JP3925212B2 - Optical deflection element control method and optical transmission system using ferroelectrics - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for controlling an optical deflection element using a ferroelectric and an optical transmission system, and in particular, in an optical communication system for transmitting a high-speed, large-capacity signal, a plurality of input ports and a plurality of output ports. For controlling an optical deflection element using a ferroelectric material characterized by an applied voltage waveform for stably driving an optical deflection element using a ferroelectric material used for switching an optical signal transmission destination between optical signals and optical transmission It is about the system.
[0002]
[Prior art]
Along with the dramatic increase in communication demand in recent years, high-speed and large-capacity coupled with wavelength division multiplexing (WDM) technology for transmitting a single optical fiber by multiplexing a plurality of signal lights having different wavelengths However, in order to construct the hardware infrastructure of the optical fiber network in the backbone communication network, an optical deflecting element for switching the transmission destination of the optical signal is required.
[0003]
Conventionally, as such an optical deflecting element, a mechanical micromirror constituted by a semiconductor processing technique has been used. In order to meet the demand for higher integration due to the recent increase in capacity, a ferroelectric substance is used. The optical deflection element used has been developed.
[0004]
In such an optical deflection element using a ferroelectric material, when switching the deflection angle of the optical deflection element in order to switch the port, a voltage in the order of milliseconds (ms) required for a predetermined deflection angle is stepped. Applied.
[0005]
FIG. 5 is a schematic plan view of a light deflecting element using a ferroelectric material. A region where the upper electrode 32 having a base length of W and a height of L is provided is a prism type light deflecting element. When a voltage is applied between the upper electrode 32 and the lower electrode formed on the back surface of the ferroelectric layer 31, the refractive index n of the ferroelectric layer 21 is decreased by Δn due to the electro-optic effect, and the emitted light from the prism inclined surface. By Snell's law
(N−Δn) × sin α = n × sin β = n × sin (α−θ ₁ )
It will be biased by the relationship.
[0006]
Note that when theta ₁ is sufficiently _{small, sinθ 1 ≒ θ 1, cosθ} 1 ≒ 1 next, since in tan [alpha = L / W,
θ ₁ ≈ (Δn / n) × (L / W)
Is approximated by
[0007]
When the deflected light is emitted from the ferroelectric layer 31, it is deflected again to the emission angle θ ₂ by the difference between the refractive index n of the ferroelectric layer 31 and the refractive index of the outside (for example, in the atmosphere). Finally, this θ ₂ becomes the deflection angle Δθ.
[0008]
[Problems to be solved by the invention]
However, in the case of an optical deflection element using a ferroelectric material, the applied voltage and deflection angle have hysteresis characteristics, so when making fine adjustments for optical axis alignment after changing the deflection angle, the change in the refractive index is the voltage. Therefore, there is a problem that fine adjustment is difficult.
[0009]
6. FIG. 6 is a graph of refractive index-voltage characteristics in a ferroelectric. As shown in the characteristic chart, butterfly-type hysteresis characteristics are obtained, and on the positive side and the negative side, that is, on the voltage increasing side and the decreasing side. It has asymmetric characteristics.
[0010]
That is, the refractive index change Δn is proportional to the applied electric field in the primary electro-optic effect, but is proportional to the square of the applied electric field in the secondary electro-optic effect, and as shown in FIG. Draws a butterfly curve and has a hysteresis loop.
[0011]
As is apparent from the figure, the amount of change in the refractive index on the return path when the voltage on the negative side is lowered differs depending on the maximum applied voltage at that time, compared to the forward path when the voltage on the positive side is increased.
[0012]
Therefore, an object of the present invention is to reduce the influence of hysteresis in the applied voltage-deflection angle characteristics based on the refractive index-voltage characteristics.
[0013]
[Means for Solving the Problems]
Here, means for solving the problems in the present invention will be described with reference to FIG. 2. FIG. 2 is a waveform diagram showing an example of an applied voltage in the embodiment of the present invention.
See FIG. 2. In order to solve the above-described problem, the present invention provides an optical deflection element that switches a deflection angle by optical switching using an electro-optic effect by a ferroelectric substance in a method for controlling an optical deflection element using a ferroelectric substance. When the deflection angle is switched to a predetermined deflection angle, a voltage (V + ΔV) larger than the voltage V required for the predetermined deflection angle is applied and then returned to the voltage V required for the predetermined deflection angle . The influence of hysteresis is reduced by using a minor loop of hysteresis possessed by the ferroelectric substance .
[0014]
In this way, after applying a voltage higher than the voltage required for the deflection angle in advance, the voltage is lowered to control the refractive index of the ferroelectric that constitutes the optical deflection element, thereby fine-tuning the hysteresis minor loop. This makes it possible to reduce the influence of the hysteresis loop of the ferroelectric substance.
