JP4339755B2 - Optical multilayer film bandpass filter - Google Patents

Description

  The present invention relates to an optical multilayer film bandpass filter composed of a dielectric multilayer film.

As is well known, a plurality of Fabry-Perot (FP) interferometers are configured by alternately laminating a high refractive index dielectric thin film having a predetermined optical thickness and a low refractive index dielectric thin film. The optical multilayer film bandpass filter having a structure generates polarized light as the incident angle of light increases and becomes parallel to the incident surface when light is obliquely incident and transmitted at a predetermined incident angle. The p-wave (TM) in the direction and the s-wave (TE) in the orthogonal direction appear separately, and as the incident angle increases, the transmission bandwidth of the p-wave increases, and conversely, the transmission bandwidth of the s-wave decreases. , Separation of the p wave and the s wave becomes large. For example, FIG. 23 shows the transmittance characteristics of an optical multilayer film bandpass filter that transmits light having a transmission region of around 1550 nm. When the incident angle of incident light is 45 degrees, transmission of p waves is performed. There is a large difference between the p-wave transmission region and the s-wave transmission region, as indicated by the solid line and the s-wave transmission rate by the dotted line.

  In addition, the optical multilayer film bandpass filter is usually used as an edge filter that uses either the rising or falling portion of the transmission band where the transmittance changes rapidly. That is, it is used as a long wave pass filter (LPF) that transmits light of a predetermined wavelength or longer and a short wave pass filter (SPF) that transmits light of a predetermined wavelength or shorter.

  For this reason, in the optical multilayer film band-pass filter, it is necessary to reduce the polarization dependency of the transmittance, particularly when the optical multilayer film filter is used corresponding to light incident obliquely. Although there are few cases where the incident angle of light is about 0 to 15 degrees, there is little problem in practical use. However, when the incident angle of light is 15 degrees or more, for example, 25 to 45 degrees, or even 50 degrees, It becomes a problem.

Furthermore, for example, as shown in FIG. 24, in an optical fiber transmission system in which a plurality of different wavelength bands of light λ ₁ , λ ₂ , λ ₃ , and λ ₄ are simultaneously propagated to _one optical cable 101, the optical multiplexer 102, The optical multilayer bandpass filter 104 used in the optical demultiplexer 103 performs multiplexing and demultiplexing with the transmitted light λ ₂ and λ ₃ and the reflected light λ ₁ and λ _4. If the two optical paths Lx and Ly at right angles are perpendicular to each other, the apparatus configuration is simple, and the design and manufacture thereof are easy. For this reason, in applications where such a device configuration is simple, it is strongly required to reduce the polarization dependency of the transmittance when light is incident obliquely at an incident angle of 45 degrees.

  In order to reduce the polarization dependency of the transmittance, a basic block in which a high refractive index layer and a low refractive index layer are alternately arranged symmetrically on both sides of a cavity layer arranged in the center, and a connecting layer is provided. The optical thickness of the cavity layer is shifted by a predetermined amount from (1/2) of the center wavelength of the transmission wavelength, or the optical thickness of the high refractive index layer and the low refractive index layer is transmitted. By shifting a predetermined amount from (1/4) of the center wavelength of the wavelength, either the rising part or the falling part is sacrificed, and the transmission band of the s-wave whose transmission bandwidth is narrowed is shifted to change the polarization dependence. There is an edge filter that eliminates (for example, see Patent Document 1 and Patent Document 2).

  However, if only the polarization dependence of the transmittance of either the rising part or the falling part is reduced, only one filter can provide appropriate characteristics when only light in a predetermined wavelength range is transmitted. Cannot be obtained. For this reason, two edge filters, LPF and SPF, whose transmittance is less dependent on polarization, are arranged separately in the optical path, and light in a predetermined wavelength range is obtained by sequentially transmitting these LPF and SPF. There must be. For this purpose, since it is necessary to dispose two filters in the optical path, two sets of disposing members must be prepared and attached in order to dispose each filter. It took time to configure.

Furthermore, as an unpolarized thin film edge filter applicable to obliquely incident light having an incident angle of 45 degrees, H and L are high refractive index layers having a refractive index n _H = 2.28, respectively, and a refractive index n _L = 1.45. When the optical film thickness is (1/4) wavelength, the basic structure Z is
Z = 1.3H, 0.6L, 1.2H, 0.6L, 1.2H, 0.6L, 1.3H
7 layers of alternating layers,
(Air) / H, 1.02Z, Z ⁴ , 1.02Z, H, L, H / (Glass)
(For example, refer to Patent Document 3). However, in such a structure, since the laminated layers are repeatedly laminated with the basic structure Z, the maximum thickness is obtained at a portion where two layers of 1.3H are formed continuously. Is 2.6H (= 1.3H × 2), and the minimum thickness is 0.6L. The optical thickness is 4.33 times larger between the maximum thickness and the minimum thickness. It will have a difference. For this reason, since the difference in layer thickness is large, it is difficult to form a film with a designed layer thickness, and it may be difficult to obtain required characteristics.
JP-A-7-104122 JP-A-7-104123 U.S. Pat. No. 4,373,782

  The present invention has been made in view of the above situation, and the object of the present invention is to eliminate the need for two edge filters even if only light in a predetermined wavelength range is transmitted, and troublesome apparatus configuration. In addition, the polarization dependency of the transmittance with respect to the obliquely incident light is small, and the incident angle is large, for example, it can cope with obliquely incident light with an incident angle of 25 to 50 degrees, It is an object of the present invention to provide an optical multilayer film bandpass filter that has no significant difference between the layer thicknesses of the laminated layers, is easy to manufacture, and easily obtains designed characteristics.

