JP2613942B2 - Waveguide type optical device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a waveguide type optical device having switching and modulation functions.

[Prior Art] A waveguide-type optical device forms a high-refractive-index portion serving as an optical waveguide on a substrate made of a ferroelectric or semiconductor material, forms an electrode on or near the optical waveguide, and applies a voltage from the outside. Is an optical device that performs switching and optical modulation by applying a voltage.

FIG. 2 shows an example of a conventional optical device of this type, in which a ferroelectric LiNb having an electro-optical effect is used.
After forming a waveguide pattern on the O ₃ substrate 1 with Ti and thermally diffusing the same to form an optical waveguide 2, an electrode 4 is further provided on or above the optical waveguide 2 with a metal film to provide the above function. Many waveguide optical devices have been researched and developed.

These waveguide-type optical devices apply a voltage to electrodes provided on or near the optical waveguide, generate an electric field in the optical waveguide, and change the refractive index of the optical waveguide by the electro-optic effect of the substrate. The desired switching and light modulation functions are obtained by changing the propagation constant and phase of the guided light. At this time, generally, the change in the refractive index of the optical waveguide is determined by the electro-optic constant of the substrate, which differs depending on the direction of the crystal axis of the substrate and the generated electric field.

Conventionally, when designing a waveguide-type optical device, the crystal axis of the substrate and the direction of electric field generation when a voltage is applied are determined so that the refractive index changes most for the same electric field strength. The linearly polarized light from the semiconductor laser is stored using a constant polarization optical fiber or the like so that the polarization direction of the guided light becomes the desired polarization state, and this is directly incident on the optical waveguide and switched within the waveguide substrate. And light modulation.

For example, in a Ti-diffused waveguide optical switch using Z-plate LiNbO ₃ , the electric field component of the light is perpendicular to the substrate surface (total reflection surface). When an electric field is generated, the electro-optic constant at this time becomes the largest value of the LiNbO ₃ crystal (r ₃₃ = 2.7 to 3.1), and the switching operation can be performed at a low voltage.

[Problems to be Solved by the Invention] In the conventional waveguide type optical device described above, the light from the semiconductor laser is deflected as shown in FIG. 2 so that the guided light of the optical waveguide is in a desired polarization state. The polarization direction is preserved by the optical fiber 7, and the polarization maintaining direction of the constant polarization optical fiber 7 is optically coupled to the polarization direction of the optical waveguide 2.

However, when trying to determine the polarization state of the guided light in the optical waveguide using only such a method, the polarization extinction ratio (the ratio of linearly polarized light propagating along the principal axis to light propagating along the principal axis and the orthogonal axis) of the constant polarization optical fiber is low. Even if the polarization extinction ratio of the polarization-maintaining optical fiber is high, if the angle between the polarization-preserving direction of the polarization-maintaining optical fiber and the polarization direction of the optical waveguide is deviated, the waveguided light of the optical waveguide is reduced. However, there is a disadvantage that the polarization orthogonal to the desired polarization is preserved, thereby deteriorating the switching characteristics and the light modulation characteristics.

In other words, in the optical switch, the guided light having the desired polarization state is completely switched, but the orthogonal polarization is not completely switched, resulting in crosstalk. Further, in the optical modulator, the polarization orthogonal to the desired polarization state does not completely operate due to the generated electric field, so that the extinction ratio at the time of modulation deteriorates. For example, in the above-described Ti diffused waveguide optical switch using the Z-plate LiNbO ₃ ,
For electric field component to the substrate surface is a horizontal polarization state of the guided light (TE mode light), sufficient switching is not performed in the switching voltage electrical optical constant becomes r ₃₃ = 1.1 to 1.4, for the TM mode light, the Minutes become crosstalk.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems and to provide a waveguide type optical device having a reduced stroke and an increased extinction ratio.

