JP2007079249A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator which is enabled to suppress a temperature drift and has a long life and high reliability. <P>SOLUTION: The optical modulator has a substrate 1 with electrooptical effect, an optical waveguide 3 which is formed on the substrate 1 and guides light, an electrode 4 which applies a voltage for modulating the light and comprises a center conductor 4a and ground conductors 4b and 4c formed on one surface side of the substrate 1, and optical waveguides 3a and 3b for interaction which modulate the phase of the light by applying the voltage between the ground conductors 4b and 4c by the optical waveguide 3, and the substrate 1 has substrate-side surfaces 1a and 1b having electric charges induced by pyroelectric effect on the outermost sides of sections perpendicular to the lengths of the optical waveguides 3a and 3b for interaction. The optical modulator has a conductive film 5 formed directly on the substrate-side surfaces 1a and 1b. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical modulator, and more particularly to an optical modulator that has a high driving speed, a low driving voltage, a low DC drift and a low temperature drift, and a good manufacturing yield.

As is well known, in an optical modulator, a substrate having a so-called electro-optic effect in which a refractive index is changed by applying an electric field, such as lithium niobate (LiNbO ₃ ) (hereinafter, a lithium niobate substrate is referred to as an LN substrate). A traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as an LN optical modulator) in which an optical waveguide and a traveling wave electrode are formed is 2.5 Gbit / s, 10 Gbit / s because of its excellent chirping characteristics. It is applied to large-capacity optical transmission systems.

  Such an LN optical modulator has recently been studied for application to an ultra large capacity optical transmission system of 40 Gbit / s, and is expected as a key device in the large capacity optical transmission system.

[Conventional technology]
FIG. 5 is a perspective view showing a configuration of an LN optical modulator configured using an x-cut LN substrate according to the prior art. 6 is a cross-sectional view taken along the line AA ′ of FIG.

In the figure, 1 is an x-cut LN substrate, 2 is a SiO ₂ buffer layer, 3 is an optical waveguide formed by thermally diffusing Ti at 1050 ° C. for about 10 hours, and a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide) is formed. It is composed.

  In the figure, reference numerals 3a and 3b denote optical waveguides (or interactive optical waveguides) in a portion where an electrical signal and light interact (referred to as an interaction portion), that is, two arms of a Mach-Zehnder optical waveguide. The optical waveguide 3 is branched by the illustrated Y-branch optical waveguide.

  In the figure, reference numeral 4 denotes a traveling wave electrode. As the traveling wave electrode 4, it is assumed that a coplanar waveguide (CPW) having one central conductor 4a and two ground conductors 4b and 4c is used. The CPW traveling wave electrode 4 has a symmetrical structure having ground conductors 4b and 4c on both sides of the center conductor 4a.

Electricity which suppresses the absorption loss which the light which guides the optical waveguide 3 receives from the metal (generally using Au) which is a traveling wave electrode, and guides the traveling wave electrode 4 which consists of the center conductor 4a and the grounding conductors 4b and 4c. Reducing the microwave equivalent refractive index of the signal (or the microwave equivalent refractive index of the traveling wave electrode) _nm and the equivalent refractive index of the light guided through the interaction optical waveguides 3a and 3b (or the equivalent refractive index of the optical waveguide) ) closer to _{n o,} in order to further approximate the characteristic impedance as possible to 50 [Omega, between the traveling wave electrode 4 and the x- cut LN substrate 1, usually thick SiO ₂ buffer layer 2 of about 400nm~1μm is deposited The

The SiO ₂ buffer layer 2 is an electrical signal or microwave equivalent refractive index n _m of the interaction optical waveguides 3a, 3b by approximating the effective refractive index n _o of the light propagating the important of expanding the optical modulation band Working.

  Next, the operation of the LN optical modulator configured as described above will be described. To operate the LN optical modulator, a direct current bias (hereinafter referred to as a DC bias) is provided between the center conductor 4a and the ground conductors 4b and 4c. And a high-frequency electrical signal (hereinafter referred to as an RF electrical signal) need to be applied.