The optical multilayer film bandpass filter of the present invention includes a substrate transparent to the target light, and the long wavelength side slope portion of the transmittance characteristic curve for the predetermined oblique incident light formed on one surface of the substrate is an optical multilayer film. The formed non-polarized short wave path filter portion and the short wavelength side slope portion of the transmittance characteristic curve for the predetermined oblique incident light formed on the other surface of the substrate are formed of an optical multilayer film. A non-polarized long wave pass filter unit, and a transmission region is formed by a long wavelength side transmission region of the short wave pass filter unit and a short wavelength side transmission region of the long wave pass filter unit. And
The short wave pass filter unit and the long wave pass filter unit include a high refractive index film formed of a dielectric and a low refractive index film formed of a dielectric having a refractive index smaller than that of the high refractive index film. Provided with a basic structure in which a plurality of basic blocks stacked alternately are stacked so that the layers formed of the same material are adjacent to each other between the adjacent basic blocks.
The refractive index films stacked in the short wave path filter section and the long wave path filter section are characterized in that the film thickness ratio of the maximum film thickness to the minimum film thickness is 3 or less. Is,
Furthermore, the short wave path filter unit or the long wave path filter unit is characterized by comprising a matching layer on at least one of the substrate side or the outer surface side ,
Further, the basic block of the short wave path filter unit is:
When the optical film thicknesses of the high refractive index film H and the low refractive index film L are represented by Ha and La in units of λa / 4 when the reference wavelength is λa,
(0.591Ha) (0.772La) (0.649Ha) (0.532La)
(0.649Ha) (0.772La) (0.591Ha)
However, λa: on the long wavelength side that shifts from the transmission region to the stop region of the transmittance characteristic curve
It is a reference wavelength at which the transmittance at the slope is 50%.
The basic block of the long wave pass filter unit is:
When the optical film thicknesses of the high refractive index film H and the low refractive index film L are expressed as Hb and Lb in units of λb / 4 when the reference wavelength is λb,
(0.976 Lb) (1.347 Hb) (1.347 Lb) (1.347 Hb)
(1.347 Lb) (1.347 Hb) (0.976 Lb)
However, λb: on the short wavelength side where the transmittance characteristic curve shifts from the transmission region to the stop region
The reference wavelength is such that the transmittance at the slope portion is 50%,
Further, the basic block of the short wave path filter unit is:
When the optical film thicknesses of the high refractive index film H and the low refractive index film L are represented by Hc and Lc in units of λc / 4 when the reference wavelength is λc,
(1.164Lc) (0.817Hc) (0.817Lc) (0.817Hc)
(0.817Lc) (0.817Hc) (1.164Lc)
However, λc: on the long wavelength side that shifts from the transmission region to the stop region of the transmittance characteristic curve
It is a reference wavelength at which the transmittance at the slope is 50%.
The basic block of the long wave pass filter unit is:
When the optical film thicknesses of the high refractive index film H and the low refractive index film L are represented by Hd and Ld in units of λd / 4 when the reference wavelength is λd,
(2.234Ld) (1.568Hd) (1.568Ld) (1.568Hd)
(1.568Ld) (1.568Hd) (2.234Ld)
However, λd: on the short wavelength side where the transmission characteristic curve shifts from the transmission region to the stop region
The reference wavelength is such that the transmittance at the slope portion is 50%,
Furthermore, the basic block formed by alternately stacking the high refractive index film and the low refractive index film of the short wave pass filter unit and the long wave pass filter unit has a total number of layers of 3 to 25, the optical thickness of the low refractive index film and the high refractive index film, Ru der those which is a 0-3 times the thickness of 1/4 the unit thickness of the reference wavelength.

  As is apparent from the above description, according to the present invention, even when only light in a predetermined wavelength range is transmitted, two edge filters are not required, the apparatus configuration is simple, and the transmittance is not biased. Even those that can deal with obliquely incident light having a small wave dependency and a large incident angle can be easily manufactured, and can provide an effect that it is easy to obtain designed characteristics.

  Embodiments of the present invention will be described below with reference to the drawings. Prior to describing the embodiments of the present invention, the background to the completion of the present invention will be described. The inventor forms both an SPF that transmits light of a predetermined wavelength or less and an LPF that transmits light of a predetermined wavelength or more into one substrate to form one optical multilayer bandpass filter. Therefore, it is considered that only light in a predetermined wavelength range can be transmitted without using two edge filters, and further, LPF and SPF formed on one substrate have a transmittance of required incident light respectively. I thought that it should be depolarized with little polarization dependency.

  However, when LPF and SPF are provided on one side of a substrate and a matching layer is provided between them to continuously form each forming layer, an error in film thickness caused when LPF and SPF are formed is reduced. Since it cannot be predicted, matching is difficult, interference ripples are likely to occur, and it is difficult to obtain the required performance. Further, by forming LPF and SPF on one side of one substrate, a thick film of each forming layer is provided only on one side of the substrate, so that the substrate is easily deformed by the stress of the film and has a required size. There is a risk that the substrate may be cracked, chipped or cracked.

  An optical multilayer film bandpass filter that has a predetermined transmission range and can transmit only light in a predetermined wavelength range, described below from the above situation, and the incident angle of light is 15 degrees or more, For example, with respect to oblique incident light as large as 25 to 45 degrees, and further 50 degrees, and particularly with respect to incident light with an incident angle of 45 degrees, the transmittance is less dependent on polarization and interference ripples are difficult to occur. Thus, the present invention was completed which is easy to manufacture and does not cause any deformation.

  First, a first embodiment will be described with reference to FIGS. 1 is a cross-sectional view, FIG. 2 is a diagram showing transmittance characteristics, FIG. 3 is a diagram showing a film configuration of the basic structure of the SPF section, and FIG. 4 is a film configuration of a basic block of the SPF section. 5 is a diagram showing the transmittance characteristics of the SPF part, FIG. 6 is a diagram showing the film configuration of the first matching layer, and FIG. 7 is a diagram showing the film configuration of the second matching layer. 8 is a diagram showing the film configuration of the basic structure of the LPF section, FIG. 9 is a diagram showing the film configuration of the basic block of the LPF section, and FIG. 10 is a diagram showing the transmittance characteristics of the LPF section. FIG. 11 is a diagram showing a film configuration of the third matching layer, and FIG. 12 is a diagram showing a film configuration of the fourth matching layer.

