JP2004347740A - Optical bandpass filter, optical communication module using the same, and manufacturing method of optical bandpass filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical bandpass filter reduced in insertion loss and excellent in optical characteristics, and to provide an optical communication module using the optical bandpass filter. <P>SOLUTION: The optical bandpass filter is made by laminating a plurality of dielectric films 84 and 85 having different refractive indexes on an optical substrate 81 and the total film thickness of the films 84 and 85 is set equal to or greater than 50 μm. Average surface roughness Ra of the surface of the substrate 81 side on which the dielectric films are provided is made equal to or less than 0.50 nm when the roughness is obtained by measuring a plurality of measurement points having measurement distances equal to or less than 100 nm. Note that the average surface roughness Ra is measured by scanning an area of 10 μmx10 μm employing an interatomic force microscope (an AFM). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００５０】
次に、上記各光学基板に、図３に模式構成図を示すイオンアシスト付き真空蒸着装置でチャンネル間隔が５０ＧＨｚ（チャンネルスペーシング、約０．４ｎｍ）用の誘電多層膜を総膜厚５４．９μｍで積層し、光学バンドパスフィルタを作成した。誘電体膜は、ＳｉＯ_２とＴａ_２Ｏ_５を用い、以下の式（１）にもとづいて交互に積層した。ＳｉＯ_２とＴａ_２Ｏ_５は、それぞれＳｉＯ_２、Ｔａ_２Ｏ_５を蒸着源として、電子式加熱法で加熱、蒸着を行った。蒸着中、イオンビーム装置からＡｒと酸素の混合イオンビームを基板に照射した。イオンビームは、加速電圧１０００Ｖ、ビーム電流６００ｍＡ、Ａｒガス流量５ｓｃｃｍ、酸素ガス流量１５ｓｃｃｍの条件で照射した。イオンビームを安定させるため、ＲＦ型電子線中和装置も使用した。
【００５１】

式中、ＬおよびＨは、低屈折率層および高屈折率層の１／４波長層を示す。Ｌ、Ｈの前の数は、１／４波長層単位の各層の長さを示す。
【００５２】
（参考例１〜５）
比較例１〜３及び実施例１、２で作成した各基板と同様に、表面研磨を施した５種の結晶化ガラス基板を作成した。各光学基板の表面粗さを表１に示す。
この各基板に、同様にイオンアシスト付き蒸着装置でチャンネル間隔が１００ＧＨｚ（チャンネルスペーシング：約０．８ｎｍ）用誘電多層膜を総膜厚４４．５μｍで積層し、光学バンドパスフィルタを作成した。なお、１００ＧＨｚでの最適化のため積層した誘電多層膜の構造は、式（２）に示す構造で設計した。
【００５３】

【００５４】
実施例１〜２、比較例１〜３及び参考例１〜５の各光学基板の表面粗さと光学バンドパスフィルタの光学特性を測定した。光学特性は、図８に示す光学特性測定装置を用いて測定した。測定のために各基板の裏面と誘電多層膜の上に、それぞれ反射防止膜を形成した。光学特性の測定には、光源にチューナブルレーザー４０１を、受光側にパワーメータ４１０を使った。発光側からの光をファイバー４０２で導き、測定箇所でコリメータレンズ４０５を使って平行光とし、５軸ゴニオ・サンプルステージに固定されたサンプルの誘電多層膜フィルタ４０６を通過後、再び同様のコリメータレンズ４０８を通して収束させてファイバー４０９に導き、最終的にパワーメータ４１０で各波長でのパワーを測定して、損失を測定した。また、各光学バンドパスフィルタのリフレクションアイソレーションは、同じ測定装置を用いて、以下の方法で測定した。フィルタ４０６で反射されて戻ってくる光を発光側コリメータレンズ４０５で再度発光側光ファイバー４０２に結合させて光を戻し、戻された反射光は、５０：５０カップラー４０４でパワーメータ４０３に分配し、その強度を測定した。リフレクションアイソレーションは、透過波長帯域内での戻り光強度比（戻り光強度の１００％反射光に対する比）の最大値で表せる。透過波長帯域内では、戻り光は小さいほど良いので、リフレクションアイソレーションは大きいほど良いことになる。
【００５５】
表１から分かるように、総膜厚が５０μｍを越す光学バンドパスフィルタでは、原子間力顕微鏡（ＡＦＭ）で測定した平均基板表面租さ（Ｒａ）を０．５０ｎｍ以下とすることで、損失が大きく改善される。図６に示したように、参考例１〜５の膜厚が４０μｍ前後の１００ＧＨｚ用光学バンドパスフィルタでは、光学基板の表面粗さによる損失はそれほど差が見られないのに対し、実施例１〜２、比較例１〜３の対比から分かるように、５０μｍ以上の総膜厚を有する光学バンドパスフィルタでは、表面粗さが損失に大きく影響しており、０．５０ｎｍ以下とすることにより、膜厚が薄い参考例の光学バンドパスフィルタに近い損失に抑えれられている。
【００５６】
特に、実施例２では原子間力顕微鏡（ＡＦＭ）で測定した表面粗さをさらに平滑化しているため、損失がさらに低減されていることが分かる。
【００５７】
なお、参考のために各基板について光学式表面粗さを測定した結果を表１に示す。表１に示すように、何れのガラスの平均基板表面粗さも０．３４〜０．３５ｎｍ程度であり、明確な差が無かった。
【００５８】
（実施例３）
実施例１において、下記式（３）で示す結合層の厚みをずらした光学バンドパスフィルタを作成した。

【００５９】
このようにして作成した光学バンドパスフィルタの光学特性と平均表面粗さを結果を表２示す。
【００６０】
【表２】

【００６１】
表２に示すように、本発明の表面粗さを有する光学基板を用いるとともに、結合層の厚さをλ／４からずらした実施例２の光学バンドパスフィルタは損失が低減されているとともに、リフレクションアイソレーションがさらに改善されていることが分かる。
【００６２】
【発明の効果】
本発明によれば、挿入損失を抑え、光学特性に優れた光学バンドパスフィルタ及びそれを用いた光通信モジュールを提供することができる。
【図面の簡単な説明】
【図１】誘電多層膜フィルタの透過及び反射プロファイルを示す図である。
