JPS6015254B2 - light switch - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method in which a light beam is guided through a transparent electro-optic crystal thin plate while being repeatedly reflected, and the P generated by the reflection is
The present invention relates to an optical switch in which the phase shift between a polarized light component and an S-polarized light component is made substantially equal, and particularly to its electrode structure.

2. Description of the Related Art Conventionally, electro-optic crystal optical waveguides have been used to construct optical modulators including, for example, optical switches.

Figure 1 shows its schematic configuration, where 1 is an electro-optic crystal thin plate;
Reference numerals 2 and 2' denote electrodes provided on, for example, the upper and lower surfaces of the thin plate, 3 a power source, 4 a polarizer, and 5 an analyzer. Such an optical modulator changes the refractive index of the plate 1 by applying a voltage V between the electrodes 2 and 2' to control polarization.

For example, in the case of an optical switch, the polarizer 4 polarizes the incident light in a predetermined direction, the thin plate 1 changes the polarization direction, and the analyzer separates the polarized light in a certain direction from the polarized light in another direction that is 9 points away from it. 5 switches the polarized light passing through the thin plate 1. In this case, in order to obtain the desired polarization with the thin plate 1, the thickness d of the thin plate 1 must be reduced to increase the electric field, the distance 1 in the longitudinal direction (light waveguide direction) must be increased, or the power source 3 It is necessary to choose a large voltage V. Generally, this type of element is operated at several thousand volts, but in order to make the device smaller and cheaper, it is desirable that the power supply voltage V be as low as possible. If an attempt is made to obtain the desired polarized light by reducing the power supply voltage, it is necessary to select the thickness d to be, for example, 60° and the distance 1 to be approximately 2°, resulting in a very large value 1/d. Under such conditions, if the light beam L is incident at right angles to the end surface la of the thin plate 1, the beam diameter in the waveguide process tends to expand in inverse proportion to the incident angle of the light beam, so it is inevitable that The light collides with both the upper and lower surfaces of the thin plate 1, is reflected there, and this process is repeated. Then, upon this reflection, a difference occurs in the phase shift between the P-polarized light component and the S-polarized light component contained in the light beam. For this reason, although it is unavoidable to set the thickness d to 4 mm in this device, the width W is made sufficiently large. FIG. 2 shows the phase shift between the incident light and reflected light of the P-polarized light component and the S-polarized light component, with the horizontal axis representing the incident angle. The reflection that occurs inside the book plate 1 has an incident angle of around 90 degrees, but as can be seen from Figure 2, even if the incident angle is about 87 degrees, a single reflection produces a P polarized light component and an S polarized light component. A difference Δ is generated in each phase difference between the two, and the above influence becomes larger as the number of reflections increases. For this reason, in the structure of FIG. 1, there is a limit to choosing the value 1/d to a large value, and in order to increase the amount of phase control, it is necessary to increase the power supply voltage.

However, lowering the voltage is of course important. In order to reduce the voltage, optical integrated circuits have been developed for single-mode light, and some of these circuits can be driven with a voltage of several volts. However, since it is difficult to apply this to the multi-molded light intended by the present invention, it is necessary to reduce the voltage by other methods. The present inventors previously proposed a low voltage drive type optical waveguide device that can be applied to multiple modes.

This is achieved by attaching a multilayered phase matching film to both the upper and lower surfaces of an electro-optic crystal thin plate so that there is no difference in the phase shifts of P waves and S waves. This principle is based on Tokuwaki Sho 55-4077.
Although it is stated in the specification of No. 6, it can be summarized as follows. As shown in Figure 3, a substance with a refractive index n and a refractive index n2
The phase shifts △p and △s due to total internal reflection between the P-polarized light component and the S-polarized light component that are incident on the boundary 6 with the substance (n, >) at an incident angle are expressed as follows. It will be done.

(However, sin.

in2/n. ) As shown in FIG. The phase shift △′ due to the addition of the optical case length during one round trip reflection when the thin film 8 of n, ) is deposited is calculated as follows: △′: object vessel〆now - other n's Si
n◇/Rei} .・・・．・Becomes a thing,
nosjnJ=n. Substituting the relational expression of sin and sorting it out, we get △'=Imaku2侭...[31].