[Means for Solving the Problems] The waveguide type optical device of the present invention has a refractive index higher than that of the optical waveguide between the end of the optical waveguide substrate surface and the electrode at the input side of the optical waveguide. And a second layer thin film having a higher refractive index than the optical waveguide is formed on the upper surface of the first layer thin film with a width wider than the width of the optical waveguide. And

This is because the polarization extinction ratio of the constant polarization optical fiber is low when light enters the optical waveguide from the constant polarization optical fiber,
Alternatively, even if an angle shift occurs between the polarization preserving direction of the polarization maintaining optical fiber and the total reflection surface of the optical waveguide, and light having a desired polarization state is not incident on the optical waveguide, the optical waveguide and the thin film of the second layer are formed. Between the first and second layers through the first layer, only the unnecessary light can be coupled to the unnecessary light, and the unnecessary light can escape to the second layer. Here, if the width of the thin film of the second layer is made wider than the width of the optical waveguide, the light once coupled to the thin film of the second layer spreads laterally in the thin film and does not couple to the optical waveguide again. .

The conditions for coupling by phase matching depend on the refractive index of the optical waveguide and the first and second layers and the thin films of the first and second layers, and differ depending on the polarization state of the guided light. Thereby, only unnecessary light can be removed.

Next, the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view of one embodiment when the present invention is applied to a 2 × 2 waveguide type optical switch, and 1 is a ferroelectric LiNbO _3.
A directional coupler pattern as shown in the figure is formed of a Ti film, and this is thermally diffused at 1050 ° C. for 8 hours to form an optical waveguide 2. In an optical switch or an optical modulator using a Z-plate LiNbO ₃ substrate, the polarization state of the light incident on the optical waveguide is
So that the electric field component is perpendicular to the substrate surface,
(TM mode light) When an optical waveguide is operated by generating an electric field from the outside, the electro-optic effect of the substrate is maximized, and operations such as switching and light modulation can be performed at a low voltage.

At this time, an electrode 4 for generating an electric field in the optical waveguide is used.
Is provided above the linear optical waveguide, absorption by the metal film constituting the electrode occurs, and the loss of guided light increases. To prevent this, the surface of the waveguide substrate 1 is usually provided with SiO ₂ having a lower refractive index than LiNbO ₃ on the surface of the waveguide substrate 1. Using the film as the buffer layer 3, the electrode 4 is formed via the buffer layer 3. Here, in order to eliminate the absorption loss of the TM mode light to the electrodes and operate at a low voltage on the surface of the waveguide substrate 1, a buffer layer 3 made of SiO _{2 of} 3000 ° is used.
, And an electrode 4 is formed on the upper surface with CrAu.

On the other hand, the end face of the waveguide substrate 1 is polished, and an optical fiber is optically coupled to and fixed to the optical waveguide 2 on the input / output end face of the optical waveguide 2. Here, the input side (light source side) of the optical switch holds the polarization state from the semiconductor laser,
A constant polarization optical fiber 7 is used to change the polarization state of the guided light of the optical waveguide 2 to TM mode light, and the polarization preserving direction is perpendicular to the surface of the waveguide substrate 1 even when coupling with the optical waveguide 2. The angle is adjusted so that The configuration of the optical switch described above is common to the configuration of the conventional optical switch.

In the optical switch of the present invention, the entire area of the substrate surface (about 5 mm) between the input side end face of the waveguide substrate 1 to which the polarization maintaining optical fiber 7 is fixed and the electrode 4 is formed.
Long), a SiO ₂ thin film 5 having a lower refractive index than the optical waveguide and a Si thin film 6 having a higher refractive index than the optical waveguide are coated. In this embodiment, in the region coated with the SiO ₂ and Si thin films, in this embodiment, TE mode light having a polarization state orthogonal to the guided light (TM mode) propagating through the optical waveguide 2 (the electric field component is parallel to the substrate surface) Need to be attenuated. The relationship between the respective thicknesses of the SiO ₂ and Si thin films and the attenuation in the TE mode is as shown in FIG. Here is sufficiently attenuated only TE mode, it is necessary to set the film thickness which does not decay for the TM mode, SiO ₂ thin film, Si film respectively 2000 Å, 900
Å

