JPH0756029A - Optical waveguide and its production - Google Patents

Abstract

PURPOSE:To provide an optical waveguide which has stable and low propagation loss, is easily produced with good reproducibility, is adequately used for optical devices and consists of a lithium niobate crystal obtd. by external diffusion of Li and the process for production of this optical waveguide. CONSTITUTION:(1) This optical waveguide consists of the lithium niobate crystal which has <=0.1dB/cm propagation loss of 0.633mum wavelength and is obtd. by external diffusion of the Li. (2) This process for production of the optical waveguide consisting of the lithium niobate crystal obtd. by external diffusion of the Li comprises subjecting the grown lithium niobate crystal pool to annealing, poling and slicing, then annealing the crystal again to adjust the proton quantity and to uniformalize the concn., and subjecting the crystal to lapping and poling to a specular surface then to a proton desorption treatment in a dry oxygen atmosphere at <=-70 deg.C dew point temp.

Description

Detailed Description of the Invention

[0001]

This invention relates to lithium niobate (L
The present invention relates to an optical waveguide obtained by Li out-diffusion, which is one of the optical waveguide forming techniques using iNbO ₃ ) crystal, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, as a method for forming an optical waveguide on a lithium niobate crystal substrate, a Ti thermal diffusion method, a proton exchange method, an LPE method (liquid phase epitaxial growth method), an ion implantation method, a Li outside diffusion method, Deposition methods (sputtering method, chemical deposition method) and the like are known.

Among these, the Ti thermal diffusion method, the proton exchange method and the LPE method are widely used industrially, and an optical modulator, an optical switch, an SHG element, an acousto-optical element are formed by these optical waveguide forming methods. , Optical devices, etc. are manufactured.

The lithium niobate crystals used for these are generally prepared by pulling a raw material prepared so as to have a congruent composition by the Czochralski method and annealing it.
After poling, it is sliced, lapped and polished to be a mirror surface, which is used as a substrate for various optical devices.

However, in the optical waveguide layer manufactured by the Ti thermal diffusion method, the proton exchange method, the LPE method, etc.,
It is necessary to include an atom different from the crystal component. For example, Ti is included in the Ti thermal diffusion method, protons (H ⁺ ) are included in the proton exchange method, and V and B are included in the LPE method. These dissimilar atoms act as light scattering centers (Rayleigh scattering), and the guided light leaks to the outside of the optical waveguide, increasing the propagation loss.

As a concrete report, in the Ti diffusion method, the propagation loss at a wavelength of 1.53 μm is 0.2 dB / c.
m, the proton exchange method has a propagation loss of 0.3 dB / cm at a wavelength of 0.633 μm, and the wavelength LPE method has a propagation loss of 0.8 dB.
The propagation loss at 3 μm is 1.0 dB / cm.

Further, as another problem, when the guided light is controlled by an external electric field, these different elements can become mobile ions, so that the electric field does not work effectively in the optical waveguide.

The above drawbacks are fatal for an optical waveguide device. If the propagation loss is high, the energy of light decreases, and the signal-to-noise ratio (signal / noise)
e), resulting in the collapse of single-mode conditions,
Multiple modes appear, leading to a reduced extinction ratio (crosstalk).

If the external electric field does not work effectively, the offset control becomes impossible and the light-electric field action length becomes long, so that unnecessary power is required.

Under these circumstances, there is a demand for an optical waveguide having a low propagation loss and containing no different elements.

On the other hand, the Li outdiffusion method is known as a technique for forming an optical waveguide without containing a different element. The feature of this method is that Li atoms, which are the composition elements of the crystal, are diffused out from the vicinity of the surface of the crystal, the Li / Nb ratio is lowered, a high refractive index layer is formed, and this is utilized as an optical waveguide. . Since the optical waveguide formed by this method does not contain a different element, there is a possibility that the above-mentioned problem can be solved.

The method of forming a Li outdiffusion optical waveguide known to date is "Applied Physics Letter,
As described in Vol. 22, 326, 1973 ”, the crystal substrate is heat-treated in vacuum at 1100 ° C. for 20 hours or more to evaporate Li atoms from the substrate. At this time, oxygen is also vaporized at the same time. The crystals turn black.
For this reason, it is necessary to continuously perform a clearing treatment at 1100 ° C. for 2 hours in the air. Furthermore, the Li outdiffusion optical waveguide obtained has a propagation loss of 1 dB / wavelength at a wavelength of 0.633 μm.
Since it is as high as cm, and about 198 propagation modes are excited, it cannot be used for an optical device.