In this case, when approximated by a linear expression with tsuba-o<<1, tan, cos, and sinJ can be regarded as constants. The combination of the phase shift given by the above equation 01 and the phase shift given by the rigid equation etc. is 6p for the P polarization component and 6p for the S polarization component.
Let it be s. When these are w/2, the ratio, n,, n2
, t. Further, the amplitude reflection coefficient rp·rS of the P polarized light component and the S polarized light component is given by: ◇=Gi-8 (8<<1) When considering up to the first order of 8, the {41st equation becomes.

Note that the above kp and ks are constants determined by no/n. The above describes the basic coefficients for one round-trip reflection with respect to the reflection shown in FIG. 4, but in reality, reflection is repeated multiple times within the thin film 7 as shown.

Therefore, we should consider the total internal reflection light obtained based on these multiple reflections, and the amplitude of the incident light should be
1'', the amplitude L of the totally reflected light is the tangent of the phase angle of the amplitude value (complex number) L of the totally reflected light given by L=r133goskin'6' for P polarized light and S polarized light respectively. (ratio of real part to imaginary part), L
= Koju (10 r2 to os60i (11 butchi in610 r
Since 20 cos 6, kasin 6 ...[7} ne n sa = bi a-・) (1 - cos 6), and if 8《1, then ka 7
Type 1 is Matsu nf=C poison raider-8 ------'8}.

kp corresponding to P polarization component and ks corresponding to S polarization component
If we introduce the above-mentioned Shoto tanf corresponding to both polarization components and find the condition that the phase shifts are equal, then
in6 graduate 1-cos8p) ...[91k
S-sin6p (1-cos6S).

Here, the left side is determined by no/n, and the right side is determined by the ratio/n,, reno n,, n, t, so select Q and t and
9} can be satisfied. The following may be considered as an example. Hizo 2,2・nl ni 2,6 n2 ni 2
, 1i ^ ni 1, 3〆, d = 3615A.

It will be like this.

FIG. 5 shows an optical waveguide device based on the above-mentioned principle.

Electrodes 2 and 2' are provided on the outermost layer, and the device operates as an optical switch by changing the voltage V applied to these electrodes. Even if the above-mentioned value 1/d is increased, this optical switch can still
Since no phase shift occurs between the polarized light and the polarized light (if a phase shift occurs, various problems will occur in terms of characteristics), the power supply voltage V can be lowered. For example, d=50 Buddhist kites, 1=1 enemy, V=
20-30V is possible. Figure 6 shows the 2 shown in Figure 5.
The gap between the phase matching films 7 and 8 (5<~<n,) having a layered structure is
It is an optical switch that solves the problem (wavelength dependence), and
Phase matching films 7 to 9 having a three-layer structure with a relationship of ~>n,,n3
has. By the way, since the optical switches shown in FIGS. 5 and 6 both have a structure in which the metal electrodes 2 and 2' are attached to the outermost layer, if the insulation resistance of any or all of the phase matching films 7 to 9 is high, the thin plate 1 The disadvantage is that a sufficient electric field is not applied to the

For example, in the structure shown in FIG. 6, the first layer 7 and the third layer 9 are silicon oxide films (Si02), and their insulation resistance is several orders of magnitude higher than that of the thin plate 1. Therefore, due to the capacitive effect, the thin plate 1 Although an electric field is generated within the battery, as time passes (charging progresses), the electric field becomes weaker (drift occurs). The present invention attempts to solve this problem from the aspect of electrode construction, and its feature is that it uses an electro-optic crystal thin plate whose length in the optical waveguide direction is larger than its thickness, and the end face of the thin plate A light beam is introduced from the thin plate, electrodes are arranged on both opposing surfaces of the thin plate, and a multilayer phase matching film made of a transparent material is coated on both surfaces.
In an optical switch in which the phase shifts of the P-polarized light component and the S-polarized light component of the light beam traveling while being repeatedly reflected on both surfaces of the thin plate and the matching film are substantially equal, at least one of the phase matching films of the multilayer structure One layer is made of a transparent conductive material and serves also as the electrode, and no insulating material layer is interposed between the phase matching film serving as the electrode and the thin plate.

