JP2016045312A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator capable of performing optical modulation at a low voltage.SOLUTION: It is known that when the thickness d of an electro-optic crystal is small, the larger the length L of the electro-optic crystal compared with d, the smaller a half wavelength voltage Vπ becomes. An optical modulator 10 includes the electro-optic crystal 1 that is single crystal fiber having electro-optic effect and a pair of electrode films 2 and 3 composed of a positive electrode and a negative electrode, and is configured such that the thickness d is small and the length L is large.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical modulator, and more particularly, to an electro-optic element that controls the phase or intensity of light using an optical material having an electro-optic effect.

  When an electric field is applied to the crystal from the outside, the refractive index of the crystal may change according to the applied electric field, and this phenomenon is called an electro-optic effect. A substance such as a crystal that exhibits this effect is called an electro-optic material.

  When light is incident on and transmitted through the electro-optic material, the phase of the emitted light changes because the refractive index changes when an electric field is applied. By utilizing this phenomenon, an element that electrically controls the phase of light and an element that electrically controls the intensity of light by further application have been manufactured (Patent Document 1). Since the electro-optic effect is sometimes called a Pockels effect, these elements are sometimes called a Pockels cell.

International Publication No. 2006/137408

  Conventionally, it has been necessary to apply a high voltage of about 1000 V in order to perform practically sufficient modulation. However, in order to reduce the load on the power supply for modulation, it is desirable to drive at a low voltage.

  An object of the present invention is to provide an optical modulator designed to address such problems.

  The present invention for solving the above-described problems includes an electro-optic crystal which is a single crystal fiber having an electro-optic effect, and a pair of electrode films formed on the surface of the single crystal fiber and comprising a positive electrode and a negative electrode.

  Here, the single crystal fiber may be formed in a cylindrical shape.

  The pair of electrode films may be arranged so that the positive electrode and the negative electrode face each other.

  According to the present invention, light modulation can be performed at a low voltage.

It is a figure for demonstrating the schematic structure of the optical modulator in embodiment of this invention. It is a figure for demonstrating a half wavelength voltage in a general light intensity modulator. It is a figure for demonstrating an example of the growth method of a single crystal fiber in the optical modulator of embodiment. It is a figure which shows the preparation aspect of the single crystal fiber made to grow by the crucible method. It is a figure which shows the structural example in case the optical modulator of embodiment is applied to a wavelength variable light source.

  Hereinafter, an optical modulator according to an embodiment of the present invention will be described. FIG. 1 is a schematic diagram illustrating a configuration example of an optical modulator 10 according to the present embodiment.

  [Configuration of optical modulator]

  As shown in FIG. 1, the optical modulator 10 includes a single crystal fiber 1 having an electrooptic effect, and a pair of electrode films formed on the surface of the single crystal fiber 1 and composed of a positive electrode 2 and a negative electrode 3. The single crystal fiber 1 is an electro-optic crystal having an electro-optic effect. In the example of FIG. 1, the single crystal fiber 1 has a cylindrical elongated shape.

  The electrode films 2 and 3 are disposed on the surface of the single crystal fiber 1 so as to face each other.

The diameter of the single crystal fiber 1 is about 100 μm to 2.5 mm, preferably about 100 μm to 1 mm. As a material of the single crystal fiber 1, an electro-optic material such as LiNbO ₃ is used.

  Usually, the electro-optic effect means the Pockels effect. The refractive index change Δn due to the Pockels effect means a refractive index change proportional to the electric field, and is represented by the following formula (1).

In Expression (1), n ₀ represents a refractive index before application of an electric field, E represents an electric field, and r represents an electro-optic coefficient.

  However, the electro-optic effect has a Kerr effect in addition to the Pockels effect described above. The Pockels effect is a primary effect, and the Kerr effect is a secondary effect. The refractive index change Δn due to the Kerr effect is proportional to the square of the electric field and is represented by the following formula (2).

  FIG. 2 shows one that is often used as a light intensity modulator among light modulators that utilize the electro-optic effect.

  In FIG. 2, an electro-optic crystal 11 made of an electro-optic material has a rectangular parallelepiped shape, and includes electrode films 12 and 13 on the upper and lower surfaces of the crystal 11. In FIG. 2, the length of the electro-optic crystal 11 is given by L and the thickness is given by d.

  The analyzer 20 is installed so as to transmit polarized light inclined by 45 ° with respect to the x-axis.

