JP3398191B2 - Waveguide type optical control device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type light control device used for an optical fiber communication system, an optical fiber sensor, an optical measurement and the like. More specifically, the present invention relates to a waveguide-type light control device in which deterioration of an extinction ratio due to an angle shift of a polarization plane generated when an optical waveguide device is connected to a polarization-maintaining optical fiber is reduced (tolerance with respect to the angle shift). 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 to practical use. In particular, in a waveguide type optical control device for the purpose of optical control, since anisotropic crystal materials having different optical characteristics depending on the crystal direction are often used as a substrate, the optical input / output optical fiber (especially the incident optical fiber) is usually polarized. Wavefront preserving optical fibers are used. This is a specific waveguide mode (TE mode in which the polarization plane is horizontal to the substrate, and TM mode in which the polarization plane is perpendicular to the substrate).
This is because it is necessary to input and output a specific linearly polarized light in order to perform desired light control on the light. As an example, FIG. 4 shows a configuration of a Mach-Zehnder interferometer type optical modulator (hereinafter, referred to as an 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 the 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 directly on the optical waveguide 2 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. In general, a TM-mode is used when a Z-cut LN substrate is used, and a TE-mode is used when an X-cut LN substrate is used. Therefore, it is necessary to use a polarization-maintaining optical fiber as the incident optical fiber 3. .
In the figure, reference numeral 4 denotes an output optical fiber. Conventionally, such a waveguide-type optical control device is connected to a polarization-maintaining optical fiber by geometrically changing the plane of polarization by observing a stress applying portion or the like from the end face or side face of the polarization-maintaining optical fiber. After it is determined, it is aligned with the optical waveguide and connected / fixed, and in the same manner as before, the silicon or ceramic member with the V-groove is formed.
A method has been adopted in which the polarization plane is fixed in a fixed state and then fixed together with the optical waveguide substrate together with this member. [0005] However, when the polarization-maintaining optical fiber is connected to and fixed to the waveguide-type optical control device as described above, if an angle shift of the polarization plane with respect to the optical waveguide occurs, As shown in FIG. 2, a periodic fluctuation occurs in the extinction level of the applied voltage-extinction characteristic. This is considered to be because the TE mode component is also excited by the angle shift of the polarization plane, is modulated by the applied electric field, and is output while being superimposed on the TM mode. When the MZ type optical modulator described above is ideally formed, that is, when the phases of the guided lights branched by the input side branching section 21 are completely coincident with each other at the output side Y branching section 23, the phases are completely matched. In the case where the optical waveguide is formed, the optical output becomes maximum at 0 V regardless of the operation in either the TM mode or the TE mode. As a result, in the case of operating in the TM mode, if any angle deviation occurs, the extinction ratio in the vicinity of 0 V used as a normal modulator is particularly deteriorated, and there is a problem that the production 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 to within ± 0.5 °. Generally, if the tolerance is about ± 3 °, the fabrication is easy, but ± 0.5 °.
In order to keep the tolerance within °, alignment takes a very long time and mass productivity is poor. In order to compensate for this, there has been proposed a method of attaching a polarizer having a high extinction ratio to an end face of an output side waveguide or an output optical fiber. However, there is a disadvantage that the loss increases as it is. In order to solve the above-mentioned problems, a waveguide type light control device according to the present invention comprises a metal stripe width and a film thickness diffused on a substrate of an anisotropic optical crystal. ,
By setting the diffusion time and the diffusion temperature, an optical waveguide formed so that only one mode of the TM mode or the TE mode is guided and the other mode is substantially cut off, It comprises a formed metal electrode for controlling guided light and at least one polarization-maintaining optical fiber arranged at one end of the optical waveguide for inputting and outputting guided light. That is, the optical waveguide is guided in one of the TM mode and the TE mode, and substantially cut off in the other mode. Here, the “substantially cutoff” means that the TE is set to such an extent that a desired extinction ratio can be obtained even if an angle shift occurs.
This is a condition in which no modal component is excited. According to the above configuration, when the polarization plane optical fiber and the optical waveguide are connected to each other, even if the polarization plane is misaligned, the TE mode is substantially cut off when guided in the TM mode. Therefore, the rate at which the TE mode component is excited is small, and the modulation efficiency of the TE mode component is also deteriorated. Therefore, the high extinction ratio in the TM mode operation can be applied not only near 0 V but also over a wide range of applied voltages. Can be maintained.
