JPH02204728A - Polarization independent optical switch - Google Patents

Abstract

PURPOSE:To offer a polarization independent optical switch obtained by loose production conditions and accuracy and easy design by forming a branch interference type optical modulation part and at least one asymmetrical Y branch part on a base and forming a static phase control part independent of an impressed electric field on the optical modulation part. CONSTITUTION:The optical switch is provided with a reference mode optical waveguide 6 to be an input side, the branch interference type optical modulation part 3 and the asymmetrical Y branch part 4 to be an output side and the modulation part 3 is provided with the static phase control part 5 for applying a static phase change to in-waveguide light. The Y branch 4 separates reference mode light and primary mode light generated on an X intersecting point O respectively into a thick optical waveguide and a thin optical waveguide. The optical modulation part 3 allows the optical waves divided into two optical waveguides to interfare with each other at the intersecting point O to generate the reference mode light and the primary mode light. The control part 5 applies a fixed phase difference to TE mode light and TM mode light independently of an opto-electric effect and the polarization independent optical switch can be obtained by controlling the phase difference to a proper value.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to optical signal processing devices for optical communication and optical measurement, and in particular, to optical signal processing devices for optical communication and optical measurement, and in particular for processing optical signals transmitted through a single mode optical fiber depending on its polarization. The present invention relates to a waveguide type optical device that performs switching without switching.

(Summary of the Invention) The present invention relates to a device that switches light transmitted through a single mode optical fiber as an optical signal, and includes a branching interference type optical modulator on a substrate such as lithium niobate or lithium tantalate. and at least one asymmetrical Y-branching section, and further includes a static phase control section that does not depend on an applied electric field in the optical modulation section.

This makes it possible to realize a polarization-independent optical switch that switches the optical path of light having an arbitrary polarization plane, and to operate it at low voltage.

(Prior Art) Conventionally, this type of optical switch uses LiNb0. A crystal is used as a substrate, and an electric field is applied in the Z-axis direction of the crystal. In this case LiNb0. Direct coupling between a conventional optical switch and a standard single-mode optical fiber has not been possible because the refractive index differs depending on the TE mode light and the 7M mode light propagating through the waveguide on the substrate. For this reason, research on polarization-independent optical switches that do not depend on polarization has recently become active. The conventional technology will be explained below.

Conventional optical switches include an optical directional coupling type optical switch (a) in which two optical waveguides are close to each other as shown in Figure 2, and an asymmetric optical switch (a) in which the optical waveguides have at least one asymmetric seven-branch path with different equivalent refractive indexes. There are a 7-minute drum type optical switch (b) and a total internal reflection type optical switch (c). Among them, optical directional coupling type optical switches are mainstream.

The optical directional coupling type optical switch uses the coupling of the leakage electric field of the guided light propagating in one optical waveguide 1a or lb to the other optical waveguide 1b.
Alternatively, light energy is transferred to 1a. However, in order to achieve a completely loss-free state, advanced manufacturing technology is required. The asymmetric 7-minute drum-shaped optical switch utilizes the principle of separating the fundamental mode light and the first mode light by changing the equivalent refractive index of the two optical waveguides on the output side, and is extremely easy to design and manufacture. It has the characteristic of being. Further, although the total internal reflection type optical switch has an extremely simple structure, it is difficult to induce a high refraction portion at the intersection of the optical waveguides by an applied electric field.

Many of these optical switches are so-called polarization-dependent optical switches that can utilize only specific linearly polarized light. On the other hand, conventional examples of polarization-independent optical switches are limited to only a few examples, most of which are optical directional coupling type optical switches, such as the following documents (i),
There is (ii). Further, there is a document (iii) regarding an asymmetric 7-minute drum-shaped optical switch, and these polarization-independent optical switches are all made of LiNbO3 crystal and apply an electric field in the Z-axis direction of the crystal.

Koko bibliography (i) K, niondo, Y, 0hta, Y,
Tanisawa, T., Aoyama.

