Mach-zehnder interferometer type optical modulator

Abstract

A Mach-Zehnder interferometer type optical modulator includes a first end facet and a reflecting portion opposing the first end facet; a single optical coupler including input and output ports, the optical coupler being disposed between the first end facet and the reflecting portion; first and second optical waveguides that are connected to the input ports of the optical coupler; third and fourth optical waveguides that are connected to the output ports of the optical coupler; and a phase shifting section disposed between the optical coupler and the reflecting portion. The phase shifting section includes a first optical waveguide structure constituting part of the third optical waveguide; a first upper electrode on the first optical waveguide structure; a second optical waveguide structure constituting part of the fourth optical waveguide; and a second upper electrode on the second optical waveguide structure.

Description

BACKGROUND OF THE INVENTION1. Field of the InventionThe present invention relates to Mach-Zehnder interferometer type optical modulators.2. Description of the Related ArtPatent Document 1 (Japanese Unexamined Patent Application Publication No. 62-183406) describes a waveguide-type optical interferometer. This waveguide-type optical interferometer includes a substrate, two optical guides composed of glass or a plastic formed on the substrate, two optical couplers that connect the optical waveguides to each other at different positions, and phase shifters disposed in the optical waveguides between the optical couplers. Each phase shifter includes a heater disposed on the optical waveguide. The optical path length of the optical waveguide is changed by controlling the temperature of the optical waveguide by heating the heater of the phase shifter.In recent years, optical modulators that modulate light in response to electric signals from outside have become one of the essential compone...

AI Technical Summary

Benefits of technology

According to this Mach-Zehnder interferometer type optical modulator, the first and second optical waveguides extend from the first end facet and are connected to the input ports of the optical coupler. Meanwhile, the third and fourth optical waveguides that extend from the reflecting portion and are connected to the output ports of the optical coupler are also provided. Incoming light enters from the first-end facet-side of the first or second optical waveguide, propagates through the optical coupler and the third and fourth optical waveguides, and is reflected by the reflecting portion so as to again propagate in the third and fourth optical waveguides. The Mach-Zehnder interferometer type optical modulator also includes the phase shifting section. The phase shifting section includes the first and second optical waveguide structures respectively constituting part of the third and fourth optical waveguides. Since the first and second upper electrodes are respectively disposed on the first and second optical waveguide structures, the phase of light guided in at least one of the third and fourth optical waveguides can be controlled by applying a modulated signal (for example, modulated voltage) to the first or second upper electrode. Lights propagating in the third and fourth optical waveguides from the phase shifting section enter the output ports of the optical coupler. Modulated light according to the modulated signal can be output from the first-end facet-side of the first or second optical waveguide through the optical coupler. A conventional Mach-Zehnder interferometer type optical modulator shown in FIG. 17, 18A or 18B has at least two optical couplers of the input optical coupler 120 and the output optical coupler 130. On the other hand, this Mach-Zehnder interferometer type optical modulator has a single optical coupler. Thus, the Mach-Zehnder interferometer type optical modulator can be operated by using only the left half of the region of the conventional Mach-Zehnder interferometer type optical modulator shown in FIG. 17, 18A or 18B. Accordingly, the device length in the waveguiding direction can be shortened. Thus, the Mach-Zehnder interferometer type optical modulator can be easily mounted in a small-size optical module.

The guided light beams that have propagated through the third and fourth optical waveguides and have been reflected by the reflecting portion may have different reflective characteristics due to the error that occurs during the device production. The difference in phase shift amount occurs at the reflecting portion due to such a difference in reflective characteristics. The difference in phase shift amount differs from one device to another. As a result, the optimum value of modulated signal (for example, modulated voltage) applied to the phase shifting section differs from one device to another, and it becomes difficult to ensure stable operation and reproduction of the device characteristics. To address this issue, the Mach-Zehnder interferometer type optical modulator may further include a phase adjusting section configured to adjust a phase of light by adjusting the refractive index of at least one of the third and fourth optical waveguides independently from the phase shifting section. The difference in phase shift amount at the reflecting portion can be compensated by adjusting a phase of guided light through the phase adjusting section. Thus, the optimum value of the modulated signal applying to the phase shifting section can be easily maintained substantially constant. The stable operation and reproduction of device characteristics can thus be easily achieved.

In the Mach-Zehnder interferometer type optical modulator according to the present invention, preferably the reflecting portion may include a plurality of distributed Bragg reflectors optically connected to the third and fourth optical waveguides. The distributed Bragg reflectors may each have a diffraction grating structure including a plurality of semiconductor portions and a plurality of air gaps that are alternately and periodically arranged in a waveguiding direction. The difference in refractive index between semiconductors and air is greater than that between semiconductors and dielectric materials. Thus, according to the Mach-Zehnder interferometer type optical modulator including such distributed Bragg reflectors, the reflecting portion achieves a high reflectivity.

In the Mach-Zehnder interferometer type optical modulator according to the present invention, preferably the phase shifting section is disposed apart from the second end facet by a predetermined gap. The second end facet is, for example, formed by cleaving. When the second end facet is terminated with the phase shifting section including the upper electrode, the electrode material near the second end facet may adhere on the second end facet during cleavage. However, according to this Mach-Zehnder interferometer type optical modulator, the second end facet is terminated with the waveguiding section not having an upper electrode and no electrode material is present near the second end facet. Therefore, adhesion of the electrode on the end facet can be suppressed.

According to the Mach-Zehnder interferometer type optical modulator, since the anti-reflection film is formed on the first end facet that transmits light, reflection of incoming light and outgoing light by the first end facet can be suppressed. Moreover, the anti-reflection film suppresses degradation of the optical coupling efficiency between the Mach-Zehnder interferometer type optical modulator and optical fibers connected to the Mach-Zehnder interferometer type optical modulator. Since the anti-reflection film is provided, outgoing light can be suppressed from being reflected by the first end facet and returning to the interior of the modulator. Accordingly, the adverse effects on the modulation operation of the modulator can be diminished.

Problems solved by technology

Therefore, it is difficult to install the Mach-Zehnder interferometer type optical modulator 100 in a small-size optical module.

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