In addition, "Radio Science, Volume 12,
537, 1977 ", the Li out diffusion heat treatment was performed at 930 to 980 ° C. for 20 to 60 minutes in an oxygen atmosphere depressurized to 5 × 10 ^-2 Torr. It is shown to anneal for 5 minutes.
As a result, a single mode Li outdiffusion optical waveguide is obtained.

As described above, in order to form a Li outdiffusion optical waveguide, a method is known in which Li, which is a component element of the crystal, is separated from the crystal in a vacuum or under reduced pressure. Oxygen is also released, resulting in oxygen deficiency, and subsequently oxygen supplement annealing in oxygen is essential.

In addition, "The IEICE General Conference, 1
984, Paper No. In 1002 ″, there is an example in which Li outdiffusion heat treatment is performed from the beginning in an oxygen atmosphere to form a Li outdiffusion optical waveguide. According to this method, oxygen supplement annealing is unnecessary, but the formed optical waveguide is used. There was a problem that the layer was thermally and temporally unstable and disappeared.

Thus, from the lithium niobate crystal, Li
The optical waveguide obtained by out-diffusion was industrially practically unusable.

[0017]

SUMMARY OF THE INVENTION An object of the present invention is to obtain niobate which is stable and has a low propagation loss, which can be easily and reproducibly manufactured, and which is suitable for an optical device and which is obtained by Li outdiffusion. An object is to provide an optical waveguide made of a lithium crystal and a manufacturing method thereof.

[0018]

The above object is achieved by desorbing the protons in the lithium niobate crystal forming the optical waveguide so that the amount of the protons is kept below a certain amount.

That is, the present invention has a wavelength of 0.633 μm.
The optical waveguide made of lithium niobate crystal obtained by Li outdiffusion has a propagation loss of 0.1 dB / cm or less.

As described above, the propagation loss at the wavelength of 0.633 μm of the optical waveguide obtained by the Li out-diffusion method is about 0.1 dB / cm.
Such an optical waveguide having a small propagation loss was first found.

In order to obtain such an optical waveguide having a small propagation loss, the amount of protons (A) in the lithium niobate crystal wafer forming the optical waveguide must be 0.05 or less.

The amount of protons (A) in the crystal wafer is 0.0
When it exceeds 5, the protons remaining in the optical waveguide layer flow by diffusion, lowering the high refractive index layer, and making the optical waveguide unstable.

The amount of protons (A) here is measured as follows. That is, it is known that the concentration of protons contained in the crystal can be estimated by measuring the light absorption coefficient at a wavelength of 2.87 μm with an infrared spectrophotometer. The proton concentration (number / cm ³ ) and the light absorption coefficient value (α: cm ⁻¹ ) are directly proportional functions. Proton concentration (pieces / cm ³ ) = 3 × 10 ¹⁹ × α (cm ⁻¹ )
(1)

The proportionality coefficient varies from reporter to reporter, and in the present invention, since the amount of protons is more important than the concentration of protons in the crystal, the absorption coefficient (α: cm ^-1 ) and the thickness of the crystal wafer ( The amount of protons (A) in the crystal wafer was represented by the product of d: cm) (α × d: dimensionless). by this,
Even crystal wafers having the same proton concentration can be distinguished when they have different thicknesses. Further, when expressing the proton concentration, only the absorption coefficient (α) is displayed.

Next, according to the manufacturing method of the present invention, the grown lithium niobate crystal boule is annealed, poled, sliced, and then annealed again to adjust the amount of protons and make the concentration uniform, and then lapping and polishing. To form a mirror surface, and then to perform a proton desorption treatment (external Li diffusion treatment) in a dry oxygen atmosphere having a dew point temperature of −70 ° C. or lower.

First, a lithium niobate crystal is generally grown by the Czochralski method. The crystal boule is annealed, poled, and then processed into a cylindrical shape. Next, the cylindrical crystal boule is sliced into a wafer having a specific crystal plane orientation to obtain a crystal wafer.

In the present invention, the sliced crystal wafer is annealed again to adjust the concentration and amount of protons in the crystal wafer. As a result, a crystal wafer having a uniform amount of protons and a uniform concentration corresponding to the amount of atmospheric water vapor can be obtained. As the annealing conditions such as temperature, time and atmosphere, generally known conditions are adopted. For example, the temperature is 700 ° C., the time is 7 hours, the oxygen gas containing water vapor, or the inert gas.

