JP2008020727A - Waveguide type optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a waveguide type optical modulator capable of applying a uniform and sufficient electric field to an optical waveguide, even if deviations in current distribution due to skin effect is generated within an electrode, through which modulation signal flows. <P>SOLUTION: In the waveguide type optical modulator 1A, the optical waveguide 2A, through which light is transmitted, is formed on a substrate 5 constituted of a semiconductor material, for example. Furthermore, a signal transmission conductive body 3A, which applies a modulation electric field for modulating the light transmitted through the optical waveguide 2A, is formed on the substrate 5. The signal transmission conductive body 3A is formed at a position, superimposed on the optical waveguide 2A and is inclined, at a prescribed angle with respect to the extending direction of the optical waveguide 2A, whereby the edge surface in the width direction intersects the optical waveguide 2A; and even if the frequency of a modulation signal is increased and the deviation in the current distribution due to skin effect is thus generated, the position at which electric field is applied is moved in the width direction of the optical waveguide 2A; and as the result, a uniform and ample electric field will be applied to the optical waveguide 2A. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a waveguide type optical modulator that performs optical modulation by arranging an optical waveguide below a signal transmission conductor through which a modulation signal flows and applying an electric field from the signal transmission conductor to the optical waveguide. Specifically, by arranging the signal transmission conductors at an angle with respect to the light traveling direction in the optical waveguide, it is possible to suppress the bias of the electric field strength distribution in the optical waveguide.

  As an external modulator for modulating light emitted from a laser diode (LD) or the like, a waveguide type optical modulator utilizing an electro-optic effect or a semiconductor electroabsorption effect has been proposed.

  FIG. 5 is a block diagram showing an example of a conventional waveguide type optical modulator. As shown in FIG. 5A, the waveguide type optical modulator 100A includes a signal transmission conductor 102 and an optical waveguide 103 on a substrate 101 made of, for example, a semiconductor material.

  In the conventional waveguide-type optical modulator 100A, a configuration in which the signal transmission conductor 102 is formed with a microstrip line type electrode structure is the mainstream, and an electric field is applied to the optical waveguide 103 disposed below the signal transmission conductor 102. Thus, the light transmission intensity of the optical waveguide 103 is changed to perform light modulation.

  In a waveguide type optical modulator having such a microstrip line type electrode structure, the frequency of the modulation signal increases with an increase in communication speed, and the frequency band of the modulation signal is in the microwave to millimeter wave band. Then, the current distribution is biased by the skin effect.

  The skin effect is a phenomenon in which when high-frequency current flows through a conductor, the current concentrates on the surface of the conductor. In the waveguide type optical modulator, if the current distribution is biased in the signal transmission conductor due to the skin effect, The electric field distribution received by the modulated light traveling inside the optical waveguide to which an electric field is applied from the transmission conductor is biased, and uniform modulation cannot be realized.

  Here, when the current distribution is biased inside the conductor due to the skin effect, the depth at which the current flows is called the skin depth, and the skin depth δ is expressed by the following equation (1).

  In the equation (1), σ is a dielectric constant, and in this example, gold (Au) is taken as an example.

  FIG. 6 is a simulation example of the electromagnetic field distribution in the vicinity of the electrode. As an example of the simulation conditions, an electric signal having a frequency of 60 GHz is passed through the microstripline electrode 104 made of gold. 6 (a) schematically shows an electric field distribution in the vicinity of the electrode, and FIG. 6 (b) schematically shows a magnetic field distribution in the vicinity of the electrode.

  FIG. 7 is a graph showing the relationship between the skin depth and the signal frequency. In this example, the electrode is made of gold.

  As the frequency of the electric signal flowing through the electrode 104 increases, the current concentrates on the edge portion of the electrode 104 due to the skin effect, so that the electric field is strong from the edge portion of the electrode 104 as shown in FIG. It can be seen that it is distributed. It can also be seen that as the signal frequency increases, the skin depth becomes shallower as shown in FIG.

  FIG. 8 is a graph showing an example of the electric field intensity distribution due to the skin effect directly under the electrode. In FIG. 8, an approximate range of a region where current is concentrated inside the electrode 104 due to the skin effect is indicated by a region K surrounded by a two-dot chain line.

