JP2007256676A - Electro-optic element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain excellent electro-optic characteristics by arranging a plurality of electrodes in a traveling direction of light and applying electric fields to the electrodes in opposite directions. <P>SOLUTION: The electro-optic element using an electro-optic crystal 40 having secondary electro-optic effect for an applied electric field has two electrode pairs 42 and 43, as electrode pairs for applying electric fields to the electro-optic crystal 40 perpendicularly to the traveling direction of the light, in the traveling direction of the light. The direction of the electric field applied to the first electrode pair 42 is made opposite from the direction of the electric field applied to the second electrode pair 43. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electro-optical element, and more particularly to an electro-optical element that changes the phase of light by an electric signal using an electro-optical crystal.

  Conventionally, various optical functional parts using electro-optic crystals have been put into practical use. These optical functional parts utilize the fact that when a voltage is applied to the electro-optic crystal, the refractive index of the crystal changes due to the electro-optic effect. For example, an optical phase modulator using an electro-optic crystal changes the phase of light by changing the speed of light passing through the crystal by changing the refractive index of the crystal. In addition, an optical switch and an optical intensity modulator can be configured using this optical phase modulator.

FIG. 1 shows a configuration of a conventional optical phase modulator (for example, Patent Document 1). Two electrodes, an anode 12a and a cathode 12b are formed on both sides of the waveguide 11 formed on the LiNbO ₃ substrate 10 (FIG. 1A). A voltage is applied between the anode 12a and the cathode 12b to change the electric field applied to the waveguide 11 (FIG. 1B). Thereby, the phase of the optical signal passing through the waveguide 11 is modulated by changing the refractive index of the electro-optic material.

In the optical phase modulator shown in FIG. 1, the phase ΔΦ of light to be modulated is defined by the length of a portion to which an electric field is applied in the light propagation direction being L, the wavelength of light being λ, If Δn,
ΔΦ = 2π × Δn × L / λ (1)
It is expressed. Furthermore, Δn is
Δn = −0.5n ³ rE (2)
It is expressed. Here, E is the electric field applied to the waveguide, r is the primary electro-optic coefficient, and n is the refractive index.

In recent years, KTa _1-x Nb _x O ₃ (KTN) crystals having a large electro-optic coefficient have attracted attention. Since the KTN crystal has a large electro-optic coefficient, an optical switch can be configured with a small size and a small driving voltage. FIG. 2 shows a configuration of an optical phase modulator using a conventional KTN crystal. In the optical phase modulator, an anode 22 a and a cathode 22 b are formed on opposite surfaces of the square KTN crystal 20. A bias electric field is applied between the anode 22a and the cathode 22b, and a signal electric field is superimposed between the two electrodes under the bias electric field. Thereby, the electric field of the KTN crystal 20 changes according to the signal, and the refractive index between the two electrodes changes, so that the phase of incident light passing through the KTN crystal 20 can be changed according to the signal. Using this optical phase modulator, optical functional elements such as a light intensity modulator, an optical switch, and a Q switch can be configured.

The bias field is used to keep the initial phase in the proper phase. In addition, a material having a secondary electro-optic effect such as a paraelectric KTN crystal has a feature that the stronger the bias electric field, the greater the electro-optic effect. FIG. 3 shows the relationship between applied voltage and refractive index in a material having a secondary electro-optic effect. The larger the bias voltage V _bias , the larger the refractive index change Δn modulated by the signal voltage V _s .

JP 05-346560 A

  However, the KTN crystal has a problem that light is deflected in the direction of the electric field when an electric field is applied. As shown in FIG. 2, when a voltage is applied to modulate the phase of incident light, the emitted light is deflected by an angle a in the direction of application of the electric field, and the light is diverted from a desired path. Uniform modulation characteristics cannot be obtained. This deflection characteristic is reversed when the polarity is changed around the voltage 0V. Therefore, if the voltage applied to the electrode is reversed, the deflection occurs in the reverse direction. Further, if the bias voltage to be applied is set low in order to suppress the influence of deflection, there is a problem that good electro-optical characteristics cannot be obtained.

