JP2000028979A - Polarization independent light control element - Google Patents

Abstract

[PROBLEMS] To provide a light control element which does not depend on the polarization state of input light at a low driving voltage. [PROBLEMS] A plurality of optical waveguides formed near the surface of an optical substrate having an electro-optic effect And a plurality of electrodes disposed on the optical substrate surface on which the plurality of optical waveguides are formed, and a voltage induced by applying microwaves to the plurality of electrodes causes mutual interaction between the optical waveguide and the plurality of electrodes. An optical control element in which an action region is formed and a polarization conversion unit for rotating an optical polarization plane substantially 90 degrees around at least one of a plurality of optical waveguides. The light control element has at least one set of phase difference adjusting electrodes, which is different from the plurality of electrodes for applying microwaves, on the plurality of optical waveguides.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization-independent light control element having a small driving voltage and independent of the polarization state of an input wave, among light control elements such as an optical modulator and an optical switch. It is.

[0002]

2. Description of the Related Art In a high-speed and large-capacity optical transmission system or optical switching system, an optical control element having a small driving voltage is useful for high-speed driving. Examples of this type of light control element include an optical switch, a phase modulator, and a light intensity modulator, and are used as a basic technology in a mechanical type that mechanically moves a prism or an optical fiber, a quartz glass waveguide, or the like. Thermo-optic effect type, Ti diffusion LiNbO ₃
It is roughly classified into an electro-optic effect type used for a waveguide and the like. Among them, the mechanical type or thermo-optical effect type optical control element has no polarization dependence, but has a problem that its response speed is as slow as about 1 msec or more.

On the other hand, the electro-optic effect type light control element has a characteristic that the response speed is extremely fast. However, even if the same voltage or electric field is applied to the optical waveguide, there is a problem that the refractive index changes depending on the polarization direction of the light, and the operation depends on the polarization direction.

Japanese Patent Laid-Open No. 7-56199 discloses a configuration example in which a thin-film wavelength plate is inserted in the middle of an optical waveguide as a means for solving the above-mentioned polarization dependence and reducing the driving voltage. I have. FIG. 5A is a top view of the Mach-Zehnder optical switch disclosed in the publication, and FIG.
FIG. 5B is a cross-sectional view along the line AA ′ in FIG. In this conventional example, a z-plate LiN having an electro-optical effect is used.
Two optical waveguides 2 are formed on a bO ₃ (LN) substrate 1 by thermal diffusion of Ti. In such an optical waveguide,
Two orthogonal polarization modes, called a TM mode having an electric field component perpendicular to the substrate surface and a TE mode having an electric field component in the horizontal direction, propagate. On the substrate (on the optical waveguide), an electrode (traveling wave electrode) 3 to which a microwave from a microwave power source 7 can be applied is formed. A groove 6 is formed in the middle of the optical waveguide, and a half-wave plate 5 for polarization conversion is inserted into the groove. Each electrode divided by the groove 6 is connected by a conductive wire 9 or the like.
8 is a terminating resistor. In order to reduce the absorption loss of the light propagating through the optical waveguide by the electrodes, the substrate 1 and the electrode 3
0.2~1μm buffer layer 16 of SiO ₂ or the like between the
It is formed with a thickness of about. The incident light 14 enters from the first input port 10 and the second input port 11, passes through two directional couplers (3 dB couplers) 4a and 4b,
Outgoing light 15 is emitted from the second output ports 12 and 13.

Here, when incident light having a constant intensity (combination of TM mode and TE mode) is made incident on the optical waveguide, the light is converted into two light guides in each mode by a 3 dB coupler 4a at the front stage constituting the Mach-Zehnder interferometer. Distribute power to the wave path. In the region where the optical waveguide and the voltage applied to the electrode interact, the phase of light in each mode changes according to the input voltage. Here, the phase change amount of the TM mode light in this region is φTM1 (V), and the phase change amount of the TE mode light is φTE1
(V). When the light propagates through the half-wave plate 5 inserted in the middle of the optical waveguide, the light propagation mode rotates by 90 °, and T
The M mode light is converted to the TE mode light, and the TE mode light is converted to the TM mode light. Again, the subsequent electrode (electrode 3
(The portion on the emission side of the half-wave plate 5) changes the phase of the light according to the input voltage. In this region, the phase change amount of the TM mode light is φTM2 (V), and the phase change amount of the TE mode light is φTE2 (V). When the total amount of phase change is φm (V) when the incident light is TM mode light and φe (V) when the incident light is TE mode light,

