JPH08211343A - Electro-optic modulator - Google Patents

Abstract

(57) [Summary] [Object] To provide an electro-optic modulator that can improve the stability of the modulation performance of the electro-optic modulator at low cost without adding a new configuration. [Structure] When the oxygen content of the ternary composition raw material composed of K ₂ O, TiO ₂ and P ₂ O ₅ for obtaining a single crystal of KTiOPO ₄ (KTP) is 58.5 atomic percent or less, the electrical resistivity is That is, the electric conductivity decreases, and when the oxygen content is 58.5 atomic percent or more, the electric resistivity decreases, that is, the electric conductivity increases. Therefore, when a KTP single crystal is prepared from a ternary composition raw material having an oxygen content of 58.5 atomic percent or less, a single crystal having low electric conductivity, that is, heat generation when a voltage is applied can be obtained. Further, when the electro-optic modulator is constructed by using this single crystal, the variation of the extinction ratio with respect to the frequency of the light modulation drive voltage is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electro-optic modulator which modulates the intensity and phase of light incident on the electro-optic crystal by utilizing the electro-optic effect generated by applying an electric field to the electro-optic crystal. .

[0002]

2. Description of the Related Art As a laser light source for optical recording, there is known a laser light generator having a light source section having, for example, a semiconductor laser for excitation, a laser medium, and a nonlinear optical crystal element.

According to the above laser beam generator, the laser beam excited by the laser beam of the semiconductor laser and taken out is introduced as the fundamental wave laser beam into the nonlinear optical crystal element, and this nonlinear optical crystal element is introduced. From
For example, high power density can be obtained with low power consumption, such as extracting a harmonic laser beam such as a second high frequency laser beam.

When the above laser light generator is used as a laser light source for optical recording, an external light modulator for modulating the intensity and phase of the output laser light from this laser light source, a so-called light modulator is required.

An electro-optic modulator as the above optical modulator is
The electro-optical modulation device is provided with an electro-optical modulation element for modulating the phase and intensity of incident light, and a voltage for performing light modulation, that is, a so-called light-modulation drive voltage is applied to the electro-optical modulation element. The phenomenon that the refractive index in the element changes,
That is, the electro-optic effect is used.

In the electro-optic modulator, an electrode layer is formed on an appropriate side surface of the electro-optic modulator, and the light modulation driving voltage is applied through the electrode layer. Also,
In an electro-optical intensity modulator that modulates the intensity of incident light, two electro-optical modulation elements are arranged so that their optical axes are orthogonal to each other, and in an electro-optical phase modulator that modulates the phase of incident light, Only one optical modulator is used.

Further, in recent years, KTiOPO ₄ (KTP), which is an electro-optic crystal, has been known as an electro-optic modulator. This is because this KTP has a large electro-optic effect and high durability against optical damage to visible light.

Further, in recent years, especially in the field of optical recording and the like, due to the development of digital recording / reproducing technology for audio and video, there has been a demand for a large capacity and a small size of the optical recording medium. As a result, in the electro-optic modulator described above, light modulation with a fast rise time is required so that high-speed writing with light of a short wavelength can be performed on the optical recording medium, and stable light modulation characteristics are required. Is desired.

Therefore, an optical modulator utilizing the acousto-optic effect has been used as the optical modulator. However, since the upper limit of the response frequency of the optical modulator utilizing the acousto-optic effect, that is, the upper limit frequency is as low as about 40 MHz, It has not been possible to meet the demand for high-speed modulation performance as described above. For this reason, an electro-optic modulator having an upper limit frequency of 500 MHz or more, which uses the electro-optic effect having a high response speed, has come to be used.

[0010]

In the electro-optic modulator, as described above, the light incident on the electro-optic modulator, that is, the direction perpendicular to the optical path of the incident light in the electro-optic modulator is used. Modulation is performed by applying a voltage from and changing the refractive index of the light traveling in this optical path. Since the magnitude of the change in the refractive index is substantially proportional to the applied voltage, it is sufficient to increase the applied voltage in order to shorten the optical path and downsize the electro-optic modulator.

