JP4138760B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulator that modulates light incident on an optical waveguide with a high-frequency electric signal using an electro-optic effect and emits the light 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, and low-cost optical modulator to be incorporated 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 an ultra-high capacity optical communication system of 40 Gbit / s is also being studied.

  Hereinafter, the characteristics of each LN optical modulator using the electro-optic effect of lithium niobate, which has been practically used or proposed, will be described in order.

(First conventional example)
4 is a perspective view of a first conventional LN optical modulator configured using a z-cut LN substrate, and FIG. 5 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, the optical waveguide 3 includes two optical waveguides 3a and 3b (or interactive optical waveguides) in an area where an electrical signal and light interact (referred to as an interaction section), that is, two Mach-Zehnder optical waveguides. An arm is formed.

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, it is assumed that a coplanar waveguide (CPW) having one central conductor 4a and two ground conductors 4b and 4c is used.

In order to suppress the absorption loss that the light guided through the optical waveguides 3a and 3b receives from the metal (generally using Au) constituting the traveling wave electrode 4 composed of the central conductor 4a and the ground conductors 4b and 4c, the traveling wave electrode 4 microwave equivalent refractive index of the electric signal for guiding (or traveling wave microwave equivalent refractive index of the electrode) n _m a reduced optical waveguide 3a, the equivalent refractive index of 3b a guiding light (or of the optical waveguide In order to make the equivalent refractive index (n0) close to n _o and the characteristic impedance as close to 50Ω as possible, a thick SiO ₂ buffer layer 2 of about 400 nm to 1 μm is usually provided between the traveling wave electrode 4 and the z-cut LN substrate 1. To deposit.

FIG. 6 shows the distribution of the lines of electric force 5 when an electric field is applied between the central conductor 4a and the ground conductors 4b and 4c of the traveling wave electrode. As can be understood from FIG. 6, the directions of the electric lines of force 5 across the two optical waveguides 3a and 3b are opposite to each other. Therefore, if the phase change amount of light propagating through the optical waveguide 3b under the central conductor 4a is Δφ ₁ , and the phase change amount of light propagating through the optical waveguide 3a under the ground conductor 4b is Δφ ₂ , then Δφ ₁ And Δφ ₂ have different signs.

In the Mach-Zehnder optical waveguide, the OFF state of the synthesized light can be realized by setting the phase difference Δφ _t (= | Δφ ₁ | + | Δφ ₂ |) of the light propagating through the two optical waveguides 3a and 3b to π. This optical modulator can form an optical signal pulse.

  However, the optical signal pulse formed by the z-cut LN optical modulator of the first conventional example has a so-called chirping in which the shape of the pulse collapses when propagating through a single mode optical fiber having a length of several tens of kilometers. There is a problem that it occurs. Next, this will be described.

As can be understood from FIG. 6, the width of the center conductor 4a is smaller than the width of the ground conductors 4b and 4c, and the width is approximately the same as the width of the optical waveguide 3b and is about 6 μm to 11 μm. Therefore, the interaction efficiency between the light propagating through the optical waveguide 3b under the central conductor 4a and the lines of electric force 5 is high. On the other hand, since the ground conductors 4b and 4c are wide, the electric force lines 5 emitted from the center conductor 4a are widely distributed in the ground conductors 4b and 4c, and the interaction between the optical waveguide 3a on the ground conductor 4b side and the electric force lines 5 is achieved. Is less efficient. The amount of phase change is approximately | Δφ ₁ | ≈5 | Δφ ₂ |.

  For this reason, chirping occurs in the optical signal pulse formed by using the LN optical modulator of the first conventional example. Incidentally, the alpha parameter (or α parameter) representing the degree of chirping is expressed as in equation (1) using the phase φ and intensity (power) E of the optical signal pulse output from this optical modulator. Yes (Non-Patent Document 1).

α = [dφ / dt] / [(1 / E) (dE / dt)] (1)
Thus, the α parameter can be expressed using the phase change amount and the intensity change amount of the output optical signal pulse.

  More specifically, the α parameter can be expressed by equation (2) obtained by developing equation (1).

α = (Γ ₁ −Γ ₂ ) / (Γ ₁ + Γ ₂ ) (2)
Γ ₁ ; Efficiency shown by overlap integral normalized by 1 between the electric signal (amplitude) and the light (power) propagating in the optical waveguide 1 (3a) Γ ₂ ; Electric signal (amplitude) and the optical waveguide 2 (3b) As described above, the efficiency of the optical signal pulse generated by the LN optical modulator of the first conventional example shown in FIGS. 4 and 5 is as follows. The cause of the chirping is that the absolute value of the phase change amount generated in the optical waveguide 3b on the center conductor 4a side and the optical waveguide 3a on the ground conductor 4b side is not the same.

