JPS6283731A - Optical switch - Google Patents

Abstract

PURPOSE:To eliminate the dependency on the polarization state of incident light by installing a crystal substrate by placing the optical axis thereof within the plane approximately perpendicular to the light transmission direction of optical waveguides and inclining the same with respect to the direction perpendicular to the surface of the crystal substrate. CONSTITUTION:The optical waveguides 12, 13 formed by thermally diffusing a Ti film pattern to several hundreds - thousands Angstrom thickness and several - several tens mum width on the LiNbO3 substrate 11 to constitute an optical directional coupler 14. The LiNbO3 substrate 11 is so cut out and shaped that the optical axis (z) of the crystal is approximately perpendicular to the light transmission direction (y) of the waveguides 12, 13 and is inclined by theta deg. with respect to the normal z' of the substrate surface. The inclination is enough with 5 deg.<=theta<=85 deg.. A buffer layer consisting of an SiO2 having 1000-3000Angstrom thickness is coated on the waveguides 12, 13. Control electrodes 15 consisting of Au, Al, etc., are installed thereon. The dependency on the polarization state of the incident light is thereby eliminated and the optical switch which has a low switching voltage and permits easy formation is obtd.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an optical switch that modulates light waves, switches optical paths, etc. in optical communications, and particularly relates to a waveguide type switch using an optical waveguide provided in a substrate. Regarding optical switches.

(Prior art and its problems) Optical communication systems are being put into practical use, and more advanced systems with large capacity and multiple functions are being developed. New functions such as switching functions of optical transmission networks and high-speed connections and switching between terminals in optical data buses are required, and the need for optical switching networks that enable these functions is increasing. It is currently in use. Optical switches currently in use mechanically move prisms, mirrors, fibers, etc., and have drawbacks such as slow speed, insufficient reliability, and large size. be. A waveguide type optical switch that uses an optical waveguide installed on a substrate is being developed as a means to solve this problem, and has features such as high speed, ability to integrate multiple elements, and high reliability. Especially strong X such as LiNbO3 crystal! Those using electric materials have features such as low light absorption and low loss, and high efficiency because they have a large electro-optical effect.

Generally, an optical switch is inserted into an optical transmission line and is often used to switch the optical path of an optical signal propagated through an optical fiber. In high-speed, large-capacity optical communication systems, single-mode optical fibers are used as optical fibers, and semiconductor lasers are used as light sources. Although a semiconductor laser beam emits linearly polarized light, a light wave propagated through a single mode optical fiber generally becomes elliptically polarized light, and its polarization state also changes over time. On the other hand, in the waveguide optical switch mentioned above,
Conventional configurations have the disadvantage that characteristics such as switch voltage and crosstalk are highly dependent on the polarization state of the incident light. FIG. 3 shows a directional coupling type optical switch, which is an example of a conventional waveguide type optical switch. Optical waveguides 32 and 33 are formed by diffusing metal such as Ti on a LiNbO3 substrate 31 cut out and shaped perpendicularly to the optical axis, that is, the Z-axis direction. The optical waveguides 32 and 33 constitute an optical directional coupler 34 by being installed close to each other at intervals of about several pm, and a buffer layer 5i is placed on the optical waveguides 32 and 33.
A control electrode 35 is installed through the 02 membrane (omitted in FIG. 3). The basic operating principle of this optical switch is that first, a light wave 3 enters from the end face of one of the optical waveguides, for example 32.
6 is a case in which the energy propagates through the optical waveguide 32 and transfers to the optical waveguide 33 adjacent to the optical directional coupler 34, and the length of the optical directional coupler 34 is made to match the perfect coupling length Lc. , almost 100% of the energy is in the optical waveguide 33
Then, the output light 37 is generated. On the other hand, when a voltage is applied to the control electrode, the optical waveguide 32, 33
The refractive index of the optical waveguide changes and the refractive index of the two becomes asymmetrical, causing a phase mismatch between the light waves propagating between the two and changing the coupling state. Under an appropriate applied voltage, energy is transferred to the original optical waveguide 32. is shifted and becomes the emitted light 38.

Here, the propagating light of the optical waveguide formed on the substrate is generally separated into two independent modes, namely, the TM mode whose polarization direction is perpendicular to the substrate surface and the TE mode whose polarization direction is perpendicular to the TE mode, and the optical direction is When a coupler is constructed, the coupling states are usually different between the two modes, and the complete coupling lengths are also correspondingly different. Furthermore, the amount of change in the refractive index caused by the electro-optic effect also differs depending on the polarization direction.
As a result, the switch voltage also varies greatly depending on the polarization direction. For example, in the case of FIG. 3, the refractive index changes obtained for TM mode and TE mode are δnTM=+r33, respectively.
ne3Ez, δnT, = 4r13noEz. Here, r33. r13 are electro-optical constants, ne, n
o is the refractive index for extraordinary light and ordinary light, respectively, and Ez is the electric field strength applied in two directions. In the case of LiNbO3 crystal, r33>3r13, so δnTM>3δnT
E, and the switch voltage in the TE mode is three times or more the switch voltage in the TM mode. Therefore, it is usually necessary to make the incident light match the polarization state of either TM or TE mode, and the optical switch with the configuration shown in Figure 3 cannot be used by inserting it into a single mode optical fiber transmission line. .

