JP2014123032A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator having a low DC bias voltage.SOLUTION: The optical modulator includes at least two optical waveguides formed in a substrate, a travelling wave electrode for a high-frequency electric signal where a high-frequency electric signal for modulating light propagates, and a bias electrode for applying a bias voltage to the light. The optical waveguide includes a high-frequency electric signal interactive part for modulating a phase of light by application of the high-frequency electric signal to the travelling wave electrode, and a DC bias interactive part for controlling a phase of light by application of a bias voltage to the bias electrode. The bias electrode comprises: a first bias electrode and a second electrode, where the above two optical waveguides are present below the electrodes; and at least another bias electrode where the two optical waveguides are not present below the electrode. A gap between the first bias electrode and the second bias electrode is narrower than a gap between the first bias electrode or the second bias electrode and the other bias electrode.

Description

  The present invention relates to an optical modulator that uses electro-optic effects to modulate light incident on an optical waveguide with a high-frequency electrical signal and emit it as an optical signal pulse, and has a small DC bias voltage together with an RF drive voltage.

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

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

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

(Conventional technology)
As a prior art optical modulator, a so-called ridge type LN optical modulator constructed using a z-cut LN substrate disclosed in Patent Document 1 is taken, and a schematic top view thereof is shown in FIG. 4A and 4B are schematic cross-sectional views taken along lines AA ′ and BB ′ in FIG.

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

An SiO ₂ buffer layer 2 is formed on the upper surface of the optical waveguide 3, and a traveling wave electrode 4 is formed on the upper surface of the SiO ₂ buffer layer 2. As the traveling wave electrode 4, a coplanar waveguide (CPW) having one central conductor 4a and two ground conductors 4b and 4c is used. Normally, the traveling wave electrode 4 is formed of Au, which is a noble metal material. The width of the center conductor 4a takes various values, but in many cases is about 7 μm, and the gap between the center conductor 4a and the ground conductors 4b and 4c also takes various values, but is often about 15 μm. In order to simplify the explanation, the present specification omits the normally used Si conductive layer for suppressing temperature drift, and proceeds with the discussion.

  In the region indicated by I in FIG. 3, the high-frequency electric signal propagating through the center conductor 4a and the ground conductors 4b and 4c interacts with the light propagating through the two interaction optical waveguides 3a and 3b. , I is referred to as a high-frequency electrical signal interaction unit. On the other hand, in the region indicated by II, the bias voltage applied to the bias electrode 5a having the bias electrodes 5a-1 and 5a-2 and the bias electrode 5b having the bias electrodes 5b-1 and 5b-2 and two mutual voltages are applied. Since the light propagating through the working optical waveguides 3a and 3b interacts, it is called a DC bias interaction section (or a DC bias section, or simply a bias section). The bias electrodes 5b-2 and 5a-1 having the interactive optical waveguides 3a and 3b below are also referred to as a first bias electrode and a second bias electrode, respectively, and there are no biases having the interactive optical waveguides 3a and 3b below. The electrodes 5a-2 and 5b-1 are also referred to as separate bias electrodes.

  As shown in FIG. 4, in this prior art, the z-cut LN substrate 1 is dug by etching or the like to form a ridge structure having ridge portions 6a and 6b. By adopting this ridge structure, it is possible to realize excellent characteristics in the effective refractive index (or microwave effective refractive index), characteristic impedance, modulation band, driving voltage, etc. of a high-frequency electric signal (or microwave). . In FIG. 4, although the height of the ridges 6a and 6b is drawn with emphasis, the depth is actually about several μm, and the thickness of the central conductor 4a and the ground conductors 4b and 4c is several tens of μm. The value is small compared to.

  In the bias interaction part of FIG. 4B, when a voltage is applied between the bias electrodes 5a-1, 5a-2 and 5b-1, 5b-2, the electric lines of force 7-1, 7- 2 and 7-3 are generated, and the electric lines of force interact with light propagating through the optical waveguides 3a and 3b.

  In this prior art, since the gaps between the bias electrodes 5a-1, 5a-2 and 5b-1, 5b-2 are all equal to W, the electric lines of force 7-1, 7-2, and 7-3 of the bias voltage There is a problem that the efficiency of interaction with the light propagating through the optical waveguides 3a and 3b is poor, and as a result, the half-wave voltage (or DCVπ, DC bias voltage, or simply bias voltage) of the bias portion is high.

