JP2006053200A - Edge filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an edge filter which is capable of reducing polarization dependence and has an excellent spectral characteristic. <P>SOLUTION: The edge filter comprises an edge filter film 4 made by alternately laminating at least two kinds of optical thin films having different refractive indexes on an optical substrate 1. The edge filter transmits signal light having the wavelength of a first wavelength band which is made incident to the edge filter film 4 and reflects signal light having the wavelength of a second wavelength band which is made incident to the edge filter 4, wherein the first wavelength band and the second wavelength band have a predetermined separation width. Further, an incident angle of the signal light for the multilayer film is determined so that a dissociation width between a spectral characteristic of S-polarization and a spectral characteristic of P-polarization becomes the separation width or below. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an edge filter that branches by transmitting a part of incident signal light and reflecting another part.

  The standardization and standardization of a new communication system using PON (Passive Optical Network) technology such as B-PON (Broadband-PON) and IEEE802.3ah has been promoted. In an access network that uses an optical fiber for an access line that connects a subscriber and a receiving station, the PON can add multiple subscribers with fewer optical fibers by inserting an optical splitter in the middle of the access line to split the signal light. It is a system that accommodates efficiently.

  In the conventional B-PON, a wavelength band of 1260 nm to 1360 nm is used in the direction from the subscriber to the accommodating station, and a wavelength band of 1480 nm to 1500 nm is used in the direction from the accommodating station to the subscriber. New standards such as B-PON and IEEE 802.3ah that are newly standardized use the 1550 nm to 1560 nm wavelength band as a new downstream communication band in addition to the conventional wavelength band described above, and perform communications such as broadcast distribution. It is assumed that the service is performed.

  In the communication device compliant with the above-mentioned new standard, an edge filter having a characteristic of separating a wavelength band of 1260 nm to 1500 nm used conventionally and a wavelength band of 1550 nm to 1560 nm for new broadcast distribution is used. It is necessary to superimpose communication using a conventional wavelength band and communication using a new wavelength band on a single-core optical fiber. For this reason, the edge filter that performs the band separation described above requires spectral characteristics that transition from transmission to reflection or from reflection to transmission in a separation width of 1500 nm to 1550 nm and 50 nm.

  As the edge filter, an optical multilayer filter in which a dielectric film is laminated on an optical substrate is used. Conventionally, the edge filter is generally used with an incident angle of signal light set to 45 degrees (for example, Patent Document 1). However, in the conventional optical multilayer filter in which the incident angle of the signal light is set to 45 degrees, the spectral characteristic depends on the polarization state of the incident light when transitioning from transmission to reflection. Since the difference in spectral characteristics from the P-polarized light becomes large, there is a problem that the transmission band and the reflection band cannot be separated when the separation width of 1500 nm to 1550 nm is 50 nm or less.

On the other hand, by reducing the incident angle, the difference between S-polarized light and P-polarized light can be reduced. However, if the incident angle is reduced, the optical paths of the incident light and the reflected light become closer. Therefore, a member such as a collimator lens must be disposed away from the edge filter, and the module size cannot be reduced during mounting. Will occur.
JP 2000-162413 A

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an edge filter that reduces polarization dependency and has good spectral characteristics.

A multilayer film formed by alternately laminating at least two types of optical thin films having different refractive indexes on an optical substrate, and transmitting signal light having a wavelength in a first wavelength band incident on the multilayer film; Reflecting signal light having a wavelength in the second wavelength band incident on the
An edge filter in which the first wavelength band and the second wavelength band are separated from a predetermined separation band, and a separation width between a spectral characteristic of S-polarized light and a spectral characteristic of P-polarized light is the separation band. The incident angle of the signal light with respect to the multilayer film is determined so as to be equal to or less than the width. As a result, it is possible to use the edge filter under the predetermined spectral characteristics while suppressing the deviation width in the spectral characteristics of the S-polarized light and the P-polarized light to be equal to or smaller than the predetermined separation width.

  Further, when the separation band is in the range of 1500 nm to 1550 nm, the incident angle is preferably 3 degrees or more and 20 degrees or less.

  Further, the first wavelength band is a wavelength band including a range of at least 1260 nm to 1360 m and a range of 1480 nm to 1500 nm, and the second wavelength band is a wavelength band including a range of at least 1550 nm to 1560 nm, or The second wavelength band may be a wavelength band including at least a range of 1260 nm to 1360 m and a range of 1480 nm to 1500 nm, and the first wavelength band may be a wavelength band including a range of at least 1550 nm to 1560 nm. With such a configuration, the wavelength band for conventional communication of 1260 nm to 1500 nm used in B-PON and the new wavelength band for broadcast distribution of 1550 nm to 1560 nm can be separated.