However, with KTP prepared by the flux method, which is a typical electro-optic material, 1 × 10
^Since a material having an electric conductivity of about ^-4 Ω ^-1 · m ^-1 is generally used, there is a problem that the KTP itself generates heat due to the current flowing in the crystal when a high voltage is applied. .

[0012] heating of the above KTP, to nonuniform refractive index in the optical path occurs, a minimum value I _min and a maximum value I _{ma x} can take the modulated output light output from the electro-optical modulator The ratio I _min / I _max (or its reciprocal), the so-called extinction ratio, deteriorates, adversely affecting the stability of the electro-optic modulator. Therefore, there is a method of improving the temperature stability of the electro-optic modulator by providing a small cooling device, but an effective cooling device and a drive circuit for this cooling device are required.

Further, there is a method of suppressing heat generation by forming an electro-optical modulator with a substance having a lower electric conductivity, for example, KTP or rubidium arsenate titanate prepared by a hydrothermal synthesis method. Because it is difficult to grow, it is effective and difficult to put into practical use.

Therefore, the present invention has been made in view of the above-mentioned circumstances, and it is possible to improve the stability of the modulation performance of the electro-optic modulator at a low cost without adding a new configuration. It is an object of the present invention to provide an electro-optic modulator that is

[0015]

In order to solve the above-mentioned problems, the present invention utilizes the electro-optical effect generated by applying an electric field to the electro-optical modulating element to enter the electro-optical modulating element. In an electro-optical modulator for modulating the intensity or phase of light, potassium oxide (K) having an oxygen content of 58.5 atomic% or less is used as the electro-optical modulator.
₂ O), titanium oxide (TiO ₂ ) and phosphorus pentoxide (P ₂ O ₅ ).
A potassium titanate phosphate (KTiOPO ₄ ) single crystal prepared from a ternary composition raw material consisting of

The ternary composition raw materials are potassium dihydrogen phosphate (KH ₂ PO ₄ ) and dipotassium hydrogen phosphate (K ₂ HP).
It may be obtained by mixing O ₄ ) and titanium dioxide (TiO ₂ ).

[0017]

According to the electro-optic modulator of the present invention, potassium titanate phosphate (KTiOPO ₄ ) or potassium oxide (K ₂ O) having an oxygen content of 58.5 atomic% or less is obtained in order to obtain KTP. By producing a single crystal using a ternary composition raw material composed of titanium oxide (TiO ₂ ) and phosphorus pentoxide (P ₂ O ₅ ), the KTP single crystal obtained has a low electric conductivity. When this KTP single crystal is used as an electro-optical modulation element, heat generation of the electro-optical modulation element caused by applying a voltage during light modulation is suppressed,
The fluctuation of the refractive index in the electro-optical modulation element caused by the heat generation in the electro-optical modulation element is suppressed, and the deterioration of the extinction rate caused by the fluctuation of the refractive index is reduced.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An electro-optic modulator according to the present invention will be described in detail below with reference to the drawings.

As shown in FIG. 1, the main part of the electro-optic modulator is potassium titanate phosphate (KTiOPO ₄ : KTP) 1 as an electro-optic modulator and a pair of parallel surfaces of the KTP 1, for example. It comprises electrode layers 2 and 3 on which a metal layer having a thickness of 20 nm of titanium and 500 nm of gold is deposited and formed, and a power source 4 connected to the electrode layers 2 and 3, for example, which generates an AC voltage.

Further, KTP1 is an entrance surface 11 and an exit surface 1.
2 and, for example, the incident laser light incident on the incident surface 11 from the incident light optical path 13 as the incident light is optically modulated, and the modulated output laser light is emitted from the emission surface 12 on the emission light optical path 14.

According to the electro-optic modulator, the KT
The laser light incident on P1 is caused by the electro-optical effect generated according to the light modulation driving voltage applied from the power source 4, that is, the phenomenon that the refractive index in the electro-optical modulation element is changed by applying the voltage, The intensity or phase of light is modulated and output as modulated output laser light.

Here, the KTP1 is obtained in the following steps.

First, as the raw material for the KTP1 component, potassium dihydrogen phosphate (KH ₂ PO ₄ ), dipotassium hydrogen phosphate (K ₂ HPO ₄ ), and titanium dioxide (TiO ₂ ) are mixed in an arbitrary ratio. For example 110 in a platinum crucible
It was obtained by keeping the temperature at 0 ° C. or higher and reacting for 8 hours or longer.