  In the prior art optical modulator shown in FIG. 5, the distribution of the desired lines of electric force when an electric field by a bias voltage is applied between the central conductor 4a and the ground conductors 4b and 4c of the traveling wave electrode is shown as DEF in FIG. Show.

  As can be seen from FIG. 7, since the direction of the electric lines of force crossing the two interactive optical waveguides 3a and 3b are opposite, the Mach-Zehnder optical waveguide 3 guides the two interactive optical waveguides 3a and 3b. The principle of the optical modulator using the Mach-Zehnder optical waveguide is to realize the OFF state of the light by shifting the phase of the light to be shifted by π.

  Next, the problem of temperature drift of the conventional LN optical modulator will be discussed with reference to FIG. Since the substrate side surfaces 1a and 1b (or z surfaces 1a and 1b) of the x-cut LN substrate 1 are a + z surface and a −z surface having a pyroelectric effect, a charge TEC is induced on the surface when the temperature of the substrate changes. .

  Since the amount of charge TEC induced on the substrate side surfaces 1a and 1b of the x-cut LN substrate 1 used in this conventional LN optical modulator varies randomly with temperature, it is induced on the substrate side surfaces 1a and 1b. Random electric lines of force EF are generated in the x-cut LN substrate 1 due to the generated electric charge TEC.

  Due to the generation of the random electric lines of force EF, the electric lines of force DEF due to the bias voltage applied between the center conductor 4a and the ground conductors 4b and 4c to operate the LN optical modulator are randomly canceled.

  In order to operate the LN optical modulator, it is necessary to apply a DC bias voltage and an RF electrical signal between the center conductor 4a and the ground conductors 4b and 4c.

In the voltage-light output characteristics shown in FIG. 9, the solid curve is the voltage-light output characteristics of the LN optical modulator in a certain state, and _Vb is the DC bias voltage at that time.

As shown in FIG. 9, the DC bias voltage _Vb is normally set at the midpoint between the peak and bottom of the light output characteristic.

However, due to the random electric lines of force EF generated by the temperature change described above, the electric lines of force DEF due to the bias voltage applied between the center conductor 4a and the ground conductors 4b and 4c are randomly canceled. This optimum bias voltage V _b greatly changes to V _b ′.

  Therefore, even in the optical modulator using the x-cut LN substrate 1 that is said to be stable with respect to temperature, a shift of the DC bias voltage due to the change in temperature, that is, a temperature drift occurs. This becomes remarkable when the distance between the substrate side surfaces 1a and 1b of the x-cut LN substrate 1 and the interaction optical waveguides 3a and 3b is relatively short.

  As described above, in an optical modulator manufactured using an LN substrate having a surface that generates the pyroelectric effect on the side surface of the substrate in the prior art, the LN substrate is caused by charges induced due to the pyroelectric effect. There was a problem that a random electric field was generated in the glass, resulting in temperature drift.

  Therefore, the present invention eliminates the temperature drift problem of the conventional technology as described above, and is suitable for a compact, high-speed and stable intensity optical modulator, phase modulator, or polarization scrambler. The object is to provide a modulator.

  In order to solve the above problems, an optical modulator according to claim 1 of the present invention modulates the light, a substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and the light. An electrode composed of a center conductor and a ground conductor formed on one surface side of the substrate, and the optical waveguide applies the voltage between the center conductor and the ground conductor. An interaction optical waveguide for modulating the phase of the light, and the substrate has a substrate side surface on which charges are induced by a pyroelectric effect on the outermost side of a cross section perpendicular to the longitudinal direction of the interaction optical waveguide In the optical modulator, a conductive film is formed directly on the side surface of the substrate.

  An optical modulator according to a second aspect of the present invention is the optical modulator according to the first aspect, wherein the conductive film is made of a semiconductor film.

