JPH07152006A - Method for adjusting initial operation point of waveguide type optical element - Google Patents

Abstract

PURPOSE:To make it possible to adjust the initial operation point of an optical element without using a DC electric field for control by applying a DC voltage from the outside on a waveguide type optical element on which external electric field has not been impressed, thereby irreversibly changing the optical modulation characteristics thereof. CONSTITUTION:The material characteristics of the optical element right after production are such that, in the state of (a), the peak value of a modulation waveform exists nearly at a zero voltage and a change of the corresponding light intensity is too small to detect a slight change of the external electric field as a change of light output intensity, and, the light output intensity changes in a decreasing direction even if the external electric field changes. The DC voltage is, thereupon, applied on this optical element and the element is left as it is. A drift having a hysteresis arises and the initial operation point (phase at the zero voltage) of the optical element moves nearly to a mid-point of the peak and valley of the modulation waveform if the voltage applied thereon is released after the application thereon and the element is left as it is (b). The large light intensity change is obtd. dependently upon the slight change of the external electric field in such state and, therefore, the initial state preferable as an electric field sensor is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for adjusting an initial operating point of a waveguide type optical element. More specifically, the present invention relates to, for example, lithium niobate (LiNbO ₃ ).
The present invention relates to a method of irreversibly adjusting an initial operating point of an optical element having an optical waveguide formed by diffusing titanium (Ti) to a desired portion of an electro-optical substrate made of, for example. By such adjustment, the phase of the output light of the optical signal propagating in the optical waveguide of the waveguide type optical element can be irreversibly adjusted and set. In this way, an optical element whose initial operating point is irreversibly adjusted and set to a desired value, for example, a lithium niobate waveguide type optical element is advantageous for an electric field sensor or the like that detects an external electric field by a phase change of output light. is there.

[0002]

2. Description of the Related Art Ferroelectric materials, especially lithium niobate (LiNbO ₃ ), and lithium tantalate (LiTa).
O ₃ ), etc. exhibit a remarkable electro-optical effect,
When an electric field is applied from the outside, its refractive index slightly changes. By utilizing this phenomenon, an optical waveguide can be formed from the above material, and an electric field can be applied from an electrode provided in the vicinity thereof to control the phase of signal light propagating in the optical waveguide.

When an optical element having an optical waveguide as described above is used in an electric field sensor or the like, in order to improve the sensitivity of the sensor, the intensity of outgoing light emitted from this optical waveguide is external to this optical waveguide. It is preferable that the voltage changes substantially linearly according to the strength of the electric field applied from the. Therefore, it is necessary to adjust the initial operating point so that the signal light emitted from the optical waveguide exhibits an appropriate phase corresponding to the applied external electric field. That is, if the intensity of emitted light is cos ² θ, it is necessary that θ = π / 2 when the intensity of the external electric field is zero.

As a conventional method for adjusting the initial operating point of the optical waveguide, a direct current (DC) electric field is continuously applied (biased) to the optical waveguide by utilizing the electro-optic effect of the optical waveguide.
However, by appropriately adjusting the strength of the applied DC electric field, the refractive index of the optical waveguide is changed, thereby adjusting the phase of the light emitted from the optical waveguide and the initial operating point of the optical element. Are known. This conventional method can be applied to an external modulator or the like. However, in the electric field sensor used for the purpose of detecting a minute external electric field by changing the intensity (phase) of the signal light, it is not allowed to use the above-mentioned electric field for controlling the emitted light phase.

As another conventional method for adjusting the initial operating point of the optical waveguide, the lengths of the bifurcated optical waveguides are formed so as to be slightly different from each other, and the light used for the bifurcated optical waveguide is adjusted. A path length difference of the order of wavelength should be generated. For example, a method is known in which two waveguides are formed in a concentric circular arc shape having different radius lengths, the signal lights emitted from the two waveguides are combined, and the phase of the emitted light is adjusted. ing.

Yet another conventional method of adjusting the initial operating point of the optical waveguide is to form the asymmetric shape by partially changing the widths of the two waveguides on the left and right sides, thereby changing the effective refractive index. A method of generating and shifting the initial operating point (Tsuchiya, Kubota, Kiyono, 1992, Autumn Meeting of the Institute of Electronics and Communication Engineers, Invention C-171) is known.

Still another known method for adjusting the initial operating point of the optical waveguide is to apply a mechanical pressure on or near the optical waveguide that is branched into two left and right parts to obtain the photoelasticity of the optical waveguide. A method (Japanese Patent Laid-Open No. 4-34,516) is known in which the effect is used to slightly change the refractive index of the optical waveguide to shift the initial operating point.

