JP2009008978A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive optical modulator with a good yield. <P>SOLUTION: The optical modulator is equipped with a substrate 1 having an electro-optical effect; an optical waveguide 2 for guiding light formed on the substrate; an electrode formed on one surface side of the substrate and including a central conductor 3a and grounding conductors 3b and 3c for applying a high frequency electric signal for modulating the phase of the light; and an electric terminal 12 for the high frequency electric signal, connected to the electrode. The electric terminal includes a bypass capacitor 17 for cutting DC component, and a damping resistance 23 is provided between the bypass capacitor and an earth so as to reduce the effect of resonance of LC resonance. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention belongs to the field of optical modulators having a low driving voltage and capable of high-speed modulation.

An optical waveguide and a traveling wave electrode are formed on a substrate having a so-called electro-optic effect (hereinafter, the lithium niobate substrate is abbreviated as an LN substrate) such as lithium niobate (LiNbO ₃ ) whose refractive index changes by applying an electric field. The formed traveling-wave electrode type lithium niobate optical modulator (hereinafter abbreviated as LN optical modulator) is a 1.55 μm band 2.5 Gbit / s, 10 Gbit / s large capacity optical transmission system because of its excellent transmission characteristics. Has been applied. Recently, application to an ultra large capacity optical transmission system of 40 Gbit / s is also being studied, and it is expected as a key device.

[Conventional technology]
This LN optical modulator includes a type using a z-cut LN substrate and a type using an x-cut LN substrate (or a y-cut LN substrate). Here, as a conventional technique, a z-cut LN optical modulator having a z-cut LN substrate and two ground conductors and using a coplanar waveguide (CPW) traveling wave electrode that is advantageous for propagation in the fundamental mode will be described.

  When an optical modulator is actually used in an optical transmission system, it is in the form of a module in which a termination resistor (or electrical termination) is mounted in a package housing (package, housing, or metal housing). So here we discuss as a module. Note that the following discussion also holds true for x-cut LN substrates and y-cut LN substrates.

FIG. 4 is a schematic top view of the z-cut LN modulator module disclosed in Patent Document 1. FIG. Reference numeral 1 denotes a z-cut LN substrate (actually, a SiO ₂ buffer layer and a Si conductive layer are formed thereon, but are omitted here). An optical waveguide 2 is formed by thermally diffusing Ti at a temperature of 1050 ° C. for about 10 hours after depositing Ti on the z-cut LN substrate 1, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). In the case of a phase modulator, a straight optical waveguide may be used. The CPW traveling wave electrode includes a central conductor 3a and ground conductors 3b and 3c. 4 is a driver which is an external circuit, 5 is a signal source, and 6 is a capacitor which cuts DC components.

  In the package 7, in addition to the LN modulator having a modulation function formed on the LN substrate 1, an electrical termination 8 is also incorporated. The detailed structure of the electrical termination 8 is shown in FIG. However, since the present specification discusses so-called LC resonance caused by inductance and capacitance at low frequency, in FIG.

  Reference numeral 9 denotes an Au wire for applying a DC bias, and 10 denotes a line connecting the Au wire 9 and the damping resistor 11. Reference numeral 12 denotes an electrical termination (termination section) for a high-frequency electrical signal (or RF signal), which is composed of high-frequency capacitors 14 and 14 'and a termination resistor (usually less than 50Ω) 15 for the high-frequency electrical signal. . Reference numeral 16 denotes a line connecting the damping resistor 11 and the bypass capacitor 17, and the line 16 and the electrical terminal 12 for high-frequency electric signals are electrically connected by a line 13.

  Reference numerals 19 and 19 ′ denote electrodes formed on the bypass capacitor 17, and reference numeral 20 denotes a dielectric portion (capacitor body) of the bypass capacitor 17. A line 18 connected to the bypass capacitor 17 is grounded to the housing. Normally, these electrical components are mounted on a dielectric substrate 1 ′ such as an alumina substrate or aluminum nitride.

  Next, the operation of the LN optical modulator configured as described above will be described. In order to operate this LN optical modulator, a DC bias (hereinafter referred to as DC bias) and a high-frequency electric signal are applied between the center conductor 3a and the ground conductors 3b and 3c.

In the voltage-light output characteristic shown in FIG. 6, the solid curve is the voltage-light output characteristic of the LN optical modulator in a certain state, and _Vb is the DC bias voltage at that time.

