JPH0228852B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a waveguide optical switch used in the fields of optical transmission and optical information processing.

As a waveguide optical switch using total internal reflection
Many examples using LiNbO ₃ have been reported. Figure 1 shows the structure of a typical optical switch. In FIG. 1, 1 is a LiNbO ₃ substrate, 2 is a waveguide, 3 is an electrode, 4 is a power source, 5 is incident light, and 6-1 and 6-2 are non-switch output light and switch output light, respectively. The waveguide 2 is divided into three parts, and the equivalent refractive index of waveguides 2-1 and 2-3 is N ₁ and N ₃ (=
NA) is formed to be larger than the equivalent refractive index N ₂ of the waveguide 2-2. When a voltage is applied to the electrode 3 from the power source 4, the equivalent refractive index N ₂ of the waveguide 2-2 increases to N ₂ +ΔN ₂ due to the electro-optic effect.

The angle between the incident light 5 and the electrode 3 is defined as an incident angle θ. The total reflection angle θ _c (OFF) when no voltage is applied is θ _c
(OFF) = cos ^-1 (N2/N1). The total reflection angle θ _c (ON) when voltage is applied is θ _c (ON) = cos ^-1 {(N ₂ +
ΔN ₂ )/N ₁ }, so θ _c (OFF) > θ _c (ON)
It is. If the incident angle θ is selected so that θ _c (OFF) > θ > _c (ON), when no voltage is applied, the incident light 5 is totally reflected and becomes non-switched light 6-1, and when a voltage is applied, the incident light 5 is reflected from the waveguide. The light passes through the light 2-2 and becomes the switch light 6-2. In this way, the optical path of the incident light 5 is switched depending on whether or not a voltage is applied.

In the optical switch using LiNbO ₃ , the change in the equivalent refractive index due to voltage application is small, so the switch angle φ between the non-switched output light 6-1 and the switched light 6-2 is small, ranging from several degrees to 12 degrees. For this reason, it is difficult to separate the switch light, making it difficult to manufacture an optical switch. Furthermore, since LiNbO ₃ is a crystalline material, there are restrictions on the size of the substrate, and the small switch angle also makes it difficult to manufacture large-scale matrix switches.

In order to solve these drawbacks, the present invention provides a waveguide type optical switch using liquid crystal, which will be explained in detail below with reference to the drawings.

FIG. 2 shows a first embodiment of the present invention, in which a is a perspective view and b is a plan view. In FIG. 2, 7 is a substrate, 8 is a lower electrode, 9 is a buffer layer, 10 is a waveguide layer, 11 is a liquid crystal layer, and 12 is an upper electrode plate.

The lower electrode 8 includes In ₂ O ₃ , Cr,
A conductive film with Ag etc. deposited on the entire surface is used. A buffer layer 9 is provided to eliminate the influence of light absorption caused by the conductivity of the lower electrode 8. Upper electrode plate 1
A conductive film similar to that of the lower electrode 8 is used on the lower surface of the electrode 2, but a diagonally patterned pattern electrode 13 is provided as shown in the shaded area in FIG.

In order to define the molecular orientation of the liquid crystal layer 11 when no voltage is applied between the patterned electrode 13 and the lower electrode 8, parallel alignment is provided on the surface of the waveguide layer 10 and the lower surface of the upper electrode plate 12 to orient the liquid crystal molecules. It has been processed. As the parallel alignment treatment method, a rubbing method or an oblique vapor deposition method that is commonly used in the manufacture of liquid crystal displays can be used. By such parallel alignment processing, the long axes of liquid crystal molecules are aligned in the x-axis direction when no voltage is applied. When a voltage is applied, as the voltage increases, the liquid crystal molecules under the pattern electrode 13 begin to rotate in the Z-axis direction with the threshold value as a boundary, and when a sufficient voltage is applied, the long axes of the liquid crystal molecules align in the Z-axis direction. As the voltage to be applied, a direct current voltage or an alternating voltage of about 1 to 10 KHz can be used, but an alternating voltage is preferable because it does not cause deterioration of the electrode. Liquid crystal molecules have different refractive indexes in the long axis and short axis directions,
The refractive index n _Le of the former is larger than the refractive index n _Lp of the latter.

The refractive indices n _b and n _g of the buffer layer 9 and waveguide layer 10 are
Select so that the relationship n _g > (n _b , n _Le ) is maintained. The thicknesses of the buffer layer 9 and the liquid crystal layer 11 are selected to be such that the influence of light absorption by the lower electrode 8 and pattern electrode 13 on propagating light passing through the waveguide layer 10 can be ignored.

