JPH09288255A - Optical wave guide element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical wave guide element which can obtain two output lights having little wavelength chirpings and in which a wavelength dependence is small and integration is easy. SOLUTION: In this element, the Y branching waveguide 21 of 1×2 is connected to the input side of two lines of parallel intermediate optical waveguides 4, 4' and the 3dB coupler 31 of 2×2 is connected to the output side of optical waveguides 4, 4'. Moreover, two pairs of electrodes 5, 5', 6, 6 for changing refractive indexes of the intermediate optical waveguides 4, 4' are provided on or in the vicinity of respective two lines of the intermediate optical waveguides 4, 4' and complementary electric signals are made to be impressed on the two pairs of electrodes.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical waveguide device applied to an optical switch, an optical modulator or the like. In recent years, with the progress of the advanced information society, it is necessary to process an enormous amount of information, and an optical communication system using an optical fiber is utilized as a means for transmitting such information. In this optical communication system as well, the transmission capacity has been increased year by year by increasing the modulation speed, and new waveguide type optical circuit devices such as an optical external modulator for increasing the speed have been used more and more. Also, attempts have been made to use various optical waveguide components in the field of measurement. Therefore, high performance and miniaturization are required.

[0002]

2. Description of the Related Art FIG. 1 shows the structure of an optical modulator disclosed in Japanese Patent Laid-Open No. 2-167524. In this device, three optical modulators 12, 13, 14 and one optical coupler 15 are formed on an optical substrate 11. The optical modulator 12 arranged on the input side connects two intermediate optical waveguides 123 and 124 to the two outputs of the 1 × 2 Y-branch waveguide 121, and further connects each intermediate optical waveguide with a directional coupling type two. An optical waveguide element having a structure in which a 3 dB coupler 122 of × 2 is connected and a pair of electrodes (consisting of a ground electrode and a strip line) 125 is provided on the two intermediate optical waveguides. Has a function of changing the refractive index of the intermediate optical waveguide to a push-pull by inputting a clock signal to the electrode of the drive circuit 16 and outputs light to the two output waveguides as a dual output modulator. Let

[0003]

In the above-mentioned conventional optical waveguide device, since the ground electrode and the strip line (signal electrode) provided on the two intermediate optical waveguides have an asymmetrical relationship, the waveguide below the ground electrode is not provided. The change in the refractive index has the opposite sign, but the magnitude becomes about ⅕ of the change in the refractive index of the waveguide below the signal line. In other words, this is an electrode configuration that performs a so-called pseudo-pseudo push-pull operation that does not result in a complete push-pull operation.Therefore, wavelength chirping occurs in the output light, and there is a problem when processing transmission characteristics or multiplexing. It was

Further, in the conventional optical waveguide device, a directional coupler is used as a 3 dB coupler, and this directional coupler has a long length and is unsuitable for integration, and
There is a problem of having large wavelength dependence, and there is a problem that a device corresponding to a wide range of wavelengths cannot be realized. Furthermore, in the conventional optical modulator, although the optical path length between the two directional couplers (3 dB coupler) on the input side and the output side is formed to be almost equal in the two lines, a slight refractive index difference of the optical waveguide is not formed. Due to the balance and temperature distribution, there is a phase difference in the wavelength level of the light, and unexpected interference occurs at the place where they are combined, causing a problem of disturbed waveform.

It is an object of the present invention to provide an optical waveguide device having two wavelengths of output light with almost no wavelength chirping and having little wavelength dependence and easy integration. Also,
It is an object of the present invention to obtain a good combined optical signal in an optical waveguide device that modulates two such output lights and combines them to function.

[0006]

The optical waveguide element according to the invention of claim 1 has 1 × on the input side of two parallel intermediate optical waveguides.
2 Y-branch waveguides or 2 × 2 3 dB couplers are connected, a 2 × 2 3 dB coupler is connected to the output side, and the refractive index of the optical waveguides is provided on or near each of the two intermediate optical waveguides. Is an optical waveguide device configured to have two sets of electrodes for changing, and to apply a complementary electric signal to the two sets of electrodes.

