Optical module

1. An optical module comprising:

an optical subassembly for converting an optical signal and an electric signal at least from one to the other;

a first multilayer substrate electrically connected to the optical subassembly; and

a second multilayer substrate partially overlapping, in a planar view, and electrically connected to the first multilayer substrate, wherein

the first multilayer substrate includes a first conductive layer and a second conductive layer,

the second multilayer substrate includes a third conductive layer, a fourth conductive layer, and a fifth conductive layer,

the first conductive layer includes a first signal wiring connected to the optical subassembly, the first signal wiring including, at a position except for an end portion, a wide portion having a width wider than that of another portion,

the second conductive layer includes a signal electrode electrically continuous with the end portion of the first signal wiring, a first ground plane overlapping the wide portion, in the planar view, and a ground terminal connected to the first ground plane,

the third conductive layer includes a second signal wiring including an end portion overlapping, in the planar view, and bonded to the signal electrode, and a ground electrode overlapping, in the planar view, and bonded to the ground terminal,

the fourth conductive layer includes a second ground plane electrically continuous with the ground electrode, the second ground plane including a through hole overlapping, in the planar view, the end portion of the second signal wiring, and

the fifth conductive layer includes a third ground plane electrically continuous with the second ground plane, the third ground plane overlapping, in the planar view, the end portion of the second signal wiring within the through hole in the planar view.

2. The optical module according to claim 1 , further comprising:

a first insulating layer intervening between the first conductive layer and the second conductive layer;

a second insulating layer intervening between the third conductive layer and the fourth conductive layer;

a third insulating layer intervening between the fourth conductive layer and the fifth conductive layer;

a first electrically continuous portion penetrating the first insulating layer to provide electrical continuity between the end portion of the first signal wiring and the signal electrode;

a second electrically continuous portion penetrating the second insulating layer to provide electrical continuity between the ground electrode and the second ground plane; and

a third electrically continuous portion penetrating the third insulating layer to provide electrical continuity between the second ground plane and the third ground plane.

3. The optical module according to claim 1 , wherein

the wide portion of the first signal wiring includes a portion protruding from the first ground plane.

4. The optical module according to claim 1 , wherein

the end portion of the second signal wiring and the signal electrode have bonding areas that are bonded to each other,

the bonding areas have a length along the first signal wiring, and

the first signal wiring includes the wide portion adjacent to the bonding area and within a range of a length smaller than or equal to the above-described length.

5. The optical module according to claim 1 , wherein

the second signal wiring includes a pair of second signal wirings, and

the through hole is formed to avoid overlapping at least a portion of an area between end portions of the pair of second signal wirings in the planar view.

6. The optical module according to claim 1 , wherein

the through hole of the second ground plane has a shape in which the width gradually narrows in a direction in which the second signal wiring extends from a tip thereof, and

an overlapping area, in the planar view, of the end portion of the second signal wiring and the second ground plane, which is adjacent to the through hole, gradually increases in the direction in which the second signal wiring extends from the tip thereof.

7. The optical module according to claim 1 , wherein

the width of the end portion of the second signal wiring is wider than that of the signal electrode.

8. The optical module according to claim 1 , wherein

the wide portion of the first signal wiring is located at a position not overlapping the through hole of the second ground plane in the planar view.