JP2005114812A - Composite dielectric multilayer film filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a film structure of a filter by combining a plurality of filters which do not affect each other. <P>SOLUTION: A required wavelength characteristic is obtained by combining a bandpass filter part 12 which transmits a predetermined wavelength band and an edge filter part 14 which transmits longer or shorter wavelength band than the predetermined wavelength. Further, a buffer layer 13 having an arbitrary film thickness represented by an integer multiple of optical film thickness of a quarter wavelength is provided between respective filter parts. Thus, a ripple due to an interference is prevented in a reflection band. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a dielectric multilayer filter used in an optical communication module, and more particularly to a composite dielectric multilayer filter in which a plurality of filters are combined.

  In the optical communication field, light in the infrared region is used, and the most used in this wavelength region is the band of 1.3 μm to 1.6 μm due to the transmission characteristics of the optical fiber. Conventional band-pass filters are mainly used for wavelength separation only in the 1.55 μm band.

Now, a dielectric multilayer structure for providing the wavelength selectivity described above will be described below. According to the optical filter film design described in Non-Patent Document 1 or the like, if the refractive indexes of the dielectric thin films H and L having two different refractive indexes are n _H and n _L , respectively, a certain design wavelength λ The film structure of the dielectric multilayer mirror having a high reflectivity centering on ₀ is given by the following equation.
Substrate / H L H L. . . . H L / Medium (1)
Here, the film thicknesses d _H and d _L of _H and _L are optical film thicknesses of a quarter wavelength, and are given by the following equations.
d _H = λ ₀ / 4n _H (2)
d _L = λ ₀ / 4n _L (3)
The uppermost layer L is in contact with the medium. In general, the medium may be air, a resin solvent, a solid substrate, or the like.
Formula (1) represents that two types of dielectric thin films H and L are alternately stacked on a certain substrate. The formula (1) can be simply expressed as the following formula.
Substrate / (HL) ⁿ / Medium (4)
Here, the equation (4) means that the layer of the HL pair is repeatedly stacked n times.
That is,
Substrate / H L H L H L / medium (5)
Is the substrate / (HL) ³ / medium (6)
Is equivalent to In order to show the film structure A of the dielectric multilayer film, it is expressed as follows.
A = (HL) ³ (7)
Next, a general film configuration of the bandpass filter will be described. First, the basic resonator structure of a bandpass filter can be expressed as: A conceptual diagram of the cross section is shown in FIG.
A = (HL) ⁿ H s L H (LH) ⁿ L (8)
Here, n is an integer (0, 1, 2, 3,...), And s is a positive even number (2, 4, 6, 8,...). A dielectric multilayer film 2 is formed on the substrate 1. (HL) ⁿ H is the first stack layer 3, sL is the spacer layer 4, and H (LH) ⁿ is the second stack layer 5. The last L layer is called a coupling layer 6.

FIG. 8 shows a conceptual diagram of a cross section of a dielectric multilayer bandpass filter that is generally used conventionally. This filter has a multiple resonator structure 8 in which basic resonator structures 2A to 2C similar to the expression (8) are superimposed on a substrate 7, and can be expressed as the following expression.
^{A = [(HL) n H} s m L H (LH) n L] m (9)
Here, n is an integer (0, 1, 2, 3,...), And m is a natural number (1, 2, 3,...). s _m is often different for each basic resonator structure, respectively positive even number (2, 4, 6, 8, ...). (HL) ⁿ H is a first stack layer, s _m L is a spacer layer, and H (LH) ⁿ is a second stack layer. The last L layer is a bonding layer.

Next, the basic film structure of the high-pass type edge filter can be expressed by the following equation.
A = [0.5H L 0.5H] ⁱ (10)
The basic film structure of the low-pass edge filter can be expressed by the following equation.
A = [0.5L H 0.5L] ⁱ (11)
In order to improve the flatness of the transmission band with these basic structures, using the commercially available film design software such as TF-Calc, the following general formula is obtained when all layers are made to have an arbitrary film thickness.
A = [α _j H β _j L] ^j (12)
Here, i and j are the number of times of lamination, and as the value increases, the rise at a predetermined wavelength becomes steeper.

