JP3602874B2 - Branch modulation type optical modulator - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to an improvement of a branch modulation type optical modulator (Mach-Zehnder type optical modulator).
[0002]
[Prior art]
In recent Mach-Zehnder type optical modulators, a Ti heat diffusion LiNbO ₃ channel type waveguide has been proposed in that the electro-optic constant of LiNbO ₃ itself and the driving voltage can be reduced.
[0003]
FIG. 7 shows this Mach-Zehnder type optical modulator 1, 2 a substrate made of a Z-cut lithium niobate plate (Z plate of LiNbO ₃ ), 3 a SiO ₂ buffer layer provided on this substrate 2, and 4 an electric A waveguide provided on the surface layer of the substrate having an optical effect, 5 is an electrode, and 6 is a polarizing plate.
[0004]
According to the optical modulator 1, the waveguide 4 includes the light incident part 4a, the demultiplexing part 4b, the phase modulating part 4c, the multiplexing part 4d, and the light emitting part 4e. The light incident on the light incident part 4a through the optical path is demultiplexed by the demultiplexing part 4b, then propagates through the phase modulation part 4c, is combined via the multiplexing part 4d, and reaches the light emission part 4e.
[0005]
An electrode 5 is formed on each phase modulator 4c, and when applied to this electrode 5, an asymmetric electric field is generated, and the electric field changes the refractive index of each phase modulator 4c. Thus, a phase difference occurs between the respective lights. Then, the amount of change in the refractive index of the phase modulation section 4c is determined by the magnitude of the electric field intensity, so that the intensity of emitted light after synthesis can be controlled by such a phase difference.
[0006]
This LiNbO ₃ crystal is optically anisotropic, and when a waveguide is formed on the surface layer of a Z-cut LiNbO ₃ substrate so that light propagates in the x direction, the refractive index depends on the polarization direction of the light. Since they are different, the TE mode light is an ordinary ray and the TM mode is an extraordinary light. Then, when the electric field is applied in the z-axis direction, the changes in the refractive indexes of the ordinary ray and the extraordinary ray are respectively
Δn _O = r ₁₃ n _O ³ _EZ / 2
^{_{Δn e = r 33 n e 3}} E Z / 2
It becomes, since r ₃₃ is increased on the order of several times that of the r _13, towards the TM mode than in the TE mode, will be greatly affected by the electric field. Therefore, when used for the modulation operation, the TM mode is preferable in that the application efficiency is higher. Therefore, in order to guide only the TM mode light, the polarizing plate 6 is provided as described above, and the Light is incident through the polarizing plate 6 so that the polarization direction and the optical axis (z-axis) of the crystal coincide.
[0007]
[Problems of the prior art]
However, in the Mach-Zehnder optical modulator 1 having the above configuration, the manufacturing conditions of the waveguide 4 (width, thickness, diffusion time, diffusion temperature, etc. of the Ti diffusion source before Ti diffusion), the waveguide 4 and the electrode 5 Is optimally designed so that the driving voltage is minimized and the TM lowest-order mode is not cut off, but the polarization direction of the polarizing plate 6 is accurately adjusted with respect to the crystal axis of the substrate 2. Is difficult to set, and therefore, the incident light may be excited not only in the TM mode but also in the TE mode, whereby the TE mode is also modulated by the electric field, and as a result, these TM and TE As shown in the extinction ratio characteristic diagram of FIG. 3 due to the superposition of both modes, there is a problem that the extinction ratio varies at each peak.
[0008]
In order to solve this problem, it is important to accurately match the polarization direction of the incident light with the optical axis of the crystal, but it is practically difficult to increase the accuracy, and a skilled technique is required.
[0009]
In addition, the configuration of the optical modulator 1 becomes complicated, resulting in an increase in cost.
[0010]
Further, when only the TM mode is excited and the condition of the waveguide for cutting off the TE mode component is selected, there is a problem that it is difficult to set the optimization for lowering the driving voltage.
[0011]
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a branch modulation optical modulator (Mach-Zehnder optical modulator) which does not impair the drive voltage characteristics and has good extinction ratio characteristics without complicating the configuration. It is in.
