JP2008009314A - Optical waveguide element, optical modulator, and optical communication device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To widely design or adjust control of a light chirp quantity. <P>SOLUTION: The optical waveguide element includes: two phase shift waveguides formed near the surface of an optical substrate; a branch waveguide connected to an input side of the two phase shift waveguides; and a merging waveguide connected to an output side of the two phase shift waveguides. The element is formed near the phase shift waveguide, and includes an electrode pair for applying an electric field to the phase shift waveguide, for varying an optical refractive index in at least the phase shift waveguide. In particular, the element unequalizes a merging ratio of optical energy in the branch waveguide or a merging ratio in the merging waveguide when absolute values of respective refractive indexes in the two phase shift waveguide are not coincident with each other. Accordingly, the chirp quantity of the optical signal output from the merging optical waveguide is controlled. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention generally relates to an optical waveguide element that can be used in the fields of optical communication, optical information processing, optical signal processing, or an optical sensor, and more particularly, an optical waveguide that performs modulation using an optical waveguide provided in a substrate. Type optical modulator.

  It is exploding in the information society including the Internet. As a result, there is an increasing need for high-speed and large-capacity optical communication systems, which are one of the key elements for building social infrastructure. An optical signal of an optical communication system with a low transmission distance and a short transmission distance is obtained by directly modulating an injection current of a semiconductor laser or a light emitting diode. This is called direct modulation. However, in this direct modulation, high-speed modulation of several GHz or more is difficult due to effects such as relaxation oscillation. Further, since wavelength fluctuations occur, it is difficult to apply direct modulation to long-distance transmission.

As means for solving this, there is an external optical modulator (Patent Document 1). An external optical modulator (hereinafter referred to as an optical modulator) modulates light having a constant intensity output from a semiconductor laser. As an optical modulator used in this method, there is a waveguide type optical modulator having an optical waveguide formed in a substrate. A waveguide-type optical modulator is generally small, suitable for mass production, and capable of high-speed operation.
JP 2003-057616 A

In particular, when a light modulator is formed using a ferroelectric material such as lithium niobate (LiNbO ₃ ) crystal, a low-loss and high-efficiency light modulator can be obtained.

FIG. 1 shows a Mach-Zehnder type optical modulator related to the present invention. In FIG. 1, 1 is a lithium niobate (LiNbO ₃ ) crystal substrate.

The input optical waveguide 2 is formed in a band shape on the crystal substrate 1. When titanium (Ti) is thermally diffused on the crystal substrate 1, the refractive index of the waveguide portion becomes relatively larger than that of the substrate 1. Utilizing this, the input optical waveguide 2 is formed. Incidentally, the refractive index of lithium niobate (LiNbO ₃ ) is about 2.14, but when titanium (Ti) is diffused, the refractive index increases by about 0.2% to about 2.144.

Thus, a good optical waveguide is formed by utilizing the difference in refractive index between the lithium niobate (LiNbO ₃ ) portion and the titanium (Ti) diffused portion. Of course, other waveguides are formed in the same manner.

  Reference numerals 4 a and 4 b denote phase shift optical waveguides having a length of about 4 mm to 80 mm branched from the input optical waveguide 2. Reference numeral 5 denotes an output optical waveguide connected to the emission side of the phase shift optical waveguides 4a and 4b. These constitute a branch interference section.

  Next, the electrode part will be described. FIG. 1 shows a traveling wave electrode 9 as an example of an electrode portion. The traveling wave electrode 9 for modulation is formed by pairing a signal electrode 9a and a ground electrode 9b on the optical waveguide 4a and the optical waveguide 4b, respectively.

  Here, the outgoing end of the traveling wave electrode 9 is terminated with a resistor R (eg, 50Ω) close to the line impedance. An electric signal s (t) for modulation is input to the incident end of the traveling wave electrode 9.

  In FIG. 1, the incident light 10 from the input optical waveguide 2 is divided in energy at a Y-shaped branch. The divided light energy passes through the phase shift optical waveguides 4 a and 4 b, rejoins at the Y-shaped joining portion, and reaches the output optical waveguide 5.

  At this time, if the light that has passed through the phase shift optical waveguides 4a and 4b joins in the same phase (phase difference 0 °), the loss becomes small, and the output light 11 becomes the maximum light amount. On the other hand, when the lights that have passed through the phase shift optical waveguides 4a and 4b are out of phase with each other (phase difference 180 °), a large loss occurs at the merged portion, so the light quantity of the output light 11 is minimized.

