JPH06110025A - External optical modulator - Google Patents

Abstract

PURPOSE:To modulate light, passing through an optical fiber, extremely efficiently since a flat substrate can be filmed with a piezoelectric element part, the piezoelectric element part with uniform characteristics can easily be manufactured, and a sound wave generated by the piezoelectric element part can be converged on the optical fiber. CONSTITUTION:The modulator consists of the optical fiber 1, the substrate 15 which has the same acoustic impedance with the clad layer of the optical fiber 1 while the optical fiber 1 is connected to one surface, and the piezoelectric element part 9 which is provided on the other surface of the substrate 15; and the substrate 15 is constituted by joining two media 15A and 15B which differ in sound wave propagation speed and a curved surface 19a which converges the sound wave emitted by the piezoelectric element part 15 on the optical fiber 1 is provided on their joined surface 19.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical external modulator for modulating light propagating in an optical fiber.

[0002]

2. Description of the Related Art Conventionally, an optical modulator used for optical communication or the like is of a direct modulation type in which an output current is directly modulated by changing an applied current to a semiconductor laser element or a light emitting diode which is a light source. Was common, but recently
An external optical modulator has been developed in which a signal is externally applied to light from a DC light source propagating in an optical fiber to indirectly modulate the signal.

As an example of such an optical external modulator,
There is an optical waveguide type in which lithium niobate or the like is used as a substrate material and an electro-optical effect is used. However, this type of external optical modulator has excellent modulation characteristics in a high frequency region, but has a drawback that it is very expensive in addition to being highly dependent on temperature.

Therefore, an optical external modulator as shown in FIG. 6 or 7 utilizing the acousto-optic effect has been proposed as a solution to such a drawback. The optical external modulator of FIG.
On the outer peripheral surface of the optical fiber 1, a lower electrode 3 made of aluminum or the like, a piezoelectric film 5 made of zinc oxide or the like, and an upper electrode 7 made of chromium-gold alloy or the like are laminated in this order to form a piezoelectric element portion 9. It is a thing. In the optical external modulator of FIG. 7, the optical fiber 1 is arranged by forming the groove 13 on one surface of the quartz glass substrate 11 having a thickness of about 1 mm, and the lower electrode 3 is formed on the other surface of the quartz glass substrate 11. The piezoelectric film 5 and the upper electrode 7 are laminated in this order to form the piezoelectric element portion 9. All of them are low in insertion loss to the optical transmission system and inexpensive.

[0005]

However, the conventional optical external modulator of this type has the following problems. That is, in the optical external modulator of FIG. 6, the lower electrode 3, the piezoelectric film 5, and the upper electrode 7 are formed by vacuum vapor deposition or sputtering, but since the surface of the optical fiber 1 is a curved surface, a uniform film is formed. Was difficult.

In the optical external modulator of FIG. 7, when the driving frequency is high, the directivity of the sound wave generated in the piezoelectric element section 9 is extremely sharp and the sound wave travels straight through the quartz glass substrate 11, so that the optical fiber 1 in the optical fiber 1 has The piezoelectric element portion that contributes to light modulation is limited to a very limited portion, and the efficiency is poor. To improve this, the aspect ratio of the piezoelectric film may be increased to increase the effective length of the optical fiber. However, since the width of the piezoelectric film is reduced, a high degree of accuracy is required for the relative position of the piezoelectric film and the optical fiber. Required and difficult to assemble.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides an optical external modulator which solves the above-mentioned problems, and has a structure in which an optical fiber and the optical fiber are bonded to one surface, A substrate having substantially the same acoustic impedance as the clad layer of the optical fiber, and a piezoelectric element portion in which the lower electrode, the piezoelectric film, and the upper electrode are laminated in this order on the other surface of the substrate, and the substrate propagates sound waves. It is characterized in that a plurality of media having different velocities are joined, and a curved surface for focusing the sound wave emitted from the piezoelectric element portion toward the optical fiber is provided on the joining surface.

[0008]

By doing so, since the lower electrode, the piezoelectric film, and the upper electrode can be formed on a flat surface, a uniform film can be formed, and at the same time, the sound wave generated from the piezoelectric element portion can be focused toward the optical fiber and the modulation efficiency can be improved. Will get better.

[0009]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows an embodiment of the present invention. This optical external modulator includes two media 15A having different propagation speeds of sound waves.
The piezoelectric element portion 9 is integrally provided on one surface of the substrate 15 that is bonded to the optical fiber 1 and the optical fiber 1 in which the clad layer is exposed by peeling off the coating layer is acoustically bonded to the other surface by the adhesive material 17. It is a thing.

Two mediums 15A and 1 constituting the substrate 15
5B has substantially the same acoustic impedance (the product of the density of the medium and the propagation velocity of the sound wave) as the cladding layer of the optical fiber 1, but one medium 15A is made of, for example, quartz glass, and the other medium 15B is made of, for example. The flint glass 15B is used, and the former has a higher sound wave propagation speed. A curved surface 19a having a convex surface on the medium 15B side is provided on the joint surface 19 between the two media 15A and 15B.

