JP2019207278A - Nonreciprocal filter - Google Patents

Abstract

To provide a nonreciprocal filter that reduces restrictions on material selection for obtaining a nonreciprocal effect.SOLUTION: An isolator being a nonreciprocal filter is represented by an equivalent circuit 10 in which a filter 20 and a nonreciprocal line 30 are connected in parallel between a first point 11 and a second point 12. The equivalent circuit 10 has a first filter characteristic related to a signal traveling from the first point 11 toward the second point 12, and a second filter characteristic related to a signal traveling from the second point 12 toward the first point 11. The first filter characteristic forms a passband including a predetermined effective band and has a first attenuation pole outside the passband. The second filter characteristic has a second attenuation pole close to the passband of the first attenuation pole in the first filter characteristic, and in the effective band, the signal is transmitted at a transmittance lower than the transmittance in the first filter characteristic.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to non-reciprocal filters.

  Isolators have non-reciprocity with different transmittances depending on the propagation direction of electromagnetic waves. In order to obtain a nonreciprocal effect, a magnetic material is generally used (see, for example, Patent Document 1).

Japanese Patent No. 412549

  The isolator is a nonreciprocal filter having nonreciprocity. Since the nonreciprocal filter is easily and inexpensively manufactured, it is desirable that the degree of freedom of material selection for the element that provides nonreciprocity is high. An object of the present disclosure is to provide a nonreciprocal filter with reduced material selection constraints for obtaining a nonreciprocal effect.

  The nonreciprocal filter of the present disclosure is represented by an equivalent circuit in which a filter and a nonreciprocal line are connected in parallel between a first point and a second point. The equivalent circuit has a first filter characteristic relating to a signal traveling from the first point toward the second point, and a second filter characteristic relating to a signal traveling from the second point toward the first point. The first filter characteristic has a first attenuation pole that forms a pass band including a predetermined effective band and appears outside the pass band according to the characteristic of the nonreciprocal line. The second filter characteristic has a second attenuation pole that appears according to the characteristic of the nonreciprocal line on the passband side of the first attenuation pole in the first filter characteristic, and the second attenuation characteristic causes The signal is transmitted with a transmittance lower than the transmittance in one filter characteristic.

  According to the embodiment of the present disclosure, it is possible to provide a nonreciprocal filter with reduced material selection restrictions for obtaining a nonreciprocal effect.

FIG. 1 is a diagram showing an equivalent circuit showing a basic configuration of an isolator according to an embodiment. FIG. 2 is a diagram showing an equivalent circuit of a first example showing the equivalent circuit of FIG. 1 in more detail. FIG. 3 is an enlarged view of a broken line part included in the equivalent circuit of FIG. FIG. 4 is a diagram illustrating frequency characteristics of insertion loss of the equivalent circuit of FIG. FIG. 5 is an enlarged view of a part of FIG. 4 in the horizontal axis direction. FIG. 6 is a diagram in which the horizontal axis of FIG. 5 is converted into a wavelength. FIG. 7 is a diagram showing an equivalent circuit of a second example showing the equivalent circuit of FIG. 1 in more detail. FIG. 8 is a diagram illustrating frequency characteristics of insertion loss of the equivalent circuit of FIG. FIG. 9 is an enlarged view of a part of FIG. 8 in the horizontal axis direction. FIG. 10 is a diagram illustrating a loss amount with respect to tan δ. FIG. 11 is a diagram showing an equivalent circuit of a third example showing the equivalent circuit of FIG. 1 in more detail. FIG. 12 is a diagram illustrating frequency characteristics of insertion loss of the equivalent circuit of FIG. FIG. 13 is an enlarged view of a part of FIG. 12 in the horizontal axis direction. FIG. 14 is a diagram showing an equivalent circuit of a fourth example showing the equivalent circuit of FIG. 1 in more detail. FIG. 15 is a diagram showing frequency characteristics of insertion loss of the equivalent circuit of FIG. FIG. 16 is an enlarged view of a part of FIG. 15 in the horizontal axis direction. FIG. 17 is a diagram showing an equivalent circuit of a fifth example showing the equivalent circuit of FIG. 1 in more detail. FIG. 18 is a diagram illustrating frequency characteristics of insertion loss of the equivalent circuit of FIG. FIG. 19 is an enlarged view of a part of FIG. 18 in the horizontal axis direction. FIG. 20 is a schematic diagram illustrating a configuration of an isolator according to an embodiment. FIG. 21 is a diagram for explaining a change in attenuation pole between the high frequency side and the low frequency side. FIG. 22 is a diagram showing a schematic configuration of an isolator according to the prior art.