In the optical waveguide device having this structure, when the switch or the modulator is operated, the change rates of the refractive index changes of the two intermediate optical waveguides can be made equal in magnitude with opposite signs. Therefore, the wavelength chirping of the output light can be set to 0 for any output light. In the optical waveguide device according to the invention of claim 2, the 2 × 2 3 dB coupler is
This is a configuration including a single waveguide to which two-input and two-output optical waveguides are connected.

In the 3 dB coupler having this structure, the element length is shortened, the 3 dB coupler functions in a wide wavelength range, and since it does not include a fine structure such as a directional coupler, it can be manufactured with good reproducibility and yield. . The optical waveguide device according to the invention of claim 3 has a construction in which means for applying intensity, phase, or frequency modulation to the output of the 2 × 2 3 dB coupler is attached.

The optical waveguide device according to a fourth aspect of the present invention is the optical waveguide device according to the second aspect.
A means for applying intensity, phase, or frequency modulation to the output of the × 2 3 dB coupler is provided, and these are 2 × 2.
In this configuration, it is multiplexed by the 3 dB coupler. For example, the light that has been intensity-modulated and merged does not cause the disturbance of the pulse waveform even if it propagates through a fiber having dispersion.

The optical waveguide device according to a fifth aspect of the present invention is the optical waveguide device according to the second aspect.
The × 2 3 dB coupler is composed of one waveguide to which two-input and two-output optical waveguides are connected. 3 of this configuration
The wavelength dependence, short element length, and yield improvement can be realized by the dB coupler. In the optical waveguide element of the invention of claim 6, the optical path length between the two 2 × 2 3 dB couplers is formed to be substantially equal in the two lines.

With this line configuration, an optical time division multiplexing function can be realized. The optical waveguide element according to the invention of claim 7 has a configuration in which a phase adjuster for adjusting the phase of light is provided on each of the two lines between the two 2 × 2 3 dB couplers. In the optical waveguide device having this phase adjuster, unexpected interference of light at the time of merging can be avoided even with respect to the difference in optical path length at the wavelength level of light, and a good merging optical signal can be obtained.

In the optical waveguide device according to the invention of claim 8, the phase adjuster is a heater utilizing a thermo-optical effect. This heater structure can be manufactured with a short element length.

[0013]

2 is a plan view showing the structure of the optical waveguide device according to the first embodiment of the present invention. The optical waveguide element is formed by forming a Ti metal pattern having a thickness of 900Å on a z-cut lithium niobate substrate 1 and thermally diffusing the Ti metal pattern at 1000 ° C. for 8 hours. Have. The width of the waveguide pattern is 7 μm, and the input / output section is a single mode waveguide. This waveguide is formed by sandwiching two linear waveguides (intermediate waveguides) 4 and 4'with one Y-branch waveguide 21 and 2 × 2 3 dB coupler 31. A 1 μm thick SiO 2 buffer layer (not shown) is formed on these intermediate waveguides, and a 20 μm thick traveling wave electrode (signal electrodes 5, 5 ′ and ground electrode 6) is formed thereon by plating. , 6 ') are formed. The signal electrode has a width of 7 μm and a length of 20 m.
m.

As shown in the figure, the two sets of traveling wave electrodes are symmetrically formed in the respective intermediate waveguides, and complementary electric signals are applied from the two sets of complementary signal power supplies 7 and 7 '. It With this configuration, the operation is performed at 5 V with respect to light having a wavelength of 1.55 μm. In this case, one signal is 100110
, The other becomes 011001, but 0 and 1 respectively.
In order to concentrate and output one light from the two output waveguides connected to the two outputs of the 3 dB coupler 31, the DC voltage is added so that a phase difference of 0 and π is generated at that voltage. Or, it is necessary to shift the optical path length of the intermediate waveguide by π / 2. Also, with complementary sine wave clocks, 1010 for each output and 0101 for the other
In the case of obtaining a periodic signal such as the above, such phase shift is not necessary. In the optical signal generated by such a configuration, the wavelength chirping due to the modulation can be set to zero.

