JP2008241912A - Wavelength filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength filter, overcoming the performance limit of a conventional flat multi-coated wavelength filter regarding a remaining transmission rate due to the refractive index of a used coated film material and a transition wave band width between a transmission wavelength band and a cut-off wavelength band. <P>SOLUTION: The wavelength filter consists of the structure having an alternative multi-coated film structure made of materials having a different refractive index in z-direction in a three-dimensional space xyz, and in which the refractive index periodically varies with a period p in the x-direction, and periodically varies with a period q (q≥p) in the y-direction, while satisfying the relations p≤λ<SB>0</SB>and q>(λ<SB>0</SB>/3), wherein λ<SB>0</SB>is the wavelength of light made incident in parallel to the z-axis in vacuum. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention belongs to the field of optical measurement technology. In particular, the present invention relates to a configuration of a so-called optical wavelength filter for extracting only a desired wavelength component from a spectrum of signal light to be measured.

  A wavelength filter is an element that extracts light in a desired wavelength range from an optical signal having various wavelength components, or removes only a light component in a specific wavelength range, such as optical image measurement, optical fiber communication, and microscopic fluorescence spectroscopy. It is frequently used in almost all engineering fields dealing with light. As a type of wavelength filter, a bandpass filter (bandpass filter) that passes only a specific wavelength range and suppresses the wavelengths before and after that, and vice versa, a band removal filter (band rejection) that suppresses only light in a specific wavelength range. Filter), and a step type filter (edge filter) in which transmission and suppression are reversed at a certain wavelength.

  Of these, especially in the band elimination filter and the staircase filter, it is required that the residual transmittance of light in the wavelength band to be suppressed is as small as possible, and that the switching from the transmission wavelength band to the suppression wavelength band is steep. . The latter can also be expressed as requiring that the width of the transition wavelength band existing between the transmission and suppression wavelength bands be as narrow as possible. These characteristics are indispensable when constructing a device in which only a specific wavelength channel must be selectively passed or removed in optical fiber communication using a plurality of laser beams having different wavelengths as signals. It is also essential when, for example, laser Raman spectroscopy is used to selectively remove only the high-intensity excitation light wavelength component from the laser from the observation field.

  As an optical wavelength filter configuration method for realizing the above-mentioned two important characteristics, that is, a low residual transmittance and a narrow transition wavelength width, a dielectric alternate multilayer film structure has been conventionally used (Patent Document 1, Patent). Reference 2).

Refractive index n _1, a film material 1 is thickness d _1, in alternate multilayer film in which the refractive index is the n _2, the film material 2 has a thickness of d _2, generally when the light wavelength is lambda _0, optical cutoff range by Bragg reflection in the vicinity of the wavelength satisfying the _{(n 1 d 1 + n 2} d 2) = mλ 0/2, i.e. the band gap arises. The order m is an integer of 1 or more. In this band gap, most of the light power is reflected from the multilayer film and cannot be transmitted, so the wavelength band of this band gap is used as a band elimination filter. The wavelength band near the end of the band gap is used as an edge filter. A deep band gap, a small residual transmittance, and a narrow transition region are almost equivalent. In the case of an alternating multilayer film composed of flat films, the band gap width expressed in units of light frequency is proportional to the light attenuation rate per period of the film at the center frequency of the band gap. Therefore, in order to provide a deep band gap, it is necessary to widen the width of the band gap.

