JPH05327107A - Optical waveguide type optical amplifire and its forming mehtod - Google Patents

Abstract

PURPOSE:To enable high concentration uniform doping of many elements, by forming a lithium niobate layer doped with rare earth elements having optical amplification action on a crystal substrate like lithium tantalate, and arranging an optical waveguide on the layer. CONSTITUTION:A lithium niobate crystal layer 12 is formed on a lithium tantalate crystal substrate or a lithium niobate crystal substrate 11 which layer is doped with rare earth element of erbium or neodymium or praseodymium and aluminum or germanium, and an optical waveguide 13 is arranged on the layer 12 by ion milling. Since lithium tantalate has the same crystal as lithium niobate, uniform epitaxial growth on the lithium tantalate crystal substrate 11 is enabled. The diffusion coefficient of element like erbium having optical amplification action is very small, and it is difficult to doped lithium niobate with erbium by thermal diffusion. Uniform doping, however, is enabled by adding raw material at the time of solvent fusing of liquid phase epitaxial.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type optical amplifier using a lithium niobate crystal for optical fiber communication.

[0002]

2. Description of the Related Art At present, in an optical waveguide type optical amplifier, a metal erbium film is formed on a lithium niobate single crystal, and thermal diffusion is carried out at about 1000 ° C. near the crystal melting point for about 80 hours, and then a titanium film is again formed. Forming an optical waveguide pattern of about 10
It has been confirmed that this can be realized by producing an optical waveguide in which a lithium niobate crystal substrate is doped with erbium by a method of thermal diffusion at 00 ° C. for 9 hours.

[0003]

However, since the optical waveguide type optical amplifier by the thermal diffusion method has a very small diffusion coefficient of erbium, it requires a high temperature heat treatment for a very long time, and it is difficult to manufacture the device. Further, since the wavelength band of incident light that can be amplified is very narrow with doping only with erbium, in the case of an optical amplifier for optical fiber communication, it is necessary to additionally diffuse aluminum or the like in order to expand the wavelength band. In addition to the complicated process, there is a problem of instability inside the crystal due to the method of manufacturing the crystal, that is, the problem of compositional homogeneity. When thermal diffusion treatment is performed several times, low propagation loss with uniform diffusion depth The optical waveguide is not reproducibly formed. Therefore,
A stable optical amplifier cannot be realized in terms of gain characteristics and the like.

[0004]

In order to solve the above problems, in the optical amplifier and the manufacturing method of the present invention, erbium and neodymium, which are materials for amplifying light, are provided on a lithium tantalate crystal substrate or a lithium niobate substrate. , Or rare earth elements such as praseodymium, aluminum and germanium for adjusting the amplification wavelength band, and liquid phase epitaxial growth of a lithium niobate crystal layer doped with titanium, iron, zinc, copper, or silver for increasing the refractive index. The crystal layer is used as an optical waveguide to form an optical amplifier.

[0005]

With this configuration, elements such as erbium, which has a light amplification effect, have a very small diffusion coefficient, and it was difficult to dope lithium niobate by thermal diffusion. By adding to the above, even a plurality of elements can be easily doped. Further, it becomes possible to realize a very compact optical waveguide type optical amplifier for optical fiber communication.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an optical amplifier according to an embodiment of the present invention. Reference numeral 11 denotes a crystal substrate of lithium tantalate. On the crystal substrate 11, a lithium niobate crystal layer 12 doped with erbium and aluminum is formed. An optical waveguide 13 is provided on the lithium niobate crystal layer 12 by ion milling. A reflection film 16 having a reflectance of 90% or more is applied to the end face of the element, and optical fibers 14 and 15 are attached for input and output.

The operation of the optical amplifier configured as described above will be described. An optical signal having a wavelength of 1.55 μm is guided from the optical fiber 14 to the optical waveguide 13. At that time, the wavelength 1.
The excitation light of 48 μm is the input optical fiber 1 at the same time as the signal light.
It is incident on 4. Separation of the amplified signal light and pumping light is extracted from the output fiber 15 through a filter.