  The graph G11 shows the electric field strength distribution at a certain depth d1 directly below the electrode 104, the graph G12 shows the electric field strength distribution at a depth d2 deeper than the depth d1, and the graph G13 shows a depth deeper than the depth d2. The electric field strength distribution at d3 is shown.

  When an optical waveguide is installed in a predetermined region L immediately below the electrode 104, as shown in FIG. 8, when current concentrates in a region K such as an edge portion of the electrode 104 due to the skin effect, a central portion (x = In the vicinity of 0), the electric field strength becomes very weak.

  FIG. 9 is a simulation example showing the relationship between the electrode width and the electric field intensity, and FIG. 9A shows the simulation conditions. In this example, when the width W of the electrode 104 is changed from 1 μm to 4 μm in 1 μm units, the electric field intensity distribution at a predetermined depth in the range of 6 μm width immediately below the electrode 104 is calculated.

  When the electrode 104 is made of gold, the skin depth at a signal frequency of 60 GHz is about 0.35 μm as shown in FIG. As a result, as shown in FIG. 9B, it is difficult to avoid the bias of the electric field intensity distribution due to the skin effect even if the width W of the electrode 104 is set to 1 μm.

  Further, when the width W of the electrode 104 is widened, as shown in FIGS. 9B to 9E, the bias of the electric field strength distribution due to the skin effect increases, and the electric field strength in the central portion immediately below the electrode becomes weak. Become.

  Further, even at a frequency of less than 60 GHz, current concentration occurs at the edge portion of the electrode due to the skin effect, and the same problem occurs when the width of the optical waveguide is several μm.

  As described above, in a waveguide type optical modulator, when an electric signal in the millimeter wave band is used as a modulation signal, a uniform electric field is applied in the waveguide unless the electrode shape takes the skin depth into consideration. It is not possible.

  In the signal transmission conductor having the microstrip line type electrode structure as shown in FIG. 5A, the electric field around the electrode spreads radially from the edge portion of the electrode as shown in FIG. 6A. Therefore, the electric field strength that can be applied to the limited solid angle portion where the optical waveguide exists is relatively weak.

  In addition, as a conventional waveguide type optical modulator, as shown in FIG. 5B, a waveguide having a coplanar waveguide type electrode structure in which ground conductors 105 are formed on both sides of the signal transmission conductor 102. A type optical modulator 100B has been proposed.

  In the waveguide type optical modulator 100B having such a coplanar waveguide type electrode structure, the main purpose is to improve the transmission characteristics by impedance matching. In the signal transmission conductor, the skin effect is increased as the signal frequency is increased. Concentration of current distribution occurs due to. For this reason, the bias of the electric field strength distribution cannot be eliminated.

  Further, a waveguide type optical modulator having a coplanar waveguide type electrode structure and having a wide region and a narrow region formed on an electrode through which a modulation signal (microwave) flows is proposed (for example, see Patent Document 1). ).

  In such a waveguide type optical modulator, the wide area and the narrow area are alternately formed on the electrode, and the electric resistance of the electrode is changed for each area by adjusting the distance between the opposing electrodes. Impedance matching is performed to ensure modulation efficiency.

  However, even if a wide region and a narrow region are formed on the electrode, current distribution is concentrated due to the skin effect as the signal frequency increases. For this reason, in the optical waveguide disposed immediately below the optical waveguide, the electric field strength applied to the central portion becomes weak, and the bias of the electric field strength distribution cannot be eliminated.

JP 2003-295141 A

  As described above, in the conventional waveguide type optical modulator, when the concentration of the current distribution due to the skin effect occurs in the electrode as the signal frequency increases, the electric field strength distribution in the optical waveguide is biased, uniform and There was a problem that a sufficient electric field could not be applied.

  The present invention has been made to solve such a problem, and can apply a uniform and sufficient electric field to an optical waveguide even when a current distribution is biased due to a skin effect in an electrode through which a modulation signal flows. An object of the present invention is to provide a waveguide type optical modulator.

