Optical modulator and optical transmitter

In an optical modulator of the invention, a signal electrode and a ground electrode are formed along a pair of branching waveguides of a Mach-Zehnder type optical waveguide, and a first region positioned on an input side of an interaction section of light and an electric signal is made a forward modulation section, and a second region positioned on an output side is made an inverse modulation section, and a spacing or the like of the signal electrode and the ground electrode is optimized so that that a loss produced in the second region is relatively greater than a loss produced in the first region, with respect to a high frequency component of an electric signal propagated through the signal electrode. As a result a wider modulation bandwidth can be realized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical modulator and an optical transmitter used for optical transmission, and in particular relates to an optical modulator of a waveguide type which modulates light using an electro-optic effect, and to an optical transmitter which uses this.

2. Description of the Related Art

For example, an optical waveguide device which uses an electro-optic crystal such as lithium niobate (LiNbO3), lithium tantalate (LiTaO2) or the like, is manufactured by forming a metal film on a part of a crystal substrate and thermal diffusing, or by proton exchanging in benzoic acid after patterning, to thereby form an optical waveguide, and afterwards providing electrodes in the vicinity of the optical waveguide. As one optical waveguide device which uses an electro-optic crystal, there is known an optical modulator as shown for example inFIG. 28.

In general, the optical modulator, depending on the shape of the optical waveguide, is divided into a phase modulator as shown at the top ofFIG. 28, and an intensity modulator as shown at the bottom ofFIG. 28. In the case of the phase modulator, a signal electrode131is formed on one optical waveguide102which is formed on a substrate101. Furthermore, in the case of the intensity modulator, the optical waveguide102comprises an input waveguide121, a branching section122, branching waveguides123and124, a multiplexing section125, and an output waveguide126, and a coplanar electrode is formed with a signal electrode131provided on one branching waveguide123, and a ground electrode132provided on the other branching waveguide124.

In such an optical modulator, for example in the case where a Z-cut substrate101is used, the refractive index change due to an electric field in the Z-direction is used. Therefore, the electrode is arranged directly above the optical waveguide102. More specifically, in the case of the intensity modulator at the bottom ofFIG. 28, the signal electrode131and the ground electrode132are respectively patterned on the branching waveguides123and124. At this time, in order to prevent the light propagated through the respective branching waveguides123and124being absorbed by the signal electrode131and the ground electrode132, a buffer layer (not shown in the figure) is provided between the substrate101and the electrodes131and132. For the buffer layer, an oxide silicon (SiO2) or the like of a thickness of 0.2 μm to 1 μm is used.

In the case of driving the above optical modulator at a high speed, the output terminal of the signal electrode131is made a traveling-wave electrode by grounding via a resistance (not shown in the figure), and a high frequency electric signal E such as a microwave is applied from an input terminal of the signal electrode131. At this time, due to the electric field generated between the signal electrode131and the ground electrode132, the refractive index of the optical waveguide102changes. Therefore, in the phase modulator at the top ofFIG. 28, the phase of the light L propagated through the optical waveguide102is modulated in accordance with the electric signal E. Furthermore, in the intensity modulator at the bottom ofFIG. 28, the refractive index of the branching waveguides123and124is changed for each, so that the phase difference of the light L propagated through each of these is changed, and an intensity modulated signal light L is output from the output waveguide126.

In the optical modulator driven at high speed as described above, it is known that a wide band optical response characteristic is obtained by controlling the effective refractive index for the electric signal E by changing the cross-section shape of the signal electrode131, and matching the propagation speed of the light L and the electric signal E. However, regarding the electric signal E propagated through the signal electrode131, if the frequency thereof becomes high, the transmission losses increase. Therefore there is a problem in that the modulation band width is limited, so that high speed modulation becomes difficult.

As previous technology related to widening the band width of the optical modulator, for example as shown inFIG. 29, a configuration has been proposed where, of the interaction section for the light L and the electric signal E, the direction of the refractive index change is reversed by inverting the polarization direction in the remaining part111(surrounded by the dotted line in the figure) with respect to the polarization direction of the substrate101(direction of the crystal axis) of the part from the input side to a certain length (refer for example to Japanese Unexamined Patent Publication Nos. 2005-284129, 2005-221874, and 2006-47746). By means of this configuration, when modulation in the non-inversed region where the polarization direction is not changed is the forward direction, then in the polarization inversed region, modulation in the opposite direction occurs. That is to say, the polarization inversed part becomes an inverse modulated section, and the other part becomes a forward modulated section. As described above, since the loss of the electric signal E is great at high frequency, the intensity of the inverse modulation in the polarization inversed region is great at low frequency, and is small at high frequency. As a result, in the overall optical modulator, the modulation at low frequency is suppressed, so that the high frequency dependency is reduced, that is, the modulation bandwidth becomes wide.

Furthermore, as another conventional technology for improving the response characteristics of the optical modulator or the like, a configuration is also proposed where the electrode width of the signal electrode and the ground electrode is changed along the light propagation direction, to thereby prevent resonance of an acoustic wave (for example surface acoustic wave) which is produced when a modulating signal of a high pulse shape is applied between the electrodes, so that occurrence of ripple is suppressed, (refer for example to Japanese Unexamined Patent Publication No. 2000-275589).