  Before describing the details of the nonreciprocal filter of the present disclosure, an isolator 100 using the Faraday effect according to the related art will be described with reference to FIG. 22 for comparison. The isolator 100 is used, for example, for optical communication using a linearly polarized light source in the infrared region.

  The isolator 100 shown in FIG. 13 is configured by disposing a Faraday rotator 101 between two polarizers 102 and 103. Each of the Faraday rotator 101 and the polarizers 102 and 103 is a plate-like member, and is arranged in parallel with the surfaces facing each other. In the following description regarding FIG. 13, the x direction and the y direction are directions along the surfaces of the Faraday rotator 101 and the polarizers 102 and 103. The z direction is a direction perpendicular to the x direction and the y direction. It is assumed that the x direction and the y direction are orthogonal to each other. With the magnetic field H applied in the z direction, the Faraday rotator 101 rotates the polarization plane of linearly polarized light traveling in the z direction by 45 degrees due to the Faraday effect. The polarizer 102 has a transmission axis in the y direction. The polarizer 103 has a transmission axis in a direction inclined 45 degrees in the optical rotation direction by the Faraday rotator 101 with respect to the y direction.

  A semiconductor laser used in many cases as a light source in applications such as optical communication has linearly polarized light. When linearly polarized light polarized in the y direction is incident from the polarizer 102 side of the isolator 100, the incident light is transmitted through the polarizer 102 having a transmission axis in the y direction. Incident light transmitted through the polarizer 102 is rotated by 45 degrees in the polarization direction by the Faraday rotator 101 to which the magnetic field H is applied. Further, the incident light transmitted through the Faraday rotator 101 is transmitted through the polarizer 103 whose transmission axis is inclined by 45 degrees with respect to the y-axis.

  On the other hand, return light can enter from the polarizer 103 side. The return light can usually have a random polarization direction, but only the linearly polarized light component inclined by 45 degrees in the y-axis direction is transmitted by the polarizer 103. The return light transmitted through the polarizer 103 is rotated by 45 degrees in the polarization direction by the Faraday rotator 101 and becomes the x direction perpendicular to the y direction. For this reason, the return light transmitted through the Faraday rotator 101 cannot pass through the polarizer 102.

  Thus, it is an indispensable requirement for the isolator 100 to obtain an optical rotation angle of 45 degrees by the Faraday rotator 101 so that the isolator 100 blocks the return light and functions as an isolator. In the infrared region, a magnetic garnet is used for the Faraday rotator 101.

  On the other hand, in the visible light region, the magnetic garnet is impermeable. For this reason, in the isolator used in the visible light region, magnetic glass is used to provide the same effect as in the case of infrared light. However, since magnetic glass has a smaller Verde constant than magnetic garnet, the size of the isolator becomes large and expensive in order to obtain an optical rotation angle of 45 degrees. Polarization-independent isolators that do not depend on the polarization of incident light are also commercially available, but this uses two birefringent crystals in addition to a Faraday rotator to change the light path between the forward path and the return path. Constructed and very expensive.

  In the microwave region, as described in Patent Document 1, it is common to use magnetic ferrite to obtain a nonreciprocal effect.

  In any case, the isolator is required to be configured using a material having high transmittance and a nonreciprocal effect in order to reduce insertion loss.

  The isolator in the present disclosure is one form of a nonreciprocal filter. The nonreciprocal filter of the present disclosure does not require an optical rotation angle of 45 degrees, and the transmittance of the magnetic material does not directly determine the transmission characteristics of the nonreciprocal filter. For this reason, the nonreciprocal filter of this indication can expand the choice of the magnetic material material for obtaining a nonreciprocal effect, and can be manufactured small and cheaply. In addition, since a nonreciprocal material can be selected from materials that can be mounted on a substrate, a waveguide structure can be easily manufactured.

  Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The drawings used in the following description are schematic. The dimensional ratios on the drawings do not necessarily match the actual ones.

