KR19980066623A - Integrated Optical Blocking Modulator - Google Patents

Abstract

The present invention relates to an integrated optical blocking optical modulator, and more particularly, to an integrated optical blocking optical modulator using an optical waveguide having a plurality of refractive index transition regions.

A block type optical modulator for modulating an optical power by changing a refractive index of the optical waveguide by a substrate, an optical waveguide formed on the substrate, an electrode for applying an electric field to the optical waveguide, and an electric field applied to a modulation region of the optical waveguide. The optical waveguide in the modulation region is an optical waveguide having a plurality of refractive index transition regions.

As described above, the shielding optical modulator according to the present invention forms a plurality of polarization inversion regions at predetermined intervals in the optical waveguide in the modulation region, thereby applying an electric field to the optical waveguide, thereby refractive index between the natural polarization region and the polarization inversion region. Since the amount of change can be made twice as large as the change in refractive index obtained in the modulation area of the block type optical modulator without polarization inversion, the optical power can be scattered more in the modulation area, so that the optical power can be modulated with a lower driving voltage. In addition, the above operating principle can be applied to an optical waveguide having a high refractive index, so that the optical waveguide of the shielding optical modulator can be brought into a high focused state, thereby reducing insertion loss.

Description

Integrated Optical Blocking Modulator

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an integrated blocking optical modulator, and more particularly, to an integrated optical blocking optical modulator using an optical waveguide having a plurality of refractive index transition regions.

Integrated optical cut-off modulators (hereinafter referred to as cut-off modulators) are one of the key components required for optical communication, such as optical power splitters, optical switches, and optical wavelength filters, and serve to convert electrical signals into optical signals. The integrated optical modulator uses the electrooptic effect of the substrate, and as the integrated optical substrate material, lithium naobate (LiNbO ₃ , LN), lithium tantalate (LiTaO ₃ , LT), electro-optic polymer (Polymer), etc. This is used representatively.

To date, various types of integrated optical modulators have been announced, including Mach-Zehnder Interferometer type modulators, Directional Coupler type optical modulators, and Cutoff type optical modulators. Since the block type optical modulator has the simplest structure among the optical modulators, it is easy to manufacture and is suitable for implementing the optical modulator at low cost. In addition, since the operating point of the device can be set to a desired position by adjusting the parameters of the optical modulator device at the time of manufacture, it is possible to obtain an operating point of a suitable device without an electric DC bias voltage as compared with the other optical modulators described above. Therefore, it is possible to prevent the operating point drift phenomenon (DC-Drift Effect) that may be generated when the operating point of the optical modulator is set by the DC bias voltage.

Figure 1a shows a three-dimensional view of a conventional Z-axis cut blocking optical modulator.

In FIG. 1A, a waveguide 8 formed on a Z-axis cut lithium nanooate or lithium tantalate substrate 7 and a buffer layer 9 formed on the waveguide 8 and the waveguide of the modulation region 10 ( And a waveguide 8 for applying an electric field to 8) and electrodes 11 and 12 formed on the substrate 7.

Here, when the electric field is applied in the optical axis (+ Z axis) direction, the waveguide 8 between the transition regions 13 and 14 decreases in the refractive index in the modulation region 10, and when the electric field is applied in the opposite direction of the optical axis, The refractive index of the waveguide 8 in the modulation region 10 is increased. The change in refractive index in the optical waveguide 8 in which the electrode 11 is located leads to the change in the waveguide mode, which causes two types of optical power modulation.

The first is optical power modulation due to mode-shape transition. FIG. 1B is a view showing a waveguide mode according to each region in the waveguide 8 of the conventional block type optical modulator shown in FIG. 1A. FIG. 1C is a view of the waveguide (b) as a boundary between the transition regions 13 and 14 shown in FIG. The refractive index decreases by Δn when the electric field is applied in the optical axis (+ Z) direction in the modulation region 10 of 8).