  In order to solve the above-described problems, a waveguide type optical modulator of the present invention includes an optical waveguide that is formed on a substrate and transmits light, and an electrode that applies a modulation electric field that modulates light transmitted through the optical waveguide. The electrode is formed on the substrate at a position overlapping the optical waveguide, tilted at a predetermined angle with respect to the extending direction of the optical waveguide, and the end face in the width direction has the optical waveguide It is characterized by having a signal transmission conductor across.

  In the waveguide type optical modulator of the present invention, when the frequency of the modulation signal flowing through the signal transmission conductor increases, an electric field is applied to the optical waveguide when current concentrates on the edge of the signal transmission conductor due to the skin effect. This position moves in a direction perpendicular to the traveling direction of light in the optical waveguide.

  Therefore, even if the current distribution in the signal transmission conductor is uneven, a sufficient electric field is applied across the entire width in the optical waveguide. As a result, the optical waveguide disposed below the signal transmission conductor is uniform with respect to the modulated light transmitted through the optical waveguide, even in a frequency band in which the bias of the electric field strength distribution due to the skin effect is significant. Even if an electric field having a sufficient strength is applied and the modulated light enters any position in the optical waveguide, the modulation is reliably performed.

  By arranging a grounded or non-grounded conductor in a predetermined shape on both sides or one side of the signal transmission conductor, an effect of confining the expansion of the electric field can be obtained, and the electric field strength can be increased.

  According to the waveguide type optical modulator of the present invention, when the frequency of the modulation signal flowing through the signal transmission conductor increases, the current concentrates on the edge of the signal transmission conductor due to the skin effect, and the electric field in the vicinity of the signal transmission conductor. Even when the intensity distribution is biased, a sufficient electric field is applied across the entire width in the optical waveguide. Thereby, even if the frequency of the modulation signal is increased, the optical signal can be reliably modulated.

  Hereinafter, embodiments of a waveguide type optical modulator of the present invention will be described with reference to the drawings.

<Configuration Example of Waveguide Type Optical Modulator of First Embodiment>
1 and 2 are configuration diagrams showing an example of a waveguide type optical modulator according to the first embodiment, and FIG. 1A shows the waveguide type optical modulator 1A according to the first embodiment. FIG. 1B is a cross-sectional view taken along line AA of the waveguide type optical modulator 1A, and FIG. 1C is a cross-sectional view taken along line BB showing a waveguide layer of the waveguide type optical modulator 1A. is there. FIG. 2 is a perspective view of the waveguide type optical modulator 1A.

  A waveguide type optical modulator 1A according to the first embodiment is an external modulator using an electro-optic effect or a semiconductor electroabsorption effect, and an optical waveguide 2A through which an optical signal is transmitted and a modulated electric field applied to the optical waveguide 2A. The substrate 5 is provided with a signal transmission conductor 3A for applying a voltage and a planar conductor 4A for adjusting the characteristic impedance of the signal transmission conductor 3A and adjusting the electric field distribution.

  In this example, in the waveguide type optical modulator 1A, the substrate 5 is made of a semiconductor material in order to use the electric field absorption effect of a semiconductor. In the substrate 5, for example, a waveguide layer 51a is formed by a semiconductor multiple quantum well active layer (MQW active layer), and an optical waveguide 2A is formed. Further, in the substrate 5, the electrode layer 51c is formed on the waveguide layer 51a via the cladding layer 51b, and the signal transmission conductor 3A and the planar conductor 4A are formed.

  The optical waveguide 2A has a refractive index slightly higher than that of the cladding layer 51b, and the light is confined and transmitted. The electroabsorption effect of a semiconductor is a phenomenon in which the energy level difference (band gap) between the conduction band and the valence band changes by applying an electric field to the quantum well structure, and the amount of light absorption changes. When an electric field is applied to the optical waveguide 2A, the light transmission intensity of the optical waveguide 2A changes and light modulation is performed.