(Second conventional example)
FIG. 7 is a cross-sectional view of the LN optical modulator of the second conventional example proposed in order to solve the problems of the first conventional example. The same parts as those of the LN optical modulator of the first conventional example shown in FIG. 5 are denoted by the same reference numerals, and detailed description of the overlapping parts is omitted.

  In the LN optical modulator of the second conventional example, two central conductors 6a and 6b and three ground conductors 7a, 7b and 7c are formed as traveling wave electrodes. That is, two CPW electrodes shown in the first conventional example are used, which is called a two-electrode type or push-pull type CPW traveling wave electrode. Therefore, the electric lines of force 8 shown in FIG. 7 are generated between the center conductors 6a, 6b and the ground conductors 7a, 7b, 7c.

  In the second conventional example, since the optical waveguides 3a and 3b are directly below the central conductors 6a and 6b of the two CPW electrodes having the same width, the phase change amount of the light propagating through the optical waveguides 3a and 3b is an absolute value. Are equal and the sign is reversed, and chirping zero should be realized in principle.

  However, as described above, the phase change amount of the light propagating through the optical waveguides 3a and 3b must have the same absolute value and the opposite sign, so that the LN optical modulator of the second conventional example is chirped to zero. In order to operate in a state, it is necessary to accurately apply positive and negative electrical signals to the center conductors 6a and 6b, which causes a very difficult problem.

  In other words, the phases when two anti-phase electrical signals output from different ports of an integrated circuit (IC) are applied to the central conductors 6a and 6b after exiting the IC are accurately set to positive and negative anti-phases. After the output from the IC, the electrical length to each interaction part in each central conductor 6a, 6b of the traveling wave electrode is made completely the same, and the rising and falling shapes of the electric waveform are made the same. In fact, there is a problem of technically much labor, time and difficulty.

(Third conventional example)
FIG. 8 is a plan view of the LN optical modulator of the third conventional example proposed in Patent Document 1 in order to solve the above-described problems of the second conventional example, and FIG. 9 is a BB ′ diagram of FIG. It is sectional drawing in a line. The same parts as those of the LN optical modulator of the first conventional example shown in FIGS. 5 and 6 are denoted by the same reference numerals, and detailed description of the overlapping parts is omitted.

  In the third conventional LN optical modulator, the optical waveguide 9 is formed with two optical waveguides 9a and 9b constituting a Mach-Zehnder optical waveguide. In addition, two central conductors 10a and 10b and three ground conductors 11a, 11b, and 11c are formed as traveling wave electrodes. Further, the z-cut LN substrate 1 is composed of a region 1a where the polarization is not reversed and a region 1b where the polarization is reversed. Specifically, the polarization of the z-cut LN substrate 1 is reversed with the vicinity of the center between the two optical waveguides 9a and 9b as a boundary. The optical waveguide 9a side corresponds to the region 1a where the polarization is not inverted, and the optical waveguide 9b side corresponds to the region 1b where the polarization is inverted. The electric lines of force 12 shown in FIG. 9 are generated between the center conductors 10a and 10b and the ground conductors 11a, 11b and 11c.

  The operating principle of the third conventional LN optical modulator will be described. First, the light incident on the optical waveguide 9 is branched into two so as to propagate through the two optical waveguides 9a and 9b constituting the Mach-Zehnder optical waveguide. On the other hand, since the central conductor 10 is also bifurcated into two central conductors 10a and 10b, an electric signal is applied to the optical waveguides 9a and 9b in the same direction as shown in FIG.

  Usually, in the z-cut optical modulator, a −z plane is used as a substrate surface from the diffusion state of metal Ti for manufacturing the optical waveguide 9. Therefore, the -z plane is used in the region 1a where the polarization is not inverted, and the + z plane is used in the region 1b where the polarization is inverted. In terms of electro-optics, when an electric field is applied to the −z plane and the + z plane, the resulting refractive index changes have the same absolute value and opposite signs.

  In the third conventional example, since the z-cut LN substrate 1 has its polarization reversed around the center of the Mach-Zehnder optical waveguide, the optical waveguides 9a and 9b are also generated by the electric force lines 12 in the same direction shown in FIG. The sign of the phase change of the light propagating through is reversed. Therefore, the phase change of the light propagating through the optical waveguides 9a and 9b has the same absolute value and the opposite sign, thereby reducing the chirping of the optical signal pulse output from the LN optical modulator.

  However, the third conventional LN optical modulator still has the following problems.

  In general, an LN optical modulator is manufactured on a z-cut LN substrate 1 having a size of 3 inches to 4 inches. Each LN optical modulator has Mach-Zehnder optical waveguides 9a and 9b having a gap of about 15 μm to 30 μm. is doing. Of the processes for manufacturing the LN optical modulator of the third conventional example shown in FIG. 8, the process for actually manufacturing the optical waveguide will be considered.