  Therefore, the refractive indexes of the optical waveguides 4a and 4b under the electrodes are changed in accordance with the magnitude of the voltage applied to the modulation electrode 9. Then, since the phase of the light passing there changes, a modulated optical signal is obtained as the output light 11.

  In FIG. 1, the impedance of the modulation electrode 9 is brought close to the impedance of the input electric signal line (usually 50Ω). That is, the traveling wave type electrode is formed by terminating the electrode 9 with a resistance R close to its impedance, thereby achieving a broad band.

The refractive index of the optical waveguide changes corresponding to the strength of the electric field (electro-optic effect). In particular, when z-plate lithium niobate (LiNbO ₃ ) as shown in FIG. 1 is used, the refractive index efficiently changes depending on the electric field component perpendicular to the substrate surface. FIG. 2 is a cross-sectional view of the optical waveguide element shown in FIG. 1 taken along the line AA ′. In the figure, the direction of the electric field (arrow) and the strength (density of electric lines of force) are also shown. Reference numeral 14 in the drawing denotes a buffer layer made of a film body that is optically transparent and has a refractive index smaller than that of the optical waveguide. It is provided mainly for the purpose of preventing light absorption by the electrodes and achieving speed matching between light and electric signals.

  Reference numeral 13 denotes an etching groove provided in the vicinity of the waveguide. This is provided for the purpose of concentrating the electric field on the waveguide or matching the speed of light and electric signals.

As apparent from FIG. 2, the direction of the electric field is opposite and the strengths of the optical waveguide 4a and the optical waveguide 4b immediately below the signal electrode 9a and the ground electrode 9b are different. For this reason, the amount of change in refractive index corresponding to the electric field (Δn _4a and Δn _4b ) has a different sign, and also has an absolute value.

Here, if the absolute values of the refractive index changes (Δn _4a and Δn _4b ) of the optical waveguide 4a and the optical waveguide 4b are different, the wavelength of the combined output light slightly varies. That is, so-called wavelength chirp occurs. Since this wavelength chirp has a great influence on the optical transmission characteristics, it is one of the important parameters when designing and constructing a current optical communication system. For this reason, it is required to design and create this amount of wavelength chirp according to the requirements of the transmission line. Incidentally, in the structure of FIG. 1, for example, Δn _4a is five times Δn _4b , so there is a large difference between the two. In this case, the α parameter that is an index of the chirp amount is 0.7.

In current optical communication systems, there are various requirements for the α parameter, and α = 0 is often required. However, in the configuration of FIG. 1, at least α cannot be 0 (| Δn _4a | = | Δn _4b |).

  As an example of a structure for realizing a chirp 0 optical modulator, there is a related technique shown in FIG. In this structure, since the corresponding optical waveguide 4a is moved away from the signal electrode 9a having a high electric field strength, the centers of both are offset. Thereby, each absolute value of the electric field strength received by the optical waveguide 4a is equal to the electric field strength received by the optical waveguide 4b.

  However, with this method, the electric field intensity itself received by the optical waveguide 4 is weakened. In order to compensate for the reduced electric field strength, it is necessary to apply a drive voltage three times that in the case of FIG. Therefore, this method is not realistic.

  As shown in FIG. 4, when the signal electrode 9a and the ground electrode 9b are formed to have substantially the same thickness, the electric field can be concentrated under the ground electrode 9b. Therefore, the absolute value of the amount of change in the refractive index under both electrodes can be made closer, and α = 0.2 can be achieved. However, even in this case, 0 cannot be achieved substantially.

  FIG. 5 shows an example of a chirp 0 modulator according to another related art. The substrate 1 is divided into two regions by broken lines in the figure. The left region (the surface is the + c plane) and the right region (the surface is the −c plane) are made so that the polarization direction as the ferroelectric (the z-axis (c-axis) direction of the crystal) is reversed. (Polarization reversal).

  In this case, even if the same electric field is applied, the amount of change in the refractive index generated in each region is positive and negative, but the magnitude is equal. For this reason, in the optical waveguide device shown in FIG. 5, the length of this region (the length of the device in the light propagation direction) is selected to a desired value. As a result, the amount of change in the refractive index of the optical waveguide 4a and the optical waveguide 4b can be made equal to the total in the two polarization regions with the opposite signs. That is, a chirp 0 modulator can be realized.