In the wavelength division multiplexing optical communication system, when the light of the specified wavelength is transmitted while cutting the light of the adjacent wavelength, the bandpass filter of the multi-resonator structure has a good rise at a predetermined wavelength and good flatness. It is used. And when trying to use the 1.3μm band and 1.55μm band at the same time, when using a band opposite to the wavelength band set as the transmission band, for example, 1.55μm band, even the reflected waveform in the 1.3μm band is flat. It is desirable to have a good filter design.
However, the conventional bandpass filter has a limit of about 100 nm in the blocking region.

  The transmission optical characteristics of the CWDM filter designed by the conventional design method are shown in FIG. 9A, and the reflection optical characteristics are shown in FIG. 9B. The horizontal axis represents wavelength [nm], and the vertical axis represents transmittance [dB] and reflectance [dB]. The incident angle is 1.25 °, and the incident medium is a resin adhesive. The graph C1 in FIG. 9A is a graph with a left scale, and C2 is a graph with a right scale. In the figure, optical communication is required to be higher than A1 (left scale), A2 (right scale), A5, and A6 and lower than A3 and A4. As shown in FIG. 9 (a), there is no problem with the conventional design method if the use is limited to around 100 nm, such as use only in the 1.55 μm band. However, when trying to use a band 100 nm or more apart, such as using the 1.3 μm band and the 1.55 μm band at the same time, a transmission band with poor flatness called a sideband is generated, making it difficult to use. For example, as shown in FIG. 9B, when the 1.55 μm band which is the wavelength band set as the transmission band is used, a sideband is generated in the other 1.3 μm band. Therefore, the 1.3 μm band and the 1.55 μm band cannot be used simultaneously.

  In the conventional method, when the stop band is wide as described above, there is a method of using a band filter optimized by combining a high-pass edge filter and a low-pass edge filter. As a special example, Patent Document 1 proposes forming filters on both sides of a substrate. However, with this method, it is impossible to achieve the wavelength accuracy required in optical communication, and it is difficult to satisfy the required value for the rise of the spectrum called crosstalk.

  As another method, there is a method of suppressing sidebands by using an absorption filter on the film formation substrate. As a special example in this case, Patent Document 2 proposes to widen the blocking region by combining with a light absorption film. However, in this case, since the blocking area is cut by absorption, the reflected light cannot be used. Thus, neither method can be used for optical communication.

Therefore, two types of films may be provided on the filter surface. However, if they are simply overlapped, the influence of interference occurs in the filter stop band, and the reflection band is not flat. If only the transmission is used, it is possible to apply these conventional examples, but the structure becomes complicated, the use of a plurality of filters, and the use of special materials become expensive. There are disadvantages.
JP-A-5-249313 JP 2000-352612 A THIN-FILM OPTICAL FILTERS (Author: HA Macleod, published in 1986)

  An object of the present invention is to realize a dielectric multilayer filter having an arbitrary wavelength characteristic such as expansion of a stop band / reflection band by combining a plurality of dielectric multilayer filters.

  According to the first aspect of the present invention, the first to n-th filter parts (an integer of n ≧ 2) configured by two or more kinds of dielectric laminated structures having different refractive indexes, and a plurality of these filter parts And a buffer layer made of a dielectric thin film material having an arbitrary thickness.

  The invention according to claim 2 of the present application is the composite dielectric multilayer filter according to claim 1, wherein the i-th filter section (i = 1 to n) transmits only a specific wavelength region. , And any one of the filters selected from the edge filter that transmits a wavelength region of a long wave or a short wave from a specific wavelength.

  According to a third aspect of the present invention, in the composite dielectric multilayer filter according to the first aspect, the number n of the filter portions is 2, and the first filter portion is a band that transmits only a specific wavelength region. The second filter unit is an edge filter unit that transmits a wavelength region longer or shorter than a specific wavelength, and the band-pass filter unit includes at least one basic resonator structure. The basic resonator structure is composed of a laminated structure of a dielectric thin film material having a film thickness that is an odd multiple of the optical film thickness of a quarter wavelength of the transmission wavelength and a dielectric thin film material having a lower refractive index. Any of the optical thin film layers sandwiched between the first and second stack layers and the first and second stack layers and having an even multiple of the optical thickness of the quarter wavelength. Spare consisting of Characterized in that it comprises a support layer.