Next, the evaluation results of the switching characteristics of the conventional optical switch and the optical switch using the configuration of the present invention will be described.
FIGS. 4A and 4B show the switching characteristics of the conventional optical switch and the optical switch according to the present invention, respectively.
As can be seen from FIG. 4, in the conventional optical switch, the polarization extinction ratio of the input side constant polarization optical fiber 7 is about 15 to 20 dB, and when the optical waveguide 2 and the constant polarization optical fiber 7 are coupled or fixed. The TM / TE polarization extinction ratio of the guided light propagating through the optical waveguide 2 is 10 to 15 d due to the misalignment (about 1 degree) in the polarization direction of
When the voltage is applied to the electrode 4 and switching is performed, the TM mode light can reduce the crosstalk by only about 20 dB even when a voltage of about 5.5 V is applied. This is because the electro-optical effect on the TE mode is small even when a voltage of about 5.5 V is applied (r ₃₃ (1/3) · r ₃₃ ), so that the TE mode hardly performs a switching operation and the polarization quenching of the guided light The switching is not enough because the ratio is as low as about 10-15dB. This is because it is affected by the TE mode and this is the amount of crosstalk.

On the other hand, in the optical switch having the configuration of the present invention, the TE mode is attenuated by about 10 dB in the region of 5 mm in length covered with the SiO ₂ or Si thin film, so that the TE mode is sufficiently switched when the 5.5 V voltage is applied. Even if it is not, the polarization extinction ratio of guided light is 20 to 25 dB
Therefore, the crosstalk during switching operation has been significantly reduced to 30 dB.

[Effects of the Invention] As described above, the present invention provides the first layer having a lower refractive index than the optical waveguide between the input side end face of the substrate surface of the waveguide type optical device and the portion where the electrode is formed. By coating a thin film of the second layer on the upper surface of the thin film and the upper surface thereof over a range having a higher refractive index than the optical waveguide and wider than the width of the optical waveguide, it is possible to actually perform switching and light modulation within the optical waveguide substrate. There is an effect that the guided light other than the guided light having the polarization state is attenuated to reduce crosstalk and increase the extinction ratio.

[Brief description of the drawings]

FIG. 1 is a perspective view of one embodiment of a waveguide type optical device according to the present invention, FIG. 2 is a perspective view of one example of a conventional waveguide type optical device, and FIG. 3 is a two-layer coating on a waveguide substrate. SiO ₂ for thin film
4 (a) and 4 (b) are diagrams showing the switching characteristics of a conventional waveguide device and an optical switch according to the present invention, showing the relationship between the film thickness and the attenuation of the guided light when Si and Si are used. is there. Explanation of symbols: 1… waveguide substrate, 2… optical waveguide, 3… buffer layer, 4… electrode, 5… SiO ₂ thin film, 6… Si thin film, 7… constant polarization optical fiber, 8 …… Single mode optical fiber.

Claims (1)

(57) [Claims] An optical waveguide is formed on a surface of a substrate, a voltage application electrode for generating an electric field in the optical waveguide, and an optical fiber optically coupled to the optical waveguide at an optical waveguide end of the optical waveguide. A waveguide type optical device comprising: a first layer thin film having a lower refractive index than the optical waveguide in a region of the substrate surface including the optical waveguide between the optical waveguide end and the electrode; A thin film of a second layer having a higher refractive index than the optical waveguide is formed on the upper surface of the thin film of the first layer with a width wider than the width of the optical waveguide. A waveguide-type optical device, wherein unnecessary light in a polarization direction is removed from light guided in the optical waveguide by coupling by phase matching with a thin film of the layer.