However, in the conventional technology which gives a wider band width by using the above inverse modulation, since the modulation component of the high frequency band in the polarization inversed region (inverse modulation section) has not become small enough, then in the high frequency band, a certain amount of inverse modulation occurs. Therefore there is a problem in that the amount of improvement in bandwidth is limited.

Furthermore, in the conventional technology for improving the response characteristics by changing the electrode width of the signal electrode and the ground electrode along the light propagation direction, the influence on the light due to the resonance of the generated acoustic wave attributable to the piezoelectricity of the substrate can be reduced, but an increase in the propagation loss of the electrical signal in the high frequency as mentioned above cannot be effectively suppressed. Therefore there is the problem that it is difficult to realize a wider band width.

SUMMARY OF THE INVENTION

The present invention addresses the abovementioned points with an object of providing an optical modulator which can realize a wider modulation bandwidth, and an optical transmitter which uses this.

In order to achieve the above object, in an optical modulator of the present invention which comprises a substrate having an electro-optic effect, an optical waveguide formed on the substrate, a signal electrode formed on the substrate, and a ground electrode formed on the substrate at a distance from the signal electrode, and in which in a first and second region set in an interaction section where light propagated through the optical waveguide, and an electrical signal propagated through the signal electrode interact with each other, a modulation direction in the first region positioned on an input side in a light propagation direction, and in the second region positioned on an output side, are reversed, and the first and second regions are configured such that a loss produced in the second region is relatively greater than a loss produced in the first region, with respect to a high frequency component of an electric signal propagated through the signal electrode.

In an optical modulator of the abovementioned configuration, the loss with respect to the high frequency component of the electrical signal in the second region on the output side of the interaction section is made relatively greater than the loss with respect to the high frequency component of the electrical signal in the first region on the input side of the interaction section. As a result the inverse modulation in the high frequency band in the second region where the modulation direction is reversed to that of the first region is suppressed more than for a conventional optical modulator which uses inverse modulation. Therefore a wider modulation bandwidth is realized.

Furthermore, the optical transmitter of the present invention, uses the abovementioned optical modulator to externally modulate and transmit an outgoing beam from a light source. In such an optical transmitter, modulated signal light can be transmitted to the outside at a faster rate.

According to the optical modulator of the present invention as described above, by making the loss of the second region with respect to the high frequency component of the electrical signal propagated through the signal electrode relatively large, the modulation band in the overall optical modulator can be made a wider band, and modulation at a higher speed than heretofore is possible. According to the optical transmitter which uses such an optical modulator, optical signals having a wider bandwidth can be transmitted, and hence a characteristic improvement of the error rate and the like on the reception side becomes possible.

Other objects, characteristics, and advantages of the present invention will become apparent from the following description of the embodiments, in conjunction with the appended drawings.

DETAILED DESCRIPTION OF THE INVENTION

Hereunder is a description of a best mode for carrying out the present invention with reference to the appended drawings. Throughout the drawings the same reference symbols denote the same or equivalent components.

FIG. 1is a plan view showing a configuration of an optical modulator according to a first embodiment of the present invention. Moreover,FIG. 2is an enlarged view of section a-a′ and section b-b′ in the optical modulator ofFIG. 1.

InFIG. 1andFIG. 2, the optical modulator of the first embodiment comprises for example a substrate1having an electro-optic effect, an optical waveguide2of a Mach-Zehnder type formed on the substrate1, and an electrode3formed on the surface of the substrate1.

The substrate1uses a crystal substrate of for example Z-cut lithium niobate (LiNbO3) or Lithium Tantalate (LiTaO2), and in a region (hereunder interaction section) where light propagated through the optical waveguide2and an electric signal E propagated through the electrode3interact, a first region A on the input side which spans from the light input end over a predetermined length in the longitudinal direction (the light propagation direction) is made a forward modulation section1A, and a second region B remaining on the output side is made an inverse modulation section1B. In the inverse modulation section1B is formed a polarization inversed region11enclosed by the dashed line in the figure. The polarization inversed region11is a region where the polarization direction (crystal axis direction) of the substrate1is inversed, and is formed for example by impressing a high pulse electric field on the substrate1which has been patterned with resist or the like.

The optical waveguide2has, for example, an input waveguide21, a branching section22, branching waveguides23and24, a multiplexing section25, and an output waveguide26, and constitutes a Mach-Zehnder interferometer. This optical waveguide2is formed on the surface of the substrate1by applying a process such as thermal diffusion of titanium (Ti) or the like, or proton exchange. Regarding the distance between the pair of branching waveguides23and24(hereunder the waveguide spacing), the waveguide spacing in the inverse modulation section1B is relatively narrower than the waveguide spacing in the forward modulation section1A. At the boundary portion spreading from the forward modulation section1A to the inverse modulation section1B, the waveguide spacing becomes gradually narrower. Furthermore, the aforementioned waveguide spacing is designed so that it becomes wider than the spacing for where coupling is produced with the light propagating through the respective branching waveguides23and24.

The electrode3has a signal electrode31and a ground electrode32. Opposite ends of the signal electrode31are positioned on one side of the substrate1, and the central portion is patterned so as to follow along above one branching waveguide23. The ground electrode32is arranged at a necessary distance apart from the signal electrode31. A buffer layer41which uses SiO2or the like, is formed between the surface of the electrode3and the substrate1in order to prevent the light propagated through the optical waveguide2from being absorbed by the electrode3.