  According to a fourth aspect of the present invention, in the composite dielectric multilayer filter according to the first aspect, the number n of the filter portions is 2, and the first filter portion has two or more kinds of dielectrics having different refractive indexes. A high-pass edge filter unit that is configured by a laminated structure of body thin films and transmits light having a wavelength shorter than a specific wavelength (λ1). The second filter unit includes two or more types having different refractive indexes. It is a low-pass type edge filter section that transmits light having a wavelength longer than another specific wavelength λ2 (λ1> λ2), which is configured by a laminated structure of dielectric thin films.

  The invention according to claim 5 of the present application is the composite dielectric multilayer filter according to any one of claims 2 to 4, wherein the optical film thickness of the buffer layer is a quarter of the transmission wavelength of the bandpass filter. It has a film thickness that is an integral multiple of the optical film thickness of the wavelength.

  The dielectric multilayer filter of the present invention having such characteristics can combine a plurality of filter portions without mutual interference, and can realize characteristics that cannot be obtained independently. High-performance dielectric multilayer suitable for coarse wavelength division multiplexing (CWDM) network systems by combining multiple filters such as edge filters such as long wavelength pass filters and short wavelength pass filters and band pass filters A membrane filter structure can be realized.

  The inventor has found a filter design in which a plurality of bands can be used simultaneously by combining a plurality of filters as shown in the following formula and forming a film structure including a buffer layer to prevent interference between them in the multilayer filter. It was. For example, when trying to use the 1.3 μm band and 1.55 μm band at the same time, when using a band opposite to the wavelength band set as the transmission band, for example, the 1.55 μm band, the flatness should be improved even in the 1.3 μm band. Can do. The film configuration is shown in Equation 12, and a conceptual diagram of the cross section is shown in FIG.

This composite multilayer filter is obtained by laminating a bandpass filter unit 12, a buffer layer 13, and an edge filter unit 14 having a multi-resonator structure on a substrate 10. The bandpass filter unit 12 is a generalization of the above-described configuration and has an m-fold resonator structure. X is a dielectric thin film of a high refractive index material or a low refractive index material having a film thickness that is an odd multiple of the optical film thickness of a quarter wavelength of the transmission wavelength of the bandpass filter, and Y is a refractive index material of other refractive index materials. It is a dielectric thin film having a film thickness that is an odd multiple of the optical film thickness of a quarter wavelength. Here, Z is a buffer layer 13, which is a dielectric thin film having an arbitrary refractive index material and a film thickness that is an integral multiple of a quarter wavelength film thickness. Similarly, H is a dielectric thin film of a high refractive index material having an optical film thickness of a quarter wavelength of the transmission wavelength of the bandpass filter, and L is a dielectric thin film having a quarter wavelength film thickness of a low refractive index material. It is a body thin film. That is, X and Y and H and L have different refractive indexes. Here, a _{1 to} a _m , b _{1 to} b _m , c, α, β, γ, e _j , f _j , m, and j have the following values.
a _{1 to} a _m 0, 1, 2, 3, 4,.
b _{1 to} b _m 0,2,4,6,8, ...
c 0, 1, 2, 3, 4, 5 ...
e _{1 to} e _j , f _{1 to} f _j 0 or more real numbers α, β, γ 0 or more real numbers m, j 1, 2, 3, 4,.
Note that a _{1 to} a _m and b _{1 to} b _m are often different for each basic resonator, and e _{1 to} e _j and f _{1 to} f _j are often different for each layer.

  In the dielectric multilayer bandpass filter of the present invention, a wide range of materials (for example, optical crystal, optical glass, quartz, transparent plastic, polymer material, etc.) that are transparent in the used wavelength range can be applied to the substrate 11. it can.