  First, the polarization is inverted with the center of the gap between the two optical waveguides 9a and 9b of the Mach-Zehnder optical waveguide 9 as a boundary. In the step of reversing the polarization, after electrodes are patterned on the entire upper surface and lower surface of the desired position in the z-cut LN substrate 1, the polarization domain is reversed by applying a high electric field, and then the electrodes formed vertically The etching is peeled off. Next, a photoresist pattern for a Mach-Zehnder optical waveguide is formed so that the boundary where the polarization is inverted is aligned with the center of the gap between the two optical waveguides 9a and 9b, and metal Ti is deposited and lifted off. Finally, the formed metal Ti pattern is thermally diffused to form the Mach-Zehnder optical waveguide 9.

  Since the physical properties of the substrate are different between the region 1a where the polarization is not reversed and the region 1b where the polarization is reversed, the state in which the metal Ti for forming the optical waveguides 9a and 9b diffuses into the z-cut LN substrate 1 also differs. . As a result, the spot size and propagation loss of light differ between the region 1a where the polarization is not inverted and the region 1b where the polarization is inverted.

  The fact that the spot size of light propagating through the optical waveguide 9a formed in the non-inverted region 1a is different from the spot size of light propagating through the optical waveguides 9a and 9b formed in the region 1b with inverted polarization is Efficiency of interaction between an electric signal propagating through a traveling wave electrode composed of the conductors 10a and 10b and the ground conductors 11a and 11b and light propagating through the two optical waveguides 9a and 9b (generally, as described above, (Which is expressed by the overlap integral of light power) is different between the two optical waveguides 9a and 9b.

  As described in the LN optical modulator of the first conventional example shown in FIG. 6, when the absolute value of the phase change amount of the light propagating through the two optical waveguides 9a and 9b is different, the light is emitted from the LN optical modulator. The optical signal pulse has a chirping characteristic, and it has not been a sufficient solution to the problem of extremely reducing the chirping.

  In addition, since the propagation loss is different between the region 1a where the polarization is not reversed and the region 1b where the polarization is reversed, a difference occurs in the power of the light propagating through the two optical waveguides 9a and 9b, resulting in the LN optical modulator. There is also a problem that the extinction ratio at the time of OFF in the optical signal pulse emitted from the laser beam deteriorates.

Further, as can be understood from FIG. 8, it is necessary to divide the central conductor 10 into two central conductors 10a and 10b at the interaction portion between the electric signal and the light, as can be understood from FIG. Here, if the 50Ω system is used in the central conductors 10a and 10b, the value is about 25Ω before the division, and if the 50Ω system is used in the divided part, the part of the central conductors 10a and 10b after the division is about 100Ω. In principle, it has a serious problem of impedance mismatch. Alternatively this can, poor workmanship on bifurcation dimensional accuracy of and on the material properties significantly affect the reflection characteristic (S ₁₁ characteristics), difficult problem that results in a big impact on the yield in the manufacture of the LN optical modulator Was holding.

(Fourth conventional example)
FIG. 10 is a plan view of a fourth conventional LN optical modulator proposed in Non-Patent Document 2.

  In the LN optical modulator of the fourth conventional example, the LN substrate, in order from the light incident side, the non-inverted region 15a, the inverted region 15b, and the uninverted polarization It is divided into regions 15c. Then, the two optical waveguides 13a and 13b constituting the Mach-Zehnder optical waveguide 13 pass through the region 15b where the polarization is inverted from the region 15a where the polarization is not inverted on the LN substrate.

  Further, as the traveling wave electrode, one center conductor 14a and two ground conductors 14b and 14c are formed. The central conductor 14a is above the optical waveguide 13b by a length L / 2 in the region 15a where the polarization is not inverted, and above the optical waveguide 13a by a length L / 2 in the region 15b where the polarization is inverted. positioned. Here, the region 15c is also a region where the polarization is not reversed.

  The optical modulator of the fourth conventional example is characterized in that the length of the central conductor 14a in the non-inverted region 15a and the length of the central conductor 14a in the inverted region 15b are equal to L / 2. That is.

  In the fourth conventional example, each light propagating through the two optical waveguides 13a and 13b has the same distance for propagating through the region 15a where the polarization is not inverted and the region 15b where the polarization is inverted (= L / 2). ) To reduce the chirping in the optical signal pulse emitted from the LN optical modulator.

  Now, in general, when the frequency of an electric signal increases, the electric signal propagating through the traveling wave electrode composed of the center conductor 14a and the ground conductors 14b and 14c becomes weak with propagation due to the conductor loss of the metal. Due to the propagation loss of the electric signal, even if the length of the non-inverted region 15a and the inverted region 15b is equal to L / 2, the polarization located on the downstream side of the electric signal The intensity of the electric signal propagating through the traveling wave electrode in the inverted region 15b is weaker than that in the region 15a where the polarization is not inverted.

  As a result, when the frequency of the electric signal applied between the center conductor 14a and the ground conductors 14b and 14c increases, the absolute value of the phase difference of the light propagating through the two optical waveguides 13a and 13b differs.

