temperature change was calculated separatelyExpansion coefficient induced wavelength shift and refractive index temperature coefficient induced wavelength shift. To obtain: 1) since the wavelength shift due to the thermal expansion coefficient is much smaller than the wavelength shift due to the temperature coefficient of the refractive index, it is most important to control the wavelength shift due to the temperature coefficient of the refractive index; 2) TiO with negative temperature coefficient of refractive index₂The film is very beneficial in reducing wavelength drift caused by temperature changes. On this basis, the utility model provides an adopt FEP plastics that coefficient of thermal expansion is very high as the basement of light filter to compensate the wavelength drift that the temperature variation arouses: when the ambient temperature rises, the linear expansion coefficient and the refractive index temperature coefficient can cause the central wavelength of the filter to shift towards the long wave direction, but the thermal expansion coefficient of the FEP substrate is higher than that of TiO₂Film and SiO₂Since the film is at least 20 times higher, the expansion of the FEP base is much greater than the expansion of the film, and the thickness of the base is much greater than the thickness of the film, so that the filter film is subjected to tensile forces resulting from the expansion of the base in all directions at the time of temperature rise, thereby reducing the thickness of the filter film and compensating for the shift to the long wavelength due to the temperature rise. On the contrary, when the ambient temperature is reduced, the optical filter film is subjected to the action of contraction force of the substrate in all directions, so that the thickness of the optical filter film is increased, and the shortwave drift generated by temperature reduction is compensated. The ingenious matching can automatically compensate the wavelength drift of the ultra-narrow band filter caused by the temperature change, and a constant temperature sealing box is not needed.

Drawings

Fig. 1 shows the main parameters of the bandpass filter.

FIG. 2 is an idealized model schematic diagram of a columnar microstructure of a thin film.

Fig. 3 is the modified equivalent refractive index curve of the three-cavity ultra-narrow band filter of the present invention.

Fig. 4 is a contribution of the anti-reflective film of the ultra-narrow band filter of the present invention to increase the peak transmittance.

Fig. 5 is a modified spectral transmittance curve of the three-cavity ultra-narrow band filter of the present invention.

Fig. 6 shows the change of optical properties before and after water absorption when the concentration density of the high and low refractive index films of the ultra-narrow band filter of the present invention is 0.99.

FIG. 7 is a low refractive index SiO₂Concentration density and ion energy of the membrane and J_I/J_MThe relationship of (1).

FIG. 8 shows high refractive index TiO of the present invention₂Concentration density and ion energy of the membrane and J_I/J_MThe relationship of (1).

Detailed Description

Bandpass filters are extremely widely used in thin film optics. FIG. 1 shows the main parameters of a bandpass filter, including the center or peak wavelength λ₀Peak transmittance T_mAnd half width Δ λ. The half-width is the width of the wavelength at one-half peak transmission, characterizing the absolute bandwidth. Using a relative bandwidth of delta lambda/lambda₀Denotes. DELTA.. lambda./lambda₀The filter with the weight percentage more than 5 percent is called a broadband filter, the filter with the weight percentage between 5 percent and 1 percent is called a narrow-band filter, and the filter with the weight percentage less than 1 percent is called an ultra-narrow-band filter. The utility model discloses an optical filter half width is 1nm, belongs to typical super narrow band filter. In the use process of the ultra-narrow band filter, the optical parameters of the ultra-narrow band filter are very sensitive to the changes of the relative humidity and the temperature of the environment, and this phenomenon is called optical instability, especially wavelength drift, and can often cause the complete failure of the filtering function of the filter.

Why are the characteristic parameters of the ultra-narrow band filter very sensitive to changes in the relative humidity of the environment? To answer this question, the microstructure of the film must be traced back. Any film prepared by a conventional thermal evaporation process is observed to have a porous columnar microstructure by electron microscopy, and fig. 2 is an idealized model schematic diagram of the columnar microstructure of the film. In the growth process of the film, the kinetic energy of deposited molecules is only 0.1-0.3 electron volt (eV), and the mobility is almost zero, so that the loose column microstructure is finally generated. The area of the inner surface 1 of the cylinder is more than ten times larger than that of the outer surface 2 of the film, and the inner surface of the cylinder is exposed to the environment atmosphere as the outer surface of the film actually, so that the cylinder has extremely strong water adsorption effect and desorption effect; meanwhile, the gaps 3 between the columns are like capillary holes penetrating through the film, and the water filling and discharging effects are extremely strong. These effects necessarily result in filter characteristics that follow changes in the relative humidity of the environment. The ideal pillar model of fig. 2 can be calculated to have a packing density P of 0.907, but the packing density P of the actual film is usually 0.75 to 0.95, which indicates that the pillar microstructure of the actual film is complicated. Different materials or different processes form different pillar microstructures, if the radius r of most pillars of the film is reduced along with the increase of the film thickness, P is less than 0.907, and conversely, if the radius r of most pillars is increased along with the increase of the film thickness, P is more than 0.907.

