Patent Application
20190025612
The present invention relates to an optical modulator, and particularly relates to an optical modulator in which an optical modulation element is mounted inside a housing, that is, an optical modulator in which at least a portion of an electric line is connected to an external circuit board through a flexible printed circuit and which is disposed on the external circuit board.
In the field of optical communication, transceivers using an optical modulator are utilized. Recently, in accordance with demands for miniaturizing optical transmission systems, RF interface connection of an optical modulator mounted in a transceiver module (transponder) also tends to be shortened.
In the flexible printed circuit 6, a signal line (signal electrode) and a ground line (ground electrode) made of a conductive material such as Au and Cu are formed on one surface or both surfaces of the circuit made of polyimide or the like. A microstrip line having a wide-width ground electrode formed on one surface and a signal electrode on a strip formed on the other surface is in wide use. In signal lines, there are cases where ground electrodes are also disposed in addition to a signal electrode as in a coplanar line such that the signal electrode is interposed therebetween. Although the flexibility of the flexible printed circuit 6 is degraded, a strip line having multiple layers of ground electrode surfaces or a grounded coplanar line is also used.
When the flexible printed circuit 6 is mounted in the optical modulator 1, a recess portion (spot facing portion) is formed on an external bottom surface of the housing as indicated with a dotted line D in
However, due to the shape of the lead pin 4, it is difficult for both the flexible printed circuit 6 and the bottom surface (surface A facing the FPC 6) forming the recess portion to be brought into completely tight contact and attached to each other. For example, in a case where the signal line and the ground electrode are configured to be coaxially disposed and a tip of the ground electrode part protrudes from a lower surface of the housing, the FPC 6 and the bottom surface A of the recess portion are separated from each other as much as the protruding height of the lead pin 4. Therefore, a gap S1 is generated between both thereof. More specifically, a gap S1 is generated between an electric wiring part B of the signal electrode, the ground electrode, and the like provided on the FPC 6, and the bottom surface A.
In addition, a gap S2 is generated between the FPC 6 and the external circuit board 7. More specifically, a gap S2 is generated between the electric wiring part B of the signal electrode, the ground electrode, and the like provided on the FPC 6, and an electric wiring part C provided on the external circuit board 7. As a reason therefor, in a case where the lead pin 4 is subjected to solder-fixing on the lower surface side of the FPC 6, the tip of the lead pin 4 protrudes from the lower surface of the FPC 6. In order to prevent this protruding lead pin 4 from coming into contact with the external circuit board 7, the depth of the recess portion is intentionally adjusted such that the gap S2 is generated.
Such gaps S1 and S2 are not necessarily air layers. For example, in a case where an insulating protective film is provided on the front surface (or both surfaces) of the FPC 6, or in a case where an insulating protective film is provided on the front surface of the external circuit board, a gap corresponding to the thickness of the protective film (in this case, a state where the gap is filled with the material of the protective film) is inevitably generated.
A case where the gap S1 or S2 is parallel to the ground electrode surface of the FPC 6 leads to generation of a parallel plate mode as described in
As illustrated in
In addition, a cavity resonance mode corresponding to the clearance, that is, the gap S1 or S2 is also generated in the gap S1 or S2 due to a microwave and a millimeter wave emitted to the space, so that a modulation signal in a particular frequency deteriorates (dips). In order to shorten a wiring distance and to ensure easiness of mounting, the gap S1 or S2 is required to be narrower. In this case, a parallel plate mode is likely to be generated.
As in a DP-BPSK optical modulator, a DQPSK optical modulator, a DP-QPSK optical modulator, and the like, in a case of a broadband optical modulator having a plurality of signal lines disposed on the FPC 6, crosstalk caused between the signal lines through a parallel plate mode leads to a severe problem. Since the crosstalk is caused due to power which has leaked from a signal line and is transferred to another signal line through a parallel plate mode having no cut-off frequency, the crosstalk appears throughout an extremely wide frequency.
