JP2009015118A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator having high performance in optical modulation characteristics and improved stability. <P>SOLUTION: The optical modulator includes a substrate 1, having an electro-optical effect, a buffer layer 2 formed on the substrate, a traveling wave electrode comprising a center conductor 4a and ground conductors 4c, 11 disposed above the buffer layer, and a ridge part, formed of a concave part provided by engraving at least a part of the substrate, in a region where an electric field intensity of high-frequency electrical signals propagating in the traveling wave electrode is strong. The ridge part comprises a ridge part 8a for the center conductor above which the center conductor is formed, and a ridge part 8b' for the ground conductor above which the ground conductor is formed, and the optical modulator has an optical waveguide 3b, in at least the ridge part for the center conductor, wherein the ground conductor formed above the ridge part for the ground conductor is mechanically independent so as not to receive stress from other grounding conductors, where the stress is in a direction perpendicular to the longitudinal direction of an interactive part and parallel to the surface of the substrate, during thermal expansion or thermal contraction. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical modulator that uses 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.

  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. 12 shows a 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. FIG. 13 is a cross-sectional view taken along the line AA ′ of FIG.

  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). Therefore, two interacting optical waveguides 3a and 3b, that is, two arms of a Mach-Zehnder optical waveguide are formed in a portion (referred to as an interacting portion) where the electrical signal and light of the optical waveguide 3 interact.

An 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. 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 made of Au. Reference numeral 5 denotes a conductive layer for suppressing temperature drift caused by a pyroelectric effect peculiar to the LN optical modulator manufactured using the z-cut LN substrate 1, and usually a Si conductive layer is used. The width S of the center conductor 4a is about 7 μm, and the gap W between the center conductor 4a and the ground conductors 4b and 4c is about 15 μm. For simplification of explanation, the Si conductive layer 5 for suppressing temperature drift shown in FIG. 12 is omitted in FIG. In the following, the Si conductive layer 5 is omitted and discussed.

  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, 10a and 10b are outer peripheral parts. The ridge portions 8a and 8b are also referred to as a central conductor ridge portion and a ground conductor ridge portion, respectively.

  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, and the like of a high-frequency electric signal. In FIG. 13, the depth of the recesses 9a, 9b and 9c (or the height of the ridges 8a and 8b) is emphasized, but it is actually about 2 to 5 μm, and the center conductor 4a and the ground The value is small compared to the thickness of the conductors 4b and 4c of about 20 μm.

  Now, it has been found that although the first prior art has high modulation characteristics as an LN optical modulator, there is a problem with stability. That is, it has been found that the temperature drift characteristic is poor despite the use of the Si conductive layer 5. The cause is thought to be due to the ridge structure that produces high modulation performance.

  The cause will be described in detail below. As can be seen from FIG. 13, the ridge portion 8a immediately below the central conductor 4a is independent of the ground conductors 4b and 4c, and therefore the ridge portion 8a is pulled in a direction parallel to the surface of the z-cut LN substrate 1. There is no power.

  However, as described above, the ridge portion 8b is formed with the 20 μm thick ground conductor 4b together with the concave portion 9c and the outer peripheral portion 10b. The Au of the ground conductor 4b and the thermal expansion coefficient of the z-cut LN substrate 1 are greatly different from each other. Further, in FIG. 13, since the ground conductor 4b is actually as wide as several millimeters, stresses such as thermal expansion and thermal contraction due to environmental changes accumulate, and a considerably large stress is applied to the ridge portion 8b.

  However, when a stress is applied to the z-cut LN substrate 1, its refractive index changes (stress birefringence). As a result, the refractive index of the interactive optical waveguide 3a changes, and the LN optical modulator operates. The DC bias point when changing is changed. This is a temperature drift phenomenon peculiar to the ridge structure, and results in impairing the stability as the LN optical modulator. Incidentally, when the environmental temperature of the LN optical modulator was changed from room temperature to 70 ° C., the change of the DC bias point in the first prior art was as large as 6V.