[0006]

## EQU1 ## φm (V) = φTM1 (V) + φTE2 (V) (1)

[0007]

## EQU2 ## φe (V) = φTE1 (V) + φTM2 (V) (2) Further, in the subsequent 3 dB coupler 4b, the same mode light in the two optical waveguides interferes with each other, and φm (V)
Alternatively, when φe (V) is an even multiple of π, the intensity of the output light of the mode at the second output port 13 becomes maximum, and when φe (V) is an odd multiple of π, the output light of the mode at the first output port 12 is obtained. Strength is maximum.

FIG. 6 is a graph showing the dependency of the light output at the second output port 13 on the applied voltage. The case of TM light incidence is shown by a solid line, and the case of TE light incidence is shown by a dotted line.
The difference between the voltage at which the intensity of the output light is maximum and the minimum output voltage adjacent to the maximum output is usually called a half-wavelength voltage Vπ (or switching voltage Vs).

Here, assuming that the ratio of the phase change amount of the TE mode light to the phase change amount of the TM mode light is η, φTE1 (V) = ηφTM1 (V), φTE2 (V) = ηφTM
2 (V), and when the interaction length is equal in the first and second stages, φTM
(V) = φTM1 (V) = φTM2 (V), φTE (V) = η
Since φTM (V) = φTE1 (V) = φTE2 (V),
The sum of the phase change amounts in the interaction regions before and after each mode is equal, and [φm (V) = φe (V) = (1+
η) φTM (V)]. Therefore, as shown in FIG. 6A, the drive voltage becomes equal for each incident mode light, and the polarization dependence of the incident mode is eliminated.

[0010]

However, in the above conventional example, this is an ideal case, and in reality, due to the manufacturing accuracy and non-uniformity of the optical waveguide and the electrodes, two regions are required in the front and rear regions. In many cases, a phase difference has already occurred between the waveguides. Assuming that the phase differences with respect to the TM-mode light and the TE-mode light in the first stage are ΔφTM1 and ΔφTE1, respectively, and those in the second stage are ΔφTM2 and ΔφTE2, respectively, the expressions (1) and (2) are replaced as follows. .

[0011]

[Equation 3] φm (V) = φTM1 (V) + φTE2 (V) −Δφm (3)

[0012]

[Expression 4] Δφm = ΔφTM1 + ΔφTE2 (4)

[0013]

Equation 5 φe (V) = φTE1 (V) + φTM2 (V) −Δφe (5)

[0014]

Δφe = ΔφTE1 + ΔφTM2 (6) In general, ΔφTM1 ≠ ΔφTM2 and ΔφTE1 ≠ ΔφTE2, and therefore, as shown in FIG.
A voltage difference corresponding to Δφm−Δφe is generated between the ON point and the OFF point, and as a result, the ON / OFF ratio (extinction ratio) is deteriorated. In addition, φTM1 (V) ≠ φTM2 where the efficiency is slightly different between the former stage and the latter stage.
In the case of (V), φTE1 (V) ≠ φTE2 (V), the extinction ratio may be similarly deteriorated.

An object of the present invention is to solve the above-mentioned conventional problems and to provide a light control element which has a low driving voltage and does not depend on the polarization state of input light.

[0016]

In order to achieve the above-mentioned object, a polarization-independent light control element according to the present invention comprises a plurality of optical waveguides formed near the surface of an optical substrate having an electro-optic effect; And a plurality of electrodes disposed on an optical substrate surface on which the plurality of optical waveguides are formed, wherein a voltage induced by applying microwaves to the plurality of electrodes causes mutual interaction between the plurality of optical waveguides and the plurality of electrodes. In an optical control element in which an action region is formed and a polarization conversion unit for rotating an optical polarization plane substantially 90 degrees around at least one position of the plurality of optical waveguides, a plurality of At least one set of phase difference adjusting electrodes different from the electrodes is provided on a plurality of optical waveguides.