[0015]
In this case, the maximum value of the voltage larger than the voltage required for the predetermined deflection angle is 1.2 times the voltage required for the predetermined deflection angle [= (V + ΔV) / V] or more. Is desirable, thereby enabling fine adjustment with high accuracy.
[0016]
As a method of applying a voltage larger than the voltage required for the predetermined deflection angle, a pulse voltage of ΔV may be superimposed on the voltage V required for the predetermined deflection angle at the initial stage of application, or the amplitude may be increased. What is necessary is just to apply the pulsating current voltage which attenuate | damps amplitude, such as the alternating voltage which attenuate | damps or the periodic pulse voltage which attenuate | damps amplitude.
[0017]
In addition, by incorporating such an optical deflection element control method in an optical transmission system, the transmission destination of an optical signal can be switched at high speed, and the configuration of the optical repeater can be reduced in size and capacity. Is possible.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Here, with reference to FIG. 1 and FIG. 2, the control method of the optical deflection element of the first embodiment of the present invention will be described. First, referring to FIG. 1, it is used in the embodiment of the present invention. The configuration of the optical deflection element will be described.
Referring to FIGS. 1A and 1B, FIG. 1A is a schematic plan view of an optical deflecting element used in an embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view. Although not shown, in practice, an optical switch is configured with a plurality of input ports, a common waveguide, an output port, and the like.
[0019]
First, on the Nb-doped SrTiO ₃ substrate 11, the cladding layer 12 made of (Pb _0.91 La _0.09 ) (Zr _0.65 Ti _0.35 ) O ₃ and having a thickness of 1 μm, for example, by using a pulse laser deposition method, and A core layer 13 made of Pb (Zr _0.52 Ti _0.48 ) O ₃ and having a thickness of, for example, 2 μm is sequentially deposited.
In this case, the refractive index n _PLZT of (Pb _0.91 La _0.09 ) (Zr _0.65 Ti _0.35 ) O ₃ is n _PLZT = 2.45, and the refractive index n _PZT of Pb (Zr _0.52 Ti _0.48 ) O ₃ is n. _{Since PZT} = 2.55, a waveguide structure is formed.
[0020]
Next, a Ti film, a Pt film, and an Au film are sequentially deposited on the back surface of the Nb-doped SrTiO ₃ substrate 11 to form the lower electrode 14, and the thickness of, for example, 200 nm ITO transparent through the metal mask on the surface of the core layer 13 By forming the conductive film and forming the prism-shaped upper electrode 15, the portion corresponding to the prism-shaped upper electrode 15 functions as a prism-type light deflection element.
[0021]
FIG. 2 is a waveform diagram of the applied voltage in the first embodiment of the present invention, in which a pulse voltage of ΔV is superimposed on a DC voltage V required for switching a predetermined deflection angle. After applying a large voltage (V + ΔV), the desired voltage V is set, and control is performed using the refractive index change in the return path.
[0022]
That is, as described above, since the secondary electro-optic effect is proportional to the square of the applied electric field, the refractive index-voltage characteristic draws a butterfly curve and has a hysteresis loop, so the voltage shown in FIG. 2 is applied. As a result, it is possible to reduce the influence of hysteresis at the time of minute control.
[0023]
Next, with reference to FIG. 3, a method for controlling the optical deflection element according to the second embodiment of the present invention will be described.
3. FIG. 3 FIG. 3 is a waveform diagram of an applied voltage in the method of controlling an optical deflection element according to the second embodiment of the present invention. The maximum amplitude of the DC voltage V required for switching a predetermined deflection angle is ΔV. An AC voltage that attenuates the amplitude is superimposed.
[0024]
In this manner, the hysteresis width can be reduced by superimposing the AC voltage that attenuates the amplitude, thereby reducing the influence of the hysteresis during a minute fluctuation after switching the deflection angle.
[0025]
Next, with reference to FIG. 4, the configuration of another optical deflection element used in the embodiment of the present invention will be described.
4 (a) and 4 (b) FIG. 4 (a) is a schematic plan view of another optical deflection element used in the embodiment of the present invention, and FIG. 4 (b) is a schematic cross-sectional view. Although not shown, the optical switch is actually configured with a plurality of input ports, a common waveguide, an output port, and the like.
[0026]
First, on the Nb-doped SrTiO ₃ substrate 21, the lower clad layer 22 made of (Pb _0.91 La _0.09 ) (Zr _0.65 Ti _0.35 ) O ₃ and having a thickness of, for example, 1 μm is formed using a pulse laser deposition method. (Zr _0.52 Ti _0.48 ) O ₃ , for example, a core layer 23 having a thickness of 2 μm and (Pb _0.91 La _0.09 ) (Zr _0.65 Ti _0.35 ) O ₃ , for example, having a thickness of 1 μm The upper cladding layer 24 is sequentially deposited.