The operation will be explained below. FIG. 2b is a top view of FIG. 2a, and the optical path is made easy to understand. A waveguide is defined as a region of the upper electrode plate 12 where the pattern electrode 13 is not provided, and a waveguide is defined as a region where the pattern electrode is attached. When the buffer layer 9 and the liquid crystal layer 11 are made thick enough to have no influence from the pattern electrode 13 and the lower electrode 8 as described above, the equivalent refractive index N is determined by the eigenvalue equation of the three-layer waveguide. (The eigenvalue equation is, for example, D.Marcuse;
Theory of Dielectric Optical Waveguide
Derived from Academic Press 1974, pp.1-19. ) tan ^-1 ζ ² ₁ γ/K+tanζ ² ₂ δ/K=KW _g −mπ (1) Here, ζ ₁ = { ¹ ...TE mode (n _g /n _b ) ² ... TM mode ζ ₁ = { ¹ ...TE mode (n _g /n _Ln ) ² ...TM mode K=k _p (n _g ² −N ² ) ^1/2 γ=k _p (n _b ² −N ² ) ^1/2 δ=k _p (n _L ² − N ² ) ^1/2 k _p = 2π/λ, where n _Ln is the refractive index of the liquid crystal layer 11 in TM mode, and n _L is the liquid crystal layer for propagating light in TE mode or TM mode. 11, λ is the wavelength of propagating light, W _g is the thickness of the waveguide layer, and m is the order of the mode.

In this embodiment, the TE mode is selected as the propagating light.
The equivalent refractive index with the waveguide is set to N ₁ (OFF) and N ₂ (OFF), respectively, when no voltage is applied, and N ₁ (ON) and N ₂ (ON), respectively, when voltage is applied. When no voltage is applied, all the liquid crystal molecules in the waveguide are oriented in the x-axis direction, so the refractive index that is effectively sensed by the TE mode is the refractive index in the long axis direction of the liquid crystal.
n _Le , and there is no difference between it and the waveguide, so
N ₁ (OFF) = N ₂ (OFF). As a result, the incident light 14 travels straight from the waveguide to become the non-switched output light 15-1.

On the other hand, when a sufficient voltage is applied, the liquid crystal molecules in the waveguide are aligned in the Z-axis direction, so that the incident light in the TE mode senses a small refractive index n _Lp in the short axis direction of the liquid crystal. Therefore, N ₁ (ON) > N ₂ (ON). The incident angle θ of the incident light 14 is the total reflection angle θ _c =
If it is smaller than cos ^-1 {N ₂ (ON)/N ₁ (ON)},
The incident light 14 is totally reflected at the boundary with the waveguide, that is, at the end of the pattern electrode 13, and becomes the switch output light 15-2. The switch angle φ is 2θ. In this way, when no voltage is applied, the incident light 14 travels straight, and when a voltage is applied, it is totally reflected and the optical path is switched.

In _the optical switch manufactured to confirm _the switch action, a SiO ₂ sputtered film (n _b =
1.46) and SiO ₂ as the waveguide layer 10 on top of it.
- A Ta ₂ O ₅ based sputtered film (n _g = 1.75) is laminated, and Merck's PCH1132 (n _Le =
1.61, n _Lp = 1.48) was sealed between the upper electrode plate 12 having the patterned electrode 13 of the Nesa film and the waveguide layer 10.

When the thickness of the buffer layer 9 was 2 μm and the thickness of the liquid crystal layer 11 was 10 μm, no influence of light absorption by the lower electrode 8 and pattern electrode 13 was observed.

Incidentally, the switch angle φ _n between the non-switch output light 15-1 and the switch output light 15-2 is twice the total reflection angle θ _c . This maximum switch angle φ _n depends on the film thickness W _g of the waveguide layer 10 .

FIG. 3 shows the relationship between the maximum switch angle φ _n and the film thickness W _g calculated using equation (1). As the film thickness W _g becomes thinner, the maximum switch angle φ _n becomes larger. However, as the film thickness W _g decreases, the propagation loss increases, so
In manufacturing the optical switch, the film thickness was set to 0.2 μm. 20V
By applying an alternating voltage of , we were able to switch the incident light to the TE _p mode, which is the fundamental mode. The maximum switch angle was 21 degrees, which showed good agreement with the results shown in Figure 3.

Liquid crystals are originally light scatterers, and when light propagates inside a liquid crystal, the light is significantly attenuated due to large scattering loss of several to 10 dB/cm. When used as a layer, a practical waveguide optical device cannot be realized. Therefore, in the present invention, most of the field distribution of light exists in the waveguide layer with extremely low loss, and only a small part of the tail of the field distribution seeps into the liquid crystal layer. The optical path is switched by sensing changes in the refractive index. Therefore, the loss of guided light is significantly reduced and can be reduced to a practical level of 1 dB/cm or less. Since only the bottom part of the field distribution of the guided light protrudes into the liquid crystal layer, it is not possible to make full use of the large refractive index change of the liquid crystal. An optical path switching angle of the same magnitude can be obtained. In other words, the value of the refractive index change of liquid crystal due to voltage application is approximately 0.1, which is 2% compared to inorganic materials such as LiNbO ₃ .
As a result, in the optical switch using total internal reflection of the present invention, the optical path switching angle is as large as about 20°, which is about 2° compared to the optical path switching angle of a conventional waveguide optical switch using an inorganic material. It's much improved compared to that.