The optical thin film material constituting the dielectric multilayer bandpass filter can be selected from dielectric materials that are transparent in the applicable wavelength range and are generally used at present. For example, calcium fluoride CaF ₂ (refractive index, hereinafter the same 1.23), magnesium fluoride MgF ₂ (1.38), silicon dioxide SiO ₂ (1.46), magnesium oxide MgO (1.80), pentoxide Tantalum Ta ₂ O ₅ (2.13), niobium pentoxide Nb ₂ O ₅ (2.25), titanium dioxide TiO ₂ (2.45), zinc selenide ZnSe (2.40), lead telluride PbTe (5 .67), aluminum nitride AlN (1.94), silicon nitride Si ₃ N ₄ (1.95), silicon Si (3.4), germanium Ge (4.0), and the like.

  These optical thin films are, for example, electron beam vapor deposition, ion assisted vapor deposition, DC magnetron sputtering, RF magnetron sputtering, ion plating, ion beam sputtering, molecular beam epitaxy, chemical vapor deposition, dip coating. A conventionally used manufacturing method can be used. In designing the multilayer film constituting the filter, commercially available film design software can be used. For example, TF-Calc (Software Spectra, Inc.), Essential Macload (Thin-Film Center, Inc.), etc. are mainly used in the optical communication industry.

  In this way, with only a normal bandpass filter having a transmission band of 1.55 μm band, the band opposite to the wavelength band set as the transmission band, for example, 1.3 μm band when using the 1.55 μm band, has poor flatness. A transmission band is generated. In order to prevent this, as shown in FIG. 1B, in addition to the band-pass filter unit 12 in the 1.55 μm band, for example, a low-pass type edge filter unit 14 having a cutoff wavelength of 1.4 μm is used to obtain 1.3 μm. Cut off the belt. In this case, the interference can be prevented by providing the buffer layer 13.

  FIG. 2 shows a configuration example of a communication WDM module as an example of use in that case. In FIG. 2A, a cylindrical capillary 23 and a GRIN lens 24 are connected to the incident optical fiber 21 and the outgoing optical fiber 22, and the bandpass filter portion and the edge filter portion described above are combined at the ends thereof. The composite multilayer filter 10 is connected. The capillary 23 is provided with an optical fiber 21 a core connected to the optical fiber 21 and an optical fiber core 22 a connected to the optical fiber 22. Further, a GRIN lens 25 and a capillary 26 are connected in the vicinity of this filter, and an optical fiber 27 for ADD and DROP is provided. A multilayer filter of a band pass filter and an edge filter is formed on one surface 10a of the filter 10 in the figure, and the transmission wavelength can be limited. An AR coat is formed on the other surface 10b of the filter. When wavelength multiplexed light is incident on the optical fiber 21, the diameter of the light is expanded by the GRIN lens 24 through the optical fiber core 21a of the capillary 23, and passes through the filter unit. Other light is reflected and emitted from the optical fiber 22. At this time, the light transmitted through the band pass filter is transmitted through the GRIN lens 25 and emitted (DROP) from the optical fiber 27. When light having the same wavelength is incident (ADD) from the optical fiber 27, it passes through the composite multilayer filter 10 through the GRIN lens 25, is focused by the GRIN lens 24, and is added to the optical fiber 22. Thus, it can be used as an optical module for ADD and DROP in wavelength division multiplexing optical communication.

  In this embodiment, a configuration in which a band pass filter unit and an edge filter unit are connected via a buffer layer is shown. However, a composite dielectric multilayer filter is configured by combining two different edge filter units. Can do. In this case, the cut-off wavelength λ1 of the high-pass type edge filter and the cut-off wavelength λ2 of the low-pass type edge filter are made different so that λ1> λ2 is selected so that light can pass in the range of λ1 and λ2. It can be configured as a band pass filter.

  In this embodiment, two filter sections are connected via a buffer layer. However, a composite filter in which three or more filter sections are connected via a buffer layer can be used. Furthermore, the buffer layer is easy to be an integral multiple of an optical thin film with a quarter wavelength of the selected wavelength, but any optical film thickness at any wavelength can be used as long as it is capable of suppressing sharp dip in the reflection band due to interference. Can be configured.