  The length of the domain-inverted region of the modulator is designed in consideration of the attenuation of the electric signal propagating through the electrode, but the attenuation constant has frequency dependence. Therefore, it is difficult to design a modulator that realizes chirp 0 over a wide range of frequencies. Further, in this case, it is necessary to add a polarization inversion process in addition to the normal manufacturing process, and there is a problem that the manufacturing cost increases.

  Future optical modulators will not only be able to realize a wide range of chirp amounts, including chirp 0, but will also be required to realize high performance such as a low driving voltage and a wide band, and also to realize low cost. However, in particular, it is difficult to realize a modulator with a chirp amount of 0, and an optical modulator that can satisfy other requirements in a well-balanced manner has not yet been realized.

  Therefore, an object of the present invention is to solve at least one of such problems and other problems. Other issues can be understood throughout the specification.

In order to solve the above problems, the present invention
An optical substrate having an electro-optic effect;
Two phase shift waveguides formed near the surface of the optical substrate;
A branching waveguide connected to the input side of the two phase-shifting waveguides;
A merging waveguide connected to an output side of the two phase-shifting waveguides;
An electrode pair formed in the vicinity of the phase shift waveguide and for applying an electric field to the phase shift waveguide in order to change at least the optical refractive index of the phase shift waveguide.

  When the absolute values of the amounts of change in the refractive indexes of the two phase shift waveguides do not match, the optical energy branching ratio in the branching waveguide or the converging ratio in the converging waveguide is made unequal. . As a result, an optical waveguide element characterized by controlling the chirp amount of the optical signal output from the merging waveguide is provided. Such an optical waveguide element may be applied to an optical modulator, an optical communication device, or the like.

  According to the present invention, it is possible to design or adjust a wide range of optical chirp control by weighting each optical power propagating through a plurality of phase shift waveguides. This makes it easier to realize a modulator with a small chirp amount and to satisfy other requirements in a well-balanced manner as compared with a conventional design and adjustment method mainly using an electrode structure.

  Several embodiments of the present invention will be described below with reference to the drawings.

FIG. 6 is a perspective view showing an example of the optical waveguide device according to the embodiment. Reference numeral 1 denotes an optical substrate having an electro-optic effect made of a lithium niobate (LiNbO ₃ ) crystal. The upper surface of the substrate 1 is the −c plane (−z axis) of the crystal. An optical waveguide is formed in the vicinity of the surface of the substrate 1 in the substantially y-axis direction.

  Reference numeral 7 denotes a branching waveguide that branches incident light at a desired branching ratio. In FIG. 6, a two-input two-output X-type waveguide is shown. The branching waveguide 7 has input waveguides 2a and 2b and output waveguides 3a and 3b.

  Reference numeral 8 denotes a merging waveguide that merges incident light at a desired merging ratio. In FIG. 6, a two-input two-output X-type waveguide is shown. The merging waveguide 8 has input waveguides 6a and 6b and output waveguides 5a and 5b. The branching waveguide 7 and the merging waveguide 8 are sometimes called merging / branching elements.

  In the case of a normal Mach-Zehnder type waveguide, the merging / branching element is a 3 dB coupler, that is, has a function of branching incident light into an equal amount of 1: 1 or merging an equal amount of light. However, the joining / branching element of the present invention is an element that branches or joins light at a ratio of c: d (c <d).

  Reference numerals 4a and 4b denote optical waveguides for phase shift formed near the surface of the optical substrate. A branching waveguide 7 is connected to the input side of the two phase shift waveguides 4a and 4b, and a merging waveguide 8 is connected to the output side thereof.

  In the vicinity of the phase shift waveguides 4a and 4b, electrode pairs 9a and 9b for applying an electric field to the phase shift waveguide are formed. The electrode pairs 9a and 9b that function in the form shown in the figure are generally called traveling wave electrodes. When an electrical signal is applied to the electrode pair 9a, 9b, the refractive index in the waveguide changes due to the electro-optic effect, causing a phase shift in the optical signal passing through the waveguide.

Let Δn _4a and Δn _4b be the amounts of change in the respective refractive indexes in the optical waveguides 4a and 4b for phase shift. In this case, when an experiment was performed on the optical waveguide device shown in FIG.
| Δn _4a |: | Δn _4b | = 3: 2
It was found that this relationship was satisfied.

