JP5145403B2 - Light modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator exhibiting high performances in optical modulation characteristics and improved in temperature drift. <P>SOLUTION: An optical modulator comprises: an optical waveguide; a progressive wave electrode for applying a high frequency electric signal; a bias electrode for applying a bias voltage to light; a high frequency electric signal interaction part for modulating a phase of the light by applying a high frequency electric signal to the progressive wave electrode; and a bias interaction part for adjusting the phase of the light by applying a bias voltage to the bias electrode. The high frequency electric signal interaction part and the bias interaction part both have ridge portions, and the bias electrode has a prescribed thickness of 3.8 &mu;m or less for suppressing temperature drift. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to an optical modulator that utilizes an electro-optic effect to modulate light incident on an optical waveguide with a high-frequency electrical signal and emit it as an optical signal pulse, with a small change in DC bias voltage with respect to ambient temperature.

  In recent years, high-speed and large-capacity optical communication systems have been put into practical use. There is a demand for the development of a high-speed, small, low-cost, and highly stable optical modulator for incorporation into such a high-speed, large-capacity optical communication system.

As an optical modulator that meets such demands, a light modulator such as lithium niobate (LiNbO ₃ ) is used for a substrate having a so-called electro-optical effect (hereinafter abbreviated as an LN substrate) whose refractive index changes by applying an electric field. There is a traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as an LN optical modulator) in which a waveguide and a traveling wave electrode are formed. This LN optical modulator is applied to a large capacity optical communication system of 2.5 Gbit / s and 10 Gbit / s because of its excellent chirping characteristics. Recently, application to a 40 Gbit / s ultra-high capacity optical communication system is also being studied.

  Hereinafter, an LN optical modulator using the electro-optic effect of lithium niobate that has been put to practical use or has been proposed will be described.

(First prior art)
FIG. 4 shows a schematic perspective view of a so-called ridge type LN optical modulator disclosed in Patent Document 1, which is configured using a z-cut LN substrate, as a first conventional optical modulator. Here, FIG. 5 is a schematic top view of FIG. 4, and FIG. 6 is a schematic cross-sectional view taken along line AA ′ of FIGS. 4 and 5.

  An optical waveguide 3 is formed on the z-cut LN substrate 1. The optical waveguide 3 is an optical waveguide formed by thermally diffusing metal Ti at 1050 ° C. for about 10 hours, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). Accordingly, two interaction optical waveguides 3a and 3b, that is, two arms of the Mach-Zehnder optical waveguide are formed at a portion where the electrical signal and light of the optical waveguide 3 interact (referred to as an interaction portion).

A SiO ₂ buffer layer 2 is formed on the upper surface of the optical waveguide 3, and a traveling wave electrode 4 is formed on the upper surface of the SiO ₂ buffer layer 2 via a conductive layer 5. The conductive layer 5 is a conductive layer for suppressing a temperature drift caused by the pyroelectric effect peculiar to the LN optical modulator manufactured using the z-cut LN substrate 1, and usually a Si conductive layer is used. As the traveling wave electrode 4, a coplanar waveguide (CPW) having one central conductor 4a and two ground conductors 4b and 4c is used. Normally, the traveling wave electrode 4 is formed of Au, which is a noble metal material. The center conductor 4a takes various values, but in many cases is about 7 μm, and the gap between the center conductor 4a and the ground conductors 4b and 4c also takes various values, but is often about 15 μm. For simplification of description, the Si conductive layer 5 for suppressing temperature drift illustrated in FIG. 4 is omitted in FIGS. 5 and 6. In the following, the Si conductive layer 5 is omitted and discussed. Reference numeral 6 denotes a high-frequency electric signal (or microwave) feeding line, which is a high-frequency connector or a microwave line. Reference numeral 7 denotes an output line for a high-frequency electric signal, which normally uses an electrical termination, but may be a high-frequency connector or a microwave line.