FIG. 4 is an explanatory diagram of the second embodiment of the present invention, and shows an example in which the incident light in the TM mode is switched. The structure of the optical switch and the alignment direction of the liquid crystal molecules are the same as in FIG. 1, but the position of the pattern electrode 13 is reversed. The incident light 14 enters the end of the waveguide at an incident angle θ as shown. When no voltage is applied, liquid crystal molecules are oriented in the x-axis direction, and the refractive index perceived by light in the fundamental mode TM _p mode is n _Lp
Since there is no difference between the waveguide and the waveguide, the equivalent refractive index N ₁ (OFF) and N ₂ (OFF) are equal.
Therefore, the incident light 14 continues straight and becomes the non-switched output light 15-1. On the other hand, when a voltage is applied, the liquid crystal in the waveguide aligns in the Z-axis direction, so
Since the refractive index for the TM _p mode increases to n _Le , the equivalent refractive index becomes N ₁ (ON) < N ₂ (ON).
Therefore, the incident angle θ of the incident light 14 is the total reflection angle θ _c =
If it is smaller than cos ^-1 {N ₁ (ON)/N ₂ (ON)},
The incident light 14 is totally reflected at the end of the waveguide and becomes the switch output light 15-2.

As described above, incident light in the TM mode travels straight when no voltage is applied, and when voltage is applied, the optical path can be switched by total reflection. The maximum switch angle φ _n depends on the waveguide layer thickness W _g as in the first embodiment.

Figure 3 shows the maximum switch angle φ _n for TM _p mode when using the same material as in the first embodiment.
The relationship between the waveguide layer thickness W g and the waveguide layer thickness W _g is shown. In the optical switch manufactured using the same material and film thickness as in the first embodiment, it was possible to switch the incident light in the TM _p mode.

In the above explanation, the orientation direction of the liquid crystal molecules is the x-axis direction, but it is easily understood that the same switching function can be obtained even if the liquid crystal molecules are oriented in the y-axis direction.

FIG. 5 is a diagram for explaining the third embodiment of the present invention, and is a plan view of a 2×3 matrix optical switch. I ₁ and I ₂ are input ports, and O ₁ , O ₂ and O ₃ are output ports. S ₁₁ to S ₁₃ and S ₂₁ to _{S 23} are switch elements, and P ₁₁ to _{P 13} and P ₂₁ to _{P 23} are switch element drive terminals. Incident light 16-1, 16-2
Switch elements S ₁₁ to S ₁₃ and S ₂₁ to S ₂₃ are arranged as shown at the intersections of the output lights 17-1, 17-2, and 17-3. The incident lights 16-1, 16-2 and the outgoing lights 17-1, 17-2, 17-3 are set to be equal to the switch angle φ. The lower electrode 8 has two conductive stripes parallel to the incident light beams 16-1 and 16-2 and formed to include switch elements S ₁₁ , S ₁₂ , S ₁₃ and S ₂₁ , S ₂₂ , S _23, respectively. It has a membrane.

On the upper electrode plate 12, pattern electrodes 31-- each made of a conductive film having a predetermined shape are provided at the positions of each switch element.
Place 1. The lead wire 13-2 is the lower electrode 8
to prevent the alignment of liquid crystal molecules from being disturbed by the lead wire when voltage is applied. Ground the lower electrode 8 and connect it to the switch element drive terminal.
By selectively applying a voltage to P ₁₁ to _{P 13} and P ₂₁ to _{P 23} , the incident light is totally reflected by a desired switch element, and a desired input port is connected. For example, if a voltage is applied to the switch element drive terminal _P12 , _I1 and _O2 are connected.

The shape of the pattern electrode 13-1 corresponds to the incident light 16-
It differs depending on whether 1 or 16-2 is in TE mode or TM mode. This will be explained using the switch element _S11 as an example.

Figure 6a is for TE mode. The liquid crystal molecules are aligned in the x-axis direction in FIG. In addition, the position of the end of the pattern electrode 13-1 is determined by the incident light 16-1.
This is the position of total reflection at the total reflection end 19-1.
shall be. If the angle formed by the incident light 16-1 and the total reflection end 19-1 is set to 1/2 of the switch angle φ, as explained in the first embodiment, when no voltage is applied, the incident light 16-1 becomes the non-switched emitted light 18, and when a voltage is applied, it becomes the switched emitted light 17.
-It becomes 3.

