JPH0358033A - Waveguide type optical device - Google Patents

Abstract

PURPOSE:To decrease crosstalks and to improve an extinction ratio by forming a thin film having the refractive index higher than the refractive index of optical waveguides and a thin film having the refractive index lower than the refractive index of the optical waveguides between an input side end face and electrodes. CONSTITUTION:A buffer layer 3 consisting of SiO2 is formed at 3000Angstrom on the surface of a waveguide substrate 1 in order to decrease the absorption loss of TM mode light on the electrodes and to make low voltage operation and the electrodes 4 consisting of CrAu are formed atop this layer. The Si thin film 6 (900Angstrom ) having the refractive index higher than the refractive index of the optical waveguides 2 and the SiO2 thin film 5 (2000Angstrom ) having the refractive index lower than the refractive index of the optical wave guides 2 are formed over the entire surface of the region (about 5mm length) on the surface of the substrate 1 between the substrate 1 and the part formed of the input side end face fixed with a constant polarization optical fiber 7 and the electrodes 4. The above-mentioned film thicknesses sufficiently attenuate only the TE mode and do not attenuate the TM mode. The crosstalks are decreased in this way and the extinction ratio is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a waveguide type optical device having switching and modulation functions.

[Conventional technology] In a waveguide type optical device, a high-refractive-index portion that serves as an optical waveguide is formed on a substrate made of a ferroelectric or semiconductor material, and an electrode is formed on or near the optical waveguide. It is an optical device that performs switching and optical modulation by applying voltage.

Figure 2 shows an example of a conventional optical device of this type. LiH, a ferroelectric material with electro-optic effect
bO, a waveguide pattern is formed on the substrate 1 using Ti. After thermally diffusing this to form an optical waveguide 2, an electrode 4 is further provided with a metal film on or near the optical waveguide 2.
Many waveguide type optical devices having the above functions have already been researched and developed.

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

For this reason, conventionally, when designing a waveguide type optical device,
The crystal axis of the substrate and the direction of electric field generation when voltage is applied are determined so that the refractive index changes the most for the same electric field strength, and the polarization direction of the guided light is adjusted accordingly to the desired polarization state. In the past, linearly polarized light from a semiconductor laser was stored using a polarization constant optical fiber, etc., and then input directly into an optical waveguide, where it was switched and optically modulated within the waveguide substrate.

For example, in a T1 diffused waveguide optical switch using two plates L i N b O s. When an electric field is generated in a direction perpendicular to the substrate surface for guided light (TM mode light) whose electric field component is perpendicular to the substrate surface (total reflection surface), the electro-optical constant at this time is L i N b O This is the largest value that the s crystal has (ri3-2.7 to 3.1). Switching operation can be performed at low voltage.

[Problems to be Solved by the Invention] The conventional waveguide type optical device described above polarizes the light from the nine semiconductor lasers at a constant polarization as shown in FIG. 2 so that the guided light in the optical waveguide has a desired polarization state. The polarization direction is preserved in the wave optical fiber 7. The polarization-maintaining direction of the polarization-constant optical fiber 7 is matched with the polarization direction of the optical waveguide 2 and optically coupled.

However, if we try to determine the polarization state of guided light in an optical waveguide using only these methods, the polarization extinction ratio (ratio of linearly polarized light propagating through the main axis and light propagating through the main axis and the orthogonal axis) of the polarization-controlled optical fiber becomes It was low. Also. Even if the polarization extinction ratio of the polarization-controlled optical fiber is high, if there is a misalignment between the polarization preservation direction of the polarization-controlled optical fiber and the polarization direction of the optical waveguide,
This has the drawback that polarized light orthogonal to the desired polarized light is stored at the waveguide end of the optical waveguide, resulting in deterioration of switching characteristics and optical modulation characteristics.