(Example 1)
As a first embodiment of the composite dielectric multilayer filter according to the present invention, a configuration example of a bandpass filter for a coarse wavelength division multiplexing (CWDM) optical communication network system is shown. This band pass filter 30 is obtained by coupling a band pass filter unit 32 and a low pass type edge filter unit 34 on a substrate 31 using a buffer layer 33. The transmission wavelength of the band pass filter unit 32 is set to 1550 nm, and the 1.3 μm band is reflected. As a substrate, a crystallized glass WMS-15 substrate manufactured by OHARA INC. Is used, and a Ta ₂ O ₅ film (refractive index) is used as a dielectric thin film H having a high refractive index in the band pass filter unit 32 and the low pass type edge filter unit 34. n _H = 2.15) and a SiO ₂ film (refractive index n _L = 1.46) was used as the dielectric thin film L having a refractive index. The film configuration of this example is shown in Formula 13.

  Here, 6H at the center of Expression 13 is the buffer layer 33, and the portion closer to the substrate than this constitutes the band-pass filter portion 32, and the outside of the buffer layer 33 constitutes the edge filter portion 34.

  At the same time, a band-pass filter 35 in Expression 13 excluding the buffer layer 33 was prepared as a comparative example. The transmission characteristics and reflection characteristics of this comparative example are shown in FIGS. 3 (a) and 3 (b). In optical communication, it is required to be higher than B1, B2, and B4 and lower than B3 and B5 in the figure. The full width at half maximum of the bandpass filter is 18 nm. The center wavelength of this filter is 1550 nm, the transmission band region of the filter is excellent in flatness, low insertion loss, and sufficient characteristics in the transmission band. However, as can be seen from FIG. 3A, in the reflection characteristics, there is a sharp dip due to interference near 1450 nm, and there is a drop in the amount of reflected light that is difficult to use in optical communications.

  Next, the transmission band characteristics and reflection characteristics of the composite dielectric multilayer filter of Example 1 are shown in FIGS. 4 (a) and 4 (b). B1 to B5 in the figure are levels required for optical communication as in FIG. The transmission characteristics were sufficient as in FIG. In addition, it can be seen that the reflection effect is reduced by including the buffer layer 33, and the 1450 nm dip is at a level causing no problem in optical communication.

(Example 2)
Next, as a second embodiment of the present invention, a configuration example of a broadband color filter that transmits light in the visible range is shown. This color filter is configured by combining a high-pass type edge filter unit and a low-pass type edge filter unit using a buffer layer to form a broadband band-pass filter. The substrate used was # 0200 (refractive index 1.53) manufactured by Matsunami Glass Co., Ltd. In the edge filter, a TiO ₂ film (refractive index n _H = 2.45) is used as the dielectric thin film H having a high refractive index, and an MgF ₂ film (refractive index n _L = 1.4) is used as the dielectric thin film L having a refractive index. 38) was used. For the buffer layer A, a SiO ₂ film (refractive index n _A = 1.46) was used. The film configuration of this example is shown in Formula 14. In Expression 14, H and L are optical film thicknesses of a quarter wavelength with respect to the center wavelength of 550 nm, respectively.

  Here, in Expression 14, the center edge filter section 42, the buffer layer 43, and the edge filter section 44 are formed on the substrate 41.

  Further, in the above-described formula 14, a composite dielectric multilayer filter 45 excluding the buffer layer 43 was prepared as a comparative example. The transmission characteristics of the filter of this comparative example are shown in FIG. In the figure, T1 is a graph with a left scale, and T2 is a graph with a right scale. In the figure, C1 to C4 are levels required to be higher than C1 and C2 and lower than C3 and C4 in the visible color filter. The characteristics of the composite dielectric multilayer filter of Example 2 are shown in FIG. In the filter 46 of the comparative example, a peak due to interference occurs in the transmission blocking region as shown in FIG.