JP-A-4-288518

  As described above, in the configuration shown as the prior art, since the gaps between the bias electrodes were all equal, the efficiency of interaction between the electric lines of force of the bias voltage and the light propagating through the optical waveguide of the bias unit was poor, As a result, there has been a problem that the half-wave voltage (DCVπ) of the bias portion becomes high.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an optical modulator in which the DC bias voltage is sufficiently reduced at the same time.

  In order to solve the above problems, an optical modulator according to claim 1 of the present invention includes a substrate having an electro-optic effect, and at least two optical waveguides for guiding light formed on the substrate. A traveling wave electrode formed on one side of the substrate and having a center electrode and a ground electrode for a high frequency electrical signal through which the high frequency electrical signal for modulating the light propagates; and a bias electrode for applying a bias voltage to the light The optical waveguide includes a high frequency electrical signal interaction unit for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode, and a bias voltage applied to the bias electrode. An optical modulator including a DC bias interaction unit for adjusting the phase of the light by applying the bias electrode in the DC bias interaction unit, A first bias electrode and a second bias electrode having a single optical waveguide, and at least one other bias electrode having no two optical waveguides below, the first bias electrode and The gap with the second bias electrode is narrower than the gap between the first bias electrode or the second bias electrode and the other bias electrode.

  An optical modulator according to a second aspect of the present invention is the optical modulator according to the first aspect, wherein the another bias electrode is composed of two, and one of the two bias electrodes is the first bias electrode. And the other of the two is disposed opposite to the second bias electrode.

  The optical modulator according to claim 3 of the present invention is the optical modulator according to claim 1 or 2, wherein the DC bias interaction portion has a ridge structure formed by digging down the substrate. It is a feature.

  An optical modulator according to a fourth aspect of the present invention is the optical modulator according to the first or second aspect, wherein the substrate for the DC bias interaction unit has a planar structure.

  In the optical modulator according to the present invention, by making the gap between the bias electrodes different in the DC bias interaction section, the efficiency of the interaction between the electric field lines of the bias voltage and the optical waveguide can be increased. There is an effect that the half-wave voltage of the bias section can be reduced.

1 is a top view showing a schematic configuration of an optical modulator according to the present invention. (A) Cross-sectional view taken along line CC 'in FIG. 1, (b) Cross-sectional view taken along line DD' in FIG. Top view showing a schematic configuration of a conventional optical modulator (A) Cross-sectional view taken along line AA 'in FIG. 3, (b) Cross-sectional view taken along line BB' in FIG.

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the prior art shown in FIGS. 3 to 4 correspond to the same functional units, description of functional units having the same reference numerals is omitted here. To do.

(Embodiment)
FIG. 1 is a top view of an optical modulator according to the present invention. 2A and 2B are cross-sectional views taken along line CC ′ and DD ′ in FIG. 1, respectively. As in the prior art, the present invention is a bias-separated optical modulator that includes a high-frequency electrical signal interaction section I and a DC bias interaction section III. As can be seen from FIG. 1, the spacing between the interaction optical waveguides 3a and 3b differs between the high frequency electrical signal interaction section I and the DC bias interaction section III.

  First, as can be seen from FIG. 2 (a) corresponding to the high frequency electrical signal interaction section I, the traveling wave electrode 4 composed of the center conductor 4a and the ground conductors 4b and 4c is propagated in the same manner as in the prior art described above. A ridge structure is employed so that the effective refractive index of the high-frequency electrical signal is close to the equivalent refractive index of light propagating through the interactive optical waveguide 3b.

In the present invention, as shown in FIG. 2B, the gap W between the bias electrodes 5b-2 'and 5a-1' above the optical waveguides 3a and 3b in the DC bias interaction section III. ₁ is made narrower than the gaps W ₂ and W ₃ between the bias electrodes 5b-2 ′ and 5a-1 ′ and the bias electrodes 5a- ₂ ′ and 5b-1 ′ having no optical waveguide below. That is, W ₁ <W ₂ and W ₃ are set. The bias electrodes 5b-2 ′ and 5a-1 ′ having the interaction optical waveguides 3a and 3b below are also referred to as a first bias electrode and a second bias electrode, respectively, and the interaction optical waveguides 3a and 3b are below. The bias electrodes 5a-2 ′ and 5b-1 ′ that are not present are also referred to as other bias electrodes.