In FIGS. 1A to 1C, when an electric field is applied to the modulation region 10 through the electrodes 11 and 12, transitions of the waveguide mode shapes a, b, and c occur according to the respective points 15, 16, and 17. . In this case, when the difference in the waveguide mode shape is large according to the transition regions 13 and 14, severe scattering occurs in which most of the mode power spreads out of the waveguide 8 and spreads to the substrate 7. This severe mode difference appears when the modulation region 10 of the waveguide 8 is in the blocking state due to the refractive index of the peripheral substrate 7 due to the electric field applied to the modulation region 10.

Second, the modulation phenomenon comes from the difference due to the amount of scattering loss occurring at the interface between the optical waveguide 8 and the cladding of the substrate 7. In the formation of the optical waveguide 8, a non-uniform pattern is usually formed on the surface between the optical waveguide 8 and the substrate 7, and scattering loss occurs through the non-uniform pattern.

At this time, the amount of scattering loss generated is largely dependent on the waveguide mode shape of the optical waveguide 8. When the optical waveguide 8 maintains a well-guiding condition, most of the waveguide mode powders are cores of the optical waveguide 8. Since it is concentrated on the core, the loss caused by the side wave unevenness of the optical waveguide can be reduced.

However, when the waveguide condition of the optical waveguide 8 is placed near the blocking region blocked by the electric field, the waveguide mode power located at the cladding of the substrate 7 is large. Scattering losses are large. Therefore, when the shape of the waveguide mode is adjusted by the applied electric field through the electrodes 11 and 12, the optical power can be modulated.

Such a block type optical modulator has a waveguide condition of the optical waveguide 8 close to the blocking region so that the modulation region 10 can be easily blocked by an electric field by the electrodes 11 and 12. Should be placed on. That is, when the refractive index of the optical waveguide 8 approaches the refractive index of the substrate 7 and a predetermined electric field is applied to the modulation region 10 through the electrodes 11 and 12, the refractive index of the modulation region 10 becomes the substrate ( It should be equal to the refractive index of 7) so that the light waves are scattered in the modulation region 10. At this time, the amount of change in the maximum refractive index which can change the applied electric field to the optical waveguide (8), usually from about 5 × 10 ^-4 is the effective refractive index of the optical waveguide (8) the substrate (7) than the refractive index of only about 5 × 10 ^-4 It should be loud. At this time, since the effective refractive index variation of the optical waveguide 11 of about 5 × 10 ^-4 is relatively small, the insertion loss from the input terminal to the output terminal of the optical modulator even when the blocking optical modulator is in an on state. There is a problem that increases).

In addition, in other types of optical modulators such as Mach-Zehnder interferometers and directional couplers, the optical voltage is modulated using phase change, while the driving voltage is small, whereas in the block type optical modulator, optical power modulation is directly performed through the electrodes 11 and 12. There is a problem that a large driving voltage is required.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide a block type optical modulator that effectively modulates light at a small insertion loss and a low driving voltage.

Figure 1a shows a three-dimensional view of a conventional block type optical modulator.

FIG. 1B is a diagram illustrating a waveguide mode according to each region in the waveguide of the block type optical modulator shown in FIG. 1A.

FIG. 1C is a diagram illustrating a change in refractive index in the modulation region of the waveguide at the boundary of the transition region shown in FIG. 1B.

2A to 2E are cross-sectional views illustrating a process of forming an optical modulator according to the present invention.

Figure 3a is a three-dimensional view of the block type optical modulator according to the present invention.

3B is a view showing a waveguide mode according to a transition region in the optical waveguide of the block type optical modulator shown in FIG. 3A.

3C is a view showing a change in refractive index when a driving voltage is applied to the transition region of the optical waveguide shown in FIG. 3A.

In order to achieve the above technical problem, the refractive index of the optical waveguide is reduced by a substrate used in the present invention, an optical waveguide formed on the substrate, an electrode for applying an electric field to the optical waveguide, and an electric field applied to the modulation region of the optical waveguide. In the block type optical modulator that is changed to modulate the optical power, the optical waveguide in the modulation region is characterized by an optical waveguide having a plurality of refractive index transition regions.

The plurality of refractive index transition regions may be formed by alternating a naturally polarized waveguide having a predetermined length and a polarized inverted waveguide at predetermined intervals.