  Here, the X axis, the Y axis, and the Z axis are the directions shown in FIGS. 1 and 2, and one end of the substrate 5 along the Z axis is the input side end 52a, and the other end is the output side end. If it is set as the part 52b, 2 A of optical waveguides will extend in the Z-axis direction linearly from the input side edge part 52a of the board | substrate 5 to the output side edge part 52b along the XZ plane. In the optical waveguide 2A, one end face is exposed at the input side end 52a of the substrate to form the incident part 21a, and the other end face is exposed to the output side end 52b to form the emission part 21b.

  The signal transmission conductor 3A is an example of an electrode and extends obliquely along the XZ plane from the input side end 52a to the output side end 52b of the substrate 5 with respect to the extending direction (Z axis) of the optical waveguide 2A. And overlaps the optical waveguide 2A via the cladding layer 51b (buffer layer). In the signal transmission conductor 3A, the end face 31L and the end face 31R in the width direction along the X axis are parallel to each other, and the width of the signal transmission conductor 3A is narrower than the width of the optical waveguide 2A.

  In the signal transmission conductor 3A, when the modulation signal becomes a high frequency, current concentrates on the edge portions of the end surface 31L and the end surface 31R in the width direction due to the skin effect. Therefore, the angle θ between the optical waveguide 2A and the signal transmission conductor 3A is set to an angle at which the end surface 31L and the end surface 31R of the signal transmission conductor 3A cross the optical waveguide 2A in the width direction.

  As a result, the signal transmission conductor 3A moves from the input side end 52a to the output side end 52b of the substrate 5 so that the position of the signal transmission conductor 3A with respect to the optical waveguide 2A travels along the Z axis in the optical waveguide 2A. It changes in the X-axis direction perpendicular to the direction.

  Accordingly, when current concentrates on the edge portion of the signal transmission conductor 3A due to the skin effect, the position where the electric field is applied in the optical waveguide 2A moves in the X-axis direction perpendicular to the light traveling direction in the optical waveguide 2A. Will do.

  Therefore, when a modulation signal is passed through the signal transmission conductor 3A, a sufficient electric field is applied across the entire width in the optical waveguide 2A even if current concentrates on the edge portion of the signal transmission conductor 3A. Modulation is performed at any position in the optical waveguide 2A.

  A wide input portion 32a is formed on the input side end portion 52a side of the signal transmission conductor 3A so that a signal line (not shown) can be connected to the output side end portion 52b side of the signal transmission conductor 3A. Is formed with an output portion 32b having a wide shape capable of connecting a signal line (not shown).

  The shape and arrangement of the signal transmission conductor 3A described above are such that the current concentrated on the edge portion of the signal transmission conductor 3A due to the skin effect is efficiently used, the skin depth at the frequency used for the modulation signal, and the signal transmission conductor 3A. It is determined according to the impedance.

  The planar conductor 4A is an example of an electrode, and is formed on both or one of the side facing the one end surface 31L and the side facing the other end surface 31R of the signal transmission conductor 3A, and in the configuration in which the planar conductor 4A is grounded. It becomes a coplanar waveguide type electrode structure. The planar conductor 4A may be ungrounded.

  The end face 41 of the planar conductor 4A facing the signal transmission conductor 3A is inclined in accordance with the signal transmission conductor 3A extending obliquely with respect to the extending direction of the optical waveguide 2A, and the planar conductor 4A is the signal transmission conductor. It is opposed to 3A with a predetermined gap 42 therebetween.

<Operation Example of Waveguide Type Optical Modulator of First Embodiment>
Next, the operation of the waveguide type optical modulator 1A according to the first embodiment will be described with reference to the drawings.

  In the waveguide type optical modulator 1A, laser light emitted from a laser diode (LD) (not shown) or the like enters the incident portion 21a of the optical waveguide 2A. Modulated light, which is laser light before modulation, incident on the optical waveguide 2A from the incident portion 21a is transmitted through the optical waveguide 2A in the direction of arrow E1 along the Z axis.

  On the other hand, in the waveguide type optical modulator 1A, a modulation signal which is an electric signal is input to the input portion 32a of the signal transmission conductor 3A. The modulation signal input from the input unit 32a to the signal transmission conductor 3A travels in the direction of the arrow E2 same as the modulated light through the signal transmission conductor 3A, and applies a modulation electric field to the optical waveguide 2A located below the signal transmission conductor 3A. Thus, it is terminated at the output portion 32b of the signal transmission conductor 3A.

