CN1996063A - Film narrow band-pass reflective filter - Google Patents

Abstract

The invention discloses a novel thin-film narrow-band reflective filter. It consists of high and low refractive index films and an extremely thin metal film. Its basic structure is: starting from the substrate side, there are alternating high and low refractive index film stacks with a high number of periods - spacer film - several periods of alternating high and low refractive index film stacks - an extremely thin metal film to the incident medium, and the incident light from the incident light The medium is incident on a thin metal film. Utilizing an extremely thin metal film, a narrow FWHM, high peak reflectivity and deep reflection cutoff are achieved. The invention has an intuitive structure, convenient design, and can freely adjust the peak reflectivity and half-width according to actual needs, and the incident light loss is small, the manufacturing process is convenient, the manufacturing cost is low, and it is easy to integrate. It is suitable for optical wavelength division multiplexing, display and other optical systems.

Description

A New Thin Film Narrow Band Reflective Filter

technical field

The invention relates to a novel thin-film narrow-band reflective filter.

Background technique

In the early 1990s, research on optical network communication systems has become the focus of national and transnational research programs. Asian, European and American countries have formulated various research programs. At the same time, various academic groups and standardization organizations have also actively carried out optical Communication, especially research on optical wavelength division multiplexing systems. In the optical wavelength division multiplexing system, the operation and processing of single wavelength light is indispensable everywhere, so the optical wave demultiplexer becomes one of the most critical devices in this system. Therefore, the dielectric film filter type wavelength division multiplexer represented by the Fabry-Perot filter has been widely used in various fields such as optics and astrophysics because of its high peak transmittance and ultra-wide cut-off area. a wide range of applications. However, since most of these filters are transmissive, the arrangement in the system takes up more space and is not easy to integrate; in addition, in order to obtain high peak transmittance and narrow half-width, these filters have to be multiplied The number of cycles of the reflective film stack and the greatly increased requirements for the symmetry of the film system have caused difficulties in plating.

Contents of the invention

The purpose of the present invention is to provide a novel thin-film narrow-band reflective filter, which can realize the demultiplexing effect on reflected light waves.

The thin-film filter is composed of an extremely thin metal film-high and low refractive index overlapping film layers, a low reflectivity reflective film stack-spacer film-high reflectivity (more cycles) high and low refractive index overlapping composition The reflective film stack constitutes a thin film device. The so-called Gires-Tournois (GT) resonant cavity is composed of a reflective film stack with low reflectivity, a spacer layer (which can be regarded as a resonant cavity) film and a high reflective film stack, so the device can be considered as a GT plated on the substrate. cavity, and then an extremely thin metal layer is added between the incident medium and the GT cavity.

The invention adopts the traditional thin film deposition process for preparation, the method is simple, the production cost is low, and the integration is easy. The whole GT cavity is completely made of high and low refractive index dielectric materials, which has the characteristics of low insertion loss, completely passive, and no need for temperature control. The invention is a reflective device, which can realize the effects of filtering, color separation and demultiplexing of reflected light waves when light beams of different wavelengths are incident, and is easy to arrange in an optical system. In addition, the present invention has an intuitive structure and convenient design, and can freely adjust the peak reflectivity and FWHM according to actual needs, so as to obtain the highest possible peak reflectivity, extremely narrow FWHM and deep broadband cutoff. The application prospects in systems such as dense wavelength division multiplexing, optoelectronic display and optical information processing will be very broad.

Description of drawings

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Fig. 1 is a structural schematic diagram of a thin-film narrow-band reflective filter;

Fig. 2 is a schematic diagram of the reflectance spectrum curve of a theoretically designed thin-film narrow-band reflective filter;

Fig. 3 is the reflectance spectrum curve schematic diagram of actual plated thin-film narrow-band reflective filter;

Fig. 4 is a schematic diagram of wavelength demultiplexing application of thin-film narrow-band reflective filter;

Fig. 5 is a schematic diagram of the application of thin-film narrow-band reflective filters in the field of color display;

In the figure: 1. Incident medium, 2. Metal film, 3. Several periods of alternating high and low refractive index film stacks, 4. Spacer film, 5. High period high and low refractive index alternating film stacks, 6. Substrate , 7, four thin-film narrow-band reflective filters with different central wavelengths, 8, thin-film narrow-band reflective filters with central wavelengths corresponding to red, green and blue primary colors respectively.