  The light R1 incident on the electro-optic crystal 11 includes both x-polarized light having an oscillating electric field in the x-axis direction and y-polarized light having an oscillating electric field in the y-axis direction. The x-polarized light and the y-polarized light are in phase.

  The x-polarized light and the y-polarized light do not interfere with each other in the electro-optic crystal 11, and each proceed independently at different speeds. The speed is different because the refractive index is different between x-polarized light and y-polarized light. This is called birefringence. For this reason, when light passes through the electro-optic crystal 11, a phase difference Δφ occurs between the x-polarized light and the y-polarized light.

refractive index n _y in the refractive indices n _x and y polarization in the x-polarized light is respectively represented by the following formula (3).

Since both the refractive index n ₀ and the electro-optic coefficient r in the equation (1) differ depending on the polarization direction, in the equation (3), they are distinguished by the subscripts x and y. These also differ depending on the type and crystal orientation of the electro-optic material. Considering this viewpoint, the phase difference Δφ is expressed by the following formula (4).

  In Expression (4), L represents the length of the electro-optic crystal 11 and λ represents the wavelength of light. In the formula (4), the electric field E in the formula (3) is expressed using the voltage V and the thickness d of the electro-optic crystal 11.

  The light R2 that has passed through the electro-optic crystal 11 passes through the analyzer 20, and the light R3 is emitted. When the light R2 passes through the analyzer 20, a polarization component inclined by 45 ° with respect to the x axis is extracted from both the x and y polarizations, and interference occurs between the same polarizations. When the phase difference Δφ is zero, the phases are matched, the two light components are strengthened, and the light intensity is maximized.

  On the other hand, when the phase difference Δφ is π, the two light components are weakened and the light intensity is minimized.

The first term of the above formula (4) indicates that a constant phase difference is generated even when the voltage is zero. The voltage required to shift the phase by π from this constant phase difference as the origin is called a half-wave voltage V _π . The half-wave voltage _Vπ is represented by the following formula (5).

In Equation (5), the half-wave voltage _Vπ is due to the Pockels effect, but the one due to the Kerr effect can also be obtained in the same manner as Equation (5). The half-wave voltage _Vπ due to the Kerr effect is expressed by the following formula (6).

In Equation (6), the half-wave voltage V _π is a voltage necessary for controlling the optical output from the minimum to the maximum, so that the lower the voltage, the more the modulator can be controlled at a lower voltage. I understand.

Here, as an electro-optical material, when using a lithium niobate (LiNbO _3), electro-optical effect becomes Pockels effect, for example, the refractive index 2.22, the effective electro-optic coefficient r _c becomes 20 pm/V. The half-wave voltage _{Vπ in} this case is given by the following formula (7).

In the equation (7), when 1 μm is given as the wavelength λ, the half-wave voltage V _π is given by the following equation (8).

In equation (8), the value of the thickness d of the electro-optic crystal 11 is small, and the half-wave voltage _Vπ becomes smaller as the length L of the electro-optic crystal 11 becomes larger than the value of d. for example aware of the beam diameter of a typical laser, when both 1mm and d and L, half-wave voltage V _[pi is a large value of 4600V. For example, L, and the when five times the 5mm of d is half-wave voltage V _[pi are obtained below 1000V.

  From the above viewpoint, in the optical modulator 10 of the present embodiment, the single crystal fiber 1 shown in FIG. 1 is used as the electro-optic material crystal. The single crystal fiber 1 is produced by a single crystal growth method shown in FIG. 3 to be described later. By this single crystal growth method, a single crystal already grown into an elongated shape is obtained at the time of crystal formation.

[Single crystal fiber growth technology]
FIG. 3 is a diagram for explaining an example of the growth technique of the single crystal fiber 1. This growth technique is called the laser melting pedestal method.

  In the laser melting pedestal method, the tip portion of the raw material rod 11 is heated and melted using laser light while the elongated raw material rod 11 is supplied from below to above. Then, the single crystal fiber 1 is pulled up from the melted portion 12 and grown. Hereinafter, the optical system will also be described.

  First, the conical mirror 20 is irradiated with the laser light. The irradiated laser light is reflected by the conical surface of the conical mirror 20. The laser light travels radially along a direction orthogonal to the cone axis of the conical mirror 20.

  The radial laser beam is reflected again by the conical mirror 30. Unlike the conical mirror 20 in which the outside of the conical surface reflects light, the conical mirror 30 reflects the light inside the conical surface. Therefore, the laser beam reflected on the conical mirror 30 has a donut-shaped beam cross section. Becomes a tubular beam.