FIG. 6b shows an example of patterned electrodes for TM mode. The liquid crystal molecules are aligned in the x-axis or y-axis direction as shown in FIG. The angle formed between the total reflection end 19-1 and the incident light 16-1 is set to be 1/2 of the switch angle φ. The input side end 19-2 and the output side end 19-3 of the pattern electrode 13-1 receive the incident light 16-1.
are perpendicular to each of the outgoing lights 17-3, and the optical paths of the incoming lights 16-1 and outgoing lights 17-3 conform to Snell's law by increasing the refractive index of the liquid crystal layer 11 when a voltage is applied. It is kept unchanged by the resulting refraction. Similarly to the second embodiment, when no voltage is applied, the incident light 16-1 changes to the non-switched output light 1.
8, and when a voltage is applied, the output light becomes switched 17-3. A 2×3 matrix optical switch was manufactured using the same materials as in the first embodiment, and
We confirmed the switch function in both TE and TM modes.

As explained above, the waveguide type optical switch of the present invention has a structure in which a liquid crystal layer is laminated on a waveguide layer, so scattering loss due to the liquid crystal can be greatly suppressed, and the total reflection effect of the optical waveguide and the large refraction of the liquid crystal can be reduced. Since it is a waveguide type optical switch that utilizes rate change, it has the advantage of being able to obtain a large switch angle of 20° or more with low loss.
The optical switch can be downsized. Therefore, there is an advantage that a large-scale optical matrix switch can be easily constructed. Also, since it uses a liquid crystal, 20V
The advantage is that the optical switch can be driven with low voltage and low power of several microwatts or less.

Furthermore, the waveguide optical switch of the present invention is different from the conventional one.
There is no need to use crystalline materials that are subject to size restrictions, such as LiNbO ₃ optical switches, and optical switches can be manufactured using large substrates made of amorphous materials such as glass. It has the advantage of being able to manufacture switches as well as large-scale matrix optical switches.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing the structure of a typical conventional optical switch, Fig. 2a is a perspective view showing the structure of the first embodiment of the present invention, Fig. 2b is a plan view of Fig. 2a, FIG. 3 is a diagram showing the relationship between the waveguide layer thickness W _g and the maximum switch angle φ _n , FIG. 4 is a plan view showing the structure of the second embodiment of the present invention, and FIG. An explanatory diagram of the third embodiment, FIG. 6a is a diagram showing an example of a pattern electrode of a switch element for TE mode, and FIG. 6b is a diagram showing an example of a pattern electrode of a switch element for TE mode.
FIG. 3 is a diagram showing an example of a pattern electrode of a switch element for TE mode. 1... LiNbO ₃ substrate, 2... Waveguide, 3... Electrode, 4... Power supply, 5... Incident light, 6-1... Non-switch output light, 6-2... Switch output light, 7...
... Substrate, 8 ... Lower electrode, 9 ... Buffer layer, 1
0... Waveguide layer, 11... Liquid crystal layer, 12... Upper electrode plate, 13... Pattern electrode, 14... Incident light,
15-1... Non-switch output light, 15-2... Switch output light, 16-1, 16-2... Incident light,
17-1, 17-2, 17-3... Outgoing light, 18
... Non-switch output light, 19-1 ... Total reflection end, 19-2 ... Incident side end, 19-3 ... Output side end.

Claims (1)

[Claims] 1. A lower electrode made of a uniform conductive film, a buffer layer, a waveguide layer having a higher refractive index than the buffer layer, and a liquid crystal layer having a lower refractive index than the waveguide layer, on a substrate. Then, upper electrode plates having patterned electrodes of a predetermined shape are sequentially laminated, and the molecular orientation of the liquid crystal under the patterned electrodes of the upper electrode plate is changed by applying a voltage, and the guided light is directed to the edge of the patterned electrode. A waveguide optical switch that switches the optical path by total reflection at the position. 2. On a substrate, a lower electrode consisting of a plurality of striped conductive films equal to the number of incident ports, a buffer layer, a waveguide layer having a higher refractive index than the buffer layer, and a liquid crystal layer having a lower refractive index than the buffer layer. and,
An upper electrode plate having a plurality of patterned electrodes of a predetermined shape arranged at a position facing the striped conductive film of the lower electrode is sequentially laminated, and a voltage is applied to the desired patterned electrode of the upper electrode to remove the pattern. A waveguide type optical switch characterized in that the molecular orientation of liquid crystal under the electrode is changed, and the guided light incident on the patterned electrode is totally reflected at the end of the patterned electrode to switch the optical path.