In this embodiment, the polarization dependency of the transmittance corresponding to oblique incident light having an incident angle of 15 to 50 degrees with respect to the vertical direction is small. 1 to 12, reference numeral 1 denotes an optical multilayer film bandpass filter, which is a high refractive index dielectric thin film having an optical film thickness of a predetermined thickness, such as Ta ₂ O ₅ having a refractive index n = 2.14. A dielectric thin film having a refractive index H and a refractive index lower than that of the high refractive index film H and having a predetermined optical thickness different from that of the high refractive index film H, for example, SiO ₂ having a refractive index n = 1.43. It has a basic structure configured by alternately laminating low refractive index films L.

  As shown in the cross-sectional view of FIG. 1, the configuration includes a short wave pass filter (SPF) unit 3 that transmits light of a predetermined wavelength or less on both surfaces of the substrate 2 and a long wave pass filter that transmits light of a predetermined wavelength or more. The (LPF) unit 4 is provided, and the transmittance characteristic thereof is, for example, as shown in FIG. 2, a characteristic having a transmission region in the range from 1475 nm to 1505 nm for obliquely incident light having an incident angle of 45 degrees. Thus, the separation of the p wave and the s wave is very small, and the polarization dependency of the transmittance is small.

  The optical multilayer bandpass filter 1 has a transmittance characteristic that shifts from the transmission region to the inhibition region on one side of the substrate 2 transparent to the target light made of glass having a refractive index n = 1.52, for example. A non-polarized SPF unit 3 is provided that transmits less light of a predetermined wavelength or less, with less polarization dependency of transmittance at the wavelength side portion. The SPF unit 3 is provided with a first matching layer 5 for matching with the substrate 2 on the substrate 2 side, and a second matching layer 6 for matching with an outer medium such as air on the outer surface side. In addition, the basic structure 7 of the SPF unit 3 is provided between the matching layers 5 and 6.

  On the other hand, on the other side of the substrate 2, the polarization dependence of the transmittance is small in the short wavelength side portion of the transmittance characteristic that shifts from the blocking region to the transmitting region, and light is not polarized so that light of a predetermined wavelength or more is transmitted. The LPF unit 4 is provided. As with the SPF unit 3, the LPF unit 4 is provided with a third matching layer 8 for matching with the substrate 2 on the substrate 2 side, and for matching with an outer medium, for example, air, on the outer surface side. The fourth matching layer 9 is provided, and the basic structure 10 of the LPF portion 4 is provided between the matching layers 8 and 9.

  As shown in FIG. 3, the basic structure 7 of the SPF unit 3 repeats the basic block 7a shown in FIG. 4 in which the high refractive index film H and the low refractive index film L are alternately laminated to form an odd layer structure a predetermined number of times. It has a laminated structure. The basic block 7a has a structure in which the optical film thicknesses (n × d) of the high refractive index film H and the low refractive index film L are in units of λa / 4 when the reference wavelength is λa. , La, the following configuration is shown. Note that n is the refractive index of the layer, and d is the physical film thickness of the layer.

That is, the configuration of the basic block 7a is a transmittance characteristic curve Sa, and the wavelength at which the transmittance at the long wavelength side slope portion Pa that shifts from the transmission region to the stop region is 50% is the reference wavelength λa.
(0.591Ha) (0.772La) (0.649Ha) (0.532La) (0.649Ha) (0.772La) (0.591Ha) (Formula 1)
It is the thing of the odd number layer shown by. At this time, the transmittance of the p wave and the s wave in the characteristic curve Sa crosses at a point where the transmittance is 50%.

By the way, at the center of symmetry of the basic block 7a in the stacking direction, the layer of (0.532La) which is the minimum layer thickness is replaced with a layer of (0.532La) = (1Lao), and (1Lao) is used as the unit layer. Rewriting (Equation 1) above,
(1.111 Hao) (1.452 Lao) (1.221 Hao) (1 Lao) (1.221 Hao) (1.452 Lao) (1.111 Hao) (Formula 2)
It becomes. Hao and Lao at this time are the optical film thicknesses of the high refractive index film H and the low refractive index film L when the reference wavelength is λao and λao / 4 is used as a unit. The reference wavelength λao is λao = 0.532 × λa.

  Since the basic structure 7 of the SPF unit 3 is configured by repeatedly stacking the basic blocks 7a configured as described above a predetermined number of times, as shown in FIG. In between, layers of (0.591 Ha) made of the same material are adjacent to each other. For this reason, the maximum layer thickness in the basic structure 7 is 1.182 Ha (= 0.591 Ha × 2), but since the minimum layer thickness for this is 0.532 La, between the maximum layer thickness and the minimum layer thickness, The optical film thickness is less than 3 times 2.22 times, and can be formed relatively easily to the designed layer thickness, and the required characteristics can be easily obtained.

  A case where the wavelength of 50% transmittance in the slope portion Pa on the long wavelength side of the characteristic curve Sa is 1520 nm and the incident light is oblique incident light with an incident angle of 45 degrees will be described. The SPF unit 3 in this case has a basic structure 7 in which the basic block 7a is repeated 17 times and 103 layers are stacked, and includes a first matching layer 5 and a second matching layer 6, and the SPF unit 3 The overall layer structure is a 141-layer structure. Further, the transmittance characteristics are as shown in FIG.

  The configuration of the first matching layer 5 on the substrate 2 side is a configuration of all 20 layers shown by calculating the layer thickness with the reference wavelength as shown in FIG. 6 being λao = 0.532 × 1520 nm = 808.64 nm. Yes, as between the basic blocks 7a, the 20th layer adjacent to the basic block 7a and the first matching layer 5 is a common layer having a double layer thickness by the outermost layer of the same material and the same thickness. It has become. The configuration of the second matching layer 6 on the outer surface side is a 20-layer configuration in which the reference wavelength as shown in FIG. 7 is set to λao = 0.532 × 1520 nm = 808.64 nm and the layer thickness is calculated. Similarly, the 122nd layer in which the basic block 7a and the second matching layer 6 are adjacent is a common layer having a double layer thickness by the outermost layer of the same material and the same layer thickness.