In addition, the distance between each transition region is characterized in that it is set to a length that can be sufficiently converted the shape of the waveguide mode converted according to each transition region.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

2A to 2E are cross-sectional views illustrating a process of forming an optical modulator according to the present invention.

2A to 2E, the substrate for blocking light modulation is a single crystal electro-optic material such as lithium naobate or lithium tantalate and has spontaneous polarization. The natural polarization region 23 and the polarization inversion region 22 are alternated on the Z-axis cut substrate 20 of the material at predetermined intervals. To this end, the natural polarization direction (+ Z) of the substrate 20 is grounded on a plurality of electrodes 21 in which the bottom surface of the substrate 20 is grounded and chromium and gold (Cr, Au) are sequentially deposited on the top surface at predetermined intervals. A positive voltage above the coercive field is applied in the opposite direction.

At this time, in order to prevent the crystals of the substrate 20 from being destroyed by the electromagnetic breakdown during the electric field application, a pulse electric field having a pulse width of several to several tens of microseconds is usually connected to the electrodes 21 on the substrate 20. It is applied between grounds to form the polarization inversion region 22 without breaking the substrate.

Alternatively, a small amount of impurities (titanium in the case of lithium naobate, hydrogen ion in the case of lithium tantalate) are inserted into the crystal of the substrate 20, and a small amount of impurity is inserted into the crystal of the substrate 20 at the phase transition temperature of the crystal ( Lithium naobate is about 1150 ℃, lithium tantalate is about 600 ℃) heat treatment method can also be applied.

In FIG. 2C, a proton exchange layer 24 is formed over the substrate 20 across the polarization inversion region 22. In forming the optical waveguide 25, care should be taken so as not to affect the structure of the polarization inversion region 22 fabricated on the substrate 20. To this end, a proton exchange process, which is a low temperature process, is applied to the substrate 20. In the proton exchange process, a metal mask (not shown) is deposited on the remaining portions except for the portion 24 on which the optical waveguide 25 is to be formed, and a dozen minutes to a proton source such as benzoic acid at about 200 ° C. A method of increasing the refractive index of an optical waveguide by immersing the substrate 20 for several hours and exchanging protons H ^{+ in the} quantum source and lithium ions Li ⁺ in the lithium naoate or lithium tantalate substrate 20. to be.

After the proton exchange optical waveguide 24 is formed, the optical waveguide 24 is annealed at a temperature of about 350 ° C. for several minutes to several hours to reduce scattering loss and recover the electro-optic coefficient. In FIG. 2D, silicon oxide (SiO ₂ ) caps are formed on the optical waveguide 24 by self-aligning method for annealing after improving the focusing speed of the optical waveguide 24 and controlling the shape of the waveguide mode. Since the silicon oxide thin film becomes the buffer layer 26 of the optical waveguide 24 after annealing, electrodes 27 and 34 to apply a driving voltage thereon without being etched after annealing are formed as shown in FIG. 2E. do. The metal thin films for the plus and ground electrodes 27 and 34 are formed by sequentially depositing chromium (Cr) and gold (Au). The electrode 27 is formed to cover all of the plurality of polarization inversion regions 22 in the modulation region 29, and the ground electrode 34 is formed next to the optical waveguide 25 to apply a voltage.

Figure 3a is a three-dimensional view of the block type optical modulator according to the present invention.

First, in order to solve the problem of large insertion loss in the optical waveguide, the initial optical waveguide 25 should be formed in a high focusing state with high refractive index. However, when the refractive index of the optical waveguide 25 is larger than the refractive index of the substrate 20 by a predetermined value or more, it becomes difficult to scatter the waveguide mode by the electric field applied by the electrodes 27 and 34. To solve this problem, a plurality of transition regions 28 are formed in the modulation region 29 to which an electric field is applied by the driving voltage according to the process illustrated in FIGS. 2A to 2E.

The region 22 of the ferroelectric domain inversion in the transition region 28 inverts the sign of the electro-optic coefficient. When the sign of the electro-optic coefficient is inverted, the refractive index of the same electric field is compared with that of the natural polarization region 23 so that the increase and decrease of the refractive index appear. In this case, when a driving voltage is applied to the electrodes 27 and 34 between the polarization inversion region 22 and the natural polarization region 23 and an electric field is applied to the modulation region 29, the difference in refractive index between the transition regions 28 is conventionally blocked. It is twice as large as the difference in refractive index between transition regions of the modulator.

