JPH0774874B2 - Waveguide optical switch - Google Patents

Description

TECHNICAL FIELD The present invention relates to a polarization-independent optical switch using a directional coupler, and particularly has a high light confinement effect in an optical waveguide,
The present invention relates to a waveguide type optical switch capable of reducing the polarization dependency and particularly reducing the device driving voltage as much as possible.

[Conventional technology]

Conventionally, in this type of waveguide type optical switch, as a directional coupler forming a main part thereof, there is one shown in a schematic configuration diagram in FIG. In the figure, the conventional waveguide type optical switch, lithium niobate (hereinafter, L _i N _b O ₃₎ substrate (1)
In this structure, two waveguides (2) and (3) are formed on the upper side, and some of the waveguides (2) and (3) are close to each other to form a coupling portion (6). When the light having the intensity P _O is incident from the left incident end face of the one waveguide (2), it changes depending on the length of the coupling portion (L) of the coupling portion (6) which is partly close to the one, and this change The light of the intensities P _A and P _B is emitted from the right emission end faces of the waveguides (2) and (3). The initial length 1 in which the respective lights of the intensities P _A and P _B emitted have the relationship of P _A / (P _A + P _B ) ≈0 is called the complete bond length. FIG. 6 (A) shows the relationship between the emitted light P _A and P _B for this complete engagement length l. Further, the complete bond length l generally takes different length depending on the polarization state of incident light, that is, TE mode or TM mode. Here, TE mode refers to a polarization state in which the electric field component is parallel to the substrate (1), and TM
The mode means a polarization state in which the electric field component is perpendicular to the substrate (1). The two waveguides (2) and (3) have the same structure.

The conventional directional coupler of the waveguide type optical switch is configured as a uniform Δβ type by providing a pair of electrodes (not shown) in the coupling portion (6), and applying an electric field to achieve an electro-optical effect. The incident light can be switched based on This switching state is shown in FIG. 6 (B) as a switching diagram of uniform Δβ. In the figure, in the state (bar state), the intensity P _{O of the} incident light becomes the intensity P _A of the emitted light, and the intensity P _B of the other emitted light is P _B = 0. In the state (cross state), the intensity P of the incident light is shown. _O indicates the intensity P _B of the emitted light and the intensity P _A of the other emitted light is 0. In addition, the electric field (ΔβL /
When π) is applied, the above state is achieved on a plurality of arcs.

Next, the ratio (L / l) between the coupling length L and the perfect coupling length 1 is set to "1" (L / l = 1) for both TM mode and TE mode, and TE / TM both modes are obtained when an electric field is applied. The switching diagram is shown in FIG. In the figure, when the electric field is applied and (ΔβL / π) is increased, the TE mode becomes (ΔβL / π).
(L / π) ≈ 5.2, and when the electric field is further applied to increase (ΔβL / π), the TM mode becomes (ΔβL / π
π) ≒ 5.9 and the state is reached. Thus, the incident light can be switched by switching the TE / TM modes from the state to the state.

In addition, as another conventional waveguide type optical switch, ELECTRONIC
S LETTERS 8th Octorber 1987 Vol.23, No.21 Page 1167-
There is one described on page 1168, which is shown in FIG.
In the figure, the conventional waveguide type optical switch, L _i N _b O ₃ substrate (1) on the two waveguides (2), is formed and (3), the two waveguides (2 ), (3), a pair of electrodes (4), (5) and another pair of electrodes (45), (55) outside the pair of electrodes (4), (5), respectively. It is a configuration.

In the above structure, the electrodes (4), (5), (45),
Apply a voltage to (55) and change the above voltage to TE / TM
The conditions for matching in both modes are determined by the electro-optic coefficients γ ₁₃ and γ ₃₃ of the substrate (1) of _Li N _b O ₃ . FIG. 9 shows the crosstalk in both TE / TM modes with respect to Δβ (corresponding to the applied voltage) in this case. In the figure, α = Δβ _TE / Δ
β _TM (Δβ _TE and Δβ _TM are phase mismatches of the directional coupler for TE and TM modes, respectively). This α is 0.25 ≦ α
When ≦ 0.34, crosstalk can be reduced to −20 dB or less in both modes by adjusting the applied voltage. Wavelength 1.
The measured value in the vicinity of 3 μm is α = 0.29, and since the above conditions are satisfied, it is possible to match the voltage in the state in both TE / TM modes.

In addition, as another conventional waveguide type optical switch, ELECTRON
ICS LETTERS 15th September 1988 Vol.24, No.19 No. 1198
These conventional waveguide type optical switches have the same electro-optical effect for both TE / TM modes in the case of an optical crystal such as L _i N _b O _3. Polarization independence was secured by using a working X-cut crystal.