【図２】３端子ＷＤＭフィルターモジュールの断面模式図である。
【図３】イオンアシスト付き真空蒸着装置の構造模式図である。
【図４】カスケード接続の説明模式図である。
【図５】光学基板における誘電体膜の総膜厚による挿入損失への影響を表すグラフである。
【図６】光学基板の表面粗さによる挿入損失への影響を表すグラフである。
【図７】本発明の光学バンドパスフィルタの製造方法の一部を示すフローチャートである。
【図８】光学特性測定装置の説明図である。
【図９】光学バンドパスフィルタの構造を示す図である。
【符号の説明】
１、８１ 光学基板
８２ スペーサ層
８３ 結合層
８４ 高屈折率膜
８５ 低屈折率膜
８６ ミラー層
８８ キャビティ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical bandpass filter in which a plurality of dielectric films having different refractive indexes are laminated on an optical substrate, and more particularly, to an optical bandpass filter with reduced insertion loss.
[0002]
[Prior art]
An optical bandpass filter used in WDM (Wavelength Division Multiplexing) technology has a structure in which dielectric films having different refractive indexes are laminated on a glass substrate, and has transmission and reflection characteristics as shown in FIG. In FIG. 1, the vertical axis indicates the transmittance and the reflectance (dB), and the horizontal axis indicates the wavelength (nm). The solid line is a transmission profile, and the dotted line is a reflection profile. In general, the center wavelength of an optical bandpass filter is matched to a channel called a grid defined by the International Telecommunication Union, and the characteristics of the filter, such as the half-value width, transmission band, and stop band, are determined in consideration of the used channel interval, transmission rate, and the like. Is set to the value of This type of filter is mainly used by being incorporated into a device shown in FIG. 2 which is called a three-terminal device. In this three-terminal device, multiplexed light enters from an input port 101, passes through an optical fiber 100 passing through a dual fiber capillary 201, and enters a collimator lens 202. Light that has passed through the collimator lens 202 enters the thin-film filter 1. As for the light incident on the thin film filter 1, only light of a predetermined band wavelength corresponding to the transmission wavelength of the thin film filter 1 is transmitted, passes through the collimator lens 202, the optical fiber 100 passing through the single fiber capillary 204, and goes to the output port 103. Come out. Other light is reflected by the thin-film filter 1 and comes out to the output port 102 via the optical fiber 100. The dual fiber capillary 201, the collimator lens 202, the thin film filter 1, and the single fiber capillary 204 are attached to a metal sleeve case 205.
[0003]
As shown in FIG. 4, when dividing the multiplexed light into respective channels, the three-terminal devices 21, 22, 23, and 24 are connected in cascade. By repeatedly inputting the light of the output port 102 reflected by the thin-film filter 1 and coming out to the input port 101 of the next device, the light of the predetermined channel that has been split from the output port 103 of each device is repeated. Is taken out. Conversely, multiplexing of multiplexed light can be performed by putting light corresponding to the transmission wavelength into each of the predetermined devices. The configuration of the optical module is disclosed, for example, in Non-Patent Document 1 below.