Now, when the film material 1 and the film material 2 are provided, the widest and deepest band gap is realized when n ₁ d ₁ = n ₂ d ₂ is satisfied. In other words, when the film material and the total number of films to be manufactured are determined in advance, a band removal filter and an edge filter with performance exceeding this cannot be realized with a flat multilayer film structure.
JP 2006-023471 A JP 2004-45853 A Japanese Patent No. 3325825 Shojiro Kawakami, Yasuo Ohtera, Takayuki Kawashima, “Preparation of Photonic Crystals and Application to Optical Devices”, Applied Physics, Vol. 68, No. 12, pp. 1335-1345, December 1999 issue C. C. Cheng, A.D. Sherer, R.A. Tyan, Y.M. Fainmann, G.M. Witzgall, and E.W. Yablonovich, “New fabrication technologies for high quality photonic crystals,” Journal of Vacuum Science Technology B, vol. 15, no. 6, pp. 2764-2767, 1997 Yasuo Ohtera, Teppei Onuki, Yoshihiko Inoue, Shojiro Kawakami, “Wavelength Filter Array for Visible to Near-Infrared Regions Using Photonic Crystals”, 2006 Spring 53rd Joint Physics Conference on Applied Physics, 24p-L-2, March 24, 2006

An object of the present invention is to solve the problem of limitations related to the band gap depth and the width of the transition region, which the above-described conventional flat multilayer film type wavelength filter has.
An object of the present invention is to provide a wavelength filter having a wide band gap and a narrow transition wavelength region.

The invention according to claim 1 has an alternate multilayer film structure made of materials having different refractive indexes in the z direction in the three-dimensional space xyz,
In the x direction, the refractive index periodically changes with a period p, and in the y direction, the refractive index periodically changes with q (q ≧ p), and is incident parallel to the z axis. is a wavelength filter which is characterized in that the _{p ≦ λ 0, q> (} λ 0/3) to meet the time of the wavelength in vacuum of optical and lambda _0.

The invention according to claim 2 has an alternate multilayer film structure made of materials having different refractive indexes in the z direction in the three-dimensional space xyz,
In the x direction, the refractive index fluctuates periodically with a period p, and in the y direction, the refractive index distribution is uniform, and the wavelength of light incident in parallel to the z axis in vacuum is determined. lambda ₀ to time (λ _0/3) <a wavelength filter which is characterized in that so as to satisfy p ≦ λ _0.

  According to a third aspect of the present invention, in the structure, two or more kinds of substances are periodically and sequentially laminated on a substrate having two-dimensional periodic unevenness, and at least a part of the whole lamination is sputtered. 3. The wavelength filter according to claim 1, wherein the wavelength filter is a structure formed by using etching alone or simultaneously with film formation.

  The invention according to claim 4 is characterized in that the structure is a structure in which a row of grooves or a row of holes periodically formed in an xy plane is formed on a flat alternating multilayer film. The wavelength filter according to claim 1 or 2.

  In the present invention, the problem is solved by adopting a multilayer film structure in which the refractive index is periodically modulated also in the in-plane direction. In a multilayer film structure having a refractive index periodicity also in the plane, a band gap is generated due to the electromagnetic field mode propagating in the in-plane direction. The frequency dependence of the attenuation constant in this band gap is compared with the conventional planar multilayer structure. It is possible to make it steeper than the attenuation constant of the film by designing the in-plane refractive index modulation period and the modulation structure.

That is, in the present invention, the multilayer film is not a flat film but a film structure whose structure periodically changes in a plane parallel to the substrate.
In the three-dimensional space xyz, when the substrate surface of the filter is the xy surface and the normal direction of the substrate is z, the refractive index distribution is changed only in the z direction in the conventional flat multilayered wavelength filter. Rate periodic structure ”(Patent Document 1, Patent Document 2).

  In contrast, in the present invention, a “two-dimensional refractive index periodic structure” or a “three-dimensional refractive index periodic structure” in which the refractive index distribution periodically fluctuates also in the x direction or the two directions x and y is used. For example, consider a two-dimensional periodic structure with refractive index modulation in the x direction. The modes of light propagating in the two-dimensional periodic structure include modes having substantially the same properties as propagating in the one-dimensional periodic structure while traveling straight in the z direction, as well as modes propagating in the x direction and the z direction. There is an in-plane propagation type electromagnetic field mode generated by coupling modes propagating to each other, and a new band gap is also generated by the influence. This in-plane propagation mode bandgap is sometimes deeper than the conventional bandgap in the one-dimensional periodic structure, and the width of the transition region can be narrowed. Therefore, this effect is utilized.