[0027]
Next, a Ti film, a Pt film, and an Au film are sequentially deposited on the back surface of the Nb-doped SrTiO ₃ substrate 21 to form the lower electrode 25, and a thickness of, for example, 200 nm ITO is passed through the surface of the upper cladding layer 24 through a metal mask. By forming a transparent conductive film to form a prism-shaped upper electrode 26, a portion corresponding to the prism-shaped upper electrode 26 is a prism-type light deflection element.
[0028]
Since this optical deflection element is a slab type waveguide in which the core layer 23 is sandwiched between the upper clad layer 24 and the lower clad layer 22, the optical waveguide loss of the optical deflection element is smaller than that of the optical deflection element shown in FIG. Can be reduced.
[0029]
Although the embodiments of the present invention have been described above, the present invention is not limited to the configurations described in the embodiments, and various modifications can be made.
For example, in the description of the above embodiment, (Pb _0.91 La _0.09 ) (Zr _0.65 Ti _0.35 ) O ₃ (PLZT) and Pb (Zr _0.52 Ti _0.48 ) O ₃ (PZT) are used as the ferroelectrics. As the composition ratio, PLZT and PZT having other composition ratios may be used as long as a predetermined refractive index difference can be formed and a predetermined refractive index change can be obtained.
[0030]
Further, the ferroelectric material is not limited to PZT / PLZT, and other known ferroelectric materials such as SBT (SrBi ₂ Ta ₂ O ₉ ) may be used.
[0031]
In the second embodiment, the AC voltage that attenuates the amplitude is applied. However, the AC voltage is not necessarily an AC voltage, and a periodic pulse voltage that attenuates the amplitude may be applied. is there.
[0032]
Further, in each of the above embodiments, although described as a single optical deflecting element, it can be used as a single component, but usually includes a plurality of input / output ports and a common waveguide. It is used by being incorporated in an optical switch.
[0033]
Further, since the deflection angle Δθ in such an optical deflecting element is not so large, it may be used in multiple stages, thereby enabling deflection to an output port at a distant facing position.
[0034]
【The invention's effect】
According to the present invention, when switching the deflection angle of an optical deflection element using a ferroelectric material, a voltage higher than the voltage necessary for switching the deflection angle is applied, and then the voltage is lowered to the necessary voltage. It is possible to reduce the influence of the hysteresis of the rate-voltage characteristic, thereby enabling fine adjustment with high accuracy, and thus greatly contributing to the development of a high-speed and large-capacity optical communication network.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of an optical deflection element used in an embodiment of the present invention.
FIG. 2 is a waveform diagram of an applied voltage in the first embodiment of the invention.
FIG. 3 is a waveform diagram of an applied voltage in the second embodiment of the present invention.
FIG. 4 is an explanatory diagram of another optical deflection element used in the embodiment of the present invention.
FIG. 5 is an explanatory diagram of a prism type optical deflection element.
FIG. 6 is an explanatory diagram of a refractive index-voltage characteristic in a ferroelectric.
[Explanation of symbols]
11 Nb-doped SrTiO ₃ substrate 12 Cladding layer 13 Core layer 14 Lower electrode 15 Upper electrode 21 Nb-doped SrTiO ₃ substrate 22 Lower cladding layer 23 Core layer 24 Upper cladding layer 25 Lower electrode 26 Upper electrode 31 Ferroelectric layer 32 Upper electrode

Claims (3)

When switching the deflection angle of the optical deflection element that switches the deflection angle by optical switching using the electro-optic effect by the ferroelectric material to a predetermined deflection angle, a voltage larger than the voltage required for the predetermined deflection angle is applied. The effect of hysteresis is reduced by using a minor loop of hysteresis possessed by the ferroelectric material by returning to a voltage necessary for the predetermined deflection angle, and the control of the optical deflection element using the ferroelectric material Method.   2. The method of controlling an optical deflection element using a ferroelectric according to claim 1, wherein a pulsating voltage with amplitude attenuation is superimposed on a DC voltage required for the deflection angle. An optical transmission system including a plurality of input ports and a plurality of output ports includes an optical deflection element made of a ferroelectric material corresponding to at least each input port, and the deflection angle of the optical deflection element is set to a predetermined deflection angle. When switching, after applying a voltage larger than the voltage necessary for the predetermined deflection angle, and returning to the voltage necessary for the predetermined deflection angle, hysteresis is achieved using a minor loop of hysteresis possessed by the ferroelectric substance. An optical transmission system comprising voltage applying means for reducing the influence of the above .

Images