CN100373185C - Channel passband filters with independently adjustable relative positions - Google Patents

Abstract

The invention provides a design method capable of independently adjusting the position of the channel and the position of the pass band in the optical filter, and adopts a symmetrical structure based on the Fabry-Perot etalon. Different from the traditional narrow-band filter and band-pass filter, it independently adjusts the position of the channel and the position of the pass-band by changing the thickness of several intermediate layers, on the basis of realizing the coexistence of the channel and the pass-band The position coherence phenomenon during the position adjustment of the two is overcome. The invention introduces the design idea and specific structural design of the optical filter, and the spectral characteristics of the channel passband optical filter calculated under the design. The designed optical filter can be used in optical detection instruments, space technology and other fields.

Description

Channel passband filters with independently adjustable relative positions

technical field

The invention relates to a design method of an optical filter device, in particular to a design of an optical filter with both channels and passbands. It has application prospects in optical instruments, astronomy, remote sensing, etc.

Background technique

Traditional multi-channel bandpass filters generally have the following two types:

1. Multi-channel bandpass filter based on Fabry-Perot etalon

The most typical multi-channel bandpass filter is the Fabry-Perot etalon structure. The filter is a symmetrical structure, with reflective layers at both ends and a spacer layer in the middle. After multiple reflections of the reflective layer, by properly selecting the physical thickness of the spacer layer, this structure can obtain a bandpass with multi-channel transmission characteristics. filter, but since the positions of all channels are related to the thickness of this spacer layer, the position changes of these channels are coherent. Therefore, it is impossible to use this structure to design an optical filter whose relative positions of the channels can be adjusted.

2. Rugate type multi-channel bandpass filter

From a design point of view, perhaps the Rugate type of multi-channel bandpass filter with a continuous refractive index structure is the most attractive, because Rugate filters have a perfect mathematical transformation form. However, since the medium used in this type of multi-channel band-pass filter is required to be a graded-index material, although it can be designed in theory, it is more expensive than multi-layer dielectric multi-channel band-pass filter in terms of plating technology. film is much more difficult.

In 1987, S.John and E.Yablonovitch et al proposed the concept of photonic crystal respectively. Since one-dimensional photonic crystals are similar in structure to optical multilayer dielectric films, from the perspective of photonic crystals, through the formation mechanism of one-dimensional photonic crystal spectra, the analysis of electromagnetic mode density and photon density of states in one-dimensional photonic crystals With research, many new technologies have been formed. Inserting a defect layer into a one-dimensional photonic crystal causes a change in the photon state density in the crystal, changes the forbidden band characteristics of the one-dimensional photonic crystal, and can form a channel in the photonic forbidden band. On this basis, Wang Li and others studied the heterojunction structure of one-dimensional photonic crystals. Two materials with different dielectric constants are used to form a one-dimensional photonic crystal with different lattice constants, and a doped heterojunction structure is formed through the coupling of defect layers, and a wide cut-off is obtained by using the band gap characteristics of the heterojunction structure. bring. Due to the modulation of the energy band of the heterojunction structure by impurities, two narrow passbands can be obtained in a wide cutoff band by doping. It overcomes the shortcoming that traditional narrow-band filters cannot obtain narrow-band filtering in a wide cut-off band. And by adjusting the position and size of the defect layer, more transmission channels can be obtained on the background of the wide band gap.

One advantage of using the photonic crystal concept to design narrow-band filters is that the working band can be pre-designed. The reason is that photonic crystals have "scale invariance". If only the lattice constant is changed and other parameters are kept constant, the overall shape of the energy band structure of the photonic crystal will not change, only the peak positions and The position of the cut-off band is shifted accordingly.

The multi-channel bandpass filter based on the Fabry-Perot etalon and the heterojunction structure of the above-mentioned one-dimensional photonic crystal are difficult to independently adjust the relative position of each channel, and the coexistence of channels and passbands cannot be achieved, thus limiting the channel bandpass The scope of application of the filter.

Contents of the invention

The purpose of the present invention is to provide a multi-channel passband filter which has both channels and passbands and can independently adjust the positions of the channels and passbands.

