An optical module having a flexible substrate having, even after actual manufacturing steps, excellent transmission characteristics of high-frequency signals and an advantage that electromagnetic field radiation is reduced even when it is connected with a package. The flexible substrate used in external connection of the package of the optical module uses a flexible substrate having a coplanar line to which a lead pin is fixedly attached, a grounded coplanar line which is in contact with the coplanar region, and a microstrip line which is in contact with the grounded coplanar line. The flexible substrate has an electrode layout in which an electromagnetic field component of a surface ground line and a signal line is more dominant than an electromagnetic field component of a back-surface ground line and the signal line in a region of the coplanar line adjacent to the grounded coplanar line.
1. An optical module comprising:
a circuit board;
a flexible substrate connected to the circuit board; and
a package including a lead pin, the lead pin being fixedly attached to the flexible substrate,
wherein:
the flexible substrate includes a transmission path having a signal line, a pair of surface ground lines, and a back-surface ground line, the signal line and the surface ground lines being provided on a first main surface of the flexible substrate, the surface ground lines being arranged on the left and right of the signal line with a gap therebetween, and the back-surface around line being provided on a second main surface of the flexible substrate opposed to the first main surface,
the transmission path includes a first region, a second region, and a third region in this order toward the package,
the first region, the second region, and the third region include the signal line and the surface around lines,
the first region includes the back-surface ground line which is overlapped with the signal line and terminated within the first region;
the third region does not include the back-surface ground line in a location overlapped with the signal line,
the second region does not include the back-surface ground line in a location overlapped with the signal line or does include the back-surface ground line being arranged so that a distance between the signal line and the back-surface ground line is longer than a distance between the signal line and the surface ground line, and
the signal line includes:
a part present in the first region and having a first width;
a part present in the third region and having a third width; and
a part present in the second region and having a width varied from the first width to the third width.
2. The optical module according to claim 1 , wherein
the surface ground line in the second region gradually gets away from a central axis of the signal line.
3. The optical module according to claim 1 , wherein
the back-surface ground line is provided below the surface ground line in the third region.
4. The optical module according to claim 3 , wherein
the back-surface ground line below the surface ground line is connected with the surface ground line by a contact hole.
5. The optical module according to claim 1 , wherein
the back-surface ground line is provided below the surface ground line in the second region.
6. The optical module according to claim 5 , wherein
the back-surface ground line below the surface ground line is connected with the surface ground line by a contact hole.
7. The optical module according to claim 1 , wherein
each of the signal line and the surface ground lines constituting the flexible substrate is connected with the lead pin in the third region.
8. The optical module according to claim 1 , wherein
the terminal of the back-surface ground line overlapped with the signal line in the first region is orthogonal to or forming a predetermined angle with respect to an extending direction of the signal line.
9. The optical module according to claim 1 , wherein
the flexible substrate has a fourth region adjacent to a side of the first region, the side being opposite to the second region;
the fourth region includes a microstrip line having the back-surface ground line overlapped with the signal line without the pair of surface ground lines; and
a characteristic impedance of the transmission path in the second region is within ±5 ohms from a characteristic impedance of the microstrip transmission path in the fourth region.
10. The optical module according to claim 1 , wherein
the signal line and the pair of surface ground lines include a part covered with an organic film or a resin.
11. The optical module according to claim 1 , wherein
a frequency of a signal propagating through the transmission path is 10 GHz or higher.
12. The optical module according to claim 11 , wherein
the frequency of the signal propagating through the transmission path is 25 GHz or higher.