JP5061876B2 - Bandpass filter - Google Patents

Description

  The present invention relates to a band-pass filter using a Fabry-Perot resonator.

  Conventionally, a bandpass filter using a one-dimensional magnetophotonic crystal which is a magneto-optical body has been devised. A one-dimensional magnetophotonic crystal has a dielectric multilayer film in which several kinds of dielectric materials are alternately stacked with regularity in thickness, and an irregular layer (defect layer) made of a magnetic material. (See Patent Document 1).

FIG. 1 is a diagram for explaining the configuration of a conventional one-dimensional magnetic photonic crystal.
The magneto-optical body 10 includes two multilayer films 13 and 14 in which two kinds of dielectrics (dielectric thin films) 11 and 12 are alternately stacked with regularity in thickness, and the two multilayer films 13 and 14 It consists of a magnetic body 15 (magneto-optic thin film) provided therebetween.

  The multilayer films 13 and 14 serve as reflecting mirrors of the Fabry-Perot resonator, and the thicknesses of the dielectrics 11 and 12 are λ / 4 (λ Is designed to be the wavelength of light. Further, the optical length of the irregular layer (defect layer) made of the magnetic material 15 in which the localization of light occurs is mλ / 2 (m is a positive integer).

The magneto-optical body 10 is disposed between a polarizer and an analyzer to constitute an optical isolator.
JP 2002-90525 A

  When the magneto-optical body 10 is configured as an optical isolator, the amount of attenuation in the pass band is determined by the positional relationship between the polarizer and the analyzer. The positional relationship between the polarizer and the analyzer is usually fixed. Therefore, conventionally, the passband attenuation is fixed and cannot be used for applications that require a function for adjusting the attenuation.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a bandpass filter that can adjust the attenuation of the passband.

  The band-pass filter of the present invention includes a Fabry-Perot resonator, a waveguide, a magnetic field applying means, and a magnetic field strength changing means. The Fabry-Perot resonator includes two reflectors and a resonance layer. The reflector is formed by alternately laminating low dielectric constant dielectric layers and high dielectric constant dielectric layers. The resonant layer is made of a magnetic material provided between the two reflectors, and generates a pass band in the stop band of the two reflectors. The waveguide accommodates a Fabry-Perot resonator, and restricts the propagating electromagnetic wave to the TE mode in which the electric field amplitude direction is a specific direction. The magnetic field applying means applies a DC magnetic field parallel to the propagation direction of the electromagnetic wave to the resonance layer from the outside of the waveguide. The magnetic field strength changing means changes the strength of the DC magnetic field.

  In this configuration, the low dielectric constant dielectric layer and the high dielectric constant dielectric layer cause Fabry-Perot interference, and a band gap occurs in the photonic crystal. As a result, the reflector acts electromagnetically as a band rejection filter. The resonant layer acts as a defect layer in the photonic crystal and causes local mode resonance. This creates a passband within the stopband. The magnetic field applying means applies a DC magnetic field parallel to the propagation direction of the electromagnetic wave to the resonance layer, and rotates the polarization plane of the electromagnetic wave propagating through the resonance layer by the Faraday effect. The magnetic field strength changing means changes the strength of the DC magnetic field and changes the turning angle of the polarization plane of the electromagnetic wave propagating through the waveguide. The waveguide restricts the mode of the electromagnetic wave propagating through the Fabry-Perot resonator to the TE mode in which the electric field amplitude direction is a specific direction. Therefore, the attenuation amount of the electromagnetic wave in the TE mode changes according to the turning angle of the polarization plane of the electromagnetic wave, and the attenuation amount of the electromagnetic wave in the passband is arbitrarily variably controlled according to the setting of the magnetic field strength changing means.

  The resonance layer preferably has an optical path length of ½ wavelength at approximately the center frequency of the passband. The resonance layer acts as the resonance layer of the Fabry-Perot resonator when the optical path length of the resonance layer is an integral multiple of ½ wavelength, where the wavelength of the passband is λ and the integer is n. The optical path length of the resonance layer is expressed by n · λ / 2, and even if n is 2, 3 or more, it functions as a resonance layer. However, when n = 1, that is, by setting the optical path length of the resonance layer to ½ wavelength, the resonance layer becomes the thinnest and the transmittance in the resonance layer is the largest, that is, the attenuation amount is the smallest. A large control range of the amount of attenuation can be secured by setting the means.