  By configuring the basic block 7a as described above, the separation of the p-wave indicated by the solid line and the s-wave indicated by the dotted line at the slope portion Pa on the long wavelength side is small, and the polarization dependency of the transmittance is small. Depolarized. Note that the slope portion Pa becomes steeper as the number of repetitions of the basic block 7a increases, and the isolation between the transmission region and the inhibition region improves.

  On the other hand, as shown in FIG. 8, the basic structure 10 of the LPF unit 4 includes the basic block 10a shown in FIG. 9 that has an odd layer configuration in which high refractive index films H and low refractive index films L are alternately stacked a predetermined number of times. It is the structure which laminated repeatedly. The basic block 10a has a structure in which the optical film thicknesses (n × d) of the high refractive index film H and the low refractive index film L are in units of λb / 4 when the reference wavelength is λb. , Lb, the following configuration is shown. Note that n is the refractive index of the layer, and d is the physical film thickness of the layer.

That is, the configuration of the basic block 10a is a characteristic curve Sb of transmittance characteristics, where the wavelength at which the transmittance at the slope portion Pb on the short wavelength side shifting from the stop band to the transmission band is 50% is the reference wavelength λb.
(0.976 Lb) (1.347 Hb) (1.347 Lb) (1.347 Hb) (1.347 Lb) (1.347 Hb) (0.976 Lb) (Formula 3)
It is the thing of the odd number layer shown by. At this time, the transmittance of the p wave and the s wave in the characteristic curve Sb intersects at a point where the transmittance is 50%.

By the way, the layer of (1.347Hb), which is the minimum layer thickness, is replaced with the layer of (1.347Hb) = (1Hbo) at the symmetry center of the basic block 10a in the stacking direction, and (1Hbo) is used as the unit layer. Rewriting (Equation 3) above,
(0.725Lbo) (1Hbo) (1Lbo) (1Hbo) (1Lbo) (1Hbo) (0.725Lbo) (Formula 4)
It becomes. Hbo and Lbo at this time are the optical film thicknesses of the high refractive index film H and the low refractive index film L when the reference wavelength is λbo and λbo / 4 is a unit. The reference wavelength λbo is λbo = 1.347 × λb.

  Since the basic structure 10 of the LPF unit 4 is configured by repeatedly stacking the basic blocks 10a configured as described above a predetermined number of times, as shown in FIG. In between, (0.976 Lb) layers of the same material are adjacent to each other. For this reason, the maximum layer thickness in the basic structure 10 is 1.952 Lb (= 0.976 Lb × 2), but the minimum layer thickness for this is 1.347 Hb, so between the maximum layer thickness and the minimum layer thickness, An optical film thickness of 1.45 times or less is 3 times or less, so that the film thickness can be formed relatively easily as designed, and the required characteristics can be easily obtained.

  The case where the wavelength of the 50% transmittance in the slope portion Pb on the short wavelength side of the characteristic curve Sb is 1470 nm and the incident light is oblique incident light with an incident angle of 45 degrees will be described. The LPF unit 4 in this case has a basic structure 10 in which the basic block 10a is repeated 12 times and 73 layers are stacked, and has a third matching layer 8 and a fourth matching layer 9, and the LPF unit 4 The overall layer structure is 97 layers. Further, the transmittance characteristics are as shown in FIG.

  The configuration of the third matching layer 8 on the substrate 2 side is a 13-layer configuration in which the reference wavelength as shown in FIG. 11 is set to λbo = 1.347 × 1470 nm = 1980.09 nm and the layer thickness is calculated. Yes, as between the basic blocks 10a, the 13th layer adjacent to the basic block 10a and the third matching layer 8 is a common layer having a double layer thickness by the outermost layers of the same material and the same layer thickness. It has become. The configuration of the fourth matching layer 9 on the outer surface side is a 13-layer configuration in which the reference wavelength as shown in FIG. 12 is set to λbo = 1.347 × 1470 nm = 1980.09 nm and the layer thickness is calculated. Similarly, the 85th layer in which the basic block 10a and the fourth matching layer 9 are adjacent is a common layer having a double layer thickness by the outermost layer of both the same material and the same layer thickness.

  By configuring the basic block 10a as described above, the separation of the p-wave indicated by the solid line and the s-wave indicated by the dotted line at the slope portion Pb on the short wavelength side is small, and the polarization dependency of the transmittance is small. It becomes a wave. As the number of repetitions of the basic block 10a increases, the slope portion Pb becomes steep as well, and the isolation between the transmission region and the blocking region is improved.

  The process of forming the SPF unit 3 and the LPF unit 4 on both surfaces of the substrate 2 is not shown, but first, for example, each layer of the SPF unit 3 is formed on one side of the substrate 2 and the other side is formed by a film forming apparatus. The film is formed so as to have a predetermined layer thickness in a state of being covered so as not to be formed. Thereafter, the substrate 2 on which the SPF portion 3 is formed is taken out of the apparatus and the cover on the other side is removed. Subsequently, each layer of the LPF unit 4 is formed on the other surface of the substrate 2 covered so as not to form a film on the formed SPF unit 3 so as to have a predetermined layer thickness. Then, the substrate 2 on which the LPF unit 4 is formed is taken out of the apparatus, and the cover of the SPF unit 3 is removed. Furthermore, processing such as cutting is performed to obtain a predetermined shape.

  As described above, the SPF portion 3 and the LPF portion 4 are formed on both surfaces of the substrate 2 by different film forming processes, so that the layer thickness of each layer of the SPF portion 3 and the LPF portion 4 is made accurate. The required performance can be easily obtained. When the SPF unit 3 and the LPF unit 4 are separately provided on both surfaces of the substrate 2 and are provided with substantially equal thicknesses, the substrate 2 is provided with each layer only on one side. Thus, there is no possibility that the substrate 2 is easily deformed without applying such a large biased film stress. Further, there is no risk of cracks, chips or cracks when the substrate 2 is cut.