  FIG. 12 shows the magnitude of chirping obtained from the above-described equation (1) or (2) when the frequency of the electric signal applied between the center conductor and the ground conductor of the LN optical modulator is changed. Changes in the α parameter (frequency characteristics of the α parameter) are shown.

  In the LN optical modulator of the fourth conventional example, as shown by the broken line characteristic in FIG. 12, chirping zero (α parameter is zero) can be substantially realized at a low frequency near DC, but chirping is performed at a high frequency. growing. As a result, it can be understood that large chirping occurs in the optical signal pulse emitted from the optical modulator.

(Fifth conventional example)
FIG. 11 is a plan view of the LN optical modulator of the fifth conventional example similarly proposed in Non-Patent Document 2 in order to solve the problems of the fourth conventional example.

  In the LN optical modulator of the fifth conventional example, the LN substrate is, in order from the light incident side, the region 18a where the polarization is not inverted, the region 18b where the polarization is inverted, and the polarization is not inverted. It is divided into regions 18c. The length of the central conductor 17a on the two optical waveguides 16a and 16b of the Mach-Zehnder optical waveguide 16 is divided into L / 4, L / 2, and L / 4, and the central conductor 17a and the ground conductors 17b and 17c are separated. A region in which the polarization is not inverted by a length L / 4 is assigned to a region where the intensity of the electric signal propagating through the traveling-wave electrode is strong and a region where the electric signal is weak, respectively. A region in which the polarization is inverted by L / 2 is assigned. In this way, the absolute value of the interaction between the light and the electric signal is made as close as possible in the two optical waveguides 16 a and 16 b of the Mach-Zehnder optical waveguide 16.

  As a result, in the LN optical modulator of the fifth conventional example, as shown by the solid line characteristic in FIG. 12, the α parameter representing the chirping amount is represented by the broken line characteristic of the fourth conventional LN optical modulator. The α parameter can be significantly reduced.

However, as can be understood from the solid line characteristics of FIG. 12, also in the LN optical modulator of the fifth conventional example, the α parameter increases as the frequency of the electric signal applied between the center conductor and the ground conductor increases. There is a problem that chirping occurs in the optical signal pulse output from the LN optical modulator when high-speed optical modulation is performed.
JP 2003-202530 A Nadege Courjal et al “Modeling and Optimization of Low Chirp LiNbO3 Mach-Zehnder Modulators With an Inverted Ferroelectric Domain Section“ Journal of Lightwave Technology vol.22 No.5 May 2004 Masaki Sugiyama et al “Compact Zero-Chirp LiNbO3 Modulator for 10-Gb / s Small-Factor Transponder” 30th European Conference on Optical Communication Post Deadline Session 2.Th 4.2.3,2004

  As described above, as a conventional example using polarization reversal for an LN substrate, the boundary between a region where polarization is not reversed and a region where the polarization is reversed is parallel to the two arms (optical waveguides) constituting the Mach-Zehnder optical waveguide. A configuration in which the polarization-inverted region is formed (third conventional example) and a configuration in which the polarization-inverted region is formed so that the boundary is perpendicular to the two arms (optical waveguide) (fourth conventional example) Example, fifth conventional example).

  In the former case, that is, when the optical waveguides are formed on LN substrates having different polarizations (third conventional example, FIGS. 8 and 9), the spot sizes of light propagating through the two arms (optical waveguides) are different. Therefore, it is difficult to make the chirping extremely small even when the frequency of the electric signal applied between the center conductor and the ground conductor is 0, that is, in the vicinity of DC, and above all, the center conductor must be bifurcated. There was a serious problem that the impedance became irregular.

  On the other hand, in the latter configuration, when the length of the interaction region where the polarization is not reversed is equal to the length of the interaction region where the polarization is reversed (fourth conventional example, FIG. 10), the chirping is zero near DC. Even so, chirping increases significantly with frequency. In the case where the length of the interaction region in which the polarization is not reversed and the length of the interaction region in which the polarization is reversed is set based on the conventional concept (fifth conventional example, FIG. 11), However, even if chirping is zero, there is a problem that chirping occurs when the frequency increases.

  The present invention has been made in view of such circumstances, and maintains a simple structure in which a traveling wave electrode is formed of one central conductor and two ground conductors, even between the central conductor and the ground conductor. An object of the present invention is to provide an optical modulator capable of suppressing the chirping generated in the optical signal pulse emitted from the optical waveguide as much as possible even if the frequency of the applied high-frequency electrical signal changes.

  In order to solve the above-mentioned problems, the present invention comprises a substrate made of a material having an electro-optic effect, formed of two types of regions, a region where polarization is not reversed and a region where polarization is reversed, and one surface side of the substrate An optical waveguide having first and second branch optical waveguides for branching and multiplexing incident light, and a high-frequency electric signal that interacts with the light propagating through each branch optical waveguide is propagated An interaction portion comprising a center conductor and a ground conductor, the center conductor facing the first and second branch optical waveguides by the first and second lengths, respectively, and a first length on the substrate The optical modulator that modulates the phase of the light propagating through each branch optical waveguide in the region of the interaction portion is formed by regions polarized in different directions from the portion facing the second length and the portion facing the second length. Adapted.