  The present invention has been made in view of such a problem, and an object of the present invention is to suppress a deflection phenomenon by arranging a plurality of electrodes in the traveling direction of light and applying an electric field in the opposite direction. An object of the present invention is to provide an electro-optic element that can obtain good electro-optic characteristics.

  In order to achieve the above object, the present invention provides an electro-optic element using an electro-optic crystal having a secondary electro-optic effect with respect to an applied electric field. An electrode pair for applying an electric field perpendicular to the traveling direction of light with respect to the electro-optic crystal, comprising two electrode pairs arranged in the traveling direction of the light, and the direction of the electric field applied to the first electrode pair And the direction of the electric field applied to the second electrode pair is opposite.

  According to a second aspect of the present invention, the first electrode pair and the second electrode pair of the first aspect are each divided into m pieces (m is a positive integer) in the light traveling direction. It is characterized by.

The electro-optic crystal is made of KTa _1-x Nb _x O ₃ (0 <x <1) or K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <0.1 The electro-optic crystal is a paraelectric phase.

  As described above, according to the present invention, the electrode pair that applies an electric field perpendicular to the light traveling direction with respect to the electro-optic crystal, the electrode pair includes two electrode pairs in the light traveling direction, Since the direction of the electric field applied to the first electrode pair is opposite to the direction of the electric field applied to the second electrode pair, deflection can be suppressed and good electro-optical characteristics can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this embodiment, two sets of electrodes are arranged in the light traveling direction, and an electric field in the opposite direction is applied to cancel the deflection. The light that has passed between the first electrode pair is deflected by an angle a and is input between the second electrode pair. Since the electric field of the same value is applied to the second electrode pair in the opposite direction to that of the first electrode pair, the second electrode pair is deflected by the angle −a to cancel the deflection by the first electrode pair.

  Similarly, 2m sets of electrodes (m is a positive integer) are arranged in the light traveling direction, and the electric field application direction of the m electrodes and the electric field application direction of the remaining m electrodes are set in opposite directions. To do. In other words, the deflection is canceled by dividing the first electrode pair and the second electrode pair into m pieces in the light traveling direction.

  FIG. 4 shows the configuration of the electro-optic element according to Example 1 of the invention. In the electro-optic element, an anode 42a and a cathode 42b, which are a first electrode pair, and an anode 43a and a cathode 43b, which are a second electrode pair, are formed on opposing surfaces of a square KTN crystal 40. The KTN crystal 40 is a crystal having a length of 5 mm in the light traveling direction, a width of 1.0 mm in a direction perpendicular to the light traveling direction, and a thickness of 0.5 mm between the two electrode pairs. The lengths in the light traveling direction of the first electrode pair 42 and the second electrode pair 43 are 1.5 mm, respectively.

The phase transition temperature of the KTN crystal 40 is 0 degree, and the temperature is maintained so as to operate at 20 degrees. A bias voltage _Vbias = 100V is applied to the first electrode pair 42, and a bias voltage _Vbias = 100V is applied to the second electrode pair 43 so that the electric field is in the opposite direction. Furthermore, the signal voltage V _s = 20V _pp is applied to the first electrode pair 42, and the signal voltage V _s = 20V _pp is applied to the second electrode pair 43 in the same phase so that the electric field is in the opposite direction. Accordingly, a voltage of 80V to 120V is applied to each electrode pair.

  When the polarity is changed around the voltage 0V, the deflection characteristic is reversed, so that the deflection generated in the first electrode pair 42 and the deflection generated in the second electrode pair 43 are offset, and there is no deflection. The light that goes straight can be taken out.

  Even if the reversal of the deflection characteristics with respect to the voltage is not complete, the majority of the deflection can be canceled if it is approximately reversed. In addition, the bias voltage and the signal voltage of the first electrode pair 42 and the second electrode pair 43 do not necessarily have the same absolute value, and the above-described difference is compensated after the application direction is reversed. It can also be adjusted.

