Driver circuit for optical transmitter

1. A driver circuit for an optical transmitter, comprising:

a multilayer board;

a differential circuit mounted on the multilayer board, and including a pair of input terminals for receiving a differential signal and a pair of output terminals for outputting an amplified differential signal;

a pair of inductor elements each having a first end and a second end;

two power supply electrodes formed on the multilayer board, and supplying a bias to the pair of output terminals; and

a pair of transmission lines formed on the multilayer board, one of the pair of transmission lines having a first connection portion electrically connected to one of the two power supply electrodes through one of the pair of inductor elements, and another of the pair of transmission lines having a second connection portion electrically connected to another of the two power supply electrodes through another of the pair of inductor elements, one end of the one of the pair of transmission lines being electrically connected to one of the pair of output terminals, one end of the another of the pair of transmission lines being electrically connected to another of the pair of output terminals,

wherein the multilayer board includes a first recessed portion between the first connection portion and the second connection portion, a second recessed portion between the first connection portion and the one of the two power supply electrodes, and a third recessed portion between the second connection portion and the another of the two power supply electrodes.

2. The driver circuit according to claim 1 , wherein the one of the pair of transmission lines has a direction parallel to a direction of the another of the pair of transmission lines on the multilayer board.

3. The driver circuit according to claim 1 , wherein each inductor element has a chip portion and two electrode portions separately formed at both ends of the chip portion, and

wherein the two electrode portions of one of the pair of inductor elements has a first electrical connection between one of the two electrode portions thereof and the one of the pair of transmission lines, and a second electrical connection between another of the two electrode portions thereof and the one of the two power supply electrodes.

4. The driver circuit according to claim 3 , wherein the first electrical connection includes a first surface contact between the one of the two electrode portions thereof and the one of the pair of transmission lines, and

wherein the second electrical connection includes a second surface contact between the another of the two electrode portions thereof and the one of the two power supply electrodes.

5. The driver circuit according to claim 1 , wherein the multilayer board has a laminated structure that includes a first insulating layer, a second insulating layer under the first insulating layer, and a third insulating layer under the second insulating layer, and has a metal layer formed between the second insulating layer and the third insulating layer,

wherein the two power supply electrodes and the pair of transmission lines are formed on the first insulating layer, and

wherein the first to third recessed portions have a depth equal to or greater than a thickness of the first insulating layer.

6. The driver circuit according to claim 5 , wherein the metal layer is electrically connected to ground.

7. The driver circuit for an optical transmitter according to claim 5 , wherein the depth of the first to third recessed portions is substantially equal to a sum of the thickness of the first insulating layer and a thickness of the second insulating layer.

8. The driver circuit according to claim 1 , wherein the second recessed portion has a width in a length direction of the pair of transmission lines, the width is equal to or greater than 0.8 times a width of the one of the pair of inductor elements in the length direction, and

wherein the third recessed portion has a width in the length direction, the width of the third recessed portion is equal to or greater than 0.8 times a width of the another of the pair of inductor elements in the length direction.

9. The driver circuit according to claim 8 , wherein the first recessed portion has a width in the length direction, the width of the first recessed portion is equal to or greater than the width of the second recessed portion in the length direction or equal to or greater than the width of the third recessed portion in the length direction.

10. The driver circuit according to claim 1 , wherein the multilayer board includes a first insulating layer having a first side on which the pair of inductor elements are mounted and a second side opposite to the first side, a second insulating layer having a first side in surface contact with the second side of the first insulating layer and a second side opposite to the first side of the second insulating layer, and a metal layer in surface contact with the second side of the second insulating layer, and

wherein the first recessed portion, the second recessed portion, and the third recessed portion have a depth substantially equal to a thickness of the first insulating layer.

11. The driver circuit according to claim 10 , wherein the first insulating layer and the second insulating layer include a ceramic material.

12. The driver circuit according to claim 11 , wherein the first recessed portion, the second recessed portion, and the third recessed portion have a dielectric constant less than a dielectric constant of the ceramic material.

13. The driver circuit according to claim 1 , wherein the multilayer board includes a first insulating layer having a first side on which the pair of inductor elements is mounted and a second side opposite to the first side, a second insulating layer having a first side in surface contact with the second side of the first insulating layer and a second side opposite to the first side of the second insulating layer, a third insulating layer having a first side in surface contact with the second side of the second insulating layer, and a metal layer formed between the second insulating layer and the third insulating layer, and

wherein the first recessed portion, the second recessed portion, and the third recessed portion have a depth substantially equal to a sum of a thickness of the first insulating layer and a thickness of the second insulating layer.

14. The driver circuit according to claim 13 , wherein the first insulating layer, the second insulating layer, and the third insulating layer include a ceramic material.

15. The driver circuit according to claim 14 , wherein the first recessed portion, the second recessed portion, and the third recessed portion have a dielectric constant less than a dielectric constant of the ceramic material.

16. An integrated driver circuit comprising:

a plurality of driver systems each including the driver circuit according to claim 1 , the driver systems being arranged parallel to each other on the multilayer board,

wherein the plurality of driver systems includes an electrical connection between one of the two power supply electrodes of one of plurality of driver systems and one of the two power supply electrodes of another of the plurality of driver systems, and the another of the plurality of driver systems is next to the one of the plurality of driver systems.