R, l5hikana: Low-Kanoive-Vol
Lage and Low-LossPolariza
tion4ndependent LiNb0:+O
pticalWaveguide 5w1tches(
Polarization independent 1x2 optical switch), Electro
nLett, Vol. 23. Na21. P,
1167, 1987 (ii) J, E, Wats
on, M., A., Milbrodt and T.
C.

R3ce: A Polarization-Ind
ependent I X16 GuidedWave
0ptical 5w1tch Integrate
d on LiNb0° (polarization independent 1×16 optical switch), Journal of Lightwave
Technology, Vol, LT-4, Na
1l, p.

1717, Nov. 1986 (iii) Y., Silberberg, P.
, Perlmutter and J.E.

Baran: Digital optical
5w1tch (digital optical switch), Appl
, Phys, Lett, 51(16), 19.
P.

1230, Oct. 1987 (Problems to be solved by the invention) It is difficult to realize a complete loss state in a directionally coupled polarization-independent optical switch, and the shape of the optical waveguide (phase constant)
It is necessary to match three parameters with high precision: , bond strength, and bond length, which requires strict manufacturing conditions and high manufacturing accuracy. Furthermore, the characteristics change due to differences in the wavelength of light and temperature. Literature (i), (ii
) In the directionally coupled polarization-independent optical switch described in
Both have drawbacks such as severe manufacturing conditions and the need for a high operating voltage of 30 to 70V. Further, an asymmetric Y-branch type polarization-independent optical switch described in document (iii) has been studied, but its major drawback is that the operating voltage is high.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned problems and provide an asymmetric Y-branch type polarization-independent optical switch that does not require strict manufacturing conditions or precision and is easy to design.

(Means for solving the problem) In order to achieve this object, the polarization-independent optical switch of the present invention is an optical switch that switches the optical path of light having an arbitrary plane of polarization. The asymmetric Y-branch is an asymmetric seven-branch path in which the two Y-branched optical waveguides have different equipolarized refractive indexes, and the asymmetric Y-branch is an asymmetric seven-branch path with different equipolarized refractive indexes. The switch further comprises means for controlling the optical modulation operating point by having a static phase control section independent of an applied electric field in one or both of the optical waveguides of the branching interference type optical modulation section. It is something.

(Function) In other words, the conventional optical switch configuration does not have a static phase control unit and performs phase control only by electro-optic effect, making it difficult to realize a polarization-independent optical switch. In the configuration of the present invention, a static phase control section is provided to impart a static phase change to the guided light without depending on the electro-optic effect. In this way, it has become easy to perform optical switching without depending on polarization.

Furthermore, according to the optical switch of the present invention, a polarization-independent optical switch and other devices can be easily configured on the same substrate, and the voltage can be reduced, making it a key device for future optical fiber communications and optical information processing. It can be.

(Embodiments) The present invention will be described in detail below by way of embodiments with reference to the accompanying drawings, but prior to explaining the embodiments, reference will be made to the operating principle of the optical switch of the present invention.

FIG. 1 shows the basic configuration of the optical switch of the present invention.

The elements shown in FIG. 3(a) include a fundamental mode optical waveguide 6 on the input side, a branching interference type optical modulator 3, and an asymmetric 7 on the output side.
The branch path 4 and the modulation section are provided with a static phase control section 5 that gives a static phase change to the guided light. The element shown in FIG. 2(b) has an asymmetric 7 input optical waveguide in FIG.
This is a configuration in which the road is replaced with a branch road.

Asymmetric 7 branch road 4 is the basic and 1 that occurred at the X intersection 0.
This is to separate the next mode light into a thick optical waveguide and a thin optical waveguide, respectively, and the branching interference type optical modulator 3 causes the light waves split into the two optical waveguides to interfere at the intersection O, and the fundamental and first mode light It is intended to generate. Further, the static phase control section 5 provides a constant phase difference of 7 between the TE mode light and the 7M mode light. By controlling this phase difference to an appropriate value, a polarization-independent optical switch can be realized.