In this case, the branching ratio of the merge branch element
c: d = 2: 3
Then, it is estimated that the difference in refractive index change amount can be offset by the difference in optical power. Actually, when the output light 11 from the output waveguide 5a of the merging waveguide 8 is evaluated by setting the branching ratio c: d of the merging / branching element to 2: 3, the chirp amount α of the output light 11 is almost equal. It was confirmed that it was zero.

  As can be seen from this example, it can be said that zero chirp can be realized when the following expression is substantially satisfied.

| Δn _4a | × c = | Δn _4b | × d
This means that the branching ratio in the branching waveguide or the merging ratio in the merging waveguide is determined based on the reciprocal of the optical loss in the two phase shifting waveguides.

For comparison, in the optical waveguide element shown in FIG.
c: d = 1: 1
As a result of evaluating the chirp amount α,
α = 0.2
It became. That is, the chirp amount α can be freely changed by appropriately adjusting the branching ratio and the merge ratio. In particular, if the above-described conditions are satisfied, it can be said that a zero-chirp optical waveguide device can be realized. Note that 0 chirp does not mean that α is completely zero, but means that the value of α is substantially zero, such as −0.02 <α <0.02. .

  In addition, since both the electrodes 9a and 9b of the optical waveguide device according to the present embodiment are thin, the electric field is efficiently concentrated under the both electrodes. Needless to say, an optical waveguide for phase shift is provided under both electrodes. Therefore, the driving voltage can be reduced by about 40% compared to the related art shown in FIG. Therefore, the present invention is advantageous with respect to the driving voltage.

  FIG. 7 is a diagram illustrating the relationship between the applied voltage and the output light 11 in the optical waveguide device according to the embodiment. As shown in the figure, it can be seen that the minimum value of the optical power is not zero. Of course, it must be desirable that the minimum value be zero. However, even in the case of c: d = 2: 3, the extinction ratio (ratio between the maximum value and the minimum value of the output light intensity) was −17 dB. With such an extinction ratio, it can be said that there is no problem in the end because it is a sufficiently small value in a normal optical communication system.

  As described above, even if a considerably unbalanced branching ratio is employed, the extinction ratio can be reduced, and the structure of the optical waveguide device according to this embodiment can be said to be widely effective.

In general, a zero chirp type optical waveguide device is required to achieve both zero chirp and a good extinction ratio. When the refractive index change amounts (Δn _4a and Δn _4b , | Δn _4a |> | Δn _4b |) of the optical waveguides 4a and 4b that are preferable from this viewpoint are examined,
1> | Δn _4b | / | Δn _4a |> 0.5
It was found that the optical waveguide and the electrode should be configured to satisfy the above. That is, if such conditions are satisfied, it can be confirmed that the chirp 0 can be achieved only by selecting the branching ratio of the optical power more suitably, and the extinction ratio can be -13 dB or less at worst.

  As shown in FIG. 6, in the optical waveguide element that gives such a change in refractive index, the portions along the waveguides of the electrodes 4a and 4b are substantially line symmetric, and the corresponding phase shift waveguides It was also confirmed that it was realized when it had a width substantially the same as the width.

  Similarly, when the output light 12 from the output waveguide 5b was examined with the configuration shown in FIG. 6, it was found that a good extinction ratio (modulation characteristic) as shown in FIG. 8 could be realized. When the wavelength chirp characteristics of the output light 12 were also examined, the effect of controlling the wavelength chirp by weighting the optical power as seen in the output light 11 was not obtained.

  From this result, it can be seen that the wavelength chirp control effect caused by the mismatch of the power of each light propagating through the two phase-shifting optical waveguides cannot be expected for any output light. That is, only the output light from a specific output waveguide among a plurality of output waveguides can obtain the effect of controlling the wavelength chirp. In FIG. 6, phase adjustment (phase adjustment is achieved by applying a voltage to the electrodes) applied to two unequal light beams incident from the input waveguides 6 a and 6 b of the merging waveguide 8. ), It can be seen that light can be concentrated and emitted to one output waveguide 5a with low loss. In FIG. 7, the maximum value of the light intensity for the output light 11 is 1, indicating that there is no loss.