The signal electrode31is made a traveling-wave electrode by grounding one end positioned at the bottom right inFIG. 1via a resistance (not shown in the figure), and an electric signal E of a high frequency corresponding to the modulation data is applied from another terminal positioned at the bottom left inFIG. 1. The distance between the signal electrode31and the ground electrode32(hereunder the electrode spacing) is greater than the electrode spacing in the forward modulation section1A, and the electrode spacing in the inverse modulation section1B is made relatively narrow. Furthermore, the cross-section shape of the signal electrode31is designed so that speed matching conditions of the light propagated through the optical waveguide2, and the electric signal E are satisfied. If the electrode spacing is made relatively narrow, the impedance drops. Therefore here, as shown by the cross-section inFIG. 2, the cross-section area of a signal electrode31B and a ground electrode32B in the inverse modulation section1B (at the bottom ofFIG. 2) are each made smaller than the cross-section of a signal electrode31A and a ground electrode32A in the forward modulation section1A (the top ofFIG. 2), so that a desired impedance (for example 50 Ω or the like) is obtained.

In the case where the cross-section of the electrode is changed in the forward modulation section1A and the inverse modulation section1B, it is necessary to pay attention to the connection with the outside. The connection with the outside is generally for example performed by bonding wire to an electrode pad provided on the substrate1. However at this time, bonding is easier if the electrode pad on the input side and the terminal side are the same shape. Furthermore, it is better to arrange each of the cross-sections in order to also easily match the impedance from the interactive portion to the electrode pad, on the input side and the terminal side. Consequently, inFIG. 1, it is preferable to design so that the cross-section of the signal electrode31from the input electrode pad which applies the electric signal E, up until the region A, and the cross-section of the signal electrode31which passes through the region B and extends to the terminal electrode pad are the same. Furthermore, in the signal electrode31of the boundary portion directed from the forward modulation section1A to the inverse modulation section1B, preferably the cross-section area is made gradually smaller, to suppress an increase in the series resistance.

Next is a description of the operation of the optical modulator of the first embodiment.

In the optical modulator of the aforementioned configuration, light Lin applied from the outside to the input waveguide21is branched into two by the branching section22and respectively sent to the branching waveguides23and24. To the branching waveguides23and24is applied an electric field generated between the signal electrode31and the ground electrode32corresponding to the electric signal E which travels on the signal electrode31, and due to the electro-optic effect of this electric field, the refractive index of the branching waveguides23and24changes. As a result, the phases of the respective beams propagated through the branching waveguides23and24are each changed.

At this time, the electric signal E input to the signal electrode31is attenuated while propagating through the signal electrode31, and the attenuation amount becomes greater at a high frequency. However, by forming the polarization inversed region11on the output side portion of the interaction section, and making the direction of modulation at the interaction section forward modulation in the input side region A, and inverse modulation in the output side region B, the intensity of the inverse modulation in the region B becomes greater at a low frequency, and is less at a high frequency. As a result, the modulation at a low frequency for the overall optical modulator is suppressed and the modulation bandwidth becomes wider.

Furthermore, in the present embodiment, by making the electrode spacing of the inverse modulation section1B (region B) narrow with respect to the electrode spacing of the forward modulation section1A (region A), the transmission loss of the high frequency component of the electrical signal E in the inverse modulation section1B due to the skin effect is intentionally increased. For example, in the case of an optical modulator of 10 Gb/s, it is good to increase the transmission loss of the electric signal E at a high frequency of 10 GHz or more. As a result, the inverse modulation in the high frequency band in the inverse modulation section1B is suppressed, and hence it is possible to realize an even wider modulation bandwidth. In addition, the spacing of the branching waveguides23and24also becomes relatively narrow corresponding to the electrode spacing of the inverse modulation section1B. Therefore, the situation where the effect of applying the electric field is reduced in the inverse modulation section1B is also avoided.

FIG. 3shows an example of change in electric field intensity in the regions A and B. Furthermore,FIG. 4shows an example of optical response characteristics of the present optical modulator.

As shown inFIG. 3, in the optical modulator, by making the electrode spacing of the region B relatively narrow, the electric field intensity corresponding to the high frequency component in the region B becomes generally greater at the boundary portion with the region A, and thereafter is attenuated considerably more than for the case of the conventional optical modulator (refer to the bottom ofFIG. 29). As a result, the intensity of inverse modulation at a high frequency in the region B is less than heretofore. On the other hand, the electric field intensity for a low frequency (here shown by the DC component) becomes greater over the whole of the region B by making the electrode spacing narrow. Therefore, the intensity of the inverse modulation at a low frequency is greater than heretofore. Consequently, as shown inFIG. 4, the modulation bandwidth of the optical modulator of the first embodiment is an even wider bandwidth than for the case of the conventional optical modulator with inverse modulation.

The modulation bandwidth of the present optical modulator becomes wider accompanying the increase in the proportion of the length of the inverse modulation section1B with respect to the overall length of the interactive portion. However, if the proportion of the length of the inverse modulation section1B becomes somewhat large, it is difficult for modulation at the low frequency. Therefore, it is necessary to optimally design the proportion of the length of the inverse modulation section1B to realize a wider band, taking into consideration the necessary degree of modulation.FIG. 5is an example showing a relation between the length of the inverse modulation section1B and error rate. The situation is apparent that when the inverse modulation section1B is short, the effect of widening the band cannot be sufficiently obtained, and the error rate worsens, while when the inverse modulation section1B is long, the degree of modulation becomes low, and hence the error rate worsens.