  Further, in Example 1, two sets of electrodes are attached to one crystal. However, it may be divided into two crystals, and one set of electrodes is attached to each of the two crystals so that light passes through the two crystals. . In the first crystal, when the deflection amount of the incident light varies depending on the distance d between the optical axis of the incident light and the anode, the distance between the optical axis of the incident light and the cathode in the second crystal. The relative position of the crystal may be adjusted so that becomes d, so that the deflection may be canceled out.

  FIG. 5 shows the configuration of an electro-optic element according to Example 2 of the present invention. In the electro-optic element, four electrode pairs, anodes 52a to 55a and cathodes 52b to 55b, are formed on opposing surfaces of the square KTN crystal 50. The KTN crystal 50 is a crystal having a length of 14 mm in the light traveling direction, a width of 1.0 mm in a direction perpendicular to the light traveling direction, and a thickness of 0.5 mm between the four electrode pairs. The lengths of the light propagation directions of the four electrode pairs are each 2.0 mm, and are arranged at intervals of 2 mm.

The phase transition temperature of the KTN crystal 50 is 0 degree, and the temperature is maintained so as to operate at 20 degrees. A bias voltage V _bias = 100 V is applied to the first and third electrode pairs 52 and 54, and a bias voltage V _bias = 100 V is applied to the second and fourth electrode pairs 53 and 55 in the reverse direction. Further, the signal voltage V _s = 20V _pp is applied to the first and third electrode pairs 52 and 54, and the signal voltage V _s = 20V _pp is the same in the opposite direction to the second and fourth electrode pairs 53 and 55. Apply in phase. Accordingly, a voltage of 80V to 120V is applied to each electrode pair.

  Similar to the first embodiment, the deflection generated in the first and third electrode pairs 52 and 54 and the deflection generated in the second and fourth electrode pairs 53 and 55 are canceled out, so that the light traveling straight without deflection can be obtained. It can be taken out.

Although this embodiment has been described using a phase modulator, the present invention can be applied to optical functional elements such as a light intensity modulator, an optical switch, and a Q switch using this phase modulator. The electro-optic crystal is not limited to a KTN crystal but can be a K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <0.1) crystal, Applies in the paraelectric phase.

It is a figure which shows the structure of the conventional optical phase modulator. It is a figure which shows the structure of the optical phase modulator using the conventional KTN crystal | crystallization. It is a figure which shows the relationship between the applied voltage and refractive index in the material which has a secondary electro-optic effect. 1 is a diagram illustrating a configuration of an electro-optic element according to Example 1 of the invention. FIG. FIG. 6 is a diagram illustrating a configuration of an electro-optic element according to Example 2 of the invention.

Explanation of symbols

10 LiNbO ₃ substrate 11 Waveguide 12a, 22a, 42a, 43a, 52a, 53a, 54a, 55a Anode 12b, 22b, 42b, 43b, 52b, 53b, 54b, 55b Cathode 20, 40, 50 KTN crystal

Claims (5)

In an electro-optic element using an electro-optic crystal having a secondary electro-optic effect with respect to an applied electric field,
An electrode pair for applying an electric field perpendicular to the traveling direction of light with respect to the electro-optic crystal, comprising two pairs of electrodes arranged in the traveling direction of the light,
An electro-optic element, wherein the direction of the electric field applied to the first electrode pair is opposite to the direction of the electric field applied to the second electrode pair.   2. The electro-optic according to claim 1, wherein the first electrode pair and the second electrode pair are each divided into m pieces (m is a positive integer) in the light traveling direction. element. The electro-optic element according to claim 1, wherein the electro-optic crystal is KTa _1-x Nb _x O ₃ (0 <x <1). _{3. The} electro-optic crystal according to claim ₁ , wherein the electro-optic crystal is K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <0.1). Electro-optic element. 5. The electro-optic element according to claim 3, wherein the electro-optic crystal is a paraelectric phase.

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