Since the above-mentioned conventional method does not require a DC electric field for phase control, it can be used for adjusting the operating point of an electric field sensor. However, in order to manufacture an optical element with desired performance with good reproducibility. It is necessary to precisely control the optical element manufacturing process. In practice, electro-optic crystal substrates such as lithium niobate used for electro-optic optical devices are
Since there are essentially many crystal defects, the in-plane characteristic distribution of the obtained optical element substrate is not necessarily uniform at the current state of the art. Therefore, when the above-mentioned conventional method is applied, the obtained optical element is obtained. This may cause a problem that there is a large variation in the initial operating point between individuals. Therefore, in the conventional method in which the initial operating point is adjusted by designing the pattern of the optical waveguide, there is a problem that the manufacturing yield of the product is lowered.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for adjusting the initial operating point of an optical waveguide type optical element without using a controlling DC electric field. Also,
The present invention is intended to provide a method capable of adjusting the initial operating point of an optical waveguide type optical element with a different adjustment amount for each optical element. Furthermore, the present invention relates to an optical element having an optical waveguide formed of a material exhibiting an electro-optical effect such as lithium niobate, when using the electric field sensor based on, for example, a Mach-Zehnder type intensity modulator. Phase adjustment of the outgoing signal light required for
That is, it is intended to provide a method for adjusting the initial operating point.

[0010]

SUMMARY OF THE INVENTION The present inventors have found that when a DC voltage is externally applied to a waveguide type optical element to which an external electric field has not been applied, its optical modulation characteristics are irreversibly changed. The inventors have found that the initial operating point of the waveguide type optical element can be adjusted by utilizing the above, and have completed the present invention.

The method of adjusting the initial operating point of a waveguide type optical element according to the present invention comprises an operation of applying a DC voltage to a waveguide type optical element which has no history of being applied with an external electric field, and the voltage application. Is performed to set the initial operating point of the waveguide type optical element to be irreversible.

In the method of the present invention, it is preferable that the DC voltage application operation is performed at a temperature of 20 ° C to 100 ° C.

In the method of the present invention, when the waveguide type optical element has a substrate made of lithium niobate and the substrate has been subjected to a treatment for reducing the hydrogen content, the direct current It is preferable that the voltage application process is performed in an atmosphere having a dew point higher than the dew point of the atmosphere for reducing the hydrogen content of the substrate.

The waveguide type optical element manufactured by adjusting the initial operating point by the method of the present invention is effective as an electric field sensor for detecting the external electric field by the phase change of the output light.

[0015]

[Function] The optical waveguide type electro-optical element is
An optical element that can modulate the phase of output light but has no history of being applied with an external electric field (immediately after manufacturing) has irreversible characteristics such as phase modulation with respect to the external electric field when the first external electric field is applied. It has the property of changing. That is, the modulation state of the output light before the external electric field is applied,
The modulation state of the output light after receiving the external electric field application is different from each other, and the modulation state of the output light of the optical element, that is, the initial operating point is set irreversibly by the first external electric field application.

The irreversible change in the characteristics of the optical element due to the application of a relatively high external electric field is such that the distribution of impurities inside the optical element and the arrangement state of ions and atoms become more stable (energy) state by the application of the electric field. It is based on a transition phenomenon, and this irreversible change is accelerated by heating as in a chemical reaction. Then, when the application of the electric field is released, the internal state of the optical element is almost frozen in the changed state.

Therefore, by applying the DC voltage to the optical element in the heating atmosphere, the irreversible change can be promoted and the required processing time can be shortened. However, practically, the optical element is subjected to an assembly process for fixing an optical fiber or the like with an adhesive, and therefore the DC voltage application process is performed at a temperature at which the adhesive does not deteriorate, that is, generally 20 to 100 ° C. Is preferably applied at a temperature of from 0 to 100 ° C., more preferably from room temperature (25 ° C.) to 100 ° C.
Is.

The waveguide type optical element to which the method of the present invention is applied preferably has an optical waveguide using lithium niobate as a substrate, and this lithium niobate substrate is used at the stage of manufacturing the optical element. It is preferable that a treatment for forcibly reducing the content of hydrogen (impurity) contained therein is performed. As described above, when the method of the present invention is applied to the waveguide type optical element having the lithium niobate substrate subjected to the hydrogen content reduction treatment, the dew point of the atmosphere in which the external electric field is applied is the hydrogen of the substrate. It is preferably higher than the dew point of the atmosphere for the content reduction treatment.
This has the advantage that moisture (hydrogen) in the atmosphere is easily injected as hydrogen ions from the electrode into the optical element through an electrochemical reaction for the operation of applying an external electric field.