  Further, in the region shown as I in FIG. 5, the high-frequency electrical signal propagating through the center conductor 4a and the ground conductors 4b and 4c interacts with the light propagating through the two interaction optical waveguides 3a and 3b. It is called an interaction part for high-frequency electric signals (or simply an interaction part).

  In the first prior art, the recesses 9a, 9b and 9c (or ridges 8a and 8b) are formed by digging the z-cut LN substrate 1 by etching or the like. Here, 12 is a side wall of the ridge portion (here, 8a).

  By adopting this ridge structure, it is possible to realize excellent characteristics in the effective refractive index (or microwave effective refractive index), characteristic impedance, modulation band, driving voltage, etc. of a high-frequency electric signal (or microwave). . In FIG. 6, the depth of the recesses 9a, 9b, and 9c (or the height of the ridges 8a, 8b) is emphasized, but in actuality, the depth is about several μm, and the central conductor 4a In comparison with the thickness of the ground conductors 4b and 4c of several tens of micrometers, the value is small.

  Next, the operation of the LN optical modulator according to the first prior art will be described. In order to operate this LN optical modulator, it is necessary to apply a DC bias voltage and a high-frequency electric signal between the center conductor 4a and the ground conductors 4b and 4c.

  The voltage-light output characteristic shown in FIG. 7 is the voltage-light output characteristic of the LN optical modulator, and Vb is the DC bias voltage at that time. As shown in FIG. 7, the DC bias voltage Vb is normally set at the midpoint between the peak and bottom of the light output characteristic. In the first prior art, a DC bias voltage is also applied to the interaction part I where the high-frequency electrical signal and light interact, so that a capacitor is provided at the electrical terminal (not shown) provided at the output part of the high-frequency electrical signal. Therefore, it is necessary to provide the function of the bias T, and the structure becomes complicated.

(Second prior art)
FIG. 8 shows the second prior art. In order to eliminate the bias T required in the first prior art, the electrical termination (not shown) includes only a resistor, and a DC bias is newly provided with a DC bias. This is an LN optical modulator having a so-called bias separation structure that is applied to the action part (or simply the DC bias part) II. This structure includes a high-frequency electrical signal interaction unit I in which a high-frequency electrical signal interacts with the interaction optical waveguides 3a and 3b, and a DC bias unit II in which a DC bias voltage is applied to the interaction optical waveguides 3a and 3b. It is called a bias separation type structure. An example thereof is disclosed in Patent Document 2.

  Sectional views taken along the lines BB ′ and CC ′ of FIG. 8 are shown in FIGS. Here, 4a ′ is a central conductor, and 4b ′ and 4c ′ are ground conductors. Reference numerals 9a ', 9b', and 9c 'denote concave portions of the DC bias portion, and ridge portions 8a' and 8b 'are formed. 11a and 11b are DC bias electrodes.

Here, the problem of the second prior art shown in FIGS. 8 and 9 will be considered. Regarding the interaction part I shown in FIG. 9A, the temperature drift can be suppressed by the structure disclosed in Patent Document 3. On the other hand, in the structure of Patent Document 3, the DC bias electrodes 11a and 11b are formed as thick as several tens of μm. Therefore, when the environmental temperature changes, since the thermal expansion coefficients of Au, the SiO ₂ buffer layer 2 and the LN substrate 1 constituting the DC bias electrodes 11a and 11b are greatly different from each other, so-called temperature drift in which the DC bias voltage changes. Occurs. In this case, the influence of Au is particularly great.

JP-A-4-288518 JP 2008-122786 A Japanese Patent No. 4170376

  As described above, suppressing the temperature drift in the DC bias section is necessary for suppressing the temperature drift of the entire LN optical modulator.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an optical modulator having a simple and effective temperature drift suppression structure in a DC bias section.