The crystal wafer annealed again in this way is lapped, polysigmed and mirror-finished, and then subjected to a proton desorption process (Li outside diffusion heat treatment).

This deprotonation treatment is performed at a dew point temperature of -70.
It is carried out in a dry oxygen atmosphere at a temperature of ℃ or below. The reason for this is
This is because the smaller the amount of water vapor in the atmosphere is, the better in order to release most of the protons from the crystal, and the gas having a dew point temperature of −70 ° C. or lower is easily available. Moreover, Li
This is because there is a concern that oxygen will also out-diffuse from the crystal in the process of out-diffusion in a gas atmosphere other than oxygen. In addition to this method, other methods such as a reduced pressure atmosphere and a dewatered solution can be used for the proton desorption treatment.

By carrying out the proton desorption process in this manner, most of the protons in the crystal wafer are desorbed, and the amount of protons becomes 0.05 or less. Further, when converted into proton concentration, the light absorption coefficient at a wavelength of 2.87 μm is preferably 0.3 cm ⁻¹ or less. By controlling the amount of protons to be 0.05 or less, the lithium niobate crystal which is easily reproducible, has stable low propagation loss, and is obtained by Li outdiffusion suitable for an optical device is formed. An optical waveguide is obtained.

As described above, protons are released because it is found that there is a strong correlation between the amount of released protons and the optical waveguide characteristics of the manufactured Li outdiffusion optical waveguide. Therefore, the amount of protons released is controlled in order to form the optical waveguide with good reproducibility, and most of the protons are released from the crystal to provide an optical waveguide suitable for a stable optical device.

[0032]

EXAMPLES The present invention will be described in detail below with reference to examples.

Test Example 1 Lithium niobate crystal wafers (Z-cut, thickness 0.6 to 2.0 m) having different proton amounts (indicated by α × d) by measuring absorption coefficient before proton desorption treatment (external Li diffusion treatment).
m, 10 × 20 mm ² ) was subjected to a proton desorption treatment in dry oxygen having a dew point temperature of −70 ° C. or lower at 1000 ° C. for 10 minutes.
The temperature rising rate was 10 ° C./min. Further, the amount of protons of the wafer after the heat treatment was 0.05 or less.

The extraordinary light equivalent refractive index of the 0th mode of the guided light excited by the prism coupler method was measured, and the result is shown in FIG. In FIG. 1, the equivalent refractive index of 2.
2020 is the refractive index of the substrate.

As a result, as shown in FIG. 1, the amount of proton desorption (difference in the amount of protons before and after the proton desorption process) and the extraordinary light equivalent refractive index of the formed Li outdiffusion optical waveguide are clearly strong. A correlation was found. This confirms the formation of the Li outdiffusion optical waveguide by releasing the protons. It was also shown that the equivalent refractive index of the formed optical waveguide depends on the amount of protons desorbed, and it is important to determine the amount of protons in the crystal before the proton desorption heat treatment. On the other hand, the ordinary light refractive index did not change and the guided light was not excited.

Further, the amount of protons before heat treatment is 0.05
In the following cases, the guided light was not excited in both ordinary light and extraordinary light.

Further, as shown in FIG. 1, the optical waveguide had optical waveguide layers formed on both sides of the crystal substrate, and the optical waveguide characteristics were the same on both sides. A conceptual view of the optical waveguide thus obtained is shown in FIG.

Test Example 2 Proton amount before proton desorption treatment (α × d) = 0.25
Lithium niobate crystal (X cut, Y cut, thickness 1
mm, 10 × 20 mm ² ) was subjected to Li external diffusion heat treatment at 1000 ° C. in dry oxygen with a dew point temperature of −70 ° C. or less at different times.

When the extraordinary light equivalent refractive index was measured by the prism coupler method, the equivalent refractive index could be controlled with the heat treatment time as shown in FIG. This means that in order to obtain a desired equivalent refractive index, in addition to the method of changing the difference in the amount of protons before and after heat treatment (dissociation amount) described in Test Example 1,
It is shown that it can also be obtained by controlling the proton desorption treatment condition.
It has been shown that it can be obtained with good reproducibility.

Further, in any of the X, Y and Z cuts of the lithium niobate crystal according to Test Examples 1 and 2,
It has been shown that a Li outdiffused optical waveguide can be manufactured.