[Problems to be Solved by the Invention]

Since the conventional waveguide type optical switch is configured as described above, as shown in the switching diagram of FIG. 7, the mode switching between TE mode and TM mode (Δ
Since βL / π) was different, there was a problem that polarization independence could not be perfected. In addition, other conventional waveguide type optical switches can obtain the matching of the complete coupling lengths of both modes of TE / TM relatively close to the cutoff condition, but since the optical confinement effect in the waveguide is weak, the waveguide propagation There is a problem that the loss and radiation loss in the bent portion of the waveguide are likely to be large, and the electro-optical effect that can be used in other conventional optical switches is weak, so the switching voltage required for optical switching becomes large. Had challenges.

The present invention has been made to solve the above problems, and provides a waveguide type optical switch that can reduce the polarization dependency with respect to a driving voltage and a switching characteristic and can secure a high optical confinement effect of the waveguide. With the goal.

[Means for Solving the Problems]

A waveguide type optical switch according to the present invention has two waveguides arranged close to each other, and a coupling length of a directional coupler for converting light intensity between the two waveguides is set to a TE mode complete coupling for a TE mode. The TM mode is formed to have a length of about 4 times, and the TM mode is formed to have a length of about 2 times the full coupling length of the TM mode. It is arranged and configured, and the voltage is applied to the pair of four-divided electrodes so that the electric field directions are alternately inverted to control the drive.

[Action]

In the waveguide type optical switch according to the present invention, the length of the coupling portion of the directional coupler is formed to be about four times the full coupling length of the TE mode and about twice the full coupling length of the TM mode, and the directional coupler is divided into four. By forming the electrodes facing each other,
A voltage that alternately inverts the electric field is applied to the four-divided electrodes to apply TE.
/ TM Transition from one mode to another mode is performed with an extremely close applied voltage value.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. FIG. 1 is a schematic configuration diagram of a waveguide type optical switch according to the present embodiment, and FIG. 2 is a configuration diagram of a four-divided electrode arrangement in the embodiment shown in FIG. In each figure, the waveguide type optical switch according to the present embodiment, L _i N _b O ₃ substrate (1) on the two waveguides (2), to form a (3), the waveguide ( 2) and a part of (3) come close to each other to form a coupling part (6), and the coupling part length L of the coupling part (6) is about 4 times the complete coupling length of the TE mode for the TE mode and TM Mode is formed as about twice the full bond length of TM mode,
A pair of four-divided electrodes (41) to (44), each of which is divided into four, on both the waveguides (2) and (3) forming the coupling portion (6),
(51) to (54) are arranged so as to face each other, and drive control is performed by applying a voltage whose electric field directions are alternately inverted to the four divided electrodes (41) to (44) and (51) to (54). It is a thing.

The four-divided electrodes (41) to (44) and (51) to (54) each have a length of L / 4 with respect to the length L of the joint portion (6) and are adjacent to each other. Electrode pair (41), (51), (42)
(52), (43), (53), (44), and (54) are applied with voltages of opposite polarities and equal to each other (Δβ inversion drive).

Next, the operation of the waveguide type optical switch according to this example based on the above configuration will be described. First, four-division electrode (41)
Voltages are applied to (44) and (51) to (54) under the condition of the Δβ inversion drive. By applying this voltage, each divided section (L /
In 4), the waveguide undergoes changes in the propagation constants of + Δβ, -Δβ, + Δβ, and -Δβ. This propagation constant change is TM, TE
The values differ depending on the mode, and the difference is shown by the following equation.

Where Δβ _TM and Δβ _TE are TM, the change in the propagation constant of the TE mode, n _e is the extraordinary refractive index, n _o is the ordinary refractive index, and γ ₃₃ · γ
₁₃ is the electro-optic constant, and Γ _TM and Γ _TE are the overlap integrals of the applied electric field for each TM / TE mode.

The above equation (1) is estimated to be about 3 and is experimental (the above-mentioned prior art document ELECTRONICS LETTERS 8th Octorber 1987 Vol.
23, No. 21)).

In the above embodiment, L / l _TE = 4, L / l _TM = 2, and a =
The relationship between the applied voltage and crosstalk when 3.0 or a = 3.6 is described in IEEE JOURNAL OF QUANTUM ELECTRONICS Vo
l.QE-12, No. 7, JULY 1976, pages 396 to 398, and the calculation results are shown in FIGS. 3 (A) and 3 (B).
In FIGS. 7A and 7B, the horizontal axis represents ΔβL / π for the TM mode and the vertical axis represents crosstalk. The crosstalk on the vertical axis is defined as the optical power of the light R _o entering the waveguide (2) on the incident side is “1”, and the optical power of the light S _o entering the waveguide (3) on the other side is When “0” is set, it is a power ratio of the light S emitted from the waveguide (3) on the emission side to the incident light R _o = “1” in each TE / TM mode. As shown in a> 1 in FIGS. 3 (A) and 3 (B), the light in each TE / TM mode becomes −20 dB or less when ΔβL / π = 0, and TE / TM when ΔβL / π = 4.5. It can be seen that the TM light in each mode is output without loss. In Fig. 3 (B) showing the case of a = 3.6, there is a slight difference between TE / TM modes, but when ΔβL / π≈5, both TE / TM modes have very small insertion loss. It can be output only by increasing.