As shown in FIG. 6, the DC bias voltage _Vb is normally set at the midpoint between the peak and bottom of the light output characteristic. Appropriate application of the DC bias _Vb is extremely important in order to effectively bring out the characteristics of the LN optical modulator.
JP 2003-295139 A

  Here, the problem of this prior art is considered. As can be seen from FIG. 5, the line 10 and the line 16 are usually composed of lines having a width of about several hundred μm. The LC resonance generated from the inductance (L) of the lines 10 and 16 and the capacitance (C) of the bypass capacitor 17 may suppress the LC resonance in order to deteriorate the characteristics at low frequencies of the LN optical modulator. It becomes important. Since the widths of the line 10 and the line 16 are defined, the damping resistor 11 for attenuating the LC resonance is also defined with the same width. Therefore, it is usually formed with a thin film resistor. As a result, in order to form the damping resistor 11, processes such as a photolithography process and a vacuum deposition are necessary, and the cost increases particularly from the viewpoint of labor costs.

  Furthermore, since the damping resistor 11 has a narrow width as described above, it may be disconnected in the process step. Further, the damping resistor 11 is made of a material having a large resistance such as nichrome or tungsten, and the material is different from that of the line 10 or the line 16 made of Au. Therefore, the damping resistor 11 may be peeled off from the line 10 or the line 16. As described above, when the line 10, the damping resistor 11, and the line 16 are not electrically connected, a DC bias cannot be applied, and the LN modulator cannot be used. It has been desired to realize a technology with a low yield that solves these problems and that has a high yield without danger of disconnection.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an optical modulator with low cost and good productivity.

  In order to solve the above problems, an optical modulator according to claim 1 of the present invention includes a substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and one of the substrates. An optical modulator formed on the surface side and comprising a center conductor and a ground conductor for applying a high-frequency electrical signal for modulating the phase of the light, and an electrical termination for the high-frequency electrical signal connected to the electrode. The electrical termination includes a capacitor that cuts a DC component, and includes a damping resistor between the capacitor and the ground so as to reduce the influence of resonance of the LC resonance.

  According to a second aspect of the present invention, the capacitor includes the high-frequency electric signal capacitor and the low-frequency bypass capacitor, and the damping resistor is provided between the bypass capacitor and the ground. Features.

  The optical modulator according to claim 3 of the present invention is characterized in that the damping resistor is at least one of a chip resistor and a thin film resistor.

  The optical modulator according to claim 4 of the present invention is characterized in that the value of the damping resistor is in the range of 5Ω to 3KΩ.

  The optical modulator according to claim 5 of the present invention is characterized in that the substrate is a semiconductor substrate.

  According to the present invention, the damping resistor for suppressing the LC resonance is installed between the low-frequency bypass capacitor having a relatively large size and the ground. Therefore, since a line for applying a DC bias to the LN modulator can be made of the same material such as Au, there is no danger of electrical disconnection or peeling. In addition, since the bypass capacitor has a relatively large size, a chip resistor can be used as a damping resistor, so that the cost is reduced and the risk of electrical disconnection is extremely small. Furthermore, when a thin film resistor is used as the damping resistor, the size can be increased, so that the risk of electrical disconnection is reduced. As described above, according to the present invention, it is extremely advantageous from the viewpoint of the cost and the production yield of the LN modulator while effectively suppressing the LC resonance.

  Hereinafter, embodiments of the present invention will be described. However, since the same numbers as those of the conventional embodiments shown in FIGS. 4 and 5 correspond to the same function units, description of the function units having the same numbers is omitted here. To do.

[Embodiment]
FIG. 1 shows a schematic top view of an LN optical modulator according to an embodiment of the present invention. Here, 21 is an electrical termination which is important in the present invention. The detailed structure is shown in FIG. In this specification, attention is paid to the LC resonance at a low frequency, and therefore, the terminal portion for the high-frequency electric signal is collectively shown as 12 as in the case of the prior art.

  As can be seen from FIG. 2, the line 22 connecting the Au wire 9 is connected to the bypass capacitor 17 and the terminal portion 12 for high-frequency electrical signals. In the present invention, the LC resonance generated by the line 22 and the bypass capacitor 17 is attenuated by the damping resistor 23 provided between the bypass capacitor 17 and the line 24 grounded to the metal package.