The phase change θ of the TE/TM mode light during static phase control and voltage application, θ8, is generally expressed as θE−a+α(1) θ8=b+βV(2). a and b are the phase changes of the TE mode light and 7M mode light due to the static phase control unit, and are the second on the right side of each equation.
The term is a phase change induced in the TE mode light and the 7M mode light due to the electro-optic effect based on the application of voltage (V). α and β are proportionality constants that depend on the electro-optic coefficient. The optical output Pill of one of the optical waveguides constituting the asymmetric seven-branch path of the optical device of the present invention is P, where P is the input light intensity.
o+ = Pisin”(θ/2) (3
) is given by The optical output P6m of the other optical waveguide is Pa2"-7cos" (θ/2) (4)
is given by However, it is θ-θ6 or θ8.

Therefore, the modulation operating point of the optical output versus applied voltage characteristic of this element for TE/TM mode light depends only on the phase changes a and b of the static phase control section.

-For example, in the configuration of FIG. 1(a), LiNb0. The operation of the device according to the present invention will be explained by taking as an example the case where light is propagated along the X-axis of the crystal and an electric field is applied in the Z-axis direction of the crystal. α: β8n113γ23 : no”γ13-2.9
: 1 (5) However, nor n
Since o is given by the ordinary refractive index and the extraordinary refractive index of the LiNbO3 crystal, the optical output versus applied voltage characteristics of the TE mode light and the TM mode light are generally given as shown in FIG. 3(a). Since α and β are in an approximately integer ratio relationship, the static phase control section is appropriately controlled to obtain the TE as shown in Figure 3(b).
, a polarization-independent optical switch can be obtained by controlling a and b so that the minimum points of the intensity of the TM mode light almost coincide with each other.

Moreover, in the device configuration shown in FIG. 1(a), LiNb0
In an element that propagates light in the Z-axis direction of a z-crystal and applies an electric field in the Y-axis direction of the crystal, the proportionality constants α and β include electro-optic coefficients that are equal in magnitude and differ only in sign. β=-α, and the optical output vs. applied voltage characteristics are shown in Figure 4 (a
), the periods match. Now, by controlling the static phase control section appropriately, as shown in Fig. 4(b), a+b
If a phase difference of -±2Nπ (N is an integer) is given, a polarization-independent optical switch is obtained. A feature of this device configuration is that each output of the seven asymmetrical branches can be continuously changed by the applied electric field.

Next, a specific method of operating the static phase control unit that provides these phase differences will be described, for example, as follows.

■ Changing the length of one or both of the two optical waveguides that make up the branching interference optical modulator.

(2) Changing the width of part of one or both of the two optical waveguides that constitute the branching interference type optical modulator.

(2) Changing the thickness of the Ti film forming part of one or both of the two optical waveguides constituting the branching interference optical modulator.

(2) A transparent insulating material having a refractive index smaller than the refractive index of the optical waveguide is loaded onto a part of one or both of the two optical waveguides that constitute the branching interference type optical modulator.

There are other means such as

The means (1) to (4) can generally provide an appropriate amount of phase change to the TE/TM mode light.

The means (2) can particularly give a large phase change to TM mode light. Note that by combining the means from (1) to (2), it is possible to realize a polarization-independent optical switch operating at zero bias. For example, the first
In the configuration shown in Figure (a), in an element that propagates light in the Z-axis direction of the LiNbOi crystal and applies an electric field in the Y-axis direction of the crystal, a+b=±2Nπ and a=π/2±2Mπ (M is an integer ) can be obtained by appropriately controlling the static phase control section so as to satisfy both conditions.

Furthermore, if the device shown in FIG. 1(a) is configured to input light from an asymmetric branch path and output light from a single fundamental mode optical waveguide, it can be used as an optical switch with two manual outputs and one output. That is, in the element having the characteristics shown in FIG. 3(b), only the light propagating through one optical waveguide of the asymmetric branch can be extracted by applying an appropriate voltage.