The sign of the alpha parameter representing the chirping magnitude of the optical signal pulse emitted from the optical waveguide within the band of the predetermined modulation frequency based on the high-frequency electrical signal is changed from plus to minus with the modulation frequency change. The first and second lengths are set so as to change from minus to plus, whereby the first length and the second length are different, and thereby the light emitted from the optical waveguide Suppresses chirping that occurs in signal pulses.

  In the optical modulator configured as described above, the traveling wave electrode of the high-frequency electric signal applied to the ground conductor, that is, the central conductor constituting the interaction portion is different from that of the first and second branch optical waveguides. The portions face each other by the first and second lengths. Further, the portion facing the first length and the portion facing the second length in the substrate are formed of different types of regions (two types of polarization non-inversion and polarization inversion).

  Then, the first optical waveguide in the central conductor is a range in which the interaction unit normally modulates the phase of the light propagating through the first and second branch optical waveguides in different directions (in different signs) from each other. The first length that faces the region that is opposite and does not reverse the polarization, and the second length that faces the second optical waveguide in the central conductor and that faces the region where the polarization is reversed are used. The sign of the α parameter representing the chirping amount is changed from plus to minus or from minus to plus within the modulation band included in the frequency.

  Therefore, chirping generated in the optical signal pulse output from the optical modulator is reduced on average.

  In another invention, in the optical modulator of the invention described above, the first and second values are set so that the integral value for the frequency of the alpha parameter in a predetermined frequency band based on the high-frequency electrical signal is substantially zero. The length is set.

  In the previous invention, the sign of the α parameter changes from plus to minus or from minus to plus, and even if the α parameter is zero at a certain frequency, the α parameter is averaged within the frequency band required for the optical pulse. It is not zero and there is a possibility that chirping will occur.

  Optical signal pulses such as NRZ, RZ, and soliton that are used in actual optical communication include a wide frequency band instead of a single frequency. Therefore, in order to extremely reduce the chirping of the optical signal pulse generated by the optical modulator, the α parameter is set to zero on average within a predetermined frequency band included in the optical signal pulse as well as a single frequency. This is very important.

  Therefore, as described above, the first and second lengths of the central conductor facing the different types of regions (two types of polarization non-inversion and polarization inversion) in the substrate are adjusted, and α in the frequency band is adjusted. The integral value for the parameter frequency is almost zero. Therefore, it is possible to provide an optical modulator with extremely small chirping in an actual optical communication system.

  In another invention, in the optical modulator of the above-described invention, there are a plurality of regions in the substrate where the polarization is not reversed and at least one of the regions where the polarization is reversed.

  In the present invention, the first and second lengths in the central conductor facing the first and second optical waveguides and facing different types of regions (two types of polarization non-inversion and polarization inversion) in the substrate. Is adjusted to change the alpha parameter representing the chirping magnitude of the optical signal pulse from plus to minus or from minus to plus as the modulation frequency changes.

  Therefore, even if the frequency of the high-frequency electrical signal applied between the center conductor and the ground conductor is changed while maintaining a simple structure in which the traveling wave electrode is formed by one center conductor and two ground conductors, The size of chirping generated in the optical signal pulse emitted from the optical waveguide can be suppressed as much as possible.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1A is a plan view schematically showing a schematic configuration of the optical modulator according to the first embodiment of the present invention, and FIG. 1B is a CC diagram of the optical modulator of FIG. FIG. 1C is a cross-sectional view taken along the line DD ′ in the optical modulator of FIG.

  An optical waveguide 19 is formed on the z-cut LN substrate 21. This optical waveguide 19 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, the optical waveguide 19 includes an introduction optical waveguide 19c that guides light incident on the leftmost incident surface of the optical modulator, and two optical waveguides 19a and 19b as branch optical waveguides in the central portion of the optical modulator. And an output optical waveguide 19d that emits an optical communication pulse from the right exit surface.

A SiO ₂ buffer layer 22 having a thickness of about 400 nm to 1 μm is formed on the upper surface of the optical waveguide 19, and a traveling wave electrode is formed on the upper surface of the SiO ₂ buffer layer 22. As the traveling wave electrode, a coplanar waveguide (CPW) having one central conductor 20a and two ground conductors 20b and 20c is employed. A high-frequency electric signal as a modulation signal is applied between the center conductor 20a and the ground conductors 20b and 20c in the traveling wave electrode.