  As described above, the effect of the present invention can be expected when two lights having unbalanced powers are combined and emitted. On the other hand, regarding the power of the output light 12 (FIG. 8), since the maximum value does not become 1, it can be said that a loss has occurred. The effect of the present invention cannot be expected for such output light.

  FIG. 9 is a diagram for explaining the concept of weighting of optical power according to the embodiment. FIG. 9A is an equivalent view of the optical waveguide element shown in FIG. The same reference numerals are given. For convenience of explanation, the power of light input to the input waveguide 2a of the optical waveguide element is assumed to be 1 here.

  As shown in FIG. 9B, in the branching waveguide 7, the power of the input light is branched to α: 1−α (α> 0.5). If the loss is ignored, light having a power of α is output from the output waveguide 3a, and light having a power of 1-α is output from the output waveguide 3b.

  As shown in FIG. 9C, light having a power of α is input to the input waveguide 6 a of the merging waveguide 8. If the branching ratio (merging ratio) of the merging waveguide 8 is α: 1−α (α> 0.5), the output waveguide 5a outputs light having a power of α · α, From the output waveguide 5b, light having a power of α · (1-α) is output.

  On the other hand, as shown in FIG. 9D, light having a power of 1−α is input to the input waveguide 6 b of the merging waveguide 8. Here, the branching ratio is 1−α: α due to the symmetry of the optical coupler that forms the merging waveguide 8. Therefore, light having a power of (1-α) · (1-α) is output from the output waveguide 5a, and light having a power of (1-α) · α is output from the output waveguide 5b. .

  Finally, as shown in FIG. 9E, from the output waveguide 5a, an optical component with power α · α and an optical component with power (1-α) · (1-α) are obtained. The synthesized light is output in consideration of each phase. Further, the output waveguide 5b outputs light synthesized by taking into account the phases of the light component with power α · (1-α) and the light component with power (1-α) · α. Will be. These phases are appropriately changed in the phase shift waveguides 4a and 4b.

  Here, it should be noted that in the output light from the output waveguide 5a, two light components having different maximum powers are added in consideration of the phase, but in the output light from the output waveguide 5b, the maximum power Two equal light components are added in consideration of the phase. Therefore, the former is effective in that the light quantity can be weighted, but the minimum value of the output light cannot be made zero. In the latter case, since the maximum powers of the two light components are equal, if the phase is shifted by 180 degrees, there is an advantage that the power of the output light becomes 0, but weighting of the light amount cannot be realized. From the viewpoint of the present invention, it goes without saying that the former is effective.

  In other words, the merging waveguide 8 is phase-adjusted to be incident on the merging waveguide 8, and two light components with non-uniform power are merged with low loss from a single output waveguide. It is desirable to have a function of emitting light. With this function, the control of the amount of optical chirp can be designed or adjusted widely. Furthermore, it can be said that a modulator with a small amount of chirp can be easily realized and other requirements can be satisfied in a well-balanced manner as compared with a conventional design and adjustment method based on an electrode structure.

  FIG. 10 is a diagram illustrating an example of an optical waveguide device according to an embodiment in which measures against induced current are taken. The traveling wave electrodes 9a and 9b shown in FIG. With this structure, if the length of the electrode in the y-axis direction (light propagation direction) in the figure is increased, an induced current tends to flow through the electrode, and the frequency characteristics may be deteriorated.

  As a countermeasure against this, as shown in FIG. 10, the frequency characteristics can be improved by strengthening the earth by using the earth electrode 9b as a ladder.

  That is, the ladder-shaped ground electrode 9b includes a first electrode portion and a second electrode portion. The first electrode portion is formed along at least one of the two phase shift waveguides. The second electrode portion is electrically connected to the first electrode portion, is formed at a position on the substrate farther from the waveguide than the first electrode portion, and is wider than the width of the first electrode portion. It is wide. According to FIG. 10, the first electrode portion is provided above the waveguide 4b. Three etching grooves are provided between the first electrode portion and the second electrode portion. Between the three etching grooves, four electrode plates for connecting the first electrode portion and the second electrode portion are provided.

  The influence of the ladder electrode 9b on the wavelength chirp was also examined. The interval between the ladders was 1 mm, and the electrode width was about 10 μm. The distance from the thin first electrode on the waveguide 4b to the pad-like second electrode was 300 μm. In this way, it has been found that the influence on the chirp amount α can be limited to a change of 0.05 or less compared to the configuration of FIG.