  As a specific form of the two-dimensional or three-dimensional periodic structure, a “self-cloning corrugated film structure” produced by depositing a multilayer film on a substrate on which periodic grooves or rows of holes are formed ( Patent Document 3, Non-Patent Document 1) and a structure in which a flat alternating multilayer film is once laminated on a substrate, and then a periodic groove or a row of holes is etched from the surface (for example, described in Non-Patent Document 2) and so on.

Incidentally, in claim 1, preferred ranges than in case of a p ≦ lambda ₀ is _{λ 0/3 ≦ p ≦ λ} 0. As a result, the effects of a transition wavelength width narrower than that of the flat film structure and a deep band gap characteristic can be obtained. The lower limit of p, λ _0/3 are preferable. When the lower limit is exceeded, the characteristics of the transition wavelength width and the band gap depth are the same as or lower than those of the flat film structure.
In claim 2, a more preferred range is _{λ 0/3 ≦ p ≦ λ} 0. It is a lower limit. In (λ _0/3) below, the characteristics of the transition wavelength width and the bandgap depth is lost greatly superior effects than the characteristic of the flat film structure.
In the present invention, a condition for causing a deep band gap by combining a mode propagating perpendicularly to a film also present in a flat multilayer film and an in-plane propagation mode appearing by forming a two-dimensional periodic structure is 2 Assuming the simplest “vacant lattice model” where the refractive index of the dimensional grating is assumed to be 1, where p is the in-plane lattice constant, λ _{0 is} the wavelength, and a is the stacking period, p = a = λ ₀ It is expressed. In an actual multilayer film structure having a refractive index distribution, the coupling between the two modes is gradually weakened, and the band gap is shallow and gentle as it deviates from this condition.
The lower limit of p is p = λ _0/3. When p becomes smaller than this, the band gap becomes equal to or shallower than that of the flat multilayer structure, and it is impossible to obtain performance superior to that of the conventional flat film.
The upper limit is p = λ ₀ , and even when p is larger than this, the coupling between the two modes is similarly weakened.
Since the band gap becomes shallow, it becomes impossible to obtain performance superior to that of the conventional flat film.

  For a given film material 1 and film material 2, a multilayer film band having a deeper optical band gap and a narrower transmission / suppression transition wavelength width than the conventional flat multilayer film structure at the same total number of film layers. A removal filter and an edge filter are provided. When the upper limit of the total number of films is determined due to manufacturing process limitations such as film thickness fluctuations due to lamination, it is possible to provide a wavelength filter with performance superior to that of a flat multilayer film structure. On the other hand, even if the total number of films is not particularly limited, the filter can be configured with a small number of layers to satisfy the same specification value. Is advantageous.

FIG. 1 shows an embodiment of the present invention. As a film material, for example, a combination of an Nb ₂ O ₅ layer 101 (refractive index of about 2.3) and a SiO ₂ layer 102 (refractive index of about 1.5) is used for visible to near-infrared wavelengths. The high refractive index film may be Ta ₂ O ₅ , TiO ₂ , HfO ₂ or the like instead of Nb ₂ O ₅ , or Si, Ge, ZnSe or the like in the infrared region having a wavelength of 1300 nm or more. Further, the material on the low refractive index side is not limited to SiO ₂ .

In the three-dimensional space xyz, the surface of the substrate 103 is xy, and a multilayer film structure is formed in the z direction. A row of grooves extending in the y direction is formed on the substrate, and a film is formed thereon by a self-cloning method to form a corrugated multilayer film. In this case, the refractive index period is provided in two directions, x and z. Light is incident from a direction parallel to the z-axis or inclined from the z-axis as necessary. For the wavelength λ ₀ to be used, the period in the in-plane direction, that is, the groove interval p is set so as to satisfy p ≦ λ ₀ <3p. The optimum value within this range is to determine the film thickness, the angle of the slope of the corrugated sheet, the total number of layers, the refractive index of the film material, etc. It asks by designing. Although Patent Document 4 and Non-Patent Document 3 also describe a method of using a self-cloning photonic crystal as various wavelength filters, the film thickness and the structural constant are not optimized. The performance exceeding the wavelength filter by the flat multilayer film is not obtained.