This will be explained in detail below with reference to the drawings. FIG. 7 is a cross-sectional structural diagram showing one embodiment of the present invention, taking as an example an optical switch having a three-layer phase matching film.

Similar to FIG. 6, there are differences between the refractive index n, ~ ratio of the three-layer phase matching films 7 to 9 and the refractive index no of the electro-optic crystal thin plate 1.
~>n, Although there is a negative relationship, the first layer phase matching film 7 is made of a transparent material with good conductivity, such as ln203 (or Sn203).
The difference is that 02) is used.

Since ln203 is n, = 2.0, the thin plate 1 is Bi,
For ZnTe, n = 2.4 (n = 3.0 for ZnTe)
), when the second layer 8 is SiH, n2=3. The third layer 9 is S
In the case of i02, n3=1.45, and the above relationship n2>n
o>n, , n3 is satisfied. Moreover, since the first layers 7, 7 can be used as electrodes due to the Sn02 good conductivity electrode, the voltage V is directly applied to the thin plate 1, and therefore the electric field generated within the thin plate 1 is maintained at a constant high electric field. , stable switching characteristics can be obtained. In addition, in the structure of FIG. 7, as an optical waveguide device, the phase matching film 7 of the first layer is n,=1.4.
5 Si02 (Fig. 6) to n. =2.0 ln203
Therefore, it is necessary to correct the phase shift of the P-polarized light component and the S-polarized light component to make them equal, but this is done by adjusting the film thickness d of layer 7 and the film thickness of layer 8. be able to. As an example, in the case of a wavelength 1.3 wavelength device, the element shown in FIG. 7 is set to d=2600A and ra=1100A. Note that the ratio of SiH in the second layer 8 can be adjusted depending on the amount added. The thin plate 1 has 132 arcs and d and 60 arcs. In the above embodiment, the first layer 7 is a phase matching film that also serves as an electrode, but it may be a layer below the second layer, thereby increasing the degree of freedom in design.

However, in this case, a layer such as SiQ must not be interposed between the phase matching film which also serves as an electrode and the thin plate 1. Therefore, for example, when the second layer 8 is used as a layer that also serves as an electrode, the first layer 7 must be formed of a material with low electrical resistance within a range that satisfies the above relational expression. In either case, it is of course possible to use Si02 for the outer layer of the phase matching film that also serves as an electrode (for example, the third layer 9 in FIG. 7). As described above, the present invention has the advantage that an optical switch can be driven at a low voltage and stable operation is expected without a drop in electric field over time due to drift.

[Brief explanation of the drawing]

Fig. 1 is a schematic configuration diagram showing an example of an optical modulator, Fig. 2 is an explanatory diagram of the phase shift between the P-polarized light component and the S-polarized light component that occurs during reflection, and Fig. 3 is an illustration of the phase shift between the P-polarized light component and the S-polarized light component that occurs at the time of reflection. An explanatory diagram of the phase shift that occurs between the P-polarized light component and the S-polarized light component due to total reflection,
Figure 4 is an explanatory diagram of the configuration of the reflecting section to prevent phase shift, Figure 5
6 and 6 are cross-sectional structural diagrams of an optical switch having a multilayered phase matching film, and FIG. 7 is a cross-sectional structural diagram showing one embodiment of the present invention. In the figure, 1 is an electro-optic crystal thin plate, and 7 to 9 are first to third layer phase matching films. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7

Claims (1)

[Claims] 1. Using an electro-optic crystal thin plate whose length in the optical waveguide direction is larger than its thickness, a light beam is introduced from the end face of the thin plate,
Electrodes are arranged on both opposing surfaces of the thin plate, and a multilayer phase matching film made of a transparent material is coated on both surfaces, and reflection is repeated on both surfaces of the thin plate and the matching film. In the optical switch in which the phase shifts of the P-polarized light component and the S-polarized light component of the light beam traveling while being made substantially equal, at least one layer of the phase matching film of the multilayer structure is made of a transparent conductive material and is also used as the electrode. An optical switch using an electro-optic crystal, characterized in that no insulating material layer is interposed between the phase matching film serving as the electrode and the thin plate.