Example 1 and Comparative Example 1 A crystal boule (Z-cut, φ3 inch, length 50 mm) grown by the Czochralski method was annealed and poled to be sliced to a thickness of 1.2 mm, lapped and polished. 10 double-sided polished wafers having a thickness of 1.1 mm were obtained. When the amount of protons on this wafer was measured, variations in the proton concentration were observed even within one wafer,
It was 0.27 ± 0.03 as a whole.

Of these wafers, 5 wafers (No. 1 to No. 1)
5) was annealed at 700 ° C. for 7 hours by introducing oxygen gas through a pure bubbler at 3 ° C. into an annealing furnace. The amount of protons after annealing is 0.21 between wafers and between wafers.
It became ± 0.01 and became uniform.

Further, in order to remove surface damage due to annealing and to make the thickness uniform, a total of 10 wafers, 5 annealed wafers and 5 unannealed wafers, are lapped and polished. Thickness is 1.0m
m.

In this way, the five wafers whose proton concentration was made uniform and the wafers which were not annealed (Nos. 6 to 10) as a comparison were simultaneously subjected to the proton desorption treatment. Processing conditions are 1000 ° C, 10 minutes, dew point temperature minus 70
It was in dry oxygen at a temperature of ℃ or below.

The extraordinary light equivalent refractive index was measured by the prism coupler method. As a result, in the annealed crystal wafer (Example 1), the 0th-order mode was 2.2037 in all the wafers.
± 0.0001, primary mode is 2.20 25 ± 0.
It became 0001. On the other hand, variations were observed in the unannealed wafers (Comparative Example 1), and the 0th-order mode was 2.2040 ± 0.0005 and the 1st-order mode was 2.2026 ± 0.0003 in all the wafers. It was Thus, in Example 1, the variation was slight.

From this, it is shown that it is effective to perform the annealing treatment for reducing the variation in the amount of protons in the crystal before the proton desorption treatment in order to obtain the optical waveguide having the desired optical waveguide characteristics. .

Further, when the propagation loss of the optical waveguide of Example 1 was measured, as shown in FIG.
m was 0.1 dB / cm, and it was confirmed that the loss was lower than that of other optical waveguides. On the other hand, the propagation loss of the optical waveguide of Comparative Example 1 is 0.4d at the wavelength of 0.633 μm.
It was B / cm. Further, the amount of protons of the optical waveguide substrate formed in this example was 0.05 or less.

Comparative Example 2 A lithium niobate crystal (Z-cut, thickness 1.0 mm, 10 × 20 mm ² ) having a proton content of 0.25 was measured at 1000 ° C. for 10 times by measurement of absorption coefficient before proton desorption treatment.
Min, the proton desorption treatment was performed in an oxygen atmosphere with a dew point temperature of minus 20 ° C. After the treatment, the extraordinary light equivalent refractive index was measured day by day by the prism coupler method. The sample was left at room temperature. The amount of protons was 0.09. It was found that the optical waveguide formed immediately after the treatment disappeared in about 1 week and the stability was not obtained. This is because the protons were not sufficiently desorbed during the proton desorption treatment, and it is preferable that the atmosphere has a dew point temperature of −70 ° C. or lower.

[0049]

INDUSTRIAL APPLICABILITY The optical waveguide of the present invention is suitable for an optical device because it has extremely small propagation loss and excellent stability. Further, the manufacturing method of the present invention makes it possible to easily obtain the optical waveguide characteristics with good reproducibility.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the equivalent refractive index and the amount of released protons.

FIG. 2 is a conceptual diagram of an optical waveguide of the present invention.

FIG. 3 is a graph showing the relationship between the equivalent refractive index and the square root (√sec) of heat treatment time.

FIG. 4 is a graph showing the measurement result of the propagation loss, and the gradient shows the magnitude of the propagation loss.

Claims (3)

[Claims] 1. The propagation loss at a wavelength of 0.633 μm is 0.1.
An optical waveguide comprising a lithium niobate crystal obtained by Li out-diffusion, characterized in that it is at most dB / cm. 2. The amount of protons (A) in the lithium niobate crystal represented by the product of the light absorption coefficient (cm ⁻¹ ) at a wavelength of 2.87 μm and the crystal thickness (cm) is 0.05 or less. The optical waveguide according to claim 1. 3. The grown lithium niobate crystal boule is annealed, poled and sliced, and then annealed again to adjust the amount of protons and to make the concentration uniform, and then lapping and polishing to form a mirror surface. A method for producing an optical waveguide comprising a lithium niobate crystal obtained by Li outdiffusion, which comprises performing proton desorption treatment in a dry oxygen atmosphere having a dew point temperature of −70 ° C. or less.