(Basic configuration of equivalent circuit)
As shown in FIG. 1, the equivalent circuit 10 according to the isolator of the present disclosure includes a filter 20 and a nonreciprocal line 30 connected in parallel between a first point 11 and a second point 12. The equivalent circuit 10 is branched into two parallel lines between the first branch point 13 and the second branch point 14, a filter is connected to one branched line, and the nonreciprocal line 30 is connected to the other line. It is connected. Hereinafter, a signal path from the first point 11 to the second point 12 is referred to as an outward path, and a signal path from the second point 12 to the first point 11 is referred to as a return path. The isolator of the present disclosure can be manufactured according to the equivalent circuit 10.

  The filter 20 includes a multistage resonator, and the filter 20 as a whole forms a bandpass filter, a lowpass filter, a highpass filter, or the like. By providing a parallel line for the filter 20, an attenuation pole is formed in the pass characteristic of the equivalent circuit 10. At this time, by using the non-reciprocal line 30 as the parallel line, the frequency of the attenuation pole appearing in the forward path (hereinafter referred to as “first attenuation pole”) according to the characteristics of the non-reciprocal line 30 and the return path The frequency of the appearing attenuation pole (hereinafter referred to as “second attenuation pole”) can be made different. Therefore, the equivalent circuit 10 may have different filter characteristics for a signal traveling on the forward path and a signal traveling on the backward path. A filter characteristic relating to a signal traveling in the forward path is defined as a first filter characteristic, and a filter characteristic relating to a signal traveling in the return path is defined as a second filter characteristic.

  In the forward path, a band that is included in the pass band of the filter 20 and through which the signal passes without being influenced by the first attenuation pole, and in the return path, a band in which the signal is attenuated by the second attenuation pole is defined as an effective band. In the first filter characteristic, the pass band formed by the equivalent circuit 10 includes the effective band, and the first attenuation pole appears outside the pass band. In the second filter characteristic, the second attenuation pole appears in the pass band of the first filter characteristic. Thereby, in the effective band, the transmittance in the second filter characteristic is lower than the transmittance in the first filter characteristic. When the transmittance in the second filter characteristic is sufficiently low, the nonreciprocal filter corresponding to the equivalent circuit 10 functions as an isolator in the effective band.

  In the equivalent circuit 10, the nonreciprocal line 30 is used only for forming an attenuation pole in the pass characteristic of the filter 20. For this reason, the signal passing between the first point 11 and the second point 12 is mainly passed through the line to which the filter 20 is connected, and the signal passing through the nonreciprocal line 30 can be reduced. This means that in the isolator corresponding to the equivalent circuit 10, most of the electromagnetic wave energy can be distributed to the filter 20 side, and the influence of the loss in the nonreciprocal line 30 can be reduced.

  In the isolator corresponding to the equivalent circuit 10, if the incident electromagnetic wave is circularly polarized, the nonreciprocity of the nonreciprocal line 30 of the equivalent circuit 10 can be embodied as a difference in refractive index between the forward path and the return path. . If the incident electromagnetic wave is linearly polarized, nonreciprocity can be embodied as a difference in Faraday rotation angle. By using these characteristics, it is possible to generate the difference between the above-described outbound path and inbound path.

(First example of equivalent circuit)
FIG. 2 is an equivalent circuit 10a of the isolator according to the first example in which the nonreciprocal line 30 has no loss. In the filter 20 of the equivalent circuit 10a, a plurality of stages of LC resonators 21a to 21l (hereinafter referred to as LC resonators 21 as appropriate) are arranged in parallel with capacitors 22a to 22k (hereinafter referred to as capacitors 22 as appropriate) sandwiched therebetween. It is a bandpass filter. As shown in FIG. 3, each LC resonator 21 has a capacitor 23 and an inductor 24 connected in parallel, and is electrically coupled by an adjacent LC resonator 21 and a capacitor 22. In addition, a lossless phase circuit 31 is arranged in parallel with the filter 20 as the nonreciprocal line 30.

  In the equivalent circuit 10a, the input / output impedance is set to 50Ω, the impedance of the lossless phase circuit 31 is set to 220Ω, the phase shift in the forward path is set to 98 °, and the phase shift in the return path is set to 125 °. The LC resonator 21 was designed to have a desired resonance frequency. The capacitance of the capacitor 22 was designed so that the coupling amount between the LC resonators 21 was a desired value. It is known that how to obtain these numerical values can be generally derived from filter theory (see, for example, Yoshihiro Konishi, “Microwave Technology Course”, Kay Lab Publishing, 2001). In the case of the present embodiment, each element value was obtained by setting the resonance frequency of the LC resonator 21 to 1.01 GHz and the bandwidth to 0.71 GHz. Various capacitances can be set for the capacitors 22 between the LC resonators 21. For example, the capacitances of all the capacitors 22 can be set to 2.26 pF.

