JPH07104122A - Band-pass filter - Google Patents

Abstract

PURPOSE:To provide the band-pass filter which is composed of.dielectric multilayered films, has a desired pass-band width, has less dependency on polarized light and is good in rising characteristic. CONSTITUTION:This band-pass filter is constituted by alternately laminating high-refractive index dielectric films and 'low-refractive index dielectric films on both sides of a cavity layer and its central wavelength of the pass band is lambda0. The optical film thicknesses of the respective high-refractive index dielectric films and the low-refractive index dielectric film are lambda0/4 and the optical film thickness of the cavity layer of integer times lambda0/2 is made large within a range larger than 0 and smaller than lambda0/2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bandpass filter composed of a dielectric multilayer film. A bandpass filter composed by alternately laminating a high-refractive index dielectric film and a low-refractive index dielectric film on both sides of a cavity layer is a dielectric material compared to a long wave pass filter or a short wave pass filter. Even if the number of layers is the same, it has a sharp rising characteristic, and thus has been widely used in optical multiplexers / demultiplexers and the like.

[0002]

2. Description of the Related Art An example of a bandpass filter is shown in FIG. In the figure, 1 is a transparent substrate made of optical glass or the like,
2 is a high-refractive-index dielectric film such as TiO ₂ (having a refractive index of 2.3) having a predetermined optical film thickness (the product of the refractive index of the dielectric and film thickness), and 3 is a predetermined optical film thickness such as SiO 2. ₂ (refractive index is 1.46) or the like, 4 is a low-refractive-index dielectric film, 4 is a predetermined optical film thickness larger than that of the low-refractive-index dielectric film 3, for example, SiO ₂ (refractive index is
1.46) etc. cavities.

Reference numeral 5 is an optical film in place of the low refractive index dielectric film 3 of the central layer, in which a desired number of high refractive index dielectric films 2 and low refractive index dielectric films 3 are alternately laminated. It is a basic block of a bandpass filter in which a cavity layer 4 having a predetermined thickness larger than that is formed.

Assuming that the center wavelength of the bandpass filter is λ ₀ (this center wavelength shifts depending on the incident angle), the optical film thickness of the high refractive index dielectric film 2 and the low film refractive index dielectric film 3 are determined. are each λ _0/4.

The optical film thickness of the cavity 4 is λ _0.
It is an integral multiple of / 2. FIG. 5 shows a bandpass filter composed of three basic blocks 5. That is, the basic block 5 of the first stage is formed on the surface of the transparent substrate 1, the low refractive index dielectric film 3 is formed on the basic block 5 of the first stage, and the basic block of the second stage is formed thereon. The block 5 is formed, and the low refractive index dielectric film 3 is formed on the second-stage basic block 5,
This is a 23-layer bandpass filter in which the third-stage basic block 5 is formed.

The bandpass filter made of the above-mentioned dielectric multilayer film has a mountain shape having a steep rise and a pass band width centered on the center wavelength λ ₀ as shown in FIG. On the other hand, in order to reduce or expand the pass band width of the bandpass filter, a dielectric film conventionally used (here, a high-refractive-index dielectric film, a low-refractive-index dielectric film, and a cavity layer are called a dielectric film) It was implemented by changing the number of layers.

FIG. 7 shows changes in the conventional pass band width. In FIG. 7, the horizontal axis indicates the number of reflective layers, and the vertical axis indicates the pass band width (unit:
nm). Conventionally, the passband width was changed by adjusting the number of layers of the dielectric film, so that as shown by the circles in FIG. 7, when the number of dielectric film layers is 19, the passband width is about
3.27 nm, when the number of dielectric film layers is 21, the pass band width is about 2.10 nm, when the number of dielectric film layers is 23, the pass band width is about 1.30 nm, the number of dielectric film layers In the case of No. 27, the pass bandwidth was about 0.55 nm, and the pass bandwidth changed discontinuously.