[0004]
Such optical band-pass filters are classified according to the used channel spacing. For example, there are filters for 200 GHz and 100 GHz. However, in order to advance multiplexing with an increase in data amount, the spacing between channels is further narrowed. Recently, a 50 GHz optical bandpass filter has been proposed.
[0005]
When multiplexing / demultiplexing of multiplexed light is performed by the above method, since there is a certain loss in both reflection and transmission of each three-terminal device itself, when used in a cascade, the amount of light decreases in the order of connection. . This decrease in light amount is mainly caused by absorption and scattering of the filter. As the total film thickness of the dielectric film increases, the decrease in light amount due to the absorption and scattering increases.
[0006]
In addition, as the channel spacing becomes narrower, the degree of interference increases in order to reduce both the transmission bandwidth and the stop bandwidth, the total number of films increases, and eventually the total film thickness increases, thereby increasing the loss due to absorption and scattering. .
[0007]
For example, in the above, the total thickness of the dielectric film in the optical band-pass filter for 200 GHz is about 30 to 35 μm, and is about 40 μm for 100 GHz, but is 50 μm or more for 50 GHz. The effect of the loss of light amount due to the increase in the amount of light increases. Therefore, when a duplexer composed of these optical bandpass filters is arranged in a cascade, the reflected light is reduced as much as possible, so that the final light amount transmitted through the optical bandpass filter is the initial transmission amount. There is a problem that the degree of loss is larger than that of
However, even in such a narrow band-pass filter having a large total film thickness, it is required that the loss is equal to or less than a predetermined amount regardless of the film thickness, and it is necessary to suppress the insertion loss to the same level as that of an optical band-pass filter of 100 GHz. There is.
[0008]
[Non-patent document 1]
Hiroshi Matsuura, 5 others, "Development of a 2-core optical module", Furukawa Electric Co., Ltd., Furukawa Electric Co., Ltd., January 2002, No. 109, p. 11-14
[0009]
[Problems to be solved by the invention]
As described above, in the conventional optical bandpass filter, there is a problem that the insertion loss is suppressed.
[0010]
An object of the present invention is to provide an optical bandpass filter which suppresses insertion loss and has excellent optical characteristics, an optical communication module using the same, and a method of manufacturing an optical bandpass filter.
[0011]
[Means for Solving the Problems]
An optical bandpass filter according to the present invention is an optical bandpass filter in which a plurality of dielectric films having different refractive indexes are laminated on an optical substrate, and the total thickness of the dielectric films is 50 μm or more. An average surface roughness Ra obtained by measuring a measurement point having a distance between measurement points of 100 nm or less on the surface of the optical substrate on which the dielectric film is provided is 0.50 nm or less. . Thereby, insertion loss can be reduced.
[0012]
An optical communication module according to the present invention is obtained by connecting multiplexing / demultiplexing devices including the above-described optical bandpass filters in a cascade. Thereby, insertion loss can be reduced.
[0013]
The method of manufacturing an optical bandpass filter according to the present invention includes manufacturing a plurality of dielectric films having different refractive indexes on an optical substrate, and manufacturing the optical bandpass filter having a total thickness of the dielectric films of 50 μm or more. A polishing step of polishing an optical substrate, and measuring a measurement point having a distance between measurement points of 100 nm or less on a surface of the optical substrate on which a dielectric film is provided after the polishing step, thereby obtaining an average surface roughness. And a determination step of determining whether or not the average surface roughness Ra obtained in the measurement step is 0.50 nm or less. The determination step determines that the average surface roughness Ra is 0.50 nm or less. Is larger, the polishing step is further performed.
[0014]
Here, the polishing step in a preferred embodiment includes at least a lapping step, a first polishing step, and a second polishing step. In the lapping step, polishing is performed by adding a polishing liquid for lapping to the optical substrate. In the first polishing step, the lapped optical substrate is polished by adding a first polishing liquid, and in the second polishing step, the optical substrate polished in the first polishing step is subjected to polishing. Polishing is performed by adding the polishing liquid of No. 2.
[0015]
In addition, the first polishing liquid is preferably a cerium oxide liquid having a particle diameter of 0.1 μm or more and 1.0 μm or less, and the second polishing liquid is preferably a particle diameter of 0.06 μm or more and 1.0 μm or less. Is preferred.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the effect of the insertion loss due to the thickness will be described. FIG. 5 shows the relationship between the total thickness of the dielectric and the insertion loss in the optical bandpass filter having the dielectric laminated on the glass substrate.