The optical amplifier of the present invention comprises a lithium niobate crystal layer 1 grown by a liquid phase epitaxial growth method.
It is characterized in that the optical waveguide 13 is formed on the surface 2.

In order to epitaxially grow the lithium niobate crystal layer 12, it is produced by using the liquid phase epitaxial device shown in FIG. 21 is a platinum crucible for flux melting, and 22 is a substrate crystal rotation shaft. A substrate holder 23 for fixing the crystal substrate with claws and a stirring plate 24 are attached to the shaft, and the structure is such that it can move up and down simultaneously with rotation. 25 is an electric furnace for heating the platinum crucible,
Reference numeral 26 indicates a heat insulating material.

The production principle is as follows. A molten flux of lead oxide, bismuth oxide, boron oxide, etc. is put in the platinum crucible 22 and kept above the melting temperature of the flux. And erbium oxide, neodymium oxide, which is a dopant,
Alternatively, praseodymium oxide is dissolved. Further, in order to broaden the wavelength band of light to be amplified, aluminum oxide, germanium oxide, and to increase the refractive index, oxides such as titanium oxide, iron oxide, zinc oxide, copper oxide, or silver oxide. To form a melt 27. After the melting, the melt 27 is usually cooled to about 600 to 800 ° C. to be in a supercooled state, and a crystal substrate is dipped in the melt, so that lithium niobate in the supercooled melt melts into the crystal substrate. Epitaxially grows on the surface of. The grown film thickness is controlled by the temperature of the solution and the dipping time. During crystal growth, the crystal substrate is rotated at a constant speed in order to make the growth uniform. Finally, in order to remove the flux, the crystal substrate is taken out from the flux and rotated at high speed, and the flux is blown off by centrifugal force. Then, the crystal substrate is cooled and taken out.

In this embodiment, lithium tantalate has the same crystal structure as lithium niobate, so that epitaxial growth is possible on a lithium tantalate crystal substrate. The lithium niobate crystal layer is doped with an erbium concentration of 750 ppm and an aluminum concentration of 4000 ppm and has a film thickness of 5 μm.

Furthermore, the optical waveguide 13 must be formed on the lithium niobate crystal layer 12. In the structure in which the lithium niobate crystal layer is grown on the lithium tantalate crystal substrate, the change in the refractive index in the substrate thickness direction is large. Therefore, a structure that allows the change in the refractive index only in the lateral direction may be adopted. Therefore, the simplest method for producing the optical waveguide is the ion milling method. In the ion milling method, an optical waveguide pattern is formed on the surface of a lithium niobate crystal layer, and unnecessary parts are removed by accelerating argon, oxygen ions and the like, and at a temperature relatively close to room temperature of 100 to 200 ° C. It can be made. A cross-sectional view of the manufactured optical modulator is shown in FIG.

31 is a crystal substrate of lithium tantalate, 3
Reference numeral 2 is a lithium niobate crystal layer doped with erbium and aluminum, and 33 is an optical waveguide. By this method, the refractive index in the optical waveguide 33 is made uniform, and it is possible to make a large change in the refractive index in the downward direction and the lateral direction of the optical waveguide. Therefore, the light confinement effect is increased and the propagation loss of light is reduced. Can be reduced. In addition, since the crystal produced by epitaxial growth has a small dislocation and a small composition change of the crystal, it is possible to reduce the propagation loss of light in that respect as well.

Further, when a lithium niobate crystal is used for the substrate crystal, the mismatch with the epitaxial crystal layer is reduced as compared with lithium tantalate crystal. In this case, in order to manufacture an optical waveguide having a refractive index higher than that of the substrate, it is possible to manufacture it by a conventional metal thermal diffusion method. After forming the erbium-doped lithium niobate crystal layer, an optical waveguide pattern of titanium, iron, zinc, copper, or silver film for increasing the refractive index is formed, and is doped by a thermal diffusion method. As an example, titanium was vapor-deposited at 500 A to obtain 10
An optical waveguide can be produced by heat treatment at 50 ° C. for 9 hours in an oxygen atmosphere. However, in that case, sufficient attention must be paid to the thermal stress.