Here, in the configuration shown in FIG.
c: d = 3: 2
As a result, the amount of chirp is
α = 0.4
It became. In the case of c: d = 1: 1, the chirp amount α was 0.2, but it was confirmed that the chirp amount was larger than that.

  That is, the technical idea of the present invention can be used not only for realizing zero chirp but also for designing and adjusting the chirp amount widely.

  FIG. 11 is a diagram illustrating an example of an optical waveguide device according to another embodiment. The branching optical waveguide 7 of this optical waveguide element has a function of branching incident light into equal amounts (one to one). As described above, there is a difference in propagation loss between the phase shift optical waveguides 4a and 4b.

  Immediately before entering the rearward merge branching element 8, the optical waveguide elements are formed so as to have c: d optical power, respectively. In this case, the merging / branching element 8 is formed so that the light is branched into the waveguides 6a and 6b at a branching ratio of c: d when light enters from the waveguide 5a. That is, the merge ratio of the waveguides 6a and 6b in the merge branching element 8 is c: d. Even if such a configuration is adopted, the same wavelength chirp control function as when the optical power is weighted by the front branch optical waveguide 7 with respect to the wavelength chirp can be realized.

  In this case, as shown in FIG. 11, if the etching groove 13 provided in the substrate 1 is provided close to the waveguide 4a, a large loss is generated in the optical signal passing through the waveguide 4a. Can do. That is, the weighting of the optical power can also be realized by changing the distance between the waveguide and the groove. In addition, the loss can be adjusted by introducing a waveguide having a curvature as the waveguides 4a and 4b.

  Various configurations can be considered as the branching / merging optical waveguide applicable to the present embodiment. For example, as a two-input / two-output type branching / merging optical waveguide, there is a directional coupler having a gap at the center as shown in FIG. There is also a directional coupler with a gap length of 0 as shown in FIG. Furthermore, there is an asymmetric X-type directional coupler as shown in FIG.

  As the one-input two-output type or two-input one-output type branching / merging optical waveguide, an asymmetric Y-branch type directional coupler as shown in FIG. 15 or an asymmetric directional coupler type as shown in FIG. There are also.

  Any directional coupler can be used to achieve different branching and merging ratios. Further, different types may be selected as the front and rear branching / merging optical waveguides as long as they are designed and created to have the same branching ratio. For example, the front stage may be a two-input / two-output type branching / merging optical waveguide, and the rear stage may be a one-input / two-output type branching / merging optical waveguide. Conversely, the latter stage may be a two-input / two-output type branching / merging optical waveguide, and the former stage may be a one-input / two-output type branching / merging optical waveguide. Of course, both may be of the 1-input 2-output type.

The foregoing has been described with a focus on lithium niobate (LiNbO ₃ ) waveguide type optical elements, but the present invention can also be effectively applied to waveguide type optical elements having other electro-optic effects.

  Further, by adjusting the ratio of the refractive index change amount in the first phase shift waveguide 4a and the refractive index change amount in the second phase shift waveguide 4b of the two phase shift waveguides. Needless to say, the amount of chirp can be further controlled. In other words, in addition to the method of weighting the optical power by the branching ratio or the merge ratio, it can be combined with the method of adjusting the ratio of the refractive index change by changing the size or position of the electrode, phase shift waveguide or groove. In this case, the chirp amount can be controlled appropriately.

  Needless to say, the optical waveguide device according to the present invention can be applied as an optical modulator. Furthermore, it goes without saying that an optical communication apparatus including the light modulator together with a light source that outputs a light beam such as a semiconductor laser can be realized.