In order to improve the polarization dependence as described above, the distance between the two optical waveguides of the optical directional coupler is changed in a tapered manner.
A directional coupling optical switch that switches with redundancy for TM and TE modes was developed by Leon Matsukouhan (L.
EONMCCAUGHAN)
E, Journal of Lightwave Teri
IEEE, Journal of
Lightwave Technology), LT-
2, No. 1, pages 51-55.

However, even with such an optical switch having a tapered coupling part, when a crystal substrate similar to that shown in FIG. 3 is used, the r33. Since the electro-optic constant r13 is used for the TE mode, the switch voltage is theoretically T
More than three times as much is required as in the case of switching only the M mode, and the switching voltage is further increased by creating a tapered structure. Conventional optical switches with no polarization dependence have had the disadvantage that the switch voltage is extremely large.

Also, usually, the refractive index difference Δn of the optical waveguide portion with respect to the substrate is
is different between ordinary light and extraordinary light, so TE and T in the state of voltage O
In order to match the complete coupling lengths of both M modes, it is necessary to adjust the optical waveguide structure to certain limited special conditions, and there is also the problem that control of the manufacturing process is extremely difficult.

In addition to the directional coupling type shown here, waveguide optical switches include total reflection type, balanced bridge type, Y-branch type, etc., but we will compare switch voltage and crosstalk, which are important for optical switches. The directional coupling type is the one that can easily lower the optical density and has a simple configuration, and the dependence on the polarization state exists in other types as well.

(Object of the present invention) An object of the present invention is to provide an optical switch that eliminates the drawbacks of the conventional waveguide optical switch described above, has no dependence on the polarization state of incident light, has a low switch voltage, and is easy to manufacture. It is in.

(Structure of the Less Invention) An optical switch of the present invention includes an optical directional coupler formed on a crystal substrate having optical anisotropy and comprising two optical waveguides close to each other, and the optical directional coupler The optical axis of the crystal substrate is set substantially perpendicular to the light transmission direction of the optical waveguide and inclined with respect to the surface of the crystal substrate.

(Function/Effect) A conventional waveguide optical switch uses a crystal substrate cut perpendicularly or parallel to the optical axis, and the deviation of the orientation of the crystal substrate from the optical axis is usually ±1 to 2 degrees or less. On the other hand, in the present invention, the optical axis is largely tilted with respect to the orientation of the crystal substrate, as described above. In the optical switch of the present invention, there is a TM mode propagating in the optical waveguide, that is, a propagation mode with a polarization component perpendicular to the substrate surface, and a TE mode with a polarization component perpendicular to the TM mode.
Both modes are affected by both the extraordinary and ordinary refractive indices. Therefore, when a voltage is applied to the electrode provided near the optical waveguide, the optical axis (2 in the case of LiNbOa crystal)
Due to the electric field component in the axial direction, the electro-optical constant r33°r1
A refractive index change is provided through both of 3 and 3. As a result,
The switching voltage can be significantly lowered compared to the conventional case where the TE mode is switched simply by the electro-optic effect via r13. Depending on how the crystal orientation is selected, other electro-optic effects (for example, in the case of LiNbO3 crystal, the light transmission direction is
If the axis is the electro-optic effect via the electro-optic constant r22)
can also be used. Furthermore, in both TM and TE modes, the isometric refractive index difference with the substrate depends on both the refractive index difference for ordinary light and extraordinary light, so the complete coupling length between both modes is shorter than in the conventional case. The difference becomes smaller. Furthermore, by appropriately selecting the angle between the optical waveguide structure and the substrate orientation with the optical axis, the guided optical mode can be made into a hybrid mode in which the TM and TE modes are periodically coupled in the light transmission direction. In that case, the period of coupling between TM-TE modes is
It is determined by the birefringence of the crystal and is usually very small compared to the complete bond length.

On the other hand, since the spatial coupling between the two optical waveguides in the optical directional coupler occurs independently of the coupling between the TM and TE modes, in the case of the above hybrid mode,
Regardless of the polarization state at the time of incidence, coupling between optical waveguides always occurs in a state in which both the TM and TE modes coexist. That is, there is no need for a tapered coupling as in conventional optical switches, and an optical switch that operates at low voltage and has no polarization dependence can be obtained.