[0017]
As can be seen from FIG. 5, when the total film thickness is 50 μm or more, the change in the insertion loss deteriorates sharply compared to the loss at a film thickness of about 40 μm used at a channel interval of 100 GHz. This is presumably because, when the incident light propagates while repeating multiple interferences in the multilayer film, the amount of light decreases due to absorption and scattering when the thickness is increased.
[0018]
In order to reduce this loss and make the insertion loss approximately the same as when the film thickness is small, it is necessary to suppress the reduction of the light amount as much as possible.
[0019]
In the present invention, as a means for reducing the loss, a pitch component of 100 nm or less, that is, a pitch component obtained by scanning an area of 10 μm × 10 μm on the side on which the dielectric film of the optical substrate is formed by an atomic force microscope (AFM). This can be improved by setting the average surface roughness Ra at a point interval of 100 nm or less to 0.50 nm or less.
[0020]
As shown in FIG. 6, the influence of the surface roughness of the optical substrate on the insertion loss is significantly different depending on the film thickness. The line (a) shows the characteristics of an optical substrate having a thickness of 44.5 μm used at a channel interval of 100 GHz, and the line (b) shows the characteristics of an optical substrate having a thickness of 54.9 μm used at a channel interval of 50 GHz. From FIG. 6, it can be seen that when the total thickness of the conventional dielectric film shown by the line (a) is about 40 μm, even if the surface properties of the substrate change, the insertion loss is not so affected. However, according to the study of the present inventors, as shown by the line (b), when the film thickness is 50 μm or more, the insertion loss greatly changes depending on the surface roughness of the optical substrate. It is predicted that the reason for this is that the thicker film thickness exceeds a certain threshold value of loss due to absorption and scattering caused by multiple interference, which appears.
[0021]
The surface roughness of the present invention is measured by using an atomic force microscope (AFM) to set a measurement area to a square area of 10 μm × 10 μm, a scanning line of 256 points × 256 points, and a tip radius of 5 to 10 nm. It is measured in a tapping mode using a needle. The average of the maximum value and the minimum value of the profile curve of the surface shape determined by each scanning line is defined as a center line L, and is surrounded by the center line L and the profile curve f (r). The value obtained by dividing the area to be obtained by the scanning line length is defined as the average surface roughness Ra. According to the measuring method according to this example, the distance between adjacent measuring points is about 39 nm. The resonance frequency in the tapping mode is 200 to 400 Hz, and the scanning frequency is 0.3 Hz.
[0022]
The surface roughness according to the present invention is different from the surface roughness measured by a conventional optical interference method. That is, it has been found that even if the average surface roughness Ra at a measurement pitch of about 1 μm measured by the optical interference method is 0.6 nm or less, the loss of the dielectric multilayer filter having a total film thickness of 50 μm or more is not improved. This is because the optical interference method has sufficient resolution in the height direction, but the resolution in the plane (resolution in the scanning direction) is about 10 times that of the AFM (that is, the distance between adjacent measurement points is about 10 times or more). ), And a small difference in surface roughness cannot be recognized. Generally, irregularities that are very small than the center wavelength of transmitted light are considered to have no effect on scattering.Thus, as a factor of insertion loss, the surface irregularities of such a fine optical substrate were not taken into account. It has been found that such minute surface fluctuations greatly affect the insertion loss as the film thickness increases. The present invention has been found to be able to suppress the insertion loss to the same degree as the case where the film thickness is small by using the optical substrate whose fine surface roughness is reduced, and it is preferable that the average surface roughness Ra is reduced. It is 0.45 nm or less, and the lower limit is not particularly limited, but is 0.20 nm or more in consideration of manufacturing.
[0023]
As the optical substrate, amorphous glass, crystallized glass, or another optical substrate can be used. Specifically, LiNbO ₃ , LiTaO ₃ , TiO ₂ , SrTiO ₃ , Al ₂ O ₃ , MgO or other oxide single crystal, polycrystalline substrate, CaF ₂ , MgF ₂ , BaF ₂ , A single crystal substrate of fluoride such as LiF, a polycrystal substrate, a single crystal of chloride such as NaCl, KBr, KCl, a single crystal of bromide, a polycrystal substrate, or the like. The optical substrate preferably has a higher transmittance, and for example, desirably has a transmittance of 99.8% or more. The thickness of the optical substrate is, for example, 3 mm or more and 20 mm or less, and more preferably 5 mm or more and 15 mm or less.