For such a substrate 15, for example, quartz glass powder is put into a mold and hardened so that a curved surface 19a is formed, and then flint glass powder is laminated thereon, and the temperature is raised while applying a high pressure. It can be made by melting. The substrate 15 has a thickness of 2 mm, for example, and the curved surface 19a has a radius of curvature of 1.25 mm and a height of 0.5 mm. The curved surface 19a may be a part of a spherical surface as shown in FIG. 2 or a part of a cylindrical surface extending in the longitudinal direction of the optical fiber 1 as shown in FIG. In short, the curved surface 19a has a shape that matches the shape of the piezoelectric element portion 9.

The piezoelectric element portion 9 is formed by laminating a lower electrode 3, a piezoelectric film 5 and an upper electrode 7 in this order on the surface of the substrate 15 on the medium (quartz glass) 15A side having a high sound wave propagation velocity. The lower electrode 3 and the upper electrode 7 are made of aluminum having a thickness of 0.2 μm, and the piezoelectric film 5 is made of zinc oxide having a thickness of 7 μm. The film is formed by vacuum vapor deposition or sputtering.

The optical fiber 1 is acoustically joined to the surface of the substrate 15 on the side of the medium (flint glass) 15B having a low sound wave propagation velocity by an adhesive 17 made of a fired body of quartz glass powder. Acoustically joining means that the acoustic impedances of both are almost equal.

Next, the operation of this optical external modulator will be described.
When the sound wave generated in the piezoelectric element portion 9 is a high frequency of about 200 MHz, the sound wave generated can be regarded as a plane wave. The sound wave traveling straight in the substrate 15 is reflected and refracted at the joint surface 19, and the incident angle and refraction angle at that time are two media 15A and 15A.
It is determined by the propagation velocity of the B sound wave. Medium 15B as shown in FIG.
When the propagation speed of the sound wave of the medium 15A is further increased and a curved surface 19a having a convex surface on the medium 15B side is provided on the joint surface 19 of both media, the sound wave is focused toward the optical fiber 1.

Therefore, when light is incident on the optical fiber 1 from the DC light source and a voltage of a predetermined frequency is applied between the lower electrode 3 and the upper electrode 7, the sound wave generated from the piezoelectric element section 9 propagates through the substrate 15. The light that is focused and passes through the optical fiber 1 can be modulated very efficiently.

The modulation output of this optical external modulator was measured by the measurement system shown in FIG.
An improvement of B or more was recognized. In FIG. 4, 21 is the optical external modulator of the above embodiment, 23 is a semiconductor laser device having a wavelength of 1.33 μm, 25 is a polarizer, and 27 is the optical external modulator 2.
1, a driving power source, 29 is an analyzer, 31 is an optical-electrical converter,
33 is a spectrum analyzer and 35 is an oscilloscope.

In this optical external modulator, by selecting the curvature of the joining curved surface 19a in the substrate 15, it is possible to slightly reduce the focusing accuracy of the sound wave and loosen the tolerance of the mounting accuracy of the optical fiber 1. .

Next, another embodiment of the present invention will be described with reference to FIG. In this optical external modulator, as the substrate 15, a medium (for example, flint glass) 15B having a low sound wave propagation speed is provided on the piezoelectric element 9 side, and a medium (for example, quartz glass) 15A having a high sound wave propagation speed is provided on the optical fiber 1 side. It was used. In this case, the sound wave can be focused toward the optical fiber 1 by providing a curved surface 19a having a convex surface on the medium 15B side on the joint surface 19 of both media as shown in the figure.

[0019]

As described above, according to the present invention, since the piezoelectric element portion can be formed on the flat substrate, the piezoelectric element portion having uniform characteristics can be easily manufactured and the piezoelectric element portion is generated. Since the sound waves can be focused toward the optical fiber,
The light passing through the optical fiber can be modulated very efficiently.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of an optical external modulator according to the present invention.

FIG. 2 is a perspective view showing an example of the shape of a joint surface in a substrate in the optical external modulator of the present invention.

FIG. 3 is a perspective view showing another example of the shape of the bonding surface in the substrate as well.

FIG. 4 is a block diagram showing a measurement system of a modulation output of an optical external modulator.

FIG. 5 is a sectional view showing another embodiment of the optical external modulator according to the present invention.

FIG. 6 is a perspective view showing an example of a conventional optical external modulator.

FIG. 7 is a perspective view showing another example of a conventional optical external modulator.

[Explanation of symbols]

 1: Optical fiber 3: Lower electrode 5: Piezoelectric film 7: Upper electrode 9: Piezoelectric element part 15: Substrate 15A: Medium with high propagation speed of sound waves 15B: Medium with low propagation speed of sound waves 17: Adhesive material 19: Bonding surface 19a: curved surface

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shigeaki Nishikawa 2-6-1, Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd.

Claims (1)

[Claims] 1. An optical fiber, a substrate having the optical fiber bonded to one surface thereof and having substantially the same acoustic impedance as the cladding layer of the optical fiber, and a lower electrode, a piezoelectric film, and an upper surface on the other surface of the substrate. The substrate is composed of a piezoelectric element portion laminated in the order of electrodes, and the substrate is formed by joining a plurality of media having different propagation speeds of sound waves, and the acoustic wave radiated from the piezoelectric element portion is directed to the optical fiber on the joint surface. An external optical modulator characterized in that a curved surface for focusing is provided.