Here, the light control element may be a Mach-Zehnder type optical switch. Preferably, the light control element is
2 inputs, 2 inputs, 2
An output optical waveguide, a polarization conversion section comprising a half-wave plate inserted in a groove formed in the optical waveguide, and a plurality of electrodes formed before and after the groove for applying microwaves. And a set of phase difference adjusting electrodes is disposed between the directional coupler and the electrode for applying microwaves.
Preferably, the light control element is connected to first and second input ports and first and second output ports formed on one end side of the substrate, and first and second input ports. Directional coupler, a second directional coupler connected to the first and second output ports, and a second direction from the first directional coupler to the other end of the substrate via the folded portion. A plurality of optical waveguides reaching the sexual coupler, and a polarization conversion unit composed of a reflection type wavelength plate and a reflection mirror formed at the other end of the substrate, and at least one set of phase difference adjustment electrodes transmits microwaves. It is provided between an electrode for application and at least one of the first and second directional couplers.

Alternatively, the light control element may be a Mach-Zehnder type light intensity modulator. Preferably, the light control element includes an input port and an output port formed on one end side of the substrate, a first Y branch connected to the input port, and a second Y branch connected to the output port. A polarization conversion comprising an optical waveguide that reaches from the first Y branch to the other end of the substrate, reaches the second Y branch through the folded portion, and a reflection type wave plate and a reflection mirror formed at the other end of the substrate. And at least one set of phase difference adjusting electrodes is provided between the electrode for applying microwaves and at least one of the first and second Y branch portions. Also preferably, the light control element includes an input port formed at one end of the substrate, a first Y branch connected to the input port, and a second Y branch formed at the other end of the substrate. Section, an output port connected to the second Y-branch, a plurality of optical waveguides connected to the first and second Y-branch, and one inserted into a groove formed in the optical waveguide. And a plurality of electrodes for applying microwaves formed before and after the groove, and a set of phase difference adjusting electrodes is formed of a directional coupler and a micro-wavelength plate. It is arranged between the electrodes for applying a wave.

Here, a DC voltage is applied to at least one set of phase difference adjusting electrodes.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a light control element according to the present invention,
In addition to a plurality of electrodes for applying microwaves in the related art, at least one set of phase difference adjusting electrodes is provided. By adjusting the voltage applied to the phase difference adjusting electrode, it is possible to reduce the deterioration of the extinction ratio due to the difference between the driving voltage and the driving point, so that it is possible to obtain a characteristic with extremely little polarization dependence.

[0021]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) FIG. 1A is a view for explaining a Mach-Zehnder type optical switch as a first embodiment of an optical control element according to the present invention, and FIG. 1A is a plan view. FIG. 1B is a cross-sectional view taken along line AA ′ of FIG. Z having an electro-optic effect as in the conventional example
To the plate LiNbO ₃ (LN) substrate 1 by Ti thermal diffusion
An optical waveguide 2 is formed. In such an optical waveguide, two orthogonal polarization modes called a TM mode having an electric field component perpendicular to the substrate surface and a TE mode having an electric field component in the horizontal direction propagate. On the substrate (on the optical waveguide), an electrode (traveling wave electrode) 3 to which a microwave from a microwave power supply 7 can be applied is formed.
A groove 6 is formed in the middle of the optical waveguide, and a half-wave plate 5 for polarization conversion is inserted into the groove. Each electrode divided by the groove 6 is connected by a conductive wire 9 or the like. 8 is a terminating resistor. In order to reduce the absorption loss of the light propagating through the optical waveguide by the electrode, the substrate 1
A buffer layer 16 of SiO ₂ or the like
It is formed with a thickness of about 1 μm. The incident light 14 enters from the first input port 10 and the second input port 11 and exits from the first and second output ports 12 and 13 through two directional couplers (3 dB couplers) 4a and 4b. Fifteen
Is emitted.