[0024]
The method for producing the optical substrate having the surface roughness of the present invention can be performed by mechanical polishing, and the following polishing method can be used.
[0025]
The polishing method will be described with reference to the flowchart of FIG. Polishing is performed in the order of wrap, intermediate polish, and final polish. In this case, the lap and the intermediate polish can be performed simultaneously by selecting polishing conditions, or one of them can be omitted.
[0026]
First, lapping is performed on a glass substrate (thickness is preferably +20 μm to 40 μm, which is the upper limit of the final thickness specification) having a predetermined shape and both surfaces of which are roughly cut (S101). At this time, a glass substrate is pressed with a weight of 5 to 25 kg on a metal platen, and a diamond slurry (diamond abrasive particle diameter: 1 to 10 μm) as a polishing liquid is sprayed, and the platen is rotated at 10 to 100 rpm. Perform polishing. This is performed on both sides, but the lap is stopped when rough cutting marks are obtained on the entire surface of the substrate and diamond slurry marks are formed.
[0027]
Next, intermediate polishing is performed (S102). For the intermediate polish, a glass substrate wrapped with a weight of 5 to 25 kg was pressed against a ceramic platen, and a cerium oxide solution (particle size: 0.1 μm to 1.0 μm) as a polishing solution was dropped (dropping amount: 10 to 200 ml / h), the platen is rotated at a rotation speed of 10 to 100 rpm to perform polishing. Polishing time is 10 to 30 minutes. This is also done on both sides, but at this point, the surface roughness can be made to a surface property of about 0.7 to 1.5 nm as measured by the method described above.
[0028]
Finally, a final polish is applied to the surface on which the dielectric multilayer film is to be formed (S103). An intermediate polished glass substrate is pressed with a weight of 5 to 25 kg against a platen on which a pad made of urethane foam is attached on a flat plate, and a polishing liquid 10 to 300 ml containing colloidal silica (particle diameter: 0.06 to 1.0 μm) The final polishing is performed by rotating the padded platen at 10 to 100 rpm while dripping at / h. The final polish is performed for about 3 to 10 minutes.
[0029]
After the final polishing, the polishing may be performed using a polishing liquid having a smaller particle diameter than the polishing liquid used in the final polishing.
[0030]
After the final polishing, the average surface roughness Ra is measured using an atomic force microscope (AFM) (S104). Next, it is determined whether the average surface roughness Ra obtained by the measurement is 0.50 nm or less (S105). If the result of the determination is that the average surface roughness Ra is 0.50 nm or less, the polishing is completed. After polishing, there are steps such as thickness measurement and correction, cutting with a lathe, and washing.
[0031]
In the above description, the lap and the intermediate polish are polished by one side, but there is no problem if both sides are polished simultaneously.
[0032]
In the optical bandpass filter according to the present invention, a dielectric film is formed on the optical substrate manufactured as described above by a vacuum film forming method. Various film formation methods such as a vacuum evaporation method, a sputtering method, a chemical vapor deposition method, and a laser ablation method can be used for the vacuum film formation method. When using the vacuum evaporation method, the substrate is irradiated with ions using an ion plating method, a cluster ion beam method, or an ion gun in which a part of the evaporation vapor stream is ionized and a bias is applied to the substrate side to improve the film quality. It is effective to use an ion assisted vapor deposition method. Examples of the sputtering method include a DC reactive sputtering method, an RF sputtering method, and an ion beam sputtering method. Examples of the chemical vapor phase method include a plasma polymerization method, a light assisted vapor phase method, a thermal decomposition method, and a metal organic chemical vapor phase method.
[0033]
As a material of the high refractive index layer, TaO _x , TiO _x , ZnS, ZnSn, GaP, InP, Si, Ge, SiGe _x , SiN _x , SiC _x , ZrO _x , NbO _x , YO _x , And a mixture thereof include SiO 2 as a material of the low refractive index layer. _x , MgF ₂ , AlO _x , SiO _x , SiO _x N _y , MgO _x , And mixtures thereof. When used as a filter material, it is preferable that the extinction coefficient be 0.001 or less in the used wavelength band in order to suppress loss. Note that there is no problem even if the material of each layer is different.
[0034]
FIG. 9 is a diagram showing an example of the optical bandpass filter of the present invention, and shows an optical bandpass filter having an N-stage cavity configuration. The cavity 88 has a spacer layer 82 made of a low-refractive-index film whose optical film thickness is an integral multiple of λ / 2, and a high-refractive-index film 84 whose optical film thickness is an odd-number multiple of λ / 4 on both sides of the spacer layer 82, and a low refractive index. It has a structure in which mirror layers 86 in which rate films 85 are alternately arranged are symmetrically combined. FIG. 9 is a schematic diagram showing the present invention for easy understanding, and its size and the like are different from actual ones.