  As shown in the above design example, the characteristic impedance of the lossless phase circuit 31 may be 220Ω with respect to the characteristic impedance of 50Ω of the equivalent circuit 10a. For this reason, it can be seen that most of the energy of the electromagnetic wave passing through the isolator based on the equivalent circuit 10a passes through the filter 20 side.

  FIG. 4 shows filter characteristics at a normalized frequency obtained by simulation as a more general design example. In FIG. 4, the broken line indicates the insertion loss with respect to the normalized frequency of the forward path. In FIG. 4, the solid line shows the insertion loss with respect to the normalized frequency of the return path. That is, the broken line indicates the first filter characteristic in the forward path of the equivalent circuit 10a. The solid line shows the second filter characteristic in the return path of the equivalent circuit 10a. The forward path has a pass band Bp with a small insertion loss indicated by a broken line. The first attenuation pole A1 in the forward path has a frequency outside the passband Bp. As shown in FIG. 4, the frequency of the attenuation pole formed differs between the forward path and the return path. The second attenuation pole A2 in the return path is located on the pass band Bp side of the first attenuation pole A1 in the filter characteristic of the forward path. The effective band Be that can be used as an isolator at this time is in the range of 0.87 to 0.94 on the normalized frequency axis, as indicated by a one-dot chain line.

  The standardized frequency is generally used in filter design, and is a filter in which the passband width of the filter is −1 to 1 and the center frequency of the filter is 0. From such a filter, a low-pass filter, a band-pass filter, a high-pass filter, and the like can be freely designed by frequency conversion. For example, in the case of a bandpass filter, it is known that the frequency can be converted from the normalized frequency F to an arbitrary real frequency f by performing frequency conversion according to the following equation.

F = A (f / f0−f0 / f) (1)
However, f0 is the center frequency of an arbitrary filter, f is an actual frequency, and A is a specific bandwidth. The conversion from the normalized frequency F to an arbitrary real frequency f can be performed by solving Equation (1) for the real frequency f.

In addition, the amount of phase shift [°] in the dielectric is obtained by using the relative permittivity ε, the relative permeability μ, and the length d of the dielectric,
√ε√μ × d / λ × 360 °
Defined by The difference in phase shift amount of θ = 98 ° and 125 ° can be easily generated by using, for example, ferrite. According to FIG. 1 of Patent Document 1, when the bias magnetic field is 39.5 kA / m (500 Oe), the saturation magnetization of ferrite is 85 mT (850 G), and the magnetic resonance half width is 158 A / m (2 Oe), the frequency is approximately 0. It is known that a difference of 60% of the maximum value of the magnetic permeability can be provided between the maximum value and the minimum value of the magnetic permeability at 5 GHz. The above-described difference in magnetic permeability is a difference of just over 25% when the phase shift amount is corrected. That is, if the ferrite having an appropriate length is used, a difference in phase shift amount of 98 ° and 125 ° can be provided.

  FIG. 5 is an enlarged view of a part of FIG. 4 including the effective band Be in the horizontal axis direction. FIG. 6 is a diagram in which the horizontal axis of FIG. 5 is converted into a wavelength. A wavelength region indicated by an alternate long and short dash line corresponds to the effective band Be. In FIG. 6, the wavelength range of the effective band Be is about 0.215 m to about 0.22 m. In this region, the forward insertion loss indicated by a broken line is −1 dB or less. In this region, the insertion loss in the return path indicated by the solid line is -20 dB or more. By forming the nonreciprocal line 30 that gives a phase change amount of 27 ° between the forward path and the return path, the insertion loss in the forward path, which is a required specification of a general optical isolator, is −1 dB or less in the effective band Be. It can be seen that it is possible to secure -20 dB or more in the return path.