  In addition, it is good also as providing the antireflection film which prevents reflection of the light of said 1st wavelength band and said 2nd wavelength band on the back surface of the said optical substrate. As a result, it is possible to prevent an increase in insertion loss caused by the light passing through the edge filter being reflected from the back surface of the optical substrate.

  The multilayer film is formed by laminating two kinds of films, a high refractive index film and a low refractive index film, and the refractive index of the high refractive index film is 1.8 to 5.0, and the low refractive index film The refractive index of the refractive index film is preferably 1.3 or more and 1.5 or less, and the number of laminated layers including the high refractive index film and the low refractive index film is preferably 30 or more and 150 or less.

Furthermore, the high refractive index film includes Ta _x O _y , TiO _x , ZnS, Si, Ge, SiGe _x , SiN _x , SiC _x , ZrO _x , NbO _x , YO _x , In _x O _y , SnO _x , The low refractive index film is made of at least one of CeO _x , HfO _x or a mixture thereof, and the low refractive index film is made of SiO _x , MgO _x , AlO _x , SiO _x C _y , SiO _x N _y , MgF _x or a mixture thereof. It should be made of at least one kind.

  According to the present invention, it is possible to provide an edge filter with reduced polarization dependency and good spectral characteristics.

Embodiment 1 of the Invention
FIG. 1 is a schematic view showing a cross section of an edge filter 10 according to the present embodiment. The edge filter film 4 is formed by alternately laminating a high refractive index film 2 and a low refractive index film 3 on a parallel flat optical substrate 1, and the number of layers and the number of layers to satisfy desired optical characteristic conditions. The film thickness is adjusted. Here, the desired optical characteristic condition for the edge filter film 4 is that the three wavelength bands used in the above-described B-PON are changed to the two wavelength bands used in the conventional communication by light transmission and reflection. This is a characteristic of separation into a new wavelength band added for broadcast distribution. Specifically, a characteristic of transmitting signal light in a wavelength band of 1260 nm to 1360 nm and 1480 nm to 1500 nm, which are conventional transmission and reception wavelengths of B-PON, and reflecting signal light in a wavelength band of 1550 nm to 1560 nm for broadcast distribution, Or, conversely, the spectral characteristics reflect signal light in the wavelength bands of 1260 nm to 1360 nm and 1480 nm to 1500 nm and transmit the signal light in the wavelength band of 1550 nm to 1560 nm.

  As shown in FIG. 1, the high refractive index film 2 is disposed immediately above the optical substrate 1 so that the difference in refractive index between the refractive index of the optical substrate 1 and the film disposed immediately above it is increased. However, a structure in which the low refractive index film 3 may be provided immediately above the planar optical substrate 1 and a high refractive index film 2 may be used as the final layer.

  Further, the edge filter 10 is provided with an antireflection film 5 formed by laminating a high refractive index film 2 and a low refractive index film 3 on a surface opposite to the optical substrate 1 in a single layer or alternately in a plurality of layers. By providing the antireflection film 5, it is possible to prevent an increase in insertion loss caused by signal light transmitted through the edge filter 10 being reflected on the back surface of the optical substrate 1. When the insertion loss caused by the back surface reflection does not become a problem, the antireflection film 5 may not be provided.

  Next, the optical characteristics of the edge filter 10 will be described. As described above, the edge filter 10 used for the communication equipment compliant with the standards such as B-PON and IEEE 802.3ah is one of the light in the wavelength band of 1260 nm to 1500 nm and the light in the wavelength band of 1550 nm to 1560 nm. Is a filter having spectral characteristics that transmits light of the other band and reflects light in the other band. There are two types of edge filters, SWPF (Short Wave Pass Filter) having a transmission band on the short wavelength side and LWPF (Long Wave Pass Filter) having a transmission band on the long wavelength side.

  As the characteristics of the edge filter 10, it is desirable that the change between the transmission band and the reflection band is steep, and the flatness of the transmittance in the transmission band is ensured. In general, it has been found that a steep band separation characteristic can be obtained as the number of stacked layers constituting the edge filter film 4 is increased. However, when the number of stacked layers is increased, ripples generated in the transmission band increase and flatness characteristics tend to be impaired. Therefore, the number of layers may be adjusted according to the band separation characteristics and flatness characteristics required for the edge filter, and the specific number of layers of the edge filter film 4 is generally selected from 30 to 150 layers. Often done.