(Second prior art)
In order to solve the problem of the first prior art, a cross-sectional view of the interaction portion of the second prior art disclosed in Patent Document 2 is shown in FIG. As can be seen from FIG. 14, the ground conductor 4b ′ formed on the ridge portion 8b and the ground conductor 4b ″ formed on the outer peripheral portion 10b are thick, but the ground conductor 4b formed in the recess 9c. The thickness of ″ ″ is reduced to 50 nm to 3 μm or less. Thus, by reducing the thickness of the ground conductor 4b ″ in the concave portion 9c, the stress applied to the ridge portion 8b by the ground conductor 4b ″ having a large area can be reduced, so that the temperature stability can be improved. This is the idea.

  However, the second prior art has a serious problem to be solved as follows. In this second prior art, although there are 4b ′, 4b ″ and 4b ″ as the ground conductors, the portion where the most current actually flows corresponding to the center conductor 4a is opposite to the center conductor 4a. The ground conductor 4b 'is as narrow as the center conductor 4a. That is, most of the current in the ground conductor flows only in the ground conductor 4b 'formed on the ground conductor ridge 8b. Here, it is clear from FIG. 14 of this specification and FIGS. 3 to 12 of Patent Document 2 that the width of the ground conductor 4b ′ is as narrow as the width of the center conductor 4a.

  Therefore, the high frequency electric signal is easily lost due to Joule heat, and the modulation band is significantly deteriorated as compared with the first prior art shown in FIG. When the authors actually confirmed through experiments, modulation of 2.5 Gbit / s was finally achieved as a transmission rate in optical communication, and modulation of 10 Gbit / s, which is currently mainstream, was difficult. In addition, 40 Gbit / s modulation, which is promising in the near future, was completely impossible.

In general, it is difficult in terms of process to form a pattern in a concave portion where a photoresist tends to accumulate, and this second prior art is difficult to manufacture. In other words, it can be said that the second prior art has a structure with a low production yield. Moreover, it can be said that it is a structure in which the cost increases in terms of labor cost from the viewpoint of the man-hour for forming the thin ground conductor in the recess and the man-hour for forming the pattern.
JP-A-4-288518 JP 2004-157500 A

  As described above, in the conventional first technique proposed as the ridge-type LN optical modulator, the stress from the ground conductor due to the difference in thermal expansion coefficient between Au and the z-cut LN substrate constituting the electrode increases with temperature. A temperature drift occurred that changed the optimum DC bias point. In the second prior art proposed to improve this temperature characteristic, most of the current flows in the ground conductor at the portion formed on the narrow ridge portion for the ground conductor. The high-frequency electric signal is easily lost due to Joule heat, and the modulation band is significantly deteriorated as compared with the first prior art. The transmission speed is finally 2.5 Gbit / s modulation, and 10 Gbit / s modulation, which is currently mainstream, is difficult, and 40 Gbit / s modulation, which is expected in the future, is completely impossible. . There is an urgent need to develop an optical modulator that can achieve temperature stabilization without sacrificing high speed and low drive voltage as an optical modulator.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an optical modulator having high performance in light modulation characteristics and improved stability.

  In order to solve the above-described problems, an optical modulator according to claim 1 of the present invention includes a substrate having an electro-optic effect, a buffer layer formed on the substrate, and a center disposed above the buffer layer. A traveling wave electrode composed of a conductor and a ground conductor, and a ridge formed by a recess provided by digging up at least a part of the substrate in a region where the electric field strength of a high-frequency electric signal propagating through the traveling wave electrode is strong. The ridge portion comprises a central conductor ridge portion with the central conductor formed upward and a ground conductor ridge portion with the ground conductor formed upward, and has an optical waveguide at least in the central conductor ridge portion. In the optical modulator, the ground conductor formed on the ridge portion for the ground conductor is perpendicular to the longitudinal direction of the interaction portion during the thermal expansion and contraction, and the substrate. In a direction parallel to the surface, characterized in that independent on mechanically so unstressed other of the ground conductor.

  The optical modulator according to claim 2 of the present invention is characterized in that there are a plurality of the ground conductors mechanically independent in a direction parallel to the surface of the substrate.

  The optical modulator according to claim 3 of the present invention is characterized in that the substrate is made of a semiconductor.