[0008]

In the conventional bandpass filter in which the number of dielectric film layers is increased or decreased to change the passband width, the passband width changes discontinuously as described above. There is a problem that the bandwidth cannot be obtained.

On the other hand, the conventional bandpass filter has no polarization dependence when the signal light enters at an incident angle of 0 degree. However, when it is used at an incident angle of 0 degree, the reflected light from the bandpass filter returns to the light source side and noise or other trouble occurs.

Therefore, the bandpass filter is generally used by projecting the signal light at an incident angle sufficiently larger than 0 degree. When used at such an incident angle, the transmittance and the reflectance of the S wave (the electric field of the incident light is polarized perpendicular to the incident surface) and the P wave (the electric field of the incident light is polarized parallel to the incident surface). As shown in FIG. 8, the pass band width of the S wave (shown by the dotted line) becomes smaller than the pass band width of the P wave (shown by the solid line).

Therefore, the rising width (wavelength band between the rising start point of the P wave and the rising end point of the S wave) becomes large as shown by D ₁ in FIG. 8, and there is a problem that there is so-called polarization dependence. there were.

The present invention was created in view of the above points, and an object thereof is to provide a bandpass filter having a desired pass bandwidth. Another purpose is that the polarization dependence is small,
Alternatively, it is applied to a long wave pass filter and has a good rising characteristic.

[0013]

In order to achieve the above object, the present invention, as illustrated in FIG. 1, alternates a high refractive index dielectric film and a low refractive index dielectric film on both sides of a cavity layer. In a band-pass filter having a center wavelength of the pass band of λ ₀ , which is formed by stacking the optical films of the high-refractive-index dielectric film and the low-refractive-index dielectric film with λ ₀ /
4 and then, the optical film thickness of lambda _0/2 integer multiple of the cavity layer, a structure which is largely in the larger lambda _0/2 smaller range than 0.

Further, the optical film thickness of the optical film thickness and a low refractive index dielectric film of each of the high refractive index dielectric film and lambda _0/4, optical of lambda _0/2 integer multiple of the cavity layer 0 for the membrane
And it was decreased configured in larger lambda _0/2 smaller range.

Alternatively, the optical thickness of the cavity layer may be set to λ _0.
/ 2 of an integral multiple, lambda _0/4 each having an optical film thickness of the optical film thickness and a low refractive index dielectric film having a high refractive index dielectric film, greater than 0 lambda _0/4 in the range smaller than It will be a larger structure.

Alternatively, the optical thickness of the cavity layer can be set to λ
_0/2 of an integral multiple, lambda _0/4 of the optical thickness of the optical film thickness and a low refractive index dielectric film of each of the high refractive index dielectric film, greater than 0 lambda _0/4 range smaller than Within this, the configuration will be made smaller.

[0017]

DETAILED DESCRIPTION OF THE INVENTION The present invention having an optical film thickness of the high refractive index dielectric film and low refractive index dielectric film as the standard value (λ _0/4), the standard value of optical thickness of the cavity layer (lambda _0/2 Than an integer multiple of
It is shifted to either the large or the small.

Further, the standard value of optical thickness of the cavity layer (λ _0/2 of integer times), and the standard value of optical thickness of the high refractive index dielectric film and low refractive index dielectric film (lambda ₀ / 4), which is either larger or smaller than / 4).

That is, one of the cavity layer and the other layer (a layer composed of a high-refractive-index dielectric film and a low-refractive-index dielectric film) has a center wavelength λ as shown in FIG. 1B. _It will be used at λ ₀₁ which is deviated from ₀ (highest reflectance), and the reflectance will be lower than the highest value.

Therefore, rather than the mountain-shaped curve 10-1 having the narrow pass band width B ₁ shown by the dotted line in FIG. 1A, the wide pass band width B ₂ shown by the solid line in which the top of the mountain is broken is shown. curve
It becomes a bandpass filter having 10-2.