(Second example of equivalent circuit)
The material used for the nonreciprocal line 30 of the first example may be selected from materials including magnetic metal such as iron (Fe) and cobalt (Co). However, the magnetic material that can be used in the infrared region has a large absorption of infrared light by the material except for the magnetic garnet. Therefore, in the equivalent circuit 10b of the second example shown in FIG. 7, it is assumed that the non-reciprocal line 30 has a loss in the equivalent circuit 10a shown in FIG. A phase circuit 32 is provided. The element value of the equivalent circuit 10b is the same as the element value of the corresponding component in FIG. At this time, the loss with respect to the value of tan δ was calculated. tan δ is a parameter corresponding to the amount of energy absorbed in the dielectric. For example, FIG. 8 shows the insertion loss with respect to the normalized frequency when tan δ = 0.0125 in the loss phase circuit 32. As in FIG. 4, in the first filter characteristic of the forward path, the first attenuation pole A1 appears on the high frequency side outside the pass band Bp, and in the second filter characteristic of the return path, the second attenuation pole A2 is in the first filter characteristic. It appears on the passband Bp side of the first attenuation pole A1. As a result, the pass band Bp includes an effective band Be in which the insertion loss is −1 dB or less in the forward path and the insertion loss is −20 dB or more in the return path.

  FIG. 9 is a diagram in which the frequency band including the effective band Be of FIG. 8 is expanded in the horizontal direction. Compared with the case where the lossless phase circuit 31 shown in FIG. 5 is used, when the loss phase circuit 32 is used, a sufficient insertion loss of −20 dB or more is obtained in the effective band Be of the same frequency bandwidth in the return path. can get. Therefore, it is considered that the isolator corresponding to the equivalent circuit 10b can be used without any practical problem.

  FIG. 10 is a graph showing a change in the loss amount at a frequency (fm in FIG. 9) at which the loss is maximum within the effective band Be with respect to the change in tan δ. As shown in FIG. 10, the loss amount hardly changes until tan δ is 0.01, and maintains about −1. Even when tan δ is 0.1, the loss is less than −2 dB. Therefore, even when the material used for the nonreciprocal line 30 has absorption, an isolator having sufficient transmission characteristics can be realized.

(Third example of equivalent circuit)
In FIG. 2, the line including the filter 20 and the line including the lossless phase circuit 31 are connected, but may be connected via a capacitor or an inductor. Further, a capacitor or an inductor may be interposed between the input and output to the filter 20. For example, in the equivalent circuit 10c of the third example shown in FIG. 11, the filter 20 and the lossless phase circuit 31 are connected via inductors 33a and 33b. The inductances of the inductors 33a and 33b between the filter 20 and the lossless phase circuit 31 are L = 0.1 nH. According to the design of the third example, the characteristic impedance of the nonreciprocal line 30 may be 210Ω with respect to the input / output impedance of 50Ω of the equivalent circuit 10c. For this reason, most of the electromagnetic wave energy passes through the filter 20 side. Further, the amount of phase shift in the forward path and the return path was set to θ = 95 ° and 125 °. FIG. 12 shows filter characteristics at a normalized frequency as a more general design. Also in FIG. 12, the first filter characteristic of the forward path has the first attenuation pole A1, and the second filter characteristic of the return path has the second attenuation pole A2 at a frequency different from that of the first attenuation pole A1. As a result, an effective band Be that functions as an isolator is generated. FIG. 13 shows an enlarged view of a part of FIG. 12 in the horizontal axis direction.

(Fourth example of equivalent circuit)
In general, it is known that in order to form an attenuation pole, jump coupling between resonators in the middle of a filter is also possible, and the nonreciprocal line 30 is branched from LC resonators 21 other than both ends of the filter 20. May be. In the equivalent circuit 10 d of the fourth example shown in FIG. 14, the nonreciprocal line 30 is branched from the second LC resonator 21 (21 b, 21 k) from both ends of the filter 20. As the nonreciprocal line 30, a lossless phase circuit 31 is used. Inductors 33a and 33b between the filter 20 and the nonreciprocal line 30 of the equivalent circuit 10d have L = 0.1 nH. According to the design of the fourth example, since the characteristic impedance of the nonreciprocal line 30 may be 350Ω with respect to the input / output impedance of 50Ω of the equivalent circuit 10d, most of the electromagnetic wave energy passes through the filter 20 side. Further, the amount of phase shift in the forward path and the return path was set to θ = 80 ° and 140 °. As a more general design, the filter characteristics at the normalized frequency are shown in FIG. FIG. 16 is an enlarged view of a part of FIG. 15 in the horizontal axis direction.