In a case of a particular frequency, crosstalk can be reduced by means of a stub circuit or a choke circuit. However, the method is not effective in a case of a broadband modulator having a signal band ranging from a MHz band to a millimeter wave band. As alternative means, crosstalk inside a wiring substrate, for example, crosstalk among a plurality of adjacent microstrip lines can be reduced by means in which a via hole is disposed between the lines, a groove is made in a dielectric substrate between the lines, or the like. However, a parallel plate mode is generated between a ground electrode surface and another ground electrode surface of the wiring substrate regardless of whether a via hole, a groove, or the like is formed inside the substrate. In addition, power which has leaked from a signal line reaches not only an adjacent signal line but also other signal lines, thereby resulting in crosstalk. The same applies not only to a microstrip line but also to a line having a different shape.
In the present invention, a phenomenon of a cavity resonance mode of a microwave and a millimeter wave, a parallel plate mode, or the like is expressed as “a resonance mode or the like”.
[Patent Literature No. 1] Japanese Laid-open Patent Publication No. 2014-165289
In order to solve the problems described above, the present invention aims to provide an optical modulator in which a phenomenon of a resonance mode or the like is prevented from being generated in a recess portion of a housing that mounts a flexible printed circuit and of which broadband characteristics are improved.
In order to solve the problems, an optical modulator of the present invention has the following technical features.
(1) There is provided an optical modulator in which an optical modulation element is mounted inside a housing, in which at least a portion of an electric line is connected to an external circuit board through a flexible printed circuit, and which is disposed on the external circuit board. A recess portion that mounts the flexible printed circuit is formed on an outer bottom surface of the housing and at a part where the flexible printed circuit is connected, and means for preventing a resonance mode or the like is provided such that a resonance mode of a microwave and a millimeter wave or a parallel plate mode is prevented from being generated among a surface A of the recess portion facing the flexible printed circuit, an electric wiring part B provided on the flexible printed circuit overlapping the surface A when seen in a plan view, and an electric wiring part C provided on the external circuit board overlapping the surface A when seen in a plan view.
(2) In the optical modulator according to (1), in the means for preventing a resonance mode or the like, at least one of a clearance or a dielectric constant between the surface A and the electric wiring part B, and a clearance or a dielectric constant between the electric wiring part B and the electric wiring part C is set to be locally varied.
(3) In the optical modulator according to (1), in the means for preventing a resonance mode or the like, at least any one of a pillar electrically connecting the surface A and the electric wiring part with each other or a pillar electrically connecting the electric wiring part B and the electric wiring part with each other is provided.
(4) In the optical modulator according to any one of (1) to (3), electric wirings are respectively formed on both surfaces of the flexible printed circuit, and a via hole electrically connecting ground wirings on both the surfaces with each other is formed.
According to the present invention, there is provided an optical modulator in which an optical modulation element is mounted inside a housing, in which at least a portion of an electric line is connected to an external circuit board through a flexible printed circuit, and which is disposed on the external circuit board. A recess portion that mounts the flexible printed circuit is formed on an outer bottom surface of the housing and at a part where the flexible printed circuit is connected, and means for preventing a resonance mode or the like is provided such that a resonance mode of a microwave and a millimeter wave or a parallel plate mode is prevented from being generated among a surface A of the recess portion facing the flexible printed circuit, an electric wiring part B provided on the flexible printed circuit overlapping the surface A when seen in a plan view, and an electric wiring part C provided on the external circuit board overlapping the surface A when seen in a plan view. Therefore, it is possible to provide an optical modulator in which a phenomenon of a resonance mode or the like is prevented and of which broadband characteristics are improved.
Hereinafter, an optical modulator of the present invention will be described in detail using preferable examples.
As illustrated in
In the optical modulator of the present invention, as the optical modulation element 2, it is possible to utilize an optical modulation element constituted by forming an optical waveguide and a modulation electrode on a substrate such as a LiNbO3 substrate (LN substrate) having an electro-optic effect. In addition, without being limited thereto, a semiconductor modulation element or the like can also be utilized. Particularly, an optical modulation element applying a high frequency signal of 25 GHz or higher is preferably utilized in the present invention.