Next, on the z-cut LiNbO ₃ substrate,
The present invention will be further described by taking an optical element having an optical waveguide manufactured by thermally diffusing titanium (Ti) as an example. An optical element using the above-mentioned waveguide, for example, a Mach-Zehnder type intensity modulator is, for example, a z-Li having a diameter of 3 inches and a thickness of 0.5 mm.
After forming a waveguide by thermally diffusing Ti into a desired pattern on an NbO ₃ wafer, a SiO ₂ film is deposited thereon by a vapor deposition method, for example, a sputtering method, and electrodes are further deposited. You can

The above-mentioned thermal diffusion of Ti is generally performed in a gas atmosphere containing water vapor in order to prevent out-diffusion of Li. In the lithium niobate substrate heat-treated in such a humid atmosphere, hydrogen (ions) are
In the form of, it is taken into the oxygen plane of the lithium niobate crystal (one of the planes perpendicular to the z-axis). In addition to hydrogen ions,
It is also taken into the crystal during the crystal pulling process and the polarization treatment. That is, the lithium niobate crystal can be considered to be energetically more stable when it contains hydrogen ions.

On the other hand, when the heat treatment of the lithium niobate substrate is performed in a dry atmosphere, the amount of hydrogen in the substrate decreases corresponding to the thermodynamic equilibrium state of the processing conditions. The amount of hydrogen in the substrate can be measured by infrared spectroscopy. For example,
OH expansion / contraction mode (up to 3500) measured on a substrate that has been subjected to heat treatment (980 ° C.) in a normal wet atmosphere in which oxygen bubbling pure water is introduced into the furnace.
The absorption coefficient of cm ^-1 ) is 2.6 cm ^-1 .
The absorption coefficient of a substrate heat-treated in an atmosphere with oxygen introduced at a dew point of about −70 ° C. is 0.70 cm ⁻¹ , and the heat treatment in a dry atmosphere reduces the amount of hydrogen contained in the substrate to about ⅓.

The solid line in FIGS. 1A and 1B is
Two waveguide type optical devices (electrode length 40 mm, distance between electrodes about 10 μm) fabricated with the amount of hydrogen contained in the lithium niobate substrate forcibly reduced by heat treatment in a dry atmosphere
FIG. 4 shows how the dc drift when a DC voltage of 5 V is applied at a temperature of 80 ° C., that is, the operating point position of the modulation waveform changes over time. Such a drift phenomenon is a phenomenon often observed in an optical element composed of a dielectric material. This is because a polarization is generated in the dielectric material by applying an electric field and the applied electric field is canceled. is there. Since the above-mentioned polarization occurs based on the displacement of ions in the dielectric, it is observed as a phenomenon having a relaxation time of about several hours.

In the example shown by the solid line in FIGS. 1A and 1B, the drift shows a saturation value at about 2V. In other words, of the applied 5V voltage, just 2
V minutes have been canceled. Next, when the applied voltage is removed, the polarization should disappear with a relaxation time comparable to the above, and a drift corresponding to the cancellation due to the polarization should occur in the opposite direction. That is, in the example of FIG.
Saturating drift at a voltage of -2V should be observed. However, as shown by the broken line in FIG. 1, when the applied voltage of 5 V is released, the saturation value becomes about −5.
A drift of V, that is, a drift whose absolute value corresponds to the applied voltage, was observed. In contrast to this sample, in the optical device manufactured through the heat treatment in a wet atmosphere, the drift due to the application of 5 V was 4 to 5 V (larger than the sample (a) in FIG. 1) and the voltage was turned off (released) in the measurement under the same conditions. ), The drift was -4 to -5V. In the present invention, the irreversible change of the modulation characteristic due to the voltage application as shown by the straight line and the broken line in FIG. 1 is utilized.

The cause of the above irreversible change in the characteristics of photohydrogen using the substrate with reduced hydrogen can be explained as follows. That is, hydrogen in the substrate crystal exists in the form of —OH in the oxygen plane. Therefore, when an electric field is applied in a direction intersecting the oxygen surface, hydrogen ions are displaced and polarized in a direction away from the oxygen surface. The cancellation of the applied electric field due to such polarization is naturally smaller as the amount of contained hydrogen ions is smaller. That is, the amount of drift of the sample manufactured by the dry atmosphere heat treatment is smaller than that of the sample manufactured by the wet atmosphere heat treatment. However, it is preferable that the equilibrium state in the environment in which the electric field is applied contains a large amount of hydrogen, or
Hydrogen (ion) diffusion in the substrate due to the electric field tends to increase the amount of hydrogen near the surface of the substrate (waveguide).