  It is known that the magnitude of the interlace coupling for generating the equivalent attenuation pole is smaller as the distance between the coupled LC resonators 21 is increased. The equivalent circuit 10d of the fourth example is different from the equivalent circuits 10a to 10c of the first example to the third example in that the difference in the amount of phase shift between the forward path and the return path is increased, and the equivalent circuit of the first example to the third example An attenuation pole equivalent to 10a to 10c is generated. In order to increase the difference in the amount of phase shift in the isolator corresponding to the equivalent circuit 10d, the thickness of the nonreciprocal element must be increased. This leads to an increase in size and price of the element. Therefore, the nonreciprocal line 30 is preferably connected to the input side end and the output side end of the filter 20 and connected in parallel to the entire filter 20. The fact that the magnitude of the interlace coupling may be small means that the attenuation pole required with a small phase difference can be realized.

  In addition, a nonreciprocal line 30 is provided between the input side end and the output side end of the filter 20, and another nonreciprocal line is provided in parallel only with some LC resonators 21 constituting the filter 20. Can also be connected. In this case, it is known that two attenuation poles are generated simultaneously.

(Fifth example of equivalent circuit)
FIG. 17 shows an equivalent circuit 10e of a fifth example in which the coupling between the LC resonators 21 is changed from the capacitors 22a to 22k to the inductors 25a to 25k. The lossless phase circuit 31 and the filter 20 are connected via capacitors 34a and 34b. The capacitance between the filter 20 and the lossless phase circuit 31 of the equivalent circuit 10e of the fifth example is C = 0.2 pF. According to this design example, the characteristic impedance of the nonreciprocal line 30 may be 520Ω with respect to the input / output impedance of 50Ω of the equivalent circuit 10e. For this reason, most of the electromagnetic wave energy of the isolator corresponding to the equivalent circuit 10e passes through the filter 20 side. Further, the amount of phase shift was set to forward path θ = 45 ° and return path θ = 59 °. FIG. 18 shows filter characteristics at a normalized frequency as a more general design. FIG. 19 shows an enlarged view of a part of FIG. 18 in the horizontal axis direction. Compared with the first to fourth examples described above, the difference in the amount of phase shift between the forward path and the return path is small. It can be seen that the effective band Be can be generated with fewer nonreciprocal lines 30.

  In the equivalent circuits 10, 10 a to 10 e, most energy passes through the filter 20 side as energy distribution through the filter 20 and the nonreciprocal line 30. Thereby, the nonreciprocal line 30 is used only for forming the attenuation pole of the filter 20, and the influence of the loss of the nonreciprocal line 30 on the equivalent circuit 10 is reduced. It is known that the reflectance when two lines having different impedances Z1 and Z2 are connected can be expressed by the following equation.

  Therefore, when the input / output impedance of the equivalent circuit 10 is Z0, the impedance Zf of the filter 20, and the impedance of the nonreciprocal line 30 is Zr, it is preferable that the following equation is established.

(Isolator corresponding to equivalent circuit)
FIG. 20 shows a schematic configuration example of an isolator 50 corresponding to the equivalent circuit 10 shown in FIG. The equivalent circuit 10 represents the behavior of an electromagnetic wave by a signal in the equivalent circuit. The isolator 50 includes a first propagation path 51, a second propagation path 52, a first branch path 54 including a filter 53, and a second branch path 56 including a nonreciprocal material 55. The first propagation path 51 is connected to the first branching path 54 and the second branching path 56 at the first demultiplexing / multiplexing unit 57. The first branch path 54 and the second branch path 56 are connected to the second propagation path 52 by the second branching / multiplexing unit 58. The end of the first propagation path 51 opposite to the first multiplexing / demultiplexing unit 57 is a first end 59. An end portion of the second propagation path 52 on the opposite side of the second multiplexing / demultiplexing portion 58 is a second end portion 60.

  The first propagation path 51, the second propagation path 52, the first branch path 54, and the second branch path 56 are paths through which electromagnetic waves of various wavelengths pass. Electromagnetic waves include microwaves, infrared rays, and visible light. The filter 53, the nonreciprocal material 55, the first end 59 and the second end 60 are respectively the filter 20, the nonreciprocal line 30, the first point 11 and the second point 12 of the equivalent circuit 10 of FIG. Corresponding to

  The isolator 50 can have various configurations as long as it is represented by the equivalent circuit 10 shown in FIG.