As described above, the respective beams which are propagated through the branching waveguides23and24of the forward modulation section1A and the inverse modulation section1B, and phase modulated, are multiplexed by the multiplexing section25, so that an intensity modulated signal Lout is output from the output waveguide26.

According to the first embodiment as described above, by making the electrode spacing of the inverse modulation section1B relatively narrow with respect to the electrode spacing of the forward modulation section1A, and increasing the transmission loss of the electrical signal of a high frequency in the inverse modulation section1B, it is possible to make the modulation bandwidth in the overall optical modulator even wider.

In the above described first embodiment, by making the electrode spacing in the inverse modulation section1B relatively narrow, the high frequency transmission loss is increased. However as shown for example in the cross-section ofFIG. 6, also by making the width of the signal electrode31B of the inverse modulation section1B relatively narrower than the width of the signal electrode31A of the forward modulation section1A, it is possible to increase the high frequency transmission loss. In this case also, a similar operational effect to the case of the first embodiment can be obtained.

Furthermore, in the first embodiment, an example was shown for preventing the drop in the impedance due to the relatively narrow electrode spacing, by respectively reducing the cross-sections of the signal electrode31B and the ground electrode32B of the inverse modulation section1B. However, as shown for example in the cross-section b-b′ ofFIG. 7, even if just the cross-section area of the ground electrode32B is reduced, but the cross-section area of the signal electrode31B is substantially the same as on the forward modulation section1A side, it is possible to realize a desired impedance. In this case, the problem of the increase in the series resistance in the boundary portion of the forward modulation section1A and the inverse modulation section1B does not arise.

Furthermore, in the first embodiment, the case was explained where the present invention was applied to the beforementioned conventional intensity modulator shown at the bottom ofFIG. 29. However, similar to this, the present invention is also applicable to the conventional phase modulator shown at the top ofFIG. 29. The configuration example for this case is shown inFIG. 8. Also in the configuration example ofFIG. 8, by making the electrode spacing of the inverse modulation section1B (region B) relatively narrower than the electrode spacing of the forward modulation section1A (region A), and increasing the transmission loss of the high frequency electric signal E in the inverse modulation section1B, a wider phase modulation bandwidth can be realized.

In addition, in the first embodiment, the example was shown for the case using a Z-cut substrate1. However also in the case where the optical modulator is configured using an X-cut substrate, then similar to the case of the first embodiment, by adapting a configuration where the electrode spacing of the inverse modulation section is made narrow, an even wider modulation bandwidth can be realized, and the present invention is thus effective.

Next is a description of a second embodiment of the present invention.

FIG. 9shows an enlarged cross-section of respective sections in an optical modulator according to the second embodiment of the present invention. The plan view showing the configuration of the overall optical modulator of the second embodiment is similar to the case of the first embodiment shown inFIG. 1. Therefore, illustration is omitted here.

The optical modulator of the second embodiment is one where, for a buffer layer41formed between the surface of the substrate1and the signal electrode and the ground electrode, so that the dielectric loss of a buffer layer41B (refer to the bottom ofFIG. 9) positioned on the inverse modulation section1B is greater than the dielectric loss of a buffer layer41A (refer to the top ofFIG. 9) positioned on the forward modulation section1A, the substance constituting the respective buffer layers41A and41B is made different. The configuration other than for the buffer layers41A and41B is the same as for the case of the first embodiment.

It is known that the dielectric loss of the buffer layer changes corresponding to the value of the dielectric loss tangent (tan δ) of the substance constituting the buffer layer. More specifically, the greater the value of tan δ, the greater the dielectric loss of the buffer layer. If the buffer layer dielectric loss becomes greater, the high frequency electric field intensity is reduced. Therefore in the region where such a buffer layer is formed, the modulation bandwidth becomes narrow.

FIG. 10is an example showing a relation of modulation bandwidth with respect to the value of tan δ of the substance constituting the buffer layer. In this manner, by configuring the buffer layer using a substance where the tan δ of the substance is large, it is possible to make the modulation intensity of the high frequency component in the region where the buffer layer is formed small. In this embodiment, by making the value of tan δ of the substance constituting the buffer layer41B on the inverse modulation section1B side greater than the value of tan δ of the substance constituting the buffer layer41A on the forward modulation section1A side, the inverse modulation intensity of the high frequency component in the inverse modulation section1B becomes smaller.

Consequently, according to the optical modulator of the second embodiment, similar to the case of the first embodiment, in addition to making the electrode spacing of the inverse modulation section1B narrow so that the propagation loss of the high frequency electric signal is increased, by increasing the dielectric loss of the buffer layer41B, it is possible to make the modulation bandwidth of the overall optical modulator even wider.

In the abovementioned second embodiment, the configuration example is shown where the dielectric loss of the buffer layer41B formed between the substrate1and the electrode3is increased. However as shown for example in the cross-section b-b′ at the top ofFIG. 11, even if instead of the buffer layer41B, a configuration is adopted where the surroundings of the signal electrode31B in the inverse modulation section1B, are covered by a substance42with a greater dielectric loss than the substrate1, or a configuration is adopted where as shown in the cross-section b-b′ at the bottom ofFIG. 11, a film42′ with a large dielectric loss is provided between the electrode3and the buffer layer41, it is possible to obtain a similar effect to the case of the second embodiment.