At this time, the film thickness (mechanical actual size) of the high-refractive-index dielectric film, the low-refractive-index dielectric film, and the cavity layer can be changed to a desired film thickness by changing the vapor deposition time when forming the film. can do.

That is, since it is possible to easily obtain a continuously changing optical film thickness, a band pass filter having a desired pass band width can be obtained. On the other hand, in the case of a bandpass filter in which the incident angle of the signal beam is sufficiently larger than 0 degree, the optical thickness of the cavity layer is made smaller than the standard value, or the optical index of the high refractive index dielectric film and the low refractive index dielectric film is reduced. If the target film thickness is made larger than the standard value, the S wave shifts to the short wavelength side, and the rising portion of the S wave approaches the rising portion of the P wave, so the rising width becomes smaller.

Therefore, it can be used as a long wave pass filter having a good rising characteristic. If the optical thickness of the cavity layer is made larger than the standard value or the optical thickness of the high refractive index dielectric film and the low refractive index dielectric film is made smaller than the standard value, the S wave shifts to the long wavelength side. As the rising portion of the S wave approaches the rising portion of the P wave, the rising width becomes smaller.

Therefore, it can be used as a short wave pass filter having a good rising characteristic.

[0025]

The present invention will be described in detail with reference to the drawings. The same reference numerals denote the same objects throughout the drawings.

FIG. 1 is a diagram showing the principle of the present invention, (A) is a wavelength characteristic characteristic diagram, (B) is a diagram explaining the action, and FIG. 2 is a diagram showing changes in the pass band width of the present invention. 3 is a rising characteristic diagram of the embodiment of the present invention, and FIG. 4 is a rising characteristic diagram of another embodiment of the present invention.

The bandpass filter of the present invention, as shown in FIG. 5, a cavity layer optical film thickness is made of lambda _0/2 of integer multiples of the dielectric film (low refractive index dielectric film) 4 on both sides, the provided high refractive index dielectric film 2 of the optical thickness is lambda _0/4, the respective optical film thickness on the high refractive index dielectric film 2 lambda ₀
/ 4 is provided a low refractive index dielectric film 3, the high refractive index dielectric film 2 and the low refractive index dielectric film 3 was formed by laminating a desired number of alternating, the center wavelength of the passband of the lambda ₀ It relates to a bandpass filter.

The reflectance characteristics of each reflecting layer of the bandpass filter are shown in FIG. 1B (horizontal axis represents wavelength, vertical axis represents reflectance). The reflectance curve L has a center wavelength λ _0.
It has an arc shape with the highest reflectance.

The wavelength characteristic is a mountain-shaped curve 10-1 having a narrow pass band width B ₁ shown by a dotted line in FIG. Here, the optical film thickness of the optical film thickness and a low refractive index dielectric film of each of the high refractive index dielectric film left as an lambda _0/4, increasing the thickness of the cavity layer (mechanical actual dimensions) to the optical thickness of lambda _0/2 integer multiple of the cavity layer,
It is greatly desired in the larger lambda _0/2 smaller range than 0.

As a result, as shown in FIG. 1B, the reflectance of the cavity layer becomes maximum at the wavelength λ ₀₁ which is shifted from the central wavelength λ ₀ to the long wavelength side. Therefore, as shown in FIG. 1 (A), the wavelength characteristic is such that the peak portion of the conventional curve 10-1 having a narrow pass band width B ₁ collapses. Then, a curve 10-2 having a wide pass band width B ₂ shown by a solid line is obtained.

[0031] Figure 2, the central wavelength lambda ₀ is 1550 nm, the dielectric film number 23 in a single cavity, a high refractive index dielectric film and the optical film thickness of the low refractive index dielectric film lambda _0/4 of the bandpass FIG. 7 is a diagram showing a change in pass band width when the optical thickness of a cavity layer is changed in the filter.