(Example) The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a perspective view showing a directional coupling type optical switch without polarization dependence, which is an embodiment of the optical switch according to the present invention. A Ti film pattern with a thickness of several hundreds to thousands of micrometers and a width of several to tens of micrometers was formed by thermal diffusion on a LiNbO3 substrate 11 having a shape similar to that of the conventional directional coupling type optical switch shown in FIG. Optical waveguides 12 and 13 are installed, and an optical directional coupler 14
It consists of However, in this example, unlike FIG. 3,
The LiNbO3 substrate 11 has an optical axis (Z) of the crystal substantially perpendicular to the light transmission direction y of the optical waveguides 12 and 13, and
It is cut out and shaped at an angle of θ0 with respect to the normal 2' to the substrate surface. In addition, this inclination is 5°≦θ≦85°
If so, it is sufficient. There is a thickness of 1 on the optical waveguide 12.13.
A buffer layer (omitted in FIG. 1) of 5i02 film of 000 to aoooA is coated, and a control electrode 15 made of Au, Al film, etc. is placed thereon. The length of the optical directional coupler 14 is set to be approximately equal to the perfect coupling length (usually several mm to several tens of mm) for both the TM and TE modes. The basic operating principle is to control the coupling state by applying a voltage to the control electrode 15 to create a phase mismatch between the light waves propagating through the two optical waveguides 12 and 13, similar to the example shown in Figure 3. It is. FIG. 2 is a sectional view of the optical directional coupler 14 of the optical switch shown in FIG. 1, viewed from the y-axis direction. Extraordinary light in the optical waveguide 12 and 13 portion,
The refractive index for ordinary light is neg.

When set to nog, the isometric refractive indexes nTM and nTE for the TE and TM modes are approximately expressed by the following formulas, respectively.

If a voltage is still applied, electric field components in two directions and in the y direction are generated, so that changes in the refractive index of δne and δno shown in the following equations occur for extraordinary light and room light, respectively. That is, nog and neg are each nog+δno.

neg+δne.

The switch voltage of this optical switch is n for TM mode.
The TM and TE modes are determined from the dependence of nTE on the applied voltage. Assuming that the magnitude of Ez is similar to that of the conventional optical switch shown in Fig. 3, the switch voltages for TM and TE modes are both the switch voltages for the TM and TE modes of the conventional optical switch shown in Fig. 3. The value is between . Therefore, in this example, the required switch voltage for both TM and TE modes is T of the conventional optical switch.
It can be made smaller than the switch voltage for E mode. Incidentally, in this example, the switch voltage was 2/3 that of the conventional one. Furthermore, since the difference in switch voltage between both modes can be made smaller than in the past, redundancy by means such as Taber coupling, which is necessary to eliminate polarization dependence, can be smaller than in the past.

In particular, when θ is around 45°, n with respect to the applied voltage
The amounts of change in TM and nTE are almost the same. Also, when choosing a structure in which the hybrid mode propagates, TE, T
The coupling between M modes has a very small period, for example, wavelength 1.
In the case of 3 pm, it occurs at a period of about 10 to 20 pm, so if you look over the normal length of the optical directional coupler 14 from several to several tens of mm, the TM and TE modes will always account for about 50% each, regardless of the polarization state of the incident light. It can be seen that they are mixed. Therefore, in these cases, the TM and TE modes can be switched with almost the same voltage without using a tapered coupling, so the voltage can be significantly reduced.

In addition, in the optical switch of this example, the position of the control electrode is set at y' with respect to the optical waveguide depending on the angle e of the optical axis (two axes).
By shifting in the directions, electric fields in two axial directions can be effectively generated in the optical waveguide.

As mentioned above, the difference in the complete bond length between the TM and TE modes in the state of zero voltage is smaller than that of the conventional method, so that process control and manufacturing are easier than the conventional method.

(Effects of the Invention) As described above, according to the present invention, there is no dependence on the polarization state of incident light, and the switch voltage is lower than that of the conventional one.
An optical switch that is easy to manufacture can be obtained.

Note that the crystal substrate used in the present invention is not limited to the above-described embodiments, and LiTaO3 crystal or the like may also be used. .

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an embodiment of the optical switch according to the present invention, FIG. 2 is a diagram for explaining the principle of the optical switch according to the invention, and FIG. 3 is a perspective view showing a conventional first switch. In the figure, 11,311t, LiNbO3 crystal substrate,
12, 13, 32° 33 are optical waveguides, 14.34 is an optical directional coupler, and 15.35 is a control electrode. Representative patent attorney Susumu Uchihara, -' 7 O l Figure 2 TE mode Figure 3

Claims (1)

[Claims] An optical directional coupler consisting of two optical waveguides close to each other formed on a crystal substrate having optical anisotropy, and a control electrode installed near the optical directional coupler, An optical switch characterized in that the optical axis is in a plane substantially perpendicular to the light transmission direction of the optical waveguide and is installed at an angle with respect to the direction perpendicular to the surface of the crystal substrate.