[0035]
This optical bandpass filter is configured by connecting N cavities 88 on an optical substrate 81 using N-1 coupling layers 83 made of a low refractive index film.
[0036]
As shown in FIG. 9, a first-stage cavity is provided immediately above the optical substrate 81, a second-stage cavity is provided thereon, and an N-th cavity is provided at the last stage. The connection layer 83 connects the cavities. It is to be noted that a matching layer 87 for the incident medium composed of one or more layers may be included directly above the N-th cavity. Further, although not shown here, between the optical substrate 81 and the first-stage cavity. May include a matching layer for the optical substrate 81 composed of one or more layers. The cavity 88 has at least two or more different structures. Here, the different structures include, for example, a case where the thickness of the spacer layer 82 is different, a case where the number of layers constituting the mirror layer 86 is different, and a case where the material of each layer constituting the cavity 88 is different.
[0037]
As described above, the mirror layer 86 of the present invention has the high refractive index films 84 adjacent to both sides of the spacer layer 82 made of the low refractive index film, and the uppermost layer and the lowermost layer of each cavity 88 are also the high refractive index films 84. . Therefore, the number of layers on one side of each mirror layer 86 is 2n + 1 layers (n is an integer of 1 or more). The low refractive index film is disposed on the coupling layer 83, and the high refractive index film 84 is disposed on a layer adjacent to the coupling layer 83. In the structure of the mirror layer 86, the number of laminations may be 2n layers and the spacer layer 82 may be a high refractive index film in order to make the coupling layer 83 a low refractive index film. It is also possible to use a combination of two different cavities 88.
[0038]
The optical bandpass filter according to the present invention is based on the above-described structure. However, when a high refractive index film is used as a spacer layer as in the related art and a 2n + 1 layer lamination is adopted, the coupling layer also has a high refractive index film. However, it has been found that in this case, the symmetry of the cavity is broken, resulting in an insertion loss, which is not suitable for an optical bandpass filter used in a narrow band.
[0039]
In the present invention, the optical film thickness of each refractive index film constituting the mirror layer is basically an odd multiple of λ / 4, but these may be changed in relation to other characteristics. In particular, it is possible to adjust the thickness of the plurality of layers at the uppermost portion of the cavity in contact with the matching layer by shifting the film thickness from λ / 4.
[0040]
When the optical bandpass filter of the present invention is used for the above-described multi-cavity filter, at least one optical film thickness C of the coupling layer between the cavities is set to 0.1λ ≦ C ≦ 0.24λ (λ Is the center wavelength of the transmission band). By employing such a configuration of the coupling layer, excellent reflection isolation can also be achieved.
[0041]
Furthermore, the optical bandpass filter of the present invention can also be used for a system that branches several channels together. In this type of optical bandpass filter, if the stop band is too wide, several channels on both sides of the transmission band cannot be used, and the channel use efficiency decreases. However, in such a case, the value of using the present invention is great.
[0042]
【Example】
Hereinafter, the present invention will be described with reference to examples. In the description of the drawings, the same reference numerals denote the same elements. However, the dimensional ratios in the drawings do not always match the actual ones. The embodiment described below is an example of the embodiment of the present invention, and the content of the invention is not limited to the embodiment.
[0043]
(Comparative Example 1)
The surface of a roughly ground crystallized glass substrate having a diameter of 100 mm and a thickness of 8.04 mm was wrapped on both surfaces by a double-sided lapping machine. Lapping was performed using a diamond slurry (average particle size: 3.0 μm) and a copper platen. The rotation speed of the platen was 50 rpm, and the weight was 10 kg. When it was confirmed by an optical microscope that the front surface was scratched by diamond slurry, the following polishing was performed. Intermediate polish was also performed on a double-side polish machine. The substrate of Comparative Example 1 was performed using a cerium oxide polishing liquid having an average particle size of 1.0 μm and a ceramic surface plate. The rotation speed of the platen was 30 rpm, and the load was 10 kg. The intermediate polish was stopped when the diamond wrap marks on the surface of the quartz substrate had completely disappeared.
[0044]
(Comparative Example 2)
In the preparation of the substrate of Comparative Example 1, intermediate polishing was performed using a cerium oxide polishing liquid having the same average particle size of 0.5 μm and a ceramic surface plate. As in Comparative Example 1, the intermediate polish was stopped when the wound on the wrap completely disappeared.