Furthermore, as an application example related to the second embodiment, even if the dielectric loss of the material constituting the signal electrode31B of the inverse modulation section1B is relatively large compared to the dielectric loss of the material constituting the signal electrode31A of the forward modulation section1A, it is possible to increase the bandwidth of the modulation bandwidth.

Moreover, in the abovementioned second embodiment, for the configuration of the first embodiment where the electrode spacing of the inverse modulation section1B is narrow, the example is shown where the dielectric loss of the buffer layer41B of the inverse modulation section1B is great. However even for the phase modulator shown inFIG. 8, or for a configuration where the width of the abovementioned signal electrode shown inFIG. 6is made narrow, by making the dielectric loss of the buffer layer of the inverse modulation section large, the modulation bandwidth can be made even wider.

In addition, in the abovementioned second embodiment, it has been considered to combine this with a configuration where the transmission loss of the high frequency electrical signal is increased by making the electrode spacing narrow. However, even if only the dielectric loss of the buffer layer of the inverse modulation section in the conventional optical modulator such as shown before inFIG. 29is made large, the conventional modulation bandwidth can be made large, and hence the present invention is effective.

Next is a description of a third embodiment of the present invention.

FIG. 12is a plan view showing a configuration of an optical modulator according to the third embodiment of the present invention. Moreover,FIG. 13is an enlarged view of section a-a′ and section b-b′ in the optical modulator ofFIG. 12.

InFIG. 12andFIG. 13, the point where the configuration of the optical modulator the third embodiment is significantly different to the configuration of the first embodiment shown inFIG. 1andFIG. 2is that regarding the distance (electrode spacing) between the signal electrode31and the ground electrode32, the electrode spacing in the inverse modulation section1B is relatively greater than the electrode spacing in the forward modulation section1A. Furthermore, since the impedance increases if the electrode spacing is relatively increased, then by making the sectional area of the signal electrode31B and the ground electrode32B in the inverse modulation section1B (the bottom ofFIG. 13) respectively greater than the sectional area of the signal electrode31A and the ground electrode32A in the forward modulation section1A, a desired impedance (for example 50 Ω or the like) is obtained. Furthermore, the spacing (waveguide spacing) of the branching waveguides23B and24B also becomes relatively wide corresponding to the electrode spacing of the inverse modulation section1B. Other configuration of the optical modulator other than the configuration described above is similar to the configuration of the first embodiment. Therefore description is omitted here.

In the optical modulator of the abovementioned configuration, by relatively widening the electrode spacing of the inverse modulation section1B, the high frequency component of the electric signal E propagated through the signal electrode31B is easily radiated to the substrate1or to the atmosphere, so that the loss is increased. By increasing the radiation loss of the high frequency electric signal E in the inverse modulation section1B, the inverse modulation in the high frequency band is suppressed. Therefore similarly to the case of the first embodiment, it is possible to realize an even wider modulation bandwidth. Furthermore, the spacing of the branching waveguides23B and24B also becomes relatively wide corresponding to the electrode spacing of the inverse modulation section1B. Therefore the situation where the effect of applying the electric field is reduced in the inverse modulation section1B is also avoided.

In the third embodiment, the example is shown for where the increase in the impedance due to making the electrode spacing relatively wider is prevented by making the sectional area of the signal electrode31B and the ground electrode32B of the inverse modulation section1B respectively greater. However, as shown for example in the cross-section b-b′ ofFIG. 14, even if only the sectional area of the ground electrode32B is made large, and the sectional area of the signal electrode31B is made the same as for the forward modulation section1A side, it is possible to realize a desired impedance.

Moreover, in the third embodiment, the case was explained where the present invention was applied to the beforementioned conventional intensity modulator shown at the bottom ofFIG. 29. However, similar to this, the present invention is also applicable to the conventional phase modulator shown at the top ofFIG. 29. The configuration example for this case is shown inFIG. 15. Also in the configuration example ofFIG. 15, by making the electrode spacing of the inverse modulation section1B relatively wider than the electrode spacing of the forward modulation section1A, and increasing the transmission loss of the high frequency electric signal E in the inverse modulation section1B, a wider phase modulation bandwidth can be realized.

Next is a description of a fourth embodiment of the present invention.

FIG. 16is a plan view showing a configuration of an optical modulator according to the fourth embodiment of the present invention.

InFIG. 16, the point where the configuration of the optical modulator of the fourth embodiment differs to the case of the first embodiment, is that pattering of the electrode3is changed so that instead of forming the polarization inversed region11on the substrate1, the signal electrode31is arranged above one branching waveguide23in the region A, and is arranged above the other branching waveguide24in the region B. Other configuration of the optical modulator other than for the above described point is similar to the configuration of the first embodiment. Therefore description is omitted here.