FIG. 3 is a diagram showing a specific example of the 3 dB coupler 31. This 3 dB coupler is composed of one waveguide to which two-input and two-output optical waveguides are connected. With this configuration, a coupler can be formed with a size of only 350 μm, and the size can be greatly reduced compared to the case where the directional coupler type requires an element length of several mm. In addition, as a result of forming the element short, the wavelength dependence is small. FIG. 4 shows the wavelength dependence of the branching ratio of the 3 dB coupler, and it can be seen that the wavelength dependence of the 3 dB coupler used in this embodiment is smaller than that of the directional coupler type.

The shape features of the coupler shown in FIG.
Since there is no fine structure such as a narrow gap (between the waveguides) as seen in the directional coupler type, it is easy to make with good reproducibility. That is, this gap is as thin as about 2 μm, and even if it is made as thin as about 0.5 μm during fabrication, it causes a large change in characteristics, but with the 3 dB coupler used in this embodiment, almost no characteristics are obtained even if the waveguide width is about 0.5 μm. Does not affect In this coupler, it is necessary to make the width of the central waveguide wider than that of the input / output waveguide. This is because the incident light from the input single-mode waveguide has power distributed to the fundamental mode (even mode) and the first-order mode (odd mode) in the center, but in the case of the same thickness as the input / output waveguide. This is because the first-order mode light is emitted outside the waveguide. Further, when the power is distributed, it is also distributed to higher-order mode light, and this component becomes unnecessary light because it reduces the performance of the 3 dB coupler. However, if the center is appropriately thinned, this higher-order mode light can be selectively Can be removed. Regarding the element length, the phase difference between the fundamental mode (even mode) and the primary mode (odd mode) is 90 °. The coupler structure in FIG. 3 is a structure that reflects these effects.

FIG. 5 shows a second embodiment of the present invention in which an optical modulator is constructed by applying the above optical waveguide element. In this device, the optical waveguide device shown in FIG. 3 is used as the first optical modulator A, and the second and third optical modulators B and C that apply intensity modulation to the output from the 3 dB coupler 31 of the optical waveguide device are provided. A 3 dB coupler 9 is provided and combined. The second and third optical modulators B and C are the same as the conventional optical modulators 13 and 14 shown in FIG. 1, but 3 dB.
The same coupler as the 3 dB coupler shown in FIG. 3 is used as the coupler. In FIG. 5, reference numerals 5-1 5′-1, 5 are used.
-2 and 5'-2 are signal electrodes, 6-1, 6'-1, 6-
Reference numerals 2 and 6'-2 represent ground electrodes, and reference numerals 7, 7 ', 8 and 8'represent signal power sources. A pair of data signals are output from the signal power supplies 8 and 8 ′ with their phases shifted by 180 ° and in synchronization with the complementary electric signals from the signal power supplies 7 and 7 ′. By adopting such a configuration, a time division multiplexed signal having a good optical level can be obtained.

Here, since the optical paths between the two 3 dB couplers 31 and 9 are formed to be substantially equal to each other in the two lines, good multiplexing can be realized. However, temperature unevenness in the substrate and the refractive index of the waveguide are realized. The phase shifts at the wavelength level of light due to the distribution of. Example 3 which improved this is shown in FIG. In the third embodiment, as shown in the drawing, the phase adjusters 10 and 1 are provided between the second and third optical modulators B and C and the 3 dB coupler 9.
0'is provided respectively. This phase adjuster has a heater utilizing a thermo-optic effect, for example, a short A with a length of 1 mm.
A u-electrode heater is used and its heating adjusts the waveguide refractive index. In this configuration, the power required for phase adjustment of π was 250 mW. Further, as shown in the figure, by placing heaters on both the intermediate waveguides, it is possible to advance or delay the phase.

When high-speed phase adjustment is required, an electrode for applying an electric field to the waveguide may be formed to change the refractive index by utilizing the electro-optical effect. In the case of being slow, a piezo element may be arranged on the waveguide and the refractive index may be changed by utilizing the photoelastic effect. In addition,
There is also an adjustment method based on the acousto-optic effect. In the above-described embodiment, lithium niobate is used as the optical substrate, but it can be similarly realized with an indium phosphide-based semiconductor waveguide device or an organic-based waveguide device.