[0045]
(Comparative Example 3)
In the preparation of the substrate of Comparative Example 1, an intermediate polish was performed using a cerium oxide polishing liquid also having an average particle diameter of 0.2 μm and a ceramic surface plate. As in Comparative Example 1, the intermediate polish was stopped when the wound on the wrap completely disappeared.
[0046]
(Example 1)
Intermediate polish was performed in the same manner as in Comparative Example 3, and after confirming that the lap mark had disappeared, final polish was performed. In the final polish, a final polish was performed for 5 minutes at a rotation speed of 30 rpm and a load of 10 kg using a platen on which a polishing liquid of colloidal silica having an average particle diameter of 0.1 μm and a urethane foam pad were attached.
[0047]
(Example 2)
An optical substrate was prepared in the same manner as in Example 1 except that the average particle size of the colloidal silica in the colloidal silica polishing liquid was 0.08 μm.
[0048]
Table 1 shows the results of measuring the surface roughness of each optical substrate prepared as described above. The surface roughness was measured five times. In the measurement, when there was a large defect, the measurement was performed while avoiding the defect.
[0049]
[Table 1]

[0050]
Next, a dielectric multilayer film having a channel spacing of 50 GHz (channel spacing, about 0.4 nm) with a total film thickness of 54.9 μm was formed on each of the above optical substrates by a vacuum deposition apparatus with ion assist as schematically shown in FIG. To form an optical bandpass filter. The dielectric film is made of SiO ₂ And Ta ₂ O ₅ And laminated alternately based on the following formula (1). SiO ₂ And Ta ₂ O ₅ Is SiO ₂ , Ta ₂ O ₅ Was used as an evaporation source, and heating and evaporation were performed by an electronic heating method. During the deposition, the substrate was irradiated with a mixed ion beam of Ar and oxygen from an ion beam device. The ion beam was irradiated under the conditions of an acceleration voltage of 1000 V, a beam current of 600 mA, an Ar gas flow rate of 5 sccm, and an oxygen gas flow rate of 15 sccm. In order to stabilize the ion beam, an RF electron beam neutralizer was also used.
[0051]

In the formula, L and H indicate quarter wavelength layers of a low refractive index layer and a high refractive index layer. The numbers before L and H indicate the length of each layer in a quarter wavelength layer unit.
[0052]
(Reference Examples 1 to 5)
In the same manner as each of the substrates prepared in Comparative Examples 1 to 3 and Examples 1 and 2, five types of crystallized glass substrates whose surfaces were polished were prepared. Table 1 shows the surface roughness of each optical substrate.
Similarly, a dielectric multilayer film having a channel spacing of 100 GHz (channel spacing: about 0.8 nm) with a total film thickness of 44.5 μm was laminated on each of the substrates by a vapor deposition apparatus with ion assist to produce an optical bandpass filter. In addition, the structure of the laminated dielectric multilayer film for optimization at 100 GHz was designed by the structure shown in Expression (2).
[0053]

[0054]
The surface roughness of each optical substrate of Examples 1 to 2, Comparative Examples 1 to 3 and Reference Examples 1 to 5 and the optical characteristics of the optical bandpass filter were measured. The optical characteristics were measured using the optical characteristic measuring device shown in FIG. For measurement, an antireflection film was formed on the back surface of each substrate and on the dielectric multilayer film. The optical characteristics were measured using a tunable laser 401 as a light source and a power meter 410 on a light receiving side. The light from the light-emitting side is guided by the fiber 402, converted into parallel light at the measurement point by using the collimator lens 405, passed through the dielectric multilayer filter 406 of the sample fixed to the 5-axis goniometer sample stage, and then returned to the same collimator lens. The light was converged through 408 and led to a fiber 409, and finally the power at each wavelength was measured with a power meter 410 to measure the loss. The reflection isolation of each optical bandpass filter was measured by the following method using the same measuring device. The light reflected by the filter 406 and returned is coupled to the light emitting side optical fiber 402 again by the light emitting side collimator lens 405 to return the light, and the returned reflected light is distributed to the power meter 403 by the 50:50 coupler 404. The strength was measured. The reflection isolation can be expressed by the maximum value of the return light intensity ratio (the ratio of the return light intensity to 100% reflected light) within the transmission wavelength band. Within the transmission wavelength band, the smaller the return light, the better, so the larger the reflection isolation, the better.