  For example, the first propagation path 51, the second propagation path 52, the first branch path 54, and the second branch path 56 are a waveguide that propagates microwaves, a dielectric line, an optical waveguide that propagates infrared light and visible light, It includes the optical path in the optical crystal, as well as free space. The waveguide includes a hollow waveguide such as a rectangular waveguide or a circular waveguide. The optical waveguide includes an optical fiber, a flat optical waveguide, and a waveguide provided on the substrate. As the filter 53, a planar filter such as a strip line, a waveguide filter, a dielectric resonator filter, a metal mesh filter, a metal thin film filter, a dielectric multilayer filter, an optical glass filter, a photo according to the wavelength of the propagating electromagnetic wave It includes a filter configured using a nick crystal resonator. In the low frequency band, a lumped constant filter can be used. A surface wave filter such as a SAW filter can also be used. As the nonreciprocal material 55, various materials can be used as long as electromagnetic waves have different characteristics between the forward path and the return path. For example, the nonreciprocal material 55 includes a magnetic material such as magnetic garnet, ferrite, iron, or cobalt. As the first demultiplexing unit 57 and the second demultiplexing unit 58, various demultiplexers and multiplexers can be adopted according to the wavelength of the propagating electromagnetic wave. The first demultiplexing unit 57 and the second demultiplexing unit 58 include a T branching waveguide, a Y branching waveguide, an optical fiber coupler, a beam splitter, a half mirror, and the like. The electromagnetic wave propagated through the isolator 50 can include single-mode and multi-mode electromagnetic waves (light). In the case of infrared light and visible light, the electromagnetic wave propagated through the isolator 50 includes coherent light and incoherent light.

  The nonreciprocal material 55 will be described more specifically. Conventional isolators generally use magnetic garnet or ferrite, but the present invention can also be realized by using the following materials, for example. In an academic paper (Kobayashi, N. et al. Optically Transparent Ferromagnetic Nanogranular Films with Tunable Transmittance. Sci. Rep. 6, 34227; doi: 10.1038 / srep34227 (2016).) It is known to develop a ferromagnetic material. According to the above paper, since a film can be formed by vapor deposition using an RF sputtering apparatus, when forming a waveguide in the present invention, a nonreciprocal line can be easily formed by a well-known technique. In addition, an academic journal (PHYSICAL REVIEW APPLIED 7, 024006 (2017)) reports that ferromagnetism develops in a thin film in which part of strontium titanate is replaced with Fe. This thin film is produced by a PLD (pulsed laser deposition) method and has a large Faraday rotation angle per unit absorption amount. When used in a nonreciprocal line in the present invention, the absorption in the nonreciprocal line portion is not considered. It is feasible.

  The isolator 50 can be represented by the equivalent circuit 10. As described above, the equivalent circuit 10 includes the first filter characteristic relating to the signal traveling from the first point 11 toward the second point 12 and the second filter characteristic relating to the signal traveling from the second point 12 toward the first point 11. And have. The first filter characteristic forms a pass band including a predetermined effective band, and has a first attenuation pole outside the pass band. The second filter characteristic has a second attenuation pole on the pass band side of the first attenuation pole in the first filter characteristic, and transmits the signal in the effective band with a transmittance lower than the transmittance in the first filter characteristic. Let For this reason, the isolator 50 represented by the equivalent circuit 10 transmits electromagnetic waves from the first end portion 59 toward the second end portion 60. In addition, the isolator 50 attenuates electromagnetic waves traveling from the second end portion 60 toward the first end portion 59.

  Furthermore, although the isolator 50 is intended for electromagnetic waves in the above description, it can be applied to an electric circuit as long as it can be represented by the equivalent circuit 10 of FIG.

  A feature of the present disclosure resides in that the frequency of the attenuation pole is moved between the forward path and the return path so that at least a part of the pass characteristic of the filter does not function in the return path. In principle, the attenuation pole can be provided on either the low frequency side or the high frequency side of the pass band of the filter in the forward path. However, it is preferable to use the attenuation pole in the high frequency range to easily affect the filter characteristics. FIG. 21 is a diagram illustrating transmission and reflection of the forward path (upper figure) and the return path (lower figure) calculated by simulation. In FIG. 21, the solid line indicates transmission and the broken line indicates reflection. On the high frequency side near 1.5 GHz, the transmission characteristics change greatly. On the other hand, there is little change in the transmission characteristics on the low frequency side near 0.7 GHz. Therefore, moving the attenuation pole included in the high frequency side of the passband according to the characteristics of the nonreciprocal line 30 changes the filter characteristics and lowers the transmittance than moving the attenuation pole on the low frequency side. Great effect. The filter can be converted to a log scale with respect to the frequency, as can be seen from Equation (1) for frequency conversion. Therefore, the band on the high frequency side is stretched with respect to the center frequency of the filter. Therefore, the influence of the attenuation pole on the high frequency side is greater than that on the low frequency side, and the use of the high frequency side requires a smaller phase difference.