[0032] As shown in FIG. 2, pass in the case of optical thickness of the cavity layer of 0.5 [lambda _0, the pass bandwidth of about 1.23nm is, the optical film thickness 0.6Ramuda ₀ The bandwidth is about 1.77 nm, and the passband width is about 3.1 nm when the optical film thickness is 0.7λ ₀ , and the passband width changes continuously even with the optical film thickness in between.

It should be noted, by reducing the thickness of the cavity layer (mechanical actual dimensions), λ _0/2 of the optical thickness of an integral multiple of the cavity layer, greater than ₀ λ 0/2 in the range smaller than Then, if it is made small as desired, the reflectance of the cavity layer becomes maximum at the wavelength λ ₀₁ which is shifted from the central wavelength λ ₀ to the short wavelength side. Therefore, the pass band width becomes large as described above.

On the other hand, the optical thickness of the cavity layer is set to λ ₀ /
It is an integer multiple of 2. And an optical film thickness of the optical film thickness and a low refractive index dielectric film having a high refractive index dielectric film of lambda _0/4, in the larger lambda _0/4 less than the range from 0 and to increase or reduce Also, the pass band width is widened as described above.

The film thickness (mechanical actual size) of the high refractive index dielectric film, the low refractive index dielectric film, and the cavity layer can be set to a desired film thickness by changing the vapor deposition time when forming the film. You can Therefore, since it is possible to easily obtain a film whose optical film thickness changes continuously, it is possible to obtain a bandpass filter having a desired pass band width.

[0036] Figure 3, the cavity layer number 3, the dielectric film number 23, an optical film thickness of lambda _0/4 of the high refractive index dielectric film and low refractive index dielectric film, an optical cavity layer Film thickness 0.47λ
The rising characteristics when set to ₀ are shown.

The incident angle of the signal beam is 35 degrees. Thus, the optical thickness of the cavity layer is set to the standard value of 0.5 λ _0.
As a result, the passband widths of the P wave and the S wave are shifted to the short wavelength side by reducing the width by 0.03 nm.

However, since the shift amount of the S wave is larger, S
The rising portion of the wave approaches the rising portion of the P wave, and the rising width (wavelength band of the rising start point of the P wave and the rising end point of the S wave) is smaller than the conventional D ₁ shown in FIG. The rising width D ₂ shown in FIG.

Therefore, it can be used as a long wave pass filter having little polarization dependence and good rising characteristics. Note that the optical thickness of the cavity layer is λ ₀ /
A second integer multiple, equal to the optical thickness of the optical film thickness and a low refractive index dielectric film of each of the high refractive index dielectric film and the optical film thickness than the standard value (λ _0/4) Also _{0 to} λ ₀
By increasing in the range of / 4, both the S wave and the P wave are shifted to the short wavelength side so that the rising part of the S wave approaches the rising part of the P wave.

Therefore, it can be used as a long wave pass filter having little polarization dependence and good rising characteristics. 4, the cavity layer number 3, the dielectric film number 23, optical film thickness of the high refractive index dielectric film and low refractive index dielectric film at lambda _0/4, the optical film thickness of the cavity layer 0.56λ
The rising characteristics when set to ₀ are shown.

The incident angle of the signal beam is 35 degrees. Thus, the optical thickness of the cavity layer is set to the standard value of 0.5 λ _0.
Therefore, by increasing by 0.06 nm, the pass band widths of the P wave and the S wave are shifted to the long wavelength side.

However, since the shift amount of the S wave is larger, S
The rising portion of the wave approaches the rising portion of the P wave, and the rising width (wavelength band between the rising start point of the P wave and the rising end point of the S wave) is the rising width D ₃ illustrated in FIG.
It will be smaller.

