Optical modulator and optical transmission apparatus
An optical modulator includes an optical waveguide device including an optical waveguide and a signal electrode for controlling a light wave propagating through the optical waveguide, a drive circuit that outputs two high-frequency signals, and two termination resistors for respectively terminating the two high-frequency signals, in which one high-frequency signal among the high-frequency signals output from the drive circuit propagates through the signal electrode of the optical waveguide device and is terminated by a first termination resistor that is one of the termination resistors, an other high-frequency signal among the high-frequency signals output from the drive circuit is terminated by a second termination resistor that is an other one of the termination resistors, a resistance value of the first termination resistor is lower than a resistance value of the second termination resistor, and the signal electrode includes a plurality of sections having constant impedances different from each other.
1 . An optical modulator comprising:
an optical waveguide device including an optical waveguide formed on a substrate and a signal electrode for controlling a light wave propagating through the optical waveguide;
a drive circuit that outputs two high-frequency signals; and
two termination resistors for respectively terminating the two high-frequency signals output from the drive circuit, wherein
one high-frequency signal out of the high-frequency signals output from the drive circuit propagates through the signal electrode of the optical waveguide device and is terminated by a first termination resistor that is one of the termination resistors,
only the one high-frequency signal out of the high-frequency signals is configured to modulate the light wave propagating through the optical waveguide,
an other high-frequency signal out of the high-frequency signals output from the drive circuit is terminated by a second termination resistor that is an other one of the termination resistors,
a resistance value of the first termination resistor is lower than a resistance value of the second termination resistor, and
the signal electrode includes a plurality of sections having constant impedances different from each other.
2 . The optical modulator according to claim 1 , wherein
the signal electrode includes at least three of the sections disposed such that impedances stepwise decrease along a propagation direction of the high-frequency signals of the signal electrode.
3 . The optical modulator according to claim 1 , wherein
at least one of the sections includes an action portion in which the high-frequency signal acts on the light wave propagating through the optical waveguide, and the section including the action portion is a lowest impedance section having a lowest impedance among the plurality of sections.
4 . The optical modulator according to claim 1 , wherein
a first electrical length of a lowest impedance section in a frequency band of the high-frequency signal, the lowest impedance section being a section having a lowest impedance among the plurality of sections, is longer than a second electrical length from a second output port that outputs the other high-frequency signal of the drive circuit to the second termination resistor in the frequency band.
5 . The optical modulator according to claim 4 , wherein
the first electrical length of the lowest impedance section is longest among respective electrical lengths of the plurality of sections in the frequency band.
6 . The optical modulator according to claim 1 , wherein
an attenuation portion that attenuates the high-frequency signal is provided between the drive circuit and the second termination resistor.
7 . The optical modulator according to claim 1 , wherein
a second electrical length from a second output port that outputs the other high-frequency signal of the drive circuit to the second termination resistor in a frequency band of the high-frequency signal is at least twice as long as a third electrical length from a first output port that outputs the one high-frequency signal of the drive circuit to the first termination resistor in the frequency band.
8 . The optical modulator according to claim 1 , wherein
an absolute value of a difference between a third electrical length L 1 from a first output port that outputs the one high-frequency signal of the drive circuit to the first termination resistor in a frequency band of the high-frequency signal and a second electrical length L 2 from a second output port that outputs the other high-frequency signal of the drive circuit to the second termination resistor in the frequency band of the high-frequency signal has a relationship of
| L 1− L 2|>λ/2
with respect to an average wavelength λ of the high-frequency signal.
9 . An optical transmission apparatus comprising:
the optical modulator according to claim 1 ; and
an electronic circuit that generates a modulation signal which is a high-frequency signal for causing the optical waveguide device to perform a modulation operation.