  As described above, according to the isolator 50 of the present disclosure, an optical rotation angle of 45 degrees is not required, and the transmittance of the nonreciprocal material 55 does not directly determine the transmission characteristics of the isolator 50. For this reason, the isolator 50 according to the present disclosure can be manufactured at low cost by expanding the options of the nonreciprocal material 55 for obtaining the nonreciprocal effect. In addition, since the nonreciprocal material 55 can be selected from materials that can be mounted on a substrate, a waveguide structure can be easily manufactured. Furthermore, since most of the electromagnetic wave passes through the filter 53 side instead of the nonreciprocal material 55 having a large loss, the loss in the forward path of the isolator 50 can be suppressed and the transmittance can be increased.

  The configuration of the isolator 50 of the present disclosure represented by the equivalent circuit 10 of FIG. 1 can be made to transmit with a predetermined transmittance lower than that of the forward path, by design, instead of blocking electromagnetic waves on the return path. That is, the present disclosure can be implemented as a nonreciprocal filter having different filter characteristics between the forward path and the return path. The isolator 50 of the present disclosure can be regarded as one form of the nonreciprocal filter of the present disclosure.

  Although the above embodiment has been described as a representative example, it will be apparent to those skilled in the art that many changes and substitutions can be made within the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited by the above-described embodiments and examples, and various modifications and changes can be made without departing from the scope of the claims. For example, a plurality of constituent blocks described in the embodiments and examples can be combined into one, or one constituent block can be divided.

DESCRIPTION OF SYMBOLS 10 Equivalent circuit 11 1st point 12 2nd point 13 1st branch point 14 2nd branch point 20 Filter 21 LC resonator 22 Capacitor 23 Capacitor 24 Inductor 25 Inductor 30 Nonreciprocal line 31 Lossless phase circuit 32 Loss phase circuit 33a 33b Inductors 34a, 34b Capacitors 50 Isolators 51 First propagation path 52 Second propagation path 53 Filter 54 First branch path 55 Nonreciprocal material 56 Second branch path 57 First branching section 58 Second branching section 58 59 First end 60 Second end 100 Isolator 101 Faraday rotator 102, 103 Polarizer H Magnetic field L1 Incident light L2 Return light A1 First attenuation pole A2 Second attenuation pole Bp Passband Be Effective band

Claims (6)

It is represented by an equivalent circuit in which a filter and a nonreciprocal line including nano-granular material are connected in parallel between the first point and the second point,
The equivalent circuit is
A first filter characteristic relating to a signal traveling from the first point toward the second point;
A second filter characteristic relating to a signal traveling from the second point toward the first point;
The first filter characteristic has a first attenuation pole that forms a pass band including a predetermined effective band, and appears outside the pass band according to the characteristic of the nonreciprocal line,
The second filter characteristic has a second attenuation pole that appears in accordance with the characteristic of the nonreciprocal line on the passband side of the first attenuation pole in the first filter characteristic. , In the effective band, the signal is transmitted with a transmittance lower than the transmittance in the first filter characteristic,
A nonreciprocal filter characterized by that.   The non-reciprocal filter according to claim 1, wherein the equivalent circuit represents a behavior of an electromagnetic wave by the signal and functions as an isolator.   The nonreciprocal filter according to claim 2, wherein the electromagnetic wave is infrared or visible light.   The first attenuation pole is included in a band on a higher frequency side than the effective band, and the second attenuation pole is obtained by moving the first attenuation pole due to characteristics of the nonreciprocal line. The nonreciprocal filter according to claim 3. When the input / output impedance is Z0, the impedance of the filter is Zf, and the impedance of the nonreciprocal line is Zr,
The nonreciprocal filter according to any one of claims 1 to 4, wherein:   The nonreciprocal filter according to claim 1, wherein the filter includes a plurality of stages of resonators, and the nonreciprocal line is connected in parallel to the entire filter.