Since the cause of such columnar microstructure is mainly due to the low molecular kinetic energy of the deposited film, the IAD technique survives the film preparation. Argon is chosen as the working gas because it has an atomic weight of up to 40 and is the least expensive of the inert gases. Argon is ionized by an ion source, the ion kinetic energy of the argon can reach hundreds of electron volts according to the beam pressure of the ion source, and the energy of the argon ions is completely transferred to the molecules of the deposited film in a momentum transfer mode through the successive collision of high-energy argon ions to the molecules of the deposited film and the successive collision among the molecules of the deposited film. As the molecular kinetic energy of the deposited film is increased, the mobility is greatly increased, the columnar microstructure disappears, and the concentration density is greatly increased, which is why IAD is selected.

The utility model discloses a reduce the absorption of super narrowband filter, choose for use SiO₂As a spacer layer, the benefits are: first, the peak transmittance of the filter can be increased, and second, if the filter is used with intense laser light, the laser damage threshold can be increased. Meanwhile, in order to increase the gradient of the ultra-narrow band filter, the membrane system adopts a modified three-cavity structure, the spacing layers of the three cavities adopt 2L-4L-2L respectively, and the cavities are connected and coupled with each other through L: s < lambda > electrically ⁴ ⁴(HL)H2LH(LH)L ⁴ ⁴(HL)H4LH(LH)L ⁴ ⁴(HL)H2LH(LH)L' | A, central wavelength λ of film system₀530nm, which is the frequency doubling wavelength of a 1060nm laser, 60 total film layers, and 4771nm total physical or geometric thickness, where S denotes the filter substrate, A denotes the incident medium air, and H denotes the quarter-wave center waveLong thickness high refractive index TiO₂Film, L represents a low refractive index SiO of quarter center wavelength thickness₂Film, L' is MgF of any thickness₂An antireflection film. It should be noted that in the three cavity filter design, the three cavity spacer layers are generally equal, and the 2L-4L-2L modified structure was found to be advantageous for index matching with the FEP substrate to improve peak transmission. Fig. 3 is the modified equivalent refractive index E curve of the three-cavity ultra-narrow band filter of the present invention. As can be seen from fig. 3, in the pass band of the filter, the equivalent refractive index E of the 2L-4L-2L structure is very close to the refractive index of the substrate FEP of 1.34, especially at the wavelengths of 529.7nm and 530.3nm, and the equivalent refractive index E is completely equal to the refractive index of the substrate FEP of 1.34, so that the transmittance curve 4 of the filter shown in fig. 4 reaches 100% at the two wavelengths. But because the equivalent refractive index E of the filter is at lambda₀Not perfectly matched to the substrate and not to air with a refractive index equal to 1, so that the curve 4 of transmittance shown in fig. 4 is not only not the highest but also has a large ripple, for which purpose a MgF denoted by L' is added₂And matching the antireflection films. MgF₂The film, when optimized in thickness, will involve TiO adjacent to it₂The film thickness was varied, and the final two films on the air side were 44.1nm and 66.5nm, respectively. FIG. 4 shows the contribution of the anti-reflective film of the ultra-narrow band filter to the peak transmittance, and curve 5 in FIG. 4 is the spectral transmittance curve of curve 4 plus the anti-reflective film, and it can be seen that MgF₂The antireflection film is useful for improving the peak transmittance. FIG. 5 is the final spectral transmittance curve of the three-cavity ultra-narrow band filter modified by the present invention, its λ₀＝530nm，T_m≈100％，△λ＝1nm。

Fig. 6 shows the change of optical characteristics of the ultra-narrow band filter before and after water absorption when the concentration density of the high and low refractive index films is 0.99, where curve 6 shows before water absorption and curve 7 shows after water absorption. The geometric thickness d of each film is not changed before and after the thin film filter absorbs water, the change is the refractive index, and the change of the refractive index can be calculated by the following formula: n ═ 1-P) n_v+pn_sWhere P is the packing density, n_vShow bookRefractive index of voids in the film, n_sRepresenting the refractive index of the solid in the film. Thus, for TiO₂A film having a refractive index n ═ (1-0.99)1.0+0.99 × 2.44 ═ 2.4256 before water absorption, and a refractive index n ═ (1-0.99)1.33+0.99 × 2.44 ═ 2.4289 after water absorption; SiO was calculated by the same method₂The refractive indices of the film before and after water absorption were 1.4554 and 1.4587, respectively. These data are substituted into the TFC program to obtain curve 6 and curve 7. As can be seen from

curves

6 and 7, the filter absorbs water and then absorbs water₀531.1nm, wavelength shifted to long wave by 1.1nm, T_m96% and 1.15 nm. Filters at λ due to wavelength drift₀530nm from about 100% transmission before water absorption to almost zero after water absorption, the wavelength shift is therefore the least tolerable. It is quite good to achieve a packing density of 0.99 for both films according to the practical experience of the film workers, but it is still completely unacceptable for the ultra-narrow band filters of the present invention. It was further calculated that a wavelength shift of less than 0.1nm could only be achieved when the aggregate density of both films reached above 0.999.