Next, a seed crystal of KTP was put into the solution of the component raw material and rotated at 60 rpm, that is, 60 rpm, while contacting the component raw material with the seed crystal at 1050.
When the above solution is cooled for about 200 hours at a temperature of about 0.1 to 950 degrees at a rate of 0.1 degrees per hour, a single crystal of KTP grows and precipitates in a molten raw material solution. A crystal thus grown and precipitated is taken out and cooled to room temperature at a rate of 50 degrees per hour to obtain a KTP single crystal.
This KTP single crystal is cut out with a predetermined size, a predetermined thickness, and a predetermined crystal plane, and each surface is subjected to optical polishing and antireflection coating to obtain the above KTP1.

Here, KH ₂ PO ₄ , K ₂ HPO ₄ and TiO
4 types of KTP by changing the ratio with ₂
A single crystal was obtained. These four types of crystals A, B, C and D are
It has the raw material composition and oxygen content shown in the following table. The raw material composition is KH ₂ PO ₄ and K ₂ HPO ₄
And TiO ₂ are mixed to obtain potassium oxide (K ₂ O
), Titanium oxide (TiO ₂ ) and phosphorus pentoxide (P ₂ O ₅ ). The oxygen content represents the ratio of oxygen atoms to the total number of atoms in these mixtures.

[0026]

[Table 1]

The relationship between the electrical resistivity (Ω · m) and the oxygen content (atomic percentage) of each crystal is shown in FIG. 2, where the inflection point is found at an oxygen content of 58.5 atomic percent. It became a curve having. This analysis is based on KT
A P single crystal was cut into a square having a thickness of about 0.2 mm and a side length of about 10 mm on the (001) plane, and a gold electrode was formed on both sides of the cut single crystal by vacuum deposition. Then, the electric conductivity obtained by inputting 100 Hz was obtained, and the electric resistivity was calculated from the electric conductivity.

According to FIG. 2, when the oxygen content is 58.5 atomic percent or less, the electrical resistivity increases, that is, the electrical conductivity decreases, and when the oxygen content is 58.5 atomic percent or more, the electrical resistivity decreases. The resistivity decreases, that is, the electric conductivity increases. Therefore, the oxygen content is 58.
K from a ternary composition raw material that is less than 5 atomic percent
When a TP single crystal is produced, a single crystal having low electric conductivity, that is, heat generation when a voltage is applied can be suppressed can be obtained.

The KTP obtained through the above-mentioned steps
An electro-optical intensity modulator that modulates the optical intensity by using a KTP crystal element that is cut out from a single crystal in a size of, for example, 1 mm × 1 mm × 6 mm, and each surface is optically mirror-polished and has an antireflection coating is shown in FIG. As shown, the first KTP 21 and the second KTP 22 as the above KTP crystal elements, and the pair of parallel surfaces of the first KTP 21 are provided with the electrode layers 31a and 31b in which gold is vacuum-deposited and the second KTP 22, respectively. Electrode layers 31c and 31d adhered to a pair of parallel surfaces
And one end is connected to the electrode layers 1a and 1c, and the other end is connected to the electrode layer 1
It has a power supply 23 connected to b and 1d.

Further, the electro-optical intensity modulator has an analyzer 24 as an optical element for passing the modulation component of the modulated output laser light emitted from the second KTP 22, and the modulation component passing through the analyzer 24. The light receiving element 25 for converting into an electric signal, the light receiving element 25 outputs the electric signal to an optical output meter (not shown) through an output terminal 36.

The first KTP 21 has an entrance surface 32, and the second KTP 22 has an exit surface 33. For example, as an incident light, an incident light optical path 34 to an incident surface 32 is used.
The incident laser light that is incident on the
The modulated output laser light is emitted from the output optical path 35. In addition, each KTP moves in the same direction as the incident light optical path 34 by each KTP.
As the optical axis of TP, the optical axis of the first KTP21 and the second optical axis of KTP21
The optical axes of the KTPs 22 are orthogonal to each other and are arranged in series.