The channel pass band filter whose channel and pass band positions can be adjusted independently proposed by the invention is a new design method based on a symmetrical multilayer dielectric film system.

In this structure, if the admittance of the medium on both sides of the intermediate layer is the same, the transmittance T is:

$\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\sqrt{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 4</span>}\sqrt{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\sqrt{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, T ₁ , T ₂ , R ₁ , and R ₂ are the transmittance and reflectance on both sides of the selected film layer, respectively, and φ ₁ , φ ₂ are the reflection phase shifts of the two reflective film layers, respectively.

It can be seen from formula (1) that if T ₁ , T ₂ , R ₁ , R ₂ and reflection phase shift φ ₁ , φ ₂ of the two reflective coating layers remain unchanged, the amount that can be changed at this time is the effective phase of the selected coating layer thickness $(\mathrm{\&delta;}=\frac{2\mathrm{\&pi;}}{\mathrm{\&lambda;}}\text{nd})\&Center\; Dot;$

When φ ₁ +φ ₂ -2δ=2kπ (k=±1, 2, 3) (2), the transmittance T of the entire film system reaches the maximum value.

In this symmetrical structure, the insertion of the middle layer causes changes in the photon state density and electromagnetic wave mode in the photonic crystal, and the multiple reflections of the reflective layers on both sides form a channel in the spectral characteristic curve. Maintaining the symmetry of the film system introduces a new film layer structure on both sides of the film system to form a new passband. Form a channel passband filter. Based on this design idea, we used two materials with different dielectric constants to form a filter with a symmetrical structure. As shown in Figure 1, where: H, L are the 1/4 wavelength optical thicknesses of high and low refractive index materials, respectively, H=n _H d _H =L=n _L d _L =λ/4, n _L =1.44, n _H =2.3 are the refractive indices of the two materials; d _H and d _L are the physical thicknesses of the two materials corresponding to the 1/4 wavelength optical thickness, respectively. Adjusting the thickness of the five intermediate layers in the double symmetric structure can change the relative position of the channel and passband in the transmission spectrum of the filter. Among them, the thickness of the four intermediate layers on the left and right sides of the two symmetrical structures is cH, which is called the intermediate layer c, and the thickness of the middle layer in the center of the symmetrical structure is dL, which is called the intermediate layer d. Adjusting c and d can independently Adjust the position of the channel and passband. The film system can be optimized by adding matching film layers.

In the present invention, SiO ₂ and TiO ₂ can be selected as two materials with different dielectric constants.

The invention is a channel passband filter device adopting an all-dielectric structure. It adopts a symmetrical structure to achieve independent and continuous changes in the positions of the channels and passbands; by adjusting the thickness of the middle layer c and d, the positions of the two channel series can be changed independently, and the position independence can be obtained by properly adjusting the thickness of the middle layer. One-dimensional photonic crystal with continuously variable channel passband. In the following, the channel-passband one-dimensional photonic crystal is taken as an example for illustration.

Description of drawings

Fig. 1 is a schematic diagram of the symmetrical film system structure of the present invention.

Fig. 2 shows the adjustment of the passband position by appropriately changing c and d while the channel position remains unchanged in the present invention.

Fig. 3 shows the adjustment of the channel position by appropriately changing c and d under the condition that the position of the passband is unchanged in the present invention.

Fig. 4 is that the present invention suitably adjusts the value of c and d, the channel that produces and the alternate change of passband position,

Detailed ways

The values of c and d of several groups of figures shown in Fig. 2 are respectively c=1.4H, d=0.8L, c=1.5H, d=0.743 5 L, c=1.6H, d=0.698L, c=1.7H , d=0.659L, c=1.8H, d=0.624L. As c is increased from 1.4 to 1.8 and d is properly adjusted, the position of the left passband gradually moves to the right, while the right channel remains unchanged at the original position.

In several groups of graphs shown in Figure 3, c=1.4H remains unchanged, and as the value of d increases from 0.2L to 0.7L, the position of the passband on the left remains basically unchanged. While the right channel moves gradually to the right.