In the optical modulator of the above described configuration, the electrical signal E applied to one end of the signal electrode31propagates along the one branching waveguide23in the region A, and propagates along the other branching waveguide24in the region B. As a result, the direction of modulation in the region B is opposite to the direction of modulation in the region A, and hence the forward modulation section1A (region A), and the inverse modulation section1B (region B) are realized similarly to the case in the first embodiment where the polarization inversed region11is formed. Furthermore, in this embodiment however, since the electrode spacing of the forward modulation section1A, the propagation loss of the high frequency electrical signal in the inverse modulation section1B is increased, and the inverse modulation in the high frequency band is suppressed. Consequently an effect similar to the case of the first embodiment can be obtained, and also the step for forming the polarization inversed region on the substrate1becomes unnecessary so that inverse modulation in the region B can be realized by simply changing the pattern design of the electrode3. Therefore, the configuration of the optical modulator becomes even simpler.

In the abovementioned fourth embodiment, the pattern of the electrode3in the configuration of the first embodiment is changed. However it is possible to apply a configuration similar to the fourth embodiment even for the configurations of the other embodiments.

Next is a description of a fifth embodiment of the present invention.

FIG. 17is a plan view showing a configuration of an optical modulator according to the fifth embodiment of the present invention.

The optical modulator of the fifth embodiment is an application example of where a reduction in wavelength chirp in the optical modulator of, for example, the first embodiment is reduced. That is to say, in the optical modulator of the first embodiment, since the electric field intensity below the signal electrode31, and the electric field intensity below the ground electrode32are different, the phase change of the respective lights propagating through the pair of branching waveguides23and24become unbalanced, and wavelength chirp (wavelength fluctuations) may occur. In order to suppress this wavelength chirp, it is effective to use movement between the branching waveguides23and24of the signal electrode31and use polarization inversion, together.

More specifically, in the optical modulator of this embodiment, as shown for example inFIG. 17, polarization inversed regions11A and11B surrounded by the dotted lines in the figure, are respectively formed on the forward modulation section1A and the inverse modulation section1B. The polarization inversed region11A has a length in the propagation direction of the light of approximately half of the overall length L of the forward modulation section1A, and is arranged in the approximate center of the forward modulation section1A. Furthermore, the polarization inversed region11B has a length in the propagation direction of the light of ½ of the overall length L′ of the inverse modulation section1B, and is arranged in the approximate center of the inverse modulation section1B.

Moreover, the signal electrode31is patterned in a desired shape such that for the forward modulation section1A, in the polarization inversed region11A it passes above the branching waveguide23, and outside of the polarization inversed region11A it passes above the branching waveguide24. Furthermore for the inverse modulation section1B, in the polarization inversed region11B it passes above the branching waveguide24, and outside of the polarization inversed region11B it passes above the branching waveguide23. On the other hand, the ground electrode32is patterned in a desired shape such that for the forward modulation section1A, in the polarization inversed region11A it passes above the branching waveguide24, and outside of the polarization inversed region11A it passes above the branching waveguide23. Furthermore, for the inverse modulation section1B, in the polarization inversed region11B it passes above the branching waveguide23, and outside of the polarization inversed region11B it passes above the branching waveguide24. The distance (electrode spacing) between the signal electrode31and the ground electrode32, similarly to the case of the first embodiment, is designed so that the electrode spacing in the inverse modulation section1B is relatively narrower than the electrode spacing in the forward modulation section1A.

In the optical modulator of the above described configuration, if this is designed so that in the length direction (overall length L) of the forward modulation section1A, the length of the polarization inversed region11A and the length of the non inversed region are approximately the same, that is, so that the lengths of the respective non inversed regions positioned before and after the polarization inversed region11A become L/4, and the length of the polarization inversed region11A becomes L/2, the phase of the light propagating through the branching waveguide23of the forward modulation section1A changes by only θ23as shown in the following equation (1), and the phase of the light propagating through the branching waveguide24of the forward modulation section1A changes by only θ24as shown by the following equation (2).

θ23=(+Δ⁢⁢nS)·L/4+(+Δ⁢⁢nG)·L/2+(+Δ⁢⁢nS)·L/4=(Δ⁢⁢nS+Δ⁢⁢nG)·L/2(1)θ24=(-Δ⁢⁢nG)·L/4+(-Δ⁢⁢nS)·L/2+(-Δ⁢⁢nG)·L/4=-(Δ⁢⁢nS+Δ⁢⁢nG)·L/2(2)
where ΔnSis the refractive index variation of the branching waveguides positioned below the signal electrode31, and ΔnGis the refractive index variation of the branching waveguides positioned below the ground electrode32.

As is apparent from the above equation (1) and equation (2), the phase of the respective lights propagating through the branching waveguides23and24of the forward modulation section1A respectively change by only (+ΔnS)·L/2, and (−ΔnG)·L/2 in the non inversed region, and respectively change by only (+ΔnG)·L/2, and (−ΔnS)·L/2 in the polarization inversed region11A. Consequently, the phases of the respective lights which pass through the branching waveguides23and24and reach to the output end of the forward modulation section1A change by only +(ΔnS+ΔnG)·L/2, and −(ΔnS+ΔnG)·L/2, giving a phase modulation where signals with the same absolute values are inverted. Therefore, in the forward modulation section1A, wavelength chirp does not occur so that there is zero chirp.

Furthermore, also for the lengthwise direction (overall length L′) of the inverse modulation section1B, similar to the abovementioned forward modulation section1A, if this is designed so that the length of the polarization inversed region11B and the length of the non inversed region are approximately the same, the phase of the light propagating through the branching waveguide23of the inverse modulation section1B changes by only θ23′ as shown in the following equation (1)′, and the phase of the light propagating through the branching waveguide24of the inverse modulation section1B changes by only θ24′ as shown by the following equation (2)′.