Therefore, even when the initial optical waveguide 25 is formed at twice the high concentration state as compared with the conventional block type optical modulator, the initial optical waveguide 25 can be driven with the same driving voltage. In order to solve the high driving voltage problem, a plurality of transition regions 28 are formed in the optical waveguide 25. As mentioned, the optical power modulation of the blocking optical modulator consists of the optical power modulation in the transition region and the scattering loss in the plane between the optical waveguide 25 and the substrate 20 cladding.

Since the optical power modulation in the transition region 28 can give a higher modulation degree in the high-concentration optical waveguide 25, a plurality of transition regions 28 are formed in the optical waveguide 25.

What is important in this method is the distance between the transition regions 28. The waveguide mode in front of the transition region 28 is divided into a waveguide mode of a new shape after the transition region and a substrate mode (light waves scattered on the substrate) after passing through the transition region 28, and the new transition region is sufficiently scattered by the substrate mode. After that, the scattering effect can be increased. Therefore, the distance between the polarization inversion region 22 and the natural polarization region 23 in the modulation region 29 is set in consideration of the point where the scattering effect is maximized and a plurality of transition regions can be formed in a given modulation region. do.

3B is a view showing a waveguide mode according to a transition region in the optical waveguide of the block type optical modulator shown in FIG. 3A.

3B, the waveguide mode 31 at the first point 30 passes through the transition region 28 in accordance with a predetermined driving voltage in the optical waveguide 25, and the waveguide mode 33 at the second point 32. Is converted to.

3C is a view showing a change in refractive index when a driving voltage is applied to the transition region of the optical waveguide shown in FIG. 3A.

In FIG. 3C, since the increase and decrease of the refractive indices of the natural polarization region 23 and the polarization inversion region 22 are Δn opposite to each other according to the same electric field, the difference in refractive index in the transition region 28 is represented by 2Δn.

Although the present invention has been described with respect to an embodiment of a block type optical modulator using a Z-axis cut substrate, a person having ordinary skill in the art can use the Y-axis or X-axis cut substrate without reference to the above embodiment without difficulty. It is well known that a blocking optical modulator operating at a low driving voltage with low insertion loss can be realized by forming a plurality of transition regions in the same modulation region in the formed optical waveguide.

As described above, the shielding optical modulator according to the present invention forms a plurality of polarization inversion regions at predetermined intervals in the optical waveguide in the modulation region, thereby applying an electric field to the optical waveguide, thereby refractive index between the natural polarization region and the polarization inversion region. Since the amount of change can be made twice as large as the change in refractive index obtained in the modulation area of the block type optical modulator without polarization inversion, the optical power can be scattered more in the modulation area, so that the optical power can be modulated with a lower driving voltage. In addition, the above operating principle can be applied to an optical waveguide having a high refractive index, so that the optical waveguide of the block type optical modulator can be made into a high focusing state, thereby reducing insertion loss.

Claims (5)

A block type optical modulator for modulating an optical power by changing a refractive index of the optical waveguide by a substrate, an optical waveguide formed on the substrate, an electrode for applying an electric field to the optical waveguide, and an electric field applied to a modulation region of the optical waveguide. The integrated optical blocking optical modulator according to claim 1, wherein the optical waveguide in the modulation region is an optical waveguide having a plurality of refractive index transition regions. The integrated optical blocking optical modulator of claim 1, wherein the plurality of refractive index transition regions are formed by alternating a naturally polarized waveguide having a predetermined length and a polarized inverted waveguide at predetermined intervals. The integrated optical blocking optical modulator according to claim 1, wherein the distance between each transition region is set to a length such that the shape of the optical power waveguide mode converted according to each transition region can be sufficiently converted. The integrated optical blocking optical modulator of claim 1, wherein a material of the substrate is lithium naobate. The lumped optical-blocking optical modulator according to claim 1, wherein the substrate is made of lithium tantalate.