  As described above, in the waveguide type optical modulator 1A, the traveling direction of the light in the optical waveguide 2A and the traveling direction of the modulation signal in the signal transmission conductor 3A are the same direction. Type light modulator.

  When a modulated electric field is applied to the optical waveguide 2A from the signal transmission conductor 3A, the amount of light absorption in this example changes according to the modulated signal. Thereby, the modulated light transmitted through the optical waveguide 2A is modulated, and the modulated light is output from the emitting portion 21b of the optical waveguide 2A.

  As described above, the signal transmission conductor 3A extends obliquely along the XZ plane at a predetermined angle θ with respect to the extending direction of the optical waveguide 2A, and the light traveling direction (Z axis) The traveling direction of the electromagnetic field is varied by an angle θ.

  Thereby, even when the modulation signal has a high frequency and current concentrates on the edge portion of the signal transmission conductor 3A due to the skin effect, the position where the electric field is applied in the optical waveguide 2A is in the traveling direction of the light in the optical waveguide 2A. Move in the vertical X-axis direction. Therefore, even if the current is concentrated on the edge portion of the signal transmission conductor 3A, a sufficient electric field is applied across the entire width in the optical waveguide 2A, and the modulated light is incident on any position in the optical waveguide 2A. However, the modulation is reliably performed.

  Further, the electric field applied from the signal transmission conductor 3A to the optical waveguide 2A is confined in a narrow range where the optical waveguide 2A exists by the planar conductor 4A disposed on both sides of the signal transmission conductor 3A with a predetermined gap 42 therebetween. Thus, the strength of the electric field applied to the optical waveguide 2A is increased.

  Further, the planar conductor 4A disposed on both sides of the signal transmission conductor 3A has a shape having a desired impedance, so that the characteristic impedance of the signal transmission conductor 3A is adjusted. As a result, the input portion 32a and the output portion 32b are formed in the signal transmission conductor 3A, so that the impedance is matched even if there is a change in the electrode shape or electrode position, such as the width of the signal transmission conductor 3A being different.

  FIG. 3 is a graph showing the change over time of the electric field strength at the central portion of the optical waveguide. A graph G1 shown by a solid line shows the change in the electric field strength due to the electrode (signal transmission conductor) to which the present invention is applied, and is shown by a one-dot chain line. Graph G2 shows a change in electric field strength due to a conventional shape electrode as a comparative example.

  In the waveguide type optical modulator using the conventional electrode as shown in FIG. 5, when the modulation signal becomes a high frequency and current is concentrated on the edge portion due to the skin effect, a sufficient electric field is generated in the central portion of the optical waveguide. The strength has not been obtained.

  On the other hand, in the waveguide type optical modulator 1A using the signal transmission conductor 3A to which the present invention is applied, even if current is concentrated on the edge portion of the signal transmission conductor 3A due to the skin effect, an electric field is generated in the optical waveguide 2A. Is moved in the X-axis direction perpendicular to the light traveling direction in the optical waveguide 2A, the electric field strength in the vicinity of the center (x = 0) of the optical waveguide 2A is greatly improved. You can see that

  As described above, in the waveguide type optical modulator 1A using the signal transmission conductor 3A to which the present invention is applied, the region to which the electric field is applied is changed in a wide range in accordance with the width of the optical waveguide 2A. Efficient electric field supply utilizing the effect is possible. In addition, the electric field strength can be greatly increased by the electric field confinement effect by the planar conductors 4A installed on both sides of the signal transmission conductor 3A.

  As a result, loss generated when the modulated signal passes through the signal transmission conductor 3A can be reduced, so that high frequency and wide band can be realized, and furthermore, a sufficient electric field can be applied with a short transmission distance. The element size can also be reduced.