Example one

As a first embodiment, the present invention is expected to solve the problem of wavelength drift caused by changes in relative humidity of the environment. The change in relative humidity induces the absorption of water by the film to cause a change in the refractive index n of the film, and thus the concentration density of the film must be increased sufficiently to completely eliminate the wavelength shift of the filter. The utility model discloses an ion-assisted deposition technique improves the gathering density of film to more than 0.999 to ensure that the optical filter wavelength drift is less than 0.1 nm. Firstly, the utility model discloses a lot of ion source parameters complicated are divided into two parameters of ion energy and ion current density, and wherein, ion energy directly obtains from ion source beam voltage, and ion current density obtains with the ion current detection target of a new installation on the same position with the basement, divides to survey the target area according to the ion current that detects and obtains ion current density. The ion energy and ion current density are fixed when the film is actually evaporated. The utility model discloses get ion energy 350eV, ion current density 50 mu A/cm². Secondly, adjusting the deposition rate of the film to control the deposition rate of the filmAnd the ion current density is classified as the ratio of the number of argon ions incident on a substrate per unit area per unit time to the number of molecules of the deposited film (J)_I/J_M). Then, at a specific argon ion energy and J_I/J_MUnder the value, the film deposition rate is changed, and SiO is tested₂And TiO₂The concentration density of the film is obtained by obtaining the ratio (J) of the ion energy of 350eV, the number of argon ions and the number of molecules of the deposited film when the concentration density of both films is 0.999 or more_I/J_M) To SiO₂Film and TiO₂The films are 0.195 and 0.284 respectively, FIG. 7 is a low refractive index SiO of the present invention₂Concentration density and ion energy of the membrane and J_I/J_MFIG. 8 shows the relationship between the refractive index of the high refractive index TiO of the present invention₂Concentration density and ion energy of the membrane and J_I/J_MThe relationship of (1). Finally, only evaporation is carried out according to the proven deposition rate, and the SiO with low refractive index is subjected to₂The deposition rate of the film was 530nm/4/1.46/120 s-0.756 nm/s; for high refractive index TiO₂The deposition rate of the film was 530nm/4/2.44/150 s-0.362 nm/s. It should be noted that since TiO is directly used₂The evaporation of the film material is difficult, and the stability of the refractive index of the film layer is poor, so the utility model uses titanium pentoxide (Ti)₂O₅) Hard film sintered material as TiO₂The initial evaporation material of the film, and the initial evaporation material of the low refractive index film is directly silicon dioxide (SiO)₂) The crystalline particles evaporate the material.

From the first example, ion energy of the ion source is 350eV, and ion current density is 50 muA/cm²Under the conditions of (1), SiO is selected₂Evaporation time of 120 seconds and TiO₂Evaporation time of 150 seconds, i.e., the ratio of the number of argon ions to the number of molecules of the deposited film (J)_I/J_M) To SiO₂Film 0.195 to TiO₂When the film is 0.284, the gathering density of the filter can be more than 0.999 by the high and low refractive index films, and the wavelength drift of the ultra-narrow band filter is less than 0.1nm when the relative humidity of the environment is changed from 10% to 95%.