  The tubular beam travels leftward in FIG. 3 and is reflected by the mirror 40. Since the mirror 40 is inclined at 45 °, the tubular beam is reflected vertically upward. In addition, since the center part of the mirror 40 has a hole part and light does not pass through the hole part, the beam reflected by the mirror 40 is also tubular.

  Next, the tubular beam reflected by the mirror 40 is incident on the concave mirror 50, reflected by the concave mirror 50, and then collected at the focal point of the concave mirror 50.

  The raw material rod 11 is supplied from below through the hole of the mirror 40, and the tip of the raw material rod 11 is melted at the focal point of the concave mirror 50.

  The seed crystal 13 is inserted from the upper part of the concave mirror 50 through the hole of the concave mirror 50 and is brought into contact with the melting part 12 that is the focal point of the concave mirror 50. Then, when the seed crystal 13 is pulled upward from the contact point, heat is removed and a crystal grows at the tip of the seed crystal 13.

  If the seed crystal 13 is continuously pulled up thereafter, a fiber-like single crystal will continue to grow. At the same time, the raw material is continuously fed to the melting portion 12 by continuously feeding the raw material rod 11 upward. Will be supplied.

In FIG. 3, LiNbO ₃ powder was first synthesized. Then, after adjusting the particle diameter of the powder with a ball mill, pressure was applied to form a pellet and fired to produce a sintered body. From this sintered body, a rod having a diameter of 3 mm was cut out to obtain a raw material rod 11. Next, this raw material rod 11 was set in the apparatus shown in FIG. 3, and the single crystal fiber 1 was produced according to the procedure described above. As the laser, a carbon dioxide laser was used.

  The diameter of the produced single crystal fiber 1 was 0.5 mm. The length of the single crystal fiber 1 was cut to 50 cm, and the cross section was optically polished. Further, a pair of electrode films 2 and 3 (FIG. 1) was deposited on the molded single crystal fiber 1 by vapor deposition to constitute an optical modulator 10. The material of the electrodes 2 and 3 is, for example, gold.

  A helium neon laser beam was incident on the optical modulator 10 with linearly polarized light that forms 45 ° with the direction of the electric field generated by the electrodes. Subsequently, the emitted light was transmitted through the polarizer. The polarization direction of the polarizer was set to be 90 ° with the polarization direction of the incident light.

  By applying a voltage between the electrodes 2 and 3, the intensity of the outgoing light R3 from the output-side polarizer could be changed. The half-wave voltage at this time was about 3V.

  The elongated single crystal that has grown is called a single crystal fiber, but it is not the same diameter as an optical fiber used for optical communication. Even if it is thin, it has a diameter of about 100 μm, and a thick one has a diameter of about 1 mm. is there.

  Such a growth method is similar to the floating zone method, but specializes in growing very thin single crystals. Since the heat is concentrated, the temperature gradient becomes very large. For this reason, the single crystal can be pulled at an ultra-high speed.

  In a general single crystal growth method, it takes several days to grow a single crystal having a length of several tens of centimeters. However, in the laser melting pedestal method shown in FIG. The single crystal fiber 1 can be manufactured.

  By configuring the optical modulator 10 (FIG. 1) using the single crystal fiber 1, the half-wave voltage can be made lower than that of the conventional one. For example, when the single crystal fiber 1 having a diameter of 500 μm and a length of 50 cm is used, the thickness d shown in FIG. 2 is 0.5 mm and the length L is 500 mm. It becomes 4.6V if it calculates according to. This indicates that the electronic device can be easily controlled by a TTL (Transistor-Transistor Logic) standard common to electronic devices, and does not require a special high-voltage amplifier.

  In addition, the growth method of the single crystal fiber 1 is not restricted to the example mentioned above. For example, FIG. 4 illustrates a production mode of the single crystal fiber 1 grown by the micro pull-down method. In FIG. 4, the raw material is melted in the crucible 60 in the same manner as a general rotational pulling method as a single crystal growth method.

  A minute hole is provided at the bottom of the crucible 60, and the melting raw material 61 oozes out little by little from this hole. Then, the melting raw material 61 is received by the seed crystal 13, and a crystal grows on the seed crystal 13.

  The single crystal fiber 1 is grown by continuously pulling down the crystal. Although the raw material 61 is melted by heating the crucible 60, the overheating of the crucible 60 may be by high frequency induction or by using a resistance heating element.