By comprising in this way, there exist the following two effects. That is, applied since the gap W ₁ between the bias electrode 5a-1'and 5b-2'is narrow, it is possible to increase the electric field of the electric power line, and the number of electric flux lines waveguide 3a, 3b, It becomes possible to do. With these two effects, as a result, DCVπ in the DC bias unit III can be reduced.

That is, in the present invention, the electric lines of force between the bias electrode 5b-2 ′ above the optical waveguide 3a and the bias electrode 5a-1 ′ above the optical waveguide 3b are set by W ₁ <W ₂ and W _3. The number of electric lines of force 7-1 ′, the number of lines of electric force 7-2 ′ between the bias electrode 5b-2 ′ and the bias electrode 5a-2 ′ where the optical waveguide does not exist below, and the bias electrode 5a-1 ′ and the light The number of electric force lines 7-3 ′ between the waveguide and the bias electrode 5b-1 ′ not present below is larger and the strength is increased.

  Further, since the electric force lines 7-2 'and the electric force lines 7-3' are present due to the presence of the bias electrode 5b-1 'and the bias electrode 5a-2', the bias electrode 5a-1 'and the bias electrode 5b are present. -2 'and the optical waveguides 3b and 3a are not misaligned (pattern deviation in the manufacturing process), the DC bias voltage does not increase significantly.

  The present invention can be applied without either the bias electrode 5b-1 'or the bias electrode 5a-2'.

(Each embodiment)
In the above description, an optical modulator having a ridge structure has been described, but it goes without saying that the present invention can also be applied to an optical modulator having a planar structure. Alternatively, it is needless to say that the high-frequency electrical signal interaction unit can be applied to a ridge structure, and the DC bias interaction unit can be applied to a planar structure (also referred to as a planar structure).

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

1: z-cut LN substrate (LN substrate)
2: SiO ₂ buffer layer (buffer layer)
3: Mach-Zehnder optical waveguide (optical waveguide)
3a, 3b: Interaction optical waveguide constituting the Mach-Zehnder optical waveguide 4: Traveling wave conductor 4a: Center conductor 4b, 4c: Ground conductor 5a, 5a ′, 5b, 5b ′: Bias electrode 5b-2, 5b-2 ′: Bias electrode (first bias electrode)
5a-1, 5a-1 ': Bias electrode (second bias electrode)
5a-2, 5a-2 ', 5b-1, 5b-1': Bias electrode (another bias electrode)
6a, 6a ', 6b, 6b': Ridge portion 7-1, 7-1 ', 7-2, 7-2', 7-3, 7-3 ': Electric field lines of DC bias voltage I: High frequency electricity Signal interaction part II, III: DC bias interaction part

Claims (4)

A substrate having an electro-optic effect, at least two optical waveguides for guiding light formed on the substrate, and a high-frequency electric signal that is formed on one surface side of the substrate and modulates the light propagates A traveling wave electrode having a center electrode and a ground electrode for high-frequency electrical signals, and a bias electrode for applying a bias voltage to the light,
The optical waveguide includes a high frequency electrical signal interaction unit for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode, and a bias voltage applied to the bias electrode. In an optical modulator comprising a DC bias interaction unit for adjusting the phase of light,
The bias electrode in the DC bias interaction unit is divided into a first bias electrode and a second bias electrode having the two optical waveguides below, and at least one separate one having no two optical waveguides below. With a bias electrode of
An optical modulation characterized in that a gap between the first bias electrode and the second bias electrode is narrower than a gap between the first bias electrode or the second bias electrode and the other bias electrode. vessel.   The another bias electrode is composed of two, one of the two is disposed opposite to the first bias electrode, and the other of the two is opposed to the second bias electrode. The optical modulator according to claim 1, wherein the optical modulator is arranged as follows.   The optical modulator according to claim 1, wherein the DC bias interaction unit has a ridge structure formed by digging down the substrate. 3. The optical modulator according to claim 1, wherein the substrate for the DC bias interaction unit has a planar structure.

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