[0012]
[Means for Solving the Problems]
The branch modulation type optical modulator according to the present invention includes a substrate made of Z-cut lithium niobate, which contains a predetermined concentration of Ti atoms in a surface layer of a −C plane to form a light incident part, a demultiplexing part, and a phase modulation part. Forming a waveguide including a multiplexing part and a light emitting part, and exciting a TM mode to a TE mode cutoff state in the light incident part or the light emitting part. Characterized in that a region having a small value is formed.
[0013]
[Action]
According to the branching modulation type optical modulator having the above-described configuration, in the light incidence portion or the light emission portion, the TM mode can be excited and the TE mode can have a refractive index distribution in which the cutoff state is obtained. TE mode components are not mixed in the emitted light, or the components can be reduced to a considerable extent. As a result, a branch modulation type optical modulator having good extinction ratio characteristics can be obtained without disposing a polarizing plate.
[0014]
If a region having a low concentration of Ti atoms is provided in the light incident portion or the light emitting portion of the waveguide as in the above configuration, when the waveguide is manufactured by the Ti thermal diffusion method, a predetermined photolithography technique is used. It can be easily formed simply by forming the mask shape of the above. Therefore, the manufacturing cost is reduced, and a low-cost branch modulation type optical modulator can be obtained.
[0015]
【Example】
FIG. 1 shows a Mach-Zehnder type optical modulator 7 according to the embodiment. The same parts as those in FIG. 7 are denoted by the same reference numerals. Reference numeral 2a denotes a substrate made of a z-cut lithium niobate plate (LiNbO ₃ Z plate), and a waveguide 8 having a width of about 5 μm was formed on the substrate 2a by a Ti thermal diffusion method. In the Ti thermal diffusion method, Ti atoms were doped at a temperature of 1000 ° C. or slightly higher. At the time of the doping, a resist pattern was formed using a mask having a predetermined shape by a photolithography technique, a Ti pattern was formed by a lift-off method, and then Ti thermal diffusion was performed.
[0016]
Since this Ti thermal diffusion method is performed at a very high temperature, polarization inversion may occur in the Ti-diffused region on the + C face of the substrate 2a. It was produced on the surface layer 9 of the surface.
[0017]
The electrode 5 provided on the waveguide 8 may have a CPW structure other than the CPS structure as shown in FIG.
[0018]
Further, when the electrode 5 is provided on the waveguide 8, the propagation loss of the guided light is apt to occur, and the phenomenon is particularly remarkable in the TM mode light. Therefore, in order to reduce the propagation loss, the SiO ₂ buffer layer 3 is provided on the substrate 2a.
[0019]
This waveguide 8 also includes a light incident portion 8a, a demultiplexing portion 8b, a phase modulation portion 8c, a multiplexing portion 8d, and a light emitting portion 8e, and the light incident on the light incident portion 8a The light is split by the splitter 8b, then propagates through the phase modulator 8c, is combined via the multiplexing unit 8d, reaches the light emitting unit 8e, and emits the light. When high-frequency power is applied between the two electrodes 5 formed on the modulation section 8c, an electric field is generated, and the electric field changes the refractive index of each phase modulation section 8c. A phase difference occurs in the light, and the degree of the refractive index of the phase modulator 8c is determined by the magnitude of the electric field intensity. Therefore, the intensity of the output light after the synthesis can be controlled by the phase difference.
[0020]
Next, a region provided in the waveguide 8 and having a low concentration of Ti atoms will be described. In this embodiment, when manufacturing the waveguide 8, a region having a low Ti atom concentration is provided in the light incident portion 8a or the light emitting portion 8e. Hereinafter, since this area is for setting the TE mode so as to be in the cutoff state, the TE mode cutoff area 10 is referred to.
[0021]
This TE mode cutoff region 10 is shown in FIGS. In these figures, the arrows indicate the direction of light propagation. In the TE mode cutoff region 10a of FIG. 2, the Ti width before diffusion is reduced to about 20 to 50% compared to the Ti width in the waveguide 8 before diffusion, and the thickness (depth) of the region 10a is reduced. It was provided so as to be the same as the waveguide 8.