Conversely, in the case of guiding in the TE mode, since the TM mode is cut off, the rate of excitation of the TM mode component is small, and the modulation efficiency of the TM mode component is deteriorated.
A high extinction ratio in the TE mode operation can be maintained even for a wide range of applied voltages. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A waveguide type optical control device according to the present invention will be described below in detail with reference to FIGS. Here, for the sake of simplicity, description will be made using the conventional MZ optical modulator shown in FIG. First, the manufacturing conditions of the waveguide type optical control device of the present invention will be described with reference to FIG. The optical waveguide is manufactured by a known technique of Ti diffusion,
Before manufacturing a device, a linear waveguide having a varied Ti stripe width as an optical waveguide is manufactured on a Z-cut LN substrate, and its spot size is measured using an NFP (near field pattern) measuring device. The conditions for single-mode waveguide were studied. Here, Ti film thickness 1
The diffusion conditions were 200 ° C., a diffusion temperature of 1050 ° C., and a diffusion time of 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 the spot size in the TM mode, and the width of the Ti stripe guided in the single mode is wider. In order to realize a low drive voltage in the TM mode operation, it is preferable to produce a device as shown in FIG. 4 having a minimum spot size of 6 μm width. FIG. 2 is a graph showing an applied voltage-optical modulation characteristic of the MZ type optical modulator manufactured under this condition. Note that the polarization plane positioning accuracy here is about ± 3 °, and does not completely coincide with 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 characteristic is considered to be due to the angular shift of the polarization plane. Variations are seen. In particular, it can be seen that the extinction ratio greatly deteriorates near 0 V. On the other hand, FIG. 1 shows an applied voltage-light modulation characteristic manufactured with a Ti stripe width of 4 μm. Under these conditions, the spot size in the TM mode becomes slightly larger,
Although the driving voltage is slightly increased, the variation in the extinction ratio due to the applied voltage is small because the TE mode is close to the cutoff condition, and the extinction ratio in the vicinity of 0 V also achieves 20 dB or more. Also, there is almost no variation in the extinction ratio over a wide applied voltage range. In the above embodiment, the description has been made using the Z-cut LN substrate. However, when similar waveguide is performed using the X-cut LN substrate, the result is opposite to that of the Z-cut LN substrate.
That is, the spot size in the TM mode is larger than the spot size in the TE mode, the wave is guided in the TE mode, the rate of excitation of the TM mode component is small, and T
Since the modulation efficiency of the M mode component is deteriorated, the cutoff is substantially cut off in the TM mode, and the TE efficiency is maintained for a wide range of applied voltages.
The high extinction ratio in the mode operation can be maintained. In the above-described embodiment, a constant Ti film thickness and a constant diffusion condition have been described. However, it goes without saying that this can be realized with a different Ti film thickness and different diffusion conditions in design. That is, the waveguide type light control device of the present invention can be manufactured with three of the four parameters of the Ti stripe width, the Ti film thickness, the diffusion temperature and the diffusion time kept constant. Further, the effectiveness of the present invention has been described using the characteristic example of the MZ type optical modulator.
As long as it is a device that performs light control that guides only the E mode, it can be implemented by appropriately modifying any device regardless of its substrate material and the like. 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 has excellent productivity can be provided. In addition, an excellent waveguide-type light control device that can maintain a high extinction ratio in the TM mode operation or the TE mode operation even for a wide range of applied voltages and has almost no variation in the extinction ratio. Can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an applied voltage-optical output characteristic diagram for explaining one embodiment of the waveguide type optical control device of the present invention. FIG. 2 is an applied voltage-optical output characteristic diagram for explaining a problem of a conventional waveguide type optical control device. FIG. 3 is a graph showing the dependency of the spot size on the Ti stripe width for explaining the difference in the manufacturing 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 light control device. [Description of Signs] 1: substrate, 2: optical waveguide, 3: incident optical fiber, 4: output optical fiber

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Claims (1)

(57) [Claims 1] Gold diffused on a substrate of an anisotropic optical crystal
Metal stripe width, film thickness, diffusion time, and diffusion temperature.
By setting, either TM mode or TE mode
Only one mode is guided and the other mode is substantially
An optical waveguide formed so as to be turned off, a metal electrode for controlling the guided light formed on the optical waveguide, and at least one polarization plane arranged at one end of the optical waveguide for inputting and outputting guided light. A waveguide-type light control device comprising a storage optical fiber.