Using each of the above-mentioned electro-optical intensity modulators configured by using the KTP crystal element obtained from the crystal B or the crystal C in the above table as each KTP, the amplitude width from the power source 23 is about 200 V, and from 100 Hz. AC voltage of up to 10 MHz is applied to each electrode to make 532 incident laser light.
The laser light linearly polarized at nm is made incident on the KTP21, and K
The intensity change of the modulated output laser light emitted from the TP22, that is, the so-called extinction ratio was measured with the analyzer 24 and the light receiving element 25.

FIG. 4 shows the relationship between the frequency (Hz) of the AC voltage applied to the KTP and the extinction ratio (%) of the modulated output laser light. The curve obtained by the electro-optical intensity modulator formed by using the KTP crystal element obtained from the crystal B is represented by a curve 51, and the electro-optical intensity modulation formed by using the KTP crystal element obtained from the crystal C is shown. The curve obtained with the instrument is represented by curve 52.

According to FIG. 4, curve 51, namely crystal B
It can be seen that the extinction ratio changed from about 78% to about 91% when the KTP crystal element derived from was used. Further, it was found that the extinction ratio changed from about 88% to about 90% in the case of using the curve 52, that is, the KTP crystal element derived from the crystal C, and was composed of K ₂ O, TiO _2, and P ₂ O _5. It can be seen that when the oxygen content in the ternary composition raw material was 58.5 atomic percent or less, the variation of the extinction ratio with respect to the frequency of the light modulation driving voltage was improved.

Although the electro-optical intensity modulator has been described above, a so-called electro-optical phase modulator for modulating the phase of incident light can also obtain the same effect by reducing the heat generation of the KTP crystal element.

With the above-mentioned structure, the KTP crystal element, which is the electro-optical modulator of the electro-optical modulator, has the oxygen content of 58.5 atomic% or less, and K ₂ O and T
KTP obtained by growing a KTP single crystal from a ternary composition raw material composed of iO ₂ and P ₂ O ₅
The crystal element has a low electric conductivity, can suppress heat generation that may occur by applying a light modulation driving voltage during light modulation, and can improve the fluctuation of the extinction ratio with respect to the frequency of the light modulation driving voltage. .

[0037]

As described above, according to the electro-optic modulator of the present invention, the ternary composition of potassium oxide having an oxygen content of 58.5 atomic percent or less, titanium oxide and phosphorus pentoxide is used. By using a KTP single crystal prepared from a raw material as an electro-optical modulation element, heat generation caused by applying a voltage during light modulation is suppressed, fluctuation of the refractive index in the electro-optical modulation element is suppressed, and The deterioration of the extinction rate is reduced without adding such a structure. In addition, it is possible to improve the stability of the light modulation performance at a low cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a main part of an electro-optical modulator of the present invention.

FIG. 2 is an oxygen content of a ternary composition raw material of KTP, which is an electro-optical modulator used in the electro-optical modulator, and the obtained K.
It is a graph which shows the relationship with the electrical resistivity of TP.

FIG. 3 is a diagram showing an example in which the electro-optic modulator is applied to an electro-optic intensity modulator.

FIG. 4 is a graph showing the relationship between the frequency of a light modulation driving voltage and the extinction ratio of modulated output laser light in the electric light intensity modulator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 KTP 2, 3 electrode layer 4 power supply 21 1st KTP 22 2nd KTP 23 power supply 31a, 31b, 31c, 31d electrode layer

Claims (2)

[Claims] 1. An electro-optic modulator for modulating the intensity or phase of light incident on the electro-optic modulation element by utilizing the electro-optic effect generated by applying an electric field to the electro-optic modulation element. The modulation element was made of a ternary composition raw material composed of potassium oxide (K ₂ O), titanium oxide (TiO ₂ ) and phosphorus pentoxide (P ₂ O ₅ ) having an oxygen content of 58.5 atomic percent or less. Potassium titanate phosphate (K
An electro-optic modulator using a TiOPO ₄ ) single crystal. 2. The ternary composition raw material is potassium dihydrogen phosphate (KH ₂ PO ₄ ) and dipotassium hydrogen phosphate (K ₂ HPO ₄ ).
The electro-optic modulator according to claim 1, wherein the electro-optic modulator is obtained by mixing titanium dioxide and titanium dioxide (TiO ₂ ).