JPH07104145A - Waveguide type optical control device - Google Patents

Abstract

PURPOSE:To lessen the deterioration in extinction ratio by a deviation in angle of a polarization plate by constituting the optical control device of an optical waveguide in which only the mode of either TM mode or TE mode is guided, metallic electrodes and a polarization plane maintaining optical fibers. CONSTITUTION:This waveguide type optical control device is composed of the optical waveguide which is formed on a substrate consisting of an anisotropic optical crystal and in which only the mode of either the TM mode or the TE mode is guided, the metallic electrodes for controlling the guided light formed on the optical waveguide and at least one piece of the polarization plane maintaining optical fibers arranged at one end of the optical waveguide in order to input and output the guided light. Then, the TE mode is approximately cut off, the ratio at which the TE mode component is excited is low and the modulation efficiency of the TE mode component is poor in the case of guiding with the TM mode even if the deviation in the angle of the polarization plane arises at the time of connecting the polarization plane optical fibers and the optical waveguide. Conversely, the TM mode is cut off, the ratio at which the TM mode component is excited is low and the modulation efficiency of the TM mode component is poor in the case of guiding with the TE mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical control device used for an optical fiber communication system, an optical fiber sensor, optical measurement and the like. More specifically, the present invention relates to a waveguide type optical control device in which deterioration of an extinction ratio due to an angle deviation of a polarization plane caused when connecting an optical waveguide device and a polarization-maintaining optical fiber is reduced (a tolerance with respect to the angle deviation is large).

[0002]

2. Description of the Related Art In recent years, various optical waveguide devices which are small and can be integrated have been proposed and put into practical use. In particular, in a waveguide type optical control device for the purpose of optical control, an anisotropic crystal material, which has different optical characteristics depending on the crystal direction, is often used as the substrate, so that it is usually used for an input / output optical fiber (especially an incident optical fiber). Wavefront preserving optical fibers are used. This is a specific guided mode (TE mode whose plane of polarization is horizontal to the substrate and TM mode whose plane of polarization is vertical to the substrate).
This is because it is necessary to input and output specific linearly polarized light in order to perform the desired light control.

As an example thereof, FIG. 4 shows the configuration of a Mach-Zehnder interferometer type optical modulator (hereinafter referred to as MZ optical modulator). As the substrate 1, lithium niobate (LiNbO3
A substrate or the like is used, and a metal such as Ti is patterned on this substrate 1 and diffused at a high temperature of about 1000 ° C., whereby the optical waveguide 2 can be easily formed. An electrode (not shown) is formed on the optical waveguide 2 directly or via a buffer layer, and an electric field is applied to a part of the optical waveguide 2 to modulate the intensity of the optical power. Generally, when the Z-cut LN substrate is used, the TM mode is used, and when the X-cut LN substrate is used, the TE mode is used. Therefore, it is necessary to use the polarization-maintaining optical fiber as the incident optical fiber 3. .
In the figure, reference numeral 4 is an output optical fiber.

Conventionally, in the connection between such a waveguide type optical control device and a polarization-maintaining optical fiber, the polarization plane is geometrically observed by observing a stress applying portion or the like from the end face or the side face of the polarization-maintaining optical fiber. After deciding, by a method similar to the method of aligning with the optical waveguide and connecting and fixing, and the same method in advance to the silicon or ceramic member with the V groove formed,
A method is employed in which the polarization plane is fixed and adhered and fixed, and thereafter, this member and the optical waveguide substrate are integrally fixed.

[0005]

However, when the polarization plane preserving optical fiber is connected / fixed to the waveguide type optical control device as described above, if an angle deviation of the polarization plane with respect to the optical waveguide occurs, FIG. As shown in (4), the applied voltage-extinction level of the extinction characteristic changes periodically. It is considered that this is because the TE mode component is also excited due to the angle shift of the plane of polarization, this is modulated by the applied electric field, and superimposed on the TM mode and output.

When the above-mentioned MZ type optical modulator is ideally formed, that is, the phases of the guided light beams branched by the input side branching section 21 are exactly the same when they are joined by the output side Y branching section 23. When the optical waveguide is formed in the optical waveguide, the optical output becomes maximum at 0 V regardless of whether the optical waveguide is operated in the TM mode or the TE mode. As a result, when the device is operated in the TM mode, if there is a slight angle deviation, the extinction ratio in the vicinity of 0 V used as a normal modulator is particularly deteriorated, and the manufacturing yield is deteriorated. By the way, 20d
In order to realize a high extinction ratio of B or more, it is necessary to suppress this angular deviation within ± 0.5 °. Generally, if it has a tolerance of ± 3 °, it is easy to manufacture, but ± 0.5
In order to keep the tolerance within °, alignment takes a very long time and mass productivity becomes poor.

In order to compensate for this, a method of attaching a polarizer having a high extinction ratio to the end face of the waveguide on the output side or to the output optical fiber has been proposed, but it not only complicates the process, but also the amount of positional deviation. However, there was a drawback that the loss would increase.

[0008]

In order to solve the above problems, a waveguide type optical control device of the present invention is formed on a substrate made of an anisotropic optical crystal and has either a TM mode or a TE mode. Optical waveguide that guides only mode, metal electrode for controlling guided light formed on the optical waveguide, and at least one polarization plane preservation arranged at one end of the optical waveguide to input / output guided light It consists of an optical fiber. That is, the optical waveguide guides in either one of the TM mode and the TE mode, and is substantially cut off in the other mode. Here, “substantially cutoff” is a condition in which the TE mode component is excited only to such an extent that a desired extinction ratio can be obtained even if an angular deviation occurs.