  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, A traveling-wave electrode formed on one surface and made up of a high-frequency electric signal center conductor and a ground conductor for applying a high-frequency electric signal for modulating the light, and a central conductor for applying a bias voltage to the light And a bias electrode made of a ground conductor, and the optical waveguide has a high frequency electrical signal interaction unit for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode, and A bias interaction unit for adjusting the phase of the light by applying a bias voltage to the bias electrode, and both the high-frequency electrical signal interaction unit and the bias interaction unit Comprises a formed ridge portion by a recess formed by digging at least a portion of a serial board, to suppress the temperature drift, the bias electrode is characterized by comprising the following predetermined thickness 3.8 .mu.m.

  In order to solve the above problems, an optical modulator according to claim 2 of the present invention includes a substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and the substrate. A traveling-wave electrode formed on one surface and made up of a high-frequency electric signal center conductor and a ground conductor for applying a high-frequency electric signal for modulating the light, and a central conductor for applying a bias voltage to the light And a bias electrode made of a ground conductor, and the optical waveguide has a high frequency electrical signal interaction unit for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode, and A bias interaction unit for adjusting the phase of the light by applying a bias voltage to the bias electrode, and both the high-frequency electrical signal interaction unit and the bias interaction unit Comprises a formed ridge portion by a recess formed by digging at least a portion of a serial board, to suppress the temperature drift, the bias electrode is characterized by comprising the following predetermined thickness 0.5 [mu] m.

  In order to solve the above-mentioned problem, an optical modulator according to a third aspect of the present invention is the optical modulator according to the first or second aspect, wherein at least the optical waveguide of the bias interaction unit is above the optical waveguide. The formed bias electrode has the predetermined thickness.

  In order to solve the above problems, an optical modulator according to a fourth aspect of the present invention is the optical modulator according to any one of the first to third aspects, wherein the predetermined thickness of the bias electrode is 100 mm. It is characterized by the above.

  In the optical modulator according to the present invention, the thickness of the DC bias electrode in the DC bias unit is set to a predetermined thickness of 3.5 μm or less, preferably 0.5 μm or less, and thus the temperature drift is remarkably suppressed and the manufacturing is extremely simple. However, there is an excellent advantage that an easy optical modulator can be provided.

1 is a top view showing a schematic configuration of an optical modulator according to a first embodiment of the present invention. (A) Cross-sectional view taken along the line DD ′ of FIG. 1, (b) Cross-sectional view taken along the line EE ′ of FIG. The figure explaining the principle of this invention The perspective view which shows schematic structure about the optical modulator of 1st prior art The top view which shows schematic structure about the optical modulator of 1st prior art Sectional view in AA 'of FIG. The figure explaining the principle of operation of an optical modulator The top view which shows schematic structure about the optical modulator of 2nd prior art (A) Sectional view taken along BB 'in FIG. 8, (b) Sectional view taken along CC' in FIG.

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the prior art shown in FIGS. 4 to 9 correspond to the same functional units, description of the functional units having the same reference numerals is omitted here. To do.

(First embodiment)
FIG. 1 is a top view of a first embodiment of the present invention. 2A and 2B are sectional views taken along line DD 'and EE' in FIG. 1, respectively. As can be seen from FIG. 2 (a) corresponding to the high-frequency electrical signal interaction section I, the center conductor 4a ′ and the ground conductors 4b ′ and 4c ′ are used in the same manner as in the first conventional technique and the second conventional technique. The center conductor 4a ′ and the ground conductors 4b ′ and 4c ′ are set so that the effective refractive index of the high-frequency electric signal propagating through the traveling wave electrode becomes close to the equivalent refractive index of the light propagating through the high-frequency electric signal interaction optical waveguide 3b. Is thick.

  On the other hand, as can be seen from FIG. 2B, the thickness of the DC bias electrodes 11a ′ and 11b ′ of the DC bias portion II is thin. FIG. 3 shows a change in DC bias when the environmental temperature of the LN optical modulator changes from 25 ° C. to 80 ° C., so-called temperature drift. As can be seen from the figure, the effect of suppressing the temperature drift is great when the thickness of the DC bias electrodes 11a ′ and 11b ′ is 3.8 μm or less, and the effect is remarkable when the thickness of the DC bias electrodes 11a ′ and 11b ′ is 0.5 μm or less. I understand that there is. On the other hand, the thickness of the DC bias electrodes 11a ′ and 11b ′ is preferably 100 mm or more in order to maintain the strength of the metal.