The center conductor 20a is located above the first by a length L ₁ first optical waveguide 19b as branched optical waveguide, and further, the optical waveguide of a second by a length L ₂ second branched optical waveguide It is located above 19a. One ground conductor 20b is located on the left side (incident surface side) of the center conductor 20a with a predetermined interval (gap) with respect to the center conductor 20a, and the other ground conductor 20c is predetermined with respect to the center conductor 20a. Are located on the right side (outgoing surface side) of the central conductor 20a.

The z-cut LN substrate 21 includes a region (a −z plane region) 21 a that is not inverted in polarization and a region (a + z plane region) 21 b that is inverted in polarization. The boundary between the region 21a where the polarization is not reversed and the region 21b where the polarization is reversed is perpendicular to the optical waveguides 19a and 19b as shown by the dotted line in FIG. The conductor 20a passes through the first length L ₁ and the second length L ₂ and the contact.

As a result, the first length L ₁ portion of the center conductor 20a is opposed to the first region 21a that does not invert the polarization of the optical waveguide 19b and z- cut LN substrate 21 as the branched optical waveguides . The second length L ₂ portion of the center conductor 20a is opposed to a region 21b which inverts the polarization of the optical waveguide 19a and z- cut LN substrate 21 as a second branched optical waveguide .

In the optical modulator having such a structure, the center conductor 20a and the ground conductor 21b, while applying a high frequency electric signal as a modulation signal to 21c, in particular from a first length L ₁ side end of the central conductor 20a When light is incident from the entrance surface at the left end of the optical modulator in a state where a microwave is input, the light passes through the introduction optical waveguide 19c and is branched into two optical waveguides 19a and 19b. In the process of propagating through the optical waveguides 19a and 19b, the branched light is phase-modulated in accordance with the high-frequency electric signal at the interaction portion composed of the center conductor 20a and the ground conductors 20b and 20c. Waveforms are synthesized by the optical waveguide 19d and output from the output end as optical signal pulses.

In the optical modulator according to the first embodiment having such an optical modulation function, the progress made up of the center conductor 20a and the ground conductors 20b and 20c in a state where a high-frequency electric signal is applied to the center conductor 20a and the ground conductors 20b and 20c. The microwave propagation loss at the frequency f of the high-frequency electric signal of the wave electrode is β _m (f), and the portion of the center conductor 20a having the first length L ₁ (facing the optical waveguide 19b and having no polarization reversed) a portion) that faces the region 21a, the second length L ₂ of the portion (the portion facing the region 21b which inverts the face vital polarization in the optical waveguide 19a), an electrical signal and an optical waveguide 19a, 19b The efficiency of the interaction with the light propagating through is expressed as I ₁ (f) and I ₂ (f), respectively. If the propagation direction of the optical and electrical signals is z as shown in FIG. 1A, the efficiency of each interaction I ₁ (f), I ₂ (f) depends on the frequency f, and (3), (4 ) Expression.

I ₁ (f) = ∫ ₀ ^L1 exp (−β _m (f) · z) dz
_{= (1-exp (-β m} (f) · L 1)) / β m (f) ... (3)
I ₂ (f) = ∫ _L1 ^L2 exp (−β _m (f) · z) dz
_{= Exp (-β m (f)} · L 1) · (1-exp (-β m (f) · L 2)) / β m (f)
…(Four)
Therefore, the conditions I ₁ (f) = I ₂ (f) (5) in which the efficiency of the interaction I ₁ (f) and I ₂ (f) are equal to each other.
By setting the first length L ₁ and the second length L ₂ in the center conductor 20a satisfying the above, chirping can be made zero at an arbitrarily designated frequency f. That is, the α parameter is zero when the equation (5) is satisfied.

From the first length L ₁ and the second length L ₂ satisfying the expression (5), the interaction efficiency I ₁ (f) of the portion of the _first length L ₁ and the second length L ₂ The interaction efficiency I ₂ (f) is obtained, and the frequency characteristic of the α parameter indicated at each frequency f of the high-frequency electric signal is obtained using the above-described equation (2).

  FIG. 2A and FIG. 2B show the frequency characteristics of the α parameter obtained. FIG. 2A shows the frequency characteristics of the α parameter in which the sign of the α parameter changes from minus (−) to plus (+) as the frequency f of the applied high-frequency electrical signal increases. ) Shows the frequency characteristics of the α parameter in which the sign of the α parameter changes from plus (+) to minus (−) as the frequency f of the high-frequency electrical signal increases.

In this way, the region 21a where the polarization of the central conductor 20a is not inverted so that the sign of the α parameter representing the chirping amount is switched between the low frequency region near DC and the high frequency region at the frequency f of the applied high frequency electrical signal. the first length L _1, and sets the second length L ₂ which faces the region 21b by inverting the polarization of the center conductor 20a opposite to. In order to realize this, L ₁ / L ₂ = 0.89. That is, the second length L ₂ may be set slightly longer than the first length L ₁ in the center conductor 20a. However, since this value varies depending on the structure of the electrode, it goes without saying that the ratio of the first length L ₁ and the second length L ₂ is not limited to this.