[0009]

With the above configuration, when the polarization plane optical fiber and the optical waveguide are connected, even if the polarization plane is misaligned, the TE mode is substantially cut off when guided in the TM mode. Since the proportion of mode components being excited is small and the modulation efficiency of TE mode components is poor, a high extinction ratio in TM mode operation is maintained not only near an applied voltage of 0 V but also in a wide range of applied voltages. be able to.
On the contrary, when the wave is guided in the TE mode, the TM mode is cut off, so that the proportion of the TM mode component excited is small and the modulation efficiency of the TM mode component is deteriorated.
It is possible to maintain a high extinction ratio in the TE mode operation for a wide range of applied voltages.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The waveguide type optical control device of the present invention will be described in detail below with reference to FIGS. Here, for simplicity, description will be given using the conventional MZ optical modulator shown in FIG. First, the manufacturing conditions of the optical waveguide control device of the present invention will be described with reference to FIG. The manufacturing method of the optical waveguide is manufactured by Ti diffusion which is a known technique.
Before manufacturing the device, a linear waveguide having a Ti stripe width changed to be an optical waveguide was prepared on a Z-cut LN substrate, and its spot size was measured using an NFP (near field pattern) measuring device. , The conditions for single-mode waveguide were examined. Here, Ti film thickness 1
The diffusion conditions were 200 Å, the diffusion temperature was 1050 ° C., and the diffusion time was 3.5 hours.

As can be seen from FIG. 3, in the Z-cut LN substrate, the spot size in the TE mode is larger than that in the TM mode, and the width of the Ti stripe guided in the single mode is also wide. In order to realize a low driving voltage in the TM mode operation, it is considered preferable to fabricate a device as shown in FIG. 4 with a width of 6 μm that minimizes the spot size. FIG. 2 is a graph showing applied voltage-optical modulation characteristics of the MZ type optical modulator manufactured under these conditions. The alignment accuracy of the polarization plane here is about ± 3 °, which does not completely match the vertical direction of the substrate. As described above, when a device is manufactured with this Ti stripe width, the spot size is reduced not only in the TM mode but also in the TE mode, and as shown in FIG. 2, the extinction which is considered to be caused by the angle deviation of the polarization plane. There are variations in characteristics. In particular, it can be seen that the extinction ratio is greatly deteriorated near 0V.

On the other hand, FIG. 1 shows applied voltage-optical modulation characteristics produced with a Ti stripe width of 4 μm. Under this condition, the TM mode spot size becomes slightly larger,
Although the driving voltage is slightly higher, the variation of the extinction ratio due to the applied voltage is small because the TE mode is close to the cutoff condition, and the extinction ratio near 0 V is 20 dB or more. Further, there is almost no variation in the extinction ratio over a wide applied voltage range.

In the above embodiment, the Z-cut LN substrate is used for explanation, but if the same waveguiding is performed by using the X-cut LN substrate, the result opposite to that of the Z-cut LN substrate is obtained.
That is, the spot size in the TM mode is larger than the spot size in the TE mode, the TE mode is guided, and the proportion of the TM mode component excited is small.
Since the modulation efficiency of the M-mode component becomes poor, the cut-off is almost made in the TM mode, and TE can be applied to a wide range of applied voltage.
It is possible to maintain a high extinction ratio in mode operation.

In the above embodiment, a constant Ti film thickness and a constant diffusion condition have been described, but it goes without saying that this can be realized under different design Ti film thicknesses and different diffusion conditions. That is, it is possible to manufacture the waveguide type optical control device of the present invention with three of the four parameters of the Ti stripe width, the Ti film thickness, the diffusion temperature and the diffusion time being constant.

Further, the effectiveness of the present invention has been described by using the characteristic example of the MZ type optical modulator.
As long as the device controls light in the E mode only, the device can be appropriately modified and implemented regardless of the substrate material or the like.

[0016]

According to the waveguide type optical control device of the present invention, a waveguide type optical control device which can be easily connected to a polarization maintaining optical fiber and is excellent in productivity can be provided. Further, it becomes possible to maintain a high extinction ratio in the TM mode operation or the TE mode operation for a wide range of applied voltages.

[Brief description of drawings]

FIG. 1 is an applied voltage-optical output characteristic diagram for explaining an example of a waveguide type optical control device of the present invention.

FIG. 2 is an applied voltage-optical output characteristic diagram for explaining the problems of the conventional waveguide type optical control device.

FIG. 3 is a graph showing the dependency of spot size on the Ti stripe width, for explaining the difference in the fabrication conditions between the present invention and the conventional waveguide type optical control device.

FIG. 4 is a perspective view for explaining an outline of a conventional waveguide type optical control device.

[Explanation of symbols]

1 ... Substrate 2 ... Optical waveguide 3 ... Incident optical fiber 4 ... Outgoing optical fiber

Claims (1)

[Claims] 1. A T formed on a substrate of an anisotropic optical crystal,
An optical waveguide that guides only one of the M mode and the TE mode, a metal electrode for controlling the guided light formed on the optical waveguide, and input / output of guided light to one end of the optical waveguide. A waveguide-type optical control device comprising at least one polarization-maintaining single-mode optical fiber arranged for this purpose.