<Modification of Waveguide Type Optical Modulator of First Embodiment>
FIG. 4 is a configuration diagram illustrating a modification of the waveguide type optical modulator according to the first embodiment. The angle difference between the traveling direction of the electromagnetic field determined by the angle that the electrode that is the signal transmission conductor has with respect to the traveling direction of the light and the traveling direction of the light by the optical waveguide does not have to be single, and FIG. A signal transmission conductor 3B having a shape meandering in a straight line across the optical waveguide 2A in the width direction as in the waveguide type optical modulator 1B shown in FIG.

  Moreover, it is good also as the signal transmission conductor 3C of the shape which meandered in the curve shape across the optical waveguide 2A across the width direction like the waveguide type optical modulator 1C shown in FIG.4 (b). Thus, by making the signal transmission conductor meander, current concentrated on the edge portion can be effectively applied to the optical waveguide 2A.

  Further, as shown in FIG. 4A, the signal transmission conductor 3C meanders linearly, thereby changing the gap with the planar electrode 4B and performing impedance matching. On the other hand, as shown in FIG. 4 (b), the signal transmission conductor 3C meanders in a curved shape to make the gap between the planar electrode 4C constant and to perform impedance matching, and to confine the electric field. Strengthen.

  Further, as in the waveguide type optical modulator 1D shown in FIG. 4 (c), the planar conductor 4D may be formed on both sides of the signal transmission conductor 3D, or the waveguide type light shown in FIG. 4 (d). Like the modulator 1E, a configuration in which the planar conductor is not formed on both sides of the signal transmission conductor 3E, or a configuration in which the planar conductor is formed on one side of the signal transmission conductor, although not shown, may be employed. Further, although not shown, a configuration in which the width of the signal transmission conductor is constant over the entire length may be employed.

  In this way, by changing the traveling direction of the light and the electromagnetic field, the current concentrated on the skin of the electrode due to the skin effect is moved on the axis perpendicular to the traveling direction of the light (on the X axis). This is within the scope of the present invention.

  The present invention is applied to an external modulator used in optical fiber communication or the like.

It is a block diagram which shows an example of the waveguide type optical modulator of 1st Embodiment. It is a block diagram which shows an example of the waveguide type optical modulator of 1st Embodiment. It is a graph which shows the time change of the electric field strength in the optical waveguide center part. It is a block diagram which shows the modification of the waveguide type optical modulator of 1st Embodiment. It is a block diagram which shows an example of the conventional waveguide type optical modulator. It is an example of a simulation of electromagnetic field distribution in the vicinity of an electrode. It is a graph which shows the relationship between skin depth and a signal frequency. It is a graph which shows an example of the electric field strength distribution by the skin effect just under an electrode. It is a simulation example which shows the relationship between the width | variety of an electrode, and electric field strength.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1A ... Waveguide type optical modulator, 2A ... Optical waveguide, 21a ... Incident part, 21b ... Output part, 3A ... Signal transmission conductor, 31L ... End face, 31R ... End face, 32a ... input part, 32b ... output part, 4A ... planar conductor, 5 ... substrate, 51a ... waveguide layer, 51b ... clad layer, 51c ... electrode Layer, 52a ... Input side end, 52b ... Output side end

Claims (5)

An optical waveguide formed on a substrate and through which light is transmitted;
In a waveguide type optical modulator comprising an electrode for applying a modulation electric field for modulating light transmitted through the optical waveguide,
The electrode includes a signal transmission conductor formed on the substrate at a position overlapping the optical waveguide, inclined at a predetermined angle with respect to the extending direction of the optical waveguide, and having an end face in the width direction crossing the optical waveguide. A waveguide type optical modulator characterized by that. The waveguide according to claim 1, wherein a conductor facing the signal transmission conductor with a predetermined gap is disposed on both sides or one side of the signal transmission conductor on the same surface as the signal transmission conductor. Type optical modulator. The waveguide type optical modulator according to claim 2, wherein the conductor is a ground conductor. The waveguide type optical modulator according to claim 1, wherein the signal transmission conductor is a traveling wave electrode to which an electric signal is supplied in the same direction as the traveling direction of light transmitted through the optical waveguide. The waveguide-type optical modulator according to claim 1, wherein the signal transmission conductor extends meandering with respect to the optical waveguide.

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