Detailed ways

The thin-film narrow-band reflective filter is first coated with alternating high and low refractive index film stacks 5 with a high number of cycles on the side of the substrate 6, and then coated with a spacer layer film 4-several periods of high and low refractive index alternating film stacks 3-extremely thin The metal film 2 enters the incident medium 1, and the incident light enters the thin metal film from the incident medium.

The invention can realize the effect of demultiplexing reflected light waves. The device is composed of alternating high and low refractive index multi-layer dielectric optical interference films and metals, and its film structure can be expressed as: substrate-(HL) ^m2 H 2L(HL) ^m1 M-incident medium. M stands for metal, and chromium (Cr) is generally used, because the real part ( _nc ) and imaginary part (k _c ) of the complex refractive index of this metal are relatively close, and the incident light will not be reflected too much when it is first incident. Go back, and can achieve strong absorption of incident light at non-central wavelengths, thereby obtaining a deep broadband cut-off. However, it should be noted that this layer of metal needs to be controlled at the nanometer level, otherwise most of the incident light will be directly absorbed by the metal, and high peak reflectivity cannot be obtained. H and L represent high and low refractive index media respectively, the optical thickness is 1/4 of the central wavelength, and the refractive indices are expressed as n _H and n _L , which can be freely selected according to actual needs. m1 and m2 represent the period numbers of the reflective film stack. For the present invention, m1 is generally small, while m2 needs to be large to achieve a sufficiently high reflectivity. In this structure, the light wave of the central wavelength continuously vibrates back and forth in the spacer film (that is, the resonant cavity) and stores a large amount of energy. After the energy is emitted, it is reflected back by the rear high-reflection film, and the returned field strength is due to The effect of thin-film interference must reach an extreme value on the interface between the metal layer and the adjacent reflective film stack. As long as the extreme value at this time is controlled to be a minimum value, that is, the field strength entering the metal layer reaches a minimum, because the metal The size of the layer is only on the order of nanometers, so it cannot strongly absorb light waves. According to the law of energy conservation, most of the light waves are reflected back to the incident interface, reaching a high peak reflectivity. For the non-central wavelength light, since the interference minimum value of the field strength cannot be reached at the interface between the metal layer and the reflective film stack, a large amount of energy is bound to be absorbed by the metal layer, thus forming a deep broadband cut-off.

With the help of thin-film optical characteristic matrix principle and perturbation theory, the peak reflectivity R ₀ and half-maximum width FWHM of thin-film narrow-band reflective filter can be approximated, expressed as:

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$\mathrm{<span\; class="notranslate">\; wxya</span>}<span\; class="notranslate">\; \&ap;</span>\frac{\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}{<span\; class="notranslate">\; [</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; twenty\; four</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="no"\; filenames="A20061005394400081.png"\; state="\{\{state\}\}">no</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="no"\; filenames="A20061005394400081.png"\; state="\{\{state\}\}">no</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; 0.5</span>}}}$

in $A=\frac{{\mathrm{no}}_L}{1+\frac{{2\mathrm{no}}_L}{{\mathrm{no}}_h-{\mathrm{no}}_L}[1-{\left(\frac{{\mathrm{no}}_h}{{\mathrm{no}}_L}\right)}^{2(m_1+1)}]}.$

In the formula, n ₀ is the refractive index of the incident medium, d is the thickness of the metal film, λ ₀ is the central wavelength, and the optical thickness of the high refractive index and low refractive index film layers is 1/4 of this wavelength. It is found that when the period number m2 of the film stack is much greater than m1, the peak reflectivity R ₀ can basically be considered to reach 100%; and when the high-low refractive index ratio n _H /n _L or the period number m1 is sufficiently large, the narrowest possible reflectivity can be achieved. width at half height. Therefore, by properly selecting various parameters in the present invention, the requirements of different usage environments can be met.