Furthermore, the element with the configuration shown in Figure 1(b) exhibits the same function as the element in Figure 1(a) when TE/TM mode light is input to only one optical waveguide of the asymmetric branch path on the input side. can do. Furthermore, by inputting TE/TM mode light to both of the asymmetric branch paths at the input end, it is possible to configure a two-manpower/two-output matrix optical switch.

Hereinafter, embodiments of the present invention will be sequentially explained.

Actual 1” [L In this example, Li, NbO:+ crystal is used as the substrate material,
A polarization-independent optical switch according to the present invention constructed on a so-called Z-axis propagation L I N b 03 element that propagates light in the Y-axis direction of the crystal and applies an electric field in the Y-axis direction will be described.

The static phase manipulation means adopted here was a method of loading an insulating material as shown in the above-mentioned means (2), and photoresist (refractive index n=1.64) was used as the material. The configuration at this time is shown in FIG. A cladding layer 8 is formed of photoresist.

Figure 6 shows the static phase difference (Δφ
= a + b), where the straight line 9 connects to the lower optical waveguide 1a of the two optical waveguides that constitute the branching interference type optical modulator shown in Fig. 5, and the straight line 10 connects to the upper optical waveguide 1b. These are the characteristics when loaded with photoresist. Both change almost linearly. There is a static phase difference (approximately 0.7π) even when no photoresist is loaded, but
This is the two optical waveguides 1 that make up the branching interference type optical modulator.
This is due to the difference in branch lengths of a, ], and b, and the above method ■
This corresponds to performing the operation in advance. In any case, by appropriately selecting the photoresist length, a polarization-independent optical switch can be constructed. Phase difference Aφ=0 or A=6
mm (in FIG. 5, a photoresist having a length of 6 cylinders is loaded on the upper optical waveguide 1b of the two branch paths), a polarization-independent optical switch is created.

Figure 7 shows the respective outputs (a), (
b) is the optical output versus applied voltage characteristic. It shows almost the same characteristics for both TE and TM mode light. That is, it operates as a polarization-independent optical switch that does not depend on polarization. The extinction ratio when circularly polarized light including both TE/TM mode light is incident is as high as 20 dB or more, and the operating voltage is as low as ±7.

As described above, according to the present invention, it is possible to configure an optical switch and a matrix optical switch that can be directly coupled to a single-mode optical fiber very easily and are indispensable for future optical fiber communications.

Fig. 8(a) shows that in the configuration of the present invention, in particular, the lengths (!1 and pz) of the two optical waveguides constituting the branching interference type optical modulation unit as the static phase control unit are changed. This is an example of a polarization-independent optical switch realized by this method.

Implementation ■Main Figure 8(b) is realized by changing the Ti film thickness D, tz) forming the optical waveguide of the branching interference type optical modulation unit as the static phase control unit in the configuration of the present invention. This is an example of a polarization-independent optical switch.

Figure 8 (c) shows that in the configuration of the present invention, in particular, by changing the widths (wl, wz) of the two optical waveguides constituting the branching interferometric optical modulator as the static phase control unit. This is an example of a realized polarization-independent optical switch.

Actual image Ll FIG. 8(d) shows the length of the two optical waveguides constituting the branching interference type optical modulator ( This is an example of a polarization-independent optical switch realized by loading an insulating material 9 on a part of the optical waveguide (I!, I and 2□) and the optical waveguide.

(Effects of the Invention) As described above in detail, the present invention has the following advantages.

First, since the optical switch according to the present invention is not an optical directional coupler but an asymmetrical Y-branch type optical switch, the manufacturing conditions and precision are not strict, and the design is easy, so it is easy to manufacture. The configuration is simple because the polarization-independent optical switch can be formed by simply providing a static phase control section. If manufacturing conditions are appropriately selected, a polarization-independent optical switch operating at zero bias can be constructed, so no bias voltage is required.