[0022]
In order to obtain the same effect as adjusting the diffusion Ti atom concentration by changing the Ti width as in the TE mode cutoff region 10b in FIG. 3, the undoped Ti is not provided inside the waveguide 8a or 8e. An area may be provided, or a dot shape may be formed as in the TE mode cutoff area 10c in FIG.
[0023]
Alternatively, in addition to the TE mode cutoff regions 10a, 10b, and 10c that do not change the Ti thickness as described above, a TE mode cut having a single-layer structure is formed in a part of the waveguide 8 having a two-layer structure as shown in FIG. An off region 10d may be provided to reduce the Ti thickness in that region.
[0024]
Thus, the provision of the TE mode cut-off regions 10 (10a, 10b, 10c, 10d) of the above-described respective structures makes the distribution of the refractive index of each region 10 different from that of the waveguide 8, thereby providing the TM. The mode was excited to set the TE mode to the cutoff state. As a result, an extinction ratio characteristic diagram as shown in FIG. 6 was obtained. According to the figure, the horizontal axis is the voltage applied to the electrode 5, the intensity of the emitted light is ideally formed, and the extinction ratio at each peak is constant.
[0025]
It should be noted that the present invention is not limited to the above-described embodiment, and various changes and improvements may be made without departing from the scope of the present invention. For example, the TE mode cutoff region 10 can adopt various shapes and structures in addition to the embodiment. In the TE mode cutoff region 10c, in addition to the spherical dots, a dot-shaped point source having an arbitrary shape can be used. It may be.
[0026]
【The invention's effect】
As described above, according to the present invention, a predetermined concentration of Ti atoms is contained in the surface layer of the −C plane of the substrate made of Z-cut lithium niobate, and the light incident portion, the demultiplexing portion, the phase modulation portion, Forming a waveguide having a multiplexing part and a light emitting part, and exciting the TM mode to cut off the TE mode in the light incident part or the light emitting part to cut off the TE mode. Since the small area is formed, the TE mode component is not mixed in the emitted light, or the component can be reduced to a considerable extent. As a result, even if the polarizing plate is not provided, the driving voltage characteristics are not impaired, and the extinction is prevented. A branch modulation type optical modulator having good ratio characteristics was obtained.
[0027]
Further, according to the present invention, when a waveguide is manufactured by a Ti thermal diffusion method, a region having a low Ti atom concentration can be easily formed only by forming a predetermined mask shape in the photolithography technique, Thus, the manufacturing cost was reduced, and a low-cost branch modulation type optical modulator was obtained.
[Brief description of the drawings]
FIG. 1 is a perspective view of a branch modulation type optical modulator according to an embodiment.
FIG. 2 is a schematic diagram illustrating a main part of a waveguide in a branch modulation type optical modulator according to an embodiment.
FIG. 3 is a schematic diagram illustrating a main part of a waveguide in a branching modulation type optical modulator according to an embodiment.
FIG. 4 is a schematic diagram showing a main part of a waveguide in a branch modulation type optical modulator according to an embodiment.
FIG. 5 is a schematic view showing a main part of a waveguide in a branch modulation type optical modulator according to an embodiment.
FIG. 6 is a diagram illustrating the extinction ratio characteristics of the branch modulation type optical modulator according to the embodiment.
FIG. 7 is a perspective view of a conventional branch modulation type optical modulator.
FIG. 8 is a diagram showing an extinction ratio characteristic of a conventional branch modulation type optical modulator.
[Explanation of symbols]
7: Mach-Zehnder type optical modulator 2a: Z-cut lithium niobate (LiNbO ₃ ) substrate 8: waveguide 5: electrode 3: SiO ₂ buffer layer 8a: light incident portion 8b: demultiplexing portion 8c: phase modulation portion 8d: Combining section 8e: emitting section 10: TE mode cut-off region

Claims (1)

The surface layer of the substrate made of Z-cut lithium niobate contains a predetermined concentration of Ti atoms in the surface layer of the -C plane , and a light incident portion, a branching portion, a phase modulation portion, a multiplexing portion, and a light emitting portion are formed. to form a waveguide having a, the light incident portion or the light emitting portion, the TE mode cutoff state to excite the TM mode, is characterized in that the formation of the small region of Ti atom concentration Branch modulation type optical modulator.

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