Figure 2 shows a schematic diagram of the reflectance spectrum curve of the theoretically designed thin-film narrow-band reflective filter. The central wavelength is 700nm, the substrate is BK7 (refractive index 1.49), M is Cr, the complex refractive index at the central wavelength is (3.84-i4.37), the thickness is 3nm, H is TiO ₂ (refractive index 2.19), L SiO ₂ (refractive index 1.45) is selected, m1 is taken as 3, m2 is taken as 7, and the physical thickness of the entire optical filter is 2.35 μm. Figure 3 shows a schematic diagram of the reflectance spectrum curve of the thin-film narrow-band reflective filter actually plated according to the theoretical design. The plated result is R ₀ =91.3%, FWHM=11nm, which is consistent with the theoretical result, and the errors are respectively 3.1% and 10%, compared with the narrow-band transmission filter that can achieve the same effect and the mature dielectric reflection filter in the past, the physical thickness of the present invention is thinner, and the requirement for structural symmetry is not very high; Compared with the metal induction filter, since the reflection peak of the present invention is not generated by the induction metal, there is no need to add an induction layer next to the metal, which requires precise control of the thickness of the film layer, which is easier to monitor and manufacture.

The thin-film narrow-band reflective filter is a reflective optical element, which can only achieve the filtering effect on reflected light waves. It should be especially noted that when the film system is designed according to the present invention, the central wavelength (LH) ^m1 2L(HL) ^m2 The admittance of the H-substrate needs to be as large as possible, so as to ensure that the interference field strength at the interface between the metal layer and the reflective film reaches a minimum value and achieve high peak reflectivity.

The invention has great application prospects in many fields such as optical communication, display, photography, optical information processing and the like.

Embodiment 1: Demultiplexing application of thin-film narrowband reflective filter in optical communication wavelength division multiplexing system

As shown in Figure 4, 7-1, 7-2, 7-3, and 7-4 represent four thin-film narrow-band reflective filters with different center wavelengths. When the wavelength of λ ₁ -λ ₄ is incident, the four filters The chip will reflect the light waves corresponding to its center wavelength respectively, thus realizing the demultiplexing effect of four channels.

Example 2: Application of thin-film narrow-band reflective filter in the field of color display

As shown in Figure 5, 8-1, 8-2, and 8-3 respectively represent narrow-band reflective filters whose central wavelengths correspond to the three primary colors of red, green, and blue. When a wide spectrum is incident, these filters reflect the three primary colors. The three primary colors can be separately modulated or combined again to achieve the required color display.

Claims (6)

1. A novel thin-film narrow-band reflective filter, characterized in that it is composed of alternating high and low refractive index dielectric films and extremely thin metal films. 2. The thin-film narrow-band reflective filter according to right 1 is characterized in that the basic structure is that from the substrate side, it is a high and low refractive index alternating film stack with a high number of cycles—spacer layer thin film—several periods of high and low refractive index Alternating film stack—an extremely thin metal film to the incident medium, the incident light is incident from the incident medium to the thin metal film, which can be expressed as: substrate-(HL) ^m2 H2L(HL) ^m1 M-form of incident medium, where H, L represents the high and low refractive index material film layer respectively, m1 and m2 are the periodic numbers of the alternating high and low refractive index film stacks respectively, m1 is small, m2 is large, (HL) ^m2 H represents the alternating high and low refractive index film with high periodic number Stack, 2L stands for spacer film, (HL) ^m1 stands for several periods of alternating high and low refractive index film stacks, and M stands for extremely thin metal film. 3. The thin-film narrow-band reflective filter according to right 1, characterized in that the real part and the imaginary part (absorption coefficient) of the metal complex refractive index are close, and the metal layer is extremely thin, which needs to be controlled at the nanometer scale. 4. The thin-film narrow-band reflective filter according to claim 1 can obtain very high peak reflectivity, very narrow FWHM and deep cut-off, and is suitable for various optical density wavelength division multiplexing, display, etc. occasion. 5. According to the thin-film narrow-band reflective filter described in right 1, adjusting the peak reflectivity mainly depends on m1 and m2 to adjust, and the half-width can be adjusted by the thickness 2L of the interval cavity, the number of cycles ml or the ratio of high and low refractive indices Adjustment. 6. The thin-film narrow-band reflective filter according to claim 1, characterized in that the device is basically composed of dielectric materials, a wide range of material selection, low insertion loss, convenient arrangement, and easy integration.

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