The present invention has a structure having performance exceeding the performance of a conventional filter (maximum band gap depth, transition wavelength band width) while satisfying an in-plane lattice spacing and operating wavelength satisfying p ≦ λ ₀ <3p. Is to provide.

  In the following, an actual embodiment will be shown.

(Self-cloning type periodic structure)
An example of an edge filter having a self-cloning corrugated multilayer structure will be described. FIG. 2 shows a self-cloning corrugated multilayer film composed of an Nb ₂ O ₅ layer 201 having a refractive index n ₁ = 2.28 and an SiO ₂ layer 202 having a refractive index n ₂ = 1.47. Here, the inclination of the slope of the corrugated plate with respect to the substrate surface was 45 °. Further, the film thickness d ₁ of the Nb ₂ O ₅ layer and the film thickness d ₂ of the SiO ₂ layer satisfy values of n ₁ d ₁ = n ₂ d ₂ , and d ₁ = 156.8 nm and d ₂ = 243.2 nm. did. The period L in the stacking direction is L = d ₁ + d ₂ = 400 nm. The period p in the x-axis direction is 420 nm. FIG. 3 shows calculated values of the light dispersion relation in this structure when it is assumed that the periodic structure continues infinitely in both the x and z directions. This calculation was performed using the finite difference time domain method. Only the mode in which the vibration direction of the electric field is parallel to the y-axis (herein referred to as TE mode) is displayed. Also, only the even symmetric mode (the electric field distribution is even symmetric with respect to the mirror symmetry plane) that can be coupled with externally perpendicular incident light is displayed in the TE mode.

FIG. 4 shows the dispersion relation of light propagating in the conventional flat multilayer film in parallel with the film normal. 3 and 4, the thickness and refractive index of each layer are the same. In each figure, the “Bloch wave number” on the horizontal axis of the left figure is the amount of phase change felt by light every time it travels through the film. In addition, the “attenuation constant” on the horizontal axis on the right side of each figure corresponds to the amount of attenuation that the amplitude of light receives each time it travels through the film. The vertical axis represents the ratio between the stacking period L and the wavelength λ ₀ and corresponds to the normalized optical frequency. A frequency region where the attenuation constant is 0 is a light passband, that is, a frequency region where light is transmitted beyond the film, and a frequency region where the attenuation constant is a finite value is a stop band. In the conventional dispersion relationship, the region indicated by 401 is a band gap that gives the maximum width and depth. In the dispersion relation of the present invention, a plurality of band gaps newly appear in each frequency band indicated by 302 due to the appearance of the band 301 in the in-plane propagation mode, which is used. The attenuation constant of the portion indicated by the special 303 has a tendency to increase rapidly with the optical frequency. This is the source of transition region characteristics between narrow passbands and stopbands. A stop band having such a steep attenuation characteristic can also occur in a high-order band gap on the high frequency side according to the period and refractive index in each direction of xz.

FIG. 5 and FIG. 6 each show the transmittance when the number of layer films is 40 layers (20 periods in terms of lamination period) in the above-described film configuration. FIG. 5 shows the self-cloning type of the present invention. A corrugated film structure, FIG. 6 shows a conventional flat film structure. The horizontal axis represents the optical frequency, and the vertical axis represents the light power transmittance in units of decibels (dB). In the structure of the present invention shown in FIG. 5, the plane of polarization of light is in the y direction, that is, the vibration direction of the electric field is in the y-axis direction. Although the frequency at which the band gap appears is different between the two structures, the transition characteristics on the low frequency side indicated by the reference numeral 501 in which the horizontal width of the horizontal axis is unified to 120 THz in the respective drawings for comparison are extremely high in the structure of the present invention. It is steep. It can also be seen that the maximum band gap depth 502 is deeper in the present invention.
In the present embodiment, the frequency band in which the band gap is generated is in the range of about 345 THz to 400 THz, and when expressed in wavelength, it is about 750 nm ≦ λ ₀ ≦ 870 nm. That is, 1.7p ≦ λ ₀ ≦ 2.1p.

