Optical modulator

#The present invention relates to an optical modulator for use in an optical communication system.

#FIG. 15 is a conceptual diagram of a conventional optical modulator. As shown, the conventional optical modulator comprises an light modulating semiconductor element 1 of an electric field absorption type (hereinafter “an electroabsorption type light modulating semiconductor element”), a high-frequency circuit substrate 2 for feeding an RF signal to the electroabsorption type light modulating semiconductor element 1, a terminating resistor substrate 3, an input optical coupling system 7a and an output optical coupling system 7b. The terminating resistor substrate 3 includes a terminating resistor 3a for impedance matching, a through hole 3b, and a transmission line connecting them. The high-frequency circuit substrate 2, the electroabsorption type light modulating semiconductor element 1, and the terminating resistor substrate 3 are electrically connected by wires 6 or the like.

#In the conventional optical modulator, the terminating resistor substrate 3 has a back side thereof serving as a grounding electrode which is electrically connected via the through hole 3b to the terminating resistor 3a. Also, the back side of the light modulating semiconductor element 1 is a grounding electrode. Hence, the light modulating semiconductor element 1 and the terminating resistor 3a are electrically connected in parallel with each other. Thus, the internal impedance of the optical modulator is a normalized impedance. The high-frequency circuit substrate 2 has a transmission line 5 for transmitting the RF signal.

#FIG. 1 is a conceptual diagram of an optical modulator according to the first embodiment of the present invention;

#FIG. 2 is a graph showing a voltage-current characteristic of a diode;

#FIG. 3 is a conceptual diagram of an optical modulator according to the second embodiment of the present invention;

#FIG. 1 is a conceptual diagram of an optical modulator showing Embodiment 1 of the present invention. FIG. 2 shows a voltage-current characteristic of a diode. The optical modulator comprises an light modulating semiconductor element 1 of an electric field absorption type (hereinafter “an electroabsorption type light modulating semiconductor element”) using the quantum confined Stark effect or the Franz-Keldysh effect, a high-frequency circuit substrate 2 for feeding an RF signal to the electroabsorption type light modulating semiconductor element 1, a terminating resistor substrate 3, an input optical coupling system 7a, and an output optical coupling system 7b. The terminating resistor substrate 3 is equipped with a terminating resistor 3a for impedance matching, a through hole 3b, and a transmission line for connecting them. The high-frequency circuit substrate 2, the light modulating semiconductor element 1, and the terminating resistor substrate 3 are connected by wires 6 or the like. The high-frequency circuit substrate 2 has a transmission line 5 mounted thereon for transmission of the RF signal and a diode 4 also mounted thereon to connect electrically to the transmission line 5.

#The terminating resistor substrate 3 has a back side thereof serving as a grounding electrode which is electrically connected via the through hole 3b to the terminating resistor 3a. Also, the back side of the light modulating semiconductor element 1 is a grounding electrode. Hence, the light modulating semiconductor element 1 and the terminating resistor 3a are electrically connected in parallel with each other so as to make the internal impedance of the optical modulator a normalized impedance.

#The operation of the modulator is now explained. As shown in FIG. 1, a continuously oscillated laser light is provided to the electroabsorption type light modulating semiconductor element 1 through the input optical coupling system 7a efficiently. Since the electroabsorption type light modulating semiconductor element 1 absorbs the laser light in response to the voltage of the RF signal applied through the high-frequency circuit substrate 2, the intensity of the laser light released from the emitting end of the light modulating semiconductor element 1 is modulated corresponding to the applied signal. The light released from the light modulating semiconductor element 1 is efficiently provided to the output optical coupling system 7b.