Example two

As a second example of the embodiment, the following example,the utility model discloses expect to alleviate the wavelength drift problem that ambient temperature arouses. Changes in the ambient temperature cause not only changes in the geometric thickness d of the film, but also changes in the refractive index n of the film, and thus wavelength shifts in the filter. Although the wavelength shift induced by the ambient temperature in the filter application is smaller than the shift caused by the concentration density, no good solution exists so far. For this reason, the present invention first proposes to use FEP organic plastic with a very high thermal expansion coefficient as a substrate of the ultra-narrow band filter. The principle is as follows: when the ambient temperature rises, the temperature coefficient of either the linear expansion coefficient or the refractive index can cause the central wavelength of the filter to shift towards the long wave direction, but the thermal expansion coefficient of the FEP substrate can reach 140 multiplied by 10^-6Degree/degree, at least in relation to SiO₂Film and TiO₂The film is more than 20 times higher, so the expansion of the FEP substrate is much greater than the expansion of the film, plus the substrate thickness is much greater than the film thickness: the thickness of the substrate is several millimeters, and the total thickness of all the films is only several micrometers, so that the thickness of the optical filter film is reduced by the tensile force of the expansion of the substrate in all directions during the temperature rise, and the shift to the long wave generated by the temperature rise is compensated. On the contrary, when the ambient temperature is reduced, the optical filter film can be subjected to the action of the shrinkage force of the substrate in all directions, so that the thickness of the optical filter film is thickened, and the shortwave drift generated by temperature reduction is compensated. Secondly, since the thermal expansion coefficient is usually an order of magnitude smaller than the temperature coefficient of refractive index, and the wavelength shift caused by the thermal expansion coefficient is much smaller than that caused by the temperature coefficient of refractive index, it is important to find a temperature coefficient of refractive index by which the high and low refractive index films can compensate each other. The maximum temperature difference of the outdoor environment temperature in winter and summer is assumed to be 80 ℃, namely the room temperature is 20 +/-40 ℃, and for TiO₂And SiO₂The calculation of the ultra-narrow band filter formed by the film shows that the change of the geometric thickness of the film caused by the rise of the ambient temperature from-20 ℃ to 60 ℃ only causes the shift of the central wavelength of the filter to the long wave by 0.1nm, but the change of the refractive index of the film caused by the same temperature change causes the shift of the central wavelength of the filter to the long wave by 0.3nm, and the shift is only SiO₂Temperature coefficient of refractive index of film 1X 10^-5Caused by/degree, thisBenefits from the utility model of TiO₂The temperature coefficient of refractive index of the film was-0.57X 10^-6The temperature coefficient of the negative refractive index at least reduces the drift of the optical filter caused by the temperature coefficient of the refractive index by one time, and the reasonable selection of the temperature coefficients of the refractive indexes of the high-refractive-index film and the low-refractive-index film is important.

The methods for improving the optical stability of the ultra-narrow band filter can be expanded to other various cut-off filters, band-pass filters and the like, and can be widely applied to optical, photoelectric and laser systems and instruments as stable signal filters.

Claims

1. The ultra-narrow band filter with high optical stability comprises a substrate and an ultra-narrow band filter film arranged on the substrate, and is characterized in that the film system structure of the ultra-narrow band filter film is a modified three-cavity structure: s < lambda > electrically ⁴ ⁴(HL)H2LH(LH)L ⁴ ⁴(HL)H4LH(LH)L ⁴ ⁴(HL)H2LH(LH)L '| a, where S denotes a filter substrate, a denotes an incident medium air, H denotes a high refractive index film of a quarter center wavelength thickness, L denotes a low refractive index film of a quarter center wavelength thickness, and L' is an antireflection film.

2. The ultra-narrow band filter of claim 1, wherein the high refractive index film is a titanium dioxide film, the low refractive index film is a silicon dioxide film, and the anti-reflective film is magnesium fluoride.

3. The ultra-narrow band optical filter of claim 2, wherein the coefficient of thermal linear expansion of the titanium dioxide film is 2.5 x 10^-6Degree, the thermal linear expansion coefficient of the silica film is 0.7 x 10^-6The coefficient of thermal linear expansion of the magnesium fluoride of the antireflection film is 18 multiplied by 10^-6Degree/deg.

4. The ultra-narrow band filter with high optical stability of claim 2,the temperature coefficient of refractive index of the titanium dioxide film is about-0.57 multiplied by 10^-6Temperature coefficient of refractive index of the silica film is 1 x 10^-5The temperature coefficient of the refractive index of the magnesium fluoride of the antireflection film is 2.3 multiplied by 10^-5Degree/deg.

5. The ultra-narrow band filter with high optical stability of claim 1, wherein the ultra-narrow band filter has a film system center wavelength λ₀＝530nm。

6. The ultra-narrow band filter of claim 2, wherein the refractive index of the titanium dioxide film at 530nm is 2.44, the refractive index of the silicon dioxide film at 530nm is 1.46, and the refractive index of the anti-reflective film at 530nm is 1.38.

7. The ultra-narrow band filter with high optical stability of claim 1, wherein the total number of the ultra-narrow band filter layers is 60, and the total thickness of the ultra-narrow band filter layers is 4771 nm.

8. The ultra-narrow band filter with high optical stability as claimed in claim 1, wherein the substrate is an optical plastic.

9. The ultra-narrow band optical filter of claim 1, wherein the substrate is a fluorinated ethylene propylene copolymer optical plastic with a refractive index of 1.34 at 530 nm.

10. The ultra-narrow band optical filter of claim 9, wherein the fluorinated ethylene propylene copolymer optical plastic has a coefficient of thermal linear expansion of 140 x 10^-6Degree/deg.