However, a major feature of the lithium niobate crystal layer produced by the liquid phase epitaxial method is that doping of the element is facilitated by adding an amount of the element that replaces a part of the constituent elements of the lithium niobate as a raw material. That is what you can do. As a dopant, erbium oxide, aluminum oxide, and other oxides such as titanium oxide, iron oxide, zinc oxide, copper oxide, lead oxide, or silver oxide are added to the crystal raw material, and titanium, iron, zinc, copper, or lead is added. Alternatively, an optical waveguide having a higher refractive index than the substrate can be manufactured by growing a lithium niobate crystal layer doped with silver or silver. As one example, when a lithium niobate crystal layer doped with an erbium concentration of 750 ppm, an aluminum concentration of 4000 ppm, and a titanium concentration of 1% is grown, the refractive index in the optical waveguide becomes uniform, and the refractive index changes downward in the optical waveguide. It is possible to take large.

[0017]

INDUSTRIAL APPLICABILITY As described above, the present invention provides a lithium niobate crystal doped with a rare earth element such as erbium, neodymium or praseodymium having a light amplification effect on a lithium tantalate crystal or a lithium niobate crystal by a liquid phase epitaxial method. A layer is prepared to be an optical waveguide. A major feature of the lithium niobate crystal produced by the liquid phase epitaxial method is that by adding it to a crystal raw material, multi-element doping can be easily performed at a high concentration and uniformly. Therefore, an optical amplifier having a wide input light band and a high gain is manufactured, and an optical amplifier and an optical IC having a wider range of applications can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical amplifier according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a liquid phase epitaxial device in the example.

FIG. 3 is a sectional view of an optical waveguide manufactured by an ion milling method in the example.

[Explanation of symbols]

11 Crystal Substrate 12 Lithium Niobate Crystal Layer 13 Optical Waveguide 14 Optical Fiber for Optical Input 15 Optical Fiber for Optical Output 16 Reflective Film 21 Platinum Crucible for Flux Heating 22 Substrate Crystal Rotating Shaft 23 Substrate Holder 24 Stirring Plate 25 Platinum Electric furnace for heating crucible 26 Heat insulating material 27 Melt 31 Crystal substrate 32 Lithium niobate crystal layer doped with erbium and aluminum 33 Optical waveguide

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H01S 3/094 3/10 Z 8934-4M

Claims (3)

[Claims] 1. A rare earth element such as erbium, neodymium, or praseodymium for amplifying light and aluminum or germanium for adjusting an amplification wavelength band are provided on a lithium tantalate crystal substrate or a lithium niobate crystal substrate. An optical amplifier comprising an epitaxial crystal layer of doped lithium niobate and an optical waveguide provided on the crystal layer. 2. A rare earth element of erbium, neodymium, or praseodymium for amplifying light and aluminum or germanium for adjusting an amplification wavelength band on a lithium tantalate crystal substrate or a lithium niobate crystal substrate. An optical amplifier comprising an epitaxial crystal layer of lithium niobate doped with titanium, iron, zinc, copper, or silver that is an element for increasing the refractive index, and an optical waveguide provided in the crystal layer. .. 3. The optical amplifier according to claim 1 or 2, together with niobium oxide and lithium carbonate which are liquid phase epitaxial crystal raw materials, and erbium oxide, neodymium oxide or praseodymium oxide which is a material for amplifying light.
Aluminum oxide, germanium oxide for adjusting the amplification wavelength band and titanium oxide, iron oxide, zinc oxide, copper oxide, or silver oxide for increasing the refractive index are dissolved, and erbium,
A method for manufacturing an optical amplifier, characterized in that neodymium or praseodymium and a lithium, niobate crystal layer doped with titanium, iron, zinc, copper or silver are epitaxially grown by a liquid phase solvent melting method.