It is a perspective view of the optical waveguide device concerning related technology. It is sectional drawing of the optical waveguide element which concerns on related technology. It is sectional drawing of the optical waveguide element for implement | achieving 0 chirp which concerns on related technology. It is sectional drawing of the optical waveguide element with a small wavelength chirp which concerns on related technology. It is a perspective view of the optical waveguide device using the polarization inversion technique which concerns on related technology. It is a perspective view of the optical waveguide device concerning an embodiment. It is a figure which shows the relationship between the output light intensity of the optical waveguide element which concerns on embodiment, and an applied voltage. It is a figure which shows the relationship between the output light discarded from the optical waveguide element which concerns on embodiment, and an applied voltage. It is a figure for demonstrating the concept of the weighting of the optical power which concerns on embodiment. It is a perspective view of the optical waveguide device which improved the frequency characteristic concerning an embodiment. It is a perspective view of an optical waveguide device realized using optical loss concerning an embodiment. 1 is a plan view of a directional coupler type waveguide branching / merging element that can be used in an optical waveguide element according to an embodiment. It is a top view of the 0-gap directional coupler which can be used for the optical waveguide element which concerns on embodiment. It is a top view of the asymmetric x-type waveguide which can be used for the optical waveguide device concerning an embodiment. It is a top view of the asymmetric Y branch type waveguide which can be used for the optical waveguide device concerning an embodiment. It is a top view of the asymmetrical directional coupler which can be used for the optical waveguide element which concerns on embodiment.

Explanation of symbols

1 ... lithium niobate (LiNbO ₃₎ substrate 2-6 ... input waveguide 7,8 ... branching element 9 ... traveling wave electrodes 10 ... input light 11, 12 output light 13 ... etching groove 14 provided on the substrate ... Buffer layer film

Claims (13)

An optical substrate having an electro-optic effect;
Two phase shift waveguides formed near the surface of the optical substrate;
A branching waveguide connected to the input side of the two phase-shifting waveguides;
A merging waveguide connected to an output side of the two phase-shifting waveguides;
An electrode pair formed in the vicinity of the phase shift waveguide and for applying an electric field to the phase shift waveguide in order to change at least the optical refractive index in the phase shift waveguide;
When the absolute values of the amounts of change in the refractive indexes of the two phase shift waveguides do not match, the split ratio of the optical energy in the branch waveguide or the merge ratio in the merge waveguide is made unequal. Thus, an optical waveguide element characterized in that the chirp amount of the optical signal output from the merging waveguide is controlled.   2. The optical waveguide device according to claim 1, wherein the branching waveguide is a one-input two-output or two-input two-output waveguide.   3. The optical waveguide device according to claim 1, wherein the merging waveguide is a two-input two-output or two-input one-output waveguide.   The merging waveguide has a function of merging two light beams of non-uniform power that are phase-adjusted and incident on the merging waveguide with low loss and emitting them from a single output waveguide. The optical waveguide device according to claim 3, wherein   The branching ratio in the branching waveguide or the joining ratio in the joining waveguide is determined based on the reciprocal of the optical loss in the two phase-shifting waveguides. Optical waveguide element.   6. The difference in optical loss between the two phase shift waveguides is realized by asymmetry of a plurality of etching grooves formed in the vicinity of the two phase shift waveguides. The optical waveguide device described. A change amount of each refractive index in the two phase shift waveguides (Δn _4a, Δn _4b , | Δn _4a |> | Δn _4b |, respectively) and the two phase shift waveguides are propagated. The relationship with the optical power (P _4a , P _4b , P _4a <P _4b respectively) is
| Δn _4a | × P _4a = | Δn _4b | × P4a
The optical waveguide device according to claim 6, wherein: About the amount of change of each refractive index in the two phase shift waveguides (assuming Δn _4a and Δn _4b , | Δn _4a |> | Δn _4b |, respectively).
1.0> | Δn _4b | / | Δn _4a |> 0.5
The optical waveguide device according to claim 1, wherein the optical waveguide device is configured to satisfy the following.   9. The optical waveguide according to claim 1, wherein the electrode pair includes a plurality of electrode plates that are substantially symmetrical and have substantially the same width as the width of the phase-shifting waveguide. element.   The electrode on the ground side of the electrode pair is electrically connected to the first electrode portion formed along at least one of the two phase shift waveguides and the first electrode portion. And a second electrode part that is formed at a position on the substrate farther from the waveguide than the first electrode part and wider than the width of the first electrode part. Item 10. The optical waveguide device according to any one of Items 1 to 9.   By adjusting the ratio of the refractive index change amount in the first phase shift waveguide and the refractive index change amount in the second phase shift waveguide of the two phase shift waveguides, the chirp amount is further increased. The optical waveguide device according to claim 1, wherein the optical waveguide device is controlled.   An optical modulator comprising the optical waveguide device according to claim 1. A light source that outputs a light beam;
An optical communication device comprising: the optical modulator according to claim 12, wherein the optical modulator modulates a light beam from the light source.

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