  The optical modulator according to claim 3 of the present invention is the optical modulator according to claim 1, wherein the conductive film is made of a conductive adhesive.

  According to a fourth aspect of the present invention, in the optical modulator according to the first to third aspects, the conductive film is electrically connected to the ground conductor.

  An optical modulator according to a fifth aspect of the present invention is the optical modulator according to any one of the first to fourth aspects, wherein the ground conductor is disposed above a center line extending in a longitudinal direction of the central conductor above the interaction optical waveguide. It has a substantially symmetric shape.

  An optical modulator according to a sixth aspect of the present invention is the optical modulator according to any one of the first to fourth aspects, wherein the ground conductor is disposed above a center line extending in a longitudinal direction of the central conductor above the interaction optical waveguide. It is characterized by having an asymmetric shape.

  In the optical modulator according to claim 1, a substrate side surface on which electric charges are induced by a pyroelectric effect is provided on the outermost side of a cross section perpendicular to the longitudinal direction of the interaction optical waveguide, and is electrically connected to the substrate side surface. By forming a conductive film on the substrate, the potential on the side surface of the substrate can be stabilized, so that temperature drift can be suppressed.

  In the optical modulator of the second aspect, since the conductive film is made of a semiconductor film, the conductive film can be formed in the optical modulator manufacturing process.

  In the optical modulator according to claim 3, since the conductive film is generally made of a conductive adhesive having a higher conductivity, temperature drift can be more effectively suppressed.

  In the optical modulator according to the fourth aspect, since the conductive film is electrically connected to the ground conductor, the potential on the side surface of the substrate is surely stabilized.

  In the optical modulator according to claim 5, since the electrode is a symmetric electrode having a ground conductor on both sides of the center conductor, it is easy to make electrical connection between the conductive film and the ground conductor. Highly effective in stabilizing

  In the optical modulator according to claim 6, although the electrode is an asymmetric electrode having a ground conductor on one side, the potential on the side surface of the substrate can be stabilized by electrically connecting the conductive film and the ground conductor. .

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the prior art shown in FIGS. To do.

[First Embodiment]
FIG. 1 shows a cross-sectional view of the optical modulator according to the first embodiment of the present invention at a position corresponding to AA ′ of the prior art shown in FIG. In this embodiment, the conductive film 5 such as Si or SiOx is formed on the substrate side surfaces 1a and 1b of the x-cut LN substrate 1 in which the charge TEC is excited by the pyroelectric effect. Further, in the present embodiment, these conductive films 5 are electrically connected to the ground conductors 4b and 4c. Since this CPW type traveling wave electrode (electrode) 4 has a symmetrical structure having ground conductors 4b and 4c on both sides of the center conductor 4a, the electrical connection between the conductive film 5 and the ground conductors 4b and 4c is easy. is there.

  In this way, by forming the conductive film 5 on the substrate side surfaces 1a and 1b, the potentials of the substrate side surfaces 1a and 1b become stable, so that the random electric lines of force EF shown in FIG. .

  Although it is effective to form the conductive film 5 on both the substrate side surfaces 1a and 1b and to electrically connect the conductive film 5 to the ground conductors 4b and 4c, the substrate side surface is slightly less effective. There are various combinations such as only one of 1a and 1b or the conductive film 5 not being electrically connected to the ground conductor 4b (and the ground conductor 4c).

[Second Embodiment]
FIG. 2 shows a cross-sectional view of the optical modulator according to the second embodiment of the present invention at a position corresponding to AA ′ of the prior art shown in FIG. In addition to the structure of the first embodiment shown in FIG. 1, in this embodiment, a conductive film 5 such as Si or SiOx is used in addition to the substrate side surfaces 1a and 1b of the x-cut LN substrate 1 and an x-cut LN substrate. 1 is also formed on the bottom surface 1c. By doing so, it becomes possible to further stabilize the potential of the substrate side surfaces 1a and 1b.