In each of FIGS. 1 (a) and 1 (b),
When a voltage of 5 V is applied, the drift is saturated to about 2 V when the polarization due to hydrogen ions in the oxygen surface is balanced with the movement of hydrogen (ions) from the outside (for example, ion conduction). It is believed that there is. When the applied voltage is released in this state, the polarized hydrogen ions return to the oxygen surface, and at the same time, even when hydrogen ions do not exist in the initial state (in the oxygen surface), the migrated hydrogen is -OH. Since it is taken in in the state of, the drift amount in the opposite direction becomes large. That is, if the amount of hydrogen ions in the final state is about the same as that of the heat treatment sample in the wet atmosphere, a drift of -4 to -5 V will occur. As a result, the hydrogen content in the substrate (at least in the region corresponding to the penetration depth of the electric field) is increased, and even if the voltage is applied again, a large “drift” (4 to 5 V) corresponding to the applied voltage is obtained. ) Is observed, and irreversible change due to voltage off (release) does not occur.

For the above reason, the change in the modulation characteristic caused by once applying the high electric field in the initial state is irreversible unless the amount of hydrogen in the substrate is changed (decreased), and the original state is applied by the subsequent application of the electric field. There is no return to (drift shows hysteresis). Therefore, the initial operating point of the optical element can be adjusted by the method of the present invention. The adjustment amount can be adjusted by the strength of the applied electric field, as long as the amount of hydrogen that is irreversibly changed (increased) by application of the electric field is sufficiently large.

Generally, in the operation of applying an electric field in the method of the present invention, an optical element having no history of application of an electric field is -10 to +.
It is preferable to apply a DC voltage of 10 V for 10 to 100 hours. After releasing the electric field application, the optical element is placed in an atmosphere at the same temperature as the electric field application operating temperature for 10 to 10 minutes.
It is preferable to leave it for about 100 hours.

[0028]

The present invention will be further described by the following examples. Example 1 z-cut T produced in a stream of dry oxygen (980 ° C.)
i: An SiO ₂ buffer layer was formed on a LiNbO ₃ waveguide substrate by a sputtering method, and the SiO ₂ buffer layer was formed in a dry oxygen stream,
An optical element for a Mach-Zehnder modulator type electric field sensor was produced through a step of heat treatment at a temperature of 00 ° C.

FIG. 2A shows the modulation characteristic (wavelength: 1.55 μm) of the optical element immediately after fabrication (no electric field applied). In this state, the peak position of the modulation waveform is almost at the zero voltage position, and to detect a minute change in the external electric field (voltage) as a change in the optical output intensity, the corresponding change in the optical intensity is small,
In addition, the output light intensity changes in the decreasing direction regardless of whether the external electric field changes positively or negatively. Such an optical element is unsuitable for an electric field sensor for detecting an external electric field.

A DC voltage of 5 V was applied to the above optical element at a temperature of 80.degree.
Is applied and left for about 1 day, then the applied voltage is released
I left it for more than a day. Then, a drift phenomenon having hysteresis occurs, and the initial operating point (phase at zero voltage) of the optical element is almost at the midpoint between the peak and the valley of the modulation waveform as shown in FIG. 2B. moved. Such a state is a preferable initial state as an electric field sensor because a large change in light intensity can be obtained depending on a minute change in the external electric field.

[0031]

According to the present invention, it becomes possible to adjust the initial operating point of an optical waveguide device without constantly applying a control voltage.

[Brief description of drawings]

1A and 1B are drifts of operating points and operating time of two waveguide type optical elements having no history of application of an electric field when a DC voltage of 5V is applied. The graph which shows the relationship (solid line) with (voltage application time), and the relationship (dashed line) with the drift after releasing the said voltage application, and operating time (standing time).

FIG. 2A is a graph showing the relationship between the intensity of output light and the applied voltage in an example of a waveguide-type optical element that has no history of application of an electric field. FIG. 2B is a graph showing the relationship between the intensity of the output light of the optical element and the applied voltage after the optical element is subjected to the initial operating point adjustment processing by the method of the present invention.

Claims (4)

[Claims] 1. An operation of applying a DC voltage to a waveguide type optical element having no history of being applied with an external electric field,
A method for adjusting an initial operating point of a waveguide type optical element, characterized in that the initial operating point of the waveguide type optical element is set irreversibly by performing an operation of releasing the voltage application. 2. The DC voltage applying operation is performed at 20 ° C. to 10 ° C.
The method according to claim 1, which is performed at a temperature of 0 ° C. 3. The waveguide type optical element has a substrate made of lithium niobate, and the substrate has been subjected to a treatment for reducing a hydrogen content, and the direct current voltage applying treatment includes: The method according to claim 1, wherein the method is performed in an atmosphere having a dew point higher than the dew point of the atmosphere for reducing the hydrogen content of the substrate. 4. An electric field sensor including a waveguide type optical element manufactured by adjusting the initial operating point by the method according to claim 1, and detecting an external electric field by a phase change of output light.