  In the present embodiment, the case where the reference wavelengths λa and λb are set to 1520 nm and 1470 nm and the incident angle of the oblique incident light is 45 degrees has been verified, but other reference wavelengths λa and λb are set and the transmission region is ultraviolet light. 200 nm to infrared light of 5000 nm, and even oblique incident light with an incident angle in the range of 15 to 50 degrees, the SPF part and the LPF part are represented by the above (formula 1) and (formula 3). Similarly, an optical multilayer film band-pass filter having a desired transmission region and low transmittance dependency of polarization can be obtained.

Next, a second embodiment will be described with reference to FIGS. 13 is a cross-sectional view, FIG. 14 is a diagram showing transmittance characteristics, FIG. 15 is a diagram showing a film configuration of the basic structure of the SPF section, and FIG. 16 is a film configuration of a basic block of the SPF section. Figure
17 is a diagram showing the film configuration of the first matching layer, FIG. 18 is a diagram showing the film configuration of the second matching layer, and FIG. 19 is a diagram showing the film configuration of the basic structure of the LPF portion. 20 is a diagram showing the film configuration of the basic block of the LPF section, FIG. 21 is a diagram showing the film configuration of the third matching layer, and FIG. 22 is a diagram showing the film configuration of the fourth matching layer. is there.

In the present embodiment, the transmittance corresponding to incident light having an incident angle of 0 to 25 degrees with respect to the vertical direction is less dependent on polarization. 13 to 22, reference numeral 11 denotes an optical multilayer film bandpass filter, which is a high refractive index dielectric thin film having a predetermined optical thickness, such as Ta ₂ O ₅ having a refractive index n = 2.14. A dielectric thin film having a refractive index H and a refractive index lower than that of the high refractive index film H and having a predetermined optical thickness different from that of the high refractive index film H, for example, SiO ₂ having a refractive index n = 1.43. It has a basic structure configured by alternately laminating low refractive index films L.

  As shown in the cross-sectional view of FIG. 13, the configuration includes a short wave pass filter (SPF) unit 13 that transmits light of a predetermined wavelength or less on both surfaces of the substrate 2 and a long wave pass filter that transmits light of a predetermined wavelength or more. (LPF) portion 14 is provided, and the transmittance characteristic thereof is a predetermined value centered on 1500 nm with respect to obliquely incident light having an incident angle of 25 degrees as shown by a transmission characteristic curve Sx in FIG. 14, for example. The transmission characteristic has a transmission range in the range of p, and the separation of the p wave and the s wave is very small, and the polarization dependency of the transmittance is small.

  The optical multilayer film bandpass filter 11 has a transmittance characteristic that shifts from the transmission region to the inhibition region on one side of the substrate 2 transparent to the target light made of glass having a refractive index n = 1.52, for example. A non-polarized wave SPF section 13 is provided that transmits light having a wavelength equal to or less than a predetermined wavelength. The SPF unit 13 is provided with a first matching layer 15 for matching with the substrate 2 on the substrate 2 side, and a second matching layer 16 for matching with an outer medium such as air on the outer surface side. The basic structure 17 of the SPF unit 13 is provided between the matching layers 15 and 16.

  On the other hand, on the other side of the substrate 2, the polarization dependence of the transmittance is small in the short wavelength side portion of the transmittance characteristic that shifts from the blocking region to the transmitting region, and light is not polarized so that light of a predetermined wavelength or more is transmitted. The LPF unit 14 is provided. Similar to the SPF unit 13, the LPF unit 14 is provided with a third matching layer 18 for matching with the substrate 2 on the substrate 2 side, and for matching with an outer medium, for example, air, on the outer surface side. The fourth matching layer 19 is provided, and the basic structure 20 of the LPF portion 14 is provided between the matching layers 18 and 19.

  The basic structure 17 of the SPF unit 13 is formed by repeatedly laminating a basic block 17a shown in FIG. 16 having a odd number of layers by alternately laminating high refractive index films H and low refractive index films L as shown in FIG. It has become the composition. The basic block 17a has a structure in which the optical film thicknesses (n × d) of the high refractive index film H and the low refractive index film L are in units of λc / 4 when the reference wavelength is λc. , Lc, the following configuration is shown. Note that n is the refractive index of the layer, and d is the physical film thickness of the layer.

That is, the configuration of the basic block 17a is a transmittance characteristic curve Sx, where the wavelength at which the transmittance at the long wavelength side slope portion Pc that shifts from the transmission region to the stop region is 50% is the reference wavelength λc.
(1.164Lc) (0.817Hc) (0.817Lc) (0.817Hc) (0.817Lc) (0.817Hc) (1.164Lc) (Formula 5)
It is the thing of the odd number layer shown by. At this time, the transmittance of the p wave and the s wave in the characteristic curve Sx intersects at a point where the transmittance is 50%.

By the way, the layer of (0.817Hc), which is the minimum layer thickness, is replaced with the layer of (0.817Hc) = (1Hco) at the center of symmetry of the basic block 17a in the stacking direction, and (1Hco) is used as the unit layer. Rewriting (Equation 5) above,
(1.425Lco) (1Hco) (1Lco) (1Hco) (1Lco) (1Hco) (1.425Lco) (Formula 6)
It becomes. Hco and Lco at this time are the optical film thicknesses of the high refractive index film H and the low refractive index film L when the reference wavelength is λco and λco / 4 is a unit. The reference wavelength λco is λco = 0.817 × λc.

  Since the basic structure 17 of the SPF unit 13 is configured by repeatedly stacking the basic blocks 17a configured as described above a predetermined number of times, as shown in FIG. In between, (1.164Lc) layers of the same material are adjacent to each other. For this reason, the maximum layer thickness in the basic structure 17 is 2.328 Lc (= 1.164 Lc × 2), but the minimum layer thickness for this is 0.817 Hc, and therefore, between the maximum layer thickness and the minimum layer thickness, The optical film thickness can be relatively easily reduced to 3 times less than 2.85 times, so that the film thickness can be formed relatively easily as designed, and the required characteristics can be easily obtained.