[Third Embodiment]
FIG. 3 shows a cross-sectional view of the optical modulator according to the third embodiment of the present invention at a position corresponding to AA ′ of the prior art shown in FIG. In the present embodiment, the conductive film 5 such as Si or SiOx used in the first or second embodiment is applied to the substrate side surfaces 1a and 1b of the x-cut LN substrate 1 in which the charge TEC is excited by the pyroelectric effect. It is fixed to a metal casing 7 as an optical modulator using a conductive adhesive 6 such as a conductive paste having higher conductivity. Furthermore, in this embodiment, these conductive adhesives 6 are electrically connected to the ground conductors 4b and 4c.

  Thus, by applying the conductive adhesive 6 to the substrate side surfaces 1a and 1b, the potential of the substrate side surfaces 1a and 1b becomes extremely stable. Therefore, the random electric lines of force EF shown in FIG. Does not occur. Since the CPW traveling wave electrode 4 has a symmetrical structure having the ground conductors 4b and 4c on both sides of the center conductor 4a, the electrical connection between the conductive adhesive 6 and the ground conductors 4b and 4c is easy.

  Although it is effective to form the conductive adhesive 6 on both the substrate side surfaces 1a and 1b and to electrically connect the conductive adhesive 6 to the ground conductors 4b and 4c, the substrate is somewhat less effective. There are various combinations such as only one of the side surfaces 1a and 1b or the electrically conductive adhesive 6 not being electrically connected to the ground conductor 4b (and the ground conductor 4c).

[Fourth Embodiment]
FIG. 4 shows a cross-sectional view of the optical modulator according to the fourth embodiment of the present invention at a position corresponding to AA ′ of the prior art shown in FIG. In addition to the structure of the third embodiment shown in FIG. 3, in this embodiment, the conductive adhesive 6 is applied to the bottom surface 1 c of the x-cut LN substrate 1 in addition to the substrate side surfaces 1 a and 1 b of the x-cut LN substrate 1. Has also formed. This makes it possible to further stabilize the potential of the substrate side surfaces 1a and 1b, and also has an advantage that the x-cut LN substrate 1 can be firmly fixed to the metal housing 7.

[About each embodiment]
The regions of the substrate side surfaces 1a and 1b on which the conductive film 5 and the conductive adhesive 6 are formed may be the entire side surface of the x-cut LN substrate 1 or a part thereof. In addition, it is preferable to form on the side surface of the x-cut LN substrate 1 corresponding to the region of the interaction optical waveguides 3a and 3b.

Here, the purpose of the present invention will be confirmed again. When the smallest value among the thickness L _H of the x-cut LN substrate 1, the length L _L in the direction along the interaction optical waveguides 3 a and 3 b, and the width L _{W in} the direction perpendicular thereto is set to L, x - resonant frequency _{f r} of the high-frequency electric signal in the cut LN substrate 1 is approximately _{f r = (c 0/2} ) · {p / (n L · L)} (1)
Is determined. Here, c ₀ is the speed of light in vacuum, p is a positive integer, and n _L is the refractive index of the x-cut LN substrate 1 in the L direction.

Generally, the thickness L _H is the smallest among the thickness L _H , the length L _L , and the width L _W of the x-cut LN substrate 1, and therefore the resonance frequency _fr is determined by the substrate thickness L _H. Therefore, in the present invention, the formation of the conductive film 5 or the conductive adhesive 6 on the substrate side surfaces 1a and 1b is not a measure for the resonance frequency of the x-cut LN substrate 1, but only for the pyroelectric effect. This is a countermeasure for the charges induced on the side surfaces 1a and 1b.

  The substrate used in the optical modulator according to the present invention has been described using the LN substrate as an example. However, other various substrates having an electro-optic effect such as lithium tantalate may be used.

  Further, although the conductive film in the optical modulator according to the present invention has been described as Si or SiOx, it goes without saying that other materials including metal oxides may be used.