A characteristic difference of the present embodiment from the conventional example shown in FIG. 5 is that two sets of phase difference adjusting electrodes 17 and 19 are provided separately from the electrode 3 for applying microwaves. 5 is disposed between the 3 dB couplers 4 a and 4 b and the electrode 3 in the former and latter stages of FIG. The DC power supply 18 (V1),
20 (V2) is connected.

Next, the operation of the optical switch having the above configuration will be described.

For the TM mode light incidence and the TE mode light incidence, the same notation as in the conventional example is given.

[0026]

(7) φm (V) = φTM1 (V) + φTE2 (V) −Δφm + δφTM1 (V1) + δφTE2 (V2) (7)

[0027]

(8) φe (V) = φTE1 (V) + φTM2 (V) −Δφe + δφTE1 (V1) + δφTM2 (V2) (8) Here, δφTM1 (V1) and δφTE1 (V
1) is the amount of phase change by the phase difference adjusting electrode 17 in the preceding stage,
δφTM2 (V2) and δφTE2 (V2) are the amounts of phase change by the phase difference adjusting electrode 19 at the subsequent stage.

Here, assuming that the ratio of the phase change amount of the TE mode light to the phase change amount of the TM mode light in the phase difference adjusting electrode is ηi, δφTE1 (V1) = ηi · δφ
TM1 (V1), δφTE2 (V2) = ηi · δφTM2 (V
2). In order to obtain the ideal state shown in FIG. 3A (equivalent to FIG. 6A),

[0029]

-Δφm + δφTM1 (V1) + ηi · δφTM2 (V2) = 0 (9)

[0030]

## EQU10 ## This is nothing but solving the simultaneous equations of -Δφe + ηi · δφTM1 (V1) + δφTM2 (V2) = 0 (10). FIG. 2 shows a case where the horizontal axis is δφTM1 (V1) and the vertical axis is δφTM2 (V2), and the straight lines of the equations (9) and (10) are drawn. If there is a wave dependence), the straight lines (9) and (10) always intersect, and the intersection point becomes the solution of the simultaneous equations. When the values of the voltages V1 and V2 of the phase difference adjusting electrode corresponding to the amount of phase change at this intersection are set, and when the microwave applied voltage is 0, in any input mode, as shown in FIG. Since the state in which the optical output of the second output port 13 is maximized can be realized, the polarization independent operation can be realized. For example, ηi = 1/3, Δφm is 2V, Δφe
Is 3V, the voltage at the intersection is V1 = 9/8 (1.125) V, V2 = 21
/ 8 (2.625) V. By applying the voltage V1 to the phase difference adjusting electrode 17 and the voltage V2 to the phase difference adjusting electrode 19, a polarization independent operation becomes possible.

From equations (9) and (10),

[0032]

１１φm−Δφe = (1−ηi) δφTM1 (V1) − (1−ηi) δφTM2 (V2) The phase difference adjusting electrode 17 is set to the values of the voltages V1 and V2 satisfying (11).
Nineteen voltages may be set. In this case, FIG.
As shown in (2), when the microwave voltage applied to the electrode 3 is other than 0, the optical output of the second output port 13 becomes maximum.
What is necessary is just to make it possible to superimpose a DC bias on the microwave application voltage and to superimpose the bias voltage Vb that maximizes the optical output on the microwave voltage to operate.

Alternatively, the state shown in FIG. 3B can be obtained by disposing only one of the phase difference adjusting electrode 17 and the phase adjusting electrode 19.

The expression (9), the expression (10) or the expression (1)
The extinction ratio does not significantly deteriorate even in a state slightly deviating from the expression (1).

(Embodiment 2) FIG. 4 is a plan view of a second embodiment of the present invention in which an intensity modulator is formed by using a reflection type wave plate for a polarization converter. 1 / in the first embodiment shown in FIG.
Instead of the two-wavelength plate, 1/4 reflective waveplate constructed using a wavelength plate 21 and the reflecting mirror 22, and z plate LiNbO ₃ in the vicinity of the end face provided with a folded optical waveguide portion 24 reflective polarization of the substrate 1 And a 2 × 2 coupler 4
This is a Mach-Zehnder type light intensity modulator in which a and b are replaced by Y-branch multiplexing units 23a and 23b. Light enters from an input port 25 and exits from an output port 26. The polarization-independent operation of this embodiment is the same as that described in the first embodiment.