  Note that the first matching layer 15 and the second matching layer 16 provided on both sides in the stacking direction of the basic structure 17 of the SPF portion 13 have a film configuration shown in FIGS. 17 and 18, for example. The configuration of the first matching layer 15 is a 13-layer configuration in which the reference wavelength as shown in FIG. 17 is set to λco = 0.817 × λc and the layer thickness is calculated, and similarly to the basic block 17a, The thirteenth layer adjacent to the basic block 17a and the first matching layer 15 is a common layer having a double layer thickness by the outermost layer of both the same material and the same layer thickness. The configuration of the second matching layer 16 on the outer surface side is a 13-layer configuration in which the reference wavelength is calculated as shown in FIG. 18 and the layer thickness is calculated by λco = 0.817 × λc. The (Nc-12) layer in which the basic block 17a and the second matching layer 16 are adjacent is a common layer having a double layer thickness by the outermost layer of both the same material and the same layer thickness. However, Nc is the total number of layers of the SPF unit 13.

  On the other hand, the basic structure 20 of the LPF unit 14 is formed by repeatedly laminating a basic block 20a shown in FIG. 20 having an odd number of layers by alternately stacking high refractive index films H and low refractive index films L as shown in FIG. It has become the composition. The basic block 20a has a structure in which the optical film thicknesses (n × d) of the high refractive index film H and the low refractive index film L are in units of λd / 4 when the reference wavelength is λd. , Ld, the following configuration is shown. Note that n is the refractive index of the layer, and d is the physical film thickness of the layer.

That is, the configuration of the basic block 20a is a transmittance characteristic curve Sx, where the wavelength at which the transmittance at the slope portion Pd on the short wavelength side shifting from the blocking region to the transmitting region is 50% is the reference wavelength λd.
(2.234Ld) (1.568Hd) (1.568Ld) (1.568Hd) (1.568Ld) (1.568Hd) (2.234Ld) (Formula 7)
It is the thing of the odd number layer shown by. At this time, the transmittance of the p wave and the s wave in the characteristic curve Sx intersects at a point where the transmittance is 50%.

By the way, the layer of (1.568Hd), which is the minimum layer thickness, is replaced with the layer of (1.568Hd) = (1Hdo) at the center of symmetry of the basic block 20a in the stacking direction, and (1Hdo) is used as the unit layer. Rewriting (Equation 7) above,
(1.425Ldo) (1Hdo) (1Ldo) (1Hdo) (1Ldo) (1Hdo) (1.425Ldo) (Equation 8)
It becomes. Hdo and Ldo at this time are optical film thicknesses of the high refractive index film H and the low refractive index film L when the reference wavelength is λdo and λdo / 4 is a unit. The reference wavelength λdo is λdo = 1.568 × λd. Note that the layer thickness of each layer is the same as that of the basic block 17a of the SPF unit 13 on the basis of the unit layer.

  Since the basic structure 20 of the LPF unit 14 is configured by repeatedly stacking the basic blocks 20a configured as described above a predetermined number of times, as shown in FIG. In between, adjacent layers of (2.234 Ld) are formed of the same material. For this reason, the maximum layer thickness in the basic structure 20 is 4.468 Ld (= 2.234 Ld × 2), but since the minimum layer thickness is 1.568 Hd, between the maximum layer thickness and the minimum layer thickness, The optical film thickness is 2.85 times, which is not more than 3 times, and the film thickness can be formed relatively easily as designed, and the required characteristics can be easily obtained.

  Note that the third matching layer 18 and the second matching layer 19 provided on both sides in the stacking direction of the basic structure 20 of the LPF portion 14 have a film configuration shown in FIGS. 21 and 22, for example. The configuration of the third matching layer 18 is a 13-layer configuration in which the reference wavelength as shown in FIG. 21 is set to λdo = 1.568 × λd and the layer thickness is calculated, and as in the basic block 20a, The 13th layer adjacent to the basic block 20a and the third matching layer 18 is a common layer having a double layer thickness by the outermost layer of both the same material and the same layer thickness. Further, the configuration of the fourth matching layer 19 on the outer surface side is a 13-layer configuration in which the reference wavelength as shown in FIG. 22 is calculated and shown by calculating the layer thickness with λdo = 1.568 × λd. The (Nd-12) layer in which the basic block 20a and the fourth matching layer 19 are adjacent to each other is a common layer having a double layer thickness by the outermost layer of both the same material and the same layer thickness. Nd is the total number of layers of the LPF unit 14.

  And about what has the said structure, the wavelength of the 50% transmittance | permeability in the long wavelength side slope part Pc of the characteristic curve Sx is 1505 nm, and the wavelength of the 50% transmittance in the short wavelength side slope part Pd is 1495 nm. The case where the incident light is oblique incident light having an incident angle of 25 degrees will be described. The SPF unit 13 in this case has a basic structure 17 in which the basic block 17a is repeated 16 times and 97 layers are stacked, and includes a first matching layer 15 and a second matching layer 16, and the SPF unit 13 The overall layer structure is 121 layers.

  In this case, the LPF unit 14 has a basic structure 20 in which the basic block 20a is repeated 16 times and 97 layers are stacked, and has a third matching layer 18 and a fourth matching layer 19, and the LPF. Similarly, the layer configuration of the entire portion 14 is 121 layers. The transmittance characteristics of the SPF section 13 and the LPF section 14 are as shown in FIG. 14. In the long wavelength side slope portion Pc and the short wavelength side slope portion Pd, The p-wave indicated by the solid line and the s-wave indicated by the dotted line are substantially coincident with each other, and the polarization dependency of the transmittance is very little depolarized. Note that the slope portions Pc and Pd become steeper as the number of repetitions of the basic blocks 17a and 20a increases, and the isolation between the transmission region and the blocking region is improved.

  Further, the transmittance characteristics for oblique incident light with an incident angle of 18 degrees and incident light from a vertical direction with an incident angle of 0 degrees are shown in FIG. 14 when the incident angle is 18 degrees and the characteristic curve Sy, and when the incident angle is 0 degrees. Is represented by the characteristic curve Sz, and the transmission region moves depending on the incident angle of light, and the p wave indicated by the solid line and the s wave indicated by the dotted line substantially coincide with each other. Wave dependence is very low. Further, with such a configuration, it is possible to provide an optical multilayer film band-pass filter having a small wavelength dependence and a large wavelength variable range with respect to each incident angle of light.