In FIG. 4, first, a powder of KTa _1-x Nb _x O ₃ was synthesized. Then, potassium carbonate was added to the synthesized powder at a ratio of 5%, and the crucible 60 was loaded. From the melted raw material, a single crystal fiber 1 of KTa _1-x Nb _x O ₃ could be produced according to the above-described procedure. A resistance heating element was used to heat the crucible 60.

  The produced single crystal fiber 1 had a diameter of 0.5 mm. The length of the single crystal fiber 1 was cut to 50 cm, and the cross section was optically polished. Further, a pair of electrode films 2 and 3 (FIG. 1) was deposited on the molded single crystal fiber 1 by vapor deposition to constitute an optical modulator 10. The material of the electrodes 2 and 3 is, for example, gold.

A helium neon laser beam was incident on the optical modulator 10 with linearly polarized light that forms 45 ° with the direction of the electric field generated by the electrodes. Subsequently, the emitted light was transmitted through the polarizer. The polarization direction of the polarizer was set to be 90 ° with the polarization direction of the incident light. Since the dielectric constant changes when the temperature of the single crystal fiber 1 of KTa _1-x Nb _x O ₃ is changed, the temperature is controlled so that the relative dielectric constant becomes 20000.

  By applying a voltage between the electrodes 2 and 3, the intensity of the outgoing light R3 from the output-side polarizer could be changed. The half-wave voltage at this time was 2.3V.

  As described above, since the optical modulator 10 of this embodiment uses the single crystal fiber 1, the half-wave voltage can be made lower than that of the conventional optical modulator.

  Further, the growth technique of the single crystal fiber 1 can grow a long crystal at a very high speed as compared with a technique for growing a normal single crystal. For this reason, mass production of the single crystal fiber is facilitated, and as a result, an inexpensive optical modulator can be provided.

  FIG. 5 shows a wavelength tunable light source as a preferred usage example of the optical modulator 10 of the present embodiment. This variable wavelength light source has a Fabry-Perot resonator structure in which an optical amplifier 73 is sandwiched between two mirrors 71 and 72. The optical modulator 10 is disposed on the optical path. The optical modulator 10 can greatly change the optical path length at a low voltage. That is, this wavelength tunable light source can change the light wavelength at a low voltage.

  Although the present embodiment has been described in detail above, the configuration of the optical modulator 1 can be changed. For example, the single crystal fiber 1 having a diameter of about 100 μm to 1 mm is preferably used, but a thinner single crystal fiber can also be manufactured. At this time, mode control such as limiting the propagation mode by depositing a clad layer on the outside of the single crystal fiber using a thin film growth method may be performed.

  Further, the outer shape of the single crystal fiber 1 is preferably a cylindrical shape as shown in FIG. 2 because the composition uniformity is excellent, but may be other outer shapes. For example, depending on the growth technique, growth conditions, or material of the single crystal fiber 1, the external shape of the single crystal fiber may be a quadrangular prism shape or the like. Also in this case, the pair of electrode films are formed so as to face each other on the surface of the single crystal fiber.

As a material for the single crystal fiber 1, LiTaO ₃ which is a material similar to LiNbO ₃ is also suitable. Alternatively, an electro-optic material having a tungsten bronze type crystal structure is also suitable for the material of the single crystal fiber 1. For example, since Sr _1-x Ba _x Nb ₂ O ₆ has a much larger electro-optic coefficient than LiNbO ₃ , it is possible to further reduce the half-wave voltage. In addition, there is a single crystal having a perovskite crystal structure, BaTiO ₃ as a representative example. BaTiO ₃ does not reach Sr _1-x Ba _x Nb ₂ O _6, but has a very large electro-optic coefficient compared to LiNbO ₃ .

In addition, KTa _1-x Nb _x O _{3, which} is a perovskite single crystal material, is a composition material having inversion symmetry, and exhibits an excellent Kerr effect instead of the Pockels effect, which is preferable.

1 Single crystal fiber 2, 3 Electrode 10 Optical modulator

Claims (3)

An optical modulator,
An electro-optic crystal which is a single crystal fiber having an electro-optic effect;
A light modulator comprising: a pair of electrode films formed on a surface of the single crystal fiber and including a positive electrode and a negative electrode.   The optical modulator according to claim 1, wherein the single crystal fiber is formed in a cylindrical shape.   3. The optical modulator according to claim 1, wherein the pair of electrode films are arranged such that the positive electrode and the negative electrode face each other.

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