In this embodiment, a polarization conversion section is formed by the wave plate and the reflection mirror on the end face of the substrate 1. A groove for inserting the wave plate is formed near the end face, and the reflection type wave plate is inserted there. Thus, the polarization converter may be configured. Although the folded portion 24 of the waveguide is shown as a V-shaped example, the folded portion may be constituted by a directional coupler or a multi-mode interferometer.

Further, the light intensity modulator can be constructed without using the reflection type polarization converter. That is, the first and second input ports 10, 11, in the configuration shown in FIG.
Instead of the first directional coupler 4a, and the first and second output ports 12, 13 and the second directional coupler 4b, FIG.
The input port 25 and the first Y branch 23 as shown in FIG.
By forming the output port 26 and the second Y branching portion 23b and the output port 26, it is possible to configure a transmission type light intensity modulator instead of a reflection type.

Further, an optical switch can be formed by using a reflection type polarization converter. That is, instead of the input port 25 and the first Y branch 23a and the second Y branch 23b and the output port 26 in the configuration shown in FIG.
The first and second input ports, the first directional coupler 4a, and the first and second output ports 1 as shown in FIG.
By forming the second directional coupler 13 and the second directional coupler 4b on one end side of the substrate, a reflection type optical switch can be configured.

In the above embodiment, the optical waveguide by Ti thermal diffusion has been described. However, the optical waveguide may be formed by, for example, an ion exchange method, or may be a ridge type optical waveguide. Also, instead of a directional coupler, a Y branch circuit or X branch
A branch circuit, a multi-mode interference optical waveguide, or the like may be used, and a directional coupler, an X-branch circuit, a multi-mode interference optical waveguide, or the like may be used instead of the Y-branch multiplexing unit. In this way, a 1 × 2 optical switch can be configured.
Further, a phase modulator can be configured by using a linear optical waveguide as the optical waveguide. Also, the case where the buffer layer material is SiO ₂ has been described, but a dielectric or semi-insulating material such as alumina or Teflon may be used. The electrode material to which the microwave is applied may be a traveling wave type electrode structure such as a coplanar stripline or a coplanar waveguide.

In the above, the light control element using the electro-optic effect in the z-plate LiNbO ₃ substrate has been described as an embodiment. In addition, an x-plate or y-plate iNbO ₃ substrate, another ferroelectric Needless to say, the configuration of the present invention is also very effective for a light control element using the electro-optic effect of an anisotropic substrate such as a semiconductor or an organic substance.

[0041]

As described above, according to the present invention,
A plurality of optical waveguides formed near the surface of the optical substrate having an electro-optic effect; and a plurality of electrodes arranged on the optical substrate surface on which the plurality of optical waveguides are formed. An interaction region between a plurality of optical waveguides and a plurality of electrodes is formed by a voltage induced by applying a wave,
And an optical control element provided with a polarization converter for rotating the optical polarization plane substantially at about 90 ° at at least one of the plurality of optical waveguides, wherein at least one of the plurality of electrodes for applying microwaves is different from the plurality of electrodes for applying microwaves. Equipped with a set of phase difference adjusting electrodes,
By controlling the phase of the propagating light by adjusting the voltage applied to the pair of phase difference adjusting electrodes, it is possible to reduce the deterioration of the extinction ratio due to the difference between the driving voltage and the driving point. A light control element having extremely low characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of a light control element according to the present invention.

FIG. 2 is a graph showing a relationship between a phase change amount of a phase difference adjusting electrode due to voltages V1 and V2 and Δφm and Δφ.

FIG. 3 is a light output characteristic diagram for explaining the operation of the light control element according to the present invention.

FIG. 4 is a view showing a second embodiment of the light control element according to the present invention.

FIG. 5 is a diagram showing an example of a conventional polarization independent element.