As the device driving voltage, ΔβL / π = 6 is required in the document described as the prior art, whereas a = 3.
Even in the case of 5, it can be operated with ΔβL / π = 5.

In the waveguide type optical switch according to the above embodiment,
Although each TE / TM mode is configured to have L / l _TE = 4 and L / l _TM = 2, it can be set with an arbitrary allowable range depending on the degree of crosstalk. For example, if the state channel output light intensity in the no-voltage state is about 75%,
1.7 ≦ L / l _TM ≦ 2.3 and 3.7 ≦ L / l _TE ≦ 4.3 can be set. When the state channel output light intensity is about 63%, it is possible to set 1.6 ≦ L / l _TM ≦ 2.4 and 3.6 ≦ L / l _TE ≦ 4.4.

Next, the waveguide type optical switch according to the above embodiment will be described in relation to the manufacturing method of one embodiment.

Generally, as a method of forming the waveguides (2) and (3) on the substrate (1) of L _i N _b O ₃ , the T _i diffusion method is most commonly used. Waveguides (2), (3) formed by this T _i diffusion method
Then, it is easier to make the refractive index change Δn _e for extraordinary light larger than the refractive index change Δn _o for ordinary rays.
That is, it is easy to be longer than L _i N _b O ₃ complete coupling length of the complete coupling length of the substrate (1) TM mode TE mode a Z-cut crystal. By properly setting the widths of the waveguides (2) and (3), the spacing between the two waveguides, and the T _i diffusion condition, the length of the directional coupler and the coupling length L can be set to TE / TM bi-mode. It is easily possible to increase the coupling length to about 3 times or 1 times the full coupling length, respectively, and such a waveguide has a sufficiently large optical confinement effect.

This concrete manufacturing method is shown in FIG. In the figure, Z-
A photoresist (1) is patterned on a substrate (1) of cut crystal L _i N _b O ₃ except a waveguide forming portion. Next, after performing _Ti vapor deposition on the photoresist patterning,
Lift off the photoresist patterning section. The vapor deposition T _i left by this lift-off is diffused to diffuse T _i Diffuse L _i
Three-dimensional waveguide of N _b O ₃ (2), to form a (3). The electrodes (4) and (5) are divided on the waveguides (2) and (3) to form a pair of four-divided electrodes (41) to (44) and (51) to (54), respectively. Manufacture optical switch.

〔The invention's effect〕

As described above, in the waveguide type optical switch according to the present invention, the coupling length of the directional coupler is set to about four times the complete coupling length of the TE mode for the TE mode and the complete coupling length of the TM mode for the TM mode. And a pair of four-divided electrodes, which are divided into four, are arranged on both waveguides of this directional coupler so as to face each other, and a voltage which is alternately inverted is applied to the four-divided electrodes to perform Δβ inversion driving. By adopting the configuration for controlling, it has the effect of making the element drive voltage as small as possible,
As a result, the polarization dependence of the switching characteristics can be reduced, and the optical confinement effect of the waveguide can be secured high.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a waveguide type optical switch according to an embodiment of the present invention, and FIG. 2 is a view of the embodiment 4 shown in FIG.
The divided electrode arrangement configuration diagram, FIGS. 3A and 3B, a = 3,
ΔβL / l-crosstalk relation graph when a = 3.6, FIG. 4 is a manufacturing process diagram of the optical switch of the embodiment shown in FIG. 1,
FIG. 5 is a schematic configuration diagram of a conventional waveguide type optical switch, and FIG.
Figure (A) is a relational graph of emitted light intensity with respect to perfect coupling length, Figure 6 (B) is a switching diagram of uniform Δβ, Figure 7 is a switching diagram of both TE / TM modes, and Figure 8 is another conventional method. FIG. 9 is a schematic configuration diagram of the waveguide type optical switch of, and FIG. 9 is a graph showing the relation between the crosstalk and the TE mode switching voltage. (1) substrate _{...... L i N b O 3 (} 2), (3) ...... waveguide (4), (5), (41) - (44), (51) - (54) ....
Electrode (6) ... Coupling portion In the drawings, the same reference numerals indicate the same or corresponding portions.

Claims (1)

[Claims] 1. A waveguide type optical switch comprising two directional waveguides arranged close to each other and a directional coupler for converting a light intensity between the both waveguides, wherein the directional couplers are coupled to each other. The section length is set to about 4 times the complete coupling length of the TE mode for the TE mode and about 2 times the full coupling length of the TM mode for the TM mode, and is formed on both waveguides of the formed directional coupler. A waveguide-type optical switch, characterized in that a pair of four-divided electrodes, each of which is divided into four, are arranged to face each other, and a voltage is applied to the pair of four-divided electrodes so that the electric field direction is alternately inverted to control the drive.