  In the optical modulator according to the present invention, when the environmental temperature of the LN optical modulator changes without increasing the propagation loss of the high-frequency electrical signal, the stress caused by the difference in the thermal expansion coefficient between the electrode and the LN substrate is light. It is prevented from being applied to the ridge formed with the waveguide. Since the ground conductor formed above the ridge portion for the ground conductor is mechanically free in the direction horizontal to the surface of the LN substrate, LN light with a small thermal drift without impairing the high performance of the ridge-type optical modulator. There is an excellent effect that a modulator can be provided.

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the prior art shown in FIGS. 12 to 14 correspond to the same functional units, description of functional units having the same reference numerals is omitted here. To do.

(First embodiment)
FIG. 1 shows a schematic perspective view of the first embodiment of the present invention. FIG. 2 is a cross-sectional view taken along the line BB ′ of the interaction unit in the first embodiment shown in FIG. As can be seen by comparing FIG. 1 and FIG. 13, instead of the ground conductor 4b in the first prior art shown in FIG. 13, in this embodiment, a ground conductor having a width Wb on the wide ridge portion 8b ′. Is formed. Here, the width S of the center conductor 4a is 7 μm, the gap W between the center conductor 4a and the ground conductor 4c or the ground conductor 11 is 15 μm, and the thickness of the center conductor 4a and the like is 20 μm.

  As can be seen from FIG. 2, in this first embodiment, there is no ground conductor 4b in the first prior art (and even the ground conductor 4b ″ 4b ″ ″ in the second prior art) The ground conductor 11 is mechanically independent (in other words, stressed) in a direction horizontal to the surface of the z-cut LN substrate 1. Therefore, when the environmental temperature changes, the stress applied to the ground conductor 11 due to the difference in thermal expansion coefficient between Au constituting the electrode and the z-cut LN substrate 1 can be remarkably reduced. I was able to suppress it.

  Now, in the first embodiment, the width Wb of the ground conductor 11 is important. FIG. 3 shows the propagation loss α of the high-frequency electric signal with respect to the width Wb of the ground conductor 11 at 1 GHz. Note that the propagation loss α of the high-frequency electrical signal increases in proportion to √f for the frequency f. As can be seen from FIG. 2, the width Wb of the ground conductor 11 is preferably 30 μm or more in order to sufficiently reduce the propagation loss α of the high-frequency electrical signal.

  As a result of designing and manufacturing various ridge-type LN optical modulators, in order to avoid current concentration on the ground conductor 11 side, at least one of the gaps W between the center conductor 4a and the ground conductor 11 is used as the width Wb. It has been found that it is preferably about 5 times, preferably about 2 times, and if it is 3 times, extremely good characteristics can be realized.

  In the second prior art concept in which current concentrates on the ground conductor 4b 'on the ridge portion 8b, the propagation loss α of the high-frequency electric signal is large, and high-speed transmission of 10 Gbit / s as well as 40 Gbit / s is difficult. This was reconfirmed from FIG.

  In the first embodiment, by setting the width of the ground conductor 11 to 30 μm, not only 10 Gbit / s but also 40 Gbit / s has been successfully transmitted, and the environmental temperature of the LN optical modulator is from room temperature to 70 ° C. Even if it was changed, the change of the DC bias point could be suppressed to a very small value of 0.5 V or less.

  Thus, in the present embodiment, unlike the second prior art shown in FIG. 14, there are no electrodes in the recesses 9a and 9c. Therefore, it is not necessary to form a thin ground conductor 4b "" as a device for relieving stress in the recess 9c, and it is not necessary to form a pattern on the thin ground conductor 4b "". Therefore, since it is easy in terms of process, the yield is high, and the cost can be reduced from the viewpoint of labor cost.

  The above numerical values for the structural parameters are examples, and various numerical values can be taken as the parameters, and the optimum value of the width Wb of the ground conductor 11 varies slightly.

  As a result of studying various requirements for the width S of the center conductor 4a of the ridge type LN optical modulator, the requirements necessary for avoiding excessive concentration of current in the ground conductor 11 are as follows. It has been found that a minimum of twice the width S of the central conductor 4a is necessary, and it is desirable that it be about 3 times or more.