  Note that (5) can be satisfied even when the α parameter is always positive or always negative and is zero only at a certain frequency, but it is important for the optical signal pulse emitted from the optical modulator. What is important is that the α parameter is averaged to be zero within a required predetermined frequency band, and such an optical modulator is adopted in an optical communication system even if it becomes zero only at a specific frequency. It is not possible.

(Second Embodiment)
Next, an optical modulator according to a second embodiment of the present invention will be described. The optical modulator of the second embodiment is an improved type of the optical modulator of the first embodiment. That is, the chirping of the optical signal pulse emitted from the optical modulator of the first embodiment is considerably reduced, but the α parameter is averaged within a predetermined frequency band necessary for the optical signal pulse transmitted in the optical communication system. Zero is more strongly required.

In the optical modulator of the second embodiment, as shown in FIGS. 3A and 3B, the maximum frequency of a predetermined frequency band required as an optical signal pulse is set to f _max, and FIG. As shown in the frequency characteristic of the α parameter in which the sign of the α parameter changes from minus (−) to plus (+) as the frequency f increases, the frequency f _{0 at} which the α parameter becomes zero from near DC. Α parameters up to α ₁ (f) and α parameters from frequency f ₀ to the maximum frequency f _max are α ₂ (f).
∫ ₀ ^f0 α ₁ (f) df = ∫ _f0 ^fmax α ₂ (f) df (6)
So that the first length L ₁ facing the region 21a where the polarization in the center conductor 20a is not inverted and the second length L ₂ facing the region 21b where the polarization is inverted in the center conductor 20a are Set. This corresponds to making the area S ₁ and the area S ₂ equal in FIG.

Further, as shown in FIG. 3B, the α parameter from the vicinity of DC is included in the frequency characteristics of the α parameter in which the sign of the α parameter changes from plus (+) to minus (−) as the frequency f increases. When but a alpha parameter to the frequency f ₀ becomes zero and α ₃ (f), the alpha parameter to the maximum frequency f _max and alpha ₄ (f) from the frequency f _0,
∫ ₀ ^f0 α ₃ (f) df = ∫ _f0 ^fmax α ₄ (f) df (7)
So that the first length L ₁ facing the region 21a where the polarization in the center conductor 20a is not inverted and the second length L ₂ facing the region 21b where the polarization is inverted in the center conductor 20a are Set. This corresponds to making the area S ₃ and the area S ₄ equal in FIG.

In order to realize this, L ₁ / L ₂ = 0.7 was a preferable value. However, since this value varies depending on the electrode structure, it is needless to say that this is not the case. Further, the maximum frequency included in the optical signal pulse differs depending on the optical transmission method, and is not limited to the embodiment illustrated as an example.

In the optical modulator of the first embodiment shown in FIGS. 2 (a) and 2 (b), the frequency at which the α parameter is made zero, and the second embodiment shown in FIGS. 3 (a) and 3 (b). Of course, the frequency f ₀ satisfying the expressions (6) and (7) in the optical modulator of FIG.

  Here, when the α parameter supplementarily described in the first embodiment of the present invention is always positive or always negative, and becomes zero only at a specific frequency, the equation (5) is satisfied, but (6), ( 7) The formula is not satisfied. From this point of view, if the α parameter described above is always positive or always negative and becomes zero only at a specific frequency, the α parameter is set within a predetermined frequency band required as an optical signal pulse transmitted in the optical communication system. It cannot be zero on average.

  Further, when an electrical signal is Fourier-transformed and the electrical signal has many components in a specific frequency region, integration may be performed as follows using a weight W (f) representing the frequency region. .

^{_{∫ 0 f0 α 1 (f)}} W (f) df = ∫ f0 fmax α 2 (f) W (f) df
… (8)
The present invention is not limited to the above-described embodiments.

  In the embodiment, the non-polarized region 21a in the z-cut LN substrate 21 is arranged on the light incident surface side, and the polarized region 21b is arranged on the optical signal pulse emitting surface side. These may be arranged at the opposite positions.

  The optical modulator of the embodiment has almost the same basic structure as the optical modulator of the fourth conventional example shown in FIG. 10 for the sake of simplicity, but the optical modulator of the fifth conventional example shown in FIG. On the other hand, the effect of the present invention can be realized by appropriately setting the length of the region where the polarization opposite to the central conductor is reversed and the region where the polarization is not reversed.

  Furthermore, even if the number of repetitions of the region where the polarization is reversed and the region where the polarization is not reversed is increased, the sign of the α parameter is changed within the predetermined frequency band necessary for the optical signal pulse, or within the predetermined frequency band. By using the idea of the present invention that the integral value with respect to the frequency of the α parameter at is substantially zero, the chirping can be remarkably reduced.