  Although it is preferable to reduce the thickness of all the DC bias electrodes, the influence of the DC bias electrodes above the interaction optical waveguides 3a and 3b is particularly large, so it is preferable to reduce the thickness thereof. . That is, most preferably, the DC bias electrode has a thickness that is about the same as that of the base electrode used for plating the central conductor 4a ′ and the ground conductors 4b ′ and 4c ′ of the so-called interaction part I. Since it is not necessary to form a DC bias electrode (to form a DC bias electrode), an optical modulator can be made with a short man-hour.

(Each embodiment)
Although the Mach-Zehnder optical waveguide is used as an example of the branched optical waveguide, it goes without saying that the present invention can be applied to other branched / multiplexed optical waveguides such as a directional coupler. The present invention is also applicable to the optical waveguide. As a method for forming the optical waveguide, various methods for forming the optical waveguide such as a proton exchange method can be applied in addition to the Ti thermal diffusion method, and various materials other than SiO ₂ such as Al ₂ O ₃ can be applied as the buffer layer.

  In addition, the present invention does not depend on the electrode structure of the DC bias unit, and it goes without saying that the present invention can be applied to various electrode structures such as a CPW structure, an asymmetric coplanar strip (ACPS) structure, and a symmetric coplanar strip (CPS) structure.

1: z-cut LN substrate (LN substrate)
2: SiO ₂ buffer layer (buffer layer)
3: Mach-Zehnder optical waveguide (optical waveguide)
3a, 3b: interaction optical waveguide constituting Mach-Zehnder optical waveguide 4: traveling wave electrode 4a, 4a ′: central conductor 4b, 4b ′, 4c, 4c ′: ground conductor 5: Si conductive layer 6: high-frequency electric signal feeder 7: High-frequency electrical signal output lines 8a, 8b: Ridge portions 8a ', 8b' in the interaction portion I: Ridge portions 9a, 9b, 9c in the DC bias portion II: Recesses 9a ', 9b', 9c in the interaction portion I ': Recessed portion in DC bias part II 11a, 11b, 11a', 11b ': DC bias electrode I: Interaction part for high frequency electric signal (interaction part)
II: DC bias interaction section (DC bias section)

Claims (4)

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 for applying a high-frequency electric signal for modulating the light A traveling wave electrode composed of a center conductor and a ground conductor for electrical signals, and a bias electrode composed of a center conductor and a ground conductor for applying a bias voltage to the light,
The optical waveguide has a high frequency electrical signal interaction unit for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode, and a bias voltage to the bias electrode to apply the light. And a bias interaction unit for adjusting the phase of
Comprising a ridge portion constituted by a recess formed by digging up at least a part of the substrate in both the high-frequency electrical signal interaction portion and the bias interaction portion;
The optical modulator, wherein the bias electrode has a predetermined thickness of 3.8 μm or less so as to suppress temperature drift. 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 for applying a high-frequency electric signal for modulating the light A traveling wave electrode composed of a center conductor and a ground conductor for electrical signals, and a bias electrode composed of a center conductor and a ground conductor for applying a bias voltage to the light,
The optical waveguide has a high frequency electrical signal interaction unit for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode, and a bias voltage to the bias electrode to apply the light. And a bias interaction unit for adjusting the phase of
Comprising a ridge portion constituted by a recess formed by digging up at least a part of the substrate in both the high-frequency electrical signal interaction portion and the bias interaction portion;
The optical modulator, wherein the bias electrode has a predetermined thickness of 0.5 μm or less so as to suppress temperature drift. 3. The optical modulator according to claim 1, wherein at least the bias electrode formed above the optical waveguide of the bias interaction unit has the predetermined thickness. 4.
  4. The optical modulator according to claim 1, wherein the predetermined thickness of the bias electrode is 100 mm or more. 5.

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