In particular, the so-called Z-axis rotation FML i N b propagates light in the Y-axis direction of LiNbO3 crystal and applies an electric field in the Y-axis direction.
A polarization-independent optical switch constructed on an O3 crystal has little damage to light (there is no outward diffusion of Ti20 when Ti is diffused, and a fundamental mode optical waveguide can be easily formed. Furthermore, it is possible to easily form a fundamental mode optical waveguide. Since each output of the branch path can be changed continuously, analog optical modulation becomes possible.

Also, especially the Z-axis rotation'tiL i N b 0! The device is made of LiNb0. Phase matching is easy because the structure avoids the effects of natural birefringence of the crystal, and a TE/TM mode optical separator can be formed on the same substrate with other optical waveguide devices by simply providing a static phase control section. This makes it possible to realize future high-performance devices (integration of elements with different functions is possible).

[Brief explanation of the drawing]

FIG. 1 shows a basic configuration diagram of the optical switch of the present invention, FIG. 2 shows various configuration diagrams of conventional optical switches, and FIGS. 3 and 4 show optical output vs. application of the optical switch element of the present invention. FIG. 5 shows an embodiment of the optical switch of the present invention, and FIGS. 6 and 7 show the photoresist length versus static position in the fifth illustrated embodiment. The phase difference characteristics and the optical output versus applied voltage characteristics are shown, respectively, and FIG. 8 shows several embodiments of the optical switch of the present invention. 1. la, Ib... optical waveguide 2... electrode 3.
・Branching interferometric optical modulator 4 ・Asymmetrical 7 branch paths 5 ・Static phase controller 6
... Input light 7.7a, 7b ... Output light 8 ... Cladding layer 11 ... Loaded insulating material patent applicant Japan Broadcasting Co., Ltd. Kintei 1 Fig. 3 O+Ah 廿fF applied voltage characteristics (b) 2λh2f1717 (b) Characteristic voltage of the main t switch depending on the intensity of the damage - Pressure of the light of ■ (v) Pressure of the light of the family (V) Figure 7 1. b> 7b length and f pressure (V)

Claims (1)

[Claims] 1. An optical switch that switches the optical path of light having an arbitrary plane of polarization, the optical switch comprising a branching interference type optical modulator and at least one asymmetric Y-branch as its optical waveguide configuration. , the asymmetric Y-branch is an asymmetric Y-branch where the two Y-branched optical waveguides have different equivalent refractive indexes, and the optical switch further applies a voltage to one or both of the optical waveguides of the branching interferometric optical modulator. 1. A polarization-independent optical switch, comprising means for controlling an optical modulation operating point by having a static phase control section that does not depend on an electric field. 2. The optical switch according to claim 1, wherein the static phase control section is a control section that changes the length of two optical waveguides constituting the branching interference type optical modulation section. light switch. 3. The optical switch according to claim 1, wherein the static phase control section is a control section that changes the equivalent refractive index of one or both of the two optical waveguides of the branching interferometric optical modulation section. Features: Polarization-independent optical switch. 4. The optical switch according to claim 3, wherein the static phase control section has a refractive index smaller than the refractive index of the optical waveguide of the interferometric optical modulation section, and a transparent insulating material is loaded on the optical waveguide. 1. A polarization-independent optical switch characterized by being a control unit for controlling polarization. 5. The polarization-independent optical switch according to claim 3, wherein the static phase control section is a control section that changes the width of the optical waveguide of the interferometric optical modulation section. 6. The optical switch according to claim 3, wherein the static phase control section is a control section that changes the thickness of a titanium film forming an optical waveguide of the branching interference type optical modulation section. switch. 7. The substrate material of the optical switch according to any one of claims 1 to 6 is lithium niobate or lithium tantalate, which propagates light in the Z-axis direction of the crystal and applies an electric field in the Y-axis direction. polarization-independent optical switch. 8. The substrate material of the optical switch according to any one of claims 1 to 6 is lithium niobate or lithium tantalate, which propagates light in the X-axis direction of the crystal and applies an electric field in the Z-axis direction. polarization-independent optical switch.