That is, in an optical switch, guided light having a desired polarization state is completely switched, but orthogonal polarized light is not completely switched, resulting in crosstalk. Also. In an optical modulator, polarized light perpendicular to the desired polarization state does not operate perfectly due to the generated electric field, resulting in a degradation of the extinction ratio during modulation. For example, in the Ti diffused waveguide optical switch using the two plates L i H b 0 3 described above, for guided light (TE mode light) whose electric field component is in a horizontal polarization state at the substrate surface. The electro-optical constant is r ss-1.1 to 1.4, and the switching voltage for TM mode light does not provide sufficient switching, resulting in crosstalk.

The present invention solves these problems of the conventional devices and provides a waveguide type optical device that reduces the stroke and increases the 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 on the input side of the optical waveguide between the end of the optical waveguide on the surface of the optical waveguide substrate and the portion where the electrode is formed. A first layer thin film with a low refractive index is coated, and a second layer thin film having a higher refractive index than the optical waveguide is formed on the top surface of the first layer thin film with a width wider than the width of the optical waveguide. It is said that

this is. When inputting light from a constant polarization optical fiber to an optical waveguide, the polarization extinction ratio of the constant polarization optical fiber may be low. Alternatively, even if an angular misalignment occurs between the polarization preserving direction of the polarization-controlled optical fiber and the total reflection surface of the optical waveguide, and light that is not in the desired polarization state enters the optical waveguide, the optical waveguide and the above jN2 layer thin film During this time, coupling by phase matching can be caused only for unnecessary light through the '1s1 layer, and this unnecessary light can be released to the second layer. here. If the width of the second layer thin film is made wider than the width of the optical waveguide, the light once coupled to the second layer thin film will spread laterally within the thin film and will not be coupled to the optical waveguide again.

The conditions for coupling by phase matching depend on the refractive index of each of the first and second layers of the optical waveguide 2 and the thin films of the first and second layers, and vary depending on the polarization state of the guided light, so these conditions should be set appropriately. By doing so, only unnecessary light can be removed.

[Example] Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a perspective view of an embodiment of the present invention applied to a 2×2 waveguide type optical switch, and 1 is a ferroelectric LiN
It is a Z plate of bO3, and a directional coupler pattern as shown in the figure is formed using a Ti film. This is thermally diffused at 1050° C. for 8 hours to form the optical waveguide 2. Z plate LiNbO,
In optical switches and optical modulators using substrates. When the polarization state of the light incident on the optical waveguide is set so that its electric field component is perpendicular to the substrate surface, (TM mode light) the optical waveguide is operated by generating an external electric field. The electro-optic effect of the substrate is the greatest, and operations such as switching and optical modulation can be performed with low voltage.

At this time, if an electrode 4 for generating an electric field in the optical waveguide 1 is provided on the top of the straight optical waveguide, absorption by the metal film composing the electrode will occur, increasing the loss of the guided light. An St02 film having a lower refractive index than LiNb03 is used as a buffer layer 3 on the surface of the substrate 1, and an electrode 4 is formed through the buffer layer 3. Here, a buffer layer 3 consisting of a waveguide substrate 1 is coated, and a CrA
The electrode 4 is formed by u.

on the other hand. The end face of the waveguide substrate 1 is polished. An optical fiber is optically connected to the input/output end face of the optical waveguide 2.
It is combined and fixed with. Here, on the input side (light source side) of the optical switch, a polarization constant optical fiber 7 is used to maintain the polarization state from the semiconductor laser and to change the polarization state of the guided light in the optical waveguide 2 to TM mode light. When coupling with the optical waveguide 2, the angle is also adjusted in such a manner that the polarization preservation direction is perpendicular to the surface of the waveguide substrate 1. The configuration of the optical switch described above is common to that of a conventional optical switch.

In the optical switch of the present invention, the entire region (approximately 5 angles) of the substrate surface between the input side end face of the waveguide substrate 1 to which the polarization-controlled optical fiber 7 is fixed and the portion where the electrode 4 is formed
In addition, Sin. has a lower refractive index than the optical waveguide. Thin 115 and
It is coated with an SL thin film 6 having a higher refractive index than the optical waveguide. Here this Sin. In the area coated with a thin film of Si,
In this embodiment, it is necessary to attenuate TE mode light (guided light whose electric field component is parallel to the substrate surface) having a polarization state perpendicular to the guided light (TM mode) propagating through the optical waveguide 2.