In
In regard to the lead pin, it is possible to used lead pins respectively corresponding to electrodes such as a signal electrode and a ground electrode. However, as illustrated in
In the flexible printed circuit 6 used for the optical modulator of the present invention, polyimide is used as a base material (substrate), and an electric line formed on the base material using Au, Cu, and the like. The thickness of an electric wiring as an electrode is 20 μm or greater and is more preferably 25 μm or greater. At least a ground wiring having the same thickness is formed together with the signal electrode on the surface where the signal electrode is formed. Even if the thickness of a ground electrode constituted of a microstrip (MS) line or a ground electrode constituted of a G-CPW line (having a coplanar line formed on one surface and a ground electrode formed on the other surface) is 20 μm or less, the thickness is sufficient to function as a ground electrode. Therefore, it is not essential to form the electrode thick.
The means for preventing a resonance mode or the like according to the present invention will be described in detail. As the means for preventing a resonance mode or the like, the following four methods can be appropriately executed in combination.
(1) Method of adjusting cavity size: The dielectric constant in a space configuring a gap or the distance between reflection surfaces is adjusted in accordance with a band to be used. A shielding reflection surface can be inserted into the gap. This method is less effective for a parallel plate mode.
(2) Unevenness of cavity: An FPC is tilted with respect to an inner surface A of the recess portion or the front surface of the external circuit board. The FPC is warped or is formed in a waveform. An inner wall surface of the recess portion is tilted with respect to the FPC or the front surface of the external circuit board. The inner wall surface is formed in a waveform, a corrugated shape, or a stepped shape.
(3) Addition of function of reflection/absorption: The inner wall surface of the recess portion is roughened. A non-metal material or a high dielectric constant material is disposed in the gap, on the inner wall surface, or on the front surface of the external circuit board.
(4) An electrical short-circuit is caused between surfaces facing each other using a via hole or a blind via hole causing a short-circuit between ground electrode surfaces of the FPC, or using a ball grid, a resistance sponge, or the like causing a short-circuit between a pillar or the ground electrode surface of the PFC and the inner surface A of the recess portion or the external circuit board.
(5) A notch is made in the ground electrode of the FPC, and the area of the ground electrode facing the inner surface A of the recess portion or the external circuit board is reduced.
Next, specific examples will be described using
As illustrated in
In addition, as a method of preventing a parallel plate mode from being generated between the FPC 6 and the external circuit board 7, it is effective to have a configuration in which no electrode such as a ground electrode is disposed on the front surface of external circuit board 7 facing the FPC 6.
In a relationship between disposition clearance in a plurality of pillars and a suppressing amount, for example, in a case where the clearance to an adjacent pillar is set to 500 μm for a microwave and a millimeter wave of 75 GHz, it is possible to realize suppression of approximately 10 dB. In a case where the pillar clearance is set to 100 μm, it is possible to realize suppression of approximately 40 dB.
In addition, in a case where the ground electrode (ground wiring) is formed on both surfaces of the FPC 6, a via hole electrically connecting both the ground electrodes with each other can be provided by disposing a conductive material in a hole penetrating the FPC in a region where both the ground electrodes face each other.
Hereinabove, preferable examples of the present invention have been described with reference to the accompanying drawings. There is no need to mention that present invention is not limited to the examples described above. The shape, the combination, and the like of each constituent members illustrated in the examples described above are examples, and various changes can be made based on the design requirement and the like within the scope not departing from the gist of the present invention. There is no need to mention that the effect can be further enhanced by appropriately combining the described examples.
As described above, according to the present invention, it is possible to provide an optical modulator in which a phenomenon of a resonance mode or the like is prevented from being generated in a recess portion of a housing that mounts a flexible printed circuit and of which the broadband characteristics are improved.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-192597 | Sep 2015 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/078429 | 9/27/2016 | WO | 00 |