  In FIG. 2, a new ground conductor may be provided on the outer peripheral portion 10 b as long as the ground conductor 11 is not mechanically affected. Also in this case, since the width of the ground conductor 11 is set so that the propagation loss α of the high-frequency electrical signal is reduced in the present invention, a high-performance LN optical modulator that cannot be realized by the second prior art can be realized. There are advantages. Needless to say, the outer peripheral portions 10b and 10a may be etched to the same height as the concave portions 9c and 9a.

(Second Embodiment)
FIG. 4 shows a second embodiment of the present invention. In the present embodiment, the width of the ground conductor 4c ′ is substantially the same as that of the ground conductor 11, thereby improving the symmetry of the structure as compared with the first embodiment shown in FIG.

(Third embodiment)
FIG. 5 shows a third embodiment of the present invention. In the present embodiment, the ridge portion 8c is formed by providing the concave portion 9d. In addition, 10d is an outer peripheral part. The ground conductor 12 has a sufficiently wide structure. In the present embodiment, symmetry is improved with respect to the ridge portions 8a, 8b ', and 8c.

(Fourth embodiment)
FIG. 6 shows a fourth embodiment of the present invention. In the present embodiment, the width of the ground conductor 12 'is substantially the same as that of the ground conductor 11, thereby improving the symmetry of the structure as compared with the third embodiment shown in FIG. Since this ground conductor 12 ′ is also mechanically independent in the direction parallel to the surface of the z-cut LN substrate 1, the influence of stress due to thermal expansion and contraction is extremely small.

  Here, in the fourth embodiment, the electrodes made of the center conductor and the ground conductor are symmetrical with respect to the center line of the ridge portion 8a. In all of the embodiments of the present invention, the electrodes and the optical waveguides 3a and 3b may have a symmetric structure with respect to a straight line perpendicular to the surface of the z-cut LN substrate 1 drawn in any place. Needless to say, the structure need not be symmetric with respect to a certain line.

  In FIG. 6, a new ground conductor may be provided on the outer peripheral portion 10 b or 10 d as long as the ground conductor 11 or 12 ′ is not mechanically affected. Also in this case, in the present invention, since the width of the ground conductors 11 and 12 'is set so that the propagation loss α of the high-frequency electrical signal is reduced, a high-performance LN optical modulator that cannot be realized by the second prior art is provided. There is an advantage that it can be realized. Needless to say, the outer peripheral portions 10b and 10d may be etched to the same height as the concave portions 9c and 9d. These apply to all embodiments of the present invention.

(Fifth embodiment)
FIG. 7 shows a fifth embodiment of the present invention. In the present embodiment, the ground conductor 13 is formed to extend to the outer peripheral portion 10b beyond the concave portion 9c. However, the ground conductor 13 may not extend to the outer peripheral portion 10b. The ground conductor 14 is assumed to be sufficiently wide. The symmetry of the structure is improved by making the widths of the ridges 8a and 8b substantially equal.

(Sixth embodiment)
FIG. 8 shows a sixth embodiment of the present invention. In the present embodiment, the width of the ground conductor 14 'is substantially the same as that of the ground conductor 13, thereby improving the symmetry of the structure as compared with the fifth embodiment shown in FIG.

(Seventh embodiment)
FIG. 9 shows a seventh embodiment of the present invention. In the present embodiment, the ridge portion 8d is formed by further providing a recess 9e in the sixth embodiment shown in FIG. In addition, 10e is an outer peripheral part. The ground conductor 15 has a sufficiently wide structure. In this embodiment, the symmetry of the structure is improved for each of the ridge portions 8a, 8b, and 8d.

(Eighth embodiment)
FIG. 10 shows an eighth embodiment of the present invention. In the present embodiment, the width of the ground conductor 15 ′ is substantially the same as that of the ground conductor 13, thereby improving the symmetry of the structure as compared with the seventh embodiment shown in FIG. 9.

(Ninth embodiment)
FIG. 11 shows a ninth embodiment of the present invention. In the present embodiment, the ground conductors 17 and 21 are separated by the groove 19. Further, the ground conductors 16 and 20 are separated by the groove 18. These groove portions 18 and 19 were formed by a dicer.

(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 directional couplers. The present invention is also applicable to a phase modulator having a single 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.