6 is an output characteristic diagram of an ideal state and an actual state in the conventional example shown in FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 LiNbO ₃ substrate 2 Optical waveguide 3 Microwave application electrode 4a, 4b Directional coupler (3 dB coupler) 5 1/2 wavelength plate 6 Wavelength plate insertion groove 7 Microwave power supply 8 Termination resistance 9 Microwave application electrode connection Unit 10 First input port 11 Second input port 12 First output port 13 Second output port 14 Incident light 15 Outgoing light 16 SiO ₂ buffer layer 17, 19 Phase difference adjusting electrode 18, 20 Phase difference adjustment Power supply 21 1/4 wavelength plate 22 Reflection mirror 23a, 23b Y-branch multiplexing section 24 Folded optical waveguide section 25 Input port 26 Output port

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kazuto Noguchi 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo F-term in Nippon Telegraph and Telephone Corporation (reference) 2H047 AA04 AB04 BB12 DD02 EE12 GG03 HH08 2H079 BA02 BA03 CA05 DA02 EA04 EB04 KA14 KA17

Claims (8)

[Claims] A plurality of optical waveguides formed near a surface of an optical substrate having an electro-optic effect; and a plurality of electrodes disposed on an optical substrate surface on which the plurality of optical waveguides are formed. A polarization converter for forming an interaction region between the plurality of optical waveguides and the plurality of electrodes by a voltage induced by applying microwaves to the plurality of electrodes, and for substantially rotating the polarization plane by about 90 ° In a light control element comprising at least one of the plurality of optical waveguides, at least one set of phase difference adjusting electrodes different from the plurality of electrodes for applying the microwave are provided on the plurality of optical waveguides. A polarization-independent light control element, comprising: 2. The polarization-independent light control element according to claim 1, wherein the light control element is a Mach-Zehnder optical switch. 3. The optical control element comprises: a two-input two-output optical waveguide in which a front-stage and a rear-stage directional coupler are formed;
A polarization conversion section comprising a half-wave plate inserted into a groove formed in the optical waveguide; and a plurality of electrodes formed before and after the groove for applying microwaves. 3. The polarization-independent light control element according to claim 2, wherein a pair of phase difference adjusting electrodes are arranged between the directional coupler and an electrode for applying the microwave. 4. The light control element is connected to first and second input ports and first and second output ports formed on one end side of the substrate and the first and second input ports. A first directional coupler, a second directional coupler connected to the first and second output ports, and a turn from the first directional coupler reaching the other end of the substrate. A plurality of optical waveguides that reach the second directional coupler via a section, and a polarization conversion section including a reflection type wave plate and a reflection mirror formed at the other end of the substrate; 3. The polarization adjusting device according to claim 2, wherein the phase difference adjusting electrode is provided between the electrode for applying the microwave and at least one of the first and second directional couplers. Wave independent light control element. 5. The polarization-independent light control element according to claim 1, wherein the light control element is a Mach-Zehnder type light intensity modulator. 6. The light control element includes an input port and an output port formed at one end of the substrate, a first Y-branch connected to the input port, and a first Y-branch connected to the output port. An optical waveguide reaching the second Y-branch from the first Y-branch to the other end of the substrate via the folded portion, and a reflection type formed at the other end of the substrate. Having a polarization plate comprising a wave plate and a reflection mirror,
6. The device according to claim 5, wherein the at least one pair of phase difference adjusting electrodes is provided between an electrode for applying the microwave and at least one of the first and second Y branch portions. The polarization-independent light control element according to claim 1. 7. The light control element includes an input port formed at one end of the substrate, a first Y-branch connected to the input port, and a second port formed at the other end of the substrate. A second Y-branch, an output port connected to the second Y-branch, a plurality of optical waveguides connected to the first and second Y-branches, and a groove formed in the optical waveguide. Having a polarization conversion portion consisting of a half-wave plate inserted in the groove, and a plurality of electrodes for applying microwaves formed before and after the groove, wherein the set of phase difference adjusting electrodes is The polarization independent light control element according to claim 5, wherein the polarization independent light control element is arranged between the directional coupler and an electrode for applying the microwave. 8. The polarization independent light control element according to claim 1, wherein a DC voltage is applied to said at least one pair of phase difference adjusting electrodes.