[0055]
As can be seen from Table 1, in an optical bandpass filter having a total film thickness of more than 50 μm, the average substrate surface roughness (Ra) measured by an atomic force microscope (AFM) is set to 0.50 nm or less, so that the loss is reduced. It is greatly improved. As shown in FIG. 6, in the optical band-pass filters for 100 GHz with the film thickness of about 40 μm in Reference Examples 1 to 5, the loss due to the surface roughness of the optical substrate is not so different. As can be seen from the comparisons of Comparative Examples 1 to 3, in an optical bandpass filter having a total film thickness of 50 μm or more, the surface roughness has a large effect on the loss. The loss is suppressed to a value close to that of the optical bandpass filter of the reference example having a small film thickness.
[0056]
In particular, in Example 2, since the surface roughness measured by an atomic force microscope (AFM) was further smoothed, it can be seen that the loss was further reduced.
[0057]
Table 1 shows the results of measuring the optical surface roughness of each substrate for reference. As shown in Table 1, the average substrate surface roughness of each glass was about 0.34 to 0.35 nm, and there was no clear difference.
[0058]
(Example 3)
In Example 1, an optical bandpass filter in which the thickness of the coupling layer was shifted by the following formula (3) was created.

[0059]
Table 2 shows the results of the optical characteristics and the average surface roughness of the optical bandpass filter thus prepared.
[0060]
[Table 2]

[0061]
As shown in Table 2, while the optical substrate having the surface roughness of the present invention was used and the thickness of the coupling layer was shifted from λ / 4, the optical bandpass filter of Example 2 had reduced loss and It can be seen that the reflection isolation is further improved.
[0062]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the insertion loss can be suppressed and the optical bandpass filter excellent in the optical characteristic and the optical communication module using the same can be provided.
[Brief description of the drawings]
FIG. 1 is a diagram showing transmission and reflection profiles of a dielectric multilayer filter.
FIG. 2 is a schematic sectional view of a three-terminal WDM filter module.
FIG. 3 is a schematic structural view of a vacuum deposition apparatus with ion assist.
FIG. 4 is a schematic diagram illustrating a cascade connection.
FIG. 5 is a graph showing the effect of the total thickness of a dielectric film on an optical substrate on insertion loss.
FIG. 6 is a graph showing the effect of the surface roughness of an optical substrate on insertion loss.
FIG. 7 is a flowchart showing a part of a method for manufacturing an optical bandpass filter according to the present invention.
FIG. 8 is an explanatory diagram of an optical characteristic measuring device.
FIG. 9 is a diagram showing a structure of an optical bandpass filter.
[Explanation of symbols]
1,81 Optical board
82 Spacer layer
83 bonding layer
84 High refractive index film
85 Low refractive index film
86 mirror layer
88 cavities

Claims (6)

An optical bandpass filter in which a plurality of dielectric films having different refractive indices are laminated on an optical substrate, and the total thickness of the dielectric films is 50 μm or more. An optical bandpass filter, wherein the surface has an average surface roughness Ra of 0.50 nm or less obtained by measuring a measurement point having a distance between measurement points of 100 nm or less. An optical communication module comprising a multiplexer / demultiplexer comprising the optical bandpass filter according to claim 1 connected in cascade. A method for manufacturing an optical bandpass filter, comprising: stacking a plurality of dielectric films having different refractive indexes on an optical substrate, wherein the total thickness of the dielectric films is 50 μm or more,
A polishing step of polishing the optical substrate,
A measuring step of measuring the average surface roughness Ra by measuring a measuring point having a distance between the measuring points of 100 nm or less on the surface of the optical substrate on which the dielectric film is provided after the polishing step;
A determination step of determining whether the average surface roughness Ra obtained in the measurement step is 0.50 nm or less,
If the average surface roughness Ra is larger than 0.50 nm in the determining step, a method of manufacturing an optical band-pass filter further performing the polishing step. The polishing step includes at least a lapping step, a first polishing step and a second polishing step,
In the lapping step, polishing is performed by adding a polishing liquid for lapping to the optical substrate,
In the first polishing step, polishing is performed by adding a first polishing liquid to the wrapped optical substrate,
6. The method for manufacturing an optical bandpass filter according to claim 5, wherein in the second polishing step, the optical substrate polished in the first polishing step is polished by adding a second polishing liquid. 5. The method according to claim 4, wherein the first polishing liquid is a cerium oxide liquid having a particle diameter of 0.1 [mu] m or more and 1.0 [mu] m or less. The method according to claim 4, wherein the second polishing liquid is colloidal silica having a particle size of 0.06 μm or more and 1.0 μm or less.

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