As shown in Figure 4, the values of c and d corresponding to the passband spectral characteristics of these channels are c=1.5H, d=0.4L, c=1.5H, d=0.5L, c=1.6H, d =0.472L, c=1.6H, d=0.6L, c=1.7H, d=0.567 7 L, c=1.7H, d=0.7L, c=1.8H, d=0.662L, c=1.8H, d=0.8L, c=1.9H, d=0.755L, with different combinations of c and d, the position of the channel and the passband can be changed alternately.

The specific method for the design and adjustment of the dual-channel position of the filter is as follows:

To design a channel band-pass filter with a symmetrical structure, firstly, according to the position of the required cut-off band, determine the size of the lattice constant, that is, the single-layer optical thickness of the reflective film stack on both sides of the middle layer of the symmetrical structure. Take the design shown in Figure 2 as an example, in this design, the cut-off band width is 260nm, the wavelength is 600nm, and the film structure of the optical filter is a double symmetrical structure based on a symmetrical multilayer dielectric film system. The structure is (HL) ² cH(LH) ² L(HL) ² cH(LH) ² dL(HL) ² cH(LH) ² L(HL) ² cH(LH) ² , where c and d represent the middle layer thickness. After the position of the cut-off band is determined, then determine the size of c and d according to the position of the required channel and passband; first determine the channel, and calculate the reflection phase shift of the reflective layers on both sides of the middle layer d of the symmetrical structure at the wavelength of the channel , by formula (2) to obtain the size of d. The same method is used to determine the passband, but the reflection phase shift calculated at this time is produced by the film system (HL) ² cH(LH) ² , that is, a symmetrical structure with a c layer on the left and right sides as the center of symmetry. of. Obtain the reflective phase shift of the wavelength where the passband is located, and obtain the size of c through formula (2). It can be found from computer simulation that the position of the channel is mainly determined by the size of d, the position of the passband is mainly determined by the size of c, and the positions of the channel and the passband can be continuously changed. Since the calculation of c is to simplify the calculation, the calculation object is the ((HL) ² cH(LH) ² symmetrical structure rather than the entire symmetrical film system. Therefore, c can be adjusted on the computer to make the position of the passband match the design requirements The thicknesses of all the designed layers have been determined, and the materials used can be selected according to actual conditions. We choose TiO ₂ and SiO ₂ , and the incident medium is air ε=1. The medium pair composed of _{TiO 2} and SiO ₂ is impurity , using the transfer matrix method, by adjusting the thickness of the intermediate layer, the optical filter with the required channel and passband parameters is obtained.

The feature of the present invention is that the positions of the two channels can be controlled by two parameters c and d, which can be changed independently, and the positions of the two channels can be adjusted arbitrarily within the cut-off band.

1. Adjust the position of the passband without changing the position of the channel:

The following one-dimensional photonic crystals with the structure (HL) ² cH(LH) ² L(HL) ² cH(LH) ² dL(HL) ² cH(LH) ² L(HL) ² cH(LH) ² are For example, the design wavelength λ=600nm, adjusting the size of c and d can be obtained as

As shown in Figure 2, the passband can be adjusted without changing the position of the channel

s position.

2. Adjust the position of the channel without changing the position of the passband:

By adjusting the size of c and d, the changes shown in Figure 3 can be obtained, and the position of the channel can be adjusted while the position of the passband remains unchanged.

3. The position of channel and passband changes alternately:

Appropriately adjusting the values of c and d can make the position of the channel and the passband change alternately, as shown in Figure 4.

Claims (2)

1. channel passband filter that relative position can independently be adjusted is characterized in that:

The dura mater based material of optical filter film is TiO ₂And SiO ₂Combination, the structure of component film system is for being the disymmetry structure on basis with symmetrical multilayer dielectric film, this disymmetry structure is:

(HL) ²CH (LH) ²L (HL) ²CH (LH) ²DL (HL) ²CH (LH) ²L (HL) ²CH (LH) ², wherein, passage and passband see through the position at peak and are adjusted by the thickness of middle layer c in the structure and middle layer d, and wherein, the thickness of middle layer c is cH, and the thickness of middle layer d is dL.

2. the channel passband filter that relative position according to claim 1 can independently be adjusted is characterized in that:

For the position that makes passage and passband coincide with design, the accurate thickness of middle layer c and middle layer d is finely tuned by computer Simulation calculation.

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