In the present invention, it is essential to use a band gap such as 302 generated by the combination of the mode propagating perpendicular to the film and the in-plane propagation mode, so that such mode coupling and attenuation characteristics are generated. It is important to determine p, d ₁ , d _{2 and the} like according to λ ₀ . That is, the structural constants and material constants of the membrane material necessary for realizing the present invention by the self-cloning multilayer film structure are not limited to the values shown in this example. In the present embodiment, the configuration example for the TE wave has been described. However, it should be understood that the configuration may operate with respect to the TM wave (opposite to the TE wave, the polarization direction in which the vibration direction of the magnetic field is parallel to the y-axis). Since a band gap due to the same in-plane propagation mode is generated, a configuration using this band gap is also included in the scope of the present invention. Although an example of a two-dimensional refractive index periodic structure in which a multilayer film is formed on a row of grooves is shown here, it is obtained by forming a multilayer film on a substrate on which periodic holes and protrusions are arranged. A dimensional self-cloning multilayer structure may be used. Since the three-dimensional structure also has a periodic modulation of the refractive index in the plane and an in-plane propagation mode is generated, by appropriately setting the distances p and q between the holes or protrusions according to λ ₀ , It is possible to obtain a sharp attenuation curve similar to that described above.

(Multilayer film structure with periodic grooves)
FIG. 7 shows an embodiment having a structure in which a row of periodic grooves is formed in a flat multilayer film so as to penetrate the film. For example, such a structure is obtained by laminating a flat multilayer film, forming a mask film with a metal or the like on the surface, forming a row of grooves in the mask by lithography, and then etching the multilayer film by reactive ion etching or the like. Can be manufactured. The film material and its refractive index were the same as in Example 1. That is, the layer 701 of the film material 1 is an Nb ₂ O ₅ layer having a refractive index n ₁ = 2.28, and the layer 702 of the film material 2 is an SiO ₂ layer having a refractive index n ₂ = 1.47. The portion of the groove 703 is air and the refractive index is 1.0. Further, the film thickness d ₁ of the Nb ₂ O ₅ layer and the film thickness d ₂ of the SiO ₂ layer are d ₁ = 196 nm and d ₂ = 304 nm as another combination satisfying n ₁ d ₁ = n ₂ d ₂ . The period L in the stacking direction is L = d ₁ + d ₂ = 500 nm. The interval between the grooves, that is, the refractive index modulation period p in the x direction was 500 nm, and the groove width w was 156.25 nm. The transmission spectrum when the total number of layers is 40 as in Example 1 is shown in FIG. Reference numeral 801 denotes a transmission spectrum of a TE wave (a linearly polarized wave whose electric field oscillation direction is parallel to the y-axis) of the structure of the present invention. Reference numeral 802 denotes a transmission spectrum of the original flat multilayer film without groove processing. In this figure, a band gap existing at a wavelength of 580 nm to 620 nm is used. In this case, the width of the band gap is narrower than that of the flat multilayer film, but the maximum depth is about 20 dB deeper than that, and the cut-off characteristic on the long wavelength side near the wavelength of 620 nm is also sharpened. In this embodiment, the relationship between the operating wavelength λ ₀ and the groove period p is 1.16p ≦ λ ₀ ≦ 1.24p.

In FIG. 9, the groove thickness is 250 nm with the same film thickness and the same groove interval p. In this case, the steep and deep band gap exists from the wavelength 535 nm to 600 nm, and the relationship between the operating wavelength λ ₀ and the groove period p is 1.07p ≦ λ ₀ ≦ 1.2p. Note here The film thickness of each layer shows an embodiment for the case that satisfies _{n 1 d 1 = n 2 d} 2 is of course the essence of the present invention is not limited to this thickness relationship. The same applies to the width of the groove. In general, the refractive index of the medium filling the groove may be other than 1.0. Even in the structure other than that shown in this embodiment, it is essential to appropriately set the groove interval and the groove width so that an in-plane propagation mode type band gap is generated according to the operating wavelength range and the film thickness of each layer. Therefore, the present invention is not limited to those having the structural constants and material constants described herein. In the present embodiment, the configuration example for the TE wave has been described. However, a configuration that operates with respect to the TM wave (the vibration direction of the magnetic field is parallel to the y-axis) is naturally a band gap caused by the same in-plane propagation mode. Is included in the scope of the present invention.