  It is preferable that the optical path lengths of the low dielectric constant dielectric layer and the high dielectric constant dielectric layer are each approximately ¼ wavelength in the stopband wavelength. In this case, the thickness of each layer can be reduced to the minimum necessary while causing Fabry-Perot interference between the low dielectric constant dielectric layer and the high dielectric constant dielectric layer. Can be secured.

  The low dielectric constant dielectric layer is preferably a resin, and the outermost layer of the Fabry-Perot resonator is preferably a resin layer. This is because the Fabry-Perot resonator can be easily formed by the optical modeling method.

  The resonance layer is preferably made of ferrite. In this case, the rotation angle of the polarization plane due to the Faraday effect with respect to a change in the DC magnetic field applied from the outside changes greatly, and the change in the attenuation of electromagnetic waves in the passband can be increased.

  According to this invention, by changing the DC magnetic field parallel to the propagation direction of the electromagnetic wave, the turning angle from the TE mode of the polarization plane of the electromagnetic wave propagating through the resonance layer is changed, and the attenuation amount of the electromagnetic wave is arbitrarily variable. Can be controlled.

First, the band pass filter according to the first embodiment will be described.
FIG. 2 is a cross-sectional view of a Fabry-Perot resonator used in the bandpass filter according to the first embodiment.
The Fabry-Perot resonator 20 according to this embodiment includes an alumina layer 21 that forms a high dielectric constant dielectric layer, an epoxy resin layer 22 that forms a low dielectric constant dielectric layer, and an epoxy resin layer 24 that forms the outermost layer. And a ferrite layer 23 forming a resonance layer. Each layer is laminated in a certain direction. Epoxy resin layers 24 are respectively provided at both ends of the Fabry-Perot resonator 20 in the stacking direction. A multilayer film 25 is provided inside each of them. The multilayer film 25 is formed by alternately arranging three alumina layers 21 and three epoxy resin layers 22. A ferrite layer 23 is provided at the center of the Fabry-Perot resonator 20 and between the multilayer films 25.

  The Fabry-Perot type resonator 20 is a one-dimensional photonic crystal, and the alumina layer 21 and the epoxy resin layer 22 cause Fabry-Perot interference to generate a band gap. Further, the ferrite layer 23 acts as a defect layer in the photonic crystal and causes local mode resonance. As a result, the Fabry-Perot resonator 20 has a pass band due to a defect layer in the band gap stop band electromagnetically.

  The thickness dimension of the alumina layer 21 and the epoxy resin layer 22 is set to a dimension that is approximately ¼ wavelength at the center frequency of the band gap of the Fabry-Perot resonator 20. Further, the thickness dimension of the ferrite layer 23 is determined to be a dimension that is approximately ½ wavelength at the center frequency of the pass band. Note that the center frequency of the band gap and the center frequency of the passband may be matched.

  In addition, since the refractive index of the epoxy resin layer is basically closer to that of air than the alumina layer, the epoxy resin layer 24 instead of the alumina layer 21 is provided at both ends of the Fabry-Perot resonator 20 in order to suppress loss due to reflection. Provided. Further, considering that it is produced by an optical modeling method to be described later, it is easier to produce the outermost layer by using the epoxy resin layer 24 instead of the alumina layer 21, so that this configuration is advantageous in terms of cost.

  In principle, the epoxy resin layer 22 which is a low dielectric constant dielectric layer and the alumina layer 21 which is a high dielectric constant dielectric layer have a thickness with an optical path length of ¼ wavelength. Therefore, the epoxy resin layer 24 may have such a thickness. However, depending on the influence such as reflection at the boundary with air, the thickness of the epoxy resin layer 24 may be preferably thin. Therefore, in this embodiment, the thickness dimension of the epoxy resin layer 24 is thinner than that of the epoxy resin layer 22. Forming.

  Here, an example of the design method of each part dimension is demonstrated.

  When the design center frequency of the band gap by the Fabry-Perot resonator is 13 GHz, the optical path length (one wavelength) in a vacuum of 13 GHz is about 23.06 mm, and the quarter wavelength is about 5.77 mm. . Therefore, it is necessary to obtain the thickness of the epoxy resin layer 22 and the thickness of the alumina layer 21 so that the optical path length (¼ wavelength) is about 5.77 mm. The required thickness dimension is a value obtained by dividing the required optical path length by the square root of the relative dielectric constant. Therefore, when the relative dielectric constant of the epoxy resin layer 22 is about 2.8, the thickness dimension of the epoxy resin layer 22 is about 3.5 mm. When the relative dielectric constant of the alumina layer 21 is about 8.4, the thickness dimension of the alumina layer 21 is about 2.0 mm. Moreover, the thickness dimension of the epoxy resin layer 24 should be thinner than the epoxy resin layer 22, and can be about 1.0 mm, for example.