  In the above, in the interaction part where the electric signal propagating through the traveling wave electrode interacts with the light propagating through the optical waveguide, the length of the region where the polarization opposite to the central conductor is inverted is different from the length of the region where the polarization is not inverted. The configuration to be shown has been described. On the other hand, even if the width of the center conductor or the gap between the center conductor and the ground conductor is widened, the propagation loss of the traveling wave electrode can be reduced, so conversely, the length of the region where the polarization is reversed and the region where the polarization is not reversed are reduced. In addition, the width of the center conductor and the width of the gap between the center conductor and the ground conductor may be made different in the propagation direction of the light or the electric signal, or these configurations may be combined.

  Normally, the length of the central conductor that overlaps the optical waveguide is equal to the length of the ground conductor that overlaps the optical waveguide, but by making one of them longer than the other, the interaction area of the photoelectric signal As a result, the modulation efficiency can be further increased.

  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. It is also applicable to. 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 can be applied as the buffer layer.

  Although the configuration using the CPW electrode has been described as the electrode configuration, other configurations such as an asymmetric coplanar strip (ACPS) or a symmetric coplanar strip (CPS) may be used.

  In addition, although the region where the polarization of the z-cut LN substrate in the interaction portion is reversed is described as one place, it may be more than that, and the structure in which the region where the polarization is not reversed and the region where the polarization is reversed is alternately combined may be used. Needless to say. The polarization may be reversed only on the surface of the substrate.

  Furthermore, a conventionally used technique for reducing the speed difference between an electric signal and light, such as increasing the traveling wave electrode or increasing the buffer layer, can be applied to the present invention as it is. Needless to say, the output side of the electric signal may be terminated by a terminator such as 40Ω or 50Ω.

The top view and sectional drawing which show schematic structure of the optical modulator concerning 1st Embodiment of this invention The figure which shows the frequency characteristic of (alpha) parameter of the optical signal pulse radiate | emitted from the optical modulator concerning the 1st Embodiment. The figure which shows the frequency characteristic of (alpha) parameter of the optical signal pulse radiate | emitted from the optical modulator concerning 2nd Embodiment of this invention. The perspective view which shows schematic structure of the optical modulator of a 1st prior art example. Sectional view of the optical modulator of the first conventional example Power line diagram showing operation of optical modulator of first conventional example Power diagram showing operation of optical modulator of second conventional example The top view which shows schematic structure of the optical modulator of a 3rd prior art example Power line diagram showing operation of optical modulator of third conventional example The top view which shows schematic structure of the optical modulator of a 4th prior art example The top view which shows schematic structure of the optical modulator of a 5th prior art example The figure which shows the frequency characteristic of alpha parameter of the optical signal pulse radiate | emitted from the optical modulator of the 4th, 5th prior art example

Explanation of symbols

1, 21 ... z-cut LN substrate, 1a, 15a, 15c, 21a ... region not reverse the polarization, 1b, 15b, 21b ... region to reverse the polarization, 2, 22 ... SiO ₂ buffer layer, 3,9,13 16, 19 ... Mach-Zehnder optical waveguides, 3a, 3b, 9a, 9b, 13a, 13b, 16a, 16b, 19a, 19b ... Optical waveguides constituting the Mach-Zehnder optical waveguides, 4 ... Traveling wave electrodes, 4a, 6a, 6b, 10a, 10b, 14a, 17a, 20a ... center conductor, 4b, 4c, 7a, 7b, 7c, 11a, 11b, 11c, 14b, 14c, 17b, 17c, 20b, 20c ... ground conductor

Claims (3)

A substrate made of a material having an electro-optic effect and formed of two types of regions, a region where polarization is not reversed and a region where polarization is reversed;
An optical waveguide formed on one surface side of the substrate and provided with first and second branch optical waveguides for branching and multiplexing incident light;
An interaction portion comprising a central conductor and a ground conductor for propagating a high-frequency electrical signal that interacts with light propagating through each of the branched optical waveguides;
The central conductor faces the first and second branch optical waveguides by the first and second lengths, respectively.
The portion facing the first length and the portion facing the second length in the substrate are formed of regions polarized in different directions,
In an optical modulator that modulates the phase of light propagating through each branch optical waveguide in the region of the interaction section,
Within a predetermined modulation frequency band based on the high-frequency electrical signal, the sign of the alpha parameter representing the chirping magnitude of the optical signal pulse emitted from the optical waveguide is changed from plus to minus as the modulation frequency changes. The first and second lengths are set so as to change from minus to plus, whereby the first length and the second length are different,
An optical modulator characterized in that chirping generated in an optical signal pulse emitted from the optical waveguide is suppressed .   2. The first and second lengths are set such that an integral value for the frequency of the alpha parameter in a predetermined frequency band based on the high-frequency electric signal is substantially zero. Light modulator.   3. The optical modulator according to claim 1, wherein there are a plurality of at least one of the number of regions in which the polarization is not reversed and the number of regions in which the polarization is reversed in the substrate.

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