  The process of forming the SPF unit 13 and the LPF unit 14 on both surfaces of the substrate 2 is not shown, but first, for example, in a state where the other surface side is covered with a film forming apparatus so that the other surface side is not formed. Each layer of the SPF unit 13 is formed to have a predetermined layer thickness. Thereafter, the substrate 2 on which the SPF portion 13 is formed is taken out of the apparatus, and the cover on the other side is removed. Subsequently, each layer of the LPF unit 14 is formed on the other surface of the substrate 2 covered so as not to form a film on the formed SPF unit 13 so as to have a predetermined layer thickness. Then, the substrate 2 on which the LPF unit 14 is formed is taken out of the apparatus, and the cover of the SPF unit 13 is removed. Furthermore, processing such as cutting is performed to obtain a predetermined shape.

  In this way, the SPF portion 13 and the LPF portion 14 are formed on both surfaces of the substrate 2 by separate film forming processes, so that the layer thickness of each layer of the SPF portion 13 and the LPF portion 14 is made accurate. The required performance can be easily obtained. When the SPF unit 13 and the LPF unit 14 are provided separately on both surfaces of the substrate 2 and are provided with substantially equal thicknesses, the substrate 2 is provided with each layer only on one side. Thus, there is no possibility that the substrate 2 is easily deformed without applying such a large biased film stress. Further, there is no risk of cracks, chips or cracks when the substrate 2 is cut.

  In the present embodiment, the transmission range of 1500 nm to 1555 nm has been described. However, the SPF portion and the LPF portion of the transmission range from 200 nm of ultraviolet light to 5000 nm of infrared light are represented by the above (formulas). 5) By adopting the configuration of (Expression 7), it is possible to depolarize light with an incident angle of 0 to 25 degrees.

  In each of the above embodiments, each of the basic blocks 7a, 10a, 17a, and 20a of the SPF units 3 and 13 and the LPF units 4 and 14 has an odd layer configuration of 7 layers. However, the range is from 3 layers to 25 layers. The high refractive index film H and the low refractive index film L are alternately laminated to constitute each basic block, and the optical film thickness of each high refractive index film H and low refractive index film L is determined by the transmittance at each slope portion. By setting the coefficient in the range of 0 to 3 appropriately and multiplying the thickness in units of (1/4) of the reference wavelength, which is 50%, it has the desired transmission region in the same way It is possible to obtain an optical multilayer film band-pass filter having less polarization dependency of transmittance. Further, the order of the high refractive index film H and the low refractive index film L in the basic blocks of the SPF part and the LPF part is not limited to the above, but the order of (high refractive index film H, low refractive index film L,...). Any of the alternating layers (low refractive index film L, high refractive index film H,...) In this order may be used.

  Further, although not shown, an intermediate refractive index film M made of different dielectric thin films having a refractive index between the refractive indices of the high refractive index film H and the low refractive index film L is provided in each of the SPF part and the LPF part. The high refractive index film H or the low refractive index film L of the basic block may be inserted and formed, and the optical film thickness of the high refractive index film H, the low refractive index film L, and the intermediate refractive index film M at that time Is a thickness obtained by multiplying the thickness in units of (1/4) of the reference wavelength at which the transmittance at each slope portion is 50% by appropriately setting a coefficient in the range of 0 to 5, The same effect can be obtained even if an optical multilayer bandpass filter is formed.

  Furthermore, in each of the above embodiments, the basic blocks 7a, 10a, 17a, and 20a of the SPF units 3 and 13 and the LPF units 4 and 14 change the reference wavelengths λa, λb, λc, and λd to the slopes of the transmittance characteristics. Although the transmittance characteristics of the p-wave and s-wave intersect at the 50% transmittance points of the portions Pa, Pb, Pc, and Pd, they are used even at wavelengths at the transmittance points in the range of 50% ± 20%. Depending on the case, it can be made non-polarized, which is practical.

As for the filter constituent members, the substrate 2 is made of, for example, white glass of non-alkali glass or borosilicate glass, quartz glass or quartz mainly composed of silicon dioxide, or, for example, BK7 (trade name), WMS-13 (trade name). And optical glass such as WMS-15 (trade name), sapphire, LiNbO ₃ , CaF ₂ , silicon, semiconductor substrate, synthetic resin, and the like.

The high refractive index film H and the low refractive index film L are, for example, TiO ₂ , Y ₂ O ₃ , Ta ₂ O ₅ , ZrO, ZrO ₂ , Si, ZnS, HfO ₂ , Ge, Nd ₂ O ₆ , Nb _{2. O 5, CeO 2, ZnO,} Fe 2 O 3, SiO 2, MgF 2, AlF 3, CaF 2, LiF, Na 3 AlF 6, Na 5 Al 3 F 14, Al 2 O 3, CeF 3, MgO, LaF ₃ , PbF ₂ , NdF _3, etc., or a mixed material thereof, and at least two types are selected, and the selected materials are deposited, sputtered, ion plating PVD (physical vapor deposition), for example, resistance heating Vapor Deposition, Electron Beam (EB) Heat Deposition, High Frequency Heat Deposition, Laser Beam Heat Deposition, Ionized Sputter, Ion Beam Sputter, Plasma Sputter, Ion Assist Law, adopts one of the radicals assisted sputtering, may be alternately stacked.