  When the design center frequency of the pass band by the Fabry-Perot resonator is 13 GHz, the half wavelength of the optical path length in a vacuum of 13 GHz is about 11.53 mm. Therefore, it is necessary to obtain the thickness of the ferrite layer at which the optical path length (1/2 wavelength) is about 11.53 mm. When the relative dielectric constant of the ferrite layer is about 11.8, the thickness dimension of the ferrite layer is about 3.2 mm.

  In addition, what was measured by S para method (Nicorson-Ross method) can be used for the dielectric constant of the above-mentioned epoxy resin, an alumina layer, and a ferrite layer.

FIG. 3 is a diagram for explaining a manufacturing process of the Fabry-Perot resonator 20.
In the actual manufacturing process of the Fabry-Perot resonator 20, an optical modeling apparatus (for example, SCS-300P manufactured by Deemec Co., Ltd.) is used, and an ultraviolet curable epoxy resin (SCR-730) is used as a material. A rectangular parallelepiped epoxy resin structure 40 having a plurality of slits (dents) 41 and 42 as shown in FIG.

  Subsequently, as shown in FIG. 3B, three alumina plates are inserted into the slit 41 to form the alumina layer 21. Two ferrite plates are inserted into the slit 42 to form the ferrite layer 23. The thickness dimension of the slits 41 and 42 is matched with the thickness dimension of the alumina layer 21 and the ferrite layer 23, and the interval between the slits 41 and 42 is matched with the thickness dimension of the epoxy resin layer. Note that the alumina plate and the ferrite plate may be formed of a single plate without forming each layer by a plurality of sheets.

  Next, as shown in FIG. 3C, an epoxy resin is continuously formed on the upper portion in the drawing, and the gap between the slits 41 and 42 and the upper portions of the alumina layer 21 and the ferrite layer 23 are resin-sealed.

  Thereafter, as shown in FIG. 3D, unnecessary resins around the edges of the alumina layer 21 and the ferrite layer 23 are cut and removed. As a result, a Fabry-Perot resonator 20 composed of the alumina layer 21, the ferrite layer 23, and the epoxy resin layers 22 and 24 with the alumina layer 21 and the ferrite layer 23 exposed on the four side surfaces is formed.

Examples of the dimensions, composition, and characteristics of the members used are as follows.
Alumina plate
Composition: 96% pure alumina
Dimensions: 18.8 × 9.3 × 0.635mm
Dielectric constant: 8.4
Ferrite plate
Composition: YIG (Y Ca) (Fe V Al In) O 12 ferrite of
Dimensions: 18.95 x 9.45 x 1.962 mm
Dielectric constant: 11.8
Epoxy resin
Dielectric constant: 2.8
As ferrite plates, spinel ferrite (Mg—Mn ferrite, Mg—Mn—Al ferrite, Ni—Zn ferrite, Ni—Al ferrite, Li—Fe ferrite) and garnet ferrite (Y—Fe ferrite, Gd) are used. -Fe ferrite) can also be used.

  FIG. 4 is a cross-sectional view showing the configuration of the entire band-pass filter including the Fabry-Perot resonator 20.

  The bandpass filter 100 includes a rectangular waveguide 50, a Fabry-Perot resonator 20, an electromagnet 60, and a magnetic field controller 70.

  The rectangular waveguide 50 propagates only the TE mode in which the electric field amplitude direction of the propagating electromagnetic wave is the vertical direction. That is, the direction of the electric field is the vertical direction in the figure, and the surface of the magnetic field loop is a direction perpendicular to the paper surface. The rectangular waveguide 50 is made of Al, Cu, or resin plated with Cu or Ag.

  The Fabry-Perot resonator 20 is inserted (filled) into the rectangular waveguide 50. Further, a probe 51 for signal input / output is disposed at both ends of the rectangular waveguide 50, and a signal is input / output by a coaxial connector.