  Further, although the z-cut LN substrate has been described, an LN substrate having other plane orientation such as x-cut and y-cut may be used, or a lithium tantalate substrate or a substrate made of a different material such as a semiconductor substrate may be used.

  In the above embodiment, the case where there are two or three ridges has been described, but the number of ridges may be one or other numbers. Needless to say, the heights of the plurality of ridges may be different. In addition, the ridge described in the present invention has a broad meaning. For example, in FIGS. 1 and 4, the portion where the z-cut LN substrate is dug down is only 9a and 9b, and the other portions are below the ground conductors 4b and 4c. Including structures that are not dug down. Conversely, more ridges may be formed. Moreover, although each recessed part is normally formed with the same width | variety, you may etch so that the recessed part near an outer peripheral part may become very wide.

  As described above, the optical modulator according to the present invention is as described above. The optical modulator according to the present invention is a high-performance ridge-type optical modulator by mechanically freeing the ground conductor. Since it prevents the application of stress due to temperature change, it is useful as a means for realizing an optical modulator with excellent temperature drift characteristics.

1 is a perspective view showing a schematic configuration of an optical modulator according to a first embodiment of the present invention. Sectional view along BB 'in FIG. Diagram explaining the propagation loss of high-frequency electrical signals Sectional drawing which shows schematic structure of the optical modulator concerning the 2nd Embodiment of this invention. Sectional drawing which shows schematic structure of the optical modulator concerning the 3rd Embodiment of this invention. Sectional drawing which shows schematic structure of the optical modulator concerning the 4th Embodiment of this invention. Sectional drawing which shows schematic structure of the optical modulator concerning the 5th Embodiment of this invention. Sectional drawing which shows schematic structure of the optical modulator concerning the 6th Embodiment of this invention. Sectional drawing which shows schematic structure of the optical modulator concerning the 7th Embodiment of this invention Sectional drawing which shows schematic structure of the optical modulator concerning the 8th Embodiment of this invention. Sectional drawing which shows schematic structure of the optical modulator concerning the 9th Embodiment of this invention The perspective view which shows schematic structure about the optical modulator of 1st prior art Sectional drawing in AA 'of FIG. Sectional drawing which shows schematic structure about the optical modulator of 2nd prior art

Explanation of symbols

1: z-cut LN substrate _2, 14, 15: SiO ₂ buffer layer 3: Mach-Zehnder optical waveguide 3a, 3b: interaction optical waveguide constituting the Mach-Zehnder optical waveguide 4: traveling wave electrode 4a: central conductor 4b, 4c, 11 , 12, 12 ', 13, 14, 14', 15, 15 ', 16, 17, 20, 21: Ground conductor 5: Si conductive layer 6: High frequency (RF) electric signal feeder 7: High frequency (RF) electric Signal output line 8a: Ridge part (ridge part for central conductor)
8b, 8b ', 8c, 8d: Ridge portion (ground conductor ridge portion)
9a, 9b, 9c, 9d, 9e: concave portions 10a, 10b, 10c, 10d, 10e: outer peripheral portions 18, 19: groove portions

Claims (3)

A substrate having an electro-optic effect; a buffer layer formed on the substrate; a traveling wave electrode comprising a central conductor and a ground conductor disposed above the buffer layer; and a high-frequency electric wave propagating through the traveling wave electrode A ridge portion formed by a recess provided by digging at least part of the substrate in a region where the electric field strength of the signal is strong, and the ridge portion includes a ridge portion for a central conductor in which the central conductor is formed above In the optical modulator comprising the ground conductor ridge portion formed above the ground conductor, and having an optical waveguide at least in the central conductor ridge portion,
The ground conductor formed on the ridge portion for the ground conductor is in the direction perpendicular to the longitudinal direction of the interaction portion and parallel to the surface of the substrate during the thermal expansion and contraction, An optical modulator characterized in that it is mechanically independent so as not to receive stress from other ground conductors.   2. The optical modulator according to claim 1, wherein there are a plurality of the ground conductors mechanically independent in a direction parallel to the surface of the substrate.   The optical modulator according to claim 1, wherein the substrate is made of a semiconductor.

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