It is sectional drawing which shows the 1st Embodiment of this invention. It is a figure which shows the transmittance | permeability characteristic of the 1st Embodiment of this invention. It is a figure which shows the film | membrane structure of the basic structure of the SPF part in the 1st Embodiment of this invention. It is a figure which shows the film | membrane structure of the basic block of the SPF part in the 1st Embodiment of this invention. It is a figure which shows the transmittance | permeability characteristic of the SPF part in the 1st Embodiment of this invention. It is a figure which shows the film | membrane structure of the 1st matching layer in the 1st Embodiment of this invention. It is a figure which shows the film | membrane structure of the 2nd matching layer in the 1st Embodiment of this invention. It is a figure which shows the film | membrane structure of the basic structure of the LPF part in the 1st Embodiment of this invention. It is a figure which shows the film | membrane structure of the basic block of the LPF part in the 1st Embodiment of this invention. It is a figure which shows the transmittance | permeability characteristic of the LPF part in the 1st Embodiment of this invention. It is a figure which shows the film | membrane structure of the 3rd matching layer in the 1st Embodiment of this invention. It is a figure which shows the film | membrane structure of the 4th matching layer in the 1st Embodiment of this invention. It is sectional drawing which shows the 2nd Embodiment of this invention. It is a figure which shows the transmittance | permeability characteristic of the 2nd Embodiment of this invention. It is a figure which shows the film | membrane structure of the basic structure of the SPF part in the 2nd Embodiment of this invention. It is a figure which shows the film | membrane structure of the basic block of the SPF part in the 2nd Embodiment of this invention. It is a figure which shows the film | membrane structure of the 1st matching layer in the 2nd Embodiment of this invention. It is a figure which shows the film | membrane structure of the 2nd matching layer in the 2nd Embodiment of this invention. It is a figure which shows the film | membrane structure of the basic structure of the LPF part in the 2nd Embodiment of this invention. It is a figure which shows the film | membrane structure of the basic block of the LPF part in the 2nd Embodiment of this invention. It is a figure which shows the film | membrane structure of the 3rd matching layer in the 2nd Embodiment of this invention. It is a figure which shows the film | membrane structure of the 4th matching layer in the 2nd Embodiment of this invention. It is a figure which shows the transmittance | permeability characteristic of the optical multilayer film band pass filter of a prior art. It is a figure which shows schematic structure of an optical fiber transmission system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 2 ... Board | substrate 3,13 ... SPF part 4,14 ... LPF part 5,15 ... 1st matching layer 6,16 ... 1st matching layer 7,17 ... SPF part basic structure 7a, 17a ... SPF part basic block 8 , 18 ... third matching layer 9, 19 ... fourth matching layer 10, 20 ... LPF part basic structure 10a, 20a ... LPF part basic block

Claims (5)

A substrate that is transparent to the target light, and a long wavelength side slope portion of the transmittance characteristic curve for a predetermined oblique incident light formed on one surface of the substrate is a non-polarized short wave formed by an optical multilayer film. A non-polarized long wave pass filter in which a short wavelength side slope portion of a transmittance characteristic curve for the predetermined oblique incident light formed on the other surface of the substrate is formed of an optical multilayer film. A transmission region is formed by a long wavelength side transmission region of the short wave pass filter unit and a short wavelength side transmission region of the long wave pass filter unit ,
The short wave pass filter unit and the long wave pass filter unit include a high refractive index film formed of a dielectric and a low refractive index film formed of a dielectric having a refractive index smaller than that of the high refractive index film. It has a basic structure in which a plurality of basic blocks stacked alternately are stacked so that layers formed of the same material are adjacent to each other between adjacent basic blocks, and the short wave path filter section and the long Each of the laminated refractive index films in the wavepass filter section has a film thickness ratio of a maximum film thickness to a minimum film thickness of 3 or less .   2. The optical multilayer film bandpass filter according to claim 1, wherein the short wavepass filter unit or the long wavepass filter unit includes a matching layer on at least one of the substrate side and the outer surface side. . The basic block of the short wave path filter unit is:
When the optical film thicknesses of the high refractive index film H and the low refractive index film L are represented by Ha and La in units of λa / 4 when the reference wavelength is λa,
(0.591Ha) (0.772La) (0.649Ha) (0.532La)
(0.649Ha) (0.772La) (0.591Ha)
However, λa: on the long wavelength side that shifts from the transmission region to the stop region of the transmittance characteristic curve
It is a reference wavelength at which the transmittance at the slope is 50%.
The basic block of the long wave pass filter unit is:
When the optical film thicknesses of the high refractive index film H and the low refractive index film L are expressed as Hb and Lb in units of λb / 4 when the reference wavelength is λb,
(0.976 Lb) (1.347 Hb) (1.347 Lb) (1.347 Hb)
(1.347 Lb) (1.347 Hb) (0.976 Lb)
However, λb: on the short wavelength side where the transmittance characteristic curve shifts from the transmission region to the stop region
2. The optical multilayer film bandpass filter according to claim 1 , wherein the transmittance is a reference wavelength at which the transmittance at the slope portion is 50%. The basic block of the short wave path filter unit is:
When the optical film thicknesses of the high refractive index film H and the low refractive index film L are represented by Hc and Lc in units of λc / 4 when the reference wavelength is λc,
(1.164Lc) (0.817Hc) (0.817Lc) (0.817Hc)
(0.817Lc) (0.817Hc) (1.164Lc)
However, λc: on the long wavelength side that shifts from the transmission region to the stop region of the transmittance characteristic curve
It is a reference wavelength at which the transmittance at the slope is 50%.
The basic block of the long wave pass filter unit is:
When the optical film thicknesses of the high refractive index film H and the low refractive index film L are represented by Hd and Ld in units of λd / 4 when the reference wavelength is λd,
(2.234Ld) (1.568Hd) (1.568Ld) (1.568Hd)
(1.568Ld) (1.568Hd) (2.234Ld)
However, λd: on the short wavelength side where the transmission characteristic curve shifts from the transmission region to the stop region
2. The optical multilayer film bandpass filter according to claim 1 , wherein the transmittance is a reference wavelength at which the transmittance at the slope portion is 50%. The basic block formed by alternately stacking the high refractive index film and the low refractive index film of the short wave pass filter unit and the long wave pass filter unit has 3 to 25 total layers, and the high refractive index 2. The optical multilayer film bandpass filter according to claim 1 , wherein the optical film thickness of the refractive index film and the low refractive index film is 0 to 3 times the thickness of which the unit thickness is 1/4 of the reference wavelength. .

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