Optical module and optical transmission device

1. An optical module comprising:

a light output element which includes a pair of input terminals, and is configured to output an optical signal modulated in accordance with an electric signal input to the light output element;

a drive circuit which includes a pair of output terminals, and is configured to output a differential signal as the electric signal;

a differential transmission line which establishes connection between the pair of input terminals and the pair of output terminals, and is formed of a pair of transmission lines,

the differential transmission line including:

a first differential transmission line which has a first characteristic impedance, and is connected to the pair of output terminals;

a second differential transmission line which has a second characteristic impedance, and is connected to the pair of input terminals, the second characteristic impedance being smaller than the first characteristic impedance; and

connecting portions configured to connect the first differential transmission line and the second differential transmission line in series with each other; and

a resistive element having both ends connected to the connecting portions of the pair of transmission lines, respectively,

the resistive element having a resistance value that is set to a value with which an absolute value of a reflection coefficient for a signal traveling from the second differential transmission line side to the first differential transmission line side is 0.10 or less.

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

a circuit board on which an integrated circuit having the drive circuit formed therein is mounted; and

flexible printed circuits connected between the light output element and the circuit board,

wherein the pair of transmission lines each include signal conductors formed of strip conductor foil formed on the circuit board and the flexible printed circuits.

3. The optical module according to claim 2 ,

wherein the connecting portions are arranged on the circuit board, and

wherein the resistive element is mounted on the circuit board.

4. The optical module according to claim 2 ,

wherein the connecting portions are arranged on the flexible printed circuits, and

wherein the resistive element is mounted on the flexible printed circuits.

5. The optical module according to claim 2 , further comprising optical subassemblies each having the light output element mounted in a package thereof,

wherein the optical subassemblies are connected to the flexible printed circuits, and

wherein the second differential transmission line includes package wirings from connection terminals of the optical subassemblies to the flexible printed circuits, to the light output element.

6. The optical module according to claim 5 ,

wherein the package is a TO-can package,

wherein the TO-can package has holes formed in a stem, and lead pins which penetrate through the holes, and

wherein the second differential transmission line includes, as the package wirings, a pair of coaxial lines including a pair of the lead pins connected to the pair of input terminals of the light output element, and a pair of the holes formed in correspondence with the lead pins.

7. The optical module according to claim 1 , wherein the drive circuit has a differential output impedance that matches the first characteristic impedance.

8. The optical module according to claim 1 ,

wherein each of a differential output impedance of the drive circuit and the first characteristic impedance is 100 ohms,

wherein the second characteristic impedance is 50 ohms, and

wherein the resistance value of the resistive element is 100 ohms.

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

a DC blocking capacitor inserted on the second differential transmission line; and

bias circuits which are connected to the second differential transmission line on the light output element side as seen from the DC blocking capacitor, and are configured to supply a bias current to the light output element.

10. The optical module according to claim 1 ,

wherein the drive circuit includes a current-mode logic circuit as an output circuit configured to output the differential signal to the pair of output terminals,

wherein the current-mode logic circuit includes a pull-up resistor having a resistance value that is ½ of the first characteristic impedance, and

wherein the optical module further comprises bias circuits each of which includes an inductance, and which are connected to the first differential transmission line.

11. An optical transmission device including the optical module of claim 1 mounted thereon.