  Since the resonance frequency of the LC resonance is a very low frequency of about 10 KHz to 10 MHz, a process is required as the damping resistor 23, and it is not a thin film resistor that is expensive from the viewpoint of labor cost.

  Unlike the prior art, the line 22 is not formed with a thin film termination resistor, and the entire line is made of the same material made of Au, so the risk of disconnection is extremely small. Further, when a chip resistor is used as the damping resistor 23, there is almost no risk of disconnection between the bypass capacitor 17 and the chip resistor. Thus, the present invention is not only inexpensive, but also has a good yield when the LN modulator is manufactured as a module. The damping resistor 23 also has an advantage of functioning as a protective resistor when the bypass capacitor 17 is short-circuited.

  The bypass capacitor 17 is a chip capacitor because it is for low frequency. That is, the dimension is relatively large on the order of millimeters, and even if the damping resistor 23 is formed of a thin film resistor, the dimension can be increased, so that there is almost no risk of disconnection or peeling, and the yield can be increased. Needless to say, chip resistors and thin film resistors may be used in combination.

  FIG. 3 shows a modulation index as an optical modulator module when the damping resistor 23 is 0Ω, 100Ω, or 1 KΩ in the present invention. From the figure, when the size of the damping resistor 23 is increased, LC resonance at a low frequency can be effectively suppressed, and the electrical effectiveness of the present invention that can significantly improve the cost and yield can be confirmed. It should be noted that if the value of the damping resistor 23 is too large, it affects the characteristics of the bypass capacitor. Therefore, a value of several hundreds Ω is suitable, but it has been confirmed that it functions effectively in the range of 5 Ω to 3 KΩ.

  In the above description, the CPW electrode has been described as an example of the traveling wave electrode. However, it goes without saying that various traveling wave electrodes such as an asymmetric coplanar strip (ACPS) and a symmetric coplanar strip (CPS), or a lumped constant electrode may be used. . In addition to the Mach-Zehnder type optical waveguide, it goes without saying that other optical waveguides such as directional couplers and straight lines may be used as the optical waveguide.

  In the above embodiment, the substrate has an x-cut, y-cut or z-cut plane orientation, that is, a crystal x-axis, y-axis or z-axis in a direction perpendicular to the substrate surface (cut plane). In addition, the plane orientation in each of the embodiments described above may be used as the main plane orientation, and other plane orientations may be mixed as the sub-plane orientation. The present invention can also be applied when the substrate is a semiconductor.

  As described above, the optical modulator according to the present invention is useful as an optical modulator that is inexpensive and has a high yield.

Schematic top view of an LN optical modulator according to an embodiment of the present invention The schematic diagram which shows the electrical termination which concerns on embodiment of this invention Graph showing the relationship between modulation index and frequency Schematic top view of a conventional optical modulator Schematic diagram showing electrical termination according to prior art The figure explaining operation | movement of the optical modulator based on a prior art

Explanation of symbols

1: z-cut LN substrate (substrate, LN substrate)
1 ': Dielectric substrate 2: Optical waveguide 3a: Center conductor 3b, 3c: Ground conductor 4: Driver 5: Signal source 6: Capacitor 7: Package 8, 21: Electrical termination 9: Au wire 10, 13, 16, 18, 22, 24: Line 11, 23: Damping resistor 12: Electrical termination (termination) for high-frequency electrical signals
14, 14 ': Capacitor for high frequency 15: Terminating resistor for high frequency 17: Bypass capacitor 19, 19': Electrode of bypass capacitor 20: Dielectric part of bypass capacitor

Claims (5)

A substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and a high-frequency electric signal formed on one surface side of the substrate and modulating the phase of the light An electrode consisting of a central conductor and a ground conductor, and an electrical modulator connected to the electrode for electrical termination to a high-frequency electrical signal,
The optical modulator includes a capacitor that cuts a DC component, and a damping resistor between the capacitor and ground so as to reduce the influence of resonance of LC resonance.   2. The optical modulator according to claim 1, wherein the capacitor includes the high-frequency electrical signal capacitor and a low-frequency bypass capacitor, and the damping resistor is provided between the bypass capacitor and the ground. .   The optical modulator according to claim 1, wherein the damping resistor is at least one of a chip resistor and a thin film resistor.   4. The optical modulator according to claim 1, wherein a value of the damping resistor is in a range of 5Ω to 3 KΩ.   The optical modulator according to claim 1, wherein the substrate is a semiconductor substrate.

Images