As is apparent from the above equation (1)′ and equation (2)′, the phase of the respective lights propagating through the branching waveguides23and24of the inverse modulation section1B respectively change by only (+ΔnG)·L′/2, and (−ΔnS). L′/2 in the non inversed region, and respectively change by only (+ΔnS)·L′/2, and (−ΔnG)·L′/2 in the polarization inversed region11B. Consequently, the phases of the respective lights which pass through the branching waveguides23and24and reach to the output end of the inverse modulation section1B change by only +(ΔnS+ΔnG)·L′/2, and −(ΔnS+ΔnG)·L′/2, giving a phase modulation where signals with the same absolute values are inverted. Therefore, in the inverse modulation section1B also, wavelength chirp does not occur so that there is zero chirp.

In addition to realizing zero chirp in the forward modulation section1A and the inverse modulation section1B as described above, in the optical modulator, similar to the case of the fourth embodiment, since the electrode3is patterned so that the modulation direction in the region B positioned on the light output side becomes the opposite with respect to the modulation direction in the region A positioned on the light input side of the interaction section, the modulation band width is widened. That is to say, the electric signal E input to the signal electrode31is attenuated while propagating through the signal electrode31, and the attenuation amount becomes larger at high frequency. However, by making the modulation direction at the interactive portion, forward modulation in the region A on the input side, and inverse modulation in the region B on the output side, the intensity of the inverse modulation in the region B becomes large at low frequency, and becomes small at high frequency. As a result, the modulation at low frequency in the overall optical modulator is suppressed, and the bandwidth is widened.

Furthermore, in the optical modulator, by making the electrode spacing of the inverse modulation section1B relatively narrow with respect to the electrode spacing of the forward modulation section1A, the transmission loss of the high frequency component of the electric signal E in the region B is increased. As a result, the inverse modulation in the high frequency band in the inverse modulation section1B is suppressed, and hence it is possible to realize an even wider modulation bandwidth. In addition the spacing of the branching waveguides23and24also becomes relatively narrow corresponding to the electrode spacing of the inverse modulation section1B. Therefore the situation where the effect of applying the electric field is reduced in the inverse modulation section1B is also avoided.

Next is a description of a sixth embodiment of the present invention.

FIG. 18is a plan view showing a configuration of an optical modulator according to the sixth embodiment of the present invention.

InFIG. 18, the point where the configuration of this embodiment is different from the case of the fifth embodiment shown inFIG. 17is that the position of the polarization inversed region in the inverse modulation section1B is changed, so that the arrangement pattern of the signal electrode31is simplified. More specifically, a polarization inversed region11B1is formed in an interval from one end of the inverse modulation section1B contacting with the forward modulation section1A to a length of L′/4, and a polarization inversed region11B2of a length L′/4 is formed with a non inversed region of a length L′/2 sandwiched between the polarization inversed regions11B1and11B2. Furthermore, the pattern of the electrode3is changed so that in the respective polarization inversed regions11B1and11B2, the signal electrode31is arranged above the branching waveguide24, and the ground electrode32is arranged above the branching waveguide23, and in the non inversed region positioned between the polarization inversed regions11B1and11B2, the signal electrode31is arranged above the branching waveguide23, and the ground electrode32is arranged above the branching waveguide24.

According to the optical modulator of the above described configuration, a similar operational effect to the case of the fifth embodiment is obtained. Also it is not necessary to change over the arrangement pattern of the signal electrode31in the border portion of the forward modulation section1A and the inverse modulation section1B, from above the branching waveguide24to above the branching waveguide23, as with the case of the fifth embodiment. Hence, the back and forth frequency of the signal electrode31between the respective branching waveguides23and24in the overall optical modulator can be reduced from three times to two times. By making the arrangement pattern of the signal electrode31simpler, an improvement effect of the propagation characteristic (for example loss or reflection and the like) of the electric signal E can be expected, and hence it becomes possible to even further widen the band width of the modulation bandwidth.

Next is a description of a seventh embodiment of the present invention.

FIG. 19is a plan view showing a configuration of an optical modulator according to the seventh embodiment of the present invention.

InFIG. 19, the optical modulator of the seventh embodiment is one where for example in the configuration of the first embodiment, a non modulation section1C where there is no phase modulation in the light propagated through the respective branching waveguides23and24, is provided in a third region C between the forward modulation section1A (region A) and the inverse modulation section1B (region B). Other configuration apart from this non modulation section1C is similar to the case of the first embodiment. Therefore description is omitted here.

In the non modulation section1C, the branching waveguides23and24are formed in a similar condition to the forward modulation section1A, whereas the signal electrode31is arranged in a position separated from above the branching waveguide23(a position separated to the upper side from the branching waveguide23inFIG. 19). The distance (electrode spacing) of the signal electrode31and the ground electrode32in the non modulation section1C is similar to the electrode spacing in the inverse modulation section1B, and narrower than the electrode spacing in the forward modulation section1A.

In the case where the non modulation section1C is provided between the forward modulation section1A and the inverse modulation section1B, it is important to match the speed of the electrical signal and the light so that the modulation bandwidth is not worsened. Therefore, the electrical length of the signal electrode31in the non modulation section1C and the optical length of the branching waveguides23and24are made to approximately coincide.