  As described above, the conductive film 5 and the conductive adhesive 6 are formed on both the substrate side surfaces 1a and 1b, and the conductive film 5 and the conductive adhesive 6 are electrically connected to the ground conductors 4b and 4c. Although this is effective, the effect is somewhat reduced, but only one of the substrate side surfaces 1a and 1b, or the conductive film 5 and the conductive adhesive 6 are not electrically connected to the ground conductor 4b (and the ground conductor 4c). There are various combinations.

  Further, as described above, the structure of the conductive layer and the optical waveguide in describing each embodiment has been mainly described as a structure that is symmetric with respect to the center of the center conductor. Needless to say.

  Furthermore, although the CPW electrode has been described as an example of the traveling wave electrode, it goes without saying that various traveling wave electrodes such as an asymmetric coplanar strip (ACPS) and a symmetric coplanar strip (CPS), or a lumped constant type electrode may be used.

  In addition to the Mach-Zehnder optical waveguide, it goes without saying that other optical waveguides such as directional couplers and straight lines may be used as the optical waveguide.

  Further, in each of the above embodiments, the substrate has an x-cut surface orientation. However, the surface orientation in each of the above-described embodiments is a main surface orientation, and other surface orientations are subordinate thereto. It goes without saying that the orientation may be mixed, and it is needless to say that the substrate may have a z-cut or y-cut plane orientation (or a mixture).

  As described above, the optical modulator according to the present invention can suppress the temperature drift, and is useful as a long-life and highly reliable optical modulator.

Sectional drawing of the optical modulator which concerns on 1st Embodiment of this invention. Sectional drawing of the optical modulator which concerns on 2nd Embodiment of this invention. Sectional drawing of the optical modulator which concerns on 3rd Embodiment of this invention. Sectional drawing of the optical modulator which concerns on 4th Embodiment of this invention. A perspective view of an LN optical modulator constructed using an x-cut LN substrate according to the prior art. Sectional view along A-A 'in FIG. The figure explaining the operation principle of the optical modulator by a prior art The figure explaining the problem of the optical modulator by a prior art The figure explaining the temperature drift of 1st prior art

Explanation of symbols

1: x-cut LN substrate 1a, 1b: side surface of x-cut LN substrate 1 1c: bottom surface of x-cut LN substrate 1 2: SiO ₂ buffer layer 3: optical waveguide 3a, 3b: interaction optical waveguide 4: progression Wave electrode 4a: Center conductor 4b, 4c: Ground conductor 5: Conductive film 6: Conductive adhesive 7: Metal casing DEF: Electric field lines due to applied voltage TEC: Charge induced by pyroelectric effect EF: Pyroelectric Electric field lines due to the charge induced by the effect

Claims (6)

A substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and a central conductor formed on one surface side of the substrate for applying a voltage for modulating the light And an electrode made of a ground conductor, and the optical waveguide has an interactive optical waveguide for modulating the phase of the light by applying the voltage between the center conductor and the ground conductor, and the substrate Is an optical modulator having a substrate side surface in which electric charges are induced by a pyroelectric effect on the outermost side of a cross section perpendicular to the longitudinal direction of the interaction optical waveguide,
An optical modulator comprising a conductive film formed directly on the side surface of the substrate.   The optical modulator according to claim 1, wherein the conductive film is made of a semiconductor film.   The optical modulator according to claim 1, wherein the conductive film is made of a conductive adhesive.   4. The optical modulator according to claim 1, wherein the conductive film is electrically connected to the ground conductor.   5. The light according to claim 1, wherein the ground conductor has a substantially symmetric shape with respect to a center line extending in a longitudinal direction of the center conductor above the interaction optical waveguide. Modulator.   5. The light modulation according to claim 1, wherein the ground conductor has an asymmetric shape with respect to a center line extending in a longitudinal direction of the center conductor above the interaction optical waveguide. vessel.

Images