  The magnetic field control unit 70 controls input power to the electromagnet 60. The electromagnet 60 applies a DC magnetic field H to the ferrite layer 23 of the Fabry-Perot resonator 20 from the outside of the rectangular waveguide 50. The direction of the DC magnetic field H is the propagation direction of the electromagnetic wave propagating through the rectangular waveguide 50. Thus, for the convenience of applying the DC magnetic field H from the outside of the rectangular waveguide 50, at least both ends of the rectangular waveguide 50 cannot basically use a magnetic shielding material such as Fe.

  The mode of electromagnetic waves propagating from the probe 51 through the rectangular waveguide is limited to the TE mode in which the electric field amplitude direction is a specific direction. On the other hand, when the DC magnetic field H is applied to the ferrite layer 23, the polarization plane of the electromagnetic wave rotates in the resonance layer due to the Faraday effect, and the attenuation of the mode changes. By controlling the magnitude of the DC magnetic field H by the magnetic field controller 70, the turning angle of the polarization plane of the electromagnetic wave due to the Faraday effect is changed, and the attenuation of the mode can be adjusted.

  Next, the results of performance simulation and actual performance measurement performed to confirm the performance of the Fabry-Perot resonator 20 of this embodiment will be described.

FIG. 5 is a sectional view of a Fabry-Perot resonator 30 shown as a comparative example.
The Fabry-Perot resonator 30 has an alumina layer instead of the above-described ferrite layer. Epoxy resin layers 34 are provided at both ends of the Fabry-Perot resonator 30. Inside thereof, a periodic structure in which 13 layers of alumina layers 31 and epoxy resin layers 32 are alternately arranged. In other words, the Fabry-Perot resonator 30 of the comparative example is equivalent to the Fabry-Perot resonator 20 described above in which the central ferrite layer 23 is replaced with an alumina layer 31 that is a high dielectric constant dielectric layer. The thickness dimension of the alumina layer 31 and the epoxy resin layers 32 and 34 of the Fabry-Perot resonator 30 of the comparative example is the same as that of the Fabry-Perot resonator 20. The Fabry-Perot resonator 30 is formed by forming an epoxy resin structure by stereolithography in the same manner as the Fabry-Perot resonator 20, and inserting an alumina layer 31 instead of a ferrite layer into the central slit. Constitute.

Next, the characteristics of the bandpass filter will be described.
FIG. 6A is a diagram showing the transmission characteristic S21 by simulation based on the transmission line theory of the bandpass filter 100. FIG. FIG. 5B is a diagram showing transmission characteristics S21 by simulation of a bandpass filter using the Fabry-Perot resonator 30 of the comparative example.

  As shown in FIG. 5B, when there is no defect layer due to the ferrite layer 23, a band gap in the photonic crystal was observed in a band of about 11.0 to 16.0 GHz. The gap center frequency was about 13.5 GHz and about -35 dB. On the other hand, when the Fabry-Perot resonator 20 having the ferrite layer 23 is inserted, as shown in FIG. 5A, the band in the photonic crystal is in the range of about 10.0 to 16.5 GHz. A gap was observed. Further, the center frequency of the pass band generated by the defect layer, that is, the defect frequency was about −12 dB at about 13.1 GHz.

  FIG. 7A is an actual measurement result showing the transmission characteristic S21 of the bandpass filter 100. FIG. FIG. 5B is an actual measurement result showing the transmission characteristic S21 of a bandpass filter using the Fabry-Perot resonator 30 of the comparative example. Characteristics similar to those in the simulation shown in FIG. 6 were observed. In the actual measurement result, a steep non-transmission spectrum occurs in the vicinity of about 16 GHz, which is due to the characteristics of the waveguide.

  Next, a case where a DC magnetic field H is applied to the Fabry-Perot resonator 20 by the electromagnet 60 will be described.

  FIG. 8 is an actual measurement result showing the transmission characteristic S21 of the band-pass filter 100 in the example in which the magnetic flux density is 0.38T as the DC magnetic field H. FIG. 9 is an actual measurement result showing the transmission characteristic S21 of the band-pass filter 100 in an example in which the magnetic flux density is 0.40T. FIG. 10 is an actual measurement result showing the transmission characteristic S21 of the bandpass filter 100 in an example in which the magnetic flux density is 0.42T. FIG. 11 is a diagram comparing each example in which the magnetic flux density is 0.00T, 0.38T, 0.40T, and 0.42T.