In an optical modulator of the abovementioned configuration, the non modulation section1C provided between the forward modulation section1A and the inverse modulation section1B is a low pass filter, and in the electric signal E propagated through the signal electrode31, the high frequency component is attenuated from after passing through the forward modulation section1A until it reaches the inverse modulation section1B.FIG. 20shows an example of the change in the electric field intensity in the respective regions A through C. Due to the skin effect due to the electrode spacing of the region C being made relatively narrow, the electric field intensity corresponding to the high frequency component in the region C, becomes temporarily great at the boundary portion of the region A, and thereafter is significantly attenuated. Therefore, the electric field intensity of the high frequency in the input end of the region B becomes smaller than for the case of the first embodiment shown inFIG. 4. As a result, the intensity of the inverse modulation at the high frequency in the region B becomes even smaller than for the case of the first embodiment. On the other hand, regarding the electric field intensity of the low frequency (here shown as a DC component), due to the electrode spacing being narrow, this becomes large over the whole of the region C and the region B. Therefore the intensity of the inverse modulation at the low frequency is increased. Consequently, as shown inFIG. 21, the modulation bandwidth of the optical modulator of the seventh embodiment is even wider than for the case of the optical modulator of the first embodiment.

Next is a description of an eighth embodiment of the present invention.

FIG. 22is a plan view showing a configuration of an optical modulator according to the eight embodiment of the present invention.

InFIG. 22, the optical modulator of the eight embodiment, is a modified example where in the configuration of the seventh embodiment shown inFIG. 19, the position of the signal electrode31in the non modulation section1C is at the center of the pair of branching waveguides23and24. In such a configuration, in the non modulation section1C, an electric field of substantially the same size is applied to the branching waveguides23and24. Therefore, the phase of the light propagating through each is maintained, and as a result there is no modulation. Consequently, according to the optical modulator of the eighth embodiment, even in the case where for example due to limitations in space on the substrate1, it is difficult to have the situation as in the seventh embodiment where the signal electrode is away from above the branching waveguide, it is possible to achieve a similar operational effect to the case of the seventh embodiment.

In the configuration of the eight embodiment, in the case where the pattern of the optical waveguide2and the electrode3is misaligned, the electric field applied to the branching waveguides23and24in the non modulation section1C becomes different, so that there is the possibility of modulation being produced. In order to prevent such a situation, for example as shown inFIG. 23, the pattern of the optical waveguide may be modified so that the branching waveguides23and24in the non modulation section1C are both positioned below the ground electrode32. In this case, it is necessary to curve the branching waveguides23and24and widen the waveguide spacing. However a drop in the yield due to process error can be avoided.

Next is a description of a ninth embodiment of the present invention.

FIG. 24is a plan view showing a configuration of an optical modulator according to the ninth embodiment of the present invention. Furthermore,FIG. 25is an enlarged view of section a-a′ and section c-c′ in the optical modulator ofFIG. 24.

InFIG. 24andFIG. 25, the optical modulator of the ninth embodiment is one where, as a separate configuration for realizing the non modulation section1C provided between the forward modulation section1A and the inverse modulation section1B, a buffer layer41C in the non modulation section1C is made thicker than the buffer layer41A in the forward modulation section1A, and an electric field applied to branching waveguides23C and24C of the non modulation section1C is decreased so that modulation does not occur. As a result, a signal electrode31C of the non modulation section1C is arranged above the branching waveguide23similar to with the other regions.

Also in the optical modulator of the above described configuration, it is possible to obtain a similar operational effect to the case of the seventh and eighth embodiments.

In the ninth embodiment, modulation was eliminated by making the buffer layer41C of the non modulation section1C thick. However for example as shown in the section c-c′ at the top ofFIG. 26, in the non modulation section1C, by providing a film43different to the buffer layer41C between the buffer layer41C and the signal electrode31C and the ground electrode32, or as shown in the section c-c′ at the bottom ofFIG. 26, in the non modulation section1C, by providing a film43which is different to the buffer layer41between the buffer layer41C and the substrate1, it is possible to realize a condition where it is even more difficult to have modulation. For the aforementioned film43, a material may be used for which the dielectric loss at a high frequency is greater than that of the buffer layer41C. In relation to applying either the configuration at the top or at the bottom ofFIG. 26, the film43and the buffer layer41C may be appropriately determined by considering the adhesion between these and the electrode3and the substrate1.

Next is a description of an example of an optical transmitter which uses any of the aforementioned optical modulators of the first through ninth embodiments.

FIG. 27is a block diagram showing a configuration of an embodiment of the optical transmitter.

InFIG. 27, an optical transmitter50comprises a light source (LD)51which generates continuous light, an LD control circuit52which controls the drive state of the light source51, an optical modulator53to which continuous light Lin output from the light source51is inputted, a signal multiplexing circuit54which multiplexes a plurality of data signals and generates a modulating signal with a high bit rate, and a driver circuit55which drives the optical modulator53in accordance with a modulation signal output from the signal multiplexing circuit54.

As the optical modulator53which is installed in the optical transmitter50of such a configuration, by applying any one of the above described optical modulators of the first through ninth embodiments, it is possible to transmit an optical signal Lout having a wider band, so that a characteristic improvement in error rate or the like on the receiving side can be achieved.