  The wavelength at which the transmittance is 50% at the rising portion of the transmission spectrum of the high-pass edge filter is called the cut-on wavelength. In FIG. 6, the cut-on wavelength of the high-pass part is 470 nm. The wavelength at which the transmittance is 50% at the falling edge of the transmission spectrum of the low-pass edge filter is called the cutoff wavelength. In FIG. 6, the cutoff wavelength of the low-pass portion is 585 nm, and the transmission bandwidth is 115 nm. As shown in FIG. 6, the filter of the second embodiment including the buffer layer 43 can be synthesized while maintaining the respective waveforms, and a wide bandpass filter can be configured.

  The composite dielectric multilayer filter according to the present invention can combine a plurality of filters without interference. For this reason, it can be used for optical communication such as a band-pass filter when the 1.3 μm band and 1.55 μm band used in communication are used together. It can also be applied to the color filter portion of the projector.

(A) is a figure which shows the composite filter structure of the m double resonator type | mold band pass filter and edge filter by embodiment of this invention, (b) is a graph which shows the wavelength characteristic. 1 is a configuration example of an optical communication WDM module according to an embodiment of the present invention. 10 is an optical characteristic of a composite multilayer filter combining a CWDM 10-layer resonator bandpass filter and a high-pass edge filter according to Comparative Example 1; FIG. 6 is an optical characteristic of a composite filter for CWDM in which a 10-fold resonator structure bandpass filter and a high-pass edge filter according to Example 1 are combined. FIG. It is an optical characteristic of the composite filter which combined the edge filter by the comparative example 2. FIG. It is an optical characteristic of the composite filter which combined the edge filter by Example 2. FIG. This is a basic resonator structure of a conventional dielectric multilayer bandpass filter. It is a conventional m-fold resonator structure dielectric multilayer bandpass filter. It is an optical characteristic of the 10-fold resonator structure band pass filter for CWDM optical communication network systems by a prior art example.

Explanation of symbols

12, 32 Band pass filter unit 11, 21, 31, 41 Substrate 13, 33, 43 Buffer layer 14, 34, 42, 44 Edge filter unit 21, 22, 27 Optical filter 23, 26 Capillary 24, 25 GRIN lens

Claims (5)

First to n-th filter parts (an integer of n ≧ 2) constituted by two or more kinds of dielectric laminated structures having different refractive indexes;
And a buffer layer made of a dielectric thin film material having an arbitrary thickness and inserted between the plurality of filter sections.   The i-th filter unit (i = 1 to n) is selected from a band-pass filter unit that transmits only a specific wavelength region and an edge filter unit that transmits a wavelength region longer or shorter than a specific wavelength. 2. The composite dielectric multilayer filter according to claim 1, wherein the filter is any one of the filters described above. The number n of the filter units is 2,
The first filter unit is a bandpass filter unit that transmits only a specific wavelength region,
The second filter unit is an edge filter unit that transmits a longer wave or a shorter wave region than a specific wavelength,
The bandpass filter unit is composed of at least one basic resonator structure,
The basic resonator structure is a first structure composed of a dielectric thin film material having a film thickness that is an odd multiple of the optical film thickness of a quarter wavelength of the transmission wavelength and a dielectric thin film material having a refractive index lower than that. , Composed of a second stack layer and any one of the optical thin film layers sandwiched between the first and second stack layers and having an even multiple of the optical thickness of the quarter wavelength. The composite dielectric multilayer filter according to claim 1, further comprising a spacer layer. The number n of the filter units is 2,
The first filter unit is a high-pass edge filter unit configured to transmit light having a wavelength shorter than a specific wavelength (λ1), which is configured by a laminated structure of two or more kinds of dielectric thin films having different refractive indexes. Yes,
The second filter section is a low-pass edge that transmits light having a wavelength longer than another specific wavelength λ2 (λ1> λ2) constituted by a laminated structure of two or more kinds of dielectric thin films having different refractive indexes. 2. The composite dielectric multilayer filter according to claim 1, wherein the composite dielectric multilayer filter is a filter section. The optical thickness of the buffer layer has a thickness that is an integral multiple of an optical thickness of a quarter wavelength of the transmission wavelength of the bandpass filter. Composite dielectric multilayer filter.

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