Further, in the structure in which the output from the 3 dB coupler is provided with means for applying phase, frequency or intensity modulation respectively, the two optical signal outputs are complementary signals and RZ (Return Zer).
Since it is also an o) signal, it can be said that this configuration is a modulator for obtaining an excellent RZ modulation signal of light. In particular, this configuration has the advantage that the necessary 0 state associated with each signal can be set independently and is not affected by the modulation due to coding.

[0021]

According to the optical waveguide element of the present invention, it is possible to obtain a modulated signal without wavelength chirping that is branched from one light source and has two time divisions. It is possible to obtain a good merged signal that does not cause waveform distortion. Since these do not have wavelength chirping especially when they are transmitted through a fiber, they have the effect of being less susceptible to waveform deterioration due to fiber dispersion.

Further, since the complementary signal type electrode is adopted, the element length can be shortened by 40% as compared with the conventional one, and the realization of such an integrated optical waveguide element is superior. The 3 dB coupler used here is composed of a 3 dB coupler consisting of a single waveguide to which two-input and two-output optical waveguides are connected, so that the size can be reduced, the wavelength dependence can be reduced, and the yield can be improved. Has realized an improvement. In addition, the introduction of a phase adjuster that adjusts the phase of the wavelength level of light at the time of merging, which has been a problem in the conventional example, has eliminated the disturbance due to the phase shift of the merging waveform. Furthermore, since this phase adjuster uses a heater that uses the thermo-optic effect, it can be manufactured with a mere 1 mm,
Effective for integration.

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional modulation device.

FIG. 2 is a diagram showing a first embodiment of the present invention.

FIG. 3 shows a 3 dB coupler according to the present invention.

FIG. 4 is a diagram for explaining the wavelength dependence of the 3 dB coupler shown in FIG.

FIG. 5 is a diagram showing a second embodiment of the present invention.

FIG. 6 is a diagram showing a third embodiment of the present invention.

[Explanation of symbols]

1 Lithium niobate substrate 2 Input optical waveguide 3,3-1,3-2 Output optical waveguide 4,4 'Intermediate waveguide 5,5', 5-1,5'-1,5-2,5'-2 Signal electrode 6,6 ', 6-1,6'-1,6-2,6'-2 Ground electrode 7,7', 8,8 'Electric signal source 9,31 3dB coupler 10,10' Phase adjuster

Claims (8)

[Claims] 1. One on the input side of two parallel optical waveguides.
A × 2 Y-branch waveguide or a 2 × 2 3 dB coupler is connected, a 2 × 2 3 dB coupler is connected to the output side, and the optical waveguide is refracted on or near each of the two intermediate optical waveguides. An optical waveguide device comprising two sets of electrodes for changing the rate and applying a complementary electric signal to the two sets of electrodes. 2. The optical waveguide element according to claim 1, wherein the 2 × 2 3 dB coupler is a 3 dB coupler formed of a single waveguide to which two-input and two-output optical waveguides are connected. 3. The optical waveguide device according to claim 1, further comprising means for applying intensity, phase or frequency modulation to the output of the 2 × 2 3 dB coupler. 4. A structure comprising means for applying intensity, phase or frequency modulation to the output of a 2 × 2 3 dB coupler, and these are multiplexed by a 2 × 2 3 dB coupler. The optical waveguide device according to claim 1 or claim 2. 5. The optical waveguide element according to claim 4, wherein the 2 × 2 3 dB coupler is a 3 dB coupler formed of a single waveguide to which two-input and two-output optical waveguides are connected. 6. The optical waveguide device according to claim 4, wherein the optical path length between the two 2 × 2 3 dB couplers is formed to be substantially equal in the two lines. 7. The optical system according to claim 4, wherein a phase adjuster for adjusting the phase of light is provided on each of the two lines between the two 2 × 2 3 dB couplers. Waveguide element. 8. The optical waveguide device according to claim 7, wherein the phase adjuster is composed of a heater utilizing a thermo-optic effect.