  In the example in which the magnetic flux density was 0.00T, the defect frequency was about −12 dB at about 13.1 GHz. In the example in which the magnetic flux density is 0.38T, the defect frequency is about −12 dB at about 13.2 GHz, and the attenuation at the frequency of 13.1 GHz is about −15 dB. In the example in which the magnetic flux density is 0.40 T, the defect frequency is about −16 dB at about 13.3 GHz, and the attenuation at the frequency of 13.1 GHz is about −26 dB. In the example in which the magnetic flux density is 0.38 T, the pass band is almost lost, and the attenuation at the frequency of 13.1 GHz is about −28 dB.

  Thus, at a defect frequency of about 13.1 GHz when the DC magnetic field H is not applied, the amount of attenuation increases as the magnetic flux density applied by the DC magnetic field H increases. It was observed that when the magnetic flux density was changed from 38T to 0.40T, the attenuation changed sharply at a certain magnetic flux density, as the attenuation changed from -15 dB to -26 dB. Similar results were confirmed by simulation. This is considered to be due to the fact that multiple reflections occur due to the periodic structure of the dielectric, and a large swirl of the polarization plane is obtained mainly by the Faraday effect in the ferrite layer, thereby preventing propagation of electromagnetic waves in the waveguide.

  In addition, although the example used in 13.0 GHz band was given in embodiment, this invention is applicable to a frequency band about 1 GHz-100 GHz. The main use of this frequency band is communication, and the frequency variable band-pass filter can be used as a filter of a multiband transmitter.

  Moreover, although the example which utilizes a rectangular waveguide was given in the embodiment, the present invention has a TE mode in which the amplitude direction of the electric field is directed in a specific direction, depending on the turning angle of the polarization plane. A waveguide having a circular shape or the like may be used as long as the attenuation can be changed, and the present invention can be applied to waveguides having various shapes.

It is a figure which shows the structure of the optical isolator shown by patent document 1. FIG. It is sectional drawing of the Fabry-Perot type | mold resonator used for the band pass filter which concerns on 1st Embodiment. It is a figure which shows the manufacturing method of the same Fabry-Perot type | mold resonator. It is sectional drawing which shows the structure of the same band pass filter. It is sectional drawing of the Fabry-Perot type | mold resonator of a comparative example. It is a figure which shows the simulation result of the transmission characteristic of a band pass filter. It is a figure which shows the measurement result of the transmission characteristic of a band pass filter. It is a figure which shows the actual measurement result of the transmission characteristic of a band pass filter in the case of applying a DC magnetic field from the outside. It is a figure which shows the actual measurement result of the transmission characteristic of a band pass filter in the case of applying a DC magnetic field from the outside. It is a figure which shows the actual measurement result of the transmission characteristic of a band pass filter in the case of applying a DC magnetic field from the outside. It is a figure which shows the measurement result of the transmission characteristic of a band pass filter at the time of changing the direct current magnetic field applied from the outside.

Explanation of symbols

DESCRIPTION OF SYMBOLS 21 ... Alumina layer 22 ... Epoxy resin layer 22, 24 ... Epoxy resin layer 23 ... Ferrite layer 24 ... Epoxy resin layer 25 ... Multilayer film 40 ... Epoxy resin structure 41, 42 ... Slit 50 ... Rectangular waveguide 51 ... Probe 60 ... Electromagnet 70 ... Magnetic field controller 100 ... Band pass filter H ... DC magnetic field

Claims (5)

It consists of two reflectors in which a low dielectric constant dielectric layer and a high dielectric constant dielectric layer are alternately stacked, and a magnetic material provided between the two reflectors, and within the stop band of the two reflectors. A Fabry-Perot resonator comprising a resonant layer that produces a passband;
A waveguide that houses the Fabry-Perot resonator and restricts the propagating electromagnetic wave to a TE mode in which the amplitude direction of the electric field is a specific direction;
A magnetic field applying means for applying a DC magnetic field parallel to the propagation direction of the electromagnetic wave to the resonance layer from the outside of the waveguide;
A band-pass filter comprising: a magnetic field intensity changing unit that changes the intensity of the DC magnetic field.   2. The bandpass filter according to claim 1, wherein the resonance layer has an optical path length of ½ wavelength at a substantially center frequency of the passband.   3. The bandpass filter according to claim 1, wherein optical path lengths of the low dielectric constant dielectric layer and the high dielectric constant dielectric layer are each ¼ wavelength at a substantially central frequency of the stop band.   The band pass filter according to any one of claims 1 to 3, wherein the low dielectric constant dielectric layer is a resin, and the outermost layer of the Fabry-Perot resonator is a resin layer.   The band-pass filter according to claim 1, wherein the resonance layer is made of ferrite.

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