Therefore, it can be used as a short wave pass filter having little polarization dependency and good rising characteristics. Note that the optical thickness of the cavity layer is λ ₀
/ 2 of an integral multiple equal to the optical thickness of the optical film thickness and a low refractive index dielectric film of each of the high refractive index dielectric film, and the standard value its optical thickness (lambda _0/4) By making it smaller than that, both the S wave and the P wave shift to the long wavelength side so that the rising part of the S wave approaches the rising part of the P wave.

Therefore, it can be used as a short wave pass filter having little polarization dependence and good rising characteristics.

[0045]

The present invention described above, according to the present invention is an optical film thickness of the high refractive index dielectric film and low refractive index dielectric film as the standard value (λ _0/4), the optical film thickness of the cavity layer the standard value (λ _0/2 of integer times) than, larger or smaller or shifted in any one, or an optical film thickness of the cavity layer and a standard value (λ _0/2 of the integral multiples), the high refractive index standard value having an optical film thickness of the dielectric film and low refractive index dielectric film (λ _0/4) than, be those shifted to any one of the greater or smaller, a band having a desired pass band width This has the effect of providing a pass filter.

Further, when the incident angle is sufficiently larger than 0 degree, the polarization dependence is reduced, and it is applied to a long wave pass filter or a short wave pass filter,
It has an excellent effect that the rising width is small.

[Brief description of drawings]

FIG. 1 is a diagram showing the principle of the present invention, (A) is a wavelength characteristic diagram, and (B) is a diagram for explaining the action.

FIG. 2 is a diagram showing changes in the pass bandwidth of the present invention.

FIG. 3 is a rising characteristic diagram of an example of the present invention.

FIG. 4 is a rising characteristic diagram of another embodiment of the present invention.

FIG. 5 is a block diagram of a bandpass filter.

FIG. 6 is a wavelength characteristic diagram of a bandpass filter.

FIG. 7 is a diagram showing a change in conventional pass bandwidth.

FIG. 8 is a diagram showing conventional polarization dependence.

[Explanation of symbols]

1 transparent substrate 2 high refractive index dielectric film 3 low refractive index dielectric film 4 cavity layer 5 basic block

Claims (4)

[Claims] _1. A bandpass filter having a center wavelength of λ _{0 in} a pass band, wherein a high-refractive index dielectric film and a low-refractive index dielectric film are alternately laminated on both sides of a cavity layer. optical film thickness of the optical film thickness and low refractive index dielectric film having a high refractive index dielectric film is lambda _0/4, the optical film thickness of lambda _0/2 integer multiple of the cavity layer, 0
Bandpass filter being greater at the larger lambda _0/2 smaller range. 2. A band pass filter having a center wavelength of λ _{0 in} a pass band, wherein a high refractive index dielectric film and a low refractive index dielectric film are alternately laminated on both sides of a cavity layer. optical film thickness of the optical film thickness and low refractive index dielectric film having a high refractive index dielectric film is lambda _0/4, the optical film thickness of lambda _0/2 integer multiple of the cavity layer, 0
Band-pass filter, wherein small in larger lambda _0/2 smaller range. 3. A band pass filter having a center wavelength of λ _{0 in} a pass band, wherein a high refractive index dielectric film and a low refractive index dielectric film are alternately laminated on both sides of a cavity layer. optical film thickness is an integral multiple of lambda _0/2, the optical thickness of the optical film thickness and low refractive index dielectric film of each of the high refractive index dielectric film of lambda _0/4 is, A bandpass filter having a large value within a range greater than _{0 and} smaller than λ 0/4. 4. A bandpass filter having a center wavelength of a pass band of λ ₀ , wherein a high-refractive-index dielectric film and a low-refractive-index dielectric film are alternately laminated on both sides of a cavity layer. optical film thickness is an integral multiple of lambda _0/2, the optical thickness of the optical film thickness and low refractive index dielectric film of each of the high refractive index dielectric film of lambda _0/4 is, A bandpass filter characterized by being small within a range larger than _{0 and} smaller than λ 0/4.