  The wavelength filter according to the present invention can be widely used in the fields of optical image measurement, optical fiber communication, laser spectroscopy, and the like for selectively removing light in a specific wavelength range with high efficiency. In particular, when used as an edge filter, it exhibits a steep cutoff characteristic that greatly exceeds the conventional flat multilayer film at the short wavelength end or the long wavelength end, so that a high-purity optical spectrum can be separated. The wavelength filter of the present invention can be widely used in all industrial fields where a flat multilayer film type wavelength filter is currently used.

It is a block diagram of the wavelength filter of this invention. It is a conceptual diagram of the wavelength filter which consists of a self-cloning type corrugated sheet multilayer film structure. It is a figure which shows the dispersion relationship of the light in the structure of FIG. It is a figure which shows the dispersion relationship of the light in a flat multilayer film structure. FIG. 3 is a transmission spectrum diagram for linearly polarized light having an electric field parallel to a groove when the total number of layers is 40 in the structure of FIG. 2. FIG. 3 is a transmission spectrum diagram when the layer thickness is the same as that of FIG. It is a conceptual diagram of the wavelength filter produced by forming a periodic groove | channel row | line | column in a flat multilayer film. 8 is a transmission spectrum of a structure having a groove width of 156.25 nm in the structure of FIG. 7 and a transmission spectrum of a flat multilayer film without grooves. FIG. 8 is a transmission spectrum of the structure of FIG. 7 having a groove width of 250 nm and a transmission spectrum of a flat multilayer film without grooves.

Explanation of symbols

101 Nb ₂ O ₅ layer 102 SiO ₂ layer 103 Substrate 201 Nb ₂ O ₅ layer 202 SiO ₂ layer 301 In-plane propagation mode dispersion relation 302 Band gap generated by in-plane propagation mode 303 Steep attenuation characteristics Dispersion curve 401 in the band gap having the maximum band gap 501 in the flat film structure The steep cutoff characteristic in the band gap which the structure of the present invention has 502 The maximum attenuation 701 in the band gap 702 of the film material 1 Film material 2 Layer 703 Periodically arranged grooves 801 Transmission spectrum according to this embodiment 802 Transmission spectrum of a conventional flat multilayer film

Claims (4)

In the three-dimensional space xyz, an alternating multilayer film structure made of materials having different refractive indexes is formed in the z direction,
In the x direction, the refractive index periodically changes with a period p, and in the y direction, the refractive index periodically changes with q (q ≧ p), and is incident parallel to the z axis. wavelength filter being characterized in that the _{p ≦ λ 0, q> (} λ 0/3) to meet the time of the wavelength in vacuum of optical and lambda _0. In the three-dimensional space xyz, an alternating multilayer film structure made of materials having different refractive indexes is formed in the z direction,
In the x direction, the refractive index fluctuates periodically with a period p, and in the y direction, the refractive index distribution is uniform, and the wavelength of light incident in parallel to the z axis in vacuum is determined. when the _{λ 0 (λ 0/3)} < wavelength filter being characterized in that so as to satisfy p ≦ λ _0. The structure is formed by sequentially laminating two or more kinds of substances on a substrate having two-dimensional periodic irregularities, and performing sputter etching alone or at least on a part of the whole lamination. The wavelength filter according to claim 1, wherein the wavelength filter is a structure formed by being used simultaneously. 3. The wavelength according to claim 1, wherein the structure is a structure in which rows of grooves or holes periodically formed in an xy plane are formed on a flat alternating multilayer film. filter.

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