Sin. The relationship between the film thickness ε of the thin film S1 and the attenuation amount of the TE mode is as shown in FIG. Here, only the TE mode is sufficiently attenuated, and the TM mode is not attenuated.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 explained. FIGS. 4(a) and 4(b) show the switching characteristics of a conventional optical switch and an optical switch according to the present invention, respectively. As can be seen from Figure 4. In a conventional optical switch, the polarization extinction ratio of the input-side polarized optical fiber 7 is about 15 to 20 dB, and there is also a difference in the angle alignment of the polarization direction when the optical waveguide 2 and the polarized optical fiber 7 are coupled or fixed. (approximately 1 degree), the TM/ of the guided light propagating through the optical waveguide 2
The TE polarization extinction ratio is about 10 to 15 dB, and even when a voltage is applied to the electrode 4 for switching, the crosstalk of TM mode light can only be reduced by about 20 dB even when a voltage of about 5.5 V is applied. This is about 5.5v
Even when voltage is applied, the electro-optical effect on the TE mode is small (rs3=(LI8)4ss").
TE mode is hardly switching.
Moreover, since the polarization extinction ratio of the guided light is as low as about 10 to 15 dB, sufficient switching is not performed. This is because it is affected by the TE7-code and becomes a crosstalk component.

On the other hand, in the optical switch configured according to the present invention. The TE mode is about 10d in a 5u long region coated with Si02, Si thin film.
Because B is attenuated. T when 5.5 V voltage is applied
Even if the E mode is not sufficiently switched, the polarization extinction ratio of the guided light is improved to 20 to 25 dB, so crosstalk during switching operation is significantly reduced to 30 dB.

[Effects of the Invention] As explained above, the present invention has... Between the input side end face of the substrate surface of the waveguide type optical device and the part where the electrode is formed, there is a first layer thin film having a lower refractive index than the optical waveguide, and a first layer thin film having a refractive index higher than the optical waveguide on the upper surface. And by coating a second layer of thin film over a wider area than the width of the optical waveguide. This has the effect of attenuating guided light other than the guided light with the desired polarization state that is actually used for switching and optical modulation within the optical waveguide substrate, reducing crosstalk and increasing the extinction ratio.

[Brief explanation of drawings]

FIG. 1 is a perspective view of an embodiment of the waveguide type optical device of the present invention. Figure 2 is a perspective view of an example of a conventional waveguide type optical device. Figure 3 shows the two-layer thin film S1 coated on the top surface of the waveguide substrate.
4(a) and 4(b) are diagrams showing the switching characteristics of a conventional waveguide type device and an optical switch according to the present invention. It is. Explanation of symbols: 1... Waveguide substrate, 2... Optical waveguide,
3... Buffer layer, 4... Electrode, 5... Sin2
Thin film, 6... St thin film, 7... Constant polarization optical fiber,
8...Single mode optical fiber. Fig. 1 11M road board Fig. 2 Fig. 3 Sj film thickness (A) Fig. 4

Claims (1)

[Claims] 1. A waveguide type optical device in which an optical waveguide is formed on the surface of a substrate and an electrode is formed on or near the optical waveguide, and an optical fiber is optically coupled to the input/output end face of the optical waveguide formed on the substrate. In the input side of the optical waveguide, a first layer thin film having a refractive index lower than that of the optical waveguide is coated between the optical waveguide end of the surface of the optical waveguide substrate and the portion where the electrode is formed, and a first layer thin film having a refractive index lower than that of the optical waveguide is coated. A waveguide-type optical device characterized in that a second layer thin film having a higher refractive index than the optical waveguide is formed on an upper surface of the layer thin film, and has a width wider than the width of the optical waveguide.