  Furthermore, in order to separate the incident signal light using the edge filter 10 and detect the reflected light, the incident angle of the signal light needs to be a certain level or more for convenience of mounting. This is because if the incident angle is too small, it is difficult to arrange a collimator lens or the like that converts reflected light into parallel light. On the other hand, in the optical multilayer filter, it is known that the spectral characteristics have polarization dependence, and the spectral characteristics differ depending on the polarization state of incident light. The deviation between the S-polarized light and the P-polarized light due to the polarization dependence increases as the incident angle increases. For this reason, in an edge filter having a narrow separation width during the transition from the transmission band to the reflection band, the degradation of the band separation characteristic due to the increase in the incident angle of the signal light becomes a serious problem.

  Therefore, the inventors set the incident angle appropriately in accordance with the required separation width between the transmission band and the reflection band (50 nm in the present invention), whereby the wavelength band of 1260 nm to 1500 nm and the wavelength of 1550 nm to 1560 nm. It has been found that an edge filter can be obtained which has good characteristics for separating the band and does not interfere with detection of reflected light.

  Further, as a result of the examination, when the angle between the normal line of the filter surface of the edge filter film 4 and the incident signal light is an incident angle, the spectral characteristics are good when the incident angle is in the range of 3 degrees to 20 degrees. It was found that can be obtained. When the incident angle is smaller than 3 degrees, a member such as a collimator lens must be disposed away from the edge filter, and the module cannot be reduced in terms of mounting. Therefore, the incident angle is desirably 3 degrees or more.

  On the other hand, when the incident angle is larger than 20 degrees, the divergence width between the spectral characteristics of S-polarized light and P-polarized light becomes large, and both S-polarized light and P-polarized light have a transmission band with a separation width of 50 nm from 1500 nm to 1550 nm. It becomes difficult to divide into the reflection band. For this reason, it is desirable that the incident angle be 20 degrees or less.

  More specifically, for example, when the edge filter is SWPF and the transmittance of the S-polarized light starts to decrease on the shorter wavelength side than the P-polarized light, the wavelength at which the transmittance of the S-polarized light starts decreasing is λ1. Assuming that the wavelength at which the transmittance of P-polarized light decreases to a predetermined value is λ2, if at least the value of λ2-λ1 is not smaller than 50 nm, both S-polarized light and P-polarized light are separated by a separation width of 50 nm from 1500 nm to 1550 nm. It cannot be divided into a transmission band and a reflection band. However, when the incident angle is larger than 20 degrees, the value of λ2−λ1 becomes 50 nm or more.

  From the above, it is desirable that the incident angle of the signal light with respect to the edge filter 10 that separates the wavelength band of 1260 nm to 1500 nm and the wavelength band of 1550 nm to 1560 nm is 3 degrees or more and 20 or less. In order to obtain better spectral characteristics, the incident angle is preferably 5 degrees or more and 15 degrees or less.

Next, the material and manufacturing method of each part of the edge filter 10 will be described. The optical substrate 1 is preferably transparent in the near-infrared wavelength band used for the B-PON signal light. For example, the optical substrate 1 desirably has a transmittance of 99.8% or more. Specifically, amorphous glass, crystallized glass, or another optical substrate can be used as the optical substrate 1. Specifically, oxide single crystals such as LiNbO ₃ , LiTaO ₃ , TiO ₂ , SrTiO ₃ , Al ₂ O ₃ and MgO, polycrystalline substrates, fluoride single crystal substrates such as CaF ₂ , MgF ₂ BaF ₂ and LiF, Polycrystalline substrates, chlorides such as NaCl, KBr, and KCl, bromide single crystals, polycrystalline substrates, and resin substrates such as fluorinated polyimide can be applied.

Examples of the material for the high refractive index film 2 include Ta _x O _y , TiO _x , ZnS, Si, Ge, SiGe _x , SiN _x , SiC _x , ZrO _x , NbO _x , YO _x , In _x O _y , SnO _x , At least one kind is selected from CeO _x , HfO _x or a mixture thereof. Further, as the material of the low refractive index film 3, at least one of SiO _x , MgO _x , AlO _x , SiO _x C _y , SiO _x N _y , MgF _x, or a mixture thereof is selected. Moreover, you may shift | deviate somewhat from said stoichiometric composition. In addition, although it is preferable to use the same kind as each refractive index film | membrane, if it is a material with a similar refractive index, it is also possible to substitute a part with refractive index film | membrane which consists of other materials.

  The edge filter 10 can be manufactured by a vacuum film forming method. As the vacuum film forming method, various film forming methods such as a vacuum deposition method, a sputtering method, a chemical vapor deposition method, and a laser brazing method can be used. When using a vacuum deposition method, ion deposition is performed by ionizing a part of the deposition air flow and applying a bias to the substrate side in order to improve the film quality, cluster ion beam method, or ion irradiation to the substrate using an ion gun. It is effective to use an ion-assisted deposition method. As the sputtering method, a DC reactive sputtering method, an RF sputtering method, an ion beam sputtering method, or the like can be used. Further, as the chemical vapor phase method, a plasma polymerization method, a light-assisted vapor phase method, a thermal decomposition method, a metal organic chemical vapor phase method, or the like can be used. In addition, the film thickness of each thin film can be made into a desired film thickness by changing the vapor deposition time etc. at the time of film formation.

Next, an embodiment of the edge filter 10 will be described. In addition, this invention is not limited to these Examples at all. The material of the edge filter described in Example 1, Example 2, and Comparative Example 1 includes Nb ₂ O ₅ (refractive index 2.30) as a high refractive index film and SiO ₂ (refractive index 1.30) as a low refractive index film. 467), and optical glass BK7 (manufactured by SCHOTT GLASS, refractive index 1.51) was used for the optical substrate. What is the design wavelength used in the following description? It means the center wavelength of the reflection band when spectral characteristics are measured at an incident angle of 0 degree.

Example 1
An optical refractive index film and a low refractive index film are alternately laminated on an optical substrate, and the transmittance is −30 dB or less in the wavelength band from 1260 nm to 1500 nm, and the transmittance is −0.1 dB or more in the wavelength band from 1550 nm to 1560 nm. The filter was designed. In addition, the design was performed with the incident angle between the normal line of the edge filter film and the signal light being 5 degrees. As a result, an edge filter having 60 layers and a design wavelength of 1327 nm was obtained. The spectral characteristics are shown in FIG. FIG. 3 shows a graph in which the band from the wavelength of 1480 nm to 1560 nm in FIG. 2 is enlarged. In FIGS. 2 and 3, the transmittance Ts of S-polarized light is indicated by a thin solid line and a thin hatched line, and the transmittance Tp of P-polarized light is indicated by a thick solid line and a thick shaded line.

  As shown in FIGS. 2 and 3, in the edge filter designed with an incident angle of 5 degrees, both S-polarized light and P-polarized light have a transmittance of −30 dB or less in the wavelength band of 1260 nm to 1500 nm, and are within the range of 1550 nm to 1560 nm. The transmittance is -0.1 dB or more. From this, it can be seen that good spectral characteristics for separating a wavelength band of 1500 nm or less and a wavelength band of 1550 nm or more are obtained.

(Example 2)
An optical refractive index film and a low refractive index film are alternately stacked on an optical substrate, and the transmittance is −0.1 dB or more in the wavelength band from 1260 nm to 1500 nm, and the transmittance is −30 dB or less in the wavelength band from 1550 nm to 1560 nm. The filter was designed. In addition, the design was performed with the incident angle between the normal line of the edge filter film and the signal light being 15 degrees. As a result, an edge filter having 60 layers and a design wavelength of 1746 nm was obtained. The spectral characteristics are shown in FIG. FIG. 5 shows a graph in which the band of wavelengths from 1480 nm to 1560 nm in FIG. 2 is enlarged. 4 and 5, the transmittance Ts of S-polarized light is indicated by a thin solid line and a thin hatched line, and the transmittance Tp of P-polarized light is indicated by a thick solid line and a thick shaded line.

  As shown in FIGS. 4 and 5, in the edge filter designed with an incident angle of 15 degrees, both the S-polarized light and the P-polarized light have a transmittance of −0.1 dB or less in the wavelength band of 1260 nm to 1500 nm, and the 1550 nm to 1560 nm Within the range, the transmittance is -30 dB or less. From this, it can be seen that good spectral characteristics for separating a wavelength band of 1500 nm or less and a wavelength band of 1550 nm or more are obtained.

(Comparative Example 1)
The incident angle between the normal of the edge filter film and the signal light is 30 degrees, and the transmittance is −0.1 dB or more in the wavelength band from 1260 nm to 1500 nm, and the transmittance is −30 dB in the wavelength band from 1550 nm to 1560 nm. The edge filter was designed to be as follows. As a result, an edge filter having 60 layers and a design wavelength of 1804 nm was obtained. The spectral characteristics are shown in FIG. FIG. 7 shows a graph in which the band of wavelengths from 1480 nm to 1560 nm in FIG. 6 is enlarged. 6 and 7, the transmittance Ts of S-polarized light is indicated by a thin solid line and a thin hatched line, and the transmittance Tp of P-polarized light is indicated by a thick solid line and a thick shaded line.

  As can be seen from FIGS. 6 and 7, in the wavelength band from 1260 nm to 1500 nm, there is a portion where the transmittance of S-polarized light and P-polarized light is −0.1 dB or less, and further, the transmittance of S-polarized light is 1550 nm. Since it is −30 dB or more, when the incident angle is 30 degrees, the edge filter of this embodiment cannot satisfy desired optical characteristics. This is because, by setting the incident angle to 30 degrees, the change in transmittance at the edge portion where the transition from transmission to reflection becomes gentle, and the difference in spectral characteristics between S-polarized light and P-polarized light becomes large. This is due to that. Specifically, as shown in FIG. 7, both the S-polarized light and the P-polarized light have a wavelength width at which the transmittance changes from −0.1 dB to −30 dB and 20 nm or more, and the S-polarized light and the P-polarized light The difference is about 30 nm. For this reason, the separation width of the transmission band and the reflection band becomes 50 nm or more, which indicates that the desired spectral characteristics cannot be satisfied.

It is a block diagram of the edge filter concerning this invention. 6 is a graph showing spectral characteristics of the edge filter of Example 1. 6 is a graph showing spectral characteristics of the edge filter of Example 1. 6 is a graph showing spectral characteristics of an edge filter of Example 2. 6 is a graph showing spectral characteristics of an edge filter of Example 2. 6 is a graph showing spectral characteristics of an edge filter of Comparative Example 1. 6 is a graph showing spectral characteristics of an edge filter of Comparative Example 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical substrate 2 High refractive index film 3 Low refractive index film 4 Edge filter film 5 Antireflection film

Claims (6)

A multilayer film formed by alternately laminating at least two optical thin films having different refractive indexes on an optical substrate;
Transmitting signal light having a wavelength of a first wavelength band incident on the multilayer film, reflecting signal light having a wavelength of a second wavelength band incident on the multilayer film;
An edge filter in which the first wavelength band and the second wavelength band separate a predetermined separation band;
An edge filter in which an incident angle of the signal light with respect to the multilayer film is determined such that a deviation width between the spectral characteristics of S-polarized light and the spectral characteristics of P-polarized light is equal to or less than the width of the separation band. The separation band is in the range of 1500 nm to 1550 nm;
The edge filter according to claim 1, wherein the incident angle is not less than 3 degrees and not more than 20 degrees.   The first wavelength band is a wavelength band including at least a range of 1260 nm to 1360 m and a range of 1480 nm to 1500 nm, and the second wavelength band is a wavelength band including a range of at least 1550 nm to 1560 nm, or The second wavelength band is a wavelength band including at least a range of 1260 nm to 1360 m and a range of 1480 nm to 1500 nm, and the first wavelength band is a wavelength band including a range of at least 1550 nm to 1560 nm. Edge filter as described in.   The edge filter according to any one of claims 1 to 3, further comprising an antireflection film that prevents reflection of light in the first wavelength band and the second wavelength band on a back surface of the optical substrate. The multilayer film is formed by laminating two kinds of films, a high refractive index film and a low refractive index film,
The refractive index of the high refractive index film is 1.8 or more and 5.0 or less, the refractive index of the low refractive index film is 1.3 or more and 1.5 or less, and the high refractive index film and the low refractive index film are low refractive index films. The edge filter according to any one of claims 1 to 4, wherein the number of laminated layers of the rate films is in the range of 30 to 150 layers. The high refractive index film includes Ta _x O _y , TiO _x , ZnS, Si, Ge, SiGe _x , SiN _x , SiC _x , ZrO _x , NbO _x , YO _x , In _x O _y , SnO _x , CeO _x , Consisting of at least one of HfO _x or a mixture thereof,
The edge filter according to claim 5, wherein the low refractive index film is made of at least one of SiO _x , MgO _x